基于模拟老化和同位素技术的生物炭砷钝化效率评估

Assessment of biochar immobilization on arsenic based on simulated aging andisotope technology

为探究生物炭钝化砷长期效果,运用实验室模拟老化和稳定同位素技术,对饱和吸附砷(As)的^(13)C标记生物炭分别进行60 d冻融老化和自然老化实验,通过老化前后生物炭材料、碳总量、碳形态、TCLP提取态As(TCLP-As)、As形态变化,评估生物炭材料老化对钝化砷稳定性的影响。结果表明,老化作用未明显改变As的价态(As^(3+),As^(5+)),但降低了生物炭对As的钝化效果,TCLP-As由老化前的46%分别增加至60%(冻融老化)和63.6%(自然老化)。这主要归因于老化作用促进生物炭中部分不稳定碳转化为溶解性有机碳(DOC),而DOC具有较强迁移性,其丰富的含氧官能团可以与As络合,形成DOC-As团聚体而发生共迁移;此外,老化后生物炭Zeta负电位、PO_(4)^(3−)数量的增加,灰分质量分数、pH的降低,导致生物炭与As间静电斥力增强,离子交换、沉淀作用减弱,促进了As的解吸释放,故生物炭对As的钝化效果降低。对比2种老化,自然老化后,生物炭与As之间静电斥力、As与PO_(4)^(3−)竞争生物炭表面吸附点位更强,沉淀作用减弱更为明显;而冻融老化后生物炭比表面积相对较大,可提供更多吸附点位,故自然老化后生物炭钝化As稳定性低于冻融老化。本研究结果可为生物炭钝化砷的应用提供参考。

Simulated aging and stable isotope technology was conducted in laboratory to investigate the longterm immobilization of biochar on arsenic(As).^(13)C-labeled biochar with As-saturated adsorption was obtained through adsorption experiment,and then the freeze-thaw aging and spontaneous aging experiments were carried out for 60 days.The carbon stability of biochar was evaluated by the change of biochar material,total carbon content and morphology of biochar,immobilization of As by TCLP extraction and Tessier sequential extraction before and after the aging.The results showed that the As valence state(As^(3+),As^(5+))was not significantly affected by aging effect.However,the TCLP-As was increased from 46%(unaged)to 60%(freeze-thaw aging)and 63.6%(spontaneous aging)respectively.This was mainly due to the transformation of unstable carbon in biochar into dissolved organic carbon(DOC)after aging,which had strong mobility and rich oxygen-containing functional groups.The DOC was easy complex with As and co-migration as DOC-As complex.In addition,the increased Zeta negative potential and the concentration of PO_(4)^(3−)and the decreased ash mass fraction and pH of biochar enhanced electrostatic repulsion between biochar and As,and then reduced the ion exchange and precipitation between biochar and As.Compared with freeze-thaw aging and spontaneous aging,the freeze-thaw aging was more conducive to pore adsorption,while for spontaneous aging,the electrostatic repulsion and competitive adsorption between biochar and As were stronger,and obviously weakening of precipitation of As and PO_(4)^(3−).Therefore,the stability of biochar immobilization of As after spontaneous aging was lower than that of freeze-thaw aging.This study could provide a good reference for the application of biochar passivation of arsenic.