Oxidation Process And Mechanism Of Organic Matter From Hyporheic Zone Sediments

Hyporheic zone,which is an important part of the environment,supplies a transition area for surface water and groundwater interacting.The bidirectional migration and mixture of surface water and ground water resulted in the complex physical,chemical and biological environment in hyporheic zone,leading to the intricate biogeochemical processes.The sediment organic matter,which has important practical significance for the control of environmental pollution and ecological protection of groundwater and surface water,is involved in almost all reactions in hyporheic zone.In order to better understand and predict the ecological environment behavior of sediment organic matter,the sediment organic matter in hyporheic zone act as the research object,and its change rule mainly explored in the process of redox.(1)Component characteristics of dissolved organic matter(DOM)from hyporheic zone sediments under redox fluctuation conditionsWith the prevalence of fluctuating redox condition in hyporheic zone,molecular characteristics of DOM from sediment and the dynamic response of redox fluctuations has been studied.Fourier transform ion cyclotron resonance ultra-high resolution mass spectrometry(FTICR MS)and excitation-emission matrix(EEM)were used to study the spectral and molecular characteristics of DOM in sediment with the dynamic response law under redox fluctuation conditions.Researches show that the redox fluctuation conditions indistinctively affected the spectral characteristics of DOM,such as SUVA254,fluorescence index(FI),biological index(BIX),whether there was significant denitrification or not.The influence of redox fluctuation conditions on the molecular proportion of each DOM component was significantly different.The redox fluctuation conditions without denitrification resulted in the opposite trend of molecular fractionations of lignin-like,lipid-like and protein-like components in DOM before and after the fluctuations.Aerobic denitrification contributed to a prominent decrease in the molecular proportions of lipid-like,protein-like,polycyclic aromatic hydrocarbon-like and unsaturated hydrocarbon-like in DOM,with a significant increase in the molecular proportions of lignin-like.The molecular weight(MW)and double bond equivalents(DBE)of DOM were obviously decreased by aerobic denitrification,and anaerobic denitrification had little effect on the MW and DBE of DOM.Regardless of the existence of denitrification,the nominal oxidation state of carbon(NOSC)of DOM in the reaction system showed a trend of increase,and finally stabilized at about-0.1 which was no longer affected by the redox fluctuation conditions.(2)Component characteristics and desorption kinetics process of DOM from hyporheic zone sediments under aerobic conditionsIn order to further study the oxidation process in the redox of sediment organic matter,a static experiment was selected.Desorption of DOM in sediments is very important in the control of DOM concentration,bioavailability of organic matter and microbial activity.Therefore,DOM molecular changes in the process of desorption kinetics of sediment organic matter from hyporheic zone in oxidation environment were studied,and the effects of diffusion mass transfer and microbial degradation in the process of organic matter release from sediments were evaluated.Researches show that the mass transfer desorption of organic matter has a great influence on the DOC concentration,but has little influence on DOM molecular components.The mass transfer process of particles and aggregates was not the only reason for the increase of DOC concentration and change of DOM molecular composition.In the desorption of organic matter from sediments,the microbial degradation of DOM effectively changed the chemical properties and molecular composition of DOM.Preferred microbial degradation of protein-like and lipid-like components in DOM resulted in effective reduction of DOC concentration and changes of DOM chemical composition.which may lead to preferential transformation of biodegradable DOM components from sediment to dissolution stage.Each component of DOM was degraded by microorganisms,but the degradation rate of different DOM components is significantly different.The degradation rate of protein-like and lipid-like compounds was significantly faster than that of lignin-like.The NOSC of biodegradable organic matter was less than-1,which reflected that protein-like and lipid-like were dominant in DOM.Biodegradable organic components,such as protein-like and lipid-like,were characterized as low NOSC,low DBE,low MW,and high H/C.This also indicated that the degradation rate or corresponding parameters of different DOM components in micro-biological degradation were different.(3)Mineralization of DOM from hyporheic zone sediments under chemical oxidation conditionsExisting researches show that microbial oxidation is dominant under oxygen oxidation conditions of sediment organic matter in hyporheic zone.However,under anaerobic conditions without oxygen,Fe(II)minerals with oxygen or H₂O₂ transferred from the redox interface can form hydroxyl radical(·OH)which can also oxidize organic matter.Therefore,the chemical oxidation of sediments organic matter in hyporheic zone should not be ignored.In order to quantify the relationship between CO₂ from sediment DOM by·OH oxidizing in hyporheic zone,Fe(II)and H₂O₂ were used as the standard reactions to produce·OH,and humic acid(HA)was used as the representative of DOM.The quantitative relationship between·OH and CO₂ was evaluated by changing the concentration of reactants in the dark.DOM in solution was characterized by EEM.The quantitative relationship between·OH and CO₂ from actual sediment DOM has also been considered.Researches show that the production of CO₂ from DOM increased with the increasing of·OH,and 0.116?0.340 m M CO₂ generated for every 1 m M·OH.when all reactants were sufficient.With cumulative·OH(x)known,the CO₂ mineralization rate(y_mr)can be estimated by y_mr=0.1673x+0.0291.It is also possible to estimate CO₂production(in the range between y_p1 and y_p2)by using the y_p1=0.036x+0.0497 and y_p2=0.0674x–0.0021.The estimated CO₂ production was slightly lower than the actual production.Complexity of DOM decreases with the increase of cumulative·OH.When·OH was sufficient,fluorescent species components in DOM was consumed by·OH.Humic acid-like components preferentially consumed,followed by tyrosine-like aromatic protein and soluble microbial byproduct-like.However,·OH was gradually consumed in oxidation process of DOM when DOM was excessed and the production of CO₂ no longer increased.The results of chemical oxidation of actual sediments were consistent with chemical oxidation of humic acid.The innovation points of this paper are as follows:(1)FTICR MS was used to semiquantitative characterize the changes of DOM molecular composition during the oxidation process of organic matter from sediments in hyporheic zone,and revealed the microscopic mechanism of microbial oxidation process of organic matter from sediments in the perspective of DOM molecular structure.(2)The relationship between·OH and CO₂ was quantified during mineralization of organic matter in dark conditions with Fe(II).