Homogeneous photocatalytic degradation of azo dye Reactive Black 5 using Fe(III) ions under visible light.

Efficient and cost-effective method to destroy complex dyes is warranted to combat increasing water pollution. In the present study, homogeneous photocatalytic oxidation (PCO) of Reactive Black 5 (RB5) dye was studied using ferric ions (Fe(III)) under visible light (VL) irradiation and sunlight (SL). In the presence of 5 mM ferric ions and at pH 2.6, more than 80% of initial 20 mg/L RB5 was decolourized in 60 min under artificial VL. Decolourization followed pseudo first-order kinetics with the reaction rate constant 0.0356 min-1. 79% of initial COD was removed at the end of 60 min, suggesting mineralization of RB5 as the main cause of decolourization. Using similar experimental conditions under SL, more than 90% RB5 was decolourized in 15 min with an almost 10-fold increase in the reaction rate constant (0.34 min-1). Rate and extent of RB5 destruction significantly decreased in the presence of •OH scavenger indicating photoreduction of Fe-hydroxo species and generation of •OH as the main mechanism of RB5 degradation. RB5 removal increased from ca. 30% to 84% with the increase in Fe(III) concentration from 0.5 to 5 mM. The corresponding 1st-order rate constants increased linearly from 0.006 to 0.036 min-1. RB5 degradation decreased linearly (R2 = 0.98) from 91.7% to 63.3% with the increase in initial RB5 concentration from 10 to 40 mg/L. Fe(III) induced homogenous PCO appears to be a reliable and low-cost method of advanced oxidation without the need for costly reagent such as H2O2.