Study on adsorption of ammonium by kaolinite in soil of ionic rare earth mining area

Ammonium pollution increases in severity during mining of ionic rare earth ores in southern China.Kaolinite is one of the main clay minerals in soils of ionic rare earth ores.It was selected as the adsorbent and the adsorption characteristics of it for ammonium were investigated.The results indicated the adsorption capacity and rate of ammonium by kaolinite increased with increasing the initial concentration of NH₄⁺-N and temperature.The adsorption capacity increased with increasing pH value when the latter was lower than 9.2;whereas it was dramatically decreased with pH value increasing when pH value was higher than 9.2.The isothermal adsorption matched the Langmuir and Freundlich isotherms.The adsorption process followed closely the pseudo-second kinetic equation.In ranges of temperature（e.g.298 Kto 310K）and pH value（e.g.3.0to 6.0）characterized the actual ionic rare earth mining area soil in several provinces in south China,the adsorption capacity of the mining soil for ammonium would increase with increasing leaching agent concentration,temperature or pH value.The ammonium removal in the polluted mining soil can be strengthened through using lower concentration of in-situ leaching solution and controlling lower pH value and temperature of the soil.