Electrochemical Oxidation of Recalcitrant Organic Compounds using Electro-Generated Sulfate-based Oxidizing Agents

Abstract

Sulfate ions have often been used as background electrolytes in electrochemical degradation of contaminants, and have generally been considered inert even when employing high oxidation power anodes such as boron-doped diamond. Boron-doped diamond (BDD) anodes are widely used in literature for investigating electrochemical oxidation of contaminants due to their excellent capability to form hydroxyl radicals as well as inorganic radicals such as chlorine, phosphate, and sulfate radicals, electrogenerated when their corresponding anions are present in the influent water. This study examined the role of sulfate ions and the potential electro- generation of sulfate-based oxidants by comparing electrooxidation rates for persistent organic contaminants at BDD anodes in the presence of sulfate or nitrate anolytes (considered as inert). The effect of the operating conditions governing this process such as anolyte concentration / conductivity, applied current density, as well as solution volume were explored in this work. The characteristics and the reactive life-time of these electro-chemically generated sulfate based oxidizing agents were further explored by studying the oxidation extent occurring after the electric current has been switched off or via different modes of intermittent electric current as an option to improve energy efficiency. The effect of chloride ions on the sulfate-based electrooxidation processes for the removal of a natural organic matter surrogate, resorcinol, was also addressed in this work because the anodic oxidation of chloride yields oxidizing species such as chlorine / hypochlorous acid as well as chlorine/chloride radicals, and may compete with sulfate-based oxidizing species, leading to the formation of toxic organic and inorganic chlorinated compounds at higher applied potentials.The detailed investigations showed that sulfate yielded 10 – 15 times higher electrooxidation rates of all target contaminants compared to the rates achieved with a nitrate anolyte at identical conductivities. The presence of specific radical quenchers (tert-butanol, methanol) reported a similar effect on diatrizoate electrooxidation rates, illustrating that hydroxyl radicals (•OH) are most likely influencing the potential anodic formation of sulfate-based oxidizing species. Thus, these results indicate the formation of strong sulfate-derived oxidant species at BDD anodes, where the energy consumption required for a 10-fold removal of diatrizoate was reduced from 45.6 to 2.44 kWh m-3 by switching from nitrate to sulfate anolyte. This electrochemical activation of sulfate was also observed at low concentrations, relevant for many wastewaters, where even only 1.56 mM sulfate anolyte solution (3.5 mS cm-1 and 100 A m-2) achieved a higher removal rate than 60 mM nitrate with correspondingly higher conductivity (9.0 mS cm-1) and operated at double the current density (200 A m-2). Chloride addition to Na2SO4 inhibited theresorcinol oxidation and mineralization, unlike its addition to NaNO3 where it enhanced the oxidation ofresorcinol. This inhibitive effect of chloride on sulfate was observed to further decrease the removal rate constants of resorcinol oxidation and mineralization with increasing the chloride concentration from 5 to 40 mM. However, the presence of sulfate is reported to inhibit the electrogeneration of chlorinated organics by ~2 – 4 folds compared to nitrate, especially at low chloride concentrations (i.e., [SO42-]:[Cl−]>1). Although increasing the pH from 2 to 7 reduced the amount of chlorinated organics formed by about half, the sulfate-based electrolysis increased the generation of undesired chlorate and perchlorate at both acidic and neutral pH.Overall findings indicate the formation of strong sulfate-derived oxidant species at BDD anodes when polarized at high potentials, even when operating at low sulfate concentrations such as 150 mg L-1, detected in natural water systems. This may have positive implications in electrooxidation of wastewaters containing sulfate because of the substantial energy savings that such systems could achieve, not only attributed to the formation of much stronger oxidants but also due to their extended reactive life-time.