Purification Mechanism Of Nitrogen Removal By LDHs Modified Zeolites Substrates In Vertical-flow Constructed Wetlands

In order to study the purification effect of nitrogen,laboratory-scale unplanted vertical-flow constructed wetland systems were constructed.Original and modified zeolites were used as substrate to treat contaminated water.Three kinds of layered double hydroxides（MgFe-LDHs,MgCo-LDHs and MgAl-LDHs）modified zeolites were synthesized with co-precipitation method under alkaline condition.The analysis of purification effect and mechanism provided a theoretical basis for the research and application of LDHs modified substrates.The purification experiment results showed that:（1）The TN removal rates were33.17%³7.87%,modified zeolites performed better than original zeolites.The NH₄⁺-N purification effect of zeolites can be improved by LDHs modification,the average removal rates were 43.05%⁵9.66%.The organic nitrogen removal rates were 39.35%⁶7.89%,the removal efficiencies were MgAl-LDHs>MgCo-LDHs>MgFe-LDHs>original zeolites.NO₂^--N and NO₃^--N were increased with different degrees in the effluent.（2）Ammonification and nitrification intensity of modified zeolites were higher than that of original zeolites.Denitrification was weak in all zeolites.（3）The mechanisms of nitrogen removal for the modified and original zeolites were adsorption and biological degradation.Physical interception property,chemical adsorption and biological effect can be improved by appropriate LDHs modification in zeolites.Through isothermal adsorption experiment,the NH₄⁺-N,NO₂^--N and NO₃^--N adsorption were studied.Isotherm data were analyzed by Langmuir and Freundlich adsorption equations.The results showed that:（1）The nitrogen adsorption behaviors on original and modified zeolites before and after purification can be described by Langmuir and Freundlich equation.（2）The NH₄⁺-N adsorption capacities of modified and original zeolites were more than 120 mg/kg.And the adsorption efficiency of nitrite and nitrate were weak,the maximum adsorption capacities were5.501⁶.075 mg/kg and 3.077³.386 mg/kg,respectively.LDHs modification can improve the NH₄⁺-N adsorption properties,while there were weak effects on NO₂^--N and NO₃^--N adsorption.（3）After purification experiment,the adsorption capacity of NH₄⁺-N,NO₂^--N and NO₃^--N decreased,indicated that modified and original zeolites adsorbed NH₄⁺-N,NO₂^--N and NO₃^--N during the purification process.To detecting the number of ammonifier,nitrosobacteria,nitrobacter and denitrifier bacteria on the substrates,MPN（most probable number）methods were used.The detection results showed that,LDHs modification had a promoting effect on the growth of these microorganisms.The surfaces of modified and original substrates were covered with a trace amount of biofilm.The numbers of microorganisms had effect on biofilm formation and nitrogen removal effect.