Co-adsorption behavior of Cr(III) and Cd(II) via direct application of argillaceous limestone

The study investigated the removal of Cr(Ⅲ) and Cd(Ⅱ) by argillaceous limestone (AL) in a binary adsorption system. The results indicate that AL's capacity to adsorb Cr(Ⅲ) is enhanced by the presence of Cd(Ⅱ). Conversely, the presence of Cr(Ⅲ) significantly decreases the adsorption of Cd(Ⅱ). The adsorption mechanisms involve precipitation, chelation, and ion exchange. AL's adsorption behavior was well described by the pseudo-second-order kinetic model, suggesting that the adsorption process was dominated by chemical interactions and the monolayer adsorption pattern. The pH affects adsorption primarily by altering the protonation degree of the AL surface. In the single-component system, the maximum adsorption capacity for Cr(III) reached 30.98 mg·g⁻¹ at pH 7.0, while that for Cd(Ⅱ) reached 38.80 mg·g⁻¹ at pH 8.0. In the binary system, Cr(Ⅲ) achieved its highest adsorption capacity (42.56 mg·g⁻¹) at pH 6.0, while Cd(Ⅱ) exhibited a maximum capacity of 19.16 mg·g⁻¹ at pH 7.0. The presence of Na⁺ and Ca²⁺ ions reduced the adsorption efficiency, with greater inhibition observed as ionic strength increased. This study clarifies the composite adsorption behavior of AL in multi-ion systems and can advance the direct application of natural minerals in environmental pollution remediation.