The influence of thermodynamic and kinetic factors on the formation and dissolution of calcium phosphates in complex geochemical systems

磷酸钙（Ca-P）的形成是从天然水体中去除磷(P)的重要过程，而碳酸盐氟磷灰石（CFAP）是海洋中活性磷的主要汇。当地条件可能有利于磷灰石的直接形成或磷灰石前体成核。然而，由于热力学和动力学因素之间的相互作用，这些矿物的原位沉淀和溶解难以预测。该项目通过结合常规沉积物和孔隙水分析、实验方法和地球化学模型以及最先进的技术来确定固相P形态，评估这些因素对Ca-P沉淀和溶解的影响。主要目标是(i)确定波罗的海沉积物中磷的形态；（ii）利用成核试验估算不同实验条件下海水和咸淡水中Ca-P沉淀的时间；（iii）利用模拟海洋、半咸淡水和淡水条件的溶解测定法，确定沉积物-水系统中Ca-P相的溶解度；（iv）确定磷灰石前体在P成岩作用中的作用；(v)利用地球化学模型和交叉验证分析方法验证结果。测定Ca-P相的技术包括化学和同步加速器方法，并附带扫描电子显微镜和电子探针分析。这些方法将与方法验证一起，有助于更好地了解复杂自然环境下的P动力学和P成岩作用。

Formation of calcium phosphate (Ca-P) is an important process in removing phosphorus (P) from natural waters, with carbonate fluorapatite (CFAP) being a major sink for reactive P in the ocean. Local conditions may either favor direct apatite formation or nucleation of apatite precursors. In situ precipitation and dissolution of these minerals are, however, difficult to predict owing to the interplay between thermodynamic and kinetic factors. This project assesses effects of these factors on Ca-P precipitation and dissolution by combining conventional sediment and pore-water analyses, experimental approaches and geochemical modelling with state-of-the-art techniques for determining solid-phase P speciation. Main objectives are to (i) determine P speciation in Baltic Sea sediments; (ii) estimate the time of Ca-P precipitation in sea and brackish water under different experimental conditions using a nucleation assay; (iii) determine solubilities of Ca-P phases in sediment-water systems using a dissolution assay that mimics marine, brackish and freshwater conditions; (iv) identify the role of apatite precursors during P diagenesis; (v) verify results using geochemical models and cross-validate analytical methods. Techniques for determining Ca-P phases include chemical and synchrotron-based methods with accompanying scanning electron microscopy and electron microprobe analysis. These approaches will, together with the method validation, contribute to better understand P dynamics and P diagenesis in complex natural environments.