Study Onuranium Speciation And Distribution In Phosphate-rich Water

Adequate supply of food needs adequate supply of fertilizers. At present, most ofthe phosphate fertilizers of the entire world are made from phosphate ores. Thedemand of phosphate fertilizers promotes the development of phosphate mines.Sedimentary phosphate ores commonly includes high amounts of radioactive elementand heavy metals. Mining of phosphate and using of phosphate fertilizer can increasethe concentrations of uranium in aqueous environments.State of the EnvironmentBulletin in China at2011has shown the growth trend of total phosphorus in the Riverwater. Strong complexation between phosphate and uranium make a difference ofuranium behavior in phosphate system from general system. Therefore, clarifyinguranium speciation distribution and mechanism controlling the migration andtransformation of uranium in phosphate system has an important significance onunderstanding uranium behavior in surficial aqueous system. Based on that, this thesiswill show the difference of uranium species between phosphate system and generalsystem, clarify the uranium speciation in natural River water and influencing factors.This study suggested:Uranium in phosphate-rich system mainly existed as UO2(HPO4)22-when neutralans oxidative. Uranium in U-P-NaCl-H2O system at oxidative environment mainlyexists as the species of UO22+, UO2(HPO4)22-and UO2(OH)n2-n. Uranium inU-CO2-P-NaCl-H2O at oxidaive environment mainly exists as the species of UO22+,UO2(HPO4)22-, UO2(CO3)34-, and UO2(OH)42-. Uranium in U-Ca-CO2-P-NaCl-H2O atoxidaive envionment mainly exists as the species of UO22+, UO2(HPO4)22-,UO2(CO3)34-, Ca2UO2(CO3)3and UO2(OH)42-.The predominance area ofUO2(HPO4)22-covers the pH range from weak acidic to weak alkaline. Thepredominance area of UO2(HPO4)22-can increase with the concentration of total phosphate in system.Many kinds of uranium minerals can be formed in phosphate-rich system atcertain conditions of Eh and pH，especially for Na-autunite.In these phosphate-richsystems, the stable minerals are U4O9, U3O8, schoepite, Na-autunite and uraninite.Compared to general system, uranium in phosphate-rich system mainly exists asUO2(HPO4)22-which had broad predominance area. The formation of Na-autuniteneeds alkaline, oxidative condition and enough concentrations of phosphate anduranium with suitable ratio. Uranium speciation distribution in the phosphate-richsystem and general system are both controlled by pH and Eh. The chemicalcomposition of system can significantly affect the speciation of uranium. Distributionof uranium speciation in Tuojiang River was controlled by the acidity andconcentration of total phosphate.Major species of uranium in Tuojiang River system included: Ca2UO2(CO3)3，UO2(HPO4)22-and UO2(CO3)34-, the distribution of which were mainly controlled byaciditity and concentration of total phosphate. Uranium in water of Mianyuan Rivermainly existed as UO2(HPO4)22-at sites of upstream near phosphate mine, whileexisted as Ca2UO2(CO3)3in other sites, e.g., dammed lake. The use of phosphatefertilizer at downstream promoted Ca2UO2(CO3)3transform to UO2(CO3)34-. Uraniumin Shiting River water mainly existed as Ca2UO2(CO3)3at most sites, except the siteST7which was located near phosphate fertilizer plant, where uranium mainly existedas UO2(HPO4)22-.The factors affecting uranium adsorption on albite in phosphate-rich systemincluded acidity, concentration of total phosphate, solid-solution ratio and ionicstrength. Acidity was major factor controlling the adsorption. Adsorption of uraniumon albite in presence of phosphate showed that adsorption percentages can be affectedby pH, total phosphate, ionic strength and solid solution ratio. The pH was majorfactor controlling the adsorption. The adsorption can reach maximum when pH at4.0,while minimum when pH at2.0. Adsorption can increase with pH in the range from2to4, but decrease with pH in the range from7to11. If solution was acidic, uraniumadsorption on albite was stronger in the presence of phosphate than situation in the absence of phosphate. If solution was alkaline, uranium adsorption on albite wasstronger in presence of low concentration of phosphate, but weaker in presence ofhigh concentration of phosphate, than situation in the absence of phosphate. Whateverin acidic or alkaline condition, adsorption can decrease with the concentration ofphosphate.Uranium adsorption on albite in the presence of phosphate was discussed atdifferent value of solid solution ratio (5g/l,10g/l,20g/l). Adsorption percentage canincrease with solid solution ratio and reach maximum at the solid solution ratio of20g/l within the pH range from2to4. Nevertheless, adsorption percentage can reachmaximum at the solid solution ratio of10g/l within the pH range from7to11.Increasing of solid solution ratio did not always promote the adsorption when pHvaried from7to11, which was consistent with result in previous literature. Uraniumadsorption on albite in the presence of phosphate was discussed at differentconcentrations of NaCl at0.02mol/l,0.1mol/l, and0.5mol/l. Ionic strength had littleimpact on uranium adsorption on albite when2<pH<4. However, ionic strength cansignificantly improve uranium adsorption when4<pH<11. Effect of phosphate, solidsolution ratio and ionic strength can be explained with the theory of surfacecomplexation and species distribution of phosphate and uranium. The speciationdistribution of uranium in phosphate-rich system had significant impact on theadsorption of uranium.Mechanisms controlling uranium distribution in river sediment of Tuojiang Riversystem included adsorption of organic materials, physical precipitation of pristinealbite, and coprecipitation of carbonate minerals. Uranium distribution in upstreamsediment of Mianyuan River was controlled by the adsorption of organic materials,while its distribution in downstream sediment was decided by the physicalpristinealbite which contained U, As and Se. The significant and positive linear correlationsamong U, As and Se was also observed in phosphate ores, which revealed that thesource uranium in sediment of Mianyuan was phosphate ores and its wall rock.Uranium distribution in sediment of Shiting River was controlled by thecoprecipitation of carbonate minerals (calcite and aragonite). There were significantand strong linear correlations among U, Ca, S and P in sediment. It suggested that the source of uranium in sediment of Shiting River was phosphogypsum at the riverbank.