Simultaneous molybdate (Mo(VI)) recovery and hazardous ions immobilization via nanoscale zerovalent iron

Nanoscale zerovalent iron (nZVI) shows great promise in valuable metal recovery from wastewater due to its high removal capacity. However, nZVI-based processes mainly focus on the sequestration step, ignoring the desorption step, which is crucial for recovery. In this study, a novel method for simultaneous Mo(VI) recovery and hazardous metal ions immobilization by nZVI was developed and the reaction mechanism was further investigated. Results shown that removal capacity of nZVI was significantly influenced by surface charge and the number of active adsorption sites. X-ray photoelectron spectroscopy analysis demonstrated that Mo(VI) reduction occurred in the inner Fe(0) core. K-edge X-ray Absorption Near Edge Structure analysis further confirmed that 5.4% and 18.0% of Mo(VI) are reduced to Mo(IV) at pH 6 and 9, respectively, suggesting that high pH favors for Mo(VI) reduction and H⁺ is responsible for the hollow-out structure at pH 6. Through adjusting the pH of wastewater from 3 to 12, over 80% of adsorbed Mo(VI) could be recovered while other metal ions remained immobilized and limited influence with common ions/anions. Overall, the proposed mechanism was significant to the research of metal reduction and competition for proton of nZVI, and the developed method had great prospects in valuable anions recovery.