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

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.

Automated summary

This study investigated the pathway and properties of uranium incorporation into magnetite crystals by simultaneously controlling sacrificial Fe-anode during electrochemical reaction. The results revealed that U(VI) enters the crystal by substituting Fe atoms and undergoes partial reduction to U(IV) by Fe(II) in the crystal. The coexistence of U(IV) and U(VI) significantly increases the incorporation capacity. Within a 400-day experiment, 10.21% and 4.23% of total uranium were released at pH = 9.0 and pH = 5.6, respectively.