Effect of Organic Acids on Release of Phosphorus From Phosphate Rocks1

The release of P from two phosphate rocks (PRs) of different reactivity with equimolar concentrations (1 or 10 mmol/L) each of 19 low-molecular-weight (LMW) organic acids was studied. The results were compared with those released with a similar concentration of one of three mineral acids (H2SO4, HNO3, and HCI). The PRs were Kodjari PR (low reactive PR) and North Florida PR (medium reactive PR). The LMW organic acids included two monocarboxylic aliphatic acids (pyruvic and glycolic acids), nine dicarboxylic aliphatic acids (oxalic, tartaric, malonic, fumaric, malic, α-ketoglutaric, maleic, oxalacetic, and succinic acids), two aliphatic tricarboxylic acids (citric andcis-aconitic acids), and six aromatic acids (phthalic, salicylic, vanillic, gallic, protocatechuic, andp-hydroxybenzoic acids). In these studies, a 1-g sample of <100-mesh-sieved PR was equilibrated with 25 ml of each acid at 25°C for 24 h. The P in the filtrate was determined. Results showed that the release of P from the PRs with the acids was affected by the chemical structure, type, and position of the functional groups of the ligands, and the concentrations of the acids. The ratios of the P-released/protons-added varied among the acids, phosphate rocks, and the concentration of the acid used. These ratios for most of the organic acids were greater than those for the mineral acids, suggesting chelation of the metals associated with P in the PRs. In general, the aliphatic acids with β-hydroxyl and α-carboxyl groups were more effective than other aliphatic or aromatic acids in releasing P from PRs. At 1 mmol/L acid, citric and oxalic acids were more effective than H2SO4 in releasing P from the two PRs; the P released from Kodjari PR was greater than that released from North Florida PR. Increasing the acid concentration from 1 to 10 mmol/L corresponded to almost a 10-fold increase in the P released from the rocks. At both concentrations of acids, the amounts of P released from the rocks with the organic and mineral acids were significantly correlated with the amounts of Ca and Mg released. The amounts of P released were negatively correlated with the pK1 values and positively correlated with the log KAI values of the organic acids.