Application Research On Ultraviolet Activated Formate For Nitrate Reduction In Groundwater

Nitrate,the most stable form of nitrogen,is easy to accumulate and cause significant negative impact on the ecological environment and public health.Nitrate in groundwater exceeds the standard seriously in China,which endangers groundwater and drinking water quality.Traditional nitrate removal methods rarely take into account the removal ratio,rate and gas selectivity,whereas Advanced Reduction Processes（ARPs）can achieve efficient,rapid and convenient pollutant degradation by activating the reduction substrate to generate reducing free radicals,which has great potential in nitrate reduction.Among reducing radicals,carbon dioxide free radical(CO₂^·-)generated from activated organic matters will be oxidized to CO₂ and emitted into the atmosphere without introducing new pollution,making it suitable for groundwater remediation.In order to meet the treatment needs of groundwater pollution,two kinds of groundwater nitrate reduction systems with UV-activated formate were constructed:basic UV/FM system and enhanced UV/H₂O₂/FM system.The reaction conditions,kinetic characteristics,reaction and product regulation mechanism of the two systems were studied,and the feasibility of engineering application of UV-activated formate to reduce groundwater nitrate was analyzed.The main conclusions are as follows:1.UV/FM advanced reduction system could produce CO₂^·-to effectively reduce NO₃^--N;For C₀（NO₃^--N）=50 mg/L and C₀（HCOO^-）=10 mmol/L,the reduction ratio and gas conversion ratio reached 98.6%and 95.4%respectively after UV illumination of 120 min.The reduction efficiency of UV/FM system was less affected by reaction conditions:formate cations and common inorganic anions in groundwater had negligible effects on NO₃^--N reduction,while HCOO^-initial concentration,initial p H value,energy input intensity,the presence of Fe²⁺and high concentration of humic acid would affect the reduction efficiency of NO₃^--N.The enhanced UV/H₂O₂/FM system could improve the reduction efficiency of NO₃^--N by coupling advanced reduction with advanced oxidation.Under the optimal ratio of C₀（H₂O₂）=10 mmol/L and C₀（HCOO^-）=20 mmol/L,NO₃^--N of 50 mg/L could be completely reduced in 120 min,and gas conversion ratio increased to 97.3%.Except for energy input of mercury lamp,UV/H₂O₂/FM system was not sensitive to the change of most reaction conditions.2.The reduction process of NO₃^--N in UV/FM system and UV/H₂O₂/FM system could be divided into two stages in kinetics.NO₂^--N generation stage was zero-order reaction,and NO₂^--N removal stage was first-order reaction.The kinetic fitting results showed that the energy input had the greatest influence on the reaction rate,followed by the initial concentration of FM and H₂O₂,and the initial p H value had the least influence.3.At the optimum concentration,NO₃^--N reduction in both systems was accompanied by NO₂^--N and NH₄⁺-N generation,but secondary pollution would not occur in the end.NO₂^--N generated in the reaction process was removed by CO₂^·-,and HCOO^-concentration determined the formation and final concentration of NO₂^--N.NH₄⁺-N was basically not generated under moderate HCOO^-.Otherwise,excessive CO₂^·-recombination into C₂O₄^2-would transform NO₃^--N into NH₄⁺-N.The introduction of H₂O₂ could consume HCOO^-,hinting CO₂^·-self-clearing,and thus inhibiting NH₄⁺-N generation.H₂O₂ also changed the effect of initial p H value on NO₃^--N reduction.Under the optimal concentration ratio,the reduction efficiency of NO₃^--N in UV/H₂O₂/FM system was better than that in UV/FM system because of the regulation of H₂O₂ on nitrogen products.The REDOX potential of the two systems decreased first and then increased slowly with final reductive states.4.The cost of UV-activated HCOO^-for NO₃^--N reduction mainly came from power consumption(EE/O_UV).In other words,the economic benefit was mainly determined by the energy input intensity.Under the same reaction conditions and NO₃^--N reduction ratio,UV/H₂O₂/FM system had lower energy consumption and better economic benefit.Under the optimal concentration ratio of each system,the TOC residual concentration exceeded the limit of TOC concentration stipulated in China’s drinking water standards at the end of reaction.Therefore,subsequent oxidation treatment should be combined to ensure the safety of the final effluent for avoiding carbon secondary pollution.In engineering application,expanding reaction scale would not affect reduction effect.UV-activated formate systems for nitrate reduction provide a promising reference for the nitrate-polluted groundwater treatment application.