In-situ Remediation Technology Based On Hematite-Fe(Ⅲ) Bioreduction For Ammonia And Organic Contaminated Groundwater In Landfills

The situation of groundwater pollution in China’s municipal solid waste landfills is not promising,so it’s urgent to develop the new in-situ groundwater pollution remediation technologies with low energy consumption and economic feasibility.The reduction process of Fe（III）mediated by microorganisms has shown broad application potential for the remediation of ammonia and organic pollutants in groundwater around landfill sites.Hematite（α-Fe₂O₃）,as one of the most widely distributed Fe（III）oxides,has low cost and high environmental friendliness.However,micro-nano sized hematite is still used in most laboratory experiments,and the material synthesis process is complex and expensive.The removal efficiency and microscopic mechanism of natural hematite Fe（III）reduction process on ammonia and organic matter are still lack of systematic research.Aiming at solving the practical environmental problems of ammonia and organic pollution of groundwater in landfills,it’s of great significance to research and develop new materials and technologies based on natural hematite reduction and to carry out engineering applications.This study explored the removal efficiency of natural hematite Fe（III）bio-reduction on ammonia and organic matter through a small test,and revealed the microscopic reaction mechanism of the mineral-microbial-pollutant interface.And then it conducted a column-scale evaluation experiment.Selecting informal landfills as research sites,based on systematically identifying the characteristics of organic ammonia pollution in groundwater,a new technology of intermittent aeration-funnel gate permeable reaction wall repair was developed.The site repair experiment was carried out to realize hematite engineering application,and the engineering problems such as difficult change and easy failure of PRB medium replacement were solved.The main conclusions are as follows:（1）When the concentration of organic matter in groundwater（in DOC）is489.60 mg/L,the removal rate of organic matter in the biological reduction process of natural hematite Fe（III）can reach 89.40%.When the concentration of ammonia is35 mg N/L,only 57.14%of ammonia is anaerobically oxidized,and the main product is nitrite nitrogen.Limited by the high thermodynamic stability of natural hematite,the biological reduction cycle is as long as 50 d,and the starch modified by Na OH is loaded onto the hematite surface at p H of 11,which can significantly improve the bioavailability of hematite Fe（III）.The hematite modified by starch was used as the reaction medium in the pilot test,the reaction medium has good permeability.The dissolved oxygen environment in the intermittent aeration control system can effectively solve the problems of incomplete ammonia,nitrite,and nitrite accumulation.（2）Natural humus in groundwater can promote the reductive dissolution of hematite Fe（III）.In the later stage of the reaction,the crystallinity of hematite Fe（III）decreases and the overall trend is amorphous.Fe（II）will deposit and complicate on the surface of hematite particles to form magnetite（the main component is Fe₃O₄）that cannot be further bio-reduced.Sphingomonadaceae,Azospirillum and Geobacter hydrogenophilus were the main driving microorganisms of hematite reduction,while Pseudomonas and Aquabacterium could simultaneously oxidize Fe（II）and reduce nitrate.Na OH modified starch contains hydrophilic groups such as hydroxyl group and carboxyl group,which are adsorbed on the surface of hematite by hydrogen bond,and starch is associated with water molecules by hydrogen bond,thus greatly improving the hydrophilicity of natural hematite and significantly increasing the bioavailability of Fe（III）.（3）In the actual landfill pollution sites where the embedded depth of groundwater is shallow,the flow rate is slow and the permeability of aquifer medium is poor,most of the pollutants in the landfill leachate,such as organic matter,ammonia,nitrate,etc.will be absorbed and intercepted by aquifer medium,which make the groundwater pollution be concentrated in the unconfined aquifer and mainly is ammonia pollution.With the gradual diffusion of pollution plume,the downstream groundwater soluble organic matter（DOM）tends to be small molecules with high exogenous heterogeneity,and the spatial migration of small subgroups of proteins is the strongest.If the pollution source is not cut off in time and the pollution path is blocked,it’s easy to lead to the continuous deterioration of groundwater quality.As one of the rapidly available hydro chemical indexes of groundwater,the conductivity is well correlated with kinetic indexes such as DOM regeneration,aromaticity and degree of humus.（4）The best hydraulic retention time of intermittent aeration-funnel wall permeability reaction of door type is 144 h,and its groundwater ammonia concentration is 4.1～6.0 mg N/L.After one-year stable operation,the spatial distribution simulation results of pollution plume show that the ammonia concentration in the repair area is lower than 0.36 mg N/L,nitrate nitrogen and nitrite nitrogen do not accumulate,and secondary organic pollution risk is low.The problem that materials are not easy to be replaced can be solved by lifting splicing and combining lifting of reaction unit.The funnel gate type two wing cutoff wall can realize the accurate capture of ammonia heavy pollution plume,and the starch modified hematite can be used as a new material for groundwater pollution remediation in the landfill with green economy,safety and efficiency.This study can provide theoretical support and engineering experience for in-situ remediation of groundwater pollution in landfills in China.In view of the concealment of groundwater pollution in landfills and the complexity of remediation and treatment work,it’s suggested that future generations can combined with the numerical simulation of groundwater to carry out continuous research which are not limited to material life evaluation and reaction dynamics calculation.