Abstract No Hung0356

No Hung0356 A P-XANES Study of Phosphate Sorption to Gibbsite and Calcium Carbonate S. Hunger, K.E. Staats, and D.L. Sparks (U. of Delaware) Beamline: X19A Introduction. Sorption and precipitation reactions of phosphate in soils are of interest as they determine phosphate mobility and bioavailability. Soils, which have continuously received P fertilizers over decades or centuries, can act as non-point sources for P in the environment with detrimental effects on water quality and aquatic biosystems. It is therefore of importance to manage nutrient inputs into soils, particularly in view of a more sustainable agriculture. In acid to neutral soils, phosphate is predominantly sorbed to Al and Fe (oxy)hydroxides, in more basic soils, phosphate sorption reactions are dominated by Ca and Mg minerals. Ca plays also an important role in phosphate reactions in limed acid soils. An important question is whether phosphate is sorbed as surface complexes or more stable surface precipitates (AlPO4, FePO4 or various calcium phosphate phases). P-XANES has been shown to be a versatile technique to identify solid phosphate phases in situ in heterogeneous systems, such as soils. It was therefore used during the past beamtime cycle to investigate phosphate sorption reactions to gibbsite (Al(OH)3) in the presence of calcium and to calcium carbonate. Materials and Methods. Samples of co-adsorbed phosphate and Ca on gibbsite were prepared at pH 6 and 8. The gibbsite suspension (ρ = 2.5 g/L), which was pre-equilibrated at the appropriate pH for 48 hours in a background electrolyte of 0.1 M NaCl, was spiked with a 50 mM CaCl2 solution to give total concentrations of 0.5 mM or 0.1 mM and allowed to equilibrate for 60 seconds. It was then spiked with a 0.1 M phosphate solution to give total concentrations of 0.25 mM or 1.0 mM and stirred for 24 hours while the pH was kept constant, first by means of an automated titrator, then by manual addition of small quantities of 0.1 M HCl. The solids were recovered by centrifugation, washed once with ethanol to remove entrained solution and freeze-dried. Samples of phosphate sorbed to calcium carbonate were prepared in a similar fashion at pH 8 using a suspension density of 5.0 g/L and initial concentrations of 5 and 1 mM phosphate. Here, reaction times of 2 days and 30 days were chosen. All P-XANES data were collected at beamline X-19A in fluorescence mode using a PIPS detector. Results and Discussion. The spectra of phosphate sorbed to calcium carbonate at pH 8 (Fig. 1) show 2.14 2.16 2.18 2.20 no rm al iz ed a bs or ba nc e (a rb itr ar y un its ) energy (keV) 0.192 mmol/g pH 8, 30 d 0.962 mmol/g pH 8, 30 d 0.132 mmol/g pH 8, 2 d 0.542 mmol/g pH 8, 2 d Ca 3 (PO 4 ) 2 Figure 11: Phosphate sorbed to calcium carbonate at pH 8, 5.0 g / L, and 0.1 M NaCl. 2.14 2.16 2.18 2.20 Ca3(PO4)2 no rm al iz ed a bs or ba nc e (a rb itr ar y un its ) energy (keV) [PO4]ini/[Ca]ini = 1 : 4 0.225 mmol/g PO4 [PO4]ini : [Ca]ini = 2 : 1 0.573 mmol/g PO4 0.364 mol/g PO4 P only, covered by mylar Figure 22: Phosphate sorbed to gibbsite at pH 8, 2.5 g / L, and 0.1 M NaCl after 24 hrs. the features of tribasic calcium phosphate (shoulder at 2.158 keV and weak peak at 2.167 keV) in addition to the white line. The varying intensity of those features in relation to the white line, however, shows that the products of phosphate sorption are apparently a mixture of tribasic calcium phosphate and a second species, the XANES spectrum of which consists mainly of the white line. A possibility is a sorption complex on the calcium carbonate surface because similar spectra have been observed for phosphate sorption complexes on gibbsite. The spectra of phosphate sorbed to gibbsite in the presence of calcium at two different initial ratios (Fig. 2) on the other hand, show none of the features of tribasic calcium phosphate or apatite, but rather resemble those of phosphate alone on gibbsite. Similar spectra, with only few features besides the white line have been observed for phosphate on gibbsite over a range of pH values (pH 4 8) and are attributed to inner-sphere surface complexes. The weak peak and shoulder in the spectrum of phosphate on gibbsite at pH 8 can be attributed to a contamination of the sample by the mylar film it was covered with during the data collection. The effect of the order of addition was investigated at pH 6 and pH 8 (Fig. 3), equilibrating the suspension with the reagent added for one hour before adding the second reagent and reacting a total of 48 hours. Again, no calcium phosphate phase is observed. Calcium adsorption is only expected to occur at pH values greater than the PZC of the solid phase, which is pH 10 for the gibbsite used. Adsorption of phosphate is expected to be mostly complete after one hour and the Ca added either reacts with the phosphate surface complexes to form ternary complexes or with the free phosphate to form solution complexes and surface precipitates. It is further attracted to the surface, which exhibits a lower PZC with the phosphate anions adsorbed, and can form outer-sphere complexes. Of the four modes of interaction listed, only the surface precipitate can be excluded when considering the spectroscopic information. No data for comparison exist for the ternary complexes, which can therefore not be excluded. Acknowledgments. The authors wish to thank Dr Wolfgang Caliebe for his support at the beamline. 2.14 2.16 2.18 2.20 no rm al iz ed a bs or ba nc e (a rb itr ar y un its ) Energy (keV) pH 6, Ca added first 0.383 mmol/g PO4 pH 6, P added first 0.722 mmol/g PO4 Hydroxyl-Apatite pH 8, Ca added first 0.432 mmol/g PO4 pH 8, P added first 0.316 mmol/g PO4 Figure 33: Phosphate on gibbsite after 48 hrs, 2.5 g/L, 0.1 M NaCl; influence of the order of addition.