Chemical Papers 70 (9) 1171–1184 (2016) DOI: 10.1515/chempap-2016-0064

ORIGINAL PAPER

Preparation of corn stalk-based adsorbents and their specific application in metal ions adsorption Ya-Ting Wang, Hou Chen*, Dong-Ju Wang, Liang-Jiu Bai, Hui Xu, Wen-Xiang Wang School of Chemistry and Materials Science, Ludong University, Yantai 264025, China Received 16 August 2015; Revised 23 December 2015; Accepted 8 February 2016

Corn stalk-based adsorbents modified from corn stalk were prepared by Cu(0)-mediated reversible-deactivation radical polymerization (Cu(0)-mediated RDRP). They were applied to remove metal ions and they exhibited good adsorption capacity, especially for Hg(II). Adsorption properties of corn stalk can be enhanced by introducing cyano, amino, amidoxime, and carboxyl groups onto its surface, which results in efficient adsorbents for different metal ions. TGA, SEM, EA, and FTIR analyses were employed to characterize the structures of corn stalk-graft-polyacrylonitrile (CS-g-PAN), corn stalk-graft-polyacrylamide (CS-g-PAM), amidoxime corn stalk-graft-polyacrylonitrile (AO CS-g-PAN) and carboxyl corn stalk-graft-poly(methyl acrylate) (CO CS-g-PMA). The maximum adsorption capacity for Hg(II) was 8.06 mmol g−1 of AO CS-g-PAN. Kinetics of the Hg(II) adsorption on AO CS-g-PAN was found to follow the pseudo-second-order model and the adsorption isotherms were well fitted with the Freundlich isotherm model. c 2016 Institute of Chemistry, Slovak Academy of Sciences  Keywords: corn stalk, modification, adsorption, metal ions

Introduction With high toxicity and non-biodegradability, heavy metal ion pollutants have been recognized as the main contributors to aquatic environment contaminations (Zhou et al., 2013). Besides environmental pollution, heavy metal ions have also greatly harmed human health (Qu et al., 2009; Tadeu et al., 2008; Du et al., 2011; Imamoglu & Tekir, 2008; Sud et al., 2008). Research shows that frequent exposure to high concentrations of Hg(II) leads to neurobehavioral disorders and developmental disabilities. Numerous efforts have been made to develop techniques for harmful pollutants removal from aqueous solutions (Sariet al., 2007; Reddad et al., 2002). Currently, the predominant method used to remove heavy metal ions from wastewater is adsorption due to the advantages of its high adsorption capacity and comprehensive target contaminants (Min et al., 2012; Crini, 2005). However, the high cost of commonly used adsorbents for regen-

eration/reactivation limits their extensive application (Aivalioti et al., 2010). Therefore, it is of great significance to develop novel adsorbents with low cost, good adsorption capacity, and with natural plant waste as raw materials. The most important evaluation parameters of the adsorbents are selectivity and capacity of adsorption. The type of the functional group essentially affects the selectivity of the adsorption via the reaction between the adsorbate and the functional group of the adsorbent (Chen et al., 2014). Agricultural residue with abundant cellulose is a well-known low-cost adsorbent of toxic pollutants (Kumar & Bandyopadhyay, 2006; Kamari et al., 2014). Cellulose contains a high amount of hydroxyl groups which can chelate with heavy metal ions (Bowes et al., 1979). However, interaction between hydroxyl groups and heavy metal ions is weak. Thus, cellulose can be considered neither high capacity nor selective adsorbent. Therefore, in order to increase its adsorption ability, specific functional groups have

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Fig. 1. Synthesis of adsorbents CS-g-PAN, CS-g-PAM, AO CS-g-PAN and CO CS-g-PMA.

to be introduced onto the surface of the agricultural residue before its application for the removal of heavy metal ions from aqueous solutions. Modification by grafting copolymerization provides a significant route of altering the physical and chemical properties of substances (Morsi et al., 2011; Š´ciban et al., 2008; Zheng et al., 2010a, 2010b, 2012). Morandi et al. (2009) increased the adsorption capacity of unmodified cellulose nanocrystals to 1,2,4-trichlorobenzene from water by grafting with polystyrene. Ma et al. (2011) enhanced the water absorption of wheat straw by grafting copolymerization of acrylic acid, acrylic amide and dimethyldiallyl ammonium chloride onto its surface. Recent advances in the reversible-deactivated radical polymerization (RDRP) have been applied to cellulose grafting (Roy et al., 2009). Compared with other RDRP methods (Daly et al., 2001; Ifuku & Kadla, 2008), Cu(0)-mediated RDRP as a novel RDRP can be successfully conducted without a vigorous deoxygenation process, allowing for an economical approach to functional macromolecules synthesis (Voepel et al., 2011; Fleischmann et al., 2010). In addition, Cu(0)-mediated RDRP as a unique reaction can be fast triggered at ambient temperature. In this work, the preparation of efficient corn stalkbased adsorbents with cyano, amino, amidoxime, and carboxyl groups, respectively. The corresponding syntheses were illustrated in Fig. 1. Adsorption performance of the above adsorbents to different metal ions was investigated in detail. CS-g-PAN can be used as a low-cost specific adsorbent to remove Cd(II). CO CS-g-PMA exhibited good adsorption for Pb(II) and Cu(II). The as-prepared adsorbent, AO CS-g-PAN, can effectively adsorb Hg(II) from aqueous solution.

Additionally, the factors influencing the Hg(II) removal performance of AO CS-g-PAN, such as the pH value, contact time, metal ions concentration, adsorbent dosage and ion strength were evaluated. Low cost and high efficiency of corn stalk-based adsorbents are their main advantages in removing different metal ions.

Experimental Corn stalk (CS) was obtained from a farm in Jining (China). Acrylonitrile (AN), acrylamide (AM) and methyl acrylate (MA) were purchased from Tianjin Fuchen Chemical Reagents (China). Hydroxylamine hydrochloride (NH2 OH · HCl) was purchased from Tianjin Ruijinte Chemical Reagents Corporation (China). Cu(0) powder, triethylamine (TEA) and N ,N -dimethylformamide (DMF) were supplied by Tianjin Chemical Reagents (China). 2-Bromoisobutyryl bromide (BriB-Br), ethyl 2-bromoisobutyrate (EBiB), 4-dimethylaminopyridine (DMAP), and N ,N ,N  ,N  -tetramethylethylenediamine (TEMED) were supplied by Aladdin Chemistry (USA). Corn stalks were crushed into granules after removing leaves. Corn stalk powder was washed with distilled water three times and dried at 50 ◦C. Then, 2.5 g of the corn stalk powder were stirred in 50 mL of an 18 mass % potassium hydroxide solution for 2 h at 22 ◦C and filtered. In order to remove residual potassium hydroxide, acetic acid was added to adjust the mixture to neutral pH and the mixture was stirred at 75 ◦C for 1 h, filtered and dried at 50 ◦C. Pretreated corn stalks were used as the substrate to prepare macroinitiators, which were acylated with

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Fig. 2. Modification of adsorbent AO CS-g-PAN from CS-g-PAN.

BriB-Br in the presence of TEA and DMAP at 25 ◦C. A typical process was as follows: 2.0 g of corn stalk powder were stirred in 30 mL of DMF for 30 min in an ice bath, and then, 4.18 mL of TEA, 2.20 g of DMAP, and 2.34 mL of BriB-Br were added into the solution. It should be noted that BriB-Br was added dropwise. The flask was finally filled with nitrogen and stirred for 1 h in the ice bath. After a defined time, the reaction proceeded at 25 ◦C for 24 h (Carlmark & Malmstr¨ om, 2003; Vlček et al., 2006). The product was thoroughly precipitated in ethanol and washed with an ethanol/water mixture. Finally, the product (CS-Br) was dried at 25 ◦C. Polymerization kinetics was determined by the following procedure: Cu powder and TEMED were mixed with DMF in a two-necked flask in ice-water. The mixture was bubbled with N2 for 10 min. Then, the appropriate macroinitiator CS-Br, sacrificial initiator EBiB, and a monomer (AN, AM, MA) were successively added into the flask. Ultimately, the flask was filled with nitrogen and the polymerization was started in an oil bath at the given temperature. After the desired polymerization time, the samples were precipitated for 24 h, and then separated by filtration. Finally, the samples were dried at 50 ◦C to a constant mass. The adsorbents were prepared by grafting different polymers onto the corn stalk surface according to the above procedure and its subsequent modification. The copolymers of corn stalk-graft-polyacrylonitrile (CS-g-PAN) reacted with hydroxylamine hydrochloride providing the adsorbent amidoxime CS-g-PAN (AO CS-g-PAN) (Zong et al., 2011). Carboxyl CS-g-PMA (CO CS-g-PMA) was obtained as shown in the following reactions (Tan et al., 2010). H+

R—COOH + CH3 OH − ←− −− −− −→ − R—COO—CH3 + H2 O

(1)

R—COO—CH3 + NaOH − ←− −→ − R—COONa + CH3 OH

(2)

Through the above process, adsorbents CS-gPAN, CS-g-PAM, AO CS-g-PAN, and CO CS-g-PMA were prepared. The modification of adsorbent AO CS-g-PAN from CS-g-PAN was described in Fig. 2. In order to explore the adsorption properties of CS-g-PAN, CS-g-PAM, AO CS-g-PAN and CO CS-g-PMA, adsorption experiments were performed individually for Hg(II), Pb(II), Cd(II), and Cu(II) at different pH values (1–6). pH of the buffer solution was regulated by the addition of a HNO3 solution. Adsorbent (10 mg) was shaken with 20 mL of 5.0 × 10−3 mol mL−1 metal ions in a 100 mL flask for 24 h at 25 ◦C. Then, a certain amount of the solution was extracted from every flask and diluted with distilled water. Different concentrations of the working and standard solutions were prepared by diluting the stock solution. An atomic adsorption spectrophotometer (AAS) was used to detect the concentration of metal ions. Adsorbed amount of the metal ion at equilibrium was calculated according to the following equation: (C0 − C) V qe = (3) W −1 where qe /(mmol g ) is the adsorbed amount at equilibrium, C0 /(mmol mL−1 ) and C/(mmol mL−1 ) are the initial and final concentrations of metal ions in the solution, W /g is the mass of the adsorbent, and V /mL is the solution volume. Adsorption capacities of CS-g-PAN, CS-g-PAM, AO CS-g-PAN, and CO CS-g-PMA for Hg(II) adsorption were further examined using distilled water (in the absence of buffer solutions). According to the adsorption capacities at equilibrium, the adsorption kinetics and isotherm of Hg(II) adsorption on AO CS-g-PAN were further investigated. Adsorption kinetics measurement was carried out by batch experiments as follow: a series of 100 mL flasks were loaded with 20 mL of Hg(II) solution (5 × 10−3 mol L−1 , pH = 6), then, 10 mg of the adsorbent were added into each flask and the mixtures were shaken. At various time intervals, 4 mL of the solution were withdrawn and diluted to 25 mL with distilled water. The adsorption isotherm was obtained by the following proce-

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Fig. 3. Synthesis mechanism of macroinitiator CS-Br.

dure: a series of 100 mL flasks were loaded with 10 mg of the adsorbent and 20 mL of the Hg(II) solution with different initial concentrations; then, the flasks were shaken for 8 h at 25 ◦C and the concentration of metal ions was determined using an atomic adsorption spectrophotometer (AAS) (Model 932B, Australia). The conversion of graft copolymers was measured by gravimetry. Functional groups introduced onto the surface of the adsorbents were detected by Fourier Transform Infrared (FTIR) spectroscopy which was recorded on a Nicolet MAGNAIR550 spectrophotometer. Surface morphologies of corn stalk-based samples were observed by scanning electron microscopy (SEM). C and N content of corn stalk-based samples was obtained using a Vario EL cube elemental analyzer. The total exchange capacity (TEC) determination was based on the content of nitrogen or oxygen grafted onto the final products. TEC was calculated according to the following equations: TEC =

N% 1.4

(4)

TEC =

O% 1.6

(5)

or

where TEC/(mmol g−1 ) is the total exchange capacity of samples, N% is the nitrogen content of CS-Br, CS-g-PAN, CS-g-PAM, AO CS-g-PAN, and O% is the oxygen content of CO CS-g-PMA.

Results and discussion Adsorbent preparation Corn stalk was used as a substrate as it contains a high amount of cellulose (Rémond et al., 2010). Pretreatment is a prerequisite process for corn stalk acylation with BriB-Br. An 18 mass % potassium hydroxide

Fig. 4. Kinetic plots of AN by Cu(0)-mediated RDRP at 65 ◦C in DMF with [AN] = 7.6 M, [AN]0 : [Cu]0 : [TEMED]0 = 200 : 0.5 : 1, mCS−Br = 0.4 g, and VEBiB = 56.76 µL.

solution was used to remove impurities, break hydrogen bonds, and make more hydroxyl groups accessible. The amounts of available hydroxyl groups and BriB-Br determine the number of initiation sites of the polymerization process. Synthesis mechanism of macroinitiator CS-Br is presented in Fig. 3. DMAP was chosen as the catalyst due to its strong alkalinity and nucleophilicity. TEA was used to provide an appropriate alkaline environment for the acylation. DMAP, TEA, and BriB-Br were used excess because the efficiency and specificity of organic reactions with corn stalk are difficult to control, especially with respect to the replacement of functional groups. Polymerization kinetics of AN, AM, and MA was investigated in detail using the molar ratio of [AN]0 : [Cu]0 : [TEMED]0 = 200 : 0.5 : 1, [AM]0 : [Cu]0 : [TEMED]0 = 50 : 1 : 2, and [MA]0 : [Cu]0 : [TEMED]0 = 200 : 0.5 : 1, respectively. The kinetic plots of AN, AM, and MA are depicted in Figs. 4, 5,

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Y. T. Wang et al./Chemical Papers 70 (9) 1171–1184 (2016)

Fig. 5. Kinetic plots of AM by Cu(0)-mediated RDRP at 65 ◦C in DMF with [AM] = 0.33 M, [AM]0 : [Cu]0 : [TEMED]0 = 50 : 2 : 2, mCS−Br = 0.1229 g, and VEBiB = 14.29 µL.

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Fig. 7. FTIR spectra of samples: pretreated corn stalk (a), CS-Br (b), CS-g-PAN (c), AO CS-g-PAN (d), CS-g-PMA (e), CO CS-g-PMA (f), and CS-g-PAM (g).

al., 2012). Comparing Figs. 4 and 6 showed that the existence of more sacrificial initiators does not increase the conversion and kpapp of AN apparently due to the low activity of AN. However, as shown in Fig. 6, the first order kinetics of MA was proved by four points only, which was attributed to the low controllability caused by MA agglomeration in the later stage. Adsorbent characterization

Fig. 6. Kinetic plots of MA by Cu(0)-mediated RDRP at 70 ◦C in DMF with [MA] = 1.1 M, [MA]0 : [Cu]0 : [TEMED]0 = 200 : 0.5 : 1, mCS−Br = 0.4 g, and VEBiB = 16.37 µL.

and 6, respectively. As depicted in Fig. 4, the conversation of acrylonitrile increased with the extension of the polymerization time and the semi-logarithmic kinetics plot was nearly linear. Figs. 5 and 6 also display similar kinetics curves. The apparent rate constants (kpapp ) of AN, AM, and MA polymerization were calculated from the curves’ slopes to be 0.0002 min−1 , 0.0019 min−1 , 0.0029 min−1 , respectively. The polymerization processes were conducted with the addition of the sacrificial initiator EBiB which enabled controlling the amount of polymer grafted onto the corn stalk surface and confirmed the ‘living’ character of Cu(0)mediated RDRP. The sacrificial initiator played an important role in the control of the polymerization process by increasing the Cu(I) concentration, thus increasing the rate of activation and subsequently the number of activation–deactivation cycles (Audouin et

Fig. 7 presented the FTIR spectra of pretreated corn stalk (a), acetylated corn stalk (CS-Br) (b), cyano groups functionalized corn stalk (CS-g-PAN) (c), amidoxime groups functionalized corn stalk (AO CS-g-PAN) (d), ester groups functionalized corn stalk (CS-g-PMA) (e), carboxyl groups functionalized corn stalk (CO CS-g-PMA) (f), and amino groups functionalized corn stalk (CS-g-PAM) (g). The spectrum of pretreated corn stalk (a) shows an ester vibration band at 1740 cm−1 in spectrum b, confirming the success of the bromine functionalization. Spectrum of CS-g-PAN (Fig. 7, spectrum c) exhibited characteristic vibration of the cyano groups at 2244 cm−1 . The success of CS-g-PAN modification is evident from the appearance of new bands at 1652 cm−1 and 929 cm−1 in the FTIR spectra corresponding to the stretching vibration of the C—N and N—O bonds of the AO groups. For CS-g-PMA (Fig. 7, spectrum e), the presence of PMA characteristic signals such as the stretching vibrations of the ester carbonyl group at 1737 cm−1 and the absorption band of C—O—C at 1061 cm−1 proved that the grafting polymerization of MA was successful. Spectrum f in Fig. 7 shows the spectrum of CO CS-g-PMA resulting from the modification of CS-g-PMA using a NaOH solution. The antisymmet-

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Fig. 8. SEM images of samples: corn stalk (A), pretreated corn stalk (B), CS-Br (C), CS-g-PAM (D), CS-g-PAN (E), AO CS-g-PAN (F), CS-g-PMA (G), CO CS-g-PMA (H).

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Table 1. Elemental analysis results of functionalized corn stalk Samples

N%

O%

TEC/(mmol g−1 )

CS-Br CS-g-PAN CS-g-PAM AO CS-g-PAN CO CS-g-PMA

0.71 17.4 6.58 13.7 –

– – – – 15.5

0.51 11.9 4.19 9.30 4.84

ric and symmetry vibration of carbonyl groups at 1570 cm−1 and 1410 cm−1 proved the appearance of carboxylate and the success of the modification. Characteristic peaks of the acylamino groups at 1667 cm−1 , 1419 cm−1 , and 611 cm−1 in the spectrum of CS-g-PAM (Fig. 7, spectrum g) corresponded with the stretching vibration of the carbonyl group, stretching vibration of the C—N bond and the swing vibration of the amino group, respectively, confirming AM grafting onto the surface of corn stalk. In order to further demonstrate the presence of functional groups on the corn stalk surface, elemental analysis was performed. The content of organic groups in functionalized corn stalks was calculated from TEC of nitrogen and oxygen as displayed in Table 1. In other words, there were 11.9 mmol g−1 of cyano groups, 2.11 mmol g−1 of amino groups, 4.65 mmol g−1 of amidoxime groups, and 4.84 mmol g−1 of carboxyl groups in CS-g-PAN, CS-g-PAM, AO CS-g-PAN, and CO CS-g-PAM, respectively. Morphologies of corn stalk (A), pretreated corn stalk (B), macroinitiator CS-Br (C), CS-g-PAM (D), CS-g-PAN (E), AO CS-g-PAN (F), CS-g-PMA (G), and CO CS-g-PMA (H) are provided in Fig. 8. It can be clearly seen that the surface of corn stalks changed after the pretreatment and the fiber bundles were made accessible (Fig. 8B). In the following reactions, the fiber bundles were disrupted and more hydroxyl groups became available, which is beneficial for the acylation, grafting polymerization and adsorption processes. Fig. 9 presents the TGA results of pretreated corn stalk and corn stalks functionalized with CS-g-PAN, CS-g-PAM, AO CS-g-PAN, and CO CS-g-PMA. The results revealed two platforms in the decomposition process of the pretreated and functionalized corn stalks. The initial decomposition in the range of 50–150 ◦C can be attributed to the in situ formation of acid and monomer molecules on the corn stalks. As shown in Fig. 9, the temperature of the thermal decomposition of corn stalk cellulose is about 357 ◦C and the relative mass loss was 82.08 %. Adsorbent CS-g-PAM had a similar plot with pretreated corn stalks, except for the higher thermal decomposition temperature and lower mass loss. In case of CS-g-PAN, AO CS-g-PAN, and CO CS-g-PMA, the

Fig. 9. TGA plots of pretreated corn stalk (1), CS-g-PAN (2), CS-g-PAM (1), AO CS-g-PAN (2), and CO CS-g-PMA (3).

major second decomposition peaks occurred at 231– 462 ◦C (mass loss of 39.84 %), 226–526 ◦C (mass loss of 30.01 %), and 123–473 ◦C (mass loss of 28.69 %), respectively. Comparing the pretreated corn stalks, the lower mass loss of CS-g-PAN, CS-g-PAM, AO CS-g-PAN, and CO CS-g-PMA indicated their higher thermal stability. Adsorption of metal cations As pH significantly influenced the electronic state of the pendant functional groups and the oxidation form of the metal ions in the medium, the effect of pH on the Hg(II), Pb(II), Cd(II), and Cu(II) adsorption onto CS-g-PAN, CS-g-PAM, AO CS-g-PAN, and CO CS-g-PMA was studied in the pH range from 1 to 6. Fig. 10 indicates that CS-g-PAN (A), CS-g-PAM (B), AO CS-g-PAN (C), and CO CS-g-PMA (D) exhibited the highest adsorption capacity for Cd(II), Hg(II), Hg(II), and Hg(II), respectively, with the corresponding maximum adsorption capacities of 1.42 mmol g−1 , 2.26 mmol g−1 , 8.04 mmol g−1 , and 3.92 mmol g−1 . Different adsorption properties of CS-g-PAN, CS-g-PAM, AO CS-g-PAN, and CO CS-g-PMA considering the four metal ions indicated that the functionalized groups of the adsorbents and the structure of the metal ions play an important role in the adsorption process. In case of CS-g-PAN, cyano groups and hydroxyl groups chelated with metal ions. However, the strong interactions between cyano groups of CS-g-PAN reduced its hydrophilicity in aqueous media and hindered such chelation, which resulted in its relatively low adsorption capacities for Hg(II), Pb(II), Cu(II) (Kolarz et al., 1988; Rabelo et al., 2003). Moreover, ionic radius of the metal ions also influences the chelation because the ions cannot coordinate with the adsorbent with

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Fig. 10. Adsorption capacity at equilibrium of CS-g-PAN (A), CS-g-PAM (B), AO CS-g-PAN (C), and CO CS-g-PMA (D) for Hg(II), Pb(II), Cd(II), and Cu(II) at optimal pH values, respectively.

confined space. However, comparing with the adsorption of Cd(II) on AGCS (22.17 mg g−1 ) (Zheng et al., 2010b) with corn stalk as the substrate for AN grafting, adsorbent CS-g-PAN (159.6 mg g−1 ) showed specific and efficient adsorption of Cd(II). The maximum adsorption of Hg(II) on AO CS-g-PAN reached 8.04 mmol g−1 as presented in Fig. 10C. Such a high adsorption capacity can be attributed to the existence of amidoxime groups that possess special chelating ability with Hg(II) (Liu et al., 2010). Also, the presence of a large number of hydroxyl groups resulted in high binding affinity to Hg(II) (Celis et al., 2000). Further experiments confirmed the above-mentioned results. Peanut shell-based adsorbent (AO peanut shell-g-PAN) and wheat strawbased adsorbent (AO WSM-g-PAN) functionalized with amidoxime groups were prepared by a similar method. Due to the low content of cellulose and amidoxime groups, the maximum adsorption capacities of AO peanut shell-g-PAN (4.45 mmol g−1 ) and AO WSM-g-PAN (4.70 mmol g−1 ) for Hg(II) (Wang et al., 2014) are lower than that of AO CS-g-PAN. Good adsorption properties of the four adsorbents for Hg(II) indicate that the coordinate chelation is the main mechanism in this process. Though Hg(II) has four empty orbits, the spatial hindrance caused by functionalized groups and the supramolecular structure of

cellulose limited the formation of four coordination compounds (Liu et al., 2011). Moreover, according to the results of elemental analysis, theoretical saturation adsorption capacity, qthe , was calculated and the most possible mechanism of inner-sphere complexation by two monodentates with one Hg(II) was proposed as depicted in Fig. 1 (Ma et al., 2009). Based on the above results, the adsorption experiments of four adsorbents for Hg(II) were further examined using distilled water (in the absence of buffer solutions). As shown in Fig. 11, adsorption capacities of CS-g-PAN (A), CS-g-PAM (B), AO CS-g-PAN (C), and CO CS-g-PMA (D) for Hg(II) were 0.54 mmol g−1 , 2.49 mmol g−1 , 2.62 mmol g−1 , and 2.67 mmol g−1 , respectively. It is obvious that the adsorption capacity of AO CS-g-PAN is apparently lower in comparison with the adsorption capacity of AO CS-g-PAN at pH 6. Therefore, the adsorption of Hg(II) on AO CS-g-PAN was further explored studying the effect of pH, ionic strength and adsorbent dosage on the adsorption time and the Hg(II) concentration at 25 ◦C. Fig. 12A introduces the effect of pH on the adsorption of Hg(II) on AO CS-g-PAN. The process of Hg(II) adsorption on AO CS-g-PAN is highly pH-dependent, which can be attributed to different charge and metal ions present at the surface of the adsorbent at differ-

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Fig. 11. Adsorption capacity at equilibrium of CS-g-PAN, CS-g-PAM, AO CS-g-PAN, and CO CS-g-PMA for Hg(II) using distilled water.

Fig. 13. Effect of adsorbent dose on the adsorption capacity for Hg(II).

ent pH values. High H3 O+ (H+ ) content leads to the protonation of the functional groups to achieve their positive charge, which impedes the Hg(II) adsorption. With the increasing pH, the deprotonated functional groups can enhance the electrostatic attraction between the adsorbent and metal ions. From Fig. 12A, it can be clearly seen that the adsorption capacity increases remarkably with the pH change from 5 to 6. However, the variation tendency was not observed when the pH increased from 6 to 7. Therefore, the presence of a certain amount of NaNO3 introduced during the process of pH adjustment resulted in the increase of the adsorption capacity. Different concentrations of NaNO3 were used to evaluate the influence of ionic strength on the adsorption of Hg(II) on AO CS-g-PAN in distilled water. As depicted in Fig. 12B, the adsorption capacity increased from 2.62 mmol g−1 to 4.04 mmol g−1 as the concentration of NaNO3 increased from 0 mol L−1

to 0.25 mol L−1 . Moreover, the adsorption capacity reached the equilibrium at the NaNO3 concentration of 0.0125 mol L−1 . Hu et al. (2013) also observed the influence of salt concentration on the adsorption; adsorption is enhanced by the addition of salt mainly due to the enhancement of the hydrophobic attraction, which was contributed in the increasing ionic strength. Effect of the AO f dosage on the adsorption capacity for Hg(II) is presented in Fig. 13. It is evident that the adsorption capacity increases with the increasing adsorbent dosage up to 10 mg, but thereafter, decreases with further increase of the adsorbent dosage. The maximum adsorption capacity was obtained at the adsorbent dosage of 10 mg in 20 mL of 5.0 × 10−3 mol mL−1 metal ions solution. The results are consistent with the experimental phenomena of excessive AO CS-g-PAN powder amount being unable to disperse completely in a solution.

Fig. 12. Effect of pH (A) and ionic strength (B, NaNO3 ) on the adsorption of Hg(II) on AO CS-g-PAN.

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Fig. 14. Adsorption kinetics (A) and pseudo second-order model (B) of Hg(II) adsorption on AO CS-g-PAN. Table 2. Kinetic parameters for Hg(II) adsorption on AO CS-g-PAN at 25 ◦C Pseudo-first-order kinetics T

qe,exp

k1

qe,cal

◦C

mmol g−1

min−1

mmol g−1

25

8.06

0.0098

6.92

Pseudo-second-order kinetics R2

0.8127

Adsorption kinetics and isotherm of Hg(II) adsorption on AO CS-g-PAN Adsorption kinetics determination is desirable as it provides information about the adsorption mechanism which is responsible for the efficiency of the adsorption process. The adsorption kinetics curve of Hg(II) adsorption on AO CS-g-PAN indicates that the equilibrium time and adsorption capacity was 480 min and 8.06 mmol g−1 , respectively. As depicted in Fig. 14A, the adsorption capacity increased rapidly at the initial time and then increased slowly until the equilibrium time of 480 min. To gain insight into the adsorption kinetics, the quantitative kinetics order and adsorption rate constant of Hg(II) adsorption on AO CS-g-PAN were determined applying the pseudo-first-order and pseudo-second-order kinetic models given by the following equations: ln

(qe − qt ) = −k1 t qe

(6)

t 1 t = + 2 qt k2 qe qe

(7)

where qe /(mmol g−1 ) is the adsorbed amount at equilibrium, k1 /(min−1 ) is the rate constant of the pseudo-first-order model, qt /(mmol g−1 ) is the adsorbed amount at time t, k2 /(g mmol−1 min−1 ) is the rate constant of the pseudo-second-order model.

k2

qe,cal

g mmol−1 min−1

mmol g−1

0.0026

8.42

R2

0.9901

Relevant parameters of the pseudo-first-order and the pseudo-second-order kinetic models obtained from fitting the results are summarized in Table 2. Good linearity (R2 > 0.9900) was observed in the plots of t/qt against t in Fig. 14B, indicating that the timedependence of Hg(II) adsorption on AO CS-g-PAN was much better fitted with the pseudo-second-order kinetic model than with the pseudo-first-order model. As a consequence, the equilibrium adsorption capacity values calculated from the pseudo-second-order kinetic model (qe,cal ) were in good agreement with the experimental data (qe,exp ). The adsorption isotherm of Hg(II) on AO CS-g-PAN was studied at 25 ◦C, revealing the relationship between the equilibrium adsorption capacity and the equilibrium concentration. As it can be seen from Fig. 15A, the adsorption capacity of AO CS-g-PAN increased with the increasing initial concentration of Hg(II). Similarly, there are two theories to describe the isothermal adsorption data: Langmuir and Freundlich adsorption isotherm model equations: Ce 1 Ce + = qe q qKL

(8)

ln Ce (9) n where Ce /(mmol mL−1 ) is the equilibrium concentration of metal ions, q/(mmol g−1 ) is the sorbent adsorption capacity, KL /(mL mmol−1 ) is the Langmuir

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ln qe = ln KF +

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Fig. 15. Adsorption isotherm (A) and Freundlich isotherm model (B) for Hg(II) adsorption on AO CS-g-PAN. Table 3. Adsorption capacities of the four studied adsorbents for Hg(II), Pb(II), Cd(II), Cu(II) adsorption and comparison with the adsorption capacities of some high efficiency adsorbents Adsorption capacity/(mmol g−1 ) Adsorbent

CS-g-PAN CS-g-PAM CO CS-g-PMA AO CS-g-PAN AO AN/MA Am-B PS-DA PS-DB R-N R-S AO WSM-g-PAN AO BSPS-g-PAN AO PAN Grape skin Grape fruit peel Oxidized corncob AGCS AMCS Titanate nanotubes SiO2 -G 2.0 CSPT Titanate nanofibers

Reference Hg(II)

Pb(II)

Cd(II)

Cu(II)

0.98 2.26 3.92 8.04 4.80 3.35 2.54 3.57 1.75 2.00 4.70 4.03 4.80 − − − − − − − − −

1.08 1.00 2.79 1.28 − − − − − − − − − − − − − − 2.51 0.80 − −

1.42 1.22 2.17 0.35 − − − − 0.69 0.77 − − − 1.20 0.98 0.49 0.20 0.11 2.12 − − −

0.45 0.30 2.96 1.98 − 2.15 1.20 0.90 − − − − − − − − − − − − 6.36 2.63

constant, KF /(mmol g−1 ) is the Freundlich constant, and n is the Freundlich exponent related to adsorption intensity (dimensionless). Experimental results were better fitted with the Freundlich model (R2 > 0.9600) as depicted in Fig. 15B, indicating that the metal ions were adsorbed onto heterogeneous surfaces (chemisorption and physisorption) (Qu et al., 2013). Comparison with other adsorbents Finally, a brief comparison of the adsorption capacity of CS-g-PAN, CS-g-PAM, AO CS-g-PAN, and CO CS-g-PMA for Hg(II), Pb(II), Cd(II), and Cu(II)

This work This work This work This work Liu et al. (2010) Hassan and El-Wakil (2003) Sun et al. (2011) Atia et al. (2003) Wang et al. (2014) Hao et al. (2014) Liu et al. (2011) Schiewer and Patil (2008) Leyva-Ramos et al. (2005) Zheng et al. (2010b) Zheng et al. (2010a) Xiong et al. (2011) Niu et al. (2013) Parmara et al. (2011) Li et al. (2011)

with other efficient adsorbents was done. As summarized in Table 3, of all the adsorbents studied (Hassan & El-Wakil, 2003; Sun et al., 2011; Atia et al., 2003; Schiewer & Patil, 2008; Leyva-Ramos et al., 2005; Xiong et al., 2011; Niu et al., 2013; Parmara et al., 2011; Li et al., 2011), adsorbent AO CS-g-PAN containing amidoxime groups showed the highest adsorption capacity for Hg(II). Moreover, the adsorption capacity of AO CS-g-PAN for Hg(II) was much higher than that of amidoxime groups functionalized adsorbents previously reported by our group (Hao et al., 2014; Liu et al., 2011). Through the introduction of amidoxime groups onto the corn stalk surface, the ad-

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sorption of Hg(II) on AO CS-g-PAN becomes more efficient than the adsorption of Hg(II) on corn stalk and PAN independently, which is consistent with our assumptions. Considering the Cd(II) adsorption, adsorbent CO CS-g-PMA had a similar adsorption capacity with titanate nanotubes and proved its high efficiency. From Table 3, it can be observed that CO CS-g-PMA also showed good adsorption capacities for Pb(II) and Cu(II), which approximated the adsorption capacity of titanate nanotubes and titanate nanofibers, respectively. The above results show that corn stalks can be used as substrates for preparing novel and efficient adsorbents by tailoring with different functional groups.

Conclusions Novel efficient corn stalk-based adsorbents: CS-g-PAN, CS-g-PAM, AO CS-g-PAN, and CO CS-g-PMA, were prepared by Cu(0)-mediated RDRP and modification processes. All corn stalk-based adsorbents exhibited good adsorption capacity for Hg(II), especially AO CS-g-PAN. The studies on the adsorption properties of AO CS-g-PAN for Hg(II) indicated that the adsorption is highly pH-dependent, and a certain amount of NaNO3 can effectively increase its adsorption capacity; the highest adsorption was achieved at the adsorbent dose of 10 mg. Good fitting with the Freundlich isotherm model of AO CS-g-PAN for Hg(II) implied that the adsorption of Hg(II) on AO CS-g-PAN is a heterogeneous process (chemisorption, physisorption). Corn stalks functionalized with cyano groups, amino groups, amidoxime groups, and carboxyl groups can be considered as efficient adsorbents of different metal ions. Acknowledgements. This study was supported by the National Natural Science Foundation of China (Nos. 51573075, 21404051, and 21404052), the Natural Science Foundation of the Shandong Province (Nos. ZR2014BQ016 and BS2014CL 040), and the Program for Scientific Research Innovation Team in Colleges and Universities of the Shandong Province.

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