Tantalum–niobium (TaNb) metallurgy produces a high‑iron slag. Large-scale stockpile of this slag raises environmental and resource-recovery concerns. The slag contains appreciable amounts of niobium (Nb), tantalum (Ta), iron (Fe), tin (Sn), uranium (U), thorium (Th) and rare-earth elements (REEs). These elements occur either as soluble sulfates or as insoluble mineral phases, and ferrous sulfate is the dominant component. Water dissolution removed ferrous sulfate and achieved an iron dissolution rate of approximately 95%. Tin, zirconium, REEs, thorium and uranium remained in the dissolution residue, which accounted for approximately 15% of the raw slag mass.X-ray diffraction (XRD) identified two hydrated phases in the raw slag: FeSO4·7H2O and FeSO4·H2O. As sulfuric acid concentration increases, the heptahydrate converts progressively to FeSO4·4H2O and finally to the stable monohydrate FeSO4·H2O. Ferrous sulfate crystals were recovered from the slag by dissolution and recrystallization. This process reveals the phase-transition behavior of ferrous sulfate in this slag system. The water dissolution–crystallization route achieves effective iron separation and reduces the solid residue substantially. It offers a promising approach to alleviate the stockpiling and disposal burden of high‑iron TaNb slag.
Indole (IND) is a representative nitrogen-containing odorant in combined sewer overflow (CSO) wastewater, whereas conventional Fenton-like processes exhibit limited efficiency under near-neutral conditions. Herein, iron-doped biochar derived from peanut shell (FeBC) was prepared to activate peroxymonosulfate (PMS) for IND degradation. Its catalytic performance was evaluated through physicochemical characterization, kinetic analysis, scavenger and probe experiments, product analysis, toxicity prediction, and actual wastewater treatment. FeBC pyrolyzed at 500 °C (FeBC500) achieved a favorable balance between iron-species incorporation and pore accessibility. At a catalyst dosage of 0.20 g L−1, a PMS concentration of 1.0 mM, and pH 7, IND removal and total organic carbon (TOC) mineralization reached 87.48% and 49.87%, respectively, within 60 min. Scavenger experiments indicated that singlet oxygen (1O2) dominated the nonradical oxidation pathway. Probe results supported the formation of high-valent iron-oxo species (Fe(IV) = O) and its possible auxiliary involvement in oxidation. The predicted risks of the transformation products were generally lower than that of IND. In actual CSO wastewater, chemical oxygen demand (COD) and IND removal efficiencies reached 92.33% and 82.15%, respectively, and FeBC500 retained high activity with modest deactivation over five cycles. This study provides experimental support for agricultural-waste valorization and odorant removal under near-neutral conditions.
