Inhibition of Hydroxyapatite Precipitation in the Presence of Fulvic, Humic, and Tannic Acids

The kinetics of precipitation of hydroxyapatite (Ca₅(PO₄)₃OH, HAP) may be important in explaining the oversaturation of many natural waters with respect to pure HAP. Consequently, the objectives of this study were to determine the rate of HAP precipitation in the absence and presence of organic acids commonly found in soil solutions, which might act as potential inhibitors of solid phase precipitation. In well-stirred solutions, the forward rate of HAP precipitation (R) between pH 7.4 to 8.4 was described with the experimentally derived rate equation: R = skfγ₂γ₃[Ca²⁺][PO³⁻₄], where s = HAP surface area (m²L⁻¹), kf = forward rate constant (L²mol⁻¹m⁻²s⁻¹), γ₂ and γ₃ = di- and trivalent ion activity coefficients calculated from the Davies equation, and brackets represent concentrations of free hydrated species (mol L⁻¹). The average kf in the absence of organic acids was 173 ± 11 L²mol⁻¹m⁻²s⁻¹. In the presence of 0.6 mM, 0.68 mM and 0.98 mM CTS (total soluble organic C) added as fulvic, humic, or tannic acid, the kf decreased to 2, 19, and 16 L²mol⁻¹m⁻²s⁻¹, respectively. The mechanism of precipitation inhibition in the presence of these acids appears to be adsorption of the organic ligand on the HAP seed crystals, thereby blocking sites for crystal growth. Smaller molecular weight organic acids such as citric, gallic, syringic, adipic, and azealic acid were not as effective at inhibiting HAP precipitation per unit CTS present, which may indicate the importance of larger molecular weight acids on actual physical coverage at the HAP surface. The inhibition of HAP precipitation in the presence of humic, fulvic, and tannic acids may explain why many soil solutions are supersaturated with respect to pure HAP, and why increases in P concentrations are observed in many agricultural studies in the presence of sludge or manure. Contribution from the Oklahoma Agric. Exp. Stn., Dep. of Agronomy, Oklahoma State Univ., Stillwater, OK 74078, and the Montana Agric. Exp. Stn., no. J-2015, Dep. of Plant and Soil Science, Bozeman, MT 59717.