柠檬酸对生物炭钝化污染土壤中重金属稳定性的影响

Effects of citric acid on the stability of immobilizaed heavy metals by biochar in contaminated soil

为了研究生物炭(BC)对重金属复合污染土壤的钝化效果以及环境条件变化后钝化产物的稳定性.在受Cd、Pb和Zn污染的土壤中添加不同比例的生物炭进行土培实验,两个月后,添加不同浓度的柠檬酸模拟植物根际环境条件,分析土壤环境条件变化后重金属钝化产物的稳定性.结果表明,与对照组相比,添加生物炭(5%和8%)显著提高了土壤的pH值、阳离子交换容量(CEC)、土壤有机质(SOM),而有效态重金属和重金属毒性浸出浓度均显著降低,且后者低于其国际标准.添加柠檬酸后,土壤pH值随柠檬酸浓度的增加呈现下降趋势;生物炭的添加比例一定时,有效态Cd(DTPA-Cd)和Cd的毒性浸出浓度(TCLP-Cd)随柠檬酸浓度的增加呈现先降低(2 mmol·kg-1)后升高(10、20 mmol·kg-1)的趋势,而有效铅(DTPA-Pb)和有效态锌(DTPA-Zn)随柠檬酸浓度的增加而上升.柠檬酸浓度一定时,有效态重金属和重金属毒性浸出浓度随生物炭的添加比例的增加而降低,当生物炭的添加比例大于5%时,TCLP-Cd和TCLP-Zn虽有所上升(与无柠檬酸相比),但均低于其国际标准.可见,生物炭可对重金属污染土壤进行有效修复,但随着环境条件的变化,被钝化的重金属会发生解吸和溶解释放,从而增强其生物有效性和环境风险,但当生物炭的添加比例较高时,会一定程度抑制重金属的解吸和溶解释放,Cd和Zn的环境风险仍处于可接受的安全水平.

Soil culture experiments were carried out to investigate the passivation effect of biochar(BC)on heavy metals contaminated soil and the stability of the immobilized products in various environments.Different proportions of biochar were added into soil polluted by Cd,Pb and Zn.Two months later,to simulate the plant rhizosphere environments,citric acid with different concentrations was added,and the stability of the immobilized products was analyzed.The results showed that,compared with the control group,biochar(5%and 8%)addition increased soil pH,cation exchange capacity(CEC)and soil organic matter(SOM)dramatically,while the contents of available heavy metal contents and the toxic leaching concentrations of heavy metals decreased significantly,and the latter were below their international standards.With the addition of citric acid,soil pH decreased with the increase of its concentration.With a fixed proportion of biochar,the content of available Cd(DTPA-Cd)and toxic leaching concentration of Cd(TCLP-Cd)showed a downward trend(2 mmol·kg-1 citric acid),and an then upward trend(10 and 20 mmol·kg-1 citric acid)with the increase of citric acid concentration while the an available content of Pb(DTPA-Pb)and available content Zn(DTPA-Zn)increased with increasing citric acid concentration.With a fixed citric acid concentration,the available content and toxic leaching concentration of heavy metals decreased with increasing proportion of biochar.When the proportion of biochar was more than 5%,the toxic leaching concentration of Cd(TCLP-Cd)and Zn(TCLP-Zn)increased(compared with that without citric acid),but both lower than their international standards.The results indicates that biochar can effectively immobilize heavy metals in contaminated soil.But with changing environmental conditions,the immobilized heavy metals would be desorbed and dissolved,thus enhancing the bioavailability and environmental risks.However,given the proportion of biochar is high enough,the desorption and dissolution of heavy metals will be inhibited,and the environmental risks of Cd and Zn would be at an acceptable level.