Effect of humic acid on nickel(II) sorption to Ca-montmorillonite by batch and EXAFS techniques study
The influence of humic acid (HA) on Ni(II) sorption to Ca-montmorillonite was examined by using a combination of batch sorption experiments and extended X-ray absorption fine structure (EXAFS) spectroscopy technique. The sorption of Ni(II) on HA–montmorillonite hybrids is strongly dependent on pH and temperature. At low pH, the sorption of Ni(II) is mainly dominated by Ni–HA–montmorillonite and outer-sphere surface complexation. The EXAFS results indicate that the first coordination shell of Ni(II) consists of ∼6 O atoms at the interatomic distances of ∼2.04 Å in an octahedral structure. At high pH, binary Ni–montmorillonite surface complexation is the dominant sorption mechanism. EXAFS analysis indicates the formation of mononuclear complexes located at the edges of Ca-montmorillonite platelets at pH 7.5, while a Ni–Al layered double hydroxide (LDH) phase at the Ca-montmorillonite surface formed with pH 8.5. At pH 10.0, the dissolved HA–Ni(II) complexation inhibits the precipitation of Ni hydroxide, and Ni–Al LDH phase forms. The rise of temperature increases the sorption capacity of Ni(II), and promotes Ni–Al LDH phase formation and the growth of crystallites. The results are important to evaluate the physicochemical behavior of Ni(II) in the natural environment.