The extractability of heavy metals with exchangeable bases is of particular importance for two reasons: (i) Ca, Mg, K, and Na form the predominant electropositive elements in the soil solution (on molar basis) and can interact with adsorbed fractions of heavy metals; and (ii) single extractions aimed at ascertaining soil mobile or bioavailable fractions of heavy metals in the soils are for a major part based on these interactions. This article assesses metal extractability by Ca(NO3)2, Mg(NO3)2, KNO3, and NaNO3, and competitiveness for adsorption to the soil matrix between the exchangeable bases and the metals Cu, Cd, Cr, Ni, and Pb. To this end, experiments were performed in which a moderately contaminated soil was extracted with varying doses of extractant: 0, 0.05, 0.1, 0.25, 0.5 M of NaNO3, 0, 0.05, 0.1, 0.2, 0.4 M of KNO3, 0, 0.05, 0.1, 0.2 M of Ca(NO3)2, and 0, 0.05, 0.1, 0.2 M of Mg(NO3)2. In addition, differences in metal extractability between 0.01 M of Ca(NO3)2 and 0.01 M of CaCl2 and between 1 M of MgCl2 and 1 M of Mg(NO3)2 were evaluated in six soils with varying soil composition and pollution level to assess the effect of the counter ion (Cl− vs. NO3−). Finally, in a third experiment, heavy metal desorption in the presence of increasing concentrations of Ca(NO3)2 (0-0.1 M) was evaluated in four polluted soils. Total soil content of a given metal does not suffice to assess its environmental impact and bioavailability. Therefore, the study of metal exchangeability with Ca2+ is proposed to grant additional and complementary insight for soil metal analysis. Extraction with sufficiently high ionic strength, such as 0.1 M of Ca(NO3)2, can be used to give an estimate of the total exchangeable pool, whereas weaker extractions at around the ionic strength of the soil solution (∼0.01 M of CaCl2) allow for the estimation of the quantitative "responsiveness" of metal release as a function of increasing Ca2+ in the extractant solution at around relevant soil solution conditions.
Department of Applied Analytical and Physical Chemistry, Laboratory of Analytical Chemistry and Applied Ecochemistry, Ghent University, Coupure Links 653, 9000 Ghent, Belgium. Dr. Meers is corresponding author. E-mail: firstname.lastname@example.org
Received July 29, 2008, and in revised form Feb. 2, 2009.
Accepted for publication Feb. 2, 2009.