Simultaneous sorption and desorption of Cd, Cr, Cu, Ni, Pb, and Zn in acid soils

In Part I of this paper we reported, for each of 11 acid soils, the rankings of six metals according to their sorption from solutions containing all six, and according to their retention under desorption conditions. Here, we analyse the same data from a different perspective: for each metal, we rank the soils by their capacities to sorb and retain it as reflected by soil/solution distribution coefficients Kd100 measured using starting solutions containing 100 mg L−1 of each metal. We also ranked the soils for overall heavy metal sorption and retention capacities using Kaplan's compound measure KdΣ, and we investigated the influence of soil characteristics on Kd100 and KdΣ100 values. Overall capacity for sorption of heavy metals was positively related to HOM, kaolinite and Fe oxides contents, and negatively related to CEC and to vermiculite and haematite contents. Overall capacity for retention of heavy metals was positively related to HOM and kaolinite contents, and negatively related to CEC and vermiculite content. The good correlation between KdΣ100(sorption) and the first component extracted in a principal components analysis of Kd100(sorption) values, and between KdΣ100(retention) and the first component extracted in a principal components analysis of Kd100(retention) values, supports the adequacy of KdΣ as a measure of the overall capacity of a soil to sorb or retain heavy metals.