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Competitive adsorption of heavy metals onto kaolinite

Prashant Srivastava1, Balwant Singh1 and Michael J. Angove2

1, 2 Environmental Soil Chemistry Group, Faculty of Agriculture, Food and Natural Resources, the University of Sydney, NSW 2006, Australia, http://www.agric.usyd.edu.au/escg/ Email: 1psri0762@mail.usyd.edu.au, 2b.singh@acss.usyd.edu.au
3
La Trobe University, PO Box 199, Bendigo, Vic 3552 Email: m.angove@latrobe.edu.au

Abstract

Polluted and contaminated soils can often contain more than one heavy metal species. It is possible that the behaviour of a particular metal species in a soil system will be affected by the presence of other metals. In this study we have investigated the adsorption of Cd(II), Cu(II), Pb(II) and Zn(II) onto kaolinite in single- and multi-element systems as a function of pH and concentration, in a background solution of 0.01 M NaNO3. In some experiments (adsorption edges) the pH was varied from 3.5 to 10.0 with total metal concentration 133.33 ÁM in the single element system, and 33.33 ÁM each of Cd(II), Cu(II), Pb(II) and Zn(II) in the multi-element system. The value of pH50 (the pH at which 50% adsorption occurs) was found to follow the sequence Cu<Pb<Zn<Cd in single-element systems, but Pb<Cu<Zn<Cd in the multi-element system. Adsorption isotherms at pH 6.0 in the multi-element system showed that there is competition between the various metals for adsorption sites on the clay. The adsorption and potentiometric titrations data for the various kaolinite-metal systems were modelled using an extended constant capacitance surface complexation model that assumed an ion-exchange process below pH 7 and the formation of inner-sphere surface complexes at higher pH. Inner-sphere complexation was more dominant for the Cu(II) and Pb(II) systems.

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