Effect Of Manganese On The Environmental Behaviors Of Divalent Mercury And Methylmercury

Methylmercury is a strong neurotoxin,and its demethylation process has attracted much attention.Mn（III）and dissolved Mn（III）complexes are ubiquitous inorganic oxidants in the environment.It is not clear whether these active Mn（III）species can directly degrade methylmercury;the adsorption behavior of methylmercury degradation product Hg（II）by manganese oxides is also unclear.In this paper,birnessite with different oxidation degrees was synthesized,and its structure and physicochemical properties were characterized by a series of advanced technologies.Then the adsorption characteristics of Hg（II）on the surface of birnessite with different oxidation degrees were investigated.Then the degradation of methylmercury by birnessite in the presence of oxalic acid was studied.Finally,Mn（III）-sodium pyrophosphate（PP）complexes was prepared,and its degradation process and mechanism of methylmercury were explored.The degradation effect of Mn（III）-PP complexes on methylmercury in actual mercury-contaminated soil was further investigated.The results clarified the important role of birnessite and Mn（III）in the degradation of methylmercury,and provided scientific basis and theoretical support for the remediation and treatment of mercury-contaminated soil.The main results are as follows:1.Hexagonal birnessite with different oxidation degrees was prepared by reducing boiling potassium permanganate with concentrated hydrochloric acid.The average oxidation degree of manganese（Mn AOS）of the samples determined by oxalic acid reduction potassium permanganate back titration method were 3.94±0.01（HB1）,3.88±0.03（HB2）and 3.84±0.02（HB3）,respectively.2.The adsorption behavior of Hg（II）on the surface of hexagonal birnessite with different oxidation degrees was investigated.The p H significantly affected the adsorption of Hg（II）by birnessite.When the p H was 3-8,the adsorption capacity increased first and then decreased with the increase of p H.At p H 5 or 6,birnessite has the strongest adsorption capacity for Hg（II）.The adsorption of Hg（II）on birnessite belongs to chemical adsorption,and its adsorption form may be heterogeneous surface adsorption.3.The degradation of methylmercury by oxalic acid-birnessite coexistence system was investigated.The addition of oxalic acid concentration significantly affected the degradation of methylmercury by birnessite.At a mineral concentration of 5 g/L,the addition of low concentration（0.1-1 m M）oxalic acid did not significantly improve the degradation of methylmercury,but high concentration（4-25 m M）oxalic acid could significantly promote the degradation of methylmercury.100μM birnessite could not degrade methylmercury,but 100-1000μM oxalic acid could significantly promote the degradation of methylmercury by birnessite.At the same time,the 1000μM oxalic acid-100μM birnessite coexistence system also has a good degradation effect on 500μg/L methylmercury.The results of free radical quenching experiments showed that Mn（III）-oxalate complexes was the main active species for the degradation of methylmercury.4.The process and mechanism of methylmercury degradation by Mn（III）-PP complexes were studied.The degradation behavior of methylmercury by Mn（III）-PP complexes was regulated by PP/Mn（III）ratio and initial p H.Under the same PP/Mn（III）ratio,the degradation ability of Mn（III）-PP complexes to methylmercury was proportional to its initial concentration.However,after adding high initial concentration of Mn（III）-PP complexes,the high PP/Mn（III）ratio will inhibit the degradation of methylmercury by Mn（III）-PP complexes.Low initial p H can significantly improve the degradation ability of Mn（III）-PP complexes to methylmercury.The Mn（III）-PP complexes also had a good degradation effect on mercaptomethylmercury at initial p H4.0.5.Three groups of experiments,original soil,original soil+Mn（III）-PP complexes,sterilized soil+Mn（III）-PP complexes,were designed to investigate the degradation ability of 45μM 2 m L Mn（III）-PP complexes to methylmercury in mercury-contaminated soil.After 168 h incubation,the concentrations of methylmercury in the three experimental soils decreased by 1.22±0.11μg Kg^-1,3.12±0.06μg Kg^-1 and 0.80±0.02μg Kg^-1,respectively.The significant difference analysis of soil methylmercury content after 0-168 h incubation in different experimental groups showed that the addition of Mn（III）-PP complexes could significantly promote the degradation of methylmercury in soil regardless of the presence or absence of microorganisms.Compared with the original soil without Mn（III）-PP complexes,the degradation amount of methylmercury increased by 1.91±0.18μg Kg^-1 after adding Mn（III）-PP complexes,and the degradation rate increased to 2.42±0.32 times.