Phosphorus Speciation in Agro-Industrial Byproducts: Sequential Fractionation, Solution ³¹P NMR, and P K- and L_2,3-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 ³¹P nuclear magnetic resonance (NMR) spectroscopy, and P K- and L_2,3-edge X-ray absorption near-edge structure (XANES) spectroscopy. The sequential P fractionation recovered 59% to almost 100% of total P (P_t), and more than 50% of P_t was extracted by H₂O and NaHCO₃ in five out of seven samples. Similarly, the NaOH + EDTA extraction for solution ³¹P NMR recovered 48−94% of P_t. The ³¹P 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_i and P_o determined in the sequential and NaOH + EDTA extractions and ³¹P NMR spectra were strongly and positively correlated (r = 0.88−1.00). Furthermore, the P K- and L_2,3-edge XANES confirmed the H₂SO₄−P_i 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.