pH Gradient at the Nanoscale Zerovalent Iron-Water Interface: Spatial Distribution, Formation Mechanism, and Impacts

Nanoscale zerovalent iron (nZVI) shows great promise for water remediation, but the nZVI-water interfacial microenvironment, particularly the local pH distribution and dynamics, remains poorly understood. Here, ATR-FTIR and scanning electrochemical microscopy (SECM) were combined to quantify pH in the Stern and inner diffusion layers, diffusion layer, and bulk phase during anoxic nZVI-water reactions. The results reveal that the nZVI-water interface became more alkaline than the bulk solution, indicating a clear interfacial-to-bulk pH contrast. At an initial pH of 3, the pH values of the Stern and inner diffusion layers, diffusion layer, and bulk phase reached 8.27, 8.49, and 6.23, respectively, after 60 min. SECM also revealed Fe²⁺ enrichment at the interface, indicating that corrosion-generated Fe²⁺ was partially surface-confined, electrostatically constraining the inward diffusion of protons or the outward transport of OH^- across the diffusion layer, thereby maintaining the interfacial alkaline microenvironment. The interfacial pH gradients consequently determine the reactivity and passivation of nZVI in water. nZVI passivates faster at an initial pH of 3 than at pH 7 or 11 due to a larger pH gradient and more pronounced interfacial Fe²⁺ accumulation at pH 3; this is unfavorable for sustained heavy-metal removal. These findings establish a combined ATR-FTIR/SECM approach for probing interfacial pH distributions and clarify how local pH gradients regulate nZVI corrosion, passivation, and durability, providing guidance for optimizing nZVI-based remediation technologies.