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Waste Management

Determination of Phosphorus Speciation in Dairy Manure using XRD and XANES Spectroscopy

^a Biological Systems Engineering Dep., Univ. of Wisconsin-Madison, 460 Henry Mall, Madison, WI 53706
^b Canadian Synchrotron Radiation Facility, Synchrotron Radiation Center, 3731 Schneider Dr., Stoughton, WI 53589-3097

^* Corresponding author (kkarthikeyan{at}wisc.edu).

Received for publication December 29, 2006. Intensive manure application is an important source of diffuse phosphorus (P) pollution. Phosphorus availability from animal manure is influenced by its chemical speciation. The major objective of this study was to investigate the P speciation in raw and anaerobically digested dairy manure with an emphasis on the calcium (Ca) and magnesium (Mg) phosphate phases. Influent and effluent from an on-farm digester in Wisconsin were sampled and sieved, and the 25 to 53 µm size fraction was dried for X-ray powder diffraction (XRD) and P K-edge X-ray absorption near edge structure (XANES) analyses. Struvite (MgNH₄PO₄·6H₂O) was identified in both the raw (influent) and anaerobically digested (effluent) manure using XRD. Qualitative analysis of P K-edge XANES spectra indicated that the Ca orthophosphate phases, except dicalcium phosphate anhydrous (DCPA) or monetite (CaHPO₄), were not abundant in dairy manure. Linear combination fitting (LCF) of the P standard compounds showed that 57.0 and 43.0% of P was associated with DCPA and struvite, respectively, in the raw manure. In the anaerobically digested sample, 78.2% of P was present as struvite and 21.8% of P was associated with hydroxylapatite (HAp). The P speciation shifted toward Mg orthophosphates and least soluble Ca orthophosphates following anaerobic digestion. Similarity between the aqueous orthophosphate (aq-PO₄), newberyite (MgHPO₄·3H₂O), and struvite spectra can cause inaccurate P speciation determination when dairy manure is analyzed solely using P K-edge XANES spectroscopy; however, XANES can be used in conjunction with XRD to quantify the distribution of inorganic P species in animal manure.

Abbreviations: ACP, amorphous calcium phosphate • ß-TCP, beta tricalcium phosphate • DCPA, dicalcium phosphate anhydrous or monetite • DCPD, dicalcium phosphate dihydrate or brushite • HAp, hydroxylapatite • OCP, octacalcium phosphate • P_i, inorganic phosphorus • P_o, organic phosphorus • PyroP, pyrophosphate • P_WEP, water-extractable phosphorus to total phosphorus percentage • TP, total phosphorus • WEP, water-extractable phosphorus • XANES, X-ray absorption near edge structure • XRD, X-ray powder diffraction

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