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The effects of cold temperature on copper ion exchange by natural zeolite for use in a permeable reactive barrier in Antarctica
Cold Regions Science and Technology 37 (2003) 159 – 168 www.elsevier.com/locate/coldregions
The effects of cold temperature on copper ion exchange by natural zeolite for use in a permeable reactive barrier in Antarctica A.Z. Woinarski a,b,*, I. Snape b, G.W. Stevens a, S.C. Stark b a
Department of Chemical Engineering, University of Melbourne, Melbourne, Victoria 3010, Australia b Human Impacts Research, Australian Antarctic Division, Kingston, Tasmania 7050, Australia Received 22 July 2002; accepted 31 March 2003
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 jC 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 jC compared to uptake in simple binary systems. These results will have significant implications on cold region barrier design. D 2003 Elsevier Science B.V. All rights reserved. Keywords: Permeable reactive barrier; Clinoptilolite; Copper; Ion exchange; Antarctica
1. 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
* Corresponding author. Human Impacts Research, Australian Antarctic Division, Kingston Tasmania 7050, Australia. E-mail address: [email protected] (A.Z. Woinarski).
surface and subsurface melt water. Indeed, leachate waters from the Thala Valley tip site have up to a 100fold 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,
0165-232X/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0165-232X(03)00038-7
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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 freedraining, 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 Amol 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.
2. 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 + results in a negative charge imbalance in the zeolite lattice that is balanced by exchangeable cations, typically hydrated Na+, K+ and Ca2 + in nature. This means a high affinity for transition metal cations such
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as heavy metals, but a low affinity for anions and nonpolar organics exists (Haggerty and Bowman, 1994). Sorption of heavy metal cations occurs through ion exchange on internal and external surfaces, molecular sieve processes and surface precipitation. The main properties of an ion exchanger that are of interest are generally considered to be its equilibrium behaviour, described in terms of equilibrium isotherms and kinetics, which are dependent on the initial solution concentration, the intrinsic characteristics of the ion exchange system, and temperature (Curkovic et al., 1997; Inglezakis et al., 2002). However, considering the current interest in natural zeolites for removing heavy metals from waters, limited data are available. Clinoptilolite equilibrium isotherms at room temperature show high degrees of heavy metal uptake and have been presented for: Pb2 + and Cd2 + (Malliou et al., 1994; Curkovic et al., 1997); Pb2 +, Ni2 +, Cd2 +, Ba2 + (Faghihian et al., 1999); Cu2 +, Zn2 +, Cd2 +, Pb2 + (Langella et al., 2000; Cincotti et al., 2001); Pb2 +, Cr2 +, Fe3 +, Cu2 + (Inglezakis et al., 2002); and Cu2 + (Panayotova, 2001; Doula et al., 2002). The ion exchange capacity is also used to describe zeolites. The total cation exchange capacity (CEC) is the number of equivalent ionogenic groups in the material. However, not all these sites are available for exchange as exclusion of cations occurs through molecular sieve processes and some of the sites may be present in mineral impurities or at inaccessible sites of the material framework (Yang et al., 2001). Maximum exchange capacities (MEC) are measured by batch or column ion exchange methods and represent the total number of exchangeable ions, a much more important parameter from an applied perspective. Heavy metal MEC data are provided for clinoptilolite in numerous studies (Ouki and Kavannagh, 1999; Cincotti et al., 2001; Inglezakis et al., 2001, 2002) and Table 1 summarises relevant data for copper. In multi-component systems, cation selectivity is determined by a complex relationship between cation charge and electronic structure, and sometimes temperature (Ames, 1960). Orders of selectivity vary considerably and have been determined for clinoptilolite as Cs+>Rb+>K+>NH4+>Ba2 + z Sr2 +>Na+> Ca 2 + >Fe 3 + >Al 3 + >Mg 2 + >Li + (Ames, 1960) and Pb2 +>NH4+>Cd2 +, Cu2 +, Sr2 +>Zn2 +>Co 2 + (Cincotti et al., 2001). Clinoptilolite pore diameters dictate the size of the molecule that can enter the channels. In this
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Table 1 Maximum Cu2 + exchange capacities for clinoptilolites MEC (mmol g 1)
Method
Sample
References
0.19
Batch equilibrium Batch equilibrium Batch equilibrium Column exhaustion Column exhaustion
Na clinoptilolite
(Cincotti et al., 2001)
0.08 0.88 0.34 – 0.42 0.08 – 0.28
Clinoptilolite Na clinoptilolite Na clinoptilolite
(Langella et al., 2000) (Mondale et al., 1995)
Clinoptilolite
molecular sieve process, ions that are too large to fit into the channels are excluded from internal surfaces and can only exchange with sites on the external surface. The clinoptilolite structure is not completely rigid and the effective pore size can vary according to the exchangeable cation incorporated into framework channels (Palmer and Gunter, 2001). It has been argued that selectivity can be interpreted from Eisenmann’s theory whereby the cation selectivity of zeolites with weak ionic fields, like clinoptilolite, is related to the cation’s free energy of hydration—it is energetically more favourable for cations with higher hydration energies to remain in solution (see Sherry, 1969). However, experimental work tends to indicate that selectivity can also be dictated by the hydrated radius of the cation (Ouki and Kavannagh, 1997). Therefore, it is apparent that a number of parameters are likely to affect the ion exchange of heavy metals such as copper. These include the clinoptilolite content, physical properties of the clinoptilolite such as particle size, crystal structure, exchange site distribution, porosity and density, chemical pre-treatment, pH, presence of competing cations, presence of nonexchange anions, temperature, and other parameters (Booker et al., 1996). This study is the first stage of an investigation into the effects that cold temperatures have on the removal of heavy metals from solution by clinoptilolite. Laboratory batch tests are used to focus on copper equilibria in simple binary systems and more complex multi-component exchange systems at 22 and 2 jC, a temperature typical of groundwaters during summer months in Antarctica. Once the effects of lower temperature on the ion exchange of copper are under-
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stood, these relationships can hopefully be translated with relative ease to other contaminants such as lead and zinc, and to clinoptilolite from other sources.
3. Materials and methods 3.1. 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 analysis was conducted on washed and unwashed clinoptilolite. The particle size distribution (see Fig. 1) reveals that even though the clinoptilolite product obtained was claimed to be in the 0.5 –2.0 mm size range, approximately 22% of the material mass has a grain size below 0.5 mm. It is likely that smaller grain sizes and dust fines are removed or lost during the washing process. While the removal of fines may slightly decrease exchange capacities due to lower unit mass, exchange kinetics are likely to be faster since dust covering the mineral surface and pore openings is removed during the washing process (Inglezakis et al., 1999). The water permeability of the zeolite was determined by using standard methods (ASTM, 1997). The saturated hydraulic conductivity was found to be 0.08 cm s 1. This value is similar to results from previous Table 2 Physicochemical properties of clinoptilolite (Zeolite Australia) Properties
Measurement
Colour Hardness (Mohs) Loss on ignition (wt.%) Thermal stability (jC) Acidic stability (pH) Bulk density (g cm 3) Molecular channel size (nm) Cation exchange capacity (meq g 1)
Light pink 7 12.5 up to 400 >2 1.1 – 1.6 0.79 0.35; 0.44 0.30 1.19
Fig. 1. Particle size distribution of Escott natural clinoptilolite.
studies on the use of clinoptilolite in PRBs (Abadzic and Ryan, 2001; Park et al., 2002). 3.2. Copper adsorption experiments 3.2.1. Materials All chemicals, salts and acids used in the study were AnalaR-grade and were supplied by BDH Chemicals Australia and APS Chemicals. All standards, solutions, and dilutions were made using distilled de-ionised water. Conductivity and pH were measured using standard meters (WTW LF197 conductivity meter and a Radiometer PHM210 standard pH meter, respectively). Copper was analysed colorimetrically at 518 nm using a GBC 916 UV – Visible Spectrophotometer. An ICP-OES (Thermo Jarrell-Ash IRIS Plasma Spectrometer) was used to analyse approximately 10% of the samples for quality control and replication purposes. Results from the two analytical methods corresponded to within F 5%. 3.2.2. Conditioning Natural zeolites can be pretreated to remove specific ions from exchange sites and replace them with more easily removed cations, usually Na+. The conversion of zeolite to a homo-ionic or near homo-ionic state has been found to improve ion exchange capacities and performance (Ouki et al., 1993; Inglezakis et al., 2001). No significant change occurs to the total cation exchange capacity or crystal structure. In this study pretreatment was achieved following methods described by Curkovic et al. (1997) and Panayotova (2001). Approximately 50 g of Milli-Q water-washed
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zeolite was contacted with a 2 mol l 1 NaCl solution under agitation from a rotary platform shaker at 150 rpm for 72 h. The NaCl solution was replaced every 24 h. The supernatant was drained and the sample was rinsed three times with Milli-Q water to remove excess NaCl, dried at 105 jC and equilibrated with the atmosphere for 7 days.
solutions, with CuCl+ complex the only significant minor species, although only at higher copper concentrations. The pH of all solutions were not adjusted, and ranged from 4.5 to 6.5.
3.2.3. Ion exchange equilibria of clinoptilolite Samples of the as-received (natural) and pretreated zeolite (Na clinoptilolite) were used to investigate the maximum exchange capacities and binary equilibria relationship with Cu2 + at room and field temperatures. Studies by the US EPA (1992) show that batch test conditions such as agitation method, material/ solution ratio, pH, and temperature can influence the apparent ion exchange equilibria. Samples consisting of 100 ml of 0.06, 0.5, 1.0, 2.5, 5, 10 and 20 mmol l 1 Cu2 + aqueous solutions, prepared from the anhydrous cupric chloride salt, were reacted with the equivalent of 0.5 g of dry clinoptilolite in 250-ml Erlenmeyer flasks. Controls (without zeolite) and blanks (zeolite and no copper) were also run. The flasks were agitated on a rotary platform shaker (150 rpm) at either 22 F 1 or 2 F 1 jC. Preliminary kinetic studies were performed to determine when batch test system equilibrium was reached using reaction times of 6, 24, 48, 72 and 96 h. After the specified time the solution was filtered through a 0.2-Am filter and acidified with HNO3 to pH < 2. A period of 48 h (the time established to reach equilibrium) was used for subsequent equilibria tests. In addition to simple binary Na+ –Cu2 + systems, equilibrium isotherms were generated in a 1% (v/v) seawater matrix—corresponding to salinity typical of field conditions in contaminated sites in the Casey region, Antarctica. An artificial seawater containing Na+, K+, Ca2 + and Mg2 + at concentrations of 4.807, 0.107, 0.105, 0.527 mol l 1, respectively, was prepared from chloride salts. This analogue seawater solution was used instead of natural seawater in order to purely investigate the detrimental effects of ion exchange competition from major cations. PHREEQC, a geochemical reaction model by Parkhurst and Appelo (2001), was used to determine the major copper species in solution at initial and equilibrium stages. The free cupric ion, Cu2 + was predicted to be the dominant species (>96%) for all
4.1. Equilibrium time
4. Results
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 jC. Equilibrium for the purposes of this study was taken as having been reached when the x/m values plateau, where x is the mass of Cu2 + adsorbed by m grams of zeolite. This occurred at a reaction time of approximately 48 h for both temperatures and zeolite forms. A reaction time of 48 h was subsequently used for all batch tests. No significant changes in solution pH or conductivity were observed over the course of the batch tests. 4.2. Conditioning Copper adsorption isotherms at 2 jC for natural and Na clinoptilolite (see Fig. 3) show that conversion
Fig. 2. Copper sorption equilibrium time for Na clinoptilolite at 22 jC (D) and 2 jC (o), and natural clinoptilolite at 2 jC (5). The initial solution was 5 mmol l 1 Cu2 +.
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Fig. 3. Copper adsorption isotherms at 2 jC for Na clinoptilolite (o) and natural clinoptilolite (5). Langmuir equations (see Eq. (2)) have been fitted to the data for Na clinoptilolite (x/ m = 0.198C/{1 + 1.637C}) and natural clinoptilolite (x/m = 0.187C/ {1 + 2.387C}).
to a Na homo-ionic state through pretreatment with an NaCl solution increases copper exchange by approximately 55% at an initial Cu2 + concentration of 20 mmol l 1. However, the increase in copper uptake approaches 20% at lower initial Cu2 + concentrations. The increase in copper uptake is similar to that found in other studies (see Table 1) and shows that pretreatment has beneficial effects for ion exchange using natural zeolites in cold temperatures. 4.3. Binary equilibria
Fig. 4. Copper adsorption isotherms for Na clinoptilolite at 22 jC (D) and 2 jC (o). Langmuir equations (see Eq. (2)) have been fitted to the data for 22 jC (x/m = 0.652C/{1 + 3.750C}) and 2 jC (x/m = 0.198C/{1 + 1.637C}).
sites. Because many contaminated sites, such as those in the Casey region, are located near the coast and have waters with relatively high salinity, the effects of seawater and temperature on Cu2 + uptake by Na clinoptilolite were investigated (see Fig. 5). There is a significant reduction in Cu2 + uptake caused by the 1% seawater matrix at both 2 and 22 jC. The decrease between maximum Cu2 + uptake in the binary system and maximum uptake in a saline solution is approximately the same for both temperatures, with a decrease of 27% at 22 jC and 23% at 2 jC. Equilibrium isotherms determined in a 1% seawater matrix indicate that the maximum Cu2 + up-
In order to understand how the unique conditions of cold regions will affect the performance of a natural zeolite barrier to remove heavy metal contaminants it is necessary to investigate the simplest system first. The effect that temperature has on the uptake of copper was initially investigated by considering the Na+ – Cu2 + binary system at 22 and 2 jC. The equilibrium isotherms generated reveal that maximum Cu2 + uptake is 0.174 mmol g 1 at 22 jC, and 0.121 mmol g 1 at 2 jC (see Fig. 4). The decrease in Cu2 + exchange due to cold temperature is 32% at Cu2 + concentrations near MEC, but approach 50% at lower Cu2 + concentrations. 4.4. Multi-component equilibria Contaminated waters will have other cations present that are known to compete with Cu2 + for exchange
Fig. 5. Copper adsorption isotherms for Na clinoptilolite in a 1% seawater matrix at 22 jC (D) and 2 jC (o). Langmuir equations (see Eq. (2)) have been fitted to the data for 22 jC. (x/m = 0.296C/ {1 + 2.337C}) and 2 jC (x/m = 0.122C/{1 + 1.312C}).
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take is 0.127 mmol g 1 at 22 jC and 0.093 mmol g 1 at 2 jC. The decrease in copper exchange due to low temperature is 27% at high Cu2 + concentrations and approaches 55% at low equilibrium concentrations. At low Cu2 + equilibrium concentrations, the effect of cold temperature on equilibria is greater in a 1% seawater matrix than in a Cu –Na binary system; however, this is not apparent at high Cu2 + levels. 4.5. Isotherms Freundlich and Langmuir isotherms are two of the most widely used equations to describe solid-solution adsorption systems (US EPA, 1992). Our data was fitted to the Freundlich isotherm (Sposito, 1980): x ¼ Kf C n m
ð1Þ
and the Langmuir isotherm (Sposito, 1979): x KL MC ¼ m 1 þ KL C
ð2Þ
where x is the millimoles of Cu2 + on the clinoptilolite, m is the mass of clinoptilolite, Kf and n are Freundlich constants, KL and M are Langmuir constants, and C is the equilibrium Cu2 + concentration in solution. It was found that the Langmuir isotherm fitted the data best. Table 3 shows Langmuir constants and coefficients of determination for various Cu2 + equilibria conditions. The constant KL may be related to the energy of adsorption and can be regarded as an ‘affinity parameter’ between the exchanging cation and zeolite for a given system, while the constant M is generally accepted as the MEC (Sposito, 1979; US EPA, 1992).
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5. Discussion 5.1. 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 excluded from many of the exchange sites accessible by Na+, possibly due to a larger hydrated radius. Nevertheless, the increase in Cu2 + exchange from conditioning described in this paper is in keeping with these other studies. Conditioning with NaCl solution causes the exchange capacity to increase because Na+ ions are more easily removed from zeolite channels than other cations, such as Ca2 +, and therefore are more easily exchanged for Cu2 +. In addition, the removal of dust by washing during pretreatment may increase Cu2 + exchange by reducing pore clogging. Alternate pretreatment methods may also lead to an increase in exchange capacity; for example, addition of NaOH to raise solution pH and expediate the conditioning process, and use of a higher pretreatment temperature (which is well known to increase the exchange capacity of clinoptilolite) (Curkovic et al., 1997). The effect of pretreatment on exchange kinetics was not apparent because both zeolites responded in a similar fashion. However, previous work has found that the kinetics of Pb2 + exchange on clinoptilolite are highly dependent on conditioning, with diffusion coefficients up to 45 times higher on Na clinoptilolite
Table 3 Langmuir constants (see Eq. (2)) and coefficients of determination (R2) Clinoptilolite
Temperature (jC)
System
KL (l mg 1)
M (mmol g 1)
R2
Na clinoptilolite Na clinoptilolite Clinoptilolite Na clinoptilolite Na clinoptilolite
22 2 2 2 22
Binary Binary Binary Seawater matrix Seawater matrix
1.980 1.271 2.387 1.312 2.337
0.180 0.123 0.078 0.093 0.127
0.999 0.999 0.999 0.993 0.993
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than natural clinoptilolite (Inglezakis et al., 2001). While a favourable change in exchange kinetics is likely, owing to the higher mobility of Na+ ions in clinoptilolite channels, the higher capacity of pretreated zeolite can also result in slower kinetics. This is possibly due to an increase in the number of ions involved in the exchange reaction, leading to the congestion (steric hindrance and electrostatic repulsion) of ions in the zeolite channels (Inglezakis et al., 2001). 5.2. System solutions No significant change in solution pH or conductivity was observed between solutions at initial and equilibrium times. While obviously dependent on test conditions, numerous studies have found that the pH of solution generally increases due to the ion exchange of protons and hydrolysis of the zeolite and other mineral phases (Curkovic et al., 1997). The lack of change in pH is probably due to the high mass ratio (1:200) between the zeolite and CuCl2 solution. Conductivity was also expected to increase from the dissolution of Na+ ions and secondary phases. While the clinoptilolite used is reportedly only 70% pure, with quartz, mordonite, smectite and mica as the secondary phases, the small increase in conductivity measured indicates that quartz is probably the only major secondary phase present because it is relatively inert and dissolution will not occur. 5.3. Temperature effects on ion exchange kinetics While batch tests were not undertaken with the aim of investigating detailed reaction kinetics, Fig. 2 shows that although equilibrium is reached at similar times, the reaction kinetics at 22 jC appear to be faster than at 2 jC, as expected. Ion exchange kinetics slow down at cold temperatures as, in general, diffusion coefficients decrease with temperature. 5.4. Temperature effects on ion exchange equilibria Several studies have investigated the effect of increasing temperature on clinoptilolite equilibria and exchange capacities. Studies have found that cation exchange is endothermic, with uptake increasing with temperature for Cs+ and Co2 + (White et al.,
1999), and Pb2 + and Cd2 + (Malliou et al., 1994; Curkovic et al., 1997). However, it is interesting to note that one study found that while Cu2 + exchange increased with temperature for natural clinoptilolite, Cu2 + exchange with a sodium form of the same zeolite slightly decreased with an increase in temperature (Panayotova, 2001). The affinity of clinoptilolite for Cs+ has also been found to decrease with increasing temperature, but only when Na+ was a competing ion (Ames, 1960). In contrast, very little work has been accomplished on the effects of cold temperatures on clinoptilolite equilibria and exchange capacities, and although the inverse of relationships found for increasing temperatures probably holds, quantification is warranted. A study using a Croatian clinoptilolite found that between 22 and 2 jC cation exchange decreased from 0.45 to 0.32 mmol g 1 ( c 29% decrease) for Pb2 + and from 0.21 to 0.14 ( c 33%) for Cd2 + (Curkovic et al., 1997). These findings are similar to the exchange capacity decrease of between 50% and 32% found in our research. Based on discussions by Palmer and Gunter (2001), this decrease in Cu2 + exchange at cold temperatures may be caused by the more difficult displacement of exchangeable cations due to increased retarding electrostatic interactions and changes in the hydrated cation radii. 5.5. Cation competition Many contaminated sites, such as abandoned waste disposal tips in the Australian Antarctic Territory, are either in close proximity to the ocean or in the intertidal zone where residual salts from tides, storm surges and sea spray can cause ground and surface waters to have relatively high salinities. As an example, the salinity of waters in contaminated sites in the Casey region has been measured at 1.5 and 0.2 for surface melt pools and groundwaters, respectively. To design a PRB system for the removal of heavy metals the uptake of contaminants must be understood not only at cold temperatures but also in waters with variable matrix conditions. Based on selectivity sequences Cu2 + should be preferentially exchanged compared to other common competitive cations present in the field such as Na+, K+, Ca2 +, Mg2 +, and Fe3 +. However, it is well known that competing cations can reduce the uptake of heavy
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metals by natural zeolites. The cation Ca2 +, in particular, competes with heavy metals such as Cu2 +, possibly because they are of similar size. Studies investigating the competition of Ca2 + and Mg2 + with Cu2 + exchange found that interference was minimal at Ca2 + and Mg2 + concentrations below 2 and 1 mmol l 1, respectively (Ouki and Kavannagh, 1997; Panayotova, 2001). However, Ouki and Kavannagh (1997) found that at Ca2 + concentrations approaching 25 mmol l 1 uptake of Cu2 + was reduced by 80%. The 1% seawater matrix solutions used in our study contain only 1.05 mmol l 1 Ca2 + and 5.27 mmol l 1 Mg2 +, which partially explains the comparatively lower reduction in Cu2 + uptake of approximately 25%. Lower uptake of Cu2 + is also likely a consequence of the higher Na+ concentration of the seawater solution, an effect possibly reflected by the findings from other studies for Cs+ and Sr2 + (Ames, 1960; Fuhrmann et al., 1995). 5.6. PRB design implications The pretreatment of clinoptilolite with NaCl solution increases Cu2 + uptake and will do so for other heavy metals. Pretreatment costs will be offset by lower material costs, due to the lower material mass required, and lower transport costs, which are of particular concern for the isolated contaminated sites in Antarctica and other cold regions. While better pretreatment methods are available, they may be more expensive and involve more complicated processing than methods similar to those used in this study. Since Cu2 + exchange capacity is significantly reduced in cold temperatures, a greater mass of zeolite will be required. This will result in higher costs for materials, materials transport and trench excavation. However, in a PRB system it is unlikely that the residence time of the contaminated water will be long enough to allow equilibrium to be obtained. Therefore, reaction kinetics during the first few minutes or hours are likely to determine the extent of contaminant removal. While overall rates of sorption and ion exchange in porous materials are generally dominated by mass transport within the pore network and not the reaction kinetics themselves, further study is required. Based on other work and results from this study it is expected that sorption kinetics will become slower
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with decreasing temperature, resulting in an increase in the required barrier thickness. Moreover, the reduction in Cu2 + exchange in slightly saline waters reveals that competing ions in solution detrimentally affect contaminant removal. It is likely that in the field where in addition to salinity, carbonates, sulphates and other major cations such as Fe3 + and Mn2 + exist, contaminant removal will be hindered further. In addition, many waste streams are polluted by inorganic and organic substances and interference in ion exchange may occur from contaminants such as petroleum fuels and oils.
6. 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 jC is significantly lower than exchange at 22 jC. 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 jC compared to Cu2 + uptake in simple binary systems. While our study only described the first stage of research into the cold region effects on a natural zeolite PRB system, it is apparent that adaptations to existing PRB technology and further quantification of material performance will be necessary for the successful treatment of heavy-metal contaminated waters in cold regions.
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taminated Land Centre Report. UK Environment Agency NC/ 01/51. Cincotti, A., Lai, N., Orru, R., Cao, G., 2001. Sardinian natural clinoptilolites for heavy metals and ammonium removal: experimental and modelling. Chem. Eng. J. 84, 275 – 282. Curkovic, L., Cerjan-Stefanovic, S., Filipan, T., 1997. Metal ion exchange by natural and modified zeolites. Water Res. 31 (6), 1379 – 1382. Deprez, P.P., Arens, M., Locher, H., 1999. Identification and assessment of contaminated sites at Casey Station, Wilkes Land, Antarctica. Polar Rec. 35 (195), 299 – 316. Doula, M., Ioannou, A., Dimirkou, A., 2002. Copper adsorption and Si, Al, Ca, Mg, and Na release from clinoptilolite. J. Colloid Interface Sci. 245, 237 – 250. Dyer, A., 2001. Zeolites. Encyclopedia of Materials: Science and Technology. Elsevier. Faghihian, H., Marageh, M.G., Kazemian, H., 1999. The use of clinoptilolite and its sodium form for removal of radioactive cesium, and strontium from nuclear wastewater and Pb2 +, Ni2 +, Cd2 +, Ba2 + from municipal wastewater. Appl. Radiat. Isotopes 50, 655 – 660. Fuhrmann, M., Aloysius, D., Zhou, H., 1995. Permeable, subsurface sorbent barrier for 90Sr: laboratory studies of natural and synthetic materials. Waste Manag. 15 (7), 485 – 493. Haggerty, G.M., Bowman, R.S., 1994. Sorption of chromate and other inorganic anions by organo-zeolite. Environ. Sci. Technol. 28 (3), 452 – 458. Inglezakis, V.J., Diamandis, N.A., Loizidou, M.D., Grigoropoulou, H.P., 1999. Effects of pore clogging on kinetics of lead uptake by clinoptilolite. J. Colloid Interface Sci. 215, 54 – 57. Inglezakis, V.J., Papadeus, C.D., Loizidou, M.D., Grigoropoulou, H.P., 2001. Effects of pretreatment on physical and ion exchange properties of natural clinoptilolite. Environ. Technol. 22, 75 – 82. Inglezakis, V.J., Loizidou, M.D., Grigoropoulou, H.P., 2002. Equilibrium and kinetic ion exchange studies of Pb2 +, Cr3 +, Fe3 + and Cu2 + on natural clinoptilolite. Water Res. 36, 2784 – 2792. Langella, A., Pansini, M., Cappelletti, P., de Gennaro, B., de Gennaro, M., Colella, C., 2000. NH4 +, Cu2 +, Zn2 +, Cd2 +, and Pb2 + exchange for Na+ in a sedimentary clinoptilolite, North Sardinia, Italy. Microporous Mesoporous Mater. 37, 337 – 343. Malliou, E., Loizidou, M., Spyrellis, N., 1994. Uptake of lead and cadmium by clinoptilolite. Sci. Total Environ. 149, 139 – 144. Mondale, K.D., Carland, R.M., Aplan, F.F., 1995. The comparative ion exchange capacities of natural sedimentary and synthetic zeolites. Miner. Eng. 8 (4/5), 535 – 548. Ouki, S.K., Kavannagh, M., 1997. Performance of natural zeolites for the treatment of mixed metal-contaminated effluents. Waste Manag. Res. 15, 383 – 394.
Ouki, S.K., Kavannagh, M., 1999. Treatment of metals-contaminated wastewaters by use of natural zeolites. Water Sci. Technol. 39 (10), 115 – 122. Ouki, S.K., Cheeseman, C., Perry, R., 1993. Effects of conditioning and treatment of chabazite and clinoptilolite prior to lead and cadmium removal. Environ. Sci. Technol. 27 (6), 1108 – 1116. Palmer, J.L., Gunter, M.E., 2001. The effects of time, temperature, and concentration on Sr2 + exchange in clinoptilolite in aqueous solutions. Am. Mineral. 86, 431 – 437. Panayotova, M.I., 2001. Kinetics and thermodynamics of copper ions removal from wastewater by use of zeolite. Waste Manag. 21, 671 – 676. Park, J.B., Lee, S.H., Lee, J.W., Lee, C.Y., 2002. Lab scale experiments for permeable reactive barriers against contaminated groundwater with ammonium and heavy metals using clinoptilolite. J. Hazard. Mater. 95 (1 – 2), 65 – 79. Parkhurst, D.L., Appelo, C.A.J., 2001. User’s guide to PHREEQC (Version 2)—a computer program for speciation, batch-reaction, one-dimensional transport, and inverse geochemical calculations. US Geological Survey Water-Resources Investigations Report. US Geological Survey. Sherry, N.S., 1969. The ion exchange properties of zeolites. In: Manisky, J.A (Ed.), Ion Exchange: A series of Advances, vol. 2. Marcel Dekker, New York, pp. 89 – 133. Snape, I., Riddle, M.J., Stark, J.S., Cole, C.M., King, C.K., Duquesne, S., Gore, D.B., 2001a. Management and remediation of contaminated sites at Casey Station, Antarctica. Polar Rec. 37 (202), 199 – 214. Snape, I., Morris, C.E., Cole, C.M., 2001b. The use of permeable reactive barriers to control contaminant dispersal during site remediation in Antarctica. Cold Reg. Sci. Technol. 32, 157 – 174. Sposito, G., 1979. Derivation of the Langmuir equation for ion exchange reactions in soils. Soil Sci. Soc. Am. J. 43, 197 – 198. Sposito, G., 1980. Derivation of the Freundlich equation for ion exchange reactions in soils. Soil Sci. Soc. Am. J. 44, 652 – 654. US EPA, 1992. Batch-Type Procedures for Estimating Soil Adsorption of Chemicals. US Environmental Protection Agency. 530SW87006F. US EPA, 1998. Permeable Reactive Barrier Technologies for Contaminant Remediation. US Environmental Protection Agency. 600R98125. White, D.A., Nattkemper, A., Rautiu, R., 1999. Evaluation of the effect of temperature on potassium clinoptilolite ion exchange. Nucl. Technol. 127, 212 – 217. Yang, S., Navrotsky, A., Wilken, R., 2001. Thermodynamics of ion-exchanged and natural clinoptilolite. Am. Mineral. 86, 438 – 447.
The effects of cold temperature on copper ion exchange by natural zeolite for use in a permeable reactive barrier in Antarctica A.Z. Woinarski a,b,*, I. Snape b, G.W. Stevens a, S.C. Stark b a
Department of Chemical Engineering, University of Melbourne, Melbourne, Victoria 3010, Australia b Human Impacts Research, Australian Antarctic Division, Kingston, Tasmania 7050, Australia Received 22 July 2002; accepted 31 March 2003
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 jC 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 jC compared to uptake in simple binary systems. These results will have significant implications on cold region barrier design. D 2003 Elsevier Science B.V. All rights reserved. Keywords: Permeable reactive barrier; Clinoptilolite; Copper; Ion exchange; Antarctica
1. 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
* Corresponding author. Human Impacts Research, Australian Antarctic Division, Kingston Tasmania 7050, Australia. E-mail address: [email protected] (A.Z. Woinarski).
surface and subsurface melt water. Indeed, leachate waters from the Thala Valley tip site have up to a 100fold 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,
0165-232X/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0165-232X(03)00038-7
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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 freedraining, 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 Amol 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.
2. 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 + results in a negative charge imbalance in the zeolite lattice that is balanced by exchangeable cations, typically hydrated Na+, K+ and Ca2 + in nature. This means a high affinity for transition metal cations such
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as heavy metals, but a low affinity for anions and nonpolar organics exists (Haggerty and Bowman, 1994). Sorption of heavy metal cations occurs through ion exchange on internal and external surfaces, molecular sieve processes and surface precipitation. The main properties of an ion exchanger that are of interest are generally considered to be its equilibrium behaviour, described in terms of equilibrium isotherms and kinetics, which are dependent on the initial solution concentration, the intrinsic characteristics of the ion exchange system, and temperature (Curkovic et al., 1997; Inglezakis et al., 2002). However, considering the current interest in natural zeolites for removing heavy metals from waters, limited data are available. Clinoptilolite equilibrium isotherms at room temperature show high degrees of heavy metal uptake and have been presented for: Pb2 + and Cd2 + (Malliou et al., 1994; Curkovic et al., 1997); Pb2 +, Ni2 +, Cd2 +, Ba2 + (Faghihian et al., 1999); Cu2 +, Zn2 +, Cd2 +, Pb2 + (Langella et al., 2000; Cincotti et al., 2001); Pb2 +, Cr2 +, Fe3 +, Cu2 + (Inglezakis et al., 2002); and Cu2 + (Panayotova, 2001; Doula et al., 2002). The ion exchange capacity is also used to describe zeolites. The total cation exchange capacity (CEC) is the number of equivalent ionogenic groups in the material. However, not all these sites are available for exchange as exclusion of cations occurs through molecular sieve processes and some of the sites may be present in mineral impurities or at inaccessible sites of the material framework (Yang et al., 2001). Maximum exchange capacities (MEC) are measured by batch or column ion exchange methods and represent the total number of exchangeable ions, a much more important parameter from an applied perspective. Heavy metal MEC data are provided for clinoptilolite in numerous studies (Ouki and Kavannagh, 1999; Cincotti et al., 2001; Inglezakis et al., 2001, 2002) and Table 1 summarises relevant data for copper. In multi-component systems, cation selectivity is determined by a complex relationship between cation charge and electronic structure, and sometimes temperature (Ames, 1960). Orders of selectivity vary considerably and have been determined for clinoptilolite as Cs+>Rb+>K+>NH4+>Ba2 + z Sr2 +>Na+> Ca 2 + >Fe 3 + >Al 3 + >Mg 2 + >Li + (Ames, 1960) and Pb2 +>NH4+>Cd2 +, Cu2 +, Sr2 +>Zn2 +>Co 2 + (Cincotti et al., 2001). Clinoptilolite pore diameters dictate the size of the molecule that can enter the channels. In this
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Table 1 Maximum Cu2 + exchange capacities for clinoptilolites MEC (mmol g 1)
Method
Sample
References
0.19
Batch equilibrium Batch equilibrium Batch equilibrium Column exhaustion Column exhaustion
Na clinoptilolite
(Cincotti et al., 2001)
0.08 0.88 0.34 – 0.42 0.08 – 0.28
Clinoptilolite Na clinoptilolite Na clinoptilolite
(Langella et al., 2000) (Mondale et al., 1995)
Clinoptilolite
molecular sieve process, ions that are too large to fit into the channels are excluded from internal surfaces and can only exchange with sites on the external surface. The clinoptilolite structure is not completely rigid and the effective pore size can vary according to the exchangeable cation incorporated into framework channels (Palmer and Gunter, 2001). It has been argued that selectivity can be interpreted from Eisenmann’s theory whereby the cation selectivity of zeolites with weak ionic fields, like clinoptilolite, is related to the cation’s free energy of hydration—it is energetically more favourable for cations with higher hydration energies to remain in solution (see Sherry, 1969). However, experimental work tends to indicate that selectivity can also be dictated by the hydrated radius of the cation (Ouki and Kavannagh, 1997). Therefore, it is apparent that a number of parameters are likely to affect the ion exchange of heavy metals such as copper. These include the clinoptilolite content, physical properties of the clinoptilolite such as particle size, crystal structure, exchange site distribution, porosity and density, chemical pre-treatment, pH, presence of competing cations, presence of nonexchange anions, temperature, and other parameters (Booker et al., 1996). This study is the first stage of an investigation into the effects that cold temperatures have on the removal of heavy metals from solution by clinoptilolite. Laboratory batch tests are used to focus on copper equilibria in simple binary systems and more complex multi-component exchange systems at 22 and 2 jC, a temperature typical of groundwaters during summer months in Antarctica. Once the effects of lower temperature on the ion exchange of copper are under-
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stood, these relationships can hopefully be translated with relative ease to other contaminants such as lead and zinc, and to clinoptilolite from other sources.
3. Materials and methods 3.1. 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 analysis was conducted on washed and unwashed clinoptilolite. The particle size distribution (see Fig. 1) reveals that even though the clinoptilolite product obtained was claimed to be in the 0.5 –2.0 mm size range, approximately 22% of the material mass has a grain size below 0.5 mm. It is likely that smaller grain sizes and dust fines are removed or lost during the washing process. While the removal of fines may slightly decrease exchange capacities due to lower unit mass, exchange kinetics are likely to be faster since dust covering the mineral surface and pore openings is removed during the washing process (Inglezakis et al., 1999). The water permeability of the zeolite was determined by using standard methods (ASTM, 1997). The saturated hydraulic conductivity was found to be 0.08 cm s 1. This value is similar to results from previous Table 2 Physicochemical properties of clinoptilolite (Zeolite Australia) Properties
Measurement
Colour Hardness (Mohs) Loss on ignition (wt.%) Thermal stability (jC) Acidic stability (pH) Bulk density (g cm 3) Molecular channel size (nm) Cation exchange capacity (meq g 1)
Light pink 7 12.5 up to 400 >2 1.1 – 1.6 0.79 0.35; 0.44 0.30 1.19
Fig. 1. Particle size distribution of Escott natural clinoptilolite.
studies on the use of clinoptilolite in PRBs (Abadzic and Ryan, 2001; Park et al., 2002). 3.2. Copper adsorption experiments 3.2.1. Materials All chemicals, salts and acids used in the study were AnalaR-grade and were supplied by BDH Chemicals Australia and APS Chemicals. All standards, solutions, and dilutions were made using distilled de-ionised water. Conductivity and pH were measured using standard meters (WTW LF197 conductivity meter and a Radiometer PHM210 standard pH meter, respectively). Copper was analysed colorimetrically at 518 nm using a GBC 916 UV – Visible Spectrophotometer. An ICP-OES (Thermo Jarrell-Ash IRIS Plasma Spectrometer) was used to analyse approximately 10% of the samples for quality control and replication purposes. Results from the two analytical methods corresponded to within F 5%. 3.2.2. Conditioning Natural zeolites can be pretreated to remove specific ions from exchange sites and replace them with more easily removed cations, usually Na+. The conversion of zeolite to a homo-ionic or near homo-ionic state has been found to improve ion exchange capacities and performance (Ouki et al., 1993; Inglezakis et al., 2001). No significant change occurs to the total cation exchange capacity or crystal structure. In this study pretreatment was achieved following methods described by Curkovic et al. (1997) and Panayotova (2001). Approximately 50 g of Milli-Q water-washed
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zeolite was contacted with a 2 mol l 1 NaCl solution under agitation from a rotary platform shaker at 150 rpm for 72 h. The NaCl solution was replaced every 24 h. The supernatant was drained and the sample was rinsed three times with Milli-Q water to remove excess NaCl, dried at 105 jC and equilibrated with the atmosphere for 7 days.
solutions, with CuCl+ complex the only significant minor species, although only at higher copper concentrations. The pH of all solutions were not adjusted, and ranged from 4.5 to 6.5.
3.2.3. Ion exchange equilibria of clinoptilolite Samples of the as-received (natural) and pretreated zeolite (Na clinoptilolite) were used to investigate the maximum exchange capacities and binary equilibria relationship with Cu2 + at room and field temperatures. Studies by the US EPA (1992) show that batch test conditions such as agitation method, material/ solution ratio, pH, and temperature can influence the apparent ion exchange equilibria. Samples consisting of 100 ml of 0.06, 0.5, 1.0, 2.5, 5, 10 and 20 mmol l 1 Cu2 + aqueous solutions, prepared from the anhydrous cupric chloride salt, were reacted with the equivalent of 0.5 g of dry clinoptilolite in 250-ml Erlenmeyer flasks. Controls (without zeolite) and blanks (zeolite and no copper) were also run. The flasks were agitated on a rotary platform shaker (150 rpm) at either 22 F 1 or 2 F 1 jC. Preliminary kinetic studies were performed to determine when batch test system equilibrium was reached using reaction times of 6, 24, 48, 72 and 96 h. After the specified time the solution was filtered through a 0.2-Am filter and acidified with HNO3 to pH < 2. A period of 48 h (the time established to reach equilibrium) was used for subsequent equilibria tests. In addition to simple binary Na+ –Cu2 + systems, equilibrium isotherms were generated in a 1% (v/v) seawater matrix—corresponding to salinity typical of field conditions in contaminated sites in the Casey region, Antarctica. An artificial seawater containing Na+, K+, Ca2 + and Mg2 + at concentrations of 4.807, 0.107, 0.105, 0.527 mol l 1, respectively, was prepared from chloride salts. This analogue seawater solution was used instead of natural seawater in order to purely investigate the detrimental effects of ion exchange competition from major cations. PHREEQC, a geochemical reaction model by Parkhurst and Appelo (2001), was used to determine the major copper species in solution at initial and equilibrium stages. The free cupric ion, Cu2 + was predicted to be the dominant species (>96%) for all
4.1. Equilibrium time
4. Results
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 jC. Equilibrium for the purposes of this study was taken as having been reached when the x/m values plateau, where x is the mass of Cu2 + adsorbed by m grams of zeolite. This occurred at a reaction time of approximately 48 h for both temperatures and zeolite forms. A reaction time of 48 h was subsequently used for all batch tests. No significant changes in solution pH or conductivity were observed over the course of the batch tests. 4.2. Conditioning Copper adsorption isotherms at 2 jC for natural and Na clinoptilolite (see Fig. 3) show that conversion
Fig. 2. Copper sorption equilibrium time for Na clinoptilolite at 22 jC (D) and 2 jC (o), and natural clinoptilolite at 2 jC (5). The initial solution was 5 mmol l 1 Cu2 +.
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Fig. 3. Copper adsorption isotherms at 2 jC for Na clinoptilolite (o) and natural clinoptilolite (5). Langmuir equations (see Eq. (2)) have been fitted to the data for Na clinoptilolite (x/ m = 0.198C/{1 + 1.637C}) and natural clinoptilolite (x/m = 0.187C/ {1 + 2.387C}).
to a Na homo-ionic state through pretreatment with an NaCl solution increases copper exchange by approximately 55% at an initial Cu2 + concentration of 20 mmol l 1. However, the increase in copper uptake approaches 20% at lower initial Cu2 + concentrations. The increase in copper uptake is similar to that found in other studies (see Table 1) and shows that pretreatment has beneficial effects for ion exchange using natural zeolites in cold temperatures. 4.3. Binary equilibria
Fig. 4. Copper adsorption isotherms for Na clinoptilolite at 22 jC (D) and 2 jC (o). Langmuir equations (see Eq. (2)) have been fitted to the data for 22 jC (x/m = 0.652C/{1 + 3.750C}) and 2 jC (x/m = 0.198C/{1 + 1.637C}).
sites. Because many contaminated sites, such as those in the Casey region, are located near the coast and have waters with relatively high salinity, the effects of seawater and temperature on Cu2 + uptake by Na clinoptilolite were investigated (see Fig. 5). There is a significant reduction in Cu2 + uptake caused by the 1% seawater matrix at both 2 and 22 jC. The decrease between maximum Cu2 + uptake in the binary system and maximum uptake in a saline solution is approximately the same for both temperatures, with a decrease of 27% at 22 jC and 23% at 2 jC. Equilibrium isotherms determined in a 1% seawater matrix indicate that the maximum Cu2 + up-
In order to understand how the unique conditions of cold regions will affect the performance of a natural zeolite barrier to remove heavy metal contaminants it is necessary to investigate the simplest system first. The effect that temperature has on the uptake of copper was initially investigated by considering the Na+ – Cu2 + binary system at 22 and 2 jC. The equilibrium isotherms generated reveal that maximum Cu2 + uptake is 0.174 mmol g 1 at 22 jC, and 0.121 mmol g 1 at 2 jC (see Fig. 4). The decrease in Cu2 + exchange due to cold temperature is 32% at Cu2 + concentrations near MEC, but approach 50% at lower Cu2 + concentrations. 4.4. Multi-component equilibria Contaminated waters will have other cations present that are known to compete with Cu2 + for exchange
Fig. 5. Copper adsorption isotherms for Na clinoptilolite in a 1% seawater matrix at 22 jC (D) and 2 jC (o). Langmuir equations (see Eq. (2)) have been fitted to the data for 22 jC. (x/m = 0.296C/ {1 + 2.337C}) and 2 jC (x/m = 0.122C/{1 + 1.312C}).
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take is 0.127 mmol g 1 at 22 jC and 0.093 mmol g 1 at 2 jC. The decrease in copper exchange due to low temperature is 27% at high Cu2 + concentrations and approaches 55% at low equilibrium concentrations. At low Cu2 + equilibrium concentrations, the effect of cold temperature on equilibria is greater in a 1% seawater matrix than in a Cu –Na binary system; however, this is not apparent at high Cu2 + levels. 4.5. Isotherms Freundlich and Langmuir isotherms are two of the most widely used equations to describe solid-solution adsorption systems (US EPA, 1992). Our data was fitted to the Freundlich isotherm (Sposito, 1980): x ¼ Kf C n m
ð1Þ
and the Langmuir isotherm (Sposito, 1979): x KL MC ¼ m 1 þ KL C
ð2Þ
where x is the millimoles of Cu2 + on the clinoptilolite, m is the mass of clinoptilolite, Kf and n are Freundlich constants, KL and M are Langmuir constants, and C is the equilibrium Cu2 + concentration in solution. It was found that the Langmuir isotherm fitted the data best. Table 3 shows Langmuir constants and coefficients of determination for various Cu2 + equilibria conditions. The constant KL may be related to the energy of adsorption and can be regarded as an ‘affinity parameter’ between the exchanging cation and zeolite for a given system, while the constant M is generally accepted as the MEC (Sposito, 1979; US EPA, 1992).
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5. Discussion 5.1. 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 excluded from many of the exchange sites accessible by Na+, possibly due to a larger hydrated radius. Nevertheless, the increase in Cu2 + exchange from conditioning described in this paper is in keeping with these other studies. Conditioning with NaCl solution causes the exchange capacity to increase because Na+ ions are more easily removed from zeolite channels than other cations, such as Ca2 +, and therefore are more easily exchanged for Cu2 +. In addition, the removal of dust by washing during pretreatment may increase Cu2 + exchange by reducing pore clogging. Alternate pretreatment methods may also lead to an increase in exchange capacity; for example, addition of NaOH to raise solution pH and expediate the conditioning process, and use of a higher pretreatment temperature (which is well known to increase the exchange capacity of clinoptilolite) (Curkovic et al., 1997). The effect of pretreatment on exchange kinetics was not apparent because both zeolites responded in a similar fashion. However, previous work has found that the kinetics of Pb2 + exchange on clinoptilolite are highly dependent on conditioning, with diffusion coefficients up to 45 times higher on Na clinoptilolite
Table 3 Langmuir constants (see Eq. (2)) and coefficients of determination (R2) Clinoptilolite
Temperature (jC)
System
KL (l mg 1)
M (mmol g 1)
R2
Na clinoptilolite Na clinoptilolite Clinoptilolite Na clinoptilolite Na clinoptilolite
22 2 2 2 22
Binary Binary Binary Seawater matrix Seawater matrix
1.980 1.271 2.387 1.312 2.337
0.180 0.123 0.078 0.093 0.127
0.999 0.999 0.999 0.993 0.993
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than natural clinoptilolite (Inglezakis et al., 2001). While a favourable change in exchange kinetics is likely, owing to the higher mobility of Na+ ions in clinoptilolite channels, the higher capacity of pretreated zeolite can also result in slower kinetics. This is possibly due to an increase in the number of ions involved in the exchange reaction, leading to the congestion (steric hindrance and electrostatic repulsion) of ions in the zeolite channels (Inglezakis et al., 2001). 5.2. System solutions No significant change in solution pH or conductivity was observed between solutions at initial and equilibrium times. While obviously dependent on test conditions, numerous studies have found that the pH of solution generally increases due to the ion exchange of protons and hydrolysis of the zeolite and other mineral phases (Curkovic et al., 1997). The lack of change in pH is probably due to the high mass ratio (1:200) between the zeolite and CuCl2 solution. Conductivity was also expected to increase from the dissolution of Na+ ions and secondary phases. While the clinoptilolite used is reportedly only 70% pure, with quartz, mordonite, smectite and mica as the secondary phases, the small increase in conductivity measured indicates that quartz is probably the only major secondary phase present because it is relatively inert and dissolution will not occur. 5.3. Temperature effects on ion exchange kinetics While batch tests were not undertaken with the aim of investigating detailed reaction kinetics, Fig. 2 shows that although equilibrium is reached at similar times, the reaction kinetics at 22 jC appear to be faster than at 2 jC, as expected. Ion exchange kinetics slow down at cold temperatures as, in general, diffusion coefficients decrease with temperature. 5.4. Temperature effects on ion exchange equilibria Several studies have investigated the effect of increasing temperature on clinoptilolite equilibria and exchange capacities. Studies have found that cation exchange is endothermic, with uptake increasing with temperature for Cs+ and Co2 + (White et al.,
1999), and Pb2 + and Cd2 + (Malliou et al., 1994; Curkovic et al., 1997). However, it is interesting to note that one study found that while Cu2 + exchange increased with temperature for natural clinoptilolite, Cu2 + exchange with a sodium form of the same zeolite slightly decreased with an increase in temperature (Panayotova, 2001). The affinity of clinoptilolite for Cs+ has also been found to decrease with increasing temperature, but only when Na+ was a competing ion (Ames, 1960). In contrast, very little work has been accomplished on the effects of cold temperatures on clinoptilolite equilibria and exchange capacities, and although the inverse of relationships found for increasing temperatures probably holds, quantification is warranted. A study using a Croatian clinoptilolite found that between 22 and 2 jC cation exchange decreased from 0.45 to 0.32 mmol g 1 ( c 29% decrease) for Pb2 + and from 0.21 to 0.14 ( c 33%) for Cd2 + (Curkovic et al., 1997). These findings are similar to the exchange capacity decrease of between 50% and 32% found in our research. Based on discussions by Palmer and Gunter (2001), this decrease in Cu2 + exchange at cold temperatures may be caused by the more difficult displacement of exchangeable cations due to increased retarding electrostatic interactions and changes in the hydrated cation radii. 5.5. Cation competition Many contaminated sites, such as abandoned waste disposal tips in the Australian Antarctic Territory, are either in close proximity to the ocean or in the intertidal zone where residual salts from tides, storm surges and sea spray can cause ground and surface waters to have relatively high salinities. As an example, the salinity of waters in contaminated sites in the Casey region has been measured at 1.5 and 0.2 for surface melt pools and groundwaters, respectively. To design a PRB system for the removal of heavy metals the uptake of contaminants must be understood not only at cold temperatures but also in waters with variable matrix conditions. Based on selectivity sequences Cu2 + should be preferentially exchanged compared to other common competitive cations present in the field such as Na+, K+, Ca2 +, Mg2 +, and Fe3 +. However, it is well known that competing cations can reduce the uptake of heavy
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metals by natural zeolites. The cation Ca2 +, in particular, competes with heavy metals such as Cu2 +, possibly because they are of similar size. Studies investigating the competition of Ca2 + and Mg2 + with Cu2 + exchange found that interference was minimal at Ca2 + and Mg2 + concentrations below 2 and 1 mmol l 1, respectively (Ouki and Kavannagh, 1997; Panayotova, 2001). However, Ouki and Kavannagh (1997) found that at Ca2 + concentrations approaching 25 mmol l 1 uptake of Cu2 + was reduced by 80%. The 1% seawater matrix solutions used in our study contain only 1.05 mmol l 1 Ca2 + and 5.27 mmol l 1 Mg2 +, which partially explains the comparatively lower reduction in Cu2 + uptake of approximately 25%. Lower uptake of Cu2 + is also likely a consequence of the higher Na+ concentration of the seawater solution, an effect possibly reflected by the findings from other studies for Cs+ and Sr2 + (Ames, 1960; Fuhrmann et al., 1995). 5.6. PRB design implications The pretreatment of clinoptilolite with NaCl solution increases Cu2 + uptake and will do so for other heavy metals. Pretreatment costs will be offset by lower material costs, due to the lower material mass required, and lower transport costs, which are of particular concern for the isolated contaminated sites in Antarctica and other cold regions. While better pretreatment methods are available, they may be more expensive and involve more complicated processing than methods similar to those used in this study. Since Cu2 + exchange capacity is significantly reduced in cold temperatures, a greater mass of zeolite will be required. This will result in higher costs for materials, materials transport and trench excavation. However, in a PRB system it is unlikely that the residence time of the contaminated water will be long enough to allow equilibrium to be obtained. Therefore, reaction kinetics during the first few minutes or hours are likely to determine the extent of contaminant removal. While overall rates of sorption and ion exchange in porous materials are generally dominated by mass transport within the pore network and not the reaction kinetics themselves, further study is required. Based on other work and results from this study it is expected that sorption kinetics will become slower
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with decreasing temperature, resulting in an increase in the required barrier thickness. Moreover, the reduction in Cu2 + exchange in slightly saline waters reveals that competing ions in solution detrimentally affect contaminant removal. It is likely that in the field where in addition to salinity, carbonates, sulphates and other major cations such as Fe3 + and Mn2 + exist, contaminant removal will be hindered further. In addition, many waste streams are polluted by inorganic and organic substances and interference in ion exchange may occur from contaminants such as petroleum fuels and oils.
6. 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 jC is significantly lower than exchange at 22 jC. 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 jC compared to Cu2 + uptake in simple binary systems. While our study only described the first stage of research into the cold region effects on a natural zeolite PRB system, it is apparent that adaptations to existing PRB technology and further quantification of material performance will be necessary for the successful treatment of heavy-metal contaminated waters in cold regions.
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