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 _2,3-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 K-edge 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_2,3- than in the K-edge XANES spectra. In conclusion, the added value of complementary P K- and L_2,3-edge XANES was clearly demonstrated, and the P fractionation and speciation results together justify using FIC and NIC as soil amendments in the tropics.