Photocatalytic removal and recovery of mercury from water using TiO2-modified sewage sludge carbon

An activated carbon was developed from municipal sewage sludge (SS) using ZnCl2 as chemical activation reagent. Combined with TiO2, the carbon was tested for photocatalytic removal and recovery of ionic mercury [Hg(II)] in the form of metallic mercury [Hg(0)] from water. Hg(II) was first photoreduced to Hg(0) which was followed by adsorption on the SS carbon and TiO2 surfaces, then was recovered on a silver trap by means of heating. Combination of the SS carbon with TiO2 and under ultraviolet irradiation could doubled the adsorption capacity of mercury on the SS carbon, and the removal rate was increased to 151 g/kg compared to 87 g/kg for SS carbon only. The optimum amount of TiO2 for the photocatalytic reaction was 5–20% of the weight of SS carbon for photoreducing 10–80 mg/L of Hg(II) solutions. The optimum illumination time was 20 min and longer illumination could not enhance the photochemical reaction. Hg(II) removal from the solution increased with the increase of pH value, and reached a plateau value at the pH range of 5–12, and furthermore, the removal increased linearly with the increase of Hg(II) concentration. Introduction of methanol into the adsorption system could greatly enhance mercury adsorption capacity of the SS carbon, and the optimum methanol addition amount was found to be 5%, which doubled the adsorption capacity. The adsorption isotherm of mercury onto the SS carbon was found to follow Freundlich isotherm model perfectly. The recovery percentage of Hg(0) from water was around 40–65%. Accordingly, it is believed that the method developed in this study is effective and practical in industrial wastewater treatment for Hg(II) disposal.