Caesalpinia bonducella Leaf Powder as Biosorbent for Cu(II) Removal

Article pubs.acs.org/IECR

Caesalpinia bonducella Leaf Powder as Biosorbent for Cu(II) Removal from Aqueous Environment: Kinetics and Isotherms Gutha Yuvaraja,† Munagapati Venkata Subbaiah,‡ and Abburi Krishnaiah*,† †

Biopolymers and Thermophysical Laboratories, Department of Chemistry, Sri Venkateswara University, Tirupati 517 502, Andhra Pradesh, India ‡ School of Chemical Engineering, Department of Bioprocess Engineering, and Department of BIN Fusion Technology, Chonbuk National University, Jeonju, Chonbuk 561-756, Korea ABSTRACT: The ability of Caesalpinia bonducella leaf powder has been utilized as an inexpensive biosorbent for the removal of Cu(II) from aqueous media. Optimum biosorption conditions were found to be pH 5.0, adsorbent dosage 0.07 g/L, agitation speed 180 rpm, and equilibrium time 120 min. Fourier transform infrared (FT-IR) spectroscopy, scanning electron microscopy, energy dispersive X-ray analysis, and X-ray diffraction were used to characterize the surface functional groups, structure, compositional analysis, and crystallinity of the biosorbent. FT-IR analysis shows the involvement of various functional groups such as hydroxyl, amide, carboxyl, and carbonyl groups in the removal of Cu(II) from aqueous environment. Kinetic data were fitted well to a pseudo-second-order kinetic model compared to pseudo-first-order and intraparticle diffusion kinetic models. The equilibrium data were well fitted by the Langmuir isotherm model by revealing the maximum sorption capacity of 76.92 mg/g. wheat shell,11 mushroom biomass (Agaricus bisporus),12 seed powder of Strychnos potatorum L.,13 macrofungus (Lactarius scrobiculatus, colimanite core waste) biomass,14,15 and Caesalpinia bonducella seed powder16 have been used to remove metal ions from wastewater. C. bonducella, commonly known as fever nut, belongs to the family Fabaceae, a prickly shrub found throughout the hotter parts of India, Myanmar, and Sri Lanka. C. bonducella leaves are disposed of as waste in several countries. Because of this, they are easily available at no cost. In this study we have utilized this waste material as a biosorbent for removal of Cu(II) from wastewater by preparing the biosorbent with a simple treatment of washing with water. Due to its simple preparation and high biosorption capacity, C. bonducella leaf powder is used as a potential biosorbent for Cu(II) ion removal in the present investigation. The present study explores the feasibility of utilizing C. bonducella leaf powder as a promising biosorbent material for the removal of Cu(II) ions from aqueous solutions. The effects of various operating parameters including initial pH, dosage, contact time, temperature, and initial metal ion concentrations were studied. The kinetic parameters were calculated to determine the biosorption mechanism.

1. INTRODUCTION Toxic heavy metal ions enter water through discharges from various industrial activities such as fertilizer, mining, metal plating, batteries, paper, alloy making, and pesticide industries and cause a serious threat to the environment.1 Copper usually occurs in nature as oxides and sulfides. In acidic environments, free aqueous Cu2+ dominates. At pH 6.0−8.0, the predominant species are Cu2+, Cu(OH)2, CuHCO3, CuCO3, and CuOH+, while at pH >10 the major species is Cu(OH)3.2 Copper is one of the most toxic heavy metals, attracting much attention from environmentalists due to its acute and chronic toxic effects in animal and human health. Copper in trace amounts may be beneficial as an activator of some enzyme systems, but Cu(II) intake over the permissible levels leads to severe mucosal irritation, hepatic and renal damage, widespread capillary damage, and central nervous system problems.3 The World Health Organization (WHO) recommended a maximum acceptable concentration of Cu(II) in drinking water of 2.0 mg/L.4 As a result, it is essential to remove Cu(II) from wastewater before disposal. A number of methods such as precipitation, coagulation, ion exchange, cementation, electrodialysis, electrowinning, electrocoagulation, and reverse osmosis are available for removal of heavy metals from liquid effluents.5 The major disadvantages of these conventional treatment methods are high capital investment, reoccurring expenses, and incomplete metal removal, generating huge volumes of sludge or waste products that require safe disposal. Biosorption is an emerging and attractive method which involves sorption of dissolved substances by a biomaterial. Biosorption has recently received a great deal of attention due to the low cost of the materials used in these applications and for the environmentally friendly impact of the treatment of exhausted sorbents. Several types of biosorbents such as bacteria,6 algae,7 wine processing waste sludge,8 Phoenix tree leaf powder,9 Moringa oleifera bark,10 © 2012 American Chemical Society

2. EXPERIMENTAL DESIGN 2.1. Preparation of Biosorbent Material. C. bonducella leaves were collected from Somala, Chittoor District, Andhra Pradesh, India. Before use, the leaves were sun-dried and washed with distilled water several times to remove surface impurities, and then dried in an oven at 323 K for 2 days. The Received: Revised: Accepted: Published: 11218

December 26, 2011 August 7, 2012 August 7, 2012 August 7, 2012 dx.doi.org/10.1021/ie203039m | Ind. Eng. Chem. Res. 2012, 51, 11218−11225

Industrial & Engineering Chemistry Research

Article

dried leaves were ground into a fine powdered form by use of a mixer. Twenty-five grams of the powder was placed in a roundbottom flask and washed with double distilled water until the washings were free of color and turbidity. Finally, it was dried in a hot air oven at 323 K for 24 h and stored in an airtight container in order to avoid moisture. The obtained product was named “CBLP” (C. bonducella leaf powder) for further biosorption studies. 2.2. Reagents. All the necessary chemicals used in this work were of analytical reagent grade purchased from S. D. Fine Chemicals, Mumbai, India, and used without any further purification. All the glassware used was washed with 10% (v/v) HCl and rinsed several times with double distilled water. 2.3. Solution Preparation. A stock solution of Cu(II) was prepared by dissolving an accurately weighed amount of 3.803 g of CuSO4·5H2O (purity >99.99%) in 1 L of distilled water. All required initial Cu(II) concentrations were obtained by successive dilutions of stock solution with the double distilled water. To the stock solution 3−6 drops of concentrated HNO3 was added to prevent hydroxide formation. 2.4. Bulk Density Determination. The bulk density of CBLP was determined by the following procedure.17 This measurement was carried out in a 25 cm3 density bottle. The biosorbent of CBLP was added to the density bottle, and all the air spaces were filled by air. The mass of the density bottle containing CBLP was determined by the following equation.

provides information on the charge state (positive or negative) of CBLP at a specific pH. The PZC of the biosorbent was determined by adopting the following procedure.18 The point of zero charge determination (pHpzc) of CBLP was carried out by adding 50 mL of double distilled water to a 125 mL Erlenmeyer flask. The flasks were capped with cotton and kept boiling for 20 min to expel the dissolved CO2. A 0.07 g sample of the biosorbent (CBLP) was placed in another 125 mL Erlenmeyer flask with 50 mL of CO2-free water. The suspensions were then mechanically shaken with a shaking incubator and allowed to equilibrate for 48 h at room temperature. After completion of shaking the samples were withdrawn from the shaker and filtered. Then the pH of the filtrate was measured, and this value has been taken as the point of zero charge. 2.8. Instrumentation. Fourier transform infrared (FT-IR) spectra were recorded on an FT-IR spectrophotometer (Thermonicolet-200 series, Germany). The surface morphology of the biosorbent was examined by scanning electron microscopy (SEM), and the elemental contents were evidenced by energy dispersive X-ray (EDX) analysis (Carl Zeiss, EVO MA 15, England). An X-ray diffraction spectrum of pure CBLP was carried out with a Seifert 3003 TT X-ray diffraction spectrometer. Pure CBLP was pressed into pellets, and it was operated in the range of 10−80 (2θ) with the 2θ length. A digital pH meter (Digisum D1-7007, India) was used for the measurements of pH. Centrifugation was done using a REMI centrifuge. Elemental analysis was carried out by a vario microelemental analyzer. The concentration of Cu(II) was determined by a flame atomic absorption spectrometer (AAS; Shimadzu AA-6300, Japan) with a deuterium background corrector. 2.9. Batch Biosorption Studies. The biosorption of Cu(II) from aqueous solutions was investigated in a batch system. All biosorption experiments were performed by mixing 0.07 g of biosorbent in 125 mL Erlenmeyer flasks containing 50 mL of Cu(II) solution. The mixtures of biosorbent and Cu(II) solutions were shaken in a thermostatic incubator at 180 rpm at the desired temperature (K) and contact time (120 min). The samples were withdrawn from the shaker at regular intervals of time until equilibrium was reached, and the reaction mixture was separated by centrifugation at 4000 rpm for 10 min. Finally, the centrifugated samples were filtered by using filter paper (Whatman filter paper no. 41) and the filtrate was analyzed by using AAS. The amount of metal bound by the biosorbent was calculated from the difference between the initial and final concentrations of the metal ions in solution. All experiments were carried out in duplicate, and average values were considered for the data analysis. The amount adsorbed per unit mass of biosorbent at equilibrium was obtained using the following equation.

bulk density =

mass of CBLP occupying 25 cm 3 density bottle volume of 25 cm 3 density bottle

(1)

2.5. Ash Content Determination. A known amount of sample (CBLP) was placed in a crucible which was previously weighed. After that the crucible was kept in a muffle furnace and the temperature was increased gradually to 723 K for a minimum of 2 h or until all the carbon was eliminated. The crucible was weighed after it was cooled to room temperature in a desiccator. The percentage of total ash content was calculated with reference to the weight of the dry CBLP biosorbent. The ash content can be calculated from the following equation. ash content =

C−A ·100 B−A

(2)

where A is the weight of the empty crucible (g), B is the weight of the sample with the crucible (g), and C is the weight after ashing (g). 2.6. Determination of Moisture Content. A certain amount of the CBLP biosorbent (wet sample) was weighed in a moisture dish and recorded as the “wet weight of sample”. The CBLP was dried at 473 K for 2 h. Then the sample was allowed to cool. The cooled sample was weighed again, and the weight was recorded as the “dry weight of sample”. All the dried and weighed samples were stored in a desiccator for further studies. The moisture content of the sample is calculated using the following equation. %W =

A−B ·100 B

qe =

(C i − Ce)V M

(4)

where qe (mg/g) is the adsorption capacity at equilibrium, Ci and Ce are the initial and equilibrium concentrations (mg/L) of Cu(II), respectively, M (g) is the biosorbent dosage, and V (L) is the volume of the solution. 2.10. Desorption Studies. Desorption studies of adsorbed Cu(II) ions from CBLP was carried out in a batch system. Desorption studies were held to determine the feasibility of regenerating the spent biosorbent and to elucidate the mechanism of biosorption. Desorption experiments were

(3)

where % W is the percentage of moisture in the sample, A is the weight of wet sample (g), and B is the weight of dry sample (g). 2.7. Determination of the Point of Zero Charge (PZC). The PZC of CBLP is an important parameter to determine as it 11219

dx.doi.org/10.1021/ie203039m | Ind. Eng. Chem. Res. 2012, 51, 11218−11225

Industrial & Engineering Chemistry Research

Article

carried out by placing 0.07 g of CBLP in a 125 mL Erlenmeyer flask containing 50 mL of Cu(II) solution at 323 K, pH 5.0, and a contact time of 120 min in a lab line shaking incubator. After filtration the filtrate was estimated to its efficiency using AAS.

3. RESULTS AND DISCUSSION 3.1. Characterization of CBLP. Characterization of biosorbent is vital to understanding the metal binding Table 1. Physicochemical Characteristics of CBLP parameter moisture content (%) bulk density (g mL−1) ash content (%) PZC

2.1 0.4 9.12 4.5

Figure 3. Effect of biosorbent dose on biosorption of Cu(II) onto CBLP (pH 5.0, Cu(II) ion concentration 45 mg/L).

Figure 1. FT-IR spectra of (a) pure CBLP and (b) Cu(II) loaded CBLP.

Figure 4. Effect of contact time plot on biosorption of Cu(II) onto CBLP (pH 5.0, biosorbent dose 0.07 g/L).

3.1.2. X-ray Diffraction (XRD) Analysis. The biosorbent C. bonducella leaf powder (CBLP) consists of appreciable amounts of cellulose, hemicellulose, and lignin components. Because of this, the biosorbent is amorphous in nature (figure not shown). 3.1.3. FT-IR Analysis. The infrared spectral analysis was carried out to determine the type of functional groups involved in biosorption of Cu(II) onto CBLP. The FT-IR spectra of unloaded biosorbent and Cu(II) loaded biosorbent were recorded in the wavenumber range 4000−400 cm−1, and the results are shown in Figure 1. A characteristic strong and broad peak at 3372 cm−1 corresponds to the O−H stretching vibration of hydroxyl groups in CBLP. The hydroxyl groups present on the CBLP biosorbent are effective binding sites for metal ions, forming stable complexes by coordination. The band located at 2925 cm−1 is attributed to the C−H symmetrical stretching vibrations of the methyl groups present on the surface of the biosorbent. The distinct peaks observed at 1741 and 1644 cm−1 characterize the stretching vibration of carbonyl groups from aldehydes and ketones. The band observed at 1112 cm−1 is assigned to C−O stretching of alcohols and carboxylic acids. The symmetrical stretching vibration at 3372 cm−1 was shifted to 3421 cm−1 after Cu(II) biosorption, indicating the involvement of free hydroxyl groups in Cu(II) biosorption. The peaks at 2927 and 1644 cm−1 were shifted to lower frequencies

Figure 2. Effect of pH on biosorption of Cu(II) onto CBLP (Cu(II) ion concentration 25 mg/L).

mechanism onto biomass. The physicochemical characteristics of the moisture content, bulk density, ash content, and PZC of the biosorbent are shown in Table 1. 3.1.1. Elemental Analysis. Elemental analysis results shows CBLP was composed of 59.21% carbon, 7.2% hydrogen, 2.2% nitrogen, and 31.39% oxygen. 11220

dx.doi.org/10.1021/ie203039m | Ind. Eng. Chem. Res. 2012, 51, 11218−11225

Industrial & Engineering Chemistry Research

Article

Table 2. Kinetic Parameters for the Biosorption of Cu(II) onto CBLP at Different Metal Ion Concentrations Lagergren first order

Pseudo second order

Weber and Morris

Cu(II) concn (mg/L)

K1 (1/min)

R2

SSE

K2 (g/mg·min)

R2

SSE

Kid (mg/g·min−0.5)

R2

SSE

50 75 100 125

0.03 0.05 0.06 0.07

0.988 0.958 0.966 0.943

0.998 0.998 0.998 0.999

0.0007 0.0003 0.0001 0.0001

0.999 0.999 0.999 0.999

0.037 0.058 0.011 0.087

2.804 4.720 7.079 7.942

0.928 0.954 0.959 0.962

0.876 0.856 0.834 0.848

Table 3. Langmuir and Freundlich Isotherm Constants and Correlation Coefficients for Cu(II) Biosorption onto CBLP at Different Temperatures Langmuir

Freundlich 2

temp (K)

qm (mg/g)

b (L/mg)

R

303 313 323

58.88 66.66 76.92

0.290 0.238 0.173

0.999 0.999 0.999

χ

Kf (mg/g)

1/n

R2

χ2

4.996 9.338 16.178

14.79 14.09 12.70

0.52 0.56 0.59

0.978 0.964 0.968

48.87 54.12 65.92

2

the biosorbent which is a rough and porous structure, making it possible for the biosorption of Cu(II) onto CBLP. The EDX spectrum did not show any characteristic signal of Cu(II) in the unloaded CBLP (figure not shown), whereas clear signals were observed in the Cu(II) loaded CBLP (figure not shown). The presence of a new signal (Cu signal) after Cu(II) biosorption shows that the metal was absorbed by CBLP and now appears as a part of its chemical composition. The disappearance of some metal cations such as Ca2+, Mg2+, and Na+ from the EDX spectrum of Cu(II) loaded CBLP suggested the involvement of an ion exchange mechanism for the adsorption of Cu(II) by CBLP. In our results, the exchange of other metal cations (Ca2+, Mg2+, Na+) by CBLP was clearly observed by the EDX analysis. 3.2. Influence of pH. The acidity of the medium affects the competition of the hydrogen ions and the Cu(II) for the active sites on the biosorbent surface. To investigate the effect of the initial solution pH on the biosorption performance, experiments were carried out by placing 50 mL of Cu(II) (25 mg/L Cu(II)) metal solution in Erlenmeyer flasks and the pH was varied from 2.0 to 7.0 at 303 K for 2 h with shaking by the mechanical shaker. The pH was adjusted using 0.1 M HCl or 0.1 M NaOH. At lower pH (pH 200 250 pHpzc the biosorbent is negatively charged. In such situations the electrostatic attractions should occur between the positive charge of Cu(II) and the negatively charged biosorbent (CBLP) surface, resulting in an increase in the biosorption of Cu(II) onto CBLP. A decrease in removal efficiency at pH < pHpzc is due to the higher concentration of hydronium ions (H3O+) in the reaction mixture. 3.3. Influence of Biosorbent Dose. The quantity of biosorbent can influence the extent of metal uptake from the solution. A 50 mL volume of Cu(II) solution was placed in a 125 mL Erlenmeyer flask. To this different amounts of biosorbent (0.01−0.08 g/L) were added by keeping the pH (5.0) and temperature (303 K) constant. It is observed that the percent removal of Cu(II) increases with the increase in biosorbent dose, which may be due to the availability of more binding sites present on the biosorbent surface. The maximum Cu(II) removal of 92% was attained with 0.07 g/L (Figure 3) biosorbent dosage, and further increase in the dose (dose > 0.07 g/L) does not significantly change the biosorption yield. This is due to the binding of almost all Cu(II) molecules to the biosorbent surface and the establishment of equilibrium between Cu(II) ions on the biosorbent and in the solution. From this the optimum biosorbent dose is taken as 0.07 g/L for subsequent batch biosorption experiments. 3.4. Effect of Contact Time and Biosorption Rate Kinetics. The biosorption rate of CBLP was studied at various initial Cu(II) concentrations (50, 75, 100, and 125 mg/L) while keeping all other parameters constant (pH 5.0, biosorbent dose 0.07 g/L, and temperature 303 K). The nature of the biosorbent and its available sorption sites affects the time needed to reach equilibrium. The biosorption efficiency increases with a rise in contact time up to 120 min. After this time there is no considerable change in Cu(II) biosorption. Therefore, the optimum contact time was selected as 120 min (Figure 4) for further experimental studies. When the initial metal concentration increases, Cu(II) biosorption increases until the binding sites are not saturated. This sorption characteristic indicates that surface saturation is dependent on

t 1 1 = + t 2 qt qe K 2qe

(8)

where qe and qt are the amounts of the Cu(II) removal per unit mass of biosorbent (mg/g) at equilibrium at any time t (min), and K2 (g/mg·min) is the pseudo-second-order rate constant. The biosorption rate constant (K2) is obtained from a linear plot of t/qt versus t (figure not shown), and the values are shown in Table 2. Based on the values of the correlation coefficients of the pseudo-first-order and pseudo-second-order kinetic models, it may be concluded that biosorption of Cu(II) follows the pseudo-second-order reaction rather than the pseudo-first-order reaction. The kinetic data were further analyzed using Weber and Morris intraparticle diffusion model.23 qt = K idt 0.5 + c

(9)

where qt (mg/L) is the amount adsorbed at time t (min), Kid is the intraparticle diffusion rate constant (mg/g·min−0.5), and C is the intercept which gives an idea about the thickness of the boundary layer. The intraparticle diffusion model coefficient values were calculated from the plot of qt (mg/g) versus t0.5 (min) (figure not shown), and the values are given in Table 2. The first sharper portion is the mass transfer of solute molecules from the bulk solution to the biosorbent surface; this is also known as instantaneous biosorption. The second portion is the gradual biosorption stage, wherein intraparticle diffusion is rate limiting. The third portion is the final equilibrium stage, wherein intraparticle diffusion starts to slow down due to the extremely low solute concentration in the solution. In addition, the sum of squares error (SSE) test was carried out to support the best fit. SSE =

∑

(qt ,e − qt ,m)2 qt ,e 2

(10)

where qt,e and qt,m are the experimental biosorption capacities of metal ions (mg/g) at time t and the corresponding values that are obtained from the kinetic models. On the basis of the low 11222

dx.doi.org/10.1021/ie203039m | Ind. Eng. Chem. Res. 2012, 51, 11218−11225

Industrial & Engineering Chemistry Research

Article

Cu(II) onto CBLP. The χ2 statistic is basically the sum of the squares of the difference between the experimental data and data obtained by calculating from models, with each squared difference divided by the corresponding data obtained by calculation from the models. The equation for evaluating the best fit model is written as

SSE values, it can be concluded that biosorption Cu(II) onto CBLP follows the pseudo-second-order model. 3.5. Effect of Temperature. Biosorption experiments were performed at different temperatures (303, 313, and 323 K) for different initial Cu(II) ion concentrations at constant pH 5.0 and a biosorbent dose of 0.07 g/L. The adsorption capacity (qm) increased from 58.88, to 66.66, to 76.92 mg/g with the rise in temperature from 303, to 313, to 323 K. It can be seen that the sorption capacity increases with the increase in temperature, mainly due to the increase in the number of sorption sites generated because of the breaking of some internal bonds near the edge of active surface sites of the biosorbent. 3.6. Isotherm Studies. The biosorption capacity of CBLP can be described by the isotherms which express the surface properties and affinity of the CBLP biosorbent. The isotherms were investigated using two models: the Langmuir24 and Freundlich25 models. 3.6.1. Langmuir Isotherm. The Langmuir isotherm model is obtained under the ideal assumption of a totally homogeneous adsorption surface, and it can be represented as follows: 1 1 ⎡1⎤ 1 = ⎢ ⎥+ qe qmK ⎣ Ce ⎦ qm

χ2 =

1 1 + bC0

(11)

(12)

where b is the Langmuir constant (L/mg) and C0 is the initial biosorbent concentration of Cu(II) ions (mg/L). The value of RL indicates the isotherm shape and whether the sorption is favorable or not, as per the following criteria: unfavorable (RL > 1); linear (RL = 1); favorable (0 < RL < 1); irreversible (RL = 0). The RL values were found to be between 0 and 1 at the temperature studied for Cu(II) biosorption. The RL values show that the biosorption of Cu(II) onto CBLP follows the Langmuir isotherm (figure not shown). 3.6.2. Freundlich Isotherm. The Freundlich isotherm is suitable for a highly heterogeneous sorption system and is expressed as log qe = log K f +

1 log Ce n

qe,m

(14)

where qe,m is the equilibrium capacity obtained from the model (mg/g) and qe is the experimental data on the equilibrium capacity (mg/g). The χ2 values of the two isotherms are comparable, and the higher χ2 values confirm that the biosorption of Cu(II) onto CBLP follows the Langmuir isotherm model. 3.7. Effect of Particle Size Influence. The particle size is an important parameter in the biosorption process. The effect of different biosorbent particle sizes (50, 75, 100, and 125 μm) on the Cu(II) biosorption capacity by CBLP (figure not shown) was studied in the present investigation. A decrease in the particle size would lead to an increase in surface area and an increase in the binding opportunities between Cu(II) and the functional groups present on the surface of the biosorbent.26 The biosorption capacity of Cu(II) decreased with increased particle size greater than 50 μm. The Cu(II) biosorption capacity decreased from 85.22, to 72.46, to 32.23% with the increased particle size from 75, to 100, to 125 μm. Therefore, the Cu(II) biosorption capacity of CBLP shows great improvement with decreasing particle size. Hence all the Cu(II) biosorption experiments such as pH, biosorbent dose, kinetics, and isotherm studies were performed with particles of size less than 50 μm because of its high removal capacity (