563376
research-article2014
WMR0010.1177/0734242X14563376Waste Management & ResearchZhang et al.
Original Article
Effective solidification/stabilisation of mercury-contaminated wastes using zeolites and chemically bonded phosphate ceramics
Waste Management & Research 2015, Vol. 33(2) 183–190 © The Author(s) 2015 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0734242X14563376 wmr.sagepub.com
Shaoqing Zhang1, Xinyan Zhang2, Ya Xiong3, Guoping Wang1 and Na Zheng1
Abstract In this study, two kinds of zeolites materials (natural zeolite and thiol-functionalised zeolite) were added to the chemically bonded phosphate ceramic processes to treat mercury-contaminated wastes. Strong promotion effects of zeolites (natural zeolite and thiolfunctionalised zeolite) on the stability of mercury in the wastes were obtained and these technologies showed promising advantages toward the traditional Portland cement process, i.e. using Portland cement as a solidification agent and natural or thiol-functionalised zeolite as a stabilisation agent. Not only is a high stabilisation efficiency (lowered the Toxicity Characteristic Leaching Procedure Hg by above 10%) obtained, but also a lower dosage of solidification (for thiol-functionalised zeolite as stabilisation agent, 0.5 g g1 and 0.7 g g-1 for chemically bonded phosphate ceramic and Portland cement, respectively) and stabilisation agents (for natural zeolite as stabilisation agent, 0.35 g g-1 and 0.4 g g-1 for chemically bonded phosphate ceramic and Portland cement, respectively) were used compared with the Portland cement process. Treated by thiol-functionalised zeolite and chemically bonded phosphate ceramic under optimum parameters, the waste containing 1500 mg Hg kg-1 passed the Toxicity Characteristic Leaching Procedure test. Moreover, stabilisation/solidification technology using natural zeolite and chemically bonded phosphate ceramic also passed the Toxicity Characteristic Leaching Procedure test (the mercury waste containing 625 mg Hg kg-1). Moreover, the presence of chloride and phosphate did not have a negative effect on the chemically bonded phosphate ceramic/thiol-functionalised zeolite treatment process; thus, showing potential for future application in treatment of ‘difficult-to-manage’ mercury-contaminated wastes or landfill disposal with high phosphate and chloride content. Keywords Chemically bonded phosphate ceramics, solidification/stabilisation, mercury-contaminated wastes, zeolites
Introduction Developing effective technologies to treat mercury-contaminated wastes is not only of great significance for environmental protection, but practically challenging owing to the limited economic benefit from mercury recycling or recovery, the high volatility, toxicity, and movability of mercury, and varied composition of mercury wastes. Mercury cannot be degraded, as an inorganic element, but its transformation into the environment could be reduced after being disposed by transforming into a less leaching state. To this end, various techniques have been developed to reduce the mobilisation of mercury and control its contamination after disposal into the environment, including roasting or retorting, stabilisation/solidification (S/S), vitrification, acid leaching, electro-remediation, etc. (Dermont et al., 2008a; Paria and Yuet, 2006; US EPA, 2007; Wang et al., 2012). Among them, S/S has been considered as best demonstrated available technology (BDAT) to treat wastes with less than 260 mg Hg kg-1, under the current Land Disposal Restrictions (LDR) programme and other
heavily contaminated mercury-mixed wastes, which are not suitable for thermal recovery and recycling (Paria and Yuet, 2006; US EPA, 2007; Wang et al., 2012). S/S processes include two parts: solidification and stabilisation; the former is to make a solidified mass by physical enclosure of the
1Northeast
Institute of Geography and Agroecology, Chinese Academy of Sciences, Changchun, People’s Republic of China 2School of Chemistry and Environmental Engineering, Changchun University of Science and Technology, Changchun, People’s Republic of China 3Beijing Municipal Research Institute of Environmental Protection, Beijing, People’s Republic of China Corresponding author: Na Zheng, Northeast Institute of Geography and Agroecology, Chinese Academy of Sciences, Changchun 130102, People’s Republic of China. Emails: [email protected]; [email protected]
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solidification agent and the latter is to utilise stabilisation agent to stabilise the contaminants by chemical reactions between them (Hekal et al., 2011; Ma et al., 2010; Minocha et al., 2003; US EPA, 2007; Zhen et al., 2012). By the type of solidification agent utilised, S/S can be classified as Portland cement, polyester resins, organic polymerisation, sulphide based, etc. (US EPA, 2007; Wang et al., 2012). Among these methods, cement-based S/S is of great importance as treatment choice for wastes owing to its low cost of material and equipment (Cullinane et al., 1986; Guha et al., 2006; Hulet et al., 2001). For stabilisation of mercury, Na2S (Hamilton and Bowers, 1997; Horpibulsuk et al., 2010), powder reactivated carbon (PAC) (Zhang and Bishop, 2002), ferric-lignin derivatives (FLD) (Zhuang et al., 2004), and thiol-functionalised zeolite (TFZ) have been studied and used as binders (Zhang et al., 2009). Basically, the materials involved in the mentioned cement processes are inexpensive and commercially available. However, these cement processes suffer serious drawbacks of significant increasing of waste mass and volume, and the questionable longevity of the solidified wastes (Dermont et al., 2008b; DOE, 1999). It has been found that mercury has a great potency to release from a cement base (Guha et al., 2006; Hulet et al., 2001). Great efforts have been paid on developing advanced processes of the conventional cement to remediate mercury-contaminated wastes. For example, chemically bonded phosphate ceramics (CBPC) has shown promising results for treating hazardous wastes (Jeong 1997; Singh and Wagh, 1998; Wagh et al., 1997). More recently, the CBPC process was studied as a cement replacement in the S/S treatment of mercury-containing light bulbs (Banerjee, 2005; Liu et al., 2008; Randall and Chattopadhyay, 2010). In this technology, the mercury-containing waste is hosted by magnesium potassium phosphate hydrate (MKP). Commonly alkali sulphide is used as a binder (Randall and Chattopadhyay, 2010). The advantage of this CBPC process is that it can treat both acidic and alkaline hazardous wastes (pH 3.5–11). Moreover, waste load of this process is higher and could be operated at a lower temperature when comparing with cement processes (DOE, 1999). In our previous work, we have demonstrated that adding zeolite material to the S/S process using Portland cement could greatly improve the stability effect of mercury in the waste (Zhang et al., 2009). In this study, we reported a more economic and effective S/S technology for mercury-contaminated wastes based on the CBPC process, and natural zeolites (NZs) and thiol-functioned zeolites (TFZ) were used as additives to improve the efficiency of the CBPC process. The aim of this study is to combine the high stabilisation efficiency of zeolites and the suitability of CBPC to treat a variety of ‘difficult-to-manage’ waste materials, since it has the advantage of treating both acidic and alkaline hazardous wastes within a wider range of pH values. The interferences of the technologies were systematically investigated and the optimum operation parameters were revealed to develop effective technologies for the treatment of mercury wastes, especially those highly mercury-contaminated ones with high salt.
Experimental Zeolites materials preparation The NZ mineral (Si/Al = 20; Jiutai Zeolite Mining Co. in Jilin, China) was used as the parent zeolite. The TFZ was prepared as reported in our previous work (Zhang et al., 2009). Basically, the thiol group was introduced by reaction between NZ with 3-mercaptopropyltriethoxysilane. Mercury adsorption and -SH content analysing were also performed as our previous work reported (Zhang et al., 2009).
S/S treatment The typical process is carried out as follows: 10.0 g of mercury surrogate waste (prepared with mercuric nitrate and sand) and 50 mL water were mixed in a 100 mL plastic bottle. Then, 1.0 g of zeolites (NZ or TFZ) was added. Then, the mixture pH was adjusted to a certain value (typically 6.0) by adding HNO3 or NaOH, and kept for 24 h before filtering through 0.45 μm filters. The total Hg concentration of the filtrate was analysed and was denoted as the stabilisation solution. Then the solid was mixed with phosphate ceramics, which was generated by reaction between MgO and KH2PO4 (mol ratio of MgO and KH2PO4 is 1:1) in solution, for solidification. The chemical equation is: MgO + KH 2 PO 4 + 5H 2O → MgKPO 4 . 6H 2O The formed MgKPO4·6H2O, MKP is a hard, dense ceramic. The ceramic paste mixtures were crushed after setting for 5 days at room temperature, and a Toxicity Characteristic Leaching Procedure (TCLP) was carried out to test the leaching of the solidified wastes. Influence of stabilisation pH. The stabilisation solution pH was adjusted and maintained at different values (from 3.0 to 12.0 at each integer). The surrogate used contained 1000 mg Hg kg-1. Zeolites and phosphate ceramics dosages were 0.02 g g-1 (mass ratio of zeolites/surrogate waste) and 1.0 g g-1 (mass ratio of phosphate ceramics/surrogate waste), respectively. Effects of dosages of zeolites and phosphate ceramics. Seven different zeolites dosages were used (zeolites/surrogate of 0, 0.05, 0.1, 0.2, 0.3, 0.4, and 0.5 g g-1) with phosphate ceramics dosages of 1.0 g g-1 to treat 10.0 g of surrogate containing 1000 mg Hg kg-1. Also, six different phosphate ceramics dosages were used (0, 0.2, 0.5, 1.0, 1.5, and 2.0 g g-1) with zeolites dosages of 0.02 g g-1. Interferences by anions. The effects of anions on S/S process were investigated; 0.5 mmol L-1 PO43− and 1 and 10 mmol L-1 Cl− were added into the stabilisation solution. Five mercury contents (100, 300, 500, 750, and 1000 mg Hg kg-1) of the surrogate were used. The dosages of zeolites and phosphate ceramics used were 0.02 g g-1 and 1.0 g g-1, respectively.
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Figure 1. The dependence of Equilibrium Hg on stabilisation pH.
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Figure 2. The dependence of the TCLP Hg of the treated surrogate on stabilisation pH.
NZ: natural zeolite; TFZ: thiol-functionalised zeolite.
NZ: natural zeolite; TCLP: Toxicity Characteristic Leaching Procedure; TFZ: thiol-functionalised zeolite.
Comparison of S/S processes under optimised operation parameters
Mercury analysis
To further evaluate these techniques, a series of surrogate waste containing different initial mercury contents were used (250, 500, 750, 1000, 1250, and 1500 mg kg-1) to investigate the influence of initial mercury contents on the S/S processes. For the process of NZ and CBPC, the optimum conditions are that pH ranges from 5.0, NZ dosage is 0.35 g g-1, and CBPC dosage is 1.75 g g-1; the optimum conditions for the process of TFZ and CBPC are pH at 6.0, zeolite loading of 0.1 g g-1, and CBPC dosage of more than 0.5 g g-1. Also, for comparison, the mercury wastes were treated by NZ and TFZ and Portland cement with the experiment conducted under optimal S/S conditions. For the process of NZ and Portland cement, the optimum conditions are pH at 6.0–7.0, NZ dosage is 0.4 g g-1, and Portland cement dosage is 1.0 g g-1; the optimum conditions for the process of TFZ and Portland cement are pH at 5.0, zeolite loading of 0.1 g g-1, and Portland cement dosage of more than 0.7 g g-1.
Treatment of real waste To further evaluate the effectiveness of S/S technologies using zeolites and phosphate ceramics, S/S treatment of real mercury wastes was performed. The real sediment wastes were obtained from two sites (in Huludao, China), which suffered heavy contamination of mercury from a surrounding zinc smelting plant. The total mercury content of the samples was 1430 and 625 mg Hg kg-1. The TCLP Hg of the sediments waste is 9.2 and 0.83 mg L-1, respectively. Consequently, they both would be categorised as hazardous wastes (TCLP>0.2 mg L-1). The S/S treatments were operated under optimal parameters obtained from mercury surrogate results.
TCLP leaching tests Hg leachability of solidified wastes was investigated with a TCLP test (US EPA Method 1311).
Mercury concentrations were analysed using a cold vapour atomic absorption spectrometer (F-732). The detection limit for the method was 0.2 μg L-1. Blanks and controls were carried out during the test for quality control. The S/S treatments were operated in triplicate (10% of the treatment). The mercury analyses were also conducted in triplicate. All the results are reported as the average values.
Results and discussion Physical characterisation and mercury adsorption of zeolites materials The as-prepared TFZ contains 0.562 mmol-SH g-1, far more than the loading in other modified zeolites reported by previous literatures (Bach et al., 2012; Bach et al., 2013; Chowdhury et al., 2012; Lou et al., 2012; Yavuz et al., 2009). Consequently, the mercury adsorption capacity of NZs increased by nearly 10 times after thiol introducing (as high as 0.445 mmol Hg g-1 for TFZ). This high mercury adsorption capacity of TFZ would greatly contribute to its stabilisation of mercury wastes on the S/S process. More detailed information about the structure and mercury adsorption behaviour of TFZ and NZ can be referred to our previous work (Zhang et al., 2009).
S/S treating of mercury surrogate PH effects. Figure 1 shows the dependence of mercury concentration in the stabilisation solution (Equilibrium Hg) on stabilisation pH; while Figure 2 shows the dependence of the TCLP Hg of the treated surrogate on stabilisation pH. As the Figure 1 shown, with TFZ used as additive, the equilibrium Hg at all stabilisation pH levels is quite low. The highest concentration occurred at pH 12, and it is below 0.4 mg L-1. Moreover, the equilibrium Hg increased greatly at a high pH region. The lowest equilibrium Hg occurred at pH 5.0 (0.013 mg L-1). This negative effect of high pH is probably caused by the interference of a strong alkaline
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Equilibrium Hg (mg/L)
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Figure 3. Effects of zeolites dosage on equilibrium Hg of treated surrogate. NZ: natural zeolite; TFZ: thiol-functionalised zeolite.
0.8 NZ TFZ
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Figure 4. Effects of zeolites dosage on TCLP Hg of treated surrogate. NZ: natural zeolite; TCLP: Toxicity Characteristic Leaching Procedure; TFZ: thiol-functionalised zeolite.
TCLP Hg (mg/L)
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results in Figure 2 have shown a similar effect of pH on the stabilisation effect. The lowest TCLP Hg (0.036 mg L-1) was also found at pH 6.0. Combining the above results, it could be concluded that the optimised stabilisation pH for CBPC and TFZ is at pH 6.0. On the other hand, for the process of NZ used as the stabilisation agent, its stabilisation effect is less significant than that of TFZ as the additive. The equilibrium Hg and TCLP Hg at all stabilisation pH levels are much higher than that for TFZ process. However, the stabilisation effect is still evident at a wide pH range of 4.0–9.0 (exhibits the lowest mercury concentration at pH 5.0, both in the stabilisation solution and the TCLP filtrate). This result indicates that NZ could also be used as an effective stabilisation agent using CBPC as solidification materials, although its stabilisation effect is less effective than TFZ, since the latter has a much higher mercury adsorption capacity. Moreover, it should be noted that the NZ process has a much wider pH range (pH 4.0–9.0) than TFZ, which may be explained by their different mercury adsorption mechanism; the former is mainly caused by the chemical reaction between -SH and mercury on the TFZ, and the latter is mainly physical adsorption on the zeolite surface (Lin et al., 2011; Rafatullah et al., 2010; Tseng, 2007; Yang et al., 2009). Hence, it could be understandable that the NZ process has a much wider pH range than TFZ, since it has been proved by previous researches that the physical adsorption process is usually less sensitive to the pH than the chemical process (Lin et al., 2011; Rafatullah et al., 2010; Tseng, 2007; Yang et al., 2009).
2.0
CBPC Dosage (g/g)
Figure 5. Effects of CBPC loading on TCLP Hg of treated surrogate (zeolite dosage of 0.02 g g-1).
NZ: natural zeolite; TCLP: Toxicity Characteristic Leaching Procedure; TFZ: thiol-functionalised zeolite.
environment on the reaction between the thiol group and Hg, and consequently decreased mercury adsorption by TFZ. The TCLP
Effect of zeolites and CBPC dosages. The effects of zeolites dosage on the mercury stabilisation during the S/S process are presented in Figure 3 and 4, respectively. With the increasing of NZ dosage from 0 (without stabilisation) to 0.2 g g-1, both the equilibrium Hg in the stabilisation solution and TCLP Hg decreased rapidly. When the NZ dosage further increased (from 0.2 to 0.5 g g-1), both equilibrium Hg and TCLP Hg decrease less obviously and keeps on quite a low level. The optimised NZ dosage is about 0.5 g g-1 for the waste with 1000 mg Hg kg-1. On the other hand, the introduction of CBPC is also effective in reducing the mobility of mercury (II). As shown in Figure 5, when the CBPC loading increased from 0.2 to 1.0 g g-1, the TCLP Hg decrease dramatically from 0.92 to 0.12 mg L-1, and then has little changes or decreases when the CBPC dosage further increased. These results indicate that both CBPC and NZ could play an important role in immobilising mercury and the optimised NZ and CBPC dosages are about 0.3–0.4 g g-1 and 1.5–2.0 g g-1 for the surrogate containing 1000 mg Hg kg-1. On the other hand, when TFZ is used as a stabilisation agent, the effects of TFZ dosages on the mercury stabilisation exhibit quite similar trends, as in the NZ/CBPC process but in a more obvious way. When the TFZ loading increased from 0 to 0.1 g g-1, both the equilibrium Hg and TCLP Hg in the filtrate decrease dramatically and keep on a much lower level at TFZ loading over 0.1 g g-1. The optimum TFZ dosage is about 0.1 g g-1 for the waste with 1000 mg Hg kg-1. Comparing these results with the NZ process, it could be concluded that the
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Equilibrium Hg (mg/L)
10 mmol/L Cl 1 mmol/L Cl 0.5 mmol/L PO34 Control
0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 0
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Initial Hg(mg/kg)
Figure 6. Interferences of anions on equilibrium Hg for surrogates using NZ as a stabilisation agent.
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TCLP Hg (mg/L)
Interferences by other anions. Under the real disposal landfill, there might be PO43- or Cl- present, or some mercury wastes with high salt content. The presence of these salt ions would increase the mercury mobilisation by forming more potentially leaching mercury chloride and phosphate complexes (Griffin and Shimp, 1978; Guevara et al., 2005; Jackson et al., 1986; Kinniburgh and Jackson, 1978; Zhang and Bishop, 2002). Treatment of these ‘difficult-to-manage’ waste materials or condition is still a scientific challenge and desires the development of more effective S/S technology under this harsh situation. Therefore, in this study, the interference or presence of chloride and phosphate on mercury stabilisation by S/S treated with CBPC and NZ and/or TFZ were investigated by treating a surrogate containing Cl- or PO43- to further evaluate the possible application of these technologies in the simulated waste or disposal situations. The results are shown as mercury concentration in stabilisation solutions and TCLP leachates, and the results for the S/S process treated with NZ and CBPC were presented in Figures 6 and 7. As shown, only a high concentration of Cl- (10 mmol L-1) has an obvious negative effect, while the influence of present phosphate and low concentration of Cl- (1 mmol L-1) is ignorable. Moreover, the negative effect of chloride is more serious for waste containing high concentration of mercury, i.e. 1000 mg kg1. The negative effect of Cl- on mercury stabilisation has been reported by several researchers (Piao and Bishop, 2006; Zhang and Bishop, 2002), which is mainly attributed to the formation of mercury chloride complexation. This would restrict the application of this S/S technology using NZ and CBPC for treatment of wastes with a high concentration of mercury and Cl-, such as brine purification sludge and chlor-alkali wastes and sludges, or the disposal landfill containing a high concentration of Cl-. The presence of chloride and phosphate, on the other hand, has a much less negative effect for the TFZ and CBPC process, as shown in Figures 8 and 9. Although a high concentration of chloride still could cause the increased equilibrium and TCLP Hg, all the TCLP Hg kept at relatively low levels (much less than 0.2 mg L-1). Remarkably, TCLP Hg for mercury waste containing 1000 mg Hg kg-1 has passed the TCLP limit, no matter at whatever level of Cl- in the experimental condition. These results indicate that this S/S technology using TFZ and CBPC performs well for serious mercury-contaminated wastes or in the landfill with high concentration of chloride and phosphate. This high
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Figure 7. Interferences of anions on TCLP Hg for surrogates using NZ as a stabilisation agent. TCLP: Toxicity Characteristic Leaching Procedure.
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introducing of TFZ is more effective in mercury stabilisation, which may be caused by its much great mercury adsorption capacity. Moreover, the fact that TFZ has a stronger stabilisation ability than NZ is even evident when comparing each of their dosages effect with the effect of CBPC dosages on Hg stabilisation. As shown in Figure 5, when the CBPC loading increased from 0.2 to 0.5 g g-1, TCLP Hg decreased, and then kept on this low level when the CBPC dosage increased from 0.5 to 2.0 g g-1. These results indicate that the contribution of CBPC on stabilising mercury is not important, especially after TFZ introduction; TFZ plays the major role in immobilising Hg for the S/S treatment by TFZ and CBPC. For this process, the optimum TFZ and CBPC dosages are about 0.1 and 0.5 g g-1 for the surrogate containing 1000 mg Hg kg-1.
10 mmol/L Cl 1 mmol/L Cl0.5 mmol/L PO43Control
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Figure 8. Interferences of anions on equilibrium Hg for surrogates using TFZ as a stabilisation agent.
tolerance to interferences of chloride and phosphate may be caused by the high mercury adsorption and low diffusion ability limited by the porous system of TFZ, which restricts mercury complexation with Cl- and PO43- (Piao and Bishop, 2006; Zhang and Bishop, 2002). Thus, it would show great potential for future
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Figure 9. Interferences of anions on TCLP Hg for surrogates using TFZ as a stabilisation agent.
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TCLP: Toxicity Characteristic Leaching Procedure.
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Figure 10. Comparison of the TCLP results of treated surrogates by different S/S technologies.
Untreated ( ); NZ and CBPC ( ); TFZ and CBPC ( ment ( ); TFZ and Cement ( ).
); NZ and Ce-
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0.8 0.6 0.4 0.2 0.0
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(625 mg/kg Hg)
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Figure 11. The TCLP results for treated real wastes.
NZ: natural zeolite; TCLP: Toxicity Characteristic Leaching Procedure; TFZ: thiol-functionalised zeolite.
application in the treatment of serious mercury-contaminated wastes and disposal in a landfill with high salt.
Comparison of S/S processes under optimised operation parameters The above results have shown treatment by zeolites and CBPC is effective to stabilise mercury surrogates. Compared with NZ, TFZ is a more efficient stabilisation agent and the S/S technology using TFZ and CBPC shows great potential for future treatment of serious mercury-contaminated wastes with high salt. Both the technologies have their optimised operation parameters, i.e. pH, zeolites dosage, and CBPC dosage, and these operation conditions would influence the stabilisation effect to different extent. For the process of NZ and CBPC, the optimum conditions are that pH ranges from 5.0, NZ dosage is 0.3–0.4 g g-1, and CBPC dosage is 1.5–2 g g-1, while for the process of TFZ and CBPC they are pH 6.0, zeolite dosage of 0.1 g g-1, and CBPC loading of more than 0.5 g g-1. To further evaluate these techniques, a series of surrogate wastes containing different initial mercury contents were used (250, 500, 750, 1000, 1250, and 1500 mg kg-1) to investigate the influence of initial mercury contents on the S/S processes. For each process, the experiments were conducted under optimum conditions to evaluate the possible application of these S/S techniques. As shown in Figure 10, after treatment by zeolites and CBPC, the TCLP Hg were greatly decreased for all the surrogate wastes containing 250~1500 mg Hg kg-1. For the S/S technology using NZ and CBPC, it could effectively immobilise mercury waste containing 500 mg Hg kg-1, as the TCLP Hg for mercury wastes below 500 mg Hg kg-1 passed the TCLP test. On the other hand, for the treatment by TFZ and CBPC, the mercury wastes below 1500 mg Hg kg-1 all passed the TCLP test, proving treatment by TFZ and CBPC is effective to stabilise mercury in the waste. Moreover, the above results also proves that TFZ is a more efficient stabilisation agent, since not only wastes with higher mercury content (1500 mg kg-1) were effectively stabilised, but also the process was conducted under the optimal S/S parameters with low dosage of zeolites and CBPC (TFZ of 0.1 g g-1 and CBPC of 0.5 g g-1) compared with the NZ process with an NZ dosage of 0.4 g g-1 and CBPC dosage of 2.0 g g-1. Moreover, compared with the S/S techniques using Portland cement and zeolites, which are demonstrated as one of the most effective S/S techniques to stabilise mercury wastes in previous literatures (Zhang et al., 2009), the S/S technologies using zeolites and CBPC reported here have shown much higher efficiency. As shown in Figure 10, no matter the amounts of NZ or TFZ used as a stabilisation agent, the TCLP Hg for the treated surrogate wastes are much lower for the CBPC and zeolites process compared with the Portland as a solidification agent, especially for the wastes with a mercury content above 1000 mg kg-1. For example, for NZ and TFZ used as stabilisation agent, the TCLP Hg of the treated surrogate with 1250 mg Hg kg-1 was reduced by 12% (from 0.74 mg L-1 for Portland cement to 0.65 mg L-1 for CBPC), and by 15% (from 0.13 mg L-1 for Portland cement to 0.11 mg L-1 for CBPC), respectively. Moreover, for the process of CBPC and zeolites, not only is a higher stabilisation efficiency (lowered the TCLP Hg by above 10%) obtained, but
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Zhang et al. also a lower dosage of solidification and stabilisation agents were used compared with the Portland cement process. For example, when TFZ is used as the stabilisation agent, optimal S/S parameters for the CBPC process are TFZ 0.1 g g-1 and CBPC 0.5 g g-1; while for the cement process, the TFZ dosage is 0.1 g g-1 and Portland cement 0.7 g g-1. These results indicate that, compared with Portland cement, CBPC is a more effective solidification agent. Moreover, as for the cost aspect, it has been estimated that the cost of the CBPC process ($15.45 per kg) is generally lower than that of conventional Portland cement stabilisation ($16.37 per kg) (both including disposal) (US EPA, 2003). Thus, the S/S technology using zeolites (especially TFZ) and CBPC is quite effective to stabilise mercury waste, and shows promising potential for the further application.
Treating of real mercury wastes To further evaluate treatment by zeolites (NZ and TFZ) and CBPC, two real mercury wastes were treated by zeolites and CBPC at the optimal S/S parameters. As shown in Figure 11, after treatment by TFZ and CBPC, TCLP Hg for both the real wastes (containing 1430 and 625 mg Hg kg-1) passed the TCLP test, while the TCLP Hg for the mercury waste containing 625 mg Hg kg-1 treated by NZ and CBPC passed the TCLP limit. These results confirm that the S/S treatment by zeolites and CBPC is effective to stabilise mercury wastes, and thus shows great potential for future application in immobilising highly contaminated mercury wastes.
Conclusions Strong promotion effects of zeolites on the stability of mercury in hazardous wastes were obtained and the technology showed certain advantages toward the traditional Portland cement process. Not only is high stabilisation efficiency obtained, but also a lower dosage of solidification and stabilisation agents were used compared with the Portland cement process. Moreover, TFZ is a more efficient stabilisation agent compared with NZ. For the process of NZ and CBPC, the optimum conditions are that pH ranges from 5.0, NZ dosage is 0.3–0.4 g g-1, and CBPC loading is 1.5–2 g g-1, while for the process of TFZ and CBPC these parameters are pH 6.0, zeolite dosage of 0.1 g g-1, and CBPC loading of more than 0.5 g g-1. Under optimum parameters, treated by TFZ and CBPC, the waste containing 1500 mg Hg kg-1 passed the TCLP test, proving treatment by TFZ and CBPC is effective to stabilise highly contaminated mercury waste. Treated by NZ and CBPC, the waste containing 625 mg Hg kg-1 passed the TCLP test. The technology using CBPC and TFZ shows potential for future application in treatment for difficult-to-manage mercury-contaminated wastes or disposal in landfill with high salt.
Declaration of conflicting interests The authors declare that there is no conflict of interest.
Funding This work was financially supported by the Key projects of science and technology development plan of Jilin province [No. 20120402], the National Natural Science Funds of China [No. 41171392], Jilin province science and technology development projects [20140520150JH], and the National Natural Science Foundation of China for Young Scholar [No. 21307007].
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research-article2014
WMR0010.1177/0734242X14563376Waste Management & ResearchZhang et al.
Original Article
Effective solidification/stabilisation of mercury-contaminated wastes using zeolites and chemically bonded phosphate ceramics
Waste Management & Research 2015, Vol. 33(2) 183–190 © The Author(s) 2015 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0734242X14563376 wmr.sagepub.com
Shaoqing Zhang1, Xinyan Zhang2, Ya Xiong3, Guoping Wang1 and Na Zheng1
Abstract In this study, two kinds of zeolites materials (natural zeolite and thiol-functionalised zeolite) were added to the chemically bonded phosphate ceramic processes to treat mercury-contaminated wastes. Strong promotion effects of zeolites (natural zeolite and thiolfunctionalised zeolite) on the stability of mercury in the wastes were obtained and these technologies showed promising advantages toward the traditional Portland cement process, i.e. using Portland cement as a solidification agent and natural or thiol-functionalised zeolite as a stabilisation agent. Not only is a high stabilisation efficiency (lowered the Toxicity Characteristic Leaching Procedure Hg by above 10%) obtained, but also a lower dosage of solidification (for thiol-functionalised zeolite as stabilisation agent, 0.5 g g1 and 0.7 g g-1 for chemically bonded phosphate ceramic and Portland cement, respectively) and stabilisation agents (for natural zeolite as stabilisation agent, 0.35 g g-1 and 0.4 g g-1 for chemically bonded phosphate ceramic and Portland cement, respectively) were used compared with the Portland cement process. Treated by thiol-functionalised zeolite and chemically bonded phosphate ceramic under optimum parameters, the waste containing 1500 mg Hg kg-1 passed the Toxicity Characteristic Leaching Procedure test. Moreover, stabilisation/solidification technology using natural zeolite and chemically bonded phosphate ceramic also passed the Toxicity Characteristic Leaching Procedure test (the mercury waste containing 625 mg Hg kg-1). Moreover, the presence of chloride and phosphate did not have a negative effect on the chemically bonded phosphate ceramic/thiol-functionalised zeolite treatment process; thus, showing potential for future application in treatment of ‘difficult-to-manage’ mercury-contaminated wastes or landfill disposal with high phosphate and chloride content. Keywords Chemically bonded phosphate ceramics, solidification/stabilisation, mercury-contaminated wastes, zeolites
Introduction Developing effective technologies to treat mercury-contaminated wastes is not only of great significance for environmental protection, but practically challenging owing to the limited economic benefit from mercury recycling or recovery, the high volatility, toxicity, and movability of mercury, and varied composition of mercury wastes. Mercury cannot be degraded, as an inorganic element, but its transformation into the environment could be reduced after being disposed by transforming into a less leaching state. To this end, various techniques have been developed to reduce the mobilisation of mercury and control its contamination after disposal into the environment, including roasting or retorting, stabilisation/solidification (S/S), vitrification, acid leaching, electro-remediation, etc. (Dermont et al., 2008a; Paria and Yuet, 2006; US EPA, 2007; Wang et al., 2012). Among them, S/S has been considered as best demonstrated available technology (BDAT) to treat wastes with less than 260 mg Hg kg-1, under the current Land Disposal Restrictions (LDR) programme and other
heavily contaminated mercury-mixed wastes, which are not suitable for thermal recovery and recycling (Paria and Yuet, 2006; US EPA, 2007; Wang et al., 2012). S/S processes include two parts: solidification and stabilisation; the former is to make a solidified mass by physical enclosure of the
1Northeast
Institute of Geography and Agroecology, Chinese Academy of Sciences, Changchun, People’s Republic of China 2School of Chemistry and Environmental Engineering, Changchun University of Science and Technology, Changchun, People’s Republic of China 3Beijing Municipal Research Institute of Environmental Protection, Beijing, People’s Republic of China Corresponding author: Na Zheng, Northeast Institute of Geography and Agroecology, Chinese Academy of Sciences, Changchun 130102, People’s Republic of China. Emails: [email protected]; [email protected]
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solidification agent and the latter is to utilise stabilisation agent to stabilise the contaminants by chemical reactions between them (Hekal et al., 2011; Ma et al., 2010; Minocha et al., 2003; US EPA, 2007; Zhen et al., 2012). By the type of solidification agent utilised, S/S can be classified as Portland cement, polyester resins, organic polymerisation, sulphide based, etc. (US EPA, 2007; Wang et al., 2012). Among these methods, cement-based S/S is of great importance as treatment choice for wastes owing to its low cost of material and equipment (Cullinane et al., 1986; Guha et al., 2006; Hulet et al., 2001). For stabilisation of mercury, Na2S (Hamilton and Bowers, 1997; Horpibulsuk et al., 2010), powder reactivated carbon (PAC) (Zhang and Bishop, 2002), ferric-lignin derivatives (FLD) (Zhuang et al., 2004), and thiol-functionalised zeolite (TFZ) have been studied and used as binders (Zhang et al., 2009). Basically, the materials involved in the mentioned cement processes are inexpensive and commercially available. However, these cement processes suffer serious drawbacks of significant increasing of waste mass and volume, and the questionable longevity of the solidified wastes (Dermont et al., 2008b; DOE, 1999). It has been found that mercury has a great potency to release from a cement base (Guha et al., 2006; Hulet et al., 2001). Great efforts have been paid on developing advanced processes of the conventional cement to remediate mercury-contaminated wastes. For example, chemically bonded phosphate ceramics (CBPC) has shown promising results for treating hazardous wastes (Jeong 1997; Singh and Wagh, 1998; Wagh et al., 1997). More recently, the CBPC process was studied as a cement replacement in the S/S treatment of mercury-containing light bulbs (Banerjee, 2005; Liu et al., 2008; Randall and Chattopadhyay, 2010). In this technology, the mercury-containing waste is hosted by magnesium potassium phosphate hydrate (MKP). Commonly alkali sulphide is used as a binder (Randall and Chattopadhyay, 2010). The advantage of this CBPC process is that it can treat both acidic and alkaline hazardous wastes (pH 3.5–11). Moreover, waste load of this process is higher and could be operated at a lower temperature when comparing with cement processes (DOE, 1999). In our previous work, we have demonstrated that adding zeolite material to the S/S process using Portland cement could greatly improve the stability effect of mercury in the waste (Zhang et al., 2009). In this study, we reported a more economic and effective S/S technology for mercury-contaminated wastes based on the CBPC process, and natural zeolites (NZs) and thiol-functioned zeolites (TFZ) were used as additives to improve the efficiency of the CBPC process. The aim of this study is to combine the high stabilisation efficiency of zeolites and the suitability of CBPC to treat a variety of ‘difficult-to-manage’ waste materials, since it has the advantage of treating both acidic and alkaline hazardous wastes within a wider range of pH values. The interferences of the technologies were systematically investigated and the optimum operation parameters were revealed to develop effective technologies for the treatment of mercury wastes, especially those highly mercury-contaminated ones with high salt.
Experimental Zeolites materials preparation The NZ mineral (Si/Al = 20; Jiutai Zeolite Mining Co. in Jilin, China) was used as the parent zeolite. The TFZ was prepared as reported in our previous work (Zhang et al., 2009). Basically, the thiol group was introduced by reaction between NZ with 3-mercaptopropyltriethoxysilane. Mercury adsorption and -SH content analysing were also performed as our previous work reported (Zhang et al., 2009).
S/S treatment The typical process is carried out as follows: 10.0 g of mercury surrogate waste (prepared with mercuric nitrate and sand) and 50 mL water were mixed in a 100 mL plastic bottle. Then, 1.0 g of zeolites (NZ or TFZ) was added. Then, the mixture pH was adjusted to a certain value (typically 6.0) by adding HNO3 or NaOH, and kept for 24 h before filtering through 0.45 μm filters. The total Hg concentration of the filtrate was analysed and was denoted as the stabilisation solution. Then the solid was mixed with phosphate ceramics, which was generated by reaction between MgO and KH2PO4 (mol ratio of MgO and KH2PO4 is 1:1) in solution, for solidification. The chemical equation is: MgO + KH 2 PO 4 + 5H 2O → MgKPO 4 . 6H 2O The formed MgKPO4·6H2O, MKP is a hard, dense ceramic. The ceramic paste mixtures were crushed after setting for 5 days at room temperature, and a Toxicity Characteristic Leaching Procedure (TCLP) was carried out to test the leaching of the solidified wastes. Influence of stabilisation pH. The stabilisation solution pH was adjusted and maintained at different values (from 3.0 to 12.0 at each integer). The surrogate used contained 1000 mg Hg kg-1. Zeolites and phosphate ceramics dosages were 0.02 g g-1 (mass ratio of zeolites/surrogate waste) and 1.0 g g-1 (mass ratio of phosphate ceramics/surrogate waste), respectively. Effects of dosages of zeolites and phosphate ceramics. Seven different zeolites dosages were used (zeolites/surrogate of 0, 0.05, 0.1, 0.2, 0.3, 0.4, and 0.5 g g-1) with phosphate ceramics dosages of 1.0 g g-1 to treat 10.0 g of surrogate containing 1000 mg Hg kg-1. Also, six different phosphate ceramics dosages were used (0, 0.2, 0.5, 1.0, 1.5, and 2.0 g g-1) with zeolites dosages of 0.02 g g-1. Interferences by anions. The effects of anions on S/S process were investigated; 0.5 mmol L-1 PO43− and 1 and 10 mmol L-1 Cl− were added into the stabilisation solution. Five mercury contents (100, 300, 500, 750, and 1000 mg Hg kg-1) of the surrogate were used. The dosages of zeolites and phosphate ceramics used were 0.02 g g-1 and 1.0 g g-1, respectively.
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Figure 1. The dependence of Equilibrium Hg on stabilisation pH.
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NZ: natural zeolite; TFZ: thiol-functionalised zeolite.
NZ: natural zeolite; TCLP: Toxicity Characteristic Leaching Procedure; TFZ: thiol-functionalised zeolite.
Comparison of S/S processes under optimised operation parameters
Mercury analysis
To further evaluate these techniques, a series of surrogate waste containing different initial mercury contents were used (250, 500, 750, 1000, 1250, and 1500 mg kg-1) to investigate the influence of initial mercury contents on the S/S processes. For the process of NZ and CBPC, the optimum conditions are that pH ranges from 5.0, NZ dosage is 0.35 g g-1, and CBPC dosage is 1.75 g g-1; the optimum conditions for the process of TFZ and CBPC are pH at 6.0, zeolite loading of 0.1 g g-1, and CBPC dosage of more than 0.5 g g-1. Also, for comparison, the mercury wastes were treated by NZ and TFZ and Portland cement with the experiment conducted under optimal S/S conditions. For the process of NZ and Portland cement, the optimum conditions are pH at 6.0–7.0, NZ dosage is 0.4 g g-1, and Portland cement dosage is 1.0 g g-1; the optimum conditions for the process of TFZ and Portland cement are pH at 5.0, zeolite loading of 0.1 g g-1, and Portland cement dosage of more than 0.7 g g-1.
Treatment of real waste To further evaluate the effectiveness of S/S technologies using zeolites and phosphate ceramics, S/S treatment of real mercury wastes was performed. The real sediment wastes were obtained from two sites (in Huludao, China), which suffered heavy contamination of mercury from a surrounding zinc smelting plant. The total mercury content of the samples was 1430 and 625 mg Hg kg-1. The TCLP Hg of the sediments waste is 9.2 and 0.83 mg L-1, respectively. Consequently, they both would be categorised as hazardous wastes (TCLP>0.2 mg L-1). The S/S treatments were operated under optimal parameters obtained from mercury surrogate results.
TCLP leaching tests Hg leachability of solidified wastes was investigated with a TCLP test (US EPA Method 1311).
Mercury concentrations were analysed using a cold vapour atomic absorption spectrometer (F-732). The detection limit for the method was 0.2 μg L-1. Blanks and controls were carried out during the test for quality control. The S/S treatments were operated in triplicate (10% of the treatment). The mercury analyses were also conducted in triplicate. All the results are reported as the average values.
Results and discussion Physical characterisation and mercury adsorption of zeolites materials The as-prepared TFZ contains 0.562 mmol-SH g-1, far more than the loading in other modified zeolites reported by previous literatures (Bach et al., 2012; Bach et al., 2013; Chowdhury et al., 2012; Lou et al., 2012; Yavuz et al., 2009). Consequently, the mercury adsorption capacity of NZs increased by nearly 10 times after thiol introducing (as high as 0.445 mmol Hg g-1 for TFZ). This high mercury adsorption capacity of TFZ would greatly contribute to its stabilisation of mercury wastes on the S/S process. More detailed information about the structure and mercury adsorption behaviour of TFZ and NZ can be referred to our previous work (Zhang et al., 2009).
S/S treating of mercury surrogate PH effects. Figure 1 shows the dependence of mercury concentration in the stabilisation solution (Equilibrium Hg) on stabilisation pH; while Figure 2 shows the dependence of the TCLP Hg of the treated surrogate on stabilisation pH. As the Figure 1 shown, with TFZ used as additive, the equilibrium Hg at all stabilisation pH levels is quite low. The highest concentration occurred at pH 12, and it is below 0.4 mg L-1. Moreover, the equilibrium Hg increased greatly at a high pH region. The lowest equilibrium Hg occurred at pH 5.0 (0.013 mg L-1). This negative effect of high pH is probably caused by the interference of a strong alkaline
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Figure 3. Effects of zeolites dosage on equilibrium Hg of treated surrogate. NZ: natural zeolite; TFZ: thiol-functionalised zeolite.
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Figure 4. Effects of zeolites dosage on TCLP Hg of treated surrogate. NZ: natural zeolite; TCLP: Toxicity Characteristic Leaching Procedure; TFZ: thiol-functionalised zeolite.
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results in Figure 2 have shown a similar effect of pH on the stabilisation effect. The lowest TCLP Hg (0.036 mg L-1) was also found at pH 6.0. Combining the above results, it could be concluded that the optimised stabilisation pH for CBPC and TFZ is at pH 6.0. On the other hand, for the process of NZ used as the stabilisation agent, its stabilisation effect is less significant than that of TFZ as the additive. The equilibrium Hg and TCLP Hg at all stabilisation pH levels are much higher than that for TFZ process. However, the stabilisation effect is still evident at a wide pH range of 4.0–9.0 (exhibits the lowest mercury concentration at pH 5.0, both in the stabilisation solution and the TCLP filtrate). This result indicates that NZ could also be used as an effective stabilisation agent using CBPC as solidification materials, although its stabilisation effect is less effective than TFZ, since the latter has a much higher mercury adsorption capacity. Moreover, it should be noted that the NZ process has a much wider pH range (pH 4.0–9.0) than TFZ, which may be explained by their different mercury adsorption mechanism; the former is mainly caused by the chemical reaction between -SH and mercury on the TFZ, and the latter is mainly physical adsorption on the zeolite surface (Lin et al., 2011; Rafatullah et al., 2010; Tseng, 2007; Yang et al., 2009). Hence, it could be understandable that the NZ process has a much wider pH range than TFZ, since it has been proved by previous researches that the physical adsorption process is usually less sensitive to the pH than the chemical process (Lin et al., 2011; Rafatullah et al., 2010; Tseng, 2007; Yang et al., 2009).
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Figure 5. Effects of CBPC loading on TCLP Hg of treated surrogate (zeolite dosage of 0.02 g g-1).
NZ: natural zeolite; TCLP: Toxicity Characteristic Leaching Procedure; TFZ: thiol-functionalised zeolite.
environment on the reaction between the thiol group and Hg, and consequently decreased mercury adsorption by TFZ. The TCLP
Effect of zeolites and CBPC dosages. The effects of zeolites dosage on the mercury stabilisation during the S/S process are presented in Figure 3 and 4, respectively. With the increasing of NZ dosage from 0 (without stabilisation) to 0.2 g g-1, both the equilibrium Hg in the stabilisation solution and TCLP Hg decreased rapidly. When the NZ dosage further increased (from 0.2 to 0.5 g g-1), both equilibrium Hg and TCLP Hg decrease less obviously and keeps on quite a low level. The optimised NZ dosage is about 0.5 g g-1 for the waste with 1000 mg Hg kg-1. On the other hand, the introduction of CBPC is also effective in reducing the mobility of mercury (II). As shown in Figure 5, when the CBPC loading increased from 0.2 to 1.0 g g-1, the TCLP Hg decrease dramatically from 0.92 to 0.12 mg L-1, and then has little changes or decreases when the CBPC dosage further increased. These results indicate that both CBPC and NZ could play an important role in immobilising mercury and the optimised NZ and CBPC dosages are about 0.3–0.4 g g-1 and 1.5–2.0 g g-1 for the surrogate containing 1000 mg Hg kg-1. On the other hand, when TFZ is used as a stabilisation agent, the effects of TFZ dosages on the mercury stabilisation exhibit quite similar trends, as in the NZ/CBPC process but in a more obvious way. When the TFZ loading increased from 0 to 0.1 g g-1, both the equilibrium Hg and TCLP Hg in the filtrate decrease dramatically and keep on a much lower level at TFZ loading over 0.1 g g-1. The optimum TFZ dosage is about 0.1 g g-1 for the waste with 1000 mg Hg kg-1. Comparing these results with the NZ process, it could be concluded that the
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Equilibrium Hg (mg/L)
10 mmol/L Cl 1 mmol/L Cl 0.5 mmol/L PO34 Control
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Figure 6. Interferences of anions on equilibrium Hg for surrogates using NZ as a stabilisation agent.
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Interferences by other anions. Under the real disposal landfill, there might be PO43- or Cl- present, or some mercury wastes with high salt content. The presence of these salt ions would increase the mercury mobilisation by forming more potentially leaching mercury chloride and phosphate complexes (Griffin and Shimp, 1978; Guevara et al., 2005; Jackson et al., 1986; Kinniburgh and Jackson, 1978; Zhang and Bishop, 2002). Treatment of these ‘difficult-to-manage’ waste materials or condition is still a scientific challenge and desires the development of more effective S/S technology under this harsh situation. Therefore, in this study, the interference or presence of chloride and phosphate on mercury stabilisation by S/S treated with CBPC and NZ and/or TFZ were investigated by treating a surrogate containing Cl- or PO43- to further evaluate the possible application of these technologies in the simulated waste or disposal situations. The results are shown as mercury concentration in stabilisation solutions and TCLP leachates, and the results for the S/S process treated with NZ and CBPC were presented in Figures 6 and 7. As shown, only a high concentration of Cl- (10 mmol L-1) has an obvious negative effect, while the influence of present phosphate and low concentration of Cl- (1 mmol L-1) is ignorable. Moreover, the negative effect of chloride is more serious for waste containing high concentration of mercury, i.e. 1000 mg kg1. The negative effect of Cl- on mercury stabilisation has been reported by several researchers (Piao and Bishop, 2006; Zhang and Bishop, 2002), which is mainly attributed to the formation of mercury chloride complexation. This would restrict the application of this S/S technology using NZ and CBPC for treatment of wastes with a high concentration of mercury and Cl-, such as brine purification sludge and chlor-alkali wastes and sludges, or the disposal landfill containing a high concentration of Cl-. The presence of chloride and phosphate, on the other hand, has a much less negative effect for the TFZ and CBPC process, as shown in Figures 8 and 9. Although a high concentration of chloride still could cause the increased equilibrium and TCLP Hg, all the TCLP Hg kept at relatively low levels (much less than 0.2 mg L-1). Remarkably, TCLP Hg for mercury waste containing 1000 mg Hg kg-1 has passed the TCLP limit, no matter at whatever level of Cl- in the experimental condition. These results indicate that this S/S technology using TFZ and CBPC performs well for serious mercury-contaminated wastes or in the landfill with high concentration of chloride and phosphate. This high
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introducing of TFZ is more effective in mercury stabilisation, which may be caused by its much great mercury adsorption capacity. Moreover, the fact that TFZ has a stronger stabilisation ability than NZ is even evident when comparing each of their dosages effect with the effect of CBPC dosages on Hg stabilisation. As shown in Figure 5, when the CBPC loading increased from 0.2 to 0.5 g g-1, TCLP Hg decreased, and then kept on this low level when the CBPC dosage increased from 0.5 to 2.0 g g-1. These results indicate that the contribution of CBPC on stabilising mercury is not important, especially after TFZ introduction; TFZ plays the major role in immobilising Hg for the S/S treatment by TFZ and CBPC. For this process, the optimum TFZ and CBPC dosages are about 0.1 and 0.5 g g-1 for the surrogate containing 1000 mg Hg kg-1.
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Figure 8. Interferences of anions on equilibrium Hg for surrogates using TFZ as a stabilisation agent.
tolerance to interferences of chloride and phosphate may be caused by the high mercury adsorption and low diffusion ability limited by the porous system of TFZ, which restricts mercury complexation with Cl- and PO43- (Piao and Bishop, 2006; Zhang and Bishop, 2002). Thus, it would show great potential for future
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TCLP Hg (mg/L)
Waste Management & Research 33(2) 0.24 0.22 0.20 0.18 0.10
TCLP Limit 10 mmol/L Cl 1 mmol/L Cl 3-
0.08
0.5 mmol/L PO4 Control
0.06 0.04 0.02 0.00 0
200
400
600
800
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Initial Hg(mg/kg)
Figure 9. Interferences of anions on TCLP Hg for surrogates using TFZ as a stabilisation agent.
TCLP Hg (mg/L)
TCLP: Toxicity Characteristic Leaching Procedure.
20 15 10 5 1.0 0.8 0.6 0.4 TCLP Limit
0.2 0.0
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500
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Figure 10. Comparison of the TCLP results of treated surrogates by different S/S technologies.
Untreated ( ); NZ and CBPC ( ); TFZ and CBPC ( ment ( ); TFZ and Cement ( ).
); NZ and Ce-
10
TCLP Hg (mg/L)
9
Untreated NZ TFZ
0.8 0.6 0.4 0.2 0.0
A
(625 mg/kg Hg)
B
(1430 mg/kg Hg)
Figure 11. The TCLP results for treated real wastes.
NZ: natural zeolite; TCLP: Toxicity Characteristic Leaching Procedure; TFZ: thiol-functionalised zeolite.
application in the treatment of serious mercury-contaminated wastes and disposal in a landfill with high salt.
Comparison of S/S processes under optimised operation parameters The above results have shown treatment by zeolites and CBPC is effective to stabilise mercury surrogates. Compared with NZ, TFZ is a more efficient stabilisation agent and the S/S technology using TFZ and CBPC shows great potential for future treatment of serious mercury-contaminated wastes with high salt. Both the technologies have their optimised operation parameters, i.e. pH, zeolites dosage, and CBPC dosage, and these operation conditions would influence the stabilisation effect to different extent. For the process of NZ and CBPC, the optimum conditions are that pH ranges from 5.0, NZ dosage is 0.3–0.4 g g-1, and CBPC dosage is 1.5–2 g g-1, while for the process of TFZ and CBPC they are pH 6.0, zeolite dosage of 0.1 g g-1, and CBPC loading of more than 0.5 g g-1. To further evaluate these techniques, a series of surrogate wastes containing different initial mercury contents were used (250, 500, 750, 1000, 1250, and 1500 mg kg-1) to investigate the influence of initial mercury contents on the S/S processes. For each process, the experiments were conducted under optimum conditions to evaluate the possible application of these S/S techniques. As shown in Figure 10, after treatment by zeolites and CBPC, the TCLP Hg were greatly decreased for all the surrogate wastes containing 250~1500 mg Hg kg-1. For the S/S technology using NZ and CBPC, it could effectively immobilise mercury waste containing 500 mg Hg kg-1, as the TCLP Hg for mercury wastes below 500 mg Hg kg-1 passed the TCLP test. On the other hand, for the treatment by TFZ and CBPC, the mercury wastes below 1500 mg Hg kg-1 all passed the TCLP test, proving treatment by TFZ and CBPC is effective to stabilise mercury in the waste. Moreover, the above results also proves that TFZ is a more efficient stabilisation agent, since not only wastes with higher mercury content (1500 mg kg-1) were effectively stabilised, but also the process was conducted under the optimal S/S parameters with low dosage of zeolites and CBPC (TFZ of 0.1 g g-1 and CBPC of 0.5 g g-1) compared with the NZ process with an NZ dosage of 0.4 g g-1 and CBPC dosage of 2.0 g g-1. Moreover, compared with the S/S techniques using Portland cement and zeolites, which are demonstrated as one of the most effective S/S techniques to stabilise mercury wastes in previous literatures (Zhang et al., 2009), the S/S technologies using zeolites and CBPC reported here have shown much higher efficiency. As shown in Figure 10, no matter the amounts of NZ or TFZ used as a stabilisation agent, the TCLP Hg for the treated surrogate wastes are much lower for the CBPC and zeolites process compared with the Portland as a solidification agent, especially for the wastes with a mercury content above 1000 mg kg-1. For example, for NZ and TFZ used as stabilisation agent, the TCLP Hg of the treated surrogate with 1250 mg Hg kg-1 was reduced by 12% (from 0.74 mg L-1 for Portland cement to 0.65 mg L-1 for CBPC), and by 15% (from 0.13 mg L-1 for Portland cement to 0.11 mg L-1 for CBPC), respectively. Moreover, for the process of CBPC and zeolites, not only is a higher stabilisation efficiency (lowered the TCLP Hg by above 10%) obtained, but
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Zhang et al. also a lower dosage of solidification and stabilisation agents were used compared with the Portland cement process. For example, when TFZ is used as the stabilisation agent, optimal S/S parameters for the CBPC process are TFZ 0.1 g g-1 and CBPC 0.5 g g-1; while for the cement process, the TFZ dosage is 0.1 g g-1 and Portland cement 0.7 g g-1. These results indicate that, compared with Portland cement, CBPC is a more effective solidification agent. Moreover, as for the cost aspect, it has been estimated that the cost of the CBPC process ($15.45 per kg) is generally lower than that of conventional Portland cement stabilisation ($16.37 per kg) (both including disposal) (US EPA, 2003). Thus, the S/S technology using zeolites (especially TFZ) and CBPC is quite effective to stabilise mercury waste, and shows promising potential for the further application.
Treating of real mercury wastes To further evaluate treatment by zeolites (NZ and TFZ) and CBPC, two real mercury wastes were treated by zeolites and CBPC at the optimal S/S parameters. As shown in Figure 11, after treatment by TFZ and CBPC, TCLP Hg for both the real wastes (containing 1430 and 625 mg Hg kg-1) passed the TCLP test, while the TCLP Hg for the mercury waste containing 625 mg Hg kg-1 treated by NZ and CBPC passed the TCLP limit. These results confirm that the S/S treatment by zeolites and CBPC is effective to stabilise mercury wastes, and thus shows great potential for future application in immobilising highly contaminated mercury wastes.
Conclusions Strong promotion effects of zeolites on the stability of mercury in hazardous wastes were obtained and the technology showed certain advantages toward the traditional Portland cement process. Not only is high stabilisation efficiency obtained, but also a lower dosage of solidification and stabilisation agents were used compared with the Portland cement process. Moreover, TFZ is a more efficient stabilisation agent compared with NZ. For the process of NZ and CBPC, the optimum conditions are that pH ranges from 5.0, NZ dosage is 0.3–0.4 g g-1, and CBPC loading is 1.5–2 g g-1, while for the process of TFZ and CBPC these parameters are pH 6.0, zeolite dosage of 0.1 g g-1, and CBPC loading of more than 0.5 g g-1. Under optimum parameters, treated by TFZ and CBPC, the waste containing 1500 mg Hg kg-1 passed the TCLP test, proving treatment by TFZ and CBPC is effective to stabilise highly contaminated mercury waste. Treated by NZ and CBPC, the waste containing 625 mg Hg kg-1 passed the TCLP test. The technology using CBPC and TFZ shows potential for future application in treatment for difficult-to-manage mercury-contaminated wastes or disposal in landfill with high salt.
Declaration of conflicting interests The authors declare that there is no conflict of interest.
Funding This work was financially supported by the Key projects of science and technology development plan of Jilin province [No. 20120402], the National Natural Science Funds of China [No. 41171392], Jilin province science and technology development projects [20140520150JH], and the National Natural Science Foundation of China for Young Scholar [No. 21307007].
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