Non-linear modeling of kinetic and equilibrium data for the adsorption of hexavalent chromium by carbon nanomaterials:Dimension and functionalization

The adsorption capacities for the removal of hexavalent chromium from aqueous solutions by six carbon nanomaterials have been evaluated. Single-walled and multi-walled carbon nanotubes as received and after oxidation treatment, graphene oxide and reduced graphene oxide are the materials with different dimension and functionalization compared in this research. Carbon nanotubes have been modified using hydrogen peroxide as oxidizing agent under microwave radiation. The oxidation treatment on carbon nanotubes has a positive effect increasing the adsorbent–adsorbate interaction. Rate-controlling mechanisms and equilibrium data are analyzed using non-linear models. Non-linear method is proposed as the most suitable method for determining the kinetic and equilibrium parameters. The values of adsorption energy(E) obtained from the Dubinin–Radushkevich isotherm,have been found around 0.371 and 0.870 k J·mol~(-1), indicating physical adsorption. Therefore, the pseudo-second order model represents better the kinetic experimental data. The results show that the Langmuir isotherm provides a slightly better fit to the experimental data compared with the Freundlich isotherm, indicating homogeneous distribution of active sites on carbon nanomaterials and monolayer adsorption. The separation factors RLare found in the range of 0–1, suggesting that the adsorption process is suitable for all adsorbents. The mechanisms for hexavalent chromium removal have been proposed as electrostatic interactions and hydrogen bonding.     

Adsorption of anionic dye on eco-friendly synthesised reduced graphene oxide anchored with lanthanum aluminate: Isotherms,kinetics and statistical error analysis

In the present study we made an effort to deploy eco-friendly synthesized reduced graphene oxide/Lanthanum Alluminate nanocomposites (RGO-LaAlO₃) and Lanthanum Alluminate (LaAlO₃) as adsorbents to remove dye from the synthetic media. XRD, SEM, BET surface area and EDX have been used to characterize the above-mentioned adsorbents. The impacts of different factors like adsorbent dosage, the concentration of adsorbate and PH on adsorption were studied. The best fit linear and nonlinear equations for the adsorption isotherms and kinetic models had been examined. The sum of the normalized errors and the coefficient of determination were used to determine the best fit model. The experimental data were more aptly fitted for nonlinear forms of isotherms and kinetic equations. Pseudo-second-order and Freundlich isotherm model fits the equilibrium data satisfactorily. Methyl orange (MO) has been used as model dye pollutant and maximum adsorption capacity was found to be 469.7 and 702.2 mg g^?1 for LaAlO₃ and RGO-LaAlO₃, respectively.     

Removal of diclofenac from aqueous solution with multi-walled carbon nanotubes modified by nitric acid☆

Modified multi-walled carbon nanotubes (MWCNTs) were used as adsorbents for removal of diclofenac. The re-action conditions were examined. Langmuir, Freundlich, Temkin, and Dubinin–Radushkevich isotherm models were applied to determine appropriate equilibrium expression. The results show that the experimental data fit the Freundlich equation well. Thermodynamic parameters show that the adsorption process is spontaneous and exothermic. The kinetic study indicates that the adsorption of diclofenac can be well described with the pseudo-second-order kinetic model and the process is controlled by multiple steps.     

New carbon composite adsorbents for the removal of textile dyes from aqueous solutions: Kinetic,equilibrium, and thermodynamic studies

New carbon composite materials were prepared by pyrolysis of mixture of coffee wastes and red mud at 700 °C with the inorganic: organic ratios of 1.9 (CC-1.9) and 2.2 (CC-2.2). These adsorbents were used to remove reactive orange 16 (RO-16) and reactive red 120 (RR-120) textile dyes from aqueous solution. The CC-1.9 and CC-2.2 materials were characterized using Fourier transform infrared spectroscopy, Nitrogen adsorption/desorption curves, scanning electron Microscopy and X-ray diffraction. The kinetic of adsorption data was fitted by general order kinetic model. A three-parameter isotherm model, Liu isotherm model, gave the best fit of the equilibrium data (298 to 323 K). The maximum amounts of dyes removed at 323 K were 144.8 (CC-1.9) and 139.5 mg g^?1 (CC-2.2) for RO-16 dye and 95.76 (CC-1.9) and 93.80 mg g^?1 (CC-2.2) for RR-120 dye. Two simulated dyehouse effluents were used to investigate the application of the adsorbents for effluent treatment.     

The adsorption of malachite green (MG) as a cationic dye onto functionalized multi walled carbon nanotubes

Synthetic dyes are widely used by several industries to color their products. The discharge of colored wastewater into the hydrosphere causes serious environmental problems. We used functionalized multi wall carbon nanotubes as an adsorbent for the adsorption of cationic dye, malachite green, from aqueous solution. Based on information provided by the Iranian Research Institute of Petroleum Industry, carbon nanotubes are produced using a chemical vapor deposition (CVD) technique. These as-received MWCNTs were functionalized by acid treatment. The remaining dye concentration was read by UV-visible absorption spectroscopy at maximum adsorption wavelength. The effect of different operational parameters such as contact time, pH of solution, adsorbent dose and initial dye concentration were studied. The results showed that by increasing of contact time, pH and adsorbent dose the removal of dye increased, but by increasing initial dye concentration, the removal efficiency decreased. Adsorption isotherms and kinetics behavior of f-MWCNTs for removal of malachite green was analyzed, and fitted to various existing models. The experimental data were well correlated with the Langmuir isotherm with a maximum adsorption capacity (q _m ) and regression coefficient (R²) of 142.85 mg/g and 0.997, respectively. The results of this study indicate that functionalized multi wall carbon nanotubes can be used as an effective adsorbent for the removal of dyes.     

Various carbon-based MgAl2O4 adsorbents and their removal efficiency of CR dye and antibiotics in aqueous media: High selective adsorption capacity,performance prediction and mechanism insight

Various carbon -based MgAl₂O₄ (MAO) adsorbents were synthesized by a simple ultrasound irradiation technology with the activated carbon (AC), carbon quantum dots (CQDs), C₃N₄ (CN) and graphene oxide (GO) as carbon sources. The optimum synthesis conditions for the carbon -based MAO adsorbents were determined by the study of the addition of different types of carbon and different mass ratios of m_{Various carbon}: m_MAO = 2.5%, 5.0%, 7.5% and 10%. The carbon -based MAO adsorbents were used for the adsorption of Congo red (CR) dye, tetracycline hydrochloride (TCH), oxytetracycline hydrochloride (OCH), ciprofloxacin (CIP), naproxen sodium (NPS) and tetrabromobisphenol A (TBBPA) and the seven factors affecting the adsorption behavior were discussed, including the type of carbon, AC content, initial dye concentration (C_CR), adsorbent dosage (C_{carbon -based MAO adsorbents}), adsorption time (A_t), pH value and reaction temperature (T). The corresponding adsorption isotherm models such as Langmuir, Tempkin, Koble-Corrigan, Toth and Freundlich models and thermodynamics of carbon -based MAO adsorbents for the adsorption of CR dye were also studied. When pH = 7.8, C_CR = 200 mg/L, A_t = 90 min, T = 287 K and C_{AC 5.0 wt%-MAO} = 1 g/L, the removal rate of CR dye reaches 89.7%. The back propagation (BP) neural network model was used to predict the adsorption percentage of carbon-based MAO adsorbents, which was consistent with the experimental results. The high adsorption capacity of AC 5.0 wt%-MAO adsorbents for the adsorption of CR dye can be assigned to the synergistic effects of electrostatic interaction, intermolecular force, n-π interaction and hydrogen bonding, while for the adsorption of TCH, OCH and CIP can be ascribed to the electrostatic interaction. This study demonstrated a huge potential of carbon -based MAO adsorbents as an eco-friendly adsorbent for the adsorption of dyes and drugs.     

Application of graphene-based adsorbents in the treatment of dye-contaminated wastewater; kinetic and isotherm studies

The adsorption of methylene blue (MB) on graphene-based adsorbents was tested through the batch experimental method. Two types of graphene-based adsorbents as graphene oxide (GO) and reduced graphene oxide (RGO) were compared to investigate the best adsorbent for MB removal. So that optimizing the MB removal for the selected type of graphene-based adsorbent, the diverse experimental factors, as pH (2–10), contact time (0–1440 min), adsorbent dosage (0.5–2 g/L), and initial MB concentration (25–400 mg/L) were analyzed. The conclusions indicated that the MB removal rised with an increase in the initial concentration of the MB and so rises in the amount of adsorbent used and initial pH. Maximum dye removal was calculated as 99.11% at optimal conditions after 240 min. Adsorption data were compiled by the Langmuir isotherm (R²: 0.999) and pseudo-second-order kinetic models (R²: 0.999). The Langmuir isotherm model accepted that the homogeneous surface of the GO adsorbent covering with a single layer. And the adsorption energy was calculated as 9.38 kJ mol⁻¹ according to the D-R model indicating the chemical adsorption occurred. The results show that GO could be utilized for the treatment of dye-contaminated aqueous solutions effectively.     

Surface modified granular activated carbon for enhancement of nickel adsorption from aqueous solution

Coal-based granular activated carbon was modified with acetates of sodium, potassium and lithium at concentrations of 10 and 15% and used as adsorbents to explore the adsorption mechanism of nickel ion in aqueous solution. Acetate treatment reduced surface area and pore volume of the activated carbons, but the adsorption amount of Ni(II) on the modified activated carbons (MAC) was greater than that on the virgin activated carbon. The adsorption depended on pH of the solution with an optimum at 4.5 and the adsorbed nickel could be fully desorbed by using 0.05M HCl solution. The maximum adsorption capacity of nickel ion on Li (15 wt%) modified activated carbon was 151.3 mg/g and the adsorption isotherm follows Langmuir, Sips, and Redlich-Peterson isotherm models better than the Freundlich isotherm model. The kinetic data was better fitted by a non-linear form of the pseudo-first order than the pseudo-second order, but the difference between two kinetic models was small.     

Magnetic Multi-Wall Carbon Nanotubes for Methyl Orange Removal from Aqueous Solutions: Equilibrium,Kinetic and Thermodynamic Studies

The magnetic iron oxide nanoparticles (SPION) were prepared using the co-precipitation method. The magnetically modified multi-wall carbon nanotubes (MWCNT) were obtained by the same method. The magnetic nanoparticles were used as adsorbents for methyl orange (MO) removal from aqueous solutions. The surface properties of adsorbents were analyzed by XRD, TGA, FTIR, and SEM. The adsorption was carried out on batch type. The kinetic, equilibrium and thermodynamics parameters were obtained by experimental results. The maximum adsorption capacity was obtained at pH 3 for both of the adsorbents. Theoretical adsorption capacities were found as 0.31 mg/g for SPION and 10.54 mg/g for MWCNT/SPION composites from Langmuir isotherm parameters. The MWCNT/SPION composites can adsorb 95% of methyl orange from water. The kinetic parameters and thermodynamic parameters were calculated.     

Hexavalent chromium adsorption on superparamagnetic multi-wall carbon nanotubes and activated carbon composites

Hexavalent chromium (Cr(VI)) adsorption from aqueous solutions on magnetically modified multi-wall carbon nanotubes (M-MWCNT) and activated carbon (M-AC) was investigated. M-MWCNT and M-AC were prepared by co-precipitation method with Fe²⁺:Fe³⁺ salts as precursors. The magnetic adsorbents were characterized by X-ray diffraction (XRD), thermogravimetric analysis (TGA) and scanning electron microscope (SEM). The effects of amount of adsorbents, contact time, initial pH, temperature and the initial concentration of Cr(VI) solution were determined. The adsorption equilibrium, kinetics, thermodynamics and desorption of Cr(VI) were investigated. Equilibrium data fitted well with the Langmuir isotherm for both of the adsorbents. The theoretical adsorption capacities are 14.28 mg/g of M-MWCNT and 2.84 mg/g of M-AC. Cr(VI) adsorption kinetics was modeled with pseudo-second order model, intra-particle diffusion model and Bangham model. Thermodynamic parameters were calculated and ΔG°, ΔH° and ΔS° indicate that the adsorption of Cr(VI) onto M-MWCNT and M-AC was exothermic and spontaneous in nature. Results revealed that M-MWCNT is an easily separated effective adsorbent for Cr(VI) adsorption from aqueous solution.     

Kinetic and isotherm studies of the adsorption of Acid Blue 92 using a low-cost non-conventional activated carbon

Adsorbents prepared from waste plants for the treatment of dyeing effluents have high significance in environmental sustainability. In this research, an attempt is made to analyze the applicability of activated carbon prepared from Euphorbia antiquorum L wood by H₃PO₄ activation method for the removal of Acid Blue 92 dye. Various kinetic models were used for the analysis of adsorption kinetics and pseudo second-order model fits well for the selected adsorbent-adsorbate system. The moderate rate of dye uptake indicates that the rate-determining step could be physisorption in nature. Langmuir, Freundlich and Dubinin-Raduskevich isotherm models were applied for the analysis of isotherm data. The positive enthalpy of adsorption substantiates that the adsorption process is endothermic in nature.     

Adsorption of Methyl Chloride on Molecular Sieves,Silica Gels,and Activated Carbon

Adsorption of methyl chloride (CH₃Cl or MeCl) on five different types of adsorbents was investigated experimentally at increasing pressures and room temperature. Prior to adsorption, all adsorbents were analyzed to assess their physical and chemical characteristics. The experimental data was then used to determine the adsorption isotherms, heats of adsorption, adsorption rates, and their respective theoretical models. The MeCl adsorption capacity was found to reasonably correlate with the adsorbent's surface area. The MeCl adsorption isotherm and adsorption rates were fitted for the first time to a Freundlich isotherm model and pseudo first-/second-order kinetic models, respectively. The range of heat of adsorption indicated a physisorption type of bonding; hence, the investigated adsorbents can potentially be regenerated for cyclic adsorption.     

Preparation of carbon aerogels with different pore structures and their fixed bed adsorption properties for dye removal

Controlled porosity carbon aerogels are prepared by a sol–gel polymerization method and the prepared materials are used as fixed bed adsorbents for dye removal. The influences of operating conditions and preparation factors on the adsorption of C. I. Reactive Red 2, as a model compound, from aqueous solution were investigated. Many column parameters were estimated at different stages and the Bed-Depth-Service-Time model was used to analyze the experimental data. The results showed that the adsorption bed capacity increased with increasing bed height, decreasing liquid flow rate and decreasing initial dye concentration. Moreover, the work indicated that the adsorption ability of the carbon aerogel could be controlled by adjusting the molar ratio of resorcinol to surfactant and carbonization conditions. In addition, the adsorption capacity could be improved when the carbon aerogel was activated by CO₂. The reasons for the difference in adsorption ability could be related to the different pore structure characteristics of various samples.     

Adsorption of Hg(II) Ions by 3-Mercaptopropyltriethoxysilane Modified Mesoporous Silica Based on Multiwalled Carbon Nanotubes: Equilibrium,Kinetic, and Thermodynamic Studies

In this study, 3-mercaptopropyltriethoxysilane (MPTS) modified mesoporous silica based on multiwalled carbon nanotubes (MWCNTs@mSiO₂-MPTS) was successfully prepared. The adsorption material was characterized and used for adsorbing Hg(II) ions from aqueous solution. The equilibrium data conformed better to the Langmuir isotherm with maximum adsorption capacity calculated 349.65 mg/g at 40ºC. The kinetics analysis revealed the adsorption process fitted well with the pseudo-second-order kinetic model. The thermodynamic parameters indicated the adsorption process was spontaneous and endothermic. All results obtained suggested that MWCNTs@mSiO₂-MPTS may be employed as an efficient material for the adsorption or preconcentration of Hg(II) ions from aqueous solution.     

Aqueous Phase Adsorption of Cephalexin onto Bentonite and Activated Carbon

The adsorption of cephalexin in aqueous solution has been investigated using bentonite and activated carbon as the adsorbents. Batch kinetics and isotherm studies were carried out to evaluate the effect of contact time, adsorbent dosage, pH, particle size, and temperature. Adsorption equilibrium data were well represented by the Langmuir and Freundlich isotherm models. The adsorption intensity was found to be increased as the aqueous phase pH increased, and had a maximum at pH = 6.1. The pseudo-first order, pseudo-second order, and intraparticle diffusion kinetic models were used to describe the kinetic data. The experimental data fitted very well with the pseudo-second-order kinetic model and also followed the simple external and intraparticle model.     

Investigation of Reactive Red Dye 198 removal using multiwall carbon nanotubes in aqueous solution

The present study evaluates the performance of multiwalled carbon nanotubes (MWCNTs) for removing Reactive Red Dye 198 (RR198) from the color wastewater. In this study, the influence of pH, adsorbent dose, initial dye concentration, and contact time on the RR198 adsorption by MWCNTs was investigated. The results showed increasing the dye concentration from 20 to 200 mg/L, removal efficiency decreased from 99.62% to 66.99%. Moreover, by increasing the pH from 3 to 10, the efficiency of dye removal decreased from 76.34% to 54.98%. Freundlich isotherm and pseudo-second-order kinetic model were the best models for describing the adsorption reactions.     

Sodic and Acidic Crystalline Lamellar Magadiite Adsorbents for the Removal of Methylene Blue from Aqueous Solutions: Kinetic and Equilibrium Studies

The present study reports the feasibility of two synthetic crystalline lamellar nano-silicates, sodic magadiite (Na-mag) and its converted acidic form (H-mag), as alternative adsorbents for the removal of the dye methylene blue (MB) from aqueous solutions. The ability of these adsorbents for removing the dye was explored through the batch adsorption procedure. Effects such as the pH and the adsorbent dosage on the adsorption capacities were explored. Four kinetic models were applied, the adsorption being best fitted to a fractionary-order kinetic model. The kinetic data were also adjusted to an intra-particle diffusion model to give two linear regions, indicating that the kinetics of adsorption follows multiple sorption rates. The equilibrium data were fitted to Langmuir, Freundlich, Sips, and Redlich-Peterson isotherm models. The maxima adsorption capacities for MB of Na-mag and H-mag were 331 and 173 mg g^?1, respectively.     

EFFECT OF TEMPERATURE ON THE ADSORPTION OF METHYLENE BLUE DYE ONTO SULFURIC ACID–TREATED ORANGE PEEL

The present study explains the preparation and application of sulfuric acid–treated orange peel (STOP) as a new low-cost adsorbent in the removal of methylene blue (MB) dye from its aqueous solution. The effects of temperature on the operating parameters such as solution pH, adsorbent dose, initial MB dye concentration, and contact time were investigated for the removal of MB dye using STOP. The maximum adsorption of MB dye onto STOP took place in the following experimental conditions: pH of 8.0, adsorbent dose of 0.4 g, contact time of 45 min, and temperature of 30°C. The adsorption equilibrium data were tested by applying both the Langmuir and Freundlich isotherm models. It is observed that the Freundlich isotherm model fitted better than the Langmuir isotherm model, indicating multilayer adsorption, at all studied temperatures. The adsorption kinetic results showed that the pseudo-second-order model was more suitable to explain the adsorption of MB dye onto STOP. The adsorption mechanism results showed that the adsorption process was controlled by both the internal and external diffusion of MB dye molecules. The values of free energy change (ΔG ^o) and enthalpy change (ΔH ^o) indicated the spontaneous, feasible, and exothermic nature of the adsorption process. The maximum monolayer adsorption capacity of STOP was also compared with other low-cost adsorbents, and it was found that STOP was a better adsorbent for MB dye removal.     

Adsorption of Tetracycline with Reduced Graphene Oxide Decorated with MnFe<Subscript>2</Subscript>O<Subscript>4</Subscript> Nanoparticles

Nanomaterials were widely used as efficient adsorbents for environmental remediation of tetracycline pollution. However, the separation of the adsorbents posed the challenge to their practical applications. In this study, we grew magnetic MnFe₂O₄ nanoparticles on the reduced graphene oxide (rGO) to form MnFe₂O₄/rGO nanocomposite with a one-step method. When used as the absorbent of Tetracycline, it exhibited an adsorption capacity of 41?mg/g. The adsorption kinetics and isotherm were fitted well with the pseudo-second order model and Freundlich model, respectively. The MnFe₂O₄/rGO nanocomposite could be easily extracted from the solution with the external magnetic field and regenerated with acid washing.     

Palladium,silver, and zinc oxide nanoparticles loaded on activated carbon as adsorbent for removal of bromophenol red from aqueous solution

The adsorption of bromophenol red (BPR) onto three adsorbents including palladium, silver and zinc oxide nanoparticles loaded on activated carbon (Pd-NP-AC, Ag-NP-AC and ZnO-NP-AC) in a batch system has been studied and the influence of various parameters has been optimized. The influence of time on removal of BPR on all adsorbent was investigated and experimental data were analyzed by four kinetic models including pseudo first and second-order, Elovich and the intraparticle diffusion equations. Following fitting the experimental data to these models, the respective parameters of each model such as rate constants, equilibrium adsorption capacities and correlation coefficients for each model were investigated and based on well known criterion their applicability was judged. It was seen that the adsorption of BPR onto all adsorbents sufficiently described by the pseudo second-order equation in addition to interparticle diffusion model. The adsorption of BPR on all adsorbent was investigated at various concentration of dye and the experimental equilibrium data were analyzed and fitted to the Langmuir, Freundlich, Tempkin, Dubinin, and Radushkevich equations. A single stage in batch process was efficient and suitable for all adsorbents using the Langmuir isotherm with maximum adsorption of 143 mg g^?1 for Pd-NP-AC, 250 mg g^?1 for Ag-NP-AC and 200 mg g^?1 for ZnO-NR-AC. Thermodynamic parameters such as ΔG°, ΔH°, and ΔS° for Pd-NP-AC adsorbent were calculated.