Precipitation of Dicalcium Phosphate Dihydrate in the Presence of Organic Acids

Inhibition of dicalcium phosphate dihydrate (DCPD) precipitation rate by organic acids may play an important role in controlling phosphate activity and availability in P-fertilized soils. The precipitation rate of DCPD was measured at pH 5.7 and 25 °C in the absence and presence of organic acids common to soil solutions using a seeded crystal growth method. Forward rate constants (kf) were determined from an integrated form of the second-order rate law expression: rate = kfS[Ca²⁺][HPO²⁻₄]γ²₂, where brackets represent concentrations, S is the surface area (m² L⁻¹), and γ₂ is the divalent ion activity coefficient calculated from the Davies equation. The average kf (L² mol⁻¹ m⁻² s⁻¹) for DCPD precipitation without organic acids was 9.7 ± 0.5. Additions of 0.26 to 2.06 mM C as humic acid and 1.1 to 3.3 mM C as fulvic acid resulted in induction periods lasting 2.0 to 632 and 1.5 to 10 min, respectively, prior to subsequent precipitation of DCPD. This indicates that DCPD precipitation was able to overgrow adsorbed humic and fulvic acids. Rates of DCPD precipitation were determined in the presence of humic, fulvic, citric, and tannic acids at concentrations ranging from 0.26 to 9.0 mM total soluble C (CTS). The highest levels of humic, fulvic, citric, and tannic acids added were 2.05, 7.8, 8.0, and 9.0 mM CTS, which resulted in decreased kf values of 1.1, 0.1, 0.6, and 2.1 L² mol⁻¹ m⁻² s⁻¹, respectively. Precipitation was inhibited by adsorption of these organic acids onto DCPD surfaces blocking sites acting as nuclei for new crystal growth. Adsorption characteristics of these organic acids were related to their functional-group content and size. The ability of DCPD to precipitate on seed crystals with adsorbed C may explain the kinetically favored formation of DCPD vs. more thermodynamically stable octacalcium phosphate (OCP) and hydroxyapatite (HAP), where precipitation rates are more strongly inhibited by soluble C.