Structure analysis of citric acid-modified phosphate fertilizer and its effects on water-soluble phosphorus fixation

Objectives  Phosphate fertilizer binds easily to Ca²⁺ and is precipitated when applied to a calcareous soil. This greatly reduces the effectiveness and availability of P for crops. In this study, we prevent phosphorus from binding to improve the efficacy of phosphate fertilizer by modifying it with citric acid.

Methods  This study employed a neutralization method to prepare modified citric acid phosphate fertilizer. Potassium hydroxide and phosphoric acid were mixed in a mass ratio of 4.67∶5.33, citric acid was added to the mixture in the mass percentage of 0, 0.2%, 0.5%, 1.0% and 10.0% before the chemical reaction. The products generated (recorded as CAP0, CAP0.2, CAP0.5, CAP1 and CAP10 in response to the percentage of citric acid addition) were dried, grounded and stored in isolated conditions for analysis. The structure and composition of the products were analyzed using Fourier transform infrared spectroscopy (FTIR), ³¹P solid-phase nuclear magnetic resonance (³¹P-NMR), and liquid chromatography-mass spectrometry (LC-MS). The water-soluble P fixation rate in the products was determined using a CaCl₂ precipitation experiment.

Results  1) Addition of citric acid changed the structure of phosphate fertilizer. Compared with the control (CAP0), a phosphate ester (P—O―C) stretching vibration in FTIR spectra at about 1088–1077 cm^–1 was observed. This indicates the formation of new substances in the modified citric acid phosphate fertilizer. The ³¹P-NMR analysis identified a K₂HPO₄ peak at 6.60 ppm in CAP0, two K₃PO₄ peaks at 12.28 ppm and 4.80 ppm, and two orthophosphoric acid monoester peaks at 12.28 ppm and 4.80 ppm in CAP10. The P in the form of orthophosphoric acid monoester accounted for 16.48% of the total P content in the modified phosphate fertilizer. The results of LC-MS analysis proved that the high-temperature dehydration reaction occured between citric acid and phosphate potassium. Specifically, the —OH in citric acid and the P—OH bond in phosphoric acid were broken. The oxygen atom in the citric acid bonded with the P atom to form orthophosphoric acid monoester (P—O—C). 2) The reaction of citric acid with phosphate reduced the water-soluble P fixation rate. Among the 5 citric acid addition ratios, 0.2% showed the most significant effect, with the water-soluble P fixation rate of 59.52% and the least fixation rate was 24.69% recorded for CAP0. The presence of orthophosphoric acid monoester was responsible for the prevention of P fixation in phosphate fertilizers.

Conclusions  The dehydration reaction caused by the addition of citric acid during the manufacturing of phosphate fertilizer modified the properties of P due to the generation of orthophosphoric acid monoester. The most significant effect in reducing water-soluble phosphorus fixation could be achieved by the addition of 0.2% citric acid to the mass base.