Purifying surface water contaminated with azo dyes using nanofiltration: Interactions between dyes and dissolved organic matter.

In this research, the interactions of two azo dyes, Methyl Orange (MO) and Eriochrome Black T (EBT), with dissolved organic matter (DOM) in surface water were studied, emphasizing their removal using nano-filtration membranes (NF-270 and NF-90). High-Performance Size Exclusion Chromatography (HPSEC) findings indicated that the dyes' molecular weight in deionized (DI) water ranged from 500 to 15k Dalton (Da), adjusting peak intensities with Jingmi River (JM) water Beijing. Notably, when dyes were diluted in JM water, ultraviolet (UV533 & 466, and UV254), together with total organic carbon (TOC) parameters, revealed color removal rates of 99.49% (EBT), 94.2% (MO), 87.6% DOM removal, and 86% TOC removal for NF-90. The NF-90 membrane demonstrated a 75% flux decline for 50 mL permeate volume due to its finer pore structure and higher rejection effectiveness. In contrast, the NF-270 membrane showed a 60% decline in flux under the same conditions. Attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR) analysis of dye-treated membranes in JM water revealed that the NF-270 showed a CC bond peak at 1660 cm-1 across various samples, while analyzing NF-90, the peaks at 1400 cm-1, 1040 cm-1, 750 cm-1, and 620 cm-1 disappeared for composite sample removal. The hydrophobicity of each membrane is measured by the contact angle (CA), which identified that initial CAs for NF-270 and NF-90 were 460 and 700, respectively, that were rapidly declined but stabilized after a few seconds of processing. Overall, this investigation shows that azo dyes interact with DOM in surface waters and enhance the removal efficiency of NF membranes.

Molecular-weight dependent promotion and competition effects of natural organic matter on dissolved black carbon removal by coagulation.

Dissolved black carbon (DBC) is the ubiquitous component of dissolved organic matter pools with the high reactivity for disinfection byproducts formation. However, it is unknown that the influence of molecular weight (MW) of natural organic matter (NOM) on the DBC removal from potable water sources. Therefore, it was studied that the DBC removal by coagulation in the presence of the NOM with various molecular weights. The DBC removal was promoted due to the presence of NOM and the promotion degree decreased with decreasing MW of NOM. Furthermore, the removal ratio of humic-like component increased as the MW of NOM decreased, suggesting that the competition between DBC and NOM increased with decreasing MW. The functional groups after coagulation were the same with that before coagulation as the MW of NOM varied, suggesting that the molecular structure was not the key factor of influencing the DBC removal. This study will give the deep insight into the prediction of the DBC removal ratio by coagulation based on the MW of NOM in water sources.

Synergistically enhancing the selective adsorption of cationic dyes through copper impregnation and amino functionality into iron-based metal-organic frameworks.

Dyes contaminating the sewages have seriously threatened the living beings and their separation from wastewater in terms of potential resource recovery is of high value. Herein, both of metal node doping and ligand group grafting were taken into account to enhance the adsorption selectivity of Fe-MOFs towards cationic dyes. The positive correlation between copper doping amount and selective coefficient (∂MOMB) for methylene blue (MB) over methyl orange (MO) within a certain range was mainly attributed to the increased surface negative charges via partial replacement of Fe(III) with Cu(II). Moreover, the amount of surface negative charges was further increased after amino functionalization and there was a synergism between Cu(II) and -NH2 in selectivity enhancement. As a result, Fe0.6Cu0.4-BDC-NH2 exhibited a 22.5-times increase in ∂MOMB and other cationic dyes including malachite green (MG) and rhodamine B (Rh. B) could also be selectively separated from binary and quaternary mixed dye systems. Moreover, Fe0.6Cu0.4-BDC-NH2 showed many superiorities like a wide pH range of 4.0-8.0, strong anti-interference ability over various inorganic ions, good recyclability, and stability. The adsorption kinetics and isotherm suggested that the MB adsorption process was a homogeneous single-layer chemisorption. Additionally, the thermodynamics manifested that the overall process was exothermic and spontaneous. According to the FT-IR and XPS spectra analysis, the electrostatic interaction and hydrogen bonding were determined as the main driving forces, and π-π interaction also contributed to the adsorption process.

Divergent Fate and Roles of Dissolved Organic Matter from Spatially Varied Grassland Soils in China During Long-Term Biogeochemical Processes.

Terrestrial dissolved organic matter (DOM) is critical to global carbon and nutrient cycling, climate change, and human health. However, how the spatial and compositional differences of soil DOM affect its dynamics and fate in water during the carbon cycle is largely unclear. Herein, the biodegradation of DOM from 14 spatially distributed grassland soils in China with diverse organic composition was investigated by 165 days of incubation experiments. The results showed that although the high humified fraction (high-HS) regions were featured by high humic-like fractions of 4-25 kDa molecular weight, especially the abundant condensed aromatics and tannins, they unexpectedly displayed greater DOM degradation during 45-165 days. In contrast, the unique proteinaceous and 25-100 kDa fractions enriched in the low humified fraction (low-HS) regions were drastically depleted and improved the decay of bulk DOM but only during 0-45 days. Together, DOM from the high-HS regions would cause lower CO2 outgassing to the atmosphere but higher organic loads for drinking water production in the short term than that from the low-HS regions. However, this would be reversed for the two regions during the long-term transformation processes. These findings highlight the importance of spatial and temporal variability of DOM biogeochemistry to mitigate the negative impacts of grassland soil DOM on climate, waters, and humans.

Exploring impacts of water-extractable organic matter on pre-ozonation followed by nanofiltration process: Insights from pH variations on DBPs formation.

This study investigated the influence of pH (4-10) on the treatment of water-extractable organic matter (WEOM), and the associated disinfection by-products (DBPs) formation potential (FP), during the pre-ozonation/nanofiltration treatment process. At alkaline pH (9-10), a rapid decline in water flux (> 50 %) and higher membrane rejection was observed, as a consequence of the increased electrostatic repulsion forces between the membrane surface and organic species. Parallel factor analysis (PARAFAC) modeling and size exclusion chromatography (SEC) provides detailed insights into the WEOM compositional behavior at different pH levels. Ozonation at higher pH significantly reduced the apparent molecular weight (MW) of WEOM in the 4000-7000 Da range by transforming the large MW (humic-like) substances into small hydrophilic fractions. Fluorescence components C1 (humic-like) and C2 (fulvic-like) exhibited a predominant increase/decrease in concentration for all pH conditions during pre-ozonation and nanofiltration treatment process, however, the C3 (protein-like) component was found highly associated with the reversible and irreversible membrane foulants. The ratio C1/C2 provided a strong correlation with the formation of total trihalomethanes (THMs) (R2 = 0.9277) and total haloacetic acids (HAAs) (R2 = 0.5796). The formation potential of THMs increased, and HAAs decreased, with the increase of feed water pH. Ozonation markedly reduced the formation of THMs by up to 40 % at higher pH levels, but increased the formation of brominated-HAAs by shifting the formation potential of DBPs towards brominated precursors.

Efficiently photocatalysis activation of peroxydisulfate by Fe-doped g-C3N5 for pharmaceuticals and personal care products degradation.

Peroxydisulfate (PDS) based advanced oxidation processes (AOPs) are widely used for the degradation of pharmaceutical and personal care products (PPCP) in wastewater treatment. In this study, a Fe-doped g-C3N5 (Fe@g-C3N5) was synthesized as a photocatalyst for catalyzing the PDS-based AOPs to degrade tetracycline hydrochloride (TH) at pH 3 and Naproxen (NPX) at pH 7. The photocatalytic performance of Fe@g-C3N5 was 19% and 67% higher than g-C3N5 and g-C3N4 for degradation of TH at pH 3, respectively, while it was 21% and 35% at pH 7. The Fe:N ratio in Fe@g-C3N5, was calculated as 1:3.79, indicating that the doped Fe atom formed a FeN4 structure with an adjacent two-layer graphite structure of g-C3N5, which improved the charge separation capacity of g-C3N5 and act as a new reaction center that can efficiently combine and catalyze the PDS to radicals. Although the intrinsic photo-degradation performance is weak, the photocatalytic performance of Fe@g-C3N5 has great room for the improvement and application in wastewater treatment.

The characters of dissolved organic matters from litter-mimic with the different humification states and their effects on drinking water treatment processes.

Dissolved organic matter (DOM) is one kind of the main pollutant in surface water that will cause many problems during drinking water treatment processes. In this study, a simulated humification process of forest litter-mimic was investigated for eight weeks continuously to study the variations in chemical properties such as DOM composition, polysaccharide/protein ratio, average molecular weight, oxidation degree, hydrophilicity, etc., as well as the impact of these variations on the coagulation, ultra-/nanofiltration (UF/NF). Results showed that the removal rate of coagulation (from 67.5 % to 37.0 %) and UF (from 14.4 % to 5.8 %) decreased significantly during the humification process as a function of time, while the removal rate of NF increased from 40.0 % to 72.9 % at first, and then decreased to 47.4 %. This study gave a deep insight into the effect of DOM with different humification ages on the drinking water treatment process with the influence of seasons and vegetation around the water source, which finally aimed to improve drinking water treatment.

Visible-light-driven iron-based heterogeneous photo-Fenton catalysts for wastewater decontamination: A review of recent advances.

Visible-light-driven heterogeneous photo-Fenton process has emerged as the most promising Fenton-derived technology for wastewater decontamination, owing to its prominent superiorities including the potential utilization of clean energy (solar light), and acceleration of ≡Fe(II)/≡Fe(III) dynamic cycle. As the core constituent, catalysts play a pivotal role in the photocatalytic activation of H2O2 to yield reactive oxidative species (ROS). To date, all types of iron-based heterogeneous photo-Fenton catalysts (Fe-HPFCs) have been extensively reported by the scientific community, and exhibited satisfactory catalytic performance towards pollutants decomposition, sometimes even exceeding the homogeneous counterparts (Fe(II)/H2O2). However, the relevant reviews on Fe-HPFCs, especially from the viewpoint of catalyst-self design are extremely limited. Therefore, this state-of-the-art review focuses on the available Fe-HPFCs in literatures, and gives their classification based on their self-characteristics and modification strategies for the first time. Two classes of representative Fe-HPFCs, conventional inorganic semiconductors of Fe-containing minerals and newly emerging Fe-based metal-organic frameworks (Fe-MOFs) are comprehensively summarized. Moreover, three universal strategies including (i) transition metal (TMs) doping, (ii) construction of heterojunctions with other semiconductors or plasmonic materials, and (iii) combination with supporters were proposed to tackle their inherent defects, viz., inferior light-harvesting capacity, fast recombination of photogenerated carriers, slow mass transfer and low exposure and uneven dispersion of active sites. Lastly, a critical emphasis was also made on the challenges and prospects of Fe-HPFCs in wastewater treatment, providing valuable guidance to researchers for the reasonable construction of high-performance Fe-HPFCs.

The migration and microbiological degradation of dissolved organic matter in riparian soils.

Riparian zones are important natural means of water purification, by decreasing the aqueous concentration of terrestrial organic matter (OM) through adsorption and microbial degradation of the organic matter within the aquatic ecosystem. Limited studies have been reported so far concerning the migration of dissolved organic matter (DOM) in the horizontal and vertical planes of riparian zones. In this study, the migration of DOM in riparian zones, from forest soil to wetland soil, and with soil depth, were explored, based on a case study reservoir. Results showed that riparian wetlands can absorb the OM from the forest soils and adjacent reservoir, and act as a major OM sink through microbial action. Methylomirabilota and GAL15 bacteria increased with soil depth for the two soil systems, and the wetland soil system also contained microbial sulfates, nitrates and carbonates. These microorganisms successfully utilize the Fe3+, SO4-, and CO3- as electron acceptors in the wetland system, resulting in enhanced OM removal. Although the variation of soil DOM in the vertical direction was the same for both forest and wetland soils, the Chemical structure of the DOM was found to be significantly different. Furthermore, the soil was found to be the main source of DOM in the forest ecosystem, with lignin as the main ingredient. The lignin structure was gradually oxidized and decomposed, with an increase in carboxyl groups, as the lignin diffused down into the soil and the adjacent reservoir. PLS-PM analysis showed that the soil physicochemical properties were the main factors affecting DOM transformation. However, microbial metabolism was still the goes deeper affecting factor. This study will contribute to the analysis that migration and transform of soil organic matter in riparian zone.

Influence of Temperature on Biofilm Formation Mechanisms Using a Gravity-Driven Membrane (GDM) System: Insights from Microbial Community Structures and Metabolomics.

A biofilm has a significant effect on water treatment processes. Currently, there is a lack of knowledge about the effect of temperature on the biofilm structure in water treatment processes. In this study, a gravity-driven membrane ultrafiltration system was operated with river feedwater at two temperatures ("low", 4 °C; "high", 25 °C) to explore the biofilm structure and transformation mechanism. The results showed that the difference in dissolved oxygen concentration might be one of the main factors regulating the structural components of the biofilm. A denser biofilm formation and reduced flux were observed at the lower temperature. The linoleic acid metabolism was significantly inhibited at low temperature, resulting in enhanced pyrimidine metabolism by Na+ accumulation. In addition, the biofilm at low temperature had a higher proportion of the metabolites of lipids and lipid-like molecules (11.25%), organic acids and derivatives (10.83%), nucleosides, nucleotides, and analogues (7.083%), and organoheterocyclic compounds (6.66%). These small molecules secrete more polysaccharides having C═O and O═C-O functional groups, which intensified the resistance of the biofilm. Furthermore, the upregulation pathway of pyrimidine metabolism also increased the risk of urea accumulation at low temperature. Limnohabitans, Deinococcus, Diaphorobacter, Flavobacterium, and Pseudomonas were identified as the principal microorganisms involved in this metabolic transformation.

The role of medium molecular weight organics on reducing disinfection by-products and fouling prevention in nanofiltration.

Nanofiltration (NF) is utilized in water treatment for controlling disinfection by-products formation potential (DBPFP) and disinfection by-products (DBPs). Attention regarding NF-based technology has been paid on membrane fouling of NF and the rejection efficiency of contaminants by NF membranes. Natural organic matter (NOM) presenting in surface waters is one main removal target in drinking water treatment by NF-based technology, and is thereby a contributor to the membrane fouling of NF. In application, pretreatments of other membrane filtration (e.g., microfiltration (MF) and ultrafiltration (UF)) has been taken prior to NF, resulting in the separation of NOM of specific molecular weight. Meanwhile, it is well known that NOM is composed of organic compounds of different molecular weights. However, the effect of NOM of specific molecular weight has been seldom investigated from the aspects of membrane fouling and the resulting DBPFP after membrane filtration. By using combinations of MF and UF (molecular weight cut-off of 100K or 20K) as pretreatment prior to NF, the NOM of various molecular weight on DBPFP and DBPs in the NF-treated water were investigated. The experiments were conducted with two real-world surface water samples and one tap water sample. It was found that medium molecular weight NOM, defined as NOM that passed UF100K but did not pass UF20K in this study, reduced fouling of the NF membrane. This is supported by the excitation and emission matrix (EEM) fluorescence spectra, size exclusion chromatography (SEC) and flux analysis. In addition, the medium molecular weight NOM also reduced the DBPFP in the NF treated water and eventually the DBPs by participating in forming a protective layer on the NF surface, blocking the transfer of small molecular weight NOM into the NF filtrate, thereby reducing the DBPFP of the NF filtrate since small molecular weight NOM was the major contributor to DBPFP in this study.

Towards microplastics contribution for membrane biofouling and disinfection by-products precursors: The effect on microbes.

Public awareness of plastic pollution and its impact on the ecosystem has increased rapidly. The microplastics in raw waters and their removal during drinking water treatment is receiving growing attention, but the impact on the efficiency of ultrafiltration has not been examined previously, especially in regard to the formation potential of disinfection by-products (DBPs-FP) in effluent water. In this study, two bench-scale continuous-flow ultrafiltration systems, with and without microplastics, were operated to examine the effect of microplastics on ultrafiltration. Results showed that the microplastics not only increased microbial growth, but also affected the microbial community (e.g. families Xanthobacteraceae, Sphingomonadaceae, Leptolyngbyaceae), which can promote the production of extracellular polymeric substances and nitrogen fixation, causing rapid membrane fouling. The formation potential of THM (TCM and BDCM) and N-DBP (TCNM) species in UF permeate increased with the presence of microplastics, due to changes in water quality. Statistical analysis indicated that tyrosine-like components (C3), ammonium (NH4+-N) and tryptophan-like component (C1) can be used as indicators of the DBPs-FP. This study provides new insights into the relationship between microplastics, membrane biofouling and DBPs-FP, and the potential adverse impact of microplastics on drinking water treatment.

Contribution of bacterial extracellular polymeric substances (EPS) in surface water purification.

Naturally present aquatic microorganisms play an important role in water purification systems, such as the self-purification of surface waters. In this study, two water sources representing polluted surface water (Olympic Green; OG) and unpolluted surface water (Jingmi river; JM), were used to explore the self-purification of surface water by bacteria under different environmental conditions. The dominant bacterial community of OG and JM waters (both are Firmicutes and Proteobacteria) were isolated, cultured, and then used to carry out flocculation tests. Results showed that the flocculation ability of the dominant bacteria and extracellular polymeric substances (EPS) obtained from OG isolation was significantly greater than that from JM. Further examination illustrated that the main components of EPS were polysaccharides, which played an important role in improving the flocculation ability of bacteria. EPS from dominant cultural bacteria strains (OG1 and JM3) isolated from the two different sources lacked hydrophilic groups (e.g. COOH) and had a networked structure which are the main reasons to enhance the flocculation ability. The bacterial diversity and redundancy analysis (RDA) results also showed that microbial community composition is determined by water quality (SS, TOC, and NH4+), and different Bacteroidetes, Actinobacteria and Proteobacteria community structures can improve the water body's ability to remove environmental pollutants (such as SS, humic acid and fulvic acid). These findings provide new information showing how bacterial communities change with environmental factors while maintaining the purity of surface water.

Iron-nickel bimetallic metal-organic frameworks as bifunctional Fenton-like catalysts for enhanced adsorption and degradation of organic contaminants under visible light: Kinetics and mechanistic studies.

Iron-nickel bimetallic organic frameworks (FeNiX-BDC, H2BDC: terephthalic acid) were developed as bifunctional materials for adsorption and photo-Fenton degradation of organic dyes with different charge properties. Significantly enhanced adsorption capacity of FeNi1/15-BDC towards methylene blue (MB) and methyl orange (MO) was achieved, 5.3 and 2.6 times higher than that of pristine Fe-BDC, which was attributed to enlarged specific surface area and pore volume and the decreased surface charges induced by Ni doping. The adsorption kinetics demonstrated that chemisorption was dominant and intra-particle diffusion was the rate-controlling step. Two-stage degradation including slow induction stage and rapid oxidation stage fitted with pseudo-zero-order kinetics well. The increased rate constants (2.472 vs. 1.188 min-1 for MB; 0.616 vs. 0.421 min-1 for MO) in the induction stage as well as the superior removal capability by asynchronism relative to synchronism jointly corroborating the improved adsorption performance was favor for subsequent degradation. Notably, this heterogeneous system not only exhibited obvious advantages like wider pH working range (3-9), better stability and reusability of catalysts, but also achieved the dual objectives of in-situ decontamination and adsorbent regeneration. The coupling of adsorption and degradation along with synergism between photocatalysis and Fenton-like process are responsible for the reinforced removal of organic contaminants.

Performance evaluation of hybrid OMBR-MD using organic and inorganic draw solutions.

The performance of two inorganic divalent salts (CaCl2, and MgCl2) and two organic salts (CH3COONa and Mg(CH3COO)2) was compared with commonly used NaCl in an osmotic membrane bioreactor (OMBR) integrated with a membrane distillation (MD) system. The system was investigated in terms of salinity buildup, flux stability, draw solution (DS) recovery and contaminants removal efficiency. Results indicated that organic DSs not only lessen the salt accumulation within the bioreactor but also increase the pollutant removal efficiency by improving biological treatment. Of all the draw solutions, NaCl and CaCl2 produced rapid declines in water flux because of the high salt accumulation in the bio-tank as compared to other salts. The DCMD system successfully recovered all organic and inorganic draw solute concentrations as per OMBR requirements. Membrane flushing frequency for the MD system followed the order Mg(CH3COO)2 > CH3COONa > CaCl2 > MgCl2 > NaCl. More than 90% removal of chemical oxygen demand (COD), NH4 +-N, and PO4 3--P was achieved in the permeate for each salt because of the dual barriers of high-retention membranes i.e., forward osmosis and MD.

Insight into the effect of organic and inorganic draw solutes on the flux stability and sludge characteristics in the osmotic membrane bioreactor.

In this study, chloride based (CaCl2 and MgCl2) and acetate based (NaOAc and MgOAc) salts in comparison with NaCl were investigated as draw solutions (DS) to evaluate their viability in the osmotic membrane bioreactor (OMBR). Membrane distillation was coupled with an OMBR setup to develop a hybrid OMBR-MD system, for the production of clean water and DS recovery. Results demonstrate that organic DS were able to mitigate the salinity buildup in the bioreactor as compared to inorganic salts. Prolonged filtration runs were observed with MgCl2 and MgOAc in contrast with other draw solutes at the same molar concentration. Significant membrane fouling was observed with NaOAc while rapid flux decline due to increased salinity build-up was witnessed with NaCl and CaCl2. Improved characteristics of mixed liquor in terms of sludge filterability, particle size, and biomass growth along with the degradation of soluble microbial products (SMP) were found with organic DS.