Microscopic level investigation of Ni(II) sorption on Na-rectorite by EXAFS technique combined with statistical F-tests

Highlights: ► The local atomic structures of Ni(II) sorption on Na-rectorite is investigated by EXAFS. ► Ni–Al LDH, Ni phyllosilicate and Ni(OH)2 as reference sample are characterized by EXAFS and compared. ► The local atomic structures of Ni(II) adsorbed differ with pH, temperature and humic substances. ► F-tests suggest that the surface precipitate is Ni–Al LDH. ► The results are quite important for the understanding of the physicochemical behavior of Ni(II). -- Abstract: Extended X-ray absorption fine structure (EXAFS) spectroscopy combined with statistical F-tests is used to investigate the local atomic structures of Ni(II) adsorbed on Na-rectorite. The EXAFS analysis results of Ni(II) sorption samples indicate that the first coordination shell consists of ∼6 O at the Ni–O interatomic distance (R) of ∼2.04 Å. The presence of Ni backscattering at RNi–Ni = ∼3.06 Å in the second coordination shell suggests the formation of Ni(II) precipitate. The results of F-tests show that the Ni(II) precipitate is Ni–Al layered double hydroxide (LDH). Our results demonstrate that Ni(II) ions are retained via different mechanisms depending on solution conditions. At low pH, Ni retention is controlled mainly by the outer-sphere surface complexation. With increasing pH, outer-sphere and inner-sphere surface complexation dominate Ni uptake. Furthermore, Ni surface loading increases with temperature increasing at pH 6.5 due to the formation of inner-sphere surface complexes and Ni–Al LDH. The formation of Ni–Al LDH becomes the dominate mechanism at the elevated pH and temperature. In the presence of humic substances, the sorption of Ni(II) on Na-rectorite is dominated by the formation of ternary surface complexes. These results are important to understand the physicochemical behavior of Ni(II) in the natural environment