| Mercury is a kind of toxic heavy metal element,it is volatile and can transport for a long distance via atmosphere,since it is considered to be a global pollutant.Mercury could transform to virulent methylmercury in natural environment,then enrich to organism through food-chain and cause heavy harm on human health.Therefore,the transfer and transformation of mercury and factors have been the hot issues study in the field of environment science.Dissolved organic matter(DOM)is an active component in ecosystem,which influence strongly the fractionation,transport and transformation,global biogeochemical behavior and bioavailability of mercury.The Gorges Reservoir is a super-large external regulation reservoir of our country,the special way of regulation lead to a huge area of water-level-fluctuating zone around it.During the period of fall of the water level,there is a large area of dense herbaceous vegetation,and partial zone is used to be agricultural land with applying organic fertilizer;When the water-level-fluctuating zone is submerged as the water level rise,with lots of DOM into water body due to the flooded plant and soil,special reservoir environment and DOM in water body may influence the methylation of mercury in reservoir.Therefore,this experiment choose the soil of The Gorges Reservoir area as the object of investigation,extracted two typical DOM(the DOM from Bermuda grass and pig manure,noted as DOMD,DOMP)in water-level-fluctuating zone and analysed their geochemical characteristics,then via indoor simulating experiment,researched the influence of DOM on the mercury methylation in different conditions,the influence of DOM sources on the mercury methylation and the influence of DOM on the mercury methylation under different concentrations of sulfate radical respectively,to more accurately understand the mechanism of DOM on the methylation of mercury.The research results as follows:(1)The result of ultraviolet-visible absorption spectra and three-dimensional fluorescence indicated that,there were differences between the two kinds of DOM extracted from Bermuda grass and pig manure,the concentration of CDOM,aromaticity,and the hydrophobic components of DOMP were all higher than DOMD.The protein-like substance was the major component of two kinds of DOM,while the bioavailability of DOMP was higher than DOMD.The humification index and the biology index of DOMD,a terrestrially derived DOM,were lower than DOMP,which was both terrestrially and autochthonous,indicated that DOMP was more humified and bioavailable.(2)The indoor simulating experiment of researching the influence of DOM on mercury methylation in soil indicated that,in the existence of microorganism,after adding DOMD,DOMP,the concentration of DOC in the suspension were reduced,the population of microorganism in soil reached to 24.50×10~6cfu/g,45.25×10~6cfu/g at 50 h,30 h,increased 14.46,36.74 times respectively,the contents of MeHg in soil increased 16.77,17.18 times at 120 h,indicated DOM could promote the methylation of mercury in soil,and the promotion was more significant in later period of incubation.The nutrient resource had been limiting and the concentration of MeHg was higher with the extension of incubation time,consequently,the population of microorganism was smaller,the time of methylation falled behind the time of peak value of microorganism population,indicated that the efficiency of mercury methylation was influenced by the growth of microorganism.In the experiment of inhibition,the concentration of DOC in the suspension were rised,the population of microorganism and the amounts of MeHg in soil were invariant almost,indicated that DOM promoted the biomethylation of mercury.(3)The indoor simulating experiment of researching the effect of DOM source on the Hg-methylation in soil indicated that,when the concentration of DOM were 50 mg/kg,100 mg/kg,200 mg/kg respectively,the amounts of MeHg in soil of adding DOMD were 2.34,3.08,6.46 times the initial amounts,and the 4.25,6.84,14.18 times of adding DOMP.It showed that two kinds of DOM could promote the methylation of mercury,and the higher concentration of DOM,the more significant promotion.While there existed difference in promotion between DOMD and DOMP.DOMP was more effective at enhancing Hg-methylation than DOMD.DOMP was higher molecular-weight,and more protein-like substance,which were conducive to adsorption and bioavailability of microorganism,so DOMP was more effective in promoting Hg-methylation.What's more,both of DOMD and DOMP could inhibit the adsorption of soil on mercury,and the higher concentration,the stronger inhibition.In addition,DOMP was stronger in inhibiting the adsorption of mercury than DOMD because of higher molecular-weight and more hydrophobic components.(4)The indoor simulating experiment of researching the effect of DOM on Hg-methylation with different concentrations of sulfate radical indicated that,in the existence of DOM,adding sulfate radical could inhibited the Hg-methylation of soil,the inhibition followed this trend:0 mg/g<0.11 mg/g<0.55 mg/g,that is the higher concentration of sulfate radical,the stronger inhibition.In all treatments,the concentration of DOC was reduced because of the utilization of DOM by microorganism,while the concentration of sulfate radical were invariant almost indicated that the activity of sulfate reducing bacteria was not the major reason for Hg-methylation,the other else microorganisms contributed to the methylation of mercury in soil. |
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