Geochemical dynamics of Cu(II) in contrasting cocoa soils: Kinetics and isotherm modeling

Understanding copper (Cu) dynamics in cocoa soils can help predict the fate, mobility, and toxicity, which is crucial for sustaining soil functionality, as well as the quality and health safety of cocoa products. Herein, the study investigated sorption-desorption characteristics of Cu(II) in six cocoa soils of varying weathering by kinetics, isothermal, and pH effect experiments in a batch system. Equilibrium sorption increased in the order of Fluvisol < Lixisol < Luvisol < Acrisol = Ferralsol = Nitisol. Kinetics data fitted well to pseudo-second-order and Elovich models. Cu(II) sorption was dominated by outer-sphere and inner-sphere complexations at low and high pH, respectively. The sorption data fitted well with Freundlich and Langmuir's models. Sorption maximum increased in the order of Nitisol (0.91 mg g⁻¹) < Ferralsol (1.21 mg g⁻¹) < Acrisol (1.29 mg g⁻¹) < Lixisol (1.61 mg g⁻¹) < Luvisol (1.68 mg g⁻¹) < Fluvisol (2.01 mg g⁻¹). The binding energy values obtained showed the highest interaction between Cu(II) and soil matrix in Fluvisol and the least in Ferralsol and Nitisol. The desorption studies consistent with the hysteresis index indicated high sorption irreversibility and less Cu(II) mobilization in Fluvisol, Lixisol, Luvisol, and Acrisol vis-à-vis Nitisol and Ferralsol. Nitisol and Ferralsol may promote the possible toxicity of Cu(II) to the soil ecosystem. The study contributes to the sensitivity of Cu(II) mobility and toxicity to the degree of soil weathering; thus, management of heavy metals in cocoa soils should be dependent on soil type.