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

Area of Science:

• Environmental Science
• Materials Science
• Electrochemistry

Background:

• Nanoscale zerovalent iron (nZVI) is a promising material for water remediation.
• The interfacial microenvironment of nZVI in water, especially pH dynamics, is poorly understood.
• Understanding interfacial pH is crucial for optimizing nZVI performance.

Purpose of the Study:

• To quantify the local pH distribution at the nZVI-water interface.
• To investigate the relationship between interfacial pH and nZVI corrosion and passivation.
• To provide insights for improving nZVI-based water treatment technologies.

Main Methods:

• Combined Attenuated Total Reflectance Fourier-Transform Infrared Spectroscopy (ATR-FTIR) and Scanning Electrochemical Microscopy (SECM).
• Quantified pH in different interfacial layers (Stern, inner diffusion, diffusion) and the bulk phase.
• Analyzed Fe2+ accumulation at the nZVI interface.

Main Results:

• The nZVI-water interface was significantly more alkaline than the bulk solution.
• At an initial pH of 3, interfacial pH reached 8.27-8.49, while bulk pH was 6.23.
• Fe2+ enrichment at the interface contributed to maintaining the alkaline microenvironment and influenced proton/hydroxide transport.
• Faster nZVI passivation occurred at pH 3 due to larger pH gradients and Fe2+ accumulation, hindering heavy metal removal.

Conclusions:

• The study established a combined ATR-FTIR/SECM approach for probing interfacial pH.
• Local pH gradients critically regulate nZVI corrosion, passivation, and durability.
• Optimizing nZVI remediation requires consideration of interfacial pH dynamics and Fe2+ accumulation.