Microbial fuel cells using natural pyrrhotite as the cathodic heterogeneous Fenton catalyst towards the degradation of biorefractory organics in landfill leachate

A critical review of approaches to enhance the performance of bio-electro-Fenton and photo-bio-electro-Fenton systems

The development of sustainable advanced energy conversion technologies and efficient pollutant treatment processes is a viable solution to the two global crises of the lack of non-renewable energy resources and environmental harm. In recent years, the interaction of biological and chemical oxidation units to utilize biomass has been extensively studied. Among these systems, bio-electro-Fenton (BEF) and photo-bio-electro-Fenton (PBEF) systems have shown prospects for application due to making rational and practical conversion and use of energy. This review compared and analyzed the electron transfer mechanisms in BEF and PBEF systems, and systematically summarized the techniques for enhancing system performance based on the generation, transfer, and utilization of electrons, including increasing the anode electron recovery efficiency, enhancing the generation of reactive oxygen species, and optimizing operational modes. This review compared the effects of different methods on the electron flow process and fully evaluated the benefits and drawbacks. This review may provide straightforward suggestions and methods to enhance the performance of BEF and PBEF systems and inspire the reader to explore the generation and utilization of sustainable energy more deeply.

Minerals as catalysts of heterogeneous Electro-Fenton and derived processes for wastewater treatment: a review

Advanced oxidation processes (AOPs) such as Fenton's reagent, which generates highly reactive oxygen species, are efficient in removing biorefractory organic pollutants from wastewater. However, Fenton's reagent has drawbacks such as the generation of iron sludge, high consumption of H₂O₂, and the need for pH control. To address these issues, Electro-Fenton (EF) and heterogeneous Electro-Fenton (HEF) have been developed. HEF, which uses solid catalysts, has gained increasing attention, and this review focuses on the use of mineral catalysts in HEF and derived processes. The reviewed studies highlight the advantages of using mineral catalysts, such as efficiency, stability, affordability, and environmental friendliness. However, obstacles to overcome include the agglomeration of unsupported nanoparticles and the complex preparation techniques and poor stability of some catalyst-containing cathodes. The review also discusses the optimal pH range and dosage of the heterogeneous catalysts and compares the performance of iron sulfides versus iron oxides. Although natural minerals appear to be the best choice for effluents at pH>4, no scale-up reports have been found. The need for further development in this field and the importance of considering the environmental impact of trace toxic metals or catalytic nanoparticles in the treated water on the receiving ecosystem is emphasized. Finally, the article acknowledges the high energy consumption of HEF processes at the lab scale and calls for their performance development to achieve environmentally friendly and cost-effective results using real wastewaters on a pilot scale.

Synchronous bio-degradation and bio-electricity generation in a Microbial Fuel Cell with aged and fresh leachate from the identical subtropical area

Seasonal leachate from both sealed and operating landfill in the identical district were employed as the sole substrate in the Microbial Fuel Cell (MFC) to evaluate the power output performance and aqueous organic waste disposal. The electrical performance was characterized to study the power generation, while the Chemical Oxygen Demand (COD) removal ratio and Coulombic Efficiency (CE) were calculated to illustrate the substrate disposal effect. In addition, Scanning Electron Microscope (SEM) on the operated anode was conducted to preliminarily explain the microbial community difference, and the phylogenetic tree constructed on the cultivated microorganism was an insight into the dominant bacteria suitable for leachate degradation. It was found that the MFCs inoculated with seasonal leachate from both sealed and operating landfill could generate electricity successfully. Although the fresh leachate-inoculated MFCs had better electrical output performance (22.7-25.6 W/m³ versus 6.61-7.48 W/m³) and COD removal efficiency (55.8%∼61.7% versus 47.7%∼51.4%), the CEs were only 4.3%∼7.6%, which were lower than the aged leachate inoculated group (5.9%∼11.3%). Based on the SEM images and the phylogenetic tree of the operated anode, the composition impacts on the microbial community and power output performance were verified, which was instructive for the leachate disposal in the MFC.

A review on bio-electro-Fenton systems as environmentally friendly methods for degradation of environmental organic pollutants in wastewater

Bio-electro-Fenton (BEF) systems have been potentially studied as a promising technology to achieve environmental organic pollutants degradation and bioelectricity generation. The BEF systems are interesting and constantly expanding fields of science and technology. These emerging technologies, coupled with anodic microbial metabolisms and electrochemical Fenton's reactions, are considered suitable alternatives. Recently, great attention has been paid to BEFs due to special features such as hydrogen peroxide generation, energy saving, high efficiency and energy production, that these features make BEFs outstanding compared with the existing technologies. Despite the advantages of this technology, there are still problems to consider including low production of current density, chemical requirement for pH adjustment, iron sludge formation due to the addition of iron catalysts and costly materials used. This review has described the general features of BEF system, and introduced some operational parameters affecting the performance of BEF system. In addition, the results of published researches about the degradation of persistent organic pollutants and real wastewaters treatment in BEF system are presented. Some challenges and possible future prospects such as suitable methods for improving current generation, selection of electrode materials, and methods for reducing iron residues and application over a wide pH range are also given. Thus, the present review mainly revealed that BEF system is an environmental friendly technology for integrated wastewater treatment and clean energy production.

Critical review of natural iron-based minerals used as heterogeneous catalysts in peroxide activation processes: Characteristics, applications and mechanisms

Recently, an increasing number of works have been reported about iron-based materials applied as catalysts in peroxide activation processes to degrade pollutants in water. Iron-based catalysts include synthetic and natural iron-based materials. However, some synthetic iron-based materials are difficult to scale up in the practical applications due to high cost and serious secondary environmental pollution. In contrast, natural iron-based minerals are more available and cheaper, and also hold a great promise in peroxide activation processes for pollutant degradation. In this review, we classify different natural iron-based materials into two categories: iron oxide minerals (e.g., magnetite, hematite, and goethite,), and iron sulfide minerals (e.g., pyrite and pyrrhotite,). Their overview applications in peroxide activation processes for pollutant degradation in wastewaters are systematically summarized for the first time. Moreover, the peroxide activation mechanisms induced by natural minerals, and the influences of reaction conditions in different systems are discussed. Finally, the application prospects and existing drawbacks of natural iron-based minerals in the peroxide activation processes for wastewater treatment are proposed. We believe this review can shed light on the application of natural iron-based minerals in peroxide activation processes and present better perspectives for future researches.

Review on electrochemical system for landfill leachate treatment: Performance, mechanism, application, shortcoming, and improvement scheme

Landfill leachate is a type of complex organic wastewater, which can easily cause serious negative impacts on the human health and ecological environment if disposed improperly. Electrochemical technology provides an efficient approach to effectively reduce the pollutants in landfill leachate. In this review, the electrochemical standalone processes (electrochemical oxidation, electrochemical reduction, electro-coagulation, electro-Fenton process, three-dimensional electrode process, and ion exchange membrane electrochemical process) and the electrochemical integrated processes (electrochemical-advanced oxidation process (AOP) and biological electrochemical process) for landfill leachate treatment are summarized, which include the performance, mechanism, application, existing problems, and improvement schemes such as cost-effectiveness. The main objective of this review is to help researchers understand the characteristics of electrochemical treatment of landfill leachate and to provide a useful reference for the design of the process and reactor for the harmless treatment of landfill leachate.

Performance evaluation of microbial fuel cell for landfill leachate treatment: Research updates and synergistic effects of hybrid systems

Over half of century, sanitary landfill was and is still the most economical treatment strategy for solid waste disposal, but the environmental risks associated with the leachate have brought attention of scientists for its proper treatment to avoid surface and ground water deterioration. Most of the treatment technologies are energy-negative and cost intensive processes, which are unable to meet current environmental regulations. There are continuous demands of alternatives concomitant with positive energy and high effluent quality. Microbial fuel cells (MFCs) were launched in the last two decades as a potential treatment technology with bioelectricity generation accompanied with simultaneous carbon and nutrient removal. This study reviews capability and mechanisms of carbon, nitrogen and phosphorous removal from landfill leachate through MFC technology, as well as summarizes and discusses the recent advances of standalone and hybrid MFCs performances in landfill leachate (LFL) treatment. Recent improvements and synergetic effect of hybrid MFC technology upon the increasing of power densities, organic and nutrient removal, and future challenges were discussed in details.

An approach to the photocatalytic mechanism in the TiO2-nanomaterials microorganism interface for the control of infectious processes

The approach of this timely review considers the current literature that is focused on the interface nanostructure/cell-wall microorganism to understand the annihilation mechanism. Morphological studies use optical and electronic microscopes to determine the physical damage on the cell-wall and the possible cell lysis that confirms the viability and microorganism death. The key parameters of the tailoring the surface of the photoactive nanostructures such as the metal functionalization with bacteriostatic properties, hydrophilicity, textural porosity, morphology and the formation of heterojunction systems, can achieve the effective eradication of the microorganisms under natural conditions, ranging from practical to applications in environment, agriculture, and so on. However, to our knowledge, a comprehensive review of the microorganism/nanomaterial interface approach has rarely been conducted. The final remarks point the ideal photocatalytic way for the effective prevention/eradication of microorganisms, considering the resistance that the microorganism could develop without the appropriate regulatory aspects for human and ecosystem safety.

Bioenergy, ammonia and humic substances recovery from municipal solid waste leachate: A review and process integration

High strength of organic matters and nitrogen are the most concerns in treatment of municipal solid waste leachate, but can be removed and recovered as bioenergy and fertilizer. A few review papers on leachate treatment technologies and single resource recovery have been published. However, none practical leachate treatment process towards multiple resources recovery has been worked out. In this paper, technologies of bioenergy, ammonia and humic substances recovery from municipal solid waste leachate are summarized. A two-stage anaerobic digestion comprising an expanded granular sludge bed reactor and an anaerobic membrane bioreactor is suggested to maximize methane production as bioenergy. Ammonia recovery by biogas recirculation with simultaneous calcium removal is proposed for the first time. Humic substances are suggested to be recovered as fertilizer from nanofiltration concentrate by membrane technology. A novel integrated leachate treatment process is proposed for resources recovery from leachate, with more environmental and economic benefits.

Current advances and trends in electro-Fenton process using heterogeneous catalysts - A review

Over the last decades, advanced oxidation processes have often been used alone, or combined with other techniques, for remediation of ground and surface water pollutants. The application of heterogeneous catalysis to electrochemical advanced oxidation processes is especially useful due to its efficiency and environmental safety. Among those processes, electro-Fenton stands out as the one in which heterogeneous catalysis has been broadly applied. Thus, this review has introduced an up-to-date collation of the current knowledge of the heterogeneous electro-Fenton process, highlighting recent advances in the use of different catalysts such as iron minerals (pyrite, magnetite or goethite), prepared catalysts by the load of metals in inorganic and organic materials, nanoparticles, and the inclusion of catalysts on the cathode. The effects of physical-chemical parameters as well as the mechanisms involved are critically assessed. Finally, although the utilization of this process to remediation of wastewater overwhelmingly outnumber other utilities, several applications have been described in the context of regeneration of adsorbent or the remediation of soils as clear examples of the feasibility of the electro-Fenton process to solve different environmental problems.

Pilot-scale bioelectrochemical system for efficient conversion of 4-chloronitrobenzene

4-Chloronitrobenzene (4-CNB) is one of the highly toxic contaminants that may lead to acute, chronic or persistent physiological toxicity to ecology and environment. Conventional methods for removing 4-CNB from aquatic environment may be problematic due to inefficiency, high cost and low sustainability. This study develops a pilot-scale bioelectrochemical system (BES, effective volume of 18 L) and examines its performance of bioelectrochemical transformation of 4-CNB to 4-chloroaniline (4-CAN) under continuous operation. The results demonstrate that the initial 4-CNB concentration in the influent and hydraulic retention time (HRT) has a significant impact on 4-CNB reduction and 4-CAN formation. Compared with the conventional anaerobic process in the absence of external power supplied, the 4-CNB conversion efficiency can be enhanced with power supplied due to microbial-mediated electron transfer at the negative cathode potential. At a voltage of 0.4 V and HRT of 48 h, the 4-CNB reduction and 4-CAN formation efficiency reached 99% and 94.1%, respectively. Based on a small external voltage applied, the pilot-scale BES is effective in the conversion of 4-CNB to 4-CAN, an intermediate that is of less toxicity and higher bioavailability for subsequent treatment. This study provides a new strategy and methods for eliminating 4-CNB, making wastewater treatment more economical and more sustainable.

A lab-scale anoxic/oxic-bioelectrochemical reactor for leachate treatments

A membraneless, liter-scale bioelectrochemical reactor with both bioanode and biocathode was established for landfill leachate treatment. Anoxic/oxic (A/O) zones at anode compartment and cathode compartment, respectively, were connected with a reflux to facilitate nitrogen removal. With raw landfill leachate of 17,500-22,600 mg L(-1) chemical oxygen demand (COD) and 1170-1490 mg L(-1) NH4(+)-N, the tested reactor removed 89.1±1.6% of chemical oxygen demand and 99.2±0.1% of NH4(+)-N at 3.0 kg COD m(-3) d(-1). The corresponding maximum power density was 2.71±0.09 W m(-3), with internal resistance of 46.7±1.6 Ω and open circuit voltage of 727±7 mV. The species of Pseudomonas, Desulfovibrio, Bacillus, Enterococcus, Pelospora, Dehalobacter dominated the anodic community, while those of methylotrophs, Rhodobacter, Verrucomicrobiaceae, Geobacter, Flavobacterium, Thauera, Desulfovibrio and Aeromonas dominated the cathodic community. The proposed A/O bioelectrochemical reactor is a prototype for practical treatment of landfill leachate at affordable costs.

Treatment of landfill leachate using microbial fuel cells: alternative anodes and semi-continuous operation

Microbial fuel cells were designed and operated to treat landfill leachate while continuously producing power. Two different anodes were tested in batch cycles using landfill leachate as a substrate without inoculation: an activated carbon anode and biochar anode. In addition, a semi-continuous serpentine design was evaluated. No significant difference of the mean was found for the peak voltage, current density or power densities between the batch cell with activated carbon or biochar. Similar COD reduction occurred at both the batch (with biochar) and semi-continuous scale (28% ± 8.8% and 21.7% ± 12.2%, respectively). The batch MFC with activated carbon anode had significantly higher COD removal (74.7% ± 5.5%). BOD was removed by the semi-continuous MFC, but ammonia was not removed in four of the five cycles. The results provide further information on the possibility of using MFCs in landfill leachate treatment systems.

Degradation of p-nitrophenol in a BES-Fenton system based on limonite

This study confirmed the feasibility of natural limonite working as the iron catalyst for the PNP wastewater treatment in the BES-Fenton system. After the start-up period of the BES-Fenton systems, air and limonite powder were injected into the cathode chamber as the original materials for manufacturing Fenton reagents of H₂O₂ and Fe(II) respectively. The experiment parameters like pH, external resistance, limonite dosage and initial PNP concentration were investigated in this research. The removal efficiency of PNP (0.25 mM) could achieve 96% in 6h under the optimal experimental conditions. A limonite dosage of 112 mg per 50 ml of PNP solution at 0.25 mM concentration each time could sustain 7 cycles of the BES-Fenton system operation with PNP removal efficiency >94%. This study suggests an efficiency and cost-effective approach for the PNP removal by using the natural limonite as the iron catalyst of the BES-Fenton system.

Three-dimensional homogeneous ferrite-carbon aerogel: one pot fabrication and enhanced electro-Fenton reactivity

This work focuses on constructing a high catalytic activity cathode of an electro-Fenton system, to overcome the defects of low activity, poor stability, and intricate fabrication of supported catalysts. A series of ferrite-carbon aerogel (FCA) monoliths with different iron/carbon ratios was synthesized directly from metal-resin precursors accompanied by phase transformation. Self-doped ferrite nanocrystals and carbon matrix were formed synchronously via moderate condensation and sol-gel processes, leading to homogeneous texture. An optimal 5% ferric content FCA was composed of coin-like carbon nano-plate with continuous porous structure, and the ferric particles with diameters of dozens of nanometers were uniformly embedded into the carbon framework. The FCA exhibited good conductivity, high catalytic efficiency, and distinguished stability. When it was used as an electro-Fenton cathode, metalaxyl degradation results demonstrated that 98% TOC elimination was realized after 4 h, which was 1.5 times higher than that of the iron oxide supported electrode. It was attributed to self-doped Fe@Fe(2)O(3) ensuring Fe(II) as the mediator, maintaining high activity via reversibe oxidation and reduction by electron transfer among iron species with different valences. Meanwhile, an abundance of independent reaction microspaces were provided for every ferric crystal to in situ decompose electrogenerated H(2)O(2). Moreover, the possible catalytic mechanism was also proposed. The FCA was a promising candidate as potential cathode materials for high-performance electro-Fenton oxidation.

Effects of sulphate addition and sulphide inhibition on microbial fuel cells

The effects of adding sulphate in: (i) standard activated sludge microbial fuel cells (MFCs) and (ii) larger-scale leachate-treating columns - both as individual units and as a system connected in cascade - are reported. S-replete power output was ∼2-fold higher than that of the S-deplete MFC. Furthermore, the effects of evolved sulphide (S(2-)) inhibition were investigated. The overall decrease in power output from the sulphide inhibitor (Na(2)MoO(4)) additions was 83% for the S-replete and 90% for the S-deplete. The second phase with the leachate treating units shows an improvement of 32-86% (depended on leachate strength) in current output as a result of adding sulphate. When leachate column MFCs were connected fluidically in series, the amount of Na(2)SO(4) made available downstream was decreasing (increase in power was 99%, 40% and 12% for columns in cascade). Results demonstrated the beneficial effects of added sulphur sources to both activated sludge and leachate-treating MFCs.