Hard and Soft Acid-Base Model Applied to Bivalent Cation Selectivity on a 2:1 Clay Mineral

Abstract

Abstract--We applied the hard and soft acid-base (HSAB) model Harsh 1990a, 1990b) to bivalent cation exchange on a purified Ca-montmorillonite. As a result, a satisfactory model is proposed to describe the gradual selectivity of exchange with Ca for 4 of the 6 metals studied (Cd, Cu, Pb, Zn). The selectivity is predicted as a function of the differences of electronegativity and softness of the metals. The deviation of Ni and Co data from the predicting model is interpreted in terms of hydration (Ni and Co being the most strongly hydrated ions). The fitting parameters of the model, et and 13, are related to the electronegativity and softness characteristics of the surface, respectively. Their ratio gives information on the nature of bonding. Results suggest that covalent bonding modifies electrostatic interactions, which in turn affect selectivity, with an increasing influence of covalent bondings in the order: Pb < Cd < Zn < Cu.To balance the lack of representativity of the model for the small molar fraction (ARM), we propose to associate to the HSAB model an equation describing the variation of the Vanselow selectivity coefficient as a function of the molar fraction of metal on clay.

RETRACTED: Competitive sorption and selective sequence of Cu(II) and Ni(II) on montmorillonite: Batch, modeling, EPR and XAS studies

“…The conclusion herein is supported by the comparability between the EXAFS-derived structural parameters of the sorption sample and those of the Cu(NO 3 ) 2 (aq) reference sample (Table 3). In a previous study, Auboiroux et al (1998) have reported that the Vanselow selectivity coefficient for the Cu 2+ /Ca 2+ exchange reaction ($2.42) is clearly higher than that for the Ni 2+ /Ca 2+ exchange reaction ($1.48). In view of this, the Cu(II) ions are expected to more preferentially enter the montmorillonite interlayer compared with the Ni(II) ions.…”

“…9A). The Vanselow selectivity coefficient for the Ni 2+ /Ca 2+ exchange reaction ($1.48) is reported to be lower than that for the Cu 2+ /Ca 2+ exchange reaction ($2.42) (Auboiroux et al, 1998). Hence, the exchange reaction between Ni(II) ions and the exchangeable Na + ions is inhibited by the coexisting Cu(II) ions.…”

Simultaneous adsorption behavior of heavy metals onto microporous olive stones activated carbon: analysis of metal interactions

“…Electronegativity values shown in Table 4 cannot explain the order of affinity for metal ions shown by COSAC, as it has been reported that metals with higher electro-negativity show higher adsorption yields (Li et al 2003;Lima et al 2007). Based on the hard soft acid-base (HSAB) theory or Pearson acid-base concept (Auboiroux et al 1998;Padilla-Ortega et al 2013), Cu(II) and Ni(II) For all the above considerations, it is evident that the affinity order Cd(II) [ Ni(II) [ Cu(II) found in this work for the studied metal ions cannot be related to the physicochemical properties of metal cations. Physicochemical properties of metal ions coupled with the steric effects of the adsorbent and physicochemical properties of its surface open the way to a multitude of interpretations that can be contradictory or even confusing.…”

Mid-infrared and infrared emission spectroscopy of Cu-exchanged montmorillonite

“…While zeolites are multicomponent catalysts with the aluminosilicate matrix, pillared clays are practically an expanded form of smectites formed by an introduction of a large interlayer cation [7,8]. Many researchers, however, are going back to basics by working with traditional clays to get a better understanding of the catalytic mechanism and substance selectivity [9][10][11].…”

“…Another significant application is of great interest in environmental and soil science [9]. In this respect, clay minerals may fix metals through ion exchange [13,14] or surface complexation [15].…”