Heavy metal biosorption by biomass of Ochrobactrum anthropi producing exopolysaccharide in activated sludge
Introduction
Alternative methods of metal removal and recovery based on biological materials have been considered. Certain types of microbial biomass can retain relatively high quantities of metals by means of a passive process known as biosorption, which is dependent on the affinity between the metallic species or its ionic forms and the binding sites on the molecular structure of the cellular wall (Manriquez et al., 1997).
The potential use of microorganisms in the treatment of heavy metal-contamined wastewaters and in the recovery of metals in mining wastes or in metallurgical effluent is of special importance (Loren, 1979; Gadd, 1996). The use of dead biomass or derived products eliminates the problem of toxicity, not only from dissolved metals but also from adverse operating conditions, and economic component of maintenance including nutrient supply (Gadd and White, 1989; Tobin et al., 1994; Saglam et al., 1999).
In this work, a floc forming bacterium Ochrobactrum anthropi, naturally present in activated sludges, was tested for removal of Cr(VI), Cd(II) and Cu(II) from pure solutions, at relatively high concentrations. This choice has been dictated by the ability of the microorganism to a polysaccharide capsule, which is responsible for the binding and accumulation metal ions in the form of superficial mucilage layer. Since exocellular polysaccharide (EPS) plays a role in the increase of heavy metal adsorption capacity for cells.
Section snippets
Isolation of microorganism
The media employed for the isolation of floc-formers were a modification of that used by Tago and Aida (1977) and formulated to enhance slime production (ESP). Purified isolates were maintained on a chemically defined medium solidified by addition of Bacto No. 1 agar (2%, w/v).
To isolate a bacterium producing the most EPS, samples (1 ml) from wastewater treatment of a petrochemical industry were passed aseptically through tenfold dilutions of Ringers solution to yield final concentrations of 10
Identification of the most exopolysaccharide producing bacterium
The API 20 E biotype of the most EPS producing bacterium was O. anthropi. The cells were cultivated in nutrient broth media amended with various concentrations of Cr(VI), Cd(II) and Cu(II). It appears that the cells were able to survive metal concentrations as high as 16 mg l−1 for Cr(VI), 30 mg l−1 for Cd(II) and 125 mg l−1 for Cu(II).
Effect of pH on the biosorption capacity
The inconsistency in literature regarding the influence of pH on biosorption seems to indicate that the way pH would alter the adsorption of metal ions to biomass
Conclusion
The aim of this work was to find the biosorption characteristics of selected one of the bacteria producing the most EPS for the removal of chromium, cadmium and copper ions. Experiments were performed as a function of pH, initial metal ion concentration and time. The obtained results showed that O. anthropi was a good adsorbing medium for metal ions and had high adsorption yields for the treatment of wastewater containing chromium, cadmium and copper ions.
The Freundlich and Langmuir adsorption
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