H₂O₂/UV-C and Fe²⁺+/H₂O₂/UV-C treatment of a commercial naphthalene sulphonate (H-acid)

The high solubility, polarity and recalcitrant nature of naphthalene sulphonates render their treatment by conventional physicochemical and biological methods a rather difficult task. The present study aimed at investigating the treatability of a commercially important naphthalene sulphonate, e.g. H-acid, using H₂O₂/UV-C and Fe²⁺/H₂O₂/UV-C photochemical oxidation processes. The H₂O₂/UV-C process was capable of 100% H-acid, 48% COD and 27% TOC removals 180 min under the experimental conditions of 468 mg/l initial COD, 60 mM initial H₂O₂ concentration and an initial pH of 5.9–6.0. A steady increase in the average oxidation state throughout the photochemical reaction revealed that more oxidized organic intermediates were formed during H₂O₂/UV-C treatment of H-acid. The effect of initial H₂O₂ concentration on H₂O₂/UV-C treatment efficiency was examined by conducting experiments at varying initial H₂O₂ concentrations up to 200 mM. An initial H₂O₂ concentration of 150 mM appeared to be the optimum where the highest rate constants and removal efficiencies were obtained in terms of H-acid, COD and TOC abatements. Fe²⁺/H₂O₂/UV-C oxidation of H-acid was appreciably faster than the H₂O₂/UV-C process causing a significant increase in the organic carbon (COD, TOC) removal efficiency when Fe²⁺ was applied as the photocatalyst. For the Photo-Fenton process carried out in the presence of 1.0 mM Fe²⁺, 96% H-acid removal was achieved in only 10 min, while 92% COD and 85% TOC abatements were attained after 60 min photochemical treatment (experimental conditions: initial COD = 463 mg/l; initial H₂O₂ = 30 mM and initial pH = 3.0 ± 0.1). Photo-Fenton degradation kinetics established on the basis of initial H-acid, COD and TOC abatement rates as well as treatment efficiencies followed the decreasing order of Fe²⁺/H₂O₂/UV-C (1.0 mM Fe²⁺) > Fe²⁺/H₂O₂/UV-C (0.2 mM Fe²⁺) > H₂O₂/UV-C.