Competitive adsorption of copper and zinc by a Bt horizon of a savanna Alfisol as affected by pH and selective removal of hydrous oxides and organic matter

Abstract

The preferential retention of heavy metals by soils is critical to their availability and mobility through the soils to contaminate groundwater. We examined the competitive adsorption of Cu and Zn by Bt horizon of a Savanna Alfisol from a dilute salt solution under a variety of conditions. Copper and Zinc adsorption from the Cu–Zn binary solution produced a characteristic H-type isotherm for Cu, and a linear isotherm for Zn. The distribution coefficient K d, which defines the affinity of the metals for the soil adsorbents, was five times greater for Cu than Zn. The adsorption envelope of the metals showed that maximum Cu adsorption occurred at pH 5.5 as against pH 6.8 for Zn adsorption probably reflecting difference in the first hydrolysis constant of the metals. The removal of organic matter from the soil reduced K d,Cu 40 times as compared to the natural soil, whereas K d,Zn was reduced by half as compared to the natural soil. Similarly, removing amorphous hydrous oxide by extraction with oxalate solution prior to the introduction of Cu and Zn reduced K d,Cu 100 times, and K d,Zn by 20 times as compared to the natural soil, but when both amorphous and crystalline hydrous oxides were removed prior to the introduction of Cu and Zn, the K d,Cu was on par with the natural soil, whereas K d,Zn increased by more than two times as compared to the natural soil. It seems that reactions with organic matter and amorphous oxides are the major controls on Cu, whereas Zn might be largely sorbed by cation exchange reaction when in competition with Cu for binding sites in soils. The metal binding sites in the amorphous hydrous oxides and organic matter were more selective for Cu than Zn.