A sustainable approach towards utilization of plastic waste for an efficient electrode in microbial fuel cell applications

When microplastics/plastics meet metal-organic frameworks: turning threats into opportunities

Significant efforts have been devoted to removal and recycling of microplastics (MPs; <5 mm) to address the environmental crises caused by their ubiquitous presence and improper treatment. Metal-organic frameworks (MOFs) demonstrate compatibility with MPs/plastics through adsorption, degradation, or assembly with the MPs/plastic polymers. Above 90% of MPs/plastic particles can be adsorbed on MOF materials via the hydrophobic interaction, electrical attraction, π-π stacking, and van der Waals forces. Meanwhile, certain MOFs have successfully converted various types of plastics into high-valued small molecules through thermocatalysis and photocatalysis. In thermocatalysis, the primary process should be C-O bond cleavage, whereas in photocatalysis it ought to be the generation of reactive oxygen species (ROS). Moreover, the construction of novel MOFs using waste MPs/plastics as the ligands was mostly accomplished through three dominant ways, including glycolysis, hydrolysis and methanolysis. Once successfully composited, the MOF@plastic materials illustrated tremendous promise for interdisciplinary research in multifunctional applications, including sewage treatment, gas adsorption/separation, and the preparation of microbial fuel cells, plastic scintillators and other sensors. The review explicated the relationships between MPs/plastics and MOF materials, as well as the challenges and perspectives for their development. It can provide a deeper understanding of how MOFs remove/degrade MP/plastic particles, how MPs/plastics are recycled to prepare MOFs, and how to build multifunctional MOF@plastic composites. Overall, this analysis is anticipated to outline future prospects for turning the threats (MPs/plastics contamination) into opportunities (e.g., as ligands to prepare MOF or MOF@plastic materials for further applications).

Applications of waste polyethylene terephthalate (PET) based nanostructured materials: A review

While polyethylene terephthalate (PET) has enjoyed widespread use, a large volume of plastic waste has also been produced as a result, which is detrimental to the environment. Traditional treatment of plastic waste, such as landfilling and incinerating waste, causes environmental pollution and poses risks to public health. Recycling PET waste into useful chemicals or upcycling the waste into high value-added materials can be remedies. This review first provides a brief introduction of the synthesis, structure, properties, and applications of virgin PET. Then the conversion process of waste PET into high value-added materials for different applications are introduced. The conversion mechanisms (including degradation, recycling and upcycling) are detailed. The advanced applications of these upgraded materials in energy storage devices (supercapacitors, lithium-ion batteries, and microbial fuel cells), and for water treatment (to remove dyes, heavy metals, and antibiotics), environmental remediation (for air filtration, CO2 adsorption, and oil removal) and catalysis (to produce H2, photoreduce CO2, and remove toxic chemicals) are discussed at length. In general, this review details the exploration of advanced technologies for the transformation of waste PET into nanostructured materials for various applications, and provides insights into the role of high value-added waste products in sustainability and economic development.

Efficient upcycling of iron scrap and waste polyethylene terephthalate plastic into Fe3O4@C incorporated MIL-53(Fe) as a novel electro-Fenton catalyst for the degradation of salicylic acid

The current research demonstrates the efficiency of a low-cost MIL-53(Fe)-metal-organic framework (MOF) derived Fe₃O₄@C (MIL-53(Fe)@Fe₃O₄@C) electrocatalyst in a batch-scale electro-Fenton (EF) process for the degradation of salicylic acid (SA) from wastewater. The electrocatalyst was prepared from the combination of polyethylene terephthalate (PET) and iron scrap wastes. The result showed 91.68 ± 3.61% degradation of 50 mg L^-1 of SA under optimum current density of 5.2 mA cm^-2, and pH of 7.0 during 180 min of electrolysis time. The degradation of SA from waste catalyst was similar to the chemical-based MIL-53(Fe)-derived Fe₃O₄@C (cFe) cathode catalyst. The presence of chloride ions (Cl^-) in the water matrix has shown a strong inhibitory effect on the elimination of SA, followed by nitrate (NO₃^-), and bicarbonate (HCO₃^-) ions. The multiple cyclic voltammetry (CV) analysis and reusability test of waste cathode catalyst showed only 8.03% drop of current density at the end of the 20th cycle and 5% drop of degradation efficiency after 6th cycle with low leaching of iron. The radical scavenging experiment revealed that the HO^• generated via electrochemical generation of H₂O₂ had a prominent contribution in the removal of SA compared to HO₂^•/O₂^•-. Besides, possible catalysis mechanism and degradation pathways were deduced. Furthermore, a satisfactory performance in the treatment of SA spiked in real water matrices was also observed by waste-derived Fe₃O₄@C cathode catalyst (wFe). Additionally, the total operating cost and toxicity analysis showed that the as-synthesized wFe cathode catalyst could be appropriate for removing organic pollutants from wastewater in the large-scale application.

A Review of Recent Advances in Microbial Fuel Cells: Preparation, Operation, and Application

The microbial fuel cell has been considered a promising alternative to traditional fossil energy. It has great potential in energy production, waste management, and biomass valorization. However, it has several technical issues, such as low power generation efficiency and operational stability. These issues limit the scale-up and commercialization of MFC systems. This review presents the latest progress in microbial community selection and genetic engineering techniques for enhancing microbial electricity production. The summary of substrate selection covers defined substrates and some inexpensive complex substrates, such as wastewater and lignocellulosic biomass materials. In addition, it also includes electrode modification, electron transfer mediator selection, and optimization of operating conditions. The applications of MFC systems introduced in this review involve wastewater treatment, production of value-added products, and biosensors. This review focuses on the crucial process of microbial fuel cells from preparation to application and provides an outlook for their future development.

Carbon-Based Sorbents for Hydrogen Storage: Challenges and Sustainability at Operating Conditions for Renewable Energy

It is estimated that all fossil fuels will be depleted by 2060 if we continue to use them at the present rate. Therefore, there is an unmet need for an alternative source of energy with high calorific value. In this regard, hydrogen is considered the best alternative renewable fuel that could be used in practical conditions. Accordingly, researchers are looking for an ideal hydrogen storage system under ambient conditions for feasible applications. In many respects, carbon-based sorbents have emerged as the best possible hydrogen storage media. These carbon-based sorbents are cost-effective, eco-friendly, and readily available. In this Review, the present status of carbon-based materials in promoting solid-state hydrogen storage technologies at the operating temperature and pressure was reported. Experimental studies have shown that some carbon-based materials such as mesoporous graphene and doped carbon nanotubes may have hydrogen storage uptake of 3-7 wt %, while some theoretical studies have predicted up to 13.79 wt % of hydrogen uptake at ambient conditions. Also, it was found that different methods used for carbon materials synthesis played a vital role in hydrogen storage performance. Eventually, this Review will be helpful to the scientific community for finding the competent material and methodology to investigate the existing hydrogen uptake issues at operating conditions.

A critical review on early-warning electrochemical system on microbial fuel cell-based biosensor for on-site water quality monitoring

The microbial fuel cell (MFC) sensor is a very promising self-powered self-sustainable system for early warning water quality detection. These sensors are cost-effective, biodegradable, compact in design, and portable in nature are favorable for real-time in situ water quality monitoring. This review represents the mechanism action behind the toxicity detection, optimization strategies, process parameters, role of biofilm, the role of external resistance, hydrodynamic study, and mathematical modeling for improving the performance of the sensor. Additionally, the techno-economic prospect of this MFC-based sensor and its challenges, limitations are addressed to make it economically more favorable for commercial use. The future direction is also explored based on the sensor's disadvantages and limitations. Comprehensively, this review covered all the possible directions of MFC sensor fabrication, their application, recent advancement, prospects challenges, and their possible solutions.

Effect of the bio-inspired modification of low-cost membranes with TiO2:ZnO as microbial fuel cell membranes

Microbial fuel cells (MFCs) are a novel technique for converting biodegradable materials into electricity. In this study, the efficiency of mixed crystal (TiO₂:ZnO) as a membrane modifier of a low-cost, antifouling and self-cleaning cation exchange membrane for MFCs was studied. The modification was prepared using polydopamine (PDA) as the bio-inspired glue, followed by gravity deposition of a mixture of catalyst nanoparticles (TiO₂:ZnO 0.03%, 1:1 ratio) as anti-biofouling agents. The effects of the membrane modification were evaluated in terms of power density, open circuit potential, coulombic efficiency, anti-biofouling properties and also color and COD removal efficiency. The results showed that the use of the PDA-modified membrane and a mixture of catalysts facilitated the transfer of cations released during the oxidation process in the anodic compartment of the MFC, which increased the power generation in the MFC by 2.5 times and 5.7 times the current compared to pristine and PDA pristine membranes, decreased the MFC operating cycle time from 5 to 3 days, doubled the lifetime of the membranes and demonstrated higher COD removal efficiency and color removal. Finally, SEM and AFM analysis showed that the modification significantly minimized surface fouling. The modified membranes in this study proved to be a potential alternative to the expensive membranes currently used in MFCs, furthermore, this modification could be an interesting alternative modification for other potential membranes for use in MFCs, due to the fact that the catalyst activation was only performed with visible light (artificial and solar), which could decrease operating costs.

Microbial electrochemical approaches of carbon dioxide utilization for biogas upgrading

Microbial electrochemical approach is an emerging technology for biogas upgrading through carbon dioxide (CO₂) reduction and biomethane (or value-added products) production. There is limited literature critically reviewing the latest scientific developments on the bioelectrochemical system (BES) based biogas upgrading technologies, including CO₂ reduction efficiency, methane (CH₄) yields, reactor operating conditions, and electrode materials tested in the BES reactor. This review analyzes the reported performance and identifies crucial parameters considered for future optimization, which is currently missing. Further, the performances of BES approach of biogas upgrading under various operating settings in particular fed-batch, continuous mode in connection to the microbial dynamics and cathode materials have been thoroughly scrutinized and discussed. Additionally, other versatile application options associated with BES based biogas upgrading, such as resource recovery, are presented. Three-dimensional electrode materials have shown superior performance in supplying the electrons for the reduction of CO₂ to CH₄. Most of the studies on the biogas upgrading process conclude hydrogen (H₂) mediated electron transfer mechanism in BES biogas upgrading.

Advancements in spontaneous microbial desalination technology for sustainable water purification and simultaneous power generation: A review

Population growth and rapid urbanization have put a lot of pressure on the already scarce freshwater around the globe. The availability of freshwater is not only limited but it is non-uniform also. Available desalination technologies help mitigate water shortage; however, these techniques are energy-intensive and unsustainable. Desalination technologies utilizing renewable energy and bio-electrochemical systems have been developed to achieve limited sustainability. With technological advancements, microbial desalination cell (MDC) has been developed which is capable of desalination, wastewater treatment, and power generation simultaneously. This review critically examined the performance of various MDC techniques concerning their stimulus parameters including COD removal, total desalination rate, total dissolved solids reduction rate, Coulombic efficiency, and power density. Limitations of MDCs have also been incorporated in the review. Work on MDC coupled with other robust desalination techniques offering advantages such as better desalination and more water recovery e.g. osmotic-MDC etc. has been included. Researchers have tremendously worked on MDCs with different electro-catalysts. Few of these are not sustainable and costly. Authors have reviewed critically with belief that it will pave a way for the commercialization of this eco-friendly technology.