Effect And Mechanism Of Thermally Activated Sodium Persulfate On Remediation Of Pahs Contaminated Soil

Polycyclic aromatic hydrocarbons（PAHs）contaminated soil area in China is huge,and the PAHs point exceeding rate is high.Meanwhile,due to their characteristics of refractory degradation,high carcinogenicity and high mutagenicity,they will eventually endanger the natural environment and human health.Therefore,the remediation of PAHs contaminated soil is imperative.Chemical oxidation has been widely used for remediating PAHs contaminated soil due to its advantages,such as short remediation cycle,high pollutant removal efficiency and low cost.Studies have shown that thermally activated persulfate is suitable for removing high concentrations of refractory organic pollutants in soil,without the problems of activator dispersion,recovery,scaling and deactivation.However,the research on the mechanism of thermal activated persulfate remediation of PAHs contaminated soil and its impact on soil physicochemical properties is relatively weak.Therefore,this paper carried out an experimental study on the remediation of high concentration of typical PAHs pollutants by thermally activated sodium persulfate,and the following main conclusions were drawn:The effects of different activation methods（FeSO₄,NaOH,H₂O₂ and Heat）on the removal of PAHs in soil were compared,and the effects of different activation methods of sodium persulfate（PS）on soil physicochemical properties（p H,organic matter,functional groups,soil morphology and metal mineral elements）were investigated.The influence of key factors（pollutant concentration,liquid to soil ratio,p H and temperature）on the removal effect of PAHs in the process of thermal activation of PS was studied.The results showed that at the optimum dosage of oxidant,the removal efficiency of PAHs was as follows:Heat>FeSO₄>H₂O₂>NaOH.Study of variations in soil properties found that p H of these four treatments decreased and the organic matter reduced most obviously after thermal activation.Due to the relatively mild activation methods,no obvious variation in the structure were observed in the SEM characterization,but infrared spectra analysis indicates that some hydrocarbons or oxygenated compounds may be produced in the soil.In the thermally activated PS system,compared with p H,liquid to soil ratio and temperature had significant impacts on the degradation efficiency of PAHs.With the increase of soil and water ratio,the removal of PAHs firstly increased and then decreased.With the increase of temperature,the removal rate of PAHs increased.When the temperature reached 90°C,the overall removal efficiency reached a maximum of99.1%.In order to further reveal the mechanism involved in the remediation of PAHs contaminated soil by thermally activated PS,the adsorption and desorption characteristics of PHE on soil samples after PS oxidation were studied,and the products after PS oxidation and the free radicals that play a dominant role in the PS oxidation process were analyzed.The results showed that after thermally activated PS oxidation,the adsorption capacity of soil to PHE decreased,indicating that the oxidation treatment destroyed the humin,fulvic acid and other components which play an important role in adsorption.However,the adsorption capacity of organic carbon per unit mass to PHE increased after the oxidation of thermally activated PS.There was hysteresis in desorption of soil samples,and the desorption hysteresis of the oxidized soil was weaker than that of the original soil.GC-MS scanning mode was used to identify PAHs products after oxidation of thermally activated PS.It was found that the main oxidation product was 3,4-dimethyl-benzaldehyde(C₉H₁₀O),which is much less toxic than PHE and Ba P.And thermally activated PS degradation can significantly reduce the concentrations of PHE and Ba P in the soil,so this method can reduce the environmental risk of the soil.The results of free radical quenching experiments and electron paramagnetic resonance tests showed that SO₄^·-was the dominant free radical in the PAHs degradation system under thermal activation of PS at 60℃and the main free radical existing in the thermally activated PS liquid system at 90°C was HO·.In order to optimize the heating method,evaluate the changes of soil physicochemical properties,study the removal mechanism of PAHs and predict the remediation process.The remediation effects of different heating methods to strengthen PS on PAHs contaminated soil were compared.A series of characterization analysis such as particle size,BET,XRD and SEM were carried out on the soil before and after remediation.The kinetic analysis of the remediation process of heat strengthened PS was carried out.The results showed that the removal effect of PAHs in the resistance heating enhanced PS was better than that in the heat conduction heating enhanced PS,which may be because the resistance heating device has the function of water replenishing and electric seepage.Some soil nutrients were lost after remediation,but other properties（particle size,morphology,minerals pore volume and pore size）had little change.Kinetic analysis shows that the exponential decay model has the best fitting accuracy to the kinetic curve in this paper.And the reaction rate（k_d value）of the resistance heating enhanced PS was higher than that of the heat conduction heating enhanced PS.