Influence Of Different Organic Components In Sediment And Fish Diet On Mercury Methylation In Water

With the increasingly serious global mercury pollution in marine water environment and the increasing number of people consuming maricultural products,it is of greater significance to investigate the environmental geochemical processes of mercury in mariculture zones.Previous studies have shown that dissolved organic matter(DOM)has an extremely important effect on the existing forms and transformation process of mercury in aquatic environment.However,from a more microscopic perspective,such as the structure of DOM,the characteristics of its components and functional groups,there is no systematic study to clarify the mechanism of DOM's influence on the existence and transformation mechanism of mercury.After the DOM in fish diet and sediment was extracted by water extraction method,6 subcomponents including HIA(hydrophilic acidic substance),HIB(hydrophilic alkaline substance),HIN(hydrophilic neutral substance),HOA(hydrophobic acidic substance),HOB(hydrophobic alkaline substance)and HON(hydrophobic neutral substance)were separated from fish diet and sediment DOM at different decomposing periods by using the tandem technology with resins with different adsorption properties,and the methylation culture test of Hg²⁺was carried out in the laboratory.The purpose of this experiment was to investigate the effects of DOM and its subcomponents in fish diet and sediment on mercury methylation in aquatic environment at different decomposing periods.1.DOM in fish diet and sediment were rich in the carboxyl,hydroxyl,methyl,methylene,C-O bond and other functional groups.Fish diet and sediment DOM exhibited higher aromaticity in the early stage of decomposition during the decomposition periods.DOM at the later stage of decomposition exhibited higher hydrophobicity at different decomposition times.Different functional groups undergo different changes under the influence of environmental factors,and the changes of functional groups would affect the interaction between them.In the anaerobic reaction,when the total mercury(THg)content in the solution was 800 ng/L,the DOM of fish diet and sediment has the highest promotion effect on mercury methylation,and the conversion amount of methyl mercury(MeHg)was 1.63 and 1.67 ng,respectively.Compared with the aerobic reaction,the sediment DOM in the anaerobic reaction will weaken the methylation capacity of mercury regardless of the mercury concentration.In the experiment of the influence of different decomposition times on mercury methylation,on the whole,the methylation ability of fish diet DOM to mercury in the aerobic and anaerobic reaction first decreased and then slightly increased with the increase of decomposition time.The methylation capacity of sediment DOM to mercury in the aerobic reaction first increased and then decreased with the increase of decay time.In anaerobic reaction,the decay time decreases first and then increases.2.The composition and number of groups contained in the hydrophobic subcomponent in the fish diet and sediment DOM are more abundant than those in the hydrophilic subcomponent.The composition of the HIB and HIN functional groups is simple and the number is small,and their ability to methylate Hg²⁺is relatively weak;similarly,the composition of the HOB and HON groups is rich and the number is more,and their ability to Hg²⁺methylation is also relatively stronger.When the concentration of mercury in the solution was low,the mercury methylation ability of each subcomponent of undecomposed fish diet and sediment DOM was increased with the increase of the concentration of mercury in the solution,and the MeHg conversion rate was higher when the concentration of mercury was lower in the solution.Among the six subcomponents,the components HOB and HON played a major role in the methylation of mercury,and the component HOB in fish food DOM had a more significant mercury methylation ability in the anaerobic reaction where the concentration of mercury in the solution was higher;The HON component and HOB component in the sediment DOM had more significant mercury methylation ability in the anaerobic reaction where the mercury concentration in the solution was lower,and the mercury methylation ability of the component HOB was stronger after the anaerobic reaction.3.Hydrophobic subcomponent is the main component of fish diet and sediment DOM,among which HOB is the most important component and always occupies the largest proportion in the whole decomposition periods.With the decomposition process,the total organic carbon(TOC)concentration of hydrophobic components decreased,while the TOC concentration of hydrophilic components increased slightly,but the increase was not significant.In terms of the overall trend,in the experiment of the influence of different decomposition periods on mercury methylation,the hydrophobic organic subcomponents in fish diet and sediment DOM will continuously enhance the ability of mercury methylation in the anaerobic reaction with the increase of decomposition time.In the anaerobic reaction,hydrophilic organic subcomponents of fish diet DOM weakened the methylation capacity of mercury with the increase of decomposition periods.In the anaerobic reaction,the hydrophilic organic subcomponents of sediment DOM first weakened and then enhanced the methylation ability of mercury with the increase of decomposition periods.