Degradation of Pesticides in Aqueous Medium by Electro-fenton and Related Methods. a Review

This paper reviews the application of indirect electro-oxidation processes such as electroFenton, photoelectro-Fenton and peroxi-coagulation to decontaminate waters containing persistent pesticides. The fundamentals of these electrochemical advanced oxidation processes (EAOPs) are described. They are environmentally friendly technologies based on the electrogeneration of H2O2 at a carbonaceous cathode from the reduction of oxygen gas. In all cases, the main oxidizing species is the powerful oxidizing agent hydroxyl radical formed from Fenton’s reaction between added Fe and electrogenerated H2O2. The characteristics of threeand two-electrode divided or undivided tank or flow cells using working cathodes such as carbon-felt, reticulated vitreous carbon or O2-diffusion and anodes such as Pt, boron-doped diamond (BDD) or iron for the treatment of pesticides in these EAOPs are analyzed. The effect of several operational parameters that have large influence on the degradation rate of these pollutants in different electro-Fenton systems is examined. Pesticides are quickly removed following pseudo first-order kinetics in most cases. Their degradation sequences are discussed from aromatic intermediates, short aliphatic acids and inorganic end products detected. Final carboxylic acids are completely destroyed using undivided Pt/carbon felt and BDD/O2 cells, whereas they are more difficultly oxidized in undivided Pt/O2 cell because of the formation of hardly oxidizable Fe(III)-oxalate complexes. These latter products can be rapidly photodecomposed by the action of UV light in photoelectro-Fenton or sunlight in solar photoelectro-Fenton, significantly enhancing the mineralization process. Peroxi-coagulation with a sacrificial Fe anode also leads to fast degradation with production of small quantities of Fe(III) complexes because organics are mainly retained in the Fe(OH)3 precipitate formed. *Corresponding author Email: [email protected] INTRODUCTION Pesticides are chemical substances intended for preventing, destroying, repelling and mitigating any pest, although they can also be used as a plant regulator, defoliant or desiccant. Herbicides, insecticides and fungicides are the most common pesticides. The widespread use of thousands of tons of these compounds in domestic, industrial and agricultural activities throughout the world, especially in developed countries, generates large volumes of contaminated wastewaters, whose direct disposal into natural channels causes their accumulation in the environment. Although very low levels of pesticides, typically concentrations < 10 μg L [1], are detected as pollutants in surface and ground waters, most of them are hardly biodegradable and toxic for human beings and animals. Therefore, aiming to avoid the adverse effects of these contaminants on living organisms, the purification of wastewaters containing pesticides is needed, thus ensuring their reuse in human activities. Unfortunately, pesticides are considered as persistent organic pollutants (POPs), since they can not be removed by conventional oxidation methods in municipal sewage treatment plants. As a response, a large variety of advanced oxida236 J. Environ. Eng. Manage., 19(5), 235-255 (2009) tion processes (AOPs) have been recently developed to remove POPs from waters. AOPs are environmentally friendly methods based on the in situ chemical, photochemical, photocatalytic or electrochemical production of OH [1-7], which is the second strongest oxidant known after fluorine, with E°(OH/H2O) = 2.8 V vs. NHE (normal hydrogen electrode). They are applied when conventional oxidation techniques become insufficient by kinetic reasons or because contaminants are refractory to chemical oxidation in aqueous medium or are partially oxidized yielding stable by-products showing even greater toxicity than the starting pollutants. In contrast, OH generated in AOPs is able to nonselectively destroy most organic and organometallic pollutants until total mineralization. These radicals react rapidly with organics mainly either by abstraction of a hydrogen atom (dehydrogenation) or by addition to a non-saturated bond (hydroxylation). The latter reaction is typical of aromatic compounds, exhibiting secondorder rate constants as high as 10-10 M s [8]. Over the last decade, electrochemical AOPs (EAOPs) based on the cathodic electrogeneration of hydrogen peroxide are being successfully tested for the treatment of acidic wastewaters containing pesticides [1,5,6,9,10]. Among these emerging indirect electro-oxidation methods, the most popular technique is electro-Fenton (EF) that can be easily applied using divided or undivided electrolytic cells. In the first case, OH is produced by the catalytic Fenton reaction between cathodically electrogenerated H2O2 and Fe 2+ whereas in the second one this radical is also formed, although to a much lesser extent, from water oxidation at the anode surface. The EF process in undivided cells can then take advantage of the oxidation reactions arising from the simultaneous participation of both anode and cathode, being more efficient for destruction of organics than classical anodic oxidation (AO). Other indirect electro-oxidation methods like photoelectro-Fenton (PEF) and peroxi-coagulation (PC) have been recently proposed to enhance the efficiency of EF from the catalytic action of UV light and the alternative use of a sacrificial iron anode, respectively. This paper presents a review of the application of the above EAOPs to the degradation of pesticides in waters. Fundamentals of EF and related indirect electro-oxidation methods are initially described to analyze their characteristics and oxidation power. FUNDAMENTALS OF ELECTRO-FENTON PROCESS Hydrogen peroxide is a “green” chemical that leaves oxygen gas and water as by-products. It is widely utilized, e.g., to bleach pulp and paper and textiles, clean electronic circuits and delignify agricultural wastes, as well as for disinfection in medical and industrial applications and as an oxidant in synthesis and wastewater treatment [11,12]. However, the direct remediation of wastewaters with H2O2 is limited by its low oxidation power, since it can only attack reduced sulphur compounds, cyanides and certain organics such as aldehydes, formic acid and some nitro-organic and sulpho-organic compounds. For this reason, H2O2 is commonly activated in acidic effluents with Fe ion as catalyst (Fenton’s reagent) to produce homogenous OH as strong oxidant of organics [2-4]. This procedure is widely developed in the traditional chemical Fenton method. It is known since 1882 that H2O2 can be accumulated in aqueous medium from the cathodic twoelectron reduction of dissolved O2 gas at carbonaceous electrodes with high surface area [13]. In acidic solutions, this reaction with Eo = 0.68 V vs. NHE can be written as follows: ( ) 2 2 2 g e + − Ο + 2Η + 2 →Η Ο (1) and takes place more easily than the four-electron reduction of this gas to water (Eo = 1.23 V vs. NHE). Hydrogen peroxide production and stability depend on factors such as cell configuration, cathode properties used and operational conditions. Electrochemical reduction at the cathode surface by reaction (2) and in much lesser extent disproportion in the bulk by reaction (3) are general parasitic reactions that result in the loss of oxidant or a lowering of current efficiency [14]: 2 2 + 2e − − Η Ο → 2ΟΗ (2) ( ) 2 2 2 2 2H O O 2H O g → + (3) When an undivided cell is utilized, H2O2 is also oxidized to O2 at the anode via hydroperoxyl radical ( •2 HO ) as intermediate by the following reactions [15]: − + + + → • e H HO O H 2 2 2 (4) − + + + → • e H O HO 2(g) 2 (5) The EF treatment of aqueous solutions of pesticides involves the continuous generation of H2O2 from O2 directly injected as pure gas or compressed air, which is efficiently reduced at carbon felt [1,16-31], reticulated vitreous carbon (RVC) [32-34] and carbonpolytetrafluoroethylene (PTFE) gas diffusion [35-47] cathodes via reaction (1). The technique becomes operative if the acidic contaminated solutions contain a small catalytic quantity of Fe that reacts with electrogenerated H2O2 to form Fe 3+ and OH according to the classical Fenton’s reaction [48]: − + + + + → + • OH OH Fe O H Fe 3 2 2 2 (6) The optimum pH for reaction (6) is 2.8, where it can be propagated by the catalytic behaviour of the Fe/Fe pair [48,51]. Table 1 collects the main reactions related to the Fenton system, along with their corresponding absolute second-order rate constant (k2) Oturan et al.: Review: Pesticides Degradation by EF 237 Table 1. Absolute second-order rate constant for the main general reactions involved in a Fenton system at pH ca. 3 [49] Reaction k2 (M -1 s) Reaction No. Initiation − + + + + → + • OH OH Fe Fe O H 3 2 2 2 55 (6) Catalysis: Fe(II) regeneration + + + + + → + • OH HO Fe Fe O H 2 2 3 2 2 (7) + + + + + → + • H O Fe HO Fe 2 2 2 3 2.0 × 10 (8) 2 2 2 3 O Fe O Fe + → + + − + • 5.0 × 10 (9) 2 2 2 2 2 3 O 2H Fe O H 2 O Fe + → + + + − + • 1.0 × 10 (10)