Nitrate removal from water using UV-M/S2O42- advanced reduction process

In this work, a new process called advanced reduction process (ARP) was used for nitrate removal from water. This ARP process combines sodium dithionite as reducing agent with ultraviolet irradiation using medium pressure lamps (UV-M) as an activating method. Experimental results showed that UV-M/S₂O₄^2- process achieved almost complete removal of nitrate from aqueous solutions containing 25 mg NO₃^-/L using stoichiometric dose of dithionite of 68.8 mg/L at neutral pH conditions. Analysis of final products and material balance confirmed that NO₃^- ions were reduced to ammonium with formation of nitrite as intermediates in addition to the formation of small amounts of volatile species, mainly ammonia and nitrogen gas. Effects of certain experimental parameters including dithionite dose, initial pH, initial nitrate concentration, and UV light source on the kinetics and efficiency of nitrate reduction were evaluated. Increasing dithionite dose augmented the rate of nitrate reduction and enhanced the efficiency of ARP process. Dithionite doses higher than stoichiometric ratios led to complete removal of nitrate in shorter reaction time. UV-M/S₂O₄^2- process was found to be effective only under neutral pH or alkaline conditions, and its removal efficiency is negligible in acidic medium (pH<4). Irradiation with UV-M was more effective than low pressure or narrow band lamps. These results can be attributed to the contribution of several mechanisms for nitrate reduction to ammonium. These include the following: direct photolysis, chemical reduction of nitrate dithionite, and mediated reduction of nitrate by free reducing radicals.

Keywords
Nitrate reduction; Advanced reduction processes; UV-M irradiation; Dithionite; Reducing radicals