Microwave assisted thermal treatment of defective coffee beans press cake for the production of adsorbents

Bioresource Technology 101 (2010) 1068–1074

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Microwave assisted thermal treatment of defective coffee beans press cake for the production of adsorbents Adriana S. Franca a,b,*, Leandro S. Oliveira a,b, Anne A. Nunes c, Cibele C.O. Alves b a

Departamento de Engenharia Mecânica, Universidade Federal de Minas Gerais, Av. Antônio Carlos 6627, 31270-901, Belo Horizonte, MG, Brazil Programa de Pós-Graduação em Ciência de Alimentos, Universidade Federal de Minas Gerais, Av. Antônio Carlos 6627, 31270-901, Belo Horizonte, MG, Brazil c Vilma Alimentos, Praça Louis Ensch, 160, Cidade Industrial, 32.210-050, Contagem, MG, Brazil b

a r t i c l e

i n f o

Article history: Received 8 June 2009 Received in revised form 26 August 2009 Accepted 27 August 2009 Available online 19 September 2009 Keywords: Adsorption Microwave activation Agri-food waste Toxic pollutants

a b s t r a c t Defective coffee press cake, a residue from coffee oil biodiesel production, was evaluated as an adsorbent for removal of basic dyes (methylene blue – MB) from aqueous solutions. The adsorbent was prepared by microwave treatment, providing a significant reduction in processing time coupled to an increase in adsorption capacity in comparison to conventional carbonization in a muffle furnace. Batch adsorption tests were performed at 25 °C and the effects of particle size, contact time, adsorbent dosage and initial solution pH were investigated. Adsorption kinetics was better described by a second-order model. The experimental adsorption equilibrium data were fitted to Langmuir, Freundlich and Tempkin adsorption models, with Langmuir providing the best fit. The results presented in this study show that microwave activation presents great potential as an alternative method in the production of adsorbents. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction Adsorption is one of the best and most employed techniques for the removal of organic and inorganic pollutants from wastewaters. Activated carbons are the most widely used adsorbents, but their widespread use is still restricted by their inherently high costs, mainly due to the costs of the precursor materials and of the energy intensive production processes (Crini, 2006). Since agricultural and food wastes are generated in large scale and are rather inexpensive, these materials are considered potential precursors for the preparation of low-cost adsorbents and have been thoroughly studied as such, with several recent reviews on the subject (Crini, 2006; Ioannidou and Zabaniotou, 2007; Suhas et al., 2007; Oliveira and Franca, 2008). Brazil is the largest coffee producer in the world, being responsible for approximately 30% of the total amount of coffee produced worldwide (MAPA, 2009). However, approximately 20% (0.5 million tons per year) of the Brazilian coffee production consists of low quality defective beans, which are commercialized in the internal market in Brazil, being used by the roasting industry in blends with non defective ones (Oliveira et al., 2006). Thus, the quality of the roasted coffee consumed in Brazil is depreciated. In

* Corresponding author. Address: Departamento de Engenharia Mecânica, Universidade Federal de Minas Gerais, Av. Antônio Carlos 6627, 31270-901, Belo Horizonte, MG, Brazil. Tel.: +55 31 34093512; fax: +55 31 34433783. E-mail address: [email protected] (A.S. Franca). 0960-8524/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2009.08.102

view of this situation, studies are currently under development in order to find alternative uses for defective coffee beans. One of the alternatives being considered is biodiesel production, using the oil extracted from such defective coffee beans (Oliveira et al., 2008a). This procedure has proven to be feasible, but it generates a solid processing residue (coffee press cake) after removal of the oil by pressing, for which there is still no profitable use (Nunes et al., 2009). A few recent studies have shown that seed press cakes can be employed as raw materials in the production of adsorbents for removal of cationic dyes, including the ones obtained by pressing sunflower seeds (Karagöz et al., 2008), Raphanus sativus (L. Var.) oilseeds (Lázaro et al., 2008) and defective coffee beans (Nunes et al., 2009), with the later demonstrating the potential of this agricultural residue for removal of methylene blue from aqueous solutions. The maximum value of uptake capacity obtained for the adsorbent based on defective coffee beans was quite high in comparison to other residue based activated carbons such as date pits and apricot stones (Aygun et al., 2003; Banat et al., 2003). However, the adsorption capacity was slightly low in comparison to other seed press cakes (Karagöz et al., 2008; Lázaro et al., 2008). Microwaves have been extensively employed as an alternative heating method and some studies have demonstrated that this technique can be successfully employed in the production of activated carbons (Hirata et al., 2002; Li et al., 2008). Some of the advantages of such technique in comparison to conventional heating for the production of adsorbents are (i) reduced processing time with consequent reduction in energy consumption, (ii) reduction or even

A.S. Franca et al. / Bioresource Technology 101 (2010) 1068–1074

elimination of the amount of gases employed for the treatment, and (iii) more efficient carbonization, given that the heating mechanism is volumetric as opposed to heat transfer from the surface towards the interior in the case of conventional processing. In view of the aforementioned, the objective of this study was to evaluate microwave thermal activation as an alternative technique for the production of adsorbents based on defective coffee press cake. Adsorption studies were conducted with methylene blue (MB) in order to verify the performance of the adsorbent for removal of cationic dyes and to compare it with the adsorbent produced by traditional carbonization in a muffle oven (Nunes et al., 2009). 2. Methods 2.1. Materials Defective coffee beans were acquired from Santo Antonio State Coffee (Santo Antônio do Amparo, MG, Brazil). MB stock solutions (1000 mg/L) were prepared in distilled water. Working solutions (100–750 mg/L) were prepared by diluting the stock solution with distilled water prior to each adsorption test. 2.2. Adsorbent preparation Defective coffee beans were screw pressed (Ecirtec, Brazil) for oil removal. The coffee press cake (CC) was submitted to preliminary carbonization tests employing a household microwave operating at 800 W (Panasonic NN6460A). After 3 min carbonization at maximum power, the samples would burn to ashes. So, several tests were conducted at varying power levels and also employing intermittent heating. The conditions that provided homogeneous carbonization without ash formation are described as follows. The CC sample (15 g) was placed in a quartz container (100 mL), which was then placed inside the microwave oven. Carbonization occurred for 2 min at 800 W and then intermittently in eight intervals of 30 s at 600 W followed by 20 s cooling. Total processing time was 8.7 min and total microwave heating time was 6 min. The produced adsorbent will be herein denominated CCMW. 2.3. Adsorbent characterization Surface functional groups determination was based on the Boehm titration method (Boehm, 1994). Solutions of NaHCO3 (0.1 mol L1), Na2CO3 (0.05 mol L1), NaOH (0.1 mol L1), and HCl (0.1 mol L1) were prepared with distilled water. A volume of 50 mL of these solutions was added to vials containing 1 g of the adsorbent, shaken for 24 h (100 rpm) and then filtered. Five solution blanks were also prepared. The excess of base or acid was then determined by back titration using NaOH (0.1 mol L1) and HCl (0.1 mol L1) solutions. Evaluation of the Point of Zero Charge (pHPZC) was based on a potentiometric titration procedure (Valdés et al., 2002). Three aqueous solutions of different pHs (3, 6 and 11) were prepared. Several amounts of adsorbent (0.05%, 0.1%, 0.5%, 1.0%, 3.0%, 7.0% and 10.0% w/w) were added to 20 mL of each solution. The aqueous suspensions were let to equilibrate for 24 h under agitation (100 rpm) at 25 °C. The pH of each solution was then measured using a digital pH meter (Micronal, São Paulo, Brazil) and the pHPZC was determined as the converging pH value from the pH vs. adsorbent mass curve. Changes in the surface structures of the activated carbon, prior to and after adsorption, were examined using Fourier Transform Infrared (FTIR) spectroscopy. The FTIR spectra were obtained and recorded on a Series 102 ABB BOMEM FTIR spectrometer (Quebec, Canada) operating in the range of 400–4000 cm1, with a resolution of 4 cm1.

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2.4. Adsorption studies Batch experiments of adsorption were performed in 250 mL Erlenmeyer flasks, with the flasks being agitated on a shaker at 100 rpm for pre-determined time intervals. The flasks were covered with aluminum foil to avoid photodegradation. In all sets of experiments, a pre-determined amount of adsorbent was thoroughly mixed with 100 mL MB solution. Preliminary tests were conducted for the following fixed parameters: initial dye concentration – 100 mg L1, initial pH 5, and adsorbent concentration = 10 g L1. Effect of particle size (diameter = D) was evaluated in the following ranges: D < 0.50 mm; 0.50 mm < D < 0.84 mm; D > 0.84 mm. Effect of initial solution pH was evaluated in the range of 3–11 and of adsorbent concentration in the range of 5–50 g L1 at a fixed initial dye concentration (500 mg L1). Effect of contact time was evaluated at time periods ranging from 5 min to 6 h and initial dye concentrations ranging from 100 to 750 mg L1, employing the best values obtained for initial solution pH, particle size and adsorbent concentration. All tests were performed in three replicates. 2.5. Analysis of methylene blue After the specified time periods, 2 mL aliquots were taken from the Erlenmeyer flasks and the dye concentration was determined by a spectrophotometer (Cole Parmer 1100 RS) at 620 nm. The amount of dye adsorbed was determined by taking the difference between the initial dye concentration and the concentration of the solution at the time of sampling. All determinations were performed in a total of three replicates per experiment and the average values were reported. 2.6. Adsorption kinetics The controlling mechanisms of the adsorption processes are usually investigated by fitting first- and second-order kinetic models to the experimental data (Ho and McKay, 1998). The kinetics equations can be represented by the following equation:

dqt ¼ kn ðqe  qt Þn dt

ð1Þ

where qe and qt correspond to the amount of dye adsorbed per unit mass of adsorbent (mg g1) at equilibrium and at time t, respectively; kn corresponds to the rate constant for nth order adsorption (kn units are min1 and g mg1min1 for first and second order, respectively). The linear integrated forms of the equations are First-order kinetics (n = 1):

lnðqe  qt Þ ¼ ln qe  k1 t

ð2Þ

Second-order kinetics (n = 2):

t 1 t ¼ þ qt k2 q2e qe

ð3Þ

Recent studies have also evaluated the application of other kinetics models, specifically developed for gas adsorption, to the removal of pollutants from aqueous solutions (Ho, 2006). The Elovich equation, which considers an exponential decrease of the adsorption rate with the increase in the amount adsorbed, is usually employed to determine the kinetics of chemisorption of gases onto heterogeneous solids. It can be represented by the following expression:

dqt ¼ aebq dt

ð4Þ

A simplified version, assuming that abt  1, can be expressed in linear form as

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qt ¼

A.S. Franca et al. / Bioresource Technology 101 (2010) 1068–1074

1 1 lnðabÞ þ ln t b b

ð5Þ

where a is the initial sorption rate (mg g1 min1) and b is the desorption constant (mg g1). Some studies have employed such equation to describe solid/liquid adsorption (Ho and McKay, 1998; Cheung et al., 2001). Ritchie’s model is based on the assumption that the adsorption rate depends solely on the number of sites that are empty at a given time, and can be represented by the following equations:

qn1 e ðqe  qt Þ

n1

¼ ðn  1Þat þ 1;

qt ¼ qe ð1  eat Þ;

n–1

n¼1

ð6Þ ð7Þ

where a is the adsorption rate constant and n is the number of surface sites occupied by each molecule. When n = 1 this model is equivalent to the Lagergren equation (first-order kinetics). A modified version of the second-order (n = 2) Ritchie model has been successfully employed to describe adsorption of Cd(II) ions by bone char and carbonized fruit peel (Cheung et al., 2001; Inbaraj and Sulochana, 2004), with the assumption that a preadsorption stage exists, i.e., there is some initial surface coverage attributed to the adsorption of impurities. The resulting model equation can be written as

qt 1 ¼1 ; b2 þ k2 t qe

1 b2 ¼ 1  h0

ð8Þ

where h0 corresponds to the surface coverage at time zero. The criteria employed for model selection were the values of the linear regression correlation coefficient (R2) and also an evaluation of the difference between experimental (qt,experimental) and estimated (qt,estimated) values for the amount of dye adsorbed:

SSq ¼

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi X ðqt;estimated  qt;experimental Þ2 =N

ð9Þ

where N corresponds to the number of experimental points. In the case of the models that provide an estimate for qe, i.e., first- and second-order models, the % difference between experimental and calculated qe values was also taken into account.

r ¼ ð1 þ K L C 0 Þ1

ð11Þ

where C0 is the highest value for initial adsorbate concentration (mg L1). Freundlich’s equation is an empirical model based on heterogeneous adsorption over independent sites. Multilayer adsorption is considered, but the model does not account for adsorbent saturation. It can be described by the following equation:

qe ¼ K F C 1=n e

ð12Þ

where KF is a constant that indicates the relative adsorption capacity (mg1(1/n) L1/n g1) and n is related to the intensity of adsorption (Hamdaoui and Naffrechoux, 2007). Freundlich isotherm has also been successfully employed to describe MB adsorption by residue based activated carbons (Hirata et al., 2002; Aygun et al., 2003; Nunes et al., 2009). The Tempkin model considers that the heat of adsorption of all the molecules in the layer decrease linearly with surface coverage due to adsorbent–adsorbate interactions. Adsorption is characterized by a uniform distribution of binding energies, up to a maximum value. The Tempkin isotherm can be written as

qe ¼

RT lnðK T C e Þ b

ð13Þ

where b is the Tempkin constant related to the heat of sorption (J mol1), KT is the Tempkin isotherm constant (L g1), R is the universal gas constant (8.314 J mol1 K1) and T is the absolute temperature (K). Applications of such model to MB adsorption have also been reported (Kavitha and Namasivayam, 2007; Demirbas et al., 2008). The best-fit model selection was based on both linear regression correlation coefficient (R2) and a non-linear v2-statistic (SS) determination test, with SS providing a measure of the difference between experimental and estimated qe values:

SS ¼

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi X ðqe;estimated  qe;experimental Þ2 =N

ð14Þ

where N corresponds to the number of experimental points. 3. Results and discussion

2.7. Adsorption isotherms

3.1. Adsorbent characterization

Langmuir, Freundlich and Tempkin models were tested for equilibrium description at ambient temperature (Hamdaoui and Naffrechoux, 2007). Langmuir isotherm has been successfully employed to describe MB adsorption by residue based activated carbons (Banat et al., 2003; Kavitha and Namasivayam, 2007; Karagöz et al., 2008). It is based on a theoretical model assuming monolayer adsorption over an energetically and structurally homogeneous adsorbent surface and does not take into account interactions between adsorbed molecules. Langmuir model can be described by the following equation:

The functional groups at the surface of the adsorbent, characterized by the Boehm method, were predominantly phenolic (1.89 ± 0.13 mmol/gsorbent), followed by basic (0.83 ± 0.01 mmol/ gsorbent). The experimental titration curves for evaluation of the point of zero charge converged to a pHPZC value of approximately 7.9 and therefore pH values should be maintained above 8 to ensure a predominant negatively charged surface. The adsorbent prepared in the muffle oven contained only basic groups and a pHPZC value of approximately 12 (Nunes et al., 2009). A comparison of these results indicates that the microwave treatment provided milder carbonization/activation, given the presence of phenolic (acid) groups in the adsorbent surface and also the lower pHPZC value. Results from the FTIR analysis, i.e., band assignments for FTIR spectra are displayed in Table 1. The main characteristic bands of the FTIR spectra were the ones at 2900 and 1660 cm1, assigned to C–H and C@C bonds, respectively. Bands in the range of 2850– 2930 cm1 have been previously associated to aliphatic C–H stretching vibration from cellulose and hemicellulose CH2 groups (Uddin et al., 2009). Wang et al. (2007) reported an absorption band near 1610 cm1 for several activated carbons, in association to the presence of carboxylic groups. Such band was also observed in our previous study (Nunes et al., 2009), but in much lower

qe ¼

qmax K L C e 1 þ K L Ce

ð10Þ

where qe (mg g1) and Ce (mg L1) correspond to the amount adsorbed per gram of adsorbent and to the solute concentration (mg L1) in the aqueous solution, respectively, after equilibrium was reached. qmax and KL are constants related to the maximum adsorption capacity (mg g1) and the adsorption energy (L mg1), respectively. Another characteristic parameter of the Langmuir isotherm is the dimensionless factor r (separation factor), related to the shape of the isotherm. Its value indicates either unfavorable (r > 1), linear (r = 1), favorable (0 < r < 1) or irreversible (r = 0) adsorption and it is evaluated as

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A.S. Franca et al. / Bioresource Technology 101 (2010) 1068–1074 Table 1 FTIR spectrum main band assignments. Band number (cm1)

Assignment

References

695 1060 1430 1660 2750 2900 3460

C–H vibrations –OCH3 groups C@C stretching in aromatic skeletal and ester C@C stretching vibration in carboxylic groups C–H stretching vibration in aromatic group C–H stretching vibration in methyl group –OH stretching vibrations in hydroxylic, carboxylic and phenolic groups

Garg et al. (2007), Wang et al. (2007) Garg et al. (2007), Nunes et al. (2009) Bouchelta et al. (2008), Nunes et al. (2009) Demirbas et al. (2009), Wang et al. (2007) Boonamnuayvitaya et al. (2005) Davila-Jimenez et al. (2005), Bouchelta et al. (2008), Nunes et al. (2009) Boonamnuayvitaya et al. (2005), Wang et al. (2007), Bouchelta et al. (2008), Nunes et al. (2009)

3.2. Influence of particle size To evaluate the effect of particle size on the adsorption process, the activated carbon was ground and sieved into three different size fractions: D < 0.50 mm; 0.50 mm < D < 0.84 mm; D > 0.84 mm. An evaluation of the experimental results showed that dye uptake increased with the decrease in particle size, due to the corresponding increase in surface area. Similar results on particle size effect have been reported for other residues (Banat et al., 2003) and also for the coffee press cake based adsorbent (CCMO) obtained by carbonization in a muffle oven (Nunes et al., 2009). 3.3. Influence of initial solution pH The initial solution pH is expected to influence the adsorptive uptake of dyes in association to its effect on the ionization process of the dye molecule (Oliveira et al., 2008b; Nunes et al., 2009). In the present study, the effect of pH was investigated for values between 3 and 11 and the results are displayed in Fig. 1. An evaluation of Fig. 1 shows that no significant differences in adsorption

100 90

% Adsorbed

intensity. This is attributed to the thermal degradation of carboxylic groups at high temperatures (>500 °C) in muffle ovens and it also confirms the milder effect of microwave carbonization on the degradation of such groups. Also, the intensity of this band decreased after MB adsorption, confirming affinity of such groups for MB. The bands observed at wavelengths ranging from 1043 to 1058 cm1 have been previously reported by Garg et al. (2007) due to the presence of –OCH3 in association the presence of lignin. This band was present in higher intensity in the microwave activated carbon in comparison to the oven activated carbon (Nunes et al., 2009), which is attributed to the thermal degradation of lignin given the high temperatures attained during conventional oven carbonization. A comparison of the IR spectra obtained for the AC’s obtained in the microwave (this study) and in the muffle (Nunes et al., 2009) indicated that muffle carbonization provided higher intensity of the absorption band near 1600 cm1 (C@O stretching vibrations) and a decrease in the intensity of absorption bands at 3330–3350 cm1, associated with stretching vibrations of the (O– H) bond. Analyses of the IR spectra of lignin, before and after carbonization (Suhas et al., 2007) have shown a gradual decrease in the intensity of vibrations of the hydroxyl-containing groups with increase in the carbonization temperature. This was attributed to part of the alcoholic hydroxyls undergoing deeper oxidation to carboxyls and aldehydes, in association with a gradual increase in the intensity of the absorption bands related to C@O stretching vibrations. The bands for the hydroxyl groups above 3000 cm1 have been suggested to be due to alcoholic or phenolic bonds. Therefore, their higher intensity after microwave carbonization in comparison to the muffle oven is in agreement with the results from both the Boehm and pHPZC analyses.

80 70

pH - 3 pH - 5 pH - 7 pH - 9 pH - 11

60 50 40 30 30

120 Time (min)

300

Fig. 1. Effect of initial solution pH on MB adsorption by CCMW (initial MB concentration 500 mg L1; adsorbent dosage 10 g L1).

capacity were observed after equilibrium was attained – 5 h adsorption (based on the Tuckey test at 5% probability). Similar results were reported by Kavitha and Namasivayam (2007) for coir pith and also by Nunes et al. (2009) for MB adsorption employing the coffee press cake based adsorbent (CCMO) obtained by carbonization in a muffle oven. The results obtained before equilibrium was reached (30 min and 2 h) show that adsorption was less efficient for pH values below the pHPZC (8) as a consequence of the positively charged adsorbent surface promoting an electrostatic repulsion to the positive MB ion in solution. However, the pH of the solutions was measured after all adsorption tests, and pH values varied during adsorption towards the pHPZC. Given that the initial solution pH did not present a significant effect on adsorption capacity after equilibrium was reached, the remaining tests were conducted at pH 5, since it is the closest value to the natural pH of the MB solution. 3.4. Influence of adsorbent dosage Experimental results regarding the influence of adsorbent dosage on the efficiency of MB removal showed that removal efficiency increased with the increase in adsorbent dosage. After 5 h, the percent removal of MB varied from 55% to 98% as the adsorbent concentration was increased from 5 to 50 g L1. This is attributed to the increase in surface area resulting from the increase in adsorbent mass, and the resulting increase in number of active adsorption sites. However, the amount of dye adsorbed per unit mass of adsorbent decreased with increasing adsorbent mass, due to the reduction in both the effective surface area and adsorbate/adsorbent ratio. After 5 h, the amount adsorbed varied from 52.7 to 9.3 mg g1 as the adsorbent concentration was increased from 5 to 50 g L1. Similar behavior for the effect of adsorbent concentrations on MB adsorption capacity was reported in the literature for other types of adsorbents such as date pits and coffee husks (Banat et al., 2003; Oliveira et al., 2008b) and also for the adsorbent obtained by conventional carbonization of the coffee press cake

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A.S. Franca et al. / Bioresource Technology 101 (2010) 1068–1074

(Nunes et al., 2009). Based on these results, the remaining experiments were conducted for the following adsorbent dosage: 10 g L1 (90% MB removal; 41 mg g1 adsorption capacity). 3.5. Adsorption kinetics The CCMW adsorption data, at room temperature (25 °C) and pH 5 (Fig. 2(a)), showed that a contact time of 4 h assured attainment of equilibrium conditions for initial dye concentrations below equal or below 500 mg L1. The average difference between adsorption capacity after 4 and 6 h was 1.1% in the concentration range of 100–500 mg L1, increasing to 6% for an initial MB concentration of 750 mg L1. Adsorption can be described by a twostage kinetic behavior, with a rapid initial adsorption during the first 30 min, followed afterwards by a much slower rate, the same qualitative behavior observed for the oven produced adsorbent (Nunes et al., 2009). The MB adsorption efficiency ranged from 43% to 92% after 30 min and from 67% to 98% after 6 h, with the highest efficiency values corresponding to the lowest initial MB concentration. A comparison of fractional adsorption data between

a

60

50

Q (mg g-1)

40

30

20

10

0 0

60

120

180

240

300

360

Time (min)

b

1.0

0.8

q/qe

0.6

0.4

0.2

0.0 0

60

120

180

240

300

360

Time (min) Fig. 2. Effect of contact time on MB adsorption by CCMW: (a) variation of the adsorption capacity with time (Initial MB concentration:  – 100 mg L1; j – 200 mg L1; N – 300 mg L1; d – 400 mg L1; h – 500 mg L1; s – 750 mg L1. Solid lines correspond to pseudo-second-order kinetics model fits); (b) variation of the fractional adsorption capacity with time in comparison with the adsorbent prepared in the oven. (Initial MB concentration: CCMO:  – 100 mg L1; j – 300 mg L1; N – 750 mg L1. CCMW: } – 100 mg L1; h – 300 mg L1; 4 – 750 mg L1).

this study and the previous one employing conventional carbonization (Nunes et al., 2009) is shown in Fig. 2(b). The effect of varying initial MB concentration on adsorption efficiency can be also perceived here. Furthermore, the data shown in Fig 2(b) show that, under the same conditions, MB adsorption occurs faster for the adsorbent produced in the oven in comparison to the one produced in the microwave. Even though the faster adsorption during the first 30 min can be perceived as an indication that MB adsorption occurs preferably on the surface of the adsorbent, as also reported by Nunes et al. (2009), the results presented in Fig. 2(b) suggest that, in the case of microwave carbonization, resistance to pore diffusion may have some importance. As expected, adsorption of MB by CCMW presented a strong dependency on MB initial concentration. An increase in the initial MB concentration led to an increase in the amount adsorbed, being attributed to the increase in driving force (concentration gradient) with the increase in the initial dye. The amount of MB adsorbed increased from 8.7 to 60 mg g1 as the initial concentration was increased from 100 to 750 mg L1, significantly higher than the results obtained by Nunes et al. (2009) employing carbonization in a conventional muffle oven (4.2–18.4 mg g1). Some aspects that might be attributed the higher adsorption capacity for the CCMW compared to that for the activated carbon produced in a muffle oven (CCMO) are discussed next. Menéndez et al. (1999) have demonstrated that microwave treatments tend to increase the aromatic character of the carbon, as observed by the decrease in the intensity of the aliphatic C–H stretching absorption bands (3000–2700 cm1). Hence, a wider distribution of delocalized p electrons on the carbon surface would favor hydrophobic interactions between the dye molecule (through the free electrons of aromatic rings and of –N@C– bonds) and the carbon matrix. However, the presence of acidic groups such as carboxylic and phenolic at the carbon surface will attract and localize the p electrons of the condensed aromatic sheets on the surface of the carbon. At pH >4, these oxygen-containing acidic groups are mostly present in their deprotonated form, thus, bearing a negative charge which, in turn, will enhance the electrostatic interaction between the carbon surface and the dye cation and, as a consequence, enhance the adsorption capacity of the carbon material. Another aspect that will enhance the adsorption capacity of the CCMW with regard to MB is the higher charge density of CCMW surface, in comparison with that of the CCMO, which favors the formation of large molecular aggregates of MB on the surface. With the formation of these aggregates at the surface, the average orientation angle of the MB molecules is increased and, thus, higher densities of adsorbed dye cations are allowed (Bujdák, 2006). In this case, the predominant mechanism of MB molecule adsorption is of the end-on type, i.e., most likely by electrostatic interaction between the negatively charged oxygenated acidic groups and the positively charged nitrogen end of the dye molecule as proposed by Pavan et al. (2008). Hydrophobic interactions are not to be ruled out though. The results regarding the ability of kinetic models to fit the data and to provide estimates for equilibrium adsorption capacity are displayed in Table 2. The Elovich model did not provide a satisfactory fit, with lower correlation coefficients especially for the adsorption tests performed at lower initial MB concentrations (<400 mg1). The first-order model, although slightly better than Elovich, also did not provide a good fit. Even though R2 values are reasonable, qe values are significantly underestimated. Ritchie’s second-order model provided a reasonable fit, both in terms of R2 and SSq values. However, such fit was based on a trial and error estimate of qe. An evaluation of R2, SSq and the differences between estimated and experimental qe values indicates that MB adsorption by CCMW can be satisfactorily described by the pseudo second-order model, thus suggesting chemisorption as the rate-controlling

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A.S. Franca et al. / Bioresource Technology 101 (2010) 1068–1074 Table 2 Kinetic parameters for MB adsorption by CCMW. Initial dye conc. (mg L1)

Pseudo first-order

100 200 300 400 500 750

Pseudo second-order a

qe

% dif.

3.23 12.47 19.62 23.75 34.97 45.60

63.9 31.6 30.2 35.9 19.4 23.5

2

k1

R

SSq

qe

% dif.a

k2

R2

SSq

0.0405 0.0416 0.0260 0.0159 0.1640 0.1050

0.9157 0.9755 0.9908 0.9784 0.9926 0.9898

17.63 18.66 29.09 42.18 29.94 39.08

9.16 19.31 28.99 38.46 46.30 62.89

2.1 5.9 3.0 3.8 6.7 5.5

0.0393 0.0067 0.0034 0.0018 0.0009 0.0006

0.9998 0.9991 0.9995 0.9996 0.9982 0.9956

0.85 2.00 4.42 4.70 5.46 10.05

Elovich

100 200 300 400 500 750 a b

Ritchie (second-order)

a

b

R2

SSq

qeb

k2

R2

SSq

533.3 10.9 25.7 11.7 5.5 7.3

1.2183 0.3179 0.2636 0.1685 0.1226 0.0926

0.9231 0.9313 0.9633 0.9757 0.9860 0.9886

0.62 2.25 3.07 3.88 4.03 4.81

9.20 19.60 28.90 38.70 46.70 67.00

0.3189 0.1095 0.1124 0.0618 0.0394 0.0223

0.9895 0.9935 0.9570 0.9989 0.9679 0.9954

0.89 2.39 4.26 5.36 5.54 16.51

Percent difference between estimated and experimental qe values. Trial and error estimate that provided the best model fit.

mechanism (Ho, 2006; Oliveira et al., 2008b; Nunes et al., 2009). The values of the rate constant decreased with increasing initial MB concentration, without significant differences for concentrations above 500 mg L1. Similar behavior was observed for the activated carbon produced by conventional heating (Nunes et al., 2009). Such results are in agreement with the discussion presented by Ho (2006), regarding the better fit of the second-order model in comparison to Lagergren (first order), Elovich and Ritchie’s equations. Furthermore, the second-order model provides a satisfactory estimate for the equilibrium adsorption capacity, thus eliminating the need for previous knowledge of this parameter from experimental data (Ho, 2006).

Table 3 Equilibrium model constants for adsorption of methylene blue by CCMW.

3.6. Adsorption equilibrium The adsorption isotherm at 25 °C is presented in Fig. 3. The shape of the curve indicates favorable adsorption, also corroborated by the range of the Langmuir separation parameter r (Table 3). Langmuir, Freundlich and Tempkin adsorption models were evaluated for description of the MB sorption isotherm and coefficients are displayed in Table 3. MB adsorption from aqueous solutions by CCMW was better described by the Langmuir and Tempkin models, indicating homogeneous and monolayer adsorption. Maximum MB uptake capacity, based on Langmuir model,

Constants

Langmuir KL (L mg1) qmax (mg g1) r R2 SS

0.0642 68.49 0.015–0.135 0.9957 3.030

Freundlich KF (mg1(1/n) L1/n g1) 1/n R2 SS

9.54 0.3735 0.8704 8.457

Tempkin KT (L g1) b (kJ mol1) R2 SS

1.072 219.06 0.9841 2.395

was 68.5 mg g1, which represents a substantial increase in adsorption capacity (4.6 times) provided by the microwave activation procedure as opposed to conventional oven thermal activation (Table 4). Adsorption capacity was high compared to other

Table 4 Monolayer maximum adsorption capacity of several residue based adsorbents for MB removal at ambient temperature (25–30 °C).

80

60 qe (mg g-1)

Model

40 Experimental Langmuir Freundlich

20

Tempkin

50

100

150

200

250

Ce (mg L-1) Fig. 3. Adsorption isotherm of MB by CCMW.

300

qmax (mg g1)

References

Waste apricota (500 °C) Defective coffee beans press cake – microwaveb (250 °C) Sunflower oil cakea (600 °C) Defective coffee beans press cake – ovenb (800 °C) Date pitsb (500 °C) Hazelnut shella (750 °C) Coir pithb (700 °C)

102.0 68.5

Basar (2006) This study

Apricot stonesa (750 °C) Walnut shella (750 °C) Almond shella (800 °C)

0 0

Residue (activation temp. °C)

350 a b

Thermal/chemical activation. Thermal activation only.

16.4 14.9

Karagöz et al. (2008) Nunes et al. (2009)

12.9 8.8 5.9

Banat et al. (2003) Aygun et al. (2003) Kavitha and Namasivayam (2007) Aygun et al. (2003) Aygun et al. (2003) Aygun et al. (2003)

4.1 3.5 1.3

1074

A.S. Franca et al. / Bioresource Technology 101 (2010) 1068–1074

5 4

ln qe

3 2 1 0 -2

-1

0

1

2

3

4

5

6

7

ln Ce Fig. 4. Freundlich plots of isotherms for MB adsorption by adsorbents based on defective coffee beans press cake.  – CCMW – microwave carbonization; j – CCMO – muffle carbonization.

adsorbents employing similar types of residues, such as sunflower oil cake and date pits (Table 4). Regarding Freundlich isotherm, the slope 1/n ranging between 0 and 1 is a measure of adsorption intensity. A value for 1/n below one indicates a normal Langmuir isotherm while 1/n above one is indicative of cooperative adsorption. An average value of 0.37 was observed for 1/n, corroborating the homogeneous nature of the adsorbent surface also consistent with the good Langmuir and Tempkin fits. The Freundlich plots for adsorption of MB onto both CCMW and CCMO are presented in Fig. 4. Notice that, in the case of MB adsorption onto CCMO, a straight line is obtained whereas in the case of MB adsorption onto CCMW, the Freundlich isotherm can be represented by two straight lines. This can be attributed to the monolayer adsorption of MB molecules occurring both on the surface and also inside micropores, as commented by Hirata et al. (2002). Hence, the adsorption of MB onto CCMO can be described as predominantly occurring on the surface of the carbon. This is also in agreement with the faster adsorption of MB by CCMO compared to CCMW. 4. Conclusions The potential of microwaves in the production of activated carbons employing defective coffee press cake was demonstrated. Equilibrium data indicated favorable and homogeneous MB adsorption. Microwave carbonization provided significant increases in the maximum uptake capacity (15?69 mg g1) and yield (10%?54%), and a significant decrease in processing time (60?8 min) compared to conventional oven carbonization. The increase in adsorption capacity obtained with microwave as opposed to conventional muffle carbonization is attributed to: (i) the presence of carboxylic groups on the adsorbent surface, (ii) development of micropores, and (iii) higher charge density on the adsorbent surface. Acknowledgements The authors acknowledge financial support from the following Brazilian Government Agencies: CNPq and FAPEMIG. References Aygun, A., Yenisoy-Karakas, S., Duman, I., 2003. Production of granular activated carbon from fruit stones and nutshells and evaluation of their physical, chemical and adsorption properties. Micropor. Mesopor. Mat. 66, 189–195. Banat, F., Al-Asheh, S., Al-Makhadmeh, L., 2003. Evaluation of the use of raw and activated date pits as potential adsorbents for dye containing waters. Process Biochem. 39, 193–202.

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