Effect of pyrolysis temperature on characteristics, chemical speciation and environmental risk of Cr, Mn, Cu, and Zn in biochars derived from pig manure

• Published in: The Science of the total environment 2020-02, Vol.704, p.135283-135283, Article 135283
• Main Authors: Shen, Xiuli, Zeng, Jianfei, Zhang, Deli, Wang, Fang, Li, Yongjun, Yi, Weiming
• Format: Article
• Language: English
• Subjects: Charcoal - chemistry; Chemical speciation; Environmental Restoration and Remediation; Environmental risk; Heavy metals; Manure - analysis; Metals, Heavy - analysis; Pig manure; Pyrolysis; Pyrolysis temperature; Soil Pollutants - analysis; Temperature
• ISSN: 0048-9697; 1879-1026
• DOI: 10.1016/j.scitotenv.2019.135283

[Display omitted] •Pyrolysis of pig manure was performed at 300–700 °C with a small interval of 50 °C.•Pyrolysis can effectively immobilize Cr, Mn, Cu, and Zn in pig manure.•Biochars derived from pig manure posed no phytotoxicity.•SBET, PV and pH may help to immobilize heavy metals and reduce bioavailability.•The optimal pyrolysis temperature for pig manure was 500 °C. The comprehensive analysis of environmental risk for heavy metals in pig manure was essential for optimization of pyrolysis conditions and scientific utilization of pig manure biochars as soil amendment. However, in previous studies, the selected pyrolysis temperature points were limited and temperature interval was large, it’s was difficult to accurately verify the effect of pyrolysis temperature on chemical speciation and environmental risk of heavy metals. Therefore, in this study, pig manure was pyrolyzed at 300–700 °C with a small interval of 50 °C to study the effect of pyrolysis temperature on characteristics and environmental risk of Cr, Mn, Cu and Zn in pig manure biochar. Results indicated that the characteristics of biochars (>500 °C) were relatively stable. The biochar obtained at 700 °C exhibited the largest surface area (8.28 m2 g−1) and pore volume (25.17 m3 kg−1), secondly is the biochar derived at 500 °C. The total percentages of exchangeable and acid fraction and reducible fraction decreased from 16.98% to 9.43% for Cr, 85.60% to 65.55% for Mn, 57.26% to 10.61% for Cu, 37.90% to 13.78% for Zn, respectively, suggesting that exchangeable and acid fraction and reducible fraction of Cr, Mn, Cu and Zn in pig manure were transformed into oxidizable and residual fractions after pyrolysis. The leaching rates, risk assessment code and potential ecological risk index values significantly decreased after pyrolysis and presented lower value at 500 and 700 °C. Biochars derived at 300–700 °C conditions posed no phytotoxicity with germination index >80%. Correlation analyses revealed that larger surface area, pore volume and pH values of biochars may help to immobilize heavy metals and reduce bioavailability. These findings demonstrated that bioavailability and toxicity of Cr, Mn, Cu and Zn in pig manure biochar were greatly reduced after pyrolysis and the optimum temperature was 500 °C considering energy cost.