A Versatile Parameter for Comparing the Capacities of Soils for Sorption and Retention of Heavy Metals Dumped Individually or Together: Results for Cadmium, Copper and Lead in Twenty Soil Horizons

Heavy metals can be immobilized by soils and their distribution among the particulate soil components depends on the identity and amount of the metal, the properties of the soil, and other environmental factors. Cd, Cu and Pb are among the most potentially toxic heavy metals, are present--often together--in numerous polluting spills and in agrochemicals. We evaluated the individual and competitive sorption and retention of Cd, Cu and Pb on 20 soil horizons. As is usual, the isotherms constructed were so irregular, especially the retention isotherms, that it was not possible to classify and compare them in terms of the conventional isotherm shapes. Nor could they be compared using Langmuir or Freundlich parameters, since not all could be fitted with either of these equations. They were therefore characterized and compared in terms of several varieties of distribution coefficient, including a novel adimensional parameter K(r) which on the basis of correlation and principal components analyses was judged to be the most coherent and generally applicable to all experimental conditions (sorption and desorption starting from single- or multi-metal solutions). K(r) proved to be mainly determined by soil pH, effective cation exchange capacity, and Mn oxides content.