Birnessite (δ-MnO2) is a layered manganese oxide widely present in the environment and actively participates in the transformation of natural organic matter (NOM) in biogeochemical processes. However, the effect of oxygen on the dynamic interface processes of NOM and δ-MnO2 remains unclear. This study systematically investigated the interactions between δ-MnO2 and fulvic acid (FA) under both aerobic and anaerobic conditions. FA was transformed by δ-MnO2 via direct electron transfer and the generated reactive oxygen species (ROS). During the 32-day reaction, 79.8% of total organic carbon (TOC) in solution was removed under anaerobic conditions, unexpectedly higher than that under aerobic conditions (69.8%), suggesting that oxygen limitation was more conducive to the oxidative transformation of FA by δ-MnO2. The oxygen vacancies (OV) on the surface of δ-MnO2 were more exposed under anaerobic conditions, thus promoting the adsorption and transformation of FA as well as regeneration of the active sites. Additionally, the reaction of FA with δ-MnO2 weakened the strongly bonded lattice oxygen (Olatt), and the released Olatt was an important source of ROS. Interestingly, a part of organic carbon (OC) was preserved by forming MnCO3, which might be a novel mechanism for carbon preservation. These findings contribute to an improved understanding of the dynamic interface processes between MnO2 and NOM and provide new insights into the effects of oxygen limitation on the cycling and preservation of OC.

Per-and polyfluoroalkyl substances (PFASs) have potential to accumulate in crops and pose health risks to humans, but it is unclear how the widely present organic matters in soil, such as humic acid (HA), affect their uptake and translocation in plants. In this study, hydroponic experiments were conducted to systematically disclose the impacts of HA on the uptake, translocation, and transmembrane transport at the subcellular level of four PFASs, including perfluorooctane sulfonic acid, perfluorooctanoic acid, perfluorohexane sulfonic acid, and 6:2 chlorinated polyfluoroalkyl ether sulfonate in wheat (Triticum aestivum L.). The results of the uptake and depuration experiments indicated that HA depressed the adsorption and absorption of PFASs in wheat roots by reducing the bioavailability of PFASs, and HA did not affect the long-range transport of PFASs to be eliminated via the phloem of wheat. However, HA facilitated their transmembrane transport in wheat roots, while the contrary effect was observed in the shoots. The inhibitor experiments coupled with transcriptomics analysis uncover that the increased transmembrane transport of PFASs stimulated by HA is mainly driven by the slow-type anion channel pathways interacting with Ca 2+ -dependent protein kinases (Ca 2+ -CDPK-SLAC1). The promoted transmembrane transport of PFASs might cause adverse effects on the plant cell wall, which causes further concerns.

Dissolved organic matter (DOM) such as fulvic acid (FA) and humic acid (HA) in soil considerably affects the fate of per- and polyfluoroalkyl substances (PFASs). However, the effect of DOM on their behavior in plants remains unclear. Herein, hydroponic experiments indicate that FA and HA reduce the accumulation of an emerging PFAS of high concern, 6:2 chlorinated polyfluoroalkyl ether sulfonate (6:2 Cl-PFESA), in wheat roots by reducing its bioavailability in the solution. Nevertheless, FA with low molecular weight (MW) promotes its absorption and translocation from the roots to the shoots by stimulating the activity and the related genes of the plasma membrane H+-ATPase, whereas high-MW HA shows the opposite effect. Moreover, in vivo and in vitro experiments indicate that 6:2 Cl-PFESA undergoes reductive dechlorination, which is regulated mainly using nitrate reductase and glutathione transferase. HA and FA, particularly the latter, promote the dechlorination of 6:2 Cl-PFESA in wheat by enhancing electron transfer efficiency and superoxide production. Transcriptomic analysis indicates that FA also stimulates catalytic activity, cation binding, and oxidoreductase activity, facilitating 6:2 Cl-PFESA transformation in wheat.

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