IONIC-STRENGTH AND PH EFFECTS ON THE SORPTION OF CADMIUM AND THE SURFACE-CHARGE OF SOILS

Two Oxisols (Mena and Malanda), a Xeralf and a Xerert from Australia a nd an Andept (Patua) and a Fragiaqualf (Tokomaru) from New Zealand wer e used to examine the effect of pH and ionic strength on the surface c harge of soil and sorption of cadmium. Adsorption of Cd was measured u sing water, 0.01 mol dm(-3) Ca(NO3)(2), and various concentrations of NaNO3 (0.01- 1.5 mol dm(-3)) as background solutions at a range of pH values (3-8). In all soils, the net surface charge decreased with an i ncrease in pH. The pH at which the net surface charge was zero (point of net zero charge, PZC) differed between the soils. The PZC was highe r for soils dominated by variable-charge components (Oxisols and Andep t) than soils dominated by permanent charge (Xeralf, Xerert and Fragia qualf). For all soils, the adsorption of Cd increased with an increase in pH and most of the variation in adsorption with pH was explained b y the Variation in negative surface charge. The effect of ionic streng th on Cd adsorption varied between the soils and with the pH. In Oxiso ls, which are dominated by variable-charge components, there was a cha racteristic pH below which increasing ionic strength of NaNO3 increase d Cd adsorption and above which the reverse occurred. In all the soils in the normal pH range (i.e. pH>PZC, the adsorption of Cd always decr eased with an increase in ionic strength irrespective of pH. If increa sing ionic strength decreases cation adsorption, then the potential in the plane of adsorption is negative. Also, if increasing ionic streng th increases adsorption below the PZC, then the potential in the plane of adsorption must be positive. These observations suggest that, depe nding upon the pH and PZC, Cd is adsorbed when potential in the plane of adsorption is either positive or negative providing evidence for bo th specific and non-specific adsorption of Cd. Adsorption of Cd was ap proximately doubled when Na rather than Ca was used as the index catio n.