Effect of co-pyrolysis of different plastics with sewage sludge on heavy metals in the biochar
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摘要: 利用高温管式炉开展城市污泥(SS)与四种塑料(PE、PP、PS和PVC)混合热解实验,分别得到四种生物炭(SSCPE、SSCPP、SSCPS和SSCPVC),研究了生物炭中重金属(Cr、Mn、Ni、Cu、Zn、As、Cd和Pb)含量、残余率、BCR形态和TCLP浸出毒性特征,并开展潜在生态风险评估。结果表明,添加不同塑料与污泥混合热解能够降低除Cd以外重金属的残余率。与污泥单独热解所得生物炭(SSC)相比,添加PE、PP和PS能够促进生物炭中的重金属向相对稳定态(F3+F4)转化,实现固化稳定;添加PVC仅对生物炭中Cr和As有固化稳定作用,对其他重金属有明显活化作用。四种生物炭中的重金属浸出量低于GB5085.3-2007浸出毒性鉴别标准规定的限值,生态风险均明显地降低至轻微风险水平,表明添加PE、PP、PS和PVC与污泥混合热解所得生物炭的应用不会带来新的环境风险,这为污泥与废塑料协同处置工艺提供了良好的理论支撑。Abstract: The experiments of sewage sludge (SS) co-pyrolysis with four kinds of plastics (PE, PP, PS and PVC) were carried out in a high temperature tubular furnace to obtain four kinds of biochar (SSCPE, SSCPP, SSCPS and SSCPVC), respectively. The contents, residual rates, BCR speciation, leaching toxicity and potential ecological risk assessment of heavy metals (Cr, Mn, Ni, Cu, Zn, As, Cd and Pb) in biochar were studied. The results show that the residues of heavy metals except Cd are reduced by adding different kinds of plastics during SS pyrolysis. Compared with the biochar (SSC) obtained by SS pyrolysis, the addition of PE, PP and PS can promote the transformation of heavy metals speciation in biochar to more stable fractions (F3+F4) and achieve the immobilization of heavy metals. The addition of PVC only promotes the immobilization of Cr and As in biochar, while exhibiting an obvious activation effect on other heavy metals. The concentrations of leaching heavy metals in four kinds of biochar are lower than the limit value of the identification standard for extraction (GB5085.3-2007), and the potential ecological risks of the four kinds of biochar are all in a slight level. This work provides a good theoretical support for the process of the cooperative disposal of SS and waste plastics.
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Key words:
- sewage sludge /
- plastics /
- co-pyrolysis /
- heavy metals /
- speciation analysis /
- leaching toxicity /
- risk assessment
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表 1 实验材料的基本性质
Table 1 Basic properties of experimental materials
Sample Proximate analysisa wa/% Ultimate analysis wa/% Qb, a/(kJ·g-1) V A FC C H N S Ob SS 29.97 68.29 1.74 15.79 3.13 1.81 0.72 10.26 11.50 PE 99.72 0.23 0.05 83.99 13.89 0.03 1.30 0.56 45.90 PP 98.19 0.82 0.99 83.91 13.55 0.56 0.92 0.24 44.00 PS 99.15 0.77 0.08 91.05 7.12 0.02 0.85 0.19 45.90 PVC 98.15 1.31 0.54 38.90 5.67 0.01 0.78 53.33 18.80 a: dry basis; b: by difference Fraction Extraction steps F1 exchangeable fraction 0.5 g of dry sample is mixed with 20 mL of 0.11 mol/L acetic acid by shaking for 16 h (25 ℃) F2 reducible fraction the dry residue of F1 is mixed with 20 mL of 0.5 mol/L hydroxylamine hydrochloride by shaking for 16 h (25 ℃) F3 oxidizable fraction the dry residue of F2 is mixed 5 mL 30% hydrogen peroxide for 1 h at room temperature and then another 5 mL of hydrogen peroxide is added after digestion for 1 h at 85 ℃; when the solution is evaporated to be near dried the residue is mixed with 25 mL of 1 mol/L ammonium acetate by shaking for 16 h (25 ℃) F4 residual fraction 0.1g dry residue of F3 is digested in an acid mixture (HNO3-HClO4-HF/HNO3-H2O2) by graphite digester Er IR Potential ecological risk Er≤40 IR≤150 low 40<Er≤80 150<IR≤300 moderate 80<Er≤160 300<IR≤600 considerate 160<Er≤320 IR>600 high Er>320 very high 表 4 污泥及生物炭中重金属的浸出量
Table 4 Leaching concentrations of heavy metals in sludge and biochars
Sample Concentration c/(mg·L-1) Cr Mn Ni Cu Zn As Cd Pb SS 0.57±0.01 5.08±1.03 39.50±5.00 4.15±0.01 44.76±8.82 0.07±0.01 0.16±0.01 0.01±0.00 SSC 0.04±0.00 2.80±0.86 0.62±0.01 6.43±0.09 16.88±3.22 0.05±0.00 0.01±0.00 0.01±0.00 SSCPE 0.13±0.00 3.60±0.55 1.30±0.00 34.73±9.09 86.48±9.38 0.03±0.00 0.05±0.01 0.00±0.00 SSCPP 0.05±0.00 3.45±0.25 1.40±0.05 35.79±1.34 91.79±7.09 0.02±0.00 0.06±0.02 0.00±0.00 SSCPS 0.03±0.00 4.95±0.28 0.75±0.09 3.54±0.51 19.35±1.29 0.05±0.01 0.01±0.00 0.00±0.00 SSCPVC 0.02±0.00 9.51±1.25 2.40±0.07 98.23±10.12 74.40±5.54 0.01±0.00 0.04±0.01 0.02±0.01 Limits
value[42]*15 - 5 100 100 5 1 5 *: from 《dentification standards for hazardous wastes-Identification for extraction toxicity》(GB5085.3—2007) 表 5 污泥及生物炭中重金属的潜在生态风险系数与指数
Table 5 Potential ecological risk factors and indices of heavy metals in sludge and biochar
Sample Er IR Potential ecological risk Cr Mn Ni Cu Zn As Cd Pb SS 51.88 3.57 67.05 40.00 3.93 4.51 131.82 0.08 302.84 considerate SSC 2.22 3.57 39.40 24.75 6.60 5.53 25.68 0.09 107.86 low SSCPE 1.04 3.52 38.86 17.69 4.72 6.36 8.99 0.09 81.27 low SSCPP 1.37 3.06 43.85 19.19 4.91 4.05 16.18 0.10 92.73 low SSCPS 4.77 3.90 56.34 26.41 5.87 3.27 4.61 0.10 105.27 low SSCPVC 0.00 4.46 24.41 28.81 5.59 2.21 5.61 0.40 71.48 low -
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