Adsorption of the Prototype Anionic Anthraquinone, Acid Blue 25, on a

Article pubs.acs.org/IECR

Adsorption of the Prototype Anionic Anthraquinone, Acid Blue 25, on a Modified Banana Peel: Comparison with Equilibrium and Kinetic Ligand−Receptor Biochemical Data Maria Giovanna Guiso,‡ Raffaela Biesuz,‡ Teresa Vilariño,† Marta López-García,† Pilar Rodríguez Barro,*,† and Manuel E. Sastre de Vicente† †

Departamento de Química Física e Enseñería Química I, Universidade da Coruña, Rúa da Fraga 10, 15008 A Coruña, Spain Dipartimento di Chimica, Università di Pavia, via Taramelli 12, 27100 Pavia, Italy

‡

S Supporting Information *

ABSTRACT: The adsorptive behavior of dye Acid Blue 25 (AB25) on the banana peel is studied with two objectives in view. First, from an environmental point of view, AB25 is considered a model of the anionic dyes, and the banana peel is a quite abundant agricultural waste which can be reused as adsorbent. Second, and on account of the recent research on possible applications of 1-aminoanthraquinone derivatives in pharmacological research, physicochemical studies on the interaction of AB25 anionic prototype and related dyes with different kinds of biomass surfaces can be useful in the basic modeling studies on the antagonist-P2 receptor interactions carried out by different researchers with 1-aminoanthraquinone dyes. A careful analysis of the acid−base properties of the biomass provides the number of weak acid groups, that was found to be 0.288(7) mmol g−1 for modified banana peel in 0.1 M KNO3. An uptake capacity value of 0.215(13) mmol g−1 is obtained when data from batch experiments are fitted to sorption isotherms. Specific surface is calculated and compared with other biosurfaces. Kinetics of the process allows calculating an intraparticle diffusion coefficient, Di, of 0.331(1) × 10−13 m2 s−1. Desorption and column experiments demonstrate the feasibility for an application for AB25 recovery in remediation. Finally, a comparison with thermodynamic and kinetic data from receptor−ligand studies is also carried out.

1. INTRODUCTION The anthraquinonic dyes represent the second most important class of commercial dyes, after azo compounds, and they are mainly used in dying wool, polyamide, and leather.1 Textbooks on color chemistry1,2 describe Acid Blue 25 (AB25)(1amino-9,10-dihydro-9,10-dioxo-4-(phenylamino) anthracenesulfonic acid monosodium salt (Figure.1), also called Acilan Direct Blue Aas the prototype of anionic anthraquinone dyes. That is the reason why AB25 often serves as a model compound for adsorption studies on different materials to remove anionic dyes from aqueous solutions (see Table S1 in the Supporting Information). Nevertheless, complete studies including not only isotherms and kinetic data but also dynamic studies in column cannot be practically found. In addition to its use as dyestuffs, AB25 has been identified as a potential antagonist for P2 receptors, of interest in pharmacology. The P2 receptors are activated by nucleotides as ATP, ADP, and so forth, and their actions are involved in diverse pathologies of nervous central system, such as Alzheimer’s, Parkinson’s, and other neurodegenerative diseases. The majority of antagonists of P2 receptors known to date are anionic molecules, and all of them share a common structure containing one or several phenylsulfonate moieties, as in AB25, whose interaction with P2 receptors is considered essential for their pharmacologic activity.3,4 Beyond this, the anthraquinonelike compounds have been found to represent a new class of potential antiviral agents.5 Therefore, prior knowledge of the physicochemical properties of AB25 should of interest for any pharmacologic study with this compound. © 2014 American Chemical Society

Figure 1. Chemical structure of Acid Blue 25 dye.

Bananas and apples are the most consumed fruits in the European market.6 The banana peel, a cellulose-based material Received: Revised: Accepted: Published: 2251

September 5, 2013 December 19, 2013 January 22, 2014 January 22, 2014 dx.doi.org/10.1021/ie402900n | Ind. Eng. Chem. Res. 2014, 53, 2251−2260

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(11.1% cellulose and 5.36% hemicellulose7) is then an important alimentary industry waste, and there exists a clear interest in possible reuses of it. In that sense, the application of agricultural solid wastes as adsorbents for removing cationic and anionic dyes has been recently reviewed comprehensively.8,9 Most studies on the binding capacity of banana peel have been oriented toward the adsorption of cationic species, either dyes, such as methyl orange, methylene blue, Rhodamine B, Congo red, methyl violet, and Amido Black 10B,10 or heavy metals.11−16 Only the adsorption of the acid dye Violet 5417 and some phenolic compounds (anionic species at high pH)18 has been investigated on banana peel. In addition, to authors’ knowledge, none of these studies include any adsorption experiments on columns, and only one work on preconcentration of copper and lead on banana peel12 has employed a column. On the one hand, the study of the adsorptive behavior of AB25 on banana peel is considered of interest from an environmental point of view, taking into account the relevance of agricultural waste of banana fruit and its possible reuse as adsorbent and that AB25 is considered a model of anionic dyes. In order to evaluate this process, a careful analysis of its acid− base behavior together with equilibrium and kinetic studies in batch has been carried out. Finally, dynamic tests on column have also been performed to assess the industrial application of the banana peel in treatments of wastewaters of dyes. On the other hand, and on account of the new area of research on possible applications of 1-aminoanthraquinone derivatives in pharmacology, physicochemical studies on the interaction of AB25, as a prototype of anionic anthraquinonic dyes, with different kind of biomass surfaces can be useful to basic modeling of the interactions antagonist−P2 receptors.

titrations. NaOH [Panreac Quımica S.A., Barcelona, Spain] and HNO 3 Suprapur [Merck KGaA, Darmstadt, Germany] solutions were used for pH adjustments. 2.3. Acid−Base Titrations. Around 0.5 g of treated banana peel was placed in a thermostatted titration cell at 25.0 ± 0.1 °C containing 100 mL of 0.1 mol L−1 KNO3 solution to keep ionic strength constant. Inert gas (nitrogen, 99.9995%) was bubbled into the solution to remove the dissolved O2 and CO2 during the titration experiment. The stirred suspension was allowed to equilibrate until the electromotive force (emf) reached a constant value. The titrating solution (0.05 mol L−1 KOH, prepared with boiled deionized water and standardized with potassium hydrogen phthalate) of constant ionic strength was added from a Crison microBu 2031 automatic buret. It is worth noting that the ionic strength was kept constant along all experiments. In this way, the total ionic strength did not change and, as a result, the liquid junction (Ej) and the activity coefficient of the proton (γH+) remained constant. Emf measurements were done by a Crison micropH 2000 m equipped with a GK2401C Radiometer combination glass electrode (saturated Ag/AgCl as reference). After each addition of titrant, the system was allowed to equilibrate until the ΔE/ Δt < 0.3 was obtained (measured for a delay time of 10 min). A complete titration typically took 6−7 h. The calibration of the glass electrode is described elsewhere.20 2.4. Sorption Experiments. For the equilibrium studies, different dye solutions were prepared from a stock solution (1000 mg L−1). Dye concentrations in the range of 0.1−2.4 mmol were studied. The experiments were performed in 100 mL conical flasks containing 0.1 g of banana peel and 40 mL of AB 25 solution at pH around 2. The mixture was shaken in a rotary shaker at 175 rpm at room temperature overnight until the equilibrium was reached. The biomass was then separated by centrifugation and the dye concentration analyzed using a spectrophotometer, UV/vis (Zuzi 4210/20) at λmax = 602 nm. The amount of sorbed dye was determined by difference from the initial concentration. The isotherms were obtained by plotting q (mmol g−1) vs Ceq (mmol L−1), where q represents mmol of adsorbed dye per gram of banana peel and Ceq is the dye equilibrium concentration in solution after biosorption. 2.5. Effect of pH. Solutions (40 mL) with different dye concentration were placed into 100 mL conical flasks. A constant mass of banana peel (0.1 g) was added to each solution and the pH, measured after equilibration at 25.0(0.1) °C, was varied in each conical flask within the range 2.0 to 8.0. For the adjustment media of pH, 0.1 M NaOH and HNO3 solutions were used. The sample was then gently stirred overnight in a rotator shaker in an incubator. The sorption profiles were obtained by plotting q (mmol g−1) vs pH. 2.6. Kinetics. A constant mass of banana peel (0.25 g) was weighed and placed into a thermostatted cell at 25.0(0.1) °C and 100 mL of dye solution were added to it. The solution pH was adjusted until a value of 2.0 was reached and the samples were then shaken at 175 rpm. Aliquots were withdrawn at various time intervals for at least 5 h. The procedure was carried out for different dye concentrations (5 × 10−2, 0.1, 0.2, and 0.55 mmol L−1). 2.7. Desorption Studies. NaOH was used as desorbing agent. The banana peel was first loaded with Acid Blue 25 in experiments conducted at constant pH (2.1) and initial dye concentration about 0.3 mmol L−1. After the adsorption process, the biomass was filtered, rinsed with deionized water, and dried in an oven at 60 °C overnight. This biomass was then

2. EXPERIMENTAL SECTION 2.1. Sorbent Preparation. The material has been pretreated with HCl because preliminary studies concerning the adsorption of metals (data not reported here) demonstrated higher sorption capacity of the modified sample with respect to the untreated one. Banana peel was rinsed with generous amounts of distilled water and dried in oven at 60 °C overnight, grounded in an analytical mill (IKA A 10), and sieved to obtain particles of a known size distribution (between 0.5 and 1 mm). This biomass was stored in polyethylene bottles until use. Thereafter, the biomass was chemically modified by pretreatment with HCl. Banana peel (about 30 g) was treated with 400 mL of 0.1 M HCl solution for 3 h at room temperature. The biomass was then filtered off and rinsed thoroughly with deionized water. The procedure was repeated three times and until a value of 10 μS of conductivity for the washing water was attained. Finally, the resulting sample was dried overnight at 60 °C. 2.2. Dye Solution Preparation. The dye used in this study is Acid Blue 25 [Sigma-Aldrich; CAS 6408-78-2]. Its chemical formula, with an anthraquinone-like structure, is shown in Figure 1. Stock solutions of AB25 were prepared by solving accurately weighed dye (without further purification) in deionized water. Working solutions were prepared by dilution and their concentration was determined using the properties of the pure standard at λ = 600 nm, where ε was found to be 12 400 M−1 cm−1.19 All reagents were of analytical grade. KNO3 [Sigma-Aldrich] was employed as a background medium during acid−base 2252

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placed in 100 mL conical flasks, it was put into contact with 40 mL of NaOH, and the samples were shaken at 175 rpm. Aliquots of solution were withdrawn at different contact times, and the dye concentration determined spectrophotometrically. Three different concentrations of NaOH were tested (10−3, 10−2, and 0.1 mol L−1). 2.8. Column Studies. The experiments were carried out using a column of 23 cm length and 1 cm internal diameter, filled with about 4 g of banana peel pretreated with HCl as described above. Glass beads (diameter: 1 mm) were introduced at the bottom and at the top of the column in order to ensure a uniform inlet flow, a good liquid distribution along the column, and a closely packed arrangement and to avoid loss of biomass (Figure 2).

sampling has been repeated at least twice, both on the raw material and on different portions of banana peel previously loaded with dye.

3. RESULTS AND DISCUSSION 3.1. Acid−Base Properties. It is possible to know the concentration of the acid functional groups (Qmax,H, mmol g−1) of any solid sorbent involved in an ionic exchange process from potentiometric titrations. The number of functional sites that constitute the polyelectrolyte material involved in the ionic exchange (Qmax,H) is calculated using the following equation: Q max ,H =

VeqC b ms

(1)

in which Veq (mL) is the volume of titrant (KOH) at the equivalence point (that is generally considered as the maximum of the first derivative of the titration curve), Cb is the concentration of titrant base (mmol/mL), and ms is the mass (g) of biomaterial. The total number of weak acid groups found for banana peel was 0.288(7) mmol g−1 in 0.1 M KNO3. The standard deviation calculated from the four replicates is very low despite the intrinsic variability of the different from material. The titration curve of a polyacid can be empirically described by two constants, pK and n′, according to the following equation proposed by Katchalsky:21 pH = pK − n′log

1−α α

(2)

where α represents the degree of ionization that is defined in eq 3 and n′ is an empirical parameter α=

Figure 2. Schematic diagram of experimental setup: (1) Reservoir tank with AB25 solution, (2) peristaltic pump, (3) porous sheet, (4) glass beads, (5) column, (6) glass beads, and (7) effluent storage.

[A−] (C0·V0)/(V0 + Vb)

(3)

where [A−] is the concentration of deionized acid groups, C0 is the initial concentration of carboxylic acid, V0 is the initial volume, and Vb is the volume of KOH added during titration. Linear fit of pH against log(1 − α)/α resulted in a value of n equal to 1.47(8), and pKα=0.5 is 4.3(1). The pK value corresponds to the expected one on the basis of the acid main composition of the banana peel, where palmitic (pKa = 4.78) and myristic (pKa= 4.9) acids are the most abundant fatty acids, as reported by Happi Emaga et al.22 Taking into account the charge balance in the acid−base titration, it is possible to calculate the total amount of proton bound (QH)20

A flow rate of 1.5 mL min−1 was guaranteed by a peristaltic pump, connected at the bottom of the column. The pH of the output solution was measured regularly. The elution’s fractions of the column were collected at the top of the column and then dye concentration was determined spectrophotometrically. 2.9. SEM EDS Measurements. The measurements were performed in the Department of Chemistry of the University of Pavia (Hydrogen Lab). The analysis with SEM was performed by placing the sample on a bi-adhesive carbon slide fixed on the aluminum sample holder. Au sputtering has been employed to have high resolution images. The SEM analysis was performed using an EvoMA10 microscope from Zeiss. The experiments were carried out using a LaB6 filament, and the tension of the beam and sample current were set to 20 kV and 50 pA, respectively. The composition of the samples was determined by energy dispersion microanalysis (EDS). Gold coating for EDS preparations was avoided in order to allow the quantitative determination of the elementary composition of the samples. Microanalysis has been performed on different areas of the sample (outer and inner surface and also in the cross-section of the pieces of banana peel). EDS was performed using an INCA Energy 350 X Max detector from Oxford Instruments equipped with a Be window. A cobalt standard was used for the calibration of the quantitative elementary analysis. The

Q H = Q max ,H −

V0 ⎛ V C − VaCa K ⎞ − w⎟ ⎜C H + b b ms ⎝ V0 CH ⎠

(4)

where Vi and Ci are, respectively, the volume and concentration of the acid and base added (subscripts a and b refer to acid and base, respectively), Kw is the ionic product of water, and CH is the concentration of protons in solution. Furthermore, equations formally equal to the classical Langmuir and Langmuir−Freundlich (LF) models have been used to describe the proton ion binding to biomass.20 In this case, the models have been referred to the proton uptake, b is substituted by KH, and the concentration of protons that remain in solution (CH) is expressed as pH (−log CH) as shown in the following equation: 2253


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Q max ,H(10 log KH − pH)1/ n 1 + (10 log KH − pH)1/ n

(5)

where n is an adjustable parameter in the LF model and equals one in Langmuir model. As can be seen in Figure 3, the LF model represents a good simulation hypothesis for experimental data; additionally, the

Figure 4. Effect of pH on the AB25 sorption by banana peel (0.1 g) treated with HCl with [AB25] = 0.1 mmol L−1.

It has been previously stated that adsorption of anionic dyes by bacterial and fungal cells is significant under acidic conditions.28 This occurs because in the case of dyes with SO3− groups, the adsorption takes place mainly when the groups of the sorbent involved in the process are protonated. These higher uptakes at lower pH values can be explained by electrostatic attractions between negatively charged dye anions and positively charged surface. Considering that fact, all the sorption experiments (kinetics, isotherms, and columns) were carried out at pH = 2. Nevertheless, the dye uptake is not negligible at basic pH, which could suggest that the sorption of AB25 does not follow a unique mechanism of electrostatic attraction between the positively charged surface of the adsorbent, under acidic conditions, and the anionic dye. In fact, van der Waals hydrogen bonding and hydrophobic and entropic considerations are energetically involved, as shown by Bird et al.,29 in studying the dye−cellulose interaction. 3.3. Equilibrium and Kinetics: Comparison with Ligand−Receptor Data. 3.3.1. Adsorption Equilibrium. The Langmuir model assumes that the sorption takes place at specific homogeneous sites within the adsorbent. The activities of the surface sites are proportional to their concentration and the number of sorption sites is fixed; no further adsorption can take place after the saturation point. The sorption process occurs in a monolayer covering the surface of the whole material and is described by

Figure 3. Proton binding by banana peel treated with HCl in 0.1 M KNO3. Circles (○) are experimental data and dashed and solid lines represent the best fit for the Langmuir−Freundlich and Langmuir equations, respectively.

best correlation coefficient was also obtained. This model consists in a continuous distribution of affinities; on the contrary, the Langmuir one represents a single discrete site model. Just an additional parameter (1/n) added to the Langmuir model, so that the chemical heterogeneity of surface is considered, allowed the model to better describe the experimental data. A value for log KH of 4.28(1) is obtained (see Table S2 in the Supporting Information), in good agreement with the value previously calculated (pKα=0.5 = 4.3(1)) with Katchalsky’s equation. 3.2. pH Influence. The study of the influence of pH on adsorbent uptake is a useful tool to understand the process involved in adsorption. The protonation of the chemical groups existing in the sorbent can influence the adsorption capacity, and consequently, the pH dependence of dye uptake is closely related to the acid−base properties of functional groups on the banana peel surface, the carboxylic and amine groups.12,16 The obtained value for the pKα=0.5 = 4.3, from the acid−base titration curve (Figure.3), suggests that the surface became neutral or positively charged at pH values lower than the pKa and, on the contrary, a negative charge was developed on the material at higher pH values. The dependence of Acid Blue 25 sorption on pH is shown in Figure 4. As it can be seen, the sorption on banana peel is about 98% at acid pH values, and then it declines sharply as the pH value increases down to a minimum value of 17% at pH = 8. Similar results were found for the sorption of AB25 on Aspergillus oryzae biomass,23 polyaminoimide hompolymer,24 ion exchange starch material,25 and egg shell membrane.26 This behavior was also observed in the removal process of anionic dyes by Aspergillus lentulus through bioaccumulation and biosorption.27

qe =

qmax ·KL ·Ce 1 + KL ·Ce

(6) −1

where qe (mmol g ) is the amount of sorbate at the equilibrium, qmax (mmol g−1) is the monolayer saturation capacity, KL (L mmol−1) is the Langmuir constant, and Ce (mmol L−1) is the sorbate concentration in the solution phase at equilibrium. Isotherm experimental data were obtained in solutions at pH 2. This pH value has been chosen in order to maximize the dye uptake (see section 2.4 for experimental details). Figure 5 shows the fitting results obtained for AB25 using the Langmuir model. The qmax obtained for adsorption of banana peel is 0.215(13) mmol g−1. Comparing this value with other values in the literature (Table S1 of Supporting Information) it can be 2254



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in molar units in order to calculate and compare adequately Gibbs free energies.34 It is observed that most values of KD representing interactions of AB25 dye with different kinds of biomaterials are within the limits mentioned above for drug− receptor equilibria. As a matter of fact, all data are greater than KD = 10 pM, which is considered a lower limit associated to a physicochemical constraint related to enthalpy−entropy compensation effect. This constraint indicates that though binding enthalpies and entropies vary within certain intervals, ΔG° = ΔH° − TΔS°, can never be smaller than around −15 kcal/mol (or KD smaller than 10 pM). A possible explanation of the origin of that constraint has been suggested by Gilli et al.33 in the sense that hydrogen bond rearrangements are the main determining force in the drug−receptor association. In Figure 6, ΔH° and ΔS° thermodynamic data from several, although few, studies on the AB25 adsorption at different Figure 5. Langmuir isotherm for the sorption of AB25 by banana peel treated with HCl. Circles (○) represent the experimental points and the continuous line modeled according with Langmuir equation.

conclude that banana peel presents a sorption capacity higher than most biomasses. The specific surface of the sorbent can be calculated from qmax obtained from Langmuir equation Ss = qmax NAA m

(7)

where Ss represents the specific surface area of the adsorbent expressed in m2 g−1, NA is Avogadro’s constant, and Am is the ionic cross-sectional area of solute. Several authors have proposed different cross-sectional molecular areas of AB25 depending on its orientation; Mui et al.30 calculated an interval of 46−162 Ǻ 2 and Giles et al. calculated 80 Ǻ 2 for an end-on orientation or 143 Ǻ 2 for a flat one.31 Consequently, values comprised between 1.06 × 105 and 1.89 × 105 (m2/g) are obtained for the specific surface area of the banana peel. In order to compare dye−biomass interactions with receptor−ligand studies, a basic concept is affinity. In drugaction theory, the affinity is defined as the ability of a drug to selectively bind to a given receptor and it is measured by the value of its drug−receptor associate ion constant or affinity constant, KA. An outstanding aim of binding studies on pharmacological receptors is to obtain reliable estimates of the affinities of selected ligands for the receptors of interest. Binding of a single ligand species to a single uniform population of receptor binding sites shows a smooth hyperbolic (saturable) dependence of the concentration of the receptor−ligand complex on the free ligand concentration32 (the form of Langmuir isotherm or the Hill equation in general). Although many effective drugs have been discovered on the basis that affinity is an appropriate substitute for in vivo effectiveness, in some cases, the kinetics of drug−receptor binding is as important as or even more important than affinity in determining drug efficacy. The inverse of affinity constant or dissociation constant KD = 1/KL is commonly employed for comparing data because it is expressed in terms of molarity. The highest-affinity drugs never display KD values smaller than 10 pM, whereas those having KD > 100 μM are not considered of practical importance.33 Values of affinity constants (KA = KL) calculated from Langmuir eq 6 for different systems involving AB25 are shown in Table S1 in the Supporting Information. Data are expressed

Figure 6. Plot of the standard enthalpies versus the standard entropies for the binding equilibria from Ramesh35 (solid line) and Gilli33 (dashed line). Points correspond to data calculated from refs 29 and 49−51.

temperatures are compared to those from Gilli’s work, on receptor−ligand data of affinity constants, and those from Ramesh’s35 study, on adsorption of dyes and heavy metals on low cost adsorbents. AB25 data are observed to fit well to the line described by Ramesh et al., but they show a shift from that of Gilli et al. An intersection point, is estimated at T = 277.6. From the obtained results, the existence of an entropy− enthalpy compensation effect, including a possible interpretation of the intersection point, cannot be confirmed in the interaction of AB25 with the biomass, and more experimental studies of the effect of temperature on the adsorption of AB25 on biomaterials have to be carried out. 3.3.2. Kinetics. Different sorption kinetic mechanistic models have been used to describe the uptake of dyes on a solid sorbent. In particular, pseudo-first-order and pseudo-secondorder equations, which are based on the sorption at vacant biosorbent surface sites, are the most widely used.36 The pseudo-second order equation, obtained in the integrated form for the boundary conditions qt = 0 at t = 0 and qt at time t, is

qt = 2255

q2k 2t 1 + qk 2t

(8)



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where k2 (g mmol−1 min−1) is the sorption rate constant, q (mmol g−1) is the amount of solute sorbed at the equilibrium, which is equivalent to qmax in eq 6, and qt (mmol g−1) is the amount of solute sorbed on the surface of the sorbent at any time. The experimental kinetic profiles, shown in Figure 7, reveal that the time needed to achieve the equilibrium strongly

with increasing dye concentration approaching a finite value at high concentrations. As pointed by Plazinski et al.,37 k2 usually depends strongly on initial concentration, decreasing as a general rule, and it is interpreted as a time scale factor; that is, when it’s value is relatively high, the time required to reach an equilibrium state by the system is relatively short and vice versa. In fact, eq 8 can be written alternatively as an hyperbolic equation, taking the product (qk2) in eq 9 equal to k3, which has time inverse units qt = q

t t + tr

(9)

where tr= 1/k3(see Table S3 in the Supporting Information) represents a relaxation time of the adsorption process; that is, an adsorption process with a very high (short) relaxation time adapts very fast to its equilibrium state. Relaxation times for the interaction AB25 with biomass of modified banana peel are also given in Table S3 in Supporting Information. Additional values for different organic molecule−adsorbent interactions can be found in Wu et al.38 This relaxation time must not be confused with the residence time in receptor−ligand studies, which is associated to the life of the receptor−ligand complex on the surface and represents the inverse of the rate constant of dissociation of the surface complex. This value cannot be obtained from pseudo-second-order rate constants expressions, contrary to what happens with pseudo-first-order rate constants (see eq 18 in Azizian’s article39). The inverse of the last mentioned constant is equivalent to the residence time in the receptor−ligand studies. Because there is no explicit equation that provides an interpretation of the dependence of k2 on the initial concentration of adsorbate, different empirical expressions (hyperbolic, exponential) have been used.40 The data obtained in this work fit acceptably well to equation

Figure 7. The kinetic profiles of AB25 by pretreated banana peel treated with HCl at different initial concentrations: 0.55 (tilted △), 0.22 (△), 0.11 (◇), and 0.052 mmol L−1 (○). Continuous lines represent the fitting obtained with the pseudo-second-order equation.

depends on the initial dye concentration and that the adsorption capacity increases with the initial dye concentration. The obtained k2 values are reported in Table S3 in the Supporting Information. Figure 8 shows pseudo-second-order rate constants k2 as a function of initial concentration of dye for different biomaterials taken from different references; data obtained in this work are also included. It is observed that k2 decreases asymptotically

k2 =

a b + C0

(10)

where a and b are constants. The analysis of kinetic data according to eq 10 shows that the dependence of k2 on C0 is essentially the same as the one proposed in certain studies about the interactions receptor− ligand when there exist kinetically relevant conformational changes in the receptor.41 These results suggest the possibility that conformational changes may occur in the biomass. There are different possible mechanisms that can control the kinetics of the adsorption process,42 but diffusion is usually the limiting one. It is common to assume, in adsorption in a batch system under rapid stirring, that the overall rate of binding depends primarily on the diffusivity of the adsorbate; namely, diffusion through the boundary layer of the fluid immediately adjacent to the external surface of the adsorbent particle (film diffusion) and diffusion through the sorbent particles (intraparticle diffusion). Weber and Morris43 have developed a graphical method to evaluate if the intraparticle diffusion is the rate-determining step in an adsorption process. According to this model, the uptake varies almost proportionately with the square root of time, and the intraparticle diffusion constant can be obtained from the slope of the plot of qt versus the square root of time by using the following equation:

Figure 8. Pseudo-second-order rate constants, k2, as a function of initial concentration of dye, for different biomaterials. The inset corresponds to the 1/k2 versus initial concentration for data obtained in this work.

qt = kit 0.5 + I 2256

(11)



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where qt is the sorption capacity at time t (mmol g−1), ki (mmol g−1min0.5) is the intraparticle diffusion constant, and I is the intercept. The intraparticle diffusion is regarded as the ratelimiting step at I = 0, whereas the film diffusion and the intraparticle diffusion are both considered as rate-limiting steps at I > 0. The intercept value also gives an idea about the boundary layer effect; the thickness of the boundary layer increased proportionally with I. As can be seen in Figure 9 none of the linear fits pass through the origin, but they are very close to it, which is related to an

3.4. Desorption studies. Desorption studies were done in order to verify the ability of the banana peel to release the dye once it has been sorbed. This point is fundamental to achieve biomass regeneration for further reuse especially in adsorption desorption processes. The AB25 desorption for three different concentrations of NaOH are shown in Figure 10. It can be noticed that 100% of

Figure 10. Desorption studies using NaOH at different concentration (0.1, 10−2, and 10−3 mol L−1). Banana peel loaded using a 3 × 10−4 M of AB25 solution. Dye desorption percentages achieved for different contact times.

Figure 9. Intraparticle diffusion model of Acid Blue 25 sorption onto HCl pretreated banana peel with different initial dye’s concentrations 0.55 (●), 0.22 (△), 0.11 (▲), and 0.052 mmol L−1 (○).

dye desorption was achieved at the lower NaOH concentration in 2 h. The percentage of desorption for the higher NaOH concentration (0.1M) did not reach 100% after 48 h. In order to verify if a high concentration of NaOH slows the desorption process, the stability of acid blue 25 in 0.1 M NaOH solution was tested and it was found not to be stable. Therefore, it can be concluded that the dye decomposition in basic media is responsible for the decrease in dye concentration in solution and not a slower desorption process. 3.5. Column Studies. The dye recovery from wastewaters is generally performed with fix-bed columns. Therefore, a column study of the adsorption process should be useful for an industrial application (or a full-scale sorption process) of the banana peel in wastewater treatments. The behavior of a continuous fixed-bed column is described by the model of the breakthrough curve. The curve is usually represented by plotting the ratio of the effluent dye concentration (C) to inlet dye concentration (C0) versus the time (or the volume of feeding solution). The results of the Acid Blue 25 column in continuous operation are shown in Figure 11. As can be seen from the figure, the breakthrough point was reached in 55.5 h with a 1.5 mL/min flow rate, so that 5 L of dye free effluent were obtained. The shape of this curve is determined by the shape of the equilibrium isotherm, and it is influenced by the individual transport processes in the column and in the sorbent. Moreover, the behavior of the breakthrough curve with respect to the time or volume depends not only on the capacity of the biomaterial but also on the initial dye concentration and flow rate.

adsorption process in which both the film and the intraparticle diffusion do contribute. The parameters for each concentration obtained from linear regression of experimental data are reported in Table S4 in the Supporting Information. During the first steps of the sorption process (at a relatively small t) and when sorption on a spherical particle with a constant diffusivity is considered, qt can be expressed by the following equation:44

6qeDi 0.5

t 0.5 (12) π 0.5r where Di is the intraparticle diffusion coefficient and r is the radius of the adsorbent particles, assumed to be spherical. The solid phase is considered to be sphere-shaped with an average radius between the radii corresponding to the upper and lower size fractions (respectively 0.5 and 1 mm). Comparing eqs 11 and 12, a linear dependence of the intraparticle rate constant on the equilibrium dye uptake can be obtained as follows: qt =

ki =

6Di 0.5 π 0.5r

qe

(13)

Equation 13, predicts a linear relationship between the intraparticle diffusion constant, ki, and the equilibrium uptake, qe. The fit of experimental data obtained for different initial dye concentrations allowed to calculate an intraparticle diffusion coefficient, Di, of 0.331(1) × 10−13 m2 s−1. Other values found in the literature for AB25 are 2 × 10−13 in activated carbon45 and 6.0 × 10−13 m2 s−1 in bagasse pith.46 2257



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where both batch and fixed-bed column can be used for pollution control. 3.6. SEM Images. SEM images of the untreated banana peel, after pretreatment with HCl and after adsorption of a loading solution of AB25, are shown in Figure 12. Comparing images b and c with the untreated material (a), it can be seen that the surface of the banana peel is clearly modified by the acid treatment. After dye adsorption, a significant change is observed in the structure of the peel. The biomaterial appears to have a rough, exfoliated texture because the surface has been partially corrupted by the dye molecules. The microanalysis (EDS) coupled with SEM gives useful information to understand the changes on the material’s surface structure. K+, P3+, and Cl− in the raw material disappear after the treatment with HCl. Moreover, the peel in contact with AB25 shows the presence of S−, a component of the dye structure (see Figure 1), which once more confirms the sorption of the molecule on the biomaterial surface.

Figure 11. Fix-bed column of Acid blue 25 onto HCl-pretreated banana peel with a dye concentration of 0.02 mmol L−1, pHin = 1.98, pHout = 1.9 with about 4 g of biosorbent. The symbol (○) represents experimental points, continuous line represents the fitting obtained using the Bohart−Adams model.

4. CONCLUSIONS The adsorption of Acid Blue 25 on modified banana peel biomass has been studied from an environmental and pharmacological perspective and the results have been compared with ligand−receptor data. The comparison, in terms of Langmuir affinity constants, suggests the existence of a possible entropy−enthalpy compensation effect. However, this result must be confirmed with more extensive experimental work on AB25 interactions with biomass. An interpretation of the variation of the pseudo-second-order kinetic constants with the initial concentration of adsorbate has been successfully carried out using equations from ligand− receptor studies, which suggests the existence of conformational changes in the receptor (biomass). There exists a close parallelism between biosorption and ligand−receptor studies, and the results from one field can be useful in the interpretation of the results in others, as shown in this work.

Several models have been adopted to fit the experimental data of adsorption columns,47 the most widely used being the Bohart−Adams one.48 This model was originally developed to study the adsorption of chlorine by charcoal. A simplified version is given by47 C 1 = kN0D ⎡ C0 1 + exp⎣ εν − kC0t ⎤⎦

(14)

where C is the outlet sorbable species concentration at eluted volume, C0 is the inlet concentration, k is the rate constant that characterizes the effect of the mass transfer between the solid and the liquid phase, N0 is the adsorption capacity of the adsorbent per unit volume of the bed, D is the total bed depth, ε is the fraction of the space in the packed bed in which the fluid flows, and ν is the average axial velocity of the flowing fluid in the interstitial spaces. The fit of experimental data to eq 14 is shown in Figure 11. Banana peel is an important alimentary industry waste; thus, it is a low-cost adsorbent suitable for removal AB25 from effluents. Besides, the stabilization process of the biomass is simple and does not imply an elaborate process. For these two reasons, its use can be considered in reactors as an alternative to activated carbon for removing dyes from industrial wastes,

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ASSOCIATED CONTENT

S Supporting Information *

Table of maximum adsorption capacity of Acid Blue 25 on various sorbents; parameters estimated for proton binding to banana peel from potentiometric titrations data, using the Langmuir and Langmuir−Freundlich models; pseudo-secondorder constants and relaxation times for Acid Blue 25 on various sorbents; intraparticle diffusion parameters obtained

Figure 12. SEM micrographs of banana peel obtained by Evo-MA10-HR microscope. (a) Untreated material, (b) HCl pretreatment, and (c) after loading with AB25. 2258



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using the Weber−Morris model. This information is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +34 98 116 7000, ext 2197. Fax 34 98 116 7065. E-mail: [email protected]. (P. R. B.) Notes

The authors declare no competing financial interest.

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Adsorption of the Prototype Anionic Anthraquinone, Acid Blue 25, on a

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