Immobilized Nanomaterials for Energy Production: Comparison
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Please note this is a comparison between Version 2 by Sirius Huang and Version 1 by Luis Adrián Ramírez Montoya.

Nanomaterials (NMs) have been extensively used in several environmental applications; however, their widespread dissemination at full scale is hindered by difficulties keeping them active in engineered systems. Thus, several strategies to immobilize NMs for their environmental utilization have been established and are described in the present rteviewxt, emphasizing their role in the production of renewable energies, the removal of priority pollutants, as well as greenhouse gases, from industrial streams, by both biological and physicochemical processes. The challenges to optimize the application of immobilized NMs and the relevant research topics to consider in future research are also presented to encourage the scientific community to respond to current needs.

  • biodegradation
  • bioenergy production
  • nanomaterials
  • greenhouse gases
  • industrial wastewater treatment
  • recalcitrant contaminants

1. Introduction

The superior catalytic properties of nanomaterials (NMs) as compared to their bulk precursors have encouraged their application in several fields, including for environmental purposes, such as the removal of persistent contaminants, stimulation of microbial processes for bioenergy production, and reduction of greenhouse gas (GHG) emissions [1][2]. Nevertheless, NMs cannot be used in suspension in engineered systems since their recovery is expensive and unreasonable in post-treatments in full-scale operations. Furthermore, the discharge of NMs to environmental reservoirs represents a serious risk, and their use in engineered systems could be counterproductive, since many of these manufactured materials may have environmental and public health consequences, such as genotoxicity and cytotoxicity [3]. Accordingly, strategies to immobilize NMs in treatment systems for several distinct applications have been developed. These immobilizing techniques are particularly relevant for environmental purposes. Among the main advantages related to the use of immobilized NMs as catalysts are: (1) facile separation and retention of the catalysts in the reaction system; (2) continuous operation proceeds smoothly; and (3) the combined effects of catalysis and adsorption with NMs cover both commitments. Nevertheless, some disadvantages could also arise from the use of immobilized NMs which need to be addressed in future works, such as deactivation, poisoning, or washout of the catalysts, as well as mass-transfer limitations [4].  The purpose of this rteviewxt is to provide an overview of the main strategies to immobilize NMs for their application in different environmental engineered processes. Particularly, this papere next section summarizes the application of immobilized NMs in the production of renewable energies, as well as the removal of recalcitrant pollutants and GHGs from industrial discharges. 

2. Immobilized NMs for Energy Production

2.1. Biofuels

Global warming caused by anthropogenic emissions associated with the conventional production of fuels has exacerbated the living conditions of our planet, putting at risk the sustainability required for future generations. Consequently, the introduction of renewable energy sources to replace fossil fuels will play a pivotal role in tackling this menace. Methane is a renewable energy source, which can be sustainably obtained in engineered systems during the treatment of industrial wastewaters, as well as solid residues, such as animal manure or the organic fraction of municipal solid waste, by a process denominated anaerobic digestion (AD) [5]. AD comprises a complex route in which several anaerobic microorganisms are responsible for converting organic substrates into biogas under anaerobic conditions. In the last few years, the role of conductive NMs on AD processes has received a lot of attention because their application to anaerobic bioreactors has significantly increased the methane production from synthetic and real wastewaters [5]. NMs applied in these studies include carbon-based materials, such as graphene oxide (GO) and carbon nanotubes (CNT), in addition to metallic NMs, such as magnetite, nano zero-valent iron (nZVI), silver, and different iron oxides [6].
Recently, direct interspecies electron transfer (DIET) has been reported involving the formation of an electric current between electron-donating and electron-accepting microorganisms. For example, DIET has been shown to occur in cocultures of Geobacter species and acetoclastic methanogens, such as Methanosarcina sp., and this electron-transferring mechanism has been pointed out as one of the reasons for the improved performance of anaerobic bioreactors for producing methane in the presence of conductive materials (CM) [7]. Certainly, it was shown that the lack of pili and other cellular structures involved in electron transfer in microbial consortia can be compensated by the presence of CM [7][8]. Nevertheless, additional studies have indicated that the input of CM on AD processes goes beyond the stimulation of DIET because electric conductivity is not the only relevant parameter [6][9]. In fact, several physical-chemical properties of NMs, such as particle size, functional group, and surface area, drive the interactions between these materials and anaerobic microorganisms, ultimately determining the performance of anaerobic bioreactors during the production of methane from wastewater and solid residues. For instance, oxidized functional groups in GO promoted strong interaction between this material and anaerobic microorganisms, in a methanogenic consortium, causing the wrapping of cells and, consequently triggering mass-transfer limitations during the conversion of complex substrates, such as starch, which was reflected in a lower production of biogas [10]. However, when GO was applied in its reduced form (GOr) to the same consortium, this CM did not cover the cells and stimulated a greater production of methane [11].
Efforts to apply immobilized NMs to improve the production of methane from wastewater have recently been reported. For instance, nano-magnetite was fixed in granular activated carbon (GAC) for its subsequent application for the anaerobic treatment of low-strength wastewater [12]. The synthesized composite was mixed with anaerobic sludge in a bioreactor, which showed superior conductivity and electron-transferring capacity, ultimately fueling 3.6-fold higher methane production, as compared to the control bioreactor. Interestingly, the application of immobilized nano-magnetite triggered a higher abundance of functional microorganisms. Additionally, a strategy for in situ formation and self-immobilization of biogenic iron oxides in anaerobic granular sludge revealed an effective improvement of the production of methane, which was attributed to increased conductivity and stimulated growth of exoelectrogens (i.e., microorganisms with the capacity to donate electrons, such as Clostridium) and endoelectrogens (i.e., microorganisms with the ability to accept electrons, such as Methanosaeta) [13]. More recently, GO was immobilized in organic xerogels, which were applied for the treatment of protein-rich wastewater. The addition of this composite promoted higher methane production and the superior removal of organic matter and ammonium, as well as the production of medium chain fatty acids [14].
Hydrogen (H2) is the only carbon-free fuel available, and is expected to play a crucial role in the future energy market due to its high energy content and clean combustion product [15]. H2 can also be obtained by anaerobic consortia under more controlled conditions (dark fermentation). Furthermore, several metallic NMs, such as nanoparticles (NPs) of Ag, Au, Ni, and Fe2O3, have been shown to enhance H2 production in terms of production rate, as well as yield [16][17][18][19]; thus, it is conceivable that these NMs could also be immobilized and applied in dark fermentation processes for improving the production of this green biofuel. Besides this, the immobilization of Enterobacter aerogenes on CNT improved the H2 production rate and yield as compared to the performance observed with free cells [20].
Magnetic nanoparticles (MNPs) represent another efficient way to keep catalysts immobilized in bioreactors, and has successfully been applied for improving AD processes [6]. Besides this, MNPs have also been extensively explored as a carrier material for immobilizing enzymes linked to biodiesel and bioethanol production [21]. Immobilized microorganisms/enzymes in MNPs can easily be recovered from fermentation broth for subsequent use in several cycles, which significantly decreases the operational costs in biofuel production. For example, immobilized Saccharomyces cerevisiae achieved an ethanol productivity of 264 g/L-h from corn starch and was effectively maintained in several cycles for more than a month [22]. Furthermore, cellulase immobilized in MNPs achieved an efficient hydrolysis of Sesbania aculeate biomass yielding up to 5.3 g/L of ethanol along with the reuse of the nano-biocatalysts several times [23]. In another study, biodiesel production from microalgal oil was sustained for up to seven regenerations with a yield reaching 94% under optimal conditions [24].

2.2. Electrochemical Energy Applications

Biogenic NPs have been integrated into bio-electrochemical systems, together with different microorganisms to produce biohydrogen or bioelectricity. For instance, Orozco et al. (2010) [25] combined Pd NPs with E. coli for the conversion of H2 into energy in a fuel cell. Similar studies were also reported in bio-electrochemical systems in which Pd or Pt NPs were immobilized, together with cultures of Pseudomonas putida [26], Desulfovidrio desulfuricans [27], Shewanella oneidensis [28], and Saccharomyces cerevisiae [29].
Graphene, jointly immobilized with NPs, has also appeared as a stirring material applied in electrochemical systems for energy storage and conversion applications in lithium batteries and supercapacitors, among other power applications [30]. In fact, the manufacturing of composites created with metallic NPs and graphene has allowed us to overcome some limitations during the application of these NMs in lithium batteries (LBs). For example, Co3S4 has the potential to be used as anode material in LBs, but shows capacity vanishing, low conductivity, and poor cyclability [31]. In contrast, the deposition of Co3S4 nanotubes on graphene allows better cycling performance and greater reversible capacity as compared to the pristine Co3S4 electrode [32]. The additional doping of anodes with elements belonging to group IV, such as Ge and Sn, has significantly enhanced Li-storage capacities, conductivity, reversible capacity, coulombic efficiency, and rate capability [30].
Supercapacitors (SCs) are electrochemical power-storage devices, which compile and discharge energy by reversible adsorption and desorption of ions at the boundaries between electrode constituents and electrolytes. Due to its large available surface, graphene represents an attractive material for electrodes in SCs [33]. In addition, the doping of graphene with Ni(OH)2 NPs resulted in better electrode–electrolyte interaction, enhancing the conductivity of ions and electrons, ultimately resulting in a superior performance of SCs [34].
Further applications of immobilized NMs in electrochemical energy devices include their employment as composite catalysts to convert solar energy into chemical energy by splitting water into hydrogen photocatalytically. Additionally, graphene-based composites have successfully been applied as photocatalysts to reduce CO2 into reactive carbon forms [30]. Table 1 summarizes the application of immobilized NMs for energy production.
Table 1. Application of immobilized nanomaterials for energy production.
Immobilized NMs Application Achievement Reference
Bioprocesses      
MNPs Methane production from low-strength wastewater 3.6-fold higher methane production as compared to the control bioreactor [12]
BIO Methanogenic process 42% higher production of methane as compared to control [13]
GO-OX Methanogenic process Higher COD and ammonium removal from protein-rich wastewater as compared to control [14]
MNPs Fermentative production of ethanol Ethanol productivity of 264 g/L-h from corn starch, maintained in several cycles [22]
MNPs Fermentative production of ethanol from Sesbania aculeate biomass Immobilized cellulase produced 5.3 g/L of ethanol along with the reuse of the nano-biocatalysts several times [23]
MNPs Biodiesel production from algal oil Yield reaching 94% was maintained several cycles under optimal conditions [24]
Electrochemical processes      
Pd, Pt Pd NPs immobilized together with different microorganisms to produce energy in BES Enhanced production of hydrogen or electricity as compared to controls without NPs [26][27][28][29]
Co3S4-graphene Lithium battery Better cycling and greater reversible capacity as compared to the pristine Co3S4 electrode [32]
Ni(OH)2-graphene Supercapacitor Better electrode–electrolyte interaction, enhancing the conductivity of ions and electrons, resulting in a higher performance [34]
Graphene-CX Aqueous supercapacitor Combination of high porosity and electrical conductivity for 25% more capacitance and 100% more power than undoped CX at high current densities [35]
S-I Pd aerogel Microfluidic fuel cell First time microwave-assisted synthesis of Pd aerogels with enhanced power densities for formic acid electro-oxidation [36]
BES, bio-electrochemical systems; BIO, biogenic iron oxide; COD, chemical oxygen demand; CX, carbon xerogel; GO-OX, graphene oxide-organic xerogel; MNPs, magnetic nanoparticles; NPs, nanoparticles; S-I, self-immobilized.
 
 

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