From brushite to hydroxylapatite: A study on phosphate mineral transformation and the fate of oxytetracycline

Livestock manure, a common fertilizer in Chinese agriculture, can lead to environmental contamination and potential health risks due to elevated antibiotic and phosphorus levels. Importantly, the high phosphorus levels initiates transformations of phosphate minerals in soils, especially calcareous soils. These variations in phosphate mineralogy can significantly impact the migration and fate of antibiotics within the soil. However, the impact of the transformation process, particularly involving the metastable phase brushite (DCPD), on the fate of antibiotics remains unclear. In this study, we synthesized DCPD and hydroxylapatite (HAP) and examined their transformation process to assess their removal capacity and investigate the migration and fate of oxytetracycline (OTC). The findings reveal that HAP exhibits a maximum immobilization capacity for OTC of 20.10 mg/g, surpassing that of DCPD by 2.56 times (7.86 mg/g). This disparity in immobilization capacity between DCPD and HAP leads to a redistribution of OTC between the solid and liquid phases during the transformation process. Notably, the introduction of OTC also inhibits the transformation process, potentially impacting the fate of other potentially harmful elements. The study highlights that the transformation process of calcium phosphorus minerals has a significant impact on the mobility and fate of antibiotics in soil, which aids in better management and mitigation of the environmental risks associated with fertilizer application.