International Journal of Biological Macromolecules 67 (2014) 409–417

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International Journal of Biological Macromolecules journal homepage: www.elsevier.com/locate/ijbiomac

Chemical modification of cellulosic biopolymer and its use in removal of heavy metal ions from wastewater A.S. Singha ∗ , Ashish Guleria Department of Chemistry, National Institute of Technology, Hamirpur HP, 177005, India

a r t i c l e

i n f o

Article history: Received 29 September 2013 Received in revised form 18 March 2014 Accepted 19 March 2014 Available online 2 April 2014 Keywords: Cellulosic biopolymer Graft copolymerization Adsorption isotherm

a b s t r a c t Use of biological macromolecules for wastewater remediation process has become the topic of intense research mostly driven by growing concerns about the depletion of petroleum oil reserves and environmental problems. So in view of technological significance of cellulosic biopolymers in various fields, the present study is an attempt to synthesize cellulosic biopolymers based graft copolymers using free radical polymerization. The resulting cellulosic polymers were characterized by Fourier transform infrared spectroscopy (FT-IR), Scanning electron microscopy (SEM), X-ray diffraction (XRD) and thermogravimetric (TGA) analysis. Furthermore, modified cellulosic biopolymer was used in removal of Cu2+ , Zn2+ , Cd2+ and Pb2+ toxic metal ions from wastewater. The effects of pH, contact time, temperature and metal ions concentration were studied in batch mode experiments. Langmuir and Dubinin–Radushkevich (D–R) models were used to show the adsorption isotherm. The maximum monolayer capacity qm calculated using Langmuir isotherm for Cu2+ , Zn2+ , Cd2+ , Pb2+ metal ions were 1.209, 0.9623, 1.2609 and 1.295 mmol/g, respectively. The thermodynamic parameters H ◦ and G◦ values for metal ions adsorption on modified cellulosic biopolymer showed that adsorption process was spontaneous as well as exothermic in nature. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Cellulose and its derivatives are the most abundant natural biomacromolecules, which are used in a number of advanced applications, such as in paper, packaging, biosorption, biomedical and in preparation of biocomposites. Among the various application of cellulose and its derivatives in different fields, the use of cellulosic biomacromolecules for wastewater remediation process has become the topic of intense research mostly driven by growing concerns about the depletion of petroleum oil reserves and environmental problems. Heavy metal ions pollution is one of the most important environmental problems being faced by the modern day society. The rapid industrialization and other mining activities have resulted in increased heavy metal concentrations in surface and ground waters. The pollution of water with heavy metal ions is considered dangerous due to their high toxicity and nonbiodegradability [1]. Further, heavy metal ions can be accumulated through the food chain even at low concentrations, leading to serious problems on aquatic, animal, plant and human health [2].

∗ Corresponding author. Tel.: +91 1972 254120; fax: +91 1972 222584. E-mail address: [email protected] (A.S. Singha). http://dx.doi.org/10.1016/j.ijbiomac.2014.03.046 0141-8130/© 2014 Elsevier B.V. All rights reserved.

The harmful and negative effect of heavy metals on living beings and ecosystem has forced the scientific community across the globe to devise new methods to avoid these ill effects. Various conventional methods such as chemical precipitation, filtration, ion exchange, electrochemical treatment, membrane technologies, adsorption on activated carbon and evaporation have been suggested for removal of metal ions from aqueous solution [3]. However, most of these methods are ineffective and also produce large quantity of sludge. Ion exchange, membrane technologies and activated carbon adsorption process are extremely expensive and hence cannot be used at large scale [4,5]. However in recent years, cellulosic biofibers are highly efficient, low cost and renewable source of biomass which can be exploited for wastewater remediation. Further these biofibers can be chemically modified for better efficiency and multiple reuses to enhance their applicability at large scale. Chemical modification of cellulosic biofibers is carried out to achieve efficient adsorption capacity for heavy metal ions [6–8]. Chemical modification by graft copolymerization of vinyl monomers onto cellulosic biofibers provides large number of functional groups which improves the adsorption capacity of cellulosic fibers significantly. Among these cellulosic biomasses, cellulosic okra fiber is another potential agriculture waste biomass which can be used as adsorbent for wastewater treatment. Okra fiber is an agricultural

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waste biomass principally composed of cellulose, hemicellulose and lignin. In continuation with our previous efforts, the current work presents the graft copolymerization of cellulosic okra fibers with acrylonitrile and methacrylic acid binary vinyl monomer is an effort to increase the graft yield as compared to grafting with single acrylonitrile monomer [9]. The structures of the modified cellulosic fibers were confirmed through FT-IR, SEM, TGA and X-ray diffraction techniques. Further graft copolymerized okra fibers have been used for removal of Cu2+ , Zn2+ , Cd2+ and Pb2+ heavy metal ions from wastewater. The adsorption parameter including effects of pH, contact time, metal ion concentration and temperature on adsorption were also investigated. 2. Experimental 2.1. Materials Okra cellulosic fibers were extracted from stem of okra plant and made free from impurities by the method described earlier in the literature [9]. Acrylonitrile (AN) and methacrylic acid (MAA) of 99% purity supplied by CDH were used as vinyl monomers. Hydrogen peroxide (H2 O2 ) supplied by Qualigens fine chemicals and ascorbic acid (Asc) supplied by (E-Merck, chemicals limited, Mumbai, India) were used as received. All other chemicals were of analytical grade and used as received without further purifications. For metal adsorption experiments, the stock solutions of Cu(II), Zn(II), Cd(II) and Pb(II) were prepared by dissolving an accurate quantity of copper sulfate (CuSO4 ·5H2 O; Merck India), zinc nitrate (Zn(NO3 )2 ·6H2 O; Loba Chemicals) and cadmium nitrate (Cd(NO3 )2 ·4H2 O; Qualigens fine chemicals) and lead nitrate (Pb(NO3 )2 ; Merck India) in double distilled water. 2.2. Synthesis of poly(acrylonitrile-co-methacrylic acid)-g-cellulosic okra polymers Free radical induced graft copolymerization synthesis onto cellulosic fibers with acrylonitrile (AN) and methacrylic acid (MAA) was carried out by immersing them in a precise amount of distilled water for 24 h. A known amount of ascorbic acid dissolved in definite amount of hydrogen peroxide was added to the reaction mixture containing 0.5 g of fibers. The optimized concentration of ascorbic acid and hydrogen peroxide were taken 3.2 × 10−2 mol/dm3 and 1.61 × 10−1 mol/dm3 , respectively, as reported in the literature [9]. The grafting of AN/MAA vinyl monomers onto cellulosic fibers has been investigated at different feed molarities ranging from 0.3 to 1.0 mol/dm3 at a fixed mole fraction of AN monomer (fAN = 0.4). The ratio of mole fraction of AN/MAA in the grafting is 0.4/0.6, respectively. The comonomer mixture was then added drop wise to the reaction kettle which was stirred for one hour with the help of electrically operated magnetic stirrer maintained at a constant temperature of 55 ◦ C. Finally when the reaction time was over, product was filtered and washed with double distilled water. To ensure complete removal of homopolymers, the crude was Soxhlet extracted for 24 h with dimethylformamide and acetone. The graft copolymers free from homopolymers were then dried in a hot air oven at 60 ◦ C to a constant weight. The grafting parameters percent graft yield (Pg ) and percent graft efficiency (Pe ) were calculated by following expression: w2 − w1 Percent graft yield (Pg ) = × 100 (1) w1 Percent graft efficiency (Pe ) =

w2 − w1 × 100 wm

(2)

where w1 , w2 and wm are the weight of raw and grafted cellulosic fiber, respectively.

The grafted okra fibers were then ball milled in ball mill apparatus, so that the average particle size of the fibers was reduced to 60 ␮m which was confirmed by SEM-EDX study. 2.3. Amidoximation of poly(acrylonitrile-co-methacrylic acid)-g-cellulosic okra fibers 2.0 g of poly(AN + MAA) vinyl monomer grafted cellulosic particle fibers were treated with an aqueous solution of hydroxylamine hydrochloride [10]. The pH 10 of solution was adjusted by adding sodium carbonate. The ratio of hydroxylamine hydrochloride and sodium carbonate in reaction mixture was 1:0.75 by weight, respectively. The reaction mixture was taken in a 500 mL round bottom flak to which 100 mL of deionized water was added and sealed. The reaction was carried out at 70 ◦ C for 90 min. The product was filtered, washed with deionized water for several times in order to remove the remaining traces of salts and finally dried at 60 ◦ C. This dried chemical product was further used as adsorbent for adsorption studies. 2.4. Characterization of chemically modified fibers The surface morphologies of cellulosic fiber before and after graft copolymerization were studied using FEI Quanta 450 SEG scanning electron microscopy machine at very high magnification. Functional groups in cellulosic fiber before and after chemical modification were determined by the FTIR Spectrometer (Perkin-Elmer spectrophotometer) in the transmittance (%) mode with a scan resolution of 4 cm−1 in the range 4000–500 cm−1 . Crystallinity index of the raw and chemically modified samples were studied through X-RD analysis using X-ray powder diffractometer (Philips 1710 X-ray diffractometer). The sample was scanned from 5 to 50◦ in step of 2◦ /min. Thermogravimetric analysis of the raw and chemically modified samples was performed using a Linseis L81-II at a heating rate of 10 ◦ C/min in temperature range of 30 to 800 ◦ C. The pH measurements of all aqueous samples were performed following standard methods with pH meter manufactured by Eutech Instruments. 2.5. Adsorption equilibrium experiments Adsorption data such as kinetic, thermodynamic and adsorption isotherm was obtained by batch techniques. The experiments were conducted in 250-mL Erlenmeyer flasks containing 0.050 g of adsorbent and 50 mL of metal ion solution in varying concentrations from 100 to 1000 mg/L. The flasks were shaken at an agitation speed of 150 ± 2 rpm at electrically thermostated reciprocating shaker for different time intervals. The metal ion concentration before and after the adsorption process was calculated by the method reported in the literature as follow. The metal ion concentration of Cu2+ in the filtrate was determined by direct titration with standard EDTA solution (1 mmol/L) at pH 10 using murexide as an indicator [11]. The concentration of Zn2+ , Cd2+ and Pb2+ ions was determined by back titration of standard EDTA solution (3 mmol/L) with aqueous Mg2+ standard solution (2.0 mmol/L) at pH 10 using Erichrome Black T as indicator [11,12]. All experiments were repeated thrice and mean values were used. The adsorption amount (q) was calculated according to Eqs. (4) and (5): q=

(C0 − Ce ) ×V W

(3)

where W is the weight of the adsorbent in gram (g), V is the volume of solution in litre (L) and C0 and Ce are the initial and equilibrium concentrations of metal ions in solution. The pH of metal solutions was adjusted with 0.1 M HNO3 and 0.1 M NaOH.

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Table 1 Effect of feed molarity and feed composition on percent graft yield and percent graft efficiency. Effect of feed molarity

Effect of feed composition

Sr. no.

AN + MAA (mol/L)

Pg

Pe

Sr. no.

fAN

Pg

Pe

1 2 3 4 5

0.3 0.5 0.7 0.9 1

15.28 26.38 46.42 36.9 33.08

3.51 3.62 4.55 2.88 2.27

1 2 3 4 5

0.25 0.35 0.45 0.55 0.65

33.7 38.64 53.78 42.48 24.76

2.43 2.93 4.31 3.58 2.21

Effect of feed molarity and feed composition on grafting parameters [Asc] = 3.25 × 10−2 mol/L, [H2 O2 ] = 1.61 × 10−1 mol/L, fAN = 0.4, Temp. = 55 ◦ C and Time = 60 min.

3. Results and discussion The mechanism of grafting reaction of vinyl monomers onto cellulosic fiber is related to the concept of accessibility of hydroxyl groups onto cellulosic fiber backbone. In cellulosic okra fiber, cellulose is the main constituent; the hydroxyl groups at C2 , C3 , C6 and C H sites at cellulose backbone are active centres for grafting of polymeric chains. For graft copolymerization onto cellulosic fibers, using ascorbic acid and hydrogen peroxide redox initiator produces hydroxyl radical which interacts with hydroxyl group present in cellulosic fiber backbone generates a radical site as reported in literature [9]. The radical site on the cellulosic fibers then initiates graft copolymerization of binary vinyl monomers mixtures which are present in the reaction mixture. Factors affecting grafting amount including effect of comonomer feed molarity and effect of feed composition have been investigated. The results of effect of comonomer feed molarity and feed composition have been reported in Table 1. The reaction scheme for chemical modification of cellulosic fiber has shown below: Amidoximation is performed to increase the functionality on the cellulosic fibers, which enhances the metal ion adsorption [13,14]. As it is clear from reaction Scheme 1, treatment of nitrile group ( CN) with hydroxylamine hydrochloride converts nitrile group

OH + n

Cellulose

CN + n

MAA

COOH

Step 1 H2O2/Ascorbic acid

Cellulosic fibers

Cellulose

AN

O COOH

CN

Cellulosic fibers

Grafted fibers

n

NH2OH.HCl / Na2CO3 Step 2 70oC

Cellulose Cellulosic fibers

O C

N

COOH

NH2 OH

n

Amiodoximated fibers Scheme 1. Reaction scheme for the chemical modification of cellulosic fibers.

Fig. 1. FT-IR spectra of (A) raw fiber (B) poly(AN) grafted fiber (C) poly(AN + MAA) grafted fiber (D) amidoximated fiber.

into amidoxime group. In case of nitrile group ( CN), which have only one nitrogen atom which can co-ordinate with metal ions. However, amidoxime functional group contain both acidic and basic parts and for coordination with metal ions two lone pairs of electrons are available on the oxygen and one lone pair one each nitrogen atom [15]. Amidoxime functional groups form stable complexes with different metal ions as compared with nitrile functional group, and consequently, polymers with amidoxime groups can be efficiently used for the removal of heavy metal ions from the aqueous solution [4]. 3.1. Characterization of chemically modified cellulosic fibers 3.1.1. FT-IR analysis Fig. 1 shows the FT-IR spectra of raw, poly(AN) grafted, poly(AN + MAA) grafted and amidoximated binary monomer grafted cellulosic fibers. The characteristic IR bands for cellulosic fiber, can be divided into four regions: the broad hydroxyl bands (3200–3600 cm−1 ), the stretching bands of CH, CH2 and CH3 (2800–3000 cm−1 ), the stretching bands of carbonyl groups (1550–1750 cm−1 ) and band at 1733 cm−1 is attributed to the stretching vibration of ester group of hemicellulose present in the raw fiber. The intense peak at 1054 cm−1 along with the weak peak at 1250 cm−1 and the shoulder at 1162 cm−1 are C O stretching vibrations of ethers and alcohols. In case of grafted fiber, the additional peak at 2245 cm−1 was found due to the cyanide group of acrylonitrile monomer and increase in intensity of the peak at 1735 cm−1 clearly indicates the evidence of grafting by binary monomer on the cellulosic fiber surface. Furthermore, the grafted fibers were treated with hydroxylamine hydrochloride which convert nitrile group into amidoxime group was confirmed by diminished cyanide peak at 2245 cm−1 . The presence of amidoxime functional group was further confirmed by the intense peaks at 1660 cm−1 and 1605 cm−1 which were due to the stretching vibrations of C N and N H groups in amidoxime, respectively [16]. 3.1.2. X-ray diffraction analysis Fig. 2 shows the XRD pattern of raw and grafted cellulosic fiber. The crystallinity index (CrI) was calculated using the intensity values corresponding to the diffraction of the crystalline structure and the amorphous fraction, according to the Segal method [17]: CrI% =

I200 − Iam × 100 I200

(4)

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Fig. 2. XRD pattern of (A) raw fiber (B) poly(AN) grafted fiber (C) poly(AN + MAA) grafted fiber (D) amidoximated fiber.

Fig. 4. TGA curves of (A) raw fiber (B) poly(AN + MAA) grafted fiber (C) amidoximated fiber.

I200 is the intensity of the crystalline peak at the maximum 2 between 22◦ and 23◦ and Iam is the intensity at the minimum at 2 between 15◦ and 18◦ . The crystallinity indexes of raw fiber and poly(AN), poly(AN + MAA) grafted fibers were 48.45, 39.19 and 26.23%, respectively. The grafted fibers show lower percent crystallinity index (C.I.). Lower crystallinity index of the grafted fiber indicates that there may be the disorientation of the cellulose crystals when poly(AN + MAA) chains are incorporated in the fiber.

were 228 ◦ C and 466 ◦ C, respectively. Raw fiber exhibited two stage decomposition with (67%) weight loss in 230–334 ◦ C range and second stage decomposition in 335–466 ◦ C range. The former stage was attributed to the loss by dehydration and volatilization processes. The second stage decomposition was attributed to the degradation of cellulose and lignin. In case of poly(AN + MAA) grafted fiber the initial and final decomposition temperatures were 235 ◦ C and 502 ◦ C, respectively. The first stage decomposition in 235–329 ◦ C range with 60.2% weight loss may be attributed to the degradation of hemicelluloses, lignin and glycosidic linkages of cellulose. The second stage decomposition from 329 to 502 ◦ C with 27.7% weight loss may be due to decomposition of grafted chains on the fiber surface. Further amidoximated fiber initial stage decomposition exists from 243 to 342 ◦ C with 58% weight loss and final stage decomposition exists from 342 to 502 ◦ C with 30% weight loss. The increase in thermal stability/resistance of grafted cellulosic fiber may be due to incorporation of more covalent bonding through inclusion of poly(AN + MAA) chains onto cellulosic fibers. The grafted product formed a crosslinked type of network on the cellulosic fibers which when heated forms an insulative carbonaceous char barrier on the surface, thus inhibiting degradation and hence increases thermal stability of grafted cellulosic fibers [18].

3.1.3. Morphological analysis Fig. 3(A and B) depicts the changes in surface morphology of fibre surfaces on being subjected to grafting with vinyl monomers. The raw cellulosic fibers exhibited a relatively smooth surface compared with that of grafted fibers. It can be observed from SEM micrograph that the surface of the grafted cellulosic fibres is highly rough in comparison with that of unmodified fiber which could be attributed to the high graft density. 3.1.4. Thermogravimetric analysis The TGA curves shown in Fig. 4 demonstrate the thermal stability of the raw fiber, grafted (AN + MAA) and amidoximated fiber. The initial and final decomposition temperatures of raw fiber

Fig. 3. SEM images of (A) raw fiber (B) poly(AN + MAA) grafted fiber.

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Fig. 5. Effect of percent graft yield, on the adsorption capacity of Cu2+ , Zn2+ , Cd2+ and Pb2+ metal ions (initial concentration 200 mg/L; contact time 3 h; shaking rate 150 rpm, 25 ◦ C).

3.2. Adsorption studies 3.2.1. Effect of role of variation of AN/MAA The effect of different percent graft yield cellulosic fibers has been studied for the adsorption capacity of the metal ions. Fig. 5 shows the effect of percent graft yield, on the adsorption capacity of the metal ions. As it has been clear from the Fig. 5, the removal of metal ions increases as percent graft yield increases. The increase in removal capacity of the graft copolymerized cellulosic fiber was due to increase in the poly(acrylonitrile-co-methacrylic acid) polymer chains which increases the cyanide and carboxyl functionality of the grafted fiber. This increases in cyanide and carboxyl functionality onto grafted fiber increases number of active sites for adsorption [6]. The low percent graft yield cellulosic fibers results in low metal removal efficiency and hence we chose maximum grafted fiber after amidoximation for removal of heavy metal ions from toxic water system. 3.2.2. Effect of pH The adsorption of toxic metal ions from aqueous solutions depend upon pH of the solution as acidity of the solution affects the ionization of the metal ions and concentration of the H+ ions on the functional groups. Adsorption of metal ions at pH 4 and attained optimum value at the pH range of 5.5–6.5 (Fig. 6). At pH 5.0, the adsorbent surface becomes less positively charged and thus metal ions with positive charge had no difficulties to approach the binding sites which facilitate greater metal ion uptake. 3.2.3. Effect of time and adsorption kinetics of metal ions Fig. 7 shows the effect of contact time on adsorption of metal ions by chemically modified fiber. From Fig. 7, it has been observed that for adsorption time of 30 min, there was increase in adsorption, however after this time interval adsorption process has been slow down. The slowdown in metal ion adsorption process was may be due to decrease in binding sites of adsorbent and also due to decrease in metal ion concentration in the solution.

413

Fig. 6. Effect of pH on the removal of Cu2+ , Zn2+ , Cd2+ and Pb2+ metal ions by chemically modified cellulosic fiber adsorbent (initial concentration 200 mg/L; contact time 3 h; shaking rate 150 rpm, 25 ◦ C).

In order to investigate the mechanism of adsorption kinetic, experimental data was fitted for pseudo-first-order and pseudosecond-order kinetic models. The pseudo-first-order kinetic model was proposed by Lagergren which can be expressed as [19]: ln (qe − qt ) = ln (qe ) − k1 t

(5)

The pseudo second order kinetic model is presented as follow [20]: 1 t = + qt k2 q2e

1 qe

t

(6)

where k1 (min−1 ), k2 (mmol/min/g) are the rate constant of pseudo-first-order and pseudo-second-order adsorption and qt is the amount of metal ion adsorbed at time t (mmol/g), qe is its value at equilibrium (mmol/g). k1 and qe can be determined from the slope and intercept of the plot ln(qe − qt ) against t (min), respectively. The second order rate equation constant k2 (mmol/min/g) can be determined by plotting t/qt versus t. The kinetic evaluations were made for the initial concentration of 200 mg/L for all metal ions studied. The values of rate order constant and qe , cal(mmol/g) for different model have been summarized in Table 2. The most suitable kinetic model can be

Fig. 7. Effect of adsorption time on the removal of Cu2+ , Zn2+ , Cd2+ and Pb2+ metal ions by chemically modified cellulosic fiber adsorbent (initial concentration 200 mg/L; shaking rate 150 rpm, 25 ◦ C).

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Table 2 Kinetic parameters for the removal of Cu2+ , Zn2+ , Cd2+ and Pb2+ heavy metal ions by chemically modified cellulosic fiber adsorbent. Pseudo-first-order kinetic model Metal ion

Equilibration time (min)

Cu2+ Zn2+ Cd2+ Pb2+

90 90 120 60

Metal ion Cu2+ Zn2+ Cd2+ Pb2+

k1 (min−1 )

qe , cal(mmol/g)

0.0319 0.507 0.0164 0.587 0.0241 0.516 0.0318 0.449 Pseudo-second-order kinetic model k2 (g/mmol/min) Equilibration qe , time (min) cal(mmol/g) 90 0.1215 0.622 90 0.1183 0.551 120 0.0515 0.726 60 0.1104 0.619

R2 0.9701 0.9974 0.9981 0.9132 R2 0.9977 0.9971 0.9975 0.9936

determined with the regressional analysis of data by comparing the experimental and calculated qe values. As seen from Table 2, pseudo-second-order kinetic model matches best with the experimental data. 3.2.4. Effect of temperature on adsorption and adsorption thermodynamics Fig. 8 shown the effect of temperature on the adsorption process studied in the temperature range of 25–60 ◦ C. Decrease in adsorption with the increase in temperature may be due to desorption of metal ions from interface to the aqueous solution [21]. Furthermore, a set of thermodynamic parameters were calculated for the metal ion adsorption, which include change in the standard free energy of adsorption (G◦ ), the heat of adsorption (H ◦ ), and standard entropy (S ◦ ). The distribution coefficient kD can be calculated as [22]: KD =

qe Ce

(7)

where qe represents equilibrium metal concentration on the adsorbent (mmol/L) and Ce is the equilibrium metal concentration in solution (mmol/L). The KD may be expressed in terms of the H ◦ (kJ/mol) and S ◦ (kJ/mol K−1 ) as a function of temperature [23]: lnKD = −

H ◦ S ◦ + RT R

(8)

The values of H ◦ and S ◦ calculated respectively from the slope and intercept of a plot of lnKD versus 1/T as given in Fig. 9. The

Fig. 9. The plot of lnKD versus 1/T.

standard free energy of adsorption (G◦ ) can be calculated from the following equation: G◦ = H ◦ − TS ◦

(9)

The calculated values of thermodynamic parameters are given in Table 3. The negative G◦ values indicated thermodynamically feasible and spontaneous nature of the adsorption. The negative values of H ◦ confirm exothermic nature of the process for all metals studied. The values of H ◦ are found in the range from −10.72 to −24.68 kJ/mol. Since typically for physical adsorption ranges from −4 to −40 kJ/mol [24], it would be suggested that physical adsorption was also contributing to the mechanism. 3.2.5. Effect of metal ion concentration and adsorption isotherm studies The initial metal ions concentration plays an important role in affecting the removal capacity of metal ions onto adsorbent surface. From Fig. 10, it has been observed that at low initial concentration, adsorption capacities of metal ions increased almost proportionally with the increase in the initial metal ion concentrations of metal ions. However, at higher concentration, the increase in metal ion uptake was slow due to saturation of binding sites on adsorbent. The adsorption isotherms were investigated using two equilibrium isotherm models, which are namely Langmuir and Dubinin–Radushkevich isotherm models. Langmuir model of adsorption predicts the existence of monolayer coverage of the heavy metal ion at the outer surface of the adsorbent. The Langmuir model in linear form can be written as [25]: Ce 1 Ce = + qe qm bqm

(10)

where qe is the equilibrium metal ion concentration on the adsorbent (mmol/g), Ce is the equilibrium metal ion concentration in Table 3 Calculated thermodynamic parameters values for removal of Cu2+ , Zn2+ , Cd2+ and Pb2+ heavy metal ions adsorption by chemically modified cellulosic fiber adsorbent. Metal ion

Fig. 8. Effect of temperature on the removal of Cu2+ , Zn2+ , Cd2+ and Pb2+ metal ions by chemically modified cellulosic fiber adsorbent (initial concentration 200 mg/L; contact time 3 h; shaking rate 150 rpm).

Cu2+ Zn2+ Cd2+ Pb2+

Thermodynamic parameters H ◦ (kJ/mol)

S ◦ (kJ/mol K−1 )

−11.914 −19.906 −19.642 −24.685

−0.0319 −0.0619 −0.0552 −0.0706

G◦ (kJ/mol) (301 K)

(308 K)

(318 K)

−2.301 −1.269 −3.041 −3.406

−2.077 −0.836 −2.655 −2.912

−1.757 −0.217 −2.103 −2.204

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Fig. 10. Effect of metal ion concentration on the removal of Cu2+ , Zn2+ , Cd2+ and Pb2+ metal ions by chemically modified cellulosic fiber adsorbent (contact time 3 h; shaking rate 150 rpm, 25 ◦ C).

the solution (mmol/L), qm is the monolayer adsorption capacity of the adsorbent (mmol/g) and b is the Langmuir adsorption constant (L/mmol) related with the free energy of adsorption. The qm and b are computed from the slope and interception of linearized plot of Ce /qe versus Ce and were summarized in Table 4. The maximum adsorption capacity of chemically modified fiber for Cu2+ , Zn2+ , Cd2+ , Pb2+ metal ions were 1.209, 0.9623, 1.2609 and 1.295 mmol/g, respectively. Dubinin–Radushkevich isotherm usually applied to determine the nature of adsorption process is either physical or chemical. The linear form D–R model is given as [26]: lnqe = lnqm − ˇε2

(11)

where qe is the amount of metal ions adsorbed on per unit weight of adsorbent (mmol/L), qm is the maximum adsorption capacity (mmol/g), ˇ is the activity coefficient related to adsorption mean free energy (mol2 /J2 ) and values of these constants were summarized in Table 4 and ε is the Polanyi potential and it can expressed as:



ε = RTln 1 +

1 Ce



(12)

where R, T and Ce represent the gas constant (8.314 J/mol K), absolute temperature (K) and adsorbate equilibrium concentration (mmol/L), respectively. The mean adsorption energy (E; kJ/mol) of adsorption gives information about adsorption mechanism and calculated from Eq. (13) given below [27]. If E value is between 8 and 16 kJ/mol, the adsorption process was followed by chemical ion exchange and Table 4 Adsorption isotherm data for the removal of Cu2+ , Zn2+ , Cd2+ and Pb2+ heavy metal ions by chemically modified cellulosic fiber adsorbent. Langmuir isotherm model Metal ion 2+

qmax (mg/g)

Cu 1.209 0.962 Zn2+ 1.261 Cd2+ 1.295 Pb2+ Dubinin–Radushkevich model qm (mmol/g) Metal ion Cu2+ 1.116 2+ 0.907 Zn 1.145 Cd2+ 1.112 Pb2+

b (L/mg)

R2

0.1732 0.2516 0.1032 0.0971

0.9985 0.9923 0.9932 0.9972

ˇ (mol2 /kJ2 ) 0.0023 0.0024 0.0025 0.0037

R2 0.9990 0.9565 0.9675 0.9318

415

Fig. 11. Effect of presence of Cu2+ ions on the removal of Zn2+ , Cd2+ and Pb2+ metal ions by chemically modified cellulosic fiber adsorbent (contact time 3 h; shaking rate 150 rpm, 25 ◦ C).

if 0.9922) for all metal ions studied from Langmuir equation indicate that this model explain metal ion adsorption by modified fibers very well. 3.2.6. Effect of presence of Cu2+ ions on the removal of Zn2+ , Cd2+ and Pb2+ metal ions The effect of presence of Cu2+ on the on the removal of Zn2+ , Cd2+ and Pb2+ has been shown in Fig. 11, which shows the decrease in the adsorption of Zn2+ , Cd2+ and Pb2+ metal ions in the presence of an increasing concentration of copper. Copper ions exerted an inhibitory effect on removal of all metal ions. In case of Pb2+ (at C0 = 400 mg/L), when we increase the copper concentration from 50 mg/L to 150 mg/L, the lead removal was decreased to 0.8 mmol/g and 0.72 mmol/g, respectively. In case of Cd2+ (at C0 = 400 mg/L), the increase in copper concentration from 50 mg/L to 150 mg/L, the removal of Cd2+ meal ion decreased to 0.9 mmol/g and 0.74 mmol/g, respectively. In case of Zn2+ (at C0 = 400 mg/L), the removal of Zn2+ metal ion decreased from 0.61 mmol/g to 0.25 mmol/g, when we increase copper concentration from 50 mg/L to 200 mg/L. It has been evident from this study that the removal capacity of the adsorbent for a particular metal ion decreased as we go on increasing the concentration of copper metal ions [29]. Thus we can say that, effect of presence of different impurities/metal ions cause a decrease in the removal of particular metal ion from waste water as compared to clean water. Further, comparative studies of graft copolymerized cellulose and various cellulosic fibers biomass as adsorbents for the removal of heavy metal ions from aqueous solution has been shown in Table 5. On comparing the results of present study with that of results reported in literature for grafted cellulose and cellulosic fibers, it has been observed that vinyl monomer graft copolymerized okra biofibers could be used as a feasible and attractive low cost adsorbent for the removal of heavy metal ions from waste water.

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Table 5 Comparative studies of graft copolymerized cellulose and cellulosic fibers biomass as adsorbents for the removal of heavy metal ions from aqueous solution. Grafted adsorbent

Monomer used ingrafting

Heavy metal ion removed

qmax (mg/g)

qmax (mmol/g)

pH

References

Cellulose

Glycidyl methacrylate Acrylamide

Banana stem

Acrylonitrile

Sunflower stalk Porous cellulose

Acrylonitrile Glycidyl methacrylate

Sawdust

Acrylic acid

Cellulose powder

Acrylic acid

Cellulose

Acrylonitrile Acrylic acid

Cellulosic okra fibers

Acrylonitrile/methacrylic acid

1.078 0.365 0.586 0.894 0.666 1.314 0.614 0.944 0.413 1.637 1.495 0.269 0.271 0.269 0.118 0.204 0.189 0.239 1.209 0.962 1.261 1.295

4.0–5.5 4.0–6.0 6.5 6.5 6.0 10.5 3.0–5.0 6.0 6.0 4.9 5.7 4.5 4.5 4.5 –

Cellulose

68.5 75.8 65.88 185.34 42.32 85.89 39.0 60 27 104 168.0 55.9 17.2 30.3 13.26 12.96 21.21 15.22 76.82 62.89 141.73 268.32

[4]

Banana (Musa paradisiaca)

Cu2+ Pb2+ Cd2+ Pb2+ Cu2+ Zn2+ Cu2+ Cu2+ Zn2+ Cu2+ Cd2+ Pb2+ Cu2+ Cd2+ Cd2+ Cu2+ Cd2+ Cu2+ Cu2+ Zn2+ Cd2+ Pb2+

[7] [13] [14] [30] [31] [32]

[33]



[33]

5.5 6.5 6.0 5.5

Present study

4. Conclusions

Acknowledgements

The recent trends in developing cellulosic biofibers based adsorbents from renewable resources are not only governed by environmental concern and demand for sources alternative of petroleum based products. The present research work assessed the possibility of obtaining new functional cellulosic adsorbent from cellulosic okra biofibers which are considered as agricultural waste biomass. The present research work explores that:

Authors are highly thankful to the Director of the National Institute of Technology Hamirpur (H.P.) for providing necessary laboratory facilities to complete this work and to MHRD New Delhi for providing the financial assistance during the course of this work.

• Graft copolymerization of binary vinyl monomer AN + MAA onto cellulosic okra fibers can be successfully obtained by free radical polymerization in the presence of ascorbic acid-hydrogen peroxide as redox initiator system. Furthermore, the grafted fibers were treated with hydroxylamine hydrochloride which convert nitrile group into amidoxime group. • The presences of functional group in different chemically modified fibers were ascertained by FT-IR spectroscopy, SEM, XRD and thermal studies. Further, chemically modified okra fibers were used in batch wise adsorption equilibrium experiments. • The adsorption kinetics was examined by using pseudo-firstorder, pseudo-second-order. Pseudo-second-order model has been successfully fitted to heavy metal ion removal as compared to other model studied. • Adsorption isotherms were demonstrated using Langmuir and Dubinin–Radushkevich (D–R) models which show that Langmuir model fits well with adsorption data obtained in the present study. • Free energy of adsorption calculated from Dubinin–Radushkevich isotherm parameter, calculated values for Cu2+ , Zn2+ , Cd2+ and Pb2+ metal ions were 14.62, 14.26, 14.15, 11.50 kJ/mol, respectively, which show that adsorption mechanism was governed by ion exchange. • The thermodynamic calculations showed the feasibility, exothermic and spontaneous nature of the adsorption of metal ions onto chemically modified cellulosic fibers at 298–333 K. • The obtained results suggested that chemically modified cellulosic fibers could be used as alternative adsorbent for removal of metal ions in wastewater effluents.

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Chemical modification of cellulosic biopolymer and its use in removal of heavy metal ions from wastewater.

Use of biological macromolecules for wastewater remediation process has become the topic of intense research mostly driven by growing concerns about t...
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