Nitrate removal from water using UV-M/S2O4 2− advanced reduction process

Abstract

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/S 2 O 4 2-process achieved almost complete removal of nitrate from aqueous solutions containing 25 mg NO 3 -/L using stoichiometric dose of dithionite of 68.8 mg/L at neutral pH conditions. Analysis of final products and material balance confirmed that NO 3 -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/S 2 O 4 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.

Destruction of Per- and Polyfluoroalkyl Substances (PFAS) with Advanced Reduction Processes (ARPs): A Critical Review

“…31 Although the term ARPs for water and wastewater treatment has been very recently adopted, the investigations on reactions of these reducing radicals with different chemicals in water began several decades ago. 32−36 For environmental applications, ARPs have proven very effective for the removal of various oxidized contaminants, such as vinyl chloride, 37,38 perchlorate, 39 bromate, 40,41 nitrate, 42 chromium(VI), 43,44 and 2,4,6-trichlorophenol. 45 Special attention has been recently directed to ARPs due to their capability of effectively destructing different PFAS chemicals in water, 31,46−59 advantageous over hydroxyl radical ( • OH)-or sulfate radical (SO 4…”

“…The reducing radicals involved typically include e aq – and H • , in addition to others such as sulfite radical anions (SO 3 •– ) and sulfur dioxide radical anions (SO 2 •– ), depending on the used activation methods and chemical solutes . Although the term ARPs for water and wastewater treatment has been very recently adopted, the investigations on reactions of these reducing radicals with different chemicals in water began several decades ago. − For environmental applications, ARPs have proven very effective for the removal of various oxidized contaminants, such as vinyl chloride, , perchlorate, bromate, , nitrate, chromium­(VI), , and 2,4,6-trichlorophenol …”

New Insights into the Mechanism of the UV/Sulfite Process: Formation of SO2•– Radicals and Their Derivatives under Acidic Conditions

“…Consequently, H• and e aq – can be efficiently consumed by dissolved oxygen, organic matter, NO 2 – , and NO 3 – , etc., hindering their pollutant removal effectiveness. , In contrast, the SO 2 •– radicals are a moderately reductive species with a standard reduction potential of −0.66 V . They are less vulnerable to interference from oxidative substances, which have been proven to be efficient in reducing pollutants such as Cr­(VI), ciprofloxacin, 1,2-dichloroethane, , vinyl chloride, nitrate, Se­(IV) and Se­(VI), , dyes, chlorate, etc. Currently, the prevalent method for producing SO 2 •– radicals is the UV/dithionite process, which has drawbacks such as the rapid decomposition of dithionite under acidic conditions, leading to a reduced efficiency in SO 2 •– production.…”

Reductive Degradation of N-Nitrosodimethylamine via UV/Sulfite Advanced Reduction Process: Efficiency, Influencing Factors and Mechanism

“…It has been proven that hydrated electrons (e aq − ) and hydrogen atoms (•H) are the main activate products formed during the ARPs, while other radicals such as sulfite radical anions (•SO 3 − ) and sulfur dioxide radical anions (•SO 2 − ) can also be generated, depending on the various activation methods (photolysis, radiolysis and sonolysis) and reductants (sulfite, dithionite and iodide). ARPs have attracted widespread attention in a short period of time due to their excellent performance on degrading various contaminants such as vinyl chloride, perchlorate, bromate, nitrate, 2,4,6-trichlorophenol and polyfluoroalkyl substances (PFAS) [15][16][17][18].…”