Preparation Of Chitosan-Based Composites For The Effective Sorption Of U(?)

Industrial wastewater produced from uranium-processing industry has brought serious harm to human health and environment.The efficient separation of uranium from wastewater is important for both the recycle of uranium resources and the reduction of environmental pollution.Sorption is an simple and highly effective method for the separation/enrichment of uranium.As the second largest natural polymer,chitosan has the advantages of wide availablity and low cost.Chitosan contains a large number of amino and hydroxyl sorption sites in its molecular structure,and thus it can effectively chelate uranyl ions.However,for the the sorption of uranium,chitosan have some limitations: First,chitosan has low chemical stability and and mechanical strength,it is easy to dissolve under acidic conditions;Second,chitosan itself has low porosity,thus it is difficult for uranium entering into the pores for sorption.By blending chitosan with carbon nano-tubes(CNT)and Na-bentonite(Na-Bt),the stability,mechanical strength and the sorption performance for U(VI)can be improved significantly.The main contents and results of this work are shown as the following:(1)The carboxyl-modified multiwalled carbon nanotubes were immobilized in chitosan-based composite membranes(CS-CNTs)which were used as efficient sorbents for U(VI)sorption.The results showed that the incorporation of CTNs with CS was mainly through physical interaction instead of chemical reaction.XPS analysis indicated that both amine and carboxyl groups are responsible for the complexation of U(VI).The sorption kinetics followed the pseudo-second order equation,whereas the sorption isotherms fitted well with Langmuir model(qm=126.7 mg/g at 298K).Moreover,the U(VI)-loaded CS-CNT8 can be effectively desorbed by 0.2 M acidified EDTA and reused.(2)The chitosan/ CNT composite aerogel was prepared by a vacuum freeze drying method.The composite aerogel was tested for U(VI)sorption.It was found that the maximum sorption capacity for U(VI)reached 585.3 mg/g.The sorption kinetic followed pseudo-second-order model.The thermodynamic parameters indicated that the sorption was an endothermic and spontaneous process.The sorption mechanism may be related to electrostatic interaction between U(VI)and CNT aerogel.(3)The carboxymethylated chitosan(CMC)/Na-bentonite(Na-Bt)composite membranes were prepared and throughly characterized.The Na-Bt/CMC mass ratio was optimized,and CB10(membrane with Na-Bt/ CMC mass ratio of 10%)was selected as the best membrane for U(VI)sorption.XPS analysis indicates that the main mechanism for UO22+ sorption onto CB10 is through inner-surface complexation.The sorption kinetics followed pseudo-second order model,indicating the chemisorption as the rate-controlling step.The U(VI)sorption on CB10 is endothermic and spontaneous,with the maximum mono-layer sorption capacity of 115.6 mg/g at p H5.0 and 298 K.Finally,the U(VI)-loaded CB10 can consecutively desorbed and resused for several cycles.The above results showed the chitosan /CNT composite aerogel was most effective for U(VI)sorption among the three sorbents,with the highest sorption rate and the largest sorption capacity,thus it can be potentially used in the treatment of radioactive waste water.