Efficiency Of Constructed Wetlands For Treating High Ammonia-nitrogen-containing Groundwater In Rare Earth Mines

Rare earth mine wastewater has high ammonia nitrogen content and strong acidity,which seriously harms the groundwater environment in mining areas and surrounding areas.The use of constructed wetlands can realize ecological sewage treatment.In this paper,zeolite,volcanic rock,and polyurethane foam（sponge ball）are used as substrates to construct three groups of small-scale constructed wetland systems to study the treatment effect of constructed wetland for rare earth mine wastewater.Through field sampling,laboratory experiments,microscopic characterization,and high-throughput sequencing,the adsorption capacity of substrates to nitrogen,characteristics of microbial population structure in wetlands,nitrogen removal efficiency of rare earth mine wastewater by constructed wetland system with different operation modes,which provides the industrial application materials,technological parameters and theoretical basis for the treatment of groundwater containing ammonia nitrogen in rare earth mine by constructed wetland.The main conclusions are as follows:（1）Si and Ca are the common main components of three kinds of matrices by XRF analysis;The theoretical adsorption capacity of ammonia nitrogen on zeolite,volcanic rock and polyurethane foam is different.The adsorption kinetics data of zeolite and volcanic rock matrix show that the theoretical adsorption capacity of NH₄⁺-N is 1557.45 mg/kg and132.178 mg/kg,respectively.And polyurethane foam has no adsorption capacity for NH₄⁺-N.Quasi-second order reaction kinetics can better describe the adsorption characteristics of the matrix for NH₄⁺-N.（2）When the hydraulic retention time（HRT）is 2 d intermittent flow operation,the temporal characteristics of nitrogen form transformation in wetland system showed that the rate of NH₄⁺-N conversion to NO₃^--N was the fastest after 1 h of water infusion.After 12 h,the removal rate of NH₄⁺-N in wetland system basically reached stabilityand.The spatial characteristics of nitrogen form transformation in wetland system showed that the removal efficiency of NH₄⁺-N and TN in the upper substrate was better than that in the middle and lower layers.（3）Under the operation mode of flooded/drained time ratio of 12 h/12 h a cycle,the removal efficiency of ammonia nitrogen and total nitrogen is as follows:zeolite（89.20%,75.69%）>volcanic rock（35.49%,20.01%）>sponge ball（27.33%,14.86%）;Under the condition of flooded/drained time ratio of 3 h/3 h for 4 cycle,the removal efficiency of ammonia nitrogen is as follows:sponge ball（48.74%）>volcanic rock（11.31%）;The removal rate of ammonia nitrogen and total nitrogen by flooded/drained time ratio of 3 h/3h a cycle tidal zeolite system decreases with the increase of experiment times.（4）The effects of different operation modes（HRT 1 d）on NH₄⁺-N removal by constructed wetlands are as follows:cascade trickling filter wetland>multi-stage tidal flow wetland>single-stage tidal flow wetland>intermittent flow wetland.Different operation modes all show that the removal effect of nitrogen pollution is positively correlated with HRT.In addition,the constructed wetland with HRT of 3 d in series trickling filtration has the best purification effect on NH₄⁺-N and TN,and the average removal rate of NH₄⁺-N by water onion and canna has no significant difference（P>0.05）.With the decrease of HRT,the removal rate of ammonia nitrogen by plants shows significant difference（P<0.05）,and the low HRT（1 d）is higher than that of high HRT（3d）（5）To the analysis of high-throughput sequencing,Proteobacteria,Nitrospirae,Planctomycetes,and Actinobacteria are the dominant phylum of each wetland system under different operation modes.Different operation modes have a great influence on microbial flora.Nitrospira is the dominant flora under tidal current operation mode.Unclassified＿Xanthomonadaceae is the most abundant bacterium in all wetlands under the operation mode of trickling constructed wetlands,followed by Nitrospira.