Mechanisms Of Organic Matter Transformation During Dissimilated Iron Reduction And Arsenic Mobilization In Aquifer Of Jianghan Plain

Geogenic high arsenic groundwater is widely distributed all over the world,being one of the most earnest environmental problems.Under anaerobic conditions,microorganisms utilize labile organic matter to reductively dissolve iron(hydro)oxides,mobilize adsorbed or coprecipitated arsenate in iron(hydro)oxides,and to enrich aqueous As in groundwater.Organic matter,as an electron donor and energy drive in the process of arsenic release from aquifers,has become a research hotspot and focus of the mechanisms of high arsenic groundwater.Since the source and composition of organic matter in aquifers are complex and diverse,and the molecular composition of organic matter poses certain challenges,the current research focuses on the characteristic components of organic matter in aquifers.However,the conversion pathway of organic matter in aquifers is not yet known.Therefore,it is necessary to find out how the use of organic matter by microorganisms in the process of iron-reducing arsenic release leads to changes in their molecular composition,to fill the gap.In addition,carry out a unified and effective parameterized description of the availability of organic matter.It is of great significance to quantitatively characterize the complex hydrological and biogeochemical processes in the high-arsenic aquifer.This study selects the typical high arsenic aquifer in the Jianghan Plain as the research area and takes the natural organic matter components in the aquifer as the research object.X-ray fluorescence(XRF),the solid phase sequential extraction and Fourier transform ion cyclotron resonance mass spectrometry were used to obtain sediment geochemistry,the arsenic and iron speciation and organic matter composition characteristics of field sediment.Through a large number of indoor anaerobic control experiments,organic matter with different molecular compositions was used as carbon source,and typical arsenic-loaded iron minerals were electron acceptors.Through comparative analysis of the physical and chemical indicators of solution redox,solid and liquid speciation of Fe/As and other elements,dissolved inorganic carbon-organic carbon and monomer organic matter content and their stable carbon isotope composition(¹³C)during the reaction process,variation of organic matter at the molecular level,we explored the transformation of organic matter components in the process of reducing and dissolving iron oxides reductive dissolution and arsenic mobilization under the participation of typical dissimilated iron-reducing bacteria(Geobacter metallireducens GS15,Shewanella putrefaciens CN32,and Shewanella oneidensis MR-1)in high-arsenic groundwater systems.This study systematically investigated how the degree of oxidation of organic matter,the type of iron minerals and environmental factors affect the mobilization of arsenic during microbially reducing iron oxides.The study mainly made the following progress:1.The forms of arsenic and iron in the sediments of the arsenic affected aquifers in the Jianghan Plain were determined.The molecular composition characteristics of the organic matter in the sediments were also clarified.Microbially reduced and dissolved arsenic-rich iron oxides in aquifers are important for the accumulation of aqueous As.The speciation of As and Fe in groundwater change synergistically,in the study area.The aquifer is partially oxidized in dry season,and the concentration of As and Fe²⁺in groundwater decreases,and the proportion of As(III)decreases;the aquifer is partially reduced in the rainy season,and the concentration of As and Fe²⁺in groundwater increases.The ratio of As(III)rises contemporaneously.In sediments,As is mainly present in the form of adsorption on the surface of amorphous and crystalline iron oxides,followed by arsenic co-precipitated in sulfides,reactive silicates and amorphous iron oxides.Water soluble organic matter(WSOM)in the sediments is dominated by terrestrial lignin and tannins.The WSOM molecular composition of the sediments at a depth of 20m is the most abundant,and the content of condensed aromatic hydrocarbons and carbohydrates is significantly higher than that of other layers.In the sediments,the degree of organic carbon oxidation is the highest,and the Nominal Oxidation State of Carbon(NOSC)value is-0.1.The comprehensive characteristics of the solid and liquid components of the aquifer indicate that the iron oxide reduction process is an important way for the mobilization of arsenic.2.Through anaerobic control experiments with organic matter of different molecular composition as carbon source,the effect of molecular composition of organic matter on the migration and transformation of iron mineral phase and arsenic in the ferrihydrite reduction process was systematically explored.The degree of oxidation of organic matter affects the respiration thermodynamics of iron-reducing bacteria.The degree of reduction of ferrihydrite in turn affects the release and fixation of arsenic.At the beginning of the experiment,ferrihydrite was reduced strongly,Fe²⁺concentration increased rapidly,solid-phase As was released,and the As concentration was as high as 60?g/L.Later,the iron reduction effect was weakened,and the final concentration of Fe²⁺reached up to 961?mol/L.The dissolved As was absorbed by the remaining ferrihydrite and siderite,magnetite,maghemite,hematite and other secondary iron minerals or complexed with organic matter on the surface of iron minerals,to scavenge As less than 10?g/L.Microorganisms preferentially use organic matter rich in¹²C.In the system,the?¹³C_DOC(Dissolved organic carbon)gradually becomes positive,and the?¹³C_DIC(Dissolved inorganic carbon)gradually becomes negative.The N20 sets have the largest DOC reaction degree,and DIC The carbon isotope fractionation values of DOC and DIC are in the range of 5.02‰to 6.71‰and-6.2‰to-10.65‰.In terms of the molecular composition of organic matter,microorganisms preferentially use organic matter with high O/C ratio and high degree of oxidation,such as tannins,condensed aromatic hydrocarbons,and highly oxidized lignin.Microorganisms produce low O/C ratio,high H/C ratio,Organic matter with a low degree of oxidation,such as lipids,low-oxidation lignin,protein,and amino acids.Under the condition of sufficient iron oxides,the oxidation state of organic matter and the colony structure control the end of the redox reaction.Controlled by the thermodynamics of microbial respiration,the ends of the redox reaction were same,and organic matter below-0.41?-0.55 was retained.Affected by the oxidation state of organic monomer carbon,the final concentration of Fe²⁺from low to high is propionate<ethanol<pyruvate<lactate<acetate,and the corresponding NOSC is-0.67,-2,0.67,0 and 0.Overall,the higher the oxidation state of organic carbon,the higher the availability of organic matter,and the stronger the effect of GS15 during reducing ferrihydrite.The carbon isotope fractionation value and organic matter NOSC are obviously positively correlated.The more negative the organic matter carbon oxidation state,the higher the thermodynamic limit during DIRB metabolism,and the greater the degree of fractionation between carbon isotops of DIC and organic carbon sources that occurs during metabolism.3.Through the control experiments,the effects of environmental factors such as the extent of crystallization of iron minerals,the buffer composition,and electron acceptors,were revealed,on iron mineral phase transformation and arsenic release in the iron reduction process with the participation of reducing bacteria.Affected by the Gibbs free energy released during the dissolution of iron oxides,the reduction intensity of DIRB on ferrihydrite is significantly greater than that of goethite.The concentration of Fe²⁺in the ferrihydrite sets is up to 900?1030?mol,and the highest in the goethite sets is only 150?mol.However,due to the lower As-fixing capacity of goethite,the dissolved As concentration of As-loaded ferrihydrite and As-loaded goethite(As:Fe=1:550)are 40?g/L and 600?1000?g/L,respectively.The mechanism of GS15 for electron transfering inhibited the release of As in goethite,and the final concentration was only 50?g/L.Affected by thermodynamics,under the same carbon source conditions,the DIC-single carbon isotope fractionation produced by the goethite reaction group is greater than that of ferrihydrite,and the difference can reach up to 3.33‰?4.02‰.When the buffering agent in the reaction system is HCO₃^-in the natural aquifer,the competitive adsorption of HCO₃^-and As promotes the arsenic migration.Especially for goethite,the dissolved As concentration increases by 600?700?g/L.AQDS not only promoted the reduction of iron oxides by DIRB,but also increased the mobilization of As,so that the highest As concentration in the goethite and ferrihydrite reaction groups reached 75?g/L and 1500?g/L,respectively.When the same organic matter is used as the carbon source,the addition of AQDS reduces the?_DIC-monomers of goethite and ferrihydrite reaction groups by 2.22‰?6.63‰and 1.01‰?3.5‰,respectively.The research features and innovations of this thesis are manifested in the following two aspects:(1)This study analyzes the transformation of the molecular composition of organic matter during reductive dissolvtion of iron oxides by Dissimilatory Iron Reducing Bacteria.It is found that the carbon oxidation state of organic matter rather than the molecular weight of organic matter is the key to controlling the availability of organic matter.It is a new mechanism that organic matter with high NOSC,such as condensed aromatics and lignin,is the key organic matter in the process of iron reduction and arsenic mobilization.(2)This study systematically explores the effects of factors such as the degree of oxidation of organic matter,the degree of crystallization of iron minerals,and electronic shuttles on the migration and transformation of arsenic from sediments.The study provides a new theoretical basis for further accurately describing the complex hydrobiogeochemical processes in arsenic affected aquifer.