Insights into modeling adsorption equilibria of single and multicomponent systems of organic and water vapors

The use of artificial neural network for modelling adsorption of Congo red onto activated hazelnut shell

Activated hazelnut shell (HSAC), an organic waste, was utilized for the adsorptive removal of Congo red (CR) dye from aqueous solutions, and a modelling study was conducted using artificial neural networks (ANNs). The structure and characteristic functional groups of the material were examined by the FTIR method. The BET surface area of the synthesized material, named HSAC, was 812 m2/g. Conducted in a batch system, the adsorption experiments resulted in a notable removal efficiency of 87% under optimal conditions. The kinetic data for hazelnut shell activated carbon (HSAC) removal of CR were most accurately represented by the pseudo-second-order kinetic model (R2 = 0.998). Furthermore, the equilibrium data demonstrated a strong agreement with the Freundlich model. The maximum adsorption capacity of HSAC for CR was determined to be 34.8 mg/g. The optimum adsorption parameters were determined to be pH 6, contact time of 60 min, 10 g/L of HSAC, and a concentration of 400 mg/L for CR. Considering the various experimental parameters influencing CR adsorption, an artificial neural network (ANN) model was constructed. The analysis of the ANN model revealed a correlation of 98%, indicating that the output parameter could be reliably predicted. Thus, it was concluded that ANN could be employed for the removal of CR from water using HSAC.

Bridging the Adsorption Data and Adsorption Process by Introducing a Polynomial Structure To Accurately Describe IUPAC Isotherms, Stepwise Isotherms, and Stepwise Breakthrough Curves

Porous heterogeneous adsorbents, those composed of multiple pore structures and surface chemical adsorption sites, can result in various gas or vapor adsorption isotherms, including five types of IUPAC adsorption isotherms and stepwise adsorption isotherms that have been difficult to model using a single adsorption equilibrium model. The limitation of the above equilibrium model further restricts the calculations of complex stepwise breakthrough curves. To bridge the adsorption data and adsorption process, it is important to first develop a simple model or method to describe these isotherms of various complex adsorption systems. In this work, assuming that the effect of the diffusion rate can be neglected under the static condition and the adsorption process is discontinuous, the number of adsorption isotherm inflection points can be used to represent the changed number of adsorption interactions. With the introduction of the polynomial structure, a series of empirical or semi-empirical polynomial adsorption models were developed. The N-site polynomial Langmuir-Freundlich equation could accurately fit common type I, II, III, IV, and V adsorption isotherms and complex stepwise adsorption isotherms covering various adsorbates, such as volatile organic compounds (VOCs), toxic industrial chemicals (TICs), water vapor, and carbon dioxide, as well as different adsorbents, such as metal/covalent organic frameworks (MOFs/COFs), zeolites, and porous carbons. Similarly, the introduction of a polynomial structure, such as the N-site polynomial Yoon-Nelson equation, was also successful in the description of interesting stepwise breakthrough curves. This work provides a more accurate adsorption equilibrium model to characterize all types of isotherms. As a foundation model, it is expected to be used to simulate the gas-solid adsorption process inside the fixed and fluidized beds packed with porous adsorbents.

Development of a Multicomponent Adsorption Isotherm Equation and Its Validation by Modeling

Researchers have made significant efforts over the past few decades to understand adsorption by developing various simple adsorption isotherm models. However, though many contaminants usually occur as multicomponent mixtures in nature, multicomponent adsorption isotherms have received limited attention and remain an area of inadequate research. We have presented here in a new multicomponent adsorption isotherm model, named the Jeppu Amrutha Manipal Multicomponent (JAMM) isotherm, that can alleviate this problem. We first developed the JAMM multicomponent isotherm using our experimental data sets of arsenic and fluoride competitive adsorption on activated carbon. We then tested the JAMM multicomponent isotherm for a case study of cadmium and zinc competitive adsorption. Next, we further assessed the JAMM isotherm using another competitive adsorption case study of copper and chromium. Through extensive validation studies and error analysis, the JAMM isotherm was able to demonstrate its efficacy in predicting the adsorption behavior in several multicomponent adsorption systems accurately. The main advantage of JAMM isotherm over other multicomponent isotherms is that it utilizes and leverages the single-component adsorption parameters to simulate multicomponent isotherms. The proposed JAMM analytical isotherm model furthermore incorporates the interaction between the components, a mole fraction parameter, and a heterogeneity index, providing a more comprehensive modeling framework for multicomponent adsorption. The mole fraction term was introduced for the distribution of adsorption sites based on the relative number of molecules of each component. An additional term for interaction coefficient was introduced for the representation of interactions. During the validation of JAMM with three experimental case studies with negligible, small, and high competition systems of adsorbates, impressive predictions were exhibited, with the average normalized absolute percentage error as 6.05% and average R2 as 0.86, highlighting the model's robustness, versatility, and reliability. We propose that the new JAMM isotherm modeling framework might profoundly help in chemical engineering, environmental engineering, and materials science applications by providing a potent tool for analyzing and predicting multicomponent adsorption systems.

Adsorption and decolorization study of reactive black 5 by immobilized metal-organic framework of UiO-66 and Gloeophyllum trabeum fungus

This study aimed to investigate immobilized metal-organic framework (MOF) UiO-66 and brown-rot fungus Gloeophyllum trabeum (GT) in PVA-SA matrices for adsorption and decolorization of reactive black 5 (RB5). Furthermore, UiO-66/GT@PVA-SA composite was successfully fabricated and obtained by immobilizing UiO-66 and GT mycelia into a mixture of PVA-SA. This composite demonstrated a decolorization ability of 80.12% for RB5 after 7 days. The composite's reusability was assessed for three cycles; at last, it only achieved 21%. This study reported that adsorption of RB5 by the composite followed a pseudo-second-order kinetic model with a correlation coefficient (R2) of 0.9997. The Freundlich model was found to be suitable for the isotherm adsorption. The process was also spontaneous and feasible, as indicated by the negative ΔG value. Subsequently, four metabolite products resulting from decolorization of RB5 by UiO-66/GT@PVA-SA composite were proposed, namely: C24H19N5Na2O13S4 (m/z = 762), C10H13N2O8S2- (m/z = 353), C12H9N4O7S2- (m/z = 384), and C10H13O8S2- (m/z = 325).

Decolorization of multicomponent dye-laden wastewater by modified waste fly ash: a parametric analysis for an anionic and cationic combination of dyes

In this research study, waste fly ash (WFA) underwent acid activation and subsequent amine functionalization using ammonia solution. This treatment improves the porosity, thermal tendency and crystallinity of WFA. Modified WFA was tested under different experimental conditions to treat the wastewater consisting of different concentrations of cationic (methylene blue and rhodamine 6G) and anionic (methyl orange) dyes. As an individual, methylene blue (MB) and rhodamine 6G (Rh) showed ~ 100% and ~ 82% removal efficiencies respectively in an alkaline medium while methyl orange (MO) exhibited only ~ 20% adsorption in the same medium. An antagonistic effect was observed in adsorption when wastewater contains both cationic dyes whereas the combination of cationic and anionic dyes in solution manifested a synergistic effect. For all individual and binary dye combinations, there is a close agreement in observed and calculated uptakes when the data was fitted to the fractional order kinetic rate equation. The adsorption of all dyes is spontaneous and endothermic in nature except for MB/MO combination where the process is exothermic in nature. 24.93 mg/g, 24.83 mg/g, and 14.95 mg/g monolayer uptake capacities of MB, Rh, and MO were found respectively from isothermal analysis of single dye adsorption data. Further, extended sips model gave higher correlation coefficient (R² = 0.99) and addressed the failed assumptions of both the Langmuir and Freundlich models. Overall, in the experimental results, the modified waste fly ash could act as successful adsorbent to treat dye bearing wastewater.

Radiation Resistance and Adsorption Behavior of Aluminum Hexacyanoferrate for Pd

Irradiation resistance is important for adsorbents used in radioactive environments such as high-level liquid waste. In this work, a silica-based composite adsorbent (KAlFe(CN)₆/SiO₂) was synthesized and γ-irradiated from 10 to 1000 kGy. The angles of the main X-ray diffraction peaks slightly decreased with the increase in irradiation dose, and a minor decomposition of CN^- occurred after irradiation to 1000 kGy, indicating that the KAlFe(CN)₆/SiO₂ adsorbent could preserve structural integrity with a dose below 100 kGy. In 1 to 7 M HNO₃, the adsorption ability of the irradiated KAlFe(CN)₆/SiO₂ remained performant, with a higher K_d than 1625 cm³ g^-1. The adsorption equilibrium of Pd(II) in 3 M HNO₃ was attained within 45 min before and after irradiation. The maximal adsorption capacity Q_e of the irradiated KAlFe(CN)₆/SiO₂ on Pd(II) ranged from 45.1 to 48.1 mg g^-1. A 1.2% relative drop in Q_e was observed after 100 kGy irradiation, showing that γ-irradiation lower than 100 kGy insignificantly affected the adsorption capacity of KAlFe(CN)₆/SiO₂. Calculating and comparing the structures and free energies of different adsorption products via the density functional theory (DFT) method showed that KAlFe(CN)₆/SiO₂ was more inclined to completely adsorb Pd(II) and spontaneously generate Pd[AlFe(CN)₆]₂.

A critical review of adsorption isotherm models for aqueous contaminants: Curve characteristics, site energy distribution and common controversies

The quantitative description of the equilibrium data by the isotherm models is an indispensable link in adsorption studies. The previous review papers focus on the underlying assumptions, fitting methods, error functions and practical applications of the isotherm models, usually ignoring their curve characteristics, selection criteria and common controversies. The main contents of this review include: (i) effect of the model parameters on the isotherm curves; (ii) determination of the site energy distribution; (iii) selection criteria of the isotherm models; and (iv) elimination of some common controversies. It is of great significance to reveal the curve characteristics for selecting a proper isotherm model. The site energy distribution is conducive to understanding the physicochemical properties of the adsorbent surface. The complete isotherm is recommended to be correlated with the experimental data. The model parameter q_max should be cautiously adopted for comparison of the adsorbent performance. The residual plot can be used to diagnose the fitting quality of the isotherm models further. This review also addresses some common mistakes and controversies and thereby avoids their propagation in future publications.

Adsorptive removal of antibiotic pollutants from wastewater using biomass/biochar-based adsorbents

This study explores adsorptive removal measures to shed light on current water treatment innovations for kinetic/isotherm models and their applications to antibiotic pollutants using a broad range of biomass-based adsorbents. The structure, classifications, sources, distribution, and different techniques for the remediation of antibiotics are discussed. Unlike previous studies, a wide range of adsorbents are covered and adsorption of comprehensive classes of antibiotics onto biomass/biochar-based adsorbents are categorized as β-lactam, fluoroquinolone, sulfonamide, tetracycline, macrolides, chloramphenicol, antiseptic additives, glycosamides, reductase inhibitors, and multiple antibiotic systems. This allows for an assessment of their performance and an understanding of current research breakthroughs in applying various adsorbent materials for antibiotic removal. Distinct from other studies in the field, the theoretical basis of different isotherm and kinetics models and the corresponding experimental insights into their applications to antibiotics are discussed extensively, thereby identifying the associated strengths, limitations, and efficacy of kinetics and isotherms for describing the performances of the adsorbents. In addition, we explore the regeneration of adsorbents and the potential applications of the adsorbents in engineering. Lastly, scholars will be able to grasp the present resources employed and the future necessities for antibiotic wastewater remediation.

A Review on Polyacrylonitrile as an Effective and Economic Constituent of Adsorbents for Wastewater Treatment

Water gets polluted due to the dumping of untreated industrial waste into bodies of water, particularly those containing heavy metals and dyes. Industrial water contains both inorganic and organic wastes. Numerous adsorbents that are inexpensive and easily available can be used to address the issue of water deterioration. This review report is focused on polyacrylonitrile as an efficient constituent of adsorbents to extract toxic ions and dyes. It discusses the various formulations of polyacrylonitrile, such as ion exchange resins, chelating resins, fibers, membranes, and hydrogels, synthesized through different polymerization methods, such as suspension polymerization, electrospinning, grafting, redox, and emulsion polymerization. Moreover, regeneration of adsorbent and heavy metal ions makes the adsorption process more cost-effective and efficient. The literature reporting successful regeneration of the adsorbent is included. The factors affecting the performance and outcomes of the adsorption process are also discussed.

Isotherm models for adsorption of heavy metals from water - A review

Adsorption is a widely used technology for removing and separating heavy metal from water, attributed to its eco-friendly, cost-effective, and high efficiency. Adsorption isotherm modeling has been used for many years to predict the adsorption equilibrium mechanism, adsorption capacity, and the inherent characteristics of the adsorption process, all of which are substantial in evaluating the performance of adsorbents. This review summarizes the development history, fundamental characteristics, and mathematical derivations of various isotherm models, along with their applicable conditions and application scenarios in heavy metal adsorption. The latest progress in applying isotherm models with a one-parameter, two-parameter, and three-parameter in heavy metal adsorption using carbon-based materials, which has gained much attention in recent years as low-cost adsorbents, is critically reviewed and discussed. Several experimental factors affecting the adsorption equilibrium, such as solution pH, temperature, ionic strength, adsorbent dose, and initial heavy metal concentration, are briefly discussed. The criteria for selecting the optimum isotherm for heavy metal adsorption are proposed by comparing various adsorption models and analyzing mathematical error functions. Finally, the relative performance of different isotherm models for heavy metal adsorption is compared, and the future research gaps are identified.

Performance evaluation of activated carbon sorbents for indoor air purification during normal and wildfire events

Volatile organic compounds (VOCs) are a significant class of indoor air pollutants and are known for their adverse effects on health. A common strategy to reduce indoor VOC levels is to use sorbents, including activated carbons (ACs). The amount of activated carbon is critical to achieving a reasonable AC filter lifetime in an air purification device. The study aims to estimate the amount of carbon needed in a typical indoor environment and in a heavy use setting such as during cooking, agriculture field fires, or wildfires. The problem is complex as various types of ACs are used, and the type and concentration of VOCs in the indoor environment also vary in different settings. Therefore, literature data on thermophysical parameters for 45 AC-VOC pairs was used to estimate the required amount of AC under a given set of conditions. The study uses modeling distributions of the footprint of suitable carbon filters for the removal of common VOCs encountered indoors for a period of 30 days. It was found that while 50% of AC-VOC pairs surveyed will require about 190-370 g at low indoor VOCs levels of 0.1-1 μmol/m³(considered a good clean indoor environment), up to 1.1 kg of ACs are needed for a carbon filter to survive 30 days in a typical indoor environment (VOCs levels of 10 μmol/m³). On the other hand, 3-15 kg or more AC will be needed in a filter to survive 30 days during adverse events such as wildfires. The objective of the present study is to aid consumers and businesses in making an informed decision on the type of AC-based indoor air filters that meet their needs. Using this data, an open-access online calculator is being developed to predict the amount of carbon needed in a filter/device at any specific indoor air condition.

Development of Free-Standing Titanium Dioxide Hollow Nanofibers Photocatalyst with Enhanced Recyclability

Titanium dioxide hollow nanofibers (THN) are excellent photocatalysts for the photodegradation of Bisphenol A (BPA) due to their extensive surface area and good optical properties. A template synthesis technique is typically employed to produce titanium dioxide hollow nanofibers. This process, however, involves a calcination procedure at high temperatures that yields powder-form photocatalysts that require post-recovery treatment before recycling. Meanwhile, the immobilization of photocatalysts on/into a membrane has been reported to reduce the active surface area. Novel free-standing TiO₂ hollow nanofibers were developed to overcome those shortcomings. The free-standing photocatalyst containing 0.75 g of THN (FS-THN-75) exhibited good adherence and connectivity between the nanofibers. The recyclability of FS-THN-75 outperformed the THN calcined at 600 °C (THN-600), which retained 80% of its original weight while maintaining excellent degradation performance. This study recommends the potential application of free-standing TiO₂ hollow nanofibers as high potential novel photocatalysts for the treatment of BPA in wastewater.

Experimental Study on the Kinetics of CO2 and H2O Adsorption on Honeycomb Carbon Monoliths under Cement Flue Gas Conditions

The main challenge of adsorption consists in the production of materials that can be used in real situations. This study comprehensively describes the CO₂ and H₂O adsorption behavior of honeycomb-shaped sorbents commonly used in rapid pressure swing adsorption cycles (RPSA). With this purpose, the kinetics and equilibrium of adsorption of CO₂/H₂O/N₂ mixtures on three honeycomb carbon monoliths (793, 932, and AM03) were assessed in a thermogravimetric analyzer (TGA) under different postcombustion capture scenarios (temperature of 50 °C and several concentrations of CO₂). The kinetics study exhibited that the single adsorption of CO₂ and H₂O can be adequately described by the Avrami and exponential decay-2 models, respectively. As expected, the three carbon monoliths presented fast adsorption of CO₂ from a CO₂/H₂O mixture. Furthermore, when humid flue gas was considered, overall adsorption kinetics were governed by CO₂. Besides, the experimental data fitting to the intraparticle diffusion model showed that gradual CO₂ and H₂O diffusion toward the micropores was the rate-limiting stage. The obtained results give a better insight into the selective adsorption of CO₂ and the potential of honeycomb carbon monoliths to separate CO₂ from humid flue gas in the context of the cement industry. Carbon monolith 793 is the best carbon monolith candidate to capture CO₂ under the evaluated conditions: a capacity of adsorption of 1 mmol of CO₂ g^-1 and favorable kinetics in 32 vol % CO₂ and 4 vol % H₂O_(v), at 50 °C and 101.3 kPa.

Thermodynamic Characteristics of the Hydrogen Sulfide Sorption Process by Ferromanganese Materials

The work analyzes hydrogen sulfide sorption from model gas mixtures containing H₂S from 1.25 × 10^-3 to 1.28 × 10^-4 mol/L under static conditions at temperatures 253 and 298 K on the raw manganese ore of the Ulu-Telyak deposit (Bashkortostan, Russia), manganese(IV) oxide, and manganese(IV) and iron(III) oxide mixtures. The thermodynamic models for calculating the equilibrium constants and Gibbs energy changes were analyzed. The sorption isotherms were described by the Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich models. The value of enthalpy of hydrogen sorption on the ore was -68.98 ± 3.45 kJ/mol and those on model mixtures Mn₄ + Fe₂O₃ and MnO₄ were ±12.20 kJ/mol and -103.826 ± 5.19 kJ/mol, respectively, and the entropies of the hydrogen sulfide sorption process on three manganese materials at 253 K were calculated. The limiting capacity values of manganese materials at 253 and 298 K were obtained. The morphological analysis of the ore samples, Mn₄ + Fe₂O₃, and MnO₄, before and after hydrogen sulfide sorption, was carried out at 253 K. The obtained thermodynamic parameters determine the advantage of using the raw manganese ore over pure oxides, which characterizes its effective practical application in the desulfurization process.

Recycling of Waste Cotton Sheets into Three-Dimensional Biodegradable Carriers for Removal of Methylene Blue

Waste cotton sheets (WCS) are promising cellulose sources due to their high content of cellulose and large amount of disposal every year, which could be recycled and employed as low-cost structural materials. The present work aims at investigating the efficacy of hydrogel adsorbents prepared from regenerated WCS as the carriers of activated carbon (AC) for treating the dye-contaminated water. Activated WCS was directly dissolved in lithium chloride/N,N-dimethylacetamide (LiCl/DMAc) solvent and then regenerated into cellulose hydrogels, which were employed as three-dimensional biodegradable matrices for loading an extremely high content of AC (up to 5000%). The morphology and properties of resultant adsorbents were studied in detail. The results showed that different washing methods and contents of AC and cellulose had obvious effects on water contents, mechanical properties, and adsorption capacities of AC/WCS hydrogels. Especially, the hydrogels containing high AC content washed by gradient ethanol solvent exhibited outstanding compressive strengths of up to 3.0 MPa at 60% strain, while the adsorption capacity of 5000%AC/0.3CS toward a model dye methylene blue (MB, initial concentration of 200 mg/L) reached 174.71 mg/g at pH 6.9 and 35 °C. This was comparable to the adsorption capacity of original AC powders, while no AC powders were released from hydrogels to water. The adsorption of MB followed the Dubinin-Astakhov model and pseudo-first-order mechanism. Thermodynamic studies showed the spontaneous and endothermic nature of the overall physical adsorption process. Therefore, this work demonstrates the feasibility to recycle WCS into biodegradable carriers of functional compounds, and the AC/regenerated cellulose hydrogels have a high potential as a promising adsorbent with low-cost and convenient separation for dye removal from wastewater.

Enthalpic and Liquid-Phase Adsorption Study of Toluene-Cyclohexane and Toluene-Hexane Binary Systems on Modified Activated Carbons

The liquid-phase adsorption of toluene in cyclohexane and hexane solutions on modified activated carbons was evaluated; the energy involved in the interaction between these solutions and the solids was determined by immersion enthalpies of pure solvents and their mixtures, and the contribution of the system constituents was calculated by differential enthalpies. The thermal treatment generated modifications that favored adsorption and interaction with the evaluated solutions, since it increased the textural parameters and the basic character of the samples. Cyclohexane could create greater competition with the adsorption sites compared to hexane, but it favored the increase in adsorption capacities (0.416 to 1.026 mmol g⁻¹) and the interactions with the solid evaluated through the immersion enthalpies. The immersion enthalpies of pure solvents (−16.36 to −112.7 J g⁻¹) and mixtures (−25.65 to −104.34 J g⁻¹) had exothermic behaviors that were decreasing due to the possible displacement of solvent molecules when increasing the solute concentration in the mixtures. The differential enthalpies for toluene were negative (−18.63 to −2.14 J), mainly due to the π–π interaction with the solid, while those of the solvent–solid component tended to be positive values (−4.25 to 55.97 J) due to the displacement of the solvent molecules by those of toluene.

Separation of biobutanol from ABE fermentation broth using lignin as adsorbent: A totally sustainable approach with effective utilization of lignocellulose

Adsorption is considered to be a promising butanol recovery method for solving the issue of inhibition in the ABE (acetone-butanol-ethanol) fermentation. As a byproduct in the second generation biobutanol industry, lignin was found to be a good adsorbent for the butanol enrichment. It is conducive to the full utilization of renewable lignocellulose biomass resource. Kinetic and equilibrium experiments indicated that lignin had a satisfactory adsorption rate and capacity that are comparable to those of many synthetic materials. Multicomponent adsorption experiments revealed that lignin had higher adsorption selectivity toward butanol than that of ethanol and acetone. The adsorption capacity of lignin for butanol first increased and then gradually decreased with increasing temperature. And maximum adsorption capacity reached 304.66 mg g^-1 at 313 K. The inflection point of temperature is close to the ABE fermentation temperature of 310 K. The condensed butanol by desorption was 145 g L^-1, with a satisfying regeneration performance. ¹H NMR and FT-IR spectra indicated that the aromatic units of lignin formed π-systems with A/B/E. The π-system is particularly significant for butanol due to its longer hydrocarbon chain. These results could contribute to the emerging lignin-based materials for butanol separation.

Designing hierarchical nanoporous membranes for highly efficient gas adsorption and storage

Nanoporous membranes with two-dimensional materials such as graphene oxide have attracted attention in volatile organic compounds (VOCs) and H₂ adsorption because of their unique molecular sieving properties and operational simplicity. However, agglomeration of graphene sheets and low efficiency remain challenging. Therefore, we designed hierarchical nanoporous membranes (HNMs), a class of nanocomposites combined with a carbon sphere and graphene oxide. Hierarchical carbon spheres, prepared following Murray's law using chemical activation incorporating microwave heating, act as spacers and adsorbents. Hierarchical carbon spheres preclude the agglomeration of graphene oxide, while graphene oxide sheets physically disperse, ensuring structural stability. The obtained HNMs contain micropores that are dominated by a combination of ultramicropores and mesopores, resulting in high VOCs/H₂ adsorption capacity, up to 235 and 352 mg/g at 200 ppmv and 3.3 weight % (77 K and 1.2 bar), respectively. Our work substantially expands the potential for HNMs applications in the environmental and energy fields.