Simultaneous adsorption of uranium(VI) and phosphate on red clay

Abstract

Combined sorption of U(VI) and phosphate ions on red clay was studied in their concentration range 0.1–1 mM/dm3. The sorption of uranium and phosphate ions was investigated and compared on several clay forms: NaCl, T-clay, H-clay, HDTMA-clay and R-clay. The order of constant distributions was logKd(Na) > logKd(T)∼logKd(H)∼logKd(R) > logKd(HDTMA) for ceq = 0.0001 mol/dm3, indicating that the modified Na-adsorbent form was the most effective in the removal of U (VI) from the aqueous phase. A huge, 8-fold increase in U(VI) sorption was observed upon the addition of phosphate ions to the initial aqueous phase. The de Job method has been successfully used to evaluate U (VI) surface complexes with phosphate anions (V). Adsorption and desorption isotherms were obtained and it was found that the desorption isotherms are located above the adsorption isotherms (for T and R-clay). This indicates the presence of chemisorption in addition to the ion exchange, as well as the contribution of surface complexes and insoluble basic salts of U (VI) in the adsorption process. The analysis of changes in the U(VI) sorption with the increasing molar ratio R = [U(VI)]/[PO43−] showed that U(VI) concentration in the sorbent phase was the highest for R = 0.6–1.7 and R = 3.1–3.8. This finding suggests that two kinds of surface complexes were probably formed, i.e. one with a high content of phosphate ions and the other with a high content of U(VI) ions. The XPS spectra of sorption products revealed the formation of U(VI)–phosphate complex on the surface of red clay. The FTIR spectra, in turn, showed a very interesting effect of phosphates and the U(VI)–phosphate complex on the solid state of the cations of HDTMA+ anchored on the clay surface.