UV light assisted degradation of acid orange azo dye by ZVI-ZnS and effluent toxicity effects

Azo dyes, the most common synthetic dyes used in the textile industry, are known xenobiotic compounds and recalcitrant to conventional degradation treatments. As consequence, such contaminants are often discharged into the effluents, treating aquatic ecosystems. Among several processes, the use of zero valent iron (ZVI) represents a suitable alternative to degrade organic molecules containing azo bonds. However, its applications are limited by corrosion and loss of reactivity over the time. To overcome these constraints, ZVI has been coupled to a suitable semiconductor (ZnS) to get a catalytic composite (ZVI-ZnS) active under UV light. The present work deals with the degradation of acid orange (AO7), used as model azo dye, by UV/ZVI-ZnS, as one step treatment and in combination with an adsorption process by biochar. The influence of ZVI-ZnS concentration (0.25, 0.5, 1 and 2 g/L) and reaction time (0-160 min) on degradation of AO7 were investigated. Intermediates formation was monitored by ESI-FT-ICR-MS analysis and the effluent toxicity was assessed by using Artemia franciscana. The experimental results showed that the UV/ZVI-ZnS process at 1 g/L of catalyst allowed to achieve a removal of AO7 up to 97% after 10 min. An increase of the dye relative concentrations as well as the toxicity related to intermediates formations has been observed for treatment time higher than 10 min. The total removal of AO7 together with effluent toxicity reduction was obtained only after the combined treatment (UV/ZVI-ZnS + biochar).