Three-dimensional Excitation-emission Matrix Fluorescence Research Articles

Synergistic effect of N doping and oxygen vacancies over TiO2 nanosheets with enhanced photocatalytic removal of tetracycline

The increasing pollution from non-biodegradable antibiotics poses a significant threat to ecosystems and human health. TiO2 is widely acknowledged as the premier photocatalyst for removing antibiotics from wastewater. To address the limitations of TiO2 in wide forbidden band width and low efficiency separation/migration of carriers, N-doping and oxygen vacancies (OVs) are co-modified within TiO2 nanosheets through the one-step calcination of a mixture of H2TiO3 and urea. The results indicate that the co-modified sample (TiO2-U5) exhibited the highest TC removal efficiency of 90% after 60 minutes of reaction, with a kinetic constant 3.6 times higher than that of pristine TiO2. The effective mineralization of TC is confirmed by three-dimensional excitation-emission matrix fluorescence spectroscopy (3D EEMs). Systematic characterization of the photocatalyst structure and monitoring of intermediates/products reveal that the synergistic effect of N-doping and oxygen vacancies (OVs) enhances band alignment, light response, and carrier separation/migration, thus improving the photocatalytic degradation performance of TC. This study highlights the significant potential of adjusting charge transport and enriching active sites through the co-introduction of doping and surface defects on TiO2 for the photocatalytic degradation of antibiotics.

Mechanistic insights of efficient aromatic organic compounds oxidation using biochar derived from coking wastewater sludge

Coking wastewater sludge possessed potential environmental hazards and pyrolysis was a desirable approach which could converted it into one functional carbonous material. The sludge-based biochar (SC-N3) originated from the feedstock of sludge from the regenerated water treatment had various iron-containing species and exhibited the effective catalytic ability. Compared with the degradation of the four pollutants (i.e., phenol, pyridine, quinoline and indole) alone, the mixed of the four pollutants in simulated coking wastewater exhibited a prominent different degradation trend, where the removal rates of indole and quinoline were enhanced from 45% and 30% to 98.9% and 82.3%, respectively. The quinone intermediates from phenol degradation, played a critical role in enhancing the removal rates of indole and quinoline as the electron transfer medium. Both the free radical pathway dominated by •OH and non-radical pathway with 1O2 play the main role in indole degradation, while •OH was the main factor in the catalytic degradation of phenol, quinoline and pyridine. By analysis the intermediates elucidated by Fourier transform ion cyclotron resonance mass spectrometry and three-dimensional excitation-emission matrix fluorescence spectra, the degradation molecular characteristics and pathways of the four pollutants in simulated coking wastewater were deciphered. The findings indicated that the catalyst SC-N3 prepared from the coking wastewater sludge presented unique catalytic properties and possessed a promising application in the degradation of refractory wastewater.

Effect of nitrogen retention composite additives Ca(H2PO4)2 and MgSO4 on the degradation of lignocellulose, compost maturation, and fungal communities in compost.

This study investigated the effects of the nitrogen retention composite additives Ca(H2PO4)2 and MgSO4 on lignocellulose degradation, maturation, and fungal communities in composts. The study included control (C, without Ca(H2PO4)2 and MgSO4), 1% Ca(H2PO4)2 + 2% MgSO4 (CaPM1), 1.5% Ca(H2PO4)2 + 3% MgSO4 (CaPM2). The results showed that Ca(H2PO4)2 and MgSO4 enhanced the degradation of total organic carbon (TOC) and promoted the degradation of lignocellulose in compost, with CaPM2 showing the highest TOC and lignocellulose degradation. Changes in the three-dimensional excitation-emission matrix fluorescence spectroscopy (3D-EEM) of dissolved organic matter (DOM) components in compost indicated that the treatment group with the addition of Ca(H2PO4)2 and MgSO4 promoted the production of humic acids (HAs) and increased the degree of compost decomposition, with CaPM2 demonstrating the highest degree of decomposition. The addition of Ca(H2PO4)2 and MgSO4 modified the composition of the fungal community. Ca(H2PO4)2 and MgSO4 increased the relative abundance of Ascomycota, decreased unclassified_Fungi, and Glomeromycota, and activated the fungal genera Thermomyces and Aspergillus, which can degrade lignin and cellulose during the thermophilic stage of composting. Ca(H2PO4)2 and MgSO4 also increased the abundance of Saprotroph, particularly undefined Saprotroph. In conclusion, the addition of Ca(H2PO4)2 and MgSO4 in composting activated fungal communities involved in lignocellulose degradation, promoted the degradation of lignocellulose, and enhanced the maturation degree of compost.

S-type heterojunction enhance the photocatalytic activity of TiO2/AgBr/Ag using loofah sponge as template towards ciprofloxacin degradation

Photocatalyst TiO2/AgBr/Ag (TBA) composites using loofah sponge as a template were successfully prepared for the first time. The photocatalytic activities of the TBA composites were evaluated by removing ciprofloxacin (CIP) under visible light. The PDOS shows that the valence band edge of TBA-30 is primarily composed of Ti 3d atom states, with small contributions from O 2p atom states. The conduction band edge is dominated by Ag 4d and O 2p atom states, with minor contributions from Br 4p atom states. TBA-30 composites demonstrate outstanding photocatalytic performance, degrading 87% of the target within 180 min. The degradation rate of CIP decreased to 70.19% after five runs, Furthermore, the TOC removal efficiency of CIP reached 84.2% in the end. CIP intermediates and degradation pathway were determined by GC-MS. Three-dimensional excitation-emission matrix fluorescence spectra (3D EEM) were used to study the degradation of CIP molecules. Trapping experiments and electron paramagnetic resonance (EPR) characterisation identified superoxide (·O2-) and photogenerated holes (h+) as the primary active radicals. Furthermore, formed S-type heterojunction and Ag as the electron capture site enhanced the separation efficiency of photoinduced electron-hole pairs. The DFT calculation was used to study the electron transport performance of TBA-30 by calculating the charge density difference.

Research on enhancing sewage sludge lysis and bioreactor initiation through composite thermophilic strains

Returning lysed sludge to biochemical tanks to achieve in-situ sludge reduction is promising. A novel approach for regulating and initiating the sludge lysis process utilizing thermophilic strains is proposed. In this study, three strains were isolated from sludge, and sequencing of the 16 S rDNA amplicon revealed that the three thermophilic strains were the closest relatives to the genus Geobacillus, and they were designated as Geobacillus sp. DX5, Geobacillus sp. DX8 and Geobacillus sp. DX11. Sludge lysis experiments demonstrated that composite thermophilic strains (CTS) were dosed at 65°C, and the sludge VSS removal efficiency after 24 h was 37.8%, significantly surpassing individual strains. Moreover, 65.8% of proteins and 52.0% of polysaccharides in Extracellular polymeric substances (EPS) of sludge were degraded. Furthermore, Three-dimensional excitation-emission matrix (3D-EEM) fluorescence spectroscopy results revealed a substantial release of dissolved organic matter during sludge lysis. Inoculation of CTS under optimal conditions (65°C, pH 7, 4 × 1010cfu/L) resulted in VSS removal efficiency and protease activity of 42.7% and 2584 U/L, respectively. After 20 days of inoculation with CTS to initiate sludge lysis reactor, VSS removal efficiency reached 46%. Additionally, it stimulated the growth of the thermophilic bacterial community, engaging indigenous thermophilic strains in the sludge lysis process.

Hydrothermal carbonization of Chinese medicine residues: Formation of humic acids and combustion performance of extracted hydrochar

Aiming at the sustainable management of high-moisture Chinese medicine residues (CMR), an alternative way integrating hydrothermal carbonization (HTC), humic acids (HAs) extraction and combustion of remained hydrochar has been proposed in this study. Effect of HTC temperature, HTC duration, and feedwater pH on the mass yield and properties of HAs was examined. The associated formation mechanism of HAs during HTC was proposed. The combustion performance of remained hydrochar after HAs extraction was evaluated. Results show that the positive correlation between hydrochar yield and HAs yield is observed. According to three-dimensional excitation emission matrix (3D EEM) fluorescence intensity, the best quality of HAs is achieved with a yield of 8.17 % at feedwater pH of 13 and HTC temperature of 200 °C. Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy analyses show abundant aromatic and aliphatic structure as well as oxygenated functional groups in HAs, which is like commercial HAs (HA-C). Besides, in terms of comprehensive combustion index (CCI), HTC can improve the combustion performance of CMR, while it becomes a bit worse after HAs extraction. Higher weighted mean apparent activation energy (Em) of hydrochar indicating its highly thermal stability. HAs extraction reduces Em and CCI of remained hydrochar. However, it can be regarded a potential renewable energy. This work confirms a more sustainable alternative way for CMR comprehensive utilization in near future.

Analysing N-nitrosamine occurrence and sources in karst reservoirs, Southwest China.

N-nitrosamines in reservoir water have drawn significant attention because of their carcinogenic properties. Karst reservoirs containingdissolved organic matter(DOM) are important drinking water sources and are susceptible to contamination because of the fast flow of various contaminants. However, it remains unclear whether N-nitrosamines and their precursor, DOM, spread in karst reservoirs. Therefore, this study quantitatively investigated the occurrence and sources of N-nitrosamines based on DOM properties in three typical karst reservoirs and their corresponding tap water. The results showed that N-nitrosamines were widely spread, with detection frequencies > 85%. Similar dominant compounds, including N-nitrosodimethylamine, N-nitrosomethylethylamine, N-nitrosopyrrolidine, and N-nitrosodibutylamine, were observed in reservoirs and tap water, with average concentrations of 4.7-8.9 and 2.8-6.7ng/L, respectively. The average carcinogenic risks caused by these N-nitrosamines were higher than the risk level of 10-6. Three-dimensional fluorescence excitation-emission matrix modeling revealed that DOM was composed of humus-like component 1 (C1) and protein-like component 2 (C2). Fluorescence indicators showed that DOM in reservoir water was mainly affected by exogenous pollution and algal growth, whereas in tap water, DOM was mainly affected by microbial growth with strong autopoietic properties. In the reservoir water, N-nitrosodiethylamine and N-nitrosopiperidine were significantly correlated with C2 and biological indicators, indicating their endogenously generated sources. Based on the principal component analysis and multiple linear regression methods, five sources of N-nitrosamines were identified: agricultural pollution, microbial sources, humus sources, degradation processes, and other factors, accounting for 46.8%, 36.1%, 7.82%, 8.26%, and 0.96%, respectively. For tap water, two sources, biological reaction processes, and water distribution systems, were identified, accounting for 75.7% and 24.3%, respectively. Overall, this study presents quantitative information on N-nitrosamines' sources based on DOM properties in typical karst reservoirs and tap water, providing a basis for the safety of drinking water for consumers.

Selenium bio-nanocomposite based on extracellular polymeric substances (EPS): Synthesis, characterization and application in alleviating cadmium toxicity in rice (Oryza sativa L.)

Selenium nanoparticles (SeNPs) have gained significant attention owing to their favorable bioavailability and low toxicity, making them widely applications in the fields of medicine, food and agriculture. In this study, bacterial extracellular polymeric substances (EPS) were used as a novel stabilizer and capping agent to prepare dispersed SeNPs. Results show that EPS-SeNPs presented negative potential (−38 mV), spherical morphologies with average particle size about 100–200 nm and kept stable at room temperature for a long time. X-ray diffraction (XRD) analysis demonstrated that the synthesized nanoparticles were pure amorphous nanoparticles, and X-ray photoelectron spectroscopy (XPS) spectrum showed a spike at 55.6 eV, indicating the presence of zero-valent nano‑selenium. Fourier-transform infrared spectroscopy (FTIR) and three-dimensional excitation-emission matrix (3D-EEM) fluorescence spectroscopy analysis confirmed proteins and polysaccharides in EPS played a crucial role in the synthesis of EPS-SeNPs. Compared to EPS or sodium selenite (Na2SeO3), EPS-SeNPs showed a relatively moderate result in terms of scavenging free radicals in vitro. In contrast, EPS-SeNPs demonstrated lower toxicity to rice seeds than Na2SeO3. Notably, the exogenous application of EPS-SeNPs effectively alleviated the growth inhibition and oxidative damaged caused by cadmium (Cd), and significantly reduced Cd accumulation in rice plants.

Humification levels of dissolved organic matter in the eastern plain lakes of China based on long-term satellite observations

Under the influence of intensive human activities and global climate change, the sources and compositions of dissolved organic matter (DOM) in the eastern plain lake (EPL) region in China have fluctuated sharply. It has been successfully proven that the humification index (HIX), which can be derived from three-dimensional excitation-emission matrix fluorescence spectroscopy, can be an effective proxy for the sources and compositions of DOM. Therefore, combined with remote sensing technology, the sources and compositions of DOM can be tracked on a large scale by associating the HIX with optically active components. Here, we proposed a novel HIX remote sensing retrieval (IRHIX) model suitable for Landsat series sensors based on the comprehensive analysis of the covariation mechanism between HIX and optically active components in different water types. The validation results showed that the model runs well on the independent validation dataset and the satellite-ground synchronous sampling dataset, with an uncertainty ranging from 30.85 % to 36.92 % (average ± standard deviation = 33.6 % ± 3.07 %). The image-derived HIX revealed substantial spatiotemporal variations in the sources and compositions of DOM in 474 lakes in the EPL during 1986–2021. Subsequently, we obtained three long-term change modes of the HIX trend, namely, significant decline, gentle change, and significant rise, accounting for 74.68 %, 17.09 %, and 8.23 % of the lake number, respectively. The driving factor analysis showed that human activities had the most extensive influence on the DOM humification level. In addition, we also found that the HIX increased slightly with increasing lake area (R2 = 0.07, P < 0.05) or significantly with decreasing trophic state (R2 = 0.83, P < 0.05). Our results provide a new exploration for the effective acquisition of long-term dynamic information about the sources and compositions of DOM in inland lakes and provide important support for lake water quality management and restoration.

The influence of micro-flocculation on membrane fouling during ultrafiltration of dissolved organic matter

This study investigated the mitigating effect of micro-flocculation as an ultrafiltration (UF) membrane pretreatment process on membrane fouling caused by organic matter in urban micro-polluted river water. The removal characteristics of organic matter by micro-flocculation were analyzed using three-dimensional fluorescence excitation emission matrix (3D-FEEM) and liquid chromatography organic carbon detector (LC-OCD). Furthermore, the variation of floc properties such as flocculation index and fractal dimension of micro-flocs formed under different flocculant dosages and their effects on UF membrane flux were investigated. The experimental results show that micro-flocculation using ferric chloride (FeCl3) can effectively remove large molecular weight organics. The features of flocs, which are influenced by the amount of FeCl3, affect the formation of the cake layer. Micro-flocculation can control the formation of a floc cake layer with a larger size and lower fractal dimension. It is helpful to effectively delay the attenuation rate of membrane flux by reducing membrane pore blockage and forming a filter cake layer with high porosity, which is of great significance for improving membrane technology. After the FeCl3 micro-flocculation treatment, the final specific flux of the membrane increased by 57.0 %. The combination of FeCl3 micro-flocculation and UF is a superior technical and economical treatment scheme for micro-polluted river water.

Unveiling the molecular responses of dissolved organic matter in landfill leachate to activated carbon adsorption

Powdered activated carbon (PAC) is an effective method for removing dissolved organic matter (DOM) in membrane bioreactor effluent of landfill leachate (MBR effluent). Among commercially available PACs, wood-based PAC (WoAC) and coal-based PAC (CoAC) are two most common ones. However, the interactions between two PACs and DOM in MBR effluent at the molecular level as well as the effect of PACs properties on the adsorption of DOM compounds are not fully understood. To address the gap, Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), three-dimensional fluorescence excitation-emission matrix (EEM) spectroscopy and other spectroscopic techniques were employed to study the change of DOM in MBR effluent when adsorbed by WoAC and CoAC. The results showed that CoAC had a better removal performance of DOM (89.2% for UV254 and 78.4% for DOC) than WoAC (80.9% for UV254 and 70.8% for DOC), which was likely due to the Fe component in CoAC playing an important role in the DOM adsorption process. Both WoAC and CoAC exhibited preferential adsorption for compounds with high unsaturation or poor oxygen. Moreover, the Fe component in CoAC may bind with some functional groups (e.g., carboxyl groups) in unsaturated compounds, which may enhance the adsorption of unsaturated compounds. These findings deepen our understanding of PAC-DOM interactions at the molecular level and provide guidance for designing cost-effective PACs for landfill leachate treatment.

Direct and Indirect Effects of Land Use on Dissolved Organic Matter in a Typical Karst River

Land use can directly affect the abundance of riverine dissolved organic matter (DOM) by transporting terrestrial organic matter to rivers and can also indirectly enhance local production of DOM by increasing riverine nutrient loading. This study investigated the characteristics and spatial distribution of DOM components in the Furong River during the rainy season (July) using ultraviolet-visible light absorption spectroscopy (UV-VIS) and three-dimensional excitation emission matrix fluorescence spectroscopy-parallel factor analysis (EEM-PARAFAC) techniques. Furthermore, correlation analysis and the partial least squares path model (PLS-PM) were used to identify and quantify the direct and indirect impacts of land use on DOM at multiple scales. The results revealed that:① the direct effects of land use on DOM were generally stronger than the indirect effects. ② The responses of different DOM components to riverine nutrient status and land use varied, with dissolved organic carbon (DOC) and colored dissolved organic matter (CDOM) components being more susceptible to riverine nutrient status and fluorescent dissolved organic matter (FDOM) being more sensitive to land use. ③ The direct impact intensity of land use on DOC and CDOM fluctuated slightly with the spatial scale, but the total impact intensity had no visible spatial scale difference, and the direct impact intensity on the FDOM component decreased with the increase in spatial scale. ④ Dryland, urban and other construction land, patch density (PD), edge density (ED), and Shannon's diversity index (SHDI) were typical land use metrics that exacerbated DOM abundance, whereas paddy field, shrubland, largest patch index (LPI), and aggregation index (AI) were typical land use metrics that effectively mitigated DOM abundance. Total nitrogen (TN), nitrate nitrogen (NO3--N), and dissolved total phosphorus (DTP) were water quality parameters that were significantly affected by land use and were closely related to DOM components, that is, nitrogen and phosphorus played an important "intermediary" role in "land use-riverine DOM." FDOM could be used as indicators to measure the strength of terrestrial organic matter directly input to rivers by land use.

Enhanced disintegration mechanism of surplus activated sludge to improve dewatering by thermally activated persulfate oxidation under mild temperature.

The dewatering treatment is an essential process for the treatment and disposal of surplus activated sludge (SAS), and improving sludge dewatering performance is still a challenge and has become a research hotspot in recent years. The oxidation and disintegration of bacterial cells and extracellular polymeric substances (EPS) by active radicals produced by advanced oxidation processes (AOPs) were extremely promising to achieve deep sludge dewatering. This paper systematically studied the efficiency and mechanism of thermally activated persulfate (TAP) oxidation technology to the improvement of SAS dewatering performance. The results showed that the relative filterability (CST0/CST) was increased 2.52 times with 2.0 mmol/g VSS potassium peroxydisulfate (PDS) at 80 °C in 90 min. Under this condition, the Zeta potential of SAS significantly decreased from - 14.8 to - 1.44 mV, while the average particle size (dp50) decreased from 52.981 to 48.259 μm. Thermal treatment disrupted the sludge structure to release large amounts of EPS including polysaccharides and protein. Meanwhile, the results of three-dimensional excitation-emission matrix (3D-EEM) fluorescence spectra showed that the TAP treatment could expedite the disintegration of sludge, facilitating the decrease of total EPS content and conversion of tightly bound EPS (TB-EPS) to loosely bound EPS (LB-EPS) and soluble EPS (S-EPS). The mechanism of TAP process to improve SAS dewatering performance was revealed, which could contribute to breaking the bottleneck of sludge depth dewatering and provide a theoretical and technical basis for its practical application.

Engineering highly dispersed AgI nanoparticles on hierarchical In2S3 hollow nanotube to construct Z-scheme heterojunction for efficient photodegradation of insecticide imidacloprid

Increasing the exposure of active sites and improving the intrinsic activity are necessary considerations for designing a highly efficient photocatalyst. Herein, an In2S3/AgI stable Z-scheme heterojunction with highly dispersed AgI nanoparticles (NPs) is synthesized by the mild self-templated and in-situ ion exchange strategy. Impressively, the optimized In2S3/AgI-300 Z-scheme heterojunction exhibits superior photodegradation activity (0.020 min−1) for the decomposition of insecticide imidacloprid (IMD), which is extremely higher than that of pure In2S3 (0.002 min−1) and AgI (0.013 min−1). Importantly, the three-dimensional excitation-emission matrix (3D EEMs) fluorescence spectra, high-resolution mass spectrometry (HRMS), the photoelectrochemical tests, radical trapping experiment, and electron spin resonance (ESR) technique are performed to clarify the possible degradation pathway and mechanism of IMD by the In2S3/AgI-300 composite. The enhanced photocatalytic performance is attributed to the highly dispersed AgI NPs on hierarchical In2S3 hollow nanotube and the construction of In2S3/AgI Z-scheme heterojunction, which can not only increase active site exposure, but also improve its intrinsic activity, facilitating rapid charge transfer rate and excellent electron-hole pairs separation efficiency. Meanwhile, the practical application potential of the In2S3/AgI-300 composite is systematically investigated. This study opens a new insight for designing catalysts with high photocatalytic performance through a convenient approach.

Ceramic membrane reactor integrated with UV/O3/Catalyst Beads for treating real textile wastewater: Enhanced effluent quality, fouling control and molecular transformations of DOM

A ceramic membrane reactor (CMR) was employed to treat real textile wastewater with the simultaneous use of UV, O3, and photocatalytic beads for dissolved organic matter (DOM) transformation and membrane fouling control. The O3 treatment exhibited high performance in color removal. Simultaneous use of UV and ozonation (UV/O3) resulted in 91% and 64% color and chemical oxygen demand (COD) removal, respectively, and delayed (3-fold) fouling time compared to UV alone. A combination of TiO2-loaded polymeric beads with UV/O3 (UV/O3/Beads) further enhanced the color (96%) and COD (86%) removal as well as improved fouling control by 9-folds compared to the control (Air alone) experiment. Three-dimensional excitation-emission matrix (3-D EEM) fluorescence spectra revealed minimum DOM and synergistic effects of UV, O3, and photocatalytic beads (UV/O3/Beads) in the degradation of fluorescent DOM. The •OH radicals were leading in degrading organic pollutants during the treatment. Transformations of heteroatomic DOMs were confirmed by the Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). S-containing compounds in the treated samples appeared in a reduced state (lower H/C and O/C ratios), indicating the transformation of R–SO3 to R–S–R. Whereas N-containing DOM was mostly oxidized. UV/O3 rapidly transformed dye molecules to colorless, creating suitable conditions for photocatalytic beads for accelerated degradation of pollutants. Also, the PTR-TOF-MS analysis revealed the successful removal of toxic VOCs (Methanethiol, Allylamine, Cyclohexenes, Butanes, Propanes, and Hydrazines) from wastewater by the UV/O3/Beads treatment. Based on these findings, DOM transformations could be better understood, and the simultaneous application of heterogeneous photocatalysis and UV/O3 could be safely recommended as a promising technology for treating industrial wastewater.

Spatial variation of dissolved organic matter in the Tuojiang River Basin in Chengdu, China: insights based on EEMs-PARAFAC analysis.

Three-dimensional excitation-emission matrix fluorescence spectroscopy coupled with parallel factor analysis was adopted to investigate the characteristics of dissolved organic matter (DOM) components in water samples collected from the Tuojiang River Basin in Chengdu, including its main stream and tributaries. Four DOM components that matched with three fluorescence peaks were identified in the whole river basin and tributaries; while three components corresponding to four fluorescence peaks were identified in the main stream. In all cases, humic-like components accounted for high proportions of the DOM. Correlation analysis revealed the same sources for four components in the whole river basin and its tributaries, whereas two components had different sources in the main stream. Ultraviolet absorbance parameters (SUVA254, SR) and fluorescence parameters (BIX, HIX, FI, β:α) indicated the dominant autochthonous sources of DOM in the whole river basin. Higher terrestrial inputs of DOM were observed in the tributaries than in the main stream. In the areas influenced by human activities (6#, 17#, 18#), the sources of DOM showed strong terrestrial characteristics and high degrees of humification and aromatization, as well as serious pollution. The results of this study have potentially far-reaching implications for environmental water management in the area.

Diurnal and seasonal variations of pCO2 and fluorescent dissolved organic matter (FDOM) in different polluted lakes.

This study aimed to assess pollution and daily-to-seasonal dynamics of the partial pressure of CO2 (pCO2) and CO2 degassing flux concerning the fluorescent dissolved organic matter (FDOM) from tropical lakes. A membrane-enclosed pCO2 sensor and water quality multimeter analyzer was deployed to continuously record daily and seasonal variations in pCO2 and CO2 degassing flux in three lakes in Savar, Dhaka. During both wet and dry seasons, all lake water was supersaturated with CO2 in contrast to the atmospheric equilibrium (~400 μatm). The pCO2 values in the lake water during the dry season were relatively low in comparison, and the pCO2 levels in the wet season were much higher due to external inputs of organic matter from watersheds and direct inputs of CO2 from soils or wetlands. The estimated water-to-air CO2 degassing flux in the different levels of polluted lakes varies with the pollution context. Study areas calculated the carbon flux and three lakes released respectively 86.75×107g CO2 year-1, 13.8×107g CO2 year-1, and 9.17×107g CO2 year-1. Three-dimensional excitation-emission matrix (3D-EEM) fluorescence spectroscopy combined with parallel factor (PARAFAC) analysis was used to investigate the distributions of fluorescent components in DOM. EEM-PARAFAC analysis identified humic-like, fulvic-like, protein-like, and more tyrosine-like FDOM components and their environmental dynamics. Terrestrial DOM may provide inputs to the terrestrial humic-like component in the lake water. In contrast, the biological activity of plankton-derived FDOM is the most likely source for the autochthonous humic-like component. FDOM and DO concentrations have negative correlations with pCO2, indicating that when the FDOM and DO level is decreased, the amount of pCO2 values increases.

Remote sensing identification of urban water pollution source types using hyperspectral data

Owing to accelerated urbanisation, increased pollutants have degraded urban water quality. Timely identification and control of pollution sources enable relevant departments to effectively perform water treatment and restoration. To achieve this goal, a remote sensing identification method for urban water pollution sources applicable to unmanned aerial vehicle (UAV) hyperspectral images was established. First, seven fluorescent components were obtained through three-dimensional excitation-emission matrix fluorescence spectroscopy of dissolved organic matter (DOM) combined with parallel factor analysis. Based on the hierarchical cluster analysis of the seven fluorescence components and three spectral indices, four pollution source (PS) types were determined, namely, domestic sewage, terrestrial input, agricultural and algal, and industrial wastewater sources. Second, several water colour and optical parameters, including the absorption coefficient of chromophoric DOM at 254 nm, humification index, chlorophyll-a concentration, and hue angle, were utilised to develop an identification method with a recognition accuracy exceeding 70% for the four PSs that is suitable for UAV hyperspectral data. This study demonstrated the potential of identifying PSs by combining the fluorescence characteristics of DOM with the optical properties of water, thus expanding the application of remote sensing technologies and providing more comprehensive and reliable information for urban water quality management.

DOM accumulation in the hyporheic zone promotes geogenic Fe mobility: A laboratory column study

Hyporheic zone (HZ) systems have a natural purification capacity, and they are commonly used to provide high quality drinking water. However, the presence of organic contaminants in HZ systems in anaerobic environments causes the aquifer sediments to release metals (e.g., Fe) at levels above drinking water standards, which affects the quality of groundwater. In this study, the effects of typical organic pollutants (dissolved organic matter (DOM)) on Fe release from anaerobic HZ sediments were investigated. Ultraviolet fluorescence spectroscopy, three-dimensional excitation-emission matrix fluorescence spectroscopy, excitation–emission matrix spectroscopy coupled with parallel factor analysis and Illumina MiSeq high-throughput sequencing were used to determine the effects of the system conditions on Fe release from HZ sediments. Compared with the control conditions (low traffic and low DOM as a baseline), the Fe release capacity was enhanced by 26.7 % and 64.4 % at low flow rate (85.8 m/d) and high organic matter concentration (1200 mg/L), which was consistent with the residence-time effect. The transport of heavy metals under different system conditions varied with the influent organic composition. The influent organic matter composition and fluorescence parameters (the humification index, biological index and fluorescence index) were closely related to the release of the Fe effluent, while these factors had less influence on Mn and As. From 16S rRNA analysis of the aquifer media at different depths at the end of the experiment, under low flow rate and high influent concentration conditions, reduction of Fe minerals by Proteobacteria, Actinobacteriota, Bacillus, and Acidobacteria promoted the release of Fe. These functional microbes play an active role in the Fe biogeochemical cycle in addition to reducing Fe minerals to promote Fe release. In summary, this study reveals the effects of the flow rate and influent DOM concentration on the release and biogeochemistry of Fe in the HZ. The results presented herein will contribute to a better understanding of the release and transport of common groundwater contaminants in the HZ and other groundwater recharge environments.

Selective associations of organic matter components with ferrihydrite: Implications for Fe-organic matter preservation in tidal flat wetlands

The association of organic matter (OM) with iron (Fe) is a critical mechanism to preserve OM in intertidal coastal ecosystems such as salt marshes and mangroves. The present study aimed to investigate the influence of the organic carbon to Fe molar ratio (OC/Fe), ionic strength, and OM composition on Fe-OM association. Fe-OM was prepared using humic acids extracted from salt marsh and mangrove sediments by the adsorption process in the OC/Fe molar ratios of 0.01–1.0 and by the coprecipitation process in the OC/Fe molar ratios of 0.1–9.9. Sample properties were studied by X-ray diffraction, scanning electron microscopy, ultraviolet–visible spectroscopy, attenuated total reflection Fourier transform infrared spectroscopy, three-dimensional fluorescence excitation emission matrix spectroscopy, and pyrolysis–gas chromatography/mass spectrometry. The results showed that selective associations of OM components with ferrihydrite were affected by the association mode (e.g., adsorption and coprecipitation), OC/Fe molar ratio, OM composition and ionic strength. Abundant aliphatic compounds could be adsorbed on ferrihydrite. Moreover, ferrihydrite was prone to preserve humic-like fluorescent components through the adsorption process. The protein-like fluorescent components and aliphatic compounds had high affinity for ferrihydrite during coprecipitation with low OC/Fe molar ratios. An increase in the OC/Fe molar ratio resulted in coprecipitation of aromatic compounds with ferrihydrite. Salinity-induced flocculation caused more amount of aromatic and polycyclic aromatic compounds to associate with ferrihydrite during the coprecipitation process. Compared to the salt marsh OM, more N-compounds and less aliphatic compounds in the mangrove OM were coprecipitated with ferrihydrite. This study provided important insights into the mechanisms of OM preservation with ferrihydrite in tidal wetland ecosystems.