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.