Polyethylene microplastic modulates lettuce root exudates and induces oxidative damage under prolonged hydroponic exposure.

Root exudates are pivotal in plant stress responses, however, the impact of microplastics (MPs) on their release and characteristics remains poorly understood. This study delves into the effects of 0.05 % and 0.1 % (w/w) additions of polyethylene (PE) MPs on the growth and physiological properties of lettuce (Lactuca sativa L.) following 28 days of exposure. The release characteristics of root exudates were assessed using UV-vis and 3D-EEM. The results indicated that PE increased leaf number but did not significantly affect other agronomic traits or pigment contents. Notably, 0.05 % PE increased the total root length and surface area compared to the 0.1 % addition, while a non-significant trend towards decreased root activity was observed with PE MPs. PE MPs with 0.1 % addition notably reduced the DOC concentration in root exudates by 37.5 %, while 0.05 % PE had no impact on DOC and DON concentrations. PE addition increased the SUVA254, SUVA260, and SUVA280 values of root exudates, with the most pronounced effect seen in the 0.05 % PE treatment. This suggests an increase of aromaticity and hydrophobic components induced by PE addition. Fluorescence Regional Integration (FRI) analysis of 3D-EEM revealed that aromatic proteins (region I and II) were dominant in root exudates, with a slight increase in fulvic acid-like substances (region III) under 0.1 % PE addition. Moreover, prolonged PE exposure induced ROS damage in lettuce leaves, evidenced by a significant increase in content and production rate of O2·-. The decrease in CAT and POD activities may account for the lettuce's response to environmental stress, potentially surpassing its tolerance threshold or undergoing adaptive regulation. These findings underscore the potential risk of prolonged exposure to PE MPs on lettuce growth.

N-doped citrate-sludge-derived carbon (NCSC) effectively promotes peroxymonosulfate activation for perfluorooctanoic acid (PFOA) removal with surface-mediated electron transfer mechanism.

In traditional wastewater treatment methods, the removal of emerging contaminants including perfluorooctanoic acid (PFOA) can be challenging. To address this, biochar is commonly utilized as an activator for peroxymonosulphate (PMS) to effectively eliminate organic pollutants. Sewage sludge has shown potential as a biochar precursor, but its complex composition and variable iron content, as well as the low specific surface area of the product limit the practical use of iron-dominated sludge-derived catalysts. To overcome this limitation, N-doped citrate-sludge-derived carbon (NCSC) was synthesized, possessing a low iron content (0.29 at%) and a large specific surface area (315.31 m2 g-1). As a comparison, Fe-/N-doped citrate-sludge-derived carbon (Fe-NCSC) was prepared by introducing exogenous iron, resulting in a higher iron content (2.12 at%) but a significantly reduced specific surface area (73.87 m2 g-1). In performance evaluation, the NCSC/PMS system achieved impressive removal efficiency, effectively eliminating 99.8% of PFOA (at an initial concentration of 2 mg L-1) within 60 min, while Fe-NCSC/PMS only achieved 84.6%. The slightly lower reaction rate per specific surface area of NCSC/PMS proved that large specific surface area was NCSC's key advantage. The lower sensitivity of NCSC to pH and water substrates than FeNCSC suggested different activation mechanisms. Further analysis of reactive sites and species showed that the main oxidation mechanism of NCSC/PMS was forming the surface-bound PMS-NCSC complexes at the N sites, followed by PFOA donating electrons to the complexes to be oxidized, which was different from the Fe/N-dominated singlet oxygen mechanism of Fe-NSC/PMS. Furthermore, the reusability of the NCSC was demonstrated, with the removal rate decreasing to only 90.1% after four cycles and recovering to 94.8% after heated regeneration. In conclusion, this study provides a viable method for the elimination of emerging contaminants such as PFOA in water remediation.

Multiple isotopes reveal the driving forces of nitrogen cycling from freshwater to brackish water.

Rivers play a crucial role in global nitrogen (N) cycling, but revealing the driving mechanism of N cycling remains challenging because of the complex natural background gradients. The Qiantang River Basin provides an opportunity to elucidate the driving mechanism due to the complex climatic and hydrological conditions. In this study, the multiple stable isotopes suggested that the conservative mixing of two end members was insufficient to explain the complex behavior of N in both seasons. In-soil processes were the primary N cycling processes that controlled riverine N loading during the wet season, whereas in-stream N biological transformation processes (nitrification and assimilation) were more prevalent during the dry season. The results of MixSIAR revealed that soil sources (soil N and N fertilizer) contributed the most to NO3- during the wet season, accounting for 64.3 %, followed by manure and sewage (31.6 %) and atmospheric precipitation (4.1 %). During the dry season, manure and sewage were the predominant contributors to NO3- (52.1 %), followed by soil N (26.6 %), N fertilizer (18.8 %), and atmospheric precipitation (2.5 %). The relationships between d-excess and δ15N-NH4+ or δ15N-NO3- suggested that both climatic and hydrological conditions would be the driving forces regulating the N transportation and transformation in this basin, leading to the high spatiotemporal heterogeneity in N loading and isotopic compositions. In the wet season, precipitation patterns served as the primary driving forces regulating in-soil biological processes and soil leaching. While the hydrological conditions, especially water residence time, were the crucial factors controlling in-stream biological processes during the dry season. This study elucidates N sources, biotransformation processes, and their driving forces from freshwater to brackish water, which has applications for understanding the N fate from terrene to ocean.

Automatic detection of suspected sewage discharge from coastal outfalls based on Sentinel-2 imagery.

Terrestrial pollution has a great impact on the coastal ecological environment, and widely distributed coastal outfalls act as the final gate through which pollutants flow into rivers and oceans. Thus, effectively monitoring the water quality of coastal outfalls is the key to protecting the ecological environment. Satellite remote sensing provides an attractive way to monitor sewage discharge. Selecting the coastal areas of Zhejiang Province, China, as an example, this study proposes an innovative method for automatically detecting suspected sewage discharge from coastal outfalls based on high spatial resolution satellite imageries from Sentinel-2. According to the accumulated in situ observations, we established a training dataset of water spectra covering various optical water types from satellite-retrieved remote sensing reflectance (Rrs). Based on the clustering results from unsupervised classification and different spectral indices, a random forest (RF) classification model was established for the optical water type classification and detection of suspected sewage. The final classification covers 14 optical water types, with type 12 and type 14 corresponding to the high eutrophication water type and suspected sewage water type, respectively. The classification result of model training datasets exhibited high accuracy with only one misclassified sample. This model was evaluated by historical sewage discharge events that were verified by on-site observations and demonstrated that it could successfully recognize sewage discharge from coastal outfalls. In addition, this model has been operationally applied to automatically detect suspected sewage discharge in the coastal area of Zhejiang Province, China, and shows broad application value for coastal pollution supervision, management, and source analysis.