Adsorption of Cadmium from Aqueous Solutions onto

Adsorption of Cadmium from Aqueous Solutions onto Coffee Grounds and Wheat Straw: Equilibrium and Kinetic Study Downloaded from ascelibrary.org by Agricultural Information Institute on 01/20/17. Copyright ASCE. For personal use only; all rights reserved.

Abhishek Dutta 1; Yudi Diao 2; Rohan Jain 3; Eldon R. Rene 4; and Susmita Dutta 5

Abstract: The adsorption kinetics of cadmium (Cd) from wastewater onto coffee grounds and wheat straw was ascertained under various process conditions. Batch tests were performed to study the effect of pH, adsorbent concentration, contact time, and initial metal concentration on the removal of Cadmium. The adsorption kinetics was envisaged by fitting the experimental data to the pseudo-first-order and pseudo-second-order kinetic models; and based on linear regression analysis, the best fit was found to be for the pseudo-second-order kinetic model for both adsorbents. Equilibrium conditions were achieved in less than 30 min. Two isotherm equations, namely Langmuir and Freundlich models, were also used to fit the experimental adsorption equilibrium data. The results showed that both physisorption and chemisorption mechanisms controlled the adsorption rate and capacity. The maximum adsorption capacity of coffee grounds and wheat straw was calculated to be 16.2 and 31.6 mg=g, respectively. DOI: 10.1061/(ASCE)EE.1943-7870.0001015. © 2015 American Society of Civil Engineers. Author keywords: Coffee grounds; Wheat straw; Cadmium; Kinetic model; Adsorption isotherms.

Introduction The importance of industrial wastewater treatment, especially for heavy metals, has become increasingly important in recent years. Heavy metals can cause serious harm to human beings, aquatic life, natural habitats, and soil–plant ecological systems, leading to severe ecological imbalance. Most heavy metal salts are soluble in water, forming aqueous solutions, and therefore cannot be separated by conventional means of physical separation (Duggal 1995). Heavy metal ions diffuse into surface water, groundwater, and soil–plant ecological systems before finally accumulating in the body of aquatic organisms, animals, plants, and human beings. Their long-term toxic effects are well documented in the literature (Smirjdkova et al. 2005). Cadmium is a highly toxic heavy metal and has a maximum permissible concentration of 5–10 ppb in industrial waters (U.S. EPA 2011). Heavy metals, including cadmium, can be removed by chemical precipitation, ion exchange, membrane filtration, adsorption, and electrochemical treatment (Fu and Wang 2011). Adsorption is an attractive technology owing to its ease of operation, flexibility in design, and ability to generate 1 Full-Time Guest Assistant Professor, Dept. of Materials Engineering (MTM), KU Leuven, Kasteelpark Arenberg 44 bus 2450, B-3001 HeverleeLeuven, Belgium; and Faculteit Industriële Ingenieurswetenschappen, KU Leuven, Campus Groep T Leuven, Andreas Vesaliusstraat 13, B-3000 Leuven, Belgium (corresponding author). E-mail: [email protected] 2 Researcher, Faculteit Industriële Ingenieurswetenschappen, KU Leuven, Campus Groep T Leuven, Andreas Vesaliusstraat 13, B-3000 Leuven, Belgium. 3 Postdoctoral Researcher, UNESCO-IHE Institute for Water Education, Westvest 7, 2611 AX Delft, Netherlands. 4 Assistant Professor, UNESCO-IHE Institute for Water Education, Westvest 7, 2611 AX Delft, Netherlands. 5 Associate Professor, Dept. of Chemical Engineering, National Institute of Technology, Durgapur 7l3209, India. Note. This manuscript was submitted on January 22, 2015; approved on June 29, 2015; published online on August 13, 2015. Discussion period open until January 13, 2016; separate discussions must be submitted for individual papers. This paper is part of the Journal of Environmental Engineering, © ASCE, ISSN 0733-9372/C4015014(6)/$25.00.

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high-quality effluent, particularly in the case of high-volume and low-metal-concentration contaminated wastewater (Fu and Wang 2011). Ideally, the adsorbent should be cost competitive and efficient; thus, a constant search is on for low-cost adsorbents that possess such qualities. Concerning the use of adsorption for the treatment of heavy metal–polluted wastewaters, different types of adsorbents, both natural and synthetic based, have been successfully tested in lab and field trials. Among them, sorption of heavy metals using low-cost natural adsorbents is widely recognized as one of the best alternatives to traditional methods of metal recovery (Say et al. 2001). It offers the following advantages: low cost, high adsorption and removal efficiency of heavy metal ions, low use of chemicals, reusability of biomaterials and nutrients, and possibility of metal recovery (Fu and Wang 2011; Owlad et al. 2009). In this line of progressive research, agricultural wastes can serve as low-cost adsorbents owing to their abundant availability. Typical examples include rice husk, peanut husk, saw dust, tamarind seeds, wheat bran, and coffee grounds. Coffee grounds are residues with fine particle sizes that are obtained during the treatment of raw coffee grounds with hot water or steam for instant coffee preparation; the final residues usually originate from cafeterias (Kyzas 2012). Almost 50% of worldwide coffee production is processed for soluble coffee preparation. Therefore, large amounts of coffee grounds are generated in the coffee industry with a worldwide annual generation of 6 million tons (Mussatto et al. 2011). Wheat straw is an agricultural byproduct, i.e., the dry stalks of wheat after the grain and chaff have been removed. The average yield of wheat straw is nearly 1.3–1.4 kg=kg wheat grain, which is a significant amount of surplus straw (Garde et al. 2002). Although wheat straw has various applications, some parts of wheat straw are still burned before discharge, thereby significantly contributing to atmospheric pollution and waste of resources (Farooq et al. 2010). The study of adsorption kinetics in wastewater treatment systems is significant from a practical viewpoint because it provides valuable insights into the reaction pathways and the mechanism of adsorption reactions. Adsorption kinetics describes the metal uptake rate, which controls the residence time and the size of the adsorption equipment (Amarasinghe and Williams 2007).

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Therefore, it is important to predict the metal uptake rate in adsorption systems so that appropriate adsorption-based techniques can be designed. The main objective of this study was to determine the maximum adsorption capacity of cadmium onto coffee grounds and wheat straw and to analyze the effect of process variables, such as pH, adsorbent concentration, contact time, and initial metal concentration, on the removal of cadmium from wastewater. The adsorption kinetics and isotherm behaviors were compared for both the adsorbents, and the different rate constants and parameters (qmax , kL , n, and K F ) were estimated.

Materials and Methods Preparation of Adsorbents Coffee grounds (medium roast, Douwe Egberts, Amsterdam, Netherlands) are usually discarded as a waste in the coffee industry and cafeterias. In this study, they were washed with distilled water and dried in an oven at 105°C for 5 h. Native wheat straw was obtained dry from a local shop (AVEVE, Leuven, Belgium) and was cut into small pieces and pressed into dry powder. Preparation of Cadmium Solution Cadmium solution was prepared by dissolving cadmium nitrate [CdðNO3 Þ2 · 4H2 O] in distilled water, and then 1 g hydrated CdðNO3 Þ2 was weighed and added to the distilled water and prepared as 1 g=L CdðNO3 Þ2 stock solution. Several solutions with pH ranging from 2.0 to 7.0 were prepared by the addition of 1 M HNO3 and 1 M NaOH. The initial concentration of CdðNO3 Þ2 was varied from 25 to 300 mg=L, prepared from the stock solution.

Effect of Contact Time Kinetic experiments were carried out by mixing 0.5 g adsorbent with 100 mL CdðNO3 Þ2 solution having an initial metal ion concentration of 100 mg=L. The suspensions were shaken at an agitation rate of 150 rpm at 25°C for 5, 10, 15, 20, 30, 60, 120, 180, and 240 min. Nine samples were taken from separate solutions that had different contact times. The adsorbents and the cadmium solutions were separated by filtration using Whatman filter paper (GE Healthcare, Eindhoven, Netherlands). Adsorption Equilibrium Studies The adsorption equilibrium study was carried out for initial cadmium concentrations varying between 25 and 300 mg=L; 5 g=L of the adsorbents were mixed with 100 mL CdðNO3 Þ2 solution at their respective initial concentrations. Estimation of Metal Uptake Capacity The metal uptake q, which refers to the amount of Cd adsorbed, was computed using the following expression: q¼

C0 − Ct V m

where C0 and Ct = cadmium concentrations (mg=L) before and after adsorption at time t, respectively; V = volume of adsorbate (L); and m = mass (g) of adsorbent added to the adsorbate. When the contact time t is equal to or greater than the equilibrium time, the metal quantity adsorbed at equilibrium can be calculated by qe ¼

Adsorption Experiments Effect of pH The effect of pH was assessed by varying the pH of the solution and keeping all other factors at constant values. pH was varied from 2.0 to 7.0 by adding HNO3 (1 M) or NaOH (1 M). During this experiment, 0.5 g adsorbent (coffee grounds and wheat straw) was added to 100 mL CdðNO3 Þ2 solution at a concentration of 100 mg=L in 250 mL Erlenmeyer flasks. The solutions were agitated for 4 h at 150 revolutions per minute (rpm) in an orbital shaker (25°C). The optimum initial pH was obtained by analyzing the percentage (%) and quantity (mg=g) of Cd adsorbed in these solutions. Effect of Adsorbent Concentration To elucidate the effect of adsorbent concentration, different amounts (0.1, 0.2, 0.3, 0.4, 0.5, 0.7, 1.0, and 1.5 g) of the adsorbent were added to 100 mL CdðNO3 Þ2 solution at a concentration of 100 mg=L in 250 mL Erlenmeyer flasks. All the solutions were adjusted to the optimal pH, obtained from the previous experiment. The suspensions were shaken for 4 h, at an agitation rate of 150 rpm (25°C). © ASCE

C0 − Ce V m

ð2Þ

To calculate the percentage adsorption of cadmium, the following equation was used:

Screening of Process Variables Screening of the different process variables (pH, initial concentration of cadmium, adsorbent concentration, and contact time) was done using one-factor-at-a-time (OFAT) analysis, which has several advantages, such as run size economy, fewer level changes, and providing protection against the risk of premature termination of experiments (Sen 1996). To estimate the influence of one factor, it was varied from low to high values, while the other factors were kept constant during the experiments.

ð1Þ

ηð%Þ ¼

C0 − Cr 100 C0

ð3Þ

where Cr = residual concentration of cadmium in solution (mg=L). Adsorption Kinetics The adsorption kinetics was studied by using two well-known models, namely, pseudo-first-order and pseudo-second-order kinetic models. The best model was selected based on the linear regression correlation coefficient R2 with a confidence interval of 95%, which is a measure of the accuracy of predicted values from a model match with the experimental data (Okewale et al. 2013). Pseudo-First-Order Model The pseudo-first-order kinetic model assumes that the rate of adsorption is proportional to the number of unoccupied sites (Lagergren 1898). The model equation can be expressed as follows: dqt ¼ k1 ðqe − qt Þ dt

ð4Þ

where k1 (1=min) = rate constant of pseudo-first-order adsorption; qe = metal quantity (mg=g) adsorbed at equilibrium; and qt = metal quantity (mg=g) adsorbed at time t. The integration of this equation with the boundary conditions (qt ¼ 0 at t ¼ 0) and (qt ¼ 0 at t ¼ t) yields k1 t ð5Þ logðqe − qt Þ ¼ log qe − 2.303

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where k1 (1=min) = rate constant of pseudo-first-order. The rate constant k1 can be obtained by a linear plot of log (qe − qt ) against time.


Pseudo-Second-Order Model The pseudo-second-order kinetic model is based on the assumption that the process may be second-order and that chemisorption occurs, which involves valence forces through sharing or exchange of electrons between the heavy metal ions and the adsorbent (Ho and McKay 1999). The model equation can be written as follows: dq ¼ k2 ðqe − qt Þ2 dt

ð6Þ

Integrating the equation with the boundary conditions (qt ¼ 0 at t ¼ 0) and (qt ¼ 0 at t ¼ t) gives t 1 1 ¼ þ qt v0 qe t

ð7Þ

v0 ¼ k2 q2e

ð8Þ

Fig. 1. Effect of solution pH on adsorption of cadmium ions on coffee grounds and wheat straw [pH ¼ 2–7, C0 ¼ 100 mg=L, adsorbent ¼ 5 g=L, T ¼ 25°C, agitation rate = 150 revolutions per minute (rpm), contact time = 4 h]

The pseudo-second-order model rate constant k2 (g=mg=min) can then be determined graphically by plotting t=qt against t.

Results and Discussions

Adsorption Isotherms

Effect of pH Langmuir Isotherm The Langmuir isotherm model is applicable in cases where only one molecular layer of adsorbate is formed at the adsorbent surface, which remains constant even at higher adsorbate concentrations (Okewale et al. 2013). The Langmuir equation can be represented as follows: Ce 1 1 ¼ þ C qe qmax kL qmax e

ð9Þ

where qmax = maximum monolayer adsorption capacity of adsorbents (mg=g); kL = Langmuir adsorption constant (L=mg); Ce = equilibrium cadmium concentration in solution (mg=L); and qe = equilibrium adsorption capacity of adsorbents (mg=g). By plotting Ce =qe against Ce , the values of qmax and kL can be calculated, respectively, from the slope and intercept of the curve. Freundlich Isotherm The Freundlich isotherm model proposes a heterogeneous energetic distribution of active sites accompanied by interaction between adsorbed molecules (Dada et al. 2012). The Freundlich linear form can be represented by the following equation: 1 log qe ¼ log K F − log Ce n

ð10Þ

where K F = Freundlich constant related to the adsorption capacity (mg=g); and 1=n = empirical parameter related to adsorption intensity. The values of 1=n and K F can be calculated from the slope and the intercept by plotting log qe against log Ce . Analytical Cadmium concentrations in solution were determined using inductively coupled plasma optical emission spectroscopy (ICP-OES, Varian) according to the standardized procedure described by Moss et al. (2010). All the experiments mentioned in this study were carried out in duplicate. © ASCE

As observed in Fig. 1, the percentage of cadmium adsorbed increased with an increase in the initial metal solution pH and reached a maximum value at an initial metal solution pH of 5. At pH 2, the removal percentages were only 5 and 7% for coffee grounds and wheat straw, respectively. The metal removal percentages by adsorption onto coffee grounds increased sharply up to pH 4.0, or 78%. From pH 4 to 5, the adsorption increased to a maximum value of 88% at pH 5.0. From this point onward, the adsorption percentage remained almost constant, despite variations in pH from 5 to 7. As with the coffee grounds, the adsorption percentage of wheat straw also showed a sharp increase from pH 2 to 3, reaching 64% at pH 3. A further increase in the pH from 3 to 5 resulted in an increase in the adsorption profiles, reaching 92%. At all the pH values tested in this study, no precipitation of cadmium was predicted by Visual MINTEQ software. The lower percentage removal of cadmium at a low pH is due to the low number of deprotonated sites, providing a smaller number of sites for the adsorption of cadmium (Dhir and Kumar 2010). Moreover, at lower pH, the presence of excess Hþ ions in the solution competes with cadmium ions for the adsorption sites. The former are adsorbed more than the latter owing to the high concentration and high mobility of Hþ ions, whereas in the higher pH range there are fewer competing Hþ ions and the surface of the adsorbent may become negatively charged owing to higher deprotonation, which enhances the adsorption of positively charged cadmium ions through electrostatic forces of attraction (Lalvani et al. 1997). In both cases, the optimal pH was found to be 5, which was used for further adsorption experiments in the present study. Effect of Initial Cadmium Concentrations Fig. 2 shows the effect of the initial cadmium concentration on the adsorption percentage, whereas, according to Fig. 2, the adsorption percentage decreased with an increase in the initial concentration. The adsorption capacity increased with an increase in the initial

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Fig. 2. Effect of initial ion concentration on adsorption percentage of cadmium onto coffee grounds and wheat straw (pH ¼ 5; ion concentration = 25, 50, 100, 150, 200, 300 mg=L; 5 g=L adsorbent; T ¼ 25°C; 150 rpm; contact time = 4 h)

Fig. 4. Kinetics of metal adsorption onto coffee grounds and wheat straw by contact time studies (pH ¼ 5; adsorbent concentration ¼ 5 g=L; initial concentration ¼ 100 mg=L, T ¼ 25°C, 150 rpm, contact time = 5–240 min)

Table 1. Pseudo-Second-Order Kinetic Model Parameter Fits Using Coffee Grounds and Wheat Straw as Adsorbent for Cadmium Adsorption Pseudo-second-order model Adsorbent Coffee grounds Wheat straw

qe (mg=g)

k2 (g=mg=min)

R2

16.45 16.75

0.049 0.042

0.9999 0.9997

Fig. 3. Effect of adsorbent concentration on adsorption percentage of cadmium onto coffee grounds and wheat straw (pH ¼ 5; adsorbent concentration = 1, 2, 3, 4, 5, 7, 10, 15 g=L; initial concentration ¼ 100 mg=L, T ¼ 25°C, 150 rpm, contact time = 4 h)

concentration, especially when the initial concentration was increased to 200 mg=L for both coffee grounds and wheat straw. A further increase in the initial metal concentration to 300 mg=L did not increase the adsorption capacity, suggesting that a plateau had been reached.

Fig. 5. Pseudo-second-order kinetics for adsorption of Cd(II) ions using coffee grounds and wheat straw at 25°C (Eq. 7) and the corresponding model fit (Table 1)

Effect of Adsorbent Dosage The effect of adsorbent concentration on the metal adsorption capacity of both coffee grounds and wheat straw is illustrated in Fig. 3. The metal uptake due to adsorption increased to a maximum value of 56.4 and 53.4 mg metal=g adsorbent for wheat straw and coffee grounds, respectively, at 1 g=L, and a further increase in the adsorbent dosage from 1 to 15 g=L led to a continuous decrease in the metal uptake. The adsorption percentage increased till the © ASCE

adsorbent dosage was 5.0 g=L; after that the value remained almost constant. The increase in the adsorption percentage is obviously due to the increase in the adsorbent quantity and, thus, the number of adsorption sites. The saturation in the adsorption profile is attributable to the high solid concentration, which blocks the available sites, resulting in a lower total number of available sites for adsorption (Arias and Sen 2009).

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Table 2. Langmuir and Freundlich Adsorption Isotherm Model Constants Using Coffee Grounds and Wheat Straw as Adsorbent for Cadmium Adsorption Langmuir Adsorbent


Coffee grounds Wheat straw

Freundlich

qmax

kL

R2

kF

n

R2

16.23 31.546

0.179 0.226

0.9727 0.9953

7.201 6.896

1.144 2.7012

0.712 0.7854

Fig. 7. Freundlich model fit to experimental data on adsorption isotherms indicating correlation coefficient for both coffee grounds and wheat straw (Table 2)

Table 3. Comparison of Adsorption Capacity of Both Coffee Grounds and Wheat Straw Obtained in this Study in Comparison to Literature Adsorption capacity (mg=g)

Fig. 6. Langmuir model fit to experimental data of adsorption isotherms indicating correlation coefficient for both coffee grounds and wheat straw (Table 2)

Adsorbent

Results Results obtained in obtained from this study literature

Coffee grounds Wheat straw

Adsorption Kinetics

Reference

16.2

15.7

Azouaou et al. (2010)

31.6

15.7 20.1 51.6

Nouri et al. (2007) Farajzadeh and Monji (2004) Nouri and Hamdaoui (2007)

Fig. 4 represents the amount of metal adsorbed (in percentage terms) onto the two adsorbents versus time at room temperature. From this figure it is quite evident that adsorption is almost instantaneous and equilibrium is reached within 30 min. For both coffee grounds and wheat straw, the majority of cadmium is removed within 5 min, with the removal percentage increasing gradually up to 30 min, at which the removal percentage reaches around 80% and remains constant until 4 h. The kinetics of metal adsorption was modeled using pseudofirst- and pseudo-second-order kinetic models. Based on R2 values, the pseudo-second-order kinetic model, which explains chemisorption as a rate-controlling mechanism, was better able to describe the adsorption mechanism of cadmium onto the two adsorbents (coffee grounds and wheat straw). Similar observations have been reported in the literature (Dhir and Kumar 2010; Bulut and Tez 2007). The model parameters, equilibrium adsorption capacity qe, and pseudosecond-order model rate constant k2 are listed in Table 1. Fig. 5 indicates the model fit for both coffee grounds and wheat straw. It can be concluded that chemisorption controls the mechanism, while the other experiments made it clear that the adsorption capacity of the adsorbent was related to the number of unoccupied sites of the adsorbent. It can therefore be inferred that both physisorption and chemisorption control the adsorption rate and capacity, respectively.

models were calculated and are represented in Table 2. By comparing the correlation coefficients of the Langmuir and Freundlich adsorption isotherms (Figs. 6 and 7), it can be easily inferred that the Langmuir model (R2 ¼ 0.969 and 0.9953 for coffee grounds and white straw, respectively) is more suitable than the Freundlich model (R2 ¼ 0.712 and 0.7854 for coffee grounds and white straw, respectively). According to the Langmuir model, the maximum adsorption capacity of wheat straw (31.6 mg=g) is almost double that of coffee grounds (16.2 mg=g). These values are comparable to the results obtained from other agricultural adsorbents, such as, for example, rice husk, grass, or tea waste (Singh and Srivastava 2001; Amarasinghe and Williams 2007; Lü et al. 2010) (Table 3). One of the plausible reasons for such behavior could be differences in the specific surface area of the adsorbent. As evident from literature reports, the Brunauer–Emmett–Teller (BET) surface area of wheat straw and coffee grounds is 30 and 4.5 m2 =g, respectively (Ballesteros et al. 2014; Külaots et al. 2010). It could also be attributed to the higher surface charge density on wheat straw compared to coffee grounds. From a rather mechanistic view-point, to understand the nature of adsorbent-metal interactions further research is required in this field.

Adsorption Equilibrium

Conclusions

The adsorption equilibrium was studied using Langmuir and Freundlich isotherm equations. The various constants of the two

The adsorption capacities of coffee grounds and wheat straw were studied using cadmium as the model heavy metal. The effect of pH,

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initial metal concentration, adsorbent concentration, and contact time on the removal of cadmium was tested. The optimal initial pH was found to be 5. The adsorption percentage decreased with an increase in the initial concentration of cadmium. The results of the kinetic study revealed that the adsorption mechanism is rapid; the equilibrium is attained within 30 min. The dominance of a chemisorption mechanism was confirmed from well-known kinetic models. Coffee grounds and wheat straw have the potential as viable alternative adsorbents to minimize the concentration of cadmium in wastewater considering the efficiency and low cost of these two adsorbents.

Acknowledgments The authors thank Britt Vandenbroeck, Department of Materials Engineering at KU Leuven, for assistance with the ICP-OES setup.

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