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 sp² 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

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 <i>m</i>-dinitrobenzene, nitrobenzene, and <i>p</i>-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

Sorption and Conformational Characteristics of Reconstituted Plant Cuticular Waxes on Montmorillonite

Plant cuticular waxes are essential barriers that regulate the transport of water and organic molecules to intact cuticular membranes. They also compose a significant fraction of the recalcitrant aliphatic components of soil organic matter (SOM). In this study, we examined the sorption and desorption of three polycyclic aromatic hydrocarbons (PAHs), naphthalene (NAPH), phenanthrene (PHEN), and pyrene (PYR), by cuticular waxes of green pepper (Capsicum annuum) that had been reconstituted by loading them onto montmorillonite (at four different loadings). The reconstituted wax samples, with and without sorbed PAHs, were characterized by solid-state 13C NMR to supply the evidence of melting transition. The sorption isotherms fit well to a Freundlich equation. Sorption isotherms were practically linear except for that of PYR sorption to the low-load wax−montmorillonite sample. The organic-carbon-normalized sorption coefficients (Koc) depended on PAH's lipophilicity (e.g., octanol−water partition coefficient) and increased with increasing wax-load on clay. Desorption was dependent on PAH's molecular sizes and sorbed amounts and on the wax load of the clay. Desorption hysteresis was observed only at high loads of NAPH and PHEN, and it decreased with both increasing wax load and molecular size (i.e. NAPH > PHEN >> PYR). Contributing to hysteresis, the melting transition of the reconstituted waxes after sorbing the PAHs was confirmed by solid-state 13C NMR data. Upon adsorption, the intensity of the NMR peak at 29 ppm (attributed to mobile amorphous paraffinic domains) increased, and a peak at 167 ppm (−COOH) appeared, reflecting the transition of solid amorphous to mobile amorphous domains in the reconstituted waxes. The intensity of melting induced by PAH adsorption decreased with increasing PAH molecular size

Dual Role of Biochars as Adsorbents for Aluminum: The Effects of Oxygen-Containing Organic Components and the Scattering of Silicate Particles

The adsorption of aluminum by biochars produced at different temperatures from rice straw (RS) and cattle manure (CM) was studied to determine the dual roles of biochar for aluminum adsorption. The compositional structures and surface charges of the biochars and ashes with and without Al loading were analyzed by Fourier-transform infrared spectroscopy, ζ-potential, scanning electron microscopy, and X-ray diffraction. The Al adsorption isotherms were fit well by the Langmuir model. The adsorption of Al to the biochars produced at 400 and 700 °C was much greater than the adsorption to the precursory materials and ashes. We found that the organic components and silicate particles within the biochars served as dual adsorptive sites for Al. The complexation of Al with organic hydroxyl and carboxyl groups and the surface adsorption and coprecipitation of Al with silicate particles (as KAlSi₃O₈) both contributed to the Al adsorption of the biochars. After the biochars were loaded with Al, the ζ-potentials of the biochars and ashes increased as a function of pH. The positive charge was maximized at pH 4.5, which is similar to the pH at which the maximum positive charge occurs for silica. The charge reversal was caused by the Stern-layer adsorption of hydrolyzed aluminum species (i.e., Al­(OH)²⁺ and Al­(OH)₂⁺) on the silicate surfaces via hydrogen bonds

Sulfonated Graphene Nanosheets as a Superb Adsorbent for Various Environmental Pollutants in Water

Graphene nanosheets, as a novel nanoadsorbent, can be further modified to optimize the adsorption capability for various pollutants. To overcome the structural limits of graphene (aggregation) and graphene oxide (hydrophilic surface) in water, sulfonated graphene (GS) was prepared by diazotization reaction using sulfanilic acid. It was demonstrated that GS not only recovered a relatively complete sp²-hybridized plane with high affinity for aromatic pollutants but also had sulfonic acid groups and partial original oxygen-containing groups that powerfully attracted positively charged pollutants. The saturated adsorption capacities of GS were 400 mg/g for phenanthrene, 906 mg/g for methylene blue and 58 mg/g for Cd²⁺, which were much higher than the corresponding values for reduced graphene oxide and graphene oxide. GS as a graphene-based adsorbent exhibits fast adsorption kinetic rate and superior adsorption capacity toward various pollutants, which mainly thanks to the multiple adsorption sites in GS including the conjugate π region sites and the functional group sites. Moreover, the sulfonic acid groups endow GS with the good dispersibility and single or few nanosheets which guarantee the adsorption processes. It is great potential to expose the adsorption sites of graphene nanosheets for pollutants in water by regulating their microstructures, surface properties and water dispersion

Interactions of Aluminum with Biochars and Oxidized Biochars: Implications for the Biochar Aging Process

Interactions of aluminum with primary and oxidized biochars were compared to understand the changes in the adsorption properties of aged biochars. The structural characteristics of rice straw-derived biochars, before and after oxidation by HNO₃/H₂SO₄, were analyzed by element composition, FTIR, and XPS. The adsorption of Al to primary biochars was dominated by binding to inorganic components (such as silicon particles) and surface complexation of oxygen-containing functional groups via esterification reactions. Oxidization (aging) introduced carboxylic functional groups on biochar surfaces, which served as additional binding sites for Al³⁺. At pH 2.5–3.5, the Al³⁺ binding was significantly greater on oxidized biochars than primary biochars. After loading with Al, the −COOH groups anchored to biochar surfaces were transformed into COO^– groups, and the negative surface charge diminished, which indicated that Al³⁺ coordinated with COO^–. Biochar is suggested as a potential adsorbent for removing Al from acidic soils

Insights on the Molecular Mechanism for the Recalcitrance of Biochars: Interactive Effects of Carbon and Silicon Components

Few studies have investigated the effects of structural heterogeneity (particularly the interactions of silicon and carbon) on the mechanisms for the recalcitrance of biochar. In this study, the molecular mechanisms for the recalcitrance of biochars derived from rice straw at 300, 500, and 700 °C (named RS300, RS500, and RS700, respectively) were elucidated. Short-term (24 h) and long-term (240 h) oxidation kinetics experiments were conducted under different concentrations of H₂O₂ to distinguish the stable carbon pools in the biochars. We discovered that the stabilities of the biochars were influenced not only by their aromaticity but also through possible protection by silicon encapsulation, which is regulated by pyrolysis temperatures. The aromatic components and recalcitrance of the biochars increased with increasing pyrolysis temperatures. The morphologies of the carbon forms in all of the biochars were also greatly associated with those of silica. Silica-encapsulation protection only occurred for RS500, not for RS300 and RS700. In RS300, carbon and silica were both amorphous, and they were easily decomposed by H₂O₂. The separation of crystalline silica from condensed aromatic carbon in RS700 eliminated the protective role of silicon on carbon. The effect of the biochar particle size on the stability of the biochar was greatly influenced by C–Si interactions and by the oxidation intensities. A novel silicon-and-carbon-coupled framework model was proposed to guide biochar carbon sequestration

Organic Pollutant Clustered in the Plant Cuticular Membranes: Visualizing the Distribution of Phenanthrene in Leaf Cuticle Using Two-Photon Confocal Scanning Laser Microscopy

Plants play a key role in the transport and fate of organic pollutants. Cuticles on plant surfaces represent the first resistance for the uptake of airborne toxicants. In this study, a confocal scanning microscope enhanced with a two-photon laser was applied as a direct and noninvasive probe to explore the in situ uptake of a model pollutant, phenanthrene (PHE), into the cuticular membrane of a hypostomatic plant, <i>Photinia serrulata</i>. On the leaf cuticle surfaces, PHE forms clusters instead of being evenly distributed. The PHE distribution was quantified by the PHE fluorescence intensity. When PHE concentrations in water varying over 5 orders of magnitude were applied to the isolated cuticle, the accumulated PHE level by the cuticle was not vastly different, whether PHE was applied to the outer or inner side of the cuticle. Notably, PHE was found to diffuse via a channel-like pathway into the middle layer of the cuticle matrix, where it was identified to be composed of polymeric lipids. The strong affinity of PHE for polymeric lipids is a major contributor of the fugacity gradient driving the diffusive uptake of PHE in the cuticular membrane. Membrane lipids constitute important domains for hydrophobic interaction with pollutants, determining significant differentials of fugacities within the membrane microsystem. These, under unsteady conditions, contribute to enhance net transport and clustering along the <i>z</i> dimension. Moreover, the liquid-like state of polymeric lipids may promote mobility by enhancing the diffusion rate. The proposed “diffusive uptake and storage” function of polymeric lipids within the membrane characterizes the modality of accumulation of the hydrophobic contaminant at the interface between the plant and the environment. Assessing the capacity of fugacity of these constituents in detail will bring about knowledge of contaminant fate in superior plants with a higher level of accuracy

Adsorption of Polycyclic Aromatic Hydrocarbons by Graphene and Graphene Oxide Nanosheets

The adsorption of naphthalene, phenanthrene, and pyrene onto graphene (GNS) and graphene oxide (GO) nanosheets was investigated to probe the potential adsorptive sites and molecular mechanisms. The microstructure and morphology of GNS and GO were characterized by elemental analysis, XPS, FTIR, Raman, SEM, and TEM. Graphene displayed high affinity to the polycyclic aromatic hydrocarbons (PAHs), whereas GO adsorption was significantly reduced after oxygen-containing groups were attached to GNS surfaces. An unexpected peak was found in the curve of adsorption coefficients (<i>K</i>_d) with the PAH equilibrium concentrations. The hydrophobic properties and molecular sizes of the PAHs affected the adsorption of G and GO. The high affinities of the PAHs to GNS are dominated by π–π interactions to the flat surface and the sieving effect of the powerful groove regions formed by wrinkles on GNS surfaces. In contrast, the adsorptive sites of GO changed to the carboxyl groups attaching to the edges of GO because the groove regions disappeared and the polar nanosheet surfaces limited the π–π interactions. The TEM and SEM images initially revealed that after loading with PAH, the conformation and aggregation of GNS and GO nanosheets dramatically changed, which explained the observations that the potential adsorption sites of GNS and GO were unusually altered during the adsorption process

Fast and Slow Rates of Naphthalene Sorption to Biochars Produced at Different Temperatures

This study investigated the sorption kinetics of a model solute (naphthalene) with a series of biochars prepared from a pine wood at 150–700 °C (referred as PW100–PW700) to probe the effect of the degree of carbonization of a biochar. The samples were characterized by the elemental compositions, thermal gravimetric analyses, Fourier transform IR spectroscopy, scanning electron microscopy, Brunauer–Emmett–Teller-N₂ surface areas (SA), and pore size distributions. Naphthalene exhibited a fast rate of sorption to PW150 owning a high oxygen content and a small SA, due supposedly to the solute partition into a swollen well-hydrated uncarbonized organic matter of PW150. The partial removal of polar-group contents in PW250/PW350, which increased the compactness of the partition medium, decreased the diffusion of the solute into the partition phase to result in a slow sorption rate. With PW500 and PW700 displaying low oxygen contents and high SA, the solute sorption rates were fast, attributed to the near exhaustion of a partition phase in the sample and to the fast solute adsorption on the carbonized biochar component. The results illustrate that the sorption rate of a solute with biochars is controlled largely by the solute’s diffusivity in the biochar’s partition phase, in which the medium compactness affects directly the solute diffusivity