Nonmicrobial mechanisms dominate the release of CO2 and the decomposition of organic matter during the short-term redox process in paddy soil slurry

Organic acid-enhanced production of hydroxyl radicals during H2O2-based chemical oxidation for the remediation of contaminated soil

Hydrogen peroxide (H2O2)-based in situ chemical oxidation (ISCO) is widely used for remediating contaminated groundwater and soil. However, its effectiveness can be limited by a low efficiency of H2O2 utilization, leading to increased costs. In this study, we showed that ascorbic acid (AA), citric acid, and hydroxylamine hydrochloride (used for comparison) significantly increased •OH production (by 2.3-108.0-fold) and chlorobenzene degradation (by 6.4-30.5-fold) in H2O2/site soil systems. Further analysis revealed that AA significantly enhanced the formation and oxidation of active Fe(II) species (e.g., 0.5 M HCl-, 5 M HCl-, and HF-Fe(II)) via the mechanisms of acid dissolution, complexation, and reduction. As a result, these processes inhibited the transformation of low-crystallinity Fe phases into high-crystallinity forms, thereby preserving the activity of the Fe phases. The different capacities of these ligands for acidification and complexation or reduction are significantly influenced by their characteristics, such as the presence of specific functional groups, as well as their concentration. This variation, in turn, affects •OH production and the degradation of contaminants in treatment systems. This study provides valuable insights into how low-molecular-weight organic acids enhance the formation of •OH and contaminant degradation during H2O2-based ISCO. These findings also contribute to the development of efficient, environmentally friendly, and cost-effective remediation technologies for the treatment of contaminated groundwater and soil.

Carbon Isotope Fractionation of Dissolved Organic Matter Due to •OH-Based Oxidation

•OH-based oxidation plays a crucial role in dissolved organic matter (DOM) transformation and carbon flux, whereas quantifying the contribution of this pathway remains challenging. Here we combined the concentration with the carbon isotope analysis of DOM and its generated CO2 to quantify the contribution of •OH-based oxidation. Results showed that the 13C enrichment factors (ε values) were -8.1‰ to -8.9‰ for benzene ring oxidation in aromatic compounds, -4.2‰ to -28.9‰ for lower-molecular-weight organic acids, and -13.0‰ for DOM from sediment. The fractionation of sediment DOM reflects the average ε value of humic substances and organic acids. These ε values were more negative than those of the photochemical and microbial processes, enabling the identification of DOM transformation mechanisms. Using an end-member mix model, we found that the proportion of •OH-based mineralization in total CO2 emission ranged from 20.9% to 39.8% for 100 g/L sediment oxidation by 5-20 mM H2O2 under pH-neutral condition within 2 h and was only 2% for oxidation by air under the same conditions. We also found that inorganic carbon degassing contributed greatly to CO2 emission during sediment oxidation. This study presents a new isotope-based tool to quantitatively assess the contribution of •OH-based oxidation to the emission of CO2 from DOM.

Developing Methylmercury-Targeted Strategies to Safeguard Rice Consumers

Mitigating mercury (Hg) risk in the rice-paddy system is crucial for safeguarding food safety and human health, as rice is a main source of human exposure to neurotoxic methylmercury (MeHg). Current mitigation strategies predominantly focus on reducing the availability of inorganic Hg (IHg) for Hg methylation, achieved primarily through Hg emission control and in situ Hg immobilization. While these IHg-targeted approaches have effectively reduced MeHg bioaccumulation and subsequent human exposure, their efficacy is largely undermined by Hg transformations and fluctuating environmental conditions due to the complex and protracted pathway linking IHg from environmental sources to MeHg at the point of human exposure. In light of recent advancements in MeHg-related transformations, we emphasize the development of MeHg-targeted strategies to improve the overall efficiency of Hg risk management in rice-paddy systems. MeHg-targeted strategies include microbial regulation to diminish net MeHg production, facilitating MeHg demethylation in soils, and promoting the in vivo MeHg degradation within rice plants. Although these approaches are still in their nascent stages, they hold significant promise due to their potential high mitigation efficacy and reduced uncertainties, owing to the shorter pathway between MeHg production and human exposure. Integrating IHg- and MeHg-targeted strategies offers a comprehensive and synergistic approach, paving the way for more effective mitigation of human exposure to MeHg in rice-paddy systems.

Production of Reactive Oxygen Species during Redox Manipulation and Its Potential Impacts on Activated Sludge Wastewater Treatment Processes

Reactive oxygen species (ROS) are ubiquitous in redox-fluctuating environments, exerting profound impacts on biogeochemical cycles. However, whether ROS can be generated during redox manipulation in activated sludge wastewater treatment processes (AS-WTPs) and the underlying impacts remain largely unknown. This study demonstrates that ROS production is ubiquitous in AS-WTPs due to redox manipulation and that the frequency and capacity of ROS production depend on the operating modes. The anaerobic/oxic continuous-flow reactor showed persistent ROS generation (0.8-2.1 μM of instantaneous H2O2), whereas the oxic/anoxic sequencing batch reactor (0.21-0.28 mM of H2O2 per cycle) and the anaerobic/anoxic digestion reactor (0.27-0.29 mM of H2O2 per cycle) exhibited periodic ROS production. Our results illustrated that ROS generated during redox manipulation can contribute to the removal of organic micropollutants. Due to their high activity, ROS can directly accelerate the abiotic oxidation of organic phenolics and Fe(II) minerals in sludges. ROS could also affect biotic nitrification by changing the microbial community composition and regulating the relative expression of functional genes, such as amoA, nrxA, and nrxB. This research demonstrates the ubiquitous production of ROS during redox manipulation in AS-WTPs, which provides new insights into pollutant removal and the abiotic and biotic elemental transformation in AS-WTPs.

Warming inhibits HgII methylation but stimulates methylmercury demethylation in paddy soils

Inorganic mercury (HgII) can be transformed into neurotoxic methylmercury (MeHg) by microorganisms in paddy soils, and the subsequent accumulation in rice grains poses an exposure risk for human health. Warming as an important manifestation of climate change, changes the composition and structure of microbial communities, and regulates the biogeochemical cycles of Hg in natural environments. However, the response of specific HgII methylation/demethylation to the changes in microbial communities caused by warming remain unclear. Here, nationwide sampling of rice paddy soils and a temperature-adjusted incubation experiment coupled with isotope labeling technique (202HgII and Me198Hg) were conducted to investigate the effects of temperature on HgII methylation, MeHg demethylation, and microbial mechanisms in paddy soils along Hg gradients. We showed that increasing temperature significantly inhibited HgII methylation but promoted MeHg demethylation. The reduction in the relative abundance of Hg-methylating microorganisms and increase in the relative abundance of MeHg-demethylating microorganisms are the likely reasons. Consequently, the net Hg methylation production potential in rice paddy soils was largely inhibited under the increasing temperature. Collectively, our findings offer insights into the decrease in net MeHg production potential associated with increasing temperature and highlight the need for further evaluation of climate change for its potential effect on Hg transformation in Hg-sensitive ecosystems.

Unrecognized Role of Organic Acid in Natural Attenuation of Pollutants by Mackinawite (FeS): The Significance of Carbon-Center Free Radicals

Organic acid is prevalent in underground environments and, against the backdrop of biogeochemical cycles on Earth, holds significant importance in the degradation of contaminants by redox-active minerals. While earlier studies on the role of organic acid in the generation of reactive oxygen species (ROS) primarily concentrated on electron shuttle or ligand effects, this study delves into the combined impacts of organic acid decomposition and Mackinawite (FeS) oxidation in contaminant transformation under dark aerobic conditions. Using bisphenol A (BPA) as a model, our findings showed that oxalic acid (OA) notably outperforms other acids in enhancing BPA removal, attaining a rate constant of 0.69 h-1. Mass spectrometry characterizations, coupled with anaerobic treatments, advocate for molecule-O2 activation as the principal mechanism behind pollutant transformation. Comprehensive results unveiled that carbon center radicals, initiated by hydroxyl radical (•OH) attack, serve as the primary agents in pollutant oxidation, accounting for at least 93.6% of the total •OH generation. This dynamic, driven by the decomposition of organic acids and the concurrent formation of carbon-centered radicals, ensures a steady supply of electrons for ROS generation. The obtained information highlights the importance of OA decomposition in the natural attenuation of pollutants and offers innovative strategies for FeS and organic acid-coupled decontamination.