Degradation of 1,2-dichloroethane from wash water of ion-exchange resin using Fenton's oxidation.

BACKGROUND, AIM, AND SCOPE: Chlorinated volatile organic compounds (CVOCs), widely used in industry as solvents and chemical intermediates in the production of synthetic resins, plastics, and pharmaceuticals, are highly toxic to the environment and public health. Various studies reported that Fenton's oxidation could degrade a variety of chlorinated VOCs in aqueous solutions. In acidic conditions, ferrous ion catalyzes the decomposition of H2O2 to form a powerful *OH radical. In this study, wastewater from wash of ion-exchange resin containing typical CVOC, 1,2-dichloroethane, was treated using Fenton's oxidation. To reduce environmental load and processing costs of wastewater, Fenton process as a simple and efficient treatment method was applied to degrade 1,2-dichloroethane of wash water.

MATERIALS AND METHODS: The water samples were collected from three different washing stages of ion-exchange resin. The degradation of 1,2-dichloroethane and total organic carbon (TOC) of wash water of ion-exchange resin by Fenton process was studied with response surface method (RSM). Design of the experiments was conducted by central composite face, and factors included in three models were Fe2+ and H2O2 doses and treatment time. Relevant quadratic and interaction terms of factors were investigated.

RESULTS: According to ANOVA, the model predicts well 1,2-dichloroethane reduction of all water samples and TOC reduction of samples 2 and 3. The Fe2+ and H2O2 doses used in the present study were most suitable when 1,2-dichloroethane concentration of the wash water is about 120 mg L(-1). In that case, Fenton's oxidation reduced 1,2-dichloroethane and TOC up to 100% and 87%, respectively, according to the RSM model. With 90-min reaction time and H2O2 dose of 1,200 mg L(-1), the required Fe2+ doses for 1,2-dichloroethane and TOC were 300 and 900 mg L(-1), respectively. The optimal H2O2/Fe2+ stoichiometric molar ratio was between 4-6. Then, concentration of Fe2+ was low enough and the amount of residual sludge can thus be reduced. It seems that most of TOC and part of 1,2-dichloroethane were removed by coagulation.

DISCUSSION: Up to a certain extent, increase of Fe2+ and H2O2 doses improved the removal of 1,2-dichloroethane and TOC. High Fe2+ doses increased the formation of ferric-based sludge, and excessive H2O2 doses in sample 2 decreased the degradation of 1,2-dichloroethane. Excess amount of hydrogen peroxide may scavenge hydroxyl radicals, thus leading to loss of oxidative power. Also, the residual hydrogen peroxide of different samples increased with increasing H2O2 dose and H2O2/Fe2+ molar ratio and decreasing treatment time probably also due to scavenging reactions. Due to the saturated nature of 1,2-dichloroethane, the oxidation mechanism involves hydrogen abstraction before addition of hydroxyl radical, thus leading to lower rate constants than for direct hydroxyl radical attack, which for one increases the treatment time.

CONCLUSIONS: Complete removal of 1,2-dichloroethane was attained with initial concentration<120 mg L(-1). Also, TOC degraded effectively. Wash water with higher concentration of 1,2-dichloroethane requires longer treatment times and higher concentrations of Fe2+ and H2O2 for sufficient 1,2-dichloroethane removal.

RECOMMENDATIONS AND PERSPECTIVES: Due to the results achieved in this study, Fenton's oxidation could be recommended to be used for organic destruction of wash water of ion-exchange resin. Residual sludge, the main disadvantage in Fenton process, can be reduced by optimizing the ferrous dose or by using heterogeneous treatment where most of the reusable iron remains in the solid phase.