Batch and fixed-bed column studies for biosorption of Zn(II) ions onto pongamia oil cake (Pongamia pinnata) from biodiesel oil extraction

Journal of Environmental Management 164 (2015) 161e170

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Research article

Batch and fixed-bed column studies for biosorption of Zn(II) ions onto pongamia oil cake (Pongamia pinnata) from biodiesel oil extraction M. Shanmugaprakash a, V. Sivakumar b, * a b

Downstream Processing Laboratory, Department of Biotechnology, Kumaraguru College of Technology, Coimbatore 641 049, India Department of Chemical Engineering, A.C Tech, Anna University, Chennai 600 025, India

a r t i c l e i n f o

a b s t r a c t

Article history: Received 22 March 2014 Received in revised form 2 July 2015 Accepted 24 August 2015 Available online xxx

The present work, analyzes the potential of defatted pongamia oil cake (DPOC) for the biosorption of Zn(II) ions from aqueous solutions in the both batch and column mode. Batch experiments were conducted to evaluate the optimal pH, effect of adsorbent dosage, initial Zn(II) ions concentration and contact time. The biosorption equilibrium and kinetics data for Zn(II) ions onto the DPOC were studied in detail, using several models, among all it was found to be that, Freundlich and the second-order model explained the equilibrium data well. The calculated thermodynamic parameters had shown that the biosorption of Zn(II) ions was exothermic and spontaneous in nature. Batch desorption studies showed that the maximum Zn(II) recovery occurred, using 0.1 M EDTA. The Bed Depth Service Time (BDST) and the Thomas model was successfully employed to evaluate the model parameters in the column mode. The results indicated that the DPOC can be applied as an effective and eco-friendly biosorbent for the removal of Zn(II) ions in polluted wastewater. © 2015 Elsevier Ltd. All rights reserved.

Keywords: Breakthrough curve Kinetics Isotherm Pongamia oil cake Regeneration Zn(II) biosorption

1. Introduction The increase in the rate of industrialization and extensive use of machines in all sectors, has contributed to a great percentage to the heavy metal pollution, by releasing the heavy metals into the effluents, causing health hazards to the biological systems. However, minimum quantities of iron, cobalt, copper, manganese, molybdenum, and zinc are required by the living systems and the heavy metals concentration beyond the certain level, damages the living system (Lane and Morel, 2000; Hall, 2002). Heavy metals such as cadmium, lead, zinc, nickel, copper, mercury and chromium or their compounds have been recognized as hazardous pollutants (Helen and Miranda, 2010). Due to their non-biodegradability and constant presence in the environment through the food chain, the removal of metallic species from waste water is significant for the protection of the environment and living organisms. Among the heavy metals, zinc is of special interest, because of its high toxicity at certain concentrations, and the wide area of industrial applications; for example, electro-plating, metal-finishing, mining and foundry activities, paint, pigments, ceramic industries, photographic paper, accelerators for rubber vulcanization, textiles,

* Corresponding author. E-mail address: [email protected] (V. Sivakumar). http://dx.doi.org/10.1016/j.jenvman.2015.08.034 0301-4797/© 2015 Elsevier Ltd. All rights reserved.

fertilizers, battery and accumulator manufacturing (Parab et al., 2006). When zinc ions are present above the tolerance level they cause serious health problems, such as respiratory distress syndrome, abdominal cramps, vomiting and nausea, prostate cancer, apoptosis, neuronal death, etc., (Plum et al., 2010). Therefore, it is very essential to treat the wastewater released from the industries, before it discharge into environment, and satisfy the environmental regulations for various bodies of water. The World Health Organization (WHO) recommends that the maximum acceptable concentration of zinc ions is 3.0 mg/L, in drinking water (WHO, 1997). Traditionally, many technologies are available for zinc removal, including chemical precipitation, conventional coagulation, reverse osmosis, ionic exchange, membrane based separation and carbon adsorption, but these are frequently inefficient when applied for the removal of metal ions in low concentrations, and require costly equipment and high cost operation and energy (Maiti et al., 2009). The disadvantages of these traditional and chemical methods have induced the urge to search for new economical, innovative and environmentally safe method. In these endeavors, the use of biological materials such as algae, fungi, yeast, bacteria and various plant biomasses, for the removal and recovery of heavy metals, at low cost operation, has gained importance during recent years (Plaza et al., 2013). One of the promising alternative methods and

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sustainable strategy is biosorption for cleaning up water that has been contaminated with heavy metals by anthropogenic activities and/or by natural processes, as it is effective, cheap and ecofriendly (Suazo-Madrid et al., 2011; Kratochvil and Volesky, 1998). There is a rapid increase in the recent years, focusing on heavy metal removal, using plant derived biomasses viz., peanut hull (Johnson et al., 2002), olive cake (Dakiky et al., 2002), Hibiscus rosasinensis (Vankar et al., 2011), Eichhornia crassipes (Mahamadi and Nharingo, 2010), Jatropha oil cake (Garg et al., 2007), Pongamia seed shell (Shanmugaprakash et al., 2014), Mustard oil cake (Khan et al., 2012). However, only a limited number of studies have been published on the biosorption of Zn(II) ions using different oil cakes. In our recent communication, Pongamia oil cake from pongamia pinnata plant (Family: Papilionaceace) is a solid waste residue, which is obtained as a by-product, after oil-extraction, is recognized as the potential biosorbent for removal of Cr(VI) ions in aqueous solutions (Shanmugaprakash et al., 2013). To date, no investigation has been reported on the removal of Zn(II) ions from waste water using DPOC as biosorbent. In the present study, the potential of defatted pongamia oil cake was exploited for the removal of zinc ions from aqueous solutions, in both the batch and continuous modes. The effects of various experimental parameters on the biosorption capacity of DPOC were investigated. Also, several isotherms, kinetic models and various thermodynamic parameters were estimated to understand the binding mechanisms. The BDST and Thomas models were applied to simulate the breakthrough curves and to determine the column capacity used in column studies. In addition, the desorptionregeneration studies were also performed to evaluate the reuse capabilities of the DPOC. 2. Experimental work 2.1. Preparation of biosorbent and synthetic waste water Pongamia oil cake was obtained from the local oil mills, and it was completely dried under the sun and then the impurities like husk, stone particles were separated manually. The dried biosorbent was converted into fine powder, by mechanical grinding and then sieved to get a uniform size of 200e250 mm. It was then dried in an oven at 90
C for 48 h and the resultant biosorbent was defatted with hexane, in a Soxhlet extractor (model no 212, Sigma Instruments Ltd., Chennai, India), in order to remove the residual oil present in the oil cake. The defatted biosorbent was then stored in a desiccator for further use. A stock zinc solution of 1000 mg/L concentration was prepared, by dissolving the required quantity of zinc sulfate in distilled water. For biosorption experiments, aqueous solutions of varying pH were prepared from the stock, by adjusting the pH with 0.1 N NaOH or 0.1 N HCl using a pH meter (LI 120, Elico Ltd., Hyderabad, India). The initial zinc ions concentration ranging from 100 to 500 mg/L was prepared, with appropriate dilution. All the chemicals used in the experiment were of analytical reagent grade and purchased from HiMedia Ltd., India. 2.2. Batch biosorption experiments Batch biosorption experiments for Zn(II) ions removal at various biosorbent doses (1.0e5.0 g/L), solution pH (3.0e7.0), temperature (303e323 K), contact time of 180 min and initial Zn(II) ions concentration (100e500 mg/L) with a stirring speed of 120 rpm were carried out. After the completion of every set of experiments, the solutions were centrifuged at 9000 rpm (Kubota, Japan) and the residual zinc ions concentration from clear supernatant was analyzed by flame atomic adsorption spectroscopy with airacetylene flame (SL 194, Elico Ltd., Hyderabad, India). In order to

study the effect of pH on removal efficiency of Zn(II) ions by DPOC, the initial pH solution was varied in pH range of 3e7, by adjusting the pH with 0.1 N NaOH or 0.1 N HCl. The initial and final pH of the solution was measured using a pH meter (LI 120, Elico Ltd., Hyderabad, India). All the experiments were carried out in duplicate and the average values are taken for the further analysis of data. In order to determine the various thermodynamic parameters, 303, 313 and 323 K temperatures were used to conduct biosorption experiments. The removal efficiency R (in percent) of Zn(II) ions was then calculated for each experiment as

Rð%Þ ¼

Co  Cf 100 Co

(1)

The biosorption capacities of the DPOC for metal ion concentration at equilibrium was calculated as

qe ðmg=g Þ ¼

Co  Ce V M

(2)

Where C0, Cf and Ce are the initial, final and equilibrium concentration of Zn(II) ions (mg/L) in the residual, respectively. V is the volume of aqueous solution (mL) and M is the mass of DPOC (g) used for the experiment (Muthusamy and Venkatachalam, 2015). 2.3. Column design and experimental procedure Column studies were carried out in a glass column (45  2.0 cm), filled with a known quantity of DPOC. In the column, 0.5-mm stainless steel mesh and 1.0 cm glass beads were kept at the bottom and the top, to support the biosorbent in the column, and also to ensure a closely packed arrangement. A 2.0 cm high layer of glass beads (1.0 mm in diameter) was placed at the column base, in order to provide a uniform inlet flow of the solution into the column. The Zn(II) solution was pumped through a peristaltic pump (model no.4651, Miclins, India) connected at the bottom of the column in an upward direction. The treated Zn(II) solution was collected from the top with the same flow rate as the feed stream and estimated for the Zn(II) ion concentration. All the experiments were performed in duplicate at 30
C temperature, pH 5 under room atmospheric pressure. The volume of the effluent, Vef (mL) was calculated using the following equation (Aksu and Gonen, 2004):

Veff ¼ Q *ttotal

(3)

where ttotal is total time (min) and Q is the volumetric flow rate of the Zn(II) ion solution inside the column (mL/min). The total amount of Zn(II) ions that passing through the column (Mtotal) was calculated by

Mtotal ¼

Co *F*te 1000

(4)

Where Co in initial zinc ion concentration (mg/mL), F is the volumetric flow rate (mL/min) and te is exhaustion time (min). The area under the breakthrough curve (A) can be determined by integrating the adsorbed concentration (Cad) versus time (t). Total quantity of zinc adsorbed (qtotal) in the column for a given feed concentration and flow rate is calculated from Eq. (5):

qtotal ¼

Q A Q ¼ 1000 1000

t¼t Z Total

CAd 100

(5)

t¼0

The total removal of Zn(II) ions (in percent) with respect to the flow volume was calculated as follows

M. Shanmugaprakash, V. Sivakumar / Journal of Environmental Management 164 (2015) 161e170

Tremoval ¼

qTotal 100 MTotal

163

(6)

Where Mad is the concentrations of the adsorbed zinc ions, (mg/L). 2.4. Characterization of the biosorbent The functional groups present on the surface of the DPOC before and after the Zn(II) ion biosorption were determined, using the Fourier transform infrared spectroscopy (FTIR) (Shimadzu, Japan). The spectra of the dry biosorbents were measured within a range of 500e4000 cm1. For this analysis, finely ground DPOC was encapsulated with KBr in the ratio of 1:50 in order to prepare translucent sample disks. The surface texture and morphology of the DPOC before and after Zn(II) biosorption was observed by the Scanning electron microscope (SEM) (JEOL Model JSM e 6390LV, Japan). The surface area was examined by N2 sorption at 77 K, using the Nova station e-series Surface area analyzer (Quantachrome, USA, Inc.). The point of zero charge (pHZPC) of the DPOC was determined, using the procedure explained by Khan et al. (2012). 2.5. Desorption and regeneration studies Initially, batch equilibrium experiments were performed at optimum conditions. Then, the feasibility of regenerating the DPOC was saturated with zinc ions is evaluated, using three different eluants, namely, hydrochloric acid (HCl) (0.05e0.1 mM), sulfuric acid (H2SO4) (0.05e0.1 mM) and ethylene diamine tetra acetic acid (EDTA) (0.01e0.1 mM). The loaded biomass was then separated from the solution, and washed with deionized water, to remove any unsorbed zinc ions. The sample was then air-dried for 24 h, and again added into an Erlenmeyer flask containing 100 mL of three different eluants for the desorption of the zinc ions. After desorption, the concentration of zinc in the eluant was analyzed, as described in ‘Batch biosorption experiments’ section, and the desorption percentage was calculated using the Equation (7)

Dpð%Þ ¼

Cdes *100 Cads

(7)

where Cdes and Cads are the concentrations of the desorbed and adsorbed zinc ions, (mg/L), respectively.

Fig. 1. FTIR spectra of native, Zn(II)-loaded and desorbed DPOC.

1288 and 1026.13 cm1. A peak at 1018 cm1 disappeared after Zn(II) biosorption on the surface of the DPOC, representing the active involvement of the CeO groups during biosorption. The peak at 3379 and 2979 shifted to 3348 and 2924 cm1 after Zn(II) biosorption, and also the intensities of the peaks were decreased. A small increase in the intensity was observed after desorption, which shows the involvement in biosorption. The reappearance of the peaks at 3379 and 2929 cm1 showed that CH, NH and OH might be involved during biosorption. From the visual observation of SEM (Fig. S1a provided as supplementary material), it was inferred that the textural morphology of the DPOC was nonuniform in shape and had number of pores (Roosta et al., 2014a,b). The surface of the Zn(II)-loaded onto the DPOC was found irregular and some pores were trapped (Fig. S1b provided as provided as supplementary material). The specific surface area of the DPOC as obtained from the t-plot was 1.862 m2/g. The Barrett, Joyner and Halenda (BJH) plot showed pores with radii lying between 6.34 and 223 nm with a distribution peak at 16.136 nm. The total pore volume of the DPOC was found to be 0.004 cm3/g. 3.2. Effect of various operating variables on the biosorption of zinc ions onto the DPOC

3. Results and discussion 3.1. Characterization of the biosorbent In order to characterize the presence of functional groups on the surface of the DPOC, the FTIR spectra were collected and analyzed. The FTIR spectra of the native biomass, loaded and desorbed biomass are shown in Fig. 1. A strong and broad band at 3379 cm1 is attributed to the lattice OeH stretching vibration of lignin, pectin, absorbed water, cellulose and hydroxyl group (Muthusamy et al., 2013). The sharp peaks at 2929 cm1 and 2854 cm1 correspond to the CeH stretching and symmetric stretching vibration of the CH2 groups, respectively (Shanmugaprakash et al., 2014). The broad peaks at 1635 cm1 and 1666 cm1 were assigned to the stretching vibrations of the NH-band and C]C- alkenes' group, respectively. The bands at 1543 cm1 and 1442 cm1 were attributed to a strong asymmetric and a weak symmetric stretching band of carboxyl groups (COOe), respectively (Khan et al., 2012). A broad band at 1018.14 cm1 is a representation of the CeO group. The FTIR spectrum after Zn(II) biosorption onto the DPOC showed a decrease in the peak intensities at 3278, 2924.09, 2206.54, 1550, 1442, 1381,

3.2.1. Effect of pH Solution pH plays a significant role in the heavy metal's biosorption. The specification of metal ions in an aqueous solution is pH dependent, and also significantly influences their interaction with the surface of the adsorbent (Cimino et al., 2000). In this study, pH of the solution was varied from 3 to 7, in order to determine the optimal value of the biosorption of Zn(II) ions onto the DPOC, by keeping the initial metal ion concentration (100 mg/L), temperature (303 K) and biosorbent dosage (4.0 g/L) as constant. The maximal removal of Zn(II) ions was observed in the pH range of 5.0; it then decreased slowly. This can be explained by the fact that the negatively charged functional groups were protonated in acidic conditions, resulting in the lower removal efficiency of the Zn(II) ions (pH < pHPZC). At the same time, an increase in the pH from 4.0 to 5.0 (pH > pHPZC) resulted in the deprotonation of the above functional groups, with a subsequent increase in the attraction sites towards the negatively charged ions, resulting in an increase in the biosorption removal efficiency (Khan et al., 2012). From Fig. S2 (provided as supplementary material), the pHPZC of the DPOC was found to be 4.0. With increasing pH > 5.0, the Zn(II) ions start to get

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hydrolyzed into Zn(OH) 3 ions, as a result of the dissolution of Zn(OH)2 (Ajjabi and Chouba, 2009). At a higher pH, the precipitation of Zn(II) ions occurs, and it results in the decrease of biosorption removal efficiency (Fig. 2a). The maximum biosorption of zinc ions was observed at pH 5.0, and it was selected for further experimental studies. 3.2.2. Effect of biosorbent dosage and initial metal ion concentration As the biosorbent dose increases from 0.05 to 0.5 g, the removal efficiency also increases from 25.62 to 60.15%; however, the biosorbent capacity starts to decrease from 251.24 to 32.03 mg/g as the biosorbent dosage increases. The decrease in the biosorption capacity may be attributed to some of the available active sites that remain unsaturated during the biosorption of Zn(II) ions onto the DPOC (Muthusamy et al., 2013). At the same time, the number of active sites increases with the increase in the adsorbent dosage, which results in an increase in the efficiency of Zn(II) ions removal. Another reason is that, as the adsorbent dose increases, it results in the adsorbenteadsorbent interactions, thereby masking the active sites present on the surface of the biosorbents (Anandkumar and Mandal, 2012). Moreover, a further increase in the adsorbent dosage did not affect the biosorbent capacity because of the unavailability of adsorbate sites due to saturation (Gupta et al., 2010). The initial Zn(II) ion concentration increases from 100 to 500 mg/L and the Zn(II) removal efficiency decreased from 81.45 to 68.42%; at the same time, the biosorption capacity of the DPOC increased from 20.36 to 85.52 mg/g. This is due to the higher availability of Zn(II) ions in the aqueous solution and the higher concentration of Zn(II) leads to an increase in the driving force, due to the development of the concentration gradient between the aqueous solution and the surface of the DPOC (Senthil Kumar et al., 2012). 3.2.3. Effect of contact time The effect of contact time on the biosorption of Zn(II) ions onto the DPOC is shown in Fig. 2b. During the initial stage, the biosorption capacity increases rapidly for the first 30 min, followed afterward by a slower rate. This is due to the fact that in the beginning stage of the biosorption process, more number of active sites are available for the biosorption of Zn(II) ions onto the DPOC (Muthusamy et al., 2013). At a later stage, the available of active sites get saturated with Zn(II) ions present in the aqueous solution

on the surface of the DPOC. Thus, the after 60 min, plateauing is takes place and equilibrium was attained within 60 min of contact time and it is used for further experiments. 3.3. Biosorption isotherm studies The Langmuir (Langmuir, 1918), Freundlich (Freundlich, 1906), Temkin (Temkin and Pyzhev, 1940) and Halsey adsorption isotherm (Halsey, 1948) were used to evaluate the effect of initial zinc ions concentration data and the results are given in Table 1. The linearised form of the Langmuir biosorption isotherm model can be represented by the following equation:

1 ¼ qe



1 Q o b Ce



 þ

1 Qo

 (8)

where qe is the amount of Zn(II) ions adsorbed per unit mass of adsorbent (mg/g), Ce is the concentration of Zn(II) ions in the aqueous solution at equilibrium (mg/L), Qo is the maximum monolayer biosorption capacity (mg/g), b is the Langmuir constant related to the affinity of the zinc ions to the DPOC (L/mg). Langmuir parameters of Qo and b are given in Table 1 and found to be 83.33 and 0.029, respectively. The biosorption capacity obtained for the DPOC is found to be better than the other biosorbents and for Hibiscus rosa sinensis (94.339 mg/g) as reported by Vankar et al. (2011); Sun et al. (2008) for aerobic granules (62.5 mg/g); Kargi and Cikla (2006) for powdered waste sludge (82 mg/g); Valdman and Leite (2000) for Sargassum sp. (118.5 mg/g); Chen et al. (2008) for chitosan (10.21 mg/g), respectively. The dimensionless separation factor, RL, in Equation (9), determined from the Langmuir isotherm, was found to be 0.199. This indicates a highly favorable sorption.

RL ¼

1 1 þ bCo

(9)

The linearised form of the Freundlich equation can be represented by the following equation:

1 ln qe ¼ ln KF þ ln Ce n

(10)

Where KF is the Freundlich constant measure of the biosorption

Fig. 2. (a) Effect of pH, (b) contact time by DPOC.

M. Shanmugaprakash, V. Sivakumar / Journal of Environmental Management 164 (2015) 161e170

165

Table 1 Various isotherm model constants and correlation coefficient (R2) for the biosorption of Zn(II) ions onto the DPOC at 30
C. Biosorbent

DPOC

Langmuir isotherm

Freundlich isotherm

Temkin isotherm

Halsey isotherm

Qo (mg/g)

b (L/mg)

R2

KF ((mg/g)(L/mg)1/n)

1/n

R2

BT (J/mol)

AT (L/mg)

R2

n

kh

R2

83.33

0.029

0.93

5.81

0.51

0.97

22.54

5.9

0.88

1.95

31.34

0.97

Experimental conditions: initial Zn(II) concentration ¼ 100e500 mg/L; solution pH ¼ 5.0; temperature ¼ 30
C; biosorbent concentration ¼ 4.0 g/L; shaking speed ¼ 120 rpm. R2 ¼ correlation coefficient.

capacity ((mg/g) (L/mg)(1/n)) and 1/n is related to the intensity of biosorption. In order to analyze the applicability of the Freundlich model lnqe was plotted against lnCe and the values are tabulated in Table 1. The values obtained for Zn(II) ions from the Freundlich model showed a maximum adsorption capacity (KF) of 31.34 with an affinity value (1/n) equal to 0.51, indicating the favorable biosorption. A similar type of Freundlich parameters are obtained by many researchers for zinc biosorption on different biosorbents (Tan et al., 2009, Khan et al., 2012). The linearised form of the Temkin isotherm is represented by:

qe ¼ B ln AT þ BT ln Ce

(11)

Where B ¼ RT/b, R is the universal gas constant (8.134 J/mol) and T (K) is the absolute temperature. The value of BT is related to the heat of biosorption (J/mol); AT is the equilibrium binding constant (L/ mg) corresponding to the binding energy. The constants AT and BT can be obtained by plotting the values of qe versus lnCe and values are shown in Table 1. The R2 values of Temkin isotherm is lower than that obtained for the Freundlich and Halsey isotherm. This indicates that Temkin isotherm model is not fit of equilibrium data as compared with the Freundlich and Halsey isotherm. Generally, the Halsey isotherm model represents a multilayer biosorption system (Halsey, 1948). The linearised form of the Halsey isotherm is represented by:

 ln qe ¼

   1 1 1 ln kh  ln n n Ce

(12)

Where kh and 1/n are the Halsey constant and exponent, respectively. The values of kh and 1/n were obtained by plotting a graph between lnqe and ln(1/Ce) and parameters are represented in Table 1. The good linear fitting by higher correlation coefficients indicates the heteroporosity of the DPOC (Anandkumar and Mandal, 2012). It could be seen that Table 1 summarizes all the constants and correlation coefficients, R2 for the given four different isotherm models. The higher coefficient of determination values (R2) indicate that the biosorption of Zn(II) ions onto the DPOC obeys the Freundlich isotherm model followed by the Halsey isotherm models. 3.4. Biosorption kinetics modeling The process of Zn(II) ions uptake rate and the rate-controlling mechanism can be analyzed by kinetic models and kinetic constants are useful in determining the biosorption rate which can be used for designing and modeling of the fixed-bed biosorption system (Gupta and Babu, 2009). In order to determine the rate at which Zn(II) ions removal onto DPOC, pseudo first-order, secondorder, Weber and Morris and Boyd model are used with the experimental data. 3.4.1. Pseudo first-order kinetics The pseudo first-order equation (Lagergren's equation) depicts biosorption in solideliquid systems, based on the sorption capacity of solids (Lagergren, 1898). The linearised form of the pseudo first

order equation can be expressed as

lnðqe  qt Þ ¼ ln qe 

k1 t 2:303

(13)

Where qe and qt (mg/g) are the biosorption capacities at equilibrium and at time t (min), respectively. A plot of ln(qeqt) versus time was drawn and the values of k1 and qe were estimated from the slope and intercept, respectively 3.4.2. Pseudo-second order kinetics The pseudo second-order rate expression has been applied for analyzing the chemisorption kinetics from liquid solutions (Ho, 2006). The linear form of the typical second-order equation can be represented as

  t 1 1 t ¼ þ qt k2 q2e qe

(14)

Where k2 is the equilibrium rate constant of the pseudo secondorder adsorption (g/mL/min) The plots of ln(qeqt)vst and t/qt vs t were drawn (Fig. 3aeb) and slopes and intercepts were used to determine the constants, k1 and k2, respectively. The value of R2 for the pseudo-first order kinetics for Zn(II) ions represents that this model had failed to estimate qe (Table 2). Hence, the biosorption mechanism cannot be well explained by the pseudo-first order kinetics. At the same time, the pseudo second order rate equation has high correlation coefficients (R2) which describe the rate controlling mechanism of the zinc biosorption of the DPOC. The trend observed is similar in this study to that reported by Anandkumar and Mandal (2012). The R2 for second order kinetics was greater than 0.98, and the estimated value of qe agreed with the experimental values, which indicates that the rate limiting step is a chemical biosorption process between the Zn(II) ions and the DPOC, and also it was suggested that the overall rate of the biosorption process (chemisorption) involved valency forces through sharing or exchange of electrons between the sorbent and sorbate (Ho, 2006; Roosta et al., 2014a,b). It is clear that the pseudo second rate constant (k2) for Zn(II) ions gets decreased with an increase in the initial metal concentration. 3.4.3. Validity of kinetic model The above two kinetic models were further subjected to check the validity by the normalized standard deviation, Dqe(%), which is given by:

Dqt ð%Þ ¼ 100

vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi uP h . i2 u qt;exp  qt;cal qt;exp t N1

(15)

Where N is the number of data points, qt,exp and qt,cal (mg/g) are the experimental and calculated biosorption capacity, respectively. The Dqt values obtained for the pseudo-first order kinetics equation ranged from 3.6% to 5.9% which was relatively high, when compared to the Dqt values of 1.9e6.6% obtained for second order kinetic equation. The highest R2 values and lowest Dqt values

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Fig. 3. Various kinetic models for Zn(II) biosorption onto the DPOC at different initial metal ion concentration (a) First order kinetic model (b) Second order kinetic model.

Table 2 Pseudo-first-order model and pseudo-second-order model and correlation coefficients for biosorption of Zn(II) ions onto the DPOC at 30
C. Initial conc. Of metal ion (mg/L)

100 200 300 400 500

qe

(exp)

21.95 37.43 54.81 71.49 81.25

(mg/g)

Pseudo I order

Pseudo II order

kI (1/min)

qe

0.041454 0.048363 0.057575 0.048363 0.059878

28.84 41.69 67.61 81.28 77.62

(cal)

(mg/g)

R2

kII(g/mg-min)

qe

0.80 0.91 0.90 0.93 0.94

0.000851 0.001126 0.001113 0.000725 0.000953

25.57 43.48 62.50 82.64 83.50

(cal)

(mg/g)

R2 0.96 0.99 0.99 0.99 0.99

Experimental conditions: Initial Zn(II) concentration ¼ 100e500 mg/L; solution pH ¼ 5.0; temperature ¼ 30
C; biosorbent concentration ¼ 4.0 g/L; shaking speed ¼ 120 rpm. R2 ¼ correlation coefficient. qe (exp) eexperimental qe values, qe (cal) ecalculated qe values.

obtained for the secondeorder kinetic equation were able to describe the biosorption kinetics of Zn(II) ions onto the DPOC and it also shows a better agreement between the experimental and calculated qe values (Dubey and Gopal, 2007). 3.4.4. Biosorption mechanism Above two kinetic models were not able to identify the exact biosorption mechanism and also the rate limiting steps take places inside the biosorption process. Therefore, it is explained by various other models, such as intra particle diffusion models and Boyd's model. Weber and Morris (1963) proposed that the intraparticle diffusion mechanism takes place on the surface of the biosorbent and it is explained as follows:

qt ¼ Kid t 0:5 þ C

(16)

Where Kid is the intraparticle diffusion rate constant (mg/g/min0.5) and C is a constant which gives an idea about the thickness of the boundary layer which can be calculated from the plot between qt and t0.5 (Figure not shown). If the rate controlling step in the biosorption of Zn(II) ions onto the DPOC is intraparticle diffusion, then the plot of qt versus t0.5 gives a straight line which passes through the origin. Any deviation from this linearity indicates that the film diffusion process is controlled for the given biosorption system. It was depicts that the plots possess multi-linear portions (i.e., two steps were involved in the biosorption of zinc ions onto the DPOC). The first linear portion is due to the film diffusion and the second linear portion is due to the intraparticle diffusion. The value of Kid can be obtained from the slope of the plot of qt versus t0.5 and given in Table 3.

The deviation of the linear plot (qt versust0.5) from the origin indicates that the intraparticle diffusion was not the only rate controlling step and the other steps may also involve simultaneously. Similar trends were observed by Senthil Kumar et al. (2012) for adsorption of cadmium ions by surface-modified Strychnos potatorum seeds. In order to interpret the ratecontrolling step during the biosorption process, the effect of the contact time data was used with the Boyd kinetic plot, and also it is used to calculate the pore diffusion coefficients of metal ions onto DPOC using following equation (Boyd et al., 1947).

" !#0:5 qt p2 De ¼ 1  exp qe R2a

(17)

With further simplifications, Equation (17) can be modified and given as

Bt ¼ 0:4977  lnð1  Ft Þ

(18)

where qe is the amount of zinc ions adsorbed onto the DPOC at equilibrium (mg/g), qt is the amount of zinc ions adsorbed onto the DPOC at time t (mg/g), Ft is the fraction of zinc ions adsorbed at any time t (min), and Bt is a mathematical function of qt/qe. If the plots of Bt versus time t, are linear and pass through the origin, then the actual slowest step in the biosorption process follows the intraparticle diffusion (internal diffusion). The effective diffusion coefficient, De was calculated using the following equationWhere

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167

Table 3 Intraparticle diffusion model constants and its correlation coefficients for biosorption of Zn(II) ions onto the DPOC at 30
C. Initial Zn(II) ions concentration

100 200 300 400 500

Intraparticle diffusion model

Boyd model

KP1 (mg/g emin0.5)

KP2 (mg/ge min0.5)

C1

C3

(R1)2

(R2)2

Diffusivity (m2/min)

1.66 4.47 7.1 9.4 11.78

4.7 4.7 6.04 8.6 9.57

0 0 0 0 0

0 33.44 52.22 66.49 78.9

0.99 0.99 0.99 0.97 0.98

0.95 0.91 0.92 0.98 0.95

8.7  107 1.16  107 1.46  106 1.62  106 1.39  106

Experimental conditions: Initial Zn(II) concentration ¼ 100e500 mg/L; solution pH ¼ 5; temperature ¼ 30
C; biosorbent concentration ¼ 4.0 g/L; shaking speed ¼ 120 rpm. R1, R2, ¼ correlation coefficients.

p2 De B¼ 2 Ra

As presented in Table 4, the Gibbs free energy (DG
), values at all the temperatures for the given biosorption system, are negative, indicating that the zinc ions involved in the biosorption process are spontaneous in nature. The Gibbs free energy value decreases with increasing temperature, indicating less driving force, resulting in the less biosorption capacity (Tan et al., 2009). The value of DH
was estimated in the range of 440.97 to 221.48 J/mol, and 9.21e10.47 kJ/mol-K for DS
. The negative values of DH
confirmed that the biosorption process is exothermic in nature and indicates that the adsorbate-adsorption process is mainly due to the electrostatic attraction (Helen and Miranda, 2010). Generally, the heat evolved during physical adsorption is of the same order of magnitude as the heat of condensation, i.e., 2.1e20.9 kJ/mol (Anandkumar and Mandal, 2012), while the heat of chemisorption generally falls into a range of 80e200 kJ/mol (Hayward and Trapnell, 1964). The positive value of DS
reflects the increased randomness at the solidesolution interface during the Zn(II) ions sorption process (Aksu and Gonen, 2004).

(19)

Where t (min), De is the effective diffusion coefficient (m2/min), Ra is the radius of spherical adsorbent particle (m) represented in Table 3. From Fig. S3 (provided as supplementary material), it is seen that the plots are linear, but they do not pass through the origin, which indicates that the biosorption process is mainly governed by external mass transport, in which the particle diffusion was the rate limiting step (Tan et al., 2009). It could be inferred from Table 3 that the diffusivity of zinc ions onto the DPOC decreases with the increase in concentration from 100 mg/L to 500 mg/L in the range of 8.7  107 m2/s to 1.39  106 m2/min, respectively, and these values agree with those in the available literature (Helen and Miranda, 2010; Muthusamy et al., 2013). 3.5. Thermodynamic studies

3.6. Continuous biosorption in packed column

The thermodynamic parameters, such as the change in the standard free energy (DG
), standard enthalpy (DH
) and standard entropy (DS
) for Zn(II) ions biosorption onto the DPOC at various temperatures (303, 313 and 323 K) were determined using the following equation:


ln Kd ¼

DS DH  R RT

The performance of biosorption in a continuous fixed bed column is an important factor in measuring the feasibility of a given biosorbent in the real time practical application. Since the batch studies only given the insight into the biosorption equilibrium and kinetics (Shanmugaprakash et al., 2013). Therefore, the experiments were carried out in a continuous reactor, in order to predict the performance of the DPOC to remove the Zn(II) ions in the continuous mode. The column performance was studied by varying the efficiency of the flow rate and bed height.




(20)

Where R is the universal gas constant (8.314 J/mol/K), T is the absolute solution temperature (in Kelvin), and Kd is the distribution coefficient calculated from the relationship of Kd ¼ (CAe/Ce) where CAe is the amount of the Zn(II) ions adsorbed under equilibrium (mg/g) and Ce is the residual concentration of Zn(II) ions in the solution under equilibrium (mg/L). The Van't Hoff plot was used to determine the values of DH
and DS
from the slope and intercepts, respectively, and the DGº value was calculated using the following equation:

DG+ ¼ RT ln Kd

3.6.1. Effect of the bed depth The uptake of the Zn(II) ions in a fixed-bed column depends mainly on the quantity of the solid biosorbent packed in the column. The experiments were performed by varying the column height as 5.0, 10.0 and 15.0 cm, by packing 6.8, 13.45 and 20.46 g of DPOC in the column, respectively and by maintaining the all other parameters in constant, such as the flow rate (15 mL/min, pH-4.5) and initial metal ion concentration (100 mg/L). With an increase

(21)

Table 4 Thermodynamic parameters for biosorption of Zn(II) ions onto the DPOC at different temperatures. Initial Zn(II) ions concentration (mg/L)

DHº (J/mol)

DSº (J/mol/K)

DGº(J/mol) 303 K

313 K

323 K

100 200 300 400 500

440.97 394.58 312.44 122.96 221.48

9.21 9.97 9.31 4.65 10.47

1537.33 1007.79 294.23 238.23 933.06

920.27 299.07 430.87 353.93 1491.84

350.67 570.91 1049.73 824.39 1961.76

Experimental conditions: Initial Zn(II) concentration ¼ 100e500 mg/L; solution pH ¼ 5; temperature ¼ 30e50 speed ¼ 120 rpm.




C; biosorbent concentration ¼ 4.0 g/L; shaking

168

M. Shanmugaprakash, V. Sivakumar / Journal of Environmental Management 164 (2015) 161e170

Fig. 4. (a) Breakthrough curve for the biosorption of Zn(II) ions onto the DPOC at different bed heights (Flow rate ¼ 10 mL/min, Co ¼ 100 mg/L, pH ¼ 5.0 and temperature ¼ 30 ± 2
C). (b) BDST model for Zn(II) ion adsorption by DPOC. (c) The experimental and predicted breakthrough curves using the Thomas models for the biosorption of Zn(II) ions onto the DPOC at different flow rate (Co ¼ 100 mg/L, pH ¼ 5.0, Bed height ¼ 15 cm and temperature ¼ 30 ± 2
C).

in the bed height, steeper breakthrough curves were obtained. From Fig. 4a, it is observed that the breakthrough time gets increased from 3.0 to 10.0 h, as the bed height increases from 5 cm to 15 cm. Moreover, the volume of metal solution also increases with the increase in the bed depth. This is may be due to the fact that the Zn(II) ions present in the aqueous solution have sufficient time to diffuse into the pores of the DPOC. Moreover, with the increase in the bed height, the residence time of the Zn(II) ions present in the solution was also increased, which makes it easy to diffuse deeper into the biosorbent. Thus, the Zn(II) ions uptake rates were found to be 78.61, 83.61 and 84.19 (mg/g) when the bed heights were maintained at 5, 10 and 15 cm, respectively. This is may be due to the increase in surface area of the DPOC which provides more number of active sites for the Zn(II) ions biosorption (Shanmugaprakash et al., 2013). The effect of the bed depth is explained using a simple model, the Bed depth service time (BDST) model (Bohart and Adams, 1920). The model ignores the intra-particle mass transfer resistance and external film resistance, such that the sorbate is sorbed onto the biosorbent surface directly, and it is linearly expressed as

t¼

  No Z 1 Co  ln 1 Co v Co ka Cb

(22)

Where No is the biosorption capacity of the bed (mg/g), v is the linear flow velocity of the metal solution through the bed (ml/min), Cb is the break through metal ion concentration (mg/L) and ka is the rate constant (L/mg/min), which characterizes the rate of solute transfer from the fluid phase to the solid phase. The plot of service time versus bed depth, at a flow rate of 10 mL/min was linear as

shown in Fig. 4b. The correlation of coefficient (R2) of 0.99 indicated the validity of the BDST model for the present system. The values of No and ka were obtained from the slope and intercept of the BDST plot, and were found to be 6678 mg/L and 1.14  103 L/mg/min respectively, assuming that the initial metal concentration Co, and linear velocity v, remained constant. The rate constant value ka which is a measure of the rate of transfer of the solute from the fluid phase to the solid phase, largely influences the breakthrough phenomenon in the column study. If ka is large, even a short bed will avoid a breakthrough, but as ka decreases a longer bed is required (Vijayaraghavan and Prabu, 2006). 3.6.2. Effect of flow rate The effect of the flow rate on biosorption of Zn(II) ions was studied, by varying the flow rates as 5, 10 and 15 mL/min, while the bed height and initial Zn(II) ions concentration were kept constant at 15 cm and 100 mg/L, respectively. The experimental breakthrough curve is illustrated in Fig. 4c and it depicts that, as the flow rate increases, both the exhaustion and breakthrough time is decreased. The uptake rate (Qo) values were found to be 49.68, 44.19 and 29.65 (mg/g) for the flow rates of 5, 10 and 15 mL/min, respectively. Under the lowest flow rate, the column performed well and the earlier breakthrough and the exhaustion times were achieved; when the flow rate was increased from 5 to 15 mL/min, the column breakthrough time reduced from 60 to 120 min. This behavior is due to the decrease in the resistance time, which is insufficient for the biosorption of the zinc ions onto the DPOC, and also the diffusion limitation of the zinc(II) ions into the pores of the DPOC at higher flow rates (Ko et al., 2000). Thomas model assumes the Langmuir adsorptionedesorption

M. Shanmugaprakash, V. Sivakumar / Journal of Environmental Management 164 (2015) 161e170

169

Table 5 Effect of Bed height for Zn(II) ions onto DPOC using BDST and Thomas Models at 30
C. BDST model parameters Bed height (cm)

Sorbent weight (g)

Breakthrough time (tb) (min)

Exhaustion time (te) (min)

Uptake rate Qo (mg/g)

5 10 15

6.8 13.45 20.46

180 360 600

540 1200 1740

78.61 83.61 84.19

Bed height (cm)

KTh (L/mg/min)

Qo (mg/g)

R2

49.68 44.19 29.65

0.96 0.91 0.94

Thomas model parameters Flow rate (mL/min) 5 10 15

15 15 15

4

0.6  10 0.5  104 0.4  104

kinetics having no axial dispersion, and sorption is the rate of the driving force, and it obeys the second order reversible reaction kinetics (Aksu and Gonen, 2004). On the basis of the above assumption, it was linearly expressed as

ln

  Co K Q M K Co  1 ¼ TH O  TH F Ce F

(23)

where Qo is the maximum solid phase concentration of the solute (mL/g); kTh is the Thomas model constant (L/mg/min), M is the mass of the biosorbent in the column (in grams), Co is the initial metal concentration (mg/L), F is the volumetric flow rate (mL/min) and Ce is the effluent metal ion concentration (mg/mL) at any time, t (h). Fig. 4c shows the comparison of the experimentally obtained and predicted breakthrough curves. Table 5 summaries the Thomas model parameters obtained at different flow rate by maintaining bed height constant (15 cm) and initial Zn(II) ions concentration (100 mg/L). The value of Kth and Qo were determined by plotting ln(Co/Ce1) versus time and tabulated in Table 5. Based on the R2 values, it can be concluded that the experimental data fitted very well to the Thomas model. From these results, it is obvious that as the flow rate increases, the values of Qo and KTh declined. These results corroborate that of Vijayarahagavan and Prabu (2006) and Plaza et al. (2013).

3.7. Desorption and regeneration studies The reusability of the given adsorbent is a significant factor in the biosorption process. From the batch studies, it is concluded that the EDTA has more efficiency than the other desorption solutions

used for the recovery of the Zn(II) ions, since EDTA is a strong hexadenate chelating agent, which is capable of a complex formation with Zn(II) ions (Table S1 provided in supplementary data). Therefore, 0.1 mM EDTA solution was used as the eluting agent to recover the zinc ions from the loaded DPOC, by keeping the flow rate of 5 mL/min and bed height of 5 cm, in constant. After the desorption of the zinc ions, the column was thoroughly washed with deionized water at a constant flow rate of 5 mL/min, and again loaded with 100 mg/L of the zinc solution to check the sorption efficiency. From the Fig. 5, it is depicted that six adsorptionedesorption studies were conducted repeatedly in the column mode, by maintaining optimum conditions. The biosorption efficiency was gradually decreased as the number of cycles increased. This might be due to the decomposition of the surface active sites present on the surface of the DPOC which can be confirmed by the FTIR results (Fig. 1). A similar result was also obtained by Suksabye et al. (2008) and Khan et al. (2012) for biosorption of Cr (VI) ions onto coir pith and Ni(II) ions onto the oil cake, respectively. 4. Conclusion The biosorption of Zn(II) onto the DPOC was investigated, by both the batch and column processes. The FTIR characterization revealed that the Zn(II) biosorption onto the DPOC was due to the various active sites and bonds present on the surface of the biosorbent. The maximum removal of Zn(II) ion occurred under optimal conditions. The experimental data were analyzed through various isotherms and kinetic models. Column studies using the DPOC revealed that column performance was significantly affected by the bed height, the flow rate and the initial Zn(II) ion concentration. Increasing flow rate and bed height resulted in the better performance of the packed bed and higher biosorption rates. The adsorbed Zn(II) ions were desorbed using 0.1 M EDTA solution, and were regenerated for further use. Acknowledgments One of authors (Dr.M.Shanmugaprakash) is grateful to the management of Kumaraguru College of Technology, Coimbatore, India for having provided research facilities. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jenvman.2015.08.034. References

Fig. 5. Regeneration studies plots for DPOC as EDTA as eluant.

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