This article was downloaded by: [University of Western Ontario] On: 12 November 2014, At: 00:02 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK
Environmental Technology Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tent20
Stabilization treatment of the heavy metals in fly ash from municipal solid waste incineration using diisopropyl dithiophosphate potassium Ying Xu
a b
b
, Yu Chen & Yueyang Feng
b
a
Key Laboratory of Integrated Regulation and Resource Development of Shallow Lakes of Ministry of Education , Hohai University , Nanjing , China b
College of Environmental Engineering , Hohai University , Nanjing , China Accepted author version posted online: 26 Nov 2012.Published online: 04 Jan 2013.
To cite this article: Ying Xu , Yu Chen & Yueyang Feng (2013) Stabilization treatment of the heavy metals in fly ash from municipal solid waste incineration using diisopropyl dithiophosphate potassium, Environmental Technology, 34:11, 1411-1419, DOI: 10.1080/09593330.2012.752871 To link to this article: http://dx.doi.org/10.1080/09593330.2012.752871
PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http:// www.tandfonline.com/page/terms-and-conditions
Environmental Technology, 2013 Vol. 34, No. 11, 1411–1419, http://dx.doi.org/10.1080/09593330.2012.752871
Stabilization treatment of the heavy metals in fly ash from municipal solid waste incineration using diisopropyl dithiophosphate potassium Ying Xua,b∗ Yu Chenb and Yueyang Fengb a Key
Laboratory of Integrated Regulation and Resource Development of Shallow Lakes of Ministry of Education, Hohai University, Nanjing, China; b College of Environmental Engineering, Hohai University, Nanjing, China
Downloaded by [University of Western Ontario] at 00:03 12 November 2014
(Received 25 July 2011; final version received 19 November 2012 ) A stabilization treatment was developed for heavy metals in fly ash from municipal solid waste incineration using the heavy metal chelator diisopropyl dithiophosphate potassium (DDP). The mechanism and effect of the DDP chelator treatment on heavy metals in the fly ash was also studied, along with the form transformation rules of the heavy metals after DDP chelator treatment. The results show that 1% DDP achieves a stabilization rate of over 95% for Pb, Zn, and Cd. The effect of DDP was better than that of inorganic stabilizers such as sodium sulphide and lime. The heavy metal concentrations in the leachate after the treatment were lower than those required by the Pollution Control Standards for Hazardous Waste Landfill (GB18598-2001). At pH 1–13, the heavy metal concentrations in the fly ash leachate were far lower than those using the inorganic stabilizers sodium sulphide and lime. DDP retains its stabilizing effect under a broader pH range. After stabilization treatment, the heavy metals in the exchangeable fraction and those bound to carbonates were mainly transformed into those bound to organic matter. This process decreases the unstable content and reduces the risk of secondary pollution of the stabilized products in the environment. Keywords: chelator; stabilization treatment; fly ash; heavy metal; heavy metal fraction
Introduction Economic development, the advanced pace of urbanization, and improvement in the material life of people have increased the amount of garbage produced by cities in China. Now, more than 150,000,000 tons of urban garbage is produced annually, with a growth rate of 8–10%, and more than 200 cities (or 30% of the nation) are surrounded by garbage [1–3]. Although incineration is an important means of garbage disposal, it produces large amounts of fly ash. The amount of fly ash produced, usually about 3–5% of the total waste, is related to the type of waste, the incineration conditions, the incinerator type, and the treatment technique. Analysis shows that fly ash is not a chemically inert material. This substance contains numerous harmful metals and salts, such as Cd, Pb, Zn, and Cr, which are likely to leach into water sources under specific situations. Incorrect treatment causes the transformation of heavy metals and pollutes the groundwater, soil, and air [4]. The heavy metals in fly ash do not degrade naturally; thus, they greatly affect the environment. Current global environmental regulations have already identified fly ash as a hazardous waste [5,6]. The search for safe and effective ways of treating the fly ash produced from municipal solid waste incineration is now an urgent social and environmental problem. ∗ Corresponding
author. Email: [email protected]
© 2013 Taylor & Francis
Solidification and stabilization technology is one of the keys in solving the problem of poisonous and hazardous waste. Solidification consists of cement solidification and asphalt solidification. This method is technically mature, easy to operate, and has a low cost. However, the increasing waste capacity ratio of solidification makes subsequent transportation difficult, and the subsequent landfilling results in tremendous land wastage. Moreover, heavy metals may leach out after weathering [7]. Stabilization is mainly achieved using chemical agents, and chemical agents commonly used include sulphides, phosphates, silicates, and lime. Through this method, heavy metals leach out when the surrounding pH changes; hence, stabilization does not satisfy the requirement for long-term security. At present, the reported organic stabilizing chemical agents are mainly dithiocarbamate and its derivatives, organic polyphosphoric acid and its salt chemicals, as well as chitosan derivatives [8,9]. Despite the adequate stabilization effect of these agents, numerous problems such as the high production cost and the lack of raw material resources make this treatment very expensive and less applicable in industry. Therefore, finding easy-to-produce and conveniently obtained raw materials and low-cost chemical agents with sufficient stabilizing effect is a hot research topic in fly-ash treatment.
Downloaded by [University of Western Ontario] at 00:03 12 November 2014
1412
Y. Xu et al.
Beneficiation chelating agents that contain dithiophosphoric acid ligands have been widely used in mineral flotation [10]. These chemicals are thiol collector agents with strong capacities for capturing heavy metals. Lotter and Bradshaw [11] reported the flotation effect of dithiophosphate on bornite, chalcocite, and pyrite; they also studied the effects of a dithiophosphate-mixed flotation agent for improving flotation efficiency and reducing the dosage of flotation agents. Güler et al. [12] used dithiophosphate in the flotation of chalcopyrite and discussed the mechanism of floating chalcopyrite with dithiophosphate. Considering dithiophosphate, chelating agents chelate and precipitate almost all free-state heavy metals in the appropriate pH value range; it is the ideal fly ash stabilizer for waste incineration. Diisopropyl dithiophosphate potassium (DDP) is a dithiophosphate chelating agent, and its performance in stabilizing the heavy metals in incineration fly ash has not been reported. The unit price of DDP chelator used for stabilization is much higher compared with the cement routinely used for solidification. However, to achieve the same stabilization effect on the heavy metals in fly ash, much less DDP than cement is required. According to the literature [13], the amount of cement for solidifying fly ash usually ranges from 25–45% (by mass fraction), whereas the amount of DDP chelating agent required ranges from 0.5–2.0% (by mass fraction). The cost of the reagents is roughly equal based on the two approaches. The process of treating fly ash through cement solidification is the same as stabilizing fly ash with DDP chelator. The difference in cost lies in adding cement or chelating agents; therefore, the total expense of the two methods is also the same. Fly ash develops a high solidification capacity after cement treatment, which leads to increased disposal cost because of the increased landfill area required for disposal. In addition, heavy metals will again percolate from the cement-solidified fly ash in acidic conditions and repollute the environment, and the solidified cement loses its original effect [14]. The DDP chelating agent-treated fly ash remains stable under a wide pH range, which reduces the risk of secondary pollution. Therefore, using DDP to stabilize fly ash is preferable to cement solidification when technical and economic factors are considered. DDP is made from isopropyl alcohol, phosphorus pentasulphide, and potassium carbonate or alkali following a simple process, with easy-to-acquire raw materials and low Table 1.
product costs, and is widely used as a flotation agent in the mineral processing industry. Flotation agents worth tens of billions of dollars are used every year, and dithiophosphates account for a large proportion. Many chemical companies in China produce DDP because its production is already established; thus, the supply is ample. Therefore, DDP can be used to process the heavy metals in the fly ash from municipal solid waste incineration. This paper discusses the mechanism of this method, and investigates the technical means and its stabilization effects. The treatment effect of DDP was compared with common inorganic chemical agents such as lime and sodium sulphide, to examine the stabilization treatment of fly ash. Materials and method Sample of fly ash The incineration-produced fly ash used in this research came from the Suzhou Waste Incineration Power Station in China. This station applies a semidry method and a bag dustcollecting method for disposing of smoke. The treatment capacity of this station is 1000 tons per day. In accordance with the Technical Specifications on Sampling and Sample Preparation from Industry Solid Waste (HJ/T20-1998), 1 kg of fly ash was collected each day for three consecutive days during the stable operation of the incinerator under normal conditions. The sample was evenly mixed, and then set aside. The elemental composition of the fly ash was analysed using X-ray fluorescence. The results show that the main elements were Ca, C1, Na, K and S, as well as heavy metal such as Zn, Pb, Cu, Cr, As, and Cd (Table 1). Determining the amount of heavy metals that leach out of the fly ash The Test Method Standard for Leachate Toxicity of Solid Wastes–Horizontal Vibration Extraction Procedure (GB5086.2-1997) was performed to test the amount of heavy metals that leach out of the fly ash. The method involves the use of deionized water as the leaching agent, mixing it with fly ash at a ratio of 10:1 (L:S). The mixture was maintained for 8 h at 23◦ C ± 2◦ C with shaking at 30 rpm ± 2 rpm, and then allowed to stand for 16 h. The mixture was then filtered with a 0.45 μm filter unit. The heavy metal content was determined through atomic absorption spectrometry.
Elemental composition of the fly ash.
Element Weight (%) Element Weight (%)
Cl
Ca
S
Na
Si
K
Al
Fe
Br
Zn
Sr
Cd
Ni
32.22 P 0.19
35.74 Mg 0.24
2.66 Ti 0.22
3.78 Pb 1.4
1.92 Cu 0.49
3.10 Sn 0.068
0.82 Ba 0.076
0.84 Mn 0.040
0.65 Sb 0.036
0.92 Cr 0.029
0.025 As 0.006
0.19 LOI 14.26
0.08
Note: LOI: loss on ignition.
Downloaded by [University of Western Ontario] at 00:03 12 November 2014
Environmental Technology Fly ash stabilization treatment DDP was tested as a chelator, with the inorganic stabilizing chemical agents sodium sulphide and lime as the chelator controls. DDP, which was purchased from a Chinese chemical material company at $2 per kg, appears as white crystals with a melting point of 110◦ C and water solubility of 100 g l−1 . The method involves dissolving the chemical agent in water to make an aqueous solution, then mixing it with fly ash at a liquid:solid ratio of 16:10 (ml:g). The mixture was thoroughly mixed into a slurry to enable the DDP chelator to react with the heavy metals. The mixture was maintained at room temperature for 24 h and then dried in an oven at 60◦ C for 12 h. The dried sample was used to test the amount of heavy metals in the leachate. The stabilization rate of the heavy metals in the treated fly ash was calculated as follows: Stabilization rate = (C0 − C1 )/C0 × 100%
1413
This method consists of five steps; each step uses different extracting agents. Based on the method, the heavy metals in the fly ash were divided into an exchangeable fraction (F1), bound to carbonates (F2), bound to Fe–Mn oxide (F3), bound to organic matter (F4), and a residual fraction (F5). The HF–HNO3 –HClO4 dissolving method was used to test the amount of Hg, Pb, Cd, Cr, Cu, Zn, Be, Ba, As, and Ni.
Experimental instruments An atomic absorption spectrophotometer TAS-986 (Beijing Purkinje General Instrument Co., Ltd), Fourier transform infrared (FT-IR) spectrometer IRPrestige-21 (Shimadzu Corporation, Japan), and X-ray fluorescence spectrometer ARL-9800 (ARL Corporation, Switzerland) were used in the experiment.
(1)
where C0 is the amount of heavy metals in the leachate of the original fly ash, C1 is the amount of heavy metals in the leachate of the fly ash after treatment. The fly ash before and after stabilization was dried at 105◦ C to a constant weight and weighed. The weight of the DDP chelating agent need not be deducted from that of the fly ash after stabilization, as the amount is so small that it is negligible. The fly ash was stabilized at 60◦ C for 12 h to shorten the time of the experiment. In practice, the stabilized fly ash can be air-dried under natural conditions, and then disposed of in a landfill. The samples obtained using the two methods were subjected to a heavy metal leaching test. The result of the test indicates that the heavy metal concentrations in the leachate were almost unchanged. The stabilized fly ash can then be transported to the landfill site for disposal, if the fly ash meets the Pollution Control Standards for Hazardous Waste Landfill (GB18598-2001) in China. The landfill site has no requirement for the compressive strength of landfill waste; therefore, the addition of cement or other coagulant to solidify the fly ash is unnecessary after DDP chelator treatment. Effects of pH on the amount of heavy metals in the leachate of the stabilized product Leaching agent liquors with pH values 1, 3, 5, 7, 9, 11, and 13 were prepared from HNO3 liquor or NaOH liquor mixed with the treated sample at a liquid:solid ratio of 10:1. The mixture was shaken for 8 h at a rate of 30 rpm ± 2 rpm at a room temperature of 23◦ C ± 2◦ C, then allowed to stand for 16 h. The mixture was filtered with a 0.45 μm filter unit to determine the heavy metal content. Analysis of the fly ash sample The heavy metals in the fly ash were extracted, using the method of Tessier et al. [15], to test their chemical form.
Quality control Parallel determination was performed three times for each experimental condition. The relative standard deviation of the samples was lower than 1%. The standard sample that contained heavy metals was added to the undetermined sample. A recovery experiment was conducted. The main recovery rate of the standard addition was 99.7%.
Results and discussion The amount of heavy metals in the fly ash leachate The Test Method Standard for Leachate Toxicity of Solid Wastes–Horizontal Vibration Extraction Procedure (GB5086.2-1997) was used to test the amount of heavy metals in the leachate of the fly ash sample. Both the Identification Standards for Hazardous Waste–Identification for Extraction Toxicity (GB5085.3-2007) and the Pollution Control Standards for Hazardous Waste Landfill Table 2. samples.
The leachate concentrations of metals in the fly ash
Metal
Leaching-out concentration (mg l−1 )
GB5085.3-2007 (mg l−1 )
GB18598-2001 (mg l−1 )
Pb Cu Zn Cr Cd Ni Hg As Ba Be
38.20 0.48 110.52 0.8910 5.21 0.84 Not detected 0.016 6.82 Not detected
5 100 100 15 1 5 0.1 5.0 100 0.02
5 75 75 12 0.5 15 0.25 2.5 150 0.2
Notes: GB5085.3-2007: Identification standard for hazardous wastes-identification for the extraction toxicity; GB18598-2001: Pollution control standards for hazardous waste landfill.
1414
Y. Xu et al.
Downloaded by [University of Western Ontario] at 00:03 12 November 2014
(GB18598-2001) specify the limits for leachate concentrations for 10 metals (Hg, Pb, Cd, Cr, Cu, Zn, Be, Ba, Ni, and As). The leachate concentrations of 10 metals in the fly ash samples are listed in Table 2. The results (Table 2) show that the concentrations of Cd, Pb, and Zn exceed the requirements in the Identification Standards for Hazardous Waste–Identification for Extraction Toxicity (GB5085.32007). These metals should be treated first through stabilization, then disposed of in a landfill. Only the Zn, Cd, and Pb in the sample fly ash in the experiment exceeded the standards; thus, experimental studies on the stabilization treatment were only conducted on these three metals. Reaction mechanism of the DDP chelator and the heavy metals DDP has sulphur in thione (=S) and thiol (-S-) groups that have a strong electron-donating ability and easily form coordinate bonds with metal ions to produce diisopropyl dithiophosphate, which is difficult to dissolve in water. While integrating with certain metal ions, the two sulphurs in the DDP chelator form a four-member ring with the following reaction formulation [16]: CH3
CH3 H3C HC O
2
+ M2+
P H3C HC O CH3
SK
CH3
H3C HC O
S
P H3C HC O CH3
P
M S
O CH CH3
S
S
S
+ + 2K (2)
O CH CH3 CH3
In this formulation, M2+ is a bivalent metal ion. The DDP chelator integrates with metal ions and becomes a polydentate ligand that then reacts with Cu, Zn, Pb, and Cd to form a chelator with a coordination number of 4. The ligand of the same metal ion chelate is very likely from different DDP chelators. The diisopropyl dithiophosphate molecule may be highly cross-linked and three-dimensional, with a molecular weight of several million Daltons. The DDP chelating agent was first mixed with water to produce an aqueous solution. The solution was mixed with the fly ash and thoroughly stirred until a mud-like consistency was reached. The heavy metals in fly ash dissolve in the water, and a coordination reaction occurs between the heavy metals and the DDP chelating agents to precipitate the chelate adsorbed onto the fly ash. This process stabilizes the heavy metals in the fly ash. The DDP chelator was added into the wastewater with 1000 mg l−1 Pb and the mixture stirred for 10 min, then the precipitate was filtered, cleaned and dried (chelated product). The dried DDP chelator and chelated product were pressed with potassium bromide. Infrared spectroscopic analysis was conducted for each, and the two spectrograms were compared. Figure 1 shows the infrared spectrogram of the DDP chelator and its product. Figure 1(a) shows that DDP has relatively strong absorbance at wave numbers 2981 cm−1 , 981 cm−1 , 796 cm−1 , 703 cm−1 , and 574 cm−1 . Comparison of these values with a standard infrared spectrogram [17]
Figure 1. FT-IR spectra of chelating agent and chelated product. (a) chelating agent; (b) chelated product.
revealed that the absorbance at 2981 cm−1 was caused by the vibration of -CH3 , that at 703 cm−1 was from the vibration of P=S, and that at 574 cm−1 was from the vibration of P-S. In contrast, the absorbance at 981 cm−1 and at 796 cm−1 was caused by the vibration of P-O-C. Comparison with the chelated product infrared spectrogram Figure 1(b) shows the location and intensity of most absorption change, indicating that the thiophosphoric acid functional group reacted with the metal ion. The DDP chelator reacts with heavy metals through the functional groups P=S and P-S to produce deposits. Figure 1(b) shows that the wave numbers of these two functional groups increase from originally 703 cm−1 and 574 cm−1 , respectively, to 790 cm−1 and 629 cm−1 after the reaction, accompanied with the change in absorption intensity of the feature peak. These changes indicate that the lone pair electron in the sulphur of the P-S and P=S functional groups participated in the reaction, forming coordinate bonds with the heavy metals and then the DDP chelator deposit. This process achieved the stabilization. The absorption of the organic metallic coordinate compound is mainly caused by the vibration of the ligand with intake locations that is unaffected by the connected metal ions [18]; thus, the absorption in the spectral range of these organic metallic coordinate compounds with different metal ions was quite similar. The spectrograms of the chelates produced by DDP reacted with Cd or Zn were similar to those shown in Figure 1(b).
Effects of the amount of added DDP chelator on the effectiveness of heavy metal stabilization Figure 2 shows the heavy metal concentrations in the leachate and the stabilization rate of the heavy metals in
Downloaded by [University of Western Ontario] at 00:03 12 November 2014
Environmental Technology
1415
Figure 2. Effects of the amount of added chelator on the heavy metal concentrations in the leachate and stabilization rate of the heavy metals in the fly ash.
the fly ash after treatment with the DDP chelator, sodium sulphide, and lime at 0%, 0.5%, 1.0%, and 2.0% (mass fraction) , respectively. The figure indicates that Pb, Zn, and Cd concentrations in the leachate decrease with increasing DDP chelator concentrations. The stabilization rate of Pb, Zn, and Cd reached 85% at a DDP chelator concentration of 0.5%. The Zn and Pb concentrations were lower than the standard requirement for landfills. The stabilization rates of Pb, Zn, and Cd reached 95% when the DDP chelator concentration was 1%. The Cd concentration in the leachate was lower than the landfill standard. The stability of various coordinate compounds formed by the metal ions decreased with increasing metal alkalinity [19]. The stability of the formed chelates was in the following order: Pb > Zn > Cd. According to coordination chemistry theory, when a number of metal ions exist together, the more stable the coordination compound (produced by the metal ion and ligand) is, the sooner the coordination compound forms. Accordingly, Pb and Zn interact first with the ligand to form stable coordination compounds under low DDP
chelator concentrations. Cd can fully react only when there is sufficient quantity of ligand. When the lime and sodium sulphide concentrations were 0.5%, the stabilization rates of Pb, Cd, and Zn were lower than 50% and 70%, respectively. The Pb concentration in the leachate satisfied the standard landfill requirement when the lime and sodium sulphide concentrations reached 2.0%. The stabilizing effect of DDP on heavy metals in the fly ash was apparently better than that of lime and of sodium sulphide. The DDP chelator was more effective than sodium sulphide, which was more effective than lime. Effects of pH on the amount of heavy metals in the fly ash leachate Figure 3 shows the changes in the heavy metal concentrations in the leachate with fluctuations in pH. The figure demonstrates that the least amounts of Pb and Zn were observed in the leachate of the untreated fly ash when the pH of the extract liquid was varied from 6 to 8. The amount
Downloaded by [University of Western Ontario] at 00:03 12 November 2014
1416
Y. Xu et al.
Figure 3. Effects of extract liquid pH on the heavy metal concentrations in the leachate.
of Cd, Pb, and Zn in the leachate quickly increased with decreasing pH at pH < 6. Considering the temperature of smoke is low, Cd may be captured by CaCO3 on the surface of the fly ash. Cd will then exist as CdCO3 on the surface of the fly ash. The fly ash used in the experiment contained a relatively higher percentage of chlorine; thus, part of the Cd would exist as a chloride. CdCl2 is easily soluble in water. Therefore, the amount of Cd in the fly ash leachate increased under low pH. CdCO3 dissolves between pH 6 and 9 [20]. However, when the pH value exceeded 8, Cd precipitated as Cd(OH)2 . The amount of Cd in the leachate decreased with increasing pH [21]. Meanwhile, the amount of Pb and Zn in the leachate gradually increased with increasing pH because the Pb(OH)2 and Zn(OH)2 produced [Pb(OH)3 ]− and [Zn(OH)4 ]2− in the presence of a high OH− concentration and dissolved in the solution [22]. Stabilizing products were prepared with 1% DDP chelator, and lime and sodium sulphide were prepared at 2%,
which were used in the subsequent experiment. The results show that the amount of Pb, Cd, and Zn in the leachate after treatment clearly decreased compared with the original fly ash when the pH was varied from 1 to 13. However, the amount of heavy metals in the leachate after DDP chelator treatment was lower than that after treatment with lime and with sodium sulphide regardless of pH. The amount of the heavy metals in the fly ash leachate stabilized by the DDP chelating agent decreased with increasing pH. At pH 1–13, the amount of Pb in the leachate ranged from 5.8–0.2 mg l−1 , Cd ranged from 0.7–0.02 mg l−1 , and Zn ranged from 78–5.1 mg l−1 . The amounts of Cd and Pb in the leachate satisfied the Pollution Control Standards for Hazardous Waste Landfill (GB18598-2001) at pH values exceeding 2. The amount of Zn in the leachate meets the Pollution Control Standards for Hazardous Waste Landfill (GB18598-2001) at pH values exceeding 3. These results show that the stability of DDP coordination compound with Pb, Zn, and Cd, respectively, abates with decrease of pH value, and the stability of the coordination compound from Zn and DDP chelator is more easily affected by acidity than that from Cd and Pb. The amount of Pb in the leachate reached the standard at pH >6, whereas that of Cd and Zn reached the standard at pH >8 in the fly ash treated with sodium sulphide. The amount of Pb, Zn, and Cd in the leachate fulfilled the standard at pH values ranging from 9–11 in the lime-treated fly ash. The amount of Pb and Zn in the leachate increased at pH >11, which indicates the duality of these heavy metals. Considering that the fly ash remains stable under a broad pH range after DDP chelator treatment, the risk of secondary pollution from the stabilized products is decreased. Figure 4 compares the different initial pH values of the extract liquid and the pH values of the leachate after stabilization treatment. The figure shows that when the pH of the extract liquid was greater than 3, the pH of the leachate will be greater than 6 because the fly ash contains large amounts of alkaline oxides of Ca and Al. Thus, when the pH of the
Figure 4. Comparison of the different initial pH of the extract liquid and the pH of the leachate after stabilization treatment.
Downloaded by [University of Western Ontario] at 00:03 12 November 2014
Environmental Technology extract liquid was relatively low, the alkaline substances dissolved from the fly ash exhibit certain acid-neutralization capacity. The pH of the leachate was still relatively high even if the extract liquid has a low pH. The experiment investigating the amount of heavy metals in the leachate from stabilized fly ash was carried out based on the experimental methods described in Test Method Standard for Leachate Toxicity of Solid Wastes – Horizontal Vibration Extraction Procedure (GB5086.21997) recommended by the Pollution Control Standards for Hazardous Waste Landfill (GB18598-2001). The method simulates the environment of hazardous waste exposed in landfills and considers the long-term stability of heavy metals in the waste with respect to rainfall leaching. After stabilization treatment, the heavy metals in the fly ash were extracted using liquids at different pH, and the heavy metal content was determined to predict the amount of dangerous substances in the leachate under different pH conditions. This method predicts and evaluates whether the stabilized products remain stable for a long time if the environmental conditions change [23]. The fly ash stabilized by the chelating agents will be ultimately disposed of in landfills. The pH of rainfall is an important factor in determining how solid waste will affect the environment. The amount of Cd, Pb, and Zn in the leachate from the fly ash stabilized by the chelating agent satisfies the Pollution Control Standards for Hazardous Waste Landfill (GB18598-2001) when the pH of the extract liquid was greater than 3. This finding indicates that the precipitate formed by the heavy metals and the DDP chelating agents was hard to dissolve under acidic conditions. The pH of rainfall will be greater than 3.0 under normal conditions. Fly ash has a good buffering capacity for acid and alkali; thus, the fly ash stabilized by the DDP chelating agents was expected to have long-term stability. Effects of reaction time on the effectiveness of stabilization The heavy metal concentrations in the leachate of the stabilized products after the incineration fly ash was treated with the DDP chelator under different reaction time are demonstrated in Figure 5. The result shows that the stabilization rate increases with increasing reaction time, at reaction times ranging from 10 min to 40 min. The stabilization rates of Pb, Zn, and Cd were 97.0%, 95.2%, and 94.6%, respectively, when the reaction time was up to 30 min. The stabilization rate remained stable for 30–40 min but slightly decreased when the reaction time reached 50 min. Therefore, reaction time influences the DDP chelator stabilizing treatment of the fly ash. On one hand, the DDP chelator did not fully react with the heavy metals when the reaction time was too short, which resulted in increasing heavy metal concentrations in the leachate. On the other hand, long-term stirring of the sample may break off the expanding deposit, which may also decrease the stabilizing effect.
1417
Figure 5. Effects of reaction time on stabilization rate of the heavy metals in the fly ash.
Consequently, we found that the best reaction time for the stabilization of the heavy metals in the fly ash was 30 min, after which the Pb, Zn, and Cd concentrations in the leachate satisfied the landfill standards.
Changes in the form of the heavy metals in fly ash after DDP chelator treatment The heavy metal toxicity of waste incineration fly ash is related to the total amount of heavy metal, which is largely determined by the metal distribution. The same total amounts of heavy metals may be distributed differently, hence, they have different biological and environmental effects [24]. To determine the mechanism of stabilization of the fly ash by chelators, the content of the different forms of the heavy metals in the fly ash were compared before and after the stabilization treatment with the DDP chelator. The result is given in Table 3. The Pb, Cd, and Zn in the fly ash after DDP chelator treatment reduced the heavy metal content in the exchangeable fraction, and those bound to carbonates. Among these metals, those in the exchangeable fraction were reduced by 0.46% to 6.92%. Furthermore, because the original fly ash has a lower content in the exchangeable fraction, the heavy metal content in the exchangeable fraction of the treated fly ash was very low. The heavy metal content bound to carbonates was also reduced by 9.66%, to 29.19%. Among the heavy metals, the Cd bound to carbonates was significantly reduced; meanwhile, the content bound to organic matter was significantly increased by 8.62% to 25.84%, wherein the Cd bound to organic matter was significantly increased. During the stabilization treatment using the DDP chelator, some metal ions combined with Fe–Mn oxides to form Fe–Mn oxidation state heavy metal, and some combined with minerals such as silicate and formed heavy metal compounds in the residual fraction. After stabilization treatment, the heavy metal content bound to Fe–Mn oxide and the residual fraction increased. The heavy metal content in the residual fraction increased by 0.7% to 1.95%, and that bound to Fe–Mn oxide increased by 0.44% to 6.1%. The Pb, Cd, and Zn content bound to carbonates, and those bound to organic matter were
1418 Table 3.
Zn Pb Cd
Y. Xu et al. The content of the different forms of the heavy metals in the fly ash. Metals
F1 (%)
F2 (%)
F3 (%)
F4 (%)
F5 (%)
F1 + F2 (%)
F3 + F4 + F5 (%)
Original After treatment Original After treatment Original After treatment
0.5 0.0401 7.03 0.112 5.22 0.52
33.78 24.12 30.69 18.38 73.95 44.76
21.78 22.22 17.85 23.38 10.89 16.99
1.99 10.61 0.83 13.83 2.53 28.37
41.95 43.01 43.60 44.30 7.41 9.36
34.28 24.16 37.72 18.49 79.17 45.28
65.72 75.84 62.28 81.51 20.83 54.72
Downloaded by [University of Western Ontario] at 00:03 12 November 2014
Notes: F1: exchangeable fraction; F2: bound to carbonates; F3: bound to Fe–Mn oxide; F4: bound to organic matter; F5: residual fraction.
significantly influenced by the DDP chelator treatment of the fly ash. Among the various forms of heavy metals, heavy metals in the exchangeable fraction were not specifically absorbed on the surface of the fly ash through the proliferation and complexation of the outer layer. Heavy metals are released through ion exchange when leached with a solution containing a large number of cations. The heavy metals bound to carbonates formed precipitates and coprecipitates in the carbonate, which can be dissolved with weak acid. The heavy metals bound to Fe–Mn oxide were absorbed and coprecipitated as Fe–Mn oxides, and are only released under reducing conditions. The heavy metals bound to organic matter were complexed and absorbed in organic compounds, which are only released under oxidizing conditions. The heavy metals in the residual fraction generally exist as native and secondary silicates and other stable minerals, and they are the most stable. They are released only with strong acids. The heavy metals in the exchangeable fraction of the fly ash are easily used by organisms. The heavy metals bound to carbonates and those bound to Fe–Mn oxides are easily affected by environmental acidity and redox situations, respectively [25]. These three forms are unstable and pose potential biological risks. Heavy metals bound to organic matter and in the residual fraction are only released under strongly oxidative and strong acidity conditions, respectively; thus, these are less harmful to the environment and they exist in a steady state. The percentage of the exchangeable heavy metal fraction in the fly ash was relatively low. However, it is this form that mainly causes the heavy metal concentrations in the fly ash leachate to exceed the landfill criteria. Based on the reaction mechanism of DDP chelating agent and heavy metals in fly ash, chelating agents stabilize heavy metals in the fly ash because the aqueous solution becomes acidic after the addition of the chelating agent. The DDP chelating agent mainly undergoes a coordination reaction with the heavy metals in the exchangeable fraction and those bound to carbonates, and increases the heavy metals bound to organic matter. The fly ash stabilization process may also be accompanied by the conversion of heavy metal into its various forms. The heavy metal content of the unstable fraction clearly decreased in the fly ash after DDP chelator treatment. The
Zn, Pb, and Cd content of the stable fraction increased by 9.68%, 13.70%, and 27.79%, respectively, compared with the content before treatment. The Pb and Zn content of the stable fraction was higher than that in the unstable fraction after treatment. This finding indicates that the DDP chelator and heavy metals in the exchangeable fraction, and those bound to the carbonates in the fly ash, transformed into stable forms bound to organic matter. Meanwhile, some of the heavy metals in the exchangeable fraction and those bound to carbonates were transformed into stable forms in the residual fraction. Thus, the biological hazard of the heavy metals in the fly ash was significantly reduced. Although the heavy metals in the unstable fraction of the fly ash constituted 41.87–62.27% of the content, the levels of heavy metals in the exchangeable content that are easily leached were much lower. The heavy metal content bound to carbonates, which is easily leached by acid, was clearly reduced, whereas the content in the more stable forms bound to Fe–Mn oxide increased. Thus, after stabilization treatment, the heavy metals in the fly ash were significantly stabilized and their migration was reduced. The Zn, Pb, and Cd contents of the stable fraction were significantly increased in the treated fly ash. This stable metal fraction is released under strongly oxidative and strongly acidic conditions, rather than under natural environments which do not fulfil either condition. Thus, treated fly ash demonstrates long-term stability under natural environments when analysed in terms of heavy metal form distribution. The loss on ignition (LOI) of the fly ash is 14.26%. The LOI value of the fly ash is related to the amount of tiny unburned organics it contains. In the original fly ash, the heavy metal bound to organic matter is mainly the product of the combination of heavy metals and organic substances in the fly ash. In the stabilized fly ash, the heavy metal bound to organic matter contains heavy metals combined with the DDP chelating agent. The LOI value of the fly ash is related to the amount of heavy metals bound to organic matter. The heavy metals bound to organic matter are in a stable form, the percentage of which will influence the heavy metal concentrations in the fly ash leachate but will not influence the stabilization effect of the chelating agent on the heavy metals in the fly ash. Similar studies have not yet reported any influence of LOI on the stabilization effect.
Downloaded by [University of Western Ontario] at 00:03 12 November 2014
Environmental Technology Conclusions The stabilization effect of DDP on heavy metals in fly ash is better than that of sodium sulphide and of lime. The stabilization rate of 1% DDP on Pb, Cd and Zn exceeds 95%, which satisfies the landfill standard (Pb: 5 mg l−1 ,Cd: 0.5 mg l−1 , and Zn: 75 mg l−1 ). The amount of Pb in the leachate satisfies the landfill standard when the sodium sulphide and lime concentrations were 2.0%. The stabilization effect of the DDP chelator on heavy metals in the fly ash was stronger than that of sodium sulphide and stronger than that of lime. The best reaction time for the DDP chelator stabilization treatment of incineration fly ash was 30 min. We found that pH minimally influences the products of DDP stabilization treatment. The amounts of Pb, Cd, and Zn in the leachate meet the standards for landfills when the pH of the extract liquid is greater than 3. The concentrations of the three heavy metals in the leachate only satisfy the landfill standard at pH >8 with sodium sulphide treatment, and pH >9 with lime treatment. The DDP chelator-treated fly ash remained stable within a broad pH range, which lessens the risk of secondary pollution from the stabilized products. The Pb, Cd, and Zn contents of the exchangeable fraction and those bound to carbonates all decreased after the fly ash was treated, whereas those bound to organic matter increased. The heavy metal content of the unstable fraction sharply decreased after the fly ash was treated, with Cd exhibiting the greatest decline in the unstable fraction. These findings reveal that the DDP chelator integrates with heavy metals in the exchangeable fraction and those bound to carbonates in the fly ash, and transforms them into relatively stable forms bound to organic matter, thereby reducing the biological risks of heavy metals in the fly ash. References [1] R.S. Bie, Cyclone furnace technology disposing flyash from MSW incineration plant, Power System Eng. 26 (2010), pp. 9–12. [2] T.M. Ye, W. Wang, X.B. Gao, X.Wan, and F.Wang, Characterization and heavy metals leaching toxicity of fly ash from municipal solid waste incinerators in China, Environ. Sci. 28 (2007), pp. 2646–2650. [3] Y.H. Jiang, B.D. Xi, X.J. Li, L. Zhang, and Z.M. Wei, Effect of water-extraction on characteristics of melting and solidification of flyash from municipal solid waste incinerator, J. Hazard. Mater. 161 (2009), pp. 871–877. [4] L. Wang, Y.Y. Jin, R.D. Li, and Y.F. Nie, Characterization of MSWI flyash, Environ. Sci. Technol. 33 (2010), pp. 21–26. [5] J.D. Chou, W.Y. Ming, H.H. Liang, and S.H. Chang, Biotoxicity evaluation of flyash and bottom ash from different municipal solid waste incinerators, J. Hazard. Mater. 168 (2009), pp. 197–202. [6] Q.H. Wang, J. Yang, Q. Wang, and T.J. Wu, Effects of waterwashing pretreatment on bioleaching of heavy metals from municipal solid waste incinerator flyash, J. Hazard. Mater. 162 (2009), pp. 812–818.
1419
[7] K.J. Hong, S. Tokunaga, and T. Kajiuchi, Extraction of heavy metals from MSW incinerator flyash by chelating agents, J. Hazard. Mater. 75 (2000), pp. 57–73. [8] K. Xu, L. Wu, and D.Z. Chen, Stabilization of heavy metals in incineration flyash with chelating agents, Energy Res. Inf. 21 (2005), pp. 82–89. [9] Sukandar, T. Padmi, T.Masaru, and I. Aoyama, Chemical stabilization of medical waste fly ash using chelating agent and phosphates: Heavy metals and ecotoxicity evaluation, Waste Manage. 29 (2009), pp. 2065–2070. [10] S.R. Fang, Y. Xu, J.L. Lu, X.Y. Wei, Z.G. Xie, and L. Xue, Experimental research on chelator for treatment of sediment contaminated by heavy metals, J. Chem. Eng. 62 (2011), pp. 231–236. [11] N.O. Lotter and D.J. Bradshaw, The formulation and use of mixed collectors in sulphide flotation, Miner. Eng. 23 (2010), pp. 945–951. [12] T. Güler, C. Hicyilmaz, G. Gökaˆgac, and Z. Ekmeci, Adsorption of dithiophosphate and dithiophosphinate on chalcopyrite, Miner. Eng. 19 (2006), pp. 62–71. [13] J.G. Jiang, Z.Z. Zhao, J. Wang, Y. Zhang, and X.J. Du, Study on cement solidification technology in treating with fly ash, Acta Sci. Circumstant. 26 (2006), pp. 230–235. [14] F. Lombardi, T. Mangialardi, L. Piga, and P. Sirini, Mechanical and leaching properties of cement solidified hospital solid waste incinerator fly ash, Waste Manage. 18 (1998), pp. 99–106. [15] A. Tessier, P.G.C. Campbell, and M. Bisson, Sequential chemical extraction procedure for the speciation of particulate trace metals, Anal. Chem. 51 (1979), pp. 844–850. [16] Y. Xu and F. Zhang, Experimental research on heavy metal wastewater treatment with dipropyl dithiophosphate, J. Hazard. Mater. B137 (2006), pp. 1636–1642. [17] R.S. Lv and Z.R. Zhou, Handbook of analytic chemistryelectrochemical analysis and optics analysis, Vol. 3, Chemical Industry Press, Beijing, 1983, pp. 647–709. [18] Q.N. Dong, Infrared Spectroscopy, Petroleum Chemical Industry Press, Beijing, 1977, pp. 202–205. [19] B. Liu, Chelate Flotation Agent, Metallurgical Industry Press, Beijing, 1982, pp. 6–14. [20] H.K. Hansen, A.J. Pedersen, L.M. Ottosen, and A. Villumsen, Speciation and mobility of cadmium in straw and wood combustion fly ash, Chemosphere 45 (2001), pp. 123–128. [21] A.T. Lima, L.M. Ottosen, A.J. Pedersen, and A.B. Ribeiro, A characterization of fly ash from bio and municipal waste, Biomass Bioenergy 32 (2008), pp. 277–282. [22] H.J. Zhang, Y. Yu, Y.W. Ni, Y.X. Li, S.Q. Wang, and J.P. Chen, Stabilization of heavy metals in municipal solid waste incineration flyash with the thiol collectors, Environ. Sci. 28 (2007), pp. 1989–1904. [23] J.G. Jiang, J. Wang, X. Xu, W. Wang, Z. Deng, and Y. Zhang, Heavy metal stabilization in municipal solid waste incineration flyash using heavy metal chelating agents, J. Hazard. Mater. B113 (2004), pp. 141–146. [24] N. Li, Q.J. Hao, C.S. Jiang, Q.P. Lu, W. Zhu, and L. Zhao, Particle size distribution and speciation analysis of heavy metals in municipal solid waste incineration flyash in Chongqing, China, Environ. Chem. 29 (2010), pp. 659–664. [25] S.J. Huang, Ch.Y. Chang, D.T. Mui, F.C. Chang, M.Y. Lee, and C.F. Wang, Sequential extraction for evaluating the leaching behavior of selected elements in municipal solid waste incineration flyash, J. Hazard. Mater. 149 (2007), pp. 180–188.
Environmental Technology Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tent20
Stabilization treatment of the heavy metals in fly ash from municipal solid waste incineration using diisopropyl dithiophosphate potassium Ying Xu
a b
b
, Yu Chen & Yueyang Feng
b
a
Key Laboratory of Integrated Regulation and Resource Development of Shallow Lakes of Ministry of Education , Hohai University , Nanjing , China b
College of Environmental Engineering , Hohai University , Nanjing , China Accepted author version posted online: 26 Nov 2012.Published online: 04 Jan 2013.
To cite this article: Ying Xu , Yu Chen & Yueyang Feng (2013) Stabilization treatment of the heavy metals in fly ash from municipal solid waste incineration using diisopropyl dithiophosphate potassium, Environmental Technology, 34:11, 1411-1419, DOI: 10.1080/09593330.2012.752871 To link to this article: http://dx.doi.org/10.1080/09593330.2012.752871
PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http:// www.tandfonline.com/page/terms-and-conditions
Environmental Technology, 2013 Vol. 34, No. 11, 1411–1419, http://dx.doi.org/10.1080/09593330.2012.752871
Stabilization treatment of the heavy metals in fly ash from municipal solid waste incineration using diisopropyl dithiophosphate potassium Ying Xua,b∗ Yu Chenb and Yueyang Fengb a Key
Laboratory of Integrated Regulation and Resource Development of Shallow Lakes of Ministry of Education, Hohai University, Nanjing, China; b College of Environmental Engineering, Hohai University, Nanjing, China
Downloaded by [University of Western Ontario] at 00:03 12 November 2014
(Received 25 July 2011; final version received 19 November 2012 ) A stabilization treatment was developed for heavy metals in fly ash from municipal solid waste incineration using the heavy metal chelator diisopropyl dithiophosphate potassium (DDP). The mechanism and effect of the DDP chelator treatment on heavy metals in the fly ash was also studied, along with the form transformation rules of the heavy metals after DDP chelator treatment. The results show that 1% DDP achieves a stabilization rate of over 95% for Pb, Zn, and Cd. The effect of DDP was better than that of inorganic stabilizers such as sodium sulphide and lime. The heavy metal concentrations in the leachate after the treatment were lower than those required by the Pollution Control Standards for Hazardous Waste Landfill (GB18598-2001). At pH 1–13, the heavy metal concentrations in the fly ash leachate were far lower than those using the inorganic stabilizers sodium sulphide and lime. DDP retains its stabilizing effect under a broader pH range. After stabilization treatment, the heavy metals in the exchangeable fraction and those bound to carbonates were mainly transformed into those bound to organic matter. This process decreases the unstable content and reduces the risk of secondary pollution of the stabilized products in the environment. Keywords: chelator; stabilization treatment; fly ash; heavy metal; heavy metal fraction
Introduction Economic development, the advanced pace of urbanization, and improvement in the material life of people have increased the amount of garbage produced by cities in China. Now, more than 150,000,000 tons of urban garbage is produced annually, with a growth rate of 8–10%, and more than 200 cities (or 30% of the nation) are surrounded by garbage [1–3]. Although incineration is an important means of garbage disposal, it produces large amounts of fly ash. The amount of fly ash produced, usually about 3–5% of the total waste, is related to the type of waste, the incineration conditions, the incinerator type, and the treatment technique. Analysis shows that fly ash is not a chemically inert material. This substance contains numerous harmful metals and salts, such as Cd, Pb, Zn, and Cr, which are likely to leach into water sources under specific situations. Incorrect treatment causes the transformation of heavy metals and pollutes the groundwater, soil, and air [4]. The heavy metals in fly ash do not degrade naturally; thus, they greatly affect the environment. Current global environmental regulations have already identified fly ash as a hazardous waste [5,6]. The search for safe and effective ways of treating the fly ash produced from municipal solid waste incineration is now an urgent social and environmental problem. ∗ Corresponding
author. Email: [email protected]
© 2013 Taylor & Francis
Solidification and stabilization technology is one of the keys in solving the problem of poisonous and hazardous waste. Solidification consists of cement solidification and asphalt solidification. This method is technically mature, easy to operate, and has a low cost. However, the increasing waste capacity ratio of solidification makes subsequent transportation difficult, and the subsequent landfilling results in tremendous land wastage. Moreover, heavy metals may leach out after weathering [7]. Stabilization is mainly achieved using chemical agents, and chemical agents commonly used include sulphides, phosphates, silicates, and lime. Through this method, heavy metals leach out when the surrounding pH changes; hence, stabilization does not satisfy the requirement for long-term security. At present, the reported organic stabilizing chemical agents are mainly dithiocarbamate and its derivatives, organic polyphosphoric acid and its salt chemicals, as well as chitosan derivatives [8,9]. Despite the adequate stabilization effect of these agents, numerous problems such as the high production cost and the lack of raw material resources make this treatment very expensive and less applicable in industry. Therefore, finding easy-to-produce and conveniently obtained raw materials and low-cost chemical agents with sufficient stabilizing effect is a hot research topic in fly-ash treatment.
Downloaded by [University of Western Ontario] at 00:03 12 November 2014
1412
Y. Xu et al.
Beneficiation chelating agents that contain dithiophosphoric acid ligands have been widely used in mineral flotation [10]. These chemicals are thiol collector agents with strong capacities for capturing heavy metals. Lotter and Bradshaw [11] reported the flotation effect of dithiophosphate on bornite, chalcocite, and pyrite; they also studied the effects of a dithiophosphate-mixed flotation agent for improving flotation efficiency and reducing the dosage of flotation agents. Güler et al. [12] used dithiophosphate in the flotation of chalcopyrite and discussed the mechanism of floating chalcopyrite with dithiophosphate. Considering dithiophosphate, chelating agents chelate and precipitate almost all free-state heavy metals in the appropriate pH value range; it is the ideal fly ash stabilizer for waste incineration. Diisopropyl dithiophosphate potassium (DDP) is a dithiophosphate chelating agent, and its performance in stabilizing the heavy metals in incineration fly ash has not been reported. The unit price of DDP chelator used for stabilization is much higher compared with the cement routinely used for solidification. However, to achieve the same stabilization effect on the heavy metals in fly ash, much less DDP than cement is required. According to the literature [13], the amount of cement for solidifying fly ash usually ranges from 25–45% (by mass fraction), whereas the amount of DDP chelating agent required ranges from 0.5–2.0% (by mass fraction). The cost of the reagents is roughly equal based on the two approaches. The process of treating fly ash through cement solidification is the same as stabilizing fly ash with DDP chelator. The difference in cost lies in adding cement or chelating agents; therefore, the total expense of the two methods is also the same. Fly ash develops a high solidification capacity after cement treatment, which leads to increased disposal cost because of the increased landfill area required for disposal. In addition, heavy metals will again percolate from the cement-solidified fly ash in acidic conditions and repollute the environment, and the solidified cement loses its original effect [14]. The DDP chelating agent-treated fly ash remains stable under a wide pH range, which reduces the risk of secondary pollution. Therefore, using DDP to stabilize fly ash is preferable to cement solidification when technical and economic factors are considered. DDP is made from isopropyl alcohol, phosphorus pentasulphide, and potassium carbonate or alkali following a simple process, with easy-to-acquire raw materials and low Table 1.
product costs, and is widely used as a flotation agent in the mineral processing industry. Flotation agents worth tens of billions of dollars are used every year, and dithiophosphates account for a large proportion. Many chemical companies in China produce DDP because its production is already established; thus, the supply is ample. Therefore, DDP can be used to process the heavy metals in the fly ash from municipal solid waste incineration. This paper discusses the mechanism of this method, and investigates the technical means and its stabilization effects. The treatment effect of DDP was compared with common inorganic chemical agents such as lime and sodium sulphide, to examine the stabilization treatment of fly ash. Materials and method Sample of fly ash The incineration-produced fly ash used in this research came from the Suzhou Waste Incineration Power Station in China. This station applies a semidry method and a bag dustcollecting method for disposing of smoke. The treatment capacity of this station is 1000 tons per day. In accordance with the Technical Specifications on Sampling and Sample Preparation from Industry Solid Waste (HJ/T20-1998), 1 kg of fly ash was collected each day for three consecutive days during the stable operation of the incinerator under normal conditions. The sample was evenly mixed, and then set aside. The elemental composition of the fly ash was analysed using X-ray fluorescence. The results show that the main elements were Ca, C1, Na, K and S, as well as heavy metal such as Zn, Pb, Cu, Cr, As, and Cd (Table 1). Determining the amount of heavy metals that leach out of the fly ash The Test Method Standard for Leachate Toxicity of Solid Wastes–Horizontal Vibration Extraction Procedure (GB5086.2-1997) was performed to test the amount of heavy metals that leach out of the fly ash. The method involves the use of deionized water as the leaching agent, mixing it with fly ash at a ratio of 10:1 (L:S). The mixture was maintained for 8 h at 23◦ C ± 2◦ C with shaking at 30 rpm ± 2 rpm, and then allowed to stand for 16 h. The mixture was then filtered with a 0.45 μm filter unit. The heavy metal content was determined through atomic absorption spectrometry.
Elemental composition of the fly ash.
Element Weight (%) Element Weight (%)
Cl
Ca
S
Na
Si
K
Al
Fe
Br
Zn
Sr
Cd
Ni
32.22 P 0.19
35.74 Mg 0.24
2.66 Ti 0.22
3.78 Pb 1.4
1.92 Cu 0.49
3.10 Sn 0.068
0.82 Ba 0.076
0.84 Mn 0.040
0.65 Sb 0.036
0.92 Cr 0.029
0.025 As 0.006
0.19 LOI 14.26
0.08
Note: LOI: loss on ignition.
Downloaded by [University of Western Ontario] at 00:03 12 November 2014
Environmental Technology Fly ash stabilization treatment DDP was tested as a chelator, with the inorganic stabilizing chemical agents sodium sulphide and lime as the chelator controls. DDP, which was purchased from a Chinese chemical material company at $2 per kg, appears as white crystals with a melting point of 110◦ C and water solubility of 100 g l−1 . The method involves dissolving the chemical agent in water to make an aqueous solution, then mixing it with fly ash at a liquid:solid ratio of 16:10 (ml:g). The mixture was thoroughly mixed into a slurry to enable the DDP chelator to react with the heavy metals. The mixture was maintained at room temperature for 24 h and then dried in an oven at 60◦ C for 12 h. The dried sample was used to test the amount of heavy metals in the leachate. The stabilization rate of the heavy metals in the treated fly ash was calculated as follows: Stabilization rate = (C0 − C1 )/C0 × 100%
1413
This method consists of five steps; each step uses different extracting agents. Based on the method, the heavy metals in the fly ash were divided into an exchangeable fraction (F1), bound to carbonates (F2), bound to Fe–Mn oxide (F3), bound to organic matter (F4), and a residual fraction (F5). The HF–HNO3 –HClO4 dissolving method was used to test the amount of Hg, Pb, Cd, Cr, Cu, Zn, Be, Ba, As, and Ni.
Experimental instruments An atomic absorption spectrophotometer TAS-986 (Beijing Purkinje General Instrument Co., Ltd), Fourier transform infrared (FT-IR) spectrometer IRPrestige-21 (Shimadzu Corporation, Japan), and X-ray fluorescence spectrometer ARL-9800 (ARL Corporation, Switzerland) were used in the experiment.
(1)
where C0 is the amount of heavy metals in the leachate of the original fly ash, C1 is the amount of heavy metals in the leachate of the fly ash after treatment. The fly ash before and after stabilization was dried at 105◦ C to a constant weight and weighed. The weight of the DDP chelating agent need not be deducted from that of the fly ash after stabilization, as the amount is so small that it is negligible. The fly ash was stabilized at 60◦ C for 12 h to shorten the time of the experiment. In practice, the stabilized fly ash can be air-dried under natural conditions, and then disposed of in a landfill. The samples obtained using the two methods were subjected to a heavy metal leaching test. The result of the test indicates that the heavy metal concentrations in the leachate were almost unchanged. The stabilized fly ash can then be transported to the landfill site for disposal, if the fly ash meets the Pollution Control Standards for Hazardous Waste Landfill (GB18598-2001) in China. The landfill site has no requirement for the compressive strength of landfill waste; therefore, the addition of cement or other coagulant to solidify the fly ash is unnecessary after DDP chelator treatment. Effects of pH on the amount of heavy metals in the leachate of the stabilized product Leaching agent liquors with pH values 1, 3, 5, 7, 9, 11, and 13 were prepared from HNO3 liquor or NaOH liquor mixed with the treated sample at a liquid:solid ratio of 10:1. The mixture was shaken for 8 h at a rate of 30 rpm ± 2 rpm at a room temperature of 23◦ C ± 2◦ C, then allowed to stand for 16 h. The mixture was filtered with a 0.45 μm filter unit to determine the heavy metal content. Analysis of the fly ash sample The heavy metals in the fly ash were extracted, using the method of Tessier et al. [15], to test their chemical form.
Quality control Parallel determination was performed three times for each experimental condition. The relative standard deviation of the samples was lower than 1%. The standard sample that contained heavy metals was added to the undetermined sample. A recovery experiment was conducted. The main recovery rate of the standard addition was 99.7%.
Results and discussion The amount of heavy metals in the fly ash leachate The Test Method Standard for Leachate Toxicity of Solid Wastes–Horizontal Vibration Extraction Procedure (GB5086.2-1997) was used to test the amount of heavy metals in the leachate of the fly ash sample. Both the Identification Standards for Hazardous Waste–Identification for Extraction Toxicity (GB5085.3-2007) and the Pollution Control Standards for Hazardous Waste Landfill Table 2. samples.
The leachate concentrations of metals in the fly ash
Metal
Leaching-out concentration (mg l−1 )
GB5085.3-2007 (mg l−1 )
GB18598-2001 (mg l−1 )
Pb Cu Zn Cr Cd Ni Hg As Ba Be
38.20 0.48 110.52 0.8910 5.21 0.84 Not detected 0.016 6.82 Not detected
5 100 100 15 1 5 0.1 5.0 100 0.02
5 75 75 12 0.5 15 0.25 2.5 150 0.2
Notes: GB5085.3-2007: Identification standard for hazardous wastes-identification for the extraction toxicity; GB18598-2001: Pollution control standards for hazardous waste landfill.
1414
Y. Xu et al.
Downloaded by [University of Western Ontario] at 00:03 12 November 2014
(GB18598-2001) specify the limits for leachate concentrations for 10 metals (Hg, Pb, Cd, Cr, Cu, Zn, Be, Ba, Ni, and As). The leachate concentrations of 10 metals in the fly ash samples are listed in Table 2. The results (Table 2) show that the concentrations of Cd, Pb, and Zn exceed the requirements in the Identification Standards for Hazardous Waste–Identification for Extraction Toxicity (GB5085.32007). These metals should be treated first through stabilization, then disposed of in a landfill. Only the Zn, Cd, and Pb in the sample fly ash in the experiment exceeded the standards; thus, experimental studies on the stabilization treatment were only conducted on these three metals. Reaction mechanism of the DDP chelator and the heavy metals DDP has sulphur in thione (=S) and thiol (-S-) groups that have a strong electron-donating ability and easily form coordinate bonds with metal ions to produce diisopropyl dithiophosphate, which is difficult to dissolve in water. While integrating with certain metal ions, the two sulphurs in the DDP chelator form a four-member ring with the following reaction formulation [16]: CH3
CH3 H3C HC O
2
+ M2+
P H3C HC O CH3
SK
CH3
H3C HC O
S
P H3C HC O CH3
P
M S
O CH CH3
S
S
S
+ + 2K (2)
O CH CH3 CH3
In this formulation, M2+ is a bivalent metal ion. The DDP chelator integrates with metal ions and becomes a polydentate ligand that then reacts with Cu, Zn, Pb, and Cd to form a chelator with a coordination number of 4. The ligand of the same metal ion chelate is very likely from different DDP chelators. The diisopropyl dithiophosphate molecule may be highly cross-linked and three-dimensional, with a molecular weight of several million Daltons. The DDP chelating agent was first mixed with water to produce an aqueous solution. The solution was mixed with the fly ash and thoroughly stirred until a mud-like consistency was reached. The heavy metals in fly ash dissolve in the water, and a coordination reaction occurs between the heavy metals and the DDP chelating agents to precipitate the chelate adsorbed onto the fly ash. This process stabilizes the heavy metals in the fly ash. The DDP chelator was added into the wastewater with 1000 mg l−1 Pb and the mixture stirred for 10 min, then the precipitate was filtered, cleaned and dried (chelated product). The dried DDP chelator and chelated product were pressed with potassium bromide. Infrared spectroscopic analysis was conducted for each, and the two spectrograms were compared. Figure 1 shows the infrared spectrogram of the DDP chelator and its product. Figure 1(a) shows that DDP has relatively strong absorbance at wave numbers 2981 cm−1 , 981 cm−1 , 796 cm−1 , 703 cm−1 , and 574 cm−1 . Comparison of these values with a standard infrared spectrogram [17]
Figure 1. FT-IR spectra of chelating agent and chelated product. (a) chelating agent; (b) chelated product.
revealed that the absorbance at 2981 cm−1 was caused by the vibration of -CH3 , that at 703 cm−1 was from the vibration of P=S, and that at 574 cm−1 was from the vibration of P-S. In contrast, the absorbance at 981 cm−1 and at 796 cm−1 was caused by the vibration of P-O-C. Comparison with the chelated product infrared spectrogram Figure 1(b) shows the location and intensity of most absorption change, indicating that the thiophosphoric acid functional group reacted with the metal ion. The DDP chelator reacts with heavy metals through the functional groups P=S and P-S to produce deposits. Figure 1(b) shows that the wave numbers of these two functional groups increase from originally 703 cm−1 and 574 cm−1 , respectively, to 790 cm−1 and 629 cm−1 after the reaction, accompanied with the change in absorption intensity of the feature peak. These changes indicate that the lone pair electron in the sulphur of the P-S and P=S functional groups participated in the reaction, forming coordinate bonds with the heavy metals and then the DDP chelator deposit. This process achieved the stabilization. The absorption of the organic metallic coordinate compound is mainly caused by the vibration of the ligand with intake locations that is unaffected by the connected metal ions [18]; thus, the absorption in the spectral range of these organic metallic coordinate compounds with different metal ions was quite similar. The spectrograms of the chelates produced by DDP reacted with Cd or Zn were similar to those shown in Figure 1(b).
Effects of the amount of added DDP chelator on the effectiveness of heavy metal stabilization Figure 2 shows the heavy metal concentrations in the leachate and the stabilization rate of the heavy metals in
Downloaded by [University of Western Ontario] at 00:03 12 November 2014
Environmental Technology
1415
Figure 2. Effects of the amount of added chelator on the heavy metal concentrations in the leachate and stabilization rate of the heavy metals in the fly ash.
the fly ash after treatment with the DDP chelator, sodium sulphide, and lime at 0%, 0.5%, 1.0%, and 2.0% (mass fraction) , respectively. The figure indicates that Pb, Zn, and Cd concentrations in the leachate decrease with increasing DDP chelator concentrations. The stabilization rate of Pb, Zn, and Cd reached 85% at a DDP chelator concentration of 0.5%. The Zn and Pb concentrations were lower than the standard requirement for landfills. The stabilization rates of Pb, Zn, and Cd reached 95% when the DDP chelator concentration was 1%. The Cd concentration in the leachate was lower than the landfill standard. The stability of various coordinate compounds formed by the metal ions decreased with increasing metal alkalinity [19]. The stability of the formed chelates was in the following order: Pb > Zn > Cd. According to coordination chemistry theory, when a number of metal ions exist together, the more stable the coordination compound (produced by the metal ion and ligand) is, the sooner the coordination compound forms. Accordingly, Pb and Zn interact first with the ligand to form stable coordination compounds under low DDP
chelator concentrations. Cd can fully react only when there is sufficient quantity of ligand. When the lime and sodium sulphide concentrations were 0.5%, the stabilization rates of Pb, Cd, and Zn were lower than 50% and 70%, respectively. The Pb concentration in the leachate satisfied the standard landfill requirement when the lime and sodium sulphide concentrations reached 2.0%. The stabilizing effect of DDP on heavy metals in the fly ash was apparently better than that of lime and of sodium sulphide. The DDP chelator was more effective than sodium sulphide, which was more effective than lime. Effects of pH on the amount of heavy metals in the fly ash leachate Figure 3 shows the changes in the heavy metal concentrations in the leachate with fluctuations in pH. The figure demonstrates that the least amounts of Pb and Zn were observed in the leachate of the untreated fly ash when the pH of the extract liquid was varied from 6 to 8. The amount
Downloaded by [University of Western Ontario] at 00:03 12 November 2014
1416
Y. Xu et al.
Figure 3. Effects of extract liquid pH on the heavy metal concentrations in the leachate.
of Cd, Pb, and Zn in the leachate quickly increased with decreasing pH at pH < 6. Considering the temperature of smoke is low, Cd may be captured by CaCO3 on the surface of the fly ash. Cd will then exist as CdCO3 on the surface of the fly ash. The fly ash used in the experiment contained a relatively higher percentage of chlorine; thus, part of the Cd would exist as a chloride. CdCl2 is easily soluble in water. Therefore, the amount of Cd in the fly ash leachate increased under low pH. CdCO3 dissolves between pH 6 and 9 [20]. However, when the pH value exceeded 8, Cd precipitated as Cd(OH)2 . The amount of Cd in the leachate decreased with increasing pH [21]. Meanwhile, the amount of Pb and Zn in the leachate gradually increased with increasing pH because the Pb(OH)2 and Zn(OH)2 produced [Pb(OH)3 ]− and [Zn(OH)4 ]2− in the presence of a high OH− concentration and dissolved in the solution [22]. Stabilizing products were prepared with 1% DDP chelator, and lime and sodium sulphide were prepared at 2%,
which were used in the subsequent experiment. The results show that the amount of Pb, Cd, and Zn in the leachate after treatment clearly decreased compared with the original fly ash when the pH was varied from 1 to 13. However, the amount of heavy metals in the leachate after DDP chelator treatment was lower than that after treatment with lime and with sodium sulphide regardless of pH. The amount of the heavy metals in the fly ash leachate stabilized by the DDP chelating agent decreased with increasing pH. At pH 1–13, the amount of Pb in the leachate ranged from 5.8–0.2 mg l−1 , Cd ranged from 0.7–0.02 mg l−1 , and Zn ranged from 78–5.1 mg l−1 . The amounts of Cd and Pb in the leachate satisfied the Pollution Control Standards for Hazardous Waste Landfill (GB18598-2001) at pH values exceeding 2. The amount of Zn in the leachate meets the Pollution Control Standards for Hazardous Waste Landfill (GB18598-2001) at pH values exceeding 3. These results show that the stability of DDP coordination compound with Pb, Zn, and Cd, respectively, abates with decrease of pH value, and the stability of the coordination compound from Zn and DDP chelator is more easily affected by acidity than that from Cd and Pb. The amount of Pb in the leachate reached the standard at pH >6, whereas that of Cd and Zn reached the standard at pH >8 in the fly ash treated with sodium sulphide. The amount of Pb, Zn, and Cd in the leachate fulfilled the standard at pH values ranging from 9–11 in the lime-treated fly ash. The amount of Pb and Zn in the leachate increased at pH >11, which indicates the duality of these heavy metals. Considering that the fly ash remains stable under a broad pH range after DDP chelator treatment, the risk of secondary pollution from the stabilized products is decreased. Figure 4 compares the different initial pH values of the extract liquid and the pH values of the leachate after stabilization treatment. The figure shows that when the pH of the extract liquid was greater than 3, the pH of the leachate will be greater than 6 because the fly ash contains large amounts of alkaline oxides of Ca and Al. Thus, when the pH of the
Figure 4. Comparison of the different initial pH of the extract liquid and the pH of the leachate after stabilization treatment.
Downloaded by [University of Western Ontario] at 00:03 12 November 2014
Environmental Technology extract liquid was relatively low, the alkaline substances dissolved from the fly ash exhibit certain acid-neutralization capacity. The pH of the leachate was still relatively high even if the extract liquid has a low pH. The experiment investigating the amount of heavy metals in the leachate from stabilized fly ash was carried out based on the experimental methods described in Test Method Standard for Leachate Toxicity of Solid Wastes – Horizontal Vibration Extraction Procedure (GB5086.21997) recommended by the Pollution Control Standards for Hazardous Waste Landfill (GB18598-2001). The method simulates the environment of hazardous waste exposed in landfills and considers the long-term stability of heavy metals in the waste with respect to rainfall leaching. After stabilization treatment, the heavy metals in the fly ash were extracted using liquids at different pH, and the heavy metal content was determined to predict the amount of dangerous substances in the leachate under different pH conditions. This method predicts and evaluates whether the stabilized products remain stable for a long time if the environmental conditions change [23]. The fly ash stabilized by the chelating agents will be ultimately disposed of in landfills. The pH of rainfall is an important factor in determining how solid waste will affect the environment. The amount of Cd, Pb, and Zn in the leachate from the fly ash stabilized by the chelating agent satisfies the Pollution Control Standards for Hazardous Waste Landfill (GB18598-2001) when the pH of the extract liquid was greater than 3. This finding indicates that the precipitate formed by the heavy metals and the DDP chelating agents was hard to dissolve under acidic conditions. The pH of rainfall will be greater than 3.0 under normal conditions. Fly ash has a good buffering capacity for acid and alkali; thus, the fly ash stabilized by the DDP chelating agents was expected to have long-term stability. Effects of reaction time on the effectiveness of stabilization The heavy metal concentrations in the leachate of the stabilized products after the incineration fly ash was treated with the DDP chelator under different reaction time are demonstrated in Figure 5. The result shows that the stabilization rate increases with increasing reaction time, at reaction times ranging from 10 min to 40 min. The stabilization rates of Pb, Zn, and Cd were 97.0%, 95.2%, and 94.6%, respectively, when the reaction time was up to 30 min. The stabilization rate remained stable for 30–40 min but slightly decreased when the reaction time reached 50 min. Therefore, reaction time influences the DDP chelator stabilizing treatment of the fly ash. On one hand, the DDP chelator did not fully react with the heavy metals when the reaction time was too short, which resulted in increasing heavy metal concentrations in the leachate. On the other hand, long-term stirring of the sample may break off the expanding deposit, which may also decrease the stabilizing effect.
1417
Figure 5. Effects of reaction time on stabilization rate of the heavy metals in the fly ash.
Consequently, we found that the best reaction time for the stabilization of the heavy metals in the fly ash was 30 min, after which the Pb, Zn, and Cd concentrations in the leachate satisfied the landfill standards.
Changes in the form of the heavy metals in fly ash after DDP chelator treatment The heavy metal toxicity of waste incineration fly ash is related to the total amount of heavy metal, which is largely determined by the metal distribution. The same total amounts of heavy metals may be distributed differently, hence, they have different biological and environmental effects [24]. To determine the mechanism of stabilization of the fly ash by chelators, the content of the different forms of the heavy metals in the fly ash were compared before and after the stabilization treatment with the DDP chelator. The result is given in Table 3. The Pb, Cd, and Zn in the fly ash after DDP chelator treatment reduced the heavy metal content in the exchangeable fraction, and those bound to carbonates. Among these metals, those in the exchangeable fraction were reduced by 0.46% to 6.92%. Furthermore, because the original fly ash has a lower content in the exchangeable fraction, the heavy metal content in the exchangeable fraction of the treated fly ash was very low. The heavy metal content bound to carbonates was also reduced by 9.66%, to 29.19%. Among the heavy metals, the Cd bound to carbonates was significantly reduced; meanwhile, the content bound to organic matter was significantly increased by 8.62% to 25.84%, wherein the Cd bound to organic matter was significantly increased. During the stabilization treatment using the DDP chelator, some metal ions combined with Fe–Mn oxides to form Fe–Mn oxidation state heavy metal, and some combined with minerals such as silicate and formed heavy metal compounds in the residual fraction. After stabilization treatment, the heavy metal content bound to Fe–Mn oxide and the residual fraction increased. The heavy metal content in the residual fraction increased by 0.7% to 1.95%, and that bound to Fe–Mn oxide increased by 0.44% to 6.1%. The Pb, Cd, and Zn content bound to carbonates, and those bound to organic matter were
1418 Table 3.
Zn Pb Cd
Y. Xu et al. The content of the different forms of the heavy metals in the fly ash. Metals
F1 (%)
F2 (%)
F3 (%)
F4 (%)
F5 (%)
F1 + F2 (%)
F3 + F4 + F5 (%)
Original After treatment Original After treatment Original After treatment
0.5 0.0401 7.03 0.112 5.22 0.52
33.78 24.12 30.69 18.38 73.95 44.76
21.78 22.22 17.85 23.38 10.89 16.99
1.99 10.61 0.83 13.83 2.53 28.37
41.95 43.01 43.60 44.30 7.41 9.36
34.28 24.16 37.72 18.49 79.17 45.28
65.72 75.84 62.28 81.51 20.83 54.72
Downloaded by [University of Western Ontario] at 00:03 12 November 2014
Notes: F1: exchangeable fraction; F2: bound to carbonates; F3: bound to Fe–Mn oxide; F4: bound to organic matter; F5: residual fraction.
significantly influenced by the DDP chelator treatment of the fly ash. Among the various forms of heavy metals, heavy metals in the exchangeable fraction were not specifically absorbed on the surface of the fly ash through the proliferation and complexation of the outer layer. Heavy metals are released through ion exchange when leached with a solution containing a large number of cations. The heavy metals bound to carbonates formed precipitates and coprecipitates in the carbonate, which can be dissolved with weak acid. The heavy metals bound to Fe–Mn oxide were absorbed and coprecipitated as Fe–Mn oxides, and are only released under reducing conditions. The heavy metals bound to organic matter were complexed and absorbed in organic compounds, which are only released under oxidizing conditions. The heavy metals in the residual fraction generally exist as native and secondary silicates and other stable minerals, and they are the most stable. They are released only with strong acids. The heavy metals in the exchangeable fraction of the fly ash are easily used by organisms. The heavy metals bound to carbonates and those bound to Fe–Mn oxides are easily affected by environmental acidity and redox situations, respectively [25]. These three forms are unstable and pose potential biological risks. Heavy metals bound to organic matter and in the residual fraction are only released under strongly oxidative and strong acidity conditions, respectively; thus, these are less harmful to the environment and they exist in a steady state. The percentage of the exchangeable heavy metal fraction in the fly ash was relatively low. However, it is this form that mainly causes the heavy metal concentrations in the fly ash leachate to exceed the landfill criteria. Based on the reaction mechanism of DDP chelating agent and heavy metals in fly ash, chelating agents stabilize heavy metals in the fly ash because the aqueous solution becomes acidic after the addition of the chelating agent. The DDP chelating agent mainly undergoes a coordination reaction with the heavy metals in the exchangeable fraction and those bound to carbonates, and increases the heavy metals bound to organic matter. The fly ash stabilization process may also be accompanied by the conversion of heavy metal into its various forms. The heavy metal content of the unstable fraction clearly decreased in the fly ash after DDP chelator treatment. The
Zn, Pb, and Cd content of the stable fraction increased by 9.68%, 13.70%, and 27.79%, respectively, compared with the content before treatment. The Pb and Zn content of the stable fraction was higher than that in the unstable fraction after treatment. This finding indicates that the DDP chelator and heavy metals in the exchangeable fraction, and those bound to the carbonates in the fly ash, transformed into stable forms bound to organic matter. Meanwhile, some of the heavy metals in the exchangeable fraction and those bound to carbonates were transformed into stable forms in the residual fraction. Thus, the biological hazard of the heavy metals in the fly ash was significantly reduced. Although the heavy metals in the unstable fraction of the fly ash constituted 41.87–62.27% of the content, the levels of heavy metals in the exchangeable content that are easily leached were much lower. The heavy metal content bound to carbonates, which is easily leached by acid, was clearly reduced, whereas the content in the more stable forms bound to Fe–Mn oxide increased. Thus, after stabilization treatment, the heavy metals in the fly ash were significantly stabilized and their migration was reduced. The Zn, Pb, and Cd contents of the stable fraction were significantly increased in the treated fly ash. This stable metal fraction is released under strongly oxidative and strongly acidic conditions, rather than under natural environments which do not fulfil either condition. Thus, treated fly ash demonstrates long-term stability under natural environments when analysed in terms of heavy metal form distribution. The loss on ignition (LOI) of the fly ash is 14.26%. The LOI value of the fly ash is related to the amount of tiny unburned organics it contains. In the original fly ash, the heavy metal bound to organic matter is mainly the product of the combination of heavy metals and organic substances in the fly ash. In the stabilized fly ash, the heavy metal bound to organic matter contains heavy metals combined with the DDP chelating agent. The LOI value of the fly ash is related to the amount of heavy metals bound to organic matter. The heavy metals bound to organic matter are in a stable form, the percentage of which will influence the heavy metal concentrations in the fly ash leachate but will not influence the stabilization effect of the chelating agent on the heavy metals in the fly ash. Similar studies have not yet reported any influence of LOI on the stabilization effect.
Downloaded by [University of Western Ontario] at 00:03 12 November 2014
Environmental Technology Conclusions The stabilization effect of DDP on heavy metals in fly ash is better than that of sodium sulphide and of lime. The stabilization rate of 1% DDP on Pb, Cd and Zn exceeds 95%, which satisfies the landfill standard (Pb: 5 mg l−1 ,Cd: 0.5 mg l−1 , and Zn: 75 mg l−1 ). The amount of Pb in the leachate satisfies the landfill standard when the sodium sulphide and lime concentrations were 2.0%. The stabilization effect of the DDP chelator on heavy metals in the fly ash was stronger than that of sodium sulphide and stronger than that of lime. The best reaction time for the DDP chelator stabilization treatment of incineration fly ash was 30 min. We found that pH minimally influences the products of DDP stabilization treatment. The amounts of Pb, Cd, and Zn in the leachate meet the standards for landfills when the pH of the extract liquid is greater than 3. The concentrations of the three heavy metals in the leachate only satisfy the landfill standard at pH >8 with sodium sulphide treatment, and pH >9 with lime treatment. The DDP chelator-treated fly ash remained stable within a broad pH range, which lessens the risk of secondary pollution from the stabilized products. The Pb, Cd, and Zn contents of the exchangeable fraction and those bound to carbonates all decreased after the fly ash was treated, whereas those bound to organic matter increased. The heavy metal content of the unstable fraction sharply decreased after the fly ash was treated, with Cd exhibiting the greatest decline in the unstable fraction. These findings reveal that the DDP chelator integrates with heavy metals in the exchangeable fraction and those bound to carbonates in the fly ash, and transforms them into relatively stable forms bound to organic matter, thereby reducing the biological risks of heavy metals in the fly ash. References [1] R.S. Bie, Cyclone furnace technology disposing flyash from MSW incineration plant, Power System Eng. 26 (2010), pp. 9–12. [2] T.M. Ye, W. Wang, X.B. Gao, X.Wan, and F.Wang, Characterization and heavy metals leaching toxicity of fly ash from municipal solid waste incinerators in China, Environ. Sci. 28 (2007), pp. 2646–2650. [3] Y.H. Jiang, B.D. Xi, X.J. Li, L. Zhang, and Z.M. Wei, Effect of water-extraction on characteristics of melting and solidification of flyash from municipal solid waste incinerator, J. Hazard. Mater. 161 (2009), pp. 871–877. [4] L. Wang, Y.Y. Jin, R.D. Li, and Y.F. Nie, Characterization of MSWI flyash, Environ. Sci. Technol. 33 (2010), pp. 21–26. [5] J.D. Chou, W.Y. Ming, H.H. Liang, and S.H. Chang, Biotoxicity evaluation of flyash and bottom ash from different municipal solid waste incinerators, J. Hazard. Mater. 168 (2009), pp. 197–202. [6] Q.H. Wang, J. Yang, Q. Wang, and T.J. Wu, Effects of waterwashing pretreatment on bioleaching of heavy metals from municipal solid waste incinerator flyash, J. Hazard. Mater. 162 (2009), pp. 812–818.
1419
[7] K.J. Hong, S. Tokunaga, and T. Kajiuchi, Extraction of heavy metals from MSW incinerator flyash by chelating agents, J. Hazard. Mater. 75 (2000), pp. 57–73. [8] K. Xu, L. Wu, and D.Z. Chen, Stabilization of heavy metals in incineration flyash with chelating agents, Energy Res. Inf. 21 (2005), pp. 82–89. [9] Sukandar, T. Padmi, T.Masaru, and I. Aoyama, Chemical stabilization of medical waste fly ash using chelating agent and phosphates: Heavy metals and ecotoxicity evaluation, Waste Manage. 29 (2009), pp. 2065–2070. [10] S.R. Fang, Y. Xu, J.L. Lu, X.Y. Wei, Z.G. Xie, and L. Xue, Experimental research on chelator for treatment of sediment contaminated by heavy metals, J. Chem. Eng. 62 (2011), pp. 231–236. [11] N.O. Lotter and D.J. Bradshaw, The formulation and use of mixed collectors in sulphide flotation, Miner. Eng. 23 (2010), pp. 945–951. [12] T. Güler, C. Hicyilmaz, G. Gökaˆgac, and Z. Ekmeci, Adsorption of dithiophosphate and dithiophosphinate on chalcopyrite, Miner. Eng. 19 (2006), pp. 62–71. [13] J.G. Jiang, Z.Z. Zhao, J. Wang, Y. Zhang, and X.J. Du, Study on cement solidification technology in treating with fly ash, Acta Sci. Circumstant. 26 (2006), pp. 230–235. [14] F. Lombardi, T. Mangialardi, L. Piga, and P. Sirini, Mechanical and leaching properties of cement solidified hospital solid waste incinerator fly ash, Waste Manage. 18 (1998), pp. 99–106. [15] A. Tessier, P.G.C. Campbell, and M. Bisson, Sequential chemical extraction procedure for the speciation of particulate trace metals, Anal. Chem. 51 (1979), pp. 844–850. [16] Y. Xu and F. Zhang, Experimental research on heavy metal wastewater treatment with dipropyl dithiophosphate, J. Hazard. Mater. B137 (2006), pp. 1636–1642. [17] R.S. Lv and Z.R. Zhou, Handbook of analytic chemistryelectrochemical analysis and optics analysis, Vol. 3, Chemical Industry Press, Beijing, 1983, pp. 647–709. [18] Q.N. Dong, Infrared Spectroscopy, Petroleum Chemical Industry Press, Beijing, 1977, pp. 202–205. [19] B. Liu, Chelate Flotation Agent, Metallurgical Industry Press, Beijing, 1982, pp. 6–14. [20] H.K. Hansen, A.J. Pedersen, L.M. Ottosen, and A. Villumsen, Speciation and mobility of cadmium in straw and wood combustion fly ash, Chemosphere 45 (2001), pp. 123–128. [21] A.T. Lima, L.M. Ottosen, A.J. Pedersen, and A.B. Ribeiro, A characterization of fly ash from bio and municipal waste, Biomass Bioenergy 32 (2008), pp. 277–282. [22] H.J. Zhang, Y. Yu, Y.W. Ni, Y.X. Li, S.Q. Wang, and J.P. Chen, Stabilization of heavy metals in municipal solid waste incineration flyash with the thiol collectors, Environ. Sci. 28 (2007), pp. 1989–1904. [23] J.G. Jiang, J. Wang, X. Xu, W. Wang, Z. Deng, and Y. Zhang, Heavy metal stabilization in municipal solid waste incineration flyash using heavy metal chelating agents, J. Hazard. Mater. B113 (2004), pp. 141–146. [24] N. Li, Q.J. Hao, C.S. Jiang, Q.P. Lu, W. Zhu, and L. Zhao, Particle size distribution and speciation analysis of heavy metals in municipal solid waste incineration flyash in Chongqing, China, Environ. Chem. 29 (2010), pp. 659–664. [25] S.J. Huang, Ch.Y. Chang, D.T. Mui, F.C. Chang, M.Y. Lee, and C.F. Wang, Sequential extraction for evaluating the leaching behavior of selected elements in municipal solid waste incineration flyash, J. Hazard. Mater. 149 (2007), pp. 180–188.
