Competitive Cu and Cd sorption and transport in soils: A combined batch kinetics, column, and sequential extraction study

The competitive effect influenced the transport behavior of Cu and Cd contrastingly in soils, as illustrated by the experimental findings obtained from column, batch kinetics, and sequential extraction tests. Of particular interest, Cd transport behavior changed from nonequilibrium in a single-metal system to equilibrium in a binary-metal system, whereas Cu exhibited a slightly greater degree of nonequilibrium transport under competition. The equilibrium time of specific sorption (∼7 days) was found to be much longer than that of nonspecific sorption (∼30 min). While there was a competitive effect on nonspecific sorption for both Cu and Cd, the majority of rate-limited specific sorption of Cd on oxide and organic matter fractions (contributing to ∼20% of total sorption) was dramatically displaced by Cu. Such a strong suppression of specific sorption of Cd by the presence of Cu resulted in a shorter equilibrium time of overall sorption, which probably accounts for its equilibrium transport. In contrast, the competitive effect on rate-limited sorption and transport behavior of Cu was less significant. This study demonstrated a correlation between the competitive effect of Cu and Cd on their nonspecific and specific sorption and the corresponding significance of rate-limited sorption and nonequilibrium transport behavior.