Journal of Hazardous Materials 283 (2015) 423–431
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Heavy metal removal and speciation transformation through the calcination treatment of phosphorus-enriched sewage sludge ash Rundong Li ∗ , Weiwei Zhao, Yanlong Li, Weiyun Wang, Xuan Zhu College of Energy and Environment, Shenyang Aerospace University, The Key Laboratory of Clean Energy in Liaoning Province, Shenyang, China
h i g h l i g h t s • • • • •
Phosphorus-enriched sewage sludge ash was calcined with organic and inorganic chlorinating agent. The removal, speciation transformation of heavy metal and the fixation of P were studied. 98.9% of Cu and 98.6% of Zn can be removed from phosphorus-enriched sewage sludge ash. Suitable conditions enabled the stable distribution of heavy metals (Cd, As, Pb, Zn, Cu, Cr, and Ni) with low bioavailability. Excessive organic chlorinating agent (PVC) addition reduced Cu removal obviously owning to the presence of carbon and hydrogen.
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Article history: Received 26 July 2014 Received in revised form 23 September 2014 Accepted 24 September 2014 Available online 2 October 2014 Keywords: Sewage sludge ash Heavy metals Removal Chemical speciation Phosphorus recovery
a b s t r a c t On the basis of the heavy metal (Cd, As, Pb, Zn, Cu, Cr, and Ni) control problem during the thermochemical recovery of phosphorus (P) from sewage sludge (SS), P-enriched sewage sludge ash (PSSA) was calcined at 1100 ◦ C. The effect of organic chlorinating agent (PVC) and inorganic chlorinating agent (MgCl2 ) on the fixed rate of P removal and the speciation transformation of heavy metal was studied. The removal of heavy metals Cd, Pb, As, Zn, and Cr exhibited an increasing tendency with the addition of chlorinating agent (PVC). However, an obvious peak under 100 g Cl/kg of PSSA appeared for Cu, owing to the presence of carbon and hydrogen in PVC. MgCl2 was found to be more effective than PVC in the removal of most heavy metals, such that up to 98.9% of Cu and 97.3% of Zn was effectively removed. Analyses of heavy metal forms showed that Pb and Zn occurred in the residue fraction after calcination. Meanwhile, the residue fraction of Cr, Ni, Cd, and Cu exhibited a decreasing tendency with the increase in the added chlorinating agent (MgCl2 ). Losses of P from PSSA were around 16.6% without the addition of chlorinating agent, which were greatly reduced to around 7.7% (PVC) and to only 1.7% (MgCl2 ). © 2014 Elsevier B.V. All rights reserved.
1. Introduction Phosphorus (P) is an essential element for biological growth and is a non-renewable resource. The world has limited phosphate resources, with forecast reserves maintained for less than 100 years [1]. Phosphate has been listed among the 20 minerals that cannot meet the demand of national economic growth in China [2]. Sewage sludge (SS) dry solids contains a vast amount of inorganic minerals and abundant nutrients, particularly large amounts of P. Related studies have demonstrated that the average P content of SS in China is 22 g P/kg SS dry solids, with the maximum reaching 37 g P/kg SS dry solids [3], which is generally considered as the largest secondary P resource both locally and internationally
∗ Corresponding author. Tel.: +86 24 89728889; fax: +86 24 89724558. E-mail addresses: [email protected], [email protected] (R. Li). http://dx.doi.org/10.1016/j.jhazmat.2014.09.052 0304-3894/© 2014 Elsevier B.V. All rights reserved.
[4], but SS is enriched in organic micropollutants (PCBs, AOX), pathogens, and various heavy metal elements [5,6]. Thus, SS is restricted for agricultural use. SS incineration technology has special advantages, including quick disposal, large volume and weight reduction ratio, and energy recovery. When organic contaminants and pathogens are effectively destroyed, 60–70% P is concentrated in the SS ash (SSA), and phosphate ore grade maximum reaches 26%, with an average of 15% (by P2 O5 ) [4,7]. During incineration, the P and heavy metal contents in SSA decrease with the addition of HCl, but increase with the addition of CaO. CaO has a fixed effect on P and heavy metals [8]. However, most heavy metals are also concentrated in SSA, which are in excess of the standards, and some are distributed among unstable fractions with high bioavailability. Some papers have studied the ecotoxicity of SSA to identify if it is a hazardous material. Lapa et al. worked that, the SSA was categorized into three types of SSA (bottom ash, fly ashes in the first and second
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Fig. 1. The experimental setup of vertical calcination furnace.
Substance
SiO2 Al2 O3 Fe2 O3 MgO CaO K2 O P2 O5 SO3
25.61 13.53 11.99 5.5 17.54 3.81 16.63 5.25
Cd Pb As Zn Cu Cr Ni
Concentration (ppm) PSSA
SD
SS
SD
4.1 104.9 48.5 3830.0 13,160.0 2201.6 843.5
0.1 2.0 1.0 46.2 42.5 36.6 20.1
3.0 86.4 51.3 2277.3 5248.2 935.0 342.5
0.1 2.4 1.8 36.2 31.4 20.4 13.9
(a)
CuCl(g) (CuCl)3(g) Cu3P(s) Cu2S(s) (CuO)(Fe2O3)(s)
100
Percentage (%)
80
cyclone). Ashes trapped in the first and second cyclones and the ashes retained in the second cyclone must be classified as ecotoxic materials due to the concentrations of Cr, Cr(VI) and As, but the bottom Ash was classified as non-ecotoxic material in the French Regulation CEMWE [9] while the study by Donatello showed SSA is close to the boundary that distinguishes between hazardous and
Fraction 2, reducible
Fraction 3, oxidizable
Fraction 4, residual
12 7. 8
10 0
1 12 20 7. 7
60
80
40
0. 00 0. 05 0. 10 2 4 6 8 10 20
A 1 ± 0.001 g sample was weighed and placed in 100 mL polypropylene centrifuge tube. Then, 0.11 mol/L HOAc (40 mL) was added. The mixture was shaken at 22 ± 5 ◦ C for 16 h, followed by centrifuging and filtering. First, 0.5 mol/L NH4 OH·HCl (40 mL) (adjusted to pH = 1.5 with HNO3 ) was added to the residue from the previous step. Second, the mixture was shaken at 22 ± 5 ◦ C for 16 h, followed by centrifuging and filtering. First, 8.8 mol/L H2 O2 (10 mL) (adjusted to pH = 1.5 with HNO3 ) was added to the residue from the previous step. Second, the mixture was digested at room temperature for 1 h with occasional manual shaking, and then 8.8 mol/L H2 O2 (10 mL) was added again, after which the tube was covered. Second, the mixture was heated at 85 ± 2 ◦ C for 1 h. Third, the cover was removed, mixture was heated at 85 ± 2 ◦ C until the volume reduced to near dryness. Finally, 1 mol/L NH4 OAc (50 mL) (adjusted to pH = 2 with HNO3 ) was added at 22 ± 5 ◦ C for 16 h. The remaining residue was digested by using the same method employed in the total digestion of PSSA.
0
Cl-Addition (g Cl/Kg ash)
(b)
CuCl(g) (CuCl)3(g) Cu2S(s) (CuO)(Fe2O3)(s)
100
80
Percentage (%)
Extraction method
Fraction 1, acid soluble
40
20
Table 2 BCR sequential extraction protocol. Fraction
60
28 0
Mass fraction (%)
24 0
Substance
16 0
Heavy metals
20 0
Major composition (PSSA)
12 7. 9
Table 1 Heavy metals content and standard deviations (averages of five replicates) of the SS and average PSSA sample and chemical composition of average PSSA sample.
non-hazardous waste and speciation of zinc was a key consideration in deterring whether or not in the EU Hazardous Waste Directive [10]. However, the high content of heavy metals must be separated from SSA for recovery of phosphorus whether it was classified as hazardous waste or not. Heavy metal chlorides have high vapor pressure. The condensation process of these metal compounds can be delayed while Cl is involved during thermal treatment. This condition accelerates the volatilization of heavy metals. Thus, SSA can be mixed with
60
40
20
0
0.0
0.1
0.2
50
100
150
200
250
300
Cl-Addition (g Cl/Kg ash) Fig. 2. Results of thermodynamic equilibrium calculations regarding the content of Cl influencing the migration properties of Cu from PSSA. (a) PVC and (b) MgCl2 .
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MgCl2 + 1/2O2 → MgO + Cl2 CuO + Cl2 → CuCl2 + 1/2O2
(a)
or MgCl2 + H2 O → MgO + 2HCl CuO + 2HCl → CuCl2 + H2 O
(b)
CaCl2 , MgCl2 and NaCl exhibit an obvious promoting effect on the heavy metal vaporization of fly ash at high temperature. MgCl2 is the most effective agent for most heavy metals. The promoting effect is more obvious in a muffle oven than in a rotary reactor [12]. The effects of MgCl2 and KCl are compared, since MgCl2 favors Zn over Cu removal, whereas KCl acts conversely. Removal rates of Cd, Cu, Pb, and Zn were notable for the two additives. In addition, MgCl2 is more effective than KCl in consolidating P because MgCl2 yields higher amounts of entrained ash and aerosols than KCl [14]. Recent XRD findings reveal that P exists in various forms, such as Ca4 Mg5 (PO4 )6 and Ca5 (PO4 )3 Cl1−X (OH), in SSA after thermal treatment at 1050 ◦ C [15]. Extensive research on heavy metals in solid waste incineration ash has generally focused on solidification and stabilization. Thermal separation technology of heavy metals mainly exists in MSWI fly ash [12,13,16,17], but seldom in SSA. Furthermore, existing studies on the thermal separation of heavy metals are mostly based on total contents and lack fraction investigations. In addition, studies
on P are mostly based on bioavailability and seldom focused on the controlling total. Thus, an in-depth study on the calcination of PSSA is conducted at 1100 ◦ C with different chlorinating agents to reveal the effect of organic chlorinating agent (PVC) and inorganic chlorinating agent (MgCl2 ) on the fixed rate of P removal and fraction transformation of heavy metals. The results provide a theoretical basis for the highly efficient and harmless recovery of P resources in SSA. 2. Materials and methods 2.1. Materials SS used in the experiment was taken from a municipal wastewater treatment plant in Dalian City, Liaoning Province, China. The Zn Cu Cr Ni
14000 12000
Mass Fraction ( mg/kg)
a certain quantity of chlorinating agent for effectively separating heavy metals from SSA. The reaction mechanism [11–13] of the inorganic chlorinating agent (such as MgCl2 ) promotes the volatilization of heavy metals (such as CuO) according to
425
2500
10000 2000 8000 1500
6000 4000
1000 2000 0
500 0
50
100
150
250
Cl-addition (Cl g/Kg ash) As Pb Cd
5
Mass Fraction ( mg/kg)
80
4 60
3
40
20 2 0
50
100
150
250
Cl-addition (Cl g/Kg ash) 1 00
0(Cl g/K g A sh ) 50 (Cl g/ K g A sh) 10 0(Cl g/ K g A sh) 15 0(Cl g/ K g A sh) 25 0(Cl g/ K g A sh)
90 80
Removal ( %)
70 60 50 40 30 20 10 0 Cd
Fig. 3. Results of thermodynamic equilibrium calculations regarding the carbon (C) and methane (CH4 ) influencing the migration properties of Cu from PSSA with 150 g Cl/kg PSSA MgCl2 addition. (a) C and (b) CH4 .
Pb
As
Zn
Elem en ts
Cu
Cr
Ni
Fig. 4. Mass fractions and removal of several heavy metals in PSSA before and after thermochemical treatment with different concentrations of PVC at 1100 ◦ C for a retention time of 30 min.
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Fig. 5. XRD of PSSA before and after calcination with the addition of chloride agent.
SS used was the combinations with 60/40 mixture of primary and secondary dewatered sludge. The SS moisture content was 79%. The SS was dried at 105 ◦ C for 24 h, drying was followed by trituration and sieving with 100 mesh, after which the SS samples were collected. The SS sample was mixed with CaO at a ratio of 20:1 SS:CaO and then placed in a muffle furnace, where it was burned for 2 h at 850 ◦ C. Trituration and sieving with 100 meshes (150 m) followed, after which the PSSA sample was collected. The chlorinating agents PVC and MgCl2 were of technical grade and supplied by Sinopharm Chemical Reagent Co., Ltd. (China). Then, 0, 50, 100, 150, and 250 g Cl/kg ash were added to the PSSA. The mixture was then homogenized. The heavy metal contents of SS and PSSA samples are given in Table 1. The chemical composition of the PSSA sample was analyzed by X-ray fluorescence (XRF). The results are compared in Table 1. 2.2. Experimental apparatus XRF (ZSX100e) was used for elemental determinations. The main crystalline compounds in ash samples were identified by X-ray diffraction (XRD, PRO/MPD), and ash agglomeration was observed by field emission scanning electron microscopy (S3400N-II). Heavy metal and P contents of the samples after digestion were analyzed by inductively coupled plasma-atomic emission spectrometry (PerkinElmer Analyst 8300). The PSSA was treated in a laboratory-scale tube furnace (length: 1100 mm, inner diameter: approximately 75 mm, high-quality aluminous material), as shown in Fig. 1. The PSSA was mixed uniformly with one of two chlorinating agents to obtain the experimental samples. Wellmixed powder samples (5 g, particle size Zn > Pb > As ≈ Cr > Cd > Ni.
5000
60 50 40 30 20 10 0 Cd
Pb
As
Zn
Elements
Cu
Cr
Ni
Fig. 6. Mass fractions and removal of several heavy metals in PSSA before (PSSA) and after thermochemical treatment with different concentrations of MgCl2 at 1100 ◦ C for a retention time of 30 min.
3.2. Effect of chlorinating agent (MgCl2 ) on heavy metal removal Compared with PVC, MgCl2 is more effective for the removal of most heavy metals. As shown in Fig. 6, the heavy metal contents (Cu, Zn, Pb, and Cd) in treated PSSA exhibited a tendency to decrease with increasing chlorinating agent (MgCl2 ) addition. This effect was the most evident for the high content of mid-volatile heavy metals Cu and Zn. Under addition conditions of 100 g Cl/kg PSSA, Cu and Zn contents decreased from 13,488 mg/kg and 3929.5 mg/kg without addition of chlorinating agent to 434.5 mg/kg and 379 mg/kg, respectively. The contents of heavy metals As and Cr exhibit a tendency to increase with increasing chlorinating agent (PVC) addition
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Fig. 7. Mass fractions and fixed rate of phosphorus in PSSA after thermochemical treatment with the chloride type at 1100 ◦ C for a retention time of 30 min. (a) PVC and (b) MgCl2 .
under addition conditions of more than 100 g Cl/kg PSSA. Similar to that in PVC, the Ni content in treated PSSA varies as a cosine curve with increasing chlorinating agent (MgCl2 ) addition. As shown in Fig. 6c, heavy metal removal rates revealed that MgCl2 has no significant effect on low-volatile Cr and Ni removal. In addition, Cr and Ni removal declined sharply under addition conditions (150 g Cl/kg PSSA) close to zero. This finding can be attributed to the formation of a certain amount of spinel (MgAl2 O4 ), as identified by the XRD analysis results of treated PSSA (150 g Cl/kg PSSA) shown in Fig. 5. Ni easily bound to the spinel, which decreased Ni removal [27]. However, limited information is available on the mechanisms involved in the sharp decline in Cr removal. MgCl2 also had no significant effect on volatile heavy metal Cd possibly because only 53% of Cd is enriched in PSSA owing to a mechanism reaction [28] shown as: Al2 O3 ·2SiO2 + CdO → CdO·Al2 O3 ·2SiO2 The reaction products CdO·Al2 O3 ·2SiO2 are stable and low in volatility. MgCl2 is more effective than PVC in the removal of most heavy metals (Cu, Zn, Pb, and As). The removal of heavy metals Cu, Zn, and Pb is positively correlated with the addition amount under addition conditions in the range of 0 g Cl/kg to 150 g Cl/kg, which increased to a high percentage of 83.2%, 35%, and 63.3%, respectively when the addition conditions just only 50 g Cl/kg PSSA. Removals of Cd, Pb, As, Zn, Cu, Cr, and Ni were 1.16%, 18.96%, 44.29%, 1%,
Fig. 8. SEM photographs of PSSA before and after calcination with chloride agent (a) PSSA, (b) 0 g Cl/kg PSSA, (c)150 g Cl/kg PSSA, (PVC) and (d) 150 g Cl/kg PSSA (MgCl2 ).
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Fig. 9. The distribution of heavy metals in SS and PSSA before and after calcination.
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1.11%, 4.04%, and 9.34% respectively, without the addition of chlorinating agent. These removals were greatly increased to 4.83%, 65.93%, 60.27%, 90.47%, 96.82%, 22.78%, and 25.64%, respectively, under the addition conditions of 100 g Cl/kg PSSA. MgCl2 obviously had positive effects on the removal of heavy metals in the order of Cu > Zn > Pb > Cr > Ni > As > Cd. 3.3. Effect of different chlorinating agents on phosphorus in PSSA During PSSA calcination, volatilization of heavy metals is inevitable while the content and chemical speciation of P is changing. Fig. 7 shows that in the case where no chlorinating agent was added, ash P content was 63 g/kg, and the fixed rate was only 83.4%. However, the rate significantly increased when a chlorinating agent was added. The P content of PSSA increased with the addition of PVC and reached its peak at 76 g/kg under addition conditions of 250 g Cl/kg PSSA. When PVC is replaced with MgCl2 , P content first showed an increasing and then a decreasing a trend, with the maximum of 72 g/kg achieved under addition conditions of 150 g Cl/kg PSSA. From the perspective of a fixed rate, both chlorinating agents aid in the fixation of P by at least 90%, but the effects of MgCl2 are better than those of PVC. Under the addition conditions of 50 g Cl/kg PSSA, the increase of fixed rate of P is especially evident, which increased to 90.6% (PVC) and 92.2% (MgCl2 ) from the same rate of 83.4%. In addition, the relationship between fixed rate of P and the added chlorinating agent is positive. For PVC, when the Cl content reached 100 g Cl/kg ash, the phosphate fixation rate tended to be stable at a value of 92.3%, whereas for MgCl2 , when the Cl content reached 150 g Cl/kg ash, the phosphate fixation rate tended to be stable at a value of 98.3%. Nonetheless, a continuous increase in the amount of chloride had a negligible effect on P fixation rate. Fig. 5 shows the XRD analysis results of PSSA relative to calcination temperature. PSSA mainly exists in three forms: Ca9 Al(PO4 )7 (2, 31.30◦ ), Ca3 (PO4 )5 OH (2, 31.74◦ ), and Al(PO4 ), (2, 26.64◦ ) before treatment. At 1100 ◦ C, P was mainly transformed into Ca9 Al(PO4 )7 without the addition of chlorinating agent. Part of P combined with the such heavy metals as Cu, Zn, and Ni, such that when a chlorinating agent was added, most ash P mainly converted into chlorapatite (Ca5 (PO4 )3 F0.09 Cl0.88 ) (2, 32.28◦ ).
3.5. Metal speciation A modified BCR sequential extraction procedure was used to obtain information on heavy metal speciation. As shown in Fig. 9, the distributions of different heavy metals exhibit significant differences in SS. Cr and Pb, which respectively account for 93% and 58% of total SS content, are mainly distributed among the reducible fraction. Cu, Zn, Ni, Cd, and As are mainly in the non-residual category (fractions 1–3: acid soluble, reducible, and oxidizable), account for 69–90% of total SS content, exhibit a strong migration behavior, and are generally considered a potential risk, especially because of the high content of Cu and Zn. During SS incineration at 850 ◦ C, the residual fraction percentage increased by 35% and 47% for Pb and Cd, respectively, and decreased by 69% for Cr. No significant change was observed for other heavy metals. The results show that without the addition of a chlorinating agent, Cu, Zn, Ni, Cd, Pb and Cr were mostly transformed into residual fraction during PSSA calcination at 1100 ◦ C and the content of residual fraction reached more than 80%. When PSSA was calcined with MgCl2 , the acid soluble and reducible As in treated PSSA still contained a certain percentage (16.2–45.6%), but the percentage of other six heavy metals was lower than 10%. Furthermore, Pb and Zn were mainly distributed among the residual fraction. Fig. 9 shows that the distributions of different heavy metals exhibit apparent differences with different MgCl2 addition contents. The residual fraction percentages for Cr, Ni, and Cd decreased constantly with increasing MgCl2 addition. These fractions mainly transformed into oxidizable forms. Under addition conditions of 250 g Cl/kg PSSA, the residual fraction percentages decreased from 92%, 84.9%, and 94.64% without the addition of a chlorinating agent to 25.12%, 43.56%, and 64%, respectively, and the oxidizable fraction percentage improved from 2.3%, 5.08%, and 4.91% without the addition of a chlorinating agent to 59.87%, 45.29%, and 15.27%, respectively. The change in the distributions of Cu and As did not exhibit certain rules. In addition, under addition conditions of 100 g Cl/kg PSSA, Cd, Pb, As, Zn, Cu, Cr, and Ni in the residual fraction respectively account for 93%, 96%, 60%, 99%, 97%, 99%, and 73% of total content. Thus, most of the heavy metals can be distributed stably by mixing PSSA with MgCl2 (100 g Cl/kg PSSA) at 1100 ◦ C.
3.4. SEM analysis of the PSSA chlorinated feature
4. Conclusions
The SEM microphotographs of PSSA before and after calcination are presented in Fig. 8. As shown in Fig. 8a and b, the microappearance of the PSSA particles was significantly altered by the melting and sintering that occurred during calcination at 1100 ◦ C. The particles aggregated, the pores disappeared, and the whole surface became smooth and dense, resembling a molten aggregate after calcination. These phenomena have negatively affected heavy metal removal. This finding confirms that the heavy metal removal (Cu, Zn, Cr, and Ni) is below 10% in PSSA without the addition of a chlorinating agent. In addition, as shown in Fig. 8b–d, when a chlorinating agent was added, the molten and glassy material on the particle surfaces disappeared, and the surface became angular. Furthermore, some grains appeared on the particle surfaces. The appearance of grains indicates that some molten materials, including heavy metals, were involved in a chlorination-volatile reaction with the chlorinating agent. The analyses of crystalline compounds by XRD (see Fig. 5) revealed that the composition became simple after calcination. Moreover, the major components were mainly phosphate ores. Heavy metal P compounds, such as 3CuO2 ·(P2 O5 ), Zn3 P2 , Zn3 (PO)2 and Ni12 P5 that were identified without a chlorinating agent were not found in treated PSSA when a chlorinating agent was added. This condition is beneficial for the environment and presents an effective method for PSSA recycling.
During the PSSA calcination, the removal of heavy metals, except for As and Pb, was below 10% without the addition of a chlorinating agent. Cu, Zn, and Ni combined with P to form low-volatile solid compounds. Chloride agents exhibited an apparent promoting effect on of heavy metal removal, especially for the mid-volatile metals Cu and Zn. A total of 84.5% (PVC) and 98.9% (MgCl2 ) of Cu and 98.6% (PVC) and 97.3% (MgCl2 ) of Zn can be removed from PSSA. The formation of CrO4 2− reduced Cr removal with the addition of chlorinating agent, and the formation of spinel (MgAl2 O4 ) with the addition of MgCl2 weakened the effect on Ni removal. The effect promoting Cu removal in PSSA exhibited an apparent difference between PVC and MgCl2 . Cu removal of Cu was positively correlated with MgCl2 addition amount, but excessive PVC addition reduced Cu removal because of the presence of C and H in PVC, which promoted the gradual transformation of some Cu to Cu3 P(s) and Cu2 S(s). During PSSA calcination with the addition of MgCl2 , the total percentage of the acid soluble and reducible fraction of Cu, Zn, Ni, Cd, Pb, and Cr was less than 10%. The residual fraction percentage for Cr, Ni, and Cd decreased constantly, whereas the distribution change in Cu and As did not exhibit certain rules with the increase in MgCl2 addition. Pb and Zn were mainly distributed among the residual fraction and account for more than 92%. Mixing PSSA with
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MgCl2 (100 g Cl/kg PSSA) enabled the stable distribution of heavy metals. Chlorinating agents apparently exhibited a promoting effect on P fixation. The P content in treated PSSA increased from 63 g/kg without the addition of chlorinating agent to 76 g/kg (PVC) and 72 g/kg (MgCl2 ), and most ash P mainly converted to chlorapatite (Ca5 (PO4 )3F0.09 Cl0.88 ). The relationship between the fixed rate of P and the added chlorinating agent added is positive. For PVC, when the Cl content reached 100 g Cl/kg ash, phosphate fixed rate tended to be stable at a value of 92.3%. For MgCl2 , when the Cl content reached 150 g Cl/kg ash, the phosphate fixed rate tended to be stable at a value of 98.3%. Most P can be enriched in PSSA, and the content of most heavy metals can be low and distributed stably with addition of a chlorinating agent and calcination at 1100 ◦ C. XRD and SEM analyses of the treated PSSA revealed that the composition became simple after calcination. Moreover, the major components were mainly phosphate ores. This finding is beneficial for the environment and presents an effective method for PSSA recycling. Acknowledgments This research was supported by the National Natural Science Foundation of China (No. 51276119) and the National Key Basic Research Program of China (2011CB201500). References [1] J. Driver, D. Lijmbach, I. Steen, Why recover phosphorus for recycling, and how, Environ. Technol. 20 (1999) 651–662. [2] J. Yuan, Development and utilization status of China’s phosphorus resources and resources guarantee system, China Min. Mag. 12 (2003) 4–9. [3] G. Guo, J. Yang, T. Chen, G. Zheng, D. Gao, B. Song, W. Di, Concentrations and variation of organic matter and nutrients in municipal sludge of China, China Water Wastewater 25 (2009) 120–121. [4] M. Franz, Phosphate fertilizer from sewage sludge ash (SSA), Waste Manag. 28 (2008) 1809–1818. [5] E.Z. Harrison, S.R. Oakes, M. Hysell, A. Hay, Organic chemicals in sewage sludge, Sci. Total Environ. 367 (2006) 481–497. [6] D. Marani, C.M. Braguglia, G. Mininni, F. Maccioni, Behaviour of Cd, Cr, Mn, Ni, Pb, and Zn in sewage sludge incineration by fluidised bed furnace, Waste Manag. 23 (2003) 117–124. [7] M. Cyr, M. Coutand, P. Clastres, Technological and environmental behavior of sewage sludge ash (SSA) in cement-based materials, Cem. Concr. Res. 37 (2007) 1278–1289. [8] J. Han, P. Kanchanapiya, T. Sakano, T. Mikuni, M. Furuuchi, G. Wang, The behavior of phosphorus and heavy metals in sewage sludge ashes, Int. J. Environ. Pollut. 37 (2009) 357–368. [9] N. Lapa, R. Barbosa, M.H. Lopes, B. Mendes, P. Abelha, I. Gulyurtlu, J. Santos Oliveira, Chemical and ecotoxicological characterization of ashes obtained from sewage sludge combustion in a fluidised-bed reactor, J. Hazard. Mater. 147 (2007) 175–183.
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Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Heavy metal removal and speciation transformation through the calcination treatment of phosphorus-enriched sewage sludge ash Rundong Li ∗ , Weiwei Zhao, Yanlong Li, Weiyun Wang, Xuan Zhu College of Energy and Environment, Shenyang Aerospace University, The Key Laboratory of Clean Energy in Liaoning Province, Shenyang, China
h i g h l i g h t s • • • • •
Phosphorus-enriched sewage sludge ash was calcined with organic and inorganic chlorinating agent. The removal, speciation transformation of heavy metal and the fixation of P were studied. 98.9% of Cu and 98.6% of Zn can be removed from phosphorus-enriched sewage sludge ash. Suitable conditions enabled the stable distribution of heavy metals (Cd, As, Pb, Zn, Cu, Cr, and Ni) with low bioavailability. Excessive organic chlorinating agent (PVC) addition reduced Cu removal obviously owning to the presence of carbon and hydrogen.
a r t i c l e
i n f o
Article history: Received 26 July 2014 Received in revised form 23 September 2014 Accepted 24 September 2014 Available online 2 October 2014 Keywords: Sewage sludge ash Heavy metals Removal Chemical speciation Phosphorus recovery
a b s t r a c t On the basis of the heavy metal (Cd, As, Pb, Zn, Cu, Cr, and Ni) control problem during the thermochemical recovery of phosphorus (P) from sewage sludge (SS), P-enriched sewage sludge ash (PSSA) was calcined at 1100 ◦ C. The effect of organic chlorinating agent (PVC) and inorganic chlorinating agent (MgCl2 ) on the fixed rate of P removal and the speciation transformation of heavy metal was studied. The removal of heavy metals Cd, Pb, As, Zn, and Cr exhibited an increasing tendency with the addition of chlorinating agent (PVC). However, an obvious peak under 100 g Cl/kg of PSSA appeared for Cu, owing to the presence of carbon and hydrogen in PVC. MgCl2 was found to be more effective than PVC in the removal of most heavy metals, such that up to 98.9% of Cu and 97.3% of Zn was effectively removed. Analyses of heavy metal forms showed that Pb and Zn occurred in the residue fraction after calcination. Meanwhile, the residue fraction of Cr, Ni, Cd, and Cu exhibited a decreasing tendency with the increase in the added chlorinating agent (MgCl2 ). Losses of P from PSSA were around 16.6% without the addition of chlorinating agent, which were greatly reduced to around 7.7% (PVC) and to only 1.7% (MgCl2 ). © 2014 Elsevier B.V. All rights reserved.
1. Introduction Phosphorus (P) is an essential element for biological growth and is a non-renewable resource. The world has limited phosphate resources, with forecast reserves maintained for less than 100 years [1]. Phosphate has been listed among the 20 minerals that cannot meet the demand of national economic growth in China [2]. Sewage sludge (SS) dry solids contains a vast amount of inorganic minerals and abundant nutrients, particularly large amounts of P. Related studies have demonstrated that the average P content of SS in China is 22 g P/kg SS dry solids, with the maximum reaching 37 g P/kg SS dry solids [3], which is generally considered as the largest secondary P resource both locally and internationally
∗ Corresponding author. Tel.: +86 24 89728889; fax: +86 24 89724558. E-mail addresses: [email protected], [email protected] (R. Li). http://dx.doi.org/10.1016/j.jhazmat.2014.09.052 0304-3894/© 2014 Elsevier B.V. All rights reserved.
[4], but SS is enriched in organic micropollutants (PCBs, AOX), pathogens, and various heavy metal elements [5,6]. Thus, SS is restricted for agricultural use. SS incineration technology has special advantages, including quick disposal, large volume and weight reduction ratio, and energy recovery. When organic contaminants and pathogens are effectively destroyed, 60–70% P is concentrated in the SS ash (SSA), and phosphate ore grade maximum reaches 26%, with an average of 15% (by P2 O5 ) [4,7]. During incineration, the P and heavy metal contents in SSA decrease with the addition of HCl, but increase with the addition of CaO. CaO has a fixed effect on P and heavy metals [8]. However, most heavy metals are also concentrated in SSA, which are in excess of the standards, and some are distributed among unstable fractions with high bioavailability. Some papers have studied the ecotoxicity of SSA to identify if it is a hazardous material. Lapa et al. worked that, the SSA was categorized into three types of SSA (bottom ash, fly ashes in the first and second
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Fig. 1. The experimental setup of vertical calcination furnace.
Substance
SiO2 Al2 O3 Fe2 O3 MgO CaO K2 O P2 O5 SO3
25.61 13.53 11.99 5.5 17.54 3.81 16.63 5.25
Cd Pb As Zn Cu Cr Ni
Concentration (ppm) PSSA
SD
SS
SD
4.1 104.9 48.5 3830.0 13,160.0 2201.6 843.5
0.1 2.0 1.0 46.2 42.5 36.6 20.1
3.0 86.4 51.3 2277.3 5248.2 935.0 342.5
0.1 2.4 1.8 36.2 31.4 20.4 13.9
(a)
CuCl(g) (CuCl)3(g) Cu3P(s) Cu2S(s) (CuO)(Fe2O3)(s)
100
Percentage (%)
80
cyclone). Ashes trapped in the first and second cyclones and the ashes retained in the second cyclone must be classified as ecotoxic materials due to the concentrations of Cr, Cr(VI) and As, but the bottom Ash was classified as non-ecotoxic material in the French Regulation CEMWE [9] while the study by Donatello showed SSA is close to the boundary that distinguishes between hazardous and
Fraction 2, reducible
Fraction 3, oxidizable
Fraction 4, residual
12 7. 8
10 0
1 12 20 7. 7
60
80
40
0. 00 0. 05 0. 10 2 4 6 8 10 20
A 1 ± 0.001 g sample was weighed and placed in 100 mL polypropylene centrifuge tube. Then, 0.11 mol/L HOAc (40 mL) was added. The mixture was shaken at 22 ± 5 ◦ C for 16 h, followed by centrifuging and filtering. First, 0.5 mol/L NH4 OH·HCl (40 mL) (adjusted to pH = 1.5 with HNO3 ) was added to the residue from the previous step. Second, the mixture was shaken at 22 ± 5 ◦ C for 16 h, followed by centrifuging and filtering. First, 8.8 mol/L H2 O2 (10 mL) (adjusted to pH = 1.5 with HNO3 ) was added to the residue from the previous step. Second, the mixture was digested at room temperature for 1 h with occasional manual shaking, and then 8.8 mol/L H2 O2 (10 mL) was added again, after which the tube was covered. Second, the mixture was heated at 85 ± 2 ◦ C for 1 h. Third, the cover was removed, mixture was heated at 85 ± 2 ◦ C until the volume reduced to near dryness. Finally, 1 mol/L NH4 OAc (50 mL) (adjusted to pH = 2 with HNO3 ) was added at 22 ± 5 ◦ C for 16 h. The remaining residue was digested by using the same method employed in the total digestion of PSSA.
0
Cl-Addition (g Cl/Kg ash)
(b)
CuCl(g) (CuCl)3(g) Cu2S(s) (CuO)(Fe2O3)(s)
100
80
Percentage (%)
Extraction method
Fraction 1, acid soluble
40
20
Table 2 BCR sequential extraction protocol. Fraction
60
28 0
Mass fraction (%)
24 0
Substance
16 0
Heavy metals
20 0
Major composition (PSSA)
12 7. 9
Table 1 Heavy metals content and standard deviations (averages of five replicates) of the SS and average PSSA sample and chemical composition of average PSSA sample.
non-hazardous waste and speciation of zinc was a key consideration in deterring whether or not in the EU Hazardous Waste Directive [10]. However, the high content of heavy metals must be separated from SSA for recovery of phosphorus whether it was classified as hazardous waste or not. Heavy metal chlorides have high vapor pressure. The condensation process of these metal compounds can be delayed while Cl is involved during thermal treatment. This condition accelerates the volatilization of heavy metals. Thus, SSA can be mixed with
60
40
20
0
0.0
0.1
0.2
50
100
150
200
250
300
Cl-Addition (g Cl/Kg ash) Fig. 2. Results of thermodynamic equilibrium calculations regarding the content of Cl influencing the migration properties of Cu from PSSA. (a) PVC and (b) MgCl2 .
R. Li et al. / Journal of Hazardous Materials 283 (2015) 423–431
MgCl2 + 1/2O2 → MgO + Cl2 CuO + Cl2 → CuCl2 + 1/2O2
(a)
or MgCl2 + H2 O → MgO + 2HCl CuO + 2HCl → CuCl2 + H2 O
(b)
CaCl2 , MgCl2 and NaCl exhibit an obvious promoting effect on the heavy metal vaporization of fly ash at high temperature. MgCl2 is the most effective agent for most heavy metals. The promoting effect is more obvious in a muffle oven than in a rotary reactor [12]. The effects of MgCl2 and KCl are compared, since MgCl2 favors Zn over Cu removal, whereas KCl acts conversely. Removal rates of Cd, Cu, Pb, and Zn were notable for the two additives. In addition, MgCl2 is more effective than KCl in consolidating P because MgCl2 yields higher amounts of entrained ash and aerosols than KCl [14]. Recent XRD findings reveal that P exists in various forms, such as Ca4 Mg5 (PO4 )6 and Ca5 (PO4 )3 Cl1−X (OH), in SSA after thermal treatment at 1050 ◦ C [15]. Extensive research on heavy metals in solid waste incineration ash has generally focused on solidification and stabilization. Thermal separation technology of heavy metals mainly exists in MSWI fly ash [12,13,16,17], but seldom in SSA. Furthermore, existing studies on the thermal separation of heavy metals are mostly based on total contents and lack fraction investigations. In addition, studies
on P are mostly based on bioavailability and seldom focused on the controlling total. Thus, an in-depth study on the calcination of PSSA is conducted at 1100 ◦ C with different chlorinating agents to reveal the effect of organic chlorinating agent (PVC) and inorganic chlorinating agent (MgCl2 ) on the fixed rate of P removal and fraction transformation of heavy metals. The results provide a theoretical basis for the highly efficient and harmless recovery of P resources in SSA. 2. Materials and methods 2.1. Materials SS used in the experiment was taken from a municipal wastewater treatment plant in Dalian City, Liaoning Province, China. The Zn Cu Cr Ni
14000 12000
Mass Fraction ( mg/kg)
a certain quantity of chlorinating agent for effectively separating heavy metals from SSA. The reaction mechanism [11–13] of the inorganic chlorinating agent (such as MgCl2 ) promotes the volatilization of heavy metals (such as CuO) according to
425
2500
10000 2000 8000 1500
6000 4000
1000 2000 0
500 0
50
100
150
250
Cl-addition (Cl g/Kg ash) As Pb Cd
5
Mass Fraction ( mg/kg)
80
4 60
3
40
20 2 0
50
100
150
250
Cl-addition (Cl g/Kg ash) 1 00
0(Cl g/K g A sh ) 50 (Cl g/ K g A sh) 10 0(Cl g/ K g A sh) 15 0(Cl g/ K g A sh) 25 0(Cl g/ K g A sh)
90 80
Removal ( %)
70 60 50 40 30 20 10 0 Cd
Fig. 3. Results of thermodynamic equilibrium calculations regarding the carbon (C) and methane (CH4 ) influencing the migration properties of Cu from PSSA with 150 g Cl/kg PSSA MgCl2 addition. (a) C and (b) CH4 .
Pb
As
Zn
Elem en ts
Cu
Cr
Ni
Fig. 4. Mass fractions and removal of several heavy metals in PSSA before and after thermochemical treatment with different concentrations of PVC at 1100 ◦ C for a retention time of 30 min.
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Fig. 5. XRD of PSSA before and after calcination with the addition of chloride agent.
SS used was the combinations with 60/40 mixture of primary and secondary dewatered sludge. The SS moisture content was 79%. The SS was dried at 105 ◦ C for 24 h, drying was followed by trituration and sieving with 100 mesh, after which the SS samples were collected. The SS sample was mixed with CaO at a ratio of 20:1 SS:CaO and then placed in a muffle furnace, where it was burned for 2 h at 850 ◦ C. Trituration and sieving with 100 meshes (150 m) followed, after which the PSSA sample was collected. The chlorinating agents PVC and MgCl2 were of technical grade and supplied by Sinopharm Chemical Reagent Co., Ltd. (China). Then, 0, 50, 100, 150, and 250 g Cl/kg ash were added to the PSSA. The mixture was then homogenized. The heavy metal contents of SS and PSSA samples are given in Table 1. The chemical composition of the PSSA sample was analyzed by X-ray fluorescence (XRF). The results are compared in Table 1. 2.2. Experimental apparatus XRF (ZSX100e) was used for elemental determinations. The main crystalline compounds in ash samples were identified by X-ray diffraction (XRD, PRO/MPD), and ash agglomeration was observed by field emission scanning electron microscopy (S3400N-II). Heavy metal and P contents of the samples after digestion were analyzed by inductively coupled plasma-atomic emission spectrometry (PerkinElmer Analyst 8300). The PSSA was treated in a laboratory-scale tube furnace (length: 1100 mm, inner diameter: approximately 75 mm, high-quality aluminous material), as shown in Fig. 1. The PSSA was mixed uniformly with one of two chlorinating agents to obtain the experimental samples. Wellmixed powder samples (5 g, particle size Zn > Pb > As ≈ Cr > Cd > Ni.
5000
60 50 40 30 20 10 0 Cd
Pb
As
Zn
Elements
Cu
Cr
Ni
Fig. 6. Mass fractions and removal of several heavy metals in PSSA before (PSSA) and after thermochemical treatment with different concentrations of MgCl2 at 1100 ◦ C for a retention time of 30 min.
3.2. Effect of chlorinating agent (MgCl2 ) on heavy metal removal Compared with PVC, MgCl2 is more effective for the removal of most heavy metals. As shown in Fig. 6, the heavy metal contents (Cu, Zn, Pb, and Cd) in treated PSSA exhibited a tendency to decrease with increasing chlorinating agent (MgCl2 ) addition. This effect was the most evident for the high content of mid-volatile heavy metals Cu and Zn. Under addition conditions of 100 g Cl/kg PSSA, Cu and Zn contents decreased from 13,488 mg/kg and 3929.5 mg/kg without addition of chlorinating agent to 434.5 mg/kg and 379 mg/kg, respectively. The contents of heavy metals As and Cr exhibit a tendency to increase with increasing chlorinating agent (PVC) addition
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Fig. 7. Mass fractions and fixed rate of phosphorus in PSSA after thermochemical treatment with the chloride type at 1100 ◦ C for a retention time of 30 min. (a) PVC and (b) MgCl2 .
under addition conditions of more than 100 g Cl/kg PSSA. Similar to that in PVC, the Ni content in treated PSSA varies as a cosine curve with increasing chlorinating agent (MgCl2 ) addition. As shown in Fig. 6c, heavy metal removal rates revealed that MgCl2 has no significant effect on low-volatile Cr and Ni removal. In addition, Cr and Ni removal declined sharply under addition conditions (150 g Cl/kg PSSA) close to zero. This finding can be attributed to the formation of a certain amount of spinel (MgAl2 O4 ), as identified by the XRD analysis results of treated PSSA (150 g Cl/kg PSSA) shown in Fig. 5. Ni easily bound to the spinel, which decreased Ni removal [27]. However, limited information is available on the mechanisms involved in the sharp decline in Cr removal. MgCl2 also had no significant effect on volatile heavy metal Cd possibly because only 53% of Cd is enriched in PSSA owing to a mechanism reaction [28] shown as: Al2 O3 ·2SiO2 + CdO → CdO·Al2 O3 ·2SiO2 The reaction products CdO·Al2 O3 ·2SiO2 are stable and low in volatility. MgCl2 is more effective than PVC in the removal of most heavy metals (Cu, Zn, Pb, and As). The removal of heavy metals Cu, Zn, and Pb is positively correlated with the addition amount under addition conditions in the range of 0 g Cl/kg to 150 g Cl/kg, which increased to a high percentage of 83.2%, 35%, and 63.3%, respectively when the addition conditions just only 50 g Cl/kg PSSA. Removals of Cd, Pb, As, Zn, Cu, Cr, and Ni were 1.16%, 18.96%, 44.29%, 1%,
Fig. 8. SEM photographs of PSSA before and after calcination with chloride agent (a) PSSA, (b) 0 g Cl/kg PSSA, (c)150 g Cl/kg PSSA, (PVC) and (d) 150 g Cl/kg PSSA (MgCl2 ).
R. Li et al. / Journal of Hazardous Materials 283 (2015) 423–431
Fig. 9. The distribution of heavy metals in SS and PSSA before and after calcination.
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1.11%, 4.04%, and 9.34% respectively, without the addition of chlorinating agent. These removals were greatly increased to 4.83%, 65.93%, 60.27%, 90.47%, 96.82%, 22.78%, and 25.64%, respectively, under the addition conditions of 100 g Cl/kg PSSA. MgCl2 obviously had positive effects on the removal of heavy metals in the order of Cu > Zn > Pb > Cr > Ni > As > Cd. 3.3. Effect of different chlorinating agents on phosphorus in PSSA During PSSA calcination, volatilization of heavy metals is inevitable while the content and chemical speciation of P is changing. Fig. 7 shows that in the case where no chlorinating agent was added, ash P content was 63 g/kg, and the fixed rate was only 83.4%. However, the rate significantly increased when a chlorinating agent was added. The P content of PSSA increased with the addition of PVC and reached its peak at 76 g/kg under addition conditions of 250 g Cl/kg PSSA. When PVC is replaced with MgCl2 , P content first showed an increasing and then a decreasing a trend, with the maximum of 72 g/kg achieved under addition conditions of 150 g Cl/kg PSSA. From the perspective of a fixed rate, both chlorinating agents aid in the fixation of P by at least 90%, but the effects of MgCl2 are better than those of PVC. Under the addition conditions of 50 g Cl/kg PSSA, the increase of fixed rate of P is especially evident, which increased to 90.6% (PVC) and 92.2% (MgCl2 ) from the same rate of 83.4%. In addition, the relationship between fixed rate of P and the added chlorinating agent is positive. For PVC, when the Cl content reached 100 g Cl/kg ash, the phosphate fixation rate tended to be stable at a value of 92.3%, whereas for MgCl2 , when the Cl content reached 150 g Cl/kg ash, the phosphate fixation rate tended to be stable at a value of 98.3%. Nonetheless, a continuous increase in the amount of chloride had a negligible effect on P fixation rate. Fig. 5 shows the XRD analysis results of PSSA relative to calcination temperature. PSSA mainly exists in three forms: Ca9 Al(PO4 )7 (2, 31.30◦ ), Ca3 (PO4 )5 OH (2, 31.74◦ ), and Al(PO4 ), (2, 26.64◦ ) before treatment. At 1100 ◦ C, P was mainly transformed into Ca9 Al(PO4 )7 without the addition of chlorinating agent. Part of P combined with the such heavy metals as Cu, Zn, and Ni, such that when a chlorinating agent was added, most ash P mainly converted into chlorapatite (Ca5 (PO4 )3 F0.09 Cl0.88 ) (2, 32.28◦ ).
3.5. Metal speciation A modified BCR sequential extraction procedure was used to obtain information on heavy metal speciation. As shown in Fig. 9, the distributions of different heavy metals exhibit significant differences in SS. Cr and Pb, which respectively account for 93% and 58% of total SS content, are mainly distributed among the reducible fraction. Cu, Zn, Ni, Cd, and As are mainly in the non-residual category (fractions 1–3: acid soluble, reducible, and oxidizable), account for 69–90% of total SS content, exhibit a strong migration behavior, and are generally considered a potential risk, especially because of the high content of Cu and Zn. During SS incineration at 850 ◦ C, the residual fraction percentage increased by 35% and 47% for Pb and Cd, respectively, and decreased by 69% for Cr. No significant change was observed for other heavy metals. The results show that without the addition of a chlorinating agent, Cu, Zn, Ni, Cd, Pb and Cr were mostly transformed into residual fraction during PSSA calcination at 1100 ◦ C and the content of residual fraction reached more than 80%. When PSSA was calcined with MgCl2 , the acid soluble and reducible As in treated PSSA still contained a certain percentage (16.2–45.6%), but the percentage of other six heavy metals was lower than 10%. Furthermore, Pb and Zn were mainly distributed among the residual fraction. Fig. 9 shows that the distributions of different heavy metals exhibit apparent differences with different MgCl2 addition contents. The residual fraction percentages for Cr, Ni, and Cd decreased constantly with increasing MgCl2 addition. These fractions mainly transformed into oxidizable forms. Under addition conditions of 250 g Cl/kg PSSA, the residual fraction percentages decreased from 92%, 84.9%, and 94.64% without the addition of a chlorinating agent to 25.12%, 43.56%, and 64%, respectively, and the oxidizable fraction percentage improved from 2.3%, 5.08%, and 4.91% without the addition of a chlorinating agent to 59.87%, 45.29%, and 15.27%, respectively. The change in the distributions of Cu and As did not exhibit certain rules. In addition, under addition conditions of 100 g Cl/kg PSSA, Cd, Pb, As, Zn, Cu, Cr, and Ni in the residual fraction respectively account for 93%, 96%, 60%, 99%, 97%, 99%, and 73% of total content. Thus, most of the heavy metals can be distributed stably by mixing PSSA with MgCl2 (100 g Cl/kg PSSA) at 1100 ◦ C.
3.4. SEM analysis of the PSSA chlorinated feature
4. Conclusions
The SEM microphotographs of PSSA before and after calcination are presented in Fig. 8. As shown in Fig. 8a and b, the microappearance of the PSSA particles was significantly altered by the melting and sintering that occurred during calcination at 1100 ◦ C. The particles aggregated, the pores disappeared, and the whole surface became smooth and dense, resembling a molten aggregate after calcination. These phenomena have negatively affected heavy metal removal. This finding confirms that the heavy metal removal (Cu, Zn, Cr, and Ni) is below 10% in PSSA without the addition of a chlorinating agent. In addition, as shown in Fig. 8b–d, when a chlorinating agent was added, the molten and glassy material on the particle surfaces disappeared, and the surface became angular. Furthermore, some grains appeared on the particle surfaces. The appearance of grains indicates that some molten materials, including heavy metals, were involved in a chlorination-volatile reaction with the chlorinating agent. The analyses of crystalline compounds by XRD (see Fig. 5) revealed that the composition became simple after calcination. Moreover, the major components were mainly phosphate ores. Heavy metal P compounds, such as 3CuO2 ·(P2 O5 ), Zn3 P2 , Zn3 (PO)2 and Ni12 P5 that were identified without a chlorinating agent were not found in treated PSSA when a chlorinating agent was added. This condition is beneficial for the environment and presents an effective method for PSSA recycling.
During the PSSA calcination, the removal of heavy metals, except for As and Pb, was below 10% without the addition of a chlorinating agent. Cu, Zn, and Ni combined with P to form low-volatile solid compounds. Chloride agents exhibited an apparent promoting effect on of heavy metal removal, especially for the mid-volatile metals Cu and Zn. A total of 84.5% (PVC) and 98.9% (MgCl2 ) of Cu and 98.6% (PVC) and 97.3% (MgCl2 ) of Zn can be removed from PSSA. The formation of CrO4 2− reduced Cr removal with the addition of chlorinating agent, and the formation of spinel (MgAl2 O4 ) with the addition of MgCl2 weakened the effect on Ni removal. The effect promoting Cu removal in PSSA exhibited an apparent difference between PVC and MgCl2 . Cu removal of Cu was positively correlated with MgCl2 addition amount, but excessive PVC addition reduced Cu removal because of the presence of C and H in PVC, which promoted the gradual transformation of some Cu to Cu3 P(s) and Cu2 S(s). During PSSA calcination with the addition of MgCl2 , the total percentage of the acid soluble and reducible fraction of Cu, Zn, Ni, Cd, Pb, and Cr was less than 10%. The residual fraction percentage for Cr, Ni, and Cd decreased constantly, whereas the distribution change in Cu and As did not exhibit certain rules with the increase in MgCl2 addition. Pb and Zn were mainly distributed among the residual fraction and account for more than 92%. Mixing PSSA with
R. Li et al. / Journal of Hazardous Materials 283 (2015) 423–431
MgCl2 (100 g Cl/kg PSSA) enabled the stable distribution of heavy metals. Chlorinating agents apparently exhibited a promoting effect on P fixation. The P content in treated PSSA increased from 63 g/kg without the addition of chlorinating agent to 76 g/kg (PVC) and 72 g/kg (MgCl2 ), and most ash P mainly converted to chlorapatite (Ca5 (PO4 )3F0.09 Cl0.88 ). The relationship between the fixed rate of P and the added chlorinating agent added is positive. For PVC, when the Cl content reached 100 g Cl/kg ash, phosphate fixed rate tended to be stable at a value of 92.3%. For MgCl2 , when the Cl content reached 150 g Cl/kg ash, the phosphate fixed rate tended to be stable at a value of 98.3%. Most P can be enriched in PSSA, and the content of most heavy metals can be low and distributed stably with addition of a chlorinating agent and calcination at 1100 ◦ C. XRD and SEM analyses of the treated PSSA revealed that the composition became simple after calcination. Moreover, the major components were mainly phosphate ores. This finding is beneficial for the environment and presents an effective method for PSSA recycling. Acknowledgments This research was supported by the National Natural Science Foundation of China (No. 51276119) and the National Key Basic Research Program of China (2011CB201500). References [1] J. Driver, D. Lijmbach, I. Steen, Why recover phosphorus for recycling, and how, Environ. Technol. 20 (1999) 651–662. [2] J. Yuan, Development and utilization status of China’s phosphorus resources and resources guarantee system, China Min. Mag. 12 (2003) 4–9. [3] G. Guo, J. Yang, T. Chen, G. Zheng, D. Gao, B. Song, W. Di, Concentrations and variation of organic matter and nutrients in municipal sludge of China, China Water Wastewater 25 (2009) 120–121. [4] M. Franz, Phosphate fertilizer from sewage sludge ash (SSA), Waste Manag. 28 (2008) 1809–1818. [5] E.Z. Harrison, S.R. Oakes, M. Hysell, A. Hay, Organic chemicals in sewage sludge, Sci. Total Environ. 367 (2006) 481–497. [6] D. Marani, C.M. Braguglia, G. Mininni, F. Maccioni, Behaviour of Cd, Cr, Mn, Ni, Pb, and Zn in sewage sludge incineration by fluidised bed furnace, Waste Manag. 23 (2003) 117–124. [7] M. Cyr, M. Coutand, P. Clastres, Technological and environmental behavior of sewage sludge ash (SSA) in cement-based materials, Cem. Concr. Res. 37 (2007) 1278–1289. [8] J. Han, P. Kanchanapiya, T. Sakano, T. Mikuni, M. Furuuchi, G. Wang, The behavior of phosphorus and heavy metals in sewage sludge ashes, Int. J. Environ. Pollut. 37 (2009) 357–368. [9] N. Lapa, R. Barbosa, M.H. Lopes, B. Mendes, P. Abelha, I. Gulyurtlu, J. Santos Oliveira, Chemical and ecotoxicological characterization of ashes obtained from sewage sludge combustion in a fluidised-bed reactor, J. Hazard. Mater. 147 (2007) 175–183.
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