Biogenic hydroxyapatite (Apatite II™) dissolution kinetics and metal removal from acid mine drainage

Apatite II (TM) is a biogenic hydroxyapatite (expressed as Ca-5(PO4)OH) derived from fish bone. Using grains of Apatite II (TM) with a fraction size between 250 and 500 mu m, batch and flow-through experiments were carried out to (1) determine the solubility constant for the dissolution reaction Ca-5(PO4)(3)(OH) double left right arrow 5Ca(2+) +3PO(4)(3-) + OH-. (2) obtain steady-state dissolution rates over the pH range between 2.22 and 714, and (3) study the Apatite II (TM)'s mechanisms to remove Pb2+, Zn2+. Mn2+, and Cu2+ from metal polluted water as it dissolves. The log K-s value obtained was -50.8 +/- 0.82 at 25 degrees C. Far-from-equilibrium fish-bone hydroxyapatite dissolution rates decrease by increasing pH. Assuming that the dissolution reaction is controlled by fast adsorption of a proton on a specific surface site that dominates through the pH range studied, probably P-O-, followed by a slow hydrolysis step, the dissolution rate dependence is expressed in mol m(-2) s(-1) as Rate(25) degrees(C) = -8.9 x 10(-10) x 9.96 x 10(5) x a(H+)/1 + 9.96 x 10(5) x a(H+) where a(H+) is the proton activity in solution. Removal of Pb2+, Zn2+, Mn2+, and Cu2+ was by formation of phosphate-metal compounds on the Apatite II (TM) substrate, whereas removal of Cd2+ was by surface adsorption. Increase in pH enhanced the removal of aqueous heavy metals. Using the kinetic parameters obtained (e.g., dissolution rate and pH-rate dependence law), reactive transport simulations reproduced the experimental variation of pH and concentrations of Ca, P and toxic divalent metal in a column experiment filled with Apatite II (TM) that was designed to simulate the Apatite II (TM)-metal polluted water interaction. (C) 2012 Elsevier B.V. All rights reserved.