Degradation of acid red 88 by the combination of sonolysis and photocatalysis

Abstract

Acid red 88 (AR88) is a mono-azo textile dye and a widely used colorant in the textile and food industries. The sonolytic, photocatalytic and sonophotocatalytic degradation of AR88 in the presence of homogeneous (Fe 3+) and heterogeneous (TiO 2) photocatalysts were studied. The effects of initial dye concentration and ultrasound power on the sonochemical degradation were also investigated. The degradation by sonolysis and photocatalysis followed first-order like kinetics with respect to the concentration of AR88. The sonophotocatalytic degradation rates using TiO 2 or Fe 3+ were found to be higher than that observed with sonolysis or photocatalysis. The synergy index calculated for TiO 2 sonophotocatalysis was 1.3, suggesting that the combination of sonolysis and TiO 2 photocatalysis resulted in a small synergetic effect. Also, the sonophotocatalysis in the presence of Fe 3+ was synergistic with a synergy index of 2.3. Total organic carbon (TOC) analysis was performed to study the extent of mineralization. TOC data showed that the mineralization using TiO 2 sonophotocatalysis was additive whereas a synergistic mineralization was observed for the sonophotocatalysis in the presence of Fe 3+. The simultaneous operation of sono-Fenton and photo-Fenton reactions is likely to be the underlying reason for the observed synergy in the presence of Fe 3+ assisted sonophotocatalysis. Electrospray mass spectrometry (ESMS) was employed for the identification of the degradation intermediates. The sonication of AR88 led to the formation of its mono and di-hydroxylated products as the primary intermediates. A possible degradation pathway has been proposed.