Efficient adsorptive removal of Cobalt(II) ions from water by dicalcium phosphate dihydrate

Although the radionuclide ⁶⁰Co is widely used, its presence in various effluents demands its removal to preclude environmental pollution and detrimental effects on human health. This study investigated the batch adsorption performance of a potential cobalt adsorbent, dicalcium phosphate dihydrate (DCPD), in immobilizing Co²⁺ from water. The influences of solution pH, contact time, initial concentration, and competing cations were examined and discussed. Stable cobalt uptake was observed at pH 4-8. The sorption kinetics showed a multi-stage uptake profile, implying that several mechanisms are involved in the adsorption process. Microscopy and structural analysis revealed that DCPD decomposes to its anhydrous form during adsorption, which explains the multistep curve over the entire adsorption period. However, the non-apatitic transformation is not exclusive to cobalt uptake. Intraparticle diffusion also contributed to the overall removal kinetics of Co²⁺ from water. Considering the Sips isotherm model, the maximum Co²⁺ adsorption capacity of DCPD was 441 mg g^-1. Cobalt uptake selectivity dropped in the presence of Ca²⁺ ions, from 1.21 × 10⁴ to 207 mL g^-1, indicating DCPD would be more applicable in treating soft ⁶⁰Co-contaminated waters. Structural analysis, elemental mapping, and qualitative analysis of solid residues confirmed that ion exchange is involved in the removal of cobalt from aqueous solutions.