Study On The Remediation Of Groundwater Polluted With Hexavalent Chromium And Nitrate Using Indigenous Microorganisms Stimulated By Emulsified Soybean Oil

Groundwater contaminated with nitrate and hexavalent chromium has become increasingly serious in plain areas,its remediation is now a research hotspots.In-situ bioremediation has drawn much attention in groundwater remediation because of its advantages such as,eco-friendly,low operation cost and avoiding of secondary pollution,while the lack of carbon source in groundwater inhibits the activity of microbial reaction,therefore,it is very important to choose an appropriate carbon source for bioremediation.Emulsified soybean oil(ESO)is an excellent slow-release carbon source,and it has the potential to remediation multiple pollutants simultaneously.Injecting emulsified soybean oil to underground can not only stimulate microorganisms to degrade pollutants directly,but also stimulate a series of biogeochemical processes which has important impacts on the migration and transformation of heavy metals in groundwater.In this study,ESO was used as carbon source and commercial river sand was utilized to simulate aquifer medium,the efficiency and remediation mechanisms of using indigenous microorganisms stimulated by ESO to remediate groundwater polluted with hexavalent chromium and nitrate was studied.The main contents of the paper are listed as follows:(1)The remediation efficiency of in-situ bioremediation of Cr(?)and nitrate single or combined pollution stimulated by ESO were explored.The results showed that the addition of ESO can effectively promote the dissimilatory iron reduction process,when the dosage of ESO was less than 40 g/L,the Fe(II)accumulation concentration increased linearly with the increasing of ESO dosage.When the initial concentrations of Cr(?)and nitrate were 5 mg/L and 20 mg/L,respectively,ESO could effectively stimulate indigenous microorganisms to remediate Cr(?)and nitrate,multiple pollutants didn't have negative effect on the microbial remediation system,the pollutants could still be removed rapidly.Particle size and p H of the aquifer medium were the main influencing factors on the remediation efficiency of Cr(?)and nitrate,the remediation efficiency increased with the decrease of particle size;p H may affect the activity of the microbial community,thus lead to the change of remediation efficiency;the removal of Cr(?)was promoted obviously by high concentration of sulfate(> 100 mg/L);both Cu²⁺ in low concentration and weak alkaline environment would promote the degradation of nitrate to a certain extent,with Cu²⁺ more effective,while the content of Mn²⁺ and natural organic matter concentration had no obvious effect on the remediation efficiency.(2)The remediation threshold of bioremediation of Cr(?)and nitrate contamination stimulated by ESO and the interplay between them was studied.When ESO dosage was 7 g/L,the remediation thresholds for nitrate or Cr(?)alone were 1600 mg/L and 10 mg/L,respectively.For combined pollution,Cr(?)would inhibit the degradation of nitrate,while nitrate would promote the removal of Cr(?),and ESO dosage has an obvious effect on the remediation threshold of nitrate and Cr(?).When ESO dosage was sufficient,the remediation threshold of nitrate and Cr(?)was 1379.79 mg/L and 12.67 mg/L,respectively;when the dosage of ESO was limited,the remediation threshold of nitrate and Cr(?)was 766.30 mg/L and 5.38 mg/L,respectively.(3)The mechanism of Cr(?)and nitrate remediation stimulated by ESO was studied by analyze the variation of pollutants component and microbial community in the aquifer medium.The results showed that ESO could effectively enhance indigenous microorganism to remediate groundwater polluted with Cr(?)and nitrate.Syntrophomonas and Ornatilinea genus could convert ESO into small molecular acids,which was then used for microbial growth and metabolism.In the Cr(?)individual pollution system,Cr(?)was reduced indirectly by Fe(II),which was generated by Anaeromyxobacter genus;while in the combined pollution system,Cr(?)was reduced by Fe(II),which was generated by Anaeromyxobacter and Geobacter genus,Cr(?)was converted to Cr(III),then Cr(III)was entrapped within secondary minerals by biogenic mineralization based on Fe(III)/Fe(II)cycle.In nitrate individual pollution system,Geobacter,Pseudomonas,Magnetospirillum and Herbaspirillum genus were all responsible for nitrate reduction,while in the combined pollution system,nitrate was reduced by Geobacter and Herbaspirillum genus.In conclusion,it is technically feasible for ESO to stimulate indigenous microorganisms to remediate Cr(?)and nitrate polluted groundwater simultaneously,and this technology have the advantage s of stable,reproducible,rapid and highly efficient,and secondary pollution was avoided during the remediation process,it is a promising technology to remediate Cr(?)and nitrate polluted groundwater.