Effects Of Humic Acids On The Reduction And Release Characteristics Of Mineral Bound Hg

As everyone knows, mercury (Hg) is one of the most dangerous environmental pollutants in the poisonous trace metals.The main sources of mercury in the soil include its natural release and anthropogenic pollution.After entering into the soil system, the mercury quickly bonded by soil is higher than 95%, which is attributed to the strong adsorption between soil minerals and mercury. Thus most of soil mercury stored, which is relatively immobile and bound by minerals into crystal lattices. Soil mercury can also emit or release into atmosphere by reduction and volatilization process, then remotely transported, and thus it is a global pollutant. Mercury and its chemical compounds in the soil can be transported with runoff into water, and then lastly through food chain and bioaccumulation consumed by human, which exposing a high hazardous risk to human health.Humics, one of the extensive natural organic matters, appears in soil, lake, river, ocean and sediment. As the important component of humics, humic acid (HA) which includes many active groups, such as phenolichydroxyl, carboxylgruppe, carbonyl (group), quinonyl and methoxy, has intense complex, adsorb and redox ability, and can change the formation and bioactivity of heavy metal ions by different interactions. In previous, most of studies have paid to concentrat on the adsorption and complexation between humic acid and metal ions. In comparison, only a few references about the role of HA in mineral bound mercury releases were reported.Thus, in this study, diffusion-absorption method was conducted to investigate the effect of HA on different types of mineral bound mercury, and compare the redox characteristics among different HAs for understanding the effect of HA on mineral bound mercury. In addition, the influence of light was also processed and the differences among HAs were analyzed. Though researching the effect of HA on the release process of mineral bound mercury, we could understand the effect of humic acid on the release processes and the corresponding mechanisms of Hg. In this study, it seeks to provide some scientific proof for understanding mercury geochemistry cycle and the pollution control of Hg. The main research results were listed as follows:The reduction of mineral bound mercurys by HAs indicated that two HAs (including commercially available from Tianjin Guangfu and Shanghai Jufeng annotated TJ and SH respectively, followed by repurify procedure) all could reduce five mineral bound mercurys, and their percent reductions increase progressively over time. Two-constant, Elovich and Exponential equation could appropriately fit for demonstration of release trend of CaCO3-Hg, Fe2O3-Hg and MnO2-Hg with high correlation coefficients (r>0.8). and Two-constant, Elovich and Parabolic equation could appropriately fit for the release trend of bentonite-Hg and kaolinite-Hg (r>0.86). Exponential equation and Parabolic equation had optimal degree of fitting, which may indicate the diffusion is an important process in mineral bound mercury releases.The reduction of HAs on mineral bound mercurys was connected with the type of mineral and its space structures. In shading and light conditions respectively, the average reduction rates of five mineral bound mercurys by HAs(SH and JY) were different (P<0.05) significantly, and the order was as follows:CaCO3-Hg(0.67%)>Fe2O3-Hg(0.42%)>kaolinite-Hg(0.35%)>MnO2-Hg(0.19%) >bentonite-Hg (0.14%). Thus from the observation, light could enhance the effect of humic acid on mineral bound mercury releases, showing that the greatest significance in Fe2O3-Hg and bentonite-Hg, and the average reduction rate was 34% and 35% respectively higher than in darkness condition. Light could affect the reduction quantity and reduction rate of mineral bound mercury, so the photochemical reduction of Hg2+ by humic acid was similar with the mechanism underlying Fe3+ and Cr6+ photo-induced reduction by humic substances.The reduction of two humic acids(SH and TJ) on five mineral bound mercurys were different (P<0.05) significantly, and the reduction capacity of SH all was greater than TJ, which maybe explained through differences of free radical groups and aromatic degree (E4/E6) resulted in various reduction capacity.Considering the above experimental results, a possible conceptual model was proposed in which the reduction of mineral bound mercury by HA could be described as three processes, consisting of the surface breakages of mercury bound mineral into more relax network or formation, and the releases of Hg by HA, ligand competition among HA, mercury and minerals, and mercury reduction by chromophoric groups in HA, then leading the Hg0 into air above aqueous system. However, future researches need to conduct for understanding the details of this process.The effect of humic aicd on mercury releases from minerals indicated that there were various speciations of Hg release by HAs, and affected by various factors.The mercury releases from five minerals by two HAs (SH and JY) increased gradually at first, then leveled out over time.The order of totle mercury release quantity from five minerals was as follows:bentonite-Hg>CaCO3-Hg> Fe2O3-Hg>kaolinite-Hg>MnO2-Hg, which showed the bonding of MnO2 and Hg was stable and was not easy release, and the effect of the mercury release from minerals by humic acid refered to the type of mineral.In five mercury bound minerals, with the increasing of HAs(SH and JY) concentrations, the totle mercury release from minerals increased, and the release capacity were all significant different (P<0.01).The release quantities of various speciations mercury from CaCO3-Hg, kaolinite-Hg and MnO2-Hg were added with the increasing of pH, but Fe2O3-Hg was over the left. The release quantities of bentonite-Hg first ascended and then descended with the adding of leaf weight by SH. and it was opposite by JY.The light promoted the release of mercury bound minerals by HAs.The difference of MnO2-Hg by SH was unconspicuous, but the totle release quantities of other minerals were different (P<0.05).The release quantities in light irradiation were higher than shading, and bentonite-Hg was the most obvious, which less about 7% was retained in minerals. The totle release quantities of CaCO3-Hg and bentonite-Hg by SH, CaCO3-Hg, kaolinite-Hg and bentonite-Hg by JY in various temperatures were different (P<0.05), which meaned we could utilize the temperature variation of different seasons to administer upon the mercury polluted soil that included different minerals. With two humic acids, the totle release quantities of CaCO3-Hg was the maximal when SO42- existed, MnO2-Hg reached the maximum without coexistence ion, and Fe2O3-Hg displayed maximum when Cl- survived together.However, the releases of kaolinite-Hg and bentonite-Hg were still concerned with the type of humic acid. From this, as for mercury polluted soil that contained different types of minerals, we could facilitate the release of soil mercury by appending different interfering ions, and repaired the contaminated soil. In all of Hg bound minerals, the influence of Cl-and SO42- was greater than Al3+, which can infer negative ion was better to promote the release of mercury bound minerals than positive ion.The effect of different HAs on mercury bound minerals existed diversity. Under the action of SH and JY, the totle release quantities of kaolinite-Hg and Fe2O3-Hg were different (P<0.05), and others were not obvious. The totle release quantities of CaCO3-Hg and bentonite-Hg by SH were higher than JY, which indicated the surface adsorptions of SH with CaCO3 and bentonite were stronger, thereby the replaced binding site between Hg and minerals was more, and the Hg release was more.Howere, the surface adsorptions of JY with kaolinite, Fe2O3 and MnO2 were stronger, so the Hg release by replaced was more. Consequently, the effect of humic acid on mercury bound minerals was related to the types of minerals. The Hg0 release quantity by JY was more than SH, showed whose reducing capacity was higher than SH. This was in line with the expressional result of their basic properties.Electronic scanning images showed that the effects of humic acids on the reduction and releases characteristics of mineral bound Hg maybe was relatived to the structural changes of minerals after adding HAs.The surfaces of mineral and mineral-Hg changed fluffily obviously after adding HAs, and their compact textures displayed obvious blowout trace and crack. There were many flocs and network structure on their surface.These changes affected the mineral's holding force to its bonding mercury, and then impacted the reduction and release of mineral bound Hg by humic acids.