Heyun Fu

Graphene Oxide-Facilitated Reduction of Nitrobenzene in Sulfide-Containing Aqueous Solutions

The main objective of this study was to test the possibility that graphene-based nanomaterials can mediate environmentally relevant abiotic redox reactions of organic contaminants. We investigated the effect of graphene oxide (GO) on the reduction of nitrobenzene by Na2S in aqueous solutions. With the presence of GO (typically 5 mg/L), the observed pseudofirst-order rate constant (kobs) for the reduction of nitrobenzene was raised by nearly 2 orders of magnitude (from 7.83 × 10(-5) h(-1) to 7.77 × 10(-3) h(-1)), strongly suggesting reaction mediation by GO. As reflected by the combined spectroscopic analyses, GO was reduced in the beginning of the reaction, and hence the reduced GO (RGO) mediated the reduction of nitrobenzene. It was proposed that the zigzag edges of RGO acted as the catalytic active sites, while the basal plane of RGO served as the conductor for the electron transfer during the catalytic process. Furthermore, changing the pH (5.9-9.1) and the presence of dissolved humic acid (10 mg TOC/L) were found to greatly influence the catalytic activity of RGO. The results imply that graphene-based nanomaterials may effectively mediate the reductive transformation of nitroaromatic compounds and can contribute to the natural attenuation and remediation of these chemicals.

Adsorption of aromatic compounds on porous covalent triazine-based framework

Covalent triazine-based frameworks (CTFs) are an emerging class of polymers whose adsorption properties of organic chemicals are not well understood. The main objective of this work was to evaluate combined effects of the functional groups of aromatic solutes and the triazine structure of a synthesized CTF on adsorption in aqueous solutions. Adsorption of the hydroxyl-, amino-, nitro-, and sulfonate-substituted monocyclic and bicyclic aromatic compounds was generally stronger than their non-substituted, nonpolar counterparts (benzene and naphthalene). When compared with Amberlite XAD-4 resin, one of the most common and widely used polymeric adsorbents, the CTF showed much stronger adsorption toward the polar and/or ionic compounds. To explain the adsorption enhancement of CTF, several specific, non-hydrophobic mechanisms were proposed, including hydrogen bonding (hydroxyl- and amino-substituted compounds), electrostatic attraction (anionized compounds), and π-π electron-donor-acceptor (EDA) interaction (nitroaromatic compounds) with the triazine structure of CTF. The hypothesized mechanisms were further supported by the observed pH dependence of adsorption. Resulting from size exclusion, adsorption of large-size dissolved humic acids on the homogeneous, nanopored (1.2 nm in size) CTF was negligible and did not affect adsorption of aromatic solutes. Additional advantages of fast adsorption/desorption kinetics and complete adsorption reversibility made CTF a superior adsorbent for aromatic compounds.

In situ hydrothermal grown silicalite-1 coating for solid-phase microextraction.

A novel fiber coated with silicalite-1 for solid-phase microextraction (SPME) was prepared by in situ hydrothermal growth method. Six substituted benzenes (nitrobenzene, p-dichlorobenzene, m-dichlorobenzene, 1,3,5-trichlorobenzene, p-chloronitrobenzene, and m-chloronitrobenzene) were employed as model analytes. The fiber exhibited high thermal stability (little weight loss up to 600 °C) and high chemical stability (no loss of function after sequential immersion in 0.1 M HCl, 0.01 M NaOH, methanol, and n-hexane each for at least 4 h). Compared with commercial fibers, 3-6 times higher extraction efficiencies were shown on the fiber for mono- and p-substituted benzenes. Under the preoptimized conditions, the fiber afforded satisfactory enhancement factors (517-1292), wide linear ranges (more than 2 orders of magnitude), low limits of detection (0.001-0.130 μg/L), and acceptable repeatability (<9.6%) and reproducibility (<8.8%). Furthermore, the fiber offered distinct shape-selectivity attributed to the uniform molecular-scale pore structure of silicalite-1. The ratios of extraction were approximately 70 between p-dichlorobenzene and 1,3,5-trichlorobenzene, 30 between p-chloronitrobenzene and m-chloronitrobenzene, and 3 between p-dichlorobenzene and m-dichlorobenzene. After pore narrowing by surface modification with SiCl(4), the selectivity for p-dichlorobenzene over m-dichlorobenzene was further enhanced by another 10 times. Finally, the fiber was successfully applied to analysis of a real water sample.

Photochemistry of Dissolved Black Carbon Released from Biochar: Reactive Oxygen Species Generation and Phototransformation

Dissolved black carbon (BC) released from biochar can be one of the more photoactive components in the dissolved organic matter (DOM) pool. Dissolved BC was mainly composed of aliphatics and aromatics substituted by aromatic C-O and carboxyl/ester/quinone moieties as determined by solid-state nuclear magnetic resonance. It underwent 56% loss of absorbance at 254 nm, almost complete loss of fluorescence, and 30% mineralization during a 169 h simulated sunlight exposure. Photoreactions preferentially targeted aromatic and methyl moieties, generating CH2/CH/C and carboxyl/ester/quinone functional groups. During irradiation, dissolved BC generated reactive oxygen species (ROS) including singlet oxygen and superoxide. The apparent quantum yield of singlet oxygen was 4.07 ± 0.19%, 2-3 fold higher than many well-studied DOM. Carbonyl-containing structures other than aromatic ketones were involved in the singlet oxygen sensitization. The generation of superoxide apparently depended on electron transfer reactions mediated by silica minerals in dissolved BC, in which phenolic structures served as electron donors. Self-generated ROS played an important role in the phototransformation. Photobleaching of dissolved BC decreased its ability to further generate ROS due to lower light absorption. These findings have significant implications on the environmental fate of dissolved BC and that of priority pollutants.

Enhanced Adsorption of Hydroxyl- and Amino-Substituted Aromatic Chemicals to Nitrogen-Doped Multiwall Carbon Nanotubes: A Combined Batch and Theoretical Calculation Study.

A large effort is being made to develop nanosorbents with tunable surface chemistry for enhanced adsorption affinity and selectivity toward target organic contaminants. Heteroatom N-doped multiwall carbon nanotubes (N-MCNT) were synthesized by chemical vapor deposition of pyridine and were further investigated for the adsorptive removal of several aromatic chemicals varying in electronic donor and acceptor ability from aqueous solutions using a batch technique. Compared with commercial nondoped multiwall carbon nanotubes (MCNT), N-MCNT had similar specific surface area, morphology, and pore-size distribution but more hydrophilic surfaces and more surface defects due to the doping of graphitic and pyridinic N atoms. N-MCNT exhibited enhanced adsorption (2-10 folds) for the π-donor chemicals (2-naphthol and 1-naphthalmine) at pH ∼6 but similar adsorption for the weak π-donor chemical (naphthalene) and even lower adsorption (up to a 2-fold change) for the π-acceptor chemical (1,3-dinitrobenzene). The enhanced adsorption of 2-naphthol and 1-naphthalmine to N-MCNT was mainly attributed to the favored π-π electron-donor-acceptor (EDA) interaction between the π-donor adsorbate molecule and the polarized N-heterocyclic aromatic ring (π-acceptor) on N-MCNT. The proposed adsorption enhancement mechanisms were further tested through the pH effects on adsorption and the density function theory (DFT) calculation. The results show for the first time that the adsorptive interaction of π-donor aromatic compounds with carbon nanomaterials can be facilitated by N-doping.

Adsorption of pharmaceuticals to microporous activated carbon treated with potassium hydroxide, carbon dioxide, and steam.

Adsorption of sulfapyridine, tetracycline, and tylosin to a commercial microporous activated carbon (AC) and its potassium hydroxide (KOH)-, CO-, and steam-treated counterparts (prepared by heating at 850°C) was studied to explore efficient adsorbents for the removal of selected pharmaceuticals from water. Phenol and nitrobenzene were included as additional adsorbates, and nonporous graphite was included as a model adsorbent. The activation treatments markedly increased the specific surface area and enlarged the pore sizes of the mesopores of AC (with the strongest effects shown on the KOH-treated AC). Adsorption of large-size tetracycline and tylosin was greatly enhanced, especially for the KOH-treated AC (more than one order of magnitude), probably due to the alleviated size-exclusion effect. However, the treatments had little effect on adsorption of low-size phenol and nitrobenzene due to the predominance of micropore-filling effect in adsorption and the nearly unaffected content of small micropores causative to such effect. These hypothesized mechanisms on pore-size dependent adsorption were further tested by comparing surface area-normalized adsorption data and adsorbent pore size distributions with and without the presence of adsorbed antibiotics. The findings indicate that efficient adsorption of bulky pharmaceuticals to AC can be achieved by enlarging the adsorbent pore size through suitable activation treatments.

Transformation and destabilization of graphene oxide in reducing aqueous solutions containing sulfide

The colloidal stability of carbon nanomaterials is a key factor controlling their fate and bioavailability in natural aquatic systems. The authors report that graphene oxide nanoparticles could be destabilized in reducing aqueous solutions containing a low concentration (0.5 mM) of sulfide, a naturally occurring reductant. Spectroscopic characterization using combined X-ray photoelectron, Fourier-transform infrared, X-ray diffraction, and Raman analyses revealed that the surface oxygen-containing groups (mainly epoxy groups) of graphene oxide were significantly reduced after reacting with sodium sulfide. The destabilization of graphene oxide was likely caused by the enhanced surface hydrophobicity of the reduced graphene oxide, whereas electrostatic repulsion played a minimal role. Solution pH was found to affect both the deoxygenation process and the aggregation behavior of graphene oxide. Coexisting humic acid reduced the reaction efficiency and stabilized graphene oxide through steric hindrance. These findings suggest for the first time that the colloidal behavior of carbon nanomaterials might change drastically when they enter natural reducing environments containing sulfide such as anaerobic aquifers and sediments.

Enhanced removal of sulfonamide antibiotics by KOH-activated anthracite coal: Batch and fixed-bed studies ☆

The presence of sulfonamide antibiotics in aquatic environments poses potential risks to human health and ecosystems. In the present study, a highly porous activated carbon was prepared by KOH activation of an anthracite coal (Anth-KOH), and its adsorption properties toward two sulfonamides (sulfamethoxazole and sulfapyridine) and three smaller-sized monoaromatics (phenol, 4-nitrophenol and 1,3-dinitrobenzene) were examined in both batch and fixed-bed adsorption experiments to probe the interplay between adsorbate molecular size and adsorbent pore structure. A commercial powder microporous activated carbon (PAC) and a commercial mesoporous carbon (CMK-3) possessing distinct pore properties were included as comparative adsorbents. Among the three adsorbents Anth-KOH exhibited the largest adsorption capacities for all test adsorbates (especially the two sulfonamides) in both batch mode and fixed-bed mode. After being normalized by the adsorbent surface area, the batch adsorption isotherms of sulfonamides on PAC and Anth-KOH were displaced upward relative to the isotherms on CMK-3, likely due to the micropore-filling effect facilitated by the microporosity of adsorbents. In the fixed-bed mode, the surface area-normalized adsorption capacities of Anth-KOH for sulfonamides were close to that of CMK-3, and higher than that of PAC. The irregular, closed micropores of PAC might impede the diffusion of the relatively large-sized sulfonamide molecules and in turn led to lowered fixed-bed adsorption capacities. The overall superior adsorption of sulfonamides on Anth-KOH can be attributed to its large specific surface area (2514 m(2)/g), high pore volume (1.23 cm(3)/g) and large micropore sizes (centered at 2.0 nm). These findings imply that KOH-activated anthracite coal is a promising adsorbent for the removal of sulfonamide antibiotics from aqueous solution.

Impact of coal structural heterogeneity on the nonideal sorption of organic contaminants

Carbonaceous geosorbents (black carbon, coal, and humin/kerogen) play a primary role in the nonideal sorption (isotherm nonlinearity, hysteresis, and multiphasic kinetics) of hydrophobic organic chemicals by soils and sediments. The present study investigated the impact of coal structural heterogeneity on sorption/desorption of two model monoaromatic compounds (1,3-dichlorobenzene and 1,3-dinitrobenzene). Due to the higher degree of aromaticity and condensation, anthracite showed stronger sorption affinity and nonlinearity and slower sorption kinetics than lignite. Removal of humic substances by alkali extraction and/or mineral fraction by acidification did not much affect organic carbon-normalized sorption coefficient to the coal, suggesting nearly complete accessibility of adsorption sites on the condensed organic carbon. However, the treatments greatly increased sorption kinetics and meanwhile alleviated hysteresis of 1,3-dinitrobenzene, as compared with the original lignite. These observations were attributed to the enhanced exposure of high-energy adsorption sites on the condensed organic carbon after exfoliating the surface coverage by humic substances and minerals. An empirical biphasic pseudo-second-order model consisting of a fast sorption phase and a slow sorption phase adequately quantified the overall sorption kinetics for the coal sorbents. The results indicated that the condensed organic carbon, in combination with other structural components, controls the nonideal sorption of unburned coal.

Strong binding of apolar hydrophobic organic contaminants by dissolved black carbon released from biochar: A mechanism of pseudomicelle partition and environmental implications

Dissolved black carbon (DBC), the soluble fraction of black carbon (BC), is an important constituent of dissolved organic matter pool. However, little is known about the binding interactions between hydrophobic organic contaminants (HOCs) and DBC and their significance in the fate process. This study determined the binding ability of DBC released from rice-derived BC for a series of apolar HOCs, including four polycyclic aromatic hydrocarbons and four chlorinated benzenes, using batch sorption and solubility enhancement techniques. Bulk BC and a dissolved soil humic acid (DSHA) were included as benchmark sorbents. The organic carbon-normalized sorption coefficient of phenanthrene to DBC was slightly lower than bulk BC, but was over ten folds higher than DSHA. Consistently, DBC was more effective than DSHA in enhancing the apparent water solubility of the tested HOCs, and the enhancement positively correlated with solute n-octanol-water partition coefficient, indicating the predominance of hydrophobic partition. The much higher binding ability of DBC relative to DSHA was mainly attributed to its higher tendency to form pseudomicellar structures as supported by the fluorescence quenching and the pH-edge data. Our findings suggest that DBC might play a significant role in the environmental fate and transport of HOCs as both sorbent and carrier.