Electrochemical crystallization transforms labile sediment phosphorus into stable crystalline vivianite.

Internal phosphorus (P) loading from sediments remains a major bottleneck for the long-term remediation of eutrophic waters. Although electrochemical strategies for vivianite (Fe3(PO4)2·8H2O) formation show promise for phosphate removal from wastewater, they typically rely on precise pH control, severely limiting their potential for in situ application in complex natural sediments. Here, a low-current-density electrochemical system is reported that directs labile sedimentary P toward crystalline vivianite under bulk aerobic laboratory conditions. Using iron electrodes at 1 mA cm-2, 32% of sedimentary P and 37% of aqueous-system P were converted into vivianite-bound P, within the range reported for authigenic vivianite in Fe-rich, low-sulfidation sediments. Increasing current density accelerated P immobilization but shifted the products toward ferrihydrite-adsorbed P and amorphous Fe-P. XANES and EXAFS showed that low current density preserved Fe(II)-rich products with higher Fe-centered connectivity, whereas higher current densities promoted Fe oxidation and structural disordering. Complementary XRD, XPS, FTIR, Raman, and SEM-EDS supported the formation of layered Fe-P particles with Fe/P ratios close to vivianite stoichiometry at low current density. The low-current condition maintained a favorable pH evolution and sustained Fe2+ supply, thereby favoring Fe(II)-phosphate crystallization over Fe hydroxide/amorphous Fe-P formation. This work demonstrates a charge-quantified, pathway-selective electrochemical mineralization strategy for converting labile sedimentary P into crystalline Fe(II)-phosphate under controlled sediment-water conditions.