Elsevier

Cold Regions Science and Technology

Volume 110, February 2015, Pages 120-128
Cold Regions Science and Technology

Removal of copper and zinc from ground water by granular zero-valent iron: A dynamic freeze–thaw permeable reactive barrier laboratory experiment

https://doi.org/10.1016/j.coldregions.2014.12.001Get rights and content

Highlights

  • A freeze–thaw assessment of ZVI was conducted for use as a PRB media.

  • Flow-through assessments were conducted after 0, 21 and 42 freeze–thaw cycles.

  • Reactive contaminants, Cu2 + and Zn2 + ions, were removed from the pore water.

  • Hydraulic retention time decreased by 15–18% after initial freeze–thaw cycling.

  • No significant change was observed in the hydraulic conductivity (Dupuit approx).

Abstract

Permeable reactive barriers (PRBs) use solution–media interactions for contaminant removal from ground and surface waters. When located in a cold region subjected to freeze–thaw cycling, these liquid–solid phase interactions may be detrimental to PRB performance. This study presents a laboratory based assessment of contaminant removal using granular zero-valent iron (ZVI) under freeze–thaw conditions. Freeze–thaw induced changes to simulated PRBs, contained within Darcy boxes, subjected to 0, 21 and 42 freeze–thaw cycles were assessed using the flow of both reactive and conservative solutions. The reactive contaminants, Cu2 + and Zn2 + ions, were removed from the pore water during solution flow and freeze–thaw cycling. The hydraulic retention time within the reactive media as assessed by a conservative tracer, decreased by 15–18% after the first set of freeze–thaw cycling and remained constant after the second set of freeze–thaw cycling. A decrease in the uniformity of the particle size distribution and the agglomeration of particles were observed; however there was no change in the hydraulic conductivity within the variance associated with the calculation method. Analysis of the solid particles suggested the contaminant metals were not concentrated in the < 212 μm fines that were generated during the experiment. The results obtained suggest that ZVI is suitable for the inclusion in sequenced PRBs for the remediation of metal contaminants in cold regions.

Introduction

Zero-valent iron (ZVI) based permeable reactive barriers (PRBs) have been constructed at contaminated sites since the early 1990s (O'Hannesin and Gillham, 1998, Warner et al., 2005). The success of some of these early projects has resulted in the installation of more than 200 PRBs worldwide with 60% of these using ZVI as the reactive media (Henderson and Demond, 2007). As the performance of these barriers continues to be discussed and designs improved, PRBs are becoming an increasingly accepted method for ground water treatment in temperate climates. A major milestone in this process was the publication of the 15-year assessment of the Elizabeth City PRB in North Carolina (Wilkin et al., 2014). This assessment showed that the PRB outlived the chromium plume and continues to treat trichloroethene (TCE) contaminated ground water. However, while these significant investigations have been conducted in temperate climates, research towards the application of PRBs in cold environments has been lacking (Camenzuli et al., 2014).

As operating PRBs consist of water flowing through a permeable material, studies of the impacts of freeze–thaw cycling on PRB media are crucial for the understanding and continued development of this in situ remediation technique for application in areas of freezing ground. The cold-climate PRB longevity concerns of reduced hydraulic conductivity and reactivity are similar to that in temperate climates (see 2.1 Hydraulic conductivity, 2.2 Iron reactivity). Additional challenges due to the climate consist of reduced kinetic rates at low temperatures (Statham et al., under review, Statham et al., in review) and physical particle–solution interactions which can result in particle disintegration and rearrangement during freeze–thaw cycling (see Section 2.3). Previous laboratory freeze–thaw assessments of potential PRB media have consisted of batch testing (Gore et al., 2006b, Li et al., 2002) or measured diesel loaded media permeability (Gore et al., 2006a). If reasonable particle stability during freeze–thaw cycling can be obtained, ZVI may be a favourable additional media for inclusion in PRBs for cold climate applications (Mumford et al., 2013).

This study presents a laboratory based freeze–thaw assessment of contaminant removal using granular ZVI. PRBs are simulated using Darcy boxes and a laboratory incubator. Solution flow changes are monitored at laboratory temperature after 0, 21 and 42 freeze–thaw cycles using both reactive and conservative tracer solutions.

Section snippets

Theory and background

A PRB acts to intercept and treat a migrating contaminant plume. The long term performance and reliability of a PRB is vital in order to provide cost effective treatment. Debate persists concerning the importance of the various mechanisms for the removal of aqueous contaminants by ZVI (Noubactep, 2008, Tratnyek and Salter, 2010). The main contaminant metal removal mechanisms by ZVI are adsorption, precipitation, cementation, co-precipitation and biological processes (Davis et al., 2007, Wilkin

Method

The impact of freeze–thaw cycling on the performance of ZVI mixed with an inert granular media was assessed using duplicated Darcy boxes subjected to 42 freeze–thaw cycles. Measuring static water levels and conducting conservative tracer tests allowed the assessment of bed hydraulics. Reaction kinetics were assessed using step increases in contaminant (copper and zinc) concentrations. All measurements were conducted before, midway and at the end of freeze–thaw cycling. Deconstruction

Heat transfer

Average temperature profiles at different depths during a sample thawing and freezing cycle are presented in Fig. 2. As shown, the seven day cycle was sufficient to ensure the media reached both the high and low temperature set points. The middle of the box was the last region to thaw (supported by individual temperature sensor data), indicating that the thickness of the insulation was insufficient for a comparison with solar induced one dimensional heat transfer. However, there is a greater

Conclusion

As a PRB consists of wet porous media, an understanding of potentially detrimental interactions during freeze–thaw cycling is vital to the design of PRB based remediation systems for application in cold climates. The results for a laboratory based freeze–thaw assessment of granular ZVI media were presented.

Freeze–thaw induced changes to simulated PRBs, contained within Darcy boxes, subjected to 0, 21 and 42 freeze–thaw cycles were assessed using the flow of both reactive and conservative

Acknowledgments

The authors wish to thank Laura Gordon and Damian Gore for XRD analysis and interpretation, Roger Curtain for his assistance to complete the SEM and EDS analyses and Andrew Lee for the photography work. The financial support of the Australian Antarctic Science Grant 4029 is gratefully acknowledged. T.S. also acknowledges the support of an Australian Postgraduate Award. The Particulate Fluid Processing Centre (PFPC) and the Advanced Microscopy Facility at the University of Melbourne are both

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