The effects of cold temperature on copper ion exchange by natural zeolite for use in a permeable reactive barrier in Antarctica
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:
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Reduction in hydraulic conductivity owing to the clogging of membranes and zeolite in the barrier by ice during freezing;
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Slow sorption kinetics at low temperatures;
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Reduction in zeolite capacity for heavy metals at low temperatures;
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Highly variable water and contaminant fluxes during diurnal freeze–thaw cycles, and weekly variations in melting associated with passing weather systems;
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Water characteristics that favour a high solubility of heavy metals such as weakly carbonic and low ionic strength;
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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.
Section snippets
Background
Zeolites are generally defined as aluminosilicates possessing three-dimensional frameworks of linked silicon–aluminium–oxygen tetrahedra. Clinoptilolite is distinguished from other zeolites of the heulandite group by a lower void volume and higher silica content (Si/Al, >4). The framework contains a network of channels defined by two eight-ring pores (0.26×0.47, 0.33×0.46 nm) and a 10-ring pore (0.30×0.76 nm) Dyer, 2001, Palmer and Gunter, 2001. The isomorphic substitution of Al3+ for Si4+
Clinoptilolite
A commercially available clinoptilolite (0.5–2.0 mm Escott natural zeolite, Zeolite Australia) was investigated. Table 2 shows the reported physicochemical properties of the clinoptilolite. The clinoptilolite is reported to contain approximately 70% clinoptilolite with minor quantities of quartz, mordonite, smectite and mica (Zeolite Australia). Analysis by X-ray diffraction confirmed that the sample contained clinoptilolite with quartz as the major secondary phase.
A particle size distribution
Equilibrium time
Ion exchange is generally regarded as a relatively rapid process, and the time to reach equilibrium in batch systems is usually found to be less than 24 h, although it can reach 72 h. While variability in clinoptilolite characteristics influences ion exchange, experimental conditions also affect the time to reach equilibrium in batch tests (US EPA, 1992). Fig. 2 shows ion exchange kinetics of natural and Na clinoptilolite for batch tests at 2 and 22 °C. Equilibrium for the purposes of this
Conditioning
The conditioning of natural zeolite through pretreatment using solutions of a single easily exchangeable cation, in this case Na+, is well known and typically increases total cation exchange capacity by 2.5–3 times. However, the increase in heavy metal exchange is much less and studies have found an increase of only 17% for Cd2+ and between 62% and 75% for Pb2+ Ouki et al., 1993, Curkovic et al., 1997. The lower treatment efficacy of heavy metal exchange indicates that these cations are
Conclusions
This study has found that cold temperatures have significant detrimental effects on the removal of Cu2+ from solution by a natural clinoptilolite. The uptake of Cu2+ at 2 °C is significantly lower than exchange at 22 °C. Exchange kinetics also appear slower at this cold temperature. The exchange of Cu2+ in slightly saline waters typical of many contaminated sites in Antarctica is diminished at both 22 and 2 °C compared to Cu2+ uptake in simple binary systems.
While our study only described the
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