Research On Compound Chemical Oxidation Remediation Of Soils Contaminated With PAHs

Polycyclic aromatic hydrocarbons（PAHs）are organic compounds,comprising of two or more benzene rings in a linear,angular or cluster arrangement,which are difficult to degrade under natural conditions.They may enter the human body through inhalation,ingestion,and dermal contact,which may cause carcinogenesis,teratogenic,and mutagenic effects.Soil problems seriously affect human health and ecological security.The study on remediating PAHs-contaminated soils is of great significance for the development and utilization of resources in contaminated lands.Presently,chemical oxidation remediation is widely used because of its high remediation efficiency,broad applicability,and simple operation.Chemical agents often choose a single oxidant,but the degradation efficiency of PAHs is low,and the application conditions are limited.Therefore,develop more efficient oxidants or use oxidant compound technology to improve the degradation efficiency of PAHs in contaminated soil,are some of the leading research directions of utilizing chemical oxidation in remediating organic contaminated soil.This thesis has explored the basic mechanism of the degradation of PAHs in soils by dual oxidants,then researched on the compound reagent package of dual oxidants with sodium persulfate as the main component,calcium peroxide as the auxiliary component,ferrous ion as the activator and oxalate ion as the complexing agent.Furthermore,in order to exploit the efficiency of the compound reagents package,the response surface methodology was combined with the optimal experiment to optimize the reagents package ratio of three agents and their applicability,and explore the effects of the degradation properties of nitrobenzene and aniline in soils by reagents.There are several main research results,as follows:（1）The mechanism of chemical oxidation degradation of PAHs in treated soil by the sodium persulfate-calcium peroxide complex was revealed.Potassium oxalate was selected as the complexing agent,and the compound system had a better degradation effect.In a single antioxidant experiment,when sodium persulfate was added from Og/kg to 47.62g/kg,HMW-PAHs degradation rate increased from 49.29%to 98.21%,and∑PAHs degradation rate increased from 60.39%to 86.95%.However,when calcium peroxide was added from 0 g/kg to 40g/kg,HMW-PAHs and ∑PAHs degradation rate portrayed a gradually increasing trend,with an initial decrease at the point where the calcium peroxide content was 10g/kg,whereby the degradation rate was at its highest,the HMW-PAHs and ∑PAHs degradation rates were 47.74%and 55.94%respectively.Based on the results of a single oxidant experiment,the following amounts of sodium persulfate,calcium peroxide,ferrous ion（ferrous sulfate）and oxalate ion（potassium oxalate）were optimized to ensure higher PAH degradation rates:47.62-95.24g/kg of sodium persulfate;5-10g/kg of calcium peroxide;20-37g/kg potassium oxalate;50 or 100g/kg of ferrous sulfate;and a reaction time of 2d.These results indicate that the compound of sodium persulfate and calcium peroxide can effectively degrade PAHs in soil.（2）Combined with the application response surface methodology（RSM）,based on the preliminary experimental results,a mathematical model was used to optimize the addition amount of each component in the sodium persulfate-calcium peroxide compound system.The addition amounts of sodium persulfate,calcium peroxide,and ferrous ion/oxalate ion addition ratio were set to three factors,which led to the degradation rates of LMW-PAHs,MMW-PAHs,HMW-PAHs and ∑PAHs as their respective responses.According to the results of the design experiment,the quadratic equation,contour line,and response surface were obtained.When calcium peroxide was 8.46g/kg,sodium persulfate was 77.13g/kg,and ferrous iron/oxalate was 3,the degradation efficacy of HMW-PAHs is optimal.The experimental results show that the prediction results of the model are satisfactory in general.（3）Sodium persulfate-calcium peroxide compound reagent package was developed,and the effects of reagent feeding method,reaction time,water/soil ratio,temperature,and the additional amount of each component on the degradation efficiency of the reagent package were analyzed.The results show that compared with adding two oxidants simultaneously,calcium peroxide was added after a reaction time of 24-36h,and reacted for 5-10d to have an increase in degradation efficiencies of 43.31%and 29.31%in HMW-PAHs and ∑PAHs,respectively.Besides,the optimal ratio of soil/water is 0.5-1,in the reaction temperature range between 4□ to 50□:when the temperature was higher,the reagent packages worked better.However,when the reaction temperature reached 65,the degradation efficiency decreased significantly.Also,calcium peroxide and ferrous sulfate had significant effects on the pH value of the reaction system.Finally,three reagent packages were optimized based on work efficacy.Reagents package 1（calcium peroxide 8g/kg,sodium persulfate 47.62g/kg,ferrous sulfate 25g/kg,potassium oxalate 7g/kg）had a degradation efficiency of some HMW-PAHs≥85%.Reagents package 2（calcium peroxide 10g/kg,sodium persulfate 71.43g/kg,ferrous sulfate 50g/kg,potassium oxalate 20g/kg）had a degradation efficiency of HMW-PAHs and some LMW-PAHs and MMW-PAHs>85%.Finally,Reagents package 3（calcium peroxide 10g/kg,sodium persulfate 71.43g/kg,ferrous sulfate 100g/kg,potassium oxalate 37g/kg）had a degradation efficiency of all PAHs≥85%.（4）The remediation effect of the reagent pack on PAHs,nitrobenzene and aniline in different soils was revealed.The reagent package was applied to different types and properties of soil to study the degradation performance of different toxic organic pollutants and the effect on the changes in soil properties.When the soil particle size was smaller,a weaker degradation performance of the reagent pack was observed for MMW-PAHs and HMW-PAHs in the soil.Meanwhile,the reagents package was more effective in PAH-contaminated soil containing a pH value of 6-7.Besides,the degradation performance of ∑PAHs in different types of soil is shown as red soil>yellow brown soil>purple soil>black soil.Additionally,the sodium persulfate-calcium peroxide compound reagent package can significantly degrade nitrobenzene and aniline in the soil,with the highest degradation rates being 88.40%and 99.51%for nitrobenzene and aniline,respectively.Furthermore,calcium peroxide alleviates the decrease in soil pH after the application of the reagent pack.When the amount of sodium persulfate in the reagent package increased,the soil total organic carbon content showed a trend of an initial increase,before a decrease was detected.