Phosphorus Speciation in Sequentially Extracted Agro-Industrial By-Products: Evidence from X-ray Absorption Near Edge Structure Spectroscopy

The phosphorus (P) in agro-industrial by-products-a potential source of freshwater eutrophication but also a valuable fertilizer-needs to be speciated to evaluate its fate in the environment. We investigated to what extent X-ray absorption near edge structure (XANES) spectroscopy at the P K- and L-2,L-3-edges reflected differences in sequentially extracted filter cakes from sugarcane (Saccharum officinarum L.) (FIC) and niger seed (Guizotia abyssinica Cass.; NIC) processing industry in Ethiopia. The P fractionation removed more labile (54%) and H2SO4-P (28%) from FIC than from NIC (18% labile, 12% H2SO4-P). For the FIC residues after each extraction step, linear combination (LC) fitting of P Kedge spectra provided evidence for the enrichment of Ca-P after the NaOH-extraction and its almost complete removal after the H2SO4 treatment. The LC fitting was unsuccessful for the NIC samples, likely because of the predominance of organic P compounds. The different proportions of Ca-P compounds between FIC (large) and NIC (small) were more distinctive in L-2,L-3- than in the Kedge XANES spectra. In conclusion, the added value of complementary P K- and L-2,L-3-edge XANES was clearly demonstrated, and the P fractionation and speciation results together justify using FIC and NIC as soil amendments in the tropics.