Study On The Influence Mechanism Of Magnetite On Nitrate Reduction Process In River-bank Infiltration Zone

The riparian infiltration zone is an active area for nitrate reduction and also one of the areas where different forms of iron and nitrogen coexist.The main processes of nitrate reduction in this zone are denitrification and nitrate allochthonous reduction to ammonium(DNRA).In a relatively closed sediment environment,the competing effects of these two nitrate reductions determine nitrogen removal and retention.During nitrate bioreduction,the presence of magnetite also influences the nitrate reduction pathway and reduction kinetics through a variety of actions and processes.However,the current study has not fully revealed the mechanism of magnetite influence on nitrate reduction process in the riparian infiltration zone.In this paper,the effect of magnetite on the nitrate reduction process was investigated by batch indoor static and dynamic experiments using the Shenyang Huangjia water source as the study area.The results of the study showed that:The effect of magnetite on the kinetic pathways and processes of nitrate reduction: magnetite played a role in inhibiting and then promoting the DNRA process.The contribution of DNRA reaction was increased by magnetite,and the contribution of DNRA reaction was increased by 2% for the reaction system with magnetite added under the condition of sufficient carbon source.Magnetite improved the nitrate reduction,which increased the nitrate reduction rate by 1.48%.The rate constant of the nitrate reduction kinetics of the reaction system with magnetite addition increased by 0.034 and the DNRA rate constant increased by 0.00023.Effect of magnetite on microbial communities during nitrate reduction: in static experiments: the relative abundance of Sphingomonas,which is mainly present in high carbon and low nitrogen environments,increased under conditions of sufficient carbon source.In the absence of a carbon source but in the presence of magnetite,the relative abundance of the iron-oxidizing and nitrate-reducing bacterium Flavobacterium increased.In dynamic experiments: the relative abundance of nitrate reducing functional bacteria such as Aquabacterium and Acinetobacter was significantly increased in the simulated column with magnetite addition.With the increase of the hydraulic gradient the microbial community abundance also increased accordingly and the nitrate reduction rate was accelerated.Effect of magnetite on microbial electron transfer activity(ETS)during nitrate reduction: magnetite significantly increased ETS.under sufficient carbon source,the ETS in the reaction system with magnetite was about twice as high as that in the system without magnetite.The effect of hydraulic retention time on the ability of magnetite to regulate the nitrate reduction process: the longer the hydraulic retention time,the more oxygen is consumed by aerobic microorganisms,the closer the reaction system is to the reduced state,resulting in a reduction in the diversity of the microbial community in the reaction system,which slows down the reduction of nitrate.