Cao Xinde, Harris Willie G, Josan Manohardeep S, Nair Vimala D
Soil and Water Science Department, University of Florida, Gainesville, FL 32611, USA.
Sci Total Environ. 2007 Sep 20;383(1-3):205-15. doi: 10.1016/j.scitotenv.2007.05.012. Epub 2007 Jun 13.
Precipitation of Ca phosphates plays an important role in controlling P activity and availability in environmental systems. The purpose of this study was to determine inhibitory effects on Ca phosphate precipitation by Mg(2+), SO(4)(2-), CO(3)(2-), humic acid, oxalic acid, biogenic Si, and Si-rich soil clay commonly found in soils, sediments, and waste streams. Precipitation rates were determined by measuring decrease of P concentration in solutions during the first 60 min; and precipitated solid phases identified using X-ray diffraction and electron microscopy. Poorly-crystalline hydroxyapatite (HAP: Ca(5)(PO(4))(3)OH) formed in control solutions over the experiment period of 24 h, following a second-order dependence on P concentration. Humic acid and Mg(2+) significantly inhibited formation of HAP, allowing formation of a more soluble amorphous Ca phosphate phase (ACP), and thus reducing the precipitation rate constants by 94-96%. Inhibition caused by Mg(2+) results from its incorporation into Ca phosphate precipitates, preventing formation of a well-crystalline phase. Humic acid likely suppressed Ca phosphate precipitation by adsorbing onto the newly-formed nuclei. Presence of oxalic acid resulted in almost complete inhibition of HAP precipitation due to preemptive Ca-oxalate formation. Carbonate substituted for phosphate, decreasing the crystallinity of HAP and thus reducing precipitation rate constant by 44%. Sulfate and Si-rich solids had less impact on formation of HAP; while they reduced precipitation in the early stage, they did not differ from the control after 24 h. Results indicate that components (e.g., Mg(2+), humic acid) producing relatively soluble ACP are more likely to reduce P stability and precipitation rate of Ca phosphate in soils and sediments than are components (e.g., SO(4)(2-), Si) that have less effect on the crystallinity.
磷酸钙的沉淀在控制环境系统中磷的活性和有效性方面起着重要作用。本研究的目的是确定土壤、沉积物和废水中常见的镁离子（Mg²⁺）、硫酸根离子（SO₄²⁻）、碳酸根离子（CO₃²⁻）、腐殖酸、草酸、生物源硅和富硅土壤黏土对磷酸钙沉淀的抑制作用。通过测量最初60分钟内溶液中磷浓度的降低来确定沉淀速率；并使用X射线衍射和电子显微镜鉴定沉淀的固相。在24小时的实验期间，对照溶液中形成了结晶度较差的羟基磷灰石（HAP：Ca₅(PO₄)₃OH），其对磷浓度呈二级依赖关系。腐殖酸和镁离子（Mg²⁺）显著抑制了HAP的形成，使得形成了更易溶的无定形磷酸钙相（ACP），从而使沉淀速率常数降低了94% - 96%。镁离子（Mg²⁺）引起的抑制作用是由于其掺入磷酸钙沉淀物中，阻止了结晶良好的相的形成。腐殖酸可能通过吸附到新形成的晶核上抑制了磷酸钙沉淀。草酸的存在由于优先形成草酸钙而几乎完全抑制了HAP沉淀。碳酸根取代了磷酸根，降低了HAP的结晶度，从而使沉淀速率常数降低了44%。硫酸根和富硅固体对HAP形成的影响较小；虽然它们在早期降低了沉淀，但在24小时后与对照无差异。结果表明，与对结晶度影响较小的成分（如硫酸根离子（SO₄²⁻）、硅）相比，产生相对易溶的ACP的成分（如镁离子（Mg²⁺）、腐殖酸）更有可能降低土壤和沉积物中磷的稳定性以及磷酸钙的沉淀速率。
