Copper and zinc sorption‐desorption studies were carried out over a range of pH values using clay fractions separated from two horizons of an acid soil from New York. In the pH range of high sorption, as much as 95% of the sorbed metal could not be desorbed by several washes with 0.05M CaCl2 and thus was considered fixed. Sorption and fixation of Cu and Zn increased rapidly above pH 4 and 5, respectively, for the whole soil clays. Following removal of the oxide fraction by oxalate and citrate‐dithionite extractions, sorption and fixation were reduced considerably at pH values below the onset of hydrolysis of the metals in bulk solution, indicating a significant role of the oxides in metal retention. Citrate‐dithionite extraction was more effective than oxalate in reducing Zn sorption and fixation, suggesting a significant influence of the more crystalline iron oxide fraction. These extraction procedures had less effect on the ability of the clays to sorb and fix Cu. Treatment of the clays by hypochlorite to remove organics tended to either enhance or have little effect on sorption and fixation of Cu and Zn. It is concluded that microcrystalline and noncrystalline oxides in the clay fraction of this soil, representing < 20% of the clay by weight, provide reactive surfaces for the chemisorption of Cu and Zn. At low pH, adsorption at these surfaces may be the dominant mechanism of heavy metal immobilization, especially in the subsoil horizons.

Samples of two New York soils, an acid and a neutral silt loam, were equilibrated with Cu2+ and Cd2+ solutions in concentrations ranging from 10−5 to 10−4M in distilled water and in 0.01M CaCl2. The extent of complexation and adsorption of the heavy metals was measured using specific ion electrodes and atomic absorption. The data were found to fit the Langmuir adsorption model, and the Cu2+ adsorption maxima were greater than those of Cd2+ for both soils. Acid soils demonstrated much less ability to retain the heavy metals than neutral soils. In the presence of 0.01M CaCl2, adsorption of the metals was much reduced, suggesting Ca2+ competition for adsorption sites. A considerable amount of Cu2+ was complexed in the soil solutions, but Cd2+ complexation was much less evident. The results support an ion exchange mechanism of adsorption in the surface soils, and suggest that precipitation occurs in calcareous subsoils.

No abstract

The concentrations of free copper and cadmium ions in soil suspensions were measured by an ion‐selective electrode as a function of pH after equilibrating metal salt solutions with the soils. Similar experiments were done with water‐extractable organic matter. Free (uncomplexed) Cu2+ concentrations were strongly pH‐dependent in the soil systems, but well below the level where precipitation could have occurred. Free Cd2+ concentrations were much less affected by pH adjustment, and the nonacid soil systems approached saturation or oversaturation with respect to CdCO3 precipitation at high pH. Similiar dependence of the free metal ion concentration on pH was found in water extracted organic matter solutions. Precipitation of CdCO3 in oversaturated solutions containing the extracted organic matter was slow, while none of the copper‐organic matter solutions attained oversaturation with respect to Cu(OH)2 or Cu2(OH)2CO3.

No abstract