A comparative study of free chlorine activated by Fe+2 and UV C light catalysts in the treatment of real and simulated textile wastewater: Optimization, reactive species and phytotoxicity assessment

Abstract

A large amount of wastewater originating from the textile industry causes serious aesthetic and ecological problems in receiving environments due to its complex structure, variability in composition and high color content. Therefore, the main aim of this study is to integrate different cost-effective and environmentally friendly treatment processes to ensure the proper treatment and reuse of dye-containing wastewater before it is finally discharged into the environment. For this purpose, different synergistic activities of chlorine oxidant under Fe+2 and UV light catalysis were investigated in the removal of Acid Black 220 (AB 220), Bemacid Red (BR) and Bemacid Blue (BB) synthetic dyes used in the dyeing of polyamide textile products. The effects of pH, initial dye concentration, chlorine and Fe+2 dosage on dye degradation were investigated. Contributions of reactive species formed by using nitrobenzene (NB), t-butanol (TBA), and isopropanol (IPA) radical scavengers as specific probes were determined. Molar ratios were determined for optimum chlorine and Fe+2 concentrations and validation experiments were performed for different initial dye concentrations. Chlorine/Fe+2 was also applied to a real textile wastewater sample to which the specified dye recipe was applied. The effects of raw and treated textile wastewater on seed germination and plant growth parameters were determined using Lepidium sativum (L. sativum). The presence of iron ions and UV light has increased the removal, Chlorine/Fe+2 combination was chosen as the most suitable process due to initial investment and operating costs. The best performance was obtained at pH 3, due to the effect of pH on the dissociation of hypochlorous acid (HClO) and hypochlorite ion (ClO−). With the increase of Fe+2 and chlorine doses to the optimal level, the degradation efficiency increased and harmful effects were observed at high doses. Removal efficiencies of 76.1 %, 94.99 % and 97.36 %, respectively, were obtained for AB 220, BR and BB under optimized conditions. It was determined that hydroxyl radicals (OH) have the highest contribution, dichloride radical (Cl2−) and chlorine (Cl) radicals are similar and effective at a good level, but chlorine oxide radical (ClO) cannot be considered as an oxidizing radical in the Chlorine/Fe+2 system. By applying Chlorine/Fe+2 to a real textile wastewater sample, high efficiency (>90 %) was obtained in terms of color removal. Considering the chemical oxygen demand (COD) removal, when compared to the efficiency of the process in color removal, it was seen that the removal was quite ineffective for AB 220 and BR, but was at an acceptable level in the real wastewater sample. The phytotoxicity experiments proved that raw wastewater is harmful and treated wastewater is much less toxic than the studied parameters and can be reused for various purposes. Therefore, the findings of this study showed that chlorination in the presence of Fe+2 is very effective, simple and fast in removing color from synthetic dye samples and real textile wastewater, and the treated wastewater is safe for reuse in terms of physicochemical parameters and toxicity.