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.