Spatiotemporal variations of dissolved organic matter in a typical multi-source watershed in northern China: a fluorescent evidence

Fluorescence fingerprint as an indicator to identify urban non-point sources in urban river during rainfall period

Nowadays, the urban non-point source (NPS) pollution gradually evolved as the main contributor to urban water contamination since the point source pollution was effectively controlled. It was imperative to perform urban NPS identification in urban river to meet the requirements of precise source governance. In this study, the real-time detection about water quality parameters and fluorescence fingerprints (FFs) was performed for BX River and its outlets during rainfall period. EEM-PARAFAC and component similarity analyses discovered that the pollution encountered by BX River mainly came from road runoff and untreated municipal wastewater (UMWW) overflow. The C1 (tryptophan-like) and C3 (terrestrial humic-like) components located at Ex/Em = ∼230(280)/340 and ∼275/430 nm were both detected in these two kinds of urban NPS. The C2 components of road runoff and UMWW overflow displayed remarkable differences, which located at Ex/Em = 250/385 and 245/365 nm, respectively, thus could be served as indicators for distinguishing them. During rainfall period, the outflow from rainwater outlets (RWOs) constantly showed similar FF features to road runoff, while the FFs of outflow from combined sewer outlets (CSOs) alternated between those of road runoff and UMWW overflow. The FF features of sections in BX River changed in response to the dynamic variations in FFs of the outlets, which revealed real-time pollution causes of BX River. This work not only realized the identification and differentiation of urban NPS, but also elucidated the dynamic variations of pollution characteristics throughout the entire process of "urban NPS-outlets-urban river", and demonstrated the feasibility of FF technique in quickly diagnosing the pollution causes of urban river during rainfall period, which provided important guidance for urban NPS governance.

Interaction and mechanistic studies of thiram and common microplastics in food and associated changes in hazard

Microplastics (MPs), an emerging pollutant in the environment and food, may adsorb other contaminants such as pesticide due to particles' properties. The adsorption behavior and their hazardous changes of four common types of MPs to thiram was investigated. The adsorption kinetics and isotherm models were fitted well using the pseudo-second-order model and the Langmuir model respectively, indicating forces such as van der Waals forces dominate, and the maximum adsorption capacity at pH values of 6-7 also indicates that electrostatic forces play a smaller role. The adsorption thermodynamic studies showed that the adsorption was a spontaneous and exothermic process. The decrease in the adsorption capacity as the concentration of MPs increases and more adsorption by aged MPs demonstrate that the adsorption process is related to the sites on the surface of MPs. Due to the influence of ionic strength, the adsorption capacity of MPs for thiram increases significantly when the adsorption process takes place in solutions with salinity or in orange and apple juices. The bioavailability of thiram adsorbed by PVC and PET increased significantly. This study highlights the strong role of MPs in food as carriers of pesticide residues in food systems and that the combined hazard should also be of concern.

Aging behavior of biodegradable polylactic acid microplastics accelerated by UV/H2O2 processes

The usage of biodegradable plastics is expanding annually due to worldwide plastic limits, resulting in a substantial number of microplastics (MPs) particles formed from biodegradable plastic products entering the aquatic environment. Until now, the environmental behaviors of these plastic product-derived MPs (PPDMPs) have remained unclear. In this work, commercially available polylactic acid (PLA) straws and PLA food bags were used to evaluate the dynamic aging process and environmental behavior of PLA PPDMPs under UV/H2O2 conditions. By combining scanning electron microscopy, two-dimensional (2D) Fourier transform infrared correlation spectroscopy (COS) and X-ray photoelectron spectroscopy, it was determined that the aging process of the PLA PPDMPs was slower than that of pure MPs. The 2D-COS analysis revealed that the response orders for the functional groups on the PLA MPs differed during the aging process. The results demonstrated that the oxygen-containing functional groups of the PLA PPDMPs were the first to react. Subsequently, the -C-H and -C-C- structural responses began, and the polymer backbone was ruptured by the aging process. However, the aging of the pure-PLA MPs started with a brief oxidation process and then breakage of the polymer backbones, followed by continuous oxidation. Moreover, compared to the PLA PPDMPs, the pure-PLA MPs exhibited a greater adsorption capacity, which was increased by 88% after aging, whereas those of the two PPDMPs only increased by 64% and 56%, respectively. This work provides new insights into the behaviors of biodegradable PLA MPs in aquatic environments, which is critical for assessing the environmental risks and management policies for degradable MPs.

Field evidences of fluorescent dissolved organic matter (FDOM) as potential fingerprints for agricultural and urban sources in river environment

Field evidences of the fluorescence differences between agricultural and urban river reaches are still lack. In this study, the middle reaches of Danhe River (DH) and Mihe River (MH) in Shouguang, China, were designed as agricultural and urban river reaches, respectively, to compare the the fluorescence differences in disparate river reaches using excitation-emission matrix coupled with parallel factor analysis (EEM-PARAFAC). Three fluorescence components were identified. C1 (Ex/Em=230,255,295 nm/420 nm) was categorized as humic-like fluorophores, C2 (Ex/Em=230,275 nm/330 nm) was recognized as tryptophan-like substances, and C3 (Ex/Em=215 nm/290 nm) was noted as tyrosine-like and phenylalanine-like compounds. The results showed that the FDOM posed significant differences between agricultural and urban river reaches (P < 0.001). The monitoring sites in DH were rich in C2 (1.90 ± 0.62 Raman Unit (RU), mean ± standard deviation), and the monitoring sites in MH were rich in C3 (1.32 ± 0.51 RU). Redundancy analysis revealed that C2 could be regarded as a fluorescence indicator of agricultural sewage in river environment, while C3 was recognized as a fluorescence indicator of domestic sewage in river environment. In conclusion, this study provided field evidences of FDOM as potential fingerprints for agricultural and urban sources in river environment.

Characterizing the effects of stormwater runoff on dissolved organic matter in an urban river (Jiujiang, Jiangxi province, China) using spectral analysis

The effect of stormwater runoff on dissolved organic matter (DOM) in rivers is one of the central topics in water environment research. Jiujiang is one of the first cities established in the green development demonstration zone of the Yangtze River Economic Belt (Jiangxi Province, China). Three-dimensional excitation-emission matrix fluorescence with parallel factor analysis (3DEEM-PARAFAC) and ultraviolet-visible (UV-Vis) spectroscopy were used to explore the effects of runoff on organic matter in Shili River (Jiujiang, Jiangxi Province, China). The results show that the runoff led to an increase of some critical pollutants and DOM concentrations, especially in the middle reaches of the river. The concentration and relative molecular weight of DOM in water increased as a result of runoff. Three humic-like (C1-C3) and two protein-like (C4 and C5) components of DOM were identified using the PARAFAC model. The sources of the three humic-like components (C1, C2, C3) were consistent, unlike those of the protein-like component C4. Compared with the pre-rainfall period, the content of humus compounds flowing into the river through the early rainwater runoff was lower, which caused the relative content and proportion of humic substances little change and protein-like species increasing. The DOM mainly derived from autochthonous sources, and runoff had limited effect on its characteristics. Jiujiang is a key demonstration city for Yangtze River conservation. Rainwater runoff is one of the pollution sources of urban rivers, which leads to the deterioration of water quality and influences the distribution characteristics of DOM in water bodies. The PARAFAC components could adequately represent different indicators and sources of DOM in urban rivers, providing an important reference for urban river management.