Experimental Study On Removal Of Nitrate From Groundwater By Coupling System Of Hydrogenotrophic Denitrification And Nano-iron Reduction

With the unorganized emissions of nitrogenous industrial wastewater and domestic sewage,as well as the continuous application of agricultural nitrogen fertilizer,nitrate pollution in groundwater is becoming more and more serious worldwide.The consumption of nitrate-containing groundwater by humans can trigger methemoglobinemia,which poses a threat to the health of the population.The removal methods for groundwater nitrate pollution include physical chemistry,chemical reduction,and biological denitrification.Since the physicochemical method only concentrates or transfers nitrates,it does not remove them from the groundwater environment;the chemical reduction method converts nitrates into ammonia nitrogen,which causes secondary pollution of groundwater;In the biological denitrification method,the dependence of heterotrophic microorganism on organic carbon source limits the application of heterotrophic denitrification in groundwater restoration.while autotrophic microorganisms in groundwater lack sufficient electron donors to reduce nitrates during denitrification.Therefore,any method that physical,chemical or biological methods has certain defects in the treatment of groundwater nitrates.The study found that the combination of chemical reduction and biological denitrification to remove groundwater nitrate can both improve the problem of high ammonia-nitrogen ratio in the individual physical chemical method,and also solve the defect of lack of electron donor in biological denitrification.Based on sodium bicarbonate as the carbon source,and hydrogen as electron donor.A hydrogen autotrophic denitrification bacteria S1 was isolated from anaerobic sludge and identified by morphological observation and phylogenetic analysis of 16S rDNA sequence.The effects of initial nitrate concentration,carbon source dosage,pH,temperature and ion concentration on the denitrification of strain S1 were studied.The results showed that the 16S rDNA sequence of strain S1 had the highest similarity?100%?with Thauera?AMXF01000339.1?.Combined with morphological characteristics,the strain S1 was identified as Thauera.The optimum carbon source dosage of strain S1 for denitrification was 0.5 g/L.The denitrification efficiency of strain S1 was inhibited under the nitrate concentration higher than 100 mg/L.The nitrate reduction activity was inhibited at pH=6.The denitrification efficiency increased continuously at pH>6.The rate of denitrification of strain S1 increased from 10 to30?.The concentration of sulfate ions higher than 90 mg/L was unfavorable for the removal of nitrate nitrogen by strain S1.The sodium oleate-coated nano-iron material was prepared by liquid-phase reduction method and characterized by the SEM,XRD and FTIR.The results showed that sodium oleate was adsorbed on nano-iron by physical action and the coated nano-iron particles were coated.Evenly,less reunion.Through the sedimentation and air stability test,it was found that nano-iron coated with sodium oleate has good dispersibility and oxidation resistance.Through the single factor test to study the influencing factors of groundwater,it was found that the sodium oleate-coated nano-iron reduced the nitrates for almost 8 days and more,which can obviously slow down the rapid reaction of nano-iron and nitrate.The removal rate of 2 g and 3 g of modified nano-iron nitrates is over 70%,but the amount of 2 g nano-iron materials is less;When the temperature was 15°C,the removal rate of nitrate by 2 g of sodium oleate.coated nano iron was about 70%,and the amount of generated ammonia nitrogen is less.The sodium nitrate-coated ferric oxide has higher nitrate removal rate at pH=7 than at pH=9,and the nitrate has a faster reduction rate at pH=5 than at pH=7,but does not provide hydrogen for the microbes.In summary,in this experiment,under the conditions of 2 g of sodium oleate modified nano iron material and pH value of 7,the sodium oleate coated nano iron can better restore simulated groundwater nitrate.The chemical reduction system of nano-iron coated with sodium oleate,the biological denitrification system of hydrogen autotrophic denitrifying bacteria,and the chemical biological denitrification combined with nano iron-hydrogen autotrophic denitrifying bacteria were compared.According to the removal effect of groundwater nitrate,the results show that the removal efficiency of groundwater nitrate can reach100%,and the NH4-N productivity is about 38%,realized by sodium oleate coated nano-iron-hydrogen autotrophic denitrifying bacteria composite system.The nitrogen removal rate reaches 62%.Therefore,the sodium oleate coated nano-hydrogen autotrophic denitrifying bacteria composite system is superior to the hydrogen autotrophic denitrifying bacteria system and the oleic acid coated nano iron single system in the process of removing groundwater nitrate.In the process of nitrate reduction in composite system,the degradation kinetics of nitrate in the denitrification process of the composite system was fitted by different temperatures,different initial pH values and different amounts of sodium oleate coated nano-iron.It was found that the mechanism of nitrate removal in the composite system is mainly divided into two stages:in the first 5 days of the composite system,the reaction time t with Ln?C/C0?at different temperatures,different initial pH values and different dosages of modified nano-iron can be well fitted with quasi-first-order kinetic equations,Ln?C/C₀?reaction time t can be fitted with a quasi first-order kinetic equation.The amount of ammonia nitrogen produced is similar to the amount of nitrate removal.The first five days can be described by the first-order kinetic equation,but the linear correlation?R²?is less than 0.92.When the environmental factors of the composite system did not change,the apparent reaction rate constant at the 5th day of the reaction changed,indicating that the mechanism of nitrate reduction in the composite system changed.The ammonia production was significantly reduced in the last 5 days,indicating that the denitrification mechanism of the composite system at the5th day of the reaction was changed from the chemical reduction of nitrate by the sodium oleate coated nano iron to the biological denitrification of the hydrogen autotrophic denitrifying bacteria.