Phosphate sorption properties of ferrihydrite precipitated in the presence of various anions
Abstract
Natural ferrihydrite contains silicate, phosphate and carbonate. Coprecipitation with these anions may prevent further crystallization of ferrihydrite and increase its phosphate sorption capacity. Synthetic ferrihydrite has been proposed as a dietary P binder for patients with chronic renal failure. Our objective was to determine whether coprecipitated silicate, phosphate, or carbonate will increase the phosphate sorption capacity of ferrihydrites without increasing their acid solubility. Two-line ferrihydrites precipitated in the presence of increasing quantities of silicate, phosphate or carbonate were characterized by phosphate sorption capacity, acid solubility, surface area, powder X-ray diffraction (XRD), and Fourier transform infrared (FTIR) spectroscopy. Powder XRD patterns indicated the presence of 2-line ferrihydrite with the addition of silicate, phosphate or carbonate, but also indicated the presence of goethite at $>$0.3 mole fraction C added. Surface area increased from 310 to 330 $\rm m\sp2g\sp{-1}$ with addition of Si, did not change with added P (average = 303 $\pm$ 12 $\rm m\sp2\ g\sp{-1}),$ and decreased from 285 to 204 $\rm m\sp2g\sp{-1}$ with addition of C. Peak areas of FTIR bands for Si-O-Fe or P-O-Fe stretch increased with increasing anion added to the silicate or phosphate coprecipitates indicating incorporation of anions in ferrihydrite. Infrared spectra of samples with added C did not differ in the region of C-O stretch indicating no incorporation of carbonate. Coprecipitation of up to 0.09 mole fraction Si, up to 0.02 mole fraction P, or addition of up to 0.5 mole fraction C had no affect on ferrihydrite P sorption capacity. Coprecipitated P blocked P sorption sites, since P sorption capacity decreased from 1.8 to 0.8 umol P m$\sp{-2}$ with increasing P incorporated. Examination of -OH stretch bands in FTIR spectra of ferrihydrite indicated that coprecipitated P facilitates H-D exchange, while P sorbed on ferrihydrite inhibits H-D exchange. Acid solubility at pH 2 and 37$\sp\circ$C for 15 or 60 min increased with increasing mole fraction Si and P incorporated and decreased with increasing mole fraction C added. Poorly crystalline goethite precipitated with added carbonate has potential as a phosphate binder since it had a high phosphate sorption capacity and low acid solubility.
Degree
Ph.D.
Advisors
Schulze, Purdue University.
Subject Area
Agronomy|Mineralogy|Geochemistry
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