title: Effect of humic acid on nickel(II) sorption to Ca-montmorillonite by batch and EXAFS techniques study.
abstract: 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.
authors.affiliation: Key Laboratory of Novel Thin Film Solar Cells, Institute of Plasma Physics, Chinese Academy of Sciences, P.O. Box 1126, Hefei, 230031, PR China.
authors.forename: Jun
authors.initials: J
authors.lastname: Hu
authors.forename: Xiaoli
authors.initials: X
authors.lastname: Tan
authors.forename: Xuemei
authors.initials: X
authors.lastname: Ren
authors.forename: Xiangke
authors.initials: X
authors.lastname: Wang