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Green synthesis and environmental applications of biogenic Pd nanoparticles

Tom Hennebel (UGent)
(2010)
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(UGent) and (UGent)
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Abstract
Over the last 100 years, the global water consumption has doubled every 20 years to result in an average annual use of 1500 m3 per capita. Due to the increasing population, the supply of safe water will be of great significance in the future. The depletion of non-contaminated groundwater sources is placing an increasing importance on water reuse for domestic, industrial and agricultural purposes and as a result, there is an acute need for sustainable water treatment technologies. Most of the current wastewater technologies are primarily designed to remove nutrients (COD, nitrogen and phosphorus) but do not effectively remove more recalcitrant xenobiotics. Moreover, because these compounds are mostly unaffected by current wastewater treatment processes, xenobiotics are expected to accumulate in water resources over the coming decades. The halogenated compounds that belong to this class of very recalcitrant and hardly biodegradable compounds can be found in every environmental compartment, where their concentration depends on their physico-chemical characteristics, the type of application and the properties of the aqueous matrix. For example, groundwater can contain high concentrations of chlorinated solvents or pesticides as a result of spills during production or leakage from underground storage tanks. At the other end of the concentration spectrum, x-ray contrast compounds, used in hospitals for imaging, are found in surface waters at a concentration range of pg L-1 to μg L-1. Furthermore, there is an increasing concern about these halogenated pollutants as many of them also exert human toxicity. It is clear that a general removal strategy, which can tackle a broad spectrum of these recalcitrant halogenated compounds at both extremely low and very high concentrations in various aqueous matrices, is required. Monometallic and bimetallic catalysts of palladium have shown their value in the dehalogenation of all kinds of pollutants. Nanoscale forms of these catalysts have demonstrated especially high activity in hydrodehalogenation reactions, presumably due to their large specific surface areas. A new biologically inspired method involves the precipitation of palladium on a bacterium to produce a nanopalladium catalyst, i.e. bio-Pd. When a hydrogen donor is provided, bacteria can reduce Pd(II) and subsequently precipitate it as Pd nanocrystals on their cell wall and in their periplasmatic space. None of the expensive or harmful compounds, currently used for the abiotic chemical synthesis of Pd nanoparticles, are required because bacteria serve as reductant for the Pd salt and as scaffold for the Pd nanoparticles. Therefore, the production of bio-Pd can be considered as a green technology. Although the production of biogenic nanoparticles and their use as a catalyst in batch tests was previously reported, their potential application for water treatment purposes was thus far not addressed. Therefore, seven research chapters were elaborated in this work to develop reactor technologies based on the bio-Pd concept to treat aqueous streams contaminated with halogenated compounds. One important focus of this work was on methods to keep the nanoparticles separated from the reactor effluents. Indeed, Pd is a very expensive metal and its loss from reactors can largely influence the costs of the sanitation technology. Moreover, since not much is known about the toxicity of these biogenic nanoparticles, precautions need to be taken to limit their release to receiving waters. The selection of the electron donor in the dehalogenation reaction was determined as one of the main reactor parameters influencing both the degradation rates and the costs of the process. Therefore, a second focus was on limiting the sustainable supply of hydrogen gas as electron donor. The studied bio-Pd applications were not only used to treat synthetic wastewater but also groundwater from natural aquifers. In a first reactor system (Chapters 2 and 8), the bio-Pd nanoparticles were retained by plate membranes with a pore size of 0.4 μm. The membranes were specifically designed to allow build up of a shallow biofilm layer at the membrane surface, allowing retention of particles with a diameter as small as 0.01 μm. Bio-Pd was effectively retained because the size of the bacteria and the bio-Pd nanocatalyst amount approximately to 1 μm and 50 nm, respectively. The bacterial carrier provided an efficient way to keep the Pd nanoparticles in suspension and in the reactor without aggregation of the Pd and hence obtaining a maximal contact between catalyst and pollutant. Removal rates up to 2.5 g TCE day-1 g-1 Pd were achieved with H2 gas as electron donor and ethane and chloride were produced as harmless endproducts. Fixed and fluidized bed reactors with encapsulated bio-Pd can offer an economical interesting alternative to membrane reactors, which have the additional drawbacks of their price, their sensitivity towards fouling and the difficulty to treat large volumes of groundwater at high flow rates. Therefore, several polymers, which are often used for the encapsulation of bacteria, were tested to produce catalyst beads for use in fixed or fluidized bed reactors (Chapters 3 and 4). The primary objective of this research was to stabilize bio-Pd while minimizing the loss of activity in dehalogenation reactions. Polyurethane cubes empowered with bio-Pd were implemented in a lab-scale fixed bed reactor for the treatment of water containing TCE. Removal rates up to 1.06 g TCE g Pd-1 d-1 were obtained in a flow through configuration of this reactor (Chapter 3). A pilot-scale (200 L) fluidized bed reactor, supplied with bio-Pd encapsulated in alginate beads, was developed for the treatment of groundwater containing an historical pollution of lindane and chlorobenzenes (Chapter 4). Removal percentages up to 75 % of the HCH isomers and 68 % of the chlorobenzenes were obtained applying a hydraulic residence time of 8 hours. The amount of hydrogen gas used in both reactor technologies (Chapter 2, 3 and 4) was much more than the stoichiometrically required amount for TCE or lindane dehalogenation. This was partially due to the low hydrogen gas solubility (0.8 mM at 20°C and 100 kPa) and partially due to the reactor setup. Therefore, two other reactor types, i.e. a membrane contactor and a microbial electrolysis cell (MEC) were constructed in order to limit the costs related to the hydrogen gas supply. In the membrane contactor, polyvinylidene fluoride membranes encapsulating bio-Pd were positioned between a gas compartment providing hydrogen gas and an aqueous compartment containing the halogenated contaminant. When operated in a fed batch configuration, a removal efficiency of 77% of the iodinated contrast medium diatrizoate was obtained after a time period of 48 h. The goal of the MEC was to produce in situ hydrogen gas in a more sustainable way. When the cathode granules were coated with 5 mg bio-Pd g-1 graphite, removal rates of 151 g TCE m-3 total cathodic compartment d-1 and 48 mg TCE g-1 Pd d-1 were obtained. Chloride and ethane were the main endproducts and formation of unwanted byproducts such as vinyl chloride was not significant. When the same setup was applied for the transformation of diatrizoate, a removal rate of 12 mg g-1 Pd d-1 with effluent concentrations under the detection limit was obtained. It is expected that the use of bioelectrochemically produced hydrogen gas can decrease the costs of the hydrogen with a factor of at least 2-4. Another sustainable source of hydrogen gas as electron donor could be microbiologically produced during the fermentation of a carbon source. It was not only shown that fermentative bacteria are capable of Pd nanoparticle formation but also of charging these nanoparticles with reactive hydrogen species (Chapters 7 and 8). In a first remediation strategy, the in situ formation of bio-Pd and the concomitant reduction of the toxic groundwater pollutant Cr(VI) was demonstrated in aquifer soil columns inoculated with Clostridium pasteurianum. In a second strategy, Citrobacter braakii was used to produce bio-Pd as catalyst and biogenic hydrogen as electron donor to remove diatrizoate. A membrane bioreactor supplied with this type of bio-Pd was capable to dehalogenate 22 mg diatrizoate mg-1 Pd over a period of 19 days before their biocatalytic activity was exhausted. This work demonstrates the general applicability of fermentative bacteria to generate catalytic Pd(0) nanoparticles and hydrogen in a one-step process. These biomass-associated nanoparticles can be used to dehalogenate iodinated contrast media and chlorinated solvents without an external source of hydrogen. The suggested one-step process can be applied in MBRs to treat aqueous streams of industrial effluent and as part of pump-and-treat systems. This work also shows the potential use of fermentative bacteria to co-generate catalytic nanoparticles and reductive hydrogen within contaminated aquifers. This work shows the general applicability of biogenic Pd nanoparticles to treat water contaminated with halogenated contaminants at large scale. Several reactor technologies, using bio-Pd as catalyst, were developed. Additionally, a process that sustainably produces hydrogen gas as electron donor was demonstrated. Currently, the largest market for these technologies is the treatment of groundwater contaminated with halogenated solvents and sludge contaminated with PCBs. Regarding this type of pollution, a pump and treat technology based on the bio-Pd system is currently the most developed in terms of economical feasibility and size (pilot-scale). However, it is believed that bio-Pd based techniques can also play a role in the search for methods to produce reusable gray water or even safe drinking by removing very recalcitrant micropollutants (e.g. diatrizoate) from water or industrial effluents. The most effective bio-Pd technology can be selected to fit a given contamination problem (Chapter 9). Furthermore, the selection of the bacterium for the bio-Pd production process and the reactor type can be based both on economical and green considerations. The implementation of bio-Pd into the market presents a number of challenges. Most significantly, further research is needed to optimize the amount of catalyst and electron donor required, given that both substrates significantly impact the process economics. Research will need to focus on more active catalysts such as biogenic bimetals to reduce the amount of Pd required. The research presented in this thesis, however, provides the foundation for the development of a bio-Pd based technology that is cost competitive and more effective than currently available clean-up strategies.

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Citation

Please use this url to cite or link to this publication:

Chicago
Hennebel, Tom. 2010. “Green Synthesis and Environmental Applications of Biogenic Pd Nanoparticles”. Ghent, Belgium: Ghent University. Faculty of Bioscience Engineering.
APA
Hennebel, T. (2010). Green synthesis and environmental applications of biogenic Pd nanoparticles. Ghent University. Faculty of Bioscience Engineering, Ghent, Belgium.
Vancouver
1.
Hennebel T. Green synthesis and environmental applications of biogenic Pd nanoparticles. [Ghent, Belgium]: Ghent University. Faculty of Bioscience Engineering; 2010.
MLA
Hennebel, Tom. “Green Synthesis and Environmental Applications of Biogenic Pd Nanoparticles.” 2010 : n. pag. Print.
@phdthesis{1040909,
  abstract     = {Over the last 100 years, the global water consumption has doubled every 20 years to result in an average annual use of 1500 m3 per capita. Due to the increasing population, the supply of safe water will be of great significance in the future. The depletion of non-contaminated groundwater sources is placing an increasing importance on water reuse for domestic, industrial and agricultural purposes and as a result, there is an acute need for sustainable water treatment technologies. Most of the current wastewater technologies are primarily designed to remove nutrients (COD, nitrogen and phosphorus) but do not effectively remove more recalcitrant xenobiotics. Moreover, because these compounds are mostly unaffected by current wastewater treatment processes, xenobiotics are expected to accumulate in water resources over the coming decades. The halogenated compounds that belong to this class of very recalcitrant and hardly biodegradable compounds can be found in every environmental compartment, where their concentration depends on their physico-chemical characteristics, the type of application and the properties of the aqueous matrix. For example, groundwater can contain high concentrations of chlorinated solvents or pesticides as a result of spills during production or leakage from underground storage tanks. At the other end of the concentration spectrum, x-ray contrast compounds, used in hospitals for imaging, are found in surface waters at a concentration range of pg L-1 to \ensuremath{\mu}g L-1. Furthermore, there is an increasing concern about these halogenated pollutants as many of them also exert human toxicity. It is clear that a general removal strategy, which can tackle a broad spectrum of these recalcitrant halogenated compounds at both extremely low and very high concentrations in various aqueous matrices, is required. 
Monometallic and bimetallic catalysts of palladium have shown their value in the dehalogenation of all kinds of pollutants. Nanoscale forms of these catalysts have demonstrated especially high activity in hydrodehalogenation reactions, presumably due to their large specific surface areas. A new biologically inspired method involves the precipitation of palladium on a bacterium to produce a nanopalladium catalyst, i.e. bio-Pd. When a hydrogen donor is provided, bacteria can reduce Pd(II) and subsequently precipitate it as Pd nanocrystals on their cell wall and in their periplasmatic space. None of the expensive or harmful compounds, currently used for the abiotic chemical synthesis of Pd nanoparticles, are required because bacteria serve as reductant for the Pd salt and as scaffold for the Pd nanoparticles. Therefore, the production of bio-Pd can be considered as a green technology.
Although the production of biogenic nanoparticles and their use as a catalyst in batch tests was previously reported, their potential application for water treatment purposes was thus far not addressed. Therefore, seven research chapters were elaborated in this work to develop reactor technologies based on the bio-Pd concept to treat aqueous streams contaminated with halogenated compounds. One important focus of this work was on methods to keep the nanoparticles separated from the reactor effluents. Indeed, Pd is a very expensive metal and its loss from reactors can largely influence the costs of the sanitation technology. Moreover, since not much is known about the toxicity of these biogenic nanoparticles, precautions need to be taken to limit their release to receiving waters. The selection of the electron donor in the dehalogenation reaction was determined as one of the main reactor parameters influencing both the degradation rates and the costs of the process. Therefore, a second focus was on limiting the sustainable supply of hydrogen gas as electron donor. The studied bio-Pd applications were not only used to treat synthetic wastewater but also groundwater from natural aquifers.
In a first reactor system (Chapters 2 and 8), the bio-Pd nanoparticles were retained by plate membranes with a pore size of 0.4 \ensuremath{\mu}m. The membranes were specifically designed to allow build up of a shallow biofilm layer at the membrane surface, allowing retention of particles with a diameter as small as 0.01 \ensuremath{\mu}m. Bio-Pd was effectively retained because the size of the bacteria and the bio-Pd nanocatalyst amount approximately to 1 \ensuremath{\mu}m and 50 nm, respectively. The bacterial carrier provided an efficient way to keep the Pd nanoparticles in suspension and in the reactor without aggregation of the Pd and hence obtaining a maximal contact between catalyst and pollutant. Removal rates up to 2.5 g TCE day-1 g-1 Pd were achieved with H2 gas as electron donor and ethane and chloride were produced as harmless endproducts.
Fixed and fluidized bed reactors with encapsulated bio-Pd can offer an economical interesting alternative to membrane reactors, which have the additional drawbacks of their price, their sensitivity towards fouling and the difficulty to treat large volumes of groundwater at high flow rates. Therefore, several polymers, which are often used for the encapsulation of bacteria, were tested to produce catalyst beads for use in fixed or fluidized bed reactors (Chapters 3 and 4). The primary objective of this research was to stabilize bio-Pd while minimizing the loss of activity in dehalogenation reactions. Polyurethane cubes empowered with bio-Pd were implemented in a lab-scale fixed bed reactor for the treatment of water containing TCE. Removal rates up to 1.06 g TCE g Pd-1 d-1 were obtained in a flow through configuration of this reactor (Chapter 3). A pilot-scale (200 L) fluidized bed reactor, supplied with bio-Pd encapsulated in alginate beads, was developed for the treatment of groundwater containing an historical pollution of lindane and chlorobenzenes (Chapter 4). Removal percentages up to 75 \% of the HCH isomers and 68 \% of the chlorobenzenes were obtained applying a hydraulic residence time of 8 hours.
The amount of hydrogen gas used in both reactor technologies (Chapter 2, 3 and 4) was much more than the stoichiometrically required amount for TCE or lindane dehalogenation. This was partially due to the low hydrogen gas solubility (0.8 mM at 20{\textdegree}C and 100 kPa) and partially due to the reactor setup. Therefore, two other reactor types, i.e. a membrane contactor and a microbial electrolysis cell (MEC) were constructed in order to limit the costs related to the hydrogen gas supply. In the membrane contactor, polyvinylidene fluoride membranes encapsulating bio-Pd were positioned between a gas compartment providing hydrogen gas and an aqueous compartment containing the halogenated contaminant. When operated in a fed batch configuration, a removal efficiency of 77\% of the iodinated contrast medium diatrizoate was obtained after a time period of 48 h. The goal of the MEC was to produce in situ hydrogen gas in a more sustainable way. When the cathode granules were coated with 5 mg bio-Pd g-1 graphite, removal rates of 151 g TCE m-3 total cathodic compartment d-1 and 48 mg TCE g-1 Pd d-1 were obtained. Chloride and ethane were the main endproducts and formation of unwanted byproducts such as vinyl chloride was not significant. When the same setup was applied for the transformation of diatrizoate, a removal rate of 12 mg g-1 Pd d-1 with effluent concentrations under the detection limit was obtained. It is expected that the use of bioelectrochemically produced hydrogen gas can decrease the costs of the hydrogen with a factor of at least 2-4.
Another sustainable source of hydrogen gas as electron donor could be microbiologically produced during the fermentation of a carbon source. It was not only shown that fermentative bacteria are capable of Pd nanoparticle formation but also of charging these nanoparticles with reactive hydrogen species (Chapters 7 and 8). In a first remediation strategy, the in situ formation of bio-Pd and the concomitant reduction of the toxic groundwater pollutant Cr(VI) was demonstrated in aquifer soil columns inoculated with Clostridium pasteurianum. In a second strategy, Citrobacter braakii was used to produce bio-Pd as catalyst and biogenic hydrogen as electron donor to remove diatrizoate. A membrane bioreactor supplied with this type of bio-Pd was capable to dehalogenate 22 mg diatrizoate mg-1 Pd over a period of 19 days before their biocatalytic activity was exhausted. This work demonstrates the general applicability of fermentative bacteria to generate catalytic Pd(0) nanoparticles and hydrogen in a one-step process. These biomass-associated nanoparticles can be used to dehalogenate iodinated contrast media and chlorinated solvents without an external source of hydrogen. The suggested one-step process can be applied in MBRs to treat aqueous streams of industrial effluent and as part of pump-and-treat systems. This work also shows the potential use of fermentative bacteria to co-generate catalytic nanoparticles and reductive hydrogen within contaminated aquifers.
This work shows the general applicability of biogenic Pd nanoparticles to treat water contaminated with halogenated contaminants at large scale. Several reactor technologies, using bio-Pd as catalyst, were developed. Additionally, a process that sustainably produces hydrogen gas as electron donor was demonstrated. Currently, the largest market for these technologies is the treatment of groundwater contaminated with halogenated solvents and sludge contaminated with PCBs. Regarding this type of pollution, a pump and treat technology based on the bio-Pd system is currently the most developed in terms of economical feasibility and size (pilot-scale). However, it is believed that bio-Pd based techniques can also play a role in the search for methods to produce reusable gray water or even safe drinking by removing very recalcitrant micropollutants (e.g. diatrizoate) from water or industrial effluents. 
The most effective bio-Pd technology can be selected to fit a given contamination problem (Chapter 9). Furthermore, the selection of the bacterium for the bio-Pd production process and the reactor type can be based both on economical and green considerations. The implementation of bio-Pd into the market presents a number of challenges. Most significantly, further research is needed to optimize the amount of catalyst and electron donor required, given that both substrates significantly impact the process economics. Research will need to focus on more active catalysts such as biogenic bimetals to reduce the amount of Pd required. The research presented in this thesis, however, provides the foundation for the development of a bio-Pd based technology that is cost competitive and more effective than currently available clean-up strategies.},
  author       = {Hennebel, Tom},
  isbn         = {9789059893931},
  language     = {eng},
  pages        = {VI, 187},
  publisher    = {Ghent University. Faculty of Bioscience Engineering},
  school       = {Ghent University},
  title        = {Green synthesis and environmental applications of biogenic Pd nanoparticles},
  year         = {2010},
}