Ping Yong

A novel electrobiotechnology for the recovery of precious metals from spent automotive catalysts

Abstract Platinum group metals are routinely used in automotive catalysts but recycle technology lags behind demand. There is no available ‘clean technology’ and leach solutions (e.g. aqua regia) to solubilise the metals from scrap are highly aggressive. A microwave‐assisted leaching method was developed which gave 80% metals recovery, with the leach time reduced from 2 h to 15 min using 50% (aq.) diluted aqua regia to give potentially a more biocompatible leachate. Desulfovibrio desulfuricans reduces soluble platinum group metals to cell‐bound insoluble base metals (e.g. Pd(II) ? Pd(0)). For use, biofilm was immobilised on a Pd‐23% Ag solid alloy membrane which delivered H” to the cells via an electrochemical chamber at the back‐side. The biomass‐coated Pd‐Ag alloy electrode was used in a flow‐through reactor for recovery of Pd, Pt and Rh from aqua regia leachates (pH 2.5) of spent automotive catalysts with up to 90% efficiency at a flow residence time of 15 minutes. Free cells did not reduce platinum group metals from the leachates but the electrobioreactor did so using biofilm‐cells preloaded with Pd(0). Reactors lacking biomass or reactors with heat‐killed biofilm removed less platinum group metals, via electrochemically‐synthesised H’ reductant alone. The use of an active biofilm layer in a flow‐through electrobioreactor provides a simple, clean and rapid potential recycle technology.

Bioremediation of uranium-bearing wastewater: Biochemical and chemical factors influencing bioprocess application

A biotechnological process for the removal of heavy metals from aqueous solution utilizes enzymatically liberated phosphate ligand which precipitates with heavy metals (M) as cell-bound MHPO(4). The enzyme, a phosphatase, obeys Michaelis-Menten kinetics in resting and immobilized cells; an integrated form of the Michaelis-Menten equation was used to calculate the apparent K(m) (K(m app.)) as operating in immobilized cells in flow-through columns by a ratio method based on the use of two enzyme loadings (E(o1), E(o2)) or two input substrate concentrations (S(o1), S(o2)). The calculated K(m app.) (4.08 mM) was substituted into an equation to describe the removal of metals by immobilized cells. In operation the activity of the bioreactor was in accordance with that predicted mathematically, within 10%. The initial tests were done at neutral pH, whereas the pH of industrial wastewaters is often low; an increase in the K(m app.) at low pH was found in previous studies. Immobilized cells were challenged with acidic mine drainage wastewaters, where the limiting factors were chemical and not biochemical. Bioreactors initially lost activity in this water, but recovered to remove uranyl ion with more than 70% efficiency under steady-state conditions in the presence of competing cations and anions. Possible reasons for the bioreactor recovery are chemical crystallization factors. (c) 1997 John Wiley & Sons, Inc.

Uptake of Sr 2+ and Co 2+ into biogenic hydroxyapatite: implications for biomineral ion exchange synthesis

Biomineral hydroxyapatite (Bio-HAp) produced by Serratia sp. has the potential to be a suitable material for the remediation of metal contaminated waters and as a radionuclide waste storage material. Varying the Bio-HAp manufacturing method was found to influence hydroxyapatite (HAp) properties and consequently the uptake of Sr(2+) and Co(2+). All the Bio-HAp tested in this study were more efficient than the commercially available hydroxyapatite (Com-HAp) for Sr(2+) and Co(2+) uptake. For Bio-HAp the uptake for Sr(2+) and Co(2+) ranged from 24 to 39 and 29 to 78 mmol per 100 g, respectively. Whereas, the uptake of Sr(2+) and Co(2+) by Com-HAp ranged from 3 to 11 and 4 to 18 mmol per 100 g, respectively. Properties that increased metal uptake were smaller crystallite size (<40 nm) and higher surface area (>70 m(2) g(-1)). Organic content which influences the structure (e.g., crystallite arrangement, size and surface area) and composition of Bio-HAp was also found to be important in Sr(2+) and Co(2+) uptake. Overall, Bio-HAp shows promise for the remediation of aqueous metal waste especially since Bio-HAp can be synthesized for optimal metal uptake properties.

From bio-mineralisation to fuel cells: biomanufacture of Pt and Pd nanocrystals for fuel cell electrode catalyst

Biosynthesis of nano-scale platinum and palladium was achieved via enzymatically-mediated deposition of metal ions from solution. The bio-accumulated Pt(0) and Pd(0) crystals were dried, applied onto carbon paper and tested as anodes in a polymer electrolyte membrane (PEM) fuel cell for power production. Up to 100% and 81% of the maximum power generation was achieved by the bio-Pt and bio-Pd catalysts, respectively, compared to commercial fuel cell grade Pt catalyst. Hence, biomineralisation could pave the way for economical production of fuel cell catalysts since previous studies have shown that precious metals can be biorecovered from wastes into catalytically active bionanomaterials.

Applications of bacterial hydrogenases in waste decontamination, manufacture of novel bionanocatalysts and in sustainable energy.

Bacterial hydrogenases have been harnessed to the removal of heavy metals from solution by reduction to less soluble metal species. For Pd(II), its bioreduction results in the deposition of cell-bound Pd(0)-nanoparticles that are ferromagnetic and have a high catalytic activity. Hydrogenases can also be used synthetically in the production of hydrogen from sugary wastes through breakdown of formate produced by fermentation. The Bio-H(2) produced can be used to power an electrical device using a fuel cell to provide clean electricity. Production of hydrogen from confectionery wastes by one organism (Escherichia coli) can be used as the electron donor for the production of Bio-Pd(0) from soluble Pd(II) by a second organism. The resulting Bio-Pd(0) can then be used as a bioinorganic catalyst in the remediation of Cr(VI)-contaminated solutions or polychlorinated biphenyls at the expense of Bio-H(2), as a hydrogenation catalyst for industry or as a component of a fuel cell electrode.

Biomineralization: linking the fossil record to the production of high value functional materials

The microbial cell offers a highly efficient template for the formation of nanoparticles with interesting properties including high catalytic, magnetic and light-emitting activities. Thus biomineralization products are not only important in global biogeochemical cycles, but they also have considerable commercial potential, offering new methods for material synthesis that eliminate toxic organic solvents and minimize expensive high-temperature and pressure processing steps. In this review we describe a range of bacterial processes that can be harnessed to make precious metal catalysts from waste streams, ferrite spinels for biomedicine and catalysis, metal phosphates for environmental remediation and biomedical applications, and biogenic selenides for a range of optical devices. Recent molecular-scale studies have shown that the structure and properties of bionanominerals can be fine-tuned by subtle manipulations to the starting materials and to the genetic makeup of the cell. This review is dedicated to the late Terry Beveridge who contributed much to the field of biomineralization, and provided early models to rationalize the mechanisms of biomineral synthesis, including those of geological and commercial potential.

Using non-invasive magnetic resonance imaging (MRI) to assess the reduction of Cr(VI) using a biofilm-palladium catalyst.

Industrial waste streams may contain contaminants that are valuable like Pd(II) and/or toxic and mutagenic like Cr(VI). Using Serratia sp. biofilm the former was biomineralized to produce a supported nanocrystalline Pd(0) catalyst, and this biofilm–Pd heterogeneous catalyst was then used to reduce Cr(VI) to less dangerous Cr(III) at room temperature, with formate as the electron donor. Cr(VI)(aq) is non‐paramagnetic while Cr(III)(aq) is paramagnetic, which enabled spatial mapping of Cr species concentrations within the reactor cell using non‐invasive magnetic resonance (MR) imaging experiments. Spatial reactivity heterogeneities were thus examined. In batch reactions, these could be attributed primarily to heterogeneity of Pd(0) distribution and to the development of gas bubbles within the reactor. In continuous flow reactions, spatial reactivity heterogeneities resulted primarily from heterogeneity of Cr(VI) delivery. Biotechnol. Bioeng. 2010;107: 11–20.

Effect of substrate concentration and nitrate inhibition on product release and heavy metal removal by a Citrobacter sp.

A Citrobacter sp. accumulates heavy metals as cell-bound metal phosphates, utilizing phosphate released by the enzymatic cleavage of a phosphomonoester substrate. The effect of increased substrate (glycerol 2-phosphate, G2P) concentration on phosphate release and heavy metal accumulation was evaluated using a stirred tank reactor (STR) and a plug flow reactor (PFR). A significant improvement in metal removal was achieved with increased substrate concentration using immobilized Citrobacter cells in the PFR, which was not observed using free cells in the STR. Nitrate is an inhibitor of the Citrobacter phosphatase. This inhibition was concentration dependent and reversible. The rate of product release was restored by increasing the concentration of substrate (G2P). The ratio of rates of phosphate release under two different conditions (different nitrate and G2P concentrations) can be described by a equation developed from Michaelis-Menten kinetics. The concentration of substrate required for restoration of maximum velocity, V(max), in a batch and continuous-flow system can be predicted by substitution and calculation; this was confirmed by an experiment in model systems using cell suspensions and polyacrylamide gel immobilized cells in a flow-though column. For use in industrial situations it may be uneconomical or infeasible to supply additional substrate. Bioreactor activity was also restored by increasing the flow residence time, in accordance with a Michaelis-Menten-based model to describe removal of lanthanum from nitrate-supplemented flow in a PFR.

Removal of lanthanum, uranium and thorium from the citrate complexes by immobilized cells of Citrobacter sp. in a flow-through reactor: implications for the decontamination of solutions containing plutonium

A Citrobacter sp., documented for heavy metal uptake, removed La and U from a flow presented to polyacrylamide gel-immobilized cells but removed little Th under the same conditions when presented alone or in combination with La and U. The poor removal of Th was attributable to the strength of the complex of tetravalent actinide species with citrate co-presented as a model chelating ligand. The implications of this for the treatment of wastes containing Pu(IV) are discussed.

Biorecovery of Precious Metals from Wastes and Conversion into Fuel Cell Catalyst for Electricity Production

Bio-manufacturing of nano-scale palladium was achieved using bacterial cells. Highly active Pd-catalyst (Bio-Pd) produced by an E. coli mutant gave power output in a fuel cell. Up to ~115% of the maximum power generation was achieved by electrodes of Bio-Pd catalysts from Escherichia coli, compared to that from a commercial-Pd electrode (~0.099 W). A bio-precious-metals (Bio-PM) catalyst made directly from an industrial reprocessing solution by the E. coli was also made into fuel cell electrodes and ~0.06W of maximum power generation was observed.