Enhanced Cr(VI) removal using zero-valent manganese nanoparticles

Hexavalent chromium (Cr(VI)) contamination poses a persistent environmental risk because of its high toxicity and mobility. In this study, zero-valent manganese nanoparticles (ZVMn) were investigated as a reactive material for Cr(VI) removal from aqueous solution and electroplating wastewater. ZVMn exhibited a high Cr(VI) removal capacity of 167.9 mg/g and maintained efficient removal under practical conditions, achieving >97% Cr(VI) removal at pH 6 with a dosage of 1.0 g/L. Kinetic and isotherm analyses showed that the removal process followed pseudo-second-order kinetics and was well described by the Freundlich model (R ² = 0.972), indicating that chemisorption and heterogeneous multilayer adsorption were involved. Mechanistic analyses revealed that ZVMn did not act as a conventional adsorbent with fixed surface sites, but underwent in situ corrosion and surface reconstruction during reaction with Cr(VI). This transformation generated a mesoporous manganese oxide/hydroxide shell, increasing the specific surface area from 40.1 to >312 m ² /g and providing abundant newly formed reactive sites for Cr immobilization. Eh–pH analysis, together with spectroscopic and microscopic characterization, confirmed that Cr(VI) removal was governed by a coupled pathway involving electrostatic attraction, redox transformation, surface complexation, and precipitation/immobilization of Cr species on the reconstructed Mn-based surface. ZVMn also showed strong tolerance to coexisting anions and retained effective performance in real electroplating wastewater. These results demonstrate that ZVMn enables a self-enhanced reactive removal mechanism and provides a promising strategy for treating Cr(VI)-contaminated water.