The Role of Hydrolysis in the Reaction of Heavy Metals with Soil‐Forming Materials

Abstract

The proposition that hydrolysis may account for the selective adsorption of heavy metals by clay surfaces is compared with an alternate explanation of specific exchange of the metal with weakly dissociable H+. It is shown that past evidence offered in support of the two hypotheses is somewhat ambiguous. Theoretical treatment is presented for the competition of ions in adsorption reactions which provides a means of distinguishing between the two alternatives through the effect of H+ on the metal‐clay reaction in the presence of a competing ion. Experimentally, the effect of H+ on the reaction of Co2+ with montmorillonite is studied in the presence and absence of Mg2+, a weakly competing ion. Ionic strength is held constant, and nonspecific reactions are minimized with an excess of CaCl2. The results are interpreted as favoring the hydrolysis hypothesis. Inherent assumptions are considered.

Characterization of aqueous lead removal by phosphatic clay: Equilibrium and kinetic studies

“…Several models have been suggested to explain the metal sorption mechanisms involving metal ions: the metal hydrolysis model, metal proton exchange model [36] and pH dependent ion exchange model based on electrostatic potential on plane of sorption and surface charge density. The following equations show the hydrolysis of metal ions resulting in the formation of metal hydroxyl species [36]:…”

Soil contamination issues: past, present and future, a personal perspective

“…In summary, the problem as described by Hodgson et al (1964) and developed further by MacNaughton and James (1974) may be that one set of experiments can be interpreted in various ways, for example, as simple ion exchange, or adsorption of the hydroxo-ion at the surface, or adsorption of the divalent ion with concomitant hydrolysis in solution or surface-encouraged hydrolysis or precipitation. There is a risk that the preferred interpretation could be largely intuitive and controlled to some extent by an arbitrary choice of model parameters which may maximise agreement between the predicted model and experimental results.…”

Adsorption of Co2+ by Oxides from Aqueous Solution

“…Hodgson et al (16) point out that the enhanced adsorption of metal ions on clay mineral surfaces with increasing p H may be explained either by a mechanism involving prior hydrolysis of the metal ions in solution (reaction 5) to give a hydrolysis product which is more strongly adsorbed (reaction 6) or by a mechanism involving direct exchange of the unhydrolyzed ion (reaction 7) with a specific group on the adsorbing surface However, the two mechanisms may be differentiated if a non-hydrolyzable bivalent competing ion is used. They have derived equations showing that 0 for the adsorption process represented by reaction 7 will be independent of [H'] in the presence of a large concentration of nonhydrolyzable competing ions.…”