Poly(acrylonitrile-co-acrylamide) [poly(AN-co- AM)] was prepared via a redox method, with sodium bisulfite and potassium persulphate as the initiators. High yield was attained in up to 76% of monomer conversion within 3 h of reaction time. The copolymers were then chemically modified with hydroxylamine hydrochloride to obtain amidoxime functional groups in the polyacrylonitrile (PAN) system. The role of the amidoxime group is to form a chelating ion-exchange network based on acrylonitrile for the adsorption of heavy metal ions in aqueous solution. Based on the Fourier transform infrared spectra, the appearance of a strong band at the regions of 1680 and 3300–3400 cm-1 due to the stretching vibration of C=O and NH2 groups, respectively, confirmed the copolymerization of acrylamide into the PAN system. In the case of modified copolymers, there was an appearance of broad bands at the regions of ∼3300 and ∼900 cm-1, which corresponded to the stretching vibrations of O-H and =N-O, respectively, from the oxime group. The scanning electron microscopy showed that the average size of the copolymer particles was ∼149 nm, and the average size increased to ∼217 nm in the case of the modified copolymer. The amidoximed poly(AN-co-AM) was analyzed by inductively coupled plasma to investigate its adsorption behavior toward Cu(II).
The abundance of heavy metal ions from chemical manufacturing and industrial effluents can have serious effects on the environment (1). These pollutants threaten the health of human populations and the natural ecosystems. Cu(II) damages the kidney and liver, causes anemia (2), damages the central nervous system and affects the enzymes in living organisms due to its high affinity towards ligands containing nitrogen and sulfur donors (2). There are various materials and methods that have been utilized to remove heavy metal ions from wastewater such as ion exchange resins and fibers (3); activated charcoal, chemical precipitation, electrode position, membrane processing (4, 5) and hydrogel materials (6). Among the methods, adsorption was reported as one of the best techniques due to the availability of different adsorbents, low processing cost, high efficiency, environmental friendliness, lower sludge generation, less equilibrium time and high selectivity for metal ions (5, 7–9).
Acrylonitrile (AN) has been utilized widely for the application of heavy metal ion uptake. AN consists of a reactive pendant group (cyano group) that can be modified by different types of reagents such as hydroxylamine, hydrazine, ethylenediamine and thiosemicarbazide through nucleophilic addition and cycloaddition reactions (1, 10). However, the degree of modification of polyacrylonitrile (PAN) is low due to the poor accessibility of the nitrile group (10). Thus, acrylamide (AM) was chosen to be incorporated into the PAN system. AM could lower the nitrile-nitrile dipolar interactions and facilitate the transformation of nitrile groups into amidoxime groups. It was reported that polyacrylamide (PAM) has good complexion ability with metal ions in solution (11). This is due to the fact that PAM can form strong coordination bonds with heavy metal ions in solution. However, the good solubility of PAM in water contributed to the poor ability of PAM to adsorb heavy metal ions in real applications, e.g., wastewater treatment (11). Hence, a lot of work has been reported to enhance the ability of PAM to form strong complexes with heavy metal ions by introducing new functional groups in PAM systems. Bentonite (BENT) was incorporated into PAM gels for the removal of Cu(II) in aqueous solution. The BENT-PAM composites showed higher adsorption capacity towards Cu(II) at 33 mg/g compared to the BENT with an adsorption capacity of 29 mg/g at pH 6.2 (11). PAM was grafted onto guar gum (GG) and further cross-linked with glutaraldehyde to obtain the hydrogel sorbent material. The Langmuir sorption capacity of hexavalent chromium ion from aqueous solution by poly(AM-graft-GG) hydrogel was found to be 588 mg/g (12). Gel beads containing hydrolyzed PAM and chitosan were prepared for the removal of Cu2+, Pb2+ and Hg2+ ions from aqueous solution. The removal order of the heavy metal ions was Pb2+>Cu2+>Hg2+ under the same conditions (13).
In this work, it is expected that the selection of an amide-containing comonomer for incorporation into the PAN system could improve the nature of sites that are capable of adsorbing heavy metal ions in aqueous solution. In addition, the presence of amide groups facilitates the formation of copper-amide linkages via interaction with aqueous Cu(II).
The poly(acrylonitrile-co-acrylamide) [poly(AN-co-AM)] was then chemically modified with hydroxylamine hydrochloride to transform the nitrile (hydrophobic) groups into amidoxime (hydrophilic) groups to form amidoximed poly(AN-co-AM) (Figure 1). The amidoxime group has high complex-forming capabilities with heavy metal ions (14). A sulfolignin-polyacrylamide graft copolymer was synthesized and further functionalized by converting its amide/carbonyl groups to hydroxamic acid/amidoxime groups by chemical modification with hydroxylamine hydrochloride for the adsorption of uranium and lead (15). Copolymers derived from the binary mixture of N-vinyl imidazole and acrylonitrile monomers were polymerized by radiation. The hydrogel was modified with hydroxylamine hydrochloride to transform the nitrile groups of AN into amidoxime functional groups in aqueous media (16) to produce chelating hydrogels. Similarly, highly functionalized hydrogel particles were prepared by modification of poly(acrylonitrile-co-1-vinylimidazole) to convert the cyano groups into amidoxime groups (17).
The advantage of the present work is the elimination of the use of organic solvent, which contributes to the ease of solvent handling with lower cost as deionized water was used as the reaction medium during the polymerization work (18). The incorporation of acrylamide into an acrylonitrile system via a redox method and further modified with hydroxylamine hydrochloride for Cu(II) uptake has not been reported elsewhere.
2.1 Synthesis and characterization
Poly(acrylonitrile-co-acrylamide) [poly(AN-co-AM)] was prepared by redox copolymerization of acrylonitrile (AN) (Merck Co., Germany, 99%) with acrylamide (AM) (R&M Marketing, Essex, UK, 99%) in deionized water. Sodium bisulfite (SBS) (Systerm Chemical, Malaysia, analytical reagent) and potassium persulfate (KPS) (Systerm Chemical, Malaysia, analytical reagent) were used to initiate the polymerization. Polymerization of poly(AN-co-AM) (90:10) was carried out in a three-necked flask at 40°C under nitrogen gas atmosphere. The flask was fitted with a condenser, and the third neck was used for nitrogen purging. A total of 200 ml of deionized water (as reaction medium) was added into the flask, and the temperature was increased to 40°C. After 30 min, 172.8 mmol of AN monomer was added to the reaction medium followed by 19.2 mmol of AM monomer and, finally, 2.09 g of SBS and 2.16 g of KPS as redox initiators. The mixture was then stirred under nitrogen at 40°C and allowed to proceed for 3 h. The polymer that formed was precipitated, filtered, washed successively with methanol (Systerm Chemical, Malaysia) and deionized water, and dried under vacuum at 45°C until a constant weight was obtained (18, 19). Polyacrylonitrile homopolymer was also prepared in the same way.
The reproducibility of the copolymerization was confirmed by several duplicate experiments. The conversion of PAN and copolymers was calculated using Equation .
The FTIR spectra of PAN and the copolymers were recorded on a Perkin Elmer 1625X infrared spectrophotometer (Perkin Elmer, USA) using KBr pellets.
2.2 Modification with hydroxylamine hydrochloride and characterization
Two grams of poly(AN-co-AM) was added to the mixture of 25 ml of methanol and 3 g of hydroxylamine hydrochloride (Systerm Chemical, Malaysia, 99%) with a magnetic stirrer and a reflux condenser. The mixture was stirred for 2 h at room temperature. Then, 6 ml of sodium hydroxide (NaOH) solution (Systerm Chemical, Malaysia) was added to the mixture to neutralize HCl and keep the pH at 8. The reaction was conducted for 6 h at 70°C under stirring. The modified poly(AN-co-AM) was filtered and washed thoroughly with deionized water. The modified poly(AN-co-AM) was then dried at 50°C in a vacuum oven until constant weight was obtained.
The Fourier transform infrared analysis of modified poly(AN-co-AM) was recorded on a Perkin Elmer 1625X infrared spectrophotometer (Perkin Elmer, USA) using KBr pellets.
2.3 Adsorption analysis in a batch experiment
A total of 4.91 g of copper (II) sulfate monohydrate (CuSO4·H2O) (Systerm Chemical, Malaysia, 99%) was dissolved in 500 ml of distilled water. This stock solution was diluted serially, as required for use in experiments and analysis. The batch experiment was carried out at room temperature (25°C) to determine the adsorption capacity towards Cu(II) by mixing each sorbent with 20 ml of 100 ppm copper solution in a centrifuge tube. The mixture was shaken by using a Hotech rotary shaker (Hotech, Taiwan) at 150 rpm. The mixture was separated by filtration, and the filtrate was collected to analyze the Cu(II) concentration by using an inductively coupled plasma (ICP) instrument. The initial and final concentration of the copper ion were recorded. The steps were repeated by varying the following parameters: pH, Cu(II) concentration, contact time and adsorbent dosage. The Cu(II) solutions used were at pH 5 unless otherwise stated.
The morphology of the unmodified and modified polymers was analyzed by variable pressure scanning electron microscopy (Leo 1450VP SEM) (Leo ZEISS, Germany) with a magnification of 20,000×, and gold-coated samples were obtained. The initial and final concentration of the Cu (II) solutions were analyzed by ICP (Perkin Elmer Emission Spectrometer Plasma 1000) (Perkin Elmer, USA).
3 Results and discussion
3.1 Monomer conversion
As shown in Table 1, PAN achieved the highest conversion (92%) as compared to the copolymers. This was expected as there were no abnormalities or defects present in the polymer chains due to the absence of a comonomer (19). The reduction of the AN concentration in the feed from 97% to 93% molar ratio resulted in the depression of the monomer conversion from 72% to 37%. This is probably due to the fact that the reduction of AN may have reduced the polymerization rate, and thus the precipitation rate of the oligomeric chains occurred slowly, which reduced the consumption of available radicals in the oligomeric phase. A non-optimized amount of monomer could reduce the rate of polymerization, thus reducing the conversion of monomers (19).
|Feed (% mol), AN/AM||Conversion (%)|
Among all copolymers, poly(AN-co-AM) (90:10) achieved the highest yield of 76%. It may be attributed to the increase in the number of monomer radicals available to react with the free radicals that formed in the polymer backbone (20) due to the optimum amount of monomers in the feed. Initially, the polymerization occurred in water due to the water solubility of the redox initiator. Oligomeric radicals formed and precipitated after achieving a certain critical molecular point. These oligomeric radicals further acted as the primary particles that propagated in the water phase. Since the oligomers are not soluble in water, the polymerization occurred according to the suspension polymerization (changed from homogeneous to heterogeneous phase). The propagation could proceed in both the oligomeric phase and the water phase. Due to the low solubility of AN in water as compared to the AM units, AN tends to be absorbed by the oligomeric radicals. In contrast, AM tends to polymerize in the water phase. Since the concentration of AM was the highest in the feed at 10% molar ratio, the possibility of AM being involved during the polymerization was high as well. During the polymerization in both the oligomeric and the water phase, the termination and the chain transfer were difficult, which resulted in high yields (21, 22). The amount of 10% molar ratio of AM was found to be the optimum concentration to obtain a high yield of poly(AN-co-AM).
3.2 Infrared spectra analysis
Figure 2 shows the IR spectra of unmodified PAN, amidoximed PAN, unmodified poly(AN-co-AM) and amidoximed poly(AN-co-AM). Both the unmodified PAN and the unmodified poly(AN-co-AM) show the absorption of the nitrile functional group at the range of 2240–2260 cm-1 and the stretching of the C-H group at the region of 3000–2850 cm-1. The IR spectrum of unmodified poly(AN-co-AM) shows the additional strong band arising from the C=O group at the region of 1680 cm-1. The stretching vibration by the NH2 band appeared at the region of 3300–3400 cm-1 due to the incorporation of the AM comonomer into the PAN system (23). The IR spectrum indicates that AM was successfully copolymerized with AN.
The amidoximed PAN and amidoximed poly(AN-co-AM) spectra had the additional broad band arising from a newly formed -C=N functional group at the region of 1640 cm-1, which supported the successful of modification of the surface. As compared to the IR spectrum of unmodified poly(AN-co-AM), the characteristic peak of the nitrile group (at the region of 2260–2240 cm-1) in the modified poly(AN-co-AM) completely disappeared, suggesting that the modification of the nitrile groups was almost complete (23). This indicates that the nitrile groups were transformed into the amidoxime group due to the amidoximation. The broad band observed in the region of 3650–3300 cm-1 of amidoximed PAN and amidoximed poly(AN-co-AM) corresponded to the O-H of oxime group (5). However, the band at the region of 3400–3300 cm-1, which is characteristic of the amidrazone group (NH2), disappeared. This is probably due to the overlap between the N-H groups and the OH stretching vibrations in the same regions (24). The band observed at 900 cm-1 was assigned to the stretching vibration of the =N-O bond from the oxime group. The IR spectrum of modified poly(AN-co-AM) indicates that the modification of poly(AN-co-AM) with hydroxylamine was achieved.
3.3 Scanning electron microscopy analysis
The scanning electron micrographs revealed that the PAN (Figure 3A) and the amidoximed PAN (Figure 3B), poly(AN-co-AM) (Figure 3C) and amidoximed poly(AN-co-AM) (Figure 3D) obtained micro-size particles. The micro-size beads are favorable because they are expected to have high specific surface areas that improve the adsorption of heavy metal ions. In addition, the microbeads appear spherical in shape, which is favorable for the adsorption process (25).
As shown in Figure 3C, the incorporation of AM into the PAN system influenced the size of the micro-size beads. The average size of poly(AN-co-AM) was ∼149 nm, which was larger than the size of the PAN microbeads (∼118 nm). This observation supports the fact that AM was successfully incorporated into the PAN copolymer.
Figure 3D shows that the average particle size of amidoximed poly(AN-co-AM) is ∼217 nm, which is larger than that of unmodified poly(AN-co-AM) (∼149 nm) (Figure 3C).
3.4 Cu(II) adsorption analysis
3.4.1 Effect of pH on adsorption
The effect of pH was investigated over the range of 1–6 while keeping all other parameters as constant. As shown in Figure 4, the adsorption of Cu(II) increased to the maximum at pH 5 and decreased at pH 6. At higher acidic condition, the presence of more hydrogen ions may result in the suppression of Cu(II) adsorption. The protonation of the amine groups at the acidic conditions reduced the number of binding sites for the adsorption of Cu(II). In addition, the competitive adsorption between the H+ ion and the Cu(II) ion for the same active adsorption site (5, 24, 26) may reduce the efficiency of heavy metal ion uptake.
As the pH increased from 2 to 5, the concentration of the H+ ion decreased or the formation of the deprotonated groups (NH2- form) increased. Thus, the electrostatic attraction between the Cu(II) and the polymer surface increased (2) and resulted in the maximum adsorption capacity at pH 5.
At pH 6, the adsorption capacity of Cu(II) decreased to the minimum value. This was expected due to the formation of soluble hydroxyl complexes as the Cu(II) started to precipitate as Cu(OH)2 at higher pH, which reduced the number of Cu(II) cations involved in the adsorption process (25).
3.4.2 Effect of contact time
Figure 5 shows the time course of the adsorption equilibrium of Cu(II) ions onto the poly(AN-co-AM). There was a rapid uptake for the first 120 min, with the adsorption equilibrium being attained within 240 min. Thus, the optimum occurred at equilibrium in between 120 and 240 min. Hence, 4 h of contact time was chosen as the optimum equilibrium time for the experimental studies. Equilibrium time is one of the important parameters for an economical wastewater treatment system (5).
3.4.3 Adsorption kinetics
The effect of contact time and initial concentration was analyzed by a kinetic point of view. Adsorption processes are controlled by various mechanisms such as mass transfer, mass diffusion, chemical reactions and particle diffusion (2). To clarify the mechanisms controlling the adsorption, several adsorption models were used to evaluate the experimental data. The adsorption kinetics was compared using two kinetic models, which are the pseudo-first-order kinetic model and the pseudo-second-order kinetic model. The pseudo-first-order kinetic equation is given in Equation .
where qe is the amount of ions adsorbed on the surface of the sorbent at equilibrium (mg/g), qt is the amount of metal ions adsorbed on the surface of the sorbent at time t (mg/g), t is time (min) and k1t is the rate constant of the pseudo-first-order adsorption (1 min-1).
Figure 6 shows the pseudo-first-order plot: log(qe-qt) against t of Cu(II) solutions. The value of k1 was calculated from the slope, while theoretical qe was calculated from the intercept at the y-axis. The data did not fit well in the pseudo-first-order reaction, with a regression coefficient (R2) value of 0.988. Hence, the adsorption of Cu(II) onto amidoximed poly(AN-co-AM) did not follow the pseudo-first-order reaction.
Figure 7 shows the plot of the pseudo-second-order kinetics for the adsorption of Cu(II) by amidoximed poly(AN-co-AM). The linear graph with regression coefficient (R2) values of up to 0.999 indicates that the kinetics of the pseudo-second-order kinetics rate model better represents the experimental data than that of the pseudo-first-order rate model. In addition, as shown in Table 2, its equilibrium adsorption capacity, qe(exp), has better agreement with the experimental values, qe(cal). It can be concluded that the pseudo-second order model is the preferred adsorption kinetic model and reflects the process of the chemical adsorption of Cu(II) onto amidoximed poly(AN-co-AM) (5). Second-order copper removal kinetics were also postulated for adsorption of copper on various adsorbents like grafted silica, tree fern, H3PO4-activated rubber wood sawdust, peat, chitin, papaya wood, etc. (7).
|First order||Second order|
|qe(exp) (mg/g)||k1 (min-1)||qe(cal) (mg/g)||R2||k2 [g/(mg min)]||qe(cal) (mg/g)||R2||H [mg/(g min)]|
|19.080||0.037||14.521||0.988||7.03 x 10-3||19.608||0.999||2.703|
3.4.4 Effect of initial metal concentration
As shown in Figure 8, as the Cu(II) concentration varied from 25 to 700 ppm, the adsorption capacity of amidoximed poly(AN-co-AM) increased from 4.93 to 83.28 mg/g and achieved constant values after 500 ppm. This indicates that the adsorption of Cu(II) is dependent on the initial concentration of Cu(II). The ratio of the initial number of Cu(II) was found to be comparable with the number of available binding sites of the adsorbent and achieved a saturation point at 500 ppm of Cu(II).
3.4.5 Adsorption isotherm
The equilibrium adsorption isotherm is one of the promising data to understand the mechanism of the adsorption and describe the relationship between the amount of heavy metal ions adsorbed and the metal ion concentration in the equilibrium solutions. The significance of the adsorption isotherm is that it reflects the distribution of adsorbate molecules between the solution and the adsorbent at equilibrium conditions (27). Two different isotherms were selected in this present study, which are Langmuir and Freundlich isotherms, to investigate the characteristic of Cu (II) adsorption onto the outer surface of amidoximed poly(AN-co-AM), whether it is a monolayer or a multilayer coverage of Cu(II).
A plot of the Langmuir isotherm for the adsorption of Cu(II) is shown in Figure 9. The initial concentration of Cu(II) for the adsorption isotherm varied from 25, 50, 100, 200, 300 and 400 ppm.
The linearized Langmuir isotherm equation is represented by Equation .
where Ce is the equilibrium concentration of the metal ion (mg/l), qe is the amount of Cu(II) adsorbed at equilibrium (mg/g), Qmax is the maximum adsorption capacity (mg/g) and b is the Langmuir constant, which is related to the binding energy between the adsorbed ion and the adsorbent.
A linearized plot of Ce/qe against Ce gives the b value. A straight line confirms the applicability of the Langmuir adsorption isotherm. The plot gives a straight line of slope and intercepts where Qmax gives the theoretical monolayer saturation (26). The Langmuir model assumes that the uptake of heavy metal ions occurs on a homogenous surface by monolayer adsorption without any interaction between adsorbed ions. In addition, the Langmuir model suggests that the equilibrium is attained when a monolayer of the adsorbate molecules saturated the adsorbent (27).
In contrast, the Freundlich model assumes that the adsorption of metal ions occurs on a heterogeneous surface (5). It relates the equilibrium liquid and solid phase capacity based on multilayer adsorption. The Freundlich isotherm model has the linear form of Equation .
where Kf is the Freundlich constant, which indicates the adsorption capacity (l/mg); n is the Freundlich constant for the intensity of adsorption; qe is the amount of Cu(II) sorbed at equilibrium (mg/g) and Ce is the equilibrium concentration of Cu(II) (mg/l).
Figure 10 shows the plot of the Freundlich isotherm for Cu(II) adsorption. The values of Kf and n could be obtained by plotting log qe against log Ce. A plot of log qe against log Ce yields a straight line, which is a confirmation of the Freundlich’s adsorption isotherm. Kf and n are indicative of the extent of the adsorption and the degree of nonlinearity between the solution concentration and the adsorption (9).
The Langmuir and Freundlich constants resulting from the equilibrium adsorption studies of Cu(II) by amidoximed poly(AN-co-AM) were determined and are listed in Table 3. It appears that the adsorption isotherm profiles of amidoximed poly(AN-co-AM) for the Cu(II) are well described by Langmuir equations (Figure 9). The higher value of the correlation coefficient (R2) of 0.993 by the Langmuir equation (as compared to an R2 of only 0.494 by Freundlich constant) is an indication of a good agreement existing between the parameters for the Cu(II). The constant Qmax, which is a measure of the adsorption capacity to form a monolayer, can be as high as 111 mg/g. The constant b, which denotes the adsorption energy, was equal to 1.795 l/mg.
|Langmuir isotherm||Freundlich isotherm|
|Qmax (mg/g)||b (l/mg)||R2||Kf (l/mg)||n||R2|
The results demonstrate that the adsorption of Cu(II) by amidoximed poly(AN-co-AM) was characterized by the monolayer coverage of the Cu(II) on the adsorbent’s outer surface. In addition, the adsorption process has a homogenous nature for each adsorbed molecules (5).
3.4.6 Effect of dosage of amidoximed poly(AN-co-AM)
The sorbent dosage is a parameter to investigate the quantitative uptake of heavy metal ions. The effect of amidoximed poly(AN-co-AM) dose on the adsorption properties for Cu(II) was studied by varying the amount of the sorbent. As shown in Figure 11, as the sorbent dose increased from 0.1 to 0.4 g, the adsorption capacity of Cu(II) per gram of amidoximed poly(AN-co-AM) decreased from 19.12 to 4.78 mg/g. This is due to the lower utilization of the adsorbent, which resulted in the less commensurate increase in the adsorption process (5).
3.5 Structure of the complexes
The expected structure of the complexes between amidoximed poly(AN-co-AM) and copper ion is shown in Figure 12. As amidoxime is supposed to be bidentate, two amidoximes may be used for the chelate formation with a metal ion, which makes a square-planar chelate. The strong coordination between the lone-pair electron of amine nitrogen and empty orbital of copper contributes to the increased adsorption capacity (7, 28).
The redox polymerization of poly(AN-co-AM) was achieved with a maximum conversion of 76%. A heavy metal ion-adsorbing polymer bearing amidoxime groups was successfully prepared by modifying poly(AN-co-AM) with NH2OH·HCl. The surface morphology of amidoximed poly(AN-co-AM) shows spherical particles with micro-size beads. The adsorption capacity of Cu(II) increased with increased pH until an optimum adsorption of Cu(II) (52.63 mg/g) at pH 5. The adsorption of Cu(II) onto poly(AN-co-AM) correlates well with the Langmuir equation. Kinetic study showed that the adsorption of Cu(II) follows the pseudo-second-order model (R2=0.999) as compared to the pseudo-first–order model. The adsorption capacity uptake of Cu(II) onto the amidoximed poly(AN-co-AM) decreased as the sorbent dosage increased.
This research project was supported by the Ministry of Education Malaysia through the scholarship of Fundamental Research Grant Scheme (02-01-13-1215FR). The authors would like to acknowledge the Universiti Putra Malaysia (UPM) and Universiti Kebangsaan Malaysia (UKM) for providing the facilities.
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