Microbe Regulates the Mineral Photochemical Activity and Organic Matter Compositions in Water

Quantitative Enhancement of Arsenate Immobilization Induced by Vacancy Defects on Various Exposed Lattice Facets of Hematite

Defects are common features in hematite that arise from deviations from the perfect mineral crystal structure. Vacancy defects have been shown to significantly enhance arsenate (As) immobilization by hematite. However, the contributions from vacancy defects on different exposed facets of hematite have not been fully quantified. In this study, hematite samples with various morphologies were pretreated with sodium borohydride (NaBH4) to generate oxygen vacancy defects (OVDs), analyzed quantitatively using extended X-ray absorption fine structure (EXAFS) and thermogravimetric analysis (TG). Batch experiments revealed that the OVDs on different exposed facets showed significant variations in improving arsenate adsorption, i.e., the quantitative enhancement of arsenate adsorption amount per unit OVD concentration (ΔQm/Cdefect) followed the sequence of (110) facet (80.05 μmol/mmoldef) > (001) facet (31.85 μmol/mmoldef) > (012) facet (13.14 μmol/mmoldef). The underlying mechanism by which OVDs affect arsenate adsorption across different exposed facets of hematite was studied. The results reveal that the tremendous improvement of arsenate adsorption caused by OVDs on the (110) facet compared to (001) and (012) facets was attributed to their stronger bonding strength of As to under-coordinated Fe atoms, thus significantly promoting the immobilization of arsenate. The findings of this study enhance our ability to precisely understand the migration and fate of As while also aiding in the design of highly efficient iron mineral materials for mitigating As pollution.

Coexisting ferrihydrite-enhanced contaminant degradation during pyrite oxygenation

Oxygenation of pyrite (Py) is known to mediate generation of reactive oxygen species (ROS) with these species capable of inducing contaminants degradation, whereas the possible participation of coexisting Fe(III) minerals in these processes is still unclear. This study finds that freshly formed ferrihydrite (Fh) significantly enhances the Py-mediated sulfamethoxazole (SMX) degradation process. Through the 56Fe isotope tracer experiment and a series of control experiments, Fh is found to be reduced by Py to form secondary solid-phase Fe(II) species (Fe(II)RF) which in turn facilitates generation of H2O2 from the O2 reduction pathway. However, Py was found to mediate rapid structural transformation of Fh to form more thermodynamically stable goethite and hematite with these less redox active minerals unable to sustainably promote the Py-mediated SMX degradation process. Therefore, the improvement of Fh on Py-mediated SMX degradation process is not readily observable in reaction systems with low concentrations of coexisting Fh. In comparison, continuing input of 10 mM Fh increased the degradation efficiency of SMX by 60 % over three days. Our results demonstrate that the oxidative degradation of organic contaminants over the oxygenation of Py when coexisting with Fh can be more significant but currently underestimated.

Polystyrene microplastics facilitate formation of refractory dissolved organic matter and reduce CO2 emissions

Microplastics, as a type of anthropogenic pollution in aquatic ecosystems, affect the carbon cycle of organic matter. Although some studies have investigated the effects of microplastics on dissolved organic matter (DOM), the impact of alterations in the chemical properties of microplastics on refractory DOM and carbon release remains unclear. Here, we observed that microplastic treatments (e.g., polystyrene, PS) altered the composition and function of microbial community, notably increasing the abundance of microbial families involved in consuming easily degradable organic matter. During the process in which microbial community decomposed organic matter into DOM, PS underwent surface oxidation. The oxidized PS aggregated with DOM and microorganisms through electrostatic interactions and chemical bonds. Moreover, these interactions between oxidized PS and microbial community affect the utilization of organic matter, resulting in a significant decrease in CO2 emissions. Specifically, total CO2 emissions decreased by approximately 23.76 % with 0.1 mg/L PS treatment and by 44.97 % with 10 mg/L PS treatment compared to those in PS-free treatments over the entire reaction. These findings underscored the significance of the chemical properties of PS in the interactions among DOM and microorganisms, emphasizing the potential impact of PS microplastics on the carbon cycle in ecosystems.

Insights into the Crystallinity-Dependent Photochemical Productions of Reactive Oxygen Species from Iron Minerals

Iron minerals are widespread in earth's surface water and soil. Recent studies have revealed that under sunlight irradiation, iron minerals are photoactive on producing reactive oxygen species (ROS), a group of key species in regulating elemental cycling, microbe inactivation, and pollutant degradation. In nature, iron minerals exhibit varying crystallinity under different hydrogeological conditions. While crystallinity is a known key parameter determining the overall activity of iron minerals, the impact of iron mineral crystallinity on photochemical ROS production remains unknown. Here, we assessed the photochemical ROS production from ferrihydrites with different degrees of crystallinity. All examined ferrihydrites demonstrated photoactivity under irradiation, resulting in the generation of hydrogen peroxide (H2O2) and hydroxyl radical (•OH). The photochemical ROS production from ferrihydrites increased with decreasing ferrihydrite crystallinity. The crystallinity-dependent photochemical •OH production was primarily attributed to conduction band reduction reactions, with the reduction of O2 by conduction band electrons being the rate-limiting key process. Conversely, the crystallinity of iron minerals had a negligible influence on photon-to-electron conversion efficiency or surface Fenton-like activity. The difference in ROS productions led to a discrepant degradation efficiency of organic pollutants on iron mineral surfaces. Our study provides valuable insights into the crystallinity-dependent ROS productions from iron minerals in natural systems, emphasizing the significance of iron mineral photochemistry in natural sites with abundant lower-crystallinity iron minerals such as wetland water and surface soils.

Natural organic small molecules promote the aging of plastic wastes and refractory carbon decomposition in water

Microplastics and nanoplastics are ubiquitous in rivers and undergo environmental aging. However, the molecular mechanisms of plastic aging and the in-depth effects of aging on ecological functions remain unclear in waters. The synergies of microplastics and nanoplastics (polystyrene as an example) with natural organic small molecules (e.g., natural hyaluronic acid and vitamin C related to biological tissue decomposition) are the key to producing radicals (•OH and •C). The radicals promote the formation of bubbles on plastic surfaces and generate derivatives of plastics such as monomer and dimer styrene. Nanoplastics are easier to age than microplastics. Pristine plastics inhibit the microbial Shannon diversity index and evenness, but the opposite results are observed for aging plastics. Pristine plastics curb pectin decomposition (an indicator of plant-originated refractory carbon), but aging plastics promote pectin decomposition. Microplastics and nanoplastics undergoing aging processes enhance the carbon biogeochemical cycle. For example, the increased carbohydrate active enzyme diversity, especially the related glycoside hydrolase and functional species Pseudomonas and Clostridium, contributes to refractory carbon decomposition. Different from the well-studied toxicity and aging of plastic pollutants, this study connects plastic pollutants with biological tissue decomposition, biodiversity and climate change together in rivers.

Highly active complexes of pyrite and organic matter regulate arsenic fate

Arsenic (As) presents high toxicity and strong carcinogenicity, and its health risks are regulated by its oxidation state and speciation. As can form complexes with the surface of minerals or organic matter through adsorption, affecting its toxicity and bioavailability. However, the regulation effect of the interaction of coexisting minerals and organic matter on As fate remains largely unknown. Here, we discovered that minerals (e.g., pyrite) and organic matter (e.g., alanyl glutamine, AG) can form pyrite-AG complexes, promoting As(III) oxidation under simulated solar irradiation. The formation of pyrite-AG was explored in terms of the interaction of surface oxygen atoms, electron transfer and crystal surface changes. From the perspective of atoms and molecules, pyrite-AG showed more oxygen vacancies, stronger reactive oxygen species (ROS) and a higher electron transport capacity than pyrite alone. Compared with pyrite, pyrite-AG effectively promoted the conversion of highly toxic As(III) to less toxic As(V) due to the enhanced photochemical properties. Moreover, quantification and capture of ROS confirmed that hydroxyl radicals (•OH) played an important role in As(III) oxidation in the pyrite-AG and As(III) system. Our results provide previously unidentified perspectives on the effects and chemical mechanisms of highly active complexes of mineral and organic matter on As fate and provide new insights into the risk assessment and control of As pollution.

FexO/FeNC modified activated carbon packing media for biological slow filtration to enhance the removal of dissolved organic matter in reused water

The biological slow filtration reactor (BSFR) process has been found to be moderately effective for the removal of refractory dissolved organic matter (DOM) in the treatment of reused water. In this study, bench scale experiments were conducted using a mixture of landscape water and concentrated landfill leachate as feed water, to compare a novel iron oxide (Fe_xO)/FeNC modified activated carbon (Fe_xO@AC) packed BSFR, with a conventional activated carbon packed BSFR (AC-BSFR), operated in parallel. The results showed that the Fe_xO@AC packed BSFR had a refractory DOM removal rate of 90%, operated at a hydraulic retention time (HRT) of 10 h at room temperature for 30 weeks, while under the same conditions the removal by the AC-BSFR was only 70%. As a consequence, the treatment by the Fe_xO@AC packed BSFR substantially reduced the formation potential of trihalomethanes, and to a less extent, haloacetic acids. The modification of Fe_xO/FeNC media raised the conductivity and the oxygen reduction reaction (ORR) efficiency of the AC media to accelerate the anaerobic digestion by consuming the electrons that are generated by anaerobic digestion itself, which lead to the marked improvement in refractory DOM removal.