Effect Of Particle Size On Distribution And Release Of Ammonium Nitrogen In Ion-adsorption Rare Earth Tailings

At present,the mining technology of ion-adsorption rare earth ore is mainly in situ leaching.However,while obtaining rare earth resources,such mining technology also causes damage to the mining area and the surrounding environment.A large number of tailings continue to release ammonium pollution to the surrounding environment after being abandoned for many years.In this paper,we collected soil samples at different depths from a rare earth tailings in Zudong deposit of Ganzhou City,Jiangxi Province,and screened their particle size.Through laboratory batch and column experiments,the influence mechanism of particle size on the occurrence and release of ammonium nitrogen was studied,and the main results were listed below:(1)Soil particle size controls the specific surface area and clay content,so the enrichment ability of ammonia nitrogen increases with smaller soil particle size,while the ratio of exchangeable to water-soluble ammonium nitrogen does not change much.With the increase of soil depth,the distribution uniformity of clay on soil particle size increases,which makes the distribution of ammonium nitrogen with different particle size less obvious.The types of clay minerals change with the increase of depth,so the ammonium nitrogen increases first and then decreases across the profile.The total ammonium nitrogen reached the highest at 11 m,while the water-soluble ammonium nitrogen reached the highest at the upper part of the whole weathered layer,indicating a greater risk of environmental release.(2)With the increase of time,the desorption capacity of both water-soluble and exchangeable ammonium nitrogen increased gradually;Through kinetic fitting of the chloride ion penetration curves of soil columns with different particle sizes,it is found that the desorption of ammonium nitrogen from soil with three particle sizes conforms to the double constant equation well,which indicates that the desorption of ammonium nitrogen from soil is a complex kinetic process.The shape of the chloride ion penetration curve of each soil column is smooth and monotonous,and there is a certain trailing phenomenon,indicating that there is a certain non-equilibrium migration of chloride ion in the process of transport.With the decrease of particle size,the penetration time of chloride ions increases,which may be due to the smaller permeability coefficient,larger specific surface area,and higher content of clay minerals and other substances in fine particle size soil.(3)Under simulated rainwater leaching,the effluent concentration of ammonium nitrogen in the three homogeneous soil columns reached the peak,then decreased rapidly,and then decreased slowly to stable;The concentration of ammonium nitrogen in heterogeneous soil column increases briefly and then decreases rapidly after water release,and the subsequent change trend is similar to that in homogeneous soil column.The release of ammonium nitrogen may be delayed to a certain extent due to the existence of non-equilibrium adsorption/desorption.In addition,as the particle size controls the specific surface area and the content of clay minerals,thus affecting the enrichment capacity of ammonium nitrogen in the soil,the effluent concentration of ammonium nitrogen increases with the decrease of particle size.The acceleration stage of ammonia leaching process is mainly the diffusion process in particles,while the deceleration stage is mainly controlled by Elovich equation.(4)There is a loss of ammonium nitrogen in each soil column after leaching,with a large proportion of the loss in water-soluble form,while only a small part of the loss in exchange form.It can be speculated that the form of ammonium nitrogen released by the actual tailings after leaching through natural rainfall is mainly water-soluble form,which has a high environmental risk.In addition,part of ammonium nitrogen desorbed from the upper part of the soil column may gradually accumulate in the lower part.It can be inferred that there is a similar cumulative effect in the actual tailings environment,which causes ammonium nitrogen and other pollutants to accumulate in the direction of rainwater migration and gradually release into the groundwater environment.Endogenous control should be strengthened to reduce the release of ammonium nitrogen into the water environment.