Conversion of hazardous waste into functional granular materials for wastewater treatment and contaminated soil remediation

The management of waste paper fly ash (WPFA) remains a major environmental challenge because of its high alkalinity and the leaching of critical elements, which may lead to its classification as hazardous waste despite its high content of reactive calcium phases. This study addresses this issue by converting WPFA into functional granular materials through an integrated Ca–P–F mineralization pathway for both phosphate removal from water and fluoride immobilization in contaminated soil. Calcium-rich industrial residues are still predominantly sent for disposal, although their reactive mineral phases could be harnessed for pollution remediation. In this work, WPFA is first stabilized by accelerated carbonation. Beyond reducing alkalinity and improving environmental compatibility, this step aims to decrease the mobility and leaching of critical metals, particularly Ba and Pb, whose leached concentrations may exceed thresholds associated with hazardous waste. The formation of carbonate phases and the transformation of reactive species contribute to lowering the concentrations measured in leachates and to securing the subsequent use of the material. The carbonated solid is then granulated, without energy-intensive thermal treatment, to produce lightweight and mechanically stable aggregates suitable for packed-bed applications. In a second step, these aggregates are used as a reactive medium for phosphate removal under continuous-flow conditions. Phosphate abatement is driven by calcium release followed by the precipitation of calcium phosphate phases, leading to the progressive formation of hydroxyapatite (HAP) on the surface and within the pore network of the granules. The aggregates thus evolve into HAP-bearing sorbents. Finally, the HAP-bearing granules are applied as amendments to a fluoride-impacted soil. Leaching tests, performed at L/S = 10, show a clear and monotonic decrease in fluoride release, even at low amendment rates. Mineralogical and microstructural analyses support a mechanistic shift from initial retention by adsorption and ion exchange toward durable stabilization within apatite-type phases, including fluorapatite, consistent with coupled Ca–P–F reactions. Sequential extractions further indicate a transfer of fluoride from labile fractions to more stable forms bound to the mineral matrix. Overall, this approach demonstrates the conversion of hazardous WPFA into a multifunctional granular material for sequential phosphate removal and fluoride stabilization, after prior immobilization of critical elements such as Ba and Pb.