title: Carbonate and magnesium interactive effect on calcium phosphate precipitation.
abstract: Precipitation of Ca phosphates, an important process in controlling P stability and activity in P-fertilized soils and P-rich wastewater, is often affected by other components. The purpose of this study was to document interactive effects of CO3(2-) and Mg2+ on Ca phosphate precipitation under conditions simulating (i) dairy manure-amended soil leachate (system I; pH 7.1) and (ii) P recovery from flushed dairy manure wastewater (system II; pH 9.2). Hydroxyapatite (HAP) and more soluble amorphous Ca phosphate (ACP) were formed in the control solutions of system I and system II, respectively. Carbonate only slightly affected the crystallinity of the precipitate, but significantly reduced the precipitation rate via CO3(2-) competition for PO4(3-) (system I) or preemptive CaCO3 precipitation (system II). Magnesium severely inhibited both precipitate crystallinity and precipitation rate, allowing formation of ACP in both systems, presumably due to Mg2+ incorporation into the Ca phosphate structure to form Mg2+ -substituted structure that crystallized to whitlockite upon heating. Coexistence of CO32- and Mg2+ in system I showed a synergistic inhibitory effect, compared to their individual presence, probably because both CO3(2-) and Mg2+ were incorporated into the precipitate. However, in system II, the individual inhibitory effect of CO3(2-) or Mg2+ was eliminated when both were present. The likely mechanism involves formation of aqueous MgCO3 (aq) which reduces free CO3(2-) and Mg2+ activities, resulting in less preemptive CaCO3 formation and enhanced Ca phosphate precipitation.
authors.affiliation: Soil and Water Science Department, University of Florida, Gainesville, Florida 32611, USA.
authors.forename: Xinde
authors.initials: X
authors.lastname: Cao
authors.forename: Willie
authors.initials: W
authors.lastname: Harris
mesh_terms: D000818:Animals; D002130:Calcium Phosphates / Q000737:chemistry*; D002254:Carbonates / Q000737:chemistry*; D002417:Cattle; D011232:Chemical Precipitation; D003460:Crystallization; D005308:Fertilizers; D008274:Magnesium / Q000737:chemistry*; D008372:Manure; D013055:Spectrophotometry, Infrared; D014865:Waste Disposal, Fluid; D014961:X-Ray Diffraction
chemical_list: D002130:Calcium Phosphates; D002254:Carbonates; D005308:Fertilizers; D008372:Manure; C020243:calcium phosphate; D008274:Magnesium