In-situ recrystallization mechanism of vivianite-phosphorus in hydrochar during Fe(III) induced hydrothermal treatment of sewage sludge

Crystallization of vivianite-phosphorus is a promising method to recover phosphate as a vital resource from sewage sludge hydrothermal carbonization (HTC) products. This study reveals the in-situ recrystallization mechanism of vivianite-phosphorus (Fe 3 (PO 4 ) 2 ·8H 2 O) regulated by FeCl 3 -assisted HTC in depth. The key finding includes the decipherment that phosphorus is mainly enriched in hydrochar (72.7 ± 2.9 %wt ∼ 84.4 ± 3.31 %wt). Further identified by HRTEM and XRD, dominant phosphorus is profiled as vivianite (Fe 3 (PO 4 ) 2 ·8H 2 O) with very few calcium phosphate crystals (Ca 3 (PO 4 ) 2 ) formation in hydrochar at near-neutral condition. Under acidic conditions, HMF oxidation triggers reductive dissolution of surface-bound Fe(III)-phosphate complexes, releasing Fe 2+ , HPO 4 2− , and PO 4 3− . The anoxic, reducing conditions at circumneutral pH created by HTC, combining with favorable Fe: P ratio (1.72) and lower calcium concentration (532 mg L −1 ), comprehensively drives the crystallization of Fe 3 (PO 4 ) 2 ·8H 2 O. This demonstration-of-practice study provides deep insights into dual organic waste resource and energy valorization.