Palladized cells as suspension catalyst and electrochemical catalyst for reductively degrading aromatics contaminants: Roles of Pd size and distribution

Biosynthesis Parameters Control the Physicochemical and Catalytic Properties of Microbially Supported Pd Nanoparticles

The biosynthesis of Pd nanoparticles supported on microorganisms (bio-Pd) is achieved via the enzymatic reduction of Pd(II) to Pd(0) under ambient conditions using inexpensive buffers and electron donors, like organic acids or hydrogen. Sustainable bio-Pd catalysts are effective for C-C coupling and hydrogenation reactions, but their industrial application is limited by challenges in controlling nanoparticle properties. Here, using the metal-reducing bacterium Geobacter sulfurreducens, it is demonstrated that synthesizing bio-Pd under different Pd loadings and utilizing different electron donors (acetate, formate, hydrogen, no e- donor) influences key properties such as nanoparticle size, Pd(II):Pd(0) ratio, and cellular location. Controlling nanoparticle size and location controls the activity of bio-Pd for the reduction of 4-nitrophenol, whereas high Pd loading on cells synthesizes bio-Pd with high activity, comparable to commercial Pd/C, for Suzuki-Miyaura coupling reactions. Additionally, the study demonstrates the novel synthesis of microbially-supported ≈2 nm PdO nanoparticles due to the hydrolysis of biosorbed Pd(II) in bicarbonate buffer. Bio-PdO nanoparticles show superior activity in 4-nitrophenol reduction compared to commercial Pd/C catalysts. Overall, controlling biosynthesis parameters, such as electron donor, metal loading, and solution chemistry, enables tailoring of bio-Pd physicochemical and catalytic properties.

Unnatural Direct Interspecies Electron Transfer Enabled by Living Cell-Cell Click Chemistry

Direct interspecies electron transfer (DIET) is essential for maintaining the function and stability of anaerobic microbial consortia. However, only limited natural DIET modes have been identified and DIET engineering remains highly challenging. In this study, an unnatural DIET between Shewanella oneidensis MR-1 (SO, electron donating partner) and Rhodopseudomonas palustris (RP, electron accepting partner) was artificially established by a facile living cell-cell click chemistry strategy. By introducing alkyne- or azide-modified monosaccharides onto the cell outer surface of the target species, precise covalent connections between different species in high proximity were realized through a fast click chemistry reaction. Remarkably, upon covalent connection, outer cell surface C-type cytochromes mediated DIET between SO and RP was achieved and identified, although this was never realized naturally. Moreover, this connection directly shifted the natural H2 mediated interspecies electron transfer (MIET) to DIET between SO and RP, which delivered superior interspecies electron exchange efficiency. Therefore, this work demonstrated a naturally unachievable DIET and an unprecedented MIET shift to DIET accomplished by cell-cell distance engineering, offering an efficient and versatile solution for DIET engineering, which extends our understanding of DIET and opens up new avenues for DIET exploration and applications.

Electrochemically assisted production of biogenic palladium nanoparticles for the catalytic removal of micropollutants in wastewater treatment plants effluent

Biogenic palladium nanoparticles (bio-Pd NPs) are used for the reductive transformation and/or dehalogenation of persistent micropollutants. In this work, H₂ (electron donor) was produced in situ by an electrochemical cell, permitting steered production of differently sized bio-Pd NPs. The catalytic activity was first assessed by the degradation of methyl orange. The NPs showing the highest catalytic activity were selected for the removal of micropollutants from secondary treated municipal wastewater. The synthesis at different H₂ flow rates (0.310 L/hr or 0.646 L/hr) influenced the bio-Pd NPs size. The NPs produced over 6 hr at a low H₂ flow rate had a larger size (D50 = 39.0 nm) than those produced in 3 hr at a high H₂ flow rate (D50 = 23.2 nm). Removal of 92.1% and 44.3% of methyl orange was obtained after 30 min for the NPs with sizes of 39.0 nm and 23.2 nm, respectively. Bio-Pd NPs of 39.0 nm were used to treat micropollutants present in secondary treated municipal wastewater at concentrations ranging from µg/L to ng/L. Effective removal of 8 compounds was observed: ibuprofen (69.5%) < sulfamethoxazole (80.6%) < naproxen (81.4%) < furosemide (89.7%) < citalopram (91.7%) < diclofenac (91.9%) < atorvastatin (> 94.3%) < lorazepam (97.2%). Removal of fluorinated antibiotics occurred at > 90% efficiency. Overall, these data indicate that the size, and thus the catalytic activity of the NPs can be steered and that the removal of challenging micropollutants at environmentally relevant concentrations can be achieved through the use of bio-Pd NPs.

Heavy-metal resistant bio-hybrid with biogenic ferrous sulfide nanoparticles: pH-regulated self-assembly and wastewater treatment application

Self-assembled bio-hybrids with biogenic ferrous sulfide nanoparticles (bio-FeS) on the cell surface are attractive for reduction of toxic heavy metals due to higher activity than bare bacteria, but they still suffer from slow synthesis and regeneration of bio-FeS and bacterial activity decay for removal of high-concentration heavy metals. A further optimization of the bio-FeS synthesis process and properties is of vital importance to address this challenge. Herein, we present a simple pH-regulation strategy to enhance bio-FeS synthesis and elucidated the underlying regulatory mechanisms. Slightly raising the pH from 7.4 to 8.3 led to 1.5-fold higher sulfide generation rate due to upregulated expression of thiosulfate reduction-related genes, and triggered the formation of fine-sized bio-FeS (29.4 ± 6.1 nm). The resulting bio-hybrid exhibited significantly improved extracellular reduction activity and was successfully used for treatment of high-concentration chromium -containing wastewater (Cr(VI), 80 mg/L) at satisfactory efficiency and stability. Its feasibility for bio-augmented treatment of real Cr(VI)-rich electroplating wastewater was also demonstrated, showing no obvious activity decline during 7-day operation. Overall, our work provides new insights into the environmental-responses of bio-hybrid self-assembly process, and may have important implications for optimized application of bio-hybrid for wastewater treatment and environmental remediation.

The influence of H 2 partial pressure on biogenic palladium nanoparticle production assessed by single‐cell ICP ‐mass spectrometry

The production of biogenic palladium nanoparticles (bio-Pd NPs) is widely studied due to their high catalytic activity, which depends on the size of nanoparticles (NPs). Smaller NPs (here defined as <100 nm) are more efficient due to their higher surface/volume ratio. In this work, inductively coupled plasma-mass spectrometry (ICP-MS), flow cytometry (FCM) and transmission electron microscopy (TEM) were combined to obtain insight into the formation of these bio-Pd NPs. The precipitation of bio-Pd NPs was evaluated on a cell-per-cell basis using single-cell ICP-MS (SC-ICP-MS) combined with TEM images to assess how homogenously the particles were distributed over the cells. The results provided by SC-ICP-MS were consistent with those provided by "bulk" ICP-MS analysis and FCM. It was observed that heterogeneity in the distribution of palladium over an entire cell population is strongly dependent on the Pd²⁺ concentration, biomass and partial H₂ pressure. The latter three parameters affected the particle size, ranging from 15.6 to 560 nm, and exerted a significant impact on the production of the bio-Pd NPs. The TEM combined with SC-ICP-MS revealed that the mass distribution for bacteria with high Pd content (144 fg Pd cell^-1 ) indicated the presence of a large number of very small NPs (D50 = 15.6 nm). These results were obtained at high cell density (1 × 10⁵  ± 3 × 10⁴ cells μl^-1 ) and H₂ partial pressure (180 ml H₂ ). In contrast, very large particles (D50 = 560 nm) were observed at low cell density (3 × 10⁴  ± 10 × 10² cells μl^-1 ) and H₂ partial pressure (10-100 ml H₂ ). The influence of the H₂ partial pressure on the nanoparticle size and the possibility of size-tuned nanoparticles are presented.

In situ engineering of highly conductive TiO2/carbon heterostructure fibers for enhanced electrocatalytic degradation of water pollutants

Rational design of nanocomposite electrode materials with high conductivity, activity, and mechanical strength is critical in electrocatalysis. Herein, freestanding, flexible heteronanocomposites were fabricated in situ by carbonizing electrospun fibers with TiO₂ nanoparticles on the surface for electrocatalytic degradation of water pollutants. The carbonization temperature was observed as a dominant parameter affecting the characteristics of the electrodes. As the carbonization temperature increased to 1000 °C, the conductivity of the electrode was significantly enhanced due to the high degree of graphitization (I_D/I_G ratio 1.10) and the dominant rutile phase. Additionally, the formation of TiO₂ protrusions and the C-Ti heterostructure were observed at 1000 °C, which contributed to increasing the electrocatalytic activity. When 1.5 V (vs. Ag/AgCl) was employed, electrocatalytic experiments using the electrode achieved 90% degradation of crystal violet and 10.9-87.5% for an array of micropollutants. The electrical energy-per-order (EEO) for the removal of crystal violet was 0.7 kWh/m³/order, indicative of low-energy requirement. The efficient electrocatalytic activity can be ascribed to the fast electron transfer and the strong ability to generate hydroxyl radicals. Our findings expand efforts for the design of highly conductive heteronanocomposites in a facile in situ approach, providing a promising perspective for the energy-efficient electrocatalytic degradation of water pollutants.

The Effect of a Hydrogen Reduction Procedure on the Microbial Synthesis of a Nano-Pd Electrocatalyst for an Oxygen-Reduction Reaction

Noble-metal electrocatalysts supported by biological-organism-derived carbons have attracted attention from the public due to the growing demands for green synthesis and environmental protection. Carbonization at high temperatures and hydrogen reduction are critical steps in this technical route. Herein, Shewanella oneidensis MR-1 were used as precursors, and the effects of the hydrogen-reduction procedure on catalysts were explored. The results showed that the performances of FHTG (carbonization followed by hydrogen reduction) displayed the best performance. Its ECSA (electrochemical surface area), MA (mass activity), and SA (specific activity) reached 35.01 m2 g−1, 58.39 A·g−1, and 1.66 A cm−2, respectively, which were 1.17, 1.75, and 1.50 times that of PHTG (prepared through hydrogen reduction followed by carbonization) and 1.56, 2.26, and 1.44 times that of DHTG (double hydrogen reduction). The high performance could be attributed to its fine particle size and rich N content, and the specific regulation mechanism was also proposed in this paper. This study opens a practical guide for effectively avoiding particle agglomeration during the fabrication process for catalysts.

In situ fabrication of gold nanoparticles into biocathodes enhance chloramphenicol removal

The development of highly conductive biofilms is a key strategy to enhance antibiotic removal in bioelectrochemical systems (BESs) with biocathodes. In this study, Au nanoparticles (Au-NPs) were in situ fabricated in a biocathode (Au biocathode) to enhance the removal of chloramphenicol (CAP) in BESs. The concentration of Au(III) was determined to be 5 mg/L. CAP was effectively removed in the BES containing a Au biocathode with a removal percentage of 94.0% within 48 h; this result was 1.8-fold greater than that obtained using a biocathode without Au-NPs (51.7%). The Au-NPs significantly reduced the charge transfer resistance and promoted the electrochemical activity of the biocathode. In addition, the Au biocathode showed a specifical enrichment of Dokdonella, Bosea, Achromobacter, Bacteroides and Petrimonas, all of which are associated with electron transfer and contaminant degradation. This study provides a new strategy for enhancing CAP removal in BESs through a simple and eco-friendly electrode design.

Biotechnological synthesis of Pd-based nanoparticle catalysts

Palladium metal nanoparticles are excellent catalysts used industrially for reactions such as hydrogenation and Heck and Suzuki C-C coupling reactions. However, the global demand for Pd far exceeds global supply, therefore the sustainable use and recycling of Pd is vital. Conventional chemical synthesis routes of Pd metal nanoparticles do not meet sustainability targets due to the use of toxic chemicals, such as organic solvents and capping agents. Microbes are capable of bioreducing soluble high oxidation state metal ions to form metal nanoparticles at ambient temperature and pressure, without the need for toxic chemicals. Microbes can also reduce metal from waste solutions, revalorising these waste streams and allowing the reuse of precious metals. Pd nanoparticles supported on microbial cells (bio-Pd) can catalyse a wide array of reactions, even outperforming commercial heterogeneous Pd catalysts in several studies. However, to be considered a viable commercial option, the intrinsic activity and selectivity of bio-Pd must be enhanced. Many types of microorganisms can produce bio-Pd, although most studies so far have been performed using bacteria, with metal reduction mediated by hydrogenase or formate dehydrogenase enzymes. Dissimilatory metal-reducing bacteria (DMRB) possess additional enzymes adapted for extracellular electron transport that potentially offer greater control over the properties of the nanoparticles produced. A recent and important addition to the field are bio-bimetallic nanoparticles, which significantly enhance the catalytic properties of bio-Pd. In addition, systems biology can integrate bio-Pd into biocatalytic processes, and processing techniques may enhance the catalytic properties further, such as incorporating additional functional nanomaterials. This review aims to highlight aspects of enzymatic metal reduction processes that can be bioengineered to control the size, shape, and cellular location of bio-Pd in order to optimise its catalytic properties.

Reduction and removal of Cr(VI) in water using biosynthesized palladium nanoparticles loaded Shewanella oneidensis MR-1

In materials science, "green" synthesis has gotten a lot of interest as a reliable, long-lasting, and ecofriendly way to make a variety of materials/nanomaterials, including metal/metal oxide nanomaterials. To accommodate various biological materials, green synthesis of metallic nanoparticles has been used (e.g., bacteria, fungi, algae, and plant extracts). In this work, Shewanella oneidensis MR-1 was used to biosynthesize palladium nanoparticles (bioPd) under aerobic conditions for the Cr(VI) bio-reduction. The size and distribution of bio-Pd are controlled by adjusting the ratio of microbial biomass and palladium precursors. The high cell: Pd ratio has the smallest average particle size of 6.33 ± 1.69 nm. And it has the lowest electrocatalytic potential (-0.132 V) for the oxidation of formic acid, which is 0.158 V lower than commercial Pd/C (5%). Our results revealed that the small size and uniformly distributed extracellular bio-Pd could achieve completely catalytic reduction of 200 mg/L Cr(VI) solution within 10 min, while the commercial Pd/C (5%) need at least 45 min. The bio-Pd materials maintain a high reduction during five cycles. Microorganisms play an important role in the whole process, which can fully disperse palladium nanoparticles, completely reduce Cr(VI), and effectively adsorb Cr(III). This work expands our understanding and provides a reference for the design and development of efficient and green bio-Pd catalysts for environmental pollution control under simple and mild conditions.

Unveiling the Surface and the Ultrastructure of Palladized Fungal Biotemplates

In this paper, two biosystems based on filamentous fungi and Pd nanoparticles (NPs) were synthesized and structurally characterized. In the first case, results concerning the integration and distribution of Pd-NPs on Phialomyces macrosporus revealed that nanoparticles are accumulated on the cell wall, keeping the cytoplasm isolated from abiotic particles. However, the Penicillium sp. species showed an unexpected internalization of Pd-NPs in the fungal cytosol, becoming a promising biosystem to further studies of in vivo catalytic reactions. Next, we report a new solution-based strategy to prepare palladized biohybrids through sequential reduction of Pd²⁺ ions over previously harvested fungus/Au-NP composites. The chemical composition and the morphology of the biohybrid surface were characterized using a combination of scanning electron microscopy, transmission electron microscopy, and photoelectron spectroscopy. The deposition of Pd⁰ over the fungal surface produced biohybrids with a combination of Au and Pd in the NPs. Interestingly, other chemical species such as Au⁺ and Pd²⁺ are also observed on the outermost wall of microorganisms. Finally, the application of A. niger/AuPd-NP biohybrids in the 3-methyl-2-buten-1-ol hydrogenation reaction is presented for the first time. Biohybrids with a high fraction of Pd⁰ are active for this catalytic reaction.

Bacterial extracellular electron transfer: a powerful route to the green biosynthesis of inorganic nanomaterials for multifunctional applications

Synthesis of inorganic nanomaterials such as metal nanoparticles (MNPs) using various biological entities as smart nanofactories has emerged as one of the foremost scientific endeavors in recent years. The biosynthesis process is environmentally friendly, cost-effective and easy to be scaled up, and can also bring neat features to products such as high dispersity and biocompatibility. However, the biomanufacturing of inorganic nanomaterials is still at the trial-and-error stage due to the lack of understanding for underlying mechanism. Dissimilatory metal reduction bacteria, especially Shewanella and Geobacter species, possess peculiar extracellular electron transfer (EET) features, through which the bacteria can pump electrons out of their cells to drive extracellular reduction reactions, and have thus exhibited distinct advantages in controllable and tailorable fabrication of inorganic nanomaterials including MNPs and graphene. Our aim is to present a critical review of recent state-of-the-art advances in inorganic biosynthesis methodologies based on bacterial EET using Shewanella and Geobacter species as typical strains. We begin with a brief introduction about bacterial EET mechanism, followed by reviewing key examples from literatures that exemplify the powerful activities of EET-enabled biosynthesis routes towards the production of a series of inorganic nanomaterials and place a special emphasis on rationally tailoring the structures and properties of products through the fine control of EET pathways. The application prospects of biogenic nanomaterials are then highlighted in multiple fields of (bio-) energy conversion, remediation of organic pollutants and toxic metals, and biomedicine. A summary and outlook are given with discussion on challenges of bio-manufacturing with well-defined controllability.

Insights into palladium nanoparticles produced by Shewanella oneidensis MR-1: Roles of NADH dehydrogenases and hydrogenases

Biologically synthesized palladium nanoparticles (bio-Pd) have attracted considerable interest as promising green catalysts for environmental remediation. However, the mechanisms by which microorganisms produce bio-Pd remain unclear. In the present study, we investigated the roles of Shewanella oneidensis MR-1 and its NADH dehydrogenases and hydrogenases (HydA and HyaB) in bio-Pd production using formate as the electron donor. The roles of NADH dehydrogenases and hydrogenases were studied by inhibiting NADH dehydrogenases and using hydrogenase mutants (ΔhydA, ΔhyaB, and ΔhydAΔhyaB), respectively. The results showed ~97% reduction of palladium by S. oneidensis MR-1 after 24 h using 250 μM palladium and 500 μM formate. Electron microscopy images showed the presence of bio-Pd on both the outer and cytoplasmic membranes of S. oneidensis MR-1. However, the inhibition of NADH dehydrogenases in S. oneidensis MR-1 resulted in only ~61% reduction of palladium after 24 h, and bio-Pd were not found on the outer membrane. The mutants lacking one or two hydrogenases removed 91-96% of palladium ions after 24 h and showed more cytoplasmic bio-Pd but less periplasmic bio-Pd. To the best of our knowledge, this is the first study to demonstrate the role of NADH dehydrogenases of S. oneidensis MR-1 in the formation of bio-Pd on the outer membrane. It also demonstrates that the hydrogenases (especially HyaB) of S. oneidensis MR-1 contribute to the formation of bio-Pd in the periplasmic space. This study provides mechanistic insights into the production of biogenic metal nanoparticles towards their possible use in industrial and environmental applications.

Insight into the electrocatalytic performance of in-situ fabricated electroactive biofilm-Pd: The role of biofilm thickness, initial Pd(II) concentration and the exposure time to Pd precursor

Biogenic palladium (bio-Pd) nanoparticles have been considered as promising biocatalyst for energy generation and contaminants remediation in water and sediment. Recently, an electroactive biofilm-Pd (EAB-Pd) network, which can be used directly as electrocatalyst and show enhanced electrocatalytic performance, has exhibited tremendous application potential. However, the information regarding to the controllable biosynthetic process and corresponding catalytic properties is scarce. This study demonstrated that the catalytic performance of EAB-Pd could be influenced by Pd loading on bacteria cells (Pd/cells), which was crucial to determine the final distribution characteristic of Pd nanocrystal on EAB skeleton. For instance, the high Pd/cells (over 0.18 pg cell^-1) exhibited almost 6-fold and 1.5-fold enhancement over EAB-Pds with Pd/cells below 0.03 in catalytic current toward hydrogen evolution reaction and nitrobenzene reduction, respectively. In addition, the Pd/cells was found to be affected by the synthesis factors, such as the ratio of biomass to initial Pd(II) concentration (cells/Pd_II) and the exposure time of EAB to Pd(II) precursor solution. The Pd/cells increased significantly as the cell/Pd_II ratio decreased from ~5.5 × 10⁷ to ~1.3 × 10⁷ cells L mg^-1 or the prolongation of exposure time from 3 h to 24 h. The findings developed in this work extensively expand our knowledge for the in-situ designing biogenic electrocatalyst and provide important information for the development of its catalytic property.

A continuous electroreduction cell composed of palladium nanocatalyst immobilized on discarded cigarette filters as an active bed for Cr(VI) removal from groundwater

In this research, a unique continuous electrochemical cell was designed and applied for the disinfection of groundwater and simultaneous Cr(VI) reduction and Cr(III) precipitation. Discarded cigarette filters (DCFs) were utilized as an efficient bed for palladium nanoparticles (PdNPs) immobilization located between porous anode and cathode made of graphite felt. The characterization of the bed was performed using FE-SEM, EDS, BET, and FT-IR analysis. The results confirmed the distribution of palladium nanoparticles on the surface of DCFs. The proposed design for electrochemical cell obviated the need to divide the anolyte and catholyte because the anode was located at the outlet of the cell, thereby avoiding the reaction between hydrogen radicals produced on the surface of PdNPs and oxygen and chlorine produced in the anode. The hydrogen gas produced in the cathode was converted to hydrogen radicals, acting as the most prominent species for the reduction. Hydroxide ions produced in the cathode increased the pH of the solution between electrodes, resulting in the precipitation of Cr (III) with an efficiency of 96%. Furthermore, free chlorine at the concentration of 1 mg L^-1 was generated through chloride ion oxidation in the anode, which can be effective for disinfection. The effect of initial Cr (VI) concentration (C₀), flow rate (Q), and current (I) was investigated, and the maximum removal efficiency (99.7%) was observed at the flow rate of 5 mL min^-1 and current of 0.05 A, respectively. No interference ensued from the various coexisting ions in groundwater. The findings of this study suggested that the proposed electrochemical cell is capable of in-situ total chromium removal and free chlorine production in groundwater simultaneously.

Shewanella oneidensis MR-1 self-assembled Pd-cells-rGO conductive composite for enhancing electrocatalysis

Biosynthesized noble metal nanoparticles (NPs) as promising green catalysts for electrochemical application has invited a lot of attention. However, effective electron transfer between biosynthesized NPs and electrode remains a challenge due to the uncontrollable and poor conductive property of cell substrates. In this study, graphene oxide (GO) was introduced into a bio-Pd synthesis process governed by Shewanella oneidensis MR-1, which was demonstrated to be simultaneously reduced with Pd(II) and transformed to reduced GO (rGO), resulting in the formation of a Pd-cells-rGO composite. Compared to the control without rGO (Pd-cells), the electrochemical conductivity of Pd-cells-rGO composite increased from almost zero to 196 μS cm^-1, indicating the rGO facilities the electron transport across the composite. Electrochemical characterizations revealed the electrochemical active surface area (ECSA) of Pd in Pd-cells-rGO was enlarged by increasing the amount of rGO in the composite, clearly indicating that the conductive network created by rGO enable the Pd NPs receive electrons from electrode and become electrochemical active. A considerable enhancement of electrocatalytic activity was further confirmed for Pd-cells-rGO as indicated by 36.7- and 17.2-fold increase (Pd-cells-rGO with Pd/GO ratio of 5/1 vs Pd-cells) of steady state current density toward hydrogen evolution and nitrobenzene reduction at -0.7 V and -0.55 V vs Ag/AgCl, respectively. We also compared the electrocatalytic performance with MWCNTs hybrids Pd-cells-CNTs. It was found that the association of Pd, cells and rGO creates an interactive and synergistic environment to allow higher conductivity and catalytic activity under the same amount of carbon nanomaterial. The strategy developed in this work activates a highly reactive NPs and proposed a designable protocol for enhancing electrocatalytic activity of biocatalysts.

Enhanced reductive removal of ciprofloxacin in pharmaceutical wastewater using biogenic palladium nanoparticles by bubbling H2

To treat waste with waste and efficiently remove the organic pollutant, waste palladiums(ii) were adsorbed and reduced on microorganism surface to catalyze the reductive removal of ciprofloxacin in pharmaceutical wastewater. By optimizing conditions such as pH and temperature, the amount of biogenic palladium adsorbed and reduced on E. coli reached 139.48 mg g⁻¹ (Pd/microorganisms). Moreover, most of the Pd(ii) was reduced to nanometer-sized Pd(0) as characterized by TEM and SEM with EDXA. Using the obtained biogenic palladium, the reductive removal of ciprofloxacin is up to 87.70% at 25 °C, 3.03 folds of that achieved in the absence of H₂. The results show that waste E. coli microorganisms can efficiently adsorb and remove waste Pd(ii) and produce Bio-Pd nanoparticle catalysts in the presence of H₂. This biogenic palladium presents high catalytic activity and great advantages in the reductive degradation of ciprofloxacin. Our method can also be applied to other waste metal ions to prepare the biogenic metals, facilitate their recovery and reuse in degrading organic pollutants in wastewater to achieve “treating waste using waste”.

A solution has been successfully introduced to three key challenges from the wastewater containing waste microorganisms, metal and ciprofloxacin, respectively.

Extracellular electron transfer of Enterobacter cloacae SgZ-5T via bi-mediators for the biorecovery of palladium as nanorods

In nature, microbes use extracellular electron transfer (EET) to recover noble metals. Most attention has been paid to the biorecovery process occurring intracellularly and on the cell surface. In this work, we report that Pd nanorods could be biosynthesized by Enterobacter cloacae SgZ-5T in the extracellular space. This bacterium possesses both a direct EET pathway through membrane redox systems and an indirect EET pathway via the self-secreted electron carrier hydroquinone (HQ). When exposed to Pd(II), the bacteria adjusted their metabolic pathway and membrane-bound proteins to secrete riboflavin (RF). However, no HQ was detected in the supernatant in presence of Pd(II). No significant change was observed through metabolomic analysis regarding the abundance of HQ in presence of Pd(II) compared to Pd(II)-free supernatant. Similar results were also obtained through transcriptomic analysis of YqjG gene encoding glutathionyl-HQ reductase synthase. X-ray photoelectron spectroscopic evidence indicated that HQ may adsorb to the surface of Pd nanorods. Moreover, the gene encoding RF synthase (ribE) was up-regulated in the present of Pd(II), suggesting that this bioreduction process induced RF synthase, which had been shown in previous results. The UV-vis spectroscopy data demonstrated that the Pd(II) reduction rate was enhanced by 5%, 5.5% and 30% by the addition of 3.33 μM HQ, 3.33 μM RF and the both, respectively. All these results revealed that the bi-mediators secreted by bacteria were beneficial for biorecovery of Pd. This work is of significance for understanding metal biorecovery processes and natural biogeochemical processes.

Bioreduction of azo dyes was enhanced by in-situ biogenic palladium nanoparticles

Biogenic nanoparticles are promising materials for their green synthesis method and good performance in stimulation on reduction of environmental contaminants. In this study, Pd(0) nanoparticles (bio-Pd) were generated by Klebsiella oxytoca GS-4-08 in fermentative condition and in-situ improved the azo dye reduction. The bio-Pd was mainly located on cell membrane with a size range of 5-20 nm by TEM and XRD data analyses. Anthraquinone-2-disulfonate (AQS) greatly increased the reduction rate of Pd(II) with a reduction efficiency as high as 96.54 ± 0.23% in 24 h. The quinone respiration theory, glucose metabolism and the biohydrogen pathway were used to explain the enhancement mechanism of the in-situ generated bio-Pd on azo dye reduction. These results indicate that the in-situ generated bio-Pd by K. oxytoca strain is efficient for azo dye reduction without complex preparation processes, which is of great significance for the removal and subsequent safe disposal of hazardous environmental compounds.

Activating electrochemical catalytic activity of bio-palladium by hybridizing with carbon nanotube as "e- Bridge"

Nano metal catalysts produced by bacteria has received increasing attention owing to its environmental friendly synthesis route. However, the formed metal nanoparticles are associated with poorly conductive cells and challenged to be electrochemically applied. In this study, Palladium (Pd) nanoparticles were synthesized by Shewanella oneidensis MR-1. We demonstrated the limitation of palladized cells (Pd-cells) serving as electro-catalysts can be relieved by hybridizing with the conductive carbon nanotubes (Pd-cells-CNTs hybrid). Compared to the Pd-cells, the electrochemical active surface area of Pd in Pd-cells-CNTs10 (the ratio of Pd/CNTs is 1/10 w/w) were dramatically increased by 68 times to 20.44 m²·g^-1. A considerable enhancement of electrocatalytic activity was further confirmed for Pd-cells-CNTs10 as indicated by a 5-fold increase of steady state current density for nitrobenzene reduction at -0.55 V vs Ag/AgCl. These results indicate that the biogenetic palladium could has been an efficient electro-catalyst but just limited due to lacking an electron transport path (e ^- Bridge). This finding may also be helpful to guide the way to electrochemically use other biogenetic metal nano-materials.