Microwave-Assisted Thermochemical Treatment of Sewage Sludge Ash for Enhanced Phosphorus Recovery and Mitigation of Cr(VI) Formation

Phosphorus (P) is an essential nutrient and a Critical Raw Material for the European Union. Sewage sludge ash (SSA), containing 5–11 wt% P, represents a promising secondary phosphorus source; however, P is mainly hosted in poorly soluble mineral phases and may coexist with hazardous elements such as chromium. This study investigates a microwave-assisted thermochemical treatment of SSA with sodium bicarbonate (NaHCO3) to enhance phosphorus bioavailability while addressing chromium speciation. Treatments were performed at 800–1000 °C for 20 min. The process promotes the conversion of whitlockite into sodium–calcium phosphates, particularly buchwaldite (NaCaPO4), resulting in phosphorus solubilization yields up to 96% under neutral ammonium citrate extraction. Experiments conducted under oxidizing conditions without a reducing agent led to significant formation of hexavalent chromium (Cr(VI)). The introduction of anthracite as a carbonaceous additive substantially reduced water-soluble Cr(VI) concentrations—by approximately 70–75%—while preserving high phosphorus availability. Although residual Cr(VI) remains above regulatory limits, the results demonstrate that carbon-assisted microwave processing is a promising strategy for simultaneously enhancing P recovery and mitigating chromium oxidation. Further optimization under controlled atmospheres is expected to enable the production of environmentally compliant phosphorus fertilizers from SSA.