Decontamination of wastewaters containing synthetic organic dyes by electrochemical methods: A general review

Effluents of a large variety of industries usually contain important quantities of synthetic organic dyes. The discharge of these colored compounds in the environment causes considerable non-aesthetic pollution and serious health-risk factors. Since conventional wastewater treatment plants cannot degrade the majority of these pollutants, powerful methods for the decontamination of dyes wastewaters have received increasing attention over the past decade. This paper presents a general review of efficient electrochemical technologies developed to decolorize and/or degrade dyeing effluents for environmental protection. Fundamentals and main applications of typical methods such as electrocoagulation, electrochemical reduction, electrochemical oxidation and indirect electro-oxidation with active chlorine species are reported. The influence of iron or aluminium anode on decolorization efficiency of synthetic dyes in electrocoagulation is explained. The advantages of electrocatalysis with metal oxides anodes and the great ability of boron-doped diamond electrodes to generate heterogeneous hydroxyl radical as mediated oxidant of these compounds in electrochemical oxidation are extensively discussed. The effect of electrode material, chloride concentration, pH and temperature on the destruction of dyestuffs mediated with electrogenerated active chlorine is analyzed. The degradation power of these pollutants with an emerging electrochemical advanced oxidation process such as electro-Fenton, based on the mediated oxidation by homogeneous hydroxyl radical formed from Fenton's reaction between cathodically produced hydrogen peroxide and catalytic Fe2+, is examined. Recent progress of emerging photoassisted electrochemical treatments with UV irradiation such as photoelectro-Fenton and photoelectrocatalysis is also described.