The influence of soil properties on the individual and competitive sorption and desorption of Cu and Cd

The sorption and retention of mixtures of heavy metals by soil are complex processes that depend on both soil properties and competition between metals for sorption sites. Cu2+ and Cd2+ are among the most potentially toxic heavy metals, they are present –often together – in numerous polluting spills and in agrochemicals. The objective was to evaluate and compare the competitive and individual sorption and desorption capacity and sorption hysteresis of Cu2+ and Cd2+ on four natural soils: a Humic Umbrisol, an Umbric Cambisol, an Endoleptic Luvisol and a Humic Cambisol. To evaluate the sorption and retention capacity, we used the parameter Kr proposed and determined in a previous work. In all the horizons Cu2+ is sorbed and retained to a greater extent than Cd2+. The individual sorption and retention of Cu2+ are most influenced by pH, CECe, and vermiculite, chlorite, and total clay content; however, Cu2+ competitive sorption and retention do not depend on chlorite content. On the other hand, the individual and competitive sorption and retention of Cd2+ are most influenced by pH, CECe and Mn oxides, vermiculite, chlorite, and total clay content. In all the horizons the greatest hysteresis corresponds to Cu2+, and in each soil the hysteresis in the A-horizon is greater than in B, except in the Bt of Endoleptic Luvisol, due to its high pH and vermiculite content. The sorption irreversibility is higher in soils of lower acidity, high levels of oxides and the presence of vermiculite and chlorite in the clay fraction. By multiple linear regression analysis, individual and competitive explanatory models of fixation capacity were obtained. In all equations the most important characteristics that explain the metal fixation are included.

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► The majority of studied soils have a high affinity for Cu2+, more than for Cd2+.
► The sorption irreversibility is higher in soils of lower acidity, high levels of oxides and the presence of vermiculite and chlorite in the clay fraction.
► The results allow to know the necessary amendments to add to soils that content these metals, avoiding that they were absorbed by the plants, or transferred to waters.