Acidic Fe(III)-Driven hydrothermal co-transformation of phosphorus and arsenic in sewage sludge: Vivianite crystallization coupled with amorphous ferric arsenate formation for phosphorus-enriched hydrochar

Addressing the challenges associated with phosphorus and arsenic coexistence is not only a scientific necessity but also a practical imperative for the growing importance of sustainable sewage sludge management. This study investigates the speciation and crystallization transformations of phosphorus and arsenic in sewage sludge under Fe(III)-catalyzed hydrothermal treatments with pH ranging from 2.5 to 8.5. Arsenic and phosphorus, both belonging to the same group of the periodic table, exhibit dynamic interactions during Fe(III)-catalyzed hydrothermal treatments. Results reveal that hydrochar yield decreases with lower pH, while the proportion of arsenic(V) in hydrochar decreases with increasing calcium ion concentration. Scanning electron microscopy shows that fibrous and needle-like structures on hydrochar surfaces enhance the adsorption and enrichment of dissolved phosphorus and arsenate, forming vivianite-P and vivianite-like mineral phases. Notably, the crystallization of scorodite (FeAsO 4 ·2H 2 O) facilitates the immobilization of As(III) in near-neutral environments. These findings deepen our understanding of the synergistic morphological processes and crystallization transformations of arsenic and phosphorus during Fe(III)-catalyzed hydrothermal treatments, offering significant implications for P-rich biochar fertilization and application.