University of Saskatchewan
Department of Soil Science
College of Agriculture and Bioresources

Environmental Soil Chemistry

Cd2+ adsorption on a thermophilic microorganism


Microbial cell walls contain many functional groups which serve as active sites for metal adsorption. The high density of these sites throughout the cell wall coupled with the large surface area/volume ratio of microorganisms make them important sorbents for metals in natural systems. Furthermore, the cell walls of microorganisms growing under extremes of pH, temperature, or ionic strength are often quite different from those growing under more typical conditions.

The adsorption behavior of thermophilic (temperature tolerant) microorganisms is of considerable interest to environmental researchers. In this study, Anoxybacillus flavithermus is used at a model thermophile for adsorption of Cd2+ from aqueous solutions.

Experimental Methods

Anoxybacillus flavithermus, isolated from the Wairakei Power Station (NZ), was selected as a model thermophile. Cells were cultured in TSB at 60C to late stationary phase. Adsorption experiments were performed at 25C with intact rinsed cells suspended in NaNO3 (0.01 M). Cadmium adsorption by the cells was quantified by batch experiments conducted as a function of pH and metal-to-bacteria concentration ratio. SCMs were developed using the computer program FITMOD, a modified version of FITEQL 2.01.

EXAFS spectra were collected at the Cd K-edge (26.711 keV) at the PNC-CAT 20-BM beamline at the Advanced Photon Source. The electron storage ring was operating at 4.5 GeV and in top up mode for all experiments. All samples were scanned in transmission mode at room temperature.

XAS Results

Cadmium adsorption increases with increasing pH, and Cd K-Edge spectroscopy was therefore conducted on samples at different pH as shown in this figure. Fits to the EXAFS data revealed that Cd was in a bonding environment consistent with a monodentate surface complex over pH 5-6. Results for the pH 6.0 sample are shown in Figure 1.


Figure 1. Fit results for the pH 6.0 adsorption sample showing the fit to the data in both k and R space. Cd-C interatomic distance is consistent with a monodentate complex with a carboxylic acid..

The Cd-O shell is due to Cd2+ existing as an octahedral hydrated cation in aqueous solution, and the Cd-C contributions arise when an inner-sphere complex forms between Cd and a carboxylic acid functional group on the microbe surface. However the Cd-C distance (and therefore the local coordination environment) is clearly different from a Cd-acetate aqueous standard. This is showin in Figure 2.


Figure 2. Comparison of RSFs of aqueous Cd2+, Cd adsorbed on A. flavithermus at different pH, and an aqueous Cd-acetate complex. Note the lack of Cd-C-C scattering and the longer Cd-C distances in the sorption samples. This is because the coordination of Cd is different, as shown in the drawings of the complexes.


Although EXAFS fitting did not detect in changes in Cd-C distances as pH changes, inspection of the Cd-O peak intensity in the RSFs or examination of Cd XANES data shows systematic decrease in Cd-O peak height (RSF) or decreased intensity of white line intensity (XANES) as pH increases. Figure 3 shows an overlay of the pH 5 and pH 6 sorption samples with a Cd2+ and a Cd-acetate standard.


Figure 3. Effect of pH on Cd EXAFS spectra (left) and Cd XANES spectra (right). Note that Cd-O intensity decreases as pH is increased.


The increased Cd-O peak height in samples at low pH has been previously attributed to complexation with phosphoryl groups. However, similar XAS spectra can be obtained by changing the % acetate complexation of a cadmium acetate solution even though there are no phosphoryl ligands in those solutions. Various Cd-acetate standards are shown in Figure 4.


Figure 4. Effect of changing solution conditions on Cd-acetate standard spectra. Note the same effect is observed with these samples as in the sorption samples of Figure 3.


This suggests that phosphoryl group complexation may not be responsible for the low pH spectral features. Alternatively, entrained Cd2+ (both in the solution and diffused into the cells of microbes) may instead be responsible. XAS studies between pH 4 and 9 are currently being planned to more fully evaluate the pH dependence of Cd adsorption.

Modeling the results

A surface complexation model (SCM) was developed to model the protonation/deprotonation of the microbial surface and the adsorption of Cd on A. flavithermus. The best fit to the experimental data was with 2 cadmium/carboxyl complexes: COO-Cd and COO-CdOH.


Figure 5. Fits of our SCM to macroscopic adsorption data. Complexes were limited in the model to only those observed by XAS experiments.


The importance of the COO-CdOH complex is somewhat unexpected as the hydrolysis constant in solution is much higher than at the surface. However, attempts to fit the data with a phosphoryl ligand were unsuccessful and we did not observe Cd-PO4 interactions in our EXAFS data. We are conducting additional EXAFS experiments at higher pH to verify the mechanism.

Conclusions

The bacteria adsorbed appreciable amounts of Cd with the adsorption increasing with increasing pH and bacteria-to-metal concentration ratio. SCMs with stability constants describing Cd adsorption by the deprotonated surface functional groups were developed. The best-fitting SCM considered adsorption of 1) the divalent metal or 2) the first metal hydrolysis product onto deprotonated carboxyl sites within the bacterial cell wall. Most other investigations of metal-bacteria adsorption have proposed SCMs involving metal adsorption to carboxyl and to phosphoryl sites within the cell wall. However, our best-fitting model, which assumes that metal adsorption involves only carboxyl sites, is confirmed by XANES and EXAFS. Further spectroscopic examination of adsorbed Cd at higher pH is required to fully validate this model.