Evidence for in-situ electric-induced uranium incorporation into magnetite crystal in acidic wastewater

• Uranium (U) can be incorporated into the magnetite crystal lattice at ambient conditions. • The pathway underlying U entry into magnetite lattice was discovered. • The relationship between U coordination states and re-release properties was determined. • Three alternative stable states of U were discovered and quantified in UDM. • Long-term immobilization of U in UDM in certain acid-base conditions was achieved. Incorporating uranium (U) from wastewater into a magnetite crystal to limit its mobility has been proposed as long-term remediation for radioactive contamination. Unfortunately, this process is accompanied by reduction or surface adsorption, thereby significantly reducing the incorporation capacity and resulting in a high probability of U release. Inducing U growth into the crystal during magnetite crystallization yields U-doped magnetite (UDM), which is an effective solution. Herein, magnetite crystallization and U incorporation were simultaneously performed by precisely controlling the sacrificial Fe-anode during the electrochemical reaction. The pathway underlying U entry into magnetite crystal, local coordination environment of U in the crystal, and release properties of U from UDM were investigated. U(VI) enters magnetite crystal through the substitution of Fe atoms in the octahedral sites, resulting in a partial reduction to U(IV) by Fe(II) in the crystal. Co-existing U(IV) and U(VI) states in the crystal significantly increased the incorporation capacity. U was present in the three following forms: surface adsorption (5%), conditionally stable incorporation (20 – 30%), and stabilized incorporation (> 65%). Within 400 days, 10.21% and 4.23% of total U were released at pH = 9.0 and pH = 5.6, respectively. The results provide new insights into reducing the risk of long-term migration contamination in wastewater by regulating U incorporation into the magnetite crystals.