Enhanced uranium(VI) adsorption by chitosan modified phosphate rock

The interactions between uranium(VI) and phosphate are known to be critical in governing the mobility of uranium(VI). However, only using phosphate compounds to remove uranium(VI) in aqueous solution is often unsatisfactory owning to insufficient active sites on their surface. The enhancement of the adsorption ability of phosphate compounds is timely important. In this study, we successfully modified a typical type of natural phosphate rock with chitosan and applied this product to remove uranium(VI). The effects of initial uranium(VI) concentration, contact time and adsorbent dosage on uranium(VI) removal behavior were systematically investigated via a series of batch experiments. The uranium(VI) adsorption on chitosan modified phosphate rock can reached equilibrium after 5 h at acidic pH (pH 2.5). The uranium(VI) removal efficiency was greatly enhanced after phosphate rock was modified by chitosan. Adsorption mechanism of uranium(VI) on modified phosphate rock was proved by FT-IR, XRD, and SEM. After modification, a new group (-NH2) and a crystalline phase of ammonium calcium hydrogen phosphate (NH4CaHP2O7) were generated. Being adsorbed uranium(VI), the morphology of adsorbents (i.e., chitosan modified phosphate rock and phosphate rock) changed markedly. XRD results showed that certain diffraction peaks for newly generated crystal structures after chitosan modification, besides these diffraction peaks became weakened and even disappeared after uranium(VI) adsorption. As to the adsorption mechanism of this modify adsorbent, results from isotherm and kinetic analyses demonstrated that uranium( VI) adsorption was more related to Freundlich model and pseudo second order equation.