Kinetics and thermodynamics of adsorption of ionizable aromatic compounds from aqueous solutions by as-prepared and oxidized multiwalled carbon nanotubes

Metaldehyde removal from aqueous solution by adsorption and ion exchange mechanisms onto activated carbon and polymeric sorbents

Metaldehyde removal from aqueous solution was evaluated using granular activated carbon (GAC), a non-functionalised hyper-cross-linked polymer Macronet (MN200) and an ion-exchange resin (S957) with sulfonic and phosphonic functional groups. Equilibrium experimental data were successfully described by Freundlich isotherm models. The maximum adsorption capacity of S957 (7.5 g metaldehyde/g S957) exceeded those of MN200 and GAC. Thermodynamic studies showed that sorption of metaldehyde onto all sorbents is endothermic and processes are controlled by entropic rather than enthalpic changes. Kinetic experiments demonstrated that experimental data for MN200 and GAC obey pseudo-second order models with rates limited by particle diffusion. Comparatively, S957 was shown to obey a pseudo-first order model with a rate-limiting step of metaldehyde diffusion through the solid/liquid interface. Results obtained suggest that metaldehyde adsorption onto MN200 and GAC are driven by hydrophobic interactions and hydrogen bonding, as leaching tendencies were high since no degradation of metaldehyde occurred. Conversely, adsorption of metaldehyde onto S957 occurs via ion-exchange processes, where sulfonic and phosphonic functionalities degrade adsorbed metaldehyde molecules and failure to detect metaldehyde in leaching studies for S957 supports this theory. Consequently, the high adsorption capacity and absence of leaching indicate S957 is promising for metaldehyde removal from source water.

Kinetic and thermodynamic studies of toluene, ethylbenzene, and m-xylene adsorption from aqueous solutions onto KOH-activated multiwalled carbon nanotubes

Multiwalled carbon nanotubes activated by KOH (CNTs-KOH) were synthesized and employed as adsorbents to study adsorption characteristics of toluene, ethylbenzene, and m-xylene (TEX) from aqueous solutions. Kinetics data were fitted by pseudo-second-order model, and intraparticle diffusion was not the only rate-controlling step. Adsorption isotherm data could fit well with Langmuir, Freundlich, and Dubinin-Radushkevich (D-R) models. The maximum adsorption capacities on CNTs-KOH are 87.12, 322.05, and 247.83 mg/g for toluene, ethylbenzene, and m-xylene, respectively. The adsorption capacities of TEX onto CNTs-KOH increased with contact time and decreased with temperature and are not significantly affected by humic acid. However, Cr(6+) could decrease adsorption of TEX by 17.66, 4.51, and 12.69% as (K(d)(1) - K(d)(2))/K(d)(1), respectively. The thermodynamics parameters indicated that adsorption was a feasible, exothermic, and spontaneous process in nature and a physisorption process. The present CNTs-KOH show a better EX adsorption performance than other adsorbents, suggesting that CNTs-KOH are promising EX adsorbents in wastewater treatment.

Fabrication of magnetic Ni nanoparticles functionalized water-soluble graphene sheets nanocomposites as sorbent for aromatic compounds removal

The magnetic Ni nanoparticles functionalized water-soluble graphene sheets nanocomposites (Ni@GSs-C(CH(3))(2)COONa) were fabricated via a facile and mild strategy. We designed a simple and efficient approach (the addition of cyano radicals) to improve the water solubility of nanocomposites. The Ni@GSs-C(CH(3))(2)COONa nanocomposites had great potential as an effective absorbent for removing aromatic compounds from waste water owing to their rapid absorption rate, high absorption capacity, convenient magnetic separation and re-use property.

Removal of U(VI) from aqueous solutions by the nano-iron oxyhydroxides

The nano-iron oxyhydroxides (α- and γ-FeOOH) were synthesized by oxidation of the various raw materials (FeSO4 and FeCl2) as iron precursors under alkaline conditions. Morphologies of nano-iron oxyhydroxides were characterized by high resolution transmission electron microscope (HRTEM), X-ray powder diffraction (XRD) and specific surface area (SSA), respectively. The removal of U(VI) from aqueous solution by nano-iron oxyhydroxides was also investigated as a function of contact time, U(VI) concentration, pH, ionic strength and temperature under ambient conditions. The occurrence of needle-like shape of nano-goethite and rod-like shape of nano- lepidocrocite were attributed to oxidization of Fe2+ under different conditions in terms of XRD analysis and HRTEM images. The sorption results indicated that the removal of U(VI) strongly depended on pH and were independent of ionic strength. Langmuir model was employed to model the sorption isotherms of U(VI) at three various temperatures of 293, 313 and 333 K. The thermodynamic parameters (Δ Hº, Δ Sº and Δ Gº) computed from the temperature showed that sorption process of U(VI) on nano-iron oxyhydroxides was endothermic and spontaneous. The mechanism of U(VI) sorption on nano-iron oxyhydroxides was assumed to outer-sphere surface complexation at low pH (pH < 5.0) and at low ionic strength conditions (IS = 0.01 M NaClO4), whereas the uptake of U(VI) was dominated by surface precipitation at high pH conditions. Thereby nano-iron oxyhydroxides can be used as an efficient backfill materials for the in situ disposal of U(VI)-bearing contaminated groundwater.

Investigation of the sequestration mechanisms of Cd(II) and 1-naphthol on discharged multi-walled carbon nanotubes in aqueous environment

The increasing exploitation of multi-walled carbon nanotubes (MWCNTs) into many industrial processes has raised considerable concerns as they are likely to be released into the environment. The interactions of the pollutants in water with discharged MWNCTs will further influence the environmental behavior and fate of these contaminants. In this context, our work aims to quantify the sequestration mechanisms and species distribution of Cd(II) and 1-naphthol on discharged MWCNTs by using batch technique. The uptake of Cd(II) on oxidized MWCNTs is greatly enhanced by the coexistent 1-naphthol, whereas the uptake of 1-naphthol on oxidized MWCNTs is not influenced by the coexistent Cd(II) in solution. Based on the experimental results, one can deduce that ion exchange or outer-sphere surface complexation mainly contributes to Cd(II) uptake on oxidized MWCNTs at low pH values, and inner-sphere surface complexation is the main uptake mechanism at high pH values. The uptake of 1-naphthol on oxidized MWCNTs is attributed to the π-π conjugate action between the aromatic ring of 1-naphthol and the graphitic structure of oxidized MWCNTs. The above-mentioned laboratorial results are further verified by the investigation conducted in actual effluent disposal system. The findings in the present study can provide more precise information on the real impact and changes in aqueous environment caused by the simultaneous presence of Cd(II), 1-naphthol and discharged oxidized MWCNTs.

Adsorption kinetics of aromatic compounds on carbon nanotubes and activated carbons

Adsorption kinetics of two organic compounds on four types of carbonaceous adsorbents (a granular activated carbon [HD4000], an activated carbon fiber [ACF10], a single-walled carbon nanotube [SWNT], and a multiwalled carbon nanotube [MWNT]) was examined in aqueous solutions. The times needed for the adsorption to reach apparent equilibrium on the four carbons followed the order of ACF10 > HD4000 > SWNT > MWNT. Ultrasonication of the carbon nanotubes (CNTs) accelerated their adsorption kinetics but had no effect on their equilibrium adsorption capacities. The pseudo-second order model (PSOM) provided good fitting for the kinetic data. The fitting of kinetic data with the intraparticle diffusion model indicated that external mass transfer controls the sorption process in the organic compound-CNT systems, whereas intraparticle diffusion dominates in the sorption of organic compounds onto activated carbons.

Retracted Article: Enhanced adsorption of ionizable aromatic compounds on humic acid-coated carbonaceous adsorbents

Adsorption of ionizable aromatic compounds (IACs) such as 1-naphthylamine and 1-naphthol on Humic acid (HA)-coated graphene oxide nanosheets (GONs), multiwalled carbon nanotubes (MWCNTs), activated carbon (AC), and flake graphite (FG) were investigated by the batch techniques.

Adsorption behavior of multi-walled carbon nanotubes for the removal of olaquindox from aqueous solutions

Multi-walled carbon nanotubes (MWCNT) were employed for the sorption of olaquindox (OLA) from aqueous solution. A detailed study of the adsorption process was performed by varying pH, ionic strength, sorbent amount, sorption time and temperature. The adsorption mechanism is probably the non-electrostatic π-π dispersion interaction and hydrophobic interaction between OLA and MWCNT. The adsorption efficiency could reach 99.7%, suggesting that MWCNT is excellent adsorbents for effective OLA removal from water. OLA adsorption kinetics were found to be very fast and equilibrium was reached within 2.0 min following the pseudo-second-order model with observed rate constants (k) of 0.169-1.048 g mg(-1)min(-1) (at varied temperatures). The overall rate process appeared to be influenced by both external mass transfer and intraparticle diffusion, but mainly governed by intraparticle diffusion. A rapid initial adsorption behavior occurred within a short period of time in this adsorption system. The sorption data could be well interpreted by the Langmuir model with the maximum adsorption capacity of 133.156 mg g(-1) (293 K) of OLA on MWCNT. The mean energy of adsorption was calculated to be 0.124 kJ mol(-1) (293 K) from the Dubinin-Radushkevich adsorption isotherm. Moreover, the thermodynamic parameters showed the spontaneous, exothermic and physical nature of the adsorption process.

Adsorption of ciprofloxacin on surface-modified carbon materials

The adsorption capacity of ciprofloxacin (CPX) was determined on three types of carbon-based materials: activated carbon (commercial sample), carbon nanotubes (commercial multi-walled carbon nanotubes) and carbon xerogel (prepared by the resorcinol/formaldehyde approach at pH 6.0). These materials were used as received/prepared and functionalised through oxidation with nitric acid. The oxidised materials were then heat treated under inert atmosphere (N2) at different temperatures (between 350 and 900°C). The obtained samples were characterised by adsorption of N2 at -196 °C, determination of the point of zero charge and by temperature programmed desorption. High adsorption capacities ranging from approximately 60 to 300 mgCPxgC(-1) were obtained (for oxidised carbon xerogel, and oxidised thermally treated activated carbon Norit ROX 8.0, respectively). In general, it was found that the nitric acid treatment of samples has a detrimental effect in adsorption capacity, whereas thermal treatments, especially at 900 °C after oxidation, enhance adsorption performance. This is due to the positive effect of the surface basicity. The kinetic curves obtained were fitted using 1st or 2nd order models, and the Langmuir and Freundlich models were used to describe the equilibrium isotherms obtained. The 2nd order and the Langmuir models, respectively, were shown to present the best fittings.

Adsorption of toluene, ethylbenzene and m-xylene on multi-walled carbon nanotubes with different oxygen contents from aqueous solutions

The purified and oxidized multi-walled carbon nanotubes (MWCNTs) with different oxygen contents are employed as adsorbents to study their physicochemical properties and adsorption behaviors of toluene, ethylbenzene and m-xylene (TEX) in aqueous solutions. The results demonstrate that adsorption capacity is significantly enhanced for 3.2% surface oxygen, but is dramatically reduced for 5.9% oxygen concentration. The adsorption kinetics is investigated and fitted with pseudo-second-order model. The adsorption isotherms are found to be fitted with Langmuir model. More interestingly, with the increasing of surface oxygen content, maximum adsorption capacities firstly increased, and then, began to decrease. In the first stage, dispersion is the most important factor. A better dispersive interaction increases the available adsorption sites, which consequently can be favorable for the aqueous phase adsorption. Therefore, maximum adsorption capacity is remarkably enhanced with the increasing of oxygen content, which is according with our results. However, in the second stage, when oxygen content increases to a certain extent, hydroxyl groups cause water clusters formation on the surface or tube end of MWCNTs, which hinder the interaction between TEX and MWCNTs. Consequently, more oxygen content leads to the decrease in maximum adsorption capacity. The decrease indicates that the formation of water clusters plays a more important role than the better dispersion of MWCNTs for TEX adsorption.

Enhanced sorption of radiocobalt from water by Bi(III) modified montmorillonite: a novel adsorbent

In this study, Ca-montmorillonite (Ca-Mt) modified with Bi(3+) was used as a novel adsorbent for the sorption of Co(II) from aqueous solutions. The sorption of Co(II) on Bi-montmorillonite (Bi-Mt) was investigated as a function of contact time, pH, ionic strength, adsorbent content, Co(II) concentrations, fulvic acid (FA) and temperature. Compared to Ca-Mt, Bi-Mt showed a higher affinity to bind Co(II) ions. The sorption percentage of Co(II) on Bi-Mt increased with increasing pH at pH 3.0-8.5, and then maintained the high level at pH 8.5-12. The sorption of Co(II) on Bi-Mt was dependent on ionic strength at low pH, and independent of ionic strength at high pH. The presence of FA enhanced Co(II) sorption at low pH, but suppressed Co(II) sorption at high pH. The thermodynamic data derived from temperature dependent sorption isotherms suggested that the sorption of Co(II) on Bi-Mt was spontaneous and endothermic process. Outer-sphere surface complexation and/or ion exchange were the main mechanisms of Co(II) sorption on Bi-Mt at low pH, whereas inner-sphere surface complexation was the main sorption mechanism at high pH. From the experimental results, it is possible to conclude that Bi-Mt is suitable for application of Co(II) removal from aqueous solutions.

Structural and proactive safety aspects of oxidation debris from multiwalled carbon nanotubes

The removal of oxidation debris from the oxidized carbon nanotube surface with a NaOH treatment is a key step for an effective functionalization and quality improvement of the carbon nanotube samples. In this work, we show via infrared spectroscopy and ultrahigh resolution and accuracy mass spectrometry that oxidation debris obtained from HNO(3)-treated multiwalled carbon nanotubes is a complex mixture of highly condensed aromatic oxygenated carbonaceous fragments. We have also evaluated their cytotoxicity by using BALB/c 3T3 mouse fibroblasts and HaCaT human keratinocytes as models. By knowing the negative aspects of dissolved organic carbon (DOC) to the water quality, we have demonstrated the removal of these carbon nanotube residues from the NaOH solution (wastewater) by using aluminium sulphate, which is a standard coagulant agent used in conventional drinking water purification and wastewater treatment plants. Our results contribute to elucidate the structural and proactive safety aspects of oxidation debris from oxidized carbon nanotubes towards a greener nanotechnology.

Adsorption of ionizable organic contaminants on multi-walled carbon nanotubes with different oxygen contents

Multi-walled carbon nanotubes (MWNTs), which are considered to be promising candidates for the adsorption of toxic organics, are released into aqueous environment with their increasing production and application. In this study, the adsorption behaviors of five structurally related ionizable organic contaminants namely perfluorooctane sulfonate (PFOS), perfluorooctanoic acid (PFOA), perfluorooctanesulfonamide (PFOSA), 2,4-dichlorophenoxyacetic acid (2,4-D) and 4-n-nonylphenol (4-NP) onto MWNTs with different oxygen contents (3.84-22.85%) were investigated. The adsorption kinetics was investigated and simulated with pseudo-second-order model. The adsorption isotherms were found to be fitted with Freundlich model and influenced by both the properties of organic chemicals and the oxygen contents of MWNTs. As adsorption capacity decreases dramatically with the increasing of oxygen contents, the MWNTs with the lowest oxygen contents possess the highest adsorption capacity among four MWNTs. For the MWNTs with the oxygen contents of 3.84%, the adsorption affinity related with hydrophobic interaction and π-electron polarizability decreased in the order of 4-NP>PFOSA>PFOS>2,4-D>PFOA. Furthermore, the adsorption characters of five contaminants were affected by solution pH and solute pK(a) considering electrostatic repulse force and hydrogen bonding, which showed the adsorption of MWNTs with lower oxygen content is much sensitive to solution chemistry.

Removal of 1-naphthylamine from aqueous solution by multiwall carbon nanotubes/iron oxides/cyclodextrin composite

The adsorption of 1-naphthylamine on multiwall carbon nanotubes/iron oxides/β-cyclodextrin composite (denoted by MWCNTs/iron oxides/CD) prepared by using plasma-induced grafting technique was investigated by batch technique under ambient conditions. The effect of contact time, pH, adsorbent content, temperature and initial 1-naphthylamine concentration, on 1-naphthylamine adsorption to MWCNTs/iron oxides/CD was examined. The adsorption of 1-naphthylamine on MWCNTs/iron oxides/CD was dependent on pH, adsorbent content, and temperature. The 1-napthylamien was adsorbed rapidly at the first 50h, and thereafter attained the adsorption saturation at 80h. The adsorption kinetic data were well described by the pseuso-second-order rate model. The adsorption isotherms were fitted by the Langmuir model better than by the Freundlich model. The maximum adsorption capacity of 1-naphthylamine was 200.0mg/g. The adsorption thermodynamic parameters of standard enthalpy (ΔH(0)), standard entropy changes (ΔS(0)), and standard free energy (ΔG(0)) were calculated from temperature dependent adsorption isotherms. The values of ΔH(0) and ΔG(0) suggested that the adsorption of 1-naphthylamine on MWCNTs/iron oxides/CD was endothermic and spontaneous. The electron-donor-acceptor interaction, Hydrophobic interaction, and Lewis acid-base interaction may play an important role in 1-naphthylamine adsorption. The results show that MWCNTs/iron oxides/CD is a promising magnetic nanomaterial for the preconcentration and separation of organic pollutants from aqueous solutions in environmental pollution cleanup.