Vol.:(0123456789) 1 3 International Journal of Environmental Science and Technology https://doi.org/10.1007/s13762-019-02401-8 ORIGINAL PAPER Mechanisms of mercury removal from aqueous solution by high‑fxation hydroxyapatite sorbents N. S. de Resende 1  · C. L. M. Camargo 2  · P. C. Reis 3  · C. A. C. Perez 4  · V. M. M. Salim 1 Received: 28 February 2019 / Revised: 4 May 2019 / Accepted: 8 May 2019 © Islamic Azad University (IAU) 2019 Abstract Hydroxyapatite is an efective 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 difusion 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 fuorescence, feld emission gun scanning electron microscopy, and X-ray powder difraction techniques. Both fresh and spent sorbent samples were analyzed using X-ray powder difraction to assess the structural changes on hydroxyapa- tite crystalline lattice during the mercury sorption process. Rietveld refnement of powder X-ray difraction data allowed insights into the mechanisms of Hg 2+ sorption on synthetic hydroxyapatite. Kinetics and Rietveld refnement 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 difusion along with its incorporation inside the hydroxyapatite crystalline lattice. Furthermore, the results on thermal stability confrmed the mer- cury immobilization by hydroxyapatite, characterizing the spent sorbent as a non-hazardous material and minimizing the environmental impacts of solid waste disposal. Keywords Hydroxyapatite · Mercury immobilization · Non-hazardous spent sorbent · Removal mechanisms · Rietveld refnement Introduction As one of the most toxic heavy metals in the environment, mercury (Hg) has its adverse effects on the ecosystem and human health aggravated due to its bioaccumulative properties (Driscoll et al. 2013; Henriques et al. 2015; Obrist et al. 2018). Indeed, mercury as a global pollutant has been widely discussed by the international government agencies and the scientifc community, demanding eforts to reduce its emissions by the development of new emission control technologies (UNEP 2013; Lin et al. 2017; Chen et al. 2018). Conventional methods for removing inorganic mercury from industrial wastewaters include sulfde precipitation (Jeong et al. 2007), coagulation (Henneberry et al. 2011), ion exchange (Asasian and Kaghazchi 2013), and adsorp- tion (Zhang et al. 2014; Xu et al. 2016; Deliz Quiñones et al. 2016; Naushad et al. 2016; Attari et al. 2017). Hydroxyapatite (HAp) has been pointed out as a promising material for heavy metal removal, especially mercury, from wastewater (Oliva et al. 2011; Kim and Lee 2014). Hex- agonal HAp—Ca 10 (PO 4 ) 6 (OH) 2 —is a crystalline structure consisting of columns of calcium and oxygen parallel to the hexagonal axis. HAp has two non-equivalent Ca sites: the site I which contains Ca 2+ ions in a columnar arrangement, coordinated to nine oxygen atoms of PO 4 3− groups, and Editorial responsibility: Fatih ŞEN. * N. S. de Resende neuman@peq.coppe.ufrj.br 1 Programa de Engenharia Química, PEQ-COPPE, Centro de Tecnologia, Universidade Federal do Rio de Janeiro– UFRJ, Bl. G/115 – Cidade Universitária, Rio de Janeiro, RJ 21941-914, Brazil 2 Escola de Química, Universidade Federal do Rio de Janeiro– UFRJ, Rio de Janeiro, RJ, Brazil 3 Instituto Nacional de Proteção Industrial– INPI, Rio de Janeiro, RJ, Brazil 4 Instituto Federal de Ciência e Tecnologia do Rio de Janeiro - IFRJ, Rio de Janeiro, RJ, Brazil