Study On Removal Pathway Of Nitrate From Water By Nano-iron-carbon And Its Kinetics

The nano zero valent iron activated carbon-?NZVI/AC?is a kind of strong reducing material which can effectively remediate the nitrate problem in groundwater.However,there are many researches on the removal efficiency of nitrate and the stability of NZVI/AC materials.The reaction pathway and kinetics of nano iron carbon microelectrolysis system have not been systematically studied.Therefore,it is necessary to further analyze the reaction pathway and kinetics of removal of nitrate from nano-zero-valent-iron system,and provide a theoretical basis for the subsequent practical application.In this paper,the microelectrolysis of nitrates was studied by studying the reduction or adsorption of nitrates by NZVI/AC,nano zero valent iron?NZVI?and activated carbon?AC?.The change of nano-iron,the change of nitrogen species,and the interaction of NO₃^-and Fe²⁺ was discussed.The emphasis was on the Langmuir-Hinshelwood model,and the pseudo first order reaction,pseudo second order reaction,n-th reaction,pseudo first order adsorption,pseudo second order adsorption et al are discussed.And the parameters are calculated.Using the method of non-linear regression,the kinetics law of the mathematical model is given to compare the kinetics of different conditions?dosage,nitrate concentration,initial pH and ionic strength?,get the following main conclusions:?1?NZVI/AC removal efficiency is significantly higher than the individual NZVI and AC,and greater than the sum of the two.These phenomenons strongly illustratethat the process of nitrate removal by NZVI/AC composite is not only a combination of AC adsorption and NZVI reduction,but also some synergetic effects exist in such a composite.?2?The effects of loading,ionic strength and the initial pH of the solution on the micro-electrolysis system were discussed.The results showed that the loading was 0.4 g,and the optimum activity was observed.?Cl-and NO₃^-?,only when Cl-had a promoting effect at 50 mg/L,when the 200 mg / L reaction was predominantly inhibited.And the NO₃^-in the NZVI/AC material can effectively remove the whole pH = 3-11,and the reaction process can continue to occur at the initial pH = 11,it was indicating that the nano-iron-carbon material the application can accommodate a wider pH range.?3?NO₃^-was first adsorbed on the surface of the NZVI/AC material during the NZVI/AC removal of the nitrate reaction,and then the surface was redoxed and the rate of adsorption was faster than the surface reaction rate.In the course of the reaction,the H+ was consumed in the solution,at higher pH,NH₄⁺ can form ammonia and then leave the solution with external oscillation or agitation.Therefore,the final product of the reaction is mainly NH₄⁺,the process does not produce N₂.NZVI/AC reduction nitrate process,The NZVI/AC was first oxidized to Fe²⁺,and as the reaction consumes H+ of the solution,the pH increases to produce Fe?OH?₂ precipitation,and finally it is oxidized to Fe₂O₃.?4?The pseudo second order adsorption kinetic model can provide good fitting results for the removal of NO₃^-and the Langmuir-Hinshelwood model for the ammonia generation in the case of NZVI/AC limiting or excess.However,for the commonly used pseudo first order reaction kinetics model cannot get a good fitting result under the condition of iron-limiting no mater NO₃^-removal or ammonia generation.The L-H model was a good way to explain the increase of total nitrogen in the reaction process,and it is proved that the removal of nitrate was a process of first adsorption and then reaction.Therefore,the process of reduction of NO₃^-to NH₄⁺ by NZVI/AC was a heterogeneous catalytic reaction.The reaction rate constant has a good correlation with the initial conditions.In the case of high NZVI/AC dosing,low ionic strength,low initial pH and low initial nitrate concentration,the nitrate removal and ammonium generation were faster than other conditions.The inhibition of the reaction at the high initial pH = 11 was greater than that at the low pH=3.The NZVI/AC surface was not affected by OH-and Fe?OH?₂ precipitate,it was proved that relative to the single NZVI,NZVI/AC micro-electrolysis system has a broader scope of application.