Adsorption of fluorobenzene onto granular activated carbon: Isotherm and bioavailability studies
Introduction
Activated carbon (AC) based biofilm reactors constitute a successfully applied technology for the treatment of aqueous effluents contaminated with aromatic pollutants (Speitel et al., 1989, Jaar and Wilderer, 1992, Klecka et al., 1996, Khodadoust et al., 1997, Carvalho et al., 2001). These reactor systems remove organic matter through a combination of physical adsorption and biological transformation, contributing to an enhanced quality of the effluent characteristics. A major benefit of AC in biological systems is related to its capacity to act as a buffer, due to the high adsorptive characteristics of this matrix. Shock loads of pollutants may be temporarily adsorbed and later biodegraded by the microbial population, thus allowing more time for the biofilm microorganisms to effectively mineralise the compounds (Abu-Salah et al., 1996, Khodadoust et al., 1997, Carvalho et al., 2001). Here, the adsorbed compounds become available for microbial degradation in a process known as bioregeneration. Bioregeneration leads to the renewal of the adsorptive capacity of AC, as microorganisms, while degrading the adsorbed compounds, release the adsorption sites, which can be then occupied by other organic molecules in the bulk solution (Rice and Robson, 1982). This process has been shown to occur in different AC types (Chudyk and Snoeying, 1984, Voice et al., 1992, Jonge et al., 1996a).
Halogenated aromatic compounds are important environmental pollutants of soil, water and air. Fluorinated compounds are among these due to their useful applications, such as aerosol propellants, surfactants, refrigerants, plastics, anesthetics, pesticides, plant growth regulators, medicines, adhesives and fire retardants (Key et al., 1997). The improper disposal together with the chemical inertness and hydrophobicity of many of these compounds led to their persistence in the environment and to the necessity of finding effective remediation technologies for their removal.
Very few studies are available on the degradation of fluorobenzene (FB), a recalcitrant fluoroaromatic compound. The potential sources of environmental release of FB are related to its main use as a solvent in the pharmaceutical industry, as an insecticide and as a reagent for plastic and resin polymers production. Only recently the microbial degradation of this compound as a single carbon and energy source was reported (Carvalho et al., 2002, Carvalho et al., 2005). The present paper reports on the adsorption capacity and equilibrium characteristics of FB onto granular activated carbon (GAC). The GAC sorption capacity for this compound was evaluated using the adsorption isotherm technique and the Langmuir, Freundlich and Redlich–Peterson models were fitted to the experimental data. In addition, the bioavailability of the adsorbed FB for further microbial degradation was evaluated.
Section snippets
Adsorbent and reagents
The experiments were conducted with thermally activated peat-based GAC (8–20 mesh, 0.85–2.4 mm particle size, surface area 600–800 m2 g−1) obtained from Sigma Chemical Co., St. Louis, USA. Prior to use, GAC was washed several times with deionised water to remove carbon fines, dried in an oven at 105 °C, and sterilised by autoclaving.
All reagents used were of the highest purity grade available (Sigma–Aldrich Chemie, Steinheim, Germany; Merck, Darmstadt, Germany).
Bacterial inoculum for bioavailability studies
A pure bacterial culture (designated
FB adsorption isotherm
The capacity of GAC to adsorb FB was evaluated through the determination of the respective isotherm. For this, different concentrations of FB were supplied to flasks containing equal amounts of GAC. A control flask, without the addition of GAC, was set-up to ascertain for losses of FB, showing no losses of this compound during the time course of the experiment. The experimental and calculated adsorption isotherms of FB onto GAC at pH 7 and 25 °C are shown in Fig. 1, with model parameters listed
Discussion
AC adsorption and biological degradation constitutes a common approach to treat recalcitrant pollutants, which are usually slowly biodegradable. Before employing such biological type systems it is important to assess whether the AC matrix has a good capacity to adsorb the pollutants, and if the adsorbed compound can be available for microbial degradation, the latter leading to the extension of the AC life through the bioregeneration process.
In this study, the capacity of GAC to adsorb FB was
Conclusion
The adsorption of FB to and bioregeneration of GAC were evaluated in this study. Based on the obtained results, the following conclusions can be drawn:
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The maximal capacity determined in this study for the adsorption of FB onto GAC was around 388 mg of FB per gram of GAC.
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FB adsorption isotherm was well described by the Redlich–Peterson model with the Freundlich model also providing a good fit. Langmuir model reasonably fitted the experimental data only for low FB concentrations, failing to
Acknowledgements
M.F. Carvalho wishes to thank a research grant from Fundação para a Ciência e Tecnologia (FCT), Portugal (BD/21839/99) and Fundo Social Europeu (III Quadro Comunitário de Apoio). This work was supported in part by the European Community’s Human Potential Programme under contract HPRTN-CT-2002-00213 [BIOSAP]. The authors are grateful to Cláudia Drumond for helpful discussions on the isotherm models.
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