Investigation Of U(?) And Th(?) Adsorption On Chloride Of Iron,Titanium And Zinc Modified Bentonite

Due to its special crystal structure and layered characteristics,bentonite adsorbs a large amount of exchangeable cations between its layers.therefor,the special layer structure not only increases its specific surface area,but also gives it good thermal conductivity,expansion ability and cation exchange capacity.Thus,bentonite has been selected as a buffer/backfill material for high-level waste geological repositories in many countries,used for seepage control in geological disposal reservoirs,conducting the decay heat of radionuclides in solidified bodies,adsorption or retardation the high-level radioactive wastes migration into the biosphere.In this work,zero-valent nano-iron?NZVI?,zero-valent zinc?ZVZ?,calcium/sodium bentonite?Ca/Na-bent?and saturated AlCl₃ modified Ca/Na-bent supported NZVI and ZVZ?NZVI/Ca/-bent,NZVI/Al/Ca-bent,ZVZ/Na/-bent,ZVZ/Al/Na-bent?had been prepared by used the NaBH₄ solution reduction Fe³⁺and Zn²⁺in aqueous solutions.TiCl₄ modified bentonite?Ti-Na-bent?and iron and titanium pillared modified bentonite?Fe/Ti-Na-bent?were prepared by cation intercalation and pillaring/calcining methods.The effects of pH value,amount of adsorbents,contact time,initial concentration of thorium or uranium solution and temperature on the efficiency for removal of uranium and thorium cation species from aqueous solution by aforementioned samples were studied in detail.SEM,EDS,XRD,XPS,ICP-OES and other characterization methods were also employed to characterize and analyze the phase composition,morphology,elemental composition of these modified bentonite and reveal the adsorption mechanism.The results showed that the uranium adsorption capacity of NZVI,NZVI/Ca/-bent and NZVI/Al/Ca-bent is higher than 1000 mg/g in two hours.Two hours later,however,the absorbed uranium began to desorption rapidly,which indicated the long-term adsorption stability of NZVI and its bentnoite composites is poor.The maximum adsorption capacities of ZVZ for uranium was 505 mg/g,ZVZ/Na/-bent and ZVZ/Al/Na-bent for uranium were approximately 250 mg/g,which are lower than NZVI,NZVI/Ca/-bent and NZVI/Al/Ca-bent,but the adsorption stability is better than the NZVI and its composities,the adsorption behavior is in accordance with the Langmuir model.The adsorption mechanism of U???by NZVI,ZVZ and their bentnoite modified is mainly chemical reduction,hydrolysis precipitation and ion exchange.The maximum adsorption capacity of iron and titanium pillared bentonite for U?VI?and Th?IV?were 356.51 mg/g and 242.8 mg/g,the adsorption capacity raised 2.33 and 1.53times compared with that before modification.The adsorption behavior of Th?IV?is consistent with the Langmuir model and the quasi-secondary kinetic model,the adsorption mechanism is mainly surface complexation and ion exchange.The maximum adsorption capacities of Ti-Na-bent for U?VI?and Th?IV?were 336.25 and 231.62 mg/g,modification raised the adsorption capacity of U???and Th???by 2.67 and 1.70 times,respectively.The adsorption processes corresponded to the Freundlich model and quasi-second-order kinetic model,and the adsorption mechanism was mainly Ion exchange.In addition to NZVI and its bentonite composites,the adsorption behaviors of U?VI?and Th?IV?by several other adsorbents are fitted well with the quasi-second-order kinetics model.The adsorption process has positive?S,?H,and negative?G,which demonstrated that the adsorption process was endothermic,entropy increase and spontaneous in nature.