Electrochemical technology for the treatment of real washing machine effluent at pre-pilot plant scale by using active and non-active anodes
Introduction
The treatment and recovery of wastewater for reuse has been a topic of interest for years, due to the large volumes of effluents generated from different industrial plants such as pharmaceutical [[1], [2], [3]], tannery [[4], [5], [6]], dairy industries [7], agriculture [[8], [9], [10]] and textile [[11], [12], [13], [14]] industries and have received significant attention from both researchers and the health and regulatory agencies. Washing machine effluents have received less attention because these are produced domestically and no prior treatment is given before the discharge of these wastes into sewage or municipal wastewater system. Nowadays, every household and industrial setup utilize washing machine for textiles and fabrics washing, which in turn, produced large volumes of wastewater. Many surfactants, microplastics, fabrics and fibers are present in the effluents, which constitute major threat to the health and safety of the organisms in receiving water/surface water bodies [15,16]. Indeed, several studies have reported the microplastics accumulation in marine habitats, and its ingestion provides a potential pathway for the transfer of pollutants, monomers, and plastic-additives to organisms with uncertain consequences for their health [15,17].
One of the sources for the occurrence of microplastics and fibers in aquatic environments has been suspected to be fibers shed from clothes/textiles during washing [18,19]. In particular, some scientists have reported that household washing machines effluents seem to be a major source of so-called “microplastics” pollution. >1900 fibers can rinse-off of a single clothing during a washing cycle and it is forecast to be more in future [20]. Ingestion of microplastics in aquatic environments by birds and turtles have been well documented in literatures and over 40% of marine bird species are known to ingest plastics [21,22]. Besides, findings have shown that microplastics are sinks for persistent organic pollutants (POPs) in ocean, which are picked up via partitioning. It is the hydrophobicity of the POPs that facilitates their concentration in microplastics at a level of several orders of magnitude higher than in the sea water [20]. Microplastics itself are toxic due to the residual monomers from manufacturing stage or additives used in compounding the plastics. Additionally, higher toxicity is exhibit by the intermediates or transformation products than parent compounds. For example, the formation of styrene and other aromatics from the burning of polystyrene [23].
Consequently, recent studies have been channeled towards the development of effective methods for removing microplastics from sewage as well as to reduce the release of microplastics and fibers into sewage/municipal water system from washing machines effluents [22]. Under the premise of protecting the environment and ensuring health and safety of ecosystem, different technologies such ozonation, Fenton's reagent [24], UV/solar heterogeneous and homogeneous photocatalysis [25,26], ultrasound (US) and electrochemical technologies [[27], [28], [29]] have been applied for the treatment of varieties of industrial and domestic effluents to reduce the presence of various pollutants, including several organic compounds. Among electrochemical technologies, electrooxidation (EO) has demonstrated to be an efficient approach for degrading POPs due to its high versatility and excellent oxidants production [[30], [31], [32], [33]] as well as it does not require chemical addition for operation, thus, it is highly safe and environmental friendly [28,34]. In EO process, organic pollutants are degraded by either direct (electron transfer to anode surface) and/or mediated (in situ strong oxidants production i.e. hydroxyl radicals) oxidation, with the latter has high potential for complete mineralization of organic pollutants [[35], [36], [37]]. The production of hydroxyl radicals (M(˙OH)) is the main objective of the EO because this radical has a very high standard reduction potential (E°(˙OH/H2O) = 2.80 V/SHE) and can indiscriminately reacts with any class of organic pollutants until their complete combustion to CO2 [38,39].
However, the nature and the quantities of the M(˙OH) generated strongly depends on the nature of anode material used. High oxygen evolution overpotential electrodes, such as BDD (“non-active” anode), generate greater quantities of highly reactive and weakly adsorbed, M(˙OH). Meanwhile, active anodes, such as DSA (metal oxide electrodes such as IrO2, RuO2 or their mixtures) or Ti/Pt, promote further oxidation of M(˙OH) to less reactive and covalently bonded chemisorbed oxygen [36]. The active anodes are more effective for the electrogeneration of other oxidant species such as active chlorine species [28].
In this context, the novelty of this work is based on the depuration of a real washing machine effluent, for first time, by using a parallel plate electrochemical flow reactor, with Ti/Pt or BDD anodes. The influence of the anode material, applied current density and supporting electrolyte on the elimination of the organic matter in the effluent during the EO treatment was carefully examined. Analysis of the initial effluent and the final treated solutions were also conducted to understand the role of the chloride content of the real effluent during EO approach.
Section snippets
Chemicals/materials
Chemical reagents used in this study were of analytical grade or higher purity. The supporting electrolyte (Na2SO4) was supplied by Andriol (Brazil). The wastewater used in this study was a mixture of effluents collected from a washing machine, after a process of washing dark jeans (initial COD = 783 mg L−1, conductivity = 1299.5 μS cm−1, pH 6.15, turbidity = 408.2 UT, initial TOC = 680 mg L−1). Studies were performed using the real effluent with or without supporting electrolyte (called,
Treatment of real effluent with or without supporting electrolyte
EO treatment of the as-received effluent (without Na2SO4 – supporting electrolyte) was first conducted in the flow reactor at applied current density of 16.6 mA cm−2 with either Ti/Pt or BDD electrode. As shown in Fig. 2, there was sharp decay in normalized COD within first 30 min of electrolysis when Ti/Pt anode was used. After that, a plateau was reached without a significant reduction in COD removal. This behavior can be explained by the adsorption of organic matter on the Ti/Pt surface,
Conclusions
EO as an alternative treatment has been investigated for the complete decontamination of real washing machine effluent. The elimination of the organic matter in the effluent was always enhanced with rise in applied current density and higher COD removal efficiency was achieved with BDD anode (up to 88%) compared Ti/Pt anode (~71%). EO treatment of as-received effluent exhibited lower COD decay as well as higher cell potential compared to the treatment of real effluent containing Na2SO4 as
Acknowledgement
Financial support from National Council for Scientific and Technological Development (CNPq – 465571/2014-0; CNPq - 446846/2014-7 and CNPq - 401519/2014-7) and FAPESP (2014/50945-4) are gratefully acknowledged. Carlos A. Martínez-Huitle acknowledges the funding provided by the Alexander von Humboldt Foundation (Germany) and Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (Brazil) as a Humboldt fellowship for Experienced Researcher (88881.136108/2017-01) at the Johannes
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