Transformation products and degradation pathway of textile industry wastewater pollutants in Electro-Fenton process

Highlights

• Continuous electrocatalytic treatment of real textile wastewater was performed.

• Most of originally present organic compounds were completely mineralized/degraded.

• Chlorine based oxidants and OH radicals mineralized/transformed the pollutants.

• Optimum steady state time was found to be 137 min with 142 min retention time.

• Toxicity bioassay test showed increased exposure from 1 h to 96 h for 100% mortality.

Abstract

In this study, pollutants from textile industry wastewater were transformed/oxidized using Ti/RuO₂ electrode by Electro-Fenton (EF) method in a continuous reactor. The performance was evaluated in terms of % COD removal, % color removal and energy consumed. Electrolysis time, retention time, current, and ferrous sulphate concentration as Fenton catalyst were selected as EF process parameters. To determine the optimum operating conditions multiple response optimization with desirability approach based on central composite design under response surface methodology was used. Spectrophotometric and GC-MS analysis were performed to identify the degraded/transformation compounds, and on this basis degradation mechanism during EF process as well as disposability of treated wastewater was analyzed. Further, bioassay test of treated textile wastewater was conducted for toxicity analysis in view of its disposal quality.

Results showed that all the components of textile wastewater were totally eliminated/transformed in lower molecular weight compounds after EF treatment of textile effluent. Further, bioassay test analysis confirmed the nontoxic nature of treated wastewater.

Introduction

Water contamination is a huge issue that the world is confronting today. It is expanding with each passing year, making grave and hopeless harm to the earth and environment. Textile industries are considered to be a major containment to the fresh water because of high toxicity and non-biodegradability of dye stuff and other chemicals. Direct discharge of textile wastewater is restricted to the environment by various environmental/pollution regulatory agencies. Unfortunately, due to biological resistance and chemical stability of dyes and other chemicals in textile wastewater, make it hardly treated by the conventional treatment methods (Georgiou et al., 2002; Meric et al., 2004; Izares et al., 2006; Chen and Liu., 2012; Bansal et al., 2013; Kaur et al., 2017), however, if treated huge volume of secondary pollutants are generated. Therefore, researchers have focused toward the treatment without generating secondary pollutants. Among all promising techniques, electro-oxidation is efficient to degrade toxic compounds in an aqueous phase, and demonstrated their great performance for the treatment of textile wastewater. However, generation of chloro-organic compounds during the treatment process is major drawback of electro-oxidation process. This drawback can be overcome by Electro-Fenton process (EF) (Brillas et al., 2009). The efficiency of the EF process predominantly depends on H₂O₂ production rate, O₂ solubility, pH, anode material, Fenton catalyst and current intensity. Fenton catalyst concentration and type significantly affect the EF treatment efficiency. Various catalyst are reported in literature but Fe²⁺ or Fe³⁺ proved good catalytic characteristics even at lower concentrations (Sirés and Brillas, 2012). EF is based on the degradation of pollutants by the action of both Fenton's reaction Eq. (2) in the bulk and anodic oxidation at the anode surface Eq. (4). H₂O₂ is continuously generated in the solution during electrolysis in EF process due to two electron oxygen reduction at cathode (Brillas et al., 2009; Sirés et al., 2014) in an acidic medium Eq. (1), and ferrous ion added into the system analogously generates the OH radicals in the classical Fenton's reaction Eq. (2). At the same time, in this process, the ferrous ion is regenerated at the cathode Eq. (3). Ferric ions generated during EF process for monomeric and polymeric hydroxyl complexes.

O₂ + 2H⁺ + 2e− → H₂O₂

Fe²⁺ + H₂O₂ → Fe³⁺ + OH⁻ + OH

Fe³⁺ + e⁻ → Fe²⁺

M(H₂O) → M(OH) + H⁺ + e⁻

Dimensionally stable anodes (DSAs) are stable during electrolysis, have high oxygen overvoltage and does not produce secondary pollutants (Brillas and Casado, 2002; Nasr et al., 2005). There are number of DSAs i.e titanium coated RuO₂, IrO₂, SnO₂, PbO₂; boron doped diamond electrodes; Pt/Ti etc. used by various researchers for electrochemical treatment processes (Wang et al., 2005; Zhou and He, 2007; Martinez-Huitle and Brillas., 2009; Salazar et al., 2012; Oturan et al., 2012; Pajootan et al., 2014; Mukimin et al., 2015; Mukimin et al., 2017). Fenton's reaction is applied in acidic pH of 2.8–3.0 (Sun and Pignatello, 1993) to efficiently produce OH. Ti/RuO₂ electrodes are highly stable in acidic medium and it is an active DSAs electrode, provide large surface area for the adsorption of pollutants (Santos et al., 2010). Ti/RuO₂ anode also favours the chlorine species-mediated oxidation in bulk. Due to this carcinogenic/toxic chlorinated organic compounds may be generated during the EF process. OH radical generation is prominent in EF process, which can mineralized chlorinated organic compounds by ^•OH mediated oxidation. Various investigations prove that EF is well able to reduce the strong nature, carcinogenic and toxic behaviour of textile wastewater (Santos et al., 2010; Salazar et al., 2012; Nidheesh and Gandhimathi, 2014; Lin et al., 2014; Pajootan et al., 2014; Ghanbari and Moradi, 2015; Asghar et al., 2015; Garcia-Segura and Brillas, 2016). In literature, no study has been reported on the textile industry wastewater treatment by continuous EF method. Researchers just examined about the treatment of simulated/model textile wastewater. Textile industry wastewater has number of components, which can advance or thwart the treatment process. Subsequently, execution examination and appropriateness of EF process require more investigation on textile industry wastewater.

In the present study, treatment of textile industry wastewater was performed by EF method in continuous mode using Ti/RuO₂ electrodes. The EF process was optimized for various process parameters i.e. elapsed time (t), retention time (R_T), current (i) and concentration of ferrous sulphate (C_Fe) (Fenton catalyst) for % chemical oxidation demand (COD) removal (X₁), % color removal (X₂) and energy consumed (X₃). Spectrophotometric and GC-MS analysis at optimum conditions was performed to identify the degraded/transformation compounds for degradation mechanism during EF process as well as disposability of treated wastewater. Further, bioassay test was performed for toxicity analysis of treated textile wastewater in view of disposal quality. The persuasive mechanism of the treatment process was also purposed by GC-MS analysis. Moreover, degradation kinetics in terms of COD removal and color removal were investigated. Operating cost analysis of the continuous mode for per Kg of COD removal was also performed to determine the economic feasibility.