Engineering Biogenic Magnetite for Sustained Cr(VI) Remediation in Flow-through Systems

In this work, we report a route to enhance the reactivity and longevity of biogenic magnetite in Cr­(VI) remediation under continuous-flow conditions by combining functionalization of the biomagnetite surface with a precious metal catalyst, nanoscale palladium, and exposure to formate. Column influent conditions were varied to simulate oxic, anoxic, and nitrate cocontaminated environments. The addition of sodium formate as an electron donor for Pd-functionalized magnetite increased capacity and longevity allowing 80% removal of Cr­(VI) after 300 h in anoxic conditions, whereas complete breakthrough occurred after 60 h in anoxic nonformate and nonfunctionalized systems. Removal of Cr­(VI) was optimized under anoxic conditions, and the presence of oxidizing agents results in a modest loss in reductive capacity. Examination of reacted Pd-functionalized magnetite reveals close association of Fe with Cr, suggesting that Pd-coupled oxidation of formate serves to regenerate the reactive surface. XMCD studies revealed that Cr­(III) is partially substituted for Fe in the magnetite structure, which serves to immobilize Cr. No evidence for a mechanistic interference by nitrate cocontamination was observed, suggesting that this novel system could provide robust, effective and sustained reduction of contaminants, even in the presence of common oxidizing cocontaminants, outperforming the reductive capacity of nonfunctionalized biogenic magnetite

Engineering a Biometallic Whole Cell Catalyst for Enantioselective Deracemization Reactions

The ability of microbial cells to synthesize highly reactive nanoscale functional materials provides the basis for a novel synthetic biology tool for developing the next generation of multifunctional industrial biocatalysts. Here, we demonstrate that aerobic cultures of Escherichia coli, genetically engineered to overproduce a recombinant monoamine oxidase possessing high enantioselectivity against chiral amines, can be augmented with nanoscale Pd(0) precipitated via bioreduction reactions. The result is a novel biometallic catalyst for the deracemization of racemic amines. This deracemization process is normally achieved by discrete sequential oxidation/reduction steps using a separate enantiomer-specific biocatalyst and metal catalyst, respectively. Here, use of E. coli cultures carrying the cloned monoamine oxidase gene and nanoscale bioreduced Pd(0) particles was used successfully for the conversion of racemic 1-methyltetrahydroisoquinoline (MTQ) to (<i>R</i>)-MTQ, via the intermediate 1-methyl-3,4-dihydroisoquinoline, with an enantiomeric excess of up to 96%. There was no loss of catalyst activity following the five rounds of oxidation and reduction, and importantly, there was minimal loss of palladium into the reaction supernatant. This first demonstration of a whole cell biometallic catalyst mixture for “single-pot”, multistep reactions opens up the way for a wide range of integrated processes, offering a scalable and highly flexible platform technology

Redox Interactions Between Cr(VI) and Fe(II) in Bioreduced Biotite and Chlorite

Contamination of the environment with Cr as chromate (Cr­(VI)) from industrial activities is of significant concern as Cr­(VI) is a known carcinogen, and is mobile in the subsurface. The capacity of Fe­(II)-containing phyllosilicates including biotite and chlorite to alter the speciation, and thus the mobility, of redox-sensitive contaminants including Cr­(VI) is of great interest since these minerals are common in soils and sediments. Here, the capacity of bacteria, ubiquitous in the surface and near-surface environment, to reduce Fe­(III) in phyllosilicate minerals and, thus, alter their redox reactivity was investigated in two-step anaerobic batch experiments. The model Fe­(III)-reducing bacterium <i>Geobacter sulfurreducens</i> was used to reduce Fe­(III) in the minerals, leading to a significant transformation of structural Fe­(III) to Fe­(II) of 0.16 mmol/g (∼40%) in biotite and 0.15 mmol/g (∼20%) in chlorite. The unaltered minerals could not remove Cr­(VI) from solution despite containing a larger excess of Fe­(II) than would be required to reduce all the added Cr­(VI), unless they were supplied in a very high concentration (a 1:10 solid to solution ratio). By contrast, even at very low concentrations, the addition of bioreduced biotite and chlorite caused removal of Cr­(VI) from solution, and surface and near surface X-ray absorption spectroscopy confirmed that this immobilization was through reductive transformation to Cr­(III). We provide empirical evidence that the amount of Fe­(II) generated by microbial Fe­(III) reduction is sufficient to reduce the Cr­(VI) removed and, in the absence of reduction by the unaltered minerals, suggest that only the microbially reduced fraction of the iron in the minerals is redox-active against the Cr­(VI)

Microbial Reduction of Arsenic-Doped Schwertmannite by <i>Geobacter sulfurreducens</i>

The fate of As­(V) during microbial reduction by <i>Geobacter sulfurreducens</i> of Fe­(III) in synthetic arsenic-bearing schwertmannites has been investigated. During incubation at pH7, the rate of biological Fe­(III) reduction increased with increasing initial arsenic concentration. From schwertmannites with a relatively low arsenic content (<0.3 wt %), only magnetite was formed as a result of dissimilatory iron reduction. However, bioreduction of schwertmannites with higher initial arsenic concentrations (>0.79 wt %) resulted in the formation of goethite. At no stage during the bioreduction process did the concentration of arsenic in solution exceed 120 μgL¹, even for a schwertmannite with an initial arsenic content of 4.13 wt %. This suggests that the majority of the arsenic is retained in the biominerals or by sorption at the surfaces of newly formed nanoparticles.Subtle differences in the As <i>K</i>-edge XANES spectra obtained from biotransformation products are clearly related to the initial arsenic content of the schwertmannite starting materials. For products obtained from schwertmannites with higher initial As concentrations, one dominant population of As­(V) species bonded to only two Fe atoms was evident. By contrast, schwertmannites with relatively low arsenic concentrations gave biotransformation products in which two distinctly different populations of As­(V) persisted. The first is the dominant population described above, the second is a minority population characterized by As­(V) bonded to four Fe atoms. Both XAS and XMCD evidence suggest that the latter form of arsenic is that taken into the tetrahedral sites of the magnetite.We conclude that the majority population of As­(V) is sorbed to the surface of the biotransformation products, whereas the minority population comprises As­(V) incorporated into the tetrahedral sites of the biomagnetite. This suggests that microbial reduction of highly bioavailable As­(V)-bearing Fe­(III) mineral does not necessarily result in the mobilization of the arsenic

A One-Pot Synthesis of Monodispersed Iron Cobalt Oxide and Iron Manganese Oxide Nanoparticles from Bimetallic Pivalate Clusters

Monodispersed iron cobalt oxide (Fe₂CoO₄) and iron manganese oxide (Mn_0.43Fe_2.57O₄) nanoparticles have been synthesized using bimetallic pivalate clusters of [Fe₂CoO­(O₂C^tBu)₆(HO₂C^tBu)₃] (<b>1</b>), Co₄Fe₂O₂(O₂C^tBu)₁₀(MeCN)₂] (<b>2</b>), and [Fe₂MnO­(O₂C^tBu)₆(HO₂C^tBu)₃] (<b>3</b>) respectively as single source precursors. The precursors were thermolyzed in a mixture of oleylamine and oleic acid with either diphenyl ether or benzyl ether as solvent at their respective boiling points of 260 or 300 °C. The effect of reaction time, temperature and precursor concentration (0.25 or 0.50 mmol) on the stoichiometry, phases or morphology of the nanoparticles were studied. TEM showed that highly monodispersed spherical nanoparticles of Fe₂CoO₄ (3.6 ± 0.2 nm) and Mn_0.43Fe_2.57O₄ (3.5 ± 0.2 nm) were obtained from 0.50 mmol of <b>1</b> or <b>3</b>, respectively at 260 °C. The decomposition of the precursors at 0.25 mmol and 300 °C revealed that larger iron cobalt oxide or iron manganese oxide nanoparticles were obtained from <b>1</b> and <b>3</b>, respectively, whereas the opposite was observed for iron cobalt oxide from <b>2</b> as smaller nanoparticles appeared. The reaction time was investigated for the three precursors at 0.25 mmol by withdrawing aliquots at 5 min, 15 min, 30 min, 1 h, and 2 h. The results obtained showed that aliquots withdrawn at reaction times of less than 1 h contain traces of iron oxide, whereas only pure cubic iron cobalt oxide or iron manganese oxide was obtained after 1 h. Magnetic measurements revealed that all the nanoparticles are superparamagnetic at room temperature with high saturation magnetization values. XMCD confirmed that in iron cobalt oxide nanoparticles, most of the Co²⁺ cations are in the octahedral site. There is also evidence in the magnetic measurements for considerable hysteresis (>1T) observed at 5 K. EPMA analysis and ICP-OES measurements performed on iron cobalt oxide nanoparticles obtained from [Fe₂CoO­(O₂C^tBu)₆(HO₂C^tBu)₃] <b>(1)</b> revealed that stoichiometric Fe₂CoO₄ was obtained only for 0.50 mmol precursor concentration. All the nanoparticles were characterized by powder X-ray diffraction (p-XRD), transmission electron microscopy (TEM), inductively coupled plasma-optical emission spectroscopy (ICP-OES), electron probe microanalysis (EPMA), X-ray magnetic circular dichroism (XMCD), and superconducting quantum interference device (SQUID) magnetometry