Adsorption Properties of Copper ( II ) Ion from Aqueous solution by starch-grafted Polyacrylamide and Crosslinked starch-grafted Polyacrylamide

Starch-grafted polyacrylamide (SA) and cross-linked starchgrafted polyacrylamide (CSA) had been synthesized via grafting polymerization from corn starch and used as adsorbents for the removal of Cu(II) ion from aqueous solution. The equilibrium, kinetics and thermodynamics of adsorption processes of SA and CSA were investigated and compared. It was found that the adsorption capacity of Cu(II) of SA and CSA could reach the maximum within 60 min when the pH was 6. The adsorption kinetics of CSA and SA for Cu(II) was favorably described by the pseudo-second-order kinetic model and the adsorption isotherm was described well with the Freundlich isotherm model. Thermodynamic studies indicated that the adsorption was a spontaneous and endothermic process with increased entropy, and the rise of temperature would benefit the adsorption; the enthalpy change(ΔH), the entropy change (ΔS) and free energy change (ΔG) of the adsorption process of Cu(II) on SA and CSA were calculated with adsorption isotherm data and basic thermodynamic relations.


Introduction
The removal of heavy-metal ions from sewage and industrial wastewater has been given lots of attention over the past few years, particularly in China where soil and water have been seriously polluted by heavy metals.Copper is present in wastewaters generated in the electroplating, cement, mining, dyeing and livestock breeding industries.Although copper is an essential trace element, high levels can cause severe environmental and public health problems.The most common adverse health effect of copper in humans is gastrointestinal discomfort.Large amounts of copper can cause hemolysis, liver necrosis and renal injury [1][2][3].The US Environmental Protection Agency (US EPA) requires that copper in drinking water doesn't exceed 1.3 mg/L.
Conventional methods for the removal of Cu from wastewater include reduction, ion exchange, evaporation, chemical precipitation and adsorption on activated carbon [4][5][6][7][8].However, most of these methods have some disadvantages, such as toxic sludge generation and high operation costs when applied for low Cu concentration wastewater on a large scale.This leads to exploration of cheaper and readily available materials for the adsorption of Cu [9,10].Starch is one of the most abundant natural polymers in the world.Because natural polymers and their derivatives are renewable, biodegradable and capable of adsorbing heavy-metal ions at low cost, some starch derivatives have been widely used for the removal of heavy metal ions.Chan and Wu [11] studied dynamic adsorption behaviors between ion and water-insoluble amphoteric starch in aqueous solutions.Kim & Lim [12] investigated the adsorption of divalent metal ions by cross-linked carboxymethyl corn starch.Guo [13] observed that water-insoluble starch phosphate carbamate had good adsorption performance for Cu(II).Xiang and Li [14] prepared cross-linked amino starch and dithiocarbamate starch and used them for the removal of Cu(II) from aqueous solutions.Dong [15] reported the adsorption behaviors of Cu(II) and Cr(VI) onto amino starch which was synthesized from dialdehyde starch.Nevertheless, these absorbents have low effective functional groups.In order to solve the problem, high contents of functional groups are needed by graft copolymerization.
Starch-grafted polyacrylamide (SA) is the copolymerization product of starch and acrylamide, which has been widely used in many fields such as papermaking, spinning, petroleum well drilling, medicine, daily chemicals, and floatation [16][17][18].Khalil and Farag [19] reported the use of SA as an adsorbent for removing heavy metal cations from aqueous solution.But because of the solubility of SA, the concentration of organic substances in aqueous solution often increased, which was unfavorable for water treatment.Crosslinked starch has infinite molecular mass resulting in its water insolubility; therefore, cross-linked starch-grafted polyacrylamide(CSA) may be reutilized as an adsorbent to remove heavy metal cations from aqueous solution.However, according to our literature survey, few investigations have been made concerning heavy metal cations adsorption onto CSA.
The objective of this study was to investigate the adsorption characteristics of Cu(II) ions from aqueous solution onto CSA.Batch experiments were conducted to compare the Cu(II) adsorption efficiencies for SA and CSA and further investigate the effects of various parameters such as initial solution pH, contact time, initial adsorbate concentration and temperature on the adsorption of Cu(II) onto CSA.The adsorption kinetic data were evaluated by pseudo-first-order and pseudo-secondorder models.The adsorption equilibrium data were analyzed by Langmuir and Freundlich isotherm models.Thermodynamic parameters of adsorption such as Gibbs free energy change (ΔG), enthalpy change (ΔH) and entropy change (ΔS) were determined.

Experiment part 2.1 Materials
Corn starch used was food grade (Tianjin DingFeng Factory, China).Epichlorohydrin (EPI), acrylamide (AM), and ceric ammonium nitrate (CAN) (AR, Shanghai Chemical Reagent Factory, Shanghai, China) were used as the cross-linking reagent, graft monomer, and initiator, respectively.CuSO 4 (AR, Shanghai Chemical Reagent Factory, Shanghai, China) was used in the adsorption experiment.Acetone, sodium chloride, potassium hydroxide, hydrochloric acid and ethanol (AR were purchased from Hangzhou Chemical Reagent Factory, Hangzhou,China.All aqueous solutions and standards were prepared using deionized water.

Preparation of crosslinked starch (CSt)
CSt was prepared via the following procedure: cornstarch (50 g) and 1% (w/v) sodium chloride solution (75 mL) were added into a 250 mL three-necked flask equipped with a stirrer and a thermometer.The slurry was heated to 30°C in a water bath.Fifteen percentage (w/w) KOH solution (20 mL) was added to the slurry, and then EPI(3.5mL) was added dropwisely for about 5 min.The reaction was kept for 4 h.Upon completion of the reaction, pH values of CSt suspension slurry was adjusted to 6-7 with 1 mol/L HCl solution.The reaction mixture was washed successively with 250 mL deionized water and 100 mL ethanol, and then filtered by vacuum filtration.Finally, the CSt product was milled and screened after drying in a vacuum oven at 60°C to a constant weight.

Preparation of SA and CSA
Preparation of SA or CSA through graft polymerization was carried out in a three-necked, round-bottom flask.3.0 g of starch (or CSt) was mixed with 50 mL of distilled water to prepare starch slurry.Then, a known amount of AM (6.0 g) monomer was added to the slurry, and a contact time of 15 min was allowed to facilitate the formation of the emulsion.Subsequently, 10 mL CAN solution (6 mmol• L −1 ) was added to form free radicals on the starch backbone.The volume of the reaction mixture was adjusted to 100 mL by adding distilled water.All the experiments were conducted under a nitrogen atmosphere with constant stirring.The graft polymerization was proceeded at 35°C for 3 h (both the grafting efficiency and the intrinsic viscosity reached the maximum).After the desired reaction period, nitrogen supply was terminated and the graft copolymer was cooled down to room temperature.The solution was precipitated with acetone, thus removing the unreacted monomer.The product was dried at 40°C.
The PAM homopolymer was removed from the precipitated graft copolymer by a Soxhlet extractor with a 40:60 (v/v) mixture of ethylene glycol and acetic acid.The purified SA (or CSA) was then washed with acetone, and was dried in a vacuum oven to a constant weight at 50°C.
The FT-IR spectra of the CSA and SA were analyzed with a NEXUS470 (Nicole Instrument Corporation, USA) using the KBr dispersion method.Compared with the FTIR spectrum of SA, there were no obvious changes in the FTIR spectrum of CSA.From the FTIR spectra of SA and CSA, it was observed that the broad peaks at 3388 cm −1 were due to the -NH stretching vibration of amide group of PAM (overlapped by -OH of starch).Smaller peaks at 2931 cm −1 were assigned to the -CH stretching vibrations and peaks at 931 cm -1 ,764 cm -1 and 704 cm -1 were attributed to carbonyl stretching vibrations of the starch substrate.There were some characteristic absorption peaks occurring at 1663 cm −1 and 1593 cm −1 which correspond to -C=O and -C=O(hydrogen bonded) stretching vibration of the -CONH 2 groups, and the peaks at 1153 cm −1 were due to C-N stretching.The presence of these additional peaks in grafted starch compared to that of non-grafted starch confirms the successful grafting of PAM chains onto the backbone of starch or the crosslinked starch.
The pH pzc values of SA and CSA measured were 5.75 and 5.81, respectively.The importance of pH pzc value is that it is expected that the removal of Cu(II) ion would be feasible below this pH at which the absorbents' surface would have a net positive charge.
The intrinsic viscosity of the graft copolymers were determined with an Ubbelohde viscometer at 30 ± 0.1°C in 1 M aqueous NaCl solution [20].The intrinsic viscosity values of SA and CSA were 238 mL/g and 413 mL/g, respectively.

Adsorption experiment
The adsorption experiments were carried out in a series of 200 mL Erlenmeyer flasks containing a certain amount of CSA (or SA) and 100 mL of CuSO 4 solutions with known Cu 2+ concentrations and initial pH (adjusted with 0.1 N hydrochloric acid or 0.1 N NaOH).The mixtures were then shaken at 150 rpm in a thermostatic water-bath shaker at 288 K.After shaking for a certain period of time, the supernatant was removed from the flask and the concentration of Cu 2+ in the aqueous phase was analyzed.All results were performed in triplicate, and the data were recorded as means.The adsorption capacity of SA (or CSA) was calculated with Eq. ( 1): where, Q is the adsorption capacity of Cu(II) on SA (or CSA) (mg/g); C 0 and C t (mg/L) are the initial concentrations of Cu(II) ions and Cu(II) concentrations at time t, respectively; V is the total volume of the aqueous solution (L); and W is the dose weight of SA (or CSA) (g).
The concentrations of Cu(II) in the aqueous solutions were measured using an atomic absorption spectrophotometer (AA800 spectrometer, PerkinElmer, USA).

Effect of pH on the adsorption process
The influence of the initial pH on the adsorption of Cu 2+ onto SA and CSA was examined in the pH range of 2.0-7.0 (Fig. 1).It can be seen from Fig. 1 that Q increased significantly with the increase of pH from 2.0 to 6.0, and then the increase slowed is slowing down as pH exceeded 6.0.As pH was higher than 7.0, the precipitation of Cu 2+ ions took place.
The influence of the solution pH on the metal ions uptake can be explained on the basis of the pH PZC of the adsorbent [21].Because the pH PZC of SA and CSA was 5.75 and 5.81, respectively, when pHs of solutions were lower than pH PZC of SA and CSA, the surface of the adsorbents was positively charged, and a high concentration of H + ions competed with Cu 2+ on the surface of adsorbents, which hindered the adsorption of Cu 2+ on the adsorbent surface.As pH value increased, the competitive adsorption of H + ions decreased [22], and the capacity adsorption of Cu 2+ was improved rapidly.When pH was higher than pH PZC , the negative charge density on the adsorbents surface was favorable for the adsorption of Cu 2+ ions through electrostatic interaction and surface complexation.
Both adsorbents had the same trends.The higher capacity of Cu 2+ ions on CSA (792 mg/g) than that of SA (680.5 mg/g), may be attributed to the higher content of nitrogen of CSA, which means CSA contained more amide groups ( functional groups).

Effect of adsorption time
The Cu 2+ adsorption capacity of SA and CSA was compared as a function of adsorption time.As shown in Fig. 2, the removal of Cu 2+ by the two adsorbents was increased with the increase in the adsorption time and the adsorption rate was initially rapid, and over 70% of the adsorption was completed within 10 min and the adsorption equilibrium was attained in 60 min.The adsorption rate of Cu 2+ ions was fast due to a large number of vacant active groups on the absorbents available for adsorption at the beginning, but became slower near the equilibrium due to quick exhaustion of the adsorption sites and the repulsive forces between Cu 2+ adsorbed on the solid adsorbents and in the aqueous phases.This result indicates that Cu 2+ ions can rapidly form chelates with the amide groups of starch.The rapid interaction of metal ions with starch would be beneficial for practical use.The result also indicates that the equilibrium time for SA and CSA was almost 60 min.For the subsequent experiments, the treatment time was maintained at 60 min to ensure that the adsorption equilibrium was achieved.

Adsorption kinetics study
Adsorption kinetics and equilibria are important for the evaluation of the basic traits of a good adsorbent [23].To investigate adsorption kinetics of Cu 2+ on SA and CSA, both pseudofirst-order and pseudo-second-order kinetic models were used to identify the rate and kinetics of the adsorption of Cu 2+ ions.
The linear form of the pseudo-first-order equation [24, 25] is given by: where, Q t and Q e (all in mg/g) are the adsorption amounts of Cu 2+ in aqueous solutions at equilibrium and at any time t, respectively.k 1 is the rate constant of the pseudo-first-order adsorption.The values of Q e (caluated), k 1 and the correlation coefficients were determined from the linear plot of ln(Q e − Q t ) versus t (Fig. 3(a)) and are shown in Table 1.
The linear pseudo-second-order equation [26,27] is given by: where k 2 is the rate constant of the pseudo-second-order kinetics.The plot of t/Q t versus t for the adsorption of Cu 2+ was plotted and is shown in Fig. 3(b).From the slope and intercept values, the values of Q e (cal.) and k 2 were calculated and are shown in Table 1.
It is clear from the results that the theoretical Qe (cal.)values more agree with the experimental ones Qe (exp.)obtained from the pseudo-second-order kinetics.Furthermore, all the correlation coefficients (R 2 ) for the pseudo-second-order kinetic model are more than 0.99(0.99965and 0.9999 for SA and CSA, respectively), implying that the adsorption of Cu(II) ions can be described more favorably by pseudo-second-order process.A similar observation was reported on the adsorption of Cu 2+ from aqueous solution using adsorbents with active groups [13,15,28].
From experimental data adsorption rate constants for SA and CSA were calculated and are shown in Table 1.It can be seen that the adsorption rate and capacity of CSA was higher than that of SA.This may be attributed to the fact that the crosslinked starch graft copolymer with a higher grafting percentage has more amide groups, which would increase the adsorption ability toward Cu(II) ions due to stronger complexation.

Effect of initial concentration of Cu2+ ion on adsorption capacity
The effect of initial Cu(II) concentration on the adsorption capacities on SA and CSA were investigated by varying the initial concentration from 30  (2) in Fig. 4, the adsorption capacities of SA and CSA increased significantly with increasing the initial Cu(II) concentration; Q of SA and CSA increased from 109.95 to 503.95 mg/g, and 127.68 to 600.3 mg/g, respectively, while the concentrations of copper ions in the gel matrix (C gel ) increased from 21.99 to 100.79mg/L , and 25.45 to 120.06 mg/L, respectively.The reason for the increase can be explained using the adsorption reactions described as follows: Eq. ( 4) shows that the increase of Cu 2+ concentration can shift the reaction to the right and cause higher adsorption.In Tab. 2, the partition coefficients decreased with increasing the initial Cu(II) concentration, which resulted from the initial Cu(II) concentration increasing faster than ions that were adsorbed on the surface of the absorbents.The increase in the adsorption capacity on CSA was much higher than SA due to more amide groups, resulting in significant adsorption differences at high copper ion concentrations.

Adsorption isotherm
The adsorption equilibrium data obtained at various initial Cu 2+ concentrations were used to assess the adsorption isotherm with the two popular adsorption models: Langmuir and Freundlich isotherm.Langmuir adsorption isotherm models the monolayer coverage on the adsorption surface.It assumes that adsorption occurs at specific homogeneous adsorption sites on the adsorbent surface and intermolecular forces decrease rapidly with the distance from the adsorption surface.The Langmuir adsorption model further assumes that all the adsorption sites are energetically identical and adsorption occurs on a structurally homogeneous adsorbent.The Langmuir isotherm equation [29] can be expressed in Eq. ( 5): where C e is the equilibrium concentration of Cu 2+ in aqueous solutions (mg/L), Q e is the adsorption amount of Cu 2+ in aqueous solutions at equilibrium(mg/g), Q m is the maximum adsorption amount of Cu 2+ in aqueous solutions at equilibrium(mg/g), and b is the Langmuir constant (L/mg).
Freundlich adsorption isotherm equation [30] is derived to model the multilayer adsorption on heterogeneous surfaces and can be described by Eq (6): where K f is the Freundlich constant which is related to the adsorption capacity of the adsorbent, and 1/n is the Freundlich constant related to adsorption intensity.
The values of Q m and b were calculated from the slope and the intercept of the linear plot of C e /Q e versus C e (Fig. 5a) and the values of K f and n were calculated from the slope and the intercept of the linear plot between lg Q e and lg C e (Fig. 5b).The values are presented in Tab. 3.
The R 2 values for the Freundlich isotherms for the adsorption of Cu 2+ on SA and CSA were above 0.99 (Table 3), indicating that adsorption of Cu 2+ on the two absorbents can be better described with the Freundlich isotherm.This observation  suggests that the two absorbents' surfaces which could adsorb copper ion were heterogeneous.Perhaps the amide groups and the -OH groups residued on starch chains along with the porous structure of the starch granule all contributed to absorption of Cu 2+ ions, therefore the physical absorption of Cu 2+ ions could be an additional mechanism for metal removal by SA or CSA.

Thermodynamics of the adsorption
The thermodynamics for adsorption of Cu 2+ ion on SA and CSA were determined at different temperatures.It is found that the adsorption capacities were increased with the increase in temperature, showing the endothermic nature of the adsorption with the temperature in the range of 283 to 303 K.
Thermodynamic parameters [31,32], such as free energy change (ΔG), enthalpy change (ΔH) and entropy change (ΔS), were estimated using the following equations.D represents the distribution coefficient (L/g), which is equal to the absorption equilibrium constant at the solid/liquid phase, and is shown as a function of the temperature.Because the relationship between lg D and 1/ T for SA and CSA is linear, the changes in the adsorption enthalpy (ΔH) and entropy (ΔS) can be calculated with the Van't Hoff equation (8): where, R is the gas constant (J/ mol•K) and T is the temperature (K).The plots of lgD against 1/T for Cu 2+ ions are shown in Fig. 6.The values of ΔH and ΔS were obtained from the slope and intercept of lgD vs 1/T plots, which were calculated by a curve-fitting program.Gibbs free energy (ΔG) was calculated with the following thermodynamic equation: where, D is the distribution coefficient (L/g) and R is the gas constant (J/ mol•K).According to Eq. ( 8), the values of ΔH and ΔS can be calculated from the slopes (ΔH/2.303R)and intercepts (ΔS /R) of the plots of lgD versus 1/T.ΔG was calculated from the thermodynamic equation ( 9).The calculated values of the thermodynamic parameters are given in Tab. 4.
As it can be seen from Tab. 4, the ΔH values are found to be positive for all cases revealing that the adsorption processes were endothermic in nature.Negative values of ΔG indicate the adsorption processes were spontaneous and thermodynamically favorable.The values of ΔG became more negative with the increase in temperature, indicating that the adsorption processes would be more favorable at high temperature.The similar results are found in the studies of Wang and Qin [33], Rakhshaee et al. [34], Xie et al. [35]and Yin et al. [36], The positive values of ΔS showed the affinity of SA or CSA for Cu 2+ ions increased the disorder at the solid/liquid interface during the adsorption process.Generally, the change in free energy for physical adsorption is between −20 kJ/mol and 0, and that for chemical adsorption is between −80 and −400 kJ/mol [37].The values of ΔG obtained in this study are in the range of physical adsorption, suggesting that the adsorption of Cu 2+ onto SA or CSA are both physical adsorption.

Conclusions
CSA exhibited higher adsorption capacity for Cu(II) ions than SA.Various factors including solution pH, contact time, initial Cu(II) concentration and temperature were studied.The results showed the adsorption capacity of Cu(II) on CSA increased with the increase of solution pH from 2 to 6.The adsorption kinetic process was well predicted by the pseudosecond-order kinetic model and the adsorption equilibrium data were correlated well with the Freundlich isotherm model.Based on the Freundlich isotherm model, the maximum Cu(II) adsorption capacity for CSA at pH=6 was 792 mg/g.The negative value of ΔG showed that the adsorption of Cu(II) onto CSA was a spontaneous and physical adsorption process.The positive value of ΔH showed the endothermic nature of adsorption.The positive values of ΔS showed the affinity of CSA for Cu(II) ions increased the disorder at the solid/liquid interface during the adsorption process.Results of this study suggest that CSA is a promising adsorbent for the removal of Cu(II) ions from aqueous solution.

Fig. 4 .
Fig. 4. Effect of initial concentration of copper ions (a)on adsorption capacity and (b)the concentration of copper ions in the gel matrix (T = 288K; pH = 6; contacted time = 60min; the highest absorbent dosage = 200mg/L) to 180 mg/L.As shown Tab. 3. Langmuir and Freundlich parameters for the adsorption of Cu 2+ ions on SA and CSA