Kinetic of the degradation of sulfanilic acid azochromotrop (SPADNS) by Fenton process coupled with ultrasonic irradiation or L-cysteine acceleration

The degradation kinetics of sulfanilic acid azochromotrop (SPADNS) in aqueous phase by Fenton process has been examined. The effect of pH, hydrogen peroxide [H₂O₂], ferrous ion [Fe²⁺], molar ratio of [H₂O₂]/[Fe^2＋] and SPADNS concentration [SPADNS] have been estimated systematically. The optimum reaction conditions were found to be pH 3.5, [H₂O₂]=1.7 10⁻³ M, [Fe^2＋] =1.010⁻⁴M for [SPADNS] 1.010⁻⁴M. Under these situations, 100% decolorization efficiency of aqueous SPAND solution was accomplished after 60 min of reaction. Also, the influence diverse inorganic anions (such as Cl, NO₃, HCO₃, etc. present in contaminated water) on the degradation efficiency of Fenton oxidation were investigated. A combination of ultrasonic irradiation and Fenton process (US/Fenton) has been used to evaluate the effect of azo bond loading factor (L_{azo bond;}0.25 to 1) on SPANDS degradation kinetic rates. The results show that the oxidation of Fenton could be enhanced by ultrasonic irradiation. For example, the color removal for (L_{azo bond;} 0.25, using Fenton and US-Fenton processes were 84.6% and 92.7% after an hour of oxidation reaction, respectively. While in the same period of time, there was no significant increment for TOC removal of chemical oxygen demand (COD) loading factor 0.25, using Fenton and US-Fenton processes, which were 44.3% and 46.0%, respectively. This may be attributed to the formation of molecules with high stability against hydroxyl free radical. Also, this study found there was no significant effect on SPANDS degradation with the introduction of L-cysteine (Cys) to Fenton system. Three kinetic models (first-order, the second-order, and Behnajady–Modirshahla–Ghanbary) were evaluated and the Behnajady–Modirshahla–Ghanbary kinetic model was found to be the best model representing the experimental kinetic data of SPADNS.