Adsorption And Transformation Characteristics Of Uranium On Sandstone Aquifer Media

During and after in-situ mining of uranium,the diffusion of uranium-containing leachate potentially has an impact on groundwater quality.The range and degree of influence are determined by natural attenuation.However,adsorption and transformation characteristics of uranium on aquifer media?especially sandstone aquifers?are unclear.This study investigates influences of solution chemical characteristics and pyrite content on adsorption and conversion on aquifer sandstone taken from Qianjiadian uranium mining area in Songliao Basin,Inner Mongolia.The main conclusions have been drawn as follows.?1?Adsorption of U?VI?on sandstone particles is a rapid process,with the adsorption equilibrium around 12 h.Adsorption of U?VI?on sandstone particles towel fits the pseudeo-second-order kinetic equation.Both Langmuir and Freundlich adsorption isotherms well fit the observed data.?2?Grain size,solution pH,coexisting ions,and ionic strength affect U?VI?adsorption to various degrees.Adsorption efficiency of U?VI?onto sandstone particles generally decreases as the grain size increases.When the grain size increases to 0.20⁰.25 mm,adsorption efficiency tends to be stable.The solution pH is the main factor affecting adsorption,by controlling the complexes of U?VI?in the solution and the surface charge of rock particles.At pH=6,adsorption reaches the maximum.HCO₃^-shows the most negative effect on U?VI?adsorption,followed by Ca²⁺,Mg²⁺,and SO₄^2-.HCO₃^-inhibits U?VI?adsorption mainly by complexing with UO₂²⁺to form negatively charged complex anions.Ionic strength shows no big effect on U adsorption.?3?Under near neutral pH condition?pH=6.5?,the increase of pyrite content significantly promotes the removal of U?VI?.This promotion is the result of adsorption and reduction processes.The reduction reaction occurs on the surface of the adsorbent.The adsorbed Fe²⁺reduces the adsorbed U?VI?by electron transfer on the surface of the adsorbent.The reduction products are UO_2+x and Fe?OH?₃.The reduction process releases adsorption sites,and therefore adsorption and reduction are mutually reinforcing processes.?4?Under weakly alkaline pH condition?pH=7.8?,it is not obvious for pyrite to promote U?VI?removal.Hydrolytic precipitation and low adsorption inhibit the reduction of U?VI?.The natural attenuation is mainly controlled by hydrolytic precipitation,keeping U?VI?concentration at a relatively low level.