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Abstract
Copper (Cu), despite being an essential micronutrient, may be toxic whenever it occurs in the soil solution at high concentrations. Chemical reactions that control copper availability in the soil–plant system are complex. Copper can be found in both organic and inorganic soil fractions. This work studied Cu adsorption into surface (0–0.2 m) and subsurface (in the B2 horizon) samples from three Brazilian soils, namely a heavy clayey-textured anionic Rhodic Acrudox (RA), a medium-textured anionic Xanthic Acrudox (XA) and a heavy clayey-textured Rhodic Hapludalf (RH), before and after the removal of organic matter (OM) and iron oxides. pH values were adjusted to 5 with either HNO3 or Ca(OH)2. In natural samples, when OM was not removed, Cu adsorption did not differ statistically among the surface samples. Cu adsorption, however, was significantly lower in the subsurface samples of XA and RA. Removal of OM influenced Cu retention only in XA and RA surface samples. In the subsoil, which had low OM contents, elimination of OM did not significantly affect adsorption. In RH, the amounts of Cu adsorbed were not significantly affected by OM removal, and the mineral fraction, basically constituted of kaolinite, contributed almost entirely to the adsorption reactions. When soil pH was below the zero point of salt effect (ZPSE), iron oxides removal increased Cu retention, likely due to a reduction in repulsion between the positively charged surface and the metal. For pH values above ZPSE, oxides effectively contributed for Cu adsorption, since their removal was followed by a decrease in Cu retention.