Limited carbon source retards inorganic arsenic release during roxarsone degradation in Shewanella oneidensis microbial fuel cells

Hydrochar-nanoparticle integration for arsenic removal from wastewater: Challenges, possible solutions, and future horizon

Arsenic (As) contamination poses a significant threat to human health, ecosystems, and agriculture, with levels ranging from 12 to 75% attributed to mine waste and stream sediments. This naturally element is abundant in Earth's crust and gets released into the environment through mining and rock processing, causing ≈363 million people to depend on As-contaminated groundwater. To combat this issue, introducing a sustainable hydrochar system has achieved a remarkable removal efficiency of over 92% for arsenic through adsorption. This comprehensive review presents an overview of As contamination in the environment, with a specific focus on its impact on drinking water and wastewater. It delves into the far-reaching effects of As on human health, ecosystems, aquatic systems, and agriculture, while also exploring the effectiveness of existing As treatment systems. Additionally, the study examines the potential of hydrochar as an efficient adsorbent for As removal from water/wastewater, along with other relevant adsorbents and biomass-based preparations of hydrochar. Notably, the fusion of hydrochar with nanoparticle-centric approaches presents a highly promising and environmentally friendly solution for achieving the removal of As from wastewater, exceeding >99% efficiency. This innovative approach holds immense potential for advancing the realms of green chemistry and environmental restoration. Various challenges associated with As contamination and treatment are highlighted, and proposed solutions are discussed. The review emphasizes the urgent need to advance treatment technologies, improve monitoring methods, and enhance regulatory frameworks. Looking outlook, the article underscores the importance of fostering research efforts, raising public awareness, and fostering interdisciplinary collaboration to address this critical environmental issue. Such efforts are vital for UN Sustainable Development Goals, especially clean water and sanitation (Goal 6) and climate action (Goal 13), crucial for global sustainability.

Optimization of soil microbial fuel cell for sustainable bio-electricity production: combined effects of electrode material, electrode spacing, and substrate feeding frequency on power generation and microbial community diversity

BACKGROUND

Microbial fuel cells (MFCs) are among the leading research topics in the field of alternative energy sources due to their multifunctional potential. However, their low bio-energy production rate and unstable performance limit their application in the real world. Therefore, optimization is needed to deploy MFCs beyond laboratory-scale experiments. In this study, we investigated the combined influence of electrode material (EM), electrode spacing (ES), and substrate feeding interval (SFI) on microbial community diversity and the electrochemical behavior of a soil MFC (S-MFC) for sustainable bio-electricity generation.

RESULTS

Two EMs (carbon felt (CF) and stainless steel/epoxy/carbon black composite (SEC)) were tested in an S-MFC under three levels of ES (2, 4, and 8 cm) and SFI (4, 6, and 8 days). After 30 days of operation, all MFCs achieved open-circuit voltage in the range of 782 + 12.2 mV regardless of the treatment. However, the maximum power of the SEC-MFC was 3.6 times higher than that of the CF-MFC under the same experimental conditions. The best solution, based on the interactive influence of the two discrete variables, was obtained with SEC at an ES of 4.31 cm and an SFI of 7.4 days during an operating period of 66 days. Analysis of the experimental treatment effects of the variables revealed the order SFI < ES < EM, indicating that EM is the most influential factor affecting the performance of S-MFC. The performance of S-MFC at a given ES value was found to be dependent on the levels of SFI with the SEC electrode, but this interactive influence was found to be insignificant with the CF electrode. The microbial bioinformatic analysis of the samples from the S-MFCs revealed that both electrodes (SEC and CF) supported the robust metabolism of electroactive microbes with similar morphological and compositional characteristics, independent of ES and SFI. The complex microbial community showed significant compositional changes at the anode and cathode over time.

CONCLUSION

This study has demonstrated that the performance of S-MFC depends mainly on the electrode materials and not on the diversity of the constituent microbial communities. The performance of S-MFCs can be improved using electrode materials with pseudocapacitive properties and a larger surface area, instead of using unmodified CF electrodes commonly used in S-MFC systems.

Phenylarsonics in concentrated animal feeding operations: Fate, associated risk, and treatment approaches

Phenylarsonics are present as additives in animal feed in some countries. As only a small fraction of these additives is metabolized in animals, they mostly end up in the environment. A comprehensive investigation of the fate of these additives is crucial for evaluating their risks. This review aims to provide a clear understanding of the transformation mechanism of phenylarsonics in vivo and in vitro and to evaluate their fate and associated risks. Degradation of phenylarsonics releases toxic As species (mainly as inorganic arsenic (iAs)). Trivalent phenylarsonics are the metabolites or biotic degradation intermediates of phenylarsonics. The cleavage of As groups from trivalent phenylarsonics catalyzed by C-As lyase or other unknown pathways generates arsenite (As(III)). As(III) can be further oxidized to arsenate (As(V)) and methylated to methyl-arsenic species. The half-lives associated with abiotic degradation of phenylarsonics ranged from a few minutes to tens of hours, while those associated with biotic degradation ranged from several days to hundreds of days. Abiotic degradation resulted in a higher yield of iAs than biotic degradation. The use of phenylarsonics led to elevated total As and iAs levels in animal products and environmental matrices, resulting in As exposure risk to humans. The oxidation of phenylarsonics to As(V) facilitated the sorptive removal of As, which provides a general approach for treating these compounds. This review provides solid evidence that the use of phenylarsonics has adverse effects on both human health and environmental safety, and therefore, supports their withdrawal from the global market.

Roxarsone transformation and its impacts on soil enzyme activity in paddy soils: A new insight into water flooding effects

The aromatic arsenical roxarsone (ROX) has been used as feed additive for decades worldwide. The past or present application of animal manure containing ROX in paddy fields results in arsenic (As) accumulation in rice grain. However, the degradation and transformation mechanisms of ROX in paddy soil which determine As bioavailability and uptake by rice are still unclear. The current study investigated the variation of As speciation and soil enzyme activities in ROX-treated soils under flooded and non-flooded conditions for six months. Our results showed that 70.2% of ROX persisted in non-flooded paddy soils after 180 d while ROX degraded completely within 7 d in flooded soils. The rapid degradation of ROX under flooded conditions owed to the enhanced biotic transformation that was caused by the low Eh and the predominant presence of Clostridium spp. and Bacillus spp. ROX was not only transformed to As(III) and As(V) in non-flooded soils but also to 3-amino-4-hydroxyphenylarsonic acid and methyl arsenicals in flooded soils. The degradation products significantly inhibited soil enzyme activities for 7-30 d, but the inhibition effects disappeared after 90 d due to the sorption of transformed As products to amorphous Fe oxides. This study provides new insights into the flooding effect on ROX fate in paddy fields, which is important for the management of animal waste and risk control on polluted sites.

Organoarsenic feed additives in biological wastewater treatment processes: Removal, biotransformation, and associated impacts

Aromatic organoarsenicals are widely used in animal feeding operations and cause arsenic contamination on livestock wastewater and manure, thereby raising the risk of surface water pollution. Biological wastewater treatment processes are often used for livestock wastewater treatment. Organoarsenic removal and biotransformation under aerobic and anaerobic conditions, and the associated impacts have received extensive attention due to the potential threat to water security. The removal efficiency and biotransformation of organoarsenicals in biological treatment processes are reviewed. The underlying mechanisms are discussed in terms of functional microorganisms and genes. The impacts associated with organoarsenicals and their degradation products on microbial activity and performance of bioreactors are also documented. Based on the current research advancement, knowledge gaps and potential research in this field are discussed. Overall, this work delivers a comprehensive understanding on organoarsenic behaviors in biological wastewater treatment processes, and provides valuable information on the control of arsenic contamination from the degradation of organoarsenicals in biological wastewater treatment processes.

The Pseudomonas putida NfnB nitroreductase confers resistance to roxarsone

Roxarsone (3-nitro-4-hydroxyphenylarsonic acid, Rox) has been used for decades as an antimicrobial growth promoter for poultry and swine. Roxarsone is excreted in chicken manure unchanged and can be microbially transformed into a variety of arsenic-containing compounds such as 3-amino-4-hydroxyphenylarsonic acid (HAPA(V)) that contaminate the environment and present a potential health hazard. To cope with arsenic toxicity, nearly every prokaryote has an ars (arsenic resistance) operon, some of which confer resistance to roxarsone. Pseudomonas putida KT2440 is a robust environmental isolate capable of metabolizing many aromatic compounds and is used as a model organism for biodegradation of aromatic compounds. Here we report that P. putida KT2440 (ΔΔars) in which the two ars operons had been deleted retains resistance to highly toxic trivalent Rox(III), the likely active form of roxarsone. In this study, a genomic library constructed from P. putida KT2440 (ΔΔars) was used to screen for resistance to Rox(III) in Escherichia coli. One gene, termed, PpnfnB, was identified that encodes a putative 6,7-dihydropteridine reductase. Cells expressing PpnfnB reduce the nitro group of Rox(III), and purified NfnB catalyzes FMN-NADPH-dependent nitroreduction of Rox(III) to less toxic HAPA(III). This identifies a key step in the breakdown of synthetic aromatic arsenicals.

Molecular density regulating electron transfer efficiency of S. oneidensis MR-1 mediated roxarsone biotransformation

Efficient extracellular electron transport is a key for sufficient bioremediation of organoarsenic pollutants such as 4-hydroxy-3-nitrobenzenearsonic acid (roxarsone). The related apparent kinetics characteristics are essential for engineering practice of bioremediation activities and for full understanding the environmental fate of roxarsone, yet remains poorly understood. We report, to our knowledge, the first study of the electron transfer characteristics between roxarsone and participating S. oneidensis MR-1. The electron transfer rate during roxarsone biotransformation was estimated up to 3.1 × 10⁶ electrons/cell/s, with its value being clearly associated with the apparent roxarsone concentration. Lowing roxarsone concentration extended the average separation distance between cells and neighboring roxarsone molecules and thereby augmented electric resistance as well as extended cell movement for foraging, thus reduced electron transfer rate. In addition, the presence of roxarsone significantly stimulated population growth of S. oneidensis MR-1 with nearly doubled maximum specific growth rate, albeit with clearly increased lag time, as compared with that of none-roxarsone scenario. These findings provide, at the first time, basic biostoichiometry of S. oneidensis MR-1 induced roxarsone biotransformation, which may shed lights for full understanding of roxarsone transformation process in waste treatment systems that are necessary for engineering practice and/or environmental risks assessment.

Electricigens in the anode of microbial fuel cells: pure cultures versus mixed communities

Microbial fuel cell (MFC) is an environmentally friendly technology for electricity harvesting from a variety of substrates. Microorganisms used as catalysts in the anodic chamber, which are termed as electricigens, play a major role in the operation of MFCs. This review provides an introduction to the currently identified electricigens on their taxonomical groups and electricity producing abilities. The mechanism of electron transfer from electricigens to electrode is highlighted. The performances of pure culture and mixed communities are compared particularly. It has been proved that the electricity generation capacity and the ability to adapt to the complex environment of MFC systems constructed by pure microbial cultures are less than the systems constructed by miscellaneous consortia. However, pure cultures are useful to clarify the electron transfer mechanism at the microbiological level and further reduce the complexity of mixed communities. Future research trends of electricigens in MFCs should be focused on screening, domestication, modification and optimization of multi-strains to improve their electrochemical activities. Although the MFC techniques have been greatly advanced during the past few years, the present state of this technology still requires to be combined with other processes for cost reduction.