Influence of Ionic Strength and Inorganic Complex Formation on the Sorption of Trace Amounts of Cd by Montmorillonite

Abstract

Cadmium sorption by montmorillonite from solutions in the 15 to 120 ppb range was studied in the presence of increasing concentrations of NaClO4, NaCl, and Na2SO4 solutions. The ionic strengths ranged from 0.01 to 1.00. Increasing ionic strengths decreased the amount of Cd sorbed on the clay surfaces. The percentage sorbed decreased from around 90% for I = 0.01 to about 50% for I = 1.00 in the ClO4 systems. The sorption of Cd in the chloride system was in the range between 25 to 50% less than the ClO4 systems for the same ionic strength. This was attributed to the presence of uncharged and negatively charged complexes of Cd with Cl ligands. This fact has some implications, especially in arid zone soils where high Cl concentrations in soil solutions are not unusual; there, Cd will behave mainly as a neutral species (CdCl20) and as an anion (CaCl3‐ and CdCl42‐), rather than as a cation (Cd2+).The SO4 systems showed a moderate decrease in the amount of Cd sorbed with respect to the ClO4 systems for the same salt concentrations. This observation was interpreted as due to the presence of a fraction of Cd in solution as the CdSO40 species.

Cadmium accumulation in crops

“…The Cl -1 complexes of Cd are negatively or neutrally charged and therefore reduce Cd sorption on cation exchange sites. Thus, under relatively high Cl concentrations, as may occur with some saline soils, the solubility and mobility of Cd may be increased substantially, although reduced sorption was also observed at Cl concentrations representative of those commonly found in soil solutions (Garcia-Miragaya and Page 1976). Increasing chloro-complexation would increase diffusion of Cd to the root surface, thereby increasing phytoavailability (McLaughlin et al 1994).…”

“…Increases in ionic strength also increased the concentration of Cd in the soil solution (Petruzzelli et al 1985;Lorenz et al 1994). Garcia-Miragaya and Page (1976) suggested that the decrease in Cd sorption with increasing ionic strength of an electrolyte was due to: (i) competition of Cd 2+ with the electrolyte cations for available clay surface sites; (ii) decreased activity of the Cd 2+ in solution and; (iii) formation of uncharged ion pairs and/or uncharged and negatively charged complexes of Cd with the anions in the electrolyte.…”

“…Phytochelatins are believed to function as detoxifying agents for Cd by virtue of their ability to bind Cd and thereby prevent it from reacting with the sulphhydryl groups of vital enzymes and proteins. Phytochelatins appear to be ubiquitous in the plant kingdom (Grill 1989) and have been isolated and purified from the roots of a number of crops including wheat (Gong et al 1990). Most research with phytochelatins has been conducted with intact plants or plant cells exposed to Cd at relatively high concentrations.…”

Copper adsorption in tropical oxisols

“…Adsorption isotherms describe the relationship between the mass of the adsorbed substance (adsorvate) by the solid (adsorvent) and the concentration of the substance in the solution in equilibrium. One of the models used to describe the adsorption of elements is the Freundlich equation: x/m = KC n , in which x = mass of the element adsorbed; m = mass of the adsorvent solid phase; C = concentration of the equilibrium of the adsorvate in solution and K and n are empirically determined constants (Garcia-Miragaya and Page, 1976;Bowman et al, 1981). Another model used to describe metal adsorption is the Langmuir equation.…”

Lead toxicity thresholds in 17 Chinese soils based on substrate-induced nitrification assay

“…Soils from Baotou and Beijing had pH values ≥ 7.8 or 8.4 (measured in deionized water without CO 2 ) and the impact of leaching is likely to be because of the effects of high ionic strength and higher concentrations of other competing cations (e.g., Ca, Mg and Na). The competing cations reduce Pb partitioning in unleached treatment soils and therefore influence Pb toxicity (Garcia-Miragaya and Page, 1976;Mattigod et al, 1979;Zhu and Alva, 1993). High ionic strength reduces Pb sorption (Puls et al, 1991) and free metal ion activity in solution (Zhu and Alva, 1993).…”