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

Although the radionuclide Co-60 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 x 10(4) to 207 mL g(-1), indicating DCPD would be more applicable in treating soft Co-60-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.

Automated summary

This study investigated the batch adsorption performance of dicalcium phosphate dihydrate (DCPD) as a potential cobalt adsorbent. Stable cobalt uptake was observed at pH 4-8, with the structure analysis revealing decomposition of DCPD during adsorption and involvement of multiple mechanisms in the process. Additionally, intraparticle diffusion contributed to the overall removal kinetics of Co2+ from water.