Hydroxyapatite as a key material for water treatment: converting surface complexity into opportunity

The hydroxyapatite (HAP) mineral is a calcium hydroxyphosphate, whose chemical formula, Ca10-x(PO4)6-x(HPO4)x(OH)2-x with 0<1, well reflects its peculiar compositional variability.1 In addition, HAP is characterized by a high structural flexibility which allows to accommodate cations (bivalent and trivalent metal ions, and anions without any significant lattice distortion.2 For this reason it has been extensively studied as sorbent material in the treatment of heavy metal polluted wastewater. However, the tendency to undergo compositional variations with ion/anion substitutions (e.g. incorporation of carbonate anions, CO32-) confers to the HAP surface layers an intrinsic complexity resulting in the co-presence of structural defects (vacancies) and of various surface groups (OH-, PO43-, HPO42-, CO32-). Far from being a mere limitation, this complexity can become a potential tool to address the metal trapping process, which is known to proceed alternatively according to three mechanisms: a) ion exchange, b) surface complexation, c) dissolution-precipitation.3 In this work HAP samples with tailored compositional and morphological properties have been synthesized. The trapping efficiency and affinity towards selected heavy metals (Cr3+, Pb2+, Cu2+, Ni2+) were evaluated in batch tests, by contacting HAP powders with single- and multi-metal solutions, so as to simulate polluted wastewaters. HAP samples have been characterized by FT-IR spectroscopy, XRPD, TEM-EDX, and N2-adsorption/desorption analyses, before and after use as sorbents in the metal capture tests. Specific attention was deserved to some relevant HAP surface features. In particular, carbonate content and location were determined by thermogravimetric analysis and infrared spectroscopy. Effective acidity/basicity (defined as the number of surface acidic/basic site number in water) was determined through liquid-solid acid-base titrations performed in a modified liquid-chromatograph (HPLC). This approach allowed to unravel the eventual structure-sensitivity of the different metal uptake mechanisms and to extrapolate useful structure-activity relationships for future material optimization.