adsroption of cationic dye onto low-cost …dl.uctm.edu/journal/node/j2017-3/9_gina_491-504.pdf ·...

Ajemba Regina Obiageli

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ADSROPTION OF CATIONIC DYE ONTO LOW-COST ADSORBENT SYNTHESIZED FROM BENTONITE CLAY

Part I. KINETIC AND THERMODYNAMIC STUDIES


Chemical Engineering Department Nnamdi Azikiwe UniversityP. M. B. 5025, Awka, Anambra, NigeriaE-mail: [email protected]

ABSTRACT

The adsorption of cationic dye, methylene blue, from an aqueous solution, was investigated using the low-cost acid activated adsorbent Ukpor clay. The latter was activated by aqueous solutions of hydrochloric acid of different concentrations, while the resulting physicochemical and structural properties were analyzed using XRF, SEM and XRD. The adsorption process was studied as a function of the adsorbent dosage, particle size, contact time, stir-ring speed and temperature. The adsorption kinetics follows the pseudo-second order kinetics with intra-particle diffusion as the rate-determining step. The thermodynamic data obtained revealed that the adsorption process is a spontaneous and endothermic one. The equilibrium modeling was carried out with the application of the Langmuir, Freundlich, Dubinin-Radushkevich, Temkin and Harkin-Jura isotherm equations. It was found that the Freundlich adsorption isotherm provides the best description of the experimental data. The generalization of all data obtained pints to the fact that the local clay mineral from Ukpor can be successfully modified and used as a good adsorbent in case of cationic dyes removal from contaminated water.

Keywords: methylene blue, cationic dyes, kinetics, thermodynamics, adsorption isotherms, low-cost adsorbents.

Received 01 July 2016Accepted 20 December 2016

Journal of Chemical Technology and Metallurgy, 52, 3, 2017, 491-504

INTRODUCTION

The pollution of water resources by industrial effluents containing organic compounds and toxic substances is a matter of great concern because of their non-biodegradable and polluting nature. Cationic dyes are widely used in acrylic, wool, nylon and silk dyeing. These dyes including different chemical structures based on substituted aromatic groups [1] are considered toxic colorants and can cause harmful effects such as allergic dermatitis, skin irritation, mutations and cancer [2]. They are also known as basic dyes depending on a positive ion, usually provided by hydrochloride or zinc chloride complexes [3]. The molecules of the cationic dyes carry a positive charge; they are water soluble yielding colored cations. Cationic functionality is found in various types of dyes, such as the cationic azo dyes and methane dyes. Basic dyes are highly visible and have high brilliance and intensity of colors [4].

Cationic dyes like crystal violet [5], methylene blue [6, 7], basic blue 41 [8] and basic red 46 [9] were requently used as models in dye adsorption studies. Methylene blue is an important basic dye and widely used in the textile industry. Acute exposure to methylene blue may cause increased heart rate, shock, vomiting, cyanosis, jaundice, quadriplegia, Heinz body formation and tissue necrosis in humans [10]. Many researchers have studied the adsorption of methylene blue dye using agricultural solid wastes such as peanut hull [11], castor seed shell [12], coconut shell [13], guava leaf [14], neem leaf [15] and gulmohar plant [16]. The dye adsorption capacities are good of values of 123.5 mg g-1, 158 mg g-1, 277.9 mg g-1, 295 mg g-1, 351 mg g-1, and 186.22 mg g-1 respectively. Natural clays are acquiring prominence as low-cost adsorbents over the last few decades due to their local and abundant availability and the capability to undergo a modification aiming to increase the surface area, the adsorption capacity and the range of

Journal of Chemical Technology and Metallurgy, 52, 3, 2017

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applicability [17 - 19]. The adsorption capacity of natural clay is hampered by its small surface area and this has led to investigations in the field of clay surfaces modification [20 - 25]. The removal of malachite green (MG) and fast green dyes is studied on montmorillonite clay adsorbent under optimized conditions. The percentage removal data show an adsorption tendency of ca 95 % and 97 %, respectively and an adsorption capacity of 1.42 mg/g and 33.45 mg/g, correspondingly [26]. The potential use of a low cost inorganic powder (persian kaolin) for removal of basic yellow 28 (BY28), methylene blue and MG from aqueous solutions is reported. The values of the adsorption capacity found range from 16 mg/g to 52 mg/g, being probably dependent on the geometry of the dye molecules [27, 28]. Methylene blue sorption has been extensively studied on perlite [29, 30], carbonised press mud [31], bagasse bottom ash [31], raw kaolin [32], pure kaolin [32], calcined raw kaoline [32], calcined pure kaoline [32], NaOH treated raw kaolin [32].

To protect the environment from contamination, the dye-stained wastewater from the industries must be treated prior to their release to the environment. Due to their resistance to photo-degradation, biodegradation and oxidizing agents [33], biological processes are not useful or efficient for dyes removal from the effluents. Most of the conventional methods of wastewater treatment such as coagulation and flocculation, sedimentation and flotation, membrane filtration, disinfection are either expensive or ineffective. These technologies very often result in phase transitions but not in elimination [34]. These inadequacies have compelled the scientists to search for novel effective and economical methods of dye removal.

Adsorption has been recognized as a potential technology for the removal of dyes from wastewater. In comparison to other physical, chemical and biological methods available for the treatment of textile industry effluents, adsorption is the most preferred technique due to the simple and flexible design applied as well as easy operation [35]. The common adsorbent, activated carbon, has good capacity of removing pollutants, but its main disadvantages refer to its high cost of treatment and difficult regeneration, which increases the wastewater treatment cost [36, 37]. Therefore, there is a necessity in finding low cost, locally available and renewable materials as sorbent for dyes removal.

This work is aimed at investigating the adsorption performance of hydrochloric acid activated bentonite

from Ukpor in removing the cationic dye, methylene blue (MB), from aqueous solutions. During this in-vestigation the effect of the initial MB concentration, solution pH and temperature, contact time, adsorbent dosage was studied. The kinetics of the process was analyzed using the well known kinetic equations, while the sorption equilibrium data was described by the Fre-undlich, Langmuir, Temkin, Dubinin-Radushkevich and Harkin-Jura isotherms.

EXPERIMENTAL

Preparation of the bentonite sorbentThe bentonite used in this study was mined from

Ukpor (N: 8°32’ 05”; E: 8°55’ 05”; A: 126 m) in An-ambra state, Nigeria. The mined clay was sun-dried for 48 hours and grinded to smaller particles using a mortar and a pestle. The ground samples were sieved to remove impurities and oven dried at 1050C. The samples were then treated with hydrochloric acid aqueous solutions of known concentrations (0.5 mol l-1, 1.5 mol l-1, 2.5 mol l-1, and 3.5 mol l-1) in a 250 cm3 flask placed in a regulated water bath. The flask was heated while con-tinuously being stirred. At the completion of the heating time, the slurry was removed from the bath and allowed to cool. After that the slurry was removed, filtered via a Buchner funnel and the clay residue was washed several times with distilled water, followed by filtration until the filtrate was neutral to pH indicator paper. The prepared wet sample was then dried in an oven at 1200C over night. The lumps of the prepared clay samples were crushed and sieved into the required particle sizes and stored for use in the adsorption studies envisaged. The raw and acid-activated Ukpor bentonite samples were labeled UBR, UB0.5, UB1.5, UB2.5, and UB3.5, where the number signified the acid concentration used during the activation process All samples were then analyzed by X-ray fluorescence (XRF), scanning electron mi-croscopy (SEM), and X-ray diffraction (XRD) aiming to determine the effect of modification on the surface properties of the clay.

Adsorption studiesThe adsorption of MB on acid-activated Ukpor

bentonite (UB2.5) was studied in a batch system. A measured quantity of the activated bentonite was added to 100 ml solution of the prepared dye solution in 250


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ml Erlenmeyer flask. The mixture was agitated in an incu-bated shaker at a pre-determined temperature and speed for the desired time. At the completion of the reaction period, the supernatant was separated by centrifugation at 3000 rpm for 15 min and the residual concentration in it was determined. The dye concentration in the raw and treated sample was determined by UV-Vis spectrophotometer (model WFJ 525). The analysis was carried out at a wave-length of 619 nm. The efficiency of MB removal by the acid-activated bentonite was calculated on the ground of:

𝑌𝑌(%) = 100 𝑐𝑐0−𝑐𝑐𝑖𝑖𝑐𝑐0

(1)

where c0 and ci are the initial and final concentrations of the dye solution.

The effect of the basic adsorption process parameters was investigated at as a function of time varying the initial dye concentration (50 mg/l - 0 mg/l), the solution pH (4 - 11), the adsorbent dosage (0.1 g/L - 1 g/L), and the temperature (303K - 343 K). The adsorbed amount at equilibrium, qe (mg/g), was calculated with the ap-plication of:

𝑞𝑞𝑒𝑒 =𝑉𝑉(𝑐𝑐0 − 𝑐𝑐𝑒𝑒)

𝑀𝑀 (2)where c0 and ce (mg/l) are the liquid-phase initial and equilibrium concentration of the dye, respectively, V (l) is the volume of the solution, while M (g) is the mass of the dry sorbent used.

Determination of pH point of zero charge (pHPZC)The pH point of zero surface charge of UB2.5 was

determined using the solid addition method [38]. 40 ml of 0.1 M NaCl solution were transferred to a series of 250 ml stoppered conical flasks. The pHi values of the solutions were adjusted between 2 and 11 by adding either 0.1 M HCl or NaOH and were measured using a Consort C931 pH meter (Belgium). The total volume of the solution in each flask was adjusted to exactly 50 cm3

by adding NaCl solution of the corresponding concentra-tion. The pHi of the solution was then accurately noted. 0.5 g of UB2.5 was added to each flask, and the flask was securely capped immediately. The suspensions were then kept shaking for 24 h and allowed to equilibrate for 0.5 h. The final pH values of the supernatant liquids were noted. The difference between the initial and final pH (pHf) values (ΔpH) was plotted versus pHi. The point of intersection of the resulting curve with the pHi axis, i.e. at ∆pH = 0, gave the pHPZC.

Kinetic and thermodynamic studiesThe contact time effect in case of MB was deter-

mined by treating the kinetic data in correspondence with the pseudo-first order, pseudo-second order, Elovich, Bangham, and intra-particle kinetic models. The adsorp-tion data referring to the relationship between the con-centration in the liquid phase and that in the adsorbent at a given temperature was analyzed on the ground of the Langmuir, Freundlich, Dubinin-Radushkevic, Temkin and Harkin-Jura isotherm models [39].

RESULTS AND DISCUSSION

Characterization of the clay adsorbents (UBR and UB2.5)

XRF AnalysisThe changes of the chemical composition of the clay

prior to and after activation are listed in Table 1. The acid treatment of the clay enhances its ion exchange ca-pability by forming Bronsted acid sites and exchanging the interlayer cations per proton with other catalytically active cations. Table 1 shows that the presence of octa-hedral cations such as Al3+, Fe3+, and Mg2+ is appreciably decreased, while that of the tetrahedral cation, like Si4+ is increased with the acid treatment. The octahedral sheet destruction provides cations release to the solu-tion, while the silica generated by the tetrahedral sheet remains in the solids, due to its insolubility. Pesquera et al [40] suggest that this silica obtained polymerizes in presence of a high acid concentrations and deposits on the undestroyed silicate fractions protecting them from further attack. The acid activation results not only in change of the octahedral and tetrahedral cations content. It also brings about a decrease in the cation exchange capacity and an increase of the surface area. The latter effect is attributed to the removal of impurities, replace-ment of exchangeable cations (Na+, Ca2+, K+) with Al3+, Mg2+, Fe3+, de-lamination of the clay and the generation of micro-porosity during the process, while the surface area decrease at higher acid concentrations is attributed to deeper penetration of the acid into the clay voids and excessive leaching of Al3+, Fe3+, and Mg2+.

Thermal analysisFig. 1 shows the differential thermal analysis (DTA)

profile of the Ukpor bentonite clay. The DTA of the clay was carried out to analyze the effect of heating on


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the surface and the structural properties of the clay. The results reveal that the dehydration stage corresponding to the removal of adsorbed and hydrated water can be seen at 106.8°C. This is the first endothermic peak. It is also seen that the maximum of the DTA curve occurs at 442.4°C. The removal of adsorbed and hydrated water provides additional adsorption sites, which results in surface area increase. The further heating is connected with a transi-tion from dehydration to de-hydroxylation stage. The second endothermic peak appears at 528.3°C, showing the maximum breaking point at 800°C. Heating beyond these temperatures results in collapsing of the interlayer spaces, and consequently in clay structure rupture, which causes morphological changes and surface area decrease.

X-ray diffraction analysis of the samplesThe XRD patterns of raw and acid activated ben-

tonite are shown in Figs. 2a and 2b, respectively. Mont-

morillonite (M) is the main mineral; however, minor amounts of quartz (Q), chyrstoballitte (C) and calcium carbonate are present and these results are in accordance with literature data [41 - 43]. The strong diffraction peaks at 2θ = 26.7° and 31.06° can be ascribed to the charac-teristic diffraction of quartz and chrystoballite impurities [44], respectively, which indicate the existence of quartz impurities. Also, the diffraction peaks at 2θ =21.6° and 36.4° reveal the presence of small amounts of calcites. The characteristic peaks of montmorillonite, calcium carbonate, and chrystoballite almost disappear upon HCl solution treatment indicating that these impurities are re-moved. The diffraction peaks at 2θ = 7.45° are assigned to the (110) characteristic peaks of sepiolites [45, 46].

Scanning electron microscopy (SEM) analysisThe raw and acid-activated bentonite samples were

examined under scanning electron microscopy (SEM) to investigate the morphological and surface characteristics as shown in Fig. 3a-d. The leaching of cations from the clay surface in the course of the acid activation creats voids on the bentonite thereby making the clay surface more porous. In case of treatment with 1.5 M acid so-lution (Fig. 3b) clumps of uneven surface can be seen along with some flat flakes of low porosity. As the acid concentration is increased to 2.5M (Fig. 3c), the clay surface becomes highly porous with even distribution of pores. Further increase in the acid concentration decreases the porosity rendering the clay surface rather flat (Fig. 3d).

pH point of the adsorbent zero chargeThe point of zero charge (pHPZC) of an adsorbent is

the pH value at which the total number of positive and

Chemical constituents (%)

Bentonite samples UBR UB0.5 UB1.5 UB2.5 UB3.5

Al2O3 26.9 20.4 13.9 5.7 8.8 SiO2 48.6 59.8 72.5 89.2 86.4 Fe2O3 17.13 13.1 8.2 0.4 0.6 MgO 0.74 0.58 0.39 0.06 0.02 Na2O 0.86 0.69 0.42 0.15 0.02 K2O 0.59 0.42 0.28 0.09 0.01 CaO 0.08 0.06 0.04 0.01 0.01 TiO2 2.06 1.87 1.21 0.75 0.15 LOI 3.05 2.76 1.76 0.92 0.52 Total 100 99.68 98.7 97.28 96.53 CEC (meq/100g) 58 51 44 28 26 Surface area (m2/g) 28.6 35.7 40.3 63.8 60.1

Table 1. XRF analysis of the raw and acid-activated Ukpor bentonite.

Fig. 1. Differential thermal analysis of the raw Ukpor bentonite.


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Fig. 2. X-ray diffraction images of (a) Raw Ukpor bentonite (b) 2.5mol activated Ukpor bentonite.

Fig. 3. SEM images of Ukpor bentonite (a) Raw (b) 1.5 M HCl (c) 2.5 M HCl (d) 3.5 M HCl.

(a)

(b)

(c)

(d)


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negative charges on its surface becomes zero. The pH at the point of adsorbent zero charge (pHPZC) was measured by using the pH drift method [47]. It can be seen from Fig 4 that the surface charge of the adsorbents UBR and UB2.5 is equal to 8.5 and 6.9, respectively, where the ∆pH values are zero. It has been reported [48] that the pHpzc of an adsorbent decreases with increase of the acidic groups on the surface. The results obtained in this study lead to the conclusion that the acid modification of the adsorbent provides a positive (acidic) surface charge as the pHpzc value of the modified surface is found lower than that of the unmodified one. The relationship between the pHpzc value and the adsorption capacity indi-cates [49] that cations adsorption is expected to increase at pH value higher than that of the pHpzc, while anions adsorption is favourable at pH values lower than that of the pHpzc. Thus, the determination of this parameter is of importance in assessing the sorption mechanism and the probable sorbate/sorbent interactions.

Effect of initial MB concentrationThe effect of initial MB concentration on the ad-

sorption process is studied by varying the values in the range from 50 mg/l to 250 mg/l. The study reveals that the adsorption capacity of the adsorbent increases, while the percentage dye removal decreases with an increase of the initial dye concentration (Fig. 5). It has been reported

[50] that the percentage dye removal decreases with an increase of the initial dye concentration, attributed most probably to the saturation of the surface adsorption sites. There will be unoccupied active sites on the surface at a low concentration, but they will start to decrease with the initial dye concentration increase [13]. On the other hand, the latter factor will cause an increase in the loading capacity of the adsorbent, probably due to the increased driving force for mass transfer [51, 52]. Garg et al. [53] studied the adsorption of MB on acid-treated sawdust and they found out that the adsorption capacity of the sawdust increased from 12.49 mg/g to 51.4 mg/g, while the percentage dye removal decreased from 99.9

Fig. 4. pH point of zero charge (pHPZC).

Fig. 5. (a) Amount of MB removed at different initial concentrations (b) Percentage dye removal at different MB concentration.

(a) (b)


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% to 82.2 % as the initial MB concentration increased from 50 mg/l to 250 mg/l.

Effect of solution pH valueThe solution pH value affects significantly the ad-

sorption process, especially in case of dye adsorption. It controls the magnitude of the electrostatic charges imparted by the ionized dye molecules. Thus the solu-tion pH increase results in increased number of hydroxyl groups, which in turn increases the number of negatively charged sites and hence facilitates the attraction between the dye and adsorbent surface. The effect of solution pH on the adsorption of MB on acid-activated Ukpor bentonite was studied in the pH range of 4 to 11. The results are shown in Fig. 6. It is seen that the adsorption capacity increases with pH value increase and decreases with this value decrease. The positive charge at the solution interface is less at a high pH value and the ad-sorbent surface appears negatively charged. This results in increased adsorption of MB due to the electrostatic attraction forces between the positively charged adsorb-ate and the negatively charged adsorbent. In contrast, the positive charge on the dye solution interface is high at a low pH value and the adsorbent surface appears posi-tively charged. This results in decreased MB adsorption. This treatment is in accord with the pHPZC value of the adsorbent. It is found equal to 6.9. As pH of the system decreases below this value, the number of the negatively charged adsorption sites decrease, while the positively charged one increase. This does not favor the cationic dyes adsorption because of electrostatic repulsion. In fact, the maximum adsorption capacity in respect to MB is found in this study at a pH value of 11. The explana-tion can be referred to findings [54], which suggest that

more negative charges are present in an alkaline environ-ment because of de-protonation of the bentonite surface hydroxyl sites ((Al-OH and Si- OH), i.e.:

- MOH + OH = - MO- + H2O (3)

Thus the number of the bentonite surface sites providing ionization increases, which in turn facilitates MB adsorption:

- MO- + BM+ = - M – O – BM (4)

Effect of adsorbent dosageThe experiments connected with adsorbent dosage

variation in the range from 2 g/l to 10 g/l were carried out under conditions described earlier. The percentage of MB removal by activated Ukpor bentonite at differ-ent adsorbent dosage is presented in Fig. 7. It is evident that the percentage of the adsorbed dye increases from 60.4 to 99.9 when the adsorbent concentration increases from 1 g/l to 10 g/l. This result can be attributed to the availability of a greater surface area and therefore, increased adsorption sites. It is also observed that the MB uptake decreases with the adsorbent concentration increase (the corresponding figure is not presented). This finding can be referred to the competition of MB ions for available adsorption sites and the agglomeration of the adsorbent particles.

Adsorption kinetic studiesThe effect of contact time on MB adsorption on acid-

activated Ukpor bentonite is studied and the results show that the adsorption increases with contact time increase. The experimental data is examined by pseudo-first-order,

Fig. 6. Effect of pH value on the amount of MB removed. Fig. 7. Percentage MB removal at different adsorbent dosage.


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pseudo-second-order, Elovich, Bangham, and intra-particle diffusion kinetic equations aiming to understand the dynamics and mechanism of the adsorption process.

A pseudo-first-order kinetic modelA simple pseudo-first order equation is used in the form:

𝑑𝑑𝑞𝑞𝑡𝑡𝑑𝑑𝑡𝑡 = 𝑘𝑘1 (𝑞𝑞𝑒𝑒 − 𝑞𝑞𝑡𝑡) (5)

where qe and qt refer to the amounts of MB adsorbed at equilibrium and at time t (min), respectively, while k1 is the corresponding rate constant [55, 56]. The linear form of the equation is given as:

log�𝑞𝑞𝑒𝑒 − 𝑞𝑞𝑡𝑡� = log𝑞𝑞𝑒𝑒 −𝑘𝑘1

2.303 𝑡𝑡 (6)

The values of k1 and qe are calculated from the slope and the intercept of the linear plot of log (qe − qt) versus t (the corresponding figure is not shown) and are given in Table 2.

A pseudo-second order kinetic modelThe corresponding pseudo-second order rate equa-

tion [57] is given as:

𝑡𝑡𝑞𝑞𝑡𝑡

=1

𝑘𝑘2𝑞𝑞𝑒𝑒2+𝑡𝑡𝑞𝑞𝑒𝑒

(7)

where k2 is the rate constant in g mg-1 min-1. The values of slope and the intercept of the plot of t/qt versus t (Fig. 8) are used to calculate those of of qe and k2 as presented in Table 2.

Elovich kinetic modelThe Elovich kinetic model is described by the fol-

lowing equation:

𝑞𝑞𝑡𝑡 = 1𝛽𝛽 ln𝛼𝛼𝛽𝛽+ 1

𝛽𝛽 ln 𝑡𝑡 (8)

where α is the initial adsorption rate (mg/g/min), while β is the desorption constant (g/mg). The slope and in-tercept of the plot of qt versus ln t are used to calculate the values of the constants α and β as shown in Table 2. The value of the regression coefficient is not high (R2 < 0.979), suggesting that the applicability of this model is not feasible.

Bangham’s equation Bangham’s model [58] verifies whether the pore

diffu sion is the only rate controlling step of the ad-sorption process studied. It is applied in the form:

log �𝑙𝑙𝑙𝑙𝑙𝑙�𝑐𝑐0

𝑐𝑐0 − 𝑞𝑞𝑡𝑡𝑚𝑚�� = log�

𝑘𝑘0𝑚𝑚2.303𝑉𝑉� + 𝜎𝜎 log 𝑡𝑡

(9)where co (mg/l) is the initial adsorbate con centration in the liquid phase, qt (mg/g) is the adsorbate concentration in the solid phase at time t (min), m (g/l ) is the adsorbent concentration, V (l) is the solution volume, while ko (l/g) and σ (σ < 1) are Bangham’s equation parameters. The values of the constants are estimated from those of the slope and the intercept of the plot of log [log (co / (co – qt m))] versus log t. They are presented in Table 2.

Intra-particle diffusion studyUsually adsorption follows three steps: (1) transport

of the adsorbate from the boundary film to the external surface of the adsorbent; (2) adsorption at a site on the surface; (3) intra-particle diffusion of the adsorbate mol-ecules to an adsorption site by a pore diffusion process. The slowest of the three steps controls the overall rate of the process. The possibility of intra-particle diffusion is explored by using the intra-particle diffusion mode [57, 59]. The intra-particle diffusion varies with the square root of time and is given as

Fig. 8. Pseudo-second order kinetic plot for MB adsorp-tion onto acid-activated Ukpor clay.

Fig. 9. Plot of Freundlich isotherm (log qe versus log Ce).


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𝑞𝑞𝑡𝑡 = 𝑘𝑘𝑖𝑖𝑑𝑑𝑡𝑡1/2 + 𝐶𝐶𝑖𝑖 (10)where qt is the amount adsorbed at time t, kid is the intra-particle diffusion rate constant (mg/g min1/2), while Ci is the intercept at stage i and is related to the thickness of the boundary layer. Large Ci values refer to great effects of the boundary layer on molecule diffusion. The intra-particle diffusion rate constant is determined from the slope of the linear plot of qt versus t1/2 (the corresponding figure is not shown) and value obtained is presented in Table 2. The intra-particle diffusion process is controlled by the dif-fusion of ions within the adsorbent. The commensurable and relatively high values of RB

2 and Ri2 for both studied

systems, calculated by the Bangham’s and the intra-par ticle diffusion models verify the significant role of the intra-particle diffusion as one of the probable rate controlling mechanisms during MB adsorption on bentonite.

The comparative consideration of all kinetic data obtained shows that the value of the regression coefficient

calculated from the plot referring to the pseudo-second order kinetic model is the highest. This leads to the conclusion that the kinetics of MB adsorption on acid-activated Ukpor bentonite is best described by this model.

Thermodynamic studiesAn adsorption isotherm is characterized by certain

constants, whose values refer to the surface properties and affinity of the adsorbent. They can be also used to compare the adsorptive capacities of the adsorbent in respect to different pollutants. MB adsorption on UB2.5 activated bentonite is studied with application of the Langmuir, Freundlich, Temkin, and Dubinin- Radushkevic models.

Langmuir isothermThe Langmuir isotherm model is based on the fact

that uptake of dye molecules occurs on an energetically homogeneous surface with no interaction between the adsorbed molecules and proceeds to a monolayer forma-

Kinetic models Parameters

Initial dye concentration

50mg/l 100mg/l 150mg/l 200mg/l 250mg/l

Pseudo-first-order

k1 (min-1) 0.0253 0.0184 0.0161 0.0161 0.0184

qe, cal (mg/g) 15.81 16.29 20.04 23.88 27.23

qe, exp (mg/g) 17.44 21.37 26.76 29.44 32.16

R12 0.973 0.950 0.965 0.944 0.932

Pseudo-second-

order

k2 (g/mg min) 4.79 x 10-4 6.09 x 10-4 4.35 x 10-4 5.59 x 10-4 9.64 x 10-4

qe, cal (mg/g) 17.54 21.28 28.57 31.25 31.25

qe, exp (mg/g) 17.44 21.37 26.76 29.44 32.16

R22 0.9994 0.9983 0.9988 0.9990 0.9990

Elovich

Β 0.287 0.248 0.188 0.162 0.148

α (mg/g min) 0.379 0.780 1.039 1.410 2.109

RE2 0.978 0.976 0.967 0.969 0.986

Bangham

k0 (mol/g) 5.81 x 10-4 3.04 x 10-4 2.64 x 10-4 1.96 x 10-4 2.11 x 10-4

σ 0.424 0.535 0.561 0.611 0.596

RB2 0.996 0.988 0.993 0.994 0.999

Intra-particle

Kp (mg/g

min0.5)

0.949

1.115 1.476 1.709 1.860

C -1.569 0.360 0.419 1.474 3.987

Ri2 0.969 0.996 0.999 0.997 0.991

Table 2. Parameters of the Pseudo-first-order, Pseudo-second-order, Elovich, Bangham, and Intra-particle kinetic models together with their regression coefficients.


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tion. It is often [60, 61] presented in the form:𝐶𝐶𝑒𝑒𝑞𝑞𝑒𝑒

= 1𝑞𝑞𝑚𝑚𝐾𝐾𝐿𝐿

+ 𝐶𝐶𝑒𝑒𝑞𝑞𝑚𝑚

(11)

where Ce (mg/L) is the equilibrium concentration (mg/L), qe (mg/g) is the amount of dye ion adsorbed, qm (mg/g) is the Langmuir constant referring to the ad-sorption capacity (mg/g), while KL is the corresponding equilibrium constant (L/g). The values of qm and KL are evaluated from the slope and the intercept of the linear plot of Ce/qe versus Ce (the figure is shown) and are given in Table 3.

A further analysis of the Langmuir equation can be made on the basis of a dimensionless equilibrium parameter, RL [62] also known as the separation factor. It is described by:

𝑅𝑅𝐿𝐿 =1

(1 + 𝐾𝐾𝐿𝐿𝐶𝐶0) (12)

The value of RL lies between 0 and 1 for favorable ad-sorption, while RL > 1 refers to unfavorable adsorption. The adsorption studied is irreversible when RL = 0. (RL = 1 rep-resents linear adsorption). RL values obtained in this study are in the range between 0.026 and 0.118, which indicates favorable adsorption of MB on activated Ukpor bentonite.

Freundlich isothermThe Freundlich isotherm assumes metal ions adsorp-

tion on a heterogeneous surface increasing infinitely with the adsorbate concentration increase. It is the most popular model for a single solute system based on the distribution of solute between the solid phase and aque-ous phase at equilibrium [63]. The Freundlich equation is usually presented in the form:

𝑞𝑞𝑒𝑒 = 𝐾𝐾𝑓𝑓𝐶𝐶𝑒𝑒1/𝑛𝑛 (13)where qe (mg/g) is the amount adsorbed, Ce (mg/l) is the equilibrium concentration of the adsorbate, while Kf and n are the Freundlich constants related to the adsorption capacity and adsorption intensity. Eq. (13) is used in its

linearized [64] form:

log𝑞𝑞𝑒𝑒 = log𝐾𝐾𝑓𝑓 +1𝑛𝑛 log𝐶𝐶𝑒𝑒

(14)The linear plot of log qe vs log Ce is presented in Fig. 9.

The values of Kf and n, estimated correspondingly from the intercept and the slope of the dependence obtained, are listed in Table 3. It is worth noting that the change of the adsorbed dye ion concentration is greater than that of the dye ion concentration in solution when 1/n >1.0.

Temkin isotherm model The Temkin isotherm model [65] has been devel-

oped on the concept of chemisorptions. It assumes that the heat of adsorbate molecules adsorption decreases linearly with adsorbent layer coverage due to adsorbate-ad sorbent interactions. The equation and its lin earized form are given as:

𝑞𝑞𝑒𝑒 =𝑅𝑅𝑅𝑅𝑏𝑏𝑅𝑅

ln(𝐾𝐾𝑅𝑅𝐶𝐶𝑒𝑒) (15)

𝑞𝑞𝑒𝑒 = 𝐵𝐵1 ln𝐾𝐾𝑅𝑅 + 𝐵𝐵1 ln𝐶𝐶𝑒𝑒 (16)where B1= RT/bT ( bT in mol kJ–1 is the Temkin isotherm constant), KT ( molg–1) is the equilib rium binding con-stant, T (K) is the temperature, while R (8.314×10‒3kJ mol–1 K–1) is the universal gas constant. The isotherm parameters, bT and KT, are calculated from the slope and the inter cept of the linear plot of qe versus ln Ce. Their values are listed in Table 3.

Dubinin-Radushkevich isothermThe Dubinin-Radushkevic (D-R) equation is widely

used to study adsorption on microporous materials based on the potential theory of adsorption. It assumes that the defining characteristic of a microporous material is its micropore volume rather than its surface area. The equation is used in the form:

ln𝑞𝑞𝑒𝑒 = ln𝑋𝑋𝑚𝑚 − 𝛽𝛽𝜀𝜀2 (17)

Langmuir isotherm Freundlich isotherm Temkin isotherm Dubinin-Radushkevich isotherm

qm

(mg/g)

KL

(L/mg) RL R2 Kf(mg/g) n R2

KT

(mol/g)

bT

(mol/kJ)

R2 Xm

(mg/g) β (10-9)

E

(kJ/mol) R2

38.46 0.149

0.026

to

0.118 0.970 8.110 2.625 0.997

4.436

5.872

0.924 8.530 2 -5 0.755

Table 3. Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich isotherm parameters for adsorption of MB on to activated Ukpor bentonite.


501

where β is the activity coefficient related to the mean adsorption energy, Xm is the maximum adsorption capac-ity, while ε is the Polanyi potential described by:

𝜀𝜀 = 𝑅𝑅𝑅𝑅 ln(1𝐶𝐶𝑒𝑒

) (18)

where R and T are the gas constant (kJ/mol/K) and the temperature (K), respectively.

The adsorption energy E is described by:

𝐸𝐸 = −1

(−2𝛽𝛽)0.5 (19)

If the value of E ranges between -1 kJ/mol and -8 kJ/mol, the adsorption process is physical, while if the value of E is within the range from -9 kJ/mol to -16 kJ/mol, the adsorption proceeding is chemical. The param-eters of the D-R equation are calculated from the slope and the intercept of the linear plot of ln qe versus ε2 and are given in Table 3. The value of E obtained is equal to -5.00 kJ/mol. This indicates that MB adsorption on activated Ukpor bentonite is a physical process.

The comparative consideration of all adsorption data obtained shows that the Freundlich isotherm model provides the best description of MB adsorption on acid-activated Ukpor bentonite.

Temperature effectThe temperature effect on MB ions adsorption in

case of using UB2.5 activated Ukpor bentonite as an adsorbent is studied. It is found that the adsorption capacity increases from 7.05 mg/g to 9.89 mg/g (Table 4) as the temperature increases from 303 K to 343 K at MB ion concentration of 100 mg/l, pH of 8.6 and an adsorbent dose of 10 g/l. This indicates that the adsorp-

tion reaction is endothermic. The adsorption capacity increase may be due to chemical interaction between the adsorbate and the adsorbent, to some new adsorption sites appearance or an increased rate of the intra-particle diffusion of MB ions in the pores of the adsorbent at higher temperatures [59]

The standard Gibb’s energy is described in this case by:∆𝐺𝐺° = −𝑅𝑅𝑅𝑅 ln𝐾𝐾𝑐𝑐 (20)

where Kc stands for the ability of the adsorbent to retain the adsorbate and the extent of the adsorbate movement within the solution [22]. The values of Kc can be deduced from the relationship:

𝐾𝐾𝑐𝑐 =𝑞𝑞𝑒𝑒𝐶𝐶𝑒𝑒

(21)

where qe is the amount adsorbed on solid phase at equi-librium, while Ce is the equilibrium concentration of the dye ion in the solution. Other thermodynamic parameters such as the change of the standard enthalpy (ΔH°) and that of the standard entropy (ΔS°) are determined using the Van’t Hoff’s equation [66],

ln𝐾𝐾𝑐𝑐 =∆𝑆𝑆𝑅𝑅 −

∆𝐻𝐻𝑅𝑅𝑅𝑅 (22)

The values of ΔH° and ΔS° are obtained from the slope and the intercept of the Van’t Hoff’s plot of ln kc versus 1/T (the corresponding figure is not shown). The values obtained are summarized in Table 5. The positive value of ΔH° indicates that the adsorption process is en-dothermic. The value of ΔS° is also positive. It refers to increased randomness at the solid/liquid interface during MB ions adsorption on activated bentonite.

Table 4. Effect of temperature on the sorption of MB on to activated Ukpor bentonite.

Temperature, (K)

Liquid-phase conc. of MB,

Ce(mg L-1)

Amount adsorbed, q (mg g-1)

MB Removed,

(%)

303 29.5 7.05 70.48

313 18.8 8.12 81.19

323 7.40 9.26 92.63

333 2.30 9.77 97.68

343 1.10 9.89 98.89

Table 5. Thermodynamic parameters of adsorption of MB on to UB2.5 activated bentonite.

Temperature,

K

∆G°,

kJ/mol

∆Η°,

kJ/mol

∆S°,

J/mol K

303 -1.688 82.242 277.7

313 -4.459

323 -7.229

333 -9.999

343 -12.769


502

CONCLUSIONS

Chemical modification of Ukpor bentonite was suc-cessfully carried out in the course of the investigation just described. It was found that the surface area, the cation exchange capacity and the sorption performance increased by the treatment applied. The modified clay showed an adequate adsorption in respect to methylene blue dye from an aqueous solution, which could be at-tributed to its porous structure and the presence of vari-ous functional groups on its surface. The study of the effect of the process variables showed that the adsorption process was highly dependent on the initial methylene blue concentration, the temperature, the solution pH and the adsorbent dosage values. The investigation of the adsorption kinetics revealed that it was best described by the pseudo-second order rate equation, while the thermo-dynamic study suggested that the adsorption of methylene blue on modified Ukpor bentonite was a spontaneous endo-thermic process best described by the Freundlich isotherm.

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