Mechanisms Of Acetochlor Degradation In Hyporheic Zone Sediment And Its Identification Using Carbon Isotope Analysis

The hyporheic zone（HZ）is an important site for contaminant storage and transformation between surface water and groundwater,which has an important impact on the fate of organochlorine herbicides in aquatic environment.The HZ has changeable redox conditions.Reduction of terminal electron acceptors（TEAs）,such as O₂、NO₃^-、Fe（Ⅲ）,and SO₄^2-,occurs in the HZ.There may be differences in the ability and mechanism for the degradation of organochlorine herbicides during different terminal electron accepting processes（TEAPs）,which has not been studied systematically.TEAs are important factor affecting the composition and metabolic functions of microbial communities in the subsurface,and have important effects on the ability and mechanism of biodegradation.However,functional microorganisms for organochlorine herbicides degradation under different TEAPs in the HZ have not been identified.Meanwhile,dissolved organic matter（DOM）in aquatic environment is composed of complex components,which can affect the biogeochemical process in the HZ as carbon source,energy source,electron shuttle,ligand and so on.However,the roles of different DOM fractions in TEAPs and degradation of organochlorine herbicides have been little studied.Compound-specific isotope analysis（CSIA）is a powerful tool to identify the degradation mechanism of organochlorine pesticides.Application of CSIA to evaluate organochlorine herbicides fate in natural environments require isotope fractionation characteristics during different potential degradation pathways.However,characteristics of the isotope fractionation during organochlorine herbicides degradation under different TEAPs have not been investigated yet.Whether CSIA can be used to analyze the degradation mechanisms of organochlorine herbicides under different TEAPs still remains unclear.This study selected acetochlor,a major organochlorine herbicide used worldwide,as model herbicide.Ten HZ sediments with different properties from various regions were used.Research on mechanisms of acetochlor degradation in HZ sediment and its identification using carbon isotope analysis was conducted.The aims of this study are to reveal the controlling factors and mechanisms of acetochlor degradation in HZ sediment,and to confirm the potential of CSIA for analysis of degradation mechanism.The main contents and conclusions are as follow:（1）Degradation capacity of acetochlor（10 mg/L）under different TEAPs in HZ sediment reaction systems（sediment:water=1:5）was investigated through laboratory experiments.Under anaerobic conditions,18 to 42.4%of acetochlor（10 mg/L）was degraded in 40 d under Fe（Ⅲ）and SO₄^2-reducing conditions.However,acetochlor was refractory during NO₃^-reducing process,and the addition of NO₃^-to the reaction system would inhibit the anaerobic degradation of acetochlor.Under aerobic conditions,O₂reacted with Fe（II）in sediment to produce a large amount of·OH.·OH accumulation exhibited a significant positive correlation with acetochlor degradation（R=0.70,P<0.05）.all nine HZ sediments degraded acetochlor effectively with a range of degradation rate from 68.4 to 100%in 40 d except for the Chang Lake sediment.（2）For identifying the role of DOM in acetochlor degradation in HZ sediment,excitation emission matrix-parallel factor analysis was performed to analyze changes of DOM fluorescent components during degradation process.Results showed that acetochlor degradation under aerobic and anaerobic conditions exhibited a significantly positive correlation with DOM content（both R values were 0.52,P<0.05）.Under anaerobic conditions,the protein-like components served as carbon and energy source to microorganisms,supporting the TEAPs and acetochlor degradation.The humic-like components acted as an electron shuttle to mediate Fe（Ⅲ）reducing process.Lack of DOM would greatly limit the TEAPs and acetochlor degradation.Under aerobic conditions,the humic-like components could promote·OH production.Besides,biogeochemical processes under anaerobic conditions preferentially consumed the protein-like components,while microbial activities under aerobic conditions accumulated large amounts of the protein-like components in HZ reaction systems.（3）16S rRNA gene sequencing was conducted to investigate functional microorganisms for acetochlor degradation in HZ sediment.Results showed that strictly anaerobic Fe（Ⅲ）and SO₄^2-reducing bacteria（Geobacter and Bacteroidetes＿vadin HA17,respectively）were key microorganisms to determine acetochlor anaerobic degradation.Sphingomonas（aerobic）and Sphingobium（facultative）were potential acetochlor-degrading bacteria under aerobic conditions.Pedobacter was potential aerobic Fe（Ⅲ）reducing bacteria,which could reduce Fe（Ⅲ）to Fe（II）under aerobic conditions.Fe（II）further reacted with O₂ to produce·OH to degrade acetochlor.（4）The dominant pathway of acetochlor degradation under different TEAPs was identified using CSIA.Under anaerobic conditions,value of the carbon isotope enrichment factor(ε_bulk,C)for acetochlor degradation during Fe（Ⅲ）reducing process was-3.7±0.4‰,indicating that biodegradation dominated the acetochlor transformation.Reduced sulfides(e.g.HS^-and S_n^2-)dominated acetochlor degradation during SO₄^2-reducing process with aε_bulk,C value of-16.4±0.4‰.Under aerobic conditions,ε_bulk,Cvalues during acetochlor degradation in HZ sediments ranged from-1.5‰to-0.5±0.3‰,demonstrating that·OH dominated acetochlor degradation（-0.5±0.1‰）and biodegradation（-5.8±0.9‰）played a secondary role.On the basis of the aforementioned carbon isotope fractionation,CSIA successfully identified the different dominant pathways during acetochlor aerobic degradation processes at the early stage（biodegradation and abiotic reduction by reduced sulfur compounds,respectively）and the late stage（·OH oxidation）in the Changjiang River and Zuili Lake sediments,and revealed that 0.5 g/L sodium propionate transformed the dominant pathway of acetochlor degradation from·OH oxidation to biodegradation.Thus,CSIA can be used to analyze pollutant transformation processes when there are multiple degradation pathways.（5）Mechanisms of different degradation pathways of acetochlor in HZ sediments were revealed using CSIA.Under anaerobic conditions,values of apparent kinetic isotope effect for carbon（AKIE_C）during acetochlor degradation under Fe（Ⅲ）and SO₄^2-reducing conditions were 1.055 and 1.172,indicating that the hydrolysis and thiolytic S_N2-type nucleophilic substitution reaction involving C-Cl bond cleavage were the first step in acetochlor degradation by anaerobic bacteria and reduced sulfides,respectively.Under aerobic conditions,the AKIE_C value for acetochlor degradation by·OH was 1.007,suggesting that degradation was carried out through S_N1 type nucleophilic substitution reaction.The AKIE_C value for acetochlor aerobic biodegradation was 1.088,indicating that aerobic biodegradation was performed by S_N2-type nucleophilic substitution reaction.AKIE_C values significantly varied during different degradation pathways,confirming the validity of the AKIE_C for identifying degradation mechanism.This study revealed that TEAs,DOM,functional microorganism,and·OH interacted with each other and jointly determined capacity and mechanism of acetochlor degradation in HZ sediment.Meanwhile,CSIA successfully identified dominant pathway and mechanism of acetochlor degradation under different TEAPs in HZ sediment.This study could provide a scientific basis for the control of organochlorine herbicides in aquatic environment,and improve the comprehending of water purification mechanism in the HZ.