Our kinetics model also reasonably predicted the effect of Ca competition on both Cu and Zn desorption kinetics. The increase in Ca concentrations enhanced both Cu and Zn release from the spiked soils (Figure S5A and S5B, SI), explained by the decrease in equilibrium partition coefficients with Ca concentrations (Figure S6, SI), although the Cu desorption kinetics was complicated by the soluble organic matter release from soils in the pre-equilibration period.