Real-time Cr(VI) monitoring and remediation using Fe3O4 nanoparticles: Insights into Fe-Cr Spinels

An analytical approach for the rapid and remote determination of Cr(VI) in water samples is demonstrated using Fe 3 O 4 nanoparticles and a cost-effective portable AC magnetometer. Cr(VI) reduction by surface Fe²⁺ leads to Fe/Cr precipitate formation, which alters the nanoparticles’ magnetic properties. Validation in natural water spiked with Cr(VI) showed a linear correlation between magnetic remanence and Cr(VI) concentrations from 0 to 10 mg/L, with a R 2 value of 0.99. Limitations of the method emerged at loadings above 10 mg/g, where extensive chromite (Cr x Fe 3-x O 4 ) coverage reduced the magnetic hysteresis area. Advanced characterization and DFT calculations revealed changes in spin-dependent properties, including magnetization reversal, electrical conductivity and magnetoresistance. This robust correlation between collective magnetic properties and Cr(VI) exposure enables, for the first time, milligram-per-liter detection via ultra-fast AC magnetometry, paving the way for real-time insight into redox processes in liquid environments and novel strategies for environmental remediation. • Magnetically-induced sensing of Cr(VI) concentration in water • Redox mechanism involving Fe 2+ and Cr(VI) electron exchange • Linear correlation of surface transformation to redox progress • Accurate determination by portable ultra-fast AC magnetometry • Analytical method applicable for water pollution online monitoring