The effect of temperature on toluene sorption by granular activated carbon and its use in permeable reactive barriers in cold regions
Introduction
There are many contaminated sites in Antarctica as a result of accidents or poor waste management. A significant proportion of the pollution is from oil and its derivatives. Although it's a common perception that hydrocarbon spills in frozen grounds are immobile, fuel components have been shown to be highly mobile in soils and sediments with low organic contents specifically during the summer melt season (Huttenloch et al., 2001, Snape et al., 2006). The low temperature and low nutrient soils in cold regions make the natural attenuation rates much slower as compared to temperate climates. Therefore, more active remediation options are often sought for such sensitive areas. Permeable reactive barriers are one such option. They are an in situ passive treatment technology that removes dissolved contaminants from polluted water through subsurface emplacement of reactive materials and are widely applied in USA and Canada (USEPA, 1998, Carey et al., 2002, Woinarski et al., 2003).
Finding suitable reactive materials for the PRB is a key issue for the successful implementation of such remediation systems. Although there has been significant work done for PRB technology in temperate regions, its application in cold regions remains little explored (Gore, 2009). The successful implementation of PRB technology in cold regions involves modifications and adaptations in the existing technology to suit the environmental and operational conditions of cold regions. The main environmental and site specific limitations to the success of a cold region PRB may be the reduction in hydraulic conductivity due to pore clogging by ice during freezing, slow sorption kinetics and reduced capacity at low temperatures, and highly variable water and contaminant fluxes (Woinarski et al., 2003). Therefore, developing an understanding of the suitability of various materials for a cold region PRB would be beneficial. A pilot scale composite PRB (made by adding various materials, such as raw St. Cloud (New Mexico, USA) zeolite, surfactant modified zeolite (raw St. Cloud zeolite was modified with a coating of the surfactant hexadecyltrimethylammonium, HDTMA), sand and granular activated carbon (GAC) into the barrier to improve the overall properties of the barrier) was installed at Casey Station, Antarctica, in 2006 in an attempt to intercept a diesel fuel plume near Casey's main power house (MPH). The objective was to arrest the migration of the plume, prevent further oil from entering the seawater, and allow time for remediation works to be conducted. The performance of each material is being tested regularly by collecting and analyzing the samples from barrier and surrounding MPH site since 2006 by Australian Antarctic Division, Kingston. From our previous work and observations from the field work, GAC was chosen for further study due to its excellent HC uptake capacity and low cost (Snape et al., 2001a). The present study investigates the effects of cold temperature on the sorption equilibria and kinetics of toluene on GAC surface. The results of this study will have significant implications for future design of PRB in cold regions.
Section snippets
Granular activated carbon (GAC)
Granular activated carbon produced from coconut husk (picafoam) was obtained from Pica Activated Carbon P/L. Activated carbons are found effective in treatment of contaminated groundwater in cold regions (Snape et al., 2001b). The adsorptive properties of GAC are due to high surface area due to its micro-porous structure, and a high degree of surface reactivity caused by surface oxide groups and inorganic impurities (Yang, 2003, Yue and Economy, 2005). The coconut shell is steam activated
Sorption isotherms
Modelling of sorption system was conducted in order to gain a greater understanding of the processes involved and to facilitate the prediction of sorption performance under various conditions. While many general purpose sorption isotherm models exist (Long et al., 2008), the most commonly used models are the Langmuir isotherm (Sposito, 1979, Sposito, 1980) and Freundlich isotherm (Freundlich, 1906). These isotherms describe a system of adsorption of solute molecules onto a surface where each
Equilibrium time
Fig. 1 shows the sorption kinetics of GAC for batch tests at 20 °C and 4 °C. Equilibrium was considered to be achieved when the ‘q’ (mass of toluene adsorbed on GAC in mmol/g) values reached a plateau. For the present study, equilibrium was reached at approximately 24 h for both the temperatures studied. Therefore, a reaction time of 24 h was subsequently used for all batch tests.
Binary equilibria
In order to understand how temperature affects the adsorption of toluene on GAC, the equilibria was studied at 20 °C and 4
Temperature effects on equilibrium
Several studies have investigated the effect of increasing temperature on GAC equilibria and sorption capacities. The process is endothermic with uptake increasing with rising temperature (Long et al., 2008, Chern and Chien, 2002). But little work has been done on the effects of low temperatures on GAC equilibria and adsorption capacities, and although the inverse relationships found for increasing temperature hold, quantification is important.
Temperature effects on sorption kinetics
This study shows that the diffusion coefficient
Conclusions
The study has found that low temperature has significant effects on the sorption of toluene on GAC. The uptake of toluene at 4 °C is significantly lower than sorption at 20 °C. The sorption kinetics studies also show that the diffusion coefficient reduces at low temperature. Therefore, though a greater mass of GAC would be required for a cold region PRB resulting in increased material and handling cost. However, the GAC has been shown to be effective for toluene uptake at cold temperature and can
Acknowledgements
This study was supported by the Particulate Fluids Processing Centre, a special Research Centre of Australian Research Council and Australian Antarctic Division.
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