Novel Polycarboxylated Starch-Based Sorbents for Cu2+ Ions

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Ind. Eng. Chem. Res. 2010, 49, 2548–2556

Novel Polycarboxylated Starch-Based Sorbents for Cu2+ Ions Kalpana Chauhan,†,‡ Ghanshyam S. Chauhan,*,† and J.-H. Ahn§ Department of Chemistry, Himachal Pradesh UniVersity, Shimla, India 171005, Dolphin PG College of Life Sciences, Chunni Kalan, Fatehgarh Sahib, Punjab, and Department of Chemical and Biological Engineering and Engineering Research Institute, Gyeongsang National UniVersity, 900 Gajwa-dong, Jinju 660-701, Republic of Korea

In this article we report use of novel starch-based functional hydrogels as Cu2+ ions sorbents. Starch was functionalized by acid hydrolysis and/or oxidized by nitrogen oxides to generate carboxylic groups at C-6. Sorption of Cu2+ ions was studied as a function of hydrogel structure and environmental factors. Hydrogels exhibit structure-property relationship in the sorption of Cu2+ ions. The hydrogel that exhibited the maximum ion uptake was used to investigate the effect of contact time, temperature, pH, and Cu2+ ions concentration on the sorption capacity. The maximum sorption capacity of 128.26 mg g-1 was obtained in 2 h at 40 °C, 7.0 pH, and 50 ppm of Cu2+ ions. Sorption data show good match both with Langmuir and Freundlich isotherms and obey pseudo-second-order kinetics. Cu2+ ions bind to sorbents by chelation. Evidence of Cu2+ uptake on hydrogels was obtained from FTIR spectrum of the ions-loaded hydrogel. 1. Introduction Starch-based functional polymers have been used in water/ environment management technologies.1,2 Starch has been modified by derivatization,3 grafting,4 network formation, and other polymer analogous reactions.5,6 The starch derivatives (polyelectrolytes) containing amide groups have been reported for the removal of heavy metal ions from their aqueous solutions.7 The effect of the derivative structure on the adsorption efficiency was reported to follow the order of carbamoylethylated starch > poly(acrylamide)-starch graft copolymer > starch carbamate. The extent of the uptake of the metal ions on these derivatives followed the order Hg2+ > Cu2+ > Zn2+ > Ni2+ > Co2+ > Cd2+ > Pb2+. The generation of active groups on starch chain and its depolymerization by partial hydrolysis are two attractive options to improve its application spectrum in metal ion uptake. Acidic hydrolysis of starch results in the reduction of molecular weight8,9 and granule size,10 while enzymatic hydrolysis increases its crystallinity.11 One of the chemical modifications of starch is by oxidization to generate active groups. The oxidation process involves conversion of the hydroxyl groups of starch into carbonyl groups and carboxyl groups.12,13 The oxidized starch has low viscosity, high stability, and excellent film forming and binding properties.14 Oxidation of starch has been reported with alkaline hypochlorite,15-17 TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl),18,19 and recently by an electrocatalytic method.20 Oxidation of starch with nitrogen oxides is a highly selective, rapid, clean, and efficient process.21 In this process oxidation at the primary hydroxyl groups is the predominant reaction.22,23 Functionalized starch is effective in the removal of metal ions1 and finds use in water purification technologies.3-6,24-29 Removal of Cu2+ has also been reported using functionalized starch having different types of active groups.30-32 The oxidized starch having carboxylic groups is of interest due to the effective chelation and antiflocculation properties of the carboxylic groups.33 * To whom correspondence should be addressed. E-mail: [email protected], [email protected]. Tel.: 091(177)2830944. Fax: 091(177)2633014. † Himachal Pradesh University. ‡ Dolphin PG College of Life Sciences. § Gyeongsang National University.

As a continuation to our earlier work on the use of biosorbents for the removal of Cu2+ under the simulated conditions,34-39 in this article we report functionalization of starch to novel hydrogels for use as Cu2+ sorbents. Starch possesses a unique molecular structure as it has linear as well as branched units. It finds uses in many low cost water purification technologies and most of the natural polymer-based flocculants consist of starch and its derivatives. It is also the most abundant biopolymer after cellulose and chitosan. In contrast to cellulose, starch is watersoluble, and that makes it an easy object for the controlled modification to incorporate active functional groups to improve its application spectrum. In the present study, starch was partially hydrolyzed/depolymerized by the acid controlled hydrolysis. Starch as well as its depolymerized form was oxidized with nitrogen oxides to generate carboxylic groups at C-6. Hydrogels of starch and its derivatives were prepared by the free radical initiation in the presence of N,N-methylene bisacrylamide. The oxidized starch and its different hydrogels are pH and temperature sensitive. These were characterized by viscosity, FTIR, and 13C NMR spectroscopy. The effect of oxidation was manifested in high Cu2+ sorption as the maximum retention capacity of 128.26 mg g-1 was obtained after six feeds of Cu2+ ions for the hydrogel prepared from the oxidized starch. Evidence of sorption was obtained by characterization of the Cu2+-loaded hydrogels by FTIR spectroscopy. To the best of our knowledge no such starch-based sorbents are reported in literature. 2. Experimental Section 2.1. Materials. Starch, sodium nitrite, copper sulfate (SD. Fines, Mumbai, India), N,N-methylenebisacrylamide (N,NMBAAm, Merck, Mumbai, India), ammonium persulfate (analytical grade, Glaxo, Mumbai, India) and other reagents of analytical grade were used as received. 2.2. Preparation of Functionalized Hydrogels. Starch (S) (10 g), 80% aqueous methanol (200 mL), and 5% w/v HCl were placed in a two-necked round-bottom flask fitted with condenser and mercury pit. The contents were refluxed for 150 min at 65 °C. The hydrolyzed strach (SH) was filtered under suction and washed with methanol. The SH (12.5 g) was dissolved in 100 mL of water, and to the resulting thick solution was

10.1021/ie9009952  2010 American Chemical Society Published on Web 02/18/2010

Ind. Eng. Chem. Res., Vol. 49, No. 6, 2010

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added 1% ammonium persulfate and 2% N,N-methylenebisacrylamide (by weight of SH). The contents were placed undisturbed in a water bath at 65 °C for 3 h. The hydrogels were extracted by filtration, washed repeatedly with water to obtain a constant weight, and dried in an air oven at 40 °C. The same procedure was followed to prepare the other starch-based hydrogels. The hydrogels obtained from starch (S) or partially hydrolyzed starch (SH) are referred as S-clN,N-MBAAm or SH-cl-N,N-MBAAm, where -cl- denotes cross-linking of S or SH with N,N-MBAAm. A 5.0 g portion of each hydrogel was placed separately in 500 mL two-necked round-bottom flasks equipped with stirrer and reflux condenser. Each flask was connected to the nitrogen oxides generating chamber (another two-necked round-bottom flask) that contained NaNO2 and a dropping funnel containing HCl. The HCl was added dropwise to the NaNO2 over a period of 70 h. The concentration of the nitrogen oxides in the oxidizing chamber was maintained by exposing the reaction set to air. The low boiling components were removed from the reaction mixture under vacuum. The oxidized products (hence after referred to as SO or SHO, SO-cl-N,N-MBAAm and SHO-cl-N,NMBAAm, where the subscripts H and O stand for hydrolyzed and oxidized, respectively) were washed with distilled water until the washings were of acidic nature and the materials obtained were dried in air. Yield of the oxidation reaction was calculated gravimetrically as %Y )

W1 100 W2

where W1 is the actual yield of reaction and W2 is the theoretical yield of the reaction for 100% conversion of the hydroxyl groups. 2.3. Determination of Degree of Generation of -COOH Groups. The carboxyl contents were estimated by the procedure reported in literature.40 The oxidized starch was stirred in 0.5 N calcium acetate solution at room temperature for 2 h. The resultant solution was titrated with 0.1 N sodium hydroxide using phenolphthalein as indicator. The carboxyl contents were calculated as follows. %COOH (by weight) ) 0.1 N NaOH used (mL) × 0.0045 × 100 × 8/W (g) × 3 where N is the normality of NaOH solution an W is the weight of the oxidized product. 2.4. Characterization of Hydrogels. Hydrogels were characterized by FTIR spectra (Nicollet 5700 spectrophotometer in KBr, over the range of 4500-500 cm-1 using OMNIC software) and solid state 13C NMR spectra (INOVA-400 spectrometer), and elemental analysis (C and H) was performed on a Carlo Erba EA-1108 instrument. 2.5. Sorption of Cu2+. Dried hydrogels (0.1 g) were separately immersed in a CuSO4 solution (Cu2+ ions ) 20 ppm) prepared in double distilled water and placed in a water bath (accuracy ( 0.01 °C) at 35 °C for 2 h. The concentration of the rejected Cu2+ ions was determined with DR 2010 spectrophotometer (Hach, Co., US) as reported elsewhere.35 The hydrogel (SO-cl-N,N-MBAAm) that exhibited the maximum uptake was used for the optimization of time (30 min to 6 h), temperature (35 to 60 °C), pH (2.5, 4.0, 6.0, and 7.0) and Cu2+ ions concentration (1-50 ppm). The maximum retention capacity (MRC) of the hydrogel was determined by repeating the sorption experiment with the same hydrogel sample using 50 ppm Cu2+ ions concentration in each feed at the conditions

Figure 1. FTIR spectra of (a) S-cl-N,N-MBAAm (b) SH-cl-N,N-MBAAm.

obtained for the maximum uptake. The sorption results were evaluated by using the following expressions: percent uptake (Pu) )

amount of metal ions sorbed 100 total ions in the feed solution

adsorption capacity (Q) (mg/g) )

(C0 - Ct)V M

where, Q is the amount of Cu2+ ions adsorbed onto the dry mass of the hydrogels (mg/g), V is the volume of the aqueous phase (L), m is weight of dry polymer (g), Co and Ct (mg L-1) are concentrations of ions in the feed solution and aqueous phase after treatment for a certain period of time t, respectively. 3. Results and Discussion 3.1. Synthesis and Characterization of Hydrogels. Controlled acidic hydrolysis of starch resulted in its depolymerization with concomitant reduction in molecular weight by an order of 5 × 103. The resultant polymer on partial hydrolysis (SH) has high solubility in water as compared to S. S, SH, or their respective hydrogels were oxidized by nitrogen oxides mainly at the primary hydroxyl groups to -CHO which were converted to -COOH in the presence of moisture. High % yield of the oxidized products (84.10-96.13) and % weight of -COOH groups (23.52-27.84) demonstrates the effectiveness of the oxidation technique used in this study. The extent of modification was more in SH than S while S-cl-N,N-MBAAm was more liable to modification than SH-cl-N,N-MBAAm (Figure 1). Partial hydrolysis opens up the polymer structure and results in some disruption of the hydrogen bonding. FTIR spectra of S and SH and their hydrogels differ in the intensity of peaks, as -OH stretching band is more intense and sharp in the spectrum of SH and its hydrogel than that of S or its hydrogels. The presence of additional peaks near 1645 and 1450

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Figure 2. 13C NMR of SO. Table 1. Effect of Polymer/Hydrogel Structure on Cu2+ Ions Uptake (Cu2+ ions ) 20 ppm, time ) 2 h, temperature ) 35 °C and initial pH ) 7.0) polymers

Pu

mg Cu2+ sorbed/gdry gel

SH SO SHO S-cl-N,N-MBAm SH-cl- N,N-MBAm SO-cl-N,N-MBAm SHO-cl- N,N-MBAm

7.26 55.13 61.00 58.11 51.96 57.36 46.55

0.78 5.92 6.56 6.24 5.58 6.16 5.00

cm-1 (COO- symmetrical and asymmetrical stretching, respectively) supports oxidation and formation of polyglucuronic acid. A small peak at 1750 cm-1 (CdO stretching of the carbonyl) indicates oxidation of secondary hydroxyl groups. The oxidation was also confirmed from the 13C NMR spectra of the oxidized starch (SO or SHO). In the spectrum of starch a signal appears at 60.3 ppm due to the absorption by the carbon bearing the primary hydroxyl group. It becomes residual in the spectra of the oxidized starch (SO or SH) which instead has a signal at 174.9 ppm. The latter also has weak signals at 217.3 and 227.0 ppm due to the oxidation of secondary hydroxyl groups to carbonyl groups (Figure 2). Evidence of the hydrolysis or/and oxidation of starch was also obtained from the elemental analysis (C and H). 3.2. Cu2+ Sorption. 3.2.1. Effect of Hydrogel Structure and Environmental Factors on Sorption Behavior: Selection of the Most Efficient Sorbent. All the functional forms of starch and their respective hydrogels were used in the sorption studies. CuSO4 was used in view of its effective dissociation in aqueous solution.41 Pu values for different forms of functional starch and their corresponding hydrogels were observed to increase from 7.26 to 61 in 2 h at 35 °C, 7.0 pH, and 20 ppm of Cu2+ ions. The effect of functionalization was evident as the highest sorption was observed for SHO (Table 1). Pu was observed to increase with the contact time and equilibrium was attained within 2 h (Figure 3). SO-cl-N,N-MBAAm was observed to be the most efficient sorbent. Its higher

Figure 3. Effect of time (temperature ) 35 °C, pH ) 7.0, Cu2+ ) 20 ppm, sorbent ) 0.1 g) on Cu2+ ions uptake.

efficiency than the other hydrogels can be ascribed to the presence of larger number of -CO2H groups than the analogous hydrogel prepared from SH. Hence, it was selected for further studies. On change of temperature, a sharp increase in the ion uptake was observed at 40 °C, and there after it increased slowly with further increase in temperature up to 55 °C (Figure 4). Metal ions uptake increases with an increase of temperature as hydrogel structure opens up and more active sites becomes accessible to the Cu2+ ions. However, at high temperature, desorption of ions takes place. The effect of change of pH was studied at 40 °C for 2 h. The highest value of Pu was observed at the technologically important pH 7.0, while the ion uptake was very low from the solutions of acidic pH (Figure 5). The results of ion uptake at the low pH are manifestation of the hydrogel structure; as in the acidic pH, the -CO2H groups of the hydrogel form a transient hydrogen-bonded structure and


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Figure 4. Effect of temperature (time ) 2 h, pH ) 7.0, Cu2+ ) 20 ppm, sorbent ) 0.1 g) on Cu2+ ions uptake.

Figure 6. Effect of Cu2+ concentration (time ) 2 h, temperature ) 40 °C, pH ) 7.0, sorbent ) 0.1 g) on Cu2+ ions uptake.

Figure 5. Effect of pH (time ) 2 h, temperature ) 40 °C, Cu sorbent ) 0.1 g) on Cu2+ ions uptake.

) 20 ppm,

Figure 7. Pu, sorption capacity, and pH as a function of Cu2+ feeds (time ) 2 h, temperature ) 40 °C, pH ) 7.0, Cu2+ ) 50 ppm each, sorbent ) 0.1 g).

these are not available for chelation with Cu2+ ions.42 Pu was observed to increase with an increase in the feed concentration of Cu2+ ions, and the maximum of 81% was observed at 50 ppm (Figure 6). 3.2.2. Evaluation of MRC. MRC of SO-cl-N,N-MBAAm was evaluated by subjecting it to different feeds each of 50 ppm of Cu2+ ions solution at different optimized conditions that include contact time of 2 h and 40 °C (Figure 7). Sorption increased with the successive feeds with the maximum Pu (98.32) observed in the fourth feed. The MRC of 128.26 mg g-1 was obtained after six feeds. The hydrogel is thus very efficient sorbent of Cu2+ as the obtained MRC value is very high. 3.2.3. Mechanism and Evidence of Ion Uptake. The large water absorption capacity of a hydrogel contributes in the partitioning of metal ions to the polymer/hydrogel phase.35 The functional groups present in SO-cl-N,N-MBAAm such as hydroxyl, carbonyl, carboxyl, and amide (from N,N-MBAAm) act as active sites. The chelation of Cu2+ ions with these groups is presented below through Scheme 1:

The formation of the Werner-type complexes with the central metal atom and polysaccharide by the ligation of the central atom involving lone electron pairs of the polysaccharide hydroxyl groups is reported in literature.1 To support this type of mechanism where H+ exchange is involved, we monitored the pH of the resultant solution after the completion of each experiment. The relationship between the final pH observed at the end of the corresponding experiment is plotted against temperature and Cu2+ ion concentration (Figure 8). The final pH was lower in all the experiments studied without exception, and the lowering of pH corroborated with Cu2+ ions uptake. Apart from the adsorption by chelation, adsorption on the amide groups of the cross-linker (N,N-MBAAm) also contributes to the overall ion uptake. Thus, the mechanism of metal ion uptake on a hydrogel is a complex process. Evidence for the participation of -CO2H and -OH groups was also elucidated from the FTIR spectrum of the Cu2+-loaded SO-cl-N,N-MBAAm which was compared with that of SO-clN,N-MBAAm. On coordination with Cu2+, the characteristic absorption peaks due to -OH and -COOH groups undergo changes both in position and intensity (Figure 9). The main

2+

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Scheme 1. Possible Interactions in Cu2+ Ion Sorption

peaks that undergo changes after ion-loading are in the regions 3500-3250, 1750-1400, and 1125-950 cm-1. Similar observations have been reported elsewhere.43 The additional peaks in the range of 500-550 cm-1 in the ions-loaded hydrogel correspond to the bridged complex of copper which has peaks at 550 and 500 cm-1 corresponding to the asymmetrical Cu-O and Cu-OH vibrations, respectively. 3.3. Modeling of Equilibrium Sorption. To investigate the adsorption capacity as a function of ion concentration, different sorption isotherms, that is, Langmuir and Freundlich were tested. Langmuir isotherm assumes monolayer adsorption over a homogeneous adsorbent surface. It does not take into consideration interactions between adsorbed molecules or ions.44 The linearized Langmuir isotherm allows the calculation of adsorption capacities and Langmuir constants and is equated by the following equation. Ceq/Q ) 1/Qmaxb + Ceq/Qmax The linear plots of Ceq/Q versus Ceq show that adsorption follows the Langmuir adsorption model (Figure 10a). The essential characteristics of the Langmuir isotherm can be expressed in terms of a dimensionless constant separation factor or equilibrium parameter RL which is defined as

isotherm that predicts whether an adsorption system is favorable or unfavorable. The RL values between 0 and 1 indicate favorable adsorption.45 Freundlich’s equation presents a nonideal sorption that involves heterogeneous adsorption over the active sites.44 The linearized form of the Freundlich adsorption isotherm was used to evaluate the sorption data, and it is represented as log Qe)log KF + (1/n) log Ce The isotherm equation was used to describe the adsorption process. and the different values obtained from it like the correlation coefficient (r2) are shown in Table 2 and Figure 10b. The r2 values show that the experimental data fit better in the linearized forms of the Langmuir than Freundlich isotherm equations over the whole Cu2+ concentration range studied, but the latter show a better match with the values of sorption capacities obtained from experimental values (Figure 10c and Table 2). Similar results are also reported elsewhere.46 YuhShan Ho47 has reported the Freundlich isotherms as the most suitable models for the cadmium sorption on tree fern; even the correlation coefficient is in favor of Langmuir (0.992) and

RL ) 1/(1 + bCo) where b is the Langmuir constant and Co is the initial concentration of Cu2+. The RL value indicates the shape of

Figure 8. The pH of the final solution after each experiment as a function of temperature and Cu2+ ions concentration.

Figure 9. FTIR spectra of SO-cl-N,N-MBAAm (a) before and (b) after Cu2+loading.


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Figure 10. Equilibrium isotherms: (a) Langmuir (Ceq/Q ) 1/Qmaxb + Ceq/Qmax); (b) Freundlich (log Qe ) log KF +(1/n) log Ce); and (c) comparison of panels a and b, with experimental results. Table 2. Correlation Coefficients and Comparison of Adsorption Capacity for Isotherm Models with Experimental Values constants 2

r RL

Langmuir

Freundlich

1 -0.820

0.999

Q (mg/g) Ce (ppm) 9.12 9.08 9.0 8.96 8.94

experimental

Langmuir

Freundlich

5.44 5.46 5.5 5.52 5.53

5.46 5.47 5.51 5.54 5.54

5.441 5.458 5.501 5.521 5.531

Freundlich (0.980). The values of RL were found to be less than zero (Table 2), indicating an irreversible adsorption process.45 The validity of Freundlich isotherm in the present case supports the heterogeneous nature of the sorption process as, apart from the -CO2H groups, sorption also takes place in the bulk of hydrogel at the cross-links where the amide groups of N,NMBAAm are the active sites. In addition to this, in the present study, -CO2H groups were generated after oxidation. These groups along with the -OH groups were not oxidized, and the

result is a heterogeneous surface and heterogeneous sorption of Cu2+ ions. 3.4. Kinetic Modeling. The kinetic of the ion uptake is related to the specificity of the interaction of heavy metal ions with the polymeric matrix. To examine the mechanism of sorption process such as mass transfer and chemical reaction, the different kinetic models were analyzed. The linear form of the pseudo-first-order equation is given below.48 log (qe,1 - qt) ) log qe,1 - (kI /2.303)t where qt and qe,1 are the amounts of Cu2+ adsorbed at time t and at equilibrium (mmol/g), respectively, and k1 is the rate constant of pseudo-first-order adsorption process (min-1). Figure 11a is a plot of log(qe,1 - qt) versus t for biosorption of Cu2+ for the pseudo-first-order equation. The value of r2 in the pseudofirst-order equation is 0.9913 (Table 3). The linear pseudosecond-order equation is given as49,50 t/Qt)1/k2qe,22 + (1/qe,2)t where k2 is the equilibrium rate constant of pseudo-second-order sorption (g/mg min). Figure 11b shows typical plots of pseudo-

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Figure 11. Kinetic behavior modeling (from left top clockwise): (a) pseudo-first-order (log(qe,1 - qt) ) log qe,1 - (kI/2.303)t), (b) pseudo-second-order (t/Qt ) 1/k2qe,22 + (1/qe,2)t), (c) Elovich equation (Qt ) (1/β) ln(Rβ) + (1/β) ln t), and (d) intraparticle diffusion model (Qt ) kintt1/2). Table 3. Correlation Coefficients for Different Kinetic Models and Comparison of Adsorption Capacity from Pseudo-first-order, Pseudo-second-order, Elovich Equation, and Intraparticle Diffusion Model with Experimental Values constants

experimental

r2

pseudofirstorder

pseudosecondorder

Elovich equation

intrapartical diffusion

Qt ) kintt1/2

0.9913

0.9928

0.9811

0.9329

where kint is the intraparticle diffusion rate constant (mg/(g min1/2)). Such plots may present a multilinearity,52 indicating that sorption process takes place in two or more steps. The first, sharper portion is the external surface adsorption or instantaneous adsorption stage. The second portion is the gradual adsorption stage, where intraparticle diffusion is rate-controlled. Out of the kinetic four models studied, the r2 value (0.9329) obtained was the lowest for this model (figure 11d). When the sorption capacity values obtained from different kinetic models were matched with the experimental values, there was significant deviation for the pseudo-first-order values and almost perfect match with those obtained for the pseudo-second order kinetics. It thus follows that sorption processes follow the pseudo-second-order kinetics that signifies the predominant contribution of the chemisorption/chelation mechanism (Figure 12 and Table 3). That means that two factors, that is, Cu2+ concentration and the active site participation of the sorbent with positive contribution of water absorption by the hydrogel in the bulk, will determine the sorption kinetics. As foresaid in subsection 3.2, this mechanism is also supported by the observations that after the completion of sorption experiment, the pH of the solution was substantially lower than the initial pH. Further, the multilayer adsorption or chemisorption is also correlated with the isotherm result, which supports the irreversible adsorption, that is, chemisorption from the RL value.

time (min)

Qt (mg/g)

2

r 30 60 120 240 360

5.44 5.46 5.5 5.52 5.53

The intraparticle diffusion model was also tested.52 The initial rate of the intraparticle diffusion is given by the following equation.

0.9913 -0.7023 -1.4997 -3.4068 -8.9126 -17.81

0.9928 6.63 5.7 5.328 5.159 5.105

0.9811 5.439 5.466 5.492 5.518 5.534

0.9329 0.03286 0.04647 0.0657 0.093 0.114

second-order equation t/Qt versus t. The plot of t/Qt versus time with a r2 value of 0.999 shows good agreement with the experimental data (Table 3). The adsorption data was also analyzed using the Elovich equation,51 which has the following linear form: (Qt ) (1/β))ln(Rβ) + (1/β)ln t where R is the initial sorption rate constant (mmol/g min), and the parameter β is related to the extent of surface coverage and activation energy for chemisorption (g/mmol). Figure 11c shows a plot of the Elovich equation. In this case, a linear relationship was also obtained between Cu2+ sorbed, Qt, and ln t over the contact time studied, with correlation coefficient 0.9811 (Table 3).


Figure 12. Comparison of pseudo-first-order, pseudo-second-order, Elovich equation, and intraparticle diffusion model with experimental values.

4. Conclusions The aim of the present study was to develop a new environmentally sensitive starch based on low cost, which is an environmentally friendly and effective sorbent for Cu2+ ions. Functionalization of starch by controlled hydrolysis and/or by oxidation and subsequent hydrogel formation has not been reported. Various characterization techniques such as FTIR and 13 C NMR were used to establish the oxidation of starch. The sorption of Cu2+ was found to be dependent on the structure of the hydrogel as well as on the external stimuli. The hydrogel synthesized from the oxidized starch exhibited the highest uptake among the whole series of functional starches and their hydrogels. The maximum percent uptake of 81.0 was obtained within 2 h, at 40 °C, and with a Cu2+ concentration of 50 ppm. It exhibited a maximum retention capacity of 128.26 mg g-1 after six feeds of Cu2+ ions. The functionalization of starch by oxidation has a positive effect on the ion uptake, as the sorption mechanism was found to occur predominantly by chelation of Cu2+ with -CO2H and -OH or -CdO groups of the hydrogel. The complex formation between ions and hydrogel was supported by the lowering of the pH of the solution to as low as 3.25 owing to the release of H+ from the sorbent. Evidence of ion uptake on the sorbent was also obtained from the changes in the intensity and position of the absorption peaks in the specific regions of the FTIR spectrum of the Cu2+-loaded hydrogel. Apart from chelation, adsorption on the cross-links of the hydrogel also contributes to the overall ion uptake. These conclusions are supported by the match of Langmuir and Freundlich isotherms and pseudo-second-order kinetics with the experimental values. Acknowledgment This study was supported by the University Grants Commission, New Delhi, India (Reference No. and Date, F. No. 30/ 47/2004(SR), Nov.1, 2004). Literature Cited (1) Ciesielski, W.; Lii, C. Y.; Yen, M. T.; Tomasik, P. Interactions of starch with salts of metals from the transition groups. Carbohydr. Polym. 2003, 51, 47.

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ReceiVed for reView June 19, 2009 ReVised manuscript receiVed February 3, 2010 Accepted February 7, 2010 IE9009952

Novel Polycarboxylated Starch-Based Sorbents for Cu2+ Ions

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