| Hexachlorocyclohexane(HCH)is a broad-spectrum organochlorine insecticide,also known as lindane.Due to its stable chemical properties and long half-life under natural conditions,lindane has relatively high residues in soil,causing continuous pollution and harm to the environment,even endangering human health through the food chain.In the long term flooding conditions,anaerobic dechlorination is the main degradation process of lindane in paddy soil.Meanwhile,the redox transformation of other biogenic elements(carbon,nitrogen,sulfur,iron)in paddy soil is also correlated with the reductive dechlorination of lindane,which made an effect on the degradation of lindane.In this paper,y-sterilized soil and the relative orginal soil were pre-cultured under flooding condition for 30 days to restore the microbial community in the sterilized soil and activate the microbial community in the original soil,so as to simulate contrasting flooded paddy soil conditions that were with different initial microflora.Then anaerobic sludge cultivation batch experiment was set to investigate the relationship between the reductive dechlorination of lindane and other typical redox processes(denitrification,iron reduction,methanogenesis)in the soil under anaerobic conditions,by adding different electron donors(acetate and methane),electron acceptors(nitrate),and organic materials(rice straw,rape straw,rabbit dung biogas slurry).The main findings are as follows:(1)In the y-sterilized soil,the iron reduction and lindane degradation were all improved relative to those in the orginal soil,with Fe(?)production enhanced 1.5-2 times,and the degradation rate of lindane increased 18.8-27.6%.Data analysis based on qPCR found that the copy numbers of Dhc gene and Geo gene in the sterilized soil were 1-2 orders of magnitude lower than that in the original soil.This suggested that after deactivation of the y sterilization treatment,the dechlorinated bacteria and iron reduction bacteria indicated by Dhc and Geo in soil were difficult to recover their original vitality in the subsequent culture process.It can be inferred that the enhanced iron reduction and lindane degradation in the sterilized soil may be mainly leaded by the abiotic reduction.The reason is that after ?-sterilization,some macromolecular organic matters,minerals as well as some dead bacteria in the soil may decompose,lead the concentrations of DOC and Fe(?)respectively increased from 1131.2 mg/kg and 285.2 mg/kg to 1743.1 mg/kg and 355.3 mg/kg.This would make the surface-bound Fe(?)increased,then the electron density surrounding Fe(?)increased,so that promoted the electron transfer of dechlorination process,thereby promoted lindane's abiotic degradation,(2)In the original soil,as compared with the control,the reduction rate of Fe(?)and degradation rate of lindane increased from about 7.5 to 25 ?M/d and from 70%to 95%,respectively,with addition of exogenous electron donor in the form of sodium acetate.Meanwhile,the results of high-throughput sequencing showed that the relative abundance of Geobacter and Anaeromyxobacter with iron reduction function increased by 0.7%and 2.2%respectively in the sodium acetate treatment group,while the functional bacteria of dechlorination did not increase.Therefore,it is speculated that the reduction process in the original soil may be dominated by biotic degradation,and the non-obligate dechlorination mediated by iron reducing bacteria coupling with iron reduction may have an important contribution in the degradation of lindane.Meanwhile,with supplement of nitrate,the degradation of lindane was decreased from 75%to 67%after 28-days'incubation,and iron reduction was also inhibited shown as a decrease in Fe(?)production by 50 ?mol/g relative to control.At the same time,narG and nirS genes showed consistent increase in the nitrate treatment,while the copy numbers of Geo gene and Dhc gene decreased.This suggested that reduction process of other electron acceptor in the soil(such as denitrification)intended to compete for electron with reductive dechlorination of lindane,thus inhibited its degradation.Additionally,in the treatment group with sodium acetate,the degradation of lindane was not significantly affected by the supplement of nitrate,with more than 90%removal of lindane achieved both in treatments with and without nitrate after 21-days'incubation.By comparing the difference between treatment groups with and without external electron donor,it can infer that the electron competition effect between the reduction processes of lindane and other electron acceptors(as NO3-)in the soil can be eliminated with supplement of external electron donor.(3)Sufficient and equivalent methane was used as electron donor to conduct further research,instead of sodium acetate.It was found that,in the treatment with simulataneous addition of methane and nitrates,the degradation rate of lindane was 88.6%;and in the treatment with addition of methane alone,the degradation rate of lindane was 95.9%.This indicated that the presence of nitrate also inhibited the degradation of lindane under methane supply conditions.And these inhibitory effects also reflected consistently in the process of iron reduction and sulfate reduction.In addition,if further comparison with the treatment without external electron donor supply,methane as electron donor could promote the degradation of lindane(degradation rate increased from 68.8-77.4%to 88.6%-95.9%),and could weaken the inhibition effect of denitrification on lindane degradation(inhibition rate decreased from 11.6%to 7.4%).But if compared with sodium acetate(inhibition rate 0.03%),the inhibition of denitrification on lindane degradation was more apparent when methane was supplyed to provide external eletron donors in the system.In this process,the gene copy number of the key functional gene of nitrate-dependent anaerobic methane oxidation mcrA was about 1.5 times in the nitrate treatment as compared with that in the non-nitrate treatment,while the intermediate product N2O,which is inevitable to be produced during traditional denitrification,was not significantly detected during the whole culture period.This suggest that nitrate-dependent anaerobic methane oxidation may exist in our incubation system.Since this methane-dependent denitrification process may reduce the consumption of universal electron donors by all reduction processes co-occurred in the incubation system,it may thus indirectly provide a new way of selective consumption of electron donors,thereby reduced the competitive effect of soil background reduction processes on reductive degradation of lindane.Therefore,compared with other electron-consuming denitrification processes,the methane-dependent denitrification(namely,nitrate-dependent anaerobic methane oxidation)may be more beneficial to reductive degradation of lindane in flooded paddy soil.However,by comparing the inhibitory effect of methane group and sodium acetate group on lindane degradation,we could infer that if the microbial availability and the non-specific utilization property of external electron donor provided are higher,methane-dependent denitrification may be not easy to occur;and under such circumstances,the effect of methane-dependent denitrification on the degradation of lindane,by providing a supplementary way for selective consumption of electron donors,would be very weak,as compared to the effect of other denitrification processes.(4)Several organic materials containing nitrogen with different C/N ratio were added to conduct anaerobic culture experiment,including rice straw(C/N=62.41),rape straw(C/N=36.75)and rabbit dung biogas slurry(C/N=0.29).After 28 days' incubation,the degradation rate of lindane in different treatment was control(55.0%)>rice straw(52.2%)>rape straw(51.8%)>rabbit dung biogas slurry(43.3%).It is suggested that addition of exogenous organic material inhibited the degradation of lindane in flooded soils,and the higher the rotten degree or easy to rotten degree of the organic materials,the greater inhibitory effect.The main reason may be that a higher rotten degree of the organic material would result in a greater release in both dissolved organic carbon(DOC)and dissolved organic nitrogen(DON)during the incubation,therefore,the competitiveness of soil background redox process(denitrification and methanogenesis)would be greater enhanced.In summary,to explore the effect of different redox processes on the degradation of lindane in paddy soil,we conducted experiment both theoretically and applicably.Our results can provide more reasonable guidance for the field management practice,and provide more theoretical support for improved remediation of soil contaminated by chlorinated organic compounds in the field. |
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