| Surfactants are usually used in remediation of subsoil and aquifers contaminated with hydrophobic organic compounds (HOCs) such as benzene, toluene, ethylbenzene and xylenes (BTEX). Among all remediation technologies for treating HOCs contaminated soil, flushing is an effective and economical method. Additionally, effluent from surfactant flushing is a pollutant in its own right and the collected wastes must be adequately disposed or treated by applying suitable physical, biological or chemical processes before discharge or reuse. Many researchers have applied different effective treatments in which the chemical treatments are destructive techniques which can simultaneously degrade surfactant and contaminants, while the other non-destructive technologies just separate contaminant from the flushing solution. Thus, the sulfate radical (SO4·-), a strong oxidant (E0=2.60V), based on activated persulfate, will be possible used by selective oxidation goal contaminants BTEX. This means that SO4·-may oxidize BTEX with a faster reaction rate than straight chain hydrocarbon surfactants which do not contain a benzene ring, and compares favorably with degradation of surfactants by advanced oxidation processes (AOPs) in surfactant wastewater. The objective of this study was to investigate the application of surfactant flushing in the treatment of toluene (as a model BTEX compound) contaminated soil. A Box-Behnken experimental design (BBD) using Response surface methodology (RSM) optimization was employed to evaluate the effects of variables such as flushing time, surfactant concentration and flow velocity on toluene removal efficiency. Moreover, in order to achieve the goal of surfactant recovery, the methods of activated persufate such as Fe2+, EC, UV, US/UV oxidation process is proposed for selective degradation of toluene with most of the surfactant remaining in solution being collected and reused. In addition, the regenerated solution from UV/S2O82-oxidation process was employed in the flushing process.(1) Laboratory soil column experiments were conducted, using the anionic surfactant sodium dodecyl sulfate (SDS), to optimize toluene removal efficiency from contaminated soil. The influence of flushing time, SDS concentration and flow velocity on toluene removal efficiency was investigated. RSM based on BBD was applied to analyze the experimental variables which all showed a positive effect on toluene removal efficiency. The interactions between SDS concentration and flushing time or flow velocity were significant, while the interaction of flushing time with flow velocity was not pronounced. The ANOVA results confirmed that the proposed models were accurate and reliable for the analysis of the variables of surfactant flushing treatment. The optimum values of selected variable and their mutual interactions were determined by RSM. The results demonstrated that toluene removal of96.2%was predicted under optimum conditions of SDS1.8%, flushing velocity4.8mL/min after270min. The predicted response was experimentally validated and resulted in a maximum degradation of91.0%which is closer to the predicted value in surfactant flushing process.(2) The Fe2+/S2O82-process was conducted for toluene degradation in aqueous systems and the degradation efficiency was optimized using RSM. Based on BBD, the optimum values of selected variable and their mutual interactions were determined. The results showed concentration of ferrous ion showed a positive effect on the toluene degradation efficiency. The interaction between PS concentration and ferrous ion concentration was significant. The ANOVA results confirmed that the proposed models were accurate and reliable for the analysis of the variables of Fe2+/S2O82-process. The results demonstrated that toluene degradation of80.8%(10min) and85.1%(30min) was predicted under optimum conditions. The predicted response was experimentally validated and resulted in a maximum degradation of81.1%(10min) and84.4%(30min) by Fe2+/S2O82-process. The main intermediates were also separated and identified by GC-MS technique and a plausible degradation pathway of toluene was proposed. Moreover, further studies of surfactant recovery were conducted in surfactant solution. The degradation efficiency was limited when Fe2+/H2O2process was introduced for oxidation of toluene in different concentration solution. However, a great enhancement of toluene degradation efficiency and lower SDS degradation efficiency were obtained during the Fe2+/S2O82-process compared with Fe2+/H2O2process. The results indicated that toluene could be more effectively degraded by selective oxidation with SO4·-in surfactant solution. (3) The EC/Fe2+/S2O82-process was conducted for toluene degradation in surfactant eluents and the pseudo second-order reaction rate constant of degradation efficiency was optimized. The results indicated that in this process, the reaction between persulfate and externally added ferrous ion results in the production of sulfate free radical, and at the same time ferrous ion could be electro-regenerated at the cathode by the reduction of ferric ion. RSM based on BBD was applied to analyze the experimental variables which concentration of PS and ferrous ion showed a positive effect on the pseudo second-order reaction rate constant of toluene degradation, while concentration of SDS showed a negative effect. The interaction between PS concentration and ferrous ion concentration was significant. The ANOVA results confirmed that the proposed models were accurate and reliable for the analysis of the variables of EC/Fe2+/S2O82-process. The shapes of3D response surfaces and contour plots showed that Fe2+concentration and PS concentration substantially affected toluene degradation efficiency. The results indicated increasing Fe2+concentration or PS concentration could increase toluene degradation, while increasing SDS concentration would decrease toluene degradation. The reaction intermediates were identified by (GC-MS) technique and a plausible degradation pathway for toluene degradation was proposed.(4) The UV/S2O82-process was conducted for toluene degradation in surfactant eluents and the degradation efficiency was optimized. The results indicated that in this process, the reaction between persulfate and UV results in the production of sulfate free radical for selective oxidation toluene. RSM based on BBD was applied to analyze the experimental variables which concentration of PS showed a positive effect on the pseudo first-order reaction rate constant of toluene degradation, while concentration of toluene and SDS showed a negative effect. The interaction between PS concentration and toluene or SDS concentration was significant. The ANOVA results confirmed that the proposed models were accurate and reliable for the analysis of the variables of UV/S2O82-process. The shapes of3D response surfaces and contour plots showed that PS concentration substantially affected toluene degradation efficiency. The results indicated increasing PS concentration could increase toluene degradation, while increasing toluene or SDS concentration would decrease toluene degradation. The reaction intermediates were identified by (GC-MS) technique and a plausible degradation pathway for toluene degradation was proposed. The results demonstrated that there were less intermediates produced in UV/S2O82-process which was been used for surfactant recovery, favorably.(5) The use of UV/S2O82-selective oxidation process was proposed for surfactant recovery and degradation of contaminant present in the solution collected. With RSM optimized parameters of SDS concentration and flushing velocity, a soil toluene removal efficiency of62.1%was obtained by the first surfactant flushing process using fresh surfactant. In order to regenerate SDS, it was found that after270min of treatment toluene was90.1%degraded for the second time the surfactant was used and94.7%for the third time. In addition, a test in which the regenerated solution was employed in the flushing process was carried out in soil column. This time, toluene removal efficiency from the contaminated soil of52.1%was achieved. When the surfactant was recovered once more (third time) after the UV/S2O82-oxidation process and the soil column flushing process,43.6%of toluene was removed from the contaminated soil. Comparing the three flushing processes, it can be seen that soil toluene removal efficiency decreased after each reuse. The results indicated that the UV/S2O82-oxidation process was potentially effective in selective degradation of toluene with most of the surfactant left for potential reuse in another polluted soil treatment. In the meanwhile, the ultrasound (US) was introduced for UV/S2O82-oxidation process and the results showed US had synergistic effects on enhancing the toluene degradation efficiency. |
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