Research On The Effect And Mechanism Of Dissolved Organic Matter On The Adsorption Of Hg²⁺ By Soils

Mercury is a key global pollutant which is non-degradable and can be bioaccumulated in the environment, and may pose the potential dangers to human health through the food chain. In terrestrial ecosystems, soil is an important part in transformation and migration of mercury, and is an important source of mercury evaporation into the atmosphere. Regardless of the mercury content in soil, it can be transported to the plant, and then into the terrestrial food chain. And the solubility and migration of mercury in the soil can eventually affect the bioaccumulation capacity and risks of mercury to surface water or groundwater, the research of solubility and mobility of mercury in terrestrial ecosystem is more important than expected. As a highly active chemical compound, dissolved organic matter (DOM) is an important natural organic ligand and migrating carrier. Therefore, DOM is one of the main factors which controls the concentration and bioavailability of mercury in terrestrial and aquatic environment. Therefore, it is significant to study the effect and mechanism of DOM on the behavior of mercury in the environment.A series of batch experiments were conducted to assess the effect of DOM obtained from wheat straw (DOMw) and pig manure (DOMp) on Hg2+ adsorption processes by the black, fluvo-aquic, and red soils in China. We also studied the adsorption of Hg2+ on 26 different types of soil and DOM on Hg2+ adsorption, and illustrated the relationship between the basic properties of soil and Hg2+ adsorption and the role of DOM on Hg2+ adsorption. Multiple methods including FTIR spectroscopy, solid-state 13C NMR spectroscopy and fluorescence spectroscopy were used to uncover the structural characterization of DOM and to elucidate the mechanism of Hg2+ adsorption on DOM. These results were then used to provide theoretical guidance for risk assessment, forecasting and management of mercury pollution, and to give the advice for more reasonable fertilizer of organic materials.The main achievements of this paper are as follows:(1) Hg2+ adsorption isotherms could be well fitted with both Langmuir and Freundlich equations, and the maximum adsorption amount of Hg followed the order:black soil (2188 mg kg-1 soil)> red soil (1196 mg kg-1 soil)> fluvo-aquic soil (1095 mg kg-1 soil).(2) Adding DOMw and DOMp to three soils, the maximum adsorption amount of Hg obviously decreased. The inhibition capacity of DOMw on Hg adsorption was higher than that of DOMp. Hg2+ adsorbed on red soil showed greatest affect by DOM, and the decrease of Hg2+ adsorption was an order of red soil> black soil> fluvo-aquic soil for the same DOM. The result of DOM concentration experiment was consistent with the conclusion made from isotherm adsorption experiments. A linear correlation between DOM concentration and Hg2+ adsorption was observed for the three soils.(3) The formation of Hg-organic complex may contribute the decrease in Hg2+ sorption. The role of DOM in reducing Hg2+ sorption could be also caused that DOM can compete adsorption sites with Hg2+ to decrease Hg2+ adsorption sites. The decrease in turn depends on the DOM and soil types.(4) The element contents of DOMs showed that two types of DOM contained C, H, N, S, the H/C and N/C of DOMp were higher than those of DOMw.(5) The Fouxier transform infrared (FTIR) spectroscopy was adopted to characterize the DOMs samples and DOMs-Hg complexes, which revealed that DOMw and DOMp contain similar groups. The complexation of DOM and Hg2+ mainly arised on C=O, COO- and O-H.(6) The CP/MAS 13C NMR spectroscopy was also used to characterize DOMs and DOMs-Hg samples. The NMR spectroscopy showed that DOMs had similar carbon functionalities but different in contents, DOMp was higher in alkyl carbon compared with DOMw, and DOMw exhibited higher aromatic carbon. The NMR spectroscopy indicated the complex reaction of Hg2+ and DOMs mainly happened on carboxyl C(7) The 3DEEM fluorescence spectroscopy was employed to study the fluoroscopy properties of DOM and the reaction with Hg2+. Both DOMp and DOMw contained aromatic protein substances and visible fulvic-like substances, DOMw was also contained UV fulvic-like substances. DOMs-Hg complexes showed a lower intensity than DOM alone and protein-like properties of DOMp and UV fulvic-like substances of DOMw participated in the complexing reaction well than the other DOM properties.(8) The adsorption of Hg2+ on 26 types of soils showed that the adsorption amount of Hg2+ was little difference, but the Kd values varied greatly. Multiple statistical analyses indicated a significant correlation between the soil CEC and SOM content and Kd value.(9) The decrease of Hg2+ adsorption when adding DOMw was significantly correlated with soil pH, SOM, Fe2O3 and CEC, and the order of relevance was SOM> pH> Fe2O3> CEC, while the effect of DOMp on Hg2+ adsorption only index related with soil Fe2O3 and Al2O3 contents.