Mechanistic understanding of Nickel(II) adsorption onto fluorapatite-based natural phosphate via Rietveld refinement combined with Monte Carlo simulations

This paper thoroughly investigated the adsorption mechanism of nickel ions (Ni²⁺) on fluorapatite-based natural phosphate (AFap) by Rietveld refinement combined with Monte Carlo simulations. The experimental results indicated that the cation exchange was the predominant mechanism in removing Ni²⁺ ions and the Pseudo first order described better the kinetic data suggesting that the adsorption rate depended on the concentration. The application of Rietveld structure refinement confirmed the preferable cation-exchange sites for Ni²⁺ in the AFap structure at low and high metal concentrations. Accordingly, Ni²⁺ occupancies within the AFap structure were estimated, and a new chemical formula was proposed. Additionally, Monte Carlo simulations gave more insights into the pH effect on the Ni²⁺ uptake behavior onto AFap surface through the energetic analysis and system configuration. The results highlight the noticeable ability of the phosphate surface to adsorb Ni²⁺ in the near-neutral environment compared to the acidic one.