Mechanisms of mercury removal from aqueous solution by high-fixation hydroxyapatite sorbents

Hydroxyapatite is an effective adsorbent for mercury removal from aqueous solutions, although capture mechanisms are not yet clearly understood. The purpose here is to investigate the type of interactions between mercury and the surface sites as well as intraparticle diffusion mechanism in this sorbent, correlating with the mercury immobilization by hydroxyapatite. Physicochemical properties of a synthetic nano-hydroxyapatite were determined by nitrogen adsorption isotherms at 77 K, X-ray fluorescence, field emission gun scanning electron microscopy, and X-ray powder diffraction techniques. Both fresh and spent sorbent samples were analyzed using X-ray powder diffraction to assess the structural changes on hydroxyapatite crystalline lattice during the mercury sorption process. Rietveld refinement of powder X-ray diffraction data allowed insights into the mechanisms of Hg2+ sorption on synthetic hydroxyapatite. Kinetics and Rietveld refinement results revealed the immobilization of mercury in the spent sorbent, suggesting that this process takes place at two steps: (1) the mercury complexation with surface phosphate sites of hydroxyapatite followed by (2) the slower mercury diffusion along with its incorporation inside the hydroxyapatite crystalline lattice. Furthermore, the results on thermal stability confirmed the mercury immobilization by hydroxyapatite, characterizing the spent sorbent as a non-hazardous material and minimizing the environmental impacts of solid waste disposal.