市政污泥生物炭稳定红壤中重金属的地球化学机制及关键影响因素

The current study conducted by the applicant shows that when the sludge produced in the oxidation ditch of manucipal wastewater treatment plants is conditioned with ferric salts, then concentrated, dewatered, and pyrolyzed into biochar at 400 oC for 2 hours, and the resulting biochar can immobilize considerable mass of Pb(II) and As(III) in the aqueous soltuion. The pyrolysis process was also found to stabilize the original black carbons and heavy metals in the sludge, reducing the emission of green house gas and heavy metals. Therefore, this sludge-derived sludge seems very promising to be an effective amendment agent for soils contaminated with both cationic and anionic heavy metals. Different from the tranditional homogeneous porous media, sludge-derived biochar is rich in inorganic minerals and organic components. When it is appplied in the soil, the soil itself components, ions in the soil solution, soil organic matter may interact with the biochars, and the invovled geochemcial mechanisms are rather complicated. The corresponding key factors and potenital environmental risk are not clear so far, which would greatly limit its wide application in practics. Therefore, the geochemcial mechanisms between these metals in aqeuous phase and biochar is proposed to be investigated in this proposal with the aid of series of tools, such as chemical blocking of organic functional groups，FTIR,TEM-XPS,EXAFS. The potenital influencing factors will be screened and quantitively determined by series of careful orthogonal experiments, to simulate the goechemical process between Pb(II)/As(III) and biochar in the red soil. The results will discover the goechemical mechanisms of the heavy metal stabilization by biochars in the red soil, and indentify the key influencing factors.

申请者已有研究发现，由市政污经氧化沟处理，产生的污泥通过铁盐调理浓缩脱水后，在400oC2小时缺氧干馏而成的污泥生物炭对Pb（II）和As(III)均具有可观的吸附量，且吸附后金属都较稳定。且干馏过程固定污泥原有碳和重金属元素，减少温室气体的排放及重金属释放。故污泥生物炭可成为阴阳离子重金属污染土壤的良好稳定剂。不同于单一组分多孔物质，污泥生物炭表面富含无机矿物和有机碳基组分，当其应用于土壤时，与土壤组分，溶液离子和有机质等相互作用，故涉及地球化学机制芜杂，其关键因素和潜在风险尚不明确，严重制约其实际应用。故本研究拟先利用水相试验和官能团遮掩、FTIR、TEM-XPS、EXAFS微观手段来确定其在水相中稳定阴阳重金属的地球化学机制。在此基础上，筛选适合的影响因子和环境条件，设计正交实验，模拟其在红壤中相关地球化学过程，从而确定污泥生物炭对稳定红壤中重金属的地球化学过程及关键因素。

通过4年的实验研究发现，由市政污泥脱水后直接干馏而成的污泥生物炭都对阳离子重金属Pb（II），阴离子重金属Cr（VI）和类金属As(III)以及有机污染物均具有可观的吸附量，且400 oC 2 小时制备的污泥生物炭比表面积最大，自身重金属溶出量最小，对上述污染物的吸附量最大。对阳离子重金属的吸附主要是内层吸附和表面沉淀为主，其中污泥炭中的磷酸盐起到主要作用，使得在酸性条件下，吸附的重金属阳离子也非常稳定。土壤中的致酸离子Al3+通过缓冲和竞争作用会抑制部分Pb在污泥炭上的吸附。对阴离子重金属Cr（VI）的吸附受到pH的影响，酸性条件有利于吸附过程。其机理在于污泥炭表面的还原物质与Cr（VI）反应生成Cr（III），土壤中的腐殖物质有利于污泥炭对Cr（VI）的氧化过程。对As（III）的吸附，在中性条件下较好，短时间以配位体交换和表面沉淀为主，随着时间的推移，吸附的As（III）被逐渐氧化成As（V）而稳定。对阿特拉津的吸附以氢键为主，并且受到其他重金属离子的抑制作用。一般情况，高温有利于重金属在污泥炭表面的稳定，而长时间的老化过程，使得污泥炭表面的含氧官能团增加，有利于Pb的稳定。并且随着时间的推移，Pb的污泥炭表面的形态也逐渐稳定。因此，污泥生物炭可以作为一种良好的土壤改良剂应用在污染土壤中，特别是重金属污染红壤中，从而实现“以废治废”。但是，因为研究期限有限，污泥生物炭在土壤中多年后是否稳定尚需在后续研究和实践中加以关注。