Natural materials for treatment of industrial effluents: comparative study of the retention of Cd, Zn and Co by calcite and hydroxyapatite. Part I: batch experiments.

This work explores the heavy metal retention capacity of materials developed from minerals that are abundant in nature, with low cost and minimum environmental impact. To accomplish this objective we have: (a) characterized commercial samples of calcite (CA) and hydroxyapatite (HAP)--including their surface properties (BET area, electrophoretic mobility, SEM, and X-ray energy dispersive spectroscopy); and, (b) qualified and quantified the interaction of Cd, Zn and Co with calcite (CaCO3) and hydroxyapatite [Ca5(PO4)3OH] through batch experiments, in a range of metal concentrations (4Zn>Co and Cd>Zn approximately Co, respectively. Retention increased with pCa and pH and could be modeled by: (a) a non-ideal ion exchange mechanism (Me/Ca) for the adsorption of Cd, Zn and Co onto CA; and, (b) a mechanism of non-ideal ion exchange and specific adsorption (Me/Ca and identical with PO4O-Me) in the case of HAP. The pH dependence is indirect in CA and is related to its solubility changes (pCa increases with pH, and so does sorption of Cd, Zn and Co). Both materials, HAP and CA, can be used for heavy metal retention. The former has better performance for water treatment due to its greater efficiency for the retention of Cd, Zn and Co (over two orders of magnitude per gram of material) and its lower solubility in a wide range of pH (6

Sorption of antimony onto hydroxyapatite.

We prepared synthetic hydroxyapatite [HAP; Ca5(PO4)(3-x)(CO3)x(OH)(1+x) (x = 0.3)] and then investigated this material's ability to remove trivalent antimony [Sb(III)] from water. The HAP was characterized by X-ray diffraction analysis, scanning electron microscopy, X-ray energy dispersive spectroscopy, X-ray photoelectron spectroscopy, and infrared spectroscopy. The sorption of Sb(III) to HAP was measured over an Sb(III) concentration range of 0.05-50 mg L(-1), at constant ionic strength (I = 0.01 mol dm(-3)) in equilibrated aqueous suspensions (34 g dm(-3)) for 5 < pH < 8 in vessels that were closed to the atmosphere. Under these conditions, we found that HAP particles were enriched in Ca after incongruent dissolution of the solid. More than 95% of the Sb(III) in solution adsorbed to the solid-phase HAP in <30 min. The equilibrium distribution of Sb(III) (solid vs liquid phase) was characterized by a Langmuir model with gamma(max) = 6.7 +/- 0.1 x 10(-8) mol m(-2) (1.4 +/- 0.2 x 10(-4) mol dm(-3) g(-1)) and K(ads) = 1.5 +/- 0.2 x 10(3) dm3 mol(-1). As Sb adsorption occurred, the pH of the isoelectric point (pH(iep)) of the HAP suspensions declined from 7.7 to 6.9. This finding supports the idea that the negative surface potential of the HAP increased due to the adsorption of Sb as a charged species. The decline in pH(iep) during Sb adsorption plus the thermodynamic description of the Sb(III)-HAP-H2O system suggest likely surface reactions for the interaction of Sb with HAP. We discuss the efficiency of Sb removal from water by HAP in the context of phosphate enrichment.