Photocatalytic removal of Hg from solid wastes of chlor-alkali plant

The removal of mercury from hazardous wastes of a chlor-alkali plant was investigated by a combined process of acid attack and photoreduction treatment. The solution obtained by acid attack of the solid wastes was treated with ultraviolet irradiation in the presence of titanium dioxide and an organic agent (citric acid), leading to the selective precipitation of reduced mercury, while the other metal compounds remained in the solution. In spite of the large excess of non-reducible metal ions (100 times or more on a molar basis), more than 99% of the mercury initially present in the solid wastes was removed. Competitive adsorption of the different metal compounds on the titanium dioxide surface led to lower rates of mercury reduction. This combined process can be an efficient alternative method for mercury separation because the final effluents have reached a quality close to that of the standards imposed by international environmental agencies.