Determination and characterization on the capacity of humic acid for the reduction of divalent mercury

Abstract

For understanding the capacity of humic acid (HA) for mercury (Hg2+) reduction, chemical reduction capacity (CRC), microbial reduction capacity (MRC), and native reduction capacity (NRC) by three types of HAs extracted from various sources (SH, TJ and JY) were measured, respectively, following the different prereductions including chemical, microbial, and control treatments. 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 being all greatly higher than those in conditions of Hg(NO3)2 and HgCl2, which suggested that the RC obtained using Fe3+ reduction method could exaggerate the real 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 being 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; and (3) HA in solution status had approximately 100-691.67 % higher reduction capacity than those as solid status. Meanwhile, through comparison of the differences among three RC indices, higher CRC and MRC values than NRC were observed, but no evident 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.