Uv-l/sulfite advanced reduction process as an efficient method for chlorate reduction in water

Abstract

The efficient reduction of disinfection by-products (DBPs) is currently a major challenge in water treatment facilities due to their high toxicity to human health and long-time residence in the environment. In this work, an advanced reduction process (ARP) that combines ultraviolet light irradiation using a low-pressure mercury lamp and sulfite reducing agent (UV-L/sulfite) was investigated for the reduction of chlorate, as a model of inorganic chlorination DBPs. The UV-L/sulfite ARP was much more effective than single processes such as sulfite alone or UV irradiation alone. The higher efficiency of UV-L/sulfite is due to the formation of free reducing radical species (sulfite anion radical (SO3 •–), hydrated electron (e– aq), and hydrogen atom (H•)). Effects of sulfite dose, initial pH, initial chlorate concentration, and UV-L light dose on the rate and extent of chlorate reduction were evaluated. Chlorate reduction was notable only in an acidic pH environment and its reduction rate was very slow at neutral and basic pH values. Increasing sulfite dose up to 180 mg/L led to increasing the efficiency and accelerated the rate of chlorate reduction. Increasing UV-light intensity up to 670 µW/cm2 (10 mg/L chlorate and 180 mg/L sulfite) improved the efficiency and accelerated the chlorate reduction rate by producing large amounts of free-reducing radicals. The major reaction product from chlorate reduction by UV-L/sulfite ARP was chloride with the formation of free chlorine as an intermediate and no chlorite was detected. The presence of SO3 •–, e– aq, and H• scavengers affected the efficiency of UV-L/sulfite ARP. A decrease of chlorate reduction yield was observed in presence of dissolved oxygen, nitrite, or nitrate confirming the contribution of SO3 •–, e– aq, and H• in the mechanism of chlorate reduction. However, the presence of Fe3+ enhanced chlorate reduction yield due to the formation of additional SO3 •– radicals from the photodecomposition of Fe(III)-HSO3 – complexes. The mechanism of chlorate reduction into chloride involves oxygen atom-abstraction and consecutive reactions with the formation of chlorite, hypochlorite as intermediates. These results point to conclude that UV-L/sulfite ARP has a great potential to be applied in water treatment to control the formation of undesirable DBPs