Microbial production and environmental applications of Pd nanoparticles for treatment of halogenated compounds

One-Dimensional Organic Conjugated Polymers as Recyclable Heterogeneous Photocatalysts

Organic conjugated polymers with long-range conjugation generally have strong light absorption capacity in visible light region and impressive performance in charge transfer, which endows them great application potential in the field of opto-electronic materials. But there are few reports on their use in photocatalytic reactions. At present, it has been reported that a variety of donor-acceptor (D-A) type organic dyes can be used in efficient organic photocatalytic transformations. We designed and synthesized one-dimensional organic conjugated polymers pPhCzBP-Th and pPhCzBP-DTh with D-A structure, and proved that they are good heterogeneous photo-redox catalysts, which can photocatalyze hydrodehalogenation reduction of α-bromoacetophenone and its derivatives. Due to the strong reducibility of the excited state, pPhCzBP-Th can also efficiently reduce α-chloroacetophenone. Furthermore, by simply wrapping the catalyst powder, high-efficient separation of products and catalysts recycling can be achieved.

Difference in toxicity of Pd (II) and mechanism of action before and after reduction by Bacillus wiedmannii MSM

At present, there are many studies on microbial reduction of Pd (II), although few of these studies assess the bacterial toxicity of Pd (II) before and after reduction by microorganisms. In this study, the toxicity of Pd (II) to Bacillus wiedmannii MSM (B. wiedmannii MSM) was assessed before and after reduction by live B. wiedmannii MSM cells (referred to as "Pd (0)-loaded cells") and after biosorption by dead B. wiedmannii MSM cells (referred to as "Pd (II)-loaded cells"). B. wiedmannii MSM is a widely occurring, nonpathogenic aerobic bacterium. Compared with Pd (II), the EC20, EC50, and EC80 of Pd (0)-loaded cells increased by 77.73-, 112.75-, and 163.91-fold, respectively, while the EC20, EC50, and EC80 of Pd (II)-loaded cells increased by only 11.55-, 9.77-, and 8.29-fold, respectively. The sterilization contribution rates of the mechanisms of action of Pd (0)-loaded cells were ranked in the order of: remaining Pd (II) > oxidative stress > physical puncture. Pd (II) was found to increase cell membrane permeability, glutathione S-transferase (GST) enzyme activity, and reactive oxygen species levels in cells and decrease the cell membrane potential. XPS results indicated that Pd (II) increased the content of polysaccharides and peptides and decreased the content of hydrocarbons in cells. These findings reveal the bactericidal mechanism of toxicity of Pd (II) and Pd (0)-loaded cells on Bacillus wiedmannii MSM and provide an environmentally friendly and inexpensive method for Pd (II) detoxification.

Electrified bioreactors: the next power-up for biometallurgical wastewater treatment

Over the past decades, biological treatment of metallurgical wastewaters has become commonplace. Passive systems require intensive land use due to their slow treatment rates, do not recover embedded resources and are poorly controllable. Active systems however require the addition of chemicals, increasing operational costs and possibly negatively affecting safety and the environment. Electrification of biological systems can reduce the use of chemicals, operational costs, surface footprint and environmental impact when compared to passive and active technologies whilst increasing the recovery of resources and the extraction of products. Electrification of low rate applications has resulted in the development of bioelectrochemical systems (BES), but electrification of high rate systems has been lagging behind due to the limited mass transfer, electron transfer and biomass density in BES. We postulate that for high rate applications, the electrification of bioreactors, for example, through the use of electrolyzers, may herald a new generation of electrified biological systems (EBS). In this review, we evaluate the latest trends in the field of biometallurgical and microbial‐electrochemical wastewater treatment and discuss the advantages and challenges of these existing treatment technologies. We advocate for future research to focus on the development of electrified bioreactors, exploring the boundaries and limitations of these systems, and their validity upon treating industrial wastewaters.

Enhanced hydrogenation catalyst synthesized by Desulfovibrio desulfuricans exposed to a radio frequency magnetic field

Desulfovibrio desulfuricans reduces Pd(II) to Pd(0)‐nanoparticles (Pd‐NPs) which are catalytically active in 2‐pentyne hydrogenation. To make Pd‐NPs, resting cells are challenged with Pd(II) ions (uptake), followed by addition of electron donor to promote bioreduction of cell‐bound Pd(II) to Pd(0) (bio‐Pd). Application of radiofrequency (RF) radiation to prepared 5 wt% bio‐Pd catalyst (60 W power, 60 min) increased the hydrogenation rate by 70% with no adverse impact on selectivity to cis‐2‐pentene. Such treatment of a 5 wt% Pd/carbon commercial catalyst did not affect the conversion rate but reduced the selectivity. Lower‐dose RF radiation (2–8 W power, 20 min) was applied to the bacteria at various stages before and during synthesis of the bio‐scaffolded Pd‐NPs. The reaction rate (μ mol 2‐pentyne converted s^‐1) was increased by ~threefold by treatment during bacterial catalyst synthesis. Application of RF radiation (2 or 4 W power) to resting cells prior to Pd(II) exposure affected the catalyst made subsequently, increasing the reaction rate by 50% as compared to untreated cells, while nearly doubling selectivity for cis 2‐pentene. The results are discussed with respect to published and related work which shows altered dispersion of the Pd‐NPs made following or during RF exposure.

Pyruvate accelerates palladium reduction by regulating catabolism and electron transfer pathway in Shewanella oneidensis

Shewanella oneidensis is a model strain of the electrochemical active bacteria (EAB) because of its strong capability of extracellular electron transfer (EET) and genetic tractability. In this study, we investigated the effect of carbon sources on EET in S. oneidensis by using reduction of palladium ions (Pd(II)) as a model and found that pyruvate greatly accelerated the Pd(II) reduction compared with lactate by resting cells. Both Mtr pathway and hydrogenases played a role in Pd(II) reduction when pyruvate was used as a carbon source. Furthermore, in comparison with lactate-feeding S. oneidensis, the transcriptional levels of formate dehydrogenases involving in pyruvate catabolism, Mtr pathway, and hydrogenases in pyruvate-feeding S. oneidensis were up-regulated. Mechanistically, the enhancement of electron generation from pyruvate catabolism and electron transfer to Pd(II) explains the pyruvate effect on Pd(II) reduction. Interestingly, a 2-h time window is required for pyruvate to regulate transcription of these genes and profoundly improve Pd(II) reduction capability, suggesting a hierarchical regulation for pyruvate sensing and response in S. oneidensis IMPORTANCE The unique respiration of EET is crucial for the biogeochemical cycling of metal elements and diverse applications of EAB. Although a carbon source is a determinant factor of bacterial metabolism, the research into the regulation of carbon source on EET is rare. In this work, we reported the pyruvate-specific regulation and improvement of EET in S. oneidensis and revealed the underlying mechanism, which suggests potential targets to engineer and improve the EET efficiency of this bacterium. This study sheds light on the regulatory role of carbon sources in anaerobic respiration in EAB, providing a way to regulate EET for diverse applications from a novel perspective.

Current Trends in the Application of Nanomaterials for the Removal of Emerging Micropollutants and Pathogens from Water

Water resources contamination has a worldwide impact and is a cause of global concern. The need for provision of clean water is becoming more and more demanding. Nanotechnology may support effective strategies for the treatment, use and reuse of water and the development of next-generation water supply systems. The excellent properties and effectiveness of nanomaterials make them particularly suitable for water/wastewater treatment. This review provides a comprehensive overview of the main categories of nanomaterials used in catalytic processes (carbon nanotubes/graphitic carbon nitride (CNT/g-C₃N₄) composites/graphene-based composites, metal oxides and composites, metal–organic framework and commercially available nanomaterials). These materials have found application in the removal of different categories of pollutants, including pharmaceutically active compounds, personal care products, organic micropollutants, as well as for the disinfection of bacterial, viral and protozoa microbial targets, in water and wastewater matrices. Apart from reviewing the characteristics and efficacy of the aforementioned nanoengineered materials for the removal of different pollutants, we have also recorded performance limitations issues (e.g., toxicity, operating conditions and reuse) for their practical application in water and wastewater treatment on large scale. Research efforts and continuous production are expected to support the development of eco-friendly, economic and efficient nanomaterials for real life applications in the near future.

Electron transfer involved in bio-Pd (0) synthesis by Citrobacter freundii at different growth phases

Gram-negative Citrobacter freundii with high Pd (II) reduction capacity was isolated from electroplating wastewater, and the electron transfer involved in Pd (II) bio-reduction by C. freundii JH was investigated in phosphate buffer saline solution with sodium formate as sole electron donor under anaerobic condition. FTIR spectra indicated that hydroxyl and amine groups on cell wall participated Pd (II) bio-sorption. TEM, XRD, XPS results confirmed that Pd (0) nanoparticles (NPs) could be bio-synthesized intra/extracellularly. Meanwhile, pH turn-over were observed owing to the reduction of cytochrome c (c-Cyt) in bio-reduction process. EPR spectra indicated that free radicals (OH) was generated from high concentration Pd (II), which would cause seriously damage to cell. Despite of the lower tolerance to Pd (II), the cells at logarithmic phase exhibited higher Pd (II) reduction capacity (72.21%) than that at stationary phase (56.21%), which might be related to the relatively stronger proton motive force (PMF) created by the substrate oxidation and the electron transfer, as evidenced by electrochemical experiments (CV, DPV, amperometric I-t curves) and protein denaturalization experiments. Additionally, c-Cyt and riboflavin were confirmed to be important participants in electron transfer. Finally, a putative synthesis mechanism of Pd (0)-NPs was deduced. This study contributed to further understanding the electron transfer in Pd (II) reduction, and provided more information for the bio-synthetic of metal nanoparticles.

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.

Toxicity assessment of palladium oxide nanoparticles derived from metallosurfactants using multi assay techniques in Allium sativum

In today's world, nanotechnology is reaching practically every ground and entering the human lifestyle by becoming a part of it. Thus, it is vital to check the cytotoxic and genotoxic effects of nanosubstances on plants, as they are the base constituent of ecosystem. The present work deals with the toxicity evaluation of metallosurfactant derived palladium oxide nanoparticles towards Allium sativum (Garlic cloves). The nanoparticles were prepared using microemulsion quenching method (a softer approach) using palladium metallosurfactants as precursors. The three ligands used were cetyltrimethylammonium chloride (CTAC), dodecylamine (DDA) and hexadecylamine (HEXA). Further, their characterization was done using TEM, Size Distribution curve, FESEM, EDS, XRD and Zeta potential. Garlic (Allium sativum) cloves were used to investigate the cytotoxicity and genotoxicity of fabricated PdO NPs. To check the cytotoxicity, optical microscopy was employed and for the genotoxic assessment, different parameters such as chromosomal aberrations in the mitosis, circular dichroism, and gel electrophoresis were utilized. From mitosis study, chromosomes aberrations were confirmed such as chromosomes stickiness, breakage, C-Mitosis, delay in anaphase, spindle fibre abnormality, laggard, vagrant and condensed chromosomes. Morphology of A. sativum clove, rooting and shooting pattern in the presence of PdO nanosuspension was observed. From all the experiments, it was concluded that all the three PdO nanosuspension are toxic in nature to both the cells and to genome, although, bishexadecyltrimethyl ammonium palladium tetrachloride (PdCTAC) Ns was found to be the most cytotoxic and genotoxic. Gel electrophoresis also confirmed the complete degradation of DNA in the presence of PdCTAC Ns.

Biofilm-Templated Heteroatom-Doped Carbon-Palladium Nanocomposite Catalyst for Hexavalent Chromium Reduction

In this study, we report an interdisciplinary and novel strategy toward biofilm engineering for the development of a biofilm-templated heteroatom-doped catalytic system through bioreduction and biofilm matrix-facilitated immobilization of the in situ-formed catalytic nanoparticles followed by controlled pyrolysis. We showed that (i) even under room temperature and bulk aerobic conditions, Shewanella oneidensis MR-1 biofilms reduced Pd(II) to form Pd(0) nanocrystals (∼10 to 20 nm) that were immobilized in the biofilm matrix and in cellular membranes, (ii) the MR-1 biofilms with the immobilized Pd(0) nanocrystals exhibited nanocatalytic activity, (iii) exposure to Pd(II) greatly increased the rate of cell detachment from the biofilm and posed a risk of biofilm dispersal, (iv) controlled pyrolysis (carbonization) of the biofilm led to the formation of a stable heteroatom-doped carbon-palladium (C-Pd) nanocomposite catalyst, and (v) the biofilm-templated C-Pd nanocomposite catalyst exhibited a high Cr(VI) reduction activity and maintained a high reduction rate over multiple catalytic cycles. Considering that bacteria are capable of synthesizing a wide range of metal and metalloid nanoparticles, the biofilm-templated approach for the fabrication of the catalytic C-Pd nanocomposite we have demonstrated here should prove to be widely applicable for the production of different nanocomposites that are of importance to various environmental applications.

The effect of biotic and abiotic environmental factors on Pd(II) adsorption and reduction by Bacillus megaterium Y-4

In this study, we screened a new aerobic bacterium (Bacillus megaterium Y-4) that can efficiently reduce Pd(II) with different electron donors. The best electron donor was sodium formate and the best reduction of Pd(II) were by log growth phase cells. The high removal capacity of Pd(II) (1658.3 mg/g) was obtained with 30 mg/L dry cell weight and 50 mg/L Pd (II) in the presence of 5 mM sodium formate. The removal amount of Pd(II) increased with initial Pd(II) concentrations ranging from 50 to 200 mg/L with 100 mg/L Pd(II) being completely removed by 148 mg/L dry cell weight in 6 h. The cell wall, periplasmic space and intracellular contents of B. megaterium Y-4 contains different kinds of enzymes for reducing Pd(II). In addition, the activity of extracellular and periplasmic enzymes was more sensitive to temperature than intracellular enzymes. XRD and XPS analysis revealed that the enzyme for reducing Pd(II) in B. megaterium Y-4 can tolerate a broad range of temperatures (20-60 °C) and pH (2.0-7.0) but was sensitive to oxygen. TEM analysis showed that biogenic palladium nanoparticles (Pd-NPs) were mainly distributed evenly in the periplasmic space of the live cells and were released from cells into aqueous solution, which reduced the toxicity of Pd(II), allowing Pd-NP recovery without cell destruction. B. megaterium Y-4 is a potential bacterium for efficient treatment and reclamation of Pd(II) pollution and formation of Pd-NPs.

Microbial synthesis of Pd-Pt alloy nanoparticles using Shewanella oneidensis MR-1 with enhanced catalytic activity for nitrophenol and azo dyes reduction

Bimetallic nanoparticles (NPs) often exhibit improved catalytic performance due to the electronic and spatial structure changes. Herein, a novel green biosynthesis method for Pd-Pt alloy NPs using Shewanella oneidensis MR-1 was proposed. The morphology, size and crystal structure of Pd-Pt alloy NPs were studied by a suite of characterization techniques. Results showed Pd-Pt alloy NPs were successfully synthesized inside and outside the cell. The biosynthesized Pd-Pt alloy NPs were polycrystalline and face-centered-cubic structure with the particle size ranged from 3-40 nm. Furthermore, the catalytic experiment demonstrated that the Pd-Pt alloy NPs exhibited the highest performance for the catalytic reduction of nitrophenol and azo dyes compared with the as-synthesized Pd and Pt monometallic NPs. This enlarged catalytic activity resulted from the synergistic effect of Pd and Pt element. Thereby, this paper provided a simple biosynthesis method for producing bimetallic alloy nanocatalyst with superior activity for contaminant degradation.

The effect of biotic and abiotic environmental factors on Pd(II) adsorption and reduction by Bacillus wiedmannii MSM

In this paper, we found a bacteria (Bacillus wiedmannii MSM) that could not only culture quickly under aerobic condition, but also can biological reduction of Pd (II) under both aerobic and anaerobic conditions. For reducing Pd (II) by Bacillus wiedmannii MSM, the best electron donor was sodium formate and the best growth time was 24 h (mid-log growth phase cells). TEM indicated that a lot of palladium nanoparticles (Pd-NPs) were mainly located in the periplasmic space of the live cells. However, the autoclaved cells could not synthesize Pd-NPs, which proved the role of enzyme in the reduction of Pd (II). A few of Pd-NPs were only formed on the surface of Cu²⁺-treated cells, which proved the main but not the only role of periplasmic hydrogenase in the reduction of Pd (II). XRD and XPS also proved that Pd-NPs could be synthesized by live cells over broad ranges of temperature (20-40 °C) and pH (pH 3.0-7.0). This may be especially useful for in situ reduction and remediation of Pd (II) for both anaerobic and aerobic wastewater.

Palladium Nanoparticles: Toxicological Effects and Potential Implications for Occupational Risk Assessment

The increasing technological applications of palladium nanoparticles (Pd-NPs) and their consequent enhancing release into the community and occupational environments, have raised public health concerns regarding possible adverse effects for exposed subjects, and particularly for workers chronically and highly exposed to these materials, whose toxico-kinetic and dynamic behavior remains to be fully understood. Therefore, this review aimed to critically analyze literature data to achieve a more comprehensive knowledge on the toxicological profile of Pd-NPs. Results from available studies demonstrated the potential for these chemicals to affect the ecosystem function, to exert cytotoxic and pro-inflammatory effects in vitro as well as to induce early alterations in different target organs in in vivo models. However, our revision pointed out the need for future studies aimed to clarify the role of the NP physico-chemical properties in determining their toxicological behavior, as well as the importance to carry out investigations focused on environmental and biological monitoring to verify and validate experimental biomarkers of exposure and early effect in real exposure contexts. Overall, this may be helpful to support the definition of suitable strategies for the assessment, communication and management of Pd-NP occupational risks to protect the health and safety of workers.

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

The palladized cell (Pd-cell) could be used as an efficient catalyst in catalyzing the degradations of a wide variety of environmental contaminants. Nevertheless, when the Pd NPs associate with the bacteria, the catalytic activity likely significantly affected by the biomass. Quantitative indicators that characterize of Pd-cell are necessary and little attention has been paid to investigate how the catalytic efficiency of Pd-cell is affected by the size and distribution of Pd NPs. To fill this gap, we explored the roles of the above-mentioned key factors on the performance of Pd-cell in catalyzing the degradations of two aromatic contaminants (nitrobenzene and p-chlorophenol) in two commonly used scenarios: (1) using Pd-cell as suspended catalyst in solution and (2) using Pd-cell as electrocatalyst directly coated on electrode. In scenario (1), the relationship of exposing area to Pd particle size and distribution factors was established. Based on theoretical estimation and catalytic performance analysis, the results indicated that adjusting the exposing area to a large value (9.3 ± 0.1 × 10⁵ nm² mg^-1 Pd) was extremely effective for improving the catalytic activity of Pd-cell used as a suspension catalyst. In scenario (2), our results showed that the best electrocatalytic performances were achieved on the electrode decorated with Pd-cells with the largest NP size (54.3 ± 16.4 nm), which exerted maximum electrochemical active surface area (10.6 m² g^-1) as well as favorable conductivity. The coverage of deposited Pd NPs (>95%) on the cell surface played a crucial role in boosting the conductivity of biocatalyst, thus determining the possibility of Pd-cell as an efficient electrocatalyst. The findings of this study provide a guidance for the synthesis and application of Pd-cell, which enables the design of Pd-cell to be suitable for different catalysis systems with high catalytic performance.

Reduction pathway and mechanism of chloronitrobenzenes synergistically catalyzed by bioPd and Shewanella oneidensis MR-1 assisted by calculation

Although microbial synthesized palladium nanoparticles (bioPd) have been demonstrated to exhibit a great activity toward dechlorination of several chlorinated pollutants, there is no systematic investigation into the substituent effect on dechlorination. Chloronitrobenzenes are widely used for manufacturing and known as persistent pollutants with recalcitrance of biodegradation for nitro groups. In this work, bioPd was synthesized by Shewanella oneidensis MR-1. The dechlorination of 2-chloronitrobenzene, 4-chloronitrobenzene and 2,4-dichloronitrobenzene catalyzed by bioPd were investigated. Simultaneous dechlorination and nitro reduction were observed by synergistic catalysis of bioPd and S. oneidensis MR-1. Pd concentration was optimized for the reduction. Producing profiles of intermediates changed with the ratio of Pd to cell, supporting a size- or shape-controlled catalytic activity of bioPd. The removal of chloro atoms at para-position was easier than that at ortho-position in 2,4-DCNB, suggesting a position effect on the reduction, which was further supported by the frontier molecular orbital and frontier electron density of 2,4-DCNB according to density functional theory.

Microbial synthesis of bimetallic PdPt nanoparticles for catalytic reduction of 4-nitrophenol

Bimetallic nanoparticles are generally believed to have improved catalytic activity and stability due to geometric and electronic changes. In this work, biogenic-Pd (bio-Pd), biogenic-Pt (bio-Pt), and biogenic-PdPt (bio-PdPt) nanoparticles were synthesized by Shewanella oneidensis MR-1 in the absence or presence of quinone. Compared with direct microbial reduction process, the addition of anthraquinone-2,6-disulfonate (AQDS) could promote the reduction efficiency of Pd(II) or/and Pt(IV) and result in decrease of particles size. All kinds of nanoparticles could catalyze 4-nitrophenol reduction by NaBH₄ and their catalytic activities took the following order: bio-PdPt (AQDS) ∼ bio-PdPt > bio-Pd (AQDS) > bio-Pd > bio-Pt (AQDS) ∼ bio-Pt. Moreover, the bio-PdPt (AQDS) nanoparticles could be reused for 6 cycles. We believe that this simple and efficient biosynthesis approach for synthesizing bimetallic bio-PdPt nanocatalysts is important for preparing active and stable catalysts.

Biogenic platinum and palladium nanoparticles as new catalysts for the removal of pharmaceutical compounds

Pharmaceutical products (PhP) are one of the most alarming emergent pollutants in the environment. Therefore, it is of extreme importance to investigate efficient PhP removal processes. Biologic synthesis of platinum nanoparticles (Bio-Pt) has been reported, but their catalytic activity was never investigated. In this work, we explored the potential of cell-supported platinum (Bio-Pt) and palladium (Bio-Pd) nanoparticles synthesized with Desulfovibrio vulgaris as biocatalysts for removal of four PhP: ciprofloxacin, sulfamethoxazole, ibuprofen and 17β-estradiol. The catalytic activity of the biological nanoparticles was compared with the PhP removal efficiency of D. vulgaris whole-cells. In contrast with Bio-Pd, Bio-Pt has a high catalytic activity in PhP removal, with 94, 85 and 70% removal of 17β-estradiol, sulfamethoxazole and ciprofloxacin, respectively. In addition, the estrogenic activity of 17β-estradiol was strongly reduced after the reaction with Bio-Pt, showing that this biocatalyst produces less toxic effluents. Bio-Pt or Bio-Pd did not act on ibuprofen, but this could be completely removed by D. vulgaris whole-cells, demonstrating that sulfate-reducing bacteria are among the microorganisms capable of biotransformation of ibuprofen in anaerobic environments. This study demonstrates for the first time that Bio-Pt has a high catalytic activity, and is a promising catalyst to be used in water treatment processes for the removal of antibiotics and endocrine disrupting compounds, the most problematic PhP.

Impact of bio-palladium nanoparticles (bio-Pd NPs) on the activity and structure of a marine microbial community

Biogenic palladium nanoparticles (bio-Pd NPs) represent a promising catalyst for organohalide remediation in water and sediments. However, the available information regarding their possible impact in case of release into the environment, particularly on the environmental microbiota, is limited. In this study the toxicity of bio-Pd NPs on the model marine bacterium V. fischeri was assessed. The impacts of different concentrations of bio-Pd NPs on the respiratory metabolisms (i.e. organohalide respiration, sulfate reduction and methanogenesis) and the structure of a PCB-dechlorinating microbial community enriched form a marine sediment were also investigated in microcosms mimicking the actual sampling site conditions. Bio-Pd NPs had no toxic effect on V. fischeri. In addition, they had no significant effects on PCB-dehalogenating activity, while showing a partial, dose-dependent inhibitory effect on sulfate reduction as well as on methanogenesis. No toxic effects by bio-Pd NPs could be also observed on the total bacterial community structure, as its biodiversity was increased compared to the not exposed community. In addition, resilience of the microbial community to bio-Pd NPs exposure was observed, being the final community organization (Gini coefficient) of samples exposed to bio-Pd NPs similar to that of the not exposed one. Considering all the factors evaluated, bio-Pd NPs could be deemed as non-toxic to the marine microbiota in the conditions tested. This is the first study in which the impact of bio-Pd NPs is extensively evaluated over a microbial community in relevant environmental conditions, providing important information for the assessment of their environmental safety.

Effect of oxidation and catalytic reduction of trace organic contaminants on their activated carbon adsorption

The combination of ozonation and activated carbon (AC) adsorption is an established technology for removal of trace organic contaminants (TrOCs). In contrast to oxidation, reduction of TrOCs has recently gained attention as well, however less attention has gone to the combination of reduction with AC adsorption. In addition, no literature has compared the removal behavior of reduction vs. ozonation by-products by AC. In this study, the effect of pre-ozonation vs pre-catalytic reduction on the AC adsorption efficiency of five TrOCs and their by-products was compared. All compounds were susceptible to oxidation and reduction, however the catalytic reductive treatment proved to be a slower reaction than ozonation. New oxidation products were identified for dinoseb and new reduction products were identified for carbamazepine, bromoxynil and dinoseb. In terms of compatibility with AC adsorption, the influence of the oxidative and reductive pretreatments proved to be compound dependent. Oxidation products of bromoxynil and diatrizoic acid adsorbed better than their parent TrOCs, but oxidation products of atrazine, carbamazepine and dinoseb showed a decreased adsorption. The reductive pre-treatment showed an enhanced AC adsorption for dinoseb and a major enhancement for diatrizoic acid. For atrazine and bromoxynil, no clear influence on adsorption was noted, while for carbamazepine, the reductive pretreatment resulted in a decreased AC affinity. It may thus be concluded that when targeting mixtures of TrOCs, a trade-off will undoubtedly have to be made towards overall reactivity and removal of the different constituents, since no single treatment proves to be superior to the other.

Platinum recovery from industrial process streams by halophilic bacteria: Influence of salt species and platinum speciation

The increased use and criticality of platinum asks for the development of effective low-cost strategies for metal recovery from process and waste streams. Although biotechnological processes can be applied for the valorization of diluted aqueous industrial streams, investigations considering real stream conditions (e.g., high salt levels, acidic pH, metal speciation) are lacking. This study investigated the recovery of platinum by a halophilic microbial community in the presence of increased salt concentrations (10-80 g L^-1), different salt matrices (phosphate salts, sea salts and NH₄Cl) and a refinery process stream. The halophiles were able to recover 79-99% of the Pt at 10-80 g L^-1 salts and at pH 2.3. Transmission electron microscopy suggested a positive correlation between intracellular Pt cluster size and elevated salt concentrations. Furthermore, the halophiles recovered 46-95% of the Pt-amine complex Pt[NH₃]₄²⁺ from a process stream after the addition of an alternative Pt source (K₂PtCl₄, 0.1-1.0 g L^-1 Pt). Repeated Pt-tetraamine recovery (from an industrial process stream) was obtained after concomitant addition of fresh biomass and harvesting of Pt saturated biomass. This study demonstrates how aqueous Pt streams can be transformed into Pt rich biomass, which would be an interesting feed of a precious metals refinery.

Biogenic FeS accelerates reductive dechlorination of carbon tetrachloride by Shewanella putrefaciens CN32

Dissimilatory metal reducing bacteria (DMRB) widely exist in the subsurface environment and are involved in various contaminant degradation and element geochemical cycling processes. Recent studies suggest that DMRB can biosynthesize metal nanoparticles during metal reduction, but it is unclear yet how such biogenic nanomaterials would affect their decontamination behaviors. In this study, we found that the dechlorination rates of carbon tetrachloride (CT) by Shewanella putrefaciens CN32 was significantly increased by 8 times with the formation of biogenic ferrous sulfide (FeS) nanoparticles. The pasteurized biogenic FeS enabled 5 times faster dechlorination than abiotic FeS that had larger sizes and irregular structure, confirming a significant contribution of the biogenic FeS to CT bioreduction resulting from its good dispersion and relatively high dechlorination activity. This study highlights a potentially important role of biosynthesized nanoparticles in environmental bioremediation.

A Post-Genomic View of the Ecophysiology, Catabolism and Biotechnological Relevance of Sulphate-Reducing Prokaryotes

Dissimilatory sulphate reduction is the unifying and defining trait of sulphate-reducing prokaryotes (SRP). In their predominant habitats, sulphate-rich marine sediments, SRP have long been recognized to be major players in the carbon and sulphur cycles. Other, more recently appreciated, ecophysiological roles include activity in the deep biosphere, symbiotic relations, syntrophic associations, human microbiome/health and long-distance electron transfer. SRP include a high diversity of organisms, with large nutritional versatility and broad metabolic capacities, including anaerobic degradation of aromatic compounds and hydrocarbons. Elucidation of novel catabolic capacities as well as progress in the understanding of metabolic and regulatory networks, energy metabolism, evolutionary processes and adaptation to changing environmental conditions has greatly benefited from genomics, functional OMICS approaches and advances in genetic accessibility and biochemical studies. Important biotechnological roles of SRP range from (i) wastewater and off gas treatment, (ii) bioremediation of metals and hydrocarbons and (iii) bioelectrochemistry, to undesired impacts such as (iv) souring in oil reservoirs and other environments, and (v) corrosion of iron and concrete. Here we review recent advances in our understanding of SRPs focusing mainly on works published after 2000. The wealth of publications in this period, covering many diverse areas, is a testimony to the large environmental, biogeochemical and technological relevance of these organisms and how much the field has progressed in these years, although many important questions and applications remain to be explored.

Recovery of critical metals using biometallurgy

The increased development of green low-carbon energy technologies that require platinum group metals (PGMs) and rare earth elements (REEs), together with the geopolitical challenges to sourcing these metals, has spawned major governmental and industrial efforts to rectify current supply insecurities. As a result of the increasing critical importance of PGMs and REEs, environmentally sustainable approaches to recover these metals from primary ores and secondary streams are needed. In this review, we define the sources and waste streams from which PGMs and REEs can potentially be sustainably recovered using microorganisms, and discuss the metal-microbe interactions most likely to form the basis of different environmentally friendly recovery processes. Finally, we highlight the research needed to address challenges to applying the necessary microbiology for metal recovery given the physical and chemical complexities of specific streams.

Synthesis and characterization of palladium nanoparticles using Catharanthus roseus leaf extract and its application in the photo-catalytic degradation

The potential effect of Catharanthus roseus leaf extract for the formation of palladium nanoparticles and its application on dye degradation was discussed. The efficiency of C.roseus leaves are used as a bio-material for the first time as reducing agent. Synthesized palladium nanoparticles were supported by UV-vis spectrometry, XRD, FT-IR and TEM analysis. The secondary metabolites which are responsible for the formation of nanoparticles were identified by GC-MS. The results showed that effect of time was directly related to synthesized nanoparticles and functional groups has a critical role in reducing the metal ions and stabilizing the palladium nanoparticles in an eco-friendly process.

Synthesis of novel palladium(0) nanocatalysts by microorganisms from heavy-metal-influenced high-alpine sites for dehalogenation of polychlorinated dioxins

In a search for new aqueous-phase systems for catalyzing reactions of environmental and industrial importance, we prepared novel biogenerated palladium (Pd) nanocatalysts using a "green" approach based on microorganisms isolated from high-alpine sites naturally impacted by heavy metals. Bacteria and fungi were enriched and isolated from serpentinite-influenced ponds (Totalp region, Parsenn, near Davos, Graubünden, Switzerland). Effects on growth dynamics were monitored using an automated assay in 96-well microtiter plates, which allowed for simultaneous cultivation and on-line analysis of Pd(II)- and Ni(II)-mediated growth inhibition. Microorganisms from Totalp ponds tolerated up to 3mM Pd(II) and bacterial isolates were selected for cultivation and reductive synthesis of Pd(0) nanocatalysts at microbial interfaces. During reduction of Pd(II) with formate as the electron donor, Pd(0) nanoparticles were formed and deposited in the cell envelope. The Pd(0) catalysts produced in the presence of Pd(II)-tolerant Alpine Pseudomonas species were catalytically active in the reductive dehalogenation of model polychlorinated dioxin congeners. This is the first report which shows that Pd(0) synthesized in the presence of microorganisms catalyzes the reductive dechlorination of polychlorinated dibenzo-p-dioxins (PCDDs). Because the "bioPd(0)" catalyzed the dechlorination reactions preferably via non-lateral chlorinated intermediates, such a pathway could potentially detoxify PCDDs via a "safe route". It remains to be determined whether the microbial formation of catalytically active metal catalysts (e.g., Zn, Ni, Fe) occurs in situ and whether processes involving such catalysts can alter the fate and transport of persistent organic pollutants (POPs) in Alpine habitats.

Applications of biosynthesized metallic nanoparticles - a review

We present a comprehensive review of the applications of biosynthesized metallic nanoparticles (NPs). The biosynthesis of metallic NPs is the subject of a number of recent reviews, which focus on the various "bottom-up" biofabrication methods and characterization of the final products. Numerous applications exploit the advantages of biosynthesis over chemical or physical NP syntheses, including lower capital and operating expenses, reduced environmental impacts, and superior biocompatibility and stability of the NP products. The key applications reviewed here include biomedical applications, especially antimicrobial applications, but also imaging applications, catalytic applications such as reduction of environmental contaminants, and electrochemical applications including sensing. The discussion of each application is augmented with a critical review of the potential for continued development.

Supported and unsupported nanomaterials for water and soil remediation: are they a useful solution for worldwide pollution?

Remediation technologies for wastes generated by industrial processes include coagulation, reverse osmosis, electrochemistry, photoelectrochemistry, advanced oxidation processes, and biological methods, among others. Adsorption onto activated carbon, sewage sludge, zeolites, chitosan, silica, and agricultural wastes has shown potential for pollutants' removal from aqueous media. Recently, nanoscale systems [nanoparticles (NPs) supported on different inorganic adsorbents] have shown additional benefits for the removal/degradation of several contaminants. According to the literature, NPs enhance the adsorption capacity of adsorbent materials and facilitate degradation of pollutants through redox reactions. In this review we analyzed relevant literature from 2011 to 2013, dealing with water and soil remediation by nanomaterials (NMs), either unsupported or supported upon inorganic adsorbents. Despite the outstanding reported results for some NMs, the analysis of the literature makes clear the necessity of more studies. There is lack of information about NMs regeneration and reusability, their large-scale application, and their efficiency in actual industrial wastewaters and contaminated soils. Additionally, little is known about NMs' life cycle, release of metal ions, disposal of pollutant loaded NMs, and their impacts on different ecosystems.

Biogenic synthesis of palladium nanoparticles using Pulicaria glutinosa extract and their catalytic activity towards the Suzuki coupling reaction

Biosynthesized Pd NPs: wonderful catalyst towards the Suzuki coupling reaction under aerobic conditions.

Biotechnologies for critical raw material recovery from primary and secondary sources: R&D priorities and future perspectives

Europe is confronted with an increasing supply risk of critical raw materials. These can be defined as materials of which the risks of supply shortage and their impacts on the economy are higher compared to most of other raw materials. Within the framework of the EU Innovation Partnership on raw materials Initiative, a list of 14 critical materials was defined, including some bulk metals, industrial minerals, the platinum group metals and rare earth elements. To tackle the supply risk challenge, innovation is required with respect to sustainable primary mining, substitution of critical metals, and urban mining. In these three categories, biometallurgy can play a crucial role. Indeed, microbe-metal interactions have been successfully applied on full scale to win materials from primary sources, but are not sufficiently explored for metal recovery or recycling. On the one hand, this article gives an overview of the microbial strategies that are currently applied on full scale for biomining; on the other hand it identifies technologies, currently developed in the laboratory, which have a perspective for large scale metal recovery and the needs and challenges on which bio-metallurgical research should focus to achieve this ambitious goal.

Microbial formation of palladium nanoparticles by Geobacter sulfurreducens for chromate reduction

Geobacter sulfurreducens was studied for the reduction of Pd(II) and production of Pd(0) nanoparticles capable of reducing Cr(VI). Transmission electronic microscopy, energy dispersive X-ray and X-ray diffraction analyses revealed that the nanoscale Pd(0) particles formed were associated with the cell surface and located inside the periplasm. The increase of cell dry weight (CDW):Pd ratio and addition of anthraquinone-2,6-disulfonate (AQDS) not only stimulated Pd(II) reduction, but also resulted in increase of nanoparticle number, decrease of particle diameter and improvement of Cr(VI) reduction efficiency. The relationship between reduction rate and initial Cr(VI) concentration (150-750 μM) followed Michaelis-Menten kinetics (Vmax=3.6 μmol h(-1) mg bio-Pd(-1) and Km=891.3 μM). These findings indicated the potential of using G. sulfurreducens cells for reclamation of palladium, formation of Pd(0) nanoparticles and efficient treatment of Cr(VI) pollution.