The need to design low-cost adsorbents for the detoxification of industrial effluents has been a growing concern for most environmental researchers. So modelling of experimental data from adsorption processes is a very important means of predicting the mechanisms of various adsorption systems. Therefore, this paper presents an overall review of the applications of adsorption isotherms, the use of linear regression analysis, nonlinear regression analysis, and error functions for optimum adsorption data analysis.

https://downloads.hindawi.com/journals/jchem/2017/3039817.pdf

Modelling and Interpretation of Adsorption Isotherms

Über die Adsorption in Lösungen

Guidelines for the use and interpretation of adsorption isotherm models: A review.

Adsorption isotherm models: Classification, physical meaning, application and solving method

Recent advancements in bioremediation of dye: Current status and challenges.

The adsorption properties of layered double hydroxide (Mg/Al-CO3) for the removal of Congo Red (CR) dye from aqueous solution were studied. The layered double hydroxide was synthesized by co-precipitation method and characterized by X-ray diffraction (XRD), Fourier Transform Infrared spectroscopic (FTIR) and Energy-Dispersive X-ray Spectroscopic (EDX). The effects of various experimental parameters such as contact time, dye concentrations and temperature variation were investigated. The results show that the amount of Congo Red adsorbed increases with increase in temperature but decreases with increase in initial dye concentration and contact time. The data were also fitted to several kinetic models: zero-order kinetic model, first-order kinetic model, second-order kinetic model, pseudo-second-order kinetic model and third-order kinetic model respectively. The adsorption process was best defined by zero-order-kinetic model (R2 = 1). Langmuir, Freundich, Temkin and Dubinin-kaganer-Radushkevich (DPK) adsorption isotherm models were applied to analyze adsorption data with Temkin isotherm being the most applicable to the adsorption process. Thermodynamic parameters e.g. △Go, △So, △Ho and △Hx of the adsorption process were found to be endothermic, spontaneous and feasible.

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Synthesis, Characterization and Application of Mg/Al Layered Double Hydroxide for the Degradation of Congo Red in Aqueous Solution

Experimental investigations were carried out using Ni/Al-CO3 layered double hydroxide as adsorbent for removal of toxic anionic dye namely Congo red from aqueous solutions. The effect of contact time, initial dye concentration and temperature were experimentally studied in batch mode to evaluate the kinetic, equilibrium and thermodynamic parameters of the adsorption process. Experimental results revealed that the degradation of the dye is mostly dependent on temperature. The dye degradation process obeyed the zero-order kinetic model, first-order kinetic model, second-order kinetic model, pseudo second order kinetic and third order kinetic model with correlation coefficient values 1, 0.9998, 0.9999, 0.9999 and 0.9997 respectively. Langmuir, Freundlich, Temkin and Dubinin-Kaganer-Radushkevic isotherms were applied to the equilibrium data and was well described by all. Thermodynamic studies showed congo red adsorption on the layered double hydroxide was endothermic and spontaneous in nature. The results indicate that layered double hydroxide could be employed as alternative for removal of anionic dyes from industrial wastewater. Keyword: Congo Red, Layered double hydroxides, Kinetic, Dye, Adsorption, isotherms, thermodynamics. decolorized once released into the aquatic environment2. Dyes can also cause allergic dermatitis and skin irritation. Some of them have been reported to be carcinogenic and mutagenic for aquatic organisms3. Therefore, it is very important to develop new systems that can be used for removing dyes from waters. 1308 AYAWEI et al., Orient. J. Chem., Vol. 31(3), 1307-1318 (2015) The treatment of dyes in industrial wastewater possesses several problems since dyes are generally difficult to biodegradate and photodegradate. Many different techniques including cloud point extraction, oxidation processes, nanofiltration, ozonation and coagulation have been used for the removal of colored dyes from wastewater4-7. However, adsorption is the most popular physicochemical treatment for the removal of dissolved organics from waters. Layered double hydroxides (LDH) or anionic clays are lamellar ionic compounds, containing a positively charged layer and exchangeable anions in the interlayer8. They consist of brucite-like layers, with a partial MII for MIII substitution, leading to an excess of positive charge compensated with anions situated in the interlayer together with water molecules. These materials can be represented by the general formula [ 1− ( )2 + / − . 2 −, abbreviated as [ − , where = 2+, 2+, 2+ ... = 3+, 3+, 3+ ... − = 23−, −, 3− .... The flexibility of the structure of these materials as well as their high anionic exchange capacity make them suitable for many applications9,10, such as the sorption of many inorganic11-14 and organic15-20 anions, potential contaminants of waters. Some authors have reported about LDH containing chelating agents such as etilendiaminetetraacetate, edta21 and nitrilotriacetate, NTA,22 as well as the metal cations uptake by these materials23,24,25. Materials and Methods Synthesis of Ni/Al -CO3 Carbonate form of Ni/Al LDH was synthesized by co-precipitation method. A 50 ml aqueous solution containing 0.3 M Ni (NO3)2.6H2O and 0.1 M Al (NO3)3.9H2O with Ni/Al ratios 2:1, was added drop wise into a 50 ml mixed solution of (NaOH (2M) + Na2CO3 (1M) with vigorous stirring and maintaining a pH of greater than 10 at room temperature. After complete addition which last between 2 hours 30 minutes to 3 hours, the slurry formed was aged at 60°C for 18 hours. The products were centrifuged at 5000 rpm for 5 minutes, with distilled water 3-4 times and dried by freeze drying. Characterization of Layered Double Hydroxide X-ray diffraction (XRD) pattern of the sample was characterized by using a Shimadzu XRD-6000 diffractometer, with Ni-filtered Cu-Kα radiation (λ= 1.54 A°) at 40 kV and 200 mA. Solid samples were mounted on alumina sample holder and basal spacing (d-spacing) was determined via powder technique. Samples scan were carried out at 10-60°, 2θ / min at 0.003° steps. FTIR spectrum was obtained using a Perkin Elmer 1725X spectrometer where samples will be were finely ground and mixed with KBr and pressed into a disc. Spectrums of samples were scanned at 2 cm-1 resolution between 400 and 4000cm-1. FESEM/EDX was obtained using Carl Zeiss SMT supra 40 VPFESEM Germany and inca penta FET x 3 EDX, Oxford. It was operated at extra high tension (HT) at 5.0 kV and magnification at 20000X. FESEM uses electron to produce images (morphology) of samples and was attached with EDX for qualitative elemental analysis. Preparation of Congo Red Solution Congo red (CI=22120) was supplied by Merck (Mumbai, India). A stock solution of CR dye was prepare (100mg/L) by dissolving a required amount of dye powder in deionized water. The stock solution was diluted with deionized water to obtain the desire concentration ranging from 20 to 40mg/ L. The concentration of CR in the experimental solution was determined from the calibration curve prepared by measuring the absorbance of different known concentrations of CR solutions at max =497 nm using a UV-vis spectrophotometer (Shimadzu, Kyoto, Japan). The pH meter using a combined glass electrode (model HI 9025C, Hanna instruments, Singapore). 1309 AYAWEI et al., Orient. J. Chem., Vol. 31(3), 1307-1318 (2015) Experimental Procedure Batch adsorption experiments were carried out to study the effect of initial congo red concentration, contact time and temperature on the adsorption of Congo red on the layered double hydroxide. Adsorption studies were carried out using 25 ml of each dye solution and 0.2g of the adsorbent. At the end of each experiment, the content of each tube was filtered using a Whatman No 14 filter paper after which the concentration of residual congo red was determined by UV-Vis spectrophotometer analysis. All experiments were carefully conducted to acquire good result. In order to determine the rate of adsorption, experiments were conducted with different initial concentrations of dyes ranging from 20 to 40 mg/L. All other factors are kept constant. The effect of period of contact on the removal of the dye on adsorbent in a single cycle was determined by time intervals of 10, 20 and 30 minutes. The adsorption experiments were performed at three different temperatures viz., 40, 60 and 80ÚC in a thermostat attached with a shaker (Remi make). The constancy of the temperature was maintained with an accuracy of ± 0.5o C. The equilibrium adsorption capacity and the yield of adsorption were calculated respectively by equations 1 and 2 below: = − ...(1) % = − 100 ...(2( where Cinit and Ceql are, respectively, the initial and equilibrium concentrations of metal ions in solution (mmol/l) and m is the layered double hydroxide dosage (g/l). Isotherms analysis To determine the equilibrium parameters of the adsorption process four isotherms namely, Freundlich, Langmuir, Temkin and Dubinin– Kaganer–Radushkevich (DKR) were applied to test the experimental data. For the Freundlich isotherm the In-In version was used: = + 1 The Langmuir model linearization (a plot of 1/qeql vs 1/Ceql) was expected to give a straight line with intercept of 1/qmax: 1 = 1 + 1 The essential characteristics of the Langmuir isotherm were expressed in terms of a dimensionless separation factor or equilibrium parameter Sf. = 1 1 + With Co as initial concentration of Congo Red in solution, the magnitude of the parameter Sf provides a measure of the type of adsorption isotherm. If Sf> 1.0, the isotherm is unfavourable; Sf = 1.0 (linear); 0< Sf < 1.0(favourable) and Sf=0 (irreversible). The DKR isotherm is reported to be more general than the Langmuir and Freundlich isotherms. It helps to determine the apparent energy of adsorption. The characteristic porosity of adsorbent toward the adsorbate and does not assume a homogenous surface or constant sorption potential26. The Dubinin–Kaganer–Radushkevich (DKR) model has the linear form = − β ...(6) where Xm is the maximum sorption 1310 AYAWEI et al., Orient. J. Chem., Vol. 31(3), 1307-1318 (2015) capacity, β is the activity coefficient related to mean sorption energy, and å is the Polanyi potential, which is equal to ε = (1 + 1 ) ....(7) where R is the gas constant (kJ/kmol) . The slope of the plot of ln e q versus ε 2 gives β (mol2/J2 ) and the intercept yields the sorption capacity, Xm (mg/g). The values of â and Xm, as a function of temperature are listed in table 1 with their corresponding value of the correlation coefficient, R2. It can be observed that the values of â increase as temperature increases while the values of Xm decrease with increasing temperature. The values of the adsorption energy, E, was obtained from the relationship [27]

Adsorption of Congo Red by Ni/Al-CO3: Equilibrium, Thermodynamic and Kinetic Studies

A complete study of adsorption processes will be less complete if the
structure and dynamics of its different elements and how they interact is not
well captured. Therefore, the extensive study of adsorption thermodynamics in
conjunction with adsorption kinetics is inevitable. Measurable thermodynamic properties such as temperature equilibrium
constant and their non-measurable counterparts such as Gibbs free energy
change, enthalpy, entropy etc. are very important design variables usually
deployed for the evaluation and prediction of the mechanism of adsorption
processes.

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Interpretation of Adsorption Thermodynamics and Kinetics

Silica was successfully functionalized with generation-3 and 5 polyamidoamine (PAMAM) dendrimers via (3-aminopropyl)triethoxysilane (APTES) linkages to obtain the G3-PAMAM and G5-PAMAM functionalized silica adsorbents designated SG3 and SG5, respectively. The SG3 and SG5 were characterized and explored for Pb(II) removal from aqueous solution. The characterization highlighted successful PAMAM silica synthesis. Preliminary Pb(II) adsorption experiments showed that the SG3 had adsorption efficiency of over 93% compared to pristine silica, silica modified with PAMAM in the absence of the APTES linkage and silica modified with APTES only, each exhibiting ≤1, ≤1 and ≈16% efficiency, respectively. The Pb(II) adsorption equilibria for both adsorbents was 90;min. with optimum adsorption pH of 6 and an experimental adsorption capacities of 121.3 and 132.8;mg/g, respectively. The adsorption process was spontaneous, feasible and endothermic. The adsorption isotherm models suggested Pb(II) ions removal mechanism involving monolayer electrostatic interactions with similar affinity occurring majorly on the SG3 and SG5 surfaces (≈90%) and within the pores (≈10%). The SG3 and SG5 adsorbents exhibited very good reusability even after three adsorption cycles maintaining ≈80% of its adsorption capacity. These fact points to the potential of the SG3 and SG5 adsorbents for removal of Pb(II) from real wastewater.

N. Ayawei

Papers

Modelling and Interpretation of Adsorption Isotherms

Synthesis, Characterization and Application of Mg/Al Layered Double Hydroxide for the Degradation of Congo Red in Aqueous Solution

Adsorption of Congo Red by Ni/Al-CO3: Equilibrium, Thermodynamic and Kinetic Studies

Interpretation of Adsorption Thermodynamics and Kinetics

Covalently bonded polyamidoamine functionalized silica used as a Pb(II) scavenger from aqueous solution