The Simultaneous Removal of Cyanide and Cadmium Ions from Electroplating Wastewater using UV/TIO2 Photocatalysis

Abstract

The simultaneous oxidation of cyanide and reduction of cadmium from electroplating wastewater using UV/TiO2 photocatalysis was investigated in this study. The investigation was performed using a batch-wise 3 L bubble-column photoreactor equipped with a 64-Watt low-pressure ultraviolet (UV) amalgam lamp (20 W at 254 nm). Preliminary experiments were conducted to identify the optimum aeration rate for ensuring the mixing of the catalyst and the wastewater. More specifically, this study focused on the two main factors that influence the effectiveness of oxidative and reductive processes, namely the TiO2 concentration (0.5–2 g/L) and the solution’s pH (11–13), at cyanide and cadmium ion concentrations of 50 and 100 ppm, respectively. A sample was taken every 30 minutes for 3 hours, and the cyanide and cadmium ion concentrations were determined using an ion-selective electrode and atomic absorption spectroscopy (AAS), respectively. It was determined that 3 L/min aeration was optimum, resulting in the removal of approximately 80% of the pollutants. A further increase in the aeration rate resulted in a decrease in the %removal rate due to competition between the oxygen and cadmium ions in terms of reacting with the electrons produced by the photocatalyst. An increase in the pH resulted in an increase in both the removal rate and the kinetics due to the high availability of the hydroxide ions needed to form the radical hydroxide that effectively oxidized the cyanide ions. It was observed that an increase in the TiO2 concentration increased both the removal rate and the kinetics until the optimum point, after which the performance of the photocatalyst decreased due to the shielding effect of the UV resulting from the excessive level of TiO2 present in the mixture. Within the experimental range, the best (most effective) condition was chosen based on the pseudo first-order rate constants. The best condition for cyanide oxidation was identified at pH 13 and 1 g/L TiO2 with kCN- 0.033 min-1, while the reduction of cadmium was found to be optimum at pH 13 and 2 g/L TiO2 with kCd2+ 0.039 min-1.