The effects of cold temperature on copper ion exchange by natural zeolite for use in a permeable reactive barrier in Antarctica

Abstract

Permeable reactive barriers (PRBs) are an in-situ passive treatment technology that removes dissolved contaminants from polluted waters through the subsurface emplacement of reactive materials such as natural zeolite. While significant work has been achieved using PRBs in temperate climates, adaptations to existing PRB technology and reactive material characteristics will be necessary for the successful treatment of heavy metal contaminated waters in cold regions.

This study investigates the effects of cold temperature on the ion exchange equilibria of copper with clinoptilolite, a common natural zeolite, in natural and pretreated sodium forms. Batch tests were conducted at 22 and 2 °C in both simple binary systems and more complex multi-component systems. Results show that cold temperatures decrease copper uptake by clinoptilolite and appear to slow reaction kinetics. The ion exchange of copper in slightly saline waters is decreased at both 22 and 2 °C compared to uptake in simple binary systems. These results will have significant implications on cold region barrier design.

Introduction

Numerous contaminated sites exist in Antarctica as a result of accidents and poor past waste management. One example of such a site is the abandoned waste disposal tip at Thala Valley near Casey Station in the Australian Antarctic Territory (Snape et al., 2001a). Contaminant dispersal from such sites occurs through particle entrainment and dissolution of heavy metals like copper, lead, cadmium, chromium and zinc, by surface and subsurface melt water. Indeed, leachate waters from the Thala Valley tip site have up to a 100-fold increase in some heavy metals above background levels, and concentrations are well above guideline levels for the protection of marine aquatic ecosystems of high conservation or ecological value Deprez et al., 1999, Snape et al., 2001a. While the physical removal of particles from the wastewater can significantly reduce the spread of pollutants, dissolved and colloidal phases must also be managed and treated.

Permeable reactive barriers (PRBs) are an in-situ passive treatment technology that removes dissolved contaminants from polluted waters through the subsurface emplacement of reactive materials US EPA, 1998, Carey et al., 2002. Several reactive materials, including zero-valent iron, calcium carbonate and granular activated carbon, were considered for use in a barrier system at Thala Valley (Snape et al., 2001b). However, the natural zeolite clinoptilolite was chosen for further study due to its low cost compared to other reactive materials and zeolites, excellent hydraulic characteristics, harmless by-products and potential ability to uptake a range of heavy metals.

While significant work has been achieved with PRB technology in temperate climates, very few studies have investigated their use in cold regions. The design of a permeable reactive barrier system suitable for use in Antarctica and possibly other cold regions will involve the adaptation of existing technology to suit the unique environmental and operational conditions. Based on a generic discussion of PRB systems in cold regions by Snape et al. (2001b), the main environmental and site-specific limitations to the efficacy of a cold region natural zeolite barrier will be:

• Reduction in hydraulic conductivity owing to the clogging of membranes and zeolite in the barrier by ice during freezing;

• Slow sorption kinetics at low temperatures;

• Reduction in zeolite capacity for heavy metals at low temperatures;

• Highly variable water and contaminant fluxes during diurnal freeze–thaw cycles, and weekly variations in melting associated with passing weather systems;

• Water characteristics that favour a high solubility of heavy metals such as weakly carbonic and low ionic strength;

• Interference from interactions between polar and non-polar contaminants varying from solvents, fuels and oils, to PCBs and heavy metals.

In addition to site-specific hydraulic and geotechnical properties, and contaminant characteristics, cold region barrier design must consider these general limitations. Barrier hydraulic conductivity must be greater than that of the surrounding subsurface for water to flow through the system. Considering that contaminated sites, such as the waste disposal site of Thala Valley near Casey Station, can be highly heterogenous, subsurface hydraulic properties are likely to vary considerably and a large particle size is essential. Additionally, the material must be free-draining, so that during freeze–thaw cycles, a frozen monolithic block does not form that might inhibit water flow longer than the surrounding subsurface. However, the large sized particles required to achieve high hydraulic conductivity will generally have lower reactivity as many contaminant removal mechanisms involve surface controlled reactions.

Natural zeolites have only recently received attention for use in the wastewater and water treatment fields. Much of the research has investigated the heavy metal contaminant removal characteristics of clinoptilolites from the United States, Hungary, Japan and Korea, and significant source-dependent variation in the ion exchange characteristics of clinoptilolite exists (Mondale et al., 1995). However, there has been very little work investigating the effect of cold temperatures on ion exchange. The potential for the clinoptilolite used in this study to remove heavy metals from waste streams under the unique conditions of Antarctica is largely unknown. Copper was chosen for study as this contaminant is a major concern in management of sites such as Thala Valley due to the high concentrations of up to 10 μmol l⁻¹ present in waste streams and its high toxicity (see Snape et al., 2001a). Clinoptilolite has a mid-range selectivity for copper and so general principles may be translated to other heavy metals of concern such as lead and zinc.