Simultaneous sorption and desorption of Cd, Cr, Cu, Ni, Pb, and Zn in acid soils II. Soil ranking and influence of soil characteristics.

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 K(d100) measured using starting solutions containing 100mgL(-1) of each metal. We also ranked the soils for overall heavy metal sorption and retention capacities using Kaplan's compound measure K(dSigma), and we investigated the influence of soil characteristics on K(d100) and K(dSigma100) 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 K(dSigma100)(sorption) and the first component extracted in a principal components analysis of K(d100)(sorption) values, and between K(dSigma100)(retention) and the first component extracted in a principal components analysis of K(d100)(retention) values, supports the adequacy of K(dSigma) as a measure of the overall capacity of a soil to sorb or retain heavy metals.