厌氧条件下可溶性有机质对汞的还原与氧化作用

Reduction and Oxidation of Mercury by Dissolved Organic Matter Under Anaerobic Conditions

在沉积物、湿地和淹水的水稻土中天然可溶性有机质(DOM),如胡敏酸(HA)和富里酸(FA)处于还原状态,其介导Hg(Ⅱ)的还原,影响汞的转化及地球化学循环.本文通过模拟黑暗厌氧环境,研究还原态HA与FA对Hg(Ⅱ)的还原.结果表明,还原态HA与FA的还原容量高于对照氧化态HA与FA.还原态HA与FA对Hg(Ⅱ)的还原最佳浓度分别是0. 2mg·L^(-1)和1. 5 mg·L^(-1).而高于最佳浓度时,由于发生巯基竞争性络合作用,抑制Hg(Ⅱ)的还原;特别是还原态HA大于5mg·L^(-1)时,不能还原Hg(Ⅱ)为Hg(0).还原态HA与FA对Hg(Ⅱ)的还原反应动力学表明:有机碳(DOC)与Hg(Ⅱ)摩尔比DOC∶Hg(Ⅱ)=400∶1时,还原反应速率IHSS-HA> FRC-HA> FRC-FA;高摩尔比[DOC∶Hg(Ⅱ)=10 000∶1]时,还原态HA对Hg(Ⅱ)的还原反应停止,甚至向相反方向进行.还原态HA与FA对Hg(0)氧化结果表明,当还原态HA与FA浓度分别增加至5 mg·L^(-1)和10 mg·L^(-1)时,样品中检测不到Hg(0),还原态HA与FA对Hg(0)发生诱导性氧化络合作用.在汞的氧化还原与络合作用中,还原态DOM扮演双重角色,影响活性汞的可利用性,进而影响微生物汞的甲基化.

In sediments,wetlands,and flooded paddy soils,natural organic matter（ DOM）,such as humic acid（ HA） and fulvic acid（ FA）,exist in a reduced state and mediate the reduction of Hg（ Ⅱ） and affect the transformation of mercury and geochemical cycles.This study simulated a dark anaerobic environment to study the reduction of Hg（ Ⅱ） by reduced HA and FA. Compared to oxidized HA and FA,the reduction capacity of the reduced HA and FA for Hg（ Ⅱ） were higher. The optimal concentrations of reduced HA and FA to reduce Hg（ Ⅱ） to Hg（ 0） were 0. 2 mg·L^-1 and 1. 5 mg·L^-1,respectively. The reduction quantity of Hg（ Ⅱ） was enhanced with the increased concentration of reduced HA and FA at a lower than optimal concentration,whereas the reduction quantity of Hg（ Ⅱ）decreased with increasing concentration of reduced HA and FA at a higher than optimal concentration due to the occurrence of thiolcompetitive complexation. Hg（ Ⅱ） was not reduced at a reduced HA concentration of 5 mg·L^-1. The kinetics results of Hg（ Ⅱ）reduction demonstrated that the reduction rates were IHSS-HA FRC-HA FRC-FA at a low molar ratio（ dissolved organic carbon（ DOC） ∶ Hg（ Ⅱ） = 400∶ 1）. The reduction of Hg（ Ⅱ） by the reduced HA stopped or processed in the opposite direction at a high molar ratio（ DOC∶ Hg（ Ⅱ） = 10 000∶ 1）. The quantity of Hg（ 0） decreased with increasing HA concentration. Furthermore,as the concentrations of reduced HA and FA were increased to 5 mg·L^-1 and 10 mg·L^-1,respectively,no Hg（ 0） was detected in the samples. The experiment of the recovery confirmed that reduced HA and FA reacted with Hg（ 0）. Additionally,reduced HA,FA,and Hg（ 0） induced oxidative complexation with sulfhydryl or disulfide bonds. Reduced DOM played a dual role in Hg redox reactions,affecting the availability of active Hg,which,in turn,affected the methylation of microbial Hg.