Remediation Of Chlordane And Mirex Contaminated Soil By Soil Washing Coupled With Photocatalytic Degradation Technology

Organochlorine pesticides?OCPs?are considered as environmental contaminants because of their high toxicity,environmental persistence and bioaccumulation,which severely threatened the ecosystem and human health.With the development of industrialization and urbanization,as well as the implementation of "Stockholm International Convention",a large number of OCPs contaminated sites have emerged during the shut down and demolition of OCPs manufacturing factories.In fact,along with the rapid progress of urbanization,most of the OCPs contaminated sites suffer a secondary development,which may extend and aggravate the pollution.Therefore,it is extremely urgent to find an effective remediation method to remove OCPs from contaminated soils.Soil washing has been extensively investigated to remove organic pollutants from contaminated soils because of its fast remediation,efficiency and low cost.A number of researches have been focus on the soil washing of OCPs-contaminated soils,whereas,few studies were related to the treatment of soil washing effluent.In the last two decades,photocatalysis technology has been developed into a more effective method in degrading organic contaminants and has the potential of treating in soil washing effluent containing OCPs.However,previous studies rarely explored the feasibility of combining soil washing and photocatalysis technology together to remediate OCPs-contaminated soils.In this study,the use of soil washing followed by photocatalysis was investigated for the remediation of soil contaminated with chlordane and mirex.It mainly includes following five parts.?1?A series of washing trials were conducted to select an optimum solvent from sixteen solubilizing agents,including hexane,dichloromethane,ethyl acetate,acetone,methanol,ethanol,propanol,isopropanol,amyl alcohol,SDS,SDBS,Triton X-100,Tween 80,rhamnolipid,hydroxypropyl-?-cyclodextrin?HPCD?and SDS/Triton X-100.?2?The photodegradation of chlordane and mirex in the washing effluent was investigated under the irradiation of a Xenon or Hg lamp,and the mechanisms responsible for the photodegradation of mirex in Triton X-100 solution were explored.?3?The removal of mirex in Triton X-100 solution was evaluated by using H₂O₂/UV,Fenton/UV and heterogeneous photo-Fenton process through analyzing the degradation products of mirex.?4?The adsorption of mirex on iron oxides including schwertmannite and goethite were investigated.?5?The photocatalytic degradation of mirex pre-adsorbed on schwertmannite and goethite were studied.The main results and conclusions can be summarized as follows:?1?Triton X-100,Tween 80 and HPCD are the optimum washing solvents.The washing efficiency of chlordane and mirex from soil increased with the concentration increase of these three solvents.The increase of washing efficiency was negligible when the concentration of solvent was higher than 10 mmol/L.An increase in the washing efficiency of chlordane and mirex was noted with the extension of ultrasonic time when it is within 60 min,and after that,the washing efficiency was kept as a constant.As the ratio of solution to soil?v:v?increased from 4:1 to 10:1,the desorption efficiency of chlordane and mirex increased.Overall,the optimum parameters for washing chlordane and mirex from contaminated soil using Triton X-100 or HPCD were 10 mmol·L-1 of concentration,60 min of washing time,and 10:1 of the volume ratio of washing solvent to soil,and for three successive washings.The optimum parameters for washing chlordane and mirex from contaminated soil using Tween 80 were 10 mmol·L-1 of concentration,20 min of washing time,and 10:1 of the volume ratio of washing solvent to soil,and for three successive washings.?2?Photodegradation could effectively degrade chlordane and mirex in the washing solvent.The degradation efficiencies of chlordane and mirex increased with the illumination time.Under irradiation of Xe lamp,the degradation efficiency of 76.7%for chlordane and 100%for mirex were found.However,under the irradiation of Hg lamp,nearly 100%of mirex was degraded in only 1 h and chlordane degradation was also completed within only 3 h.The degradation of chlordane and mirex was more rapid in the presence of Hg lamp than that of Xe lamp.The reductive dechlorination occurred during the photodegradation of mirex,and the major product was photomirex.?3?During the photocatalysis of mirex by H₂O₂/UV,the major product was photomirex.When using the Fenton/UV method,only 7.6%degradation efficiency of mirex was obtained.Therefore,H₂O₂/UV and Fenton/UV were not effective in degrading mirex in the simulated soil washing eluent.?4?Biosynthesis-schwertmannite/H₂O₂/UV,chemosynthesis-goethite/H₂O₂/UV and chemosynthesis-goethite/UV techniques were found to be effective in degrading mirex?>70%?.In the heterogeneous photo-Fenton systems,the presence of H₂O₂ restricted the removal of mirex and decreased its degradation efficiency.The adsorption of mirex onto iron-based catalysts occurred during the heterogeneous photo-Fenton process.Photomirex,10,10-dihydro mirex and 2,8-dihydro mirex were detected as degradation products when using schwertmannite as a catalyzer.In the heterogeneous photo-Fenton systems of chemosynthesis-goethite/UV,the reductive dechlorination and oxidative cleaving the cage carbon of mirex occurred,and as a result degradation products including photomirex,10,10-dihydro mirex,2,8-dihydro mirex and tetrachloro-p-benzoquinone were found.?5?The sorption isotherms of mirex onto chemosynthesis-schwertmannite and chemosynthesis-goethite fitted well with the Langmuir equation,with r2 values>0.99.Schwertmannite and goethite exhibited excellent adsorption of mirex,with the maximum adsorption capacities of 153 and 770 mg/g,respectively.The adsorption of mirex onto these two iron oxides increased when Triton X-100 concentration increased.However,after reaching 0.1 CMC?Critical Micelle Concentration?,the adsorption began to decline with increasing the concentration of Triton X-100.An increase in the adsorption of mirex was found when the pH value of solution decreased.?6?Mirex adsorbed onto schwertmannite and goethite can be degradated by UV.Approximately 25.2%,19.2%and 76.3%of the mirex pre-adsorped onto biosynthesis-schwertmannite,chemosynthesis-schwertmannite and chemosynthesis-goethite were degradated during 4h UV irradiation,respectively.The amount of mirex desorbed from three iron oxides can be neglected because of its low concentration.Photomirex,10,10-dihydro mirex and 2,8-dihydro mirex were detected as the degradation products in the degradation process of mirex adsorbed onto schwertmannite.Photomirex,10,10-dihydro mirex and 2,8-dihydro mirex,pentachloro-2-propanone,hexachloroacetone,tetrachloro-4-cyclopentene-1,3-dione and hexachloro-2-cyclopenten-l-one were detected as degradation products after 4 h of the reaction time in the degradation process of mirex adsorbed onto goethite.Therefore,the soils contaminated with both chlordane and mirex can be washed firstly using Triton X-100 solution,and then the chlordane and mirex in the washed eluent can be degraded by the heterogeneous photocatalytic system using chemosynthesis-goethite as a catalyzer.In this process,both chlordane and mirex can be effectlively removed from contaminated soil and deeply degraded through both reductive dechlorination and oxidative cleaving the cage carbon mechanisms.