胡敏酸对汞还原能力的测定和表征

还原容量(RC)是表征胡敏酸(HA)氧化还原特性的重要指标.采用饱和H2振荡法和土壤溶液法对HA分别进行化学和微生物预处理,结合对照(未经任何还原前处理),在Fe3+还原法测定RC基础上,以柠檬酸铁(FeCit)为参照,分别以氯化汞(HgCl2)及硝酸汞Hg(NO3)3]作电子受体,测定了3种HA(上海巨枫SH、天津光复TJ、缙云山JY)对汞的化学还原容量(CRC)、微生物还原容量(MRC)、本底还原容量(NRC),以了解HA对Hg2+的还原能力.结果表明,①不同电子受体对HA还原汞能力影响显著,FeCit条件下测得RC值远高于Hg(NO3)2和HgCl2(1～2个数量级),因此采用Fe3+还原法会过高估计HA对Hg2+的实际还原能力;②受自身结构和基团的影响,3种胡敏酸对汞的还原能力差异明显,以JY最高,分别(以C计)为(0.95±0.03)mmolc.mol-1(NRC)、(5.95±0.63)mmolc.mol-1(CRC)和(6.26±0.51)mmolc.mol-1(MRC);③溶液态HA还原Hg能力明显高于固态HA,增幅在100%～691.67%之间.同时,通过对比3个还原容量指标发现,CRC和MRC均显著大于NRC,而CRC和MRC之间无明确大小关系,因此,CRC并不能完全代表HA在微生物还原条件下对Hg的真实还原容量.

Determination and Characterization on the Capacity of Humic Acid for the Reduction of Divalent Mercury

Reduction capacity (RC) has been recognized as an important parameter for evaluating the redox role of humic acid (HA). Thus, for understanding the capacity of HA for the reduction of mercury(Hg²⁺), chemical reduction capacity (CRC), microbial reduction capacity (MRC) and native reduction capacity (NRC) for mercury reduction by three types of HAs extracted from various sources (SH, TJ and JY) were measured respectively, following the pre-treatment of the HA samples by saturated hydrogen oscillation, soil solution incubation and the control (without any pretreatment). Three electron acceptors including mercuric chloride (HgCl₂), mercuric nitrate Hg(NO₃)₃] and ferric citrate (FeCit) as a reference were adapted respectively based on the Fe³⁺ reduction method. The principal results indicated that: 1 the capacity of HA for the reduction of Hg was significantly affected by various electron acceptors, with the RC values obtained under FeCit condition were all greatly higher than those in the conditions of Hg(NO₃)₂ and HgCl₂, which suggested that the RC obtained using Fe³⁺ reduction method could exaggerate the actual capacity of HA for the reduction of Hg²⁺; 2 significant differences existed for the reduction capacity of Hg²⁺ by different HAs, with those of JY were the highest, which were (0.95±0.03)mmol_c·mol^-1 (NRC), (5.95 ±0.63)mmol_c·mol^-1 (CRC) and (6.26±0.51)mmol_c·mol^-1 (MRC) respectively; 3 HA in solution forms had approximately 100%-691.67% higher reduction capacity than those as solid forms. Meanwhile, through comparison of the differences among three RC indices, higher CRC and MRC values than NRC were observed, but no significant difference between CRC and MRC was concluded. Thus, CRC may not be applicable to comprehensively represent the real reduction capacity of HA for Hg reduction under microbial condition.