Study On Remediation Of Petroleum Hydrocarbon Contaminated Groundwater By NZVI@BC Activated Persulfate

Total Petroleum Hydrocarbon（TPH）pollution of groundwater in organically polluted sites is a common problem,which causes serious harm to the surrounding environment and human health.Remidiation of TPH contaminated groundwater has attracted more and more attention and has become a hot research topic for scientific reserch and application in environmental science and engineering.This study selected a TPH contaminated sites in a estuarine intertidal zone,invistigated its hydrogeological characteristics and TPH pollution in soil and groundwater.The distribution characteristics and transport mechanisms of TPH in the contaminated sites were also analyzed.Aimed to break through the technical bottlenecks of ineffective decomposition,low activity and low utilization of persulfate（PS）in the process of in situ chemical oxidation of TPH.A typical agricultural waste in Guangxi,bagasse,was used to produce biochar（BC）,nano zero-valent iron（nZVI）was prepared by a liquid phase reduction method,and BC was used as carriers to systhesis the BC supported nZVI composite（nZVI@BC）.The activation process of PS by nZVI@BC and the degradation efficiency and mechanism were investigated using batch experiments.In order to clarify the degradation,mechanism,and derivative environmental impact of TPH in groundwater,a field experimental study on the remediation of TPH contaminated groundwater by nZVI@BC/PS system was carried out.The optimization of petroleum hydrocarbons degradation in groundwater by nZVI@BC/PS based on numerical simulation was carried out.According to the objectives of TPH restoration duration-engineering economy-restoration effect of groundwater,a strategy of extraction treatment-in situ chemical oxidation（ISCO）suitable for the restoration of TPH in groundwater in this contaminated site was proposed.This study is conducive to the popularization of ISCO technology from theoretical research to engineering application and provides reliable reference basis and theoretical support for the remediation of groundwater contamination.The main conclusions are as follows:（1）The TPH in groundwater of the investigated area exceeded the standard limit seriously and the maximum concentration exceeded 95.23 times of the standard.The center of pollution plume was consistent with the area of TPH pollution in soil.Due to the effect of tides,the groundwater level fluctuated frequently and there was no long-term stable groundwater flow direction,which affected the dynamic transport of TPH between groundwater and aquifer rock and soil,thus expanding the scope of TPH pollution plume.The non-carcinogenic risk hazard quotient of the carbon chain in each segment of TPH in groundwater exceeded the acceptable level.The higher the carbon chain concentration value,the higher the hazard quotient.The hazard quotients was in orders of C₁₆-C₂₁>C₁₂-C₁₆ and C₁₀-C₁₂>C₂₁-C₃₅,and the maximum hazard quotient was 76.4 times of the acceptable level.The concentration of petroleum hydrocarbon in groundwater was greater than its risk screening value,and the hazard quotient of petroleum hydrocarbon in each segment was much higher than the acceptable level of non-carcinogenic hazard quotient.It is therefore necessary to control the pollution of petroleum hydrocarbon in groundwater in the study area.（2）The degradation rate of TPH in nZVI@BC/PS system was significantly higher than that in PS,BC,nZVI@BC,BC/PS and nZVI/PS systems.The nZVI@BC dose,persulfate concentration,and initial p H of the reaction system were key factors affecting TPH degradation in nZVI@BC/PS system.Under nZVI@BC dose of 1.0 g/L,PS concentration of80 g/L,and p H of 5.81,the TPH removal rate was 91.56%.The nZVI@BC showed high recycling performance and induced free radical and non-free radical activation in nZVI@BC/PS,mainly SO₄-·,·OH,and ¹O₂.Among them,Fe⁰ was the source of SO₄-·and·OH generated by activated PS,and BC was the main source of ¹O₂ generated by activated PS.The interaction between nZVI@BC and PS resulted in the change of electron distribution in the nZVI@BC/PS system,which made the length of O-O bond become larger and easier to break and generated free radicals.The lower energy difference（ΔE=0.65e V）from the initial state to the transition state of nZVI@BC/PS enhanced the break of peroxide bond on PS faster,and the smaller initial configuration energy level of nZVI@BC/PS facilitated the electron transfer process of organic matter to the PS complex,which was conducive to free radical generation and organic matter degradation.The central part of linear alkanes in petroleum hydrocarbons was easily attacked by the active species SO₄^-·,·OH,and ¹O₂,and was broken to produce short-chain alkanes.The cyclic and heterocyclic hydrocarbons in petroleum hydrocarbons produced low carbon epoxides and organic esters as by-products.（3）The nZVI@BC/PS system followed the quasi-first-order reaction kinetics for the TPH degradation process in groundwater of the contaminated site,and the kinetic rate constant was 0.035-0.040 d^-1.After 42 days of injection,the TPH concentration in the test area was fully up to the standard,and the degradation rate was up to 91.3%.The nZVI@BC/PS had a remarkable effect on the remediation of groundwater in TPH contaminated sites.Influenced by the medium and groundwater dynamics in the site,free radicals generated by the nZVI@BC/PS system were consumed by non-target pollutants in the groundwater environment,resulting in slower degradation kinetics of TPH,and the degradation efficiency was much lower than that of in laboratories.Groundwater dynamics was the main factor affecting the transport and diffusion of remediation agents.The hydrodynamic conditions led to the inconsistency between the change of TPH concentration and the remediation effect in the monitoring Wells in the test area.Correlation analysis showed that the parameters of p H,EC,Fe³⁺,and SO₄^2-had no significant correlation with TPH concentration before the application of metarials.However,these parameters fluctuated greatly after drug injection and were significantly correlated with TPH concentration within28 days after application.The changes could reflect the degradation process of TPH.（4）Based on the TPH pollution of groundwater in the studied area and parameters from the field experiment using nZVI@BC/PS system to degrade TPH,numerical simulation on the optimization of TPH degradation in groundwater and a groundwater flow-solute transport model were used to evaluate different remediation strategies.The simulation results showed that,by setting up 6 pumping wells,when a single pumping treatment technology was applied,the repair process would completed after a total operation time of 1980 days,and55740 m³ of groundwater has been treated;when ISCO was applied with the injection of remediation agent of 100 kg/m²,the pollutant concentration dropped below the remediation target value after 140 days,effectively speeding up the repair process.In a comprehensive consideration of remediation period,engineering economy,and remediation effect,the extraction treatment-in situ chemical oxidation strategy was selected.Firstly,the TPH contaminated plume and high concentration areas were extracted for 300 days,and the TPH concentration decreased by 17.96%-55.18%.The ISCO of nZVI@BC/PS system was then used to continue the remediation process.Under this condition,the amount of PS was greatly reduced and the groundwater pollutants were treated within a reasonable time limit.The remediation has high technical feasibility and economic feasibility.The remediation strategy and technical method can provide scientific basis and reference for the remediation of groundwater in organically polluted sites such as petroleum hydrocarbons.