The specific reactive surface area of granular zero-valent iron in metal contaminant removal: Column experiments and modelling

Abstract

A series of dynamic-flow kinetic experiments were conducted to assess the removal rates of aqueous Cu2+ and Zn2+ ions by zero-valent iron (ZVI), a promising material for inclusion in cold-climate remediation applications. The influence of experimental parameters on contaminant removal rates, including aqueous flow rate, operating temperature, and the concentrations of ZVI, salt and dissolved oxygen, was investigated.A mass transport model has been developed that accounts (i) aqueous-phase dispersion processes, (ii) film diffusion of contaminant ions to the reactive ZVI surface and (iii) the reactive removal mechanism itself. Regression to the experimental data indicated that when oxygen is present in the solution feed Cu2+ and Zn2+ removal processes were limited by film diffusion. In de-aerated solutions film diffusion still controls Cu2+ removal but a first-order surface reaction provides a better model for Zn2+ kinetics.Using air as the equilibrium feed gas, the reactive proportion of the total surface area for contaminant removal was calculated to be 97% and 64% of the active spherically-assumed geometric area associated with ZVI media for Cu2+ and Zn2+, respectively.Relative to a gas absorption area, determined in previous studies, the reactive proportion is less than 0.41% of the unreacted ZVI total surface area. These findings suggest that only part of the iron oxyhydroxide surface is reacting during ZVI based metal contaminant removal.