Journal
ENVIRONMENTAL SCIENCE AND POLLUTION RESEARCH
Volume 21, Issue 14, Pages 8573-8584Publisher
SPRINGER HEIDELBERG
DOI: 10.1007/s11356-014-2787-x
Keywords
Dairy wastewater; DSA electrode; Electrocoagulation; Electron spin resonance; Electro-oxidation; Indole; Sequential water treatment
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Funding
- CONACYT (Mexico)
- MICINN (Spain) [CTQ2010-16164/BQU]
- FEDER funds
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Dairy wastewater is characterized by a high content of hardly biodegradable dissolved, colloidal, and suspended organic matter. This work firstly investigates the performance of two individual electrochemical treatments, namely electrocoagulation (EC) and electro-oxidation (EO), in order to finally assess the mineralization ability of a sequential EC/EO process. EC with an Al anode was employed as a primary pretreatment for the conditioning of 800 mL of wastewater. A complete reduction of turbidity, as well as 90 and 81 % of chemical oxygen demand (COD) and total organic carbon (TOC) removal, respectively, were achieved after 120 min of EC at 9.09 mA cm(-2). For EO, two kinds of dimensionally stable anodes (DSA) electrodes (Ti/IrO2-Ta2O5 and Ti/IrO2-SnO2-Sb2O5) were prepared by the Pechini method, obtaining homogeneous coatings with uniform composition and high roughness. The (OH)-O-center dot formed at the DSA surface from H2O oxidation were not detected by electron spin resonance. However, their indirect determination by means of H2O2 measurements revealed that Ti/IrO2-SnO2-Sb2O5 is able to produce partially physisorbed radicals. Since the characterization of the wastewater revealed the presence of indole derivatives, preliminary bulk electrolyses were done in ultrapure water containing 1 mM indole in sulfate and/or chloride media. The performance of EO with the Ti/IrO2-Ta2O5 anode was evaluated from the TOC removal and the UV/Vis absorbance decay. The mineralization was very poor in 0.05 M Na2SO4, whereas it increased considerably at a greater Cl- content, meaning that the oxidation mediated by electrogenerated species such as Cl-2, HClO, and/or ClO- competes and even predominates over the (OH)-O-center dot-mediated oxidation. The EO treatment of EC-pretreated dairy wastewater allowed obtaining a global 98 % TOC removal, decreasing from 1,062 to < 30 mg L-1.
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