Bioreduction and biocrystallization of palladium by Desulfovibrio desulfuricans NCIMB 8307

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Abstract
The reduction of Pd(II) to Pd(0) was accelerated by using the sulfate‐reducing bacterium Desulfovibrio desulfuricans NCIMB 8307 at the expense of formate or H2 as electron donors at pH 2–7. With formate no reduction occurred at pH 2, but with H2 50% of the activity was retained at pH 2, with the maximum rate (1.3–1.4 μ mol min−1 mg dry cells−1) seen at pH 3–7, which was similar to the rate with formate at neutral pH. Excess nitrate was inhibitory to Pd(II) reduction using formate, but not H2. Chloride ion was inhibitory as low as 100 mM using formate but with H2 only ca. 25% inhibition was observed at 500 mM Cl− and H2 was concluded to be the electron donor of choice for the potential remediation of industrial wastes. Deposited Pd was visible on the cells using transmission and scanning electron microscopy and analysis by energy dispersive X‐ray microanalysis (EDAX) identified the deposit as Pd, confirmed as Pd(0) by X‐ray powder diffraction analysis (XRD). The crystal size of the biodeposited Pd(0) was determined to be only 50% of the size of Pd(0) crystals manufactured chemically from Pd(II) at the expense of H2 and, unlike the chemically manufactured material, the biocrystal size was independent of the pH. The “biological” Pd(0) functioned as a superior chemical catalyst in a test reaction which liberated hydrogen from hypophosphite. Pd, and also Pt and Rh, could be recovered by resting cell suspensions under H2 from an industrial processing wastewater, suggesting a possible future application of bioprocessing technology for precious metals. © 2002 Wiley Periodicals, Inc. Biotechnol Bioeng 80: 369–379, 2002.