Effect of pH, ionic strength, temperature and humic substances on the sorption of Ni(II) to Na–attapulgite

Sorption of Ni(II) on Na–attapulgite as a function of pH and ionic strength is investigated under ambient conditions. The results indicate that sorption of Ni(II) is strongly dependent on ionic strength at pH < 8, and independent of ionic strength at pH > 8. Results of the desorption tests of Ni(II) from bare and HA bound attapulgite hybrids indicate that the sorption is reversible at pH 6.0. Outer-sphere complexation or ion exchange may be the main sorption mechanism of Ni(II) to attapulgite at pH < 8, whereas the uptake of Ni(II) at pH > 8 is mainly dominated by inner-sphere complexation. The presence of FA and HA decreases Ni(II) sorption to FA/HA–attapulgite hybrids at pH > 7 obviously, and the influence at pH < 7 is weakly. Sorption of Ni(II) to Na–attapulgite increases with increasing temperature. The thermodynamic data (ΔH0, ΔS0, ΔG0) are calculated from the temperature-dependent sorption isotherms, and the results suggest that sorption process of Ni(II) on attapulgite is spontaneous and endothermic.