Aromatic Pollutants In Water Research Articles

Dual defect regulation of BiOCl halogen layer enables photocatalytic O2 activation into singlet oxygen for refractory aromatic pollutant removal

The generation of singlet oxygen (1O2) based on photocatalytic activation O2 is considered to have important application prospects in purifying refractory organic pollutants in water. However, the uncertain dual pathway transformation of activated O2 severely limits the generation of 1O2. In this work, we show a robust BiOCl with dual defects (adjacent I-substitution defect and Cl vacancy) in halogen layer for the selective activation of O2 to generate 1O2. Combining experiments and theoretical calculations, we confirm that dual defects are beneficial in optimizing band structures, improving carrier separation efficiency, and promoting O2 adsorption and activation. More importantly, it is confirmed that dual defects can directionally convert O2 into 1O2 by increasing the thermodynamic conversion energy barrier of non-1O2 conversion pathways and serving as a necessary site for 1O2 generation with dual functions of oxidation and reduction. Applying dual defect modified BiOCl to the removal of refractory aromatic pollutants in water, it is found that it has efficient and stable photocatalytic degradation efficiency and broad environmental adaptability. This work not only provides in-depth insights into the mechanism of photocatalytic activation of O2 to selective produce 1O2, but also lays the foundation for further development of highly active photocatalysts for environmental remediation and energy conversion.

Preparations of cyclodextrin polymer and MgO jointly entrapped iron(III)-TAML catalysts for the removal of aromatic pollutants in water

Iron(III)-tetra-amido-macrocyclic (Fe(III)-TAML) activators generally activate H2O2 to efficiently degrade electron-rich pollutants (e.g., phenols) in water under basic conditions, though Fe(III)-TAML is readily demetallized and inactivated under acidic or nearly neutral conditions. Fe(III)-TAML/H2O2 systems often display low reactivity toward electron-deficient pollutants (benzenes) widespread in water. Fe(III)-TAML catalysts with improved pH adaptations and substrate spectra are highly desirable. Herein, we proposed one-pot preparations of cyclodextrin polymer (CDP) and MgO jointly supported Fe(III)-TAML catalysts for the removal of various aromatic contaminants in water. The composite catalysts exhibited typical polymeric network structure of CDP, embedded with basic MgO/Mg(OH)2 particles and three different Fe(III)-TAML species. The composites repeatedly used for 3–4 times still activated H2O2 to completely remove 4-chlorophenol in water during initial pH 2.0–9.0 conditions or in the presence of main water constituents. Despite low Fe(III)-TAML loadings (0.03%–0.10%), these composites displayed great pH adaptations and diverse reactive regions toward organic compounds, compared to other supported catalysts containing 0.24%–1.27% Fe(III)-TAML. Column experiments demonstrated that the composite/H2O2 completely removed electron-rich 4-chlorophenol and electron-deficient benzenes in simulated groundwater. This study indicates that taking CDP as a support may open new prospects for the design of supported catalysts.

Controlled synthesis of graphene oxide/silica hybrid nanocomposites for removal of aromatic pollutants in water

The remarkable characteristics of graphene make it a model candidate for boosting the effectiveness of nano-adsorbents with high potential owing to its large surface area, π–π interaction, and accessible functional groups that interact with an adsorbate. However, the stacking of graphene reduces its influence adsorption characteristics and also its practical application. On the other hand, the widespread use of aromatic compounds in the industry has aggravated the contamination of the water environment, and how to effectively remove them has become a research hotspot. Herein, we develop the functionalization of silica nanoparticles on graphene oxide nanosheet (FGS) by a facile, cheap, and efficient synthesis protocol for adsorption of Trypan Blue (TB) and Bisphenol A (BPA). It was demonstrated that chemical activation with KOH at high autoclaving temperature successfully transformed rice husk ash (RHA) into FGS. The graphene oxide layered interlamination was kept open by using SiO2 to expose the interlayers' strong adsorption sites. XRD, EDX, FTIR, Raman spectroscopy, SEM, HR-TEM, and BET surface area are used to investigate the chemical composition, structure, morphology, and textural nature of the as-produced FGS hybrid nanocomposite. The various oxygen-containing functional groups of the hybrid nanocomposites resulted in a significantly increased adsorption capacity, according to experimental findings. In addition, FGS2, the best composite, has a specific surface area of 1768 m2g−1. Based on Langmuir isotherms, the maximal TB dye and BPA removal capacity attained after 30 min were 455 and 500 mg/g, respectively. The Langmuir isotherm model, a pseudo-second-order kinetic model, and an intraparticle diffusion model have all been used to provide mechanistic insights into the adsorption process. This suggests that BPA and TB adsorption on FGS2 is mostly chemically regulated monolayer adsorption. Due to its unique sp2-hybridized single-atom-layer structure, the exposed graphene oxide nanosheets' extremely hydrophobic effect, hydrogen bonding, and strong—electron donor–acceptor interaction contributed to their improved adsorption of BPA and TB. According to adsorption thermodynamics, FGS2 adsorption of TB and BPA is a spontaneous exothermic reaction that is aided by lowering the temperature. For adsorption-based wastewater cleanup, the produced nanocomposites with a regulated amount of carbon and silica in the form of graphene oxide and silica can be used. These findings suggest that functionalized GO/SiO2 hybrid nanocomposites could be a viable sorbent for the efficient and cost-effective removal of aromatic chemicals from wastewater.

Sensing aromatic pollutants in water with catalyst-sensitized water-gated transistor

Some materials that are active heterogeneous catalysts for the breakdown of non-ionic aromatic solutes in water are found to act as potentiometric sensitizers for same solutes. As an example, here the aromatic water pollutant, benzyl alcohol, was sensed with a limit of detection below its potability limit of 19 μM. Our findings are rationalized on the grounds that both catalysis and sensing rely on adhesion of analyte/substrate on the sensitizer/catalyst. Specifically, a set of powdered transition metal-doped zeolites and related frameworks that catalyze the oxidation of waterborne aromatic pollutants were dispersed in phase transfer matrices. Matrices were introduced into water-gated thin film transistors that act as potentiometric transducers. Potentiometric sensing of non-ionic waterborne pollutants is limited to molecules with a ‘free’ molecular dipole, i.e., a dipole that is not locked in the molecular plane. The present work establishes an application for catalysts beyond catalysis itself. The use of catalysts as sensitizers is recommended for wider uptake and in reverse, to screen candidate catalysts.

Tunable mid-infrared graphene-titanium nitride plasmonic absorber for chemical sensing applications

In this paper, using double-layer graphene nanograting arrays on top of a titanium nitride ground plane, a tunable plasmonic sensor has been presented for mid-infrared chemical sensing applications. Utilizing the plasmonic field enhancement and near-field coupling of the double-layer graphene scheme, narrowband absorption spectra in the mid-infrared region have been achieved for the required selective characteristic of the proposed sensor. The large surface area and atomic level thickness of graphene result in high surface sensitivity, leading to the tunability of the resonant wavelength of the sensor by the chemical potential variation. Moreover, employment of titanium nitride as the ground plane benefits from its abundance and low cost, fabrication stability, high melting point, and biocompatibility compared to metallic plates. Using finite-difference time-domain numerical simulations, it has been shown that the proposed sensor yields high sensitivity and a figure of merit of 3188.8 nm/RIU and 9.1RIU−1, respectively, in the refractive index range of 1.31–1.39. To prove the feasibility of the design for chemical sensing applications, the sensor response in contact with organic aromatic pollutants in water has also been investigated, demonstrating a high sensitivity of 29,250 nm/RIU and a figure of merit of 83.5RIU−1.

Defect-Abundant Covalent Triazine Frameworks as Sunlight-Driven Self-Cleaning Adsorbents for Volatile Aromatic Pollutants in Water.

Covalent triazine frameworks (CTFs) with high adsorption potential and photocatalytic ability features are expected to be designed as a new class of adsorbents that can regenerate themselves just by harnessing sunlight. To simultaneously improve both the adsorption and photocatalytic regeneration performance, a defect-abundant CTF-m was designed and tuned effectively by varying the lengths of benzene ring chains incorporated into the CTF backbone. It has been demonstrated that two kinds of defects in terms of broken benzene rings and pyrrole nitrogen were newly generated, other than the normal benzene rings and triazine units in the CTF-m skeleton. Benefiting from these defects, the adsorption sites with high energy for adsorbing volatile aromatic pollutants were significantly increased, which are reflected by higher saturated adsorption capacities of CTF-m (3.026 mmol/g for benzene (BEN), 1.490 mmol/g for naphthalene (NAP), and 0.863 mmol/g for phenol (PHE)) compared with those of CTF-1 and CTF-2. Furthermore, these defects narrowed the band structure and facilitated the separation of photogenerated charge carries, thus promoting photocatalytic regeneration. The percentage of CTF-m regenerated was still higher than 90% in the fourth cycle. These experimental results, together with the density functional theory (DFT) studies, soundly corroborated that the defects could optimize the adsorption and regeneration property of CTF-m. The present work highlights the potential of fabrication of defective CTFs as solar-driven self-cleaning adsorbents to remove pollutants from water.

Versatile Processing of Metal-Organic Framework-Fluoropolymer Composite Inks with Chemical Resistance and Sensor Applications.

We report a new class of metal-organic framework (MOF) inks with a water-repellent, photocurable fluoropolymer (PFPE) having up to 90 wt % MOF loading. These MOF inks are enabled to process various MOFs through spray coating, pen writing, stencil printing, and molding at room temperature. Upon UV curing, the hydrophobic PFPE matrix efficiently blocks water permeation but allows accessibility of chemicals into the MOF pores, thereby freeing the MOF to perform its unique function. Moreover, by introducing functional MOFs we successfully demonstrated a water-tolerant chemosensor for a class of aromatic pollutants in water and a chemical-resistant thermosensor for visualizing temperature image. This approach would open up innumerable opportunities for those MOFs that are otherwise dormant.

Thiol-/thioether-functionalized porous organic polymers for simultaneous removal of mercury(ii) ion and aromatic pollutants in water

Two novel thiol-/thioether-functionalized porous organic polymers were prepared for simultaneous removal of Hg(ii) and aromatic pollutants in water with high binding ability and fast uptake kinetics.

Rapid detection of aromatic pollutants in water using swellable micelles of fluorescent polymers

We constructed a kind of novel swellable fluorescent micelles for rapid and sensitive detection of toxic aromatic pollutants (APs) in water based on capture-report strategy. An amphiphilic triblock copolymer capped with an aggregation induced emission (AIE) chromophore self assembled into micelles with core-shell structures in aqueous solution. Hydrophobic segments of block polymers organized into cores with the AIE chromophore near the core/shell interface, which were stabilized by a corona of water-soluble polymer segments. Water-soluble polymer segments captured APs in water. The captured pollutants were subsequently transported to hydrophobic cores of micelles. The cores swelled after absorbing APs, leading to fluorescence quenching of the AIE chromophores. The fluorescent micelles allowed rapid detection of APs in the order of seconds at a concentration of 1 ug/L. Compared with commercial GC–MS, the fluorescent micelles require a much smaller amount sample and are much quicker with comparable sensitivity. With the merits of high sensitivity, rapid response, simple operation procedures, and low cost, the fluorescent micelles provide an ideal candidate for the facile detection of toxic aromatic pollutants in water.

Inside Cover: Mechanistic Studies of TiO2 Photocatalysis and Fenton Degradation of Hydrophobic Aromatic Pollutants in Water (Chem. Asian J. 24/2016)

Inside Cover: Mechanistic Studies of TiO₂ Photocatalysis and Fenton Degradation of Hydrophobic Aromatic Pollutants in Water (Chem. Asian J. 24/2016)

Mechanistic Studies of TiO2 Photocatalysis and Fenton Degradation of Hydrophobic Aromatic Pollutants in Water.

HO-adduct radicals have been investigated and confirmed as the common initial intermediates in TiO2 photocatalysis and Fenton degradations of water-insoluble aromatics. However, the evolution of HO-adduct radicals to phenols has not been completely clarified. When 4-d-toluene and p-xylene were degraded by TiO2 photocatalysis and Fenton reactions, respectively, a portion of the 4-deuterium or 4-CH3 group (18-100 %) at the attacked ipso position shifted to the adjacent position of the ring in the formed phenols (NIH shift; NIH is short for the National Institutes of Health, to honor the place where this phenomenon was first discovered). The results, combined with the observation of a key dienyl cationic intermediate by in situ attenuated total reflectance FTIR spectroscopy, indicate that, for the evolution of HO-adduct radicals, a mixed mechanism of both the carbocation intermediate pathway and O2 -capturing pathway occurs in both aqueous TiO2 photocatalysis and aqueous Fenton reactions.

Graphene-coated materials using silica particles as a framework for highly efficient removal of aromatic pollutants in water.

The substantial aggregation of pristine graphene nanosheets decreases its powerful adsorption capacity and diminishes its practical applications. To overcome this shortcoming, graphene-coated materials (GCMs) were prepared by loading graphene onto silica nanoparticles (SiO2). With the support of SiO2, the stacked interlamination of graphene was held open to expose the powerful adsorption sites in the interlayers. The adsorption of phenanthrene, a model aromatic pollutant, onto the loaded graphene nanosheets increased up to 100 fold compared with pristine graphene at the same level. The adsorption of GCMs increased with the loading amount of the graphene nanosheets and dramatically decreased with the introduction of oxygen-containing groups in the graphene nanosheets. The highly hydrophobic effect and the strong π-π stacking interactions of the exposed graphene nanosheets contributed to their superior adsorption of GCMs. An unusual GCM peak adsorption coefficient (Kd) was observed with the increase in sorbate concentration. The sorbate concentration at peak Kd shifted to lower values for the reduced graphene oxide and graphene relative to the graphene oxide. Therefore, the replacement of water nanodroplets attached to the graphene nanosheets through weak non-hydrogen bonding with phenanthrene molecules via strong π-π stacking interactions is hypothesized to be an additional adsorption mechanism for GCMs.

Hollow Co@C prepared from a Co-ZIF@microporous organic network: magnetic adsorbents for aromatic pollutants in water.

This work shows the new engineering strategy of magnetic adsorbents by the combination of zeolitic imidazolate framework (ZIF) and microporous organic network (MON) chemistry. ZIF-67 nanoparticles containing Co(2+) ions were coated with MON. The thermolysis of ZIF-67@MON under argon resulted in hollow carbon materials bearing cobalt nanoparticles which showed promising performance as magnetic adsorbents for aromatic pollutants in water.

Multivariate Adaptive Regression Splines for Prediction of Rate Constants for Radical Degradation of Aromatic Pollutants in Water

A quantitative structure–activity relationship was developed to predict the rate constants for radical degradation of aromatic pollutants in water. A set of 1,508 zero-to three-dimensional descriptors was used for each molecule in the data set. Multiple linear regression was used as a descriptor selection method and the multivariate adaptive regression spline (MARS) method was successfully applied for the mapping model. The root-mean-square error and coefficient of determination were obtained as 0.0996 and 0.8998, respectively. In comparison with other models, the results show that MLR–MARS can be used as a powerful model for prediction of rate constants for radical degradation of aromatic pollutants in water.

Modeling of photodegradation kinetics of aromatic pollutants in water matrix

In this study photodegradation kinetics of aromatic pollutants in water was simulated by combining two approaches: kinetic modeling and structural-relationship modeling. Semiempirical model based on Lambert's law (“LL model”) was used to determine quantum yields (Φ) of 28 single-benzene ring pollutants. QSAR/QSPR modeling, using Genetic Algorithm (GA) and multiple linear regression analysis (MLRA) to select the descriptors and to generate the best correlation models, was applied to establish structure–property relationship of molar absorption coefficient (ɛ) and quantum yield (Φ). In both cases the most predictive models consistent of 4 variables, i.e. descriptors, possessing the good ratios of the number of variables and the predictive ability to avoid overfitting. It was found that ɛ depends on the overall number of bonds, particular atom-centered fragment related to substituents possessing double and single bonded heteroatoms (O or N) to C atom, as well as the shape and size of the molecule and polarizibility. Molecular symmetry was shown as the most influential factor toward Φ. The applied GA-MLRA approach using DRAGON generated descriptors yielded good results, which was also confirmed by external validation. “LL model” simulation for two additional single-benzene ring pollutants, using predicted Φ and ɛ by QSAR/QSPR, slightly deviated from the experimental results (RMSD ≤ 0.75%).

Facile On-Site Detection of Substituted Aromatic Pollutants in Water Using Thin Layer Chromatography Combined with Surface-Enhanced Raman Spectroscopy

A novel facile method for on-site detection of substituted aromatic pollutants in water using thin layer chromatography (TLC) combined with surface-enhanced Raman spectroscopy (SERS) was explored. Various substituted aromatics in polluted water were separated by a convenient TLC protocol and then detected using a portable Raman spectrometer with the prepared silver colloids serving as SERS-active substrates. The effects of operating conditions on detection efficacy were evaluated, and the application of TLC-SERS to on-site detection of artificial and real-life samples of aromatics/polluted water was systematically investigated. It was shown that commercially available Si 60-F(254) TLC plates were suitable for separation and displayed low SERS background and good separation efficiency, 2 mM silver colloids, 20 mM NaCl (working as aggregating agent), 40 mW laser power, and 50 s intergration time were appropriate for the detection regime. Furthermore, qualitative and quantitative detection of most of substituted aromatic pollutants was found to be readily accomplished using the developed TLC-SERS technique, which compared well with GC-MS in terms of identification ability and detection accuracy, and a limit of detection (LOD) less than 0.2 ppm (even at ppb level for some analytes) could be achieved under optimal conditions. The results reveal that the presented convenient method could be used for the effective separation and detection of the substituted aromatic pollutants of water on site, thus reducing possible influences of sample transportation and contamination while shortening the overall analysis time for emergency and routine monitoring of the substituted aromatics/polluted water.

Ozone, Electron Beam and Ozone-Electron Beam Degradation of Phenol. A Comparative Study

The efficacy of electron beam (EB), ozone (O3) and the combined EB/O3 treatment on the removal of phenol, as a prototype for aromatic pollutants in water, is compared on the base of degradation, chemical oxygen demand (COD), total organic carbon (TOC) and toxicity. Complete decomposition of phenol (47 mg/L, 500 µM) was obtained with 14 kGy. Applying simultaneously 27 mg O3/L a dose of 10.5 kGy was sufficient. By the same amount of only ozone a phenol concentration of 45% remained. A TOC reduction of more than 70% was attained with EB/O3 (21 kGy/54 mg O3/L), whereas the identical, separate conditions solely led to 24% (EB) and 14% (O3). The EB/O3 treatment showed also the best results in COD decrease (79%, 21 kGy/54 mg O3/L) and detoxification (7 kGy/18 mg O3/L).

Photodegradation of aromatic pollutants in water over TiO2 supported on molecular sieves

TiO2 was supported on porous materials, including NaY and Na-mordenite zeolites, as well as mesoporous MCM-41 molecular sieve by using impregnation method with organic solvents. The products were characterized with powder XRD, BET surface area measurement, TEM, IR, Raman and UV–VIS spectroscopies. The supported TiO2 was crystallized in anatase structure and the intensity of its X-ray diffraction peaks increased with TiO2 loading. In contrast, the total surface area of the supported catalyst decreased with TiO2 loading. A blue shift of the absorption edge in the UV–VIS spectra was observed when TiO2 particle size decreased, a phenomenon corresponding to the particle size quantization effect. For photodegradation of aromatic pollutants in water, the activity was found strongly influenced by the chemical nature of the pollutant and the surface property of the support. For volatile pollutants such as benzene and chlorobenzenes, molecular sieve supports facilitated the photodegradation reaction by providing high surface area for adsorption. Moreover, there is an optimal loading of TiO2 to achieve the best photocatalytic activity on various supports. The supports in contrast did not show positive contribution in degradation of hydrophilic pollutants such as phenol.

Probing the TiO2 Photocatalytic Mechanisms in Water Purification by Use of Quinoline, Photo-Fenton Generated OH• Radicals and Superoxide Dismutase

In an attempt to improve our understanding of the basic mechanisms of the degradation of aromatic pollutants in water by TiO2 photocatalysis, quinoline (benzo[b]pyridine) was selected as a molecular probe, principally because of the difference in electron density over its two rings. This study was based on the identification and quantification of the primary products or principal secondary products of quinoline degradation either by TiO2 photocatalysis at pH 3 and 6 or by OH• radicals generated via the photo-Fenton reaction (Fe(II/III)−H2O2−UV) at pH 3. In this latter case, the three major products were those expected from the preferential electrophilic attack of OH• radicals on the electron-richer benzene moiety, viz., 5-, and 8-hydroxyquinolines and quinoline-5,8-dione derived from them. TiO2 photocatalysis did not yield this dione, and at the same percentages of degraded quinoline, the amounts of 5-hydroxyquinoline were lower by a factor of ca. 2 at pH 3 and ca. 10 at pH 6 (those of the 8-isomer were also decreased but no accurate measurements were obtained). In addition, at pH 6, we observed marked increases in the amounts of products corresponding to the oxidation of the pyridine moiety, viz., 4-quinolinone and especially 2-aminobenzaldehyde (the major product) and its N-formyl derivative. These results show that oxidative steps in TiO2 photocatalysis do not involve only OH• radicals. It was also observed that, at pH 6, superoxide dismutase (SOD), which catalyzes the elimination of O2•- species, decreased the TiO2 photocatalytic rate of quinoline disappearance, almost suppressed the formation of 2-aminobenzaldehyde, and lowered the amount of 4-quinolinone. The SOD and pH effects suggest a mechanism involving quinoline activation by hole transfer, followed by superoxide addition to the resulting radical cation. The nucleophilic character of superoxide implies addition to the pyridine moiety, i.e., with a regioselectivity opposite that of the OH• radical pathway.

The GC-MS identification of some aliphatic intermediates from the TIO2 photocatalytic degradation of dimethoxybenzenes in water

In an attempt to answer the as yet unsolved question of the structures of the aliphatic compounds resulting from the ring opening of aromatic pollutants in water during a photocatalytic treatment, we have carried out a GC-MS study (with or without silylation) of aqueous solutions of each of the dimethoxybenzenes (DMBs) UV-irradiated in the presence of powder TiO2. The aliphatic intermediates are esters or acids which were identified either by comparison with authentic compounds (as is indicated by an asterisk in the following list) or by interpretation of the mass spectra (EI and CI). From 1,4-DMB, they are: (E)* and (Z)* H3COOC-CH =CH-COOCH3. From 1,3-DMB, they are: H3COOC-CH=CH-CO-COOH3; H3COOC-CO-CH(OCH3)-CH2OH; H3COOC-CH=CH-CHO; H3COOC-CH=CHOCH3. From 1,2-DMB, they are: (E,E) H3COOC-CH =CH-CH=CH-COOCH3*; HOOC-CH=CH-CH=CH-COOCH3 isomers; H3COOC-CH=CH-CHO; (E) HOOC-CH=CH-COOH*; H3COOC-CHOH-CHO; H3COOC-COOCH3*, In the case of 1,2-DMB, additional analyses showed the presence of CH3OH. Its formation was suggested to result from the cleavage of the methoxy groups of DMBs. These identifications show the complexity of the degradation pathways. The dimethyl esters of (E,E) hexadienedioic acid and of (E) and (Z) butenedioic acids could result from the opening of 1,2- or 1,4-DMB, respectively, before these pollutants are hydroxylated. This suggests that hydroxyl radicals are not the only oxidizing species involved. The fact that all aliphatics contain a number of C atoms ≤ to that in DMBs bears witness to the bond cleavages which lead to the mineralization.