Sorption behavior of U(VI) onto Chinese bentonite: Effect of pH, ionic strength, temperature and humic acid

Abstract

Herein, a local bentonite from Huangshan county (Anhui province, China) was purified and the purge bentonite sample was characterized by using FTIR and XRD to determine its chemical constituents and microstructures. The sample was chosen as an adsorbent to remove uranium ions from aqueous solutions as a function of various environmental parameters such as contact time, pH, ionic strength, foreign ions, humic acid and temperature under ambient conditions. The results indicated that the sorption of U(VI) on the bentonite was strongly dependent on pH and ionic strength. At pH<6.5, the sorption of U(VI) on the bentonite increased with increasing pH, whereas the sorption of U(VI) on bentonite decreased with increasing pH at pH>6.5. The presence of humic acid (HA) enhanced the sorption of U(VI) on bentonite obviously at low pH while it reduced the U(VI) sorption on bentonite at high pH values. The enhancement of U(VI) sorption on HA-bentonite hybrids at low pH was attributed to the strong surface complexation of surface adsorbed HA with U(VI) on bentonite surface, whereas the decrease of U(VI) sorption on bentonite at high pH was attributed to the formation of free HA-U complexes in solution. The Langmuir, Freundlich and D–R models were applied to simulate the sorption isotherms of U(VI) at three different temperatures of 298, 318 and 338K, respectively. The thermodynamic parameters (i.e., ∆H0, ∆S0 and ∆G0) calculated from the temperature dependent sorption isotherms indicated that the sorption process of U(VI) on bentonite was an endothermic and spontaneous process. At low pH, the sorption of U(VI) was mainly dominated by outer-sphere surface complexation and ion exchange with H+/Na+ on bentonite surfaces, whereas inner-sphere surface complexation was the main sorption mechanism at high pH values. Based on the experimental results, bentonite is a very suitable material for the preconcentration and solidification of U(VI) ions from large volumes of aqueous solutions in U(VI) pollution cleanup.