| Resource & environmental sciences |
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| Research progress in effects of biochar on transport of inorganic pollutants in soil |
| ZHANG Dong1, LIU Xingyuan2, ZHAO Hongting1* |
| (1. Institute of Environmental Materials & Applications, College of Materials & Environmental Engineering, Hangzhou Dianzi University, Hangzhou 310018, China; 2. Guangdong Dazhong Agriculture Science Co., Ltd., Dongguan 523169, Guangdong, China) |
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Abstract Biochar is a carbon-rich product obtained from thermal treatment and pyrolysis of various plant- and animal-based biomass. The biomass for preparation of biochar had extensive sources, and the treatment is usually easy operation, mainly thermochemical decomposition under a poor-oxygen condition. Biochar has been considered as a low cost and high efficiency sorbent for both organic and inorganic contaminants including heavy metals, radioactive elements, nitrogen and phosphate, due to its abundant O-containing functional groups and surface charges, advanced micro- and macro- pore structures, and rich carbon content.
In this paper, recent research progress on biochar with regards to its mechanisms and potential applications in remediation of inorganic contaminated soils was reviewed. The key parameters controlling biochar’s properties include pyrolysis temperatures and feedstock types, resulting in biochar with great difference in surface areas, pore size distribution, pH, H/C ratio, ion exchange capacity, and carbon content. Therefore, the sorption mechanisms of inorganic pollutants varied with different properties of biochar. The sorption mechanisms of inorganic pollutants such as heavy metal, radioactive elements, nitrogen and phosphate were summarized as well as their potential applications in real soil condition. Several different possible mechanisms were proposed: 1) electrostatic outer-sphere complexation due to surface cationic exchange; 2) surface complexation with active O-containing functional groups such as carboxyl and hydroxyl groups; 3) electrostatic attraction of anionic inorganic pollutants such as phosphate and arsenic to protonated groups under alkaline pH; 4) coprecipitation of heavy metal and phosphate with organic matter and mineral oxides on surface of the biochar or presorbed metal ions; 5) specific binding of iodide with aromatic carbon in biochar; 6) to donate electrons for mitigating/reducing heavy metal such as chromium; 7) physical adsorption of heavy metals onto biochar’s surface; 8) changing the pH of point of zero charge (pHpzc) to immobilize or mobilize heavy metals.
Generally and undoubtedly, the use of biochar as an environmental sorbent can have strong implications. It can effectively sorb various organic and inorganic contaminants in aqueous solutions. However, due to soil complexity, whether biochar is suitable for the remediation of inorganic contaminated soil is still unclear. These confused results could attribute to: 1) high dissolved organic carbon contents of soil at the increased pH induced by biochar addition may mobilize heavy metal leaching and/or form high available species; 2) electrostatic repulsion between anionic heavy metal ions and negatively charged biochar surface may enhance the desorption of heavy metal from soil-biochar matrix; 3) changing soil pH may result in mobilization or immobilization of heavy metals; 4) the transportation of biochar in soil system may influence the mitigation of sorbed heavy metals; 5) the availability of heavy metal sorbed by biochar to soil microorganism or plants; 6) the stability and biodegradation of biochar is also an uncertain factor for the application of biochar in the remediation of inorganic contaminated soil.
Based on the limited information, we proposed that biochars, especially those pyrolyzed at high temperature were suitable for the remediation of the low pH and/or low dissolved organic carbon soil contaminated with cadmium, lead, copper, zinc and other heavy metals. Furthermore, further researches on interactions among soil-biochar-pollutants and field applications for remediation of contaminated soil are urgently needed.
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Published: 20 July 2016
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生物质炭对土壤无机污染物迁移行为影响研究进展
生物质炭材料来源广泛，制备工艺相对简单，且具备丰富的含氧官能团、发达的孔隙结构和表面电荷，对有机污染物和各类无机污染物（重金属、氮磷、放射性元素）均具有良好的潜在吸附能力，被认为是一种低成本、高效的新型环境功能吸附材料。本文针对重金属、氮磷等土壤无机物，在介绍生物质炭基本性质的基础上，综述了生物质炭吸附无机污染物的机制，探讨了应用于无机污染土壤缓解和修复的可行性，并指出了相应的发展趋势。生物质炭的基本特性受来源材料性质、裂解温度等主要因子的影响，其碳含量和结构、H/C比值、孔隙结构、pH等性质有较大差异，这也导致生物质炭对重金属、氮磷等无机污染物的吸附机制包含了表面物理吸附、络合作用、静电引力、阳离子交换、共沉淀、碘-碳特殊作用等多种机制。然而，受土壤复杂理化性质和生物活性、生物质炭迁移性和稳定性等因素影响，生物质炭在无机污染土壤缓解和修复中的应用有很大潜力，但尚存在不确定性、调控性差等问题，甚至反而会活化土壤中的污染物。因此，在应用生物质炭缓解和修复重金属污染土壤时，应充分考虑土壤性质、污染程度和类型与生物质炭性质的匹配度。生物质炭更适合pH和有机质含量较低的镉、铅、铜、锌等重金属污染土壤；与低温生物质炭相比，高温生物质炭的适用范围更广。
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