Journal of Colloid and Interface Science 284 (2005) 83–88 www.elsevier.com/locate/jcis Adsorption of benzoic acid onto high specific area activated carbon cloth Erol Ayranci ∗ , Numan Hoda, Edip Bayram Chemistry Department, Akdeniz University, 07058 Antalya, Turkey Received 20 August 2004; accepted 19 October 2004 Available online 18 December 2004 Abstract The adsorption of benzoic acid from aqueous solution onto high area carbon cloth at different pH values has been studied Over a period of 125 the adsorption process was found to follow a first-order kinetics and the rate constants were determined for the adsorption of benzoic acid at pH 2.0, 3.7, 5.3, 9.1, and 11.0 The extents of adsorption and the percentage coverage of carbon cloth surfaces were calculated at 125 of adsorption Adsorption isotherms at pH values of 2.0, 3.7, and 11.0 were derived at 25 ◦ C Isotherm data were treated according to Langmuir and Freundlich equations and the parameters of these equations were evaluated by regression analysis The fit of experimental isotherm data to both equations was good It was found that both the adsorption rate and the extent of adsorption at 125 were the highest at pH 3.7 and decreased at higher or lower pH values The types of interactions governing in the adsorption processes are discussed considering the surface charge and the dissociation of benzoic acid at different pH values 2004 Elsevier Inc All rights reserved Keywords: Adsorption; Benzoic acid; Carbon cloth; Surface charge Introduction Adsorption of organic molecules from aqueous solution on activated carbon is a widely used method in raw and wastewater treatments and in food, beverage, pharmaceutical, and chemical industries [1] The adsorption capacity of activated carbon is related to its surface area, pore structure, and surface chemistry The surface chemistry of activated carbon is characterized by heteroatoms that compose the surface such as oxygen, nitrogen, hydrogen, sulfur, and phosphorous [2] Those heteroatoms are in the form of functional groups such as ketones, carboxyls, phenols, ethers, lactones, or nitro groups and they have a significant effect on the chemical character, acidity, and degree of hydrophobicity of the carbon surface [3,4] The characteristics of the adsorbate also influence the adsorption process These characteristics are molecular size, solubility, pK, and the nature of adsorbate molecules Ionic strength and pH of the medium affect the adsorption process by controlling electrostatic interac* Corresponding author Fax: +90-242-227-89-11 E-mail address: eayranci@akdeniz.edu.tr (E Ayranci) 0021-9797/$ – see front matter 2004 Elsevier Inc All rights reserved doi:10.1016/j.jcis.2004.10.033 tions between the adsorbent and the adsorbate The carbon surface charge and the dissociation or protonation of the adsorbate are determined mainly by the pH of the solution The carbon surface charge will be positive when the pH is lower than the pH at the point of zero charge of the surface (pHpzc ) and will be negative when pH is higher than pHpzc [5] Benzoic acid constitutes a simple model for complex matrices that may be present in the aqueous phase Therefore there are many reports in the literature on the adsorption of benzoic acid from the aqueous phase on various materials such as activated carbon, synthetic calcite, soil, metal hydroxides, mineral surfaces, silica, calcite, dolomite, and some metal oxides [6–14] In recent literature the use of high specific area carbon cloth appears to be an attractive alternative for selection of the adsorbent For example, studies on adsorption and electrosorption at high area carbon cloth have been reported for various adsorbates such as inorganic S-containing anions [15], ethylxanthate and thiocyanate [16,17], phenol, phenoxide and chlorophenols [18], some aromatic heterocyclic compounds [19], pyridine [20], and some pesticides [21,22] in relation to wastewater purification 84 E Ayranci et al / Journal of Colloid and Interface Science 284 (2005) 83–88 The aim of the present study is to determine the kinetics and equilibrium states of adsorption of benzoic acid onto high specific surface area activated carbon cloth A systematic study is conducted in order to observe the influence of pH on the adsorption process Materials and methods 2.1 Materials The carbon cloth used in the present work was obtained from Spectra Corp (MA) coded as Spectracarb 2225, having a specific area of 2500 m2 g−1 (measured using the Kr, BET method by the manufacturer) Benzoic acid, hydrochloric acid, and sodium hydroxide were obtained from Merck (Germany) Deionized water was used in the adsorption experiments 2.2 Treatment of the carbon cloth The carbon cloth material was found [15] to provide spontaneously a small but significant quantity of ions into the conductivity water used, probably due to its complex structure originating from its somewhat unknown proprietary preparation procedure A deionization cleaning procedure was therefore applied, as described previously to avoid desorption of ions during the adsorption measurements [15] In this procedure, a carbon cloth sample was placed in a flow-through washing cup and eluted with L of warm (60 ◦ C) conductivity water in a kind of successive batch operation for days with N2 bubbling in order to avoid possible adsorption of CO2 that might have been dissolved in water The outflow water from each batch was tested conductometrically for completeness of the washing procedure The washed carbon cloth modules were then dried under vacuum at 120 ◦ C and kept in a desiccator for further use The carbon cloth was cut to desired dimensions (about 0.5 × 1.5 cm) and weighed accurately 2.3 Adsorption cell A specially designed cell was used to carry out the adsorption studies and simultaneously to perform in situ concentration measurements by means of UV absorption spectrophotometry The cell (Fig 1) was V shaped with one arm containing the carbon cloth attached to a short Pt wire sealed to a glass rod and the other arm containing a thin glass tube through which N2 gas was passed for the purposes of mixing and eliminating any dissolved CO2 The two arms were connected to a glass joint leading to a vacuum pump at the upper part of the V-shaped cell in order to provide the opportunity for initial outgassing of the carbon adsorbent, and the cell and solution A quartz spectrophotometer cuvette was sealed to the bottom of the adsorption cell Fig Diagram of the adsorption cell 2.4 Optical absorbance measurements Adsorbate solutions were prepared by dissolving a fixed amount of benzoic acid in water and adjusting the pH to 2.0, 3.7, 5.3, 9.1, and 11.0 by additions of 0.1 M HCl or 0.1 M NaOH and diluting to a final volume to keep the benzoic acid concentration the same in all solutions pH values of solutions were measured with a pH meter (Jenway 3040 ion analyzer) Calibration curves were prepared at each pH value to convert the absorbance data of kinetic and equilibrium experiments to concentration data A Shimadzu 160A UV/Vis spectrophotometer was used for optical absorbance measurements The absorbance measurements were conducted in situ during the study of the kinetics of adsorption process as follows In all experiments, the size and the mass of the carbon cloth were kept as constant as possible (about 18.0 ± 0.1 mg) Its mass was accurately measured and recorded each time for calculation of fractional coverage, θ , or the amount of adsorption per unit area, M, of the carbon cloth Carbon cloth pieces were prewetted by leaving in water for 24 h before use The idea of using prewetted carbon cloth originates from our previous findings that prewetting enhances the adsorption process [15,16] Carbon cloth was dipped into the adsorption cell initially containing only water and vacuum was applied to remove all air in the pores of the carbon cloth Then wetted and degassed carbon cloth was removed from the cell for a short time and water in the cell was replaced with a known volume of sample solution (20 mL) The sliding door of the sample compartment of the spectrophotometer was left half open and the quartz cuvette fixed at the bottom of the adsorption cell (which now contained the sample solution) was E Ayranci et al / Journal of Colloid and Interface Science 284 (2005) 83–88 inserted into the front sample compartment A Teflon tube connected to the tip of a thin N2 -bubbling glass tube was lowered from one arm of the adsorption cell down the UV cell to a level just above the light path to provide effective mixing Finally, the carbon cloth, which was removed temporarily after wetting and degassing, was inserted from the other arm of the adsorption cell into the solution Then, quickly, an opaque curtain was spread above the sample compartment of the spectrophotometer, over the cell, to prevent interference from external light The program for monitoring the absorbance at the specific wavelength of maximum absorbance predetermined by taking the whole spectrum of benzoic acid was then run on the built-in microcomputer of the spectrophotometer Absorbance data were recorded in programmed time intervals of over a period of 125 Absorbance data were converted into concentration data using calibration relations predetermined at the wavelength of interest for the corresponding pH of benzoic acid 2.5 Determination of adsorption isotherms The adsorption isotherms of benzoic acid on carbon cloth were determined on the basis of batch analysis Carboncloth pieces of varying masses were allowed to equilibrate with benzoic acid solutions of known initial concentration at 25 ◦ C for 48 h Preliminary tests showed that the concentration of benzoic acid remained unchanged after 8–10 h of contact with the carbon cloth So, the allowed contact time of 48 h ensures the equilibration The initial concentration of benzoic acid solutions was 1.96 × 10−4 M at pH 3.7 and 2.0 and 1.96 × 10−5 M at pH 11.0 The equilibrium concentrations of benzoic acid solutions were measured spectrophotometrically The amount of benzoic acid adsorbed per unit mass of carbon cloth, qe , was calculated by V (c0 − ce ) , (1) m where V is the volume of benzoic acid solution in L, c0 and ce are the initial and equilibrium concentrations, respectively, of the benzoic acid solutions in mmol L−1 and m is the mass of the carbon cloth in grams Equation (1) gives qe in millimoles benzoic acid adsorbed per gram carbon cloth qe = 85 which this difference is zero, i.e., initial and final pH are the same, was determined to be the pHpzc value The pHpzc value of the carbon cloth used in this study was found to be 7.4 Results and discussion 3.1 Absorption characteristics and calibration data for benzoic acid at different pH In Table 1, absorption characteristics and calibration data for benzoic acid are given at different pH Absorption maximum gradually shifts from 230 nm at pH 2.0 to 224 nm at pH 5.3 and remains the same up to pH 11.0 while the molar absorptivity varies between 10,800 and 8000 in this pH range Calibration data were evaluated according to Lambert-Beer law by the method of least-squares analysis with excellent correlations as indicated by the regression coefficients given in the last column of Table 3.2 Kinetics and extents of adsorption of benzoic acid at different pH The initial concentrations of benzoic acid and the amount of carbon cloth used were kept constant for kinetic studies of the adsorption process at different pH in order to make a comparative study The initial concentration of benzoic acid was 1.96 × 10−4 M and the amount of carbon cloth module was 18.0 ± 0.1 mg The decrease in concentration of benzoic acid with time as it is adsorbed onto carbon cloth at different pH values is shown in Fig The natural pH of 1.96 × 10−4 M benzoic acid was 3.7 Solution of benzoic acid at pH 2.0 was prepared by adding required amounts of 0.1 M HCl and solutions of benzoic acid at pH 5.3, 9.1, and 11.0 were prepared by adding required amounts of 0.1 M NaOH while monitoring from the pH meter It is seen from Fig that the extent of adsorption during the period of 125 is the highest at the natural pH of 3.7 and decreases at higher or lower pH values A more quantitative comparison can be made in the extent of adsorption of benzoic acid at different pH by introducing two related terms: the amount of adsorbate adsorbed per unit area of the carbon cloth, M, given by Eq (2) and the percent- 2.6 Determination of pHpzc of carbon cloth The batch equilibrium method described by Babiˇc et al [23] for the determination of pH at the point of zero charge was used Carbon cloth samples (0.08 g) were shaken in Erlenmeyer flasks for 24 h with 50 ml of 0.1 M NaNO3 at different initial pH values, which were adjusted by adding NaOH or HNO3 solutions At the end of contact period, the H+ and OH− ion concentrations were measured with the pH meter Then the amounts of OH− and H+ ions adsorbed were calculated by subtracting the last measured concentrations of H+ and OH− ions from the initial concentrations The pH at Table Spectrophotometric and calibration parameters for benzoic acid at different pH values pH λmax (nm) ε (au cm−1 M−1 )a rb 2.0 3.7 5.3 9.1 11.0 230 228 224 224 224 10,800 9200 8150 8000 8100 0.9982 0.9999 0.9994 0.9999 a ε is the molar absorptivity and au stands for absorbance unit b r is the correlation coefficient for fit of data to Lambert-Beer’s law 86 E Ayranci et al / Journal of Colloid and Interface Science 284 (2005) 83–88 Fig Adsorption behaviors of benzoic acid from aqueous solutions at different pH values onto the carbon cloth The initial concentration is 1.96 × 10−4 M The mass of the carbon cloth is 18.0 ± 0.1 mg age coverage at the carbon cloth surface, θ , given by Eq (3), M = (c0 − ct )V /2500m, θ = (c0 − ct )V NA × 100 (2) (4 × 10 19 × 2500m), carbon cloth calculated from Eqs (2) and (3) at 125 of adsorption are given in Table at different pH values The extent of adsorption of benzoic acid according to the θ and M values at 125 decreases in the pH order of 3.7 > 2.0 > 5.3 > 9.1 > 11.0 For the kinetic investigation of adsorption of benzoic acid onto carbon cloth, the concentration versus time data were treated according first-order law by plotting ln(c0 /ct ) as a function of time The linearity of such plots supports the validity of the assumption of first-order law and furthermore the slopes of the lines provide the first-order rate constant (k) for the adsorption process Linear regression analysis of the data provided the rate constants given in the fifth column of Table for the adsorption of benzoic acid at different pH values The last column of Table shows the regression coefficients, r The closeness of r to supports the idea that the adsorption process follows the first-order kinetics When the rate constants are examined, it can clearly be seen that the adsorption rate is the highest at the natural pH of 3.7 and very slow at pH values greater than 5.3 It should be noted that the rate constant also decreases from a natural pH of 3.7 to a more acidic pH of 2.0 The interactions between benzoic acid and the carbon surface, which may explain these kinetic results, will be discussed in Section 3.4 (3) where c0 and ct are the concentrations of the solutions at the beginning and at a specific time during the adsorption process, respectively V is the volume of the solution, m the mass of the carbon cloth module, and NA Avogadro’s number The calculations are based on the known specific surface area of 2500 m2 g−1 for the carbon cloth provided by the manufacturer, corresponding to an approximate value of × 1019 carbon sites/m2 of the surface determined by the atomic radius of carbon but dependent on the actually unknown geometry of surface carbon-atom packing The θ and M values for the adsorption of benzoic acid onto 3.3 Adsorption isotherms of benzoic acid at different pH The equilibrium adsorption isotherms for benzoic acid at different pH values were derived at 25 ◦ C The qe data versus equilibrium concentration (ce ) were treated according to well-known isotherm equations of Langmuir and Freundlich The linear forms of Langmuir and Freundlich equations are given in Eqs (4) and (5), respectively, ce ce = + , qe bqm qm ln qe = ln K + (1/n) ln ce , (4) (5) Table Parameters for the extent of adsorption (M, θ) and for the kinetics of adsorption (k, r) of benzoic acid at various pH values pH c0 (mol L−1 ) M (10−8 mol (m2 C cloth)−1 ) θ k (min−1 ) r 2.0 3.7 5.3 9.1 11.0 1.96 × 10−4 1.96 × 10−4 1.96 × 10−4 1.96 × 10−4 1.96 × 10−4 6.68 7.40 3.69 3.04 2.00 0.101 0.111 0.056 0.046 0.030 0.0108 0.0143 0.0038 0.0031 0.0017 0.9964 0.9954 0.9925 0.9773 0.9598 Table The parameters of Langmuir and Freundlich isotherm equations for the adsorption of benzoic acid at three pH pH 2.0 3.7 11.0 Langmuir Freundlich qm (mmol g−1 ) b (L mmol−1 ) r K (mmol g−1 )(mmol L−1 )1/n 1/n r 0.93 1.38 0.07 181.7 40.76 108.2 0.9970 0.9863 0.9804 2.70 6.24 0.38 0.35 0.59 0.53 0.9807 0.9887 0.9804 E Ayranci et al / Journal of Colloid and Interface Science 284 (2005) 83–88 87 Fig The fit of experimental adsorption data (Q) to Langmuir ( -) and Freundlich (—) models for benzoic acid at pH 2.0 where qe is the amount of solute adsorbed per unit mass of adsorbent (mmol g−1 ), ce is the equilibrium concentration of solute (mmol L−1 ), qm is the amount of solute adsorbed per unit mass of adsorbent required for monolayer coverage of the surface (mmol g−1 ), b is a constant related to the heat of adsorption (L mmol−1 ), K ((mmol g−1 )(L mmol−1 )1/n ) and 1/n are constants which can be related to adsorption capacity and the strength of adsorption or to the surface heterogeneity, respectively The parameters of these equations obtained by linear regression analysis of the experimentally derived data are given in Table together with regression coefficients, r The fits of experimental data for benzoic acid at pH 2.0, 3.7, and 11.0 to Langmuir and Freundlich models are shown in Figs 3, 4, and 5, respectively Visual observation from these figures and the regression coefficients, r, given in the forth and seventh columns of Table show that the experimental adsorption data fit almost equally well to both isotherm models of Langmuir and Freundlich The adsorption capacity of carbon cloth for benzoic acid is the highest at pH 3.7 as indicated by the qm parameter of the Langmuir model and the K parameter of the Freundlich model (Table 3) This result is in agreement with the conclusion reached upon the kinetic treatment discussed in the previous section According to the classification of Giles et al [24] all the isotherms at the three pH values are of L type A characteristic of L-type isotherms is that there is no strong competition between the solvent and the adsorbate to occupy the adsorbent surface sites Observation of L-type isotherms also implies that the adsorbate molecule is not vertically oriented at the surface [24] 3.4 Interactions of benzoic acid with carbon surface It is known that at pH values lower than pHpzc the surface is positively charged [2,5,6,25,26] Since pHpzc for the Fig The fit of experimental adsorption data (Q) to Langmuir ( -) and Freundlich (—) models for benzoic acid at pH 3.7 Fig The fit of experimental adsorption data (Q) to Langmuir ( -) and Freundlich (—) models for benzoic acid at pH 11.0 carbon cloth is 7.4, the carbon surface is positively charged below this pH value On the other hand the natural pH of 1.96 × 10−4 M benzoic acid solution is 3.7 at which benzoic acid is ∼80% in anionic form of benzoate (pKa : 4.2) Therefore at this pH the main interaction between the carbon surface and the adsorbate is expected to be electrostatic attraction in nature This explains the greatest extents of adsorption observed at this pH (Fig 2, Table 2) At pH 2.0 the calculations show that benzoic acid is mainly in neutral molecular form with only ∼0.6% dissociation Although the surface is more positively charged at this pH than that at pH 3.7, because of the loss of negative charge on the adsorbate there is a slight decrease in the extents of adsorption 88 E Ayranci et al / Journal of Colloid and Interface Science 284 (2005) 83–88 (Fig 2, Table 2) The types of interactions between the surface and the adsorbate at this pH (2.0) are expected to be mainly dispersion interactions between π electrons of benzoic acid and π electrons in basal plane of the carbon cloth in addition to some residual electrostatic interactions At pH 5.3, although the benzoic acid is almost completely in anionic benzoate form, due to the decrease in surface positive charge compared to that at pH 2.0 or pH 3.7, there is a decrease in the extents of adsorption (Fig 2) Both electrostatic and dispersion forces are expected to be effective for the interaction between adsorbate and carbon surface at this pH (5.3) At pH 9.1 and 11.0 (which are greater than pHpzc ) the carbon surface is negatively charged and benzoic acid is in anionic form Electrostatic repulsion between the adsorbate and the carbon surface must be operative at these pH values leading to the observation of the lowest extents of adsorption (Fig 2, Table 2) Very small amounts of adsorption observed at these pH values may result from π –π dispersion interactions Conclusions Adsorption of benzoic acid onto high specific area carbon cloth was found to follow first-order kinetics at different pH values Experimental adsorption isotherm data at 25 ◦ C fitted well to both Langmuir and Freundlich equations The rate and extent of adsorption were found to be the highest at pH 3.7, which corresponds to the natural pH of 1.96 × 10−4 M benzoic acid, in the pH range from 2.0 to 11.0 The very small rates and extents of adsorption observed at pH 9.0 and 11.0 were attributed to increased electrostatic repulsions between benzoate anion and the negative surface charge at these pH values Acknowledgments The authors thank the Scientific 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