Sodium

Research Article pubs.acs.org/journal/ascecg

Ultrasonic Method to Synthesize Glucan‑g‑poly(acrylic acid)/Sodium Lignosulfonate Hydrogels and Studies of Their Adsorption of Cu2+ from Aqueous Solution Xiaohong Wang,* Yingying Wang, Haiqian Hou, Junjie Wang, and Chen Hao* School of Chemistry and Chemical Engineering, Jiangsu University, Xuefu Road 301#, Zhenjiang 212013, China ABSTRACT: In the paper, a green ultrasonic method was applied to prepare glucan-g-poly(acrylic acid) (GL-g-PAA), sodium lignosulfonate-g-poly(acrylic acid) (SLS-g-PAA), and glucan-g-poly(acrylic acid) /sodium lignosulfonate (GL-gPAA/SLS) hydrogels with the participation of initiator ammonium persulfate (APS) and cross-linker N′,N-methylenebis(acrylamide) (NMBA), and these hydrogels were taken as absorbents to remove the Cu2+ ion from aqueous solutions. The structure, morphology, and stability of hydrogels were confirmed by Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and thermogravimetric analysis (TGA). The various experimental conditions that influence the adsorption capacity were investigated such as temperature (25−50 °C), pH (1.0−6.0), adsorbent dosage (10−60 mg), foreign ions (300 mg·L−1), and contact time (0−180 min) as well as the initial concentration of the Cu2+ ion solution (100−600 mg·L−1). In addition, the experimental results indicated that the adsorption isotherms of the hydrogels for Cu2+ ions was in line with the Freundlich model, and the adsorption kinetics of the lyogels for Cu2+ ions were in good agreement with the pseudo-second-order model. The maximum adsorption capacities were 195.6, 188.5, and 221.4 mg·g−1 for GL-g-PAA, SLS-g-PAA, and GL-g-PAA/SLS, respectively. The thermodynamic parameters of Cu2+ ion adsorption onto preceding hydrogels were calculated. The positive ΔS° value reflected that adsorption is a process of entropy increase. The ΔG° was negative, revealing that the adsorption was a spontaneous process, and the ΔH° was positive value, suggesting that the adsorption was endothermic in nature. KEYWORDS: Sodium lignosulfonate, Glucan, Hydrogels, Adsorption, Ultrasonic synthesis

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extraction.8−10 Among them, adsorption is getting one of the most popular wastewater treatment technologies because of its low cost, high efficiency, and recyclable characteristics, as well as the ability to enrich and recover heavy metals. So far, adsorption technology, as a significant separation method, has been successfully applied in the fields of chemistry, biology, analysis, and environment.11−13 Recently, polymers have received increasing attention in wastewater treatment because of their efficient removal of heavy metals in wastewater, especially those polymeric hydrogels with cross-linked network structure and the functions of adsorption, water absorption, water retention, and slowrelease.14−16 Polymeric hydrogels can be synthesized with various hydrophilic functional groups such as carboxyl groups, sulfonic acid groups, and hydroxyl groups. They tend to expand rapidly in solution because of their superhydrophilicity characteristics which can shorten the time to reach the equilibrium.17 Nowadays, application of hydrogels has been distributed in many fields. One is that hydrogels were as adsorbent materials due to their many advantages such as low-cost, fast responsive

INTRODUCTION Water contamination by heavy metals has already posed a serious threat to humans and animals because of potential toxicity, even at very low concentrations.1 Furthermore, heavy metals, unlike organic pollutants, are nonbiodegradable and are easily enriched in living organisms, which can give rise to severe health problems.2,3 Hence, how to deal with water polluted by heavy metal ions is still an urgent environmental problem.4 Copper, as a heavy metal, is extensively applied in various fields such as copper polishing, fertilizer manufacturing, paints and pigments, and others,5 but it can be hazardous when a given concentration is surpassed. It has been currently determined that the toxic effects of copper can include gastrointestinal distress with short-term exposure and kidney damage with long-term exposure.6 At present, the government has also established the strict regulations to control the amount of copper released in some fields. For instance, The World Health Organization and the U.S. Environmental Protection Agency have limited the amount of copper in drinking water to 4.0 and 1.3 mg·L−1, respectively.7 Besides, many methods have been widely used to remove these elements in industrial effluents in view of potential environmental hazards and toxic effects on organisms such as ion exchange, adsorption, coagulation and flocculation, precipitation, membrane separation, and solvent © 2017 American Chemical Society

Received: February 2, 2017 Revised: June 11, 2017 Published: June 24, 2017 6438

DOI: 10.1021/acssuschemeng.7b00332 ACS Sustainable Chem. Eng. 2017, 5, 6438−6446

Research Article

ACS Sustainable Chemistry & Engineering and high-capacity.18,19 With the rapid development of polymer hydrogels, many natural polymeric materials such as starch,20 cellulose,21 and chitosan22 have been used for hydrogel synthesis because of the low cost and resource-rich and biodegradable properties. It is well-known that lignin existing in wood biomass is an extremely abundant natural polymer on Earth. At present, many lignin-applications are based on its derivatives like lignosulfonates.23 Sodium lignosulfonate as a byproduct in the pulping and papermaking process contains lots of functional groups, and they are inexpensive, renewable, and readily available relative to other materials. Therefore, a value-added utilization method of sodium lignosulfonate can be beneficial to not only the biorefinery process, but also to the pulping industry.24,25 Glucan, a group of polysaccharides, is present in the cell walls and well-tolerated by humans and animals.26 It has the effect of scavenging free-radicals, antiradiation, dissolving cholesterol and preventing hyperlipidemia and is widely used in medicine, food, cosmetics and other industries.27 The study is aimed to synthesize GL-g-PAA, SLS-g-PAA, and GL-g-PAA/SLS hydrogels via ultrasonic solution polymerization and use them as adsorbents to remove the Cu2+ ion from aqueous solutions. The parameters that influence the adsorption capacity of the hydrogels were studied such as pH, temperature, adsorbent dosage, and contact time as well as initial concentration. In addition, the adsorption mechanisms like adsorption kinetics and isotherms were also investigated toward heavy metal Cu2+ ions.

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EXPERIMENTAL SECTION

Material. Sodium lignosulfonate (SLS) was produced by Fu Chen Chemical Reagent Factory located in Tianjin, China. Glucan (GL) was derived from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Acrylic acid (AA) was a polymeric monomer that was obtained from the Shanghai Macklin Biochemical Co., Ltd. Ammonium persulfate (APS) used as initiator was purchased from Shanghai Suran Chemical Reagent Co., Ltd., and the cross-linking agent N′,N-methylenebis acrylamide (NMBA) was obtained from Sinopharm Chemical Reagent Co., Ltd. Other reagents are analytical pure. Deionized water was used in all experiments. Preparation of GL-g-PAA/SLS Hydrogels. GL-g-PAA/SLS hydrogels were synthesized by ultrasound reaction by the following steps: With the continuous stirring by glass rod, 5 mL of a 10.5 mol NaOH solution was added dropwise to a beaker containing 5 mL of AA placed in an ice−water bath to partially neutralize AA. Then, other reagents such as SLS (0.05g), GL (0.2 g), APS (1.6%), and NMBA (0.17%) were sequentially added into the above-mentioned solution under constant stirring condition, and the solution was transferred to a KQ-2200 CNC ultrasonic-cleaner, keeping it at 60 °C for 2 h to achieve this system. Later, synthetic hydrogels were soaked for 24 h using absolute ethanol for the purpose of removing unreacted monomers. Eventually, the hydrogels were placed in vacuum oven to dry at 80 °C, and the xerogel was thoroughly pulverized to obtain small particles that the particle size was located in the range of 50−70 mesh. The SLS-G-PAA lyogel was synthesized by a similar process without adding GL. For preparing GL-g-PAA hydrogel, SLS was eliminated on the basis of the above-mentioned synthetic method. The schematic illustration of the preparation of GL-g-PAA/SLS hydrogel by ultrasound solution polymerization was shown in Figure 1. Adsorption Studies. A specified quality of xerogel was put into the solution containing 50 mL of 300 mg·L−1 Cu2+, and the solution was then placed in the thermostat oscillator in the shock to achieve the adsorption balance. The residual Cu2+ concentration in solution was measured by TAS-986 atomic absorption spectrophotometer, and the adsorption capacity and percentage adsorption of Cu2+ ion were

Figure 1. Schematic illustration of the preparation of GL-g-PAA/SLS hydrogel by ultrasound solution polymerization. calculated by the following equations:28

qe =

(C0 − Ce)V M

% removal =

(1)

(C0 − Ce) × 100 C0 −1

(2) −1

where C0 (mg·L ) and Ce (mg·L ) represent the initial concentration and the equilibrium concentration of Cu2+ in solution, qe (mg·g−1) is the amount of the Cu2+ adsorbed on the adsorbent, V (L) is the volume of the solution used, and M (g) is the weight of the adsorbent used. Adsorption kinetic experiments were carried using 40 mg of hydrogel in 50 mL of 300 mg·L−1 Cu2+ aqueous solution with different intervals (0−180 min) at 25 °C until the adsorption equilibrium was achieved. For the adsorption isotherms, a series of Cu2+ solutions with different concentrations (100−600 mg·L−1) were kept in contact with 40 mg of hydrogel for 180 min. The influence of pH (1.0−6.0) on Cu2+ adsorption was investigated under the conditions given initial concentration of Cu2+ (300 mg·L−1) and adsorption time (180 min), and the pH value of Cu2+ ion solution was controlled by adding 0.1 mol L−1 hydrochloric acid or sodium hydroxide solution. The influence of temperature was investigated (temperature range, 25−50 °C; temperature interval, 5 °C) on the adsorption behavior of Cu2+ ion (initial concentration, 300 mg·L−1). The impact of sorbent dose on the adsorption efficiency of Cu2+ ion was estimated in 300 mg·L−1 Cu2+ solution with different amount of hydrogel (10−60 mg). The effects of competitive ions were also evaluated at same 300 mg·L−1 concentration with various metal ions such as Fe3+, Cr3+, Co2+, Zn2+, Ni2+, and Pb2+. 6439


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ACS Sustainable Chemistry & Engineering Characterization. Fourier transform Infrared spectroscopy (FTIR) of samples were measured by KBr tableting method using the U.S.-made Nicolet Nexus 470 spectrometer (wavenumber range: 400−4000 cm−1). The microstructure of the sample was analyzed using a JSM-7001F scanning electron microscope (SEM) manufactured by Jeol Ltd., Japan (accelerating voltage, 15.0 kV). The hydrogel samples were subjected to thermogravimetric analysis (TGA) in a nitrogen atmosphere using a STA 449C integrated thermal analyzer from Netzsch, Germany, at a temperature range of 20−800 °C (heating rate 5 °C·min−1).

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RESULTS AND DISCUSSION Fourier Transforms Infrared Spectroscopy. The FTIR spectra of GL-g-PAA, SLS-g-PAA, and GL-g-PAA/SLS hydrogel composites were showed in the Figure 2. In curve (a), the

Figure 3. TGA and DTG curves of GL-g-PAA, SLS-g-PAA, and GL-gPAA/SLS hydrogel composite.

Figure 3 shows the TGA and DTG curves of GL-g-PAA, SLS-gPAA, and GL-g-PAA/SLS hydrogel composites in different temperatures. The hydrogel samples all showd three weightlessness stages: At the first stage below 100 °C, weightlessness corresponds to the loss of adsorbed water. The second weight loss stage, which occurred at a temperature range of 200−400 °C, is due to the decomposition of the PAA copolymer. The decomposition temperature of the third weight loss stage occurs at temperatures higher than 400 °C, which may be due to the disintegration of the polymer molecular skeleton, indicating that the decomposition process is over. From the DTG curves, it can be seen that these hydrogels appeared all three endothermic peaks. In the medium region (200−400 °C) and high-temperature region (400−600 °C), the decomposition rates of the samples are larger, and the weight loss rate is higher, which corresponds to the decomposition of the major structure of the hydrogel. The trends of the thermal decomposition weight loss curves of the three kinds of hydrogels were basically the same. However, the endothermic peak of GL-gPAA/SLS hydrogel was higher than that of GL-g-PAA and SLSg-PAA hydrogels in high temperature region (400−600 °C), and these peaks appeared at 446, 447, and 449 °C respectively. Besides, it can be also discovered from their TGA curves showing a decreasing trend that 47% of the residues of GL-gPAA/SLS hydrogel were retained at 800 °C, while 41 and 42% of the residues of GL-g-PAA and SLS-g-PAA hydrogels were retained at this temperature, respectively. These distinctions demonstrated that GL-g-PAA/SLS hydrogel composites owns better thermal stability relative to GL-g-PAA and SLS-g-PAA hydrogels. Scanning Electron Microscope. The SEM images of GLg-PAA, SLS-g-PAA, and GL-g-PAA/SLS hydrogels were represented in Figure 4. It can be seen from the figure that there is an obvious difference among the surface of their images. The surface of GL-g-PAA hydrogel was smooth in the Figure 4a, while the surface of SLS-g-PAA showed plenty of wrinkles in the Figure 4b. In addition, GL-g-PAA/SLS hydrogel contains a great amount of the pore structure in the Figure 4c which is beneficial to absorb heavy metal ions. Therefore, it can be

Figure 2. FTIR spectra of GL-g-PAA-a, SLS-g-PAA-b, and GL-g-PAA/ SLS-c hydrogel composites.

absorption peak at 3300−3500 cm−1 is due to the O−H stretching vibration. At 1720 and 1640 cm−1, the absorption peaks are attributed to the CO stretching vibrations of carboxylic ether (−COOR) and carboxylic acid (−COOH) groups, respectively. In addition, the characteristic absorption peak of glycosidic bond of glucan was found at 849 cm−1. In curve (b), the absorption peaks of −COOR and −COOH groups could be also observed, illustrating that acrylic acid exists in the prepared hydrogels. Besides, the characteristic absorption peak of the −SO3H group of SLS was discovered at 1160 cm−1. In curve (c), the absorption peak of the O−H group appears at 3420 cm−1, and the absorption peak of the CO stretching of carboxylic acid ester (−COOR) and carboxylic acid (−COOH) groups appear at 1710 and 1610 cm−1, respectively. Besides, the characteristic peaks of GL and SLS occur at 850 and 1160 cm−1, respectively. It may be due to the steric hindrance effect of different modification groups of PAA that the absorption bands of some characteristic groups of hydrogels incur red- or blue-shift, such as the difference of −COOR absorption bands of the three kinds of hydrogels. Hence, the conclusion could be drawn that GL and SLS have been successfully grafted onto PAA polymer. Thermogravimetric Analysis. Thermal stability is a major index for hydrogel materials in practical application. 6440


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ACS Sustainable Chemistry & Engineering

Figure 4. SEM images of GL-g-PAA (a), SLS-g-PAA (b), and GL-g-PAA/SLS (c) hydrogel composites.

concluded that GL-g-PAA/SLS hydrogel composite possesses better adsorption property compared with those of GL-g-PAA and SLS-g-PAA hydrogels. Effect of the Adsorbent Dose on Adsorption. In order to achieve the optimum adsorbent dose, the adsorption capacities of the prepared hydrogels on Cu2+ ions were studied at different adsorbent doses. As shown in Figure 5, the adsorption

Figure 6. Effect of pH on the adsorption of Cu2+ ions onto the prepared hydrogels. Reaction conditions: contact time: 3 h; temperature: 25 °C; initial concentration of Cu2+ ions: 300 mg·L−1; adsorbent dosage: 40 mg; solution volume: 50 mL.

functional groups on the surface of adsorbents. Figure 6 shows the adsorption capacities of the prepared hydrogels on Cu2+ ions in various solution pHs ranging from 1 to 6. It can be discovered from the figure that in the pH range of 1−2, the loading capacity of the prepared hydrogels on Cu2+ ions increased with smaller amplitude due to the fact that in high acid medium the ionic groups such as −COO− and −SO3− in the hydrogel are severely protonated, decreasing the electrostatic repulsion among the internal anions of the lyogel, limiting the expansion of the hydrogel internal network structure, and leading to a reduction in the loading capacity of Cu2+ ions.31 When the solution pH increased from 2 to 4, the adsorption capacity of GL-g-PAA, SLS-g-PAA, and GL-g-PAA/SLS for Cu2+ ions appeared to jump and increased from 101.7 to 189.5 mg·g−1, from 93.2 to 178.6 mg·g−1, and from 117.1 to 214.3 mg·g−1, respectively. This may be due to the fact that the −COOH and −SO3H groups are predominantly in the forms of −COO− and −SO3−. The ionization of these groups increases the binding sites of Cu2+ ions to the anions and leads to a rapid increase in the adsorption capacity of Cu2+ ions.32 When solution pH is greater than 4, the H+ concentration in the solution basically can be neglected with respect to the concentration of Cu2+ ions. Therefore, the adsorption capacity of the prepared hydrogels on Cu2+ ions tended to be smooth.33 Influence of Temperature on Adsorption and Thermodynamic Study. Temperature is also an important element in the process of heavy metal ion adsorption. It can be determined that the adsorption behavior is exothermic or endothermic reaction through the research, providing theoretical support for practical applications. Figure 7 shows the relattionship between the temperature and the adsorption

Figure 5. Influence of the amount of adsorbent on the removal efficiency of Cu2+ ions onto the prepared hydrogels. Reaction conditions: initial pH: 4; contact time: 3 h; temperature: 25 °C; initial concentration of Cu2+ ions: 300 mg·L−1; solution volume: 50 mL.

capacities of Cu2+ ions presented a clear growth trend with the increasing of adsorbent dose at the beginning. When the amount of the adsorbents was increased to 40 mg, the adsorption capacities of GL-g-PAA, SLS-g-PAA, and GL-g-PAA/SLS on Cu2+ ions reached the maximum and were 195.3, 188.4, and 221.7 mg·g−1, respectively. However, the adsorption capacities of Cu2+ ions clearly decreased as the adsorbent dose increased from 40 to 60 mg. This phenomenon could be explained by the number of active adsorption sites decreasing by virtue of the high dosage, resulting in aggregation of the adsorbents.29 Besides, it can be seen from Figure 5 that the removal percent of Cu2+ ions gradually increased with augmenting the adsorbent dose. The results showed that the concentration of adsorbate decreased gradually with the increase of adsorbent dosage. According to the eq 2, it was concluded that the removal percent of the prepared hydrogels Cu2+ ions shows a gradually increasing trend.30 Therefore, the dose was selected as an optimized dose in subsequent experiments. Effect of Initial pH on Adsorption. The solution pH can exert profound influence in the efficiency of the adsorption process because the acidity of the aqueous solution affects the physical and chemical properties of pollutants and the 6441


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positive, reflecting that the adsorption on Cu2+ ions is a process of increasing entropy. This phenomenon could be attributed be the reason that the prepared hydrogels contain a large amount of Na+ and that these ions are exchanged with Cu2+ ions in solution which causes the chaos of the particle system leading to the entropy increase.35 Besides, ΔG° was negative, illustrating that the adsorption behavior on Cu2+ ions was a spontaneous process.36 Effect of initial concentration on adsorption and isotherm study. The initial concentration of adsorbent solution has an obvious influence on the loading capacity of adsorbent. Correspondingly, the hydrogels were put into 50 mL of Cu2+ ion solution with different concentrations for adsorption experiments. As shown in Figure 8, the uptake capacity of the prepared hydrogels showed a gradual upward tendency when the concentration of the adsorbate solution increased from 100 to 600 mg L−1. This phenomenon could be attributed to both a large number of accessible Cu2+ ions being located near the adsorbents and that the gradient produced between the adsorbate and surface of the adsorbent became higher when the initial concentration gradually increased, resulting in the enhancement of the adsorption capacity.37 In addition, it can be seen from the visual images of the prepared composites before and after Cu2+ ion adsorption that the color of the prepared hydrogel showed an obvious change from self-color to deeply blue. This phenomenon occurred due to the fact that a large number of Cu2+ ions entered into the surface and interior of the prepared hydrogels, suggesting that the prepared hydrogels own an excellent adsorption effect on Cu2+ ions from aqueous solution.38 To understand the adsorption mechanism of the prepared hydrogels on Cu2+ ions, the adsorption data were constructed by two adsorption isotherm models (Langmuir and Freundlich adsorption isotherm models). The Langmuir model assumes that the adsorption is a monolayer adsorption occurring on the homogeneous surface of the adsorbent without any side effects, whereas the Freundlich equation is based on empirical equation of the heterogeneous nature of the adsorbent, considering the existence of a multilayered structure. The two adsorption isotherm equations are described as follows:39

Figure 7. Effect of temperature on the adsorption of Cu2+ ions onto the prepared hydrogels. Reaction conditions: contact time: 3 h; initial pH: 4; Initial concentration of Cu2+ ions: 300 mg·L−1; adsorbent dosage: 40 mg; solution volume: 50 mL.

capacities of the prepared hydrogels on Cu2+ ions. When the temperature of batch adsorption increased from 25 to 50 °C, the adsorption capacities of the prepared GL-g-PAA, SLS-gPAA, and GL-g-PAA/SLS hydrogels on Cu2+ ions increased from 195.4 to 248.6 mg·g−1, from 188.2 to 243.5 mg·g−1, and from 221.7 to 272.1 mg·g−1, respectively. A possible reason is that plenty of Cu2+ ions can take sufficient energy to interact with adsorption sites with the enhancement of the temperature. Thermodynamic parameters (ΔG°, ΔS°, and ΔH°) were calculated by the following equations:34 q ΔS° ΔH ° ln e = − ce R RT (3) ΔG° = − RT ln

qe ce

−1

(4) −1

where qe (mg·g ) and ce (mg·L ) are the adsorption quantity of Cu2+ and the equilibrium concentration of adsorbent in solution, respectively, R (8.134 J·K−1·mol−1) is the universal gas constant, T (K) is the temperature in Kelvin. The thermodynamic parameters obtained from the calculation according to the aforementioned formulas are listed in Table 1. The plot

Langmuir model: ce c 1 = + e qe KLqm qm

Table 1. Thermodynamic Parameters of Cu (II) Adsorption by the Synthesized Lyogels GL-g-PAA

SLS-g-PAA

298 303 308 313 318 323 ΔH° (kJ·mol−1) ΔS° (J·mol−1 K−1)

Freundlich model:

GL-g-PAA/SLS −1

ΔG° (kJ·mol )

temperature (K) −0.7512 −1.088 −1.412 −1.7501 −1.989 −2.396

−0.5661 −0.8715 −1.2167 −1.607 −1.839 −2.238 18.43 59.36

19.28 66.59

(5)

ln qe = ln KF +

−1.448 −1.867 −2.134 −2.511 −2.881 −3.207

1 ln ce n

(6)

where qe (mg·g−1) and ce (mg·L−1) are the amounts of Cu2+ ions adsorbed and the adsorbate concentration in solution at equilibrium, respectively, qm (mg·g−1) is the theoretical maximum adsorption capacity, KL (L·mg−1) and KF (L·g−1) are the constants of the Langmuir and Freundlich model respectively, and n is the heterogeneity factor. Adsorption isotherm parameters are listed in Table 2. The maximum Cu2+ ions adsorption capacities of the prepared GL-g-PAA, SLS-gPAA, and GL-g-PAA/SLS hydrogels using the Langmuir isotherm model were 526.1, 628.9, and 526.3 mg·g−1, respectively. These results represented that there is a large difference between the theoretical maximum adsorption capacity and the actual value obtained in this experiment. Besides, the Freundlich isotherm model showed more significant correlation

21.27 78.16

of ln(qe/ce) versus 1/T is shown in the inset in Figure 7. From the slope (−ΔH°/R), ΔH° is positive, indicating that the adsorption process was endothermic in nature. Raising the temperature is beneficial to the adsorption behavior of the prepared hydrogels on Cu2+ ions. In addition, the ΔS° was 6442


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Figure 8. Effect of initial concentration on the adsorption of Cu2+ ions onto the prepared hydrogels (a); Langmuir (b) and Freundlich (c) adsorption isotherm models of the prepared hydrogels. Visual images of the prepared composites before and after Cu2+ ions adsorption (d). Reaction conditions: contact time: 3 h; initial pH: 4; temperature: 25 °C; adsorbent dosage: 40 mg; solution volume: 50 mL.

Table 2. Isotherm Parameters for Cu2+ Adsorption Using the Prepared Hydrogels models samples GL-g-PAA SLS-g-PAA GL-g-PAA/SLS

Langmuir −1

qmax (mg g ) 526.1 628.9 526.3

Freundlich −1

KL (mL g g )

R

−3

5.435 × 10 3.156 × 10−3 7.542 × 10−3

2

0.7982 0.8407 0.8585

1/n

KF (L g−1)

R2

0.5387 0.6681 0.4969

14.99 6.773 21.54

0.9652 0.9908 0.9737

Figure 9. (A) Effect of contact time on Cu2+ adsorption (B) Pseudo-first-order and pseudo-second-order kinetic models for Cu2+ adsorption by the prepared hydrogels. Reaction conditions: initial concentration of Cu2+ ions: 300 mg·L−1; initial pH: 4; temperature: 25 °C; adsorbent dosage: 40 mg; solution volume: 50 mL.

in Figure 9A, the loading capacity of the adsorbent increased rapidly as time went on, and the adsorption capacities of GL-gPAA, SLS-g-PAA, and GL-g-PAA/SLS reached 184.3, 172.6, and 204.2 mg·g−1 after only 60 min, respectively. Thereafter, the adsorption capacity tended to be almost stable, with the continued extension of the adsorption time and the adsorption dynamic equilibrium achieved between the action ions. These results occurred due to the large amount of spatially

coefficient relative to that of the Langmuir isotherm model. Therefore, the Freundlich isotherm model is suitable for describing Cu2+ ions adsorption, suggesting that the Cu2+ ions adsorption is multimolecular layer adsorption and occurs on a heterogeneous surface.40 Effect of Contact Time on Adsorption and Kinetic Study. The adsorption capacity of the prepared hydrogels on Cu2+ ions was examined in different adsorption times. As shown 6443


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ACS Sustainable Chemistry & Engineering active sites in the surface area of the adsorbent at the initial stage, and these sites are gradually occupied by the Cu2+ ions as time goes by so that adsorption behavior became slower by virtue of reduction of the adsorption sites and reached the equilibrium after adsorption saturation.41 In order to investigate the adsorption efficiency and adsorption mechanism of the aforementioned hydrogels on Cu2+ ions, the two kinetics models (pseudo-first-order and pseudosecond-order kinetic models) were applied to test the experimental data obtained with the change of contact time. The two kinetics equations were represented by following equations:42 ln(qe − qt) = ln qe − k1t

(7)

t 1 t = + 2 qt qe k 2qe

Figure 10. Influence of external interfering ions on Cu2+ adsorbing performance of the prepared lyogels. Reaction conditions: initial concentration of Cu2+ ions: 300 mg·L−1; contact time: 3 h; initial pH: 4; temperature: 25 °C; adsorbent dosage: 40 mg; solution volume: 50 mL.

(8)

−1

2+

where qt (mg·g ) is the amount of Cu ions removed at t time, qe (mg·g−1) is the equilibrium adsorption capacity, and k1 (min−1) and k2 (g·mg−1 min−1) are the adsorption rate constants of pseudo-first-order and pseudo-second-order kinetic models, respectively. The parameters fitted by the two adsorption kinetic models are listed in Table 3.

capacities as a result of the interferences of foreign ions. This phenomenon can be explained by the fact that extraneous interfering ions compete with Cu2+ ions on the surface of the adsorbent in order to combine with adsorbent.45 Compared with other interfering ions, Fe3+ and Cr3+ with higher charge radius ratio are more easily adsorbed by adsorbed hydrogels, which leads to a lower Cu2+ adsorption capacity. In addition, it can be concluded that the adsorption of metal ions by hydrogel polymers was not selective, so the hydrogel adsorbent can be used to remove various metal ion pollutants in wastewater. It is noteworthy that the hydrogels synthesized in this study have better adsorptivity for Cu2+ ions than that for other adsorbents, as listed in Table 4.

Table 3. Kinetic Parameters for the Adsorption of Cu2+ Ions by the Aforesaid Hydrogels samples qe (mg g−1) k1 (min−1) R2 qe (mg g−1) k2 (g mg−1min−1) R2

GL-g-PAA

SLS-g-PAA

Pseudo-First-Order Model 95.6 103.4 2.873 × 10−2 2.767 × 10−2 0.9293 0.9693 Pseudo-Second-Order Model 207.4 200.3 4.823 × 10−4 4.699 × 10−4 0.9986 0.9991

GL-g-PAA/SLS 110.2 2.736 × 10−2 0.9671

Table 4. Comparison of Adsorption Capacities on Cu (II) of Some Adsorbents

232.6 4.908 × 10−4 0.9994

It is evident from Figure 9B and Table 3 that the pseudosecond-order model has a greater correlation coefficient (R2) than does the pseudo-first-order model, and the theoretical equilibrium adsorption capacities based on the pseudo-secondorder model are closer to the experimental data. These results suggested that the adsorption rate is controlled via the chemical adsorption.43 Effect of Foreign Ions on Adsorption. There are usually a large number of various heavy metal ions in industrial wastewater. Consequently, it is extremely necessary to consider the influence of foreign ions in adsorption research. The charge, charge radius ratio, and electronegativity of external interfering ions as well as the physicochemical properties of the adsorbent have an influence on the final adsorption of the target ion.44 The adsorption of the prepared hydrogels of Cu2+ ions was studied in the existence of extraneous interfering ions such as Fe3+, Cr3+, Co2+, Zn2+, Ni2+, and Pb2+ with same concentrations (300 mg·L−1). As shown in Figure 10, because of the interference of other metal ions the adsorption ability of the hydrogels of Cu2+ ions was obviously decreased, and the adsorption capacity of Cu2+ showed an obvious deviation due to the difference in the properties of the ions. The electrostatic attraction and chelation occurred between metal ions and hydrophilic group of the prepared hydrogels, and the degrees of the electrostatic attraction and chelation were not same for every metal ion. These experimental results revealed that the prepared hydrogels can only take up Cu2+ ions around half their

adsorbent

adsorption capacity (mg/g)

ref

chitosan-bound Fe3O4 clinoptilolite Fe3O4-DES/EDTA MNPs-EDTA P(AA-co-AM) biochars St-g-PAA/5%SH SLS-g-PAA GL-g-PAA GL-g-PAA/SLS

21.50 25.76 28.00 46.27 121.0 137.0 182.3 188.5 195.6 221.4

46 47 48 49 50 51 52 this study this study this study

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CONCLUSION In this study, GL-g-PAA, SLS-g-PAA, and GL-g-PAA/SLS were synthesized by ultrasonic polymerization technique. The capacity of these hydrogels to adsorb Cu2+ ions was investigated under different experiment conditions. Results revealed that the adsorption capacity reached the optimum when the adsorbent dosage was increased to 40 mg and boosted with the increase of experimental temperature and initial solution pH. The kinetic data of the adsorption indicated that the adsorption behavior took place quickly and reached to equilibrium between the adsorbent and metal ions after about 80 min. In addition, it was discovered that the adsorption kinetic model of the aforementioned hydrogels for Cu2+ ions is in line with the pseudosecond-order model with maximum adsorption capacity of 207.4, 200.3, and 232.6 mg·g−1 for GL-g- PAA, SLS-g-PAA, and GL-g-PAA/SLS, respectively. Besides, the Freundlich 6444


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isothermal model is in good agreement with the experimental data which are both spontaneous and thermodynamically beneficial to the adsorption process. Furthermore, the adsorption capacity of GL-g-PAA/SLS hydrogel was better than those of GL-g-PAA and SLS-g-PAA hydrogels. The reason may be that GL-g-PAA/SLS has abundant pore structure, and the number and species of hydrophilic oxygen groups which can be chelated with metal ions are more abundant than those in GL-g-PAA and SLS-g-PAA. Hence, the findings suggested that the prepared hydrogels are low-cost, highly efficient, and technically feasible adsorbents and potential materials for heavy metal removal in the water treatment industries.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel.: +86 511 88791800. Fax: +86 511 88791800. *E-mail: [email protected]. ORCID

Xiaohong Wang: 0000-0002-1315-8195 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the Open Project Program of State Key Laboratory of Analytical Chemistry for Life Science (Nanjing University) (KLACLS1010), the Program of Department of Education of Jiangsu Province (12KJD610003), the Natural Science Foundation of Jiangsu Province (BK20131249), the Senior Personnel Scientific Research Foundation of Jiangsu University (15JDG084), Natural Science Fund Project of Colleges in Jiangsu Province (16KJB430008), and the College Students’ Innovative Practice Fund of Jiangsu University (JSDXDC201661) for financial support of this research.

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REFERENCES

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