Variable-charge minerals

Violante A, Gianfreda L (2000) Role of biomolecules in the formation and reactivity towards nutrients and organics of variable charge minerals and organo-mineral complexes in soil environment. In Bollag J-M, Stotzky G (eds) Soil biochemistry, vol 10. Marcel Dekker, New York, USA, pp 207-270... 

The effect of other inorganic anions (sulfate, molybdate, silicate), low molecular mass organic ligands (LMMOLs, such as oxalate, malate, citrate, tartrate and succinate), and fulvic or humic acid on the sorption of arsenate and arsenite onto variable charge minerals and soils has been studied (Roy et al. 1986 Grafe et al. 2001 Liu et al. 2001 Violante et al. 2005a,b). 

Oxides and hydroxides of Al, Fe, Mn, and Si may exist in the subsurface mainly as a mixture (known also as a solid solution) rather than as pure mineral phases. They are considered amphoteric materials, characterized by no permanent surface charge. Their cation and anion exchange capacities reflect adsorption of potentialdetermining ions such as H+ and OH". Different surfaces have a diverse affinity for H+ and OH" ions and thus exhibit various points of zero charge (PZC). Details of various models for variable charge minerals may be found in the extensive review of McBride (1989). 

Barrow, N.J. Bowden, J.W. Posner, A.M. Quirk, J.P. (1981) Describing the adsorption of copper, zinc and lead on a variable charge mineral surface. Aust. J. Soil Res. 19 309-321... 

Rietra, R.P.J.J. Hiemstra,T. Van Riemsdijk, W.H. (1999 a) The relationship between molecular structure and ion adsorption on variable charge minerals. Geochim. Cosmochim. Acta 63 3009-3015... 

Bowden, J. W., Nagarajah, S., Barrow, N. J., Posner, A. M. Quirk, J. P. 1980. Describing the adsorption of phosphate, citrate and selenite on a variable charge mineral surface. Australian Journal of Soil Research, 18, 49-60. 

Bowden, J. W., A. M. Posner, and J. P. Quirk. 1977. Ionic adsorption on variable charge mineral surfaces Theoretical charge development and titration curves. Aust. J. Soil. Res. 15 121-136. 

Sorption of anions onto variable-charge minerals and soils varies with pH. With increasing pH, within a certain range, sorption decreases (due to a decrease of positive charge of minerals), or increases to a maximum close to the pKa for anions of monoprotic conjugate acids and then decreases. Slope breaks have... 

I.I. Effect of Residence Time on Desorption Some researchers found that trace elements [Ni, Pb, As(V)] reacted with metal oxides and pyrophyllite over longer times resulted in either irreversible or reversible sorption mechanisms. Violante et al. (2003) studied the effect of residence time on the sorption of Zn onto ferrihydrite in the presence of Cu. As Cu has a greater affinity than Zn for tire surfaces of ferrihydrite, Cu was added from 1 to 336 hours after Zn at a Zn/Cu molar ratio of 2. Zinc sorption increased, particularly when Cu was added 6 to 336 hours after Zn. A possible explanation of these findings is that trace elements initially sorbed on the surfaces of variable-charge minerals slowly form precipitates with time. As discussed before, sorption is considered to be the predominant sorption mechanism responsible for trace element uptake on mineral surfaces within the first few hours, while surface precipitation is considered to be a much slower process, occurring on a time scale of hours to days (McBride, 1994 Scheidegger et al., 1997 Sparks, 1999 Borda and Sparks, Chapter 3, this volume). Clearly, Cu added many hours or days after Zn addition cannot replace Zn ions that have formed precipitates on the surfaces of the ferrihydrite. 

He has contributed to research on the interface between soil chemistry and mineralogy and soil biology. His special areas of research include the formation mechanisms of aluminum hydroxides and oxyhydroxides, the surface chemistry and reactivities of short-range-ordered precipitation products of Al and Fe, the influence of biomolecules on the sorption and desorption of nutrients and xenobiotics on and from variable charge minerals and soils, the factors that influence the sorption and residual activity of enzymes on phyllosilicates, variable charge minerals, organomineral complexes, and soils and the chemistry of arsenic in soil environments. 

MeUs, P. et al.. Describing the adsorption of potential determining ions on variable charge mineral srrrfaces, Studi Sassar., 30, 137, 1983. 

Rietra, R.P., Hiemstra, T.. and van Riemsdijk, W.H., Electrolyte anion affinity and its effect on oxyanion adsorption on goethite, J. Colloid Interf. Sci., 229, 199, 2000. Rietra, R.P., Hiemstra, T, and van Riemsdijk, W.H., The relationship between molecular structure and ion adsorption on variable charge minerals, Geochim. Cosmochim. Acta, 63. 3009, 1999. 

Barrow, N.J. and Bowden, J.W., A comparison of models for describing the adsorption of anions on a variable charge mineral surface, J. Colloid Interf. Sci., 119, 236, 1987. 

ION EXCHANGE ON OXIDES AND OTHER VARIABLE-CHARGE MINERALS... 

A simple chemical model can be used to show how the charge of oxides and other variable-charge minerals depends on pH and ionic strength. Consider the chemisorption of an acid HX ... 

Addition of neutral salts to suspensions of variable-charge minerals (or soils containing these minerals predominantly) shifts the pH of the suspensions toward the PZC. 

There are good reasons to believe that the application of the Nemst equation and the DDL model to oxides and other variable-charge mineral surfaces is inappropriate. To begin with, cations and anions may adsorb on oxides by direct coordination to the charged surface group. Even for monovalent cations and anions, a high percentage of the bonds with oxide surfaces are believed to be of the inner-sphere type. 

This experiment is not commonly done because it is difficult to maintain a measurable concentration of the soluble metal at higher pH. It could conceivably pertain to situations in soils where cation exchange on silicate clays buffers the metal ion concentration at the same time that adsorption on variable-charge minerals occurs. It is an unlikely situation that will not be analyzed further. 

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