Study On Efficient Reduction Of Nitrite By Carbon Dioxide Radical Based On Photochemical Process Of Iron Ion And Small Molecular Carboxylic Acid

Nitrite(NO2-)is a common pollutant in water.It is very easy to form carcinogenic compounds.A small amount of NO2-can cause biological poisoning.Among the many methods to solve nitrite pollution,the chemical reduction method using Fe0,formic acid and Fe(Ⅱ)as reducing agents has attracted more and more attention because of its advantages of simple operation and high efficiency,but there is still the shortage that the main product of NO2-reduction is ammonia nitrogen or nitrogen oxide;As a green reducing agent,carbon dioxide radical(CO2·-)can selectively reduce NO2-to nitrogen.How to produce CO2·-efficiently is a key problem to be solved.In view of the photochemical activity of iron ions and the characteristics of small molecular carboxylic acids as CO2·-precursors,this paper proposes a strategy based on photochemical activation of small molecular carboxylic acids by iron ions to produce CO2·-.In this paper,three reduction systems(UV/Fe(Ⅲ)/FA,UV/Fe(Ⅲ)/OA and UV/Fe(Ⅱ)/H3Cit)were constructed with iron ion,UV and formic acid(FA),oxalic acid(OA)and citric acid(H3Cit),respectively,for the selective reduction of NO2-in water.The effects of iron ion valence and concentration,small molecule carboxylic acid concentration and initial pH on nitrite reduction were discussed.The generation mode of free radicals and the reduction mechanism of nitrite were analyzed by means of electron spin resonance detection of free radicals and their quenching experiment,infrared spectrum analysis before and after system reaction and TOC change measurement.The specific research contents and results are as follows:(1)The UV/Fe(Ⅲ)/FA system was used to reduce and remove NO2-in water.The results showed that when the initial concentration of nitrite was 30 mg N·L-1,the concentration of Fe(Ⅲ)was 1 mmol·L-1,the concentration of FA was 16 mmol·L-1,and the initial pH was 3.0(unadjusted),after 60 minutes of reaction,the removal rate of nitrite was 100%,the removal rate of total nitrogen was 98.80%,and the selectivity of product nitrogen was 98.51%.The reduction rate of nitrite increased with the increase of Fe(Ⅲ)concentration and FA concentration,and decreased significantly with the increase of pH.The good photochemical activity of FA and the promotion of Fe(Ⅲ)improved the yield of CO2·-and obtained efficient NO2-removal rate and nearly 100%nitrogen selectivity of the product.(2)The UV/Fe(Ⅲ)/OA system was used to reduce and remove NO2-in water.The results showed that when the initial concentration of nitrite was 30 mg N·L-1,the concentration of Fe(Ⅲ)was 8 mmol·L-1,the concentration of OA was 16 mmol·L-1 and the initial pH was 1.7(unadjusted).After 180 minutes of reaction,the removal rate of nitrite was 100%,the removal rate of total nitrogen was 94.72%,the selectivity of product nitrogen was 94.72%and the mineralization rate of OA was 88.54%.In UV/Fe(Ⅲ)/OA system,there are free radicals such as CO2·-and·Oh,and CO2·-plays a key role in the reduction of NO2-.It is mainly that the efficiency of CO2·-production by OA photoactivation alone is low,but when Fe(Ⅲ)and ultraviolet light act together,the yield of CO2·-and the reduction efficiency of NO2-are greatly improved due to photoinduced electron transfer.(3)The UV/Fe(Ⅱ)/H3Cit system was used to reduce and remove NO2-in water.The results showed that when the initial concentration of nitrite was 30 mg N·L-1,the concentration of Fe(Ⅱ)was 3 mmol·L-1,the concentration of H3cit was 3 mmol·L-1,and the initial pH was 2.2(unadjusted).After 60 minutes of reaction,the removal rate of nitrite was almost 100%,the removal rate of total nitrogen was 87.50%,and the selectivity of product nitrogen was 81.60%.The reduction rate of nitrite increased with the increase of Fe(Ⅱ)concentration and H3cit concentration;The increase of pH is not conducive to the remo val of nitrite and total nitrogen.Fe(Ⅱ)-H3Cit complex can well capture no produced by the reaction of Fe(Ⅱ)and NO2-and NOx produced by the photolysis of NO2-,and then promote the activation of H3cit to generate CO2·-through Fe(Ⅲ)in the system,so as to realize the selective reduction of NO2-.(4)Under the same conditions(the initial concentration of NO2-is 30 mg N·L-1,the concentration of Fe(Ⅱ)is 3 mmol·L-1,the concentrations of FA,OA and H3Cit are all 3 mmol·L-1,and the initial pH is 2.2),after 60 minutes of reaction,the three reduction systems of UV/Fe(Ⅱ)/FA,UV/Fe(Ⅱ)/OA and UV/Fe(Ⅱ)H3Cit nave a certain removal rate of NO2-and nitrogen selectivity of the product.UV/Fe(Ⅱ)/FA system has the lowest removal efficiency of NO2-and total nitrogen;74.87%and 63,86%respectively.UV/Fe(Ⅱ)/OA and UV/Fe(Ⅱ)/H3Cit systems can remove nearly 100%of NO2-,and the removal rates of total nitrogen are 91.20%and 87.50%respectively.The concentrations of NOx in FA,OA and H3Cit systems are 5.70 mg N·L-1,4.46 mg N·L-1 and 1.78 mg N·L1 respectively,which may be because H3Cit system can better complex no,so there is less NOx in the product.Due to the different number of carboxyl groups,reducibility and ionization constants contained in the molecules,the three small molecule carboxylic acids have different complexation with iron ions and the performance of reducing NO2-,resulting in different reduction efficiency of NO2-in UV/Fe(Ⅱ)/NO2-system.In the strategy proposed in this paper,which is based on the photochemical process of iron ions and small molecular carboxylic acids to produce CO2·-efficiently reduce nitrite to nitrogen,iron ions have a wide range of sources,low toxicity and easy recovery;Small molecule carboxylic acids are easy to mineralize in the environment and decompose into carbon dioxide and water without secondary pollution.This makes the reduction process of NO2-efficient and environmentally friendly,has high application potential,and provides a new idea for the reduction and removal of nitrite in water.