Aqueous adsorption and removal of organic contaminants by carbon nanotubes

Removal of emerging contaminants from the environment by adsorption

Emerging contaminants (EC's) are pollutants of growing concern. They are mainly organic compounds such as: pesticides, pharmaceuticals and personal care products, hormones, plasticizers, food additives, wood preservatives, laundry detergents, surfactants, disinfectants, flame retardants, and other organic compounds that were found recently in natural wastewater stream generated by human and industrial activities. A majority of ECs does not have standard regulations and could lead to lethal effects on human and aquatic life even at small concentrations. The conventional primary and secondary water treatment plants do not remove or degrade these toxic pollutants efficiently and hence need cost effective tertiary treatment method. Adsorption is a promising method worldwide for EC removal since it is low initial cost for implementation, highly-efficient and has simple operating design. Research has shown that the application of different adsorbents such as, activated carbons(ACs), modified biochars (BCs), nanoadsorbents (carbon nanotubes and graphene), composite adsorbents, and other are being used for EC's removal from water and wastewater. The current review intends to investigate adsorption process as an efficient method for the treatment of ECs. The mechanism of adsorption has also been discussed.

pH-dependent KOW provides new insights in understanding the adsorption mechanism of ionizable organic chemicals on carbonaceous materials

The dominant adsorption mechanism of ionizable organic chemicals (IOCs) on carbonaceous materials is still unclear. This study used benzoic acid (BA), o-chlorobenzoic acid (2-CBA) and p-chlorobenzoic acid (4-CBA) as representatives of IOCs to investigate the dominant adsorption mechanism on graphene oxide (GO) and graphite (GP), with a specific emphasis on the impact of pH-dependent K_OW. The order of apparent adsorption of the investigated chemicals changed with pH, which could not be explained by their constant K_OW. According to our measurement, K_OW varied greatly with pH. The pH-dependent adsorption of BA, 2-CBA and 4-CBA were significantly correlated with pH-dependent K_OW for both GO and GP. Our observations indicated that the hydrophobic effects may dominate the apparent adsorption of BA, 2-CBA and 4-CBA on GO and GP. Electrostatic repulsion played a minor role in their reduced adsorption at high pH. Negative charge-assisted H-bond and π-π interactions may explain the adsorption of negatively charged BA, 2-CBA and 4-CBA on negatively charged GO. This study implied that pH-dependent K_OW rather than constant K_OW should be incorporated to understand IOC adsorption as affected by pH.

Attenuation of bacterial cytotoxicity of carbon nanotubes by riverine suspended solids in water

The impact of solid particles on ecotoxicity of nanomaterials in water environments is poorly understood. This study investigated the effect of natural riverine suspended solids (SPS) on the cytotoxicity of single-walled carbon nanotubes (SWCNTs) towards a bacterium, Ochrobactrum sp. in water. Compared with SWCNT suspension without SPS, the presence of SPS at different concentrations ranging from 20 to 400 mg L^-1 markedly increased the survival rates of bacteria exposed to 50 mg L^-1 SWCNTs and bacterial survival rates increased with SPS concentrations by a power law. Sedimentation experiments and field emission scanning electron microscopy revealed the occurrence of heteroaggregation between SWCNTs and SPS, probably responsible for the reduced SWCNT toxicity. Furthermore, the extended Derjaguin-Landau-Verwey-Overbeek (ExDLVO) calculation showed the mitigated toxicity might also result from the decreased SWCNT-bacterium interaction energy with the increased SPS concentrations and the stronger SPS-SWCNT interaction than the SWCNT-bacterium interaction. This work provides new insights into our understanding of environmental hazards of engineered nanomaterials in aquatic systems.

Induced structural changes of humic acid by exposure of polystyrene microplastics: A spectroscopic insight

The occurrence of microplastics (MPs) as emerging contaminants in the environment may cause changes in water or sediment characteristics, and further affect their biogeochemical cycles. Thus, insights into the interactions between dissolved organic matter (DOM) and MPs are essential for the assessment of environmental impacts of MPs in ecosystems. Integrating spectroscopic methods with chemometric analyses, this work explored the chemical and microstructural changes of DOM-MP complex to reveal the mechanism of DOM-MP interaction at a molecular level. MPs were found to interact with the aromatic structure of DOM via π-π conjugation, then be entrapped in the DOM polymers by the carboxyl groups and C=O bonds, constituting a highly conjugated co-polymer with increased electron density. This induced the fluorescence intensity increase in DOM. The interaction affinity of DOM-MP was highly dependent on the MP size and solution pH. This work offers a new insight into the impact of MP discharge on environment and may provide an analytical framework for evaluating MP hetero-aggregation and the roles of MPs in the transportation of other contaminants. Furthermore, the integrated methods used in this work exhibit potential applications in exploring the fragmentation processes of MPs and formation of secondary MPs under natural conditions.

Amine-functionalized magnetic bamboo-based activated carbon adsorptive removal of ciprofloxacin and norfloxacin: A batch and fixed-bed column study

Amine-functionalized magnetic bamboo-based activated carbon (AFM-BAC) derived from bamboo products wastes were employed for effective adsorption of fluoroquinolone antibiotics ciprofloxacin (CIP) and norfloxacin (NOR) through batch processing. The effects of factors on the adsorption of both antibiotics were studied. The studies of various factors influencing the adsorption behavior indicated that the maximum adsorption capacities for two antibiotics adsorption were observed in weakly acidic condition and the adsorption amounts of two antibiotics increased with the increase of zwitterionic form, implying the importance of zwitterionic form, and the adsorption process is spontaneous and endothermic. The result of date indicated that adsorption of both two antibiotics onto the AFM-BAC better fits Langmuir isotherm model. The saturated magnetization of AFM-BAC reached 8.55 emu g^-1. A fixed-bed column adsorption with a bench-scale was carried out. Desorption and regeneration experiments showed that the AFM-BAC for both antibiotics could remain above 80% after five consecutive recycling cycles.

Designer carbon nanotubes for contaminant removal in water and wastewater: A critical review

The search for effective materials for environmental cleanup is a scientific and technological issue of paramount importance. Among various materials, carbon nanotubes (CNTs) possess unique physicochemical, electrical, and mechanical properties that make them suitable for potential applications as environmental adsorbents, sensors, membranes, and catalysts. Depending on the intended application and the chemical nature of the target contaminants, CNTs can be designed through specific functionalization or modification processes. Designer CNTs can remarkably enhance contaminant removal efficiency and facilitate nanomaterial recovery and regeneration. An increasing number of CNT-based materials have been used to treat diverse organic, inorganic, and biological contaminants. These success stories demonstrate their strong potential in practical applications, including wastewater purification and desalination. However, CNT-based technologies have not been broadly accepted for commercial use due to their prohibitive cost and the complex interactions of CNTs with other abiotic and biotic environmental components. This paper presents a critical review of the existing literature on the interaction of various contaminants with CNTs in water and soil environments. The preparation methods of various designer CNTs (surface functionalized and/or modified) and the functional relationships between their physicochemical characteristics and environmental uses are discussed. This review will also help to identify the research gaps that must be addressed for enhancing the commercial acceptance of CNTs in the environmental remediation industry.

Evaluation of the sorption mechanism of ionic liquids onto multi-walled carbon nanotubes

The knowledge of the sorption mechanism of different chemicals onto third generation carbon sorbents such as carbon nanotubes (CNTs) is needed in order to project systems for the effective removal of pollutants from the environment. This paper reports evaluation of the sorption mechanism of selected ionic liquids (ILs), being considered as potential pollutant in environment, onto various CNTs. CNTs characterized by the smallest diameter and the biggest surface area showed the highest sorption capacity to isolate ILs from an aqueous solution. CNTs with a bigger diameter, a functionalized surface and particularly a helical shape showed a lower sorption capacity. The sorption mechanism has been defined as complex, including van der Waals, π-π and electrostatic interactions with dominating π-π interactions. Due to the relatively high sorption coefficient (355.98 ± 20.69-6397.10 ± 355.42 L kg^-1 depending on the IL) the study showed that multi-walled carbon nanotubes can potentially be used to effectively isolate ILs from an aqueous solution. Moreover, proved in this study, the fast sorption kinetic, and uncomplicated regeneration process, leading to an even higher sorption capacity, means that CNTs are promising material which could find potential applications in the treatment of water contaminated by ILs.

Green synthesis of graphene from recycled PET bottle wastes for use in the adsorption of dyes in aqueous solution

Polyethyleneterephthalate (PET) is an important component of post-consumer plastic waste. This study focuses on the potential of utilizing "waste-treats-waste" by synthesis of graphene using PET bottle waste as a source material. The synthesized graphene is characterized by SEM, TEM, BET, Raman, TGA, and FT-IR. The adsorption of methylene blue (MB) and acid blue 25 (AB25) by graphene is studied and parameters such as contact time, adsorbent dosage were optimized. The Response Surface Methodology (RSM) is applied to investigate the effect of three variables (dye concentration, time and temperature) and their interaction on the removal efficiency. Adsorption kinetics and isotherm are followed a pseudo-second-order model and Langmuir and Freundlich isotherm models, respectively. Thermodynamic parameters demonstrated that adsorption of dye is spontaneous and endothermic in nature. The plastic waste can be used after transformation into valuable carbon-based nanomaterials for use in the adsorption of organic contaminants from aqueous solution.

Sorption of ionizable and ionic organic compounds to biochar, activated carbon and other carbonaceous materials

The sorption of ionic and ionizable organic compounds (IOCs) (e.g., pharmaceuticals and pesticides) on carbonaceous materials plays an important role in governing the fate, transport and bioavailability of IOCs. The paradigms previously established for the sorption of neutral organic compounds do not always apply to IOCs and the importance of accounting for the particular sorption behavior of IOCs is being increasingly recognized. This review presents the current state of knowledge and summarizes the recent advances on the sorption of IOCs to carbonaceous sorbents. A broad range of sorbents were considered to evaluate the possibility to read across between fields of research that are often considered in isolation (e.g., carbon nanotubes, graphene, biochar, and activated carbon). Mechanisms relevant to IOCs sorption on carbonaceous sorbents are discussed and critically evaluated, with special attention being given to emerging sorption mechanisms including low-barrier, charge-assisted hydrogen bonds and cation-π assisted π-π interactions. The key role played by some environmental factors is also discussed, with a particular focus on pH and ionic strength. Overall the review reveals significant advances in our understanding of the interactions between IOCs and carbonaceous sorbents. In addition, knowledge gaps are identified and priorities for future research are suggested.

Modeling caffeine adsorption by multi-walled carbon nanotubes using multiple polynomial regression with interaction effects

Permanent monitoring of environmental issues demands efficient, accurate, and user-friendly pollutant prediction methods, particularly from operating variables. In this research, the efficiency of multiple polynomial regression in predicting the adsorption capacity of caffeine (q) from an experimental batch mode by multi-walled carbon nanotubes (MWCNTs) was investigated. The MWCNTs were specified by scanning electron microscope, Fourier transform infrared spectroscopy and point of zero charge. The results confirmed that the MWCNTs have a high capacity to uptake caffeine from the wastewater. Five parameters including pH, reaction time (t), adsorbent mass (M), temperature (T) and initial pollutant concentration (C) were selected as input model data and q as the output. The results indicated that multiple polynomial regression which employed C, M and t was the best model (normalized root mean square error = 0.0916 and R² = 0.996). The sensitivity analysis indicated that the predicted q is more sensitive to the C, followed by M, and t. The results indicated that the pH and temperature have no significant effect on the adsorption capacity of caffeine in batch mode experiments. The results displayed that estimations are slightly overestimated. This study demonstrated that the multiple polynomial regression could be an accurate and faster alternative to available difficult and time-consuming models for q prediction.

Sorption of triclosan to carbon nanotubes: The combined effects of sonication, functionalization and solution chemistry

The sonication effect on the sorption behavior of carbon nanotubes (CNTs) with different functional groups has been poorly understood in previous studies, especially when combined with solution chemistry that may affect both the sorption and dispersion of CNTs. Our results show that sonication accelerated sorption of triclosan and increased the sorption capacities of CNTs with and without functionalizations at a neutral pH. Regardless of how sonication on CNTs was applied, the sorption decreased in the following order: pristine CNTs>CNTs-OH>CNTs-COOH. Sorption decreased with the increase of pH for all types of CNTs due to the electrostatic repulsion between triclosan and CNTs. The pH effect is greater for pristine CNTs than functionalized CNTs, while the sonication effect is greater for functionalized CNTs than pristine CNTs. Sonication increased sorption at a low pH (i.e., <9.5) but decreased sorption at a high pH (i.e., >9.5) for all types of CNTs, which may be due to the difference in the structure of CNTs at different pHs. Within 0.001-0.1M NaCl, sorption increased and then decreased for all types of CNTs with and without sonication, which can be explained by the balance between the salting-out effect and electrostatic screening effect.

Removal of Bisphenol A aqueous solution using surfactant-modified natural zeolite: Taguchi's experimental design, adsorption kinetic, equilibrium and thermodynamic study

In this study, surfactant-modified natural zeolite was used to remove Bisphenol A (BPA) from aqueous solutions. Kinetics, equilibrium and thermodynamics of BPA adsorption on the adsorbent surfaces were investigated. The experimental data were described with the Temkin isotherm and the pseudo-second- order kinetic model. Taguchi's robust design approach was used to optimize adsorption of BPA. Experimentation was planned as per Taguchi's L27 orthogonal array. Tests were conducted with different adsorbate amount, pH, time, initial concentration of BPA, temperature and agitation speed. The optimum levels of control factors for maximum total organic carbon removal were defined (adsorbate amount at 0.25 g, pH at 7, time at 30 min, initial concentration of BPA at 50 mg/L, temperature at 30°C and agitation speed at 200 rpm). The ANOVA analysis shown that the most effective control factor is adsorbent dosage; its contribution is 56.4%. Contribution of pH and mixing rate are 7.5% and 7.6%, respectively. A confirmation experiment was conducted to verify the feasibility and effectiveness of the optimal combination. The observed value of S/N (η_obs = 39) ratio is compared with that of the predicted value (η_opt = 48). The prediction error, that is, η_opt- η_obs = 9, is within CI value.

Assessment of carbon nanotubes and silver nanoparticles loaded clays as adsorbents for removal of bacterial contaminants from water sources

This work evaluated the antimicrobial efficacy of kaolin clay and its loaded forms with carbon nanotubes (CNTs) and silver nanoparticles (AgNPs) against bacterial isolates from different water supplies (tap, underground and surface water) in addition to wastewater. A total of 160 water samples were collected from different water sources in the investigated districts. Samples were cultured for isolation and serological identification of pathogenic bacteria. AgNPs were synthesized by a typical one-step synthesis protocol, where CNTs were carried out in a reactor employing the double bias-assisted hot filament chemical vapor deposition method. Both were characterized using transmission electron microscopy, infrared and X-ray fluorescence (XRF) spectroscopy. The antimicrobial efficacy of each of natural kaolin clay, AgNPs- and CNTs-loaded clays were evaluated by their application in four concentrations (0.01, 0.03, 0.05 and 0.1 ppm) at different contact times (5 min, 15 min, 30 min and 2 h). AgNPs-loaded clays at concentrations of 0.05 and 0.1 mg/l for 2 h contact time exhibited a higher bactericidal efficacy on Escherichia coli and Salmonella spp. (70, 70, 80 and 90%, respectively) compared to CNTs-loaded clay. Concluding, the application of AgNPs-loaded clay for removal of water bacterial contaminants at a concentration of 0.1 ppm for 2 h contact times resulted in highly effective removals.

Advanced Nanomaterials for the Removal of Chemical Substances and Microbes From Contaminated and Waste Water

The development of cost-effective, efficient and stable materials helps to provide the affordable solutions to get safe and fresh water to increasing population with health guidelines of emerging contaminants. Nanomaterials (NMs)-based techniques involve the design, synthesis, manipulation, characterization and exploitation of materials for adsorption and separation of target species from the contaminated and waste water. NMs show better adsorption capacity and catalytic for number chemical species and microbes because of their small size and large surface area that favors the purification and treatment of waste or contaminated environmental water. Here, we present the chemical properties, adsorption/removal mechanism and applications of advanced NMs such as magnetic nanoparticles (MNPs), carbon nanotubes (CNTs), graphene and graphene oxide (GO), titanium oxide (TiO2), silica (SiO2), silver (Ag), gold (Au) NPs and zeolites in effective and efficient removal of toxic metal ions, organic and inorganic chemical substances and disease-causing microbes from contaminated and wastewater.

Influence of the Gastrointestinal Environment on the Bioavailability of Ethinyl Estradiol Sorbed to Single-Walled Carbon Nanotubes

Recent evidence suggests that, because of their sorptive nature, if single-walled carbon nanotubes (SWCNTs) make their way into aquatic environments, they may reduce the toxicity of other waterborne contaminants. However, few studies have examined whether contaminants remain adsorbed following ingestion by aquatic organisms. The objective of this study was to examine the bioavailability and bioactivity of ethinyl estradiol (EE2) sorbed onto SWCNTs in a fish gastrointestinal (GI) tract. Sorption experiments indicated that SWCNTs effectively adsorbed EE2, but the chemical was still able to bind and activate soluble estrogen receptors (ERs) in vitro. However, centrifugation to remove SWCNTs and adsorbed EE2 significantly reduced ER activity compared to that of EE2 alone. Additionally, the presence of SWCNTs did not reduce the extent of EE2-driven induction of vitellogenin 1 in vivo compared to the levels in organisms exposed to EE2 alone. These results suggest that while SWCNTs adsorb EE2 from aqueous solutions, under biological conditions EE2 can desorb and retain bioactivity. Additional results indicate that interactions with gastrointestinal proteins may decrease the level of adsorption of estrogen to SWCNTs by 5%. This study presents valuable data for elucidating how SWCNTs interact with chemicals that are already present in our aquatic environments, which is essential for determining their potential health risk.

Co2SnO4 nanoparticles as a high performance catalyst for oxidative degradation of rhodamine B dye and pentachlorophenol by activation of peroxymonosulfate

Spinel Co₂SnO₄ NPs, prepared by hydrothermal process, are applied for PMS activation for organic pollutants degradation.

Removal of dye molecules from aqueous solution by carbon nanotubes and carbon nanotube functional groups: critical review

Scheme for the adsorption of dye molecules by CNTs and CNT functionality.

Application and characterization of electroactive membranes based on carbon nanotubes and zerovalent iron nanoparticles

Carbon nanotube (CNT) membranes were produced from multi-walled CNTs by a filtration technique and used for the removal of the betablocker metoprolol by adsorptive and reactive processes. The reactivity of CNT membranes was enhanced by nanoparticulate zero-valent iron (NZVI) which was deposited on the CNT membranes by pulsed voltammetry applying defined number of pulses (Fe-CNT (100) and Fe-CNT (400) membranes). Surface analysis with SEM showed iron nanoparticle sizes between 19 and 425 nm. Pore size distribution for the different membranes was determined by capillary flow porometry (Galwick fluid). Pore size distribution for all membranes was similar (40 nm), which resulted in a water permeability typical for microfiltration membranes. Metoprolol was removed by the CNT membrane only by sorption, whereas the Fe-CNT membrane revealed also metoprolol degradation due to Fenton type reactions. Further application of electrochemical potentials on both the CNT and the Fe-CNT membranes improved the removal efficiencies to 74% for CNT membranes at 1 V and to 97% for Fe-CNT (400) membranes at 1 V. Seven transformation products have been identified for metoprolol by high-resolution mass spectrometry when electrochemical degradation was performed with CNT and Fe-CNT membranes. Additionally, two of the identified transformation products (TPs) were also observed for Fe-CNT membranes without the application of electrochemical potential. However, only 10% of the degraded metoprolol could be explained by the formation of TPs.

Removal of triazine-based pollutants from water by carbon nanotubes: Impact of dissolved organic matter (DOM) and solution chemistry

Adsorption of organic pollutants by carbon nanotubes (CNTs) in the environment or removal of pollutants during water purification require deep understanding of the impacts of the presence of dissolved organic matter (DOM). DOM is an integral part of environmental systems and plays a key role affecting the behavior of organic pollutants. In this study, the effects of solution chemistry (pH and ionic strength) and the presence of DOM on the removal of atrazine and lamotrigine by single-walled CNTs (SWCNTs) was investigated. The solubility of atrazine slightly decreased (∼5%) in the presence of DOM, whereas that of lamotrigine was significantly enhanced (by up to ∼70%). Simultaneous introduction of DOM and pollutant resulted in suppression of removal of both atrazine and lamotrigine, which was attributed to DOM-pollutant competition or blockage of adsorption sites by DOM. However the decrease in removal of lamotrigine was also a result of its complexation with DOM. Pre-introduction of DOM significantly reduced pollutant adsorption by the SWCNTs, whereas introduction of DOM after the pollutant resulted in the release of adsorbed atrazine and lamotrigine from the SWCNTs. These data imply that DOM exhibits higher affinity for the adsorption sites than the triazine-based pollutants. In the absence of DOM atrazine was a more effective competitor than lamotrigine for adsorption sites in SWCNTs. However, competition between pollutants in the presence of DOM revealed lamotrigine as the better competitor. Our findings help unravel the complex DOM-organic pollutant-CNT system and will aid in CNT-implementation in water-purification technologies.

Phenanthrene Bioavailability and Toxicity to Daphnia magna in the Presence of Carbon Nanotubes with Different Physicochemical Properties

Studies investigating the effect of carbon nanotubes (CNTs) on the bioavailability and toxicity of hydrophobic organic compounds in aquatic environments have generated contradictory results, and the influence of different CNT properties remains unknown. Here, the adsorption of the polycyclic aromatic hydrocarbon phenanthrene (70-735 μg/L) to five types of CNTs exhibiting different physical and chemical properties was studied. The CNTs were dispersed in the presence of natural organic matter (nominally 20 mg/L) in order to increase the environmental relevance of the study. Furthermore, the bioavailability and toxicity of phenanthrene to Daphnia magna in the absence and presence of dispersed CNTs was investigated. Both CNT dispersion and adsorption of phenanthrene appeared to be influenced by CNT physical properties (diameter and specific surface area). However, dispersion and phenanthrene adsorption was not influenced by CNT surface chemical properties (surface oxygen content), under the conditions tested. Based on nominal phenanthrene concentrations, a reduction in toxicity to D. magna was observed during coexposure to phenanthrene and two types of CNTs, while for the others, no influence on phenanthrene toxicity was observed. Based on freely dissolved concentrations, however, an increased toxicity was observed in the presence of all CNTs, indicating bioavailability of CNT-adsorbed phenanthrene to D. magna.

The interactions of UV and/or H2O2 treated CNTOH and CNTCOOH with environmental fulvic acids

The fate of carbon nanotubes (CNT) in the environment will be governed by the presence of natural dissolved organic matter (DOM). Many studies indicate that CNT create stabilized suspensions in the presence of DOM. Easier transport in the environment may indicate their greater hazard. However these studies describe the interactions of DOM with as produced CNT. In the present studies the interactions of UV and/or H2O2 treated wastewater containing CNTOH or CNTCOOH with the naturally occurred fulvic acids (FA) were presented. FA sorption, both kinetics and mechanism, were described using batch regime. The sorption of FA followed a pseudo-second order kinetics and was described with the highest accuracy by Langmuir or Dubinin-Radushkevich model for CNTOHs and Langmuir, Temkin or Dubinin-Radushkevich - for CNTCOOHs. The mechanism of FA sorption onto CNTOHs was ascribed to π-π, heterogeneous and electrostatic interactions. The π-π and electrostatic interactions can the mostly defined FA adsorption onto CNTCOOHs. The parameters affecting FA sorption were combination of porosity and dispersity.

Photochemical behavior of carbon nanotubes in natural waters: reactive oxygen species production and effects on •OH generation by Suwannee River fulvic acid, nitrate, and Fe (III)

The photochemical activities of three kinds of carbon nanotubes (CNTs) were investigated in the present study. Efficient procedures of dispersing the three kinds of carbon nanotubes in water were established, and the quantitative analysis methods were also developed by TOC-absorbance method. High pH value or low ionic strength of the colloidal solutions facilitated the dispersion of CNTs. The suspensions of three kinds of CNTs could generate singlet oxygen ((1)O2) and hydroxyl radical (•OH) under irradiation of simulated sunlight, while superoxide radical (O2 (•-)) was not detected. The steady-state concentrations of (1)O2 and •OH generated by these CNTs were also determined. The presence of CNTs in natural waters can affect the photochemical behavior of water constituents, such as nitrate, dissolved organic matter, and Fe(3+). Specifically, in nitrate solution, the presence of CNTs could inhibit the generation of •OH by nitrate through light screening effect, while the quenching effect of hydroxyl radicals by CNTs was not observed. Besides light screening effect, the three kinds of CNTs used in the experiments also have a strong inhibiting effect on the ability of DOM to produce •OH by binding to the active sites. Moreover, the adsorption of Fe(3+) on MWCNT-OH and MWCNT-COOH could lead to its inactivation of formation of •OH in acidic conditions. However, the presence of the three kinds of CNTs did not affect the ligand-to-metal charge transfer (LMCT) reaction of DOM-Fe (III) complex.

Carbon Nanotube Based Groundwater Remediation: The Case of Trichloroethylene

Adsorption of chlorinated organic contaminants (COCs) on carbon nanotubes (CNTs) has been gaining ground as a remedial platform for groundwater treatment. Applications depend on our mechanistic understanding of COC adsorption on CNTs. This paper lays out the nature of competing interactions at play in hybrid, membrane, and pure CNT based systems and presents results with the perspective of existing gaps in design strategies. First, current remediation approaches to trichloroethylene (TCE), the most ubiquitous of the COCs, is presented along with examination of forces contributing to adsorption of analogous contaminants at the molecular level. Second, we present results on TCE adsorption and remediation on pure and hybrid CNT systems with a stress on the specific nature of substrate and molecular architecture that would contribute to competitive adsorption. The delineation of intermolecular interactions that contribute to efficient remediation is needed for custom, scalable field design of purification systems for a wide range of contaminants.

Environmental application of nanotechnology: air, soil, and water

Global deterioration of water, soil, and atmosphere by the release of toxic chemicals from the ongoing anthropogenic activities is becoming a serious problem throughout the world. This poses numerous issues relevant to ecosystem and human health that intensify the application challenges of conventional treatment technologies. Therefore, this review sheds the light on the recent progresses in nanotechnology and its vital role to encompass the imperative demand to monitor and treat the emerging hazardous wastes with lower cost, less energy, as well as higher efficiency. Essentially, the key aspects of this account are to briefly outline the advantages of nanotechnology over conventional treatment technologies and to relevantly highlight the treatment applications of some nanomaterials (e.g., carbon-based nanoparticles, antibacterial nanoparticles, and metal oxide nanoparticles) in the following environments: (1) air (treatment of greenhouse gases, volatile organic compounds, and bioaerosols via adsorption, photocatalytic degradation, thermal decomposition, and air filtration processes), (2) soil (application of nanomaterials as amendment agents for phytoremediation processes and utilization of stabilizers to enhance their performance), and (3) water (removal of organic pollutants, heavy metals, pathogens through adsorption, membrane processes, photocatalysis, and disinfection processes).

Adsorption mechanism of different organic chemicals on fluorinated carbon nanotubes

Multi-walled carbon nanotubes (MC) were fluorinated by a solid-phase reaction method using polytetrafluoroethylene (PTFE). The surface alteration of carbon nanotubes after fluorination (MC-F) was confirmed based on surface elemental analysis, TEM and SEM. The incorporation of F on MC surface was discussed as F incorporation on carbon defects, replacement of carboxyl groups, as well as surface coating of PTFE. The adsorption performance and mechanisms of MC-F for five kinds of representative organic compounds: sulfamethoxazole (SMX), ofloxacin (OFL), norfloxacin (NOR), bisphenol a (BPA) and phenanthrene (PHE) were investigated. Although BET-N2 surface area of the investigated CNTs decreased after fluorination, the adsorption of all five chemicals increased. Because of the glassification of MC-F surface coating during BET-N2 surface area measurement, the accessible surface area of MC-F was underestimated. Desorption hysteresis was generally observed in all the sorption systems in this study, and the desorption hysteresis of MC-F were stronger than the pristine CNTs. The enhanced adsorption of MC-F may be attributed the pores generated on the coated PTFE and the dispersed CNT aggregates due to the increased electrostatic repulsion after fluorination. The rearrangement of the bundles or diffusion of the adsorbates in MC-F inner pores were the likely reason for the strong desorption hysteresis of MC-F. The butterfly structure of BPA resulted in its high sorption and strong desorption hysteresis. The exothermic sorption character of OFL on CNTs resulted in its strong desorption hysteresis.

Microwave-induced carbon nanotubes catalytic degradation of organic pollutants in aqueous solution

In this study, a new catalytic degradation technology using microwave induced carbon nanotubes (MW/CNTs) was proposed and applied in the treatment of organic pollutants in aqueous solution. The catalytic activity of three CNTs of 10-20nm, 20-40nm, and 40-60nm diameters were compared. The results showed that organic pollutants such as methyl orange (MO), methyl parathion (MP), sodium dodecyl benzene sulfonate (SDBS), bisphenol A (BPA), and methylene blue (MB) in aqueous solution could be degraded effectively and rapidly in MW/CNTs system. CNTs with diameter of 10-20nm exhibited the highest catalytic activity of the three CNTs under MW irradiation. Further, complete degradation was obtained using 10-20nm CNTs within 7.0min irradiation when 25mL MO solution (25mg/L), 1.2g/L catalyst dose, 450W, 2450MHz, and pH=6.0 were applied. The rate constants (k) for the degradation of SDBS, MB, MP, MO and BPA using 10-20nm CNTs/MW system were 0.726, 0.679, 0.463, 0.334 and 0.168min(-1), respectively. Therefore, this technology may have potential application for the treatment of targeted organic pollutants in wastewaters.

Adsorption and desorption of dissolved organic matter by carbon nanotubes: Effects of solution chemistry

Increasing use of carbon nanotubes (CNTs) has led to their introduction into the environment where they can interact with dissolved organic matter (DOM). This study focuses on solution chemistry effects on DOM adsorption/desorption processes by single-walled CNTs (SWCNTs). Our data show that DOM adsorption is controlled by the attachment of DOM molecules to the SWCNTs, and that the initial adsorption rate is dependent on solution parameters. Adsorbed amount of DOM at high ionic strength was limited, possibly due to alterations in SWCNT bundling. Desorption of DOM performed at low pH resulted in additional DOM adsorption, whereas at high pH, adsorbed DOM amount decreased. The extent of desorption conducted at increased ionic strength was dependent on pre-adsorbed DOM concentration: low DOM loading stimulated additional adsorption of DOM, whereas high DOM loading facilitated release of adsorbed DOM. Elevated ionic strength and increased adsorbed amount of DOM reduced the oxidation temperature of the SWCNTs, suggesting that changes in the assembly of the SWCNTs had occurred. Moreover, DOM-coated SWCNTs at increased ionic strength provided fewer sites for atrazine adsorption. This study enhances our understanding of DOM-SWCNT interactions in aqueous systems influenced by rapid changes in salinity, and facilitates potential use of SWCNTs in water-purification technologies.

Multi-walled carbon nanotubes with selected properties for dynamic filtration of pharmaceuticals and personal care products

In this study, multi-walled carbon nanotubes (MWCNT) with selected properties, including pristine MWCNT, hydroxylated MWCNT (H-MWCNT), thin-walled MWCNT with large inner diameter (L-MWCNT), aminated MWCNT, and high-purity MWCNT were investigated for dynamic removal of eight pharmaceuticals and personal care products (PPCP). The removal ratios of different PPCP by the pristine MWCNT followed a decreasing order of triclosan (0.93) > prometryn (0.71) > 4-acetylamino-antipyrine (0.67) > carbendazim (0.65) > caffeine (0.42) > ibuprofen (0.34) > acetaminophen (0.29) at 100 min of filtration. Similar or even higher PPCP removals were obtained for all PPCP as the influent concentration decreased, suggesting potential consistent PPCP removals at environmental PPCP concentrations. The removal ratio of acetaminophen was increased to 0.74 by using H-MWCNT. SRFA (Suwannee River fulvic acid) suppressed PPCP adsorption to MWCNT, to greater extents with increasing SRFA concentrations. The L-MWCNT, despite a large inner diameter of 52 ± 3 nm, did not provide better resistance to the competitive adsorption of SRFA than MWCNT with a small inner diameter of 10 ± 2 nm. Future research will be conducted to minimize the effect of SRFA and facilitate application of MWCNT to the treatment of PPCP-contaminated water.

Linear solvation energy relationship for the adsorption of synthetic organic compounds on single-walled carbon nanotubes in water

The linear solvation energy relationship (LSER) was applied to predict the adsorption coefficient (K) of synthetic organic compounds (SOCs) on single-walled carbon nanotubes (SWCNTs). A total of 40 log K values were used to develop and validate the LSER model. The adsorption data for 34 SOCs were collected from 13 published articles and the other six were obtained in our experiment. The optimal model composed of four descriptors was developed by a stepwise multiple linear regression (MLR) method. The adjusted r(2) (r(2)adj) and root mean square error (RMSE) were 0.84 and 0.49, respectively, indicating good fitness. The leave-one-out cross-validation Q(2) ([Formula: see text]) was 0.79, suggesting the robustness of the model was satisfactory. The external Q(2) ([Formula: see text]) and RMSE (RMSEext) were 0.72 and 0.50, respectively, showing the model's strong predictive ability. Hydrogen bond donating interaction (bB) and cavity formation and dispersion interactions (vV) stood out as the two most influential factors controlling the adsorption of SOCs onto SWCNTs. The equilibrium concentration would affect the fitness and predictive ability of the model, while the coefficients varied slightly.

Removal of tetracycline antibiotic from contaminated water media by multi-walled carbon nanotubes: operational variables, kinetics, and equilibrium studies

Multi-walled carbon nanotubes (MWCNTs) were purified and oxidized by a 4 mol L(-1) mixture of H2SO4:H2O2 and then were used as adsorbent for tetracycline (TC) adsorption from aqueous solutions. The purified MWCNTs were characterized using Fourier transform infrared spectroscopy, field emission scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction, and N2 adsorption/desorption isotherms. The adsorption of TC onto the MWCNT was investigated as a function of the initial pH of the solution, adsorbent dosage, and background electrolyte cations and anions. The results of the one-way analysis of variance (ANOVA) showed that Fe(3+) ion significantly affected and decreased TC adsorption onto the MWCNT (P-value < 0.05), while other studied cations and anions did not affect TC adsorption (P-value>0.05). Nonlinear pseudo-first-order, pseudo-second-order, general order, and Avrami fractionary-order kinetic models were used to investigate the kinetics of TC adsorption. The fractionary-order kinetic model provided the best fit to experimental data. In addition, the adsorption isotherms data were well described by nonlinear equation of the Liu isotherm model with the maximum adsorption capacity of 253.38 mg g(-1). The results of this study indicate that the oxidized MWCNTs can be used as an effective adsorbent for TC removal from aqueous solutions.

Effect of humic acid on the sulfamethazine adsorption by functionalized multi-walled carbon nanotubes in aqueous solution: mechanistic study

The presence of humic acid (HA) inhibited sulfamethazine (SMZ) adsorption by three types of multi-walled carbon nanotubesviacompetitive interactions, molecular sieving, and pore blockage in solution (pH < 9).

The effects of microwave regeneration on adsorptive performance of functionalized carbon nanotubes

In this study, the microwave regeneration method was applied to investigate the properties and adsorptive performance of functionalized carbon nanotubes (f-CNTs) in different cycles of regeneration/reuse. For this purpose, an organic and hazardous dye (Reactive Blue 19) was chosen as a widely used pollutant. N2 adsorption/desorption isotherms, scanning electron microscopy and Fourier transform infrared spectroscopy were used to characterize f-CNTs during the regeneration/reuse procedure. The morphology, specific surface area and pore volume of f-CNT samples were not significantly altered. However, the functional groups present on the f-CNTs' surface were gradually removed after successive cycles of regeneration/reuse. A sudden decrease of adsorption capacity (about 20%) after the first cycle of regeneration/reuse was attributed to the elimination of functional groups interacting with the dye molecules because of the molecular-level heating. Relatively high regeneration efficiencies (73.30 to 80.16%) proved that the microwave regeneration method was successful. Very high step stripping efficiencies (80.16 to 98.02%) in four cycles of regeneration/reuse demonstrated that the microwave regeneration method could be utilized in consecutive cycles. After four cycles of regeneration/reuse, the CNTs could not be considered as functionalized.

Utilization of a pyrrole derivative based antimicrobial functionality impregnated onto CaO/g-C3N4 for dyes adsorption

A novel functionalization of CaO/g-C₃N₄ based nanocomposite using 4,5-diphenyl-2-thioxo-2,5-dihydro-1H-pyrrole-3-cabonitrile (P3C@CaO–HCN) was fabricated for wastewater remediation from organic dyes and microbial pollutants.

Single-Walled Carbon Nanotubes (SWCNTs), as a Novel Sorbent for Determination of Mercury in Air

BACKGROUND

Based on the noticeable toxicity and numerous application of mercury in industries, removal of mercury vapor through sorbent is an important environmental challenge.

PURPOSE OF THE STUDY

Due to their highly porous and hollow structure, large specific surface area, light mass density and strong interaction, Single-Walled Carbon Nanotubes (SWCNTs) sorbent were selected for this investigation.

METHODS

In this study, instrumental conditions, method procedure and different effective parameters such as adsorption efficiency, desorption capacity, time, temperature and repeatability as well as retention time of adsorbed mercury were studied and optimized. Also, mercury vapor was determined by cold vapor atomic absorption spectrometry (CV-AAS).Obtained data were analyzed by Independent T- test, Multivariate linear regression and one way-ANOVA finally.

RESULTS

For 80 mg nanotubes, working range of SWCNT were achieved 0.02-0.7 mg with linear range (R2=0.994).Our data revealed that maximum absorption capacity was 0.5 mg g-1 as well as limit of detection (LOD) for studied sorbent was 0.006 mg. Also, optimum time and temperature were reported, 10 min and 250 °C respectively. Retention time of mercury on CNTs for three weeks was over 90%. Results of repeated trials indicated that the CNTs had long life, so that after 30 cycles of experiments, efficiency was determined without performance loss.

CONCLUSION

Results showed that carbon nanotubes have high potential for efficient extraction of mercury from air and can be used for occupational and environmental purposes. The study of adsorption properties of CNTs is recommended.