The Molecular Environment of Phosphorus in Sewage Sludge Ash: Implications for Bioavailability

Producing a P fertilizer from sewage sludge ash (SSA) is a strategy to efficiently recycle P from a secondary raw material. The P speciation in four SSAs was characterized before and after the removal of heavy metals by a thermo-chemical treatment that involved CaCl2 addition. We chose complementary techniques to determine the direct P speciation, including X-ray powder diffraction, solid-state P-31 direct-polarization magic-angle spinning nuclear magnetic resonance, and X-ray absorption near edge structure. Results from these techniques were compared with operational and functional speciation information obtained from a sequential P extraction and a plant biotest with Italian ryegrass grown on a soil-sand mixture with little available P. The speciation of P in untreated and thermo-chemically treated SSAs depended on their elemental composition. At a molar ratio of Ca:P <= 2, SSAs contained combinations of polymorphs of AlPO4, beta-tricalcium phosphate, and apatite-like P species. In SSAs with a molar ratio of Ca:P > 2, an apatite-like molecular environment was predominant. The thermo-chemical process induced an increase in crystalline phases and enhanced the crystallinity of the P species. The structural order of the bulk sample was the most decisive parameter in controlling the P availability of the studied SSAs to plants. We conclude that, to produce a high-quality fertilizer and despite of the successful heavy metal removal, the thermo-chemical process requires further development toward enhanced P bioavailability.