| Mercury is a highly toxic global pollutant.Rice can be enriched with mercury(Hg),and the consumption of rice in mercury-contaminated areas is one of the main ways in which mercury is exposed to mercury,thus understanding the geochemical cycle of mercury for mercury-contaminated paddy soils,controlling mercury pollution in paddy ecosystems,protecting ecosystems and food safety is of great significance.Mercury is a kind of pro-sulfur elements,and its biogeochemical behavior is closely related to sulfur status..In this paper,firstly,the synergistic transformation of mercury and sulfur in paddy field ecosystem was studied by flooding the soil in paddy field and applying different sulfur.Furthermore,adsorption and desorption processes between Hg2+ and the main components of paddy soil were investigated.The effects of environmental factors such as pH,salinity and organic matter on the adsorption and desorption processes were also discussed The main results are as follows:(1).The geochemical morphology of mercury in flooded soil was studied by continuous step extraction method.The results showed that the addition of sulfur could promote the decrease of organic bound mercury in soil and the increase of residual mercury Sodium thiosulfate treatment group was more obvious;the transformation of exchangeable mercury and carbonate-bound mercury was closely related to pH,Eh,etc;the distribution of elements in iron,manganese and mercury in soil colloid was studied by synchrotron radiation X-ray fluorescence.The results showed that exogenous sulfur affected the micro-area distribution of Fe-Mn and Hg and inhibited the binding between mercury and iron and manganese.The K-edge absorption spectrum of sulfur was collected and the results were obtained.It indicated that the sulfur in the soil gradually changed from high-valence to low-valence sulfur with the prolongation of time,and S2-was combined with mercury in soil to promote the transformation of other forms of mercury to residual mercury in the soil.The continuous extraction of sulfur from soil at different time indicated that the extracted sulfur in the soil was gradually converted to the adsorbed sulfur in the soil with the prolonged time.(2).To study the effect of environmental factors(pH,salinity,humic acid concentration)on the adsorption and desorption of Hg2+ on a single mineral.The results showed that the adsorption of Hg2+ on montmorillonite and kaolinite were associated with pH,Hg2+ on montmorillonite pH increased from 4 to 6 the adsorption rate increased,pH increased from 6 to 8,the adsorption rate decreases.On kaolinite pH increased from 4 to 8,the adsorption rate decreased.The high concentration of background electrolyte inhibits Hg2+ adsorption;humic acid can promote Hg2+ adsorption on kaolin.In addition,humic acid can also promoted the desorption of mercury from kaolin..The desorption process of humic acid to kaolin adsorption was related to pH,the pH increased from 4 to 5,its desorption rate decreasesd,pH increases from 5 to 8,and the desorption rate is increased.Sodium sulfide and sodium thiosulfate could promote the desorption of kaolin adsorption of mercury,sodium sulfide desorption better.(3).The effects of pH,humic acid concentration and other factors on the adsorption of Hg2+ on iron oxides were studied.The results showed that the adsorption of Hg2+ on iron oxides increased with pH in the acidic range(pH4～7).In the acidic system,humic acids with a concentration of less than 100 mg/kg can promote the adsorption of Hg2+,and when the humic acid concentration was more than 100mg/kg,theadsorption of Hg2+ was inhibited. |
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