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

Highlights

• Column studies of Cu²⁺ and Zn²⁺ removal by granular ZVI under anticipated Antarctic field conditions.

• Studies include varying flow rate, temperature, and the concentrations of ZVI, salt and dissolved oxygen.

• A mass transport model, using a differential transport equation, was developed.

• Film diffusion controls Cu²⁺ and Zn²⁺ removal under aerobic conditions.

• Results suggest that only a small percentage of the iron oxyhydroxide surface is active during reactive metal contaminant removal.

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.

Introduction

Zero-valent iron (ZVI) is a widely used reactive material for contaminant removal in remediation and water treatment technologies (Gillham, 2010). Despite its successful commercial application, the relative importance of different removal mechanisms is known to vary with type and concentration of ZVI, contaminants, and other medium components (Alowitz and Scherer, 2002, Wilkin and McNiel, 2003, Rangsivek and Jekel, 2005, Noubactep, 2008). A detailed understanding of removal mechanisms is required in order to optimise the design of in situ permeable reactive barriers (PRBs) and other low-cost water treatment systems (Gillham, 2010, Rangsivek and Jekel, 2011, Hussam and Munir, 2007). Whether contaminant removal is (i) dominated by reactions at localised defects in a developing iron oxyhydroxide layer or (ii) distributed across the entire iron oxyhydroxide film has broad implications for technology development (Gaspar et al., 2002).

To date, experimental investigation of contaminant removal mechanisms has focused on qualitative understanding of the reaction site and quantitative characterisation of kinetic rate constants. Surface analysis techniques, such as auger electron microscopy, have identified localised regions of reactive sites and removal products on the iron oxyhydroxide surface (Gaspar et al., 2002, Rangsivek and Jekel, 2005). Results suggest that contaminant removal may occur at defects in an iron oxyhydroxide layer surrounding an elemental iron core (Gaspar et al., 2002). Multi-site models have been proposed to conceptualise the iron surface area as a collection of different reactive sites (Bandstra and Tratnyek, 2004, Burris et al., 1998, Bi et al., 2010). Bi et al. (2010) investigated reactive and non-reactive sorption sites for the treatment of trichloroethene by ZVI using the transient and steady-state sections of column breakthrough curves. The modelling results indicated that only 2% of sorption sites controlled reactivity (Bi et al., 2010); which could further emphasise the importance of variations in the iron oxyhydroxide layer. Contaminant treatment kinetic rate constants have also been determined using rotating-disc cementation (Lee et al., 1978, Ku and Chen, 1992) and column experiments with varying solution velocities and/or multiple sampling points (Komnitsas et al., 2007, Wüst et al., 1999). These observations have aided in the qualitative understanding of reactive surfaces, however quantification of ZVI reactive proportion of the total surface area from experimental data has not yet been undertaken.

Transport models have also been applied to predict contaminant removal by ZVI (Mayer et al., 2002, Yabusaki et al., 2001, Li et al., 2006, Jeen et al., 2007, O et al., 2009, Carniato et al., 2012). These models use laboratory-defined rate constants scaled to reactive media surface areas (Mayer et al., 2002). Currently, due to varying solution parameters and ZVI source, calibration of some rate constants (based on gas generation and other reaction parameters) is required to model different systems (Jeen et al., 2007, Carniato et al., 2012). Developing new methods of determining reactive surface areas may help in the further understanding of contaminant removal and development of these predictive models.

This study forms part of a broader research program investigating the application of ZVI-based PRBs at contaminated coastal Antarctic sites (Statham et al., 2015a, Statham et al., 2015b), where design of remediation technologies is limited by environmental and logistical considerations. Antarctic PRB systems have been implemented previously (Mumford et al., 2013), using media other than ZVI, in the context of hydrocarbon remediation. These previous studies have elucidated unique design challenges, for example diurnal and seasonal variation in water flow rates and associated variations in the degree of contaminant loading (Mumford et al., 2014). Hence, this study emulates site-specific parameters, including operating temperature, soil salinity, and dissolved oxygen concentration, and quantifies their influence on contaminant removal performance. This process will aid placement, media selection and sizing of future ZVI remediation technologies. While the current research focuses on melt water management, general insights into ZVI behaviour can potentially aid in the development of nano-ZVI based remediation techniques (Zhang, 2003, Tratnyek and Johnson, 2006). The model contaminants Cu²⁺ and Zn²⁺ have been selected as these represent potential metals of concern at coastal Antarctic sites (Snape et al., 2001, Northcott et al., 2003); the selection also enables a comparison of contaminants that are, respectively, reducible and non-reducible by metallic iron. A combination of experimental and numerical modelling techniques is used to evaluate the contributions of species diffusion and reaction mechanisms to net contaminant removal rates. Comparison of model-determined reactive surface areas with the total column ZVI surface area can provide further insight into contaminant interaction with iron oxyhydroxide surfaces. While the current findings are readily applicable to cold-climate PRB design, the fundamental results also outline the quantification of ZVI reactive surface areas, which will aid in the general development of ZVI-based water treatment systems.