The Mechanisms Of Soil Microbial And Enhanced Bioremediation For HCHs Contamination

Hexachlorocyclohexane?HCH?was used globally in various pesticide formulations to obtain high crop yields and to protect trees and seedlings;from 1947 to 1997,?33%of the total global production occurred in China.Although HCH use has been banned or restricted since the 1970s,many former pesticide manufacturing plant sites are still heavily contaminated due to the inappropriate disposal or storage of HCH.This contamination poses an enormous threat both to the environment and public health due to the persistence,toxicity,and bioaccumulation of HCH in the environment.Although?-HCH?or lindane?is the only isomer that shows specific insecticidal activity,both?-and?-HCH are carcinogenic to humans.Therefore,an effective,economical,and safe remediation technique is needed.This study firstly compared the degradation efficiency of HCHs by persulfate oxidation and zero-valent iron reduction under different conditions.Anaerobic microcosms were then established through n ZVI reduction coupled with anaerobic dechlorination.Incubations without n ZVI pretreatment were set up by adding?i?deoxygenated Milli-Q water only as control groups and?ii?acetate solution to stimulate microbial activity.There were three n ZVI-amended treatments with?iii?Milli-Q water;?iv?acetate;and?v?sodium azide to inhibit microbial activity.Logistic modeling was employed to examine the impacts of n ZVI and other additions on HCH isomers degradation.Quantitative real-time PCR?q PCR?and 16S r RNA amplicon sequencing was employed to study the response of the microbial communities to the n ZVI treatment and to the biostimulation with sodium acetate.The main findings are as follows:?1?The effect of different activators and persulfate additions and activation methods on the removal of HCHs was investigated.The experimental results showed that persulfate only had a removal effect on?-HCH,with removal rates of 73.2-76.7%at a SPS concentration of 300 m M.The?-HCH removal rate was less than 20%in different treatments,and the activator addition facilitated the removal of?-HCH but still less than 40%.Increased concentrations of HCHs were observed under different activation and in time-dynamic experiment.This may be due to the release of pollutants caused by the reaction between SPS and the soil organic matter,making the final HCHs extraction concentration higher than that of the control group.In the persulfate oxidation field test conducted in Shenyang,it was found that the effect of persulfate on the removal of HCHs was not stable,and that ferrous,alkaline and hydrogen peroxide activation treatments did not reach the restoration targets.In summary,persulfate oxidation is not suitable for the removal of the soil contaminants used in this study.?2?The effect of different types of zero-valent iron?nano-zero-valent iron,micron-zero-valent iron,and commercial zero-valent iron?and dosage on the removal effect of HCHs was compared.The results showed that the removal of total HCHs by the three zero-valent iron species reached an average rate of about 70%,mainly for?-and?-HCH.The experimental results showed that n ZVI was better than the other two zero-valent iron with 83.5%,61.5%and 28.6%removal of?-,?-and?-HCHs,respectively.The degradation reaction of HCH isomers was terminated after 6 h at an optimal concentration of 4.2 g kg^-1.The results of the field test in Wuhan also showed that zero-valent iron had a better removal effect on HCHs,with no other isomers except?-HCH being detected at different additions,while about 60%of?-HCH could be removed at 3.6%addition.?3?Based on the results of the first two experiments,n ZVI was selected for anaerobic incubation batch experiments to investigate the effects of n ZVI and bio-stimulation on the degradation of historical residual HCHs at high concentrations.The results showed that n ZVI combined with bio-stimulation?n ZVI-Acetate?removed at least 25%more HCHs than the other treatments,especially the removal rate of refractory?-HCH is as high as 78.2%,almost twice as much as n ZVI or the natural attenuation treatment that relies on indigenous microorganisms.The addition of n ZVI increased the p H of the system and decreased Eh,which facilitated the hydrolysis of HCHs and the growth of anaerobic bacteria.Logistic equation fitting showed that n ZVI and microbial reduction dechlorination dominated the degradation of?-HCH and?-HCH,respectively.In addition,M?ssbauer analysis showed that reducing iron ore was generated in the n ZVI-Acetate treatment.?4?The mechanism of n ZVI enhanced microorganisms to remove HCHs was explored by quantitative PCR?q PCR?and 16S r RNA gene amplicon sequencing.The results revealed that the addition of n ZVI and acetate changed the bacterial community composition,and Desulfotomaculum,Dehalobacter,Geobacter,and Desulfuromonas likely contributed to the depletion of HCH isomers.In combination with other physicochemical indicators,we hypothesize that n ZVI provided a suitable growth environment?<-250 m V?as well as an electron donor for the Dehalobacter,and converted the highly chlorinated HCHs to low-chlorinated ones,making it easier for dechlorinators to achieve complete dehalogenation.It also promoted the growth of IRB and SRB,which directly metabolize a variety of halogenated organic compounds,and also promoted the generation of reductive iron ore for the chemical dechlorination of HCHs.The above results show that persulfate,which has been shown to degrade HCHs in many investigations,is not suitable for the soil samples in our study,and zero-valent iron can achieve a better removal effect,but for long-term aging of heavily contaminated soils,the large amount of addition may limit the application of this technology.Based on this,the n ZVI-enhanced soil microbial remediation technique was proposed in this study by adding n ZVI and sodium acetate to stimulate the growth of indigenous dechlorinating bacteria,and it compensates for simple chemical reduction or natural attenuation,achieves efficient degradation of high concentrations of HCHs in historically contaminated soil.The results of this study research provide an efficient and economical green remediation technique and an effective theoretical basis for the remediation of contaminated sites.