Ecological Implications of Calcium Adsorption–Desorption on Maize-Derived Biochar for Sustainable Soil Nutrient Retention

Biochar has emerged as a sustainable material for improving soil quality and nutrient cycling in agroecosystems. This study investigated the adsorption–desorption behaviour of calcium ions (Ca²⁺) on maize-derived biochar produced at 400 °C and evaluated its potential role in enhancing soil nutrient retention. The adsorption characteristics were analyzed using Langmuir and Freundlich isotherm models, while adsorption kinetics were assessed using pseudo-first-order and pseudo-second-order models. The Freundlich model provided the best fit to the experimental data, indicating multilayer adsorption on a heterogeneous biochar surface. Kinetic analysis revealed that the pseudo-second-order model most accurately described the adsorption process, suggesting that chemisorption was involved during the initial sorption stage. The adsorption performance was strongly influenced by the physicochemical properties of the biochar, including porous structure, elevated cation exchange capacity, and the presence of oxygen-containing functional groups such as hydroxyl and carboxyl groups. These properties promoted Ca²⁺ retention through electrostatic attraction, ion exchange, and surface complexation mechanisms. Desorption analysis showed a hysteresis index (HI) close to unity, indicating a moderately reversible sorption–desorption process that enabled both nutrient retention and gradual nutrient release. This balanced interaction between adsorption and desorption is beneficial for sustaining nutrient availability while reducing nutrient leaching losses in soil systems. The findings highlight the ecological significance of maize-derived biochar as a sustainable soil amendment for improving nutrient cycling, enhancing soil fertility, and supporting resilient agroecosystem management.