Abstract Phosphorus (P) availability in calcareous soils is limited by the low solubility of calcium phosphate minerals, such as hydroxyapatite (HAP). Carboxylic acid‐promoted dissolution of HAP is well‐known. However, it is not clearly understood how the composition and orientation of functional groups (e.g., carboxyl and β‐hydroxyl group) in carboxylic acids affect the HAP dissolution. Accordingly, six carboxylic acids (monocarboxylic acid: acetic acid, dicarboxylic acid: fumaric acid, maleic acid, and tartaric acid, and tricarboxylic acid: tricarballylic acid and citric acid) that promoted HAP dissolution were investigated in a batch mode as a function of ligand concentration (0–6 mM) at a constant pH of 8. The extent of HAP dissolution by ligands was varied from 53 to 484 µmol of P L −1 . Under equal concentrations of total carboxylic functional groups, the kinetic rate constant estimated by the exponential model was not similar among all ligands. Instead, the highest rate was with citric acid (120.55–343.10 µmol P h −B , where B is 0.06–0.08), followed by tartaric acid, tricarballylic acid, maleic acid, fumaric acid, and acetic acid. This suggests that the composition and orientation of functional groups in each ligand are more important factors in explaining the ligand‐specific dissolution order. Two mechanisms associated with the composition and orientation of functional groups in ligands were discussed. They are (1) disequilibrium through ligand‐Ca aqueous complexation and (2) ring size (five‐ > six‐ > seven‐membered) of chelated–ligand surface complex on the mineral surface. The finding adds insight into the carboxylic ligand‐promoted dissolution of HAP in alkaline soils.
Freiberg et al. (Sun,) studied this question.