Study On Degradation Of Hexachlorocyclohexanes In Groundwater Of Chemo-contaminated Site Using CSIA-ESIA

Hexachlorocyclohexanes(HCHs)were persistent organic pollutants,widely detected in the environment,and highly toxic,semi-volatile and persistent.China was once a major producer and user of HCHs in the world and also one of the most seriously HCHs polluted countries.Although the production and use of HCHs was banned in 1983,they can still be detected in atmosphere,surface water,groundwater and soil due to their environmental persistence;particularly relocation of chemical industry of HCHs revealed the main sites contaminated by HCHs.Therefore,it is of great theoretical and practical significance to study the biodegradation and chemical remediation mechanisms of HCHs in groundwater of contaminated sites.Taking HCHs in groundwater of a contaminated site as the research object,this study integrated the theory and methodology of contaminant hydrogeology,environmental chemistry,molecular biology,and CSIA-ESIA technologies.The main work line was set to analyze and characterize the HCHs site background and its biodegradability through contaminated site investigation—laboratory experiment—mechanism analysis.The effect and mechanism of sodium acetate anaerobic biostimulation and CaO2/Fe(Ⅱ)on degradation of HCHs in groundwater were undertaken.Main outcomes were achieved as follows:(1)The methods of determination of HCHs concentration and carbon isotope in groundwater samples were developed.The recovery rates of liquid-liquid extraction and isotopic fractionation after extraction were studied.The recovery rates of HCHs in groundwater were 83.9-109.3%,which can meet the requirements of testing of HCHs in groundwater samples.The detection limits of the GC-IRMS method and its corresponding standard sample concentration were determined by the dynamic average method.The δ13C standard deviation of HCHs standard solution above the detection limits were 0.25‰-0.46‰,which were within the instrument error range(±0.5‰),indicating that the analysis method was highly accurate.The δ13C differences between the extraction and the standard solution were 0‰～0.19‰,indicating that the extraction process didn’t result in carbon isotope fractionation of HCHs.(2)The carbon isotopic compositions of the HCHs standard solutions from three manufacturers had been tested using the developed carbon isotope analysis method.Theδ13C of HCHs ranged from-24.63‰ to-35.52‰.HCHs from different manufacturers showed different carbon isotope compositions,while different isomers from the same manufacturer showed different carbon isotope compositions.The main reasons were the differences in raw materials and synthesis processes.The different carbon isotope compositions of HCHs can be used to identify the source of pollutants in the contaminated site.(3)The concentrations of the four isomers in the groundwater were consistent,with the highest concentration of γ-HCH and the lowest concentration of β-HCH.The presence of a hyperanaerobic zone with DO less than 5 mg·L-1 in the site indicated that aerobic degradation had occurred,while the presence of a low-value zone for NO3-near the source indicated that denitrification may have occurred.The microbial community structure in the underground of the site indicated that there were anaerobic degrading bacteria of HCHs in the groundwater of the site,indicating the possibility of anaerobic biodegradation.The carbon isotope of α-HCH,γ-HCH,(+)α-HCH and(-)α-HCH were enriched significantly in groundwater of typical monitoring wells,and the difference ofδ13C is greater than 2‰,indicating that α-HCH and y-HCH have undergone in-situ microbial degradation.(4)The groundwater in the contaminated site was selected as the experimental water for the anaerobic microcosm experiment.Under the anaerobic stimulation of sodium acetate,the degradation rate of α-HCH was 98.10%on the 80th day of the experiment,indicating that the addition of sodium acetate could promote the degradation of α-HCH.The concentrations of NO3-and SO42-decreased with time,which proved that denitrification and sulfate reduction occurred.The addition of sodium acetate significantly increased the relative abundances of Dhb and Geo genes,the Firmicutes and the Clostridia,which were favorable for α-HCH degradation.Carbon isotope fraction was found in the biostimulation and causing the isotope enrichment εC and AKIE were-2.9±0.4‰ and 1.029±0.007 respectively.The mechanism was inferred to be a reduction reaction resulting in two-electron dechchlorination.(5)The α-HCH was degraded by CaO2/Fe(Ⅱ)Fenton-like system.When the initial concentration of α-HCH was 0.5 mg·L-1,and the pH was 2,and the dosage of CaO2/Fe(Ⅱ)were 1 mM,the degradation rate of α-HCH was the highest,reaching 97.57%at 180 min.The degradation of α-HCH was greatly affected by pH,causing degradation rate decreased gradually with pH from 2,5,7 to 9.The Cl-,HCO3-and HA had strong inhibitory effects on the degradation of α-HCH,while the NO3-and SO42-had little.The-OH played a major role in the degradation of α-HCH according to the results of free radical masking and EPR detection.Carbon isotope fraction was found in pH 2,5,7 and 9,and causing the isotope enrichment εc were-2.0±0.1 ‰,-2.4±0.1‰,-1.7±0.2‰ and7.9±1.1‰,respectively.The corresponding AKIE were 1.012±0.003,1.014±0.001,1.010±0.003 and 1.049±0.006,respectively for the pH 2,5,7 and 9.The possible degradation pathways of α-HCH were proposed,including hydrogen abstraction and dehydrochlorination.The CaO2/Fe(Ⅱ)was used to perform degradation the groundwater of contaminated site.The results showed the CaO2/Fe(Ⅱ)can be used to repair HCHs in groundwater of contaminated site.