Determination and Characterization on the Capacity of Humic Acid for the Reduction of Divalent Mercury
胡敏酸对汞还原能力的测定和表征

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)mmolc·mol-1 (NRC), (5.95 ±0.63)mmolc·mol-1 (CRC) and (6.26±0.51)mmolc·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.