As the environment preservation gradually becomes a matter of major social concern and more strict legislation is being imposed on effluent discharge, more effective processes are required to deal with non-readily biodegradable and toxic pollutants. Synthetic organic dyes in industrial effluents cannot be destroyed in conventional wastewater treatment and consequently, an urgent challenge is the development of new environmentally benign technologies able to mineralize completely these non-biodegradable compounds. This review aims to increase the knowledge on the electrochemical methods used at lab and pilot plant scale to decontaminate synthetic and real effluents containing dyes, considering the period from 2009 to 2013, as an update of our previous review up to 2008. Fundamentals and main applications of electrochemical advanced oxidation processes and the other electrochemical approaches are described. Typical methods such as electrocoagulation, electrochemical reduction, electrochemical oxidation and indirect electro-oxidation with active chlorine species are discussed. Recent advances on electrocatalysis related to the nature of anode material to generate strong heterogeneous OH as mediated oxidant of dyes in electrochemical oxidation are extensively examined. The fast destruction of dyestuffs mediated with electrogenerated active chlorine is analyzed. Electro-Fenton and photo-assisted electrochemical methods like photoelectrocatalysis and photoelectro-Fenton, which destroy dyes by heterogeneous OH and/or homogeneous OH produced in the solution bulk, are described. Current advantages of the exposition of effluents to sunlight in the emerging photo-assisted procedures of solar photoelectrocatalysis and solar photoelectro-Fenton are detailed. The characteristics of novel combined methods involving photocatalysis, adsorption, nanofiltration, microwaves and ultrasounds among others and the use of microbial fuel cells are finally discussed.

Decontamination of wastewaters containing synthetic organic dyes by electrochemical methods. An updated review

Direct And Mediated Anodic Oxidation of Organic Pollutants

Advanced oxidation processes (AOPs) constitute important, promising, efficient, and environmental-friendly methods developed to principally remove persistent organic pollutants (POPs) from waters and wastewaters. Generally, AOPs are based on the in situ generation of a powerful oxidizing agent, such as hydroxyl radicals (•OH), obtained at a sufficient concentration to effectively decontaminate waters. This critical review presents a precise and overall description of the recent literature (period 1990–2012) concerning the main types of AOPs, based on chemical, photochemical, sonochemical, and electrochemical reactions. The principles, performances, advantages, drawbacks, and applications of these AOPs to the degradation and destruction of POPs in aquatic media and to the treatment of waters and waste waters have been reported and compared.

Advanced Oxidation Processes in Water/Wastewater Treatment: Principles and Applications. A Review

In recent years, there has been increasing interest in finding innovative solutions for the efficient removal of contaminants from water, soil and air. The present tutorial review summarizes the results of an extensive selection of papers dealing with electrochemical oxidation, which is proposed as an alternative for treating polluted wastes. Both the direct and indirect approaches are considered, and the role of electrode materials is discussed together with that of other experimental parameters. Apart from discussing the possibility of removing selected contaminants from water using different anodes, efficiency rates for pollutant removal have been collected, the dependence of these rates on operational conditions advantages and disadvantages determining the further full-scale commercial application.

Electrochemical oxidation of organic pollutants for the wastewater treatment: direct and indirect processes

Photoreduction of CO2 into sustainable and green solar fuels is generally believed to be an appealing solution to simultaneously overcome both environmental problems and energy crisis. The low selectivity of challenging multi-electron CO2 photoreduction reactions makes it one of the holy grails in heterogeneous photocatalysis. This Review highlights the important roles of cocatalysts in selective photocatalytic CO2 reduction into solar fuels using semiconductor catalysts. A special emphasis in this review is placed on the key role, design considerations and modification strategies of cocatalysts for CO2 photoreduction. Various cocatalysts, such as the biomimetic, metal-based, metal-free, and multifunctional ones, and their selectivity for CO2 photoreduction are summarized and discussed, along with the recent advances in this area. This Review provides useful information for the design of highly selective cocatalysts for photo(electro)reduction and electroreduction of CO2 and complements the existing reviews on various semiconductor photocatalysts.

Cocatalysts for Selective Photoreduction of CO2 into Solar Fuels.

Coagulation and electrocoagulation of oil-in-water emulsions.

The electrodissolution of aluminum electrodes in aqueous solutions containing sulfate or chloride ions is studied in this work. The results obtained are important in order to obtain a better understanding of the electrocoagulation process, as the electrodissolution of the anode surface is its first step. It has been determined that both chemical and electrochemical dissolution play an important role in the aluminum generation. The chemical dissolution of aluminum is strongly influenced by the pH. Alkaline pHs increase the dissolution rate by orders of magnitude. Within the experimental conditions used, the supporting media does not seem to influence greatly the chemical dissolution process. The electrochemical dissolution process depends mainly on the specific electrical charge passed. Salinity does not significantly affect the electrodissolution rate. Good fittings between experimental and modeled data are obtained by modeling the system with a simple model based on two assumptions:  a highly segregated ...

Electrodissolution of Aluminum Electrodes in Electrocoagulation Processes

Dyes are common pollutants in a large variety of industrial wastewaters, and the treatment of these wastes by coagulation has been extensively studied in the literature. This work is focused on the comparison of the efficiencies of the chemical and the electrochemical coagulation processes with hydrolyzing aluminum salts, and it tries to determine the similarities or differences that exist between the two coagulation processes. To do this, Eriochrome Black T solutions were used as a model of dye-polluted wastewater, and experiments of both coagulation technologies were planned to meet the same operation conditions. The pH, the aluminum concentration, the type of electrolyte, and the mode of dosing of aluminum were found to influence the process. Moreover, the speciation of aluminum was found to be the key parameter to explain the results, in terms of the mechanisms previously proposed in the literature for dissolved organic matter coagulation.

Coagulation and electrocoagulation of wastes polluted with dyes.

In this work, the electrocoagulation process using aluminum and iron electrodes has been used to treat synthetic wastewaters polluted with three different types of pollutant models:  kaolin suspensions, dye solutions, and oil-in-water emulsions. It was obtained that both electrodes can achieve high efficiencies (above 80%) in the treatment of the three wastes. However, there are strong differences in the electrochemical coagulation or breakup mechanism that can be explained in terms of the speciation of the dissolved metals and especially in terms of the significant concentrations of monomeric and polymeric ionic species that appear in the treatment with aluminum electrodes. In every case, sweep coagulation explains the coagulation of kaolin suspension with both aluminum and iron electrodes. However, in the case of aluminum, the neutralization charge mechanism should also be considered for low reagent doses. The coagulation of EBT (Eriochrome Black T) solutions and the breakup of O/W emulsions (oil-in-wat...

Fabiola Martínez

Papers

Coagulation and electrocoagulation of oil-in-water emulsions.

Electrodissolution of Aluminum Electrodes in Electrocoagulation Processes

Coagulation and electrocoagulation of wastes polluted with dyes.

Study of the Electrocoagulation Process Using Aluminum and Iron Electrodes

The pH as a key parameter in the choice between coagulation and electrocoagulation for the treatment of wastewaters.