Toluene sorption by granular activated carbon and its use in cold regions permeable reactive barrier: Fixed bed studies
Highlights
► Studied the effect of temperature on hydrocarbon sorption on GAC. ► The experiments were conducted in batch and column mode. ► Axial dispersion of the solute was determined experimentally and simulated. ► The breakthrough curves show significant drop in sorption capacity of GAC at low temperatures. ► The studied material would be used in Permeable Reactive Barrier in Cold regions.
Introduction
Permeable reactive barriers (PRB) are considered one of the most practicable approaches to clean up oil spill sites in cold regions due to their minimal impact on the environment and minimal power requirements and attention needed (Snape et al., 2001). But finding suitable materials for the cold region PRB is the key to its successful implementation in such remote areas. Various studies have shown that GAC can successfully remove hydrocarbons from water (Arora et al., 2010, Chatzopoulos and Varma, 1995, Hindarso et al., 2001, Valderrama et al., 2008). Although batch equilibrium and kinetic studies are excellent means to describe the ion exchange characteristics of GAC, understanding the contaminant transport characteristics of flow through a porous medium is imperative to the design of a PRB. While the understanding of exchange processes gained from batch tests is theoretically sufficient to describe and predict exchange performance, in practice this understanding is not directly applicable to dynamic systems of flow through porous media. Column testing is generally considered to be a more realistic simulation of field conditions and facilitate the generation of realistic PRB design parameters and allow the development of a model to predict the performance of PRB (USEPA, 1998). Furthermore, parameters characterizing an adsorbent determined from batch tests can differ significantly compared to those measured from column techniques (Inglezakis and Grigoropoulou, 2003, Plassard et al., 2000). As column tests are more costly, time consuming and difficult to perform than batch test methods, and processes affecting the behavior of solutes flowing through a column are complicated and not as well understood as for batch conditions, the theoretical understanding and modeling of solute transport is less well developed than for batch systems (Helfferich, 1962).
Therefore, it's important to understand the solute transport mechanism in GAC fixed beds and to develop a model describing solute transport in real situations to facilitate the GAC PRB design and implementation in cold regions. This study investigates the impact of temperature and axial flow on the GAC column performance to adsorb hydrocarbons by studying breakthrough point and breakthrough curve at various flow rates and development of a model to predict the solute transport parameters. Toluene has been used as a representative soluble aromatic hydrocarbon in this study.
Section snippets
Granular activated carbon
The granular activated carbon (GAC), produced from coconut husk was obtained from Pica Activated Carbon P/L. Activated carbons are very effective in treatment of contaminated groundwater. 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).
Reagents
All reagents used in this study were AnalaR grade and were used as received
Axial dispersion coefficient
Axial dispersion coefficients (Dl) and porosity (n) at various flow rates were determined at 20° and 4 °C by solving Eq. (2) using a computer program, CXTFIT. The data generated by CXTFIT showed an excellent fit to the experimental (R2 = 0.993 to 0.998) data (Figs. 1 and 2). The axial dispersion coefficient increased with increase in flow rate and varied from 0.812 × 10−6 to 4.88 × 10−6 m2/s at various flow rates and temperatures (Table 1). Table 1 also shows that column Peclet numbers were moderately
Discussion
For a given material and the solvent system, theoretically the equilibrium and exchange parameters determined from batch and fixed bed studies should be the same (Helfferich, 1962). However, the column capacities of GAC are approx. 30–60% lower than the batch-estimated values which are in line with the findings of other researchers (Abusafa and Yucel, 2002, Inglezakis et al., 2002). The lower values of the Langmuir constant, Kl in fixed bed systems can be attributed to the differences in local
Conclusions
This study shows that low temperature detrimentally affects the sorption of toluene from water in fixed bed columns and therefore will have a significant influence on the design of a cold region PRB for hydrocarbon removal. Fixed bed breakthrough capacities and saturation capacities at 4° are significantly lower than at 20 °C. The saturation capacity decreases with increased flow rates. Fixed-bed sorption parameters are significantly different from batch-estimated values, with the capacity of
Acknowledgments
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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