Targeting phosphorus transformation to hydroxyapatite through sewage sludge pyrolysis boosted by quicklime toward phosphorus fertilizer alternative with toxic metals compromised

Advancing pyrolysis practice for phosphorus (P) recovery and recycling from sewage sludge arouses intensive research interests due to the forthcoming threat of global P sustainability. However, how to both in situ manipulate P targeted transformation to a designed form and accommodate downstream use while compromise adverse impacts of concomitant toxic metals (TMs) challenges the worldwide initiative of P reclamation from sewage sludge. Here, a P targeted transformation to hydroxyapatite (P2tHAP) technique to bridge the discrepancy of P recovery and subsequent use was developed. Through manipulating calcium-to-P molar ratio (Ca/P) of sludge feedstock by calcium oxide, the sludge P can be targetedly transformed to HAP during pyrolysis, a bioavailable form delivered as sewage sludge char-derived phosphorus fertilizer (SCPF). The P supply competency of the SCPF for Ca/P below or above 1.67 with a risk abatement of TMs was comparable to conventional P fertilizer, while the Ca/P = 1.67 was prone to the occurrence of sludge ash sintering, detrimental to the HAP formation. The lowered bioavailability of TMs was orchestrated by the root-secreted oxalic acid, due to the triggered phosphate starvation response. The random forest analysis indicated the driving force in the evolution of rhizobacterial communities, dominantly arising from those TMs after the SCPF soil application. Thermodynamic and kinetic calculations validated that HAP with the stable molar Gibbs energy gHAP (∼−7.23×103kJ/mol) underpinned the progress of P2tHAP during sludge pyrolysis. The outcome prospects to propel the integrative design of an innovative reclamation of P-rich wastes and simultaneous compromise of concomitant contaminants.