Sorption Speciation of Nickel(ii) onto Ca-Montmorillonite: Batch, EXAFS Techniques and Modeling

Abstract

The sorption speciation of Ni(II) on Ca-montmorillonite was evaluated using a combination of batch experiments, extended X-ray absorption fine structure (EXAFS) spectroscopy and modeling. The pH and temperature at the aqueous-montmorillonite interface affects both the extent of Ni(II) sorption as well as the local atomic structure of the adsorbed Ni(II) ions. At 0.001 mol/L Ca(NO 3 ) 2 and low pH, the study reveals that the majority of Ni(II) is adsorbed in the interlayers of Ca-montmorillonite coordinated by six water molecules in an octahedron as an outer-sphere complex. At higher pH, inner-sphere surface complexes are formed. The Ni-Si/Al distances (R Ni-Al =3.00 Å, R Ni-Si1 =3.10 Å and R Ni-Si2 =3.26 Å) determined by EXAFS confirm the formation of mononuclear complexes located at the edges of Ca-montmorillonite platelets at pH 7.5 and 8.5. At pH 10.0, the Ni-Ni/Si distances (R Ni-Ni =3.07 Å and R Ni-Si =3.26Å) indicates the formation of Ni-phyllosilicate precipitates. A rise in temperature promotes inner-sphere complexation, which in turn leads to an increase in Ni(II) sorption on Ca-montmorillonite.Sorption edges are fitted excellently by surface complexation model (SCM) with the aid of surface species determined from EXAFS spectroscopy.

Interaction mechanism of radionickel on Na-montmorillonite: Influences of pH, electrolyte cations, humic acid and temperature

“…At high Ni(II) loadings, the formation of precipitation/co-precipitation phases on minerals might be expected [14,15]. Several macroscopic studies showed the strong Ni retention on mineral surfaces [9][10][11][12][13][14][15]. The sorption mechanisms of Ni(II) varied with different mineral surfaces and the environmental conditions, and then the desorption and migration of Ni(II) pollutant were also different.…”

“…There is no denying that the environmental condition has more or less influence on Ni(II) sorption and speciation. It was reported that the uptake mechanisms of Ni(II) onto minerals were dominated by cation exchange or outer-sphere surface complexation at low pH, and by inner-sphere surface complexation at high pH, which were dependent on the environmental conditions such as ionic strength, pH and temperature [9][10][11][12][13]. At high Ni(II) loadings, the formation of precipitation/co-precipitation phases on minerals might be expected [14,15].…”

“…The aforementioned researches were mainly focused on the investigation of sorption capability of minerals [7][8][9][10][11][12][13][14][15][16]. However, desorption reactions of radionuclides from mineral surface were the most relevant retention process for radionuclides with the surrounding conditions changing [17].…”

“…Sorption of Ni(II) onto mineral surfaces has been extensively studied since this process has significant effect on the transport properties [7][8][9][10][11][12][13]. There is no denying that the environmental condition has more or less influence on Ni(II) sorption and speciation.…”

Adsorption of nickel by Indian soils

“…The presence of Al 3+ in acidic soils and its competition for exchange sites might have resulted in reduced Ni adsorption in these soils like Cd (Cavallaro and McBride, 1978). According to Tan (2011), at low pH, majority of Ni (II) was adsorbed in the inter layers of Ca-montmorillonite co-ordianted by six water molecules in an octahedron as an outer-sphere complex; on the other hand, at higher pH, inner-sphere surface complexes are formed. Also, the Fe-oxides present in acidic soils may lead to decreased Ni adsorption by blocking access of Ni to exchange sites as reported by Cowan et al (1992) and Zachara et al (1992) for Cd.…”