Modeling and optimization of fast degradation of contaminated soil with 2-methylpropane-2-thiol by modified Fenton process

Authors
P. Roohi 1
E. Fatehifar 2
E. Fatehifar 2
R. Alizadeh 3
1 Researcher in environmental Engineering Research center
2 Head of Environmental Engineering Research Center
3 Head of chemical engineering department
Abstract
The pollution of soil with 2-methylpropane-2-thiol as an odorant hydrocarbon is an environmental problem. It also causes secondary impacts such as social dissatisfaction and economic problems due to tourist revenue reduction. 2-methylpropane-2-thiol is a hazardous material and remediation of soil polluted by this material with a fast method is important to study.
In this study, modified Fenton treatment is investigated for oxidation of 2-methylpropane-2-thiol. Central Composite Design (CCD) based on Response Surface Methodology (RSM) was used to obtain appropriate effects of the main factors (initial H2O2 concentration, FeSO4 to soil ratio and stirring time interval percentage) and their interactions on the removal efficiency. Treatments were set up to monitor 2-methylpropane-2-thiol removal efficiency for initial contaminant concentration of 64690 part per million by weight. Samples were analyzed by gas chromatograph equipped with FID and TCD detector and HP-Plot Q column. Design of experiment in the three-factor with five-level matrix include 20 experiment. Randomization technique is used to guard against unknown and uncontrolled factors as lurking nuisance factors. Moreover, blocking technique is used for investigation of probable effect of initial soil temperature on results.
Analysis of variance and Pareto analysis show that all main factor are effective. Also, stirring time interval percentage was the most influential factors on 2-methylpropane-2-thiol removal efficiency. Results of the experiments shows that at low concentration levels removal efficiency increases with hydrogen peroxide concentration up to the certain level. For higher concentration of hydrogen peroxide concentration, the removal efficiencies decreased which could be due to scavenging. Also, increasing in FeSO4:Soil ratio increases removal efficiency up to the certain level because Fe2+ is an alternative to enable more extensive and greater contaminant oxidation; however a greater ratio (greater than 0.0040) causes decrease in the removal efficiency. This phenomena could be due to side reactions which affect reactive radicals such as OH• radicals. Furthermore, investigation of the results demonstrates that 2-methylpropane-2-thiol removal efficiency rises with increasing stirring time interval percentage. This phenomena could be due to uniform distribution of oxidation agent and Fe2+ and better desorption of contaminant from soil to liquid phase.
Moreover, based on analysis of variance, the interaction between hydrogen peroxide and FeSO4: Soil ratio was significant with positive effect on the removal efficiency. This interaction could be the result of reaction between H2O2 and Fe2+. By considering main and interaction effects, with the raising H2O2 and Fe2+ concentration up to a certain level, the removal efficiency increase and with further concentration increasing the removal efficiency will be dropped. Analysis of variance indicate that initial soil temperature (21 and 25 0C) were not effective factors during the time interval of the experiments which could be due to the exothermic reaction between hydrogen peroxide, FeSO4 and contaminant. P-value of lack-of-fit (0.064) indicates that suggested model adequately fits the data with good correlation coefficient (R2=95.12%). Optimum condition suggested for maximum 2-methylpropane-2-thiol removal efficiency (94.412%) shows that concentration of H2O2 and Fe2+ ion must be at the certain level and maximum stirring time for remediation in the studied intervals.
CCD model predict 94.084% for the removal efficiency at optimum condition which is good agreement with the predicted value.

Keywords

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Modares Civil Engineering journal
Volume 17, Issue 4
November and December 2017
Pages 77-88