Inhibition of nitrobenzene adsorption by water cluster formation at acidic oxygen functional groups on activated carbon

Adsorption behavior of organic pollutants on microplastics

Microplastics (MPs) are emerging pollutants that act as a carrier of toxic pollutants, release toxic substances, and aggregate in biota. The adsorption behavior of MPs has recently become a research hot spot. The objective of this study was to summarize the main mechanisms by which MPs adsorb organic pollutants, introduce some mathematical models commonly used to study the adsorption behavior of MPs, and discuss the factors affecting the adsorption capacity from three perspectives, i.e., the properties of MPs and organic pollutants, and environmental factors. Adsorption kinetics and isothermal adsorption models are commonly used to study the adsorption of organic pollutants on MPs. We observed that hydrophobic interaction is the most common mechanism by which MPs adsorb organic pollutants, and also reportedly controls the portion of organic pollutants. Additionally, electrostatic interaction and other non-covalent forces, such as hydrogen bonds, halogen bonds, and π-π interactions, are also mechanisms of organic pollutant adsorption on MPs. The particle size, specific surface area, aging degree, crystallinity, and polarity of MPs, and organic pollutant properties (hydrophobicity and dissociated forms) are key factors affecting adsorption capacity. Changes in the pH, temperature, and ionic strength also affect the adsorption capacity. Current research on the adsorption behavior of MPs has mainly been conducted in laboratories, and in-depth studies on the adsorption mechanism and influencing factors are limited. Therefore, studies on the adsorption behavior of MPs in the environment are required, and this study will contribute to a better understanding of this topic.

Size effect of polystyrene microplastics on sorption of phenanthrene and nitrobenzene

Microplastics can have strong sorption capacity for many contaminants, thus greatly influencing the fate, transport and bioavailability of those contaminants in the environment. However, the effect of particle size on contaminant sorption by microplastics is still poorly understood. This study investigated the sorption of phenanthrene and nitrobenzene to micron-, submicron- and nano- sized polystyrene microplastics of 170 µm, 102 µm, 50 µm, 30 µm, 800 nm, 235 nm or 50 nm. All phenanthrene sorption isotherms and most nitrobenzene sorption isotherms were linear because of the strong sorption capacity of microplastics and the hydrophobic partitioning. The log K_d values ranged between 3.07-4.20 and 1.58-3.14 log (L/kg) for phenanthrene and nitrobenzene, respectively. The log K_d values of phenanthrene and nitrobenzene both increased with decreasing particle size for micron-sized polystyrenes (micro-polystyrene) and submicron-sized polystyrenes (submicro-polystyrene). However, in comparison with 235 nm submicro-polystyrene, the log K_d values of 50 nm nano-polystyrene were significantly lower for phenanthrene and comparable for nitrobenzene because its aggregation greatly reduced the effective surface area accessible for sorption. The results improved our understanding of the fate and risks of microplastics associated with the two typical organic contaminants in the micrometer to nanometer scale.

Boehm Titration Revisited (Part II): A Comparison of Boehm Titration with Other Analytical Techniques on the Quantification of Oxygen-Containing Surface Groups for a Variety of Carbon Materials

The use of the Boehm titration (BT) method as an analytical tool for the quantification of oxygen-containing surface groups is systematically investigated for oxidized carbon black, carbon nanotubes and two active carbons with specific surface areas between 60 and 1750 m2 g−1. The accuracy of the BT method is quantitatively compared with results from elemental analysis (EA), temperature programmed desorption (TPD), and X-ray photoelectron spectroscopy (XPS). Overall, the results from TPD are in line with the values obtained by BT. Both show the equal ratio of the oxygen groups to each other. Within the series of carbon samples, all methods provide similar trends for the total oxygen content yet the absolute numbers are deviating significantly. Reasons for these discrepancies are discussed and linked to the specific characteristics of the different methods. As the BT method is a solution based method, it only probes the surface fraction of the carbon that is accessible to the base solution. That means, it probes the relevant fraction for applications where carbon is in contact to aqueous solutions. Overall, the BT method can be conveniently applied to a broad range of carbon materials as long as the samples are sufficiently hydrophilic and of the enough sample amount is provided.

Boehm Titration Revisited (Part I): Practical Aspects for Achieving a High Precision in Quantifying Oxygen-Containing Surface Groups on Carbon Materials

Practical aspects of the Boehm titration method are evaluated for obtaining reliable results in the quantification of oxygen-containing surface groups in a short time. Analytical criteria such as accuracy, repeatability, precision, and robustness are applied. Oxidized multi-walled carbon nanotubes (MWCNTs) are used as the model substance. Different reaction bases (NaHCO3(aq), Na2CO3(aq), NaOH(aq)) are applied and treatment times are studied. We also show that smaller amounts of carbon material can be reliably analyzed by using an autotitrator combined with a pH electrode. We find that indirect titration with Na2CO3 results in the highest titration precision and accuracy despite the lower base strength compared with NaOH. Therefore, CO2 impurities do not have to be removed and only 7 min is necessary for one titration. The titration error with respect to the proposed method is 0.15% of the aliquot volume. The mixing method during the carbon treatment with bases (stirring, shaking, ultrasound treatment) has no influence on the result as long as one allows a few hours for the reaction to complete. Finally, we provide a standard operating procedure for obtaining results with high precision during Boehm titration.

Linear free energy relationships for the adsorption of volatile organic compounds onto multiwalled carbon nanotubes at different relative humidities: comparison with organoclays and activated carbon

Accurate prediction of the sorption coefficients of volatile organic compounds (VOCs) on carbon nanotubes (CNTs) is of major importance for developing an effective VOC removal process and risk assessment of released nanomaterial-carrying contaminants. The linear free energy relationship (LFER) approach was applied to investigate the adsorption mechanisms of VOCs on multiwalled CNTs (MWCNTs). The gas-solid partition coefficients (log K_d) of 17 VOCs were determined at 0%, 55%, and 90% relative humidity (RH). The cavity/dispersion interaction is generally the most influential adsorption mechanism for all RH cases. The hydrogen-accepting interactions declined but with constant hydrogen-donating interactions during the increase of RH, suggesting that the acidity of VOC was important in forming sorptive interaction with the MWCNT surface. Moreover, the comparison of log K_d of VOCs on MWCNTs and other sorbents revealed that the sorption performance of MWCNTs is much more stable over a wider range of RHs due to better site availability and site quality. Furthermore, for all 6 adsorbents in all RHs, the positive contribution of hydrogen bonding ability was found as compared to the negative one found for sorbents completely in water, indicating that the hydrogen-bond donor and acceptor on the sorbent surface contribute to the sorption in the gas phase. In conclusion, the LFER-derived coefficients can be useful in predicting the performance of VOC adsorption on adsorbents and in facilitating the design of efficient VOC removal systems.

Bamboo (Acidosasa edulis) shoot shell biochar: Its potential isolation and mechanism to perrhenate as a chemical surrogate for pertechnetate

In this work, a biochar was prepared from bamboo (Acidosasa edulis) shoot shell through slow pyrolysis (under 300-700 °C). Characterization with various tools showed that the biochar surface was highly hydrophobic and also had more basic functional groups. Batch sorption experiments showed that the biochar had strong sorption ability to perrhenate (a chemical surrogate for pertechnetate) with maximum sorption capacity of 46.46 mg/g, which was significantly higher than commercial coconut shell activated carbon and some adsorbents reported previously. Desorption experiments showed that more than 94% of total perrhenate adsorbed could be recovered using 0.1 mol/L KOH as a desorption medium. Pearson correlation analysis showed that the recovery of perrhenate by the biochars was mainly through surface adsorption mechanisms involving both high hydrophobicity and high basic sites of biochar surface.

Characterization of volatile organic compound adsorption on multiwall carbon nanotubes under different levels of relative humidity using linear solvation energy relationship

Multiwall carbon nanotubes (MWCNTs) have been used as an adsorbent for evaluating the gas/solid partitioning of selected volatile organic compounds (VOCs). In this study, 15 VOCs were probed to determine their gas/solid partitioning coefficient (LogKd) using inverse gas chromatography at different relative humidity (RH) levels. Interactions between MWCNTs and VOCs were analyzed by regressing the observed LogKd with the linear solvation energy relationship (LSER). The results demonstrate that the MWCNT carbonyl and carboxyl groups provide high adsorption capacity for the VOCs (LogKd 3.72-5.24g/kg/g/L) because of the π-/n-electron pair interactions and hydrogen-bond acidity. The increasing RH gradually decreased the LogKd and shifted the interactions to dipolarity/polarizability, hydrogen-bond basicity, and cavity formation. The derived LSER equations provided adequate fits of LogKd, which is useful for VOC-removal processes and fate prediction of VOC contaminants by MWCNT adsorption in the environment.

Effect of Polarity of Activated Carbon Surface, Solvent and Adsorbate on Adsorption of Aromatic Compounds from Liquid Phase

In this study, introduction of acidic functional groups onto a carbon surface and their removal were carried out through two oxidation methods and outgassing to investigate the adsorption mechanism of aromatic compounds which have different polarity (benzene and nitrobenzene). Adsorption experiments for these aromatics in aqueous solution and n-hexane solution were conducted in order to obtain the adsorption isotherms for commercial activated carbon (BAC) as a starting material, its two types of oxidized BAC samples (OXs), and their outgassed samples at 900 °C (OGs). Adsorption and desorption kinetics of nitrobenzene for the BAC, OXs and OGs in aqueous solution were also examined. The results showed that the adsorption of benzene molecules was significantly hindered by abundant acidic functional groups in aqueous solution, whereas the adsorbed amount of nitrobenzene on OXs gradually increased as the solution concentration increased, indicating that nitrobenzene can adsorb favourably on a hydrophilic surface due to its high dipole moment, in contrast to benzene. In n-hexane solution, it was difficult for benzene to adsorb on any sample owing to the high affinity between benzene and n-hexane solvent. On the other hand, adsorbed amounts of nitrobenzene on OXs were larger than those of OGs in n-hexane solution, implying that nitrobenzene can adsorb two adsorption sites, graphene layers and surface acidic functional groups. The observed adsorption and desorption rate constants of nitrobenzene on the OXs were lower than those on the BAC due to disturbance of diffusion by the acidic functional groups.

Adsorption of synthetic organic contaminants by carbon nanotubes: a critical review

In last ten years, a large number (80⁺) of articles regarding aqueous phase adsorption of a variety of synthetic organic compound (SOC) by CNTs were published in peer-reviewed journals. Adsorption depends upon the physicochemical properties of the adsorbates and CNTs as well as the background water chemistry. Among all properties reported in the literature, no parameter was reported as solely controlling SOC adsorption by CNTs. In this article, these contributing parameters were reviewed and the associated explanations were discussed. This comprehensive literature survey provides (i) a thorough CNT characterization summary, (ii) a discussion of adsorption mechanisms of SOCs by CNTs and (iii) a summary of the statistical adsorption model development efforts. It also includes discussions of agreements and differences in the literature, and identifies some research needs.

Synthesis and characterization of cubic mesoporous bridged polysilsesquioxane for removing organic pollutants from water

Hexane, octane, phenyl, and biphenyl-bridged bis(triethoxysilyl) precursors were used in synthesizing cubic mesoporous bridged polysilsesquioxane (BPS) copolymers. Structural characterization was carried out by FTIR, small angle XRD, Brunauer-Emmett-Teller-N2 sorption, (1)H NMR, and TEM. We successfully synthesized both "rigid" and "soft" 3D cubic mesoporous BPS with high surface area and pore volume, as attested by the comprehensive characterization data. Adsorption of pyrene, phenanthrene, nitrobenzene, and 2,4-dichlorophenol on BPS was greatly affected by adsorbate properties, i.e., Kow, solvation properties and molecular size. Hydrophobic interaction dominantly controlled organic pollutants' sorption on BPS. Other interactions, e.g., π-π and H-bond interactions, also have effects on sorption as indicated by Kow normalized sorption isotherms. Rigid aromatic BPS (phenyl and biphenyl) showed a higher sorption capacity than soft aliphatic BPS (hexane and octane). A conceptual model was proposed to further explain the phenomena. This study suggests a promising application of cubic mesoporous BPS in wastewater treatment.