Removal of Copper (Cu2+) from Water Using Novel Hybrid

Article pubs.acs.org/jced

Removal of Copper (Cu2+) from Water Using Novel Hybrid Adsorbents: Kinetics and Isotherms Yaping Zhang,† Xin Wang,‡ Junsheng Liu,*,‡ and Linlin Wu‡ †

Engineering Research Center of Biomass Materials, Ministry of Education, Southwest University of Science and Technology, Mianyang, Sichuan 621010, China ‡ Key Laboratory of Membrane Materials & Processes, Department of Chemical and Materials Engineering, Hefei University, 99 Jinxiu Road, Hefei Economic and Technological Development Zone, Hefei 230601, China ABSTRACT: The mercapto group (−SH group) has a high ability to adsorb heavy-metal ions, this unique characteristic reveals that the group has potential application in the removal of heavy-metal ions from wastewater. For such purpose, novel hybrid adsorbents containing−SH groups were prepared via sol−gel process. The existence of −SH groups in the prepared hybrid adsorbents was confirmed by Fourier transform infrared spectra. The thermal stability of these hybrid adsorbents was detected via thermogravimetric analysis and differential scanning calorimetry studies. The adsorption behaviors of Cu2+ ions on these adsorbents were examined, and the obtained adsorption data were modeled using conventional theoretical models. It is showed that the thermal degradation temperature of hybrid adsorbents can arrive at 350 °C. Adsorption experiments revealed that the optimal adsorption temperature was near 40 °C and the Langmuir constant Qm can arrive at 0.396 mmol g−1 at pH 4. Meanwhile, it is found that their adsorptions for Cu2+ ions fitted well with the Lagergren first-order kinetic and Langmuir isotherm models. Moreover, testing results indicated that Cu2+ adsorption on samples A to C was solely controlled by intraparticle diffusion. Desorption experiments indicated that these samples can be recovered using aqueous HNO3 solution as a desorbent. These results demonstrate that hybrid adsorbents containing −SH groups are promising in the separation and recovery of heavy-metal ions from wastewater for environmental application. maximum adsorption capacity of nanoSe0 for copper could arrive at 0.89 g g−1 at 298.15 K. Chen and coauthors9 synthesized the graphene oxide/Fe3O4 (GO/Fe3O4) composites as adsorbents to remove Cu(II) from aqueous solution. It is found that the adsorption capacity of Cu(II) can reach 18.26 mg g−1 at 293 K. From these examples, it can be seen that adsorption technique can effectively remove copper ions from Cu-bearing water. Recently, an attempt was made to prepare silica-based hybrid adsorbents for the utility of heavy-metal removal from water.10−13 Our continuing interest in such types of hybrid adsorbents makes us to do more. Consequently, to search a new strategy to preparing hybrid adsorbents and examining their adsorption properties for toxic heavy-metal ions, herein, a new approach to hybrid adsorbents containing mercapto groups (−SH groups) via sol−gel reaction was established. By comparison with the previous publications,10−13 the novelty of this strategy can be seen as follows: (1) the silica was inserted into the hybrid matrix to prepare the hybrid adsorbents via sol−gel process, (2) the −SH group, which has a high ability to adsorb heavy-metal ions,14,15 will be introduced into polyethylene glycol (PEG) chains through the cross-linking

1. INTRODUCTION The pollution from toxic heavy-metals has attracted much attention. These toxic heavy-metal ions are severely contaminating our drinking water and threatening our health. Because some heavy-metals, such as copper (Cu2+), lead (Pb2+), cadmium (Cd2+), etc., are easily accumulated in human body throughout the food chain and cannot be biodegraded completely. As a result, various diseases or health problems of human body are induced.1−3 Especially, copperware has been widely used in our daily life, which also is a pollution source of copper ions. It is reported that excessive intake of Cu2+ by humans will cause gastrointestinal distress, even liver or kidney damage.4 To restrain or delete the contamination from heavymetal ions, an extremely severe maximum contaminant level (MCL) in water resources is established by many countries. For example, the MCL public health goal of Cu2+ stipulated by U.S. Environmental Protection Agency (EPA) is 1.3 mg L−1.4 The MCL of Cu2+ has been set as 1.0 mg L−1 in Chinese standards for drinking water quality.5 Consequently, Cu2+ removal from aqueous medium becomes important and urgent. Presently, many innovative strategies are newly proposed for Cu2+ removal from aqueous solution and some typical adsorbents for the removal of heavy-metal ions can be found in literature.6−9 For instance, Bai et al.8 prepared the elemental selenium nanoparticles (nanoSe0) as adsorbents to remove copper from aqueous solutions. It is reported that the © 2013 American Chemical Society

Received: October 20, 2012 Accepted: April 2, 2013 Published: April 12, 2013 1141

dx.doi.org/10.1021/je301168m | J. Chem. Eng. Data 2013, 58, 1141−1150

Journal of Chemical & Engineering Data

Article

of −OH groups in PEG molecules with acetaldehyde solution, (3) both kinetic and isotherm data will be modeled using conventional theoretical modes to examine the adsorption behaviors of Cu2+ ions on these hybrid adsorbents. Different from the previous job,13 the adsorption capacity of these hybrid adsorbents for Cu2+ ions was highly increased, demonstrating that they are promising in the removal of toxic divalent heavymetal ions from aqueous medium.

The adsorption capacity (qCu2+) of Cu2+ ions is calculated using eq 1

qCu2+ =

Table 1. Composition of Samples A, B, and C PEG

MPTMS

acetaldehyde

g

mL

mL

A B C

5.2 5.2 5.2

5 10 40

2 2 2

(1)

where V (mL) is the volume of aqueous Cu(NO3)2 solution, C0 and CR (mol dm−3) are the concentrations of initial and remaining Cu(NO3)2, respectively, and W (g) is the weight of samples. To optimize adsorption condition for Cu2+ ions, both pH value and solution temperature were first determined to select an optimal pH scale and solution temperature. To maintain pH value located at the desirable level during adsorption experiments, 0.01 mol dm−3 NaOH or HCl was used to adjust the initial pH of solution. For adsorption kinetic studies, the sample was dipped into 0.01 mol dm−3 aqueous Cu(NO3)2 solution at 40 °C for various contact times at pH 4. At the same time, the adsorption isotherm can be obtained from the dependency of adsorption capacity on initial solution concentration, in which the initial concentration ranged from (0.001 to 0.1) mol dm−3 at 40 °C for 24 h at pH 4. Moreover, desorption efficiency (%) of these samples was measured by titrimetric analysis using HCl and HNO3 acidic solutions (0.05 mol dm−3) as desorbents, respectively.

2. EXPERIMENTAL SECTION 2.1. Materials. 3-Mercaptopropyl trimethoxysilane (MPTMS, purity: ≥ 95.0 %) was purchased from Silicone New Material Co. Ltd. of Wuhan University (Wuhan, China) and used without further purification. Polyethylene glycol (PEG) (MW: 10000) was purchased from National Pharmaceutical Group Corp. of China (Shanghai, China) and used as received. Acetaldehyde (40 % wt in aqueous medium) and other reagents were of analytical grade. 2.2. Preparation of Hybrid Adsorbents. The preparation of hybrid adsorbents (named as samples A, B, and C; their compositions are listed in Table 1) in this case is similar to that

sample

(C 0 − C R )V W

3. RESULTS AND DISCUSSION 3.1. Preparation of Samples. As discussed in section 2.2, the coating solution for samples A, B, and C was prepared via a sol−gel reaction, which was conducted by cross-linking the −OH groups in PEG molecular chains with the Si−OH groups in the molecular chains of MPTMS to produce the O−Si−O linkage in the presence of acetaldehyde. Samples A, B, and C can thus be obtained by casting the above-prepared coating solution on a flat Teflon plastic-board and then which suffered a subsequent desiccation process in a furnace. Furthermore, it should be emphasized that in this new designed approach, the creation of functionalized groups was dissimilar with that proposed in our earlier articles.12,13 Previously, the functionalized groups were chiefly produced via the ring-opening of a lactone and the effect of steric hindrance of phenyl groups12 or the sulfonation reaction of phenyl groups using a sulfonating agent.13 In this work, the functionalized groups will be directly introduced into the hybrid materials via a sol−gel reaction. The formation of functionalized hybrid materials is presented in Scheme 1. Since samples A, B, and C were synthesized at low reaction temperature via

in the previous articles.10−13,16,17 In this current work, samples A, B, and C were chiefly synthesized from the reaction of organic PEG with inorganic MPTMS in a N,N-dimethylformamide (DMF) solution via sol−gel process. As a typical example, the procedure for sample A was described briefly as follows. First, 5.2 g of PEG was dissolved in 20 mL of a DMF solution and agitated vigorously for 1 h at 80 °C in the presence of HCl solution (near 0.5 mL); and then 5 mL of a MPTMS solution was added dropwise within 1 h into the prepared PEG mixture. Second, the mixture was agitated strongly for an additional 4 h at 80 °C to conduct the sol−gel process. Third, the reaction product was cross-linked with acetaldehyde (around 2 mL) at 80 °C for an additional 6 h. Subsequently, a homogeneous coating solution could be obtained. Finally, the coating solution was spread on a Teflon plate to produce a membrane in air at room temperature for an additional 3−4 days. And then the membrane was dried at 40 °C for an additional 3 days to acquire the final functionalized hybrid material as an adsorbent. 2.3. Sample Characterizations. Fourier transform infrared (FTIR) spectra of the prepared samples A, B, and C were obtained using a Shimadzu FTIR-8400S spectrometer in the region of (4000 to 400) cm−1 at a resolution of 0.85 cm−1. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) thermal analyses of the prepared samples A, B, and C was recorded with a Netzsch STA 409 PC/PG thermogravimetry analyzer, under a nitrogen flow using a heating rate of 10 °C min−1 from (40 to 500) °C. 2.4. Adsorption Experiments. The adsorption experiments of samples A, B, and C for copper (Cu2+) were performed in a manner similar to that of our previous studies, in which the hybrid adsorbents produced were used to separate Cu2+ or Pb2+ ions from aqueous solution.16,17

Scheme 1. The Formation of Functionalized Hybrid Materials

1142



Article

sol−gel process, the functionalized groups of −SH groups can be kept in intact. The existence of −SH groups can be further proved by FTIR spectra (see Figure 1, hereinafter).

Figure 2. TGA curves of samples A (solid line), B (dash line), and C (dash dot dot).

Table 2. Thermal Analysis Data of Samples A, B, and C in TGA Curves Figure 1. FTIR spectra of (a, b, and c) for samples A, B, and C.

3.2. FTIR Spectra. To confirm the existence of −SH groups and investigate the formation of hybrid product, FTIR spectroscopy was performed and illustrated in Figure 1. From the curves a−c in Figure 1, it can be observed that the change trends in adsorption peaks of samples A, B, and C are similar. The peak at 3400 cm−1 was the stretching of −OH groups. The C−H stretching of CH3 and CH2 groups appeared at 2900 cm−1. The band at 1110 cm−1 was the stretching vibration of Si−O−Si, Si−O−C, and C−O−C,18 which confirmed the formation of hybrid products. Moreover, it can be noted in curves a−c that the stretching vibration of −SH groups occurred at 2550 cm−1,18,19 which proved the existence of functionalized −SH groups in the above-prepared hybrid adsorbents. This observation suggests that samples A, B, and C really contain −SH groups, which is expected to be applied to adsorb toxic heavy-metals from aqueous solution. 3.3. TGA Study. Thermal stability is the dominating performance of a hybrid material. Investigating the thermal stability of samples A, B, and C can get a new insight into their temperature endurances. For such a purpose, the TGA thermal study was performed and showed in Figure 2. Meanwhile, the thermal data of samples A, B, and C in TGA curves are analyzed and summarized in Table 2. From Figure 2, it can be probed that from samples A to C, their degradation curves in weight loss (%) exhibit similar trends. Such degradation curve can be further separated into two main stages, which are individually located within the temperature region of (40 to 350), and (350 to 500) °C. According with these degradation steps, two distinctly exothermic peaks were detected in DSC curves (see Figure 3, hereinafter). The first degradation step in weight loss (%) from (40 to 350) °C was the disintegrating of organic components and functionalized groups. The second one in weight loss (%) higher than 350 °C can be assigned to the further degraded of polymer chains and the formation of hybrid matrix. Furthermore, it can also be probed that the temperature of degradation (Td) at (5 and 10) % weight loss (i.e., Td5 and Td10) shows different change trends. Among them, sample B has the highest value in Td5 and Td10. Meanwhile, these change trends disagree with those in the composition of samples A to

sample

Td5/°C

Td10/°C

R500/wt %

A B C

314.35 329.93 318.01

343.79 353.88 350.73

30.26 38.93 51.87

Figure 3. DSC curves of samples A (solid line), B (dash line), and C (dash dot dot).

C (cf., Table 1). This observation suggests that a transformation in thermal degradation behaviors of samples A, B, and C had occurred when MPTMS was incorporated into the hybrid matrix. Moreover, it can be spotted that the weight loss (%) of samples A, B, and C has a quick increase as the Td value exceeds Td10, demonstrating that the decomposition process has been highly lifted. In addition, it can be detected that the residue weight (wt %) at 500 °C (R500) was increased from samples A to C (cf., Table 2). To explain such phenomenon, much attention will be paid to the effect of inorganic addition on the product performance of samples A, B, and C. The above trend is logical in theory if the influence of inorganic ingredient in the sample is considered, that is, the higher the inorganic component in a hybrid material, the larger the residue in the sintered product. From samples A to C, the content of MPTMS was elevated orderly, the amount of residual SiO2 in the sintered product accordingly followed the same direction. 1143



Article

3.5.1. The Effect of Initial pH. The initial pH of a solution has a strong effect on the adsorption of heavy-metal ions on an adsorbent. To optimize the pH scale for the adsorption of Cu2+ ions on samples A, B, and C, the initial pH from 2.0 to 5.6 at a concentration of 0.01 mol dm−3 for 24 h was measured to investigate pH effect. The effect of initial pH on Cu2+ adsorption was presented in Figure 4.

The above-mentioned results suggest that the thermal stability of a hybrid material can be modified via incorporating inorganic composition into a hybrid matrix. While, the higher thermal degradation temperature of these hybrid adsorbents indicated that they can be potentially applied in some harsh circumstances, such as Cu2+ removal from industrial wastewater in high temperature media. 3.4. DSC Analysis. To examine the crystallization transformation behavior of samples A, B, and C, DSC analysis was fulfilled and the related graphs are shown in Figure 3. Meanwhile, the peak temperatures in the DSC curves are summarized in Table 3. Table 3. Peak Temperature of Samples A to C in DSC Curves exothermic peak /°C

endothermic peak /°C

sample

first

second

first

second

third

A B C

64.8 65.6 63.9

381.7 377.7 370.1

50.6 45.5 55.3

299.7 320.1 314.7

422.8 423.4 421.8

As illustrated in Figure 3, three endothermic and two exothermic peaks are spotted in the curves for samples A to C. Chang et al.20 reported that in the DSC graph, single or multiple endothermic peaks will be probed when the crystal melts; in contrast, an exothermic peak can be detected as the sample starts to form a crystalline structure. On the basis of such a conclusion, it is easily reasoned out that for samples A, B, and C, the temperature of melting point (Tm) will be situated at (50.6, 45.5, and 55.3) °C, which evidence the influence of MPTMS addition in these samples. Moreover, it will be found that for samples A, B, and C, the temperature of first crystallization (Tc1) (i.e., the first exothermic peak) is centered at (64.8, 65.6, and 63.9) °C. Such a change trend in T c1 implies the transformation of crystallization from amorphous to crystalline. Furthermore, it can be noted that the temperature of the second crystallization (Tc2) (i.e., the second exothermic peak) is located at (381.9, 377.7, and 370.1) °C, which displays a decreasing trend from samples A to C. Such downward trend demonstrates the further crystallization of the hybrid matrix in the samples. This outcome clearly evidences the effect of an inorganic additive in a functionalized hybrid material on its property of crystallization transformation. The above-mentioned trends can be explained as follows. One dominating factor can be accredited to the difference of cross-linking degree in the organic and inorganic moieties, which will vary the structure of sample although the same amount of formaldehyde was used to cross-link −OH groups in PEG and MPTMS. Another chief factor is associated with the further formation and completion of a hybrid matrix as the thermal degradation temperature is elevated, leading to an advance in the crystallization temperature of samples A, B, and C. 3.5. Adsorption for Copper Ions. To inspect the adsorption properties of samples A, B, and C, adsorption experiments were performed using copper (Cu2+) as a typical example of toxic heavy-metal ions. The major influencing factors including initial pH, solution temperature, contact time, and initial solution concentration are examined. The obtained adsorption data were modeled using Lagergren first-order and second-order kinetic equations, intraparticle diffusion, Langmuir and Freundlich isotherm equations.

Figure 4. Initial pH versus the adsorption capacity of Cu2+ ions on samples A (solid square), B (solid circle), and C (solid uptriangle).

From Figure 4, it can be discovered that the adsorption capacity of Cu2+ ions increases gradually as the initial pH value elevated from 2.0 to 5.6, suggesting the influence of pH on Cu2+ adsorption. However, it should be emphasized that sediment of Cu(OH)2 will also be detected in solution if the initial pH is larger than 5.6. Considering such a problem, the adsorption experiments at pH > 5.6 were not carried out. Chen and coauthors9 reported that when the pH of the solution is lower than 8, the predominant Cu(II) species is Cu2+ and it is removed primarily by adsorption technique. Consequently, to overcome the appearance of unwanted species in solution and precisely determine the adsorption capacity of Cu2+ ions, aqueous Cu(NO3)2 solution at pH 4 is thus selected as the adsorption medium to examine the adsorption behaviors of Cu2+ ions on samples A, B, and C, hereafter. To explain the effect of initial pH on Cu2+ adsorption in the studied pH range, much attention should be paid to the reaction sites in −SH groups at different pH values. It is wellknown that the bond of S−H is weaker than that of O−H, the −SH group is thus easily dissociated into an −S− group in aqueous solution, and the extent of dissociation increases as the pH of solution is elevated. In contrast, the extent of dissociation of the −SH group decreases as the pH of the solution is reduced. As a result, the adsorption capacity of the Cu2+ ions on these samples varies with the change of dissociation degree of −SH group at different pH ranges. For example, at low pH, the amount of H+ is relatively high; the dissociation of the −SH group will be blocked, resulting in a decrease in the adsorption capacity of Cu2+ ions on these samples due to the effect of coions. However, at high pH, the amount of H+ is relatively lower and the amount of −OH is relatively higher, thus the dissociation of the −SH group will be accelerated. In this case, the −S− group is easily combined with Cu2+ ions because of the electrostatic attraction between the opposite charges, leading to an increase in the adsorption capacity of Cu2+ ions. Moreover, at middle pH, the dissociation of the −SH group is very low and might arrive at an equilibrium state. In this case, 1144



Article

The above phenomena can be explained as follows. It is well accepted that the adsorption of metal ions is an endothermic process. Therefore, the adsorption efficiency of Cu2+ ions increased from 30 to 40 °C. As for the reason why the adsorption capacity of Cu2+ ions decreased as temperature was beyond 40 °C, it is difficult to exclusively explain such phenomenon within the authors’ knowledge. The possible reason may be related to the impact of water evaporation on Cu2+ adsorption. This is because the evaporation of water from aqueous Cu(NO3)2 solution was also accelerated when the temperature of solution was beyond 40 °C. Thus it will impact Cu2+ adsorption on these samples. As a result, the adsorption efficiency of Cu2+ ions decreased slightly when the temperature of the solution moved past 40 °C. A similar phenomenon was also found in a previous article,12 in which the adsorption capacity of Cu2+ ions decreased as the solution temperature was elevated to 55 °C. 3.5.3. Adsorption Kinetics. The dependency of copper adsorption capacity (qCu2+) on contact time (t) was illustrated in Figure 6.

the ion exchange might be the main influencing factor. Consequently, the dissociation and ion-exchange mechanism will be the predominating control step and can be used to explain the effect of pH on Cu2+ adsorption. The proposed reaction mechanism at different pH ranges can be briefly described as follows. At low pH, H+

RSH + Me 2 + XooY RS−Me 2 + + H+

(2)

At middle pH, RSH + Me 2 + ⇄ RS−Me 2 + + H+

(3a)

RS−Me 2 + + RSH ⇄ RS−···Me 2 +···S−R + H+ (3b)

At high pH, OH−

RSH XooooY RS− + H 2O

(4a)

RS− + Me 2 + ⇄ RS−Me 2 +

(4b)

RS−Me 2 + + RS− ⇄ RS−···Me 2 +···S−R

(4c)

3.5.2. The Determination of Solution Temperature. Solution temperature is a major factor that will influence the adsorption of heavy-metal ions on an adsorbent in water. To choose a desirable solution temperature for Cu2+ adsorption on samples A to C, adsorption experiments from (30 to 50) °C were performed and illustrated in Figure 5.

Figure 6. Adsorption kinetic curves of Cu2+ ions on samples A (solid square), B (solid circle), and C (solid uptriangle), the concentration of aqueous Cu(NO3)2 solution was 0.01 mol dm−3 at pH 4.

From Figure 6, it can be detected that for samples A, B, and C, the adsorption capacity of Cu2+ ions all increased with the elapsed contact time. However, considering the individual sample, the trend is different. For example, sample B has the highest adsorption capacity of Cu2+ ions among them. This outcome suggests that proper addition of inorganic composition MPTMS into a hybrid polymer will favor Cu2+ adsorption. But, excess addition of MPTMS into a hybrid material does not conduce to its adsorption for heavy-metal ions. The formation of the hybrid matrix and the cross-linking of functionalized groups might be responsible for such trends. Presently, the Lagergren kinetic model is a helpful tool to evaluate the adsorption performances of a material.21,22 Typically, Lagergren first-order and second-order kinetic models can be linearly denoted as eqs 5b and 6b, respectively.

Figure 5. Solution temperature versus the adsorption capacity of Cu2+ ions on samples A (solid square), B (solid circle), and C (solid uptriangle).

From Figure 5, it can be seen that solution temperature plays an important role in the adsorption of Cu2+ ions on samples A to C. For example, for samples A to C, the adsorption capacity of Cu2+ ions elevates from (30 to 40) °C. It reached the peak at 40 °C and then decreased as the solution temperature went beyond 40 °C. However, if the individual sample was considered, it can be realized that the adsorption of Cu2+ ions on these samples indicated a different change trend. Among them, sample B exhibits the highest adsorption capacity of Cu2+ ions at 40 °C. Obviously, Cu2+ adsorption on these samples can obtain optimal result at 40 °C. The temperature at 40 °C was therefore selected as the adsorption temperature of aqueous Cu(NO3)2 solution in this case.

qt = qe(1 − e−k1t )

(5a)

or log(qe − qt ) = log qe − 1145

k1 t 2.303

(5b)


Journal of Chemical & Engineering Data qt =

Article

the contrary, linear regression coefficient (R2) of Lagergren second-order model for Cu2+ adsorption fitted worse (R2 < 0.9) (see Table 4). This finding suggests that the adsorption of Cu2+ ions on these hybrid adsorbents can be described using a Lagergren first-order kinetic model. In addition, by comparison the values of the rate constant of the Lagergren first-order kinetic model k1, it can be observed that these k1 values indicate a downward trend from sample A to C, which is inconsistent with that of MPTMS content being incorporated into these samples (cf., Table 1). This finding reveals that the incorporation of MPTMS into the adsorbents does not favor the improvement of the rate constant of Lagergren first-order kinetic model. Moreover, it is well accepted that the intraparticle diffusion model is a useful tool to evaluate the adsorption performances of metal ions on an adsorbent. The metal ions, which are adsorbed by an adsorbent, usually can transport from the solution through the interface layer of solution and adsorbent into the particle pores. Such interface transport characteristics of metal ions from the main solution to the interior of an adsorbent can be elaborated using an intraparticle diffusion model. Presently, it is reported that the influence of intraparticle diffusion on the adsorption rate can be described on the basis of dependency of adsorption capacity and contact time, which can be calculated using eq 723

qe2k 2t (1 + qek 2t )

(6a)

or t 1 t = + qt qe k 2qe2 −1

(6b) −1

−1

where k1 (h ) and k2 (g h mmol ) are the rate constant of first-order and second-order kinetic model, respectively; qt (mmol g−1) and qe (mmol g−1) are the adsorption capacity of metal ions (Me2+) at time t (h) and at the equilibrium state, respectively. The Lagergren kinetic model for Cu2+ adsorption was calculated and exhibited in Figure 7a,b.

qt = xi + k pt 0.5

(7)

where qt (mmol g−1) is the amount of adsorbed metal ions at time t (h), kp (mmol g−1 h−1/2) is the rate constant of intraparticle diffusion, and xi (mmol g−1) is the intercept of the coordinate axis, which reveals the effect of boundary layer thickness on the adsorption of metal ions. Generally accepted, if the diagram of qt versus t0.5 produces the correlation of straight line, the adsorption process can be considered as solely controlled by intraparticle diffusion. In contrast, if the diagram of qt versus t0.5 forms a multilinear relationship, such an adsorption process will be controlled by two or more steps.23,24 The intraparticle diffusion plot of Cu2+ adsorption on samples A to C was illustrated in Figure 8. From Figure 8, it can be seen that the linear regression coefficient (R2) gives a better adaptability for Cu2+ adsorption on these samples (see Table 4). Since the plot nearly passes through the origin, it can thus be considered that intraparticle diffusion is the rate-limiting step during the whole adsorption process; that is, Cu2+ adsorption on samples A to C is solely controlled by intraparticle diffusion and the effect of boundary layer thickness on Cu2+ adsorption can be ignored.25 This outcome is in disagreement with the finding obtained from ionic groups in a previous article.12 The dominating factors for such phenomena can be assigned to the further functionaliza-

Figure 7. Lagergren kinetic model for Cu2+ adsorption on samples A (solid square), B (solid circle), and C (solid uptriangle): (a) firstorder, (b) second-order model.

From Figure 7, it is found that the linear regression coefficient (R2) of Lagergren first-order model for Cu2+ adsorption gave a better fitting (R2 > 0.9) (see Table 4). On Table 4. Kinetic Model Parameters for Cu2+ Adsorption Lagergren first-order model

Lagergren second-order model

intraparticle diffusion model

k1

qcal

k2

qe

kp

xi

sample

h−1

mmol g−1

R2

g h−1 mmol−1

mmol g−1

R2

mmol g−1 h−1/2

mmol g−1

R2

A B C

0.127 0.0918 0.00525

0.205 0.186 0.980

0.936 0.955 0.953

0.961 1.540 0.308

0.211 0.216 0.186

0.834 0.906 0.908

0.0422 0.0450 0.0230

−0.00674 0.00490 −0.00415

0.985 0.978 0.976

1146



Article

Figure 8. Intraparticle diffusion plot of Cu2+ adsorption on samples A (solid square), B (solid circle), and C (solid uptriangle).

Figure 9. Adsorption capacity of Cu2+ ions on samples A (solid square), B (solid circle), and C (solid uptriangle) vs initial solution concentration.

tion of the prepared hybrid adsorbents and the formation of the hybrid matrix. 3.5.4. Adsorption Isotherms. To further investigate the adsorption behaviors of Cu2+ ions on the prepared hybrid adsorbents, both Langmuir and Freundlich isotherm equations are applied to model the measured data. The Langmuir isotherm model usually belongs to monolayer adsorption on the active sites of an adsorbent, which can be calculated using eq 8:22,26

Ce C 1 = e + qe Qm Q mb −1

(8) −3

where qe (mmol g ) and Ce (mol dm ) are the equilibrium concentrations of metal ions in the adsorbed and liquid phases, respectively. Qm (mmol g−1) and b (dm3 mol−1) are the Langmuir constants, which can be obtained from the intercept and slope of testing point linearity based on the relationship of Ce/qe and Ce. Unlike the Langmuir isotherm model, Freundlich isotherm model mainly focuses on the adsorption of the heterogeneous surface with uniform energy; which can be calculated using eqs 9a and 9b22,26 qe = kFCe1/ n

(9a)

log qe = log kF + −1

1 log Ce n

(9b) −3

where qe (mmol g ) and Ce (mol dm ) are the equilibrium concentrations of metal ion in the adsorbed and liquid phases, respectively; kF [(mmol g−1) (mol dm−3)−1/n] and n are the Freundlich constants, which can be obtained from the slope and intercept of testing point linearity on the basis of the function of log qe on log Ce. The dependency of adsorption capacity of Cu2+ ions on initial solution concentration (i.e., adsorption isotherm graph) was given in Figure 9. As expected, the adsorption capacity of Cu2+ ions rose as the initial solution concentration increased. Figure10 (panels a and b) presents the Langmuir and Freundlich isotherms of Cu2+ ions. The parameters obtained from the plot of Langmuir and Freundlich isotherms are tabulated in Table 5. Obviously, the experimental data give a better linearity with the Langmuir isotherm model (R2 > 0.99). On the contrary, linear regression coefficient (R2) of the Freundlich isotherm

Figure 10. Adsorption isotherm of Cu2+ ions on samples A (solid square), B (solid circle), and C (solid uptriangle): (a) Langmuir and (b) Freundlich models.

model (R2 < 0.90) does not meet the required region. This result implies that Cu2+ adsorption on these samples can be depicted using Langmuir monolayer adsorption. The affinity of −SH groups in the molecular chains for Cu2+ ions might be responsible for such trend. Furthermore, Table 5 lists the Langmuir constant Qm value of samples A to C. It can be found that the Qm value increases from sample A to B and then decreases from sample B to C. Clearly, such change trend is different with that of MPTMS content incorporating into these samples (cf., Table 1). The 1147



Article

Table 6. The Calculated RL Values Using the Parameters Obtained from the Langmuir Isotherm

Table 5. Parameters Obtained from the Plot of Langmuir and Freundlich Isotherms for Cu2+ Adsorption Langmuir Qm sample mmol g−1 A B C

0.381 0.396 0.144

RL value

Freundlich

b

kF

C0/(mol dm−3)

A

B

C

dm3 mol−1

R2

(mmol g−1) (mol dm−3)−1/n

n

R2

175.029 281.517 274.475

0.995 0.996 0.992

1.269 1.227 0.291

2.327 2.642 3.823

0.900 0.802 0.905

0.001 0.005 0.01 0.05 0.1

0.851 0.533 0.363 0.102 0.0540

0.780 0.415 0.262 0.0663 0.0343

0.784 0.421 0.267 0.0679 0.0351

Table 7. Comparison of Qm for Cu2+ Adsorption with Those Reported in References

result demonstrates that proper incorporation of MPTMS into the hybrid adsorbents can evidently improve the adsorption capacity of Cu2+ ions. However, excess introduction of MPTMS into these hybrid adsorbents will reduce its adsorption capacity for heavy-metal ions. To further insight into the adsorption natures of the prepared samples, determining the reaction site of −SH group in the above-prepared hybrid adsorbents is important. For such purpose, the relationship of Langmuir constant Qm with the content of MPTMS in these samples is determined. It is found that the amount of −SH group per amount of Cu2+ in the samples A, B, and C is 1.97, 4.11, and 5.98. This finding reveals that the binding of divalent metal Cu2+ ions with the −SH group was not completely conducted. There still remain unutilized reaction sites in the molecular chains (see Scheme 1). For example, for sample A, around one Cu2+ ion is possibly bound by two −SH groups and another two −SH groups remain intact (in this case, the electrostatic attraction effect might be the dominating reaction). With regard to sample B, about one Cu2+ ion is adsorbed by one −SH group (in this case, ion exchange might be the major reaction and control the reaction sites). Taking sample C into account, one Cu2+ ion is combined with six −SH groups (in this case, the complex effect might occur and predominate the reaction sites). From these trends, it can be deduced that the capture of the −SH group for Cu2+ ions is a complicated process. For the complete utilization of the reaction sites, a better experimental system needs to be designed. Moreover, with regard to the Langmuir isotherm model, it is reported that the favorability of adsorption can be estimated using the separation factor or equilibrium parameter (RL). Its value can be calculated using eq 1022,26 1 RL = (1 + bC0) (10)

sorbent type activated nylon-based membrane carboxymethyl-βcyclodextrin modified Fe3O4 nanoparticles nanoSe0 graphene oxide/Fe3O4 hybrid adsorbent

Qm/(mg g−1)

solution temperature/°C

pH

10.794

25

4

6

47.2

25

6

7

890 (0.89 g g−1) 18.26 25.16 (0.396 mmol g−1)

25 (298.15 K)

3

8

20 (293 K) 40

5.3 4

9 this work

ref

constant Qm for Cu2+ adsorption in this job with those reported in some references. From Table 7, it is found that the Qm value in this work is higher than that of activated nylon-based membrane, graphene oxide/Fe3O4, etc. if the difference of experimental conditions designed is ignored, From such comparison, it can be concluded that the prepared hybrid adsorbents have an advantage over those reported in some literature. Notice that such a direct comparison is hard to give a satisfactory result due to the existence of crucial distinction in physicochemical properties of various adsorbents and the equilibrium condition designed. 3.5.5. Gibbs Free Energy. Understanding the thermodynamic parameter is vitally important for the removal of heavymetal ions via adsorption. Considering that the Cu2+ adsorption on these samples is the Langmuir adsorption process, the Gibbs free energy (ΔG) can be calculated using eq 11.27,28 The results are tabulated in Table 8. (11)

ΔG = −RT ln b

where b (dm3 mol−1) is the Langmuir adsorption equilibrium constant, R is the gas constant (8.314 J mol−1 K−1), and T (K) is the absolute temperature.

where C0 (mol dm−3) is the initial solution concentration and b (dm3 mol−1) is the equilibrium constant of Langmuir adsorption. Typically, when the RL value is centered on a range such as 0 < RL < 1, it will be conducive to to metal adsorption, otherwise the adsorption of metal ions on the surface of adsorbent will be unfavorable.22,26 Table 6 lists the calculated RL values using the parameters obtained from the Langmuir isotherm. From Table 6, it can be detected that the RL values are all centered on the range of 0 < RL < 1, demonstrating that these samples can easily adsorb Cu2+ ions from water. This result evidence that these hybrid adsorbents have promising applications in the removal of Cu2+ ions from wastewater for environmental purposes. To evaluate the advantage of these hybrid adsorbents with others reported in literature, Table 7 summarizes the Langmuir

Table 8. The Calculated Gibbs Free Energy (ΔG) Values at 40 °C sample

T/K

ln b

ΔG/(kJ mol−1)

A B C

313.15 313.15 313.15

5.16495 5.64019 5.61486

−13.4471 −14.6844 −14.61845

As listed in Table 8, the ΔG values are all negative for samples A to C. This finding demonstrates that Cu 2+ adsorption on these samples is spontaneous in nature, further corroborating the result gained from the equilibrium parameter (RL). 3.6. Desorption Experiment. Currently, it is well accepted that regeneration and recovery of spent adsorbents and metals, 1148



Article

(2) Mahamadi, C.; Nharingo, T. Competitive adsorption of Pb2+, Cd2+, and Zn2+ ions onto Eichhornia crassipes in binary and ternary systems. Bioresour. Technol. 2010, 101, 859−864. (3) Zhang, J. P.; Wang, A. Q. Adsorption of Pb(II) from aqueous solution by chitosan-g-poly(acrylic acid)/attapulgite/sodium humate composite hydrogels. J. Chem. Eng. Data 2010, 55, 2379−2384. (4) National primary drinking water regulations. EPA 816-F-09-004; U.S. Environmental Protection Agency: Washington, DC, 2009. (5) Standards for drinking water quality. GB 5749-2006; Ministry of Health: P.R. China, 2006. (6) He, Z.-Y.; Nie, H.-L.; Branford-White, C.; Zhu, L.-M.; Zhou, Y.T.; Zheng, Y. Removal of Cu2+ from aqueous solution by adsorption onto a novel activated nylon-based membrane. Bioresour. Technol. 2008, 99, 7954−7958. (7) Badruddoza, A. Z. M.; Tay, A. S. H.; Tan, P. Y.; Hidajat, K.; Uddin, M. S. Carboxymethyl-β-cyclodextrin conjugated magnetic nanoparticles as nano-adsorbents for removal of copper ions: Synthesis and adsorption studies. J. Hazard. Mater. 2011, 185, 1177−1186. (8) Bai, Y.; Rong, F.; Wang, H.; Zhou, Y.; Xie, X.; Teng, J. Removal of Copper from Aqueous Solutions by Adsorption on Elemental Selenium Nanoparticles. J. Chem. Eng. Data 2011, 56, 2563−2568. (9) Li, J.; Zhang, S. W.; Chen, C. L.; Zhao, G. X.; Yang, X.; Li, J. X.; Wang, X. K. Removal of Cu(II) and Fulvic Acid by Graphene Oxide Nanosheets Decorated with Fe3O4 Nanoparticles. ACS Appl. Mater. Interfaces 2012, 4, 4991−5000. (10) Liu, J. S.; Si, J. Y.; Zhang, Q.; Zheng, J. H.; Han, C. L.; Shao, G. Q. Preparation of negatively charged hybrid adsorbents and their applications for Pb2+ removal. Ind. Eng. Chem. Res. 2011, 50, 8645− 8657. (11) Liu, J. S.; Song, L.; Shao, G. Q. Novel zwitterionic inorganic− organic hybrids: Kinetic and equilibrium model studies on Pb2+ removal from aqueous solution. J. Chem. Eng. Data 2011, 56, 2119− 2127. (12) Dong, Q.; Liu, J. S.; Song, L.; Shao, G. Q. Novel zwitterionic inorganic−organic hybrids: Synthesis of hybrid adsorbents and their applications for Cu2+ removal. J. Hazard. Mater. 2011, 186, 1335− 1342. (13) Hu, K. Y.; Liu, J. S.; Wo, H. Z.; Li, T. Novel negatively charged hybrids. 2. Preparation of silica-based hybrid copolymers and their applications for Cu2+ removal. J. Appl. Polym. Sci. 2010, 118, 42−51. (14) Bois, L.; Bonhommé, A.; Ribes, A.; Pais, B.; Raffin, G.; Tessier, F. Functionalized silica for heavy metal ions adsorption. Colloids Surf., A 2003, 221, 221−230. (15) Machidaa, M.; Fotoohi, B.; Amamo, Y.; Ohba, T.; Kanoh, H.; Mercier, L. Cadmium(II) adsorption using functional mesoporous silica and activated carbon. J. Hazard. Mater. 2012, 221− 222, 220− 227. (16) Liu, J. S.; Wang, X. H.; Xu, T. W.; Shao, G. Q. Novel negatively charged hybrids. 1. Copolymers: Preparation and adsorption properties. Sep. Purif. Technol. 2009, 66, 135−142. (17) Liu, J. S.; Ma, Y.; Xu, T. W.; Shao, G. Q. Preparation of zwitterionic hybrid polymer and its application for the removal of heavy metal ions from water. J. Hazard. Mater. 2010, 178, 1021−1029. (18) Wu, C. M.; Xu, T. W.; Yang, W. H. A new inorganic−organic negatively charged membrane: Membrane preparation and characterizations. J. Membr. Sci. 2003, 224, 117−125. (19) Nagarale, R. K.; Gohil, G. S.; Shahi, V. K.; Rangarajan, R. Organic−inorganic hybrid membrane: Thermally stable cationexchange membrane prepared by the sol−gel method. Macromolecules 2004, 37, 10023−10030. (20) Chang, J.-H.; Park, D.-K.; Ihn, K. J. Polyimide nanocomposite with a hexadecylamine clay: Synthesis and characterization. J. Appl. Polym. Sci. 2002, 84, 2294−2301. (21) Kumar, G. P.; Kumar, P. A.; Chakraborty, S.; Ray, M. Uptake and desorption of copper ion using functionalized polymer coated silica gel in aqueous environment. Sep. Purif. Technol. 2007, 57, 47−56. (22) Ramesh, A.; Hasegawa, H.; Maki, T.; Ueda, K. Adsorption of inorganic and organic arsenic from aqueous solutions by polymeric Al/ Fe modified montmorillonite. Sep. Purif. Technol. 2007, 56, 90−100.

respectively, are significantly important in industrial processes. For such a purpose, a desorption experiment was performed, and the corresponding data are listed in Table 9. Table 9. Desorption Efficiency of Samples A to C for Cu2+ Ions in Various Desorbents efficiency/% sample

concentration of acid/(mol dm−3)

desorption time/h

HCl

HNO3

A B C

0.05 0.05 0.05

12 12 12

16.8 14.4 40.6

41.1 18.8 55.3

It is found that the desorption efficiency (%) of these samples using aqueous HNO3 solution as a desorbent is higher than that of using an aqueous HCl solution, suggesting that the adsorbed samples can be effectively regenerated or reused using aqueous HNO3 solution.

4. CONCLUSIONS Silicone MPTMS was used to capture Cu2+ ions. The kinetic and isotherm data of Cu2+ adsorption on the prepared hybrid adsorbents were determined. It is observed that the incorporation of MPTMS into the hybrid adsorbents does not favor the improvement of Lagergren kinetic rate constant. Meanwhile, it is found that proper incorporation of MPTMS into the adsorbents can highly improve the Langmuir constant Qm value. However, excess introduction of MPTMS into these hybrid adsorbents will reduce the Qm value. These findings suggest that the adsorption performances of the prepared hybrid adsorbents can be effectively adjusted via the incorporation of silicone, indicating that the hybrid of inorganic composition with organic polymers is a promising approach to modify the adsorption behaviors of an adsorbent.

■

AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 551 62158439. Fax: +86 551 62158437. E-mail: [email protected]. Funding

Financial support from the Natural Science Foundation of China (No. 21076055), Significant Foundation of Educational Committee of Anhui Province (No. ZD2008002-1), Science and Technology Innovation Fund for Students of Hefei University (No. 11XSKY02), and Open Fund from Engineering Research Center of Biomass Materials, Ministry of Education, Southwest University of Science and Technology (No. 12zxbk10) is highly appreciated. Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS Special thanks are given to the anonymous reviewers for their insightful comments and suggestions. REFERENCES

(1) Zhao, X.; Jia, Q.; Song, N.; Zhou, W.; Li, Y. Adsorption of Pb(II) from an aqueous solution by titanium dioxide/carbon nanotube nanocomposites: Kinetics, thermodynamics, and isotherms. J. Chem. Eng. Data 2010, 55, 4428−4433. 1149



Article

(23) Atia, A. A.; Donia, A. M.; Yousif, A. M. Removal of some hazardous heavy metals from aqueous solution using magnetic chelating resin with iminodiacetate functionality. Sep. Purif. Technol. 2008, 61, 348−357. (24) Guibal, E.; Milot, C.; Tobin, J. M. Metal−anion sorption by chitosan beats: Equilibrium and kinetic studies. Ind. Eng. Chem. Res. 1998, 37, 1454−1463. (25) Chung, J.; Chun, J.; Lee, J.; Lee, S. H.; Lee, Y. J.; Hong, S. W. Sorption of Pb(II) and Cu(II) onto multi-amine grafted mesoporous silica embedded with nano-magnetite: Effects of steric factors. J. Hazard. Mater. 2012, 239− 240, 183−191. (26) Wang, J.; Kuo, Y. Preparation of fructose-mediated (polyethylene glycol/chitosan) membrane and adsorption of heavy metal ions. J. Appl. Polym. Sci. 2007, 105, 1480−1489. (27) Karatas, M. Removal of Pb(II) from water by natural zeolitic tuff: Kinetics and thermodynamics. J. Hazard. Mater. 2012, 199− 200, 383−389. (28) Mohan, D.; Singh, K. P. Single- and multi-component adsorption of cadmium and zinc using activated carbon derived from bagasse−an agricultural waste. Water Res. 2002, 36, 2304−2318.

1150


Removal of Copper (Cu2+) from Water Using Novel Hybrid

Recommend Documents