Competitive Adsorption of Neutral Red and Cu2+ onto Pyrolytic Char

Article pubs.acs.org/jced

Competitive Adsorption of Neutral Red and Cu2+ onto Pyrolytic Char: Isotherm and Kinetic Study Weihua Zou,* Hongjuan Bai, and Shuaipeng Gao School of Chemical Engineering and Energy, Zhengzhou University, No. 100 of Kexue Road, Zhengzhou, 450001, P. R. China ABSTRACT: Pyrolytic char treated with muffle furnace (TPC) was used as an adsorbent for the removal of neutral red (NR) and Cu2+ from aqueous solutions in a batch system. The adsorption behavior of NR and Cu2+ in single and binary systems on the TPC was studied. For a single system, the NR and Cu2+ adsorption isotherm followed Langmuir and Redlich−Peterson models with a maximum adsorption capacity of (0.216 and 0.261) mmol·g−1, respectively. The adsorption kinetics of NR and Cu2+ followed both the pseudosecond-order and Elovich equations. The diffusion process with two stages for NR and Cu2+ can describe the adsorption process. For a binary system, NR and Cu2+ showed competitive adsorption on the TPC. The adsorption isotherm of NR and Cu2+ still followed the Langmuir model, and the adsorption kinetic models of NR and Cu2+ still followed the pseudosecond-order kinetic model, while the diffusion process of NR was single-stage and two-stage for Cu2+. The adsorption capacity of NR and Cu2+ is reduced compared with that in a single system, but the total adsorption capacity is higher. TPC has a higher affinity toward Cu2+ over NR.

1. INTRODUCTION Many industries discharge dyes and heavy metals in their effluents, such as leather, paper, textile, and dyeing, and so forth. Dyes and heavy metal ions can cause environmental and health problems to aquatic animals and human beings.1 Hence, it is essential to remove them from wastewater before they are discharged into water bodies. Various techniques for removing dyes and heavy metal ions from the wastewater have been developed, including coagulation,2 adsorption,3 reverse osmosis,4 oxidation,5 and so forth. Among these techniques, adsorption is an attractive and efficient process due to its low cost, effectiveness, and environment-friendliness. Commercial activated carbon is an effective adsorbent, but its high cost limits its large-scale application.6 So there is a need to find a substitute low-cost alternative. Pyrolytic char (PC), also called biochar, is a pyrolytic byproduct during bio-oil production, and it is a carbonaceous product of pyrolyzing biomass without any activation.7 As biomass pyrolysis to gas and liquid fuels grows in importance, it is predicted that large amounts of these chars will be available, making PC production much less expensive, compared to activated carbon. This would add value to PC and provide a potentially alternative to activated carbons. Considering that the surface of PC is abundant in oxygen/nitrogen functional groups,8 the PC can be used as a modification-free adsorbent for the removal of pollutants and has good environmental and economical prospects.9 To our best knowledge, few studies have been reported using PC as an adsorbent and investigating the effects of dye and heavy ion on the adsorption performance of PC so far. Although a lot of work have been focused on the application of various adsorbents for the single component system (dye or © 2012 American Chemical Society

metal ion) removal from the aqueous solution in recent years,10−17 however, few studies have been reported the competitive adsorption between dye and metal ion in binary systems. Some investigations on metal ions in binary- or ternarycomponent adsorption have been reported.18,19 In fact, it is necessary to study the simultaneous adsorption process involving two or more components because sole dye or toxic metal ions rarely exist in wastewater. So, it is necessary to study the competitive adsorption between the dye and heavy metal ions, and this would be helpful to make the best of PC properties for tertiary treatment. In this paper, PC treated with muffle furnace (TPC) was used as an adsorbent for the removal of neutral red (NR) and Cu2+ from aqueous solution. The competitive adsorption of NR and Cu2+ was also investigated. The objective of this work is to compare their adsorption in single and binary adsorption systems to study the adsorption process and determine the adsorption equilibrium and kinetics.

2. MATERIALS AND METHODS 2.1. Adsorbent and Adsorbate. 2.1.1. Adsorbent. In this study, PC was obtained from a key laboratory in the College of Chemistry and Molecular Engineering of Zhengzhou University. The PC was obtained from the combustion of biomass which was composed of pine wood. First, the sample was washed several times with 0.1 mol·kg−1 HCl and then washed with deionized water until the washing solution pH reached neutral. Then the Received: June 15, 2012 Accepted: August 26, 2012 Published: September 10, 2012 2792

dx.doi.org/10.1021/je300686u | J. Chem. Eng. Data 2012, 57, 2792−2801

Journal of Chemical & Engineering Data

Article

2.3. Adsorption Experiments. 2.3.1. Adsorption Isotherms. The batch adsorption experiments were conducted at 293 K. For a single adsorption system, a series of 150 mL flask with 0.025 g of TPC and 10 mL of dye or Cu2+ solution at different initial concentrations was shaked by the thermostatshaking equipment at 100 rpm for 4.32·104 s. NR was measured by Shimadzu UV-3000 UV spectrophotometer at λmax = 532 nm, and Cu2+ was measured by an AAanalyst 300 flame atomic absorption spectrometer at λmax = 324.8 nm. The effect of the initial solution pH on the removal of dye or heavy ion was analyzed over a (2 to 7) pH value. The initial solution pH value was adjusted with 0.1 mol·kg−1 NaOH or HNO3 solutions. For a binary adsorption system, the target species NR was varied over the range of (0.161 to 1.041) mmol·kg−1 with a certain gradient, which is the same as the initial concentrations in single adsorption system, while the concentration of the interferential species Cu2+ was (0.251, 0.392, and 0.573) mmol·kg−1, respectively. Similarly, the target species Cu2+ was varied over the range of (0.274 to 1.672) mmol·kg−1 with a certain gradient, which is the same as initial concentrations in single adsorption system, while the concentration of the interferential species NR was (0.455, 0.883, and 1.523) mmol·kg−1, respectively. The adsorption amount qe, the distribution coefficients Kd, and the decreasing ratio Dr, were calculated according to eq 1, eq 2, and eq 3,21 respectively.

sample was dried in the oven at 373 K and sieved to obtain particle size distribution (198 to 245) μm. Then, 5 g of PC was treated with muffle furnace at 573 K for 3600 s. Finally, the heattreated PC (TPC) was used as the adsorbent for further adsorption experiments. The chemical constituents of PC and TPC according to the XRF analyses are listed in Table 1. Table 1. Chemical Constituents of PC and TPC According to XRF Analyses mass percent/% constituent

PC

TPC

SiO2 CaO Fe2O3 K2O Al2O3 P2O5 SO3 MgO Na2O Cl TiO2

41.40 20.48 11.61 8.45 4.76 5.33 1.96 1.90 1.88 0.743 0.703

55.91 13.83 7.245 5.72 5.71 4.65 2.10 1.66 1.78 0.46 0.526

2.1.2. Adsorbate. The adsorbates used in this work are listed in Table 2. Neutral red (NR) was used as an adsorbate without further purification. Stock solution was prepared by dissolving 0.5 g of NR into 1000 mL of distilled water. The dye standards were obtained by diluting the dye stock solution in accurate proportions to the needed concentration. Stock solution of 31.25 mmol·kg−1 Cu2+ was prepared from Cu(NO3)2·3H2O in deionized water, and to prevent the precipitation of Cu2+ by hydrolysis, the stock solution of Cu2+ contained a few drops of 0.1 mol·kg−1 HNO3. The working solution was obtained by diluting the stock solution. 2.2. Characterization. The Fourier transform infrared spectrum of TPC were obtained with a PE-1710 FTIR (USA) instrument with a KBr pellet at a resolution of 4 cm−1, in the range of (4000 to 400) cm−1. Photomicrography of the exterior surface of PC and TPC was obtained by a JSM-7500 F (Japan) scanning electron microscope. The specific surface area and pore size distribution of PC and TPC were measured with NOVA 1000 High-Speed Automated Surface Area and Pore Size Analyzer (USA), and the Branuaer−Emmett−Teller (BET) adsorption model was used in the calculation. According to implemented software routines calculation of pore size followed the method of Barrett−Joyner−Halenda (BJH). The pH of zero point charge (pHpzc) was obtained to evaluate the TPC surface charge at different pH. The determination of the pHpzc of the TPC was carried out according to ref 20.

qe =

V (C0 − Ce) m

(1)

Kd =

1000V (C0 − Ce) mCe

(2)

Dr =

qe − q′e qe

·100 % (3)

where C0 and Ce are the initial and equilibrium concentrations of NR or metal ion solution (mmol·kg−1), m is the mass of TPC (g), and V is the volume of solution (L). qe is the equilibrium adsorption capacity (mmol·g−1), Kd is the distribution coefficient (mL·g−1); Dr is the decreasing ratio induced by the competition of the secondary species, and qe′ (mmol·g−1) is the equilibrium adsorption capacity in the binary adsorption system. 2.3.2. Adsorption Kinetics. For a kinetic study in single component system, a series of 150 mL flask with 0.025 g of TPC and 10 mL of dye or Cu2+ solution at different initial concentrations were shaked by the thermostat-shaking equipment at 293 K under 100 rpm. The flasks were then taken out of the thermostat-shaking equipment at predetermined time intervals, and the adsorption amounts were also calculated with eq 1. For the binary adsorption system, TPC at loadings of 2.5

Table 2. General Data of the Adsorbates Chosen for This Present Study

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Figure 1. FT-IR spectrum of TPC.

kg·m−3 was mixed with NR and Cu2+ mixture solution at the same as initial concentrations in a single adsorption system.

3. RESULTS AND DISCUSSION 3.1. Characterization of TPC. Figure 1 shows the FT-IR spectrum of TPC. The absorption peak at ca. 3420 cm−1 indicated the existence of bonded hydroxy groups on the TPC surface. Similarly, the peaks at (1704 and 1605) cm−1 indicated the existence of carbonyl group stretching in aldehydes and ketones.22 The peak at ca. (1419 and 1089) cm−1 was due to the stretch vibration of C−O associated with the carboxyl group. These groups may function as proton donors, and so deprotonated hydroxy and carboxyl groups may be involved in coordination with positive dye or heavy ions. When NR or Cu2+ ions were dissolved in deionized water, they were positively charged and will show attraction to the TPC structure. Based on this, it is expected that TPC will show an adsorption affinity toward NR or Cu2+ ions. Figure 2 shows the SEM photographs of PC and TPC which were taken at 500× magnifications. It was found that the PC’s surface texture was smooth. Comparing the images of PC, the TPC surface is rougher, which could be indicated that the surface area of TPC is higher than that of PC. The morphology of TPC can promote the adsorption of dyes or heavy metal ions due to the irregular structure and make the adsorption of dyes or heavy metal ions in different parts of TPC possible. PC, the particle size distribution of which is (198 to 245) μm, has a specific surface area of 0.516 m2·g−1. After treating with muffle furnace, the surface of TPC increases the specific surface area of the raw PC to 27.42 m2·g−1. The SEM micrograph of the two materials reinforced this result. The average pore diameters of PC and TPC were (19.18 and 18.87) nm, respectively.

Figure 2. SEM of PC (a) and TPC (b).

Treatment with a muffle furnace may lead to the larger surface area and smaller pore diameter. Figure 3 illustrates the point of zero charge (pzc) of TPC. The pHpzc of TPC was 5.93. When the value of pH is below 5.93, the surface of TPC was positively charged, while it will acquire a negative charge above. 3.2. Effect of Initial pH on NR and Cu2+ Adsorption in a Single Adsorption System. It is well-known that the initial pH of the solution is an important factor influencing the adsorption process. That is because the charge of the adsorbate and the adsorbent often hinges on the solution pH, and the surface charge of TPC is also dependent on the pH of the solution. As NR or Cu2+ is precipitated when pH is over (6.5 or 7), the effect of pH was conducted in the range of pH (2.09 to 6.2) or (2.31 to 6.94) for NR and Cu2+, respectively. After adsorption, the pH values of the solution were also analyzed, and the variations were all within one unit. Figure 4 shows the result. Besides, the NR solution pH without adjustment by 0.1 mol·kg−1 HNO3 or NaOH is about 5.5 in this paper. So pH 5.5 was selected in the experiment for NR. 2794



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decreasing, the adsorption capacity increased indicates that one of major adsorption process is ion exchange.21 When the pH is beyond 6, one can observe that there is a sharp increase in the adsorption. It is common knowledge that metal ions react with hydroxide ions of basic solution. With the pH increasing, more hydroxyl ions would be presented, which could lead to surface precipitation. So when pH is too high, the effective mechanisms of the Cu2+ removal in aqueous solution might be adsorption and precipitation. When at higher pH, Cu2+ converted to various hydrolysate in aqueous solution. The previous report has shown similar results for Cu2+.23 3.3. Adsorption Equilibrium Isotherms. 3.3.1. Adsorption Isotherms for the Single System. Equilibrium data are important for the design of adsorption system. In this work, the Langmuir, Freundlich, and Redlich−Peterson models were applied to describe the adsorption equilibrium. The Langmuir model24 supposes that adsorption takes place at specific homogeneous sites within the adsorbent, so the saturated monolayer isotherm can be represented as:

Figure 3. Variation of zero charges with equilibrium pH of TPC suspensions. ◧, pHinitial; ◭, pHfinal.

qe =

qmKLCe 1 + KL ·Ce

(4)

where Ce (mmol·kg−1) is the equilibrium concentration, qe (mmol·g−1) is the amount of NR or Cu2+ adsorbed onto per unit mass of TPC at equilibrium; qm (mmol·g−1) is the maximum adsorption capacity which is correlated with monolayer coverage; and KL (L·mmol−1) is the Langmuir constant which is correlated with the affinity of the binding sites and adsorption energy, the higher the KL value, the higher the affinity of the binding sites. The Freundlich model25 can be expressed as: qe = KFCe1/ n 2+

Figure 4. Effect of solution pH on NR and Cu adsorption on TPC. (C0,NR: 0.596 mmol·kg−1; C0,Cu2+: 0.938 mmol·kg−1; TPC dosage: 2.5 kg·m−3; contact time: 4.32·104 s; 293 K). ◧, Cu2+; ◭, NR.

(5)

where KF is the Freundlich constant which is correlated with the capacity of adsorption and 1/n is a indicator of adsorption intensity. The Freundlich model is employed to describe a heterogeneous system and reversible adsorption and is not confined to the monolayers.26 The higher value for KF indicates a higher affinity for adsorbate onto adsorbent and the values of 1/n (0.1 < 1/n < 1), indicating that the adsorption process is favorable.26 The Redlich−Peterson27 model is given by eq 6:

For NR, when the pH below 3, the adsorption capacity increased with the pH increasing, and then when the pH was between (3.5 and 6.2), the adsorption capacity almost kept constant. The overall charge of the adsorbent affected the pH dependence of dye adsorption. In this paper, when the adsorption capacity increased with the pH increasing, this could be ascribed to electrostatic attraction of the positive charge of NR. The FT-IR spectrum of TPC evidenced that there were plenty of hydroxyl groups contained in TPC. With the pH increasing from (2 to 3.5), the electrostatic attraction between adsorption sites and NR cations increased in NR removal. With the pH decreasing, the negatively charged surface of TPC had a tendency to be saturated with protons, and there was competition between NR cation and hydrogen ions on the adsorption sites; therefore, the adsorption capacity of NR decreased. When the pH was 2, the TPC surface is positively charged, and the NR adsorption was unfavorable. For Cu2+, adsorption increases with increasing pH. At low pH, the adsorption capacity of Cu2+ was inhibited, and this possibly because there was a competition between hydrogen and metal ions among the adsorption sites, and the hydrogen ions had an apparent preponderance. With the pH increasing, the negative charge on TPC surface increased because of the deprotonation of the metal binding sites, and thus the metal ions adsorption increased. With the H+ ion concentration (at high value pH)

qe =

ACe 1 + BCe g

(6)

where A, B, and g are the Redlich−Peterson parameters, g is between 0 and 1. Redlich−Peterson equation is the compromise between Langumuir and Frenudlich models. For g = 1, eq 6 converts to the form of Langmuir model. The relevant parameters were obtained on the basis of the three-parameter model at different temperatures. The relative parameters for isotherm and kinetic equation were calculated employing the variation χ2 between the experimental data and the calculated data using nonlinear regression analysis. The expression for χ2 can be given as: 2

χ =

∑

(qe,exp − qe,calc)2 qe,calc

(7)

where qe,calc is the calculated adsorption capacity of NR or Cu2+ adsorbed onto TPC which is correlated with the various 2795



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Figure 5. Adsorption isotherms of NR and Cu2+ on TPC in single and binary systems using a nonlinear regressive method (TPC dosage: 2.5 kg·m−3; contact time: 4.32·104 s; 293 K): ···, Langmuir fitted curve; , Freundlich fitted curve; ---, Redlich−Peterson fitted curve. ⧫, NR (C0,Cu2+ = 0 mmol·kg−1); ◧, NR (C0,Cu2+ = 0.455 mmol·kg−1); ◭, NR (C0,Cu2+ = 0.883 mmol·kg−1); ◐, NR(C0,Cu2+ = 1.523 mmol·kg−1); ▲, Cu2+ (C0,NR = 0 mmol·kg−1); ○ with ×, Cu2+ (C0,NR = 0.251 mmol·kg−1); □ with ×, Cu2+ (C0,NR = 0.392 mmol·kg−1); ☆, Cu2+ (C0,NR = 0.573 mmol·kg−1).

adsorption models and qe,exp is correlated with the experimental data of adsorption capacity. Figure 5 shows the fitting curves of the experimental data. The parameters of the three models calculated on the basis of eqs 4, 5, and 6 are listed in Table 3. It was found that regression coefficients R2 obtained from the Langmuir and Redlich− Peterson models were bigger than that of the Freundlich model for NR and Cu2+, and χ2 obtained from the Langmuir and Redlich−Peterson model were lower than that of the Freundlich model, which suggested both the Langmuir and the Redlich−

Peterson models appeared to be the better models for the adsorption of NR and Cu2+ onto TPC. Therefore, it indicated that, after adsorption, a NR or Cu2+ monolayer covered the surface of adsorbent, and the maximum adsorption of NR and Cu2+ is (0.216 and 0.261) mmol·g−1 on the basis of the Langmuir isotherm. Besides, in this study, the constant g of Redlich− Peterson isotherm is near to 1, which also indicates that Langmuir isotherm conformed to the isotherm better than the Freundlich isotherm. 2796



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Table 3. Parameters for Adsorption Isotherms of NR and Cu2+ on TPC in Single and Binary Systems Cu2+

NR 2+

C0,Cu /mmol·kg 0

0.455

qm/mmol·g−1 KL/L·mmol−1 R2 χ2·104

0.216 ± 0.009 23.74 ± 4.15 0.9207 2.2

0.173 ± 0.014 6.74 ± 1.67 0.9307 1.0

KF 1/n R2 χ2·104

0.250 ± 0.015 0.263 ± 0.034 0.8827 3.2

0.169 ± 0.013 0.363 ± 0.068 0.8684 1.9

A B g R2 χ2·104

6.375 ± 2.664 28.13 ± 10.74 0.929 ± 0.097 0.9256 2.3

0.833 ± 0.259 5.07 ± 1.46 1.276 ± 0.341 0.9393 1.0

−1

0.883

C0,NR/mmol·kg−1 1.523

0

Langmuir Isotherm 0.151 ± 0.012 0.149 ± 0.019 0.261 ± 0.009 6.12 ± 1.47 4.49 ± 1.57 22.20 ± 3.77 0.9314 0.8805 0.9430 0.7 1.2 1.7 Freundlich Isotherm 0.143 ± 0.011 0.133 ± 0.013 0.268 ± 0.014 0.364 ± 0.072 0.407 ± 0.098 0.195 ± 0.04 0.8528 0.7980 0.8540 1.5 2.0 4.3 Redlich−Peterson Isotherm 0.633 ± 0.150 0.393 ± 0.066 5.928 ± 1.983 4.42 ± 1.01 3.06 ± 0.54 22.64 ± 7.52 1.364 ± 0.312 1.963 ± 0.605 0.994 ± 0.075 0.9501 0.9350 0.9431 0.6 0.8 2.0

3.3.2. Adsorption Isotherms for the Binary Adsorption System. The competitive adsorption equilibrium isotherm for NR and Cu2+ binary adsorption system was obtained through fixing the initial concentration of interferential components. Figure 5 also shows the adsorption isotherms in the binary adsorption system and fitted curves by the Langmuir, Freundlich, and Redlich−Peterson models of NR and Cu2+. The parameters of the three models are listed in Table 3. On the basis of the values of R2, χ2 and the experimental results depicted in Table 3, both the Langmuir isotherm and the Redlich−Peterson model fit the experimental data. However, the constant g of the Redlich− Peterson isotherm was bigger than 1 in binary adsorption system, which means that the isotherm was not suitable in the coadsorption system. Thus, one can concluded that adsorption isotherms of NR and Cu2+ in the binary adsorption system would only follow the Langmuir isotherm. On the basis of the Langmuir isotherm, the adsorption capacity of NR or Cu2+ was lower than that in the single adsorption system. In the binary adsorption system, with the initial concentration increasing, the adsorption capacity of NR increased under a fixed concentration of Cu2+, and the tendency was the same as that of the single adsorption system. However, with the initial concentration of Cu2+ increasing, the adsorption capacity of NR decreased obviously at higher concentration. The maximum adsorption capacity of NR decreased from (0.216 to 0.149) mmol·g−1 with the coexistence of 1.523 mmol·L−1 Cu2+, and the decreasing ratio value Dr was about 31.02 %. The inhibitive phenomenon was also found when the target component was Cu2+. The maximum capacity of Cu2+ in single adsorption system was 0.261 mmol·g−1, while it decreased to 0.167 mmol·g−1 when the initial concentration of NR was 0.573 mmol·L−1. Compared to Dr, the inhibitory extent was about 36.02 % by NR, which is bigger than that by Cu2+. In the binary adsorption system, the lower adsorption capacity suggested the inhibitive phenomenon between the dye and the metal ion. The similar tendency was also found in the ternary metal solution of Cu(II)/Pb(II)/Cd(II),28 Cd(II)/Zn(II)/Ni(II),18 and in the binary metal solution of Cu(II)/Cd(II), Pb(II)/Cd(II) and Cu(II)/Pb(II),21 the coexistent component in the system leads to the adsorption capacity of the primary component reduced.29 The reason for the

0.251

0.392

0.573

0.200 ± 0.006 12.53 ± 1.75 0.9637 0.5

0.186 ± 0.004 13.26 ± 1.50 0.9748 0.3

0.167 ± 0.006 11.67 ± 2.07 0.9367 0.5

0.191 ± 0.009 0.211 ± 0.042 0.8394 2.4

0.177 ± 0.007 0.202 ± 0.036 0.8703 1.5

0.157 ± 0.006 0.205 ± 0.039 0.8467 1.3

1.980 ± 0.371 9.82 ± 1.99 1.082 ± 0.068 0.9714 0.5

2.224 ± 0.430 11.96 ± 2.47 1.030 ± 0.054 0.9761 0.3

1.789 ± 0.593 10.69 ± 3.84 1.024 ± 0.093 0.9375 0.6

inhibitive phenomenon may be the competitive adsorption on the active adsorption sites of TPC. The distribution coefficients Kd (mL·g−1) is a potential mobility index of one component in the binary adsorption system, which also proved the interaction of the two components in the solutions due to the competitive adsorption. Due to the coexistence of NR or Cu2+, Kd decreased. The results are seen in Figure 6. The similar phenomenon was also found in the Cd(II)/ Ni(II)/Zn(II) competitive adsorption onto sludge-amended soil.30 The reason was probably attributed as follows. Each species was mainly adsorbed onto specific sites at a lower concentration, and the adsorption sites could be partly overlapped with the total concentration increasing in binary adsorption systems.31,32 Figure 7 displays the three-dimensional adsorption surfaces for the dye and metal adsorpiton capacity of each component against the equilibrium concentrations. From Figure 7a, one can see that the shapes of the surface of NR component were influenced by the concentration of Cu2+ at equilibrium. The dye uptakes decreased at elevated equilibrium concentrations of Cu2+. From Figure 7a, it was also found that with the initial concentration of NR increased at (0.251, 0.392, and 0.573) mmol·kg−1, the maximum dye adsorption capacity was (0.0856, 0.117, and 0.141) mmol·g−1, respectively. When concentration of Cu2+ increased from (0.274 to 1.672) mmol·kg−1, the decreasing ratio value was about (16.20, 32.23, and 36.02) %, respectively. As illustrated in Figure 7b, similar uptake surfaces were observed when the initial concentration of Cu2+ increased at (0.455, 0.883, and 1.523) mmol·kg−1, the maximum adsorption capacity of Cu2+ was (0.160, 0.208, and 0.292) mmol·g−1, respectively. When concentration of NR increased from (0.161 to 1.041) mmol·kg−1, the decreasing ratio value was about (37.32, 40.09, and 40.84) %, respectively. One can also observe that Cu2+ uptakes decreased at elevated equilibrium concentration of NR. This result coincided with the conclusion derived from Figure 5. 3.4. Adsorption Kinetics. 3.4.1. Single System. To investigate the adsorption kinetics of NR and Cu2+ on TPC, four kinetic models, such as pseudofirst-order equation, pseudosecond-order equation, Elovich equation, and intraparticle diffusion model, were employed. 2797



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Figure 6. Distribution coefficients Kd (mL·g−1) of NR and Cu2+ in binary solute. (TPC dosage: 2.5 kg·m−3; contact time: 4.32·104 s; 293 K). ⧫, NR (C0,Cu2+ = 0 mmol·kg−1); ◧, NR (C0,Cu2+ = 0.455 mmol·kg−1); ◭, NR (C0,Cu2+ = 0.883 mmol·kg−1); ◐, NR (C0,Cu2+ = 1.523 mmol·kg−1); ▲, Cu2+ (C0,NR = 0 mmol·kg−1); ○ with ×, Cu2+ (C0,NR = 0.251 mmol·kg−1); □ with ×, Cu2+ (C0,NR = 0.392 mmol·kg−1); ☆, Cu2+(C0,NR = 0.573 mmol·kg−1).

Figure 7. Three-dimensional isotherm surfaces of binary system: (a) The adsorpiton capacity of NR is plotted as a function of the equilibrium concentrations of NR and Cu2+ (at NR initial concentrations of (0.251, 0.392, and 0.573) mmol·kg−1 from bottom surface to top surface). (b) The adsorpiton capacity of Cu2+ is plotted as a function of the equilibrium concentrations of NR and Cu2+ (at Cu 2+ initial concentrations of (0.455, 0.883, and 1.523) mmol·kg−1 from bottom surface to top surface; TPC dosage: 2.5 kg·m−3; contact time: 4.32·104 s; 293 K).

The pseudofirst-order equation33 may be given as the following equation: qt = qe(1 − e−k1t )

(8) −1

2+

where qe and qt (mmol·g ) are the NR or Cu adsorption capacity at equilibrium and at time t (s), respectively, and k1 (s−1) is the rate constant of the pseudofirst-order. The pseudosecond-order kinetic model34 is given by the following equation: qt =

and the equilibrium was approached after 4.32·104 s. Figure 8 presents the nonlinear curves of the pseudofirst-order, the pseudosecond-order and the Elovich kinetic models fitted with the experimental data of NR and Cu2+ adsorption. The nonlinear fitting curves were produced by Origin 7.5 software. The kinetic parameters calculated from models were listed in Table 4. Based on the values of R2 and χ2 given in Table 4 and the experimental data depicted in Figure 8, it was shown that the pseudosecond-order model and the Elovich equations were good in fitting the experimental data. The Elovich equation can be successfully employed to describe the adsorption kinetics of ion exchange system, so it could be deduced that the adsorption process is a chemical process, especially an ion exchange

k 2qe2t 1 + k 2qet

(9)

−1 −1

where k2 (g·mmol ·s ) is the rate constant of pseudosecondorder adsorption. The Elovich equation35 is given as the following equation: qt = A + B ln t

(10)

where A and B are the Elovich constants. The adsorption rate of NR or Cu2+ onto TPC was fast at the first 6·103 s, after that the adsorption rate dropped down slowly 2798



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qt = ktt 0.5 + C

(11)

where kt (mmol·g−1·s−0.5) is the intraparticle diffusion rate constant and C is a constant related to the thickness of the boundary layer, which is in direct ratio to the effect of the boundary layer. Generally, the plot of qt against t0.5 may show a multilinearity, and this indicated that the adsorption processes contained two or more steps. The adsorption of a solute from solution by porous adsorbents is essentially relevant to three consecutive steps. The external surface adsorption or the instantaneous adsorption is the first step. The second step is gradual adsorption stage where intraparticle diffusion is rate-limiting. The third step is the final equilibrium stage where intraparticle diffusion started to slow down due to the extremely low adsorbate concentrations left in the solutions.37 If the line passed through the origin, the intraparticle diffusion would be the sole rate-limiting step. If the line did not pass through the origin, it implied that intraparticle diffusion was not the sole rate control step, and other processes may control the adsorption rate.38 The plot of qt versus t0.5 for NR and Cu2+ adsorption on TPC in a single system is shown in Figure 9, and the values of kt and C are listed in Table 4. From Figure 9, it was found that the experimental data points showed two linear sections, which indicates the different steps in adsorption process. From Figure 9, the intercept values of the first linear segments were different from zero for all two-linear plots, this consistent with the value of C showed in Table 4, which suggested that pore diffusion was not be the sole rate-limiting step at the beginning of batch adsorption. Film-diffusion control may also be occurred in these early stages of the adsorption process and it may also been

Figure 8. Time profiles of NR and Cu2+ adsorption by TPC under noncompetitive and competitive conditions (TPC dosage: 2.5 kg·m−3; 293 K; C0,NR: 0.597 mmol·kg−1; C0,Cu2+: 0.911 mmol·kg−1): ···, pseudofirst-order model; , pseudosecond-order model; ---, Elovich model; ◧, NR in a binary system; ◐, Cu2+ in a binary system; □ with ×, NR in a single system; ○ with ×, Cu2+ in a single system; ⧫, NR and Cu2+ in a binary system.

process,35 and if the pseudosecond-order rate equation could describe the single adsorption system with higher R2, it shows that the chemisorption is the rate-controlling mechanism.36 So the adsorption process may be consist of both ion exchange and chemisorption rate-controlling process, which is a complex nature. The intraparticle diffusion model35 was also used to investigate the kinetic adsorption of a single adorption system. It is given by the following equation:

Table 4. Parameters for Adsorption Kinetics of NR and Cu2+ on TPC in Single and Binary Systems Cu2+

NR adsorbate

single system

qe(exp)/mmol·g−1 qe(cal)/mmol·g−1 k1·103/s−1 R2 χ2·103

0.143 0.130 ± 0.004 0.390 ± 0.073 0.7464 9.528

k2·103/g·mmol−1·s−1 qe(cal)/mmol·g−1 R2 χ2·103

3.830 ± 0.680 0.143 ± 0.004 0.9223 2.892

A·102 B·102 R2 χ2·103

−7.541 ± 0.786 2.065 ± 0.083 0.9874 0.427

kt1·103/mmol·g−1·s−0.5 C1·102/mmol·g−1 R SD·103 kt2·104/mmol·g−1·s−0.5 C2·102/mmol·g−1 R SD·103

0.953 ± 0.116 3.372 ± 0.667 0.9855 5.17 2.863 ± 0.234 8.714 ± 0.351 0.9805 2.63

binary system Pseudofirst-Order Model 0.128 0.109 ± 0.046 0.320 ± 0.050 0.7137 6.356 Pseudosecond-Order Model 3.520 ± 0.670 0.122 ± 0.004 0.8801 13.700 Elovich Model −9.490 ± 0.829 2.050 ± 0.091 0.9734 2.788 Intraparticle Diffusion Model 0.421 ± 0.014 4.594 ± 0.167 0.9915 3.26

2799

single system

binary system

0.231 0.223 ± 0.006 0.640 ± 0.070 0.8905 62.185

0.207 0.169 ± 0.006 0.490 ± 0.080 0.6458 35.369

3.880 ± 0.460 0.244 ± 0.005 0.9545 14.118

3.710 ± 0.710 0.186 ± 0.006 0.8447 14.646

−10.866 ± 3.055 3.470 ± 0.352 0.8818 18.927

−9.174 ± 1.230 2.677 ± 0.134 0.9659 2.834

1.850 ± 0.118 6.249 ± 0.694 0.9821 9.74 0.483 ± 0.292 22.29 ± 0.405 0.5953 3.19

1.200 ± 0.071 5.963 ± 0.323 0.9931 2.08 5.053 ± 0.175 9.886 ± 0.230 0.9935 3.00



Article

one positive charge, while Cu2+ possesses two positive charges, and the ion size of Cu2+ is smaller than NR, which made it much easier for ion exchange and had a higher rate of adsorption. The relative adsorption of NR and Cu2+ on TPC in a binary adsorption system was obtained by the following equation:1

Ar =

[qt ]B [qt ]S

(12)

where [qt]B and [qt]S are the adsorption capacity of specific adsorbate in binary system and single system at time t, respectively. Also for the binary adsorption system, the selectivity of adsorption on TPC is calculated by the following equation: S= Figure 9. Intraparticle diffusion model of adsorption of NR and Cu2+ on TPC in single and binary systems. (TPC dosage: 2.5 kg·m−3; 293 K). ◧, NR in a binary system; ◐, Cu2+ in a binary system; □ with ×, NR in a single system; ○ with ×, Cu2+ in a single system.

(A r )Cu (A r )NR

(13)

Figure 10 shows the adsorption selectivity in binary adsorption system and the variation of relative adsorption of NR and Cu2+

the rate limiting step during the time period of the first linear segment.39 It suggested that the actual adsorption process may contain both the surface adsorption and intraparticle diffusion. 3.4.2. Binary Component. Figure 8 also shows the competitive kinetic results in NR and Cu2+ binary system. For the two species, with the time increasing the adsorption capacity was increased and then followed by a plateau which was the same as in single adsorption system. But the competition adsorption behavior of the two components could still be found evidently from Figure 8. For the adsorption capacity of a certain species, comparing the profiles in single and binary adsorption systems, the latter was always lower during the adsorption process. The possible mechanism may be probably due to the lots of ions competing against the confined adsorption sites, so the adsorption capacity of each species in binary adsorption system is lower than that in single adsorption system. Based on the adsorption equilibrium, the individual adsorption of NR and Cu2+ is reduced. However, the total adsorption capacity of NR and Cu2+ on TPC is higher than that of any NR and Cu2+ adsorption capacity at equilibrium in single adsorption system. From Figure 8 and Table 4, it was found that the Elovich equations were good in fitting the experimental data, and the R2 are much higher. Thus, one can conclude that the kinetics of NR and Cu2+ in the binary adsorption system may also follow the Elovich kinetic model. Figure 9 also shows the intraparticle diffusion model of NR and Cu2+ in binary adsorption systems. The adsorption of NR showed a one-stage diffusion process, unlike the case of NR in a single adsorption system. This may be because of the competitive adsorption with Cu2+. From Table 4, one can see that Cu2+ had a higher diffusion rate at the first sharper portion, and it was easily diffused into the inner pore of TPC, which occupied the active adsorption sites of TPC and prevented the further adsorption of NR to the inner pore of TPC. Thus, the adsorption of NR would be occurring in the surface of TPC and show a single-stage process in the binary adsorption system. Compared to the adsorption rate constants in single system, the diffusion rate constants in binary adsorption system were lower. The difference in electronic structure and molecular size led to the higher rate of Cu2+ than NR in a binary adsorption system. It has been proved that a higher charge density of ions would show a higher ion exchange rate and adsorption phenomena depend on the charge density of cations.40 For NR and Cu2+, the NR ion possesses only

Figure 10. Relative adsorption of NR and Cu2+ or selectivity in a binary system. ◧, Cu2+; ◭, NR; □ with ×, selectivity.

with time. It was found that the relative adsorption of NR did not change too much during the overall adsorption process while the relative adsorption of Cu2+ increased slowly. The selectivity of adsorption on TPC shows an increasing tendency and the value approached 1. It suggested that TPC had a higher affinity to Cu2+ in a binary adsorption system. When the adsorption achieves the equilibrium, the selectivity of adsorption would be the same for NR and Cu2+.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Funding

This work was supported by the Education Department of Henan Province in China (No. 2010A610003) and Henan Science and Technology Department in China (No. 122300410163). Notes

The authors declare no competing financial interest. 2800



■

Article

ions onto a novel IDA-chelating resin: Isotherm and kinetic modeling. Water Res. 2011, 45, 1177−1188. (22) Tarley, C. R. T.; Arruda, M. A. Z. Biosorption of heavy metals using rice milling by-products. Characterisation and application for removal of metals from aqueous effluents. Chemosphere 2004, 54, 987− 995. (23) Han, R. P.; Zou, W. H.; Zhang, Z. P.; Shi, J.; Yang, J. J. Removal of copper (II) and lead (II) from aqueous solution by manganese oxide coated sand I. Characterization and kinetic study. J. Hazard. Mater. B 2006, 137, 384−395. (24) Langmuir, L. The constitution and fundamental properties of solids and liquids. J. Am. Chem. Soc. 1916, 38, 2221−2295. (25) Freundlich, H. M. F. Over the adsorption in the solution. J. Phys. Chem. 1906, 57, 385−470. (26) Ö zcan, A. S.; Erdem, B.; Ö zcan, A. Adsorption of Acid Blue 193 from aqueous solutions onto Na-bentonite and DTMA-bentonite. J. Colloid Interface Sci. 2004, 280, 44−54. (27) Redlich, O.; Peterson, D. L. A useful adsorption isotherm. J. Phys. Chem. 1959, 63, 1024−1026. (28) Sheng, P. X.; Ting, Y. P.; Chen, J. P. Biosorption of heavy metal ions (Pb, Cu, and Cd) from aqueous solutions by the marine alga Sargassum sp. in single- and multiple-metal systems. Ind. Eng. Chem. Res. 2007, 46, 2438−2444. (29) Balasubramanian, R.; Perumal, S. V.; Vijayaraghavan, K. Equilibrium isotherm studies for the multicomponent adsorption of lead, zinc, and cadmium onto Indonesian Peat. Ind. Eng. Chem. Res. 2009, 48, 2093−2099. (30) Antoniadis, V.; Tsadilas, C. D.; Ashworth, D. J. Monometal and competitive adsorption of heavy metals by sewage sludge-amended soil. Chemosphere 2007, 68, 489−494. (31) Saha, U. K.; Taniguchi, S.; Sakurai, K. Simultaneous adsorption of cadmium, zinc, and lead on hydroxyaluminum- and hydroxyaluminosilicate-montmorillonite complexes. Soil Sci. Soc. Am. J. 2002, 66, 117− 128. (32) Papini, M. P.; Saurini, T.; Bianchi, A.; Majone, M.; Beccari, M. Modeling the competitive adsorption of Pb, Cu, Cd, and Ni onto a natural heterogeneous sorbent material (Italian Red soil). Ind. Eng. Chem. Res. 2004, 43, 5032−5041. (33) Ho, Y. S.; Ng, J. C. Y.; McKay, G. Kinetics of pollutant sorption by biosorbents: Review. Sep. Purif. Methods 2000, 29, 189−232. (34) Ho, Y. S.; McKay, G. Sorption of dye from aqueous solution by peat. Chem. Eng. J. 1998, 70, 115−124. (35) Cheung, C. W.; Porter, J. F.; Mckay, G. Sorption kinetics for the removal of copper and zinc from effluents using bone char. Sep. Purif. Technol. 2000, 19, 55−64. (36) Reddad, Z.; Gerente, C.; Andres, Y.; Le Cloirec, P. Adsorption of several metal ions onto a low-cost biosorbent: kinetic and equilibrium studies. Environ. Sci. Technol. 2002, 36, 2067−2073. (37) Wu, F. C.; Tseng, R. L.; Juang, R. S. Comparisons of porous and adsorption properties of carbons activated by steam and KOH. J. Colloid Interface Sci. 2005, 283, 49−56. (38) Liu, L.; Wan, Y. Z.; Xie, Y. D.; Zhai, R.; Zhang, B.; Liu, J. D. The removal of dye from aqueous solution using alginate-halloysite nanotube beads. Chem. Eng. J. 2012, 187, 210−216. (39) Srivastava, V. C.; Swamy, M. M.; Mall, I. D.; Prasad, B.; Mishra, I. M. Adsorptive removal of phenol by bagasse fly ash and activated carbon: Equilibrium, kinetics and thermodynamics. Colloids Surf. 2006, 272, 89−104. (40) Erdem, E.; Karapinar, N.; Donat, R. The Removal of Heavy Metal Cations by Natural Zeolites. J. Colloid Interface Sci. 2004, 280, 309−314.

REFERENCES

(1) Wang, S. B.; Ariyanto, E. Competitive adsorption of malachite green and Pb ions on natural zeolite. J. Colloid Interface Sci. 2007, 314, 25−31. (2) Zahrim, A. Y.; Tizaoui, C.; Hilal, N. Coagulation with polymers for nanofiltration pre-treatment of highly concentrated dyes: A review. Desalination 2011, 266, 1−16. (3) Zou, W. H.; Li, K.; Bai, H. J.; Shi, X. L.; Han, R. P. Enhanced cationic dyes removal from aqueous solution by oxalic acid modified rice husk. J. Chem. Eng. Data 2011, 56, 1882−1891. (4) Nataraj, S. K.; Hosamani, K. M.; Aminabhavi, T. M. Nanofiltration and reverse osmosis thin film composite membrane module for the removal of dye and salts from the simulated mixtures. Desalination 2009, 249, 12−17. (5) Rocha, J. H. B.; Gomes, M. M. S.; Fernandes, N. S.; da Silva, D. R.; Martínez-Huitle, C. A. Application of electrochemical oxidation as alternative treatment of produced water generated by Brazilian petrochemical industry. Fuel Process. Technol. 2012, 96, 80−87. (6) Gu, L.; Xu, J. L.; Liu, B.; Zhang, H. N.; Yu, X.; Luo, Z. X. Dissolved organic nitrogen (DON) adsorption by using Al-pillared bentonite. Desalination 2011, 269, 206−213. (7) Qiu, Y. P.; Zheng, Z. Z.; Zhou, Z. L.; Sheng, G. D. Effectiveness and mechanisms of dye adsorption on a straw-based biochar. Bioresour. Technol. 2009, 100, 5348−5351. (8) Cheng, H. N.; Wartelle, L. H.; Klasson, K. T.; Edwards, J. C. Solidstate NMR and ESR studies of activated carbons produced from pecan shells. Carbon 2010, 48, 2455−2469. (9) Liu, W. J.; Zeng, F. X.; Jiang, H.; Zhang, X. S. Preparation of high adsorption capacity bio-chars from waste biomass. Bioresour. Technol. 2011, 102, 8247−8252. (10) Kyzas, G. Z.; Lazaridis, N. K.; Mitropoulos, A. C. Removal of dyes from aqueous solutions with untreated coffee residues as potential lowcost adsorbents: Equilibrium, reuse and thermodynamic approach. Chem. Eng. J. 2012, 189−190, 148−159. (11) Han, R. P.; Wang, Y.; Sun, Q.; Wang, L. L.; Song, J. Y.; He, X. T.; Dou, C. C. Malachite green adsorption onto natural zeolite and reuse by microwave irradiation. J. Hazard. Mater. 2010, 175, 1056−1061. (12) Song, J. Y.; Zou, W. H.; Bian, Y. Y.; Su, F. Y.; Han, R. P. Adsorption characteristics of methylene blue by peanut husk in batch and column modes. Desalination 2011, 265, 119−125. (13) Han, R. P.; Zhang, L. J.; Song, C.; Zhang, M. M.; Zhu, H. M.; Zhang, L. J. Characterization of modified wheat straw, kinetic and equilibrium study about copper ion and methylene blue adsorption in batch mode. Carbohydr. Polym. 2010, 79, 1140−1149. (14) Tanyildizi, M. Ş. Modeling of adsorption isotherms and kinetics of reactive dye from aqueous solution by peanut hull. Chem. Eng. J. 2011, 168, 1234−1240. (15) Zou, W. H.; Bai, H. J.; Zhao, L.; Li, K.; Han, R. P. Characterization and properties of zeolite as adsorbent for removal of uranium(VI) from solution in fixed bed column. J. Radioanal. Nucl. Chem. 2011, 288, 779− 788. (16) Gupta, N.; Kushwaha, A. K.; Chattopadhyaya, M. C. Adsorptive removal of Pb2+, Co2+ and Ni2+ by hydroxyapatite/chitosan composite from aqueous solution. J. Taiwan Inst. Chem. E 2012, 43, 125−131. (17) Ahmed, S. A. Batch and fixed-bed column techniques for removal of Cu(II) and Fe(III) using carbohydrate natural polymer modified complexing agents. Carbohydr. Polym. 2011, 83, 1470−1478. (18) Srivastava, V. C.; Mall, I. D.; Mishra, I. M. Antagonistic competitive equilibrium modeling for the adsorption of ternary metal ion mixtures from aqueous solution onto bagasse fly ash. Ind. Eng. Chem. Res. 2008, 47, 3129−3137. (19) Xiao, B.; Thomas, K. M. Competitive adsorption of aqueous metal ions on an oxidized nanoporous activated carbon. Langmuir 2004, 20, 4566−4578. (20) Faria, P. C. C.; Ó rfão, J. J. M.; Pereira, M. F. R. Adsorption of anionic and cationic dyes on activated carbons with different surface chemistries. Water Res. 2004, 38, 2043−2052. (21) Li, L. J.; Liu, F. Q.; Jing, X. S.; Ling, P. P.; Li, A. M. Displacement mechanism of binary competitive adsorption for aqueous divalent metal 2801


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