Wednesday, November 15, 2006 - 3:00 PM

Inhibition of Calcium Phosphate Precipitation under Conditions Typical of Manure-Amended Soil Solutions.

Xinde Cao, Willie Harris, Manohardeep Josan, and Vimala Nair. Univ of Florida, Soil & Water Science Dept, 2169 McCarty Hall A, Gainesville, FL 32611

Soils amended with manure tend to leach high levels of phosphorus (P) despite conditions that appear favorable for stable Ca phosphate formation. The objective of this study was to identify possible inhibitors for the precipitation of Ca phosphates in the manure-amended soils. Thus, a chemically defined system was applied to investigate effects of manure-derived components (e.g., Mg, sulfate, carbonate, dissolved organic carbon) on Ca phosphate precipitation under conditions simulating those in the manure-amended soil solution. X-ray diffraction (XRD) and scanning electron microscopy equipped with energy dispersive X-ray spectroscopy (SEM-EDS) were used to characterize the solids formed in the solution. Visual MINTEQ modeling was conducted to calculate mineral solubility for treatment solutions. Poorly-crystallized hydroxyapatite (Ca5(PO4)5OH) was identified in all solutions except those containing Mg, humic acid, and oxalic acid in which only amorphous Ca phosphate was formed. Sulfate, carbonate, biogenic silicon, and low-density clay had little impact on formation of Ca phosphates, but Mg, humic acid, and oxalic acid significantly inhibited the precipitation of Ca phosphates. Over 24 hours, Mg, humic acid, and oxalic acid reduced P precipitation rates by 72.2 %, 66.7 %, and 69.4 %, respectively, compared to that in the control system. Inhibition caused by Mg may result from Mg incorporation into Ca phosphate precipitates, preventing development of long-range order of Ca phosphate formation. Complexation of humate with Ca would reduce activities of free Ca and P and hence hindered precipitation of Ca phosphates. Oxalate-induced inhibition was attributed to kinetically favored formation of Ca oxalate precipitate prior to formation of Ca phosphates. Our finding that manure-derived components Mg, humic acid, and oxalic acid hindered Ca phosphate precipitation provides strong evidence to explain high leachability of P occurring in manure-amended soils.