Adsorption isotherms, kinetics and mechanism for the adsorption of cationic and anionic dyes onto carbonaceous particles prepared from Juglans regia shell biomass

In the present study, Juglans regia shells were used to prepare activated carbon by acid treatment method. J. regia shell-based activated carbon was used for the adsorption of two synthetic dyes namely, a basic dye malachite green and an acid dye amido black 10B. The prepared adsorbent was crushed and sieved to three different mesh sizes 100, 600 and 1,000 μm. The adsorbent was characterized by scanning electron microscopy, surface acidity and zero-point charge. Batch experiments were carried out by varying the parameters like initial aqueous phase pH, adsorbent dosage and initial dye concentration. The equilibrium data were tested with Langmuir, Freundlich, Redlich–Peterson and Sips isotherm at three different temperatures 293, 300 and 313 K and it was found that the Freundlich isotherm best fitted the adsorption of both the dyes. Kinetic data were tested with pseudo first-order model and pseudo second-order model. The mechanism for the adsorption of both the dyes onto the adsorbent was studied by fitting the kinetic data with intraparticle diffusion model and Boyd plot. External mass transfer was found to be the rate-determining step. Based on the ionic nature of the adsorbates, the extent of film diffusion and intraparticle diffusion varied; both being system specific. Thermodynamic parameters were also calculated. Finally, the process parameters of each adsorption system were compared to develop the understanding of the best suitable system.     

Chromium(VI) adsorption from aqueous solution by prepared biochar from <Emphasis Type="Italic">Onopordom Heteracanthom</Emphasis>

In this research, the stems of Onopordom Heteracanthom which is a kind of weed were converted to biochar particles, and their characteristics were investigated. The morphology and purity of these particles were examined by SEM and EDX techniques, respectively. Specific surface area was obtained as 5.73 m² g^?1 by BET method. The biochar particles obtained from Onopordom Heteracanthom were evaluated as an adsorbent to remove Cr(VI) from aqueous environments. The effect of some parameters such as initial concentration of Cr(VI), dosage of adsorbent, and pH were investigated on the adsorption capacity of Cr(VI) onto the adsorbent. The equilibrium data were analyzed by various isotherm models. The results revealed that in this process, the adsorption isotherm and kinetics have more conformity with Langmuir isotherm and pseudo-second-order kinetics, respectively. The multi-linearity of the Weber and Morris adsorption kinetic model indicates that the intra-particle diffusion is not merely the rate-controlling step for the whole adsorption process.     

Linearized and non-linearized isotherm models comparative study on adsorption of aqueous phenol solution in soil

Use of native soil in adsorption of phenol from industrial wastewater has been one of the attractive option for dephenolation, especially in view of low cost and ease in accessibility, as well as scope for regeneration (or, at least reuse). However, an effective usage of the adsorbent necessitates a deeper understanding of the adsorption characteristics. Most of the study of adsorption characteristics are confined to analysis of mono- and bi- parametric isotherm models (and rarely, linearized multi-parametric isotherm models), due to the difficulties in solving higher parametric models, as well as fairly satisfying results by lower-parametric models. In the present study, adsorption batch studies were carried out using a naturally and widely available common soil of south India (namely, Adhanur soil), for removal of phenol from the aqueous solution, with an explicit objective of comparison of linear and non-linear regression methods for finding variation in isotherm coefficients and fitness of the models. Six linearized isotherm models (including four linearized Langmuir models) and three non-linear isotherm model were discussed in this paper, and their coefficients were estimated. Although all the studied isotherm models showed fairly good fit to the experimental data, but Redlich—Peterson isotherm was found to be the best representative for phenol-sorption on the used soil adsorbent. Besides, it was observed that to determine the isotherm parameters non-linear isotherm models were found to be the best representative of adsorption characteristics, than their linearized counter-parts.     

Utilization of agro-waste for removal of toxic hexavalent chromium: surface interaction and mass transfer studies

Abundantly available agricultural waste materials (banana bunch, sorghum stem and casuarinas fruit) are processed with negligible cost and are found to be highly suitable as biosorbents for chromium(VI) removal from aqueous environment due to high surface area and functional groups of adsorbents. The equilibrium data have been analyzed for the adsorbate–adsorbate/adsorbent interactions and found to be fitted to the data in the order, Hill–de Boer ≥ Fowler–Guggenheim ? Frumkin > Kiselev. To determine the characteristic parameters for process design, mass transfer studies have been carried out using two-parameter isotherm models (Harkins–Jura, Halsey, Smith, El-Awady and Flory–Huggins) and three-parameter isotherm models (Redlich–Peterson and Sips) which are applied to the experimental data. The fitness of the isotherms describes that both mono- and multilayer adsorptions occur in the present studied three biosorbents in preference to the latter. The mechanism of adsorption has been studied using diffusion kinetic models (viz. liquid film diffusion, Dunwald–Wagner intra-particle diffusion model and moving boundary model) and described the possibility of diffusion in the order of banana bunch–stem powder > sorghum stem powder > casuarinas fruit powder in terms of diffusion coefficients. In essence of all the results, the selected adsorbents can be used as a potential adsorbent for the removal of Cr(VI) from aqueous solutions.     

Adsorption of methyl tert-butyl ether using granular activated carbon: Equilibrium and kinetic analysis

The adsorption of methyl tert-butyl ether by granular activated carbon was investigated. The experimental data were analyzed using the Freundlich isotherm and the Langmuir isotherm. Although equilibrium data were found to follow Freundlich isotherm model, it were fitted better by the Langmuir model with a maximum adsorption capacity of 204.1 mg/g. The kinetic data obtained at different concentrations were analyzed to predict the constant rate of adsorption using three common kinetic models: pseudo-first-order, pseudo-second-order equation and intraparticle diffusion equation. The pseudo-second-order model was suitable for describing the adsorption kinetics for the removal of methyl tert-butyl ether from aqueous solution onto granular activated carbon. Both the Lagergren first-order rate constant k ₁ and pseudo-second-order rate constant k ₂ decrease with increasing initial concentrations of methyl tert-butyl ether and the intraparticle diffusion rate constant k _p shows the reverse characteristic. Analysis of sorption data using a boyd plot confirmed that external mass transfer is the main rate-limiting step at the initial stage of adsorption. Results illustrate that granular activated carbon is an effective adsorbent for methyl tert-butyl ether and also provide specific guidance into adsorption of methyl tert-butyl ether on granular activated carbon in contaminated groundwater.     

Adsorption kinetics and thermodynamics of organophosphorus profenofos pesticide onto Fe/Ni bimetallic nanoparticles

Bimetallic Fe/Ni nanoparticles were synthesized and used for the removal of profenofos organophosphorus pesticide from aqueous solution. These novel bimetallic nanoparticles (Fe/Ni) were characterized by scanning electron microscopy, energy-dispersive X-ray analysis spectroscopy, X-ray diffraction, and Fourier transform infrared spectroscopy. The effect of the parameters of initial pesticide concentration, pH of the solution, adsorbent dosage, temperature, and contact time on adsorption was investigated. The adsorbent exhibited high efficiency for profenofos adsorption, and equilibrium was achieved in 8 min. The Langmuir, Freundlich, and Temkin isotherm models were used to determine equilibrium. The Langmuir model showed the best fit with the experimental data (R ² = 0.9988). Pseudo-first-order, pseudo-second-order, and intra-particle diffusion models were tested to determine absorption kinetics. The pseudo-second-order model provided the best correlation with the results (R ² = 0.99936). The changes in the thermodynamic parameters of Gibb’s free energy, enthalpy, and entropy of the adsorption process were also evaluated. Thermodynamic parameters indicate that profenofos adsorption using Fe/Ni nanoparticles is a spontaneous and endothermic process. The value of the activation energy (E _a = 109.57 kJ/mol) confirms the nature of the chemisorption of profenofos onto Fe/Ni adsorbent.     

Single and binary adsorption of lead and cadmium ions from aqueous solution using the clay mineral beidellite

Beidellite, a low-cost, locally available and natural mineral was used as an adsorbent for the removal of lead and cadmium ions from aqueous solutions in batch experiments. The kinetics of adsorption process was tested for the pseudo first-order, pseudo second-order reaction and intra-particle diffusion models. The rate constants of adsorption for all these kinetic models were calculated. Comparison amongst the models showed that the sorption kinetics was best described by the pseudo second-order model. Langmuir and Freundlich isotherm models were applied to the experimental equilibrium data for different temperatures. The adsorption capacities (Q°) of beidellite for lead and cadmium ions were calculated from the Langmuir isotherm. It was found that adsorption capacity was in the range of 83.3–86.9 for lead and 42–45.6 mg/g for cadmium at different temperatures. Thermodynamic studies showed that the metal uptake reaction by beidellite was endothermic in nature. Binary metal adsorption studies were also conducted to investigate the interactions and competitive effects in binary adsorption process. Based on the optimum parameters found, beidellite can be used as adsorbent for metal removal processes.     

Ionic solid-impregnated sulphate-crosslinked chitosan for effective adsorption of hexavalent chromium from effluents

Microwave-assisted tetrabutyl ammonium-impregnated sulphate-crosslinked chitosan was synthesized for enhanced adsorption of hexavalent chromium. The adsorbent obtained was extensively characterized using Fourier transform infrared, X-ray diffraction, scanning electron microscopy and energy-dispersive X-ray studies. Various isotherm models such as Langmuir, Freundlich and Dubinin–Radushkevich were studied to comprehend the adsorption mechanism of hexavalent chromium by the adsorbent. Maximum adsorption capacity of 225.9 mg g^?1 was observed at pH 3.0 in accordance with Langmuir isotherm model. The sorption kinetics and thermodynamic studies revealed that adsorption of hexavalent chromium followed pseudo-second-order kinetics with exothermic and spontaneous behaviour. A column packed with 1 g of adsorbent was found to give complete adsorption of Cr(VI) up to 900 mL of 200 mg L^?1 solution which discerns the applicability of the adsorbent material for higher sample volumes in column studies. The effective adsorption results were obtained due to both ion exchange and ion pair interaction of adsorbent with hexavalent chromium. Greener aspect of overall adsorption was regeneration of the adsorbent which was carried out using sodium hydroxide solution. In the present study, the regenerated adsorbent was effectively reused up to ten adsorption–desorption cycles with no loss in adsorption efficiency.     

Preparation and characterization of biosorbents and copper sequestration from simulated wastewater

This paper reports the potential of chemically treated wood chips to remove copper (II) ions from aqueous solution a function of pH, adsorbent dose, initial copper (II) concentration and contact time by batch technique. The wood chips were treated with (a) boiling, (b) formaldehyde and (c) concentrated sulphuric acid and characterized by Fourier transform infrared spectroscopy, scanning electron microscopy and energy dispersive analysis X-ray. pH 5.0 was optimum with 86.1, 88.5 and 93.9 % copper (II) removal by boiled, formaldehyde-treated and concentrated sulphuric acid-treated wood chips, respectively, for dilute solutions at 20 g L^?1 adsorbent dose. The experimental data were analysed using Freundlich, Langmuir, Dubinin–Radushkevich and Temkin isotherm models. It was found that Freundlich and Langmuir models fitted better the equilibrium adsorption data and the adsorption process followed pseudo-second-order reaction kinetics. The results showed that the copper (II) is considerably adsorbed on wood chips and it could be an economical option for the removal of copper from aqueous systems.     

Removal of dyes by adsorption on magnetically modified activated sludge

The ability of magnetically modified activated sludge affected by thermal treatment to remove water-soluble organic dyes was examined. Twelve different dyes were tested. Based on the results of the initial sorption study, four dyes (namely aniline blue, Nile blue, Bismarck brown Y and safranin O) were chosen for further experiments due to their promising binding onto magnetic activated sludge. Significant factors influencing adsorption efficiency such as dependence of contact time, initial pH or temperature were studied in detail. The adsorption process was very fast; more than 88 % of dye content (55 mg/L) was adsorbed within 15 min under experimental conditions used. The equilibrium adsorption data were analyzed by Freundlich, Langmuir and Sips adsorption isotherm models, and the fitting of each isotherm model to experimental data was assessed on the basis of error functions. The maximum adsorption capacities of magnetic activated sludge were 768.2, 246.9, 515.1 and 326.8 mg/g for aniline blue, Bismarck brown Y, Nile blue and safranin O, respectively. The kinetic studies indicated that adsorption of all selected dyes could be well described by the pseudo-second-order kinetic model, and the thermodynamic data suggested the spontaneous and endothermic process.     

Removal of dexamethasone from aqueous solutions using modified clinoptilolite zeolite (equilibrium and kinetic)

The aim of this study was to remove a known pharmaceutics, dexamethasone, from an aqueous solution using clinoptilolite zeolite (CP). CP is a natural, versatile and inexpensive mineral, which has been investigated and applied in the last few decades. Herein, the experiments were carried out in the common conditions of a batch system in room temperature, and the effects of some parameters such as pH of the solution, initial concentration of dexamethasone, adsorbent dose and contact time were studied. Kinetic and isotherm of adsorption processes of dexamethasone on CP were surveyed in the current study. Results revealed that the maximum efficiency (78 %) occurred in pH = 4. The adsorption process followed a pseudo-second-order kinetic model as well as Freundlich and Sips isotherm models fitted with the experimental data well.     

Evans blue removal from wastewater by rarasaponin–bentonite

The feasibility of natural bentonite and rarasaponin–bentonite for Evans blue removal from aqueous solution was studied. Rarasaponin is a natural surfactant obtained from Sapindus rarak DC was used as modifying agent for natural bentonite modification. Adsorption experiments were conducted in a batch system at various temperatures. Several temperature-dependent isotherm models (Langmuir, Freundlich, Sips and Toth) were used to represent equilibrium data. It was found that Toth model represents the adsorption equilibrium data better than other models. Kinetic data were best represented by the pseudo-first order model. The controlling mechanism of the adsorption of Evans blue onto natural bentonite and rarasaponin–bentonite was physical adsorption.     

Adsorption of brilliant green from aqueous solution by unmodified and chemically modified Tarap (<Emphasis Type="Italic">Artocarpus odoratissimus</Emphasis>) peel

Tarap peel (TP) and oxalic-acid-modified Tarap peel (TP-OX) were used to remove brilliant green (BG) dye from aqueous solution. Surface modification of TP suggested that functional groups such as carboxyl, hydroxyl and amino were involved in the adsorption of BG onto TP. Parameters such as effects of pH, contact time, ionic strength, initial dye concentration and temperature were included to study the adsorption of BG onto TP and TP-OX. Adsorption isotherm models were used to investigate the adsorption process, while kinetics models were used to provide insight into the adsorption mechanisms. Optimised contact time of 2 h with no pH adjustment was used. Adsorption of BG onto TP was best fitted to the Freundlich model, while experimental data for TP-OX are best described by the Tempkin model. The maximum adsorption capacities were determined as 174 and 275 mg g^?1 for TP and TP-OX, respectively. Thermodynamics study indicated the endothermic nature of adsorptions of BG onto both adsorbents. According to kinetics study, the adsorption mechanisms on both adsorbents followed pseudo-second-order model, and film diffusion might have major role in the adsorption process.     

<Emphasis Type="Italic">Vitis vinifera</Emphasis> leaf litter for biosorptive removal of nitrophenols

Vitis vinifera (grape) leaf litter, an abundant agricultural waste in South Africa was chemically modified with H₃PO₄ and carbonized for use as biosorbent. Characterization and the potential application of the adsorbent in simultaneous removal of 4-nitrophenol and 2-nitrophenol from aqueous solutions were investigated. The adsorbent was characterized using FTIR, SEM and EDX elemental microanalysis. The EDX and FTIR analysis revealed the presence of surface oxygen moieties capable of binding to adsorbate molecules while the SEM micrographs showed the development of pores and cavities in the adsorbent. Batch adsorption experiments were conducted at a varying contact time, adsorbent dosage, pH and initial adsorbate concentration to investigate optimal conditions. The maximum adsorption capacity of the adsorbent was 103.09 and 103.10 mg/g for 4-nitrophenol and 2-nitrophenol, respectively. The adsorption process was best fitted into Freundlich isotherm while the adsorption kinetics followed a pseudo-second-order model. Liquid film and intra-particle diffusion contributed to the adsorption process. Thermodynamic parameters of ΔG°, ΔH° and ΔS° were evaluated. The adsorption was exothermic, feasible and spontaneous. The results suggest a possible application of grape leaf litter as a precursor for activated carbon and for cheaper wastewater treatment technologies.     

Characterization and efficacy of a new generation scavenger of heavy metal pollutant: a green method of remediation of wastewater

The presence of heavy metals in diverse forms in the environment (which is having a direct and continuous interface with the health of living beings) demands an efficient removal technique which would be cost effective, eco-friendly, easily operable and free of secondary waste. Hence, the useless and discarded leaves of Cissus rotundifolia have been valorized by utilizing it for the removal of heavy metal pollutant after its modification. The developed adsorbent is then characterized through X-ray diffraction study, scanning electron microscope analysis, Fourier transform infrared spectroscopy and application of mathematical models. The adsorption process was found to follow Langmuir isotherm model and pseudo-first-order kinetics. The intraparticle diffusion was found to be the rate limiting step. Besides being usable in batch method, this adsorbent is also applicable to fixed bed column technique, due to its faster kinetics, high hydrophilicity, and easy recovery with mild mineral acid (like 0.002 mol per liter of hydrochloric acid).     

Adsorption of Fe~(3+) Ions from Aqueous Solution by Natural and Synthetic Apatites: Kinetic,Equilibrium and Thermodynamic Study

In this study, the adsorption kinetics, equilibrium and thermodynamics of Fe3+ ions on natural (NAP) and synthetic (HAP) apaties were examined. The adsorption efficiency of Fe3+ onto the NAP and HAP was increased with increasing temperature. The kinetics of adsorption of Fe3+ ions was discussed using three kinetic models, the pseudo-first-order, the pseudo-second-order and the intra-particle diffusion model. The experimental data fitted very well the pseudo-second-order kinetic model. The initial sorption rate and the activation energy were also calculated. The activation energy of the sorption was calculated as 37.15 and 49.84 kJ·mol 1 for NAP and HAP, respectively. Experimental results were also analysed by the Langmuir, Freundlich and Dubinin–Redushkevich (D–R) isotherm equations at different temperatures. RL separation factor for Langmuir and the n value for Freundlich isotherm show that Fe3+ ions are favorably adsorbed by NAP and HAP. Various thermodynamic parameters such as enthalpy (ΔH), Gibbs free energy (ΔG) and entropy (ΔS) changes were computed and the results showed that the adsorption of Fe3+ ions onto NAP and HAP were spontaneous and endothermic in nature.     

Removal of cadmium (II) from aqueous solution and natural water samples using polyurethane foam/organobentonite/iron oxide nanocomposite adsorbent

A novel polyurethane foam/organobentonite/iron oxide nanocomposite adsorbent was successfully prepared via in situ polymerization of toluene diisocyanate and polyol in presence of 5 wt% organobentonite/iron oxide. The obtained nanocomposite was characterized in detail, and the results revealed that the clay layers are exfoliated and/or intercalated in the polymer matrix forming a nanocomposite structure. The application of the prepared nanocomposite for adsorption of cadmium ions from aqueous solution was tested as a function of various experimental parameters using batch procedures. Adsorptive removal of Cd(II) onto the nanocomposite attained maximum at adsorbent content 1.5 g/L, pH 6, and the equilibrium was established within 60 min. Kinetic studies showed that the experimental data fit very well to pseudo-second-order model, and the adsorption process proceeds through three steps. It was found that external liquid film and intraparticle diffusion steps deeply affect the rate of Cd²⁺ ions adsorption onto the synthesized nanocomposite. Langmuir isotherm model fitted the adsorption data better than Freundlich with a maximum adsorption capacity (q _m) for Cd(II) equal to 78 mg/g under the specified experimental conditions. The synthesized nanocomposite afforded effective extraction for Cd²⁺ ions from natural water samples and excellent reusability feature. This study declares the potential efficiency of a new clay/polymer nanocomposite as alternative for wastewater remediation.     

Adsorption of diethyl and dibutyl phthalates onto activated carbon produced from <Emphasis Type="Italic">Albizia julibrissin pods:</Emphasis> kinetics and isotherms

The adsorption behavior study of diethyl and dibutyl phthalates was investigated onto a new activated carbon prepared from an abundant biomass “Albizia julibrissin pods,” treated chemically by H₃PO₄. A series of experiments were conducted in a batch system to estimate the effect of operating conditions such as the adsorbent nature, the dose of adsorbent, the contact time, the initial concentration and the temperature on the adsorption efficiency. The optimum operating conditions were found to be 0.1 and 0.05 g of adsorbent for diethyl and dibutyl phthalates, respectively, at 30 min equilibrium time, 150 mg g^?1 and 293 K. The adsorption isotherms for both phthalates were fit at different temperatures using the nonlinear regression of Langmuir, Freundlich, Dubinin–Radushkevich and Redlich–Peterson. The pseudo-first order, pseudo-second order by nonlinear regression and intraparticle diffusion models were used to describe the adsorption kinetic. The results show that the intraparticle diffusion model is not the limiting step governing the adsorption mechanism. The structural and textural characteristics of adsorbent surface were investigated. FTIR analysis of unloaded and phthalates-loaded adsorbent revealed that the aliphatic groups attached to phthalate esters are involved in adsorption mechanism.     

Dye removal from aqueous solutions using novel nanocomposite hydrogel derived from sodium montmorillonite nanoclay and modified starch

A novel nanocomposite hydrogels (NCHs)-based polysaccharide was synthesized by incorporating Na-montmorillonite (NaMMT) nanoparticles (NPs) into poly acrylic acid and polyacrylamide grafted onto starch [P(AA–AAm)-g-starch] hydrogel. The conditions applied in this synthesis were optimized using the Taguchi orthogonal experimental design method. The characterization of NaMMT NPs/P(AA–AAm)-g-starch NCH was performed by Fourier transform infrared, thermogravimetric analysis, scanning electron microscopy/energy-dispersive X-ray, and atomic force microscopy analyses. The resulting optimized NCH showed improved pH-dependent swelling and enhanced safranin adsorption capacity compared to pure hydrogel and NaMMT NPs. Its adsorption process could be described very well by pseudo-second-order and Elovich kinetic models. The adsorption isotherm was followed by Freundlich isotherm models. The conclusion confirms the prospect of NCH as an effective adsorbent for the adsorption of dyes from aqueous solutions.