Optimization of electrochemical oxidation for the degradation of acid yellow 99 via sulfate and hydroxyl radicals activation: A study using Plackett–Burman and Box–Behnken designs

Abstract

The synthetic azo dye acid yellow 99 (AY99) is a nonbiodegradable and carcinogenic pollutant widely used in dyeing paper, leather, textiles, protein fibers, and other industries. The degradation of AY99 in an aqueous solution by the electrochemical oxidation process EOP over a platinum grid anode and graphite cathode was studied. The results indicated that the degradation of AY 99 by the EOP process followed second-order reaction kinetics (R² > 0.98). To optimize a range of operating parameters, such as pH (1–4), temperature (20–40 °C), electrolyte concentration (50–500 mM), supporting electrolyte species (Na₂SO₄, NaCl), applied current (60–500 mA), initial concentration of AY99 (0.05–0.2 mM), Plackett–Burman design and Box–Behnken statistical experimental design (BBD) were applied. The proposed quadratic model was validated by a high adjusted correlation coefficient (R²_adj = 0.987) and a high correlation coefficient (R² = 0.994). The order of the main factors affecting the degradation efficiency was as follows: pH > current intensity > [Na₂SO₄] > temperature. On the basis of the optimization results, a maximum removal efficiency > 98 % was obtained under the following conditions: pH = 2.9, temperature = 39.5 °C, [Na₂SO₄] = 500 mM, and current intensity = 77.77 mA. Comparative mineralization processes with different cathode materials (carbon felt and graphite) and various electrolyte concentrations were also studied under optimum conditions. This study demonstrated that complete removal of AY 99 could be achieved through this electrochemical oxidation (Pt anode/graphite cathode/(Na₂SO₄ or K₂S₂O₈) electrolytes) by sulfate (SO₄⁻) and hydroxyl (OH) radicals generated during the process.