Phosphorus Speciation in Agro-Industrial Byproducts: Sequential Fractionation, Solution <sup>31</sup>P NMR, and P <i>K</i>- and <i>L</i><sub>2,3</sub>-Edge XANES Spectroscopy

Little is known about P species in agro-industrial byproducts from developing countries, which may be either pollutants or valuable soil amendments. The present study speciated P in dry (COD) and wet (COW) coffee, sisal (SIS), barley malt (BEB) and sugar cane processing (FIC) byproducts, and filter cakes of linseed (LIC) and niger seed (NIC) with sequential fractionation, solution <sup>31</sup>P nuclear magnetic resonance (NMR) spectroscopy, and P <i>K</i>- and <i>L</i><sub>2,3</sub>-edge X-ray absorption near-edge structure (XANES) spectroscopy. The sequential P fractionation recovered 59% to almost 100% of total P (P<sub>t</sub>), and more than 50% of P<sub>t</sub> was extracted by H<sub>2</sub>O and NaHCO<sub>3</sub> in five out of seven samples. Similarly, the NaOH + EDTA extraction for solution <sup>31</sup>P NMR recovered 48−94% of P<sub>t</sub>. The <sup>31</sup>P NMR spectra revealed orthophosphate (6−81%), pyrophosphate (0−10%), and orthophosphate monoesters (6−94%). Orthophosphate predominated in COD, COW, SIS, and FIC, whereas BEB, LIC, and NIC were rich in orthophosphate monoesters. The concentrations of P<sub>i</sub> and P<sub>o</sub> determined in the sequential and NaOH + EDTA extractions and <sup>31</sup>P NMR spectra were strongly and positively correlated (<i>r</i> = 0.88−1.00). Furthermore, the P <i>K</i>- and <i>L</i><sub>2,3</sub>-edge XANES confirmed the H<sub>2</sub>SO<sub>4</sub>−P<sub>i</sub> detected in the sequential fractionation by unequivocal identification of Ca−P phases in a few samples. The results indicate that the combined use of all four analytical methods is crucial for comprehensive P speciation in environmental samples and the application of these byproducts to soil.