Soil Environmental Behavior Of Perfluorooctanoic Acid And Its Catalytic Degradation Mechanism In Soil And Water Environment

Perfluorooctanoic acid(PFOA)is a persistent organic pollutant that has been banned for a long time.However,there are still large residues in soil and groundwater,causing environmental pollution and posing a serious threat to human health.Therefore,this paper investigates the soil environmental behavior of this compound and its catalytic degradation mechanism in soil and water environment,which is of great theoretical significance and practical value for the in-depth understanding of its fate in the environment,effective control of its pollution spread and remediation of contaminated sites.The main research results are as follows.(1)The test soil was divided into three different particle size fractions:sand,coarse silt and fine silt(380-180 μm,180-48 μm and 48 μm),and the adsorption and desorption study of PFOA was carried out.The results showed that about 50%of PFOA(2 mg/L)could be adsorbed after contact with the soil,and less than 10%of PFOA would be desorbed from the contaminated soil sample.The adsorption and desorption equilibrium was reached after 24 h.The diffusion of PFOA in different soil components was analyzed according to pseudo first kinetics and pseudo second kinetics,intra-particle diffusion equation as well as Boyd’s equation.It was found that PFOA diffuses rapidly in contact with the soil surface within 2 h,followed by intra-particle diffusion,and intra-particle diffusion is an important rate-limiting step.The study of the adsorption of PFOA and its short-chain PFCAs in soil showed that the former one would inhibate the adsorption rate and amount of the latter one.In addition,when the initial concentration of pollutants was greater than 10 mg/L,the high concentration of PFOA was not easily adsorbed by soil;the decrease of pH,the increase of temperature and the increase of coexisting ion concentration promoted the adsorption of PFOA in soil;the growth of soil strains in the process of soil adsorption was reduced after 24 h,but no sudden lethality of strains was observed.The mechanism of PFOA adsorption in soil indicated that hydrophobic interaction was the main mechanism force in PFOA adsorption in soil,followed by electrostatic interaction and surface complexation.Finally,by comparing the uptake characteristics of PFOA in different soil samples from nine Chinese provinces,the adsorption rates or amounts of PFOA differed significantly(about 10%)due to the completely different physicochemical properties of the nine soils.A linear regression equation was used to calculate the correlation of the influencing parameters that may affect the uptake of PFOA in soils:OM>CEC>pH>soil fraction.(2)Advanced oxidation(AOP)was subsequently proposed for the degradation of PFOA in solution,and the chemical degradation performance of activated carbon(FAC)and nitrogen-doped activated carbon(NAC)as heterogeneous catalysts for the removal of PFOA from solution by thermally activated peroxymonosulfate(PMS)was investigated and compared.The physicochemical properties of FAC and NAC were compared and studied in detail by a series of characterization.The amination modification resulted in NAC with more nitrogen-containing and oxygen-containing functional groups,although the specific surface area(SBET)was significantly lower than that of FAC and the adsorption capacity was reduced,the multilayer structure of NAC and the nitrogen-containing functional groups would accelerate the electron transfer process.During the degradation of PFOA,when the concentration of PMS was 5.0 mM and the amount of FAC/NAC was 4 g/L,the degradation rates of PFOA(1.0 mg/L)after 6 h were 93.5%and 88.9%,while the defluorination efficiencies were 72.3%and 55.9%.respectively.This result indicates that NAC has a stronger ability to catalyze the degradation of PFOA in aqueous solution than FAC,and the two catalysts showed different catalytic activities for the degradation of PFOA in the presence of coupling with PMS.Electron paramagnetic resonance(EPR)assays confirmed that both catalytic systems produced SO4·-and ·OH to attack PFOA during the oxidation process.The methods could not only be applied in a wide pH range to effectively remove PFOA,FAC/PMS and NAC/PMS could also be reused,which provide a feasible and cost-effective method for the degradation of PFOA in situ actual ground water remediation.(3)Both FAC/PMS and NAC/PMS systems were subsequently applied to the AOP method for the removal of PFOA from soil.The effect of OC,as an important component of soil,on the removal of PFOA in soil by the AOP method was investigated for the first time.Two homogeneous soils,but with retained and removed OC components were provided as soil samples,respectively.In the removal of soil PFOA by FAC/PMS and NAC/PMS,after optimizing the reaction temperature,catalyst and oxidant dosing,reaction time and soil-to-solution volume ratio,the results showed that 76%and 70%removal of PFOA(1.0 mg/kg)could be achieved at 60℃ after 12 h in FAC/PMS and NAC/PMS system.In addition,density functional theory(DFT)calculations combined with experimental data verified that the adsorption capacity of FAC and NAC for PFOA was three times stronger than that of soil particles.This stronger adsorption can "aggregate" the PFOA molecules in the soil first and shorten the distance of radical and electron transfer from the catalyst surface to the pollutant,thus speeding up the attack of SO4·-and·OH on PFOA.(4)In order to improve the surface properties and structural integrity of the catalyst,a carbonaceous catalyst containing Al and Ni was prepared by calcination in a tube furnace,with the contents of Al,C and Ni being 65%,15.4%and 13.08%,respectively.The obtained AlCNi has a good catalytic performance without causing metal leakage during PFOA degradation.98%of PFOA was rapidly removed with AlCNi within 3 h.The defluorination efficiency was as high as 70.43%,and no oxidant(PMS)or reducing agent(Na2SO3)was needed to be added.Combined with a series of characterization analysis of AlCNi before and after the reaction,it can be concluded that PFOA was degraded by dehalogenation reduction,among them,graphite C provided adsorption sites,Ni acted as a catalyst to accelerate electron transfer,while more valence electrons of Al were converted to zero electrons after the reaction and the binding energy was converted to higher energy,which could easily gain or lose electrons on the surface of the reduced material to attack the contaminant.For AlCNi,the surface-OH,Lewis acid sites and Al2O3 surface formed oxygen vacancies under the action of mechanical force,which not only released vacant electrons,but also 10%superoxide radicals were generated,therefore acted together on the reduction degradation of PFOA to achieve good removal effect.In summary,this study mainly investigated the sorption-desorption behaviour of PFOA and its short-chain PFCAs in soil,thus recognising the migration and transformation behaviour of PFOA after entering the soil medium.We then used the AOPs method to study the removal of PFOA from solution and soil samples,respectively.Based on the activity and structure of the modified catalysts,the catalysts were amine modified and bimetallic modified,respectively,and finally,a reductive dehalogenation method was selected to degrade PFOA,and the degradation process can be compared with that of the oxidation method for the degradation pathway.