Cr(VI) formation during ozonation of Cr-containing materials in aqueous suspension – implications for water treatment

Ozonation, or advanced oxidation processes (utilising ozone decomposition products as oxidants) are widely used in industrial wastewater and drinking water treatment plants. In these applications the use of ozone is based on ozone and its decomposition by-products being strong oxidants. In this paper, the possible oxidation of non-Cr(VI) Cr-containing materials suspended in water during ozonation, is presented. This study is of particular interest within the South African context, considering that South Africa holds the majority of global chromium ore resources and has the largest Cr-related industry in the world. Chromium also occurs commonly in other industrial waste materials and is a naturally-occurring element in the crust of the earth. Results indicated that in situ formation of Cr(VI) is possible during aqueous ozonation. pH had a significant influence, since the decomposition products of aqueous O3, i.e. hydroxyl radicals formed at higher pH levels, were found to be predominantly responsible for Cr(VI) formation. Increased ozonation contact time, water temperature and solid loading also resulted in higher Cr(VI) concentrations being formed.