Adsorption Properties Of Uranium On Carboxymethyl Chitosan Materials

The pollution of uranium containing waste water is becoming a serious problemdue to the rapid development of nuclear industry. Uranium could not be naturaldegraded, but could be enriched through the food chain and eventually reached thehuman-body and thus to be a serious threat to the environment and human health.Chitosan could be chelated with variety of metal ions adsorption as its molecularchain containing a large number of amino and hydroxyl functional groups. Chitosanwas biocompatible, biodegradable and non-toxic, and could be used as adsorbents foruranium ions. However, chitosan has poor water solubility and chemical stability, andlow adsorption capacity, and for this its application was limited.Chitosan could be functionally modified because of the excellent chemicalreactivity of its-NH2and–OH groups. Carboxymethyl chitosan was a water-solublederivative of chitosan after the introduction of carboxymethyl groups to the molecularchain of chitosan. The carboxymethylation of chitosan could improve its watersolubility and the cross-linked modification could enhance its chemical stability,moreover, the introduction of carboxymethyl groups into chitosan could damage thecrystal structure of chitosan and form the multiple adsorption sites such ascarboxymethyl, amine and hydroxyl groups, thus increasing its uranium adsorptioncapacity.For the considerations mentioned above, firstly, we synthesized water-solublecarboxymethyl chitosan using a microwave-heating method; then, usingcarboxymethyl as a raw material, we prepared three kinds of adsorbents for uranium adsorption such as crosslinked carboxymethyl chitosan resins, ion-imprintedcarboxymethyl chitosan nanoparticles and magnetic carboxymethyl nanoparticles. Thepreparation process of carboxymethyl chitosan resin was optimized by the responsesurface optimization.The above adsorbents were used to adsorb the uranium ions in the simulatedwastewater, the different factors such as pH, uranium initial concentration,temperature, and adsorption time were investigated, The adsorption isotherm model,adsorption thermodynamic, regenerative capacity, adsorption selectivity, andadsorption mechanism was discussed.The results showed that the main influencing factors of CMCR weremicrowave-heating time, chloroacetic dosage and cross-linker dosage, with thechloroacetic dosage as the most significant influencing factor. Meanwhile, themicrowave-heating time had a interaction with the crossliner dosage. The optimalvalues were determined to be:2.09mL of chloroacetic dosage,21.75min ofmicrowave-heating time and5.78g of corsslinked dosage, under these conditions, theadsorption capacity was found to be196.63mg/g.The kinetic calculations indicated that the adsorption of uranium ions could bewell-fitted with pseudo-second konetic equation. From the adsorption isotherms,ICMCNPs, CMCR, and NICMCNPs could be better described with Langmuiradsorption model, indicate the monolayer adsorption for these resins, however,MCMCNPs could be fitted with both with Langmuir and Freundlich models, whichindicated that the adsorption of uranium onto MCMCNPs was a combination of bothmonolayer and multilayer adsorption. It was also found that the D-R adsorptionisotherms could not be well-fitted with all adsorbents, maybe due to the poor porousstructures for these adsorbents. Thermodynamic calculations showed that theadsorption process was exothermic and higher temperature is not favorable for theadsorption of the uranium ions onto the adsorbents. The results also showed that alladsorbents had good regeneration ability using suitable eluting solutions, moreover, the ion-imprinted resins such as ICMCNPs also showed excellent selectivity foruranium ion adsorption.