Adsorption of Polycyclic Aromatic Hydrocarbons on Graphene Oxides and Reduced Graphene Oxides

Bioavailability of Carbon Nanomaterial-Adsorbed Polycyclic Aromatic Hydrocarbons to Pimphales promelas: Influence of Adsorbate Molecular Size and Configuration

Despite carbon nanomaterials' (CNMs) potential to alter the bioavailability of adsorbed contaminants, information characterizing the relationship between adsorption behavior and bioavailability of CNM-adsorbed contaminants is still limited. To investigate the influence of CNM morphology and organic contaminant (OC) physicochemical properties on this relationship, adsorption isotherms were generated for a suite of polycyclic aromatic hydrocarbons (PAHs) on multiwalled carbon nanotubes (MWCNTs) and exfoliated graphene (GN) in conjunction with determining the bioavailability of the adsorbed PAHs to Pimphales promelas using bile analysis via fluorescence spectroscopy. Although it appeared that GN adsorbed PAHs indiscriminately compared to MWCNTs, the subsequent bioavailability of GN-adsorbed PAHs was more sensitive to PAH morphology than MWCNTs. GN was effective at reducing bioavailability of linear PAHs by ∼70%, but had little impact on angular PAHs. MWCNTs were sensitive to molecular size, where bioavailability of two-ringed naphthalene was reduced by ∼80%, while bioavailability of the larger PAHs was reduced by less than 50%. Furthermore, the reduction in bioavailability of CNM-adsorbed PAHs was negatively correlated with the amount of CNM surface area covered by the adsorbed-PAHs. This study shows that the variability in bioavailability of CNM-adsorbed PAHs is largely driven by PAH size, configuration and surface area coverage.

Macroscopic, Spectroscopic, and Theoretical Investigation for the Interaction of Phenol and Naphthol on Reduced Graphene Oxide

Interaction of phenol and naphthol with reduced graphene oxide (rGO), and their competitive behavior on rGO were examined by batch experiments, spectroscopic analysis and theoretical calculations. The batch sorption showed that the removal percentage of phenol or naphthol on rGO in bisolute systems was significantly lower than those of phenol or naphthol in single-solute systems. However, the overall sorption capacity of rGO in bisolute system was higher than single-solute system, indicating that the rGO was a very suitable material for the simultaneous elimination of organic pollutants from aqueous solutions. The interaction mechanism was mainly π-π interactions and hydrogen bonds, which was evidenced by FTIR, Raman and theoretical calculation. FTIR and Raman showed that a blue shift of C═C and -OH stretching modes and the enhanced intensity ratios of I_D/I_G after phenols sorption. The theoretical calculation indicated that the total hydrogen bond numbers, diffusion constant and solvent accessible surface area of naphthol were higher than those of phenol, indicating higher sorption affinity of rGO for naphthol as compared to phenol. These findings were valuable for elucidating the interaction mechanisms between phenols and graphene-based materials, and provided an essential start in simultaneous removal of organics from wastewater.

The role of graphene oxide and graphene oxide-based nanomaterials in the removal of pharmaceuticals from aqueous media: a review

In this review paper, the ill effects of pharmaceuticals (PhAs) on the environment and their adsorption on graphene oxide (GO) and graphene oxide-based (GO-based) nanomaterials have been summarised and discussed. The adsorption of prominent PhAs discussed herein includes beta-blockers (atenolol and propranolol), antibiotics (tetracycline, ciprofloxacin and sulfamethoxazole), pharmaceutically active compounds (carbamazepine) and analgesics such as diclofenac. The adsorption of PhAs strictly depends upon the experimental conditions such as pH, adsorbent and adsorbate concentrations, temperature, ionic strength, etc. To understand the adsorption mechanism and feasibility of the adsorption process, the adsorption isotherms, thermodynamics and kinetic studies were also considered. Except for some cases, GO and its derivatives show excellent adsorption capacities for PhAs, which is crucial for their applications in the environmental pollution cleanup.

Mechanical investigation of U(VI) on pyrrhotite by batch, EXAFS and modeling techniques

The interaction mechanism of U(VI) on pyrrhotite was demonstrated by batch, spectroscopic and modeling techniques. Pyrite was selected as control group in this study. The removal of U(VI) on pyrite and pyrrhotite significantly decreased with increasing ionic strength from 0.001 to 0.1mol/L at pH 2.0-6.0, whereas the no effect of ionic strength was observed at pH >6.0. The maximum removal capacity of U(VI) on pyrite and pyrrhotite calculated from Langmuir model was 10.20 and 21.34mgg^-1 at pH 4.0 and 333K, respectively. The XPS analysis indicated the U(VI) was primarily adsorbed on pyrrhotite and pyrite and then approximately 15.5 and 9.8% of U(VI) were reduced to U(IV) by pyrrhotite and pyrite after 20 days, respectively. Based on the XANES analysis, the adsorption edge of uranium-containing pyrrhotite located between U^IVO₂(s) and U^VIO₂²⁺ spectra. The EXAFS analysis demonstrated the inner-sphere surface complexation of U(VI) on pyrrhotite due to the occurrence of U-S shell, whereas the U-U shell revealed the reductive co-precipitates of U(VI) on pyrrhotite/pyrite with increasing reaction times. The surface complexation modeling showed that outer- and inner-surface complexation dominated the U(VI) removal at pH<4 and pH>5.0, respectively. The findings presented herein play a crucial role in the removal of radionuclides on iron sulfide in environmental cleanup applications.

Recent advances in nanomaterials for water protection and monitoring

Nanomaterials (NMs) for adsorption, catalysis, separation, and disinfection are scrutinized. NMs-based sensor technologies and environmental transformations of NMs are highlighted.

Direct Observation, Molecular Structure, and Location of Oxidation Debris on Graphene Oxide Nanosheets

The presence of oxidation debris (OD) complicates the structures and properties of graphene oxide (GO) nanosheets, thereby impacting their potential applications. However, the origin of OD is still in dispute. Moreover, characterizing the structure and location of supposed OD on nanosheets of GO produced during the oxidation process is difficult. Herein, the attached state and size of OD on graphene oxide nanosheets were directly observed using HRTEM, the molecular structure of OD was initially proposed based on the spectroscopic characterization and Q-TOF mass spectrometry, and the locations of OD on the GO nanosheets were detected through the adsorption of probe molecules onto as-prepared GO (a-GO) and base-washed GO (bw-GO). The results indicated that OD possesses a highly crystalline structure and can be defined as several nanometre-sized polyaromatic molecules with a considerable number of oxygen-containing functional groups attached on the edges. The dark nanodot seated on a-GO was clearly observed in the HRTEM images, whereas it appeared as a clean nanosheet in the image of bw-GO, indicating that OD is removed by base-washing treatment. Following the base-washing treatment, the contents of carboxyl groups on bw-GO unexpectedly increased and subsequently contributed to the desorption of OD from a-GO due to the electrostatic repulsion being stronger than primary π-π interactions. Compared with a-GO, the adsorption of phenanthrene, as an aromatic probe, onto bw-GO increased by 6-fold via π-π stacking interactions, whereas the increase in the adsorption of m-dinitrobenzene, as a defect probe, was not as remarkable as that of phenanthrene. Reasonably, the OD nanoparticles were primarily located at the sp(2) structures on the GO nanosheets through π-π interactions rather than attached on defects/edges. The insights regarding the existence, molecular structures and attached sites of OD nanoparticles on GO nanosheets provide a theoretical basis for preparing OD-free GO for optimizing the potential applications of GO nanosheets.

Characteristic Evaluation of Graphene Oxide for Bisphenol A Adsorption in Aqueous Solution

This paper investigates the characteristics of graphene oxide (GO) for Bisphenol A (BPA) adsorption in water. Batch experiments on the influence of significant parameters were performed. While an improvement of the adsorption capacity of BPA was obtained by the increment of contact time and the initial BPA concentration, the increment of pH above 8, GO dosage, and temperature showed the reverse results. The thermodynamic study suggested that BPA adsorption on GO was an exothermic and spontaneous process. The kinetics was explained by the pseudo-second-order model which covers all steps of adsorption. The fit of the results with the Langmuir isotherm indicated the monolayer adsorption. At 298 K, the adsorption reached equilibrium within 30 min with the maximum adsorption capacity of 49.26 mg/g. The low BPA adsorption capacity of GO can be interpreted by the occurrence of oxygen-containing functional groups (OCFGs) that are able to form hydrogen bonds with the surrounding OCFGs and water molecules. This effect inhibited the role of π–π interactions that are mainly responsible for the adsorption of BPA.

Structural benefits of bisphenol S and its analogs resulting in their high sorption on carbon nanotubes and graphite

Bisphenol S (BPS), a new bisphenol analog, is considered to be a potential replacement for bisphenol A (BPA), which has gained concern because of its potentially adverse health impacts. Therefore, studies are needed to investigate the environmental fate and risks of this compound. In this study, the adsorption of BPS and four structural analogs on multi-walled carbon nanotubes (MWCNTs) and graphite (GP) were investigated. When solid-phase concentrations were normalized by the surface areas, oxygen-containing functional groups on the absorbents showed a positive impact on phenol sorption but inhibited the sorption of chemicals with two benzene rings. Among BPS analogs, diphenyl sulfone showed the lowest sorption when hydrophobic effects were ruled out. Chemicals with a butterfly structure, formed between the two benzene rings, showed consistently high sorption on MWCNTs, independent of the substituted electron-donating or accepting functional groups. This study emphasizes the importance of chemical conformation on organic, contaminant sorption on engineered, carbonaceous materials.

Wrinkles and Folds of Activated Graphene Nanosheets as Fast and Efficient Adsorptive Sites for Hydrophobic Organic Contaminants

To create more wrinkles and folds as available adsorption sites, graphene nanosheets (GNS) were thermally treated with KOH for morphological alteration. The surface structures and properties of the activated graphene nanosheets (AGN) were characterized by BET-N2, SEM, TEM, Raman, XRD, XPS, and FTIR. After KOH etching, the highly crystal structure was altered, self-aggregation of graphene layers were evidently relieved, and more single to few layer graphene nanosheets were created with wrinkles and folds. Also both specific surface area and micropore volume of AGN increased relative to GNS. The adsorption of AGN toward p-nitrotoluene, naphthalene and phenanthrene were greatly enhanced in comparison with GNS, and gradually promoted with increasing degree of KOH etching. Adsorption rate of organic contaminants on AGN was very fast and efficient, whereas small molecules showed higher adsorption rates due to the more porous surface of graphene. In addition to π-π interaction, the high affinities of p-nitrotoluene to AGN are suggested from strong electron charge transfer interactions between nitro groups on p-nitrotoluene and defect sites of AGN. A positively linear correlation between organic molecule uptake and the micropore volume of AGN indicated that pore-filling mechanism may play an important role in adsorption. Morphological wrinkles and folds of graphene nanosheets can be regulated to enhance the adsorption capability and kinetics for efficient pollutant removal and to selectively preconcentrate adsorbates with different sizes for detection.

Polycyclic aromatic hydrocarbons (PAHs) removal by sorption: A review

Polycyclic aromatic hydrocarbons (PAHs) are organic micro pollutants which are persistent compounds in the environment due to their hydrophobic nature. Concerns over their adverse effects in human health and environment have resulted in extensive studies on various types of PAHs removal methods. Sorption is one of the widely used methods as PAHs possess a great sorptive ability into the solid media and their low aqueous solubility property. Several adsorbent media such as activated carbon, biochar, modified clay minerals have been largely used to remove PAHs from aqueous solution and to immobilise PAHs in the contaminated soils. According to the past studies, very high removal efficiency could be achieved using the adsorbents such as removal efficiency of activated carbon, biochar and modified clay mineral were 100%, 98.6% and >99%, respectively. PAHs removal efficiency or adsorption/absorption capacity largely depends on several parameters such as particle size of the adsorbent, pH, temperature, solubility, salinity including the production process of adsorbents. Although many studies have been carried out to remove PAHs using the sorption process, the findings have not been consolidated which potentially hinder to get the correct information for future study and to design the sorption method to remove PAHs. Therefore, this paper summarized the adsorbent media which have been used to remove PAHs especially from aqueous solutions including the factor affecting the sorption process reported in 142 literature published between 1934 and 2015.

Ruthenium molecular complexes immobilized on graphene as active catalysts for the synthesis of carboxylic acids from alcohol dehydrogenation

Green efficient synthesis of carboxylic acids catalyzed by molecular ruthenium complexes supported on graphene using water as solvent and with the formation of molecular hydrogen as a by-product.

Polyaromatic N-heterocyclic carbene ligands and π-stacking. Catalytic consequences

This article highlights how π-stacking interactions have an important influence on the catalytic properties of transition metal complexes decorated with rigid polyaromatic ligands.

Competitive adsorption of three phenolic compounds to hydrophilic urea-formaldehyde macroporous foams derived from lignin-based Pickering HIPEs template

Hydrophilic urea-formaldehyde macroporous foams (UFMF) were simply synthesized by templating oil-in-water Pickering high internal phase emulsions (HIPEs) solely stabilized by lignin particles.

Effect of Microbes on the Adsorption of Naphthalene by Graphene Oxide

The adsorption of naphthalene on graphene oxide (GO) nanosheets in presence of Paecilomyces cateniannulatus (P. cateniannulatus) was conducted by the batch techniques. The morphology and nanostructure of GO were characterized by SEM, TEM, FTIR, XPS, and Raman. The adsorption kinetics indicated that the adsorption of naphthalene on GO and GO + P. catenlannulatus can be satisfactorily fitted pseudo-first-order and pseudo-second-order kinetic model, respectively. P. catenlannulatus inhibited the adsorption of naphthalene on GO at pH<4.0, whereas the increased adsorption was observed at pH>4.0. The adsorption of naphthalene on GO and GO + P. catenlannulatus can be better fitted by Langmuir and Freundlich model, respectively. The change in the conformation of GO was responsible to the increased adsorption of naphthalene by SEM and TEM images. According to FTIR analysis, naphthalene was absorbed by the oxygen-containing functional groups of GO, especially for –COOH. The finding in the study provides the implication for the preconcentration and removal of polycyclic aromatic hydrocarbons from environment cleanup applications.

Adsorption of 4-n-Nonylphenol and Bisphenol-A on Magnetic Reduced Graphene Oxides: A Combined Experimental and Theoretical Studies

Adsorption of 4-n-nonylphenol (4-n-NP) and bisphenol A (BPA) on magnetic reduced graphene oxides (rGOs) as a function of contact time, pH, ionic strength and humic acid were investigated by batch techniques. Adsorption of 4-n-NP and BPA were independent of pH at 3.0- 8.0, whereas the slightly decreased adsorption was observed at pH 8.0-11.0. Adsorption kinetics and isotherms of 4-n-NP and BPA on magnetic rGOs can be satisfactorily fitted by pseudo-second-order kinetic and Freundlich model, respectively. The maximum adsorption capacities of magnetic rGOs at pH 6.5 and 293 K were 63.96 and 48.74 mg/g for 4-n-NP and BPA, respectively, which were significantly higher than that of activated carbon. Based on theoretical calculations, the higher adsorption energy of rGOs + 4-n-NP was mainly due to π-π stacking and flexible long alkyl chain of 4-n-NP, whereas adsorption of BPA on rGOs was energetically favored by a lying-down configuration due to π-π stacking and dispersion forces, which was further demonstrated by FTIR analysis. These findings indicate that magnetic rGOs is a promising adsorbent for the efficient elimination of 4-n-NP/BPA from aqueous solutions due to its excellent adsorption performance and simple magnetic separation, which are of great significance for the remediation of endocrine-disrupting chemicals in environmental cleanup.

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 (K_d) was observed with the increase in sorbate concentration. The sorbate concentration at peak K_d 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.

Macroscopic and spectroscopic investigations of the adsorption of nitroaromatic compounds on graphene oxide, reduced graphene oxide, and graphene nanosheets

The surface properties and adsorption mechanisms of graphene materials are important for potential environmental applications. The adsorption of m-dinitrobenzene, nitrobenzene, and p-nitrotoluene onto graphene oxide (GO), reduced graphene oxide (RGO), and graphene (G) nanosheets was investigated using IR spectroscopy to probe the molecular interactions of graphene materials with nitroaromatic compounds (NACs). The hydrophilic GO displayed the weakest adsorption capability. The adsorption of RGO and G was significantly increased due to the recovery of hydrophobic π-conjugation carbon atoms as active sites. RGO nanosheets, which had more defect sites than did GO or G nanosheets, resulted in the highest adsorption of NACs which was 10-50 times greater than the reported adsorption of carbon nanotubes. Superior adsorption was dominated by various interaction modes including π-π electron donor-acceptor interactions between the π-electron-deficient phenyls of the NACs and the π-electron-rich matrix of the graphene nanosheets, and the charge electrostatic and polar interactions between the defect sites of graphene nanosheets and the -NO2 of the NAC. The charge transfer was initially proved by FTIR that a blue shift of asymmetric -NO2 stretching was observed with a concomitant red shift of symmetric -NO2 stretching after m-dinitrobenzene was adsorbed. The multiple interaction mechanisms of the adsorption of NAC molecule onto flat graphene nanosheets favor the adsorption, detection, and transformation of explosives.

PAMAM dendrimers and graphene: materials for removing aromatic contaminants from water

We present results from experiments and atomistic molecular dynamics simulations on the remediation of naphthalene by polyamidoamine (PAMAM) dendrimers and graphene oxide (GrO). Specifically, we investigate 3rd-6th generation (G3-G6) PAMAM dendrimers and GrO with different levels of oxidation. The work is motivated by the potential applications of these emerging nanomaterials in removing polycyclic aromatic hydrocarbon contaminants from water. Our experimental results indicate that GrO outperforms dendrimers in removing naphthalene from water. Molecular dynamics simulations suggest that the prominent factors driving naphthalene association to these seemingly disparate materials are similar. Interestingly, we find that cooperative interactions between the naphthalene molecules play a significant role in enhancing their association to the dendrimers and GrO. Our findings highlight that while selection of appropriate materials is important, the interactions between the contaminants themselves can also be important in governing the effectiveness of a given material. The combined use of experiments and molecular dynamics simulations allows us to comment on the possible factors resulting in better performance of GrO in removing polyaromatic contaminants from water.

Environmental applications of three-dimensional graphene-based macrostructures: adsorption, transformation, and detection

Just as graphene triggered a new gold rush, three-dimensional graphene-based macrostructures (3D GBM) have been recognized as one of the most promising strategies for bottom-up nanotechnology and become one of the most active research fields during the last four years. In general, the basic structural features of 3D GBM, including its large surface area, which enhances the opportunity to contact pollutants, and its well-defined porous structure, which facilitates the diffusion of pollutant molecules into the 3D structure, enable 3D GBM to be an ideal material for pollutant management due to its excellent capabilities and easy recyclability. This review aims to describe the environmental applications and mechanisms of 3D GBM and provide perspective. Thus, the excellent performance of 3D GBM in environmental pollutant adsorption, transformation and detection are reviewed. Based on the structures and properties of 3D GBM, the removal mechanisms for dyes, oils, organic solvents, heavy metals, and gas pollutants are highlighted. We attempt to establish "structure-property-application" relationships for environmental pollution management using 3D GBM. Approaches involving tunable synthesis and decoration to regulate the micro-, meso-, and macro-structure and the active sites are also reviewed. The high selectivity, fast rate, convenient management, device applications and recycling utilization of 3D GBM are also emphasized.