Adsorption Enthalpy Change Research Articles

Adsorption selectivity of cubic nano-CeO2 and effect of particle size on adsorption thermodynamics

Nanoparticles can effectively remove organic dyes from wastewater due to their excellent adsorption performances which are closely related to particle size. Herein, the relations between the thermodynamic properties and the particle size were deduced, and the influence mechanisms were also discussed. Then the adsorptions of four dyes on cubic nano-CeO2 were investigated. We found that cubic nano-CeO2 has strong adsorption selectivity for four dyes and the size dependence of the adsorption thermodynamic properties. This strong selectivity is attributed to electrostatic interaction and hydrogen bonding. Furthermore, with the particle size decreasing, the logarithm of standard adsorption equilibrium constant (lnKo) increase, while the standard molar Gibbs energy change of adsorption (ΔadsGmθ), the standard molar adsorption enthalpy change (ΔadsHmθ) and the standard molar adsorption entropy change (ΔadsSmθ) decrease, and there are good linear relationships between this thermodynamic properties and the reciprocal of the particle size. The essences of effect of particle size on adsorption thermodynamics are that lnKθ and ΔadsGmθ are influenced by both the molar surface area and the difference in surface tensions after adsorption, ΔadsHmθ by the molar surface area, the differences in surface tensions and in temperature coefficients of surface tensions, ΔadsSmθ by the molar surface area and the difference in temperature coefficients of the surface tensions.

Efficient removal of cadmium(II) with SBA-15 nanoporous silica: studies on equilibrium, isotherm, kinetics and thermodynamics

Nanomesoporous silica SBA-15 was prepared by hydrothermal method. The samples were characterized by powder X-ray diffraction and scanning electron microscopy. The fibrous particles with a diameter of 333 ± 10 nm were obtained. Nanoporous mesoporous SBA-15 was used as an adsorbent to adsorb Cd2+ in water body. The effects of conditions such as acidity were studied, and the optimized adsorption conditions were obtained. The results showed that when the pH value was 5.0, the temperature was 25 ± 1 °C and the adsorption time was 40 min, the nanomesoporous SBA-15 had the best adsorption effect for Cd2+ and the adsorption rate was reached 73.28% and the adsorption capacity was 9.84 mg/g. Study found that the adsorption is the quasi-second-order kinetic model, which belongs to the Freundlich adsorption type. The results of adsorption thermodynamics showed that when the temperature is 25–55 °C, the adsorption is spontaneous exothermic reaction, ∆G0 < 0, the adsorption enthalpy change ΔH0 = − 54.732 kJ/mol, the adsorption entropy change ∆S0 = − 129.343 J/(mol K). The effects of nitric acid and hydrochloric acid on desorption were investigated, respectively. The results showed that the desorption effect of 0.1 mol/L nitric acid as desorbent was better than that of hydrochloric acid. At desorption for 10 h, the desorption rate was the highest, reaching 65.56%.

Thermodynamics of multi-walled carbon nanotube biofunctionalization using nisin: The effect of peptide structure

A detailed adsorption thermodynamic study of two nisin variants (nisA and nisZ) on multi-walled carbon nanotubes (MWCNT) was conducted, and the antimicrobial properties of biofunctionalized MWCNT were evaluated. The MWCNT adsorption capacity for peptides (qp) increased with pH increase, with greater values for nisZ (qp up to 250 mg g−1) than for nisA (qp up to 180 mg g−1) for all studied conditions. Zeta potential measurements (ζ) showed that the electrostatic repulsion between the MWCNT-peptide complexes determined the dispersion features that were stable at pH 2.0 and 3.0, with ζ reaching 45.0 mV at the lowest pH. Despite the similar ζ values for both peptides, slightly greater stabilization of MWCNT dispersions was exhibited in presence of nisZ at pH 3. At pH 4 and 5, peptide adsorption was not able to promote MWCNT dispersibility. Isothermal titration calorimetry revealed that the adsorption process was driven by enthalpy for both peptides, as the adsorption enthalpy changes (ΔHads) were less negative than −99.7 kJ mol−1. Despite the large dependence of ΔHads on the pH andqp values, indicating the important role of electrostatic interactions on the adsorption process, a change in only one amino acid residue in the nisin structure promoted intense changes in the adsorption thermodynamic parameters. We have suggested that the more hydrophobic character of nisZ at the lower pH values caused this peptide to interact with the MWCNT surface through its two domains. Interesting, the antimicrobial properties of both peptides were not damaged, and the functionalized MWCNT showed antibacterial activity against the indicator micro-organism Lactococcus lactis in the agar diffusion and solution assays.

Use of SBA-15 ordered nano mesoporous silica for removal of copper(II) from aqueous media: Studies on equilibrium, isotherm, kinetics and thermodynamics

Copper is a heavy metal. When its content is low, it is good to the health of biological body. However, when its content is high, it has the toxic effect on biological body. At this time, removal of the toxicity of copper is very important. SBA-15 molecular sieve was prepared by hydrothermal synthesis method. The sample was characterized by powder X-ray diffraction, scanning electron microscopy, transmission electron microscopy, 77 K adsorption-desorption isotherm of nitrogen. The fibrous particles with a diameter of 333 ± 10 nm were obtained. Nano mesoporous SBA-15 with Brunner-Emmett-Teller specific surface area 503 m2 g−1 was used as an adsorbent to adsorb Cu(II) from solution. The effects of acidity, temperature, contact time, amount of adsorbent and initial concentration of Cu(II) on the adsorption were studied. Under the optimized adsorption conditions, the maximum adsorption rate of Cu(II) by nano-mesoporous SBA-15 was 89.0% and the maximum adsorption capacity was 11.39 mg/g. By studying kinetics, it was found that the adsorption belongs to the quasi-second-order kinetic model, simultaneously belonging to the Freundlich adsorption isotherm. The results of adsorption thermodynamics showed that when temperature is 25–55 °C, △G° < 0, the adsorption enthalpy change ΔH° = −50.185 kJ/mol, the adsorption entropy change △S° = −127.703 J /(mol K), the adsorption is spontaneous exothermic entropy reduction process. Desorption studies were carried out by using nitric acid and hydrochloric acid, respectively. The results showed that the desorption effect using 0.1 mol/L nitric acid as desorbent is better than that of hydrochloric acid. The desorption rate is the highest at desorption for 10 h, reaching 73.0%. This method has a potential to remove other heavy metals.

Comparative study of adsorption for m‐phenylenediamine in aqueous solution onto chemically modified resins

ABSTRACTFour kinds of hypercrosslinked resins (acidic resins labeled SAMR and TAMR and heterocyclic resins IDLMR and PRLMR) were synthesized as adsorbents to remove m‐phenylenediamine (MPD) from aqueous solution comparably. The Brunauer–Emmett–Teller specific surface area and pore size distribution of the two types of resins have been compared, and the infrared spectra of the resins confirm the presence of the modified groups. The adsorption isotherms illustrate that the Freundlich equation fits more appropriately than the Langmuir equation or the Temkin equation. In addition, the kinetic curves indicate that the pseudo‐second‐order model is a better fit than the pseudo‐first‐order model. The thermodynamic studies show that the isosteric adsorption enthalpy changes (ΔH) and the Gibbs free energy change (ΔG) are both negative, revealing that all of the adsorption processes are spontaneous and exothermic. All of the modified resins exhibit the best adsorption capacity at pH = 6.24. The results show that the acidic resins have better adsorption of MPD. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 47378.

Selective removal of nicotine from the main stream smoke by using a surface-imprinted polymer monolith as adsorbent.

Using molecularly imprinted polymer as a selective adsorbent for gaseous toxicants is a novel attempt. In present work, a nicotine surface-imprinted monolith (MIM) was used for the selective removal of nicotine from smoke. First, the retention capacity and selectivity for this MIM was tested by using it as the stationary phase in gas chromatography and chromatographic conditions optimized. Then, the gas phase adsorption isotherms of MIM were constructed and the adsorption thermodynamics explored. At last, the applicability for MIM in the removal of nicotine in smoke was explored. Results indicated a stronger retention capacity and a higher selectivity of MIM toward the template vapor, with a capacity factor (87.88) and a selectivity factor (10.15) under the optimized conditions. A higher standard adsorption enthalpy change for this MIM toward the template (ΔHa0 = 65.53 kJ mol-1) than that for the non-imprinted monolith (NIM) column (ΔHa0 = 47.46 kJ mol-1) was observed. The adsorption isotherm for MIM appears the BET type II shape, while that for the NIM was approximately linear. When this MIM was used as the adsorbent, it exhibited a high performance in the selective removal of nicotine from the main stream smoke, with an adsorption percentage of 99.43%.

The potential of nonliving Sargassum hemiphyllum as a biosorbent for nickel(II) removal—isotherm, kinetics, and thermodynamics analysis

The ability of non‐living Sargassum hemiphyllum to remove nickel(II) from aqueous solutions was investigated. The optimum adsorption condition was occurred at a pH range from 4 to 8, a biomass of 0.5 g L−1, and an initial nickel(II) concentration of 250 mg L−1 with a contact time of 20 min. The Langmuir, Freundlich, and Dubinin‐Radushkevich isotherm models were used to fit the adsorption isotherms. The fitting results were more in line with the Langmuir model (R2 = 0.9915), and the maximum adsorption capacity obtained by Langmuir model was 42.95 mg g−1 at 308 K, respectively. The biosorption kinetics were analyzed using the pseudo‐first‐order, pseudo‐second‐order and intraparticle diffusion, and it was determined that they followed pseudo‐second‐order kinetics (R2 = 0.9996) at 308 K. The enthalpy (ΔH°) and entropy (ΔS°) change of adsorption was 3.95 kJ mol−1 and 20.28 J mol−1 K−1, respectively, which indicated that the adsorption was spontaneous and endothermic. The biosorbents were also characterized by Fourier infrared spectroscopy (FT‐IR), Scanning electron microscopy and Energy disperse spectroscopy (SEM‐EDS), and X‐ray photoelectron spectroscopy (XPS). The characterization analysis demonstrated that the functional groups such as hydroxyl, carboxyl, amino, and methyl on the algal surface involved in the adsorption for nickel(II). Our results suggested that the nonliving S. hemiphyllum could be used as an effective biosorbent for the removal of nickel(II) from wastewater. © 2018 American Institute of Chemical Engineers Environ Prog, 38: S250–S259, 2019

An evaluation of coal fly ash as an adsorbent for the removal of methylene blue from aqueous solutions: kinetic and thermodynamic studies

In this study the effect of the dose and particle size of the adsorbent, initial dye concentration, initial pH, contact time and temperature were investigated for the removal of by means of fly ash (FA) methylene blue (MB) from an aqueous solution. The FA dose was found to be 2.0 g and the under 270 mesh sized particles were found to be effective particles for adsorption. The adsorption process reached its maximum value at 0.5 mg/L dye concentration and attained equilibrium within 10 minutes. The adsorption isotherm was found to follow the Langmuir model. The estimated adsorption free energy (ΔGo), enthalpy change (ΔHo), and entropy change (ΔSo) for the adsorption process were −37.77 kJ mol−1, −13.44 kJ mol−1 and 122 J mol−1 K−1 respectively at 298 K. The maximum adsorption capacity is 0,12 mg g−1 at 298 K and 0,07 mg g−1 at 398 K. The adsorption process was exothermic, feasible and spontaneous. The positive value of ΔSo shows the affinity of FA for MB while the low value of ΔGo suggests a physical adsorption process.

Trimellitated sugarcane bagasse: A versatile adsorbent for removal of cationic dyes from aqueous solution. Part I: Batch adsorption in a monocomponent system

Trimellitated-sugarcane bagasse (STA) was used as an environmentally friendly adsorbent for removal of the basic dyes auramine-O (AO) and safranin-T (ST) from aqueous solutions at pH 4.5 and 7.0. Dye adsorption was evaluated as a function of STA dosage, agitation speed, solution pH, contact time, and initial dye concentration. Pseudo-first- and pseudo-second-order, Elovich, intraparticle diffusion, and Boyd models were used to model adsorption kinetics. Langmuir, Dubinin-Radushkevich, Redlich-Peterson, Sips, Hill-de Boer, and Fowler-Guggenheim models were used to model adsorption isotherms, while a Scatchard plot was used to evaluate the existence of different adsorption sites. Maximum adsorption capacities for removal of AO and ST were 1.005 and 0.638 mmol g−1 at pH 4.5, and 1.734 and 1.230 mmol g−1 at pH 7.0, respectively. Adsorption enthalpy changes obtained by isothermal titration calorimetry (ITC) ranged from −21.07 ± 0.25 to −7.19 ± 0.05 kJ mol−1, indicating that both dyes interacted with STA by physisorption. Dye desorption efficiencies ranged from 41 to 51%, and re-adsorption efficiencies ranged from 66 to 87%, showing that STA can be reused in new adsorption cycles. ITC data combined with isotherm studies allowed clarification of adsorption interactions.

Synthesis and application of a new carboxylated cellulose derivative. Part III: Removal of auramine-O and safranin-T from mono- and bi-component spiked aqueous solutions

In the third part of this series of studies, the adsorption of the basic textile dyes auramine-O (AO) and safranin-T (ST) on a carboxylated cellulose derivative (CTA) were evaluated in mono- and bi-component spiked aqueous solutions. Adsorption studies were developed as a function of solution pH, contact time, and initial dye concentration. Adsorption kinetic data were modeled by monocomponent kinetic models of pseudo-first- (PFO), pseudo-second-order (PSO), intraparticle diffusion, and Boyd, while the competitive kinetic model of Corsel was used to model bicomponent kinetic data. Monocomponent adsorption equilibrium data were modeled by the Langmuir, Sips, Fowler-Guggenhein, Hill de-Boer, and Konda models, while the IAST and RAST models were used to model bicomponent equilibrium data. Monocomponent maximum adsorption capacities for AO and ST at pH 4.5 were 2.841 and 3.691 mmol g−1, and at pH 7.0 were 5.443 and 4.074 mmol g−1, respectively. Bicomponent maximum adsorption capacities for AO and ST at pH 7.0 were 1.230 and 3.728 mmol g−1. Adsorption enthalpy changes (ΔadsH) were obtained using isothermal titration calorimetry. The values of ΔadsH ranged from −18.83 to −5.60 kJ mol−1, suggesting that physisorption controlled the adsorption process. Desorption and re-adsorption of CTA was also evaluated.

Methylene blue removal using Salix babylonica (Weeping willow) leaves powder as a low-cost biosorbent in batch mode: Kinetic, equilibrium, and thermodynamic studies

In this article, Salix babylonica leaves powder (SBLP) has been used for the removal methylene blue (MB) from aqueous solutions by adsorption process. The surface of SBLP analyzed by scanning electron microscopy (SEM) and Fourier transform infrared spectrometer (FTIR). The effects of changing parameters such as adsorbent dosage, contact time, pH, temperature, and initial dye concentration on methylene blue adsorption were investigated. Three isotherm equations, Langmuir, Freundlich, and Temkin, have been tested in order to describe the equilibrium characteristics of adsorption. The experimental data fitted well by Langmuir isotherm, with maximum adsorption capacity 60.97mg/g. The experimental data were fitted using pseudo-first-order and pseudo- second-order and intraparticle diffusion models. The experimental results were fitted better with pseudo-second-order model compared to the other two models. The amount of thermodynamic parameters such as Gibbs free energy (∆G°), change in enthalpy of adsorption (∆H°), and change in entropy (∆S°) were calculated and this parameter showed that MB adsorption on SBLP is spontaneous and exothermic in nature. The result of experimental shows that SBLP is suitable low-cost adsorbent for removal of methylene blue from aqueous solutions.

Effect of solvent composition on the van’t Hoff enthalpic curve using amylose 3,5-dichlorophenylcarbamate–based sorbent

Van’t Hoff plots have been widely used for investigating the thermodynamic properties of adsorption processes in various chromatography systems. By measuring the retention factor k over a certain temperature range, the plot of ln k versus 1/T often yields a straight line with a slope of −ΔHvH0/R. Although this method provides information on adsorption enthalpy changes, its theoretical basis does not account for the effect of the solvent. In this paper, the relationship between apparent enthalpy changes determined directly from van’t Hoff plots and solvent modifier concentrations is systematically investigated using three simple solutes—tetrahydrofuran, acetone, and tert-butanol—in an n-hexane-methyl tert-butyl ether (MTBE) mobile phase with an amylose 3,5-dichlorophenylcarbamate–based sorbent. The apparent enthalpy changes of solutes are strongly dependent on MTBE concentration, increasing rapidly with MTBE content at low concentrations but leveling off after ΔH0 reaches approximately −15kJ/mol. These data cannot be explained by the thermodynamic model currently used in the literature. A new three-equilibrium-constant thermodynamic model is developed herein to account for solute–sorbent, solvent–sorbent, and solute–solvent interactions. The thermodynamic parameters of the model are estimated from the apparent enthalpy changes at different MTBE concentrations. The results reveal that two key dimensionless groups control the van’t Hoff enthalpic curves: the fractions of solute molecules bound to modifier molecules and adsorption sites occupied by modifier molecules. As a result, the shapes of van’t Hoff enthalpic curves reflect the adsorption isotherm of MTBE without complexation or information regarding solute–MTBE complexation without MTBE competitive adsorption. The new model is thus demonstrated to be more reliable than the current model for examining the thermodynamic properties of retention mechanisms.

Adsorption of red azo dyes on multi-walled carbon nanotubes and activated carbon: A thermodynamic study

Carbonaceous materials have been extensively studied as highly efficient adsorbents for the removal of dyes from wastewater. However, investigations of thermodynamic aspects of the interactions between these materials and dyes remain scarce. This paper describes the thermodynamics of the interactions between the Ponceau 4R (PR), Congo Red (CR), and Allura Red (AR) dyes and multi-walled carbon nanotubes (MWCNTs). The interactions between the dyes and activated carbon (AC) were also evaluated for comparison. The investigation used a combination of adsorption isotherms and isothermal titration nanocalorimetry (ITC) measurements, and a thermodynamic approach provided full characterization of the adsorption process. For both MWCNT and AC, the amount of adsorbed dye (ΓD) increased in the order ΓD (PR)<ΓD (AR)<ΓD (CR), and the adsorption capacity normalized by the adsorbent specific area was up to 5.6 times higher for MWCNT. The maximum amount adsorbed (ΓD,max) reached values of up to 2.00μmol m−2 for CR adsorption on MWCNT. For both adsorbents, the process of dye adsorption was enthalpically driven and entropically unfavorable. All the thermodynamic parameters depended on the surface coverage and the structures of both dye and adsorbent. The adsorption enthalpy change (ΔadsH) and entropy change (TΔadsSref) values were higher than −137.0kJ mol−1 and −114.2kJ mol−1, respectively. In addition to the structural differences between MWCNT and AC, the ITC data suggested that the adsorption sites are heterogeneous and that the dyes preferentially adsorb on the more hydrophilic sites on the adsorbent surfaces.

Dual-mode adsorption of cochineal natural dye on wool fibers: Kinetic, equilibrium, and thermodynamic studies

The dual-mode adsorption model was used to investigate the adsorption behavior of cochineal natural dye on wool fibers. Kinetic, thermodynamic, and equilibrium characteristics were investigated in terms of the adsorption isotherm, affinity, enthalpy and entropy changes, dyeing rate, diffusion coefficient, and activation energy of dye diffusion. The results revealed the prominent role of dyeing pH in determination of dyeing mechanism and dye adsorption isotherm type. At pH 4, dual Langmuir-Nernst model with the highest correlation coefficient was found as the most appropriate isotherm model to describe the adsorption behavior of cochineal onto wool fibers while at pH 6, the adsorption isotherm was the Nernst type. Cochineal adsorption onto wool was exothermic which resulted in progressive fall in affinity and equilibrium dye up-take values with increasing temperature. Moreover, dyeing rate steadily increased with temperature. At pH 4, affinity, dyeing rate, and diffusion coefficient demonstrated higher values compared to pH 6 whereas enthalpy and entropy changes, and activation energy showed lower values. Additionally, negative value for activation energy was obtained at pH 6. The results are deliberated based on the different possible interactions between cochineal dye and wool fiber.

One novel humidity-resistance formaldehyde molecular probe based hydrophobic diphenyl sulfone urea dry-gel: Synthesis, sensing performance and mechanism

The work designs a novel hydrophobic organic dry-gel named diamino diphenyl sulfone (DDS) urea for selectively detecting formaldehyde in air. It is synthesized by using octadecylisothiocyanate and diamino diphenyl sulfone as reactants in tetrahydrofuran solution at 70°C. The dry-gel is obtained by freezing drying. Sulfanilamide urea dry-gel acts as the contrastive material which is synthesized in a similar condition. The dry-gel coated Quartz Crystal Microbalance (QCM) sensor exhibits a rapid and reversible selective response towards formaldehyde gas. Repeated measurement results show high sensitivity and low detection limit (1ppm) for formaldehyde detection. The gas sensing and contact angle tests indicate its stability under different humidity conditions. Based on the adsorption isotherm experiments, it reveals that adsorption enthalpy change between DDS urea dry-gel and HCHO molecules locates the range of chemical adsorption, endowing the sensor satisfactory selectivity and reversibility. Sensing mechanism of formaldehyde sensor has been proved by a simulation calculation based on a quantum chemistry software Gaussian though comparing the interaction between amine group or carbamido and formaldehyde. The calculation results reveal that Schiff base interaction between the amine group and aldehyde group is more responsible for formaldehyde sensing instead of hydrogen bond interaction between carbamido and aldehyde group.

In English

Extensive use of synthetic dyes in different industries has created a major pollution problem. Among various treatments, adsorption has been considered as an effective method due to its simplicity, availability and effectiveness in removing of dyes from wastewaters. In the current work, the composite “corn stalks-рolyaniline” (CS-PAN) is proposed to be used as alternative adsorbent to expensive activated carbons. We have synthesized the new composite by the chemical polymerization of aniline on the surface of corn stalks using (NH 4 ) 2 S 2 O 8 as an oxidation agent and H 3 PO 4 as a dopant. The composite was characterized by using Fourier infrared spectroscopy, specific surface area analysis, pH of point zero charge, bulk density, and total pore volume. In comparison with corn stalks, the composite CS-PAN had a high adsorption capacity and efficiency toward anionic dyes (Acid Red and Acid Orange) in the aqueous medium. The adsorption process attains equilibrium within 120–150 min. The adsorption of the acid dyes increased on CS-PAN composite with increase in temperature. It indicates that the adsorption is an endothermic process. Kinetic studies revealed the adsorption process was best described by the reaction model of pseudo-second order than pseudo-first order. The activation energy of adsorption was calculated as 13.1 and 7.8 kJ/mol for Acid Red and Acid Orange, respectively. Equilibrium adsorption studies of the anionic dyes on CS-PAN indicated the Langmuir equation fit better than the Freundlich equation. Maximum adsorption capacity CS-PAN was calculated at different temperatures (303, 313, and 328 K) as 7.0·10 –5 , 8.3·10 –5 , 10.7·10 –5 and 9.8·10 –5 , 12.2·10 –5 , 16.0·10 –5 mol/g toward Acid Red and Acid Orange, respectively. The values of the standard free energy change have been calculated as -29.6, -30.7, -32.6 for Acid Red and -30.1, -31.1, -32.7 kJ/mol for Acid Orange at 303, 313, and 323 K, respectively, which indicating that a spontaneous process occurred. Calculations indicated that t he enthalpy changes of adsorption were 6.7 and 1.3 kJ/mol for Acid Red and Acid Orange, respectively. Besides, the calculated standard entropy changes of adsorption were 107 and 85 J/mol K for Acid Red and Acid Orange, respectively. Consequently, the composite CS-PAN, an inexpensive, easily synthesized, and efficient adsorbent, could be an alternative for more costly adsorbents used for removal of anionic dyes in wastewater treatment.

Retention models and interaction mechanisms of benzene and other aromatic molecules with an amylose-based sorbent

Stoichiometric displacement models have been widely used for understanding the adsorption mechanisms of solutes in chromatography systems. Such models are used for interpreting plots of solute retention factor versus concentrations of polar modifier in an inert solvent. However, these models often assume that dispersion forces are negligible and they are unable to account for solutes with significant aromatic interactions. In this study, a systematic investigation of the relationship between retention behavior and aromatic groups was performed using five simple aromatic molecules—benzene, naphthalene, mesitylene, durene, and toluene—with a commercially available amylose tris(3,5-dimethylphenylcarbamate)-based sorbent. The enthalpy changes of adsorption, determined from van’t Hoff plots, were obtained separately in pure n-hexane and in pure isopropanol (IPA). In pure n-hexane, the solute adsorptions were driven by electrostatic interactions, favoring a T-shaped binding configuration (edge-to-face π-π interaction). The order of enthalpy change indicated the amount of effective T-shaped π-interactions. In pure IPA, solute adsorption was dominated by dispersion forces, favoring a sandwich binding configuration (face-to-face π-π interaction). The adsorption isotherms of toluene revealed that in pure IPA and in pure n-hexane, the isotherms were linear. The results suggested that the high solvent strength of IPA weakened the interactions between aromatic molecules. The retention behavior of the benzene, naphthalene, mesitylene, and durene as a function of IPA concentration was investigated. U-shaped retention curves were found for all aromatic solutes. A new retention model for monovalent aromatic solutes was developed for describing the U-shaped curves. Three key dimensionless groups were revealed to control the retention behavior. The models suggested that solvophobic interactions should be accounted for in the retention models used to investigate the retention behaviors of solutes associated with aromatic groups.

Microwave‐synthesized Pepper Peduncle Carbon, Characterization, and Its Adsorption Studies

Adsorption on activated carbon is an efficient method for the removal of toxic dyes. However, since commercially available charcoal is quite expensive, activated carbon obtained from agricultural by‐products may serve as a good replacement. In this study, activated carbon was prepared from pepper peduncle, an agricultural waste product, by microwave activation. The synthesized carbon was characterized by X‐ray diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy, and thermal gravimetric analysis techniques. It was then used for the adsorption of methylene blue dye from an aqueous solution, which was studied as a function of the dye concentration, contact time, and temperature. The adsorption data were fitted to Freundlich and Langmuir isotherm models. The adsorption kinetics was studied by employing first‐ and second‐order kinetic models, and it was found that the adsorption of methylene blue on the synthesized activated carbon follows a second‐order kinetic model. Effect of temperature on the adsorption process was studied, and the thermodynamic parameters such as activation energy, change in enthalpy, entropy, and free energy of adsorption were calculated on the basis of the absolute theory of reaction rate expressions. About 99.5–91.8% of the dye was removed for an initial dye concentration in the range 20–100 mg/g in 1 h. Thus the synthesized activated carbon was found to be very efficient in adsorbing the dye.

Synergistic removal of Cu(II) and nitrazine yellow dye using an eco‐friendly chitosan‐montmorillonite hydrogel: Optimization by response surface methodology

ABSTRACTA comprehensive feasibility study on adsorption of Cu(II) and a water‐soluble nitrazine yellow (NY) dye by chitosan‐montmorillonite (CS‐MMT) hydrogel as the biosorbent was investigated as a function of biosorbent dosage, initial concentration, pH, temperature, and the presence of salts. Box–Behnken methodology was applied to optimize the adsorption experiments. Maximum adsorption values were determined as 132.74 mg/g and 144.41 mg/g at pH = 5.0, for Cu(II) and NY dye, respectively. Equilibrium isotherms of Langmuir and Freundlich were analyzed by the non‐linear regression model. The equilibrium data were well described by Freundlich model and the adsorption process well fitted pseudo‐second order kinetics. The enthalpy change of adsorption (ΔH°) were calculated as −3.78 kJ/mol and −5.75 kJ/mol for Cu(II) and NY dye, respectively, indicating that the adsorption processes were exothermic. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 43664.

Effects of Zeolite Structural Confinement on Adsorption Thermodynamics and Reaction Kinetics for Monomolecular Cracking and Dehydrogenation of n-Butane.

The effects of zeolite structure on the kinetics of n-butane monomolecular cracking and dehydrogenation are investigated for eight zeolites differing in the topology of channels and cages. Monte Carlo simulations are used to calculate enthalpy and entropy changes for adsorption (ΔHads-H+ and ΔSads-H+) of gas-phase alkanes onto Brønsted protons. These parameters are used to extract intrinsic activation enthalpies (ΔHint‡), entropies (ΔSint‡), and rate coefficients (kint) from measured data. As ΔSads-H+ decreases (i.e., as confinement increases), ΔHint‡ and ΔSint‡ for terminal cracking and dehydrogenation decrease for a given channel topology. These results, together with positive values observed for ΔSint‡, indicate that the transition states for these reactions resemble products. For central cracking (an earlier transition state), ΔHint‡ is relatively constant, while ΔSint‡ increases as ΔSads-H+ decreases because less entropy is lost upon protonation of the alkane. Concurrently, selectivities to terminal cracking and dehydrogenation decrease relative to central cracking because ΔSint‡ decreases for the former reactions. Depending on channel topology, changes in the measured rate coefficients (kapp) with confinement are driven by changes in kint or by changes in the adsorption equilibrium constant (Kads-H+). Values of ΔSint‡ and ΔHint‡ are positively correlated, consistent with weaker interactions between the zeolite and transition state and with the greater freedom of movement of product fragments within more spacious pores. These results differ from earlier reports that ΔHint‡ and ΔSint‡ are structure-insensitive and that kapp is dominated by Kads-H+. They also suggest that ΔSads-H+ is a meaningful descriptor of confinement for zeolites having similar channel topologies.