Reactivity, Exchangeability, and Solubility of Cu, Zn, and Cd in Various Soil Materials: Concepts and Evaluation

Six extractants (0.005 mole DTPA dm^–3, pH 7.3; 1 mole CH₃COONH₄ dm^–3, pH 7.0; 0.10 mole HNO₃ dm^–3; 0.10 mole HCl dm^–3; 1 mole HNO₃ dm^–3 and 1 mole HCl dm^–3) and three ratios (1 : 2, 1 : 5, 1 : 10) were applied for characterizing the reactivity, exchangeability, and solubility of Cu, Zn and Cd in media of contrasting origin: lacustrine sediments (LS), urban soils (US) and those impacted by copper ore processing, namely metallurgically impacted soils (MIS). Data revealed that soil materials differed in pH as follows: LS (6.99 ± 0.11–7.79 ± 0.28) > US (6.49 ± 0.32–7.23 ± 0.29) > MIS (5.68 ± 1.03–6.30 ± 0.66), for pH_KCl and \({\text{p}}{{{\text{H}}}_{{{{{\text{H}}}_{{\text{2}}}}{\text{O}}}}}\), respectively. The DTPA test (reactivity) applied at ratios 1 : 5 and 1 : 10 showed the best extraction for Cu and Zn as confirmed by the coefficients of correlations (r) in the range 0.85*** < r < 0.96**** (P < 0.005 and 0.001). Exchangeability represented by CH₃COONH₄ was best described (0.94*** < r < 0.99**** at P < 0.005 and 0.001) by the amounts of Cu and Zn determined at the ratio 1 : 5. The narrowest (1 : 2) and largest (1 : 10) ratios showed an acceptable prediction for Cu, Zn and Cd. The relationships of the pairs 1 mole HCl dm^–3 versus 1 mole HNO₃ dm^–3 (solubility test) applied at 1 : 5 resulted in r values varying between 0.94*** and 0.99****, (P < 0.005 and 0.001). Linear regressions with Cd tests gave low r values (r < 0.50) and significance (P > 0.05). This implies that both extractants may be used interchangeably for extracting Cu and Zn in contrasting environmental media. On the basis of algorithms from the stepwise analysis, the evaluation of trace element (TE) extractability of ecosystems potentially contaminated by TE should pertinently involve two factors: acid-based extractants and cation exchange capacity (CEC). Organic matter appeared as a facultative parameter.