Copper retention kinetics in acid soils

Abstract

Retention and release kinetics of Cu on four acid Typic haplumbrepts developed on two different types of parent rock material (granite and amphibolite) were studied with a stirred-flow chamber (SFC) method. The granitic soils were lower in organic material and lower in Fe and Al oxides than the soils formed in amphibolite. The kinetic parameters were assessed in four consecutive Cu retention–release cycles by alternately applying pulses of solutions with and without Cu. Granite soils showed lower total Cu retention (7–12 mmol kg−1) than amphibolite soils (16–21 mmol kg−1) after one single pulse application of 0.0787 mmol Cu L−1 at pH 5.5, which may be due to differences in their organic and oxides compositions. The amount of Cu retained diminished to 40 to 25% in the third retention cycle relative to the first, suggesting that the soils’ Cu retention depends on the previous metal loading. Conversely, the released Cu was approximately 20% of that retained in the first cycle, but the amounts released were similar for all cycles and all soils. The results obtained were fitted using a first-order equation for both retention and release of Cu. In the first cycle, first order rate coefficients of retention ranged from 0.084 to 0.56 min−1, and increased by about a factor of two in the next cycle. Release rate coefficients were more than 10 times lower than those of retention, and less dependent on the previous metal loading. The process of retention and release of Cu was found to be hysteretic, which suggest that the desorption mechanism or path is not an exact opposite of the adsorption mechanism or path.