Adsorption behavior of heavy metals onto chemically modified sugarcane bagasse

Adsorption behavior of heavy metals onto chemically modified sugarcane bagasse

Bioresource Technology 101 (2010) 2067–2069 Contents lists available at ScienceDirect Bioresource Technology journal homepage: www.elsevier.com/loca...

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Bioresource Technology 101 (2010) 2067–2069

Contents lists available at ScienceDirect

Bioresource Technology journal homepage: www.elsevier.com/locate/biortech

Short Communication

Adsorption behavior of heavy metals onto chemically modified sugarcane bagasse Puspa Lal Homagai a, Kedar Nath Ghimire a,*, Katsutoshi Inoue b a b

Central Department of Chemistry, Tribhuvan University, Kirtipur, Kathmandu, Nepal Department of Applied Chemistry, Saga University, Saga 840-8502, Japan

a r t i c l e

i n f o

Article history: Received 9 September 2009 Received in revised form 17 November 2009 Accepted 17 November 2009

Keywords: Charred sugarcane bagasse Adsorption Xanthation Adsorbent

a b s t r a c t A new process for the xanthation of sugarcane (Saccharum officinarum) bagasse was investigated for the separation of cadmium, lead, nickel, zinc and copper from their aqueous solutions. Adsorption capacity of the charred xanthated sugarcane bagasse (CXSB) was found to be significantly more than the several biosorbents reported in the literatures. The modified material was characterized by FTIR and elemental analysis. The kinetics of sorption of the tested metals was fast, reaching equilibrium within 20–40 min. The maximum adsorption capacities evaluated in terms of mol/kg dry gel were 1.95 for Cd(II), 1.58 for Pb(II), 2.52 for Ni(II), 2.40 for Zn(II) and 2.91 for Cu(II), respectively. The high adsorption capacity and the kinetics results indicated that CXSB can be used as the selective adsorbent for the removal of these respective metal ions from wastewater. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction Heavy metal pollution in water and soil is a matter of great public concern these days. Recently, much attention is given to prepare adsorbents from various wastes generated from forestry (Horsfall et al., 2006), fishery (Inoue and Yoshizuka, 1999) and by-products of agriculture (Ghimire et al., 2008; Lokesh and Tare, 1989). In Nepal, sugarcane industries produce a large amount of sugarcane bagasse (SB) that requires little processing to increase its sorptive capacity and its cost is supposed to be low even after chemical modification. The SB contains cellulose (46.0%), hemicellulose (24.5%), lignin (19.95%), fat and waxes (3.5%), ash (2.4%), silica (2.0%) and other element (1.7%) (Sene et al., 2002). The polysaccharides found in sugarcane bagasse are biopolymers having many hydroxyl and/or phenolic groups that can be chemically modified to form new compounds with changed properties (Navarro et al., 1996). Although there are some reports of raw sugarcane bagasse as the adsorbent (Bassso et al., 2002), however, owing to its low adsorption capacity, we have explored a simple means of chemical modification to enhance its metal adsorption properties in the present study. The choice of xanthate group is due to the presence of sulfur atom and it is well known that sulfur group has a very strong affinity for most of the heavy metals, and the metal sulfur complex is very stable in basic medium (Chauhan and Sankararamkrishnan, 2008; Kumar et al., 2000; Sankararamakrishnan et al., 2006; Tare and Chaudhari, 1987). In view of this regard, xanthation of sugarcane bagasse was investigated in the present study. In

* Corresponding author. Tel.: +977 1 4333945, +977 9841830466 (mobile). E-mail address: [email protected] (K.N. Ghimire). 0960-8524/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2009.11.073

contrast to the ion exchange chelating resins made of plastic, this adsorbent is free from any refractory post treatment after its use since the main component of the adsorbents are natural polysaccharides, that is very easy to be incinerated. 2. Methods 2.1. Chemicals Standard stock solutions of 1000 mg/L were prepared appropriately Cd+2 and Pb+2 from their nitrate salts, Ni+2, Zn+2 and Cu+2 from their chloride salts (Wako Chem. Ltd. Japan), respectively. The pH of working solution was adjusted using 0.1 M NaOH and 0.1 M HCl and 0.1 M 2-[4-(2-hydroxymethyl)-1-piperazinyl] ethanesulfonic acid (HEPES) was also used as a buffer reagent. All the required experimental solutions were diluted using 0.1 M HNO3. 2.2. Preparation of adsorbent materials Sugarcane bagasse (SB) was collected from local juice centre situated in Kathmandu Metropolitan city. It was dried in air oven at 70 °C for 24 h and grounded into fine particles with the help of an electric grinder. It was sieved to pass uniform size of 212 lm. Then 100 g of SB was treated with 200 ml concentrated H2SO4 and stirred for 30 min and left for overnight. It is well known that acid treatment with such biopolymer creates a suitable environment for its ring opening (Morrison and Boyd, 1994). It was washed with deionized water to remove excess acid and any other soluble substances until neutrality and then dried. This material is referred as charred sugarcane bagasse (CSB). 25 g of the CSB was

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added into 200 ml of 4 M NaOH solution and shaken for 1 h. Further 25 ml of CS2 was added and stirred for 3 h then left for overnight. It was filtered and washed repeatedly until the pH of the suspension became neutral. This material is ready for the experiments and called as charred xanthated sugarcane bagasse (CXSB).

2.3. Adsorption experiments The test solutions of cadmium (II), lead (II), nickel (II), zinc (II) and copper (II) were prepared from corresponding standard stock solution by diluting with 0.1 M nitric acid and 0.1 M of HEPES as a buffering agent, respectively. The optimum pH of the solution was maintained by adding small amount of nitric acid or sodium hydroxide. In the batch-wise tests, 20 mg of dried adsorbent was taken into 50 ml conical flask with 15 ml of corresponding prepared diluted solution. The flasks were shaken vigorously in a thermostated shaker at 303 K at 150 rpm for 24 h to ensure the equilibrium to be attained. The initial and equilibrium concentrations of the metal ions were measured by using Shimadzu AA6650 atomic absorption spectrophotometer. All experiments were performed in duplicate at least and mean values were presented in all the cases studied. The sorption capacity of metal ions is the concentration of the metal ions on the adsorbent and can calculated based on the mass balance principle where



ci  c e L  1000 W

ð1Þ

In the above equations, q represents the amount of metal up taken per unit mass of the adsorbent (mol kg1), L is the volume of the test solution (mL), W is the dry mass of the adsorbent (kg), Ci and Ce the initial and final concentrations (mol dm3), respectively.

3. Results and discussion 3.1. Characterization of the adsorbents The FTIR spectra of the CXSB were recorded on a FTIR/IR-410 (JASCO, Japan) with KBr dispersion method. The peak observed at 3434 cm1 is due to the stretching vibration of the –OH groups. The bands around 1186 and 3045 cm1 are assigned to [email protected] stretching and C–H stretching vibration, respectively. Absorption peaks appeared at 1556 cm1 corresponding to the [email protected] stretching vibration of the xanthate unit and it may be attributed to the – CS2H deformation suggesting that CXSB has been successfully xanthated. The peaks at 1176, 1022 and 460 cm1 in the spectrum of charred bagasse represents the [email protected], [email protected] and S–S stretching vibration, respectively and were strong indicative of the presence of xanthate group bonded to the charred materials. Prior to modification of CSB the band at 1752 cm1 is due to the –CHO group formed during the charring process. This peak disappears after modification of CSB into CXSB and an intense broad band is observed at 1556 cm1 revealing that xanthate group had been introduced onto CSB. The major absorption bands characteristic of the [email protected] groups lay in the region 1563 700 cm1 (Silverstein et al., 1981). This region is much more intense for CXSB compared to CSB. The peak corresponding to C–S–S and C–O–C symmetric stretching seemed to have merged into a broad band at 1556 cm1.The asymmetric stretching vibration of C–O–C is also observed at 1176 cm1. The amount of carbon, hydrogen and nitrogen in CXSB were found to be 52.35, 2.12, and 0.22% whether raw sugarcane contained 46.58, 6.11 and 0.11%, respectively. After xanthation, the percentage of sulfur had been determined to be as 4.92% which confirmed the proper modification of charred sugarcane bagasse by the introduction of xanthate group.

3.2. Effect of pH The pH of a solution played an important role for adsorption of metal ions. At low pH, there is high concentration of H+ that has high mobility as compared to metal ions and competition between H+ with metal ions decreases their adsorption. On the other hand, as the pH value of the solution increases, adsorption also increases due to lesser number of H+ and greater number of surface ligands with negative charges. The low adsorption of metal ions at low pH may be due to sorbate lyophobic behavior (Volesky and Schiewer, 1999). The same way, the solubility of metal in solution decreases with increasing pH and the sorption increases with increasing pH. The optimum pH for Cd, Pb, Ni, Zn, and Cu biosorption was found to be 5, 4, 4, 6 and 5, respectively (figure not shown). The selectivity order in the removal of heavy metals at pH around four follows the order Pb > Cu > Ni > Cd > Zn. The xanthate group is known to be unstable in acid solution and to be able to dissociate from the CXSB. Decomposition is a two step process in which the first is the protonation of the hydroxyl group and the second is the elimination of carbon disulfide. Since the metal ions are bound to the sulfur atoms of xanthate groups, any loss of sulfur could lead to a reduction in the adsorption capacity. 3.3. Adsorption isotherms The main objective of isotherm is to evaluate the capacity of the modified biomass to sequester heavy metals from an aqueous solution. It was done by characterizing the equilibrium state of the functionalized adsorbent that had been allowed to react with aqueous solution of the metal of interest. Isotherm studies were performed by batch-wise method using various concentrations of metal ions ranging from 25 to 1000 mg dm3; and optimum pH was maintained for all synthetic solutions. Sorption isotherms were evaluated using linearized Langmuir model represented by Eq. (2).

ce 1 ce ¼ þ qe qmb qm

ð2Þ

where qe (mol/kg) is the concentration of adsorbed metal ions per gram of adsorbent; Ce (mol/L) is the concentration of metal ion in aqueous solution at equilibrium, qm (mol/kg) the ultimate capacity, b (L/mg) the binding constant. To examine the relationship between the metal sorption capacity (qe) and the metal ions concentration at equilibrium (Ce), the sorption equilibrium data for cadmium, lead, nickel, zinc and copper were compared to Langmuir model. The experimental qmax and Langmuir adsorption isotherm parameters evaluated from the isotherm plots for Cd+2, Pb+2, Ni+2, Zn+2 and Cu+2 are given in Table 1. The maximum adsorption capacity based on experimental results as shown in Fig. 1 to be was 1.95, 1.58, 2.52, 2.40 and 2.91 mol/kg, respectively. On the other hand, the theoretical monolayer capacity (qmax) of the above respective metal ions based on Langmuir adsorption equation was determined to be 2.04, 1.69, 2.80, 2.54 and 3.13 mol/kg, respectively. The complexation mechanism of the metal ions with the xanthate group has taken place through ion exchange process. This is confirmed by the fact that the initial pH of the solution after adsorption has been decreased due to the release of proton from the CXSB into the aqueous medium. 3.4. Sorption kinetics The sorption kinetics of the metal ions onto CXSB was analyzed as the function of time at an initial concentration of 100 mg/L solutions. The concentrations of the metal ions were analyzed keeping their optimum pH in a regular and certain interval of time by AAS.

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5.21 kg mol1 min1, respectively based on kinetic model as shown in Eq. (3).

Table 1 Langmuir adsorption isotherm model parameters and experimental qmax. Metal ions

qmax (mol /kg) Langmuir model

qmax (mol /kg) experimental

b (1/ kg)

R2

Cd (II) Pb (II) Ni (II) Zn (II Cu (II)

2.04 1.69 2.80 2.54 3.13

1.95 1.58 2.52 2.40 2.91

3.10 4.44 0.58 1.36 0.87

0.99 0.99 0.99 0.99 0.99

4. Conclusion A novel adsorbent CXSB was prepared by treating the charred sugarcane bagasse with CS2 under alkaline condition. The equilibrium sorption data fitted well with Langmuir adsorption isotherm and the adsorbent–adsorbate kinetics exhibited pseudo-second order model. The maximum adsorption capacity onto the CXSB was evaluated to be higher than the reported values in various literatures. This signifies that the introduction of xanthate group onto charred sugarcane bagasse is of importance in enhancing the adsorption capacities of raw sugarcane bagasse. Such chemically modified xanthated sugarcane bagasse might find potential use as adsorbent in industrial wastewater treatment. The regeneration and desorption aspect of CXSB needs further studies for its real cost effectiveness. Acknowledgements One of the researchers Mr. Puspa Lal Homagai is grateful to NAST, Nepal, for bestowing PhD Fellowship to undertake this research work. References

Fig. 1. Adsorption isotherm plot for adsorption of several metal ions onto CXSB.

Table 2 Sorption kinetics of pseudo-second order for several metal ions onto CXSB. Metal ions

R2

K2 (kg/mol/min)

Cd (II) Pb (II) Ni (II) Zn (II) Cu (II)

0.99 0.99 0.99 0.99 0.99

4.14 6.59 7.90 1.09 5.20

From the experimental data, it is observed that the percentage of adsorption increased with the increase in time from 5 to 40 min and then it became constant in the range 20–240 min. The pseudo-second order kinetics model has often been used to fit the experimental kinetic adsorption data. The linearized pseudo-second order kinetic equation is used as the following form:

t 1 t ¼ þ qt k2 q2e qe

ð3Þ

where qt (mol kg1) is the amount of adsorption at time t (min), K2 (kg mol1 min1) is the rate constant of the pseudo-second order kinetic adsorption. The values of K2 and qe can be obtained from the intercept and slope of the plot of the experimental t/qt versus t. The experimental data are shown in Table 2, which can be explained by the pseudo-second order kinetic model with the correlation coefficient R2 being almost unity (0.99) for all the metal ions. The experimental value of K2 was found to be 4.14, 6.60, 7.91, 1.09 and

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