Red mud, a Fe-rich waste generated from the aluminum industry, was recovered as an adsorbent for wastewater treatment. The separation process of red mud from water after adsorption, including centrifugation and filtration, was complicated. This study demonstrated an alternative option to recycle red mud for preparing magnetic adsorbent via a facile hydrothermal route using ascorbic acid as reductant. Red mud is weakly magnetized and consists of andradite, muscovite, hematite, and cancrinite. After hydrothermal treatment, andradite in red mud was reductively dissolved by ascorbic acid, and transformed into magnetite and morimotoite. With increasing hydrothermal temperature, the dissolution of andradite accelerated, and the crystallite size of magnetite increased. When the hydrothermal temperature reached 200 °C, the prepared adsorbent P-200 showed a desirable saturation magnetization of 4.1 Am²/kg, and could be easily magnetically separated from water after adsorption. The maximum adsorption capacity of P-200 for Zn²⁺ was 89.6 mg/g, which is eight-fold higher than that of the raw red mud. The adsorption of Zn²⁺ by P-200 fitted the Langmuir model, where cation exchange was the main adsorption mechanism. The average distribution coefficient of Zn²⁺ at low ppm level was 16.81 L/g for P-200, higher than those of the red mud (0.3 L/g) and the prepared P-120 (1.48 L/g) and P-270 (5.48 L/g), demonstrating that P-200 had the best adsorption capacity for Zn²⁺ and can be served as a practical adsorbent for real-world applications. To our knowledge, this is the first study to report the conversion of red mud into a magnetic adsorbent under mild conditions.

When reclaimed water was recharged into groundwater containing a low content of ferrous iron, Fe-rich particles were generated and started to transport into the porous medium. X-ray powder diffraction and Mössbauer spectroscopy analysis showed that the generated Fe-rich particles were in a poorly crystallized ferrihydrite phase. After the formation of ferrihydrite particles, the mass loss of contaminants was calculated, which was 88.2% for Al, 93.3% for Zn, and 41.6% for chemical oxygen demand (COD). Protein-like compounds were predominant in the removed COD. Bovine serum albumin (BSA) was used as a model to investigate the effect of proteins on the transport of ferrihydrite particles in porous medium. Results showed that the attachment efficiency of ferrihydrite particles on the porous medium decreased from 100% to 75% with the increase of BSA concentration from 10 mg/L to 100 mg/L. BSA was attached to the surface of ferrihydrite particles via electrostatic adsorption. Thus, the zeta potential of ferrihydrite particles changed from positive to negative, and the particles became less aggregated, thereby enhancing particle mobility. This observation provided evidence that protein residues in reclaimed water enhance the transport of Fe-rich particles in saturated porous medium.