Mechanisms of 1,4-Dioxane Biodegradation and Adsorption by Bio-Zeolite in the Presence of Chlorinated Solvents: Experimental and Molecular Dynamics Simulation Studies

Biologically active filtration (BAF) for metabolic 1,4-dioxane removal from contaminated groundwater

1,4-Dioxane is a persistent contaminant that is not effectively removed by conventional water treatment processes. In this study, bench-scale granular activated carbon (GAC)-based biologically active filtration (BAF) systems were developed to metabolically degrade 1,4-dioxane at environmentally relevant levels (<1000 μg L-1). BAF was established using predeveloped biologically activated carbon particles by mixing a 1,4-dioxane-degrading microbial community with granular activated carbon. 1,4-Dioxane removal performance was examined at a range of 1,4-dioxane concentrations (100-1000 μg L-1), hydraulic loading rates (3.6-14 cm h-1), and with the presence of co-contaminants (natural organic matter (NOM) and 1,1-DCE). BAFs achieved 69 ± 7 % removal with an influent 1,4-dioxane concentration of 100 μg L-1 and hydraulic loading rates of 3.6-14 cm h-1, with the lowest effluent concentration of 21 μg L-1. The presence of NOM and 1,1-DCE negatively and irreversibly impacted 1,4-dioxane removal performance of BAF, and pretreatment processes to remove co-contaminants are crucial to maintain the 1,4-dioxane removal efficiency. Microbial analysis revealed the enrichment of 1,4-dioxane degrading species (CB1190-like bacteria) and functional genes responsible for 1,4-dioxane biodegradation (dxmB and aldh) at the top 12 cm of the columns, suggesting the effectiveness of biological 1,4-dioxane removal within short column lengths. This study demonstrated effective metabolic 1,4-dioxane removal at environmentally relevant concentrations by the BAFs, and can provide insights into designing better 1,4-dioxane remediation strategies.

A Phototautomeric 3D Covalent Organic Framework for Ratiometric Fluorescence Humidity Sensing

Photoinduced proton transfer is an essential photochemical process for designing photocatalysts, white light emitters, bioimaging, and fluorescence sensing materials. However, deliberate control of the excited/ground states and meticulous manipulation of the excited state intramolecular proton transfer (ESIPT) pathway constitute a significant challenge in liquids and dense solids. Here, we present the integration of a hydronaphthoquinone fluorophore into a crystalline, porous, phototautomeric dynamic 3D covalent organic framework (COF) to show guest-induced fluorescence turn-on, emission redshift enhancement, and shortened lifetimes for ratiometric fluorescence humidity sensing. Theoretical and spectroscopic studies provide mechanistic insights into the conformational dynamics, charge transfer coupled with local excitation, and ground-state uphill regulation for the multiple tautomers. We illustrate the sensitive, rapid, steady, and self-calibrated ratiometric fluorescence sensing for a wide range of humidity benefiting from the architectural and chemical robustness and crystallinity of such a phototautomeric 3D COF. These findings provide molecular insights into the design of functional porous materials that integrate host-guest mutual recognition and photoelectronic response for multiplex molecular sensing for environmental monitoring and biomedical diagnostics applications.

Molecular Simulation Study of All-Silica Zeolites for the Adsorptive Removal of Airborne Chloroethenes

Chloroethenes (C2H4-xClx with x = 1, 2, 3, 4) are produced and consumed in various industrial processes. As the release of these compounds into air, water, and soils can pose significant risks to human health and the environment, different techniques have been exploited to prevent or remediate chloroethene pollution. Although several previous experimental and computational studies investigated the removal of chloroethenes using zeolite adsorbents, their structural diversity in terms of pore size and pore topology has hardly been explored so far. In this work, molecular simulations using validated empirical force field parameters were used to study the gas-phase adsorption of chloroethenes in 16 structurally distinct zeolite frameworks. As all of these frameworks are synthetically accessible in high-silica form, the simulations used purely siliceous zeolite models. In the most relevant concentration range (0.1 to 10 ppm by volume), substantial uptakes of tri- and tetrachloroethene were computed for several zeolite frameworks, prominently EUO, IFR, MTW, MOR, and BEA. In contrast, vinyl chloride uptakes were always too low to be of practical relevance for adsorptive removal. For selected frameworks, simulation snapshots were analyzed to investigate the impact of pore shape and, at higher uptakes, guest-guest interactions on the adsorption behavior. Hence, this study not only identifies zeolites that should be prioritized in future investigations but also contributes to the microscopic understanding of chloroethene adsorption in crystalline microporous materials.

Chlorinated volatile organic compounds (CVOCs) and 1,4-dioxane kinetics and equilibrium adsorption studies on selective macrocyclic adsorbents

Chlorinated volatile organic compounds (CVOCs) are often found in combination with 1,4-dioxane which has been used as a solvent stabilizer. It would be desirable to separate these compounds since biodegradation of 1,4-dioxane follows an aerobic pathway while anaerobic conditions are needed for biodegrading CVOCs. Conventional adsorbents such as activated carbon (AC) and carbonaceous resins have high adsorption capacities for 1,4-dioxane and CVOCs but lack selectivity, limiting their use for separation (Liu et al., 2019). In the current work, two macrocyclic adsorbents, β-CD-TFN and Res-TFN, were examined for selective adsorption of chlorinated ethenes in the presence of 1,4-dioxane. Both adsorbents exhibited rapid adsorption of the CVOCs and minimal adsorption of 1,4-dioxane. Res-TFN had a higher adsorption capacity for CVOCs than β-CD-TFN (measured linear partition coefficient, Kd 2140 -9750 L⋅kg-1 versus 192-918 L⋅kg-1 for 1,1, DCE, cis-1,2-DCE and TCE, respectively) and was highly selective for CVOCs(TCE Kd ~117 Kd for 1,4-dioxane). By comparison, TCE and 1,4-dioxane adsorption on AC was approximately equal at 100 µg⋅L-1 and approximately 1/3 of the adsorption of TCE on the Res-TFN. The greater adsorption and selectivity of Res-TFN suggest that it can be used as a selective adsorbent to separate CVOCs from 1,4-dioxane to allow separate biodegradation.

Fe-ZSM-5 zeolite catalyst for heterogeneous Fenton oxidation of 1,4-dioxane: effect of Si/Al ratios and contributions of reactive oxygen species

Heterogeneous Fenton oxidation using traditional catalysts with H2O2 for the degradation of 1,4-dioxane (1,4-DX) still presents challenge. In this study, we explored the potential of Fe-ZSM-5 zeolites (Fe-zeolite) with three Si/Al ratios (25, 100, 300) as heterogeneous Fenton catalysts for the removal of 1,4-DX from aqueous solution. Fe2O3 or ZSM-5 alone provided ineffective in degrading 1,4-DX when combined with H2O2. However, the efficient removal of 1,4-DX using H2O2 was observed when Fe2O3 was loaded on ZSM-5. Notably, the Brønsted acid sites of Fe-zeolite played a crucial role during the degradation of 1,4-DX. Fe-zeolites, in combination with H2O2, effectively removed 1,4-DX via a combination of adsorption and oxidation. Initially, Fe-zeolites demonstrated excellent affinity for 1,4-DX, achieving adsorption equilibrium rapidly in about 10 min, followed by effective catalytic oxidative degradation. Among the Fe-ZSM-5 catalysts, Fe-ZSM-5 (25) exhibited the highest catalytic activity and degraded 1,4-DX the fastest. We identified hydroxyl radicals (·OH) and singlet oxygen (1O2) as the primary reactive oxygen species (ROS) responsible for 1,4-DX degradation, with superoxide anions (HO2·/O2·-) mainly converting into 1O2 and ·OH. The degradation primarily occurred at the Fe-zeolite interface, with the degradation rate constants proportional to the amount of Brønsted acid sites on the Fe-zeolite. Fe-zeolites were effective over a wide working pH range, with alkaline pH conditions favoring 1,4-DX degradation. Overall, our study provides valuable insights into the selection of suitable catalysts for effective removal of 1,4-DX using a heterogeneous Fenton technology.

Adsorption Behavior and Kinetics of 1,4-Dioxane by Carbon Aerogel

1,4-dioxane is a potential carcinogen in water and is difficult to deal with due to its robust cycloether bond and complete miscibility with water. To remove 1,4-dioxane in an economically viable and environmentally friendly way, a series of carbon aerogels were synthesized as adsorbents for 1,4-dioxane. The experiment results showed that adsorption performances were closely related to the preparation conditions of carbon aerogels, such as the molar ratio, heating rate, pyrolysis temperature and residence time, which were carefully controlled. Scanning electron microscope analysis revealed the presence of a three-dimensional porous network structure in carbon aerogels. Brunauer-Emmett-Teller analysis results demonstrated an increase in specific surface area (673.89 m2/g) and total pore volume after carbonization, with an increase in mesoporous porosity and a decrease in microporosity. When considering each variable individually, the highest specific surface area of prepared carbon aerogels was achieved at a pyrolysis temperature of 800 °C, a holding time of 1 h, and a heating rate of 2 °C/min. Under optimal experimental conditions, the adsorption removal of 1,4-dioxane by carbon aerogels exceeded 95%, following quasi-second-order kinetics and Langmuir isothermal adsorption isotherms, indicating that monolayer adsorption on the surface of carbon aerogels occurred. The maximum adsorption capacity obtained was 67.28 mg/g at a temperature of 318 K, which was attributed to the presence of a large proportion of mesopores and abundant micropores simultaneously in carbon aerogels. Furthermore, with the interference of chlorinated solvents such as trichloroethylene (TCE), the removal efficiency of 1,4-dioxane had no obvious inhibition effect. Regeneration experiments showed that after five continuous cycles, the carbon aerogels still kept a comparable adsorption capacity, which illustrates its potential application in 1,4-dioxane-polluted water purification.

A Review of Challenges and Opportunities for Microbially Removing 1,4-Dioxane to Meet Drinking-Water and Groundwater Guidelines

1,4-Dioxane is an emerging contaminant in drinking-water sources and contaminated sites. Microbial removal of 1,4-dioxane has attracted a lot of attention, but faces a challenge: being not able to continuously metabolize 1,4-dioxane to below most drinking-water and groundwater guidelines. The 1,4-dioxane concentrations in most drinking-water sources and contaminated sites are too low to sustain biomass growth. This minireview discusses strategies that may potentially address the challenge. The strategies include: 1) finding oligotrophs for which the minimum 1,4-dioxane concentrations to sustain biomass are low, 2) determining conditions that maximize 1,4-dioxane co-metabolism or co-oxidation, 3) creating novel materials as biomass carriers and contaminant concentrators, and 4) lowering the life-cycle costs of technologies that combine biodegradation with (electro)chemical oxidation or phytoremediation.

Releasing SiO tetrahedron and AlO octahedron from montmorillonite to enhance the adsorption performance of carbon@chitosan@montmorillonite nanosheet for cationic dyes: Coupling quantum chemistry simulations with experiments

A novel adsorbent of carbon@chitosan@montmorillonite nanosheet (C@CS@ MTN) was successfully fabricated layer-by-layer assembly method to deal with cationic dye wastewater. Batch adsorption experiments showed that the adsorption capacities of MB and RhB were higher than 325 mg·g^-1 and 236 mg·g^-1, respectively, indicating that C@CS@MTN exhibited an excellent adsorption performance. Through quantum chemistry simulations, the molecular electrostatic potential, electron density, differential charge density, molecular orbital distribution and adsorption binding energy were analyzed to reveal the adsorption reaction mechanism between C@CS@MTN and cationic dyes. The results indicated that SiO tetrahedron ring and AlO octahedron ring released from montmorillonite with inherent periodic structure was beneficial to electrostatic attraction, while cation-π interaction benefitted from the interaction between Al atoms of AlO octahedron ring and benzene ring. It was noteworthy that the electron transfer direction of electrostatic attraction was from O atoms of SiO tetrahedron ring to the benzene ring of dye molecules, but the electron transfer direction of cation-π interaction was from benzene ring of dye molecules to Al atoms of AlO octahedron ring. These results provide fundamental theoretical support for the functional design of mineral-based adsorbents and the efficient removal of cationic dyes.

Interface engineering strategy of a Ti4O7 ceramic membrane via graphene oxide nanoparticles toward efficient electrooxidation of 1,4-dioxane

Although Ti₄O₇ ceramic membrane has been recognized as one of the most promising anode materials for electrochemical advanced oxidation process (EAOP), it suffers from relatively low hydroxyl radical (^•OH) production rate and high charge-transfer resistance that restricted its oxidation performance of organic pollutants. Herein, we reported an effective interface engineering strategy to develop a Ti₄O₇ reactive electrochemical membrane (REM) doped by graphene oxide nanoparticles (GONs), GONs@Ti₄O₇ REM, via strong GONs-O-Ti bonds. Results showed that 1% (wt%) GON doping on Ti₄O₇ REM significantly reduced the charge-transfer resistance from 73.87 to 8.42 Ω compared with the pristine Ti₄O₇ REM, and yielded ^•OH at 2.5-2.8 times higher rate. The 1,4-dioxane (1,4-D) oxidation rate in batch experiments by 1%GONs@Ti₄O₇ REM was 1.49×10^-2 min^-1, 2 times higher than that of the pristine Ti₄O₇ REM (7.51×10^-3 min^-1) and similar to that of BDD (1.79×10^-2 min^-1). The 1%GONs@Ti₄O₇ REM exhibited high stability after a polarization test of 90 h at 80 mA/cm², and within 15 consecutive cycles, its oxidation performance was stable (95.1-99.2%) with about 1% of GONs lost on the REM. In addition, REM process can efficiently degrade refractory organic matters in the groundwater and landfill leachate, the total organic carbon was removed by 54.5% with a single-pass REM. A normalized electric energy consumption per log removal of 1,4-D (EE/O) was observed at only 0.2-0.6 kWh/m³. Our results suggested that chemical-bonded interface engineering strategy using GONs can facilitate the EAOP performance of Ti₄O₇ ceramic membrane with outstanding reactivity and stability.

How β-cyclodextrin- loaded mesoporous SiO2 nanospheres ensure efficient adsorption of rifampicin

In this study, β-CD@mesoporous SiO₂ nanospheres (β-CD@mSi) were prepared by loading β-cyclodextrin (β-CD) onto mesoporous silica nanospheres through an in situ synthesis. This not only solved the defect of β-CD being easily soluble in water, but also changed the physical structure of the mesoporous silica nanospheres. FTIR and XPS results showed that β-CD was successfully loaded onto mesoporous silica nanospheres (mSi), while enhancing the adsorption effect. β-CD@mSi with a monomer diameter of about 150 nm were prepared. At a temperature of 298k, the removal efficiency of a 100 mg/L solution of rifampicin can reach 90% in 4 h and the adsorption capacity was 275.42 mg g^-1 at high concentration. Through the calculation and analysis of adsorption kinetics, adsorption isotherms and adsorption thermodynamics based on the experimental data, the reaction is a spontaneous endothermic reaction dominated by chemical adsorption. The electron transfer pathway, structure-activity relationship and energy between β-CD@mSi and rifampicin were investigated by quantum chemical calculations. The accuracy of the characterization test results to judge the adsorption mechanism was verified, to show the process of rifampicin removal by β-CD@mSi more clearly and convincingly. The simulation results show that π-π interaction plays a major interaction in the reaction process, followed by intermolecular hydrogen bonding and electrostatic interactions.

Insights into the structure-performance relationships of extraction materials in sample preparation for chromatography

Sample preparation is one of the most crucial steps in analytical processes. Commonly used methods, including solid-phase extraction, dispersive solid-phase extraction, dispersive magnetic solid-phase extraction, and solid-phase microextraction, greatly depend on the extraction materials. In recent decades, a vast number of materials have been studied and used in sample preparation for chromatography. Due to the unique structural properties, extraction materials significantly improve the performance of extraction devices. Endowing extraction materials with suitable structural properties can shorten the pretreatment process and improve the extraction efficiency and selectivity. To understand the structure-performance relationships of extraction materials, this review systematically summarizes the structural properties, including the pore size, pore shape, pore volume, accessibility of active sites, specific surface area, functional groups and physicochemical properties. The mechanisms by which the structural properties influence the extraction performance are also elucidated in detail. Finally, three principles for the design and synthesis of extraction materials are summarized. This review can provide systematic guidelines for synthesizing extraction materials and preparing extraction devices.

Non-Thermal Plasma-Modified Ru-Sn-Ti Catalyst for Chlorinated Volatile Organic Compound Degradation

Chlorinated volatile organic compounds (CVOCs) are vital environmental concerns due to their low biodegradability and long-term persistence. Catalytic combustion technology is one of the more commonly used technologies for the treatment of CVOCs. Catalysts with high low-temperature activity, superior selectivity of non-toxic products, and resistance to chlorine poisoning are desirable. Here we adopted a plasma treatment method to synthesize a tin-doped titania loaded with ruthenium dioxide (RuO2) catalyst, possessing enhanced activity (T90%, the temperature at which 90% of dichloromethane (DCM) is decomposed, is 262 °C) compared to the catalyst prepared by the conventional calcination method. As revealed by transmission electron microscopy, X-ray diffraction, N2 adsorption, X-ray photoelectron spectroscopy, and hydrogen temperature-programmed reduction, the high surface area of the tin-doped titania catalyst and the enhanced dispersion and surface oxidation of RuO2 induced by plasma treatment were found to be the main factors determining excellent catalytic activities.

Effects of Additional Carbon Sources in the Biodegradation of 1,4-Dioxane by a Mixed Culture

A mixed culture utilizing 1,4-dioxane as a sole carbon and energy source was obtained from the activated sludge at a textile wastewater treatment plant. The biodegradation of 1,4-dioxane was characterized by a model based on the Monod equation. The effects of the presence of easily degradable carbon sources other than 1,4-dioxane were investigated using dextrose. Structural analogs commonly found in 1,4-dioxane-containing wastewater such as tetrahydrofuran (THF), 2-methyl-1,3-dioxolane, and 1,4-dioxene were also evaluated for their potential effects on 1,4-dioxane biodegradation. The presence of dextrose did not show any synergetic or antagonistic effects on 1,4-dioxane biodegradation, while the structural analogs showed significant competitive inhibition effects. The inhibitory effects were relatively strong with heptagonal cyclic ethers such as THF and 2-methyl-1,3-dioxolane, and mild with hexagonal cyclic ethers such as 1,4-dioxene. It was also shown that the treatment of 1,4-dioxane in the raw textile wastewater required 170% more time to remove 1,4-dioxane due to the co-presence of 2-methyl-1,3-dioxolane, and the extent of delay depended on the initial concentration of 1,3-doxolane.