Treatment Of Organic Pollutants In Aqueous Solution By Eletro-Fenton And Electro-Fenton-Like Processes

In recent years, more and more refractory and toxic organic contaminants are detected in wastewater, surface water and ground water. Many of these organic polluants can hardly be degraded by conventional water treatments. Advanced oxidation processes (AOPs), which based on the generation of hydroxyl adicals (OH), have been applied to treat various non-biodegrable organic compounds in water. Conventional Fenton technology is a promising AOPs for the treatment of organic contaminants in water. In Fenton Process, hydroxyl radicals, which are a kind of strong oxidant, is formed through Fenton's reaction and then degrade organic pollutants. Similar to conventional Fenton process, transition metal ions (Fe2+, Co2+, Ag+, etc.) can also activate persulfate (PS) and generate sulfate radicals (SO4·-). Sulfate radical is a powerful oxidant and can oxidize most of organic pollutants. This process is named Fenton-like process. There are some disadvantages existed in conventional Fenton and Fenton-like process. For example, a high concentration of Fe2+ is required and a large amount of iron sludge is generated. In order to solve these problems, electro-Fenton and sulfate radical-based electro-Fenton-like processes are employed. Fe2+ can be regenerated via cathodic reduction in electro-Fenton and electro-Fenton-like processes. Therefore, the Fe2+ concentration used in these processes is much lower than that in Fenton and Fenton-like processes. In this paper, electro-Fenton and sulfate radical-based electro-Fenton-like processes were used to degrade artificial sweeteners and azo dye. The removal efficiency, the oxidation mechanism, degradation pathway and toxicity evolution of target pollutants were investigated.(1) A detailed discussion on the oxidative degradation of artificial sweeteners aspartame (ASP) in acidic aqueous solution containing catalytic amount of Fe2+ by using eletro-Fenton process is reported. In elecro-Fenton process, ASP could be completely removed in a 30 min reaction and the removal of ASP followed pseudo-first-order kinetics. The increase of Fe2+ concentration and applied current to certain extent could increase the removal efficiency of ASP, while further increasing the Fe2+ concentration and applied current could lead to the decrease of removal efficiency. Absolute rate constant of hydroxylation reaction of ASP was determined as (5.23±0.02)×109 M-1 S-1. When boron-doped diamond (BDD) was used as anode, ASP could be completely mineralized in a 360 min reaction. Short-chain aliphatic acids such as oxalic, oxamic and maleic acid were identified as aliphatic intermediates in the electro-Fenton process. The bacteria luminescence inhibition showed the toxicity of ASP solution increased at the beginning of electrolysis, and then it declined until lower than the untreated ASP solution at the end of the reaction.(2) The removal of artificial sweeteners saccharin (SAC) in aqueous solution by electro-Fenton processes was performed. Experiments were carried out in an undivided cylindrical glass cell with a carbon-felt cathode and a DSA, Pt or boron-doped diamond (BDD) anode. The removal of SAC by electrochemically generated hydroxyl radicals followed pseudo-first order kinetics with all the anodes. The absolute rate constant of the SAC hydroxylation reaction was (1.85 ± 0.01) x 109 M-1 s-1, which was determined by using the competition kinetic method. The comparative study of TOC removal efficiency during electro-Fenton treatment indicated a higher mineralization rate with BDD than Pt anode. Oxalic, formic, and maleic acid were observed during electro-Fenton process. The evolution of toxicity of SAC and/or its reaction byproducts based on the V. fischeri bacteria luminescence inhibition was studied.(3) Orange II was degraded in a divided electrolytic cell in which a salt bridge was used to connect the anode and cathode. Hydrogen peroxide and ferrous ion are electrogenerated at the carbon-felt (CF) cathode. Cathode reduction contributes to the decolorization in cathodic compartment. The decay of COD and TOC was attributed to hydroxyl radicals produced by Fenton's reaction in cathodic compartment. Anodic oxidation of Orange II was much less pronounced than cathodic Fenton's reaction. The effects of some important reaction parameters in cathodic compartment indicated that the optimal conditions for Orange II degradation were current density 1.78 mA/cm2, initial pH 3.0 and Fe3+ concentration 0.2 mM. The intermediate products were determined by GC-MS analysis and the plausible degradation pathway in cathodic compartment was proposed. Toxicity test with Daphnia magna showed that the acute toxicity of the solution increased during the first stage of the reaction, and then gradually declined with the progress of cathodic Fenton's reaction.(4) The removal of Orange II by an electro/a-FeOOH/peroxydisulfate process is reported in this study. In electro/a-FeOOH/peroxydisulfate process, sulfate radicals were generated by activating peroxydisulfate (PDS) with goethite (a-FeOOH). When combined with electrochemical process, the Fe(III) on the surface of a-FeOOH converts to Fe(II) by cathodic reduction. The effect of initial pH on the decolorization of Orange II was investigated. Response surface methodology (RSM) based on Box-Behnken statistical experiment design (BBD) was applied to analyze the experimental variables. The positive and negative effects on the decolorization of Orange II were determined. The response surface methodology models were derived based on the results of the pseudo-first-order decolorization rate constant and the response surface plots were developed accordingly. The results indicated the applied current showed a positive effect on the decolorization rate constant of Orange II. The interaction of a-FeOOH dosage and PDS concentration was significant. The ANOVA results confirmed that the proposed models were accurate and reiable for the analysis of the varibles of EC/a-FeOOH/PDS process.(5) The decolorization of Orange II in aqueous solution by Fe3O4 activated peroxydisulfate (PDS) oxidation in an electrochemical reactor (EC/Fe3O4/PDS process) was performed in this study. Various parameters were investigated to optimize the process, including initial pH, current density, PDS concentration and Fe3O4 dosage. The stability of Fe3O4 particles was observed by recycle experiments. The X-ray photoelectron spectroscopy (XPS) was applied to investigate the surface properties of Fe3O4 before and after reaction. GC-MS analysis was employed to identify the intermediate products and a plausible degradation pathway of Orange II was proposed. The change of acute toxicity during the treatment was investigated by activated sludge inhibition test. The TOC removal efficiency was 30.0% in a 90 min reaction.