Characteristics And Potential Electron Donating Capacities Of Dissolved Organic Matter In High Arsenic Groundwater

Dissolved organic matter（DOM）,as the main carbon source of microorganisms in groundwater,plays an important role as an electron donor in the process of microbial-mediated reductive dissolution of iron（Fe）and manganese（Mn）oxides and release of arsenic（As）.However,the origin of DOM in high As groundwater and its synergistic action with microorganisms are still unclear.The effect of redox characteristics of DOM on As remains to be clarified.In view of the above two problems,64 groundwater samples with different As concentrations were collected along the groundwater streamline in Hetao Basin,Inner Mongolia.Five spectral indices were measured,including biological index（BIX）,humification index（HIX）,fluorescence index（FI）,aromatic index(SUVA₂₅₄)and spectral slope(S_275-295).Three-dimensional fluorescence spectroscopy combined with parallel factor analysis（EEM-PARAFAC）was used to characterize the components and sources of DOM in groundwater.By chemically quantifying the reduced capacity of DOM in groundwater,the redox characteristics of DOM and its influence on the formation of high As groundwater are studied in depth,which could provide a more detailed basis for the mechanism of As enrichment in groundwater.The following main understandings were obtained:（1）Arsenic concentrations in the study area range from 2.28 to 854μg/L and As（III）is the major species,and characterized by high distribution and high content.In the deep groundwater,As concentrations of the plain area are higher than that of the transition zone.As for shallow groundwater,it is widely distributed in the plain area and As concentrations vary widely.（2）FI values ranging from 1.5 to 1.8 indicate the mixture of terrestrial and microbial sources of groundwater DOM in the study area.The results of EEM-PARAFAC revealed that DOM in the groundwater of the study area comprised three major components,namely terrestrial humic-like component（C1）,microbial humic-like component（C2,C3）and protein-like component（C4）.The relative abundance of C2 and C3 accounted for more than 60%of the total fluorescent components,indicating that microbial humic-like components made the major contribution to DOM in groundwater.Compared with deep groundwater,HIX values of DOM in shallow groundwater are relatively higher and S_275–295 values are relatively smaller,which indicate that shallow groundwater DOM has higher humification degree and higher molecular weight.（3）Compared with native anaerobic groundwater samples（AGS）,fluorescence intensities of reduced groundwater samples（RGS）decrease and peak positions show a blue shift,indicating that theπ-π*bond cleavage occurs in the HS molecular structure during the chemical reduction process.As for the PARAFAC components,the C1（%）of the RGS becomes low and C4（%）increases.Compared with AGS,RGS have lower HIX and SUVA₂₅₄ values,and higher BIX,FI and S_275-295 values.These results suggest that the structural of terrestrial humic-like components may be changed during the chemical reduction process.This may be caused by the double bond of the conjugated structure becoming a single bond,interrupting the conjugation,resulting in the decrease of the fluorescence intensity of terrestrial humic-like components.（4）The positive correlations between S_275-295 and SUVA₂₅₄ and the reducing capacity of DOM indicate that small molecular weight organic matter or organic matter containing more aromatic structures have stronger electron transfer capacities,which can serve as an electron shuttle to provide more electrons to Fe oxides.（5）The positive correlation between the protein to humic index（PHI）and potential electron donating capacities indicates that humic-like component promote electron transfer,which is an effective electron shuttle.In areas where As concentration is relatively low,DOM acts as an electron shuttle to play the role of electron transfer.However,in areas with relatively high As concentration,in addition to the electronic shuttle of DOM,other effects may jointly promote the release of As,such as complexation,competitive adsorption and DOM directly as an electron donor.