The Effects Of Humic Acid On Hg(Ⅱ) Abiotic Reduction And Its Influencing Factors

Humic substances (HS) such as humic acid (HA) and fulvic acid (FA) are ubiquitously exited in various environmental compartments. Due to the occurrence of various functional groups in their molecular structure, HS can interact with components of soil or water and organic and inorganic pollutants, thus HS have an important impact on the behavior of pollutants. Mercury is a kind of heavy metals elements which can greatly harm the environment, in the environment interface the migration of gaseous Hg0 is an important biogeochemistry process which can be different from other heavy metals element. So far, many studies have been completed for understanding interaction of some types of HA and Hg2+, which mainly concentrated on complexation and adsorption mechanism. Currentlu,only a few studies reported on the redox properties of humic acid and the effect factors of Hg2+ reduction. Thus, in this paper, studies were processed to understand reduction capacity of different HA and its influencing factors was discussed, which resulted in following conclusion:The reduction capacity of HA was gotten through the method of Fe3+ reduction,. Comparison with the reduction capacity of three types of HAs, results suggested that JY had the greatest values of reduction capacity (RC), which were far higher than SH and JY, indicating that JY had the greatest reduction capacity. The reduction capacity of HA is affected by the type of electron acceptor, which suggested that three RC values obtained from Fe(NO3)3 as electron acceptor were significantly higher than RC values obtained from using FeCit as electron acceptor. In addition, through comparison with two statuses of HAs, it suggested that RC of HA in aqueous status was higher than in solid status, and the differences were significant (p<0.05). Furthermore, based on comparison of three types of RC, results showed while HA was treated by H2 and soil solution.extraneous source of chemical and microbial electron acceptors existed, the reduction capacity of HA significantly was enhanced, which indicated that CRC and MRC both were higher than NRC.,but there is not obvious regulation between the CRC and the MRC. Through studies of reduction capacity of different HA using Hg2+ as electron acceptor, the reduction capacity of HA to the actual pollutant was disscussed. It suggested that:when the electron acceptor is HgCl2 or Hg(NO3)2, among different Has, the differences of the reduction capcity of HA were significant(p<0.05), of which JY is maximum, SH is the second, TJ is minimum. Three RC values obtained from Hg(NO3)3 as electron acceptor were higher than RC values obtained from using HgCl2 as electron acceptor. Using the method of Fe3+ reduction for determination and measurement of three RC values of HA (including NRC, CRC and MRC) can rationally express the reduction characteristics of humic acid, and understand the capacity details of effect of HA on Hg2+reduction qualitatively instead of quantificationally.In aqueous system, the effect of HA on Hg2+ reduction was significant. Through consecutive ventilation method, experiment for understanding the effect of HA on Hg2+ was processed. Based on kinetics experiment, in time range of 600min, the production of Hg0 was more obviously increasing, following a lower slope increasing. While reaction time exceeded 1200min, all experiment was processed to equilibrium. Meanwhile, using of kinetics equation fitting, first-order equation was best for fitting, which showed the initial rate constants (K) ranked as follows:JY (9.6×10-3 min-1)> SH (8.8×10-3 min-1)> TJ (6.2×10-3min-1). Through the experiments under various abiotic factors including initial concentrations of Hg2+ and HA, temperature, pH, and light radiation, which suggested that the Hg2+ reduction rate increased then decreased with increasing Hg2+ initial concentrations and HA concentrations. Meanwhile, natural light radiation and increasing temperature can promote Hg0 production. In addition, under strong acid (pH3.6) and base (pH8.1) conditions, the effect of HA on Hg2+ reduction was inhibited. In addition, through the studies of difference of RC under two different HA initial condition:aqueous and solid status, HA in aqueous solution can increase Hg2+ reduction rather than in solid status, which has significant difference (p<0.05). Furthermore, through analysis of active functional groups in three types of HA in both of status, HA in aqueous solution had more active functional groups than in solid status, which resulted in Hg2+ reduction by HA in was much more obviously in aqueous system. Finally, from comparison of the reduction characteristics of three types of HA (TJ, SH and JY) on Hg2+ reduction, which has significant difference (p<0.05). Of all HAs, JY has the greatest RC, then SH, and following TJ. In natural system, especially in aquatic environment, there exists two roles of HA interacts with Hg2+ including complexation and reduction, thus which indicates that it can adsorb mercury in its existing media through complexation, but also can affect the interface migration of mercury through reduction.