Biosorption of Cr(III) and Cr(VI) onto the cell surface of Pseudomonas aeruginosa

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Abstract

Biosorption of the chromium ions Cr(III) and Cr(VI) onto the cell surface of Pseudomonas aeruginosa was investigated. Batch experiments were conducted with various initial concentrations of chromium ions to obtain the sorption capacity and isotherms. It was found that the sorption isotherms of P. aeruginosa for Cr(III) were described well by Langmuir isotherm models, while Cr(VI) appeared to fit Freundlich models. The results of FT-IR analysis suggested that the chromium binding sites on the bacterial cell surface were most likely carboxyl and amine groups. The bacterial surface of P. aeruginosa seemed to engage in reductive and adsorptive reactions with respect to Cr(VI) biosorption.

Introduction

Toxic heavy metals are frequently contained in wastewaters produced by many industrial processes, such as those employed in the electroplating, metal finishing, metallurgical, tannery, chemical manufacturing, mining, and battery manufacturing industries [1], [2]. The existence of heavy metals in the environment represents a very significant and long-term environmental hazard. Even at low concentrations these metals can be toxic to organisms, including humans. In particular, chromium is a contaminant that is a known mutagen, teratogen and carcinogen [3]. Chromium is generally found in electroplating and metal finishing industrial effluents, as well as sewage and wastewater treatment plant discharges [4]. Among the several oxidation states (di, tri, penta and hexa), trivalent chromium, Cr(III), together with the hexavalent state, Cr(VI), can be the main forms present in aquatic environments [5]. Chromate (CrO42−) is the prevalent species of Cr(VI) in natural aqueous environments, and is the major pollutant from chromium-related industries [6]. Although Cr(III) is less toxic than Cr(VI), long-term exposure to Cr(III) is known to cause allergic skin reactions and cancer [7]. As a result, the total chromium level in effluent is strictly regulated in many countries. In the USA, the concentration of chromium in drinking water has been regulated with a maximum level of 0.1 mg/l for total chromium [8].

The removal of heavy metals from aqueous solutions has therefore received considerable attention in recent years. However, the practical application of physicochemical technology such as chemical precipitation, membrane filtration and ion exchange is sometimes restricted due to technical or economical constraints. For example, the ion exchange process is very effective but requires expensive adsorbent materials [9], [10]. The use of low-cost waste materials as adsorbents of dissolved metal ions provides economic solutions to this global problem and can be considered an eco-friendly complementary [11], [12]. At present, emphasis is given to the utilization of biological adsorbents for the removal and recovery of heavy metal contaminants.

Biomass involving pure microbial strains has shown high capacities for the selective uptake of metals from dilute metal-bearing solutions. Several investigations have reported that Pseudomonas aeruginosa displays efficiency for metal uptake [13], [14], [15]. Chang and Hong [16] found that the amount of mercury adsorbed by a P. aeruginosa biomass sample (180 mg Hg/g dry cells) was higher than that bound to a cation exchange resin (100 mg Hg/g dry resin). Hu et al. [17] reported that P. aeruginosa strain CSU showed the highest affinity and maximal capacity for uranium (100 mg U/g dry weight), and that it was also competitive when compared to commercial cation exchange resins.

Previous studies on biosorption using microorganisms have generally focused on the removal of metal ions from aqueous solutions. However, a few studies were undertaken to interpret and establish the mechanisms involved in metal ion binding. Furthermore, the binding sites for chromium have not been specifically identified.

The objective of the present work was to assess the potential of P. aeruginosa for the biosorption of chromium. The functional groups involved in chromium biosorption were identified using FT-IR analysis. These results would contribute to a better understanding of biosorption phenomena and aid in the development of potential biosorbents that possess high capacities for heavy metal uptake from aqueous environments.

Section snippets

Preparation of the biomass

P. aeruginosa PAO1 (courtesy of Dr. Beveridge, University of Guelph, Canada) was used as a biosorbent in these experiments. P. aeruginosa is a bacterium commonly isolated from various environmental sources such as soil, water and plant surfaces [18]. The bacteria were cultured following the procedure outlined in Kang et al. [19]. In brief, the cells were grown aerobically with agitation in a growth medium (TSB; Trypticase Soy Broth, Difco) at 37 °C and set at 180 rpm in a shaking incubator. After

Surface characterization of P. aeruginosa

The bacterial species used in this study was P. aeruginosa, a gram-negative aerobic species that is commonly found in near-surface systems [18]. The cell wall of this bacterium is composed of peptidoglycan and teichoic acids, and possesses carboxyl, phosphoryl, hydroxyl, and amino functional groups at the surface [22]. Before a study of the biosorption of metal, an investigation of the surface characteristics of P. aeruginosa was required to understand the mechanisms of metal biosorption.

The

Conclusions

This study shows that P. aeruginosa can be applied to chromium-contaminated wastewater. The sorption of chromium ions by P. aeruginosa was modeled well by the Langmuir and Freundlich sorption isotherms. The data of potentiometric titration indicated the presence of two major functional groups on the cell wall, corresponding to pKa values of 5.2 and 9.5. FT-IR spectrometry showed bindings of chromium ions were dominated by complexation to the carboxyl and amine groups on the biomass surface. In

Acknowledgements

This research was supported by the Gwangju Institute of Science and Technology (GIST) Research Fund and National Research Laboratory Project (Arsenic Geoenvironment Lab.) to K.-W. Kim.

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