Phosphate adsorption from aqueous solutions by disused adsorbents: Chitosan hydrogel beads after the removal of copper(II)

Phosphate adsorption from aqueous solutions by disused adsorbents: Chitosan hydrogel beads after the removal of copper(II)

Chemical Engineering Journal 166 (2011) 970–977 Contents lists available at ScienceDirect Chemical Engineering Journal journal homepage: www.elsevie...

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Chemical Engineering Journal 166 (2011) 970–977

Contents lists available at ScienceDirect

Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej

Phosphate adsorption from aqueous solutions by disused adsorbents: Chitosan hydrogel beads after the removal of copper(II)夽 Jie Dai a , Hu Yang a,∗ , Han Yan a , Yonggang Shangguan b , Qiang Zheng b , Rongshi Cheng a,c a Key Laboratory for Mesoscopic Chemistry of MOE, Department of Polymer Science & Technology, School of Chemistry & Chemical Engineering, Nanjing University, Nanjing 210093, China b Department of Polymer Science and Engineering, Key Laboratory of Macromolecule Synthesis and Functionalization of MOE, Zhejiang University, Hangzhou 310027, China c Polymer Institute, College of Material Science and Engineering, South China University of Technology, Guangzhou 510640, China

a r t i c l e

i n f o

Article history: Received 27 September 2010 Received in revised form 20 November 2010 Accepted 22 November 2010 Keywords: Disused adsorbents Cu(II)-loaded chitosan hydrogel beads Phosphate adsorption Adsorption mechanism

a b s t r a c t The traditional treatment for disused adsorbents is usually recovery for recycling or direct discarding. In this paper, a more efficient and economical method is introduced to treat disused adsorbents. Chitosan hydrogel beads (CS) are proven to be effective biosorbents for the removal of many heavy metal ions from aqueous solutions. The CS after the saturated adsorption of metal ions are usually recovered in dilute acidic solutions. However, in this work, the disused CS after the removal of copper(II) (Cu(II)) were not recovered but were directly applied to adsorb phosphate from the aqueous solutions. The CS after the adsorption of Cu(II) were stable and suitable for phosphate adsorption within a wide pH range. The results of pH effects on the adsorption capacities of phosphate indicated that the maximum was achieved at around pH 5.0 near 28.86 mgP/g. In the various forms of phosphates, the Cu(II)-loaded CS preferred to adsorb dihydrogen phosphate. Further adsorption equilibrium and kinetics study showed that the adsorption behaviors were mainly ascribed to chemical adsorption. The adsorption isotherm was well described by the Langmuir equation. The adsorption kinetics followed both pseudo first-order and second-order models. The effects of electrolytes on the adsorption of phosphate indicated that the additional NaCl and Na2 SO4 both slightly decreased the adsorption capacities for the anions’ competition of adsorption active sites. © 2010 Elsevier B.V. All rights reserved.

1. Introduction With the rapid development of modern industries, water pollution has worsened day by day. In view of the characteristics of the current state of water pollution, soluble organic compounds, heavy metal ions, and non-biodegradable matter have rapidly increased in water [1,2]. Therefore, various methods including chemical precipitation, ion exchange, biological treatment, and adsorption method have been employed for the removal of these pollutants from wastewater [3]. Compared with other techniques, adsorption is an economical and efficient method for the removal of the aforementioned contaminations, particularly those from very dilute solutions [4,5]. Adsorption may probably be used for the collection and recycling of valuables from wastewater also. In this regard, many kinds of novel adsorbents have been developed and widely applied in wastewater

夽 Supported by the Key Natural Science Foundation of China (Grant Nos. 50633030 and 51073077). ∗ Corresponding author. Tel.: +86 25 83686350; fax: +86 25 83686350. E-mail address: [email protected] (H. Yang). 1385-8947/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2010.11.085

treatment. However, many disused adsorbents have been produced in companies with the rapid development of adsorbents. There are two normal methods to treat disused adsorbents: one is to recover them from dilute acidic or alkaline solutions for recycling, and the other is to discard and directly incinerate them. Based on adsorption mechanisms, the effects of adsorptions mainly depend on the molecular structures of the adsorbents and the contaminations themselves. However, after reaching adsorptive saturation, the original surface structures of the adsorbents distinctly change due to a layer of embedded contaminations whose structures are stable enough in a certain condition. These disused adsorbents can also be used as new adsorbents for their altered structures to adsorb other contaminations, which have good affinity with the previously adsorbed pollutants. Therefore, the third method to treat disused adsorbents is to use them as new adsorbents, which seems to be the most significant method among all three. Over the years, excess phosphate has drawn substantial research attention because it is believed to be the cause of eutrophication and water quality problems [6]. The potential sources of phosphate in water include detergents, pigment formulation, the electronic industry, mineral processing, and the over-application of both

J. Dai et al. / Chemical Engineering Journal 166 (2011) 970–977

synthetic and animal-based fertilizers [7]. Excessive phosphate can lead to the intense accumulation of algae, which results in the decline of aquatic life and threatened human health [5–8]. Therefore, strict discharge standards of phosphate (0.5–1.0 mgP/L) are applied to control its level in water [9]. To achieve levels below the limits of discharge standards, the adsorption method has been employed for the removal of phosphate [10]. A variety of adsorbents have been developed such as cationic metal–EDA complexes anchored inside mesoporous silica MCM-41 supports [5], the chitosan-g-poly(acrylic acid)/vermiculite ionic hybrid [11], slag [12], red mud [13], palygorskite [14], iron-based components [15], zirconium [16], coal fly ash [17], crab shells [18], lithium [19], and MgMn-layered double hydroxides [20]. Among these adsorbents, metal ions play an important role in the adsorption of phosphate because they are facile in binding with phosphate. An experiment [16] regarding the loading of Zr(IV) and Fe(III) ions onto collagen fiber was conducted, and the metal ion loaded materials showed high phosphate uptakes. In recent years, chitosan hydrogel beads (CS) have been given much attention because they are environment-friendly and lowcost biosorbents which have been proven effective for the removal of some toxic metal ions from aqueous solutions including Cu(II) and Pb(II) [21–27]. For these CS after the saturated adsorption of metal ions, the traditional method to treat them is to recover them in EDTA or acidic solutions for recycling [26–28]. However, they are also a kind of metal ions loaded adsorbents. Recently, Perez-Novo et al. [29] reported that phosphate can efficiently bind with Cu(II). Cu(II) can be effectively adsorbed and loaded onto CS from aqueous solutions. In this paper, the disused CS after the removal of Cu(II) were used for further phosphate adsorption from aqueous solutions. The fundamental adsorption behaviors of Cu(II)-loaded CS for the removal of phosphate, including the effect of pH, additional electrolytes, adsorption equilibrium, and kinetics, were investigated.

2. Experimental 2.1. Materials Chitosan, which had a 90.5% degree of deacetylation and a viscosity average molecular weight of 3.0 × 105 g/mol, was purchased from Shandong Aokang Biological Co. Ltd., China. Glutaraldehyde (GLA) solution (25%, w/w) was supplied by Sinopharm Chemical Reagent Co. Ltd., China. CuSO4 ·5H2 O, hydrochloric acid (HCl), sodium hydrate (NaOH), KH2 PO4 , NaCl, Na2 SO4, and other reagents used in this work were all A.R.-grade reagents. Distilled water was used in all experiments.

2.2. Preparation of the adsorbents: CS and Cu(II)-loaded CS (CS–Cu) 2.2.1. Preparation of CS The desired amount of chitosan powders was dissolved in 200 ml 0.5% (w/w) HCl aqueous solution. The viscous solution was left overnight for mixing homogenously before adding dropwise into a 3.0 mol/L NaOH alcohol–water solution (1/2, v/v) to prepare CS. The CS were then filtered and rinsed with distilled water until the solution pH became the same as that of the fresh distilled water. The 25 g CS were suspended into 200 ml 0.0938 mol/L GLA solutions and were left standing for 3.5 h at room temperature for cross-linking reaction under continuous stirring. Finally, the CS were washed with distilled water until the pH became neutral. The beads were stored in distilled water for further use.

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2.2.2. Preparation of CS–Cu The CS were weighed and immersed into Cu2 SO4 aqueous solution at pH 5.0 under continuous stirring at room temperature for 24 h to prepare the Cu(II)-loaded CS sample. The initial concentration of Cu(II) was 385.0 mg/L. In the Cu2 SO4 solution, no precipitation of Cu(OH)2 was observed to interfere in the adsorption process. The initial and final Cu(II) concentrations of the solutions were analyzed with ethylenediamine as a developing agent at a wavelength of 546 nm by a Vis spectrophotometer (Type 7200; Unico Corp.; China). The amount of Cu(II) loaded onto CS was then measured using the following equation: q=

(C0 − Ce )V m

(1)

where C0 and Ce (mg/L) are the initial and final solute concentrations of the solutions, V (L) is the volume of the solution, and m (g) is the dried weight of the adsorbents. 2.3. Characterization of materials The Fourier transform infrared spectra and X-ray photoelectron spectroscopy of the CS before and after adsorption of the Cu(II) ions were recorded by a Fourier transform infrared spectrometer (Type TENSOR 27; Bruker Co.; Germany) and an X-ray photoelectron spectrometer (Type K-Alpha; Thermo Scientific Corp.; America) with Al K␣ X-ray source, respectively. 2.4. Effect of pH on the adsorption of phosphate on CS–Cu Stock solution of KH2 PO4 was prepared with distilled water and further diluted to the desired concentrations for the experiments. The range of the initial pH values was 2.0–12.0, adjusted by 0.1 mol/L HCl or 0.1 mol/L NaOH solutions, respectively. The initial concentration of the phosphate aqueous solution was 100.0 mgP/L. CS–Cu were weighed and immersed into phosphate solutions at different pH values under continuous stirring at 25 ◦ C for 24 h. The initial and final phosphate concentrations of the solutions were analyzed based on the method of the reduction of molybdophosphate (GB1576-2001; China) and detected at a wavelength of 660 nm by a Vis spectrophotometer (Type 7200; Unico Corp.; China). The amount of adsorption, q (mgP/g), was also calculated using Eq. (1). The initial and final pH values of these phosphate solutions were all detected by a pH meter (Type Delta 320; Mettler Toledo Corp.; Switzerland). 2.5. Effect of additional electrolytes on the adsorption of phosphate Using NaCl or Na2 SO4 as background electrolytes, the effects of various anions on the adsorption of phosphate by CS–Cu were studied at various initial pH values from 2.0 to 12.0 at 25 ◦ C. The initial concentration of the phosphate aqueous solution was 100.0 mgP/L, and the concentration of each electrolyte was 0.1 mol/L. 2.6. Adsorption equilibrium study The adsorbent of CS–Cu was suspended in phosphate solution, with phosphate concentrations ranging from 10 to 180 mgP/L. All experiments were conducted at 25 ◦ C and around pH 5.0. The adsorption and detection procedures followed the previous method. The same analysis method as mentioned in Section 2.4 was employed to detect the initial and final phosphate concentrations of the solutions by a Vis spectrophotometer. The amount of adsorption was calculated based on Eq. (1).

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taken out at the desired time intervals to analyze the current phosphate concentration. Meanwhile, the same volume of distilled water with a pH equal to 5.0 was added into the bulk solutions to keep the volume constant. The adsorption of phosphate at time ti , q(ti ) (mgP/g), was calculated using the following equation: q(ti ) =

(C0 − Cti )V0 −

i−1 2

Cti−1 Vs

(2)

m

where C0 and Cti (mgP/L) are the initial phosphate concentration and the phosphate concentrations at time ti , respectively. V0 and Vs (L) are the volume of the phosphate solution and that of the sample solution taken out every time for phosphate concentration analysis, respectively. In this study, Vs is equal to 0.1 ml, and m (g) is the dried weight of the CS–Cu. 2.8. Reusability experiments The phosphate-loaded CS–Cu were recovered from the 0.1 mol/L NaOH aqueous solution. They were collected from the solutions by filtration, washed with distilled water, and then reused in the next cycle of adsorption experiments. The adsorption–desorption experiments were conducted for six cycles. All experiments were performed at room temperature. Fig. 1. Macroscopic image of CS–Cu.

3. Results and discussion

a

3.1. Preparation of the CS–Cu adsorbent

1157

b

1649

The biosorbent of CS was prepared according to a reported method [25,27]. The detailed adsorption behaviors of Cu(II) from the aqueous solutions by CS were described in a previous work [27]. From the macroscopic image of the Cu(II)-loaded CS as shown in Fig. 1, their average diameter was approximately 2.5 mm. The appearance of the adsorbent was spherical, and the surface color turned from primrose yellow to glaucous, which indicated that the Cu(II) ions were adsorbed on the surface. For further characterization, the FTIR and X-ray photoelectron spectra of the CS before and after the adsorption of the Cu(II) ions are shown in Figs. 2 and 3, respectively. Comparing the FTIR spectroscopy of the CS as shown in Fig. 2a with that after the adsorption of the Cu(II) ions in Fig. 2b, the characteristic peaks at around 1649 and 1064 cm−1 assigned to the –NH2 and –OH groups, respectively, both shifted to a lower wave number, and the intensities of the two peaks became stronger. The chemical bonds involving the chelating effects of the –OH and –NH2 groups with Cu(II) ions were found. Furthermore, in the typical XPS wide scan spectra as shown in Fig. 3, a new peak at the binding energy (BE) of about 935.0 eV appeared after the adsorption of the Cu(II) ions, which was attributed to the Cu 2p orbital. The new peak provided evidence of Cu(II) being adsorbed on the surface of the CS, which were in agreement with

1064

1390 1257

3421

1060

1637

3423

4000

3500

3000

2500

2000

1500

1000

500

wave number (cm-1) Fig. 2. FTIR spectra for the CS: before (a) and after (b) adsorption of Cu(II).

2.7. Adsorption kinetics The initial concentration of the phosphate solutions and the pH value were fixed at 100.0 mgP/L and 5.0, respectively. The beads were weighed and immersed into phosphate solutions under continuous stirring at 25 ◦ C. Then 0.1 ml of the sample solutions were O1s

a

O1s

b C1s

C1s

Cu2p

N1S

1400 1200 1000 800

600

400

Binding Energy (eV)

N1S

200

0

1400 1200 1000 800

600

400

200

Binding Energy (eV)

Fig. 3. Typical XPS wide scan spectra for the CS: before (a) and after (b) adsorption of Cu(II).

0

J. Dai et al. / Chemical Engineering Journal 166 (2011) 970–977

NH2

Cu2+

NH2

Cu2+

H+

+

NH3+

H2PO4-

+

+

973

Cu2+

OH-

(1) Cu2+

NH2

OH-

+

H2PO4- (2)

NH2 represents CS Scheme 1. Model of pH effect on phosphate adsorption by CS–Cu.

100

CS-Cu in pure water CS-Cu in 0.1 mol/L NaCl CS-Cu in 0.1 mol/L Na2SO4

25 99

qe (mgP/g)

2+

Percentage of desorbed Cu ( % )

30

98 30

CS in pure water

20 15 10

20 5 10 0 0 2 1

2

3

4

5

6

7

8

9

10

11

4

6

12

8

10

12

pH

pH Fig. 4. Effect of pH on Cu(II) desorption from CS–Cu at 25 ◦ C.

the FTIR findings. From the molecular structure of chitosan, there were abundant free –OH and –NH2 groups on its backbone, Cu(II) ions were facile to be chelating with these functional groups, and the saturated adsorption capacity of the Cu(II) ions was 66.55 mg/g at 25 ◦ C with a pH equal to 5.0 [27]. For further application of the Cu(II)-loaded CS in the removal of phosphate, the stability of CS–Cu in various conditions is very important. The desorption behavior of Cu(II) from CS–Cu was simultaneously investigated in solutions with different pH conditions in the presence of phosphate at 25 ◦ C. The pH effect on Cu(II) desorption from CS–Cu is shown in Fig. 4. Cu(II) ions would be released largely in a lower pH (pH < 3) for excessive protons substituting Cu(II) ions from the surface of CS, which may be the basis for recovering the Cu(II)-loaded CS for recycling in an acidic solution [27]. The detailed desorption process of Cu(II) from CS–Cu is described in Scheme 1(1). When the pH was higher than 4.0, there was very little Cu(II) desorption due to the chemical bonds involving the chelating effects of the –OH and –NH2 groups of chitosan with Cu(II) ions. This indicated that the adsorbent was stable enough and suitable for phosphate adsorption at a pH higher than 4.0.

Fig. 5. Effects of pH on phosphate adsorption by CS–Cu and CS adsorbents at 25 ◦ C under varied conditions: CS–Cu in pure water (), CS–Cu in 0.1 mol/L NaCl (), CS–Cu in 0.1 mol/L Na2 SO4 (), and CS in pure water ().

and then decreased after reaching the maximum about 28.86 mgP/g at around pH 5.0. At lower pH conditions, Cu(II) largely desorbed from the adsorbents, and CS had no effect on the adsorption of phosphate, as mentioned above. Therefore, the adsorption capacity of phosphate onto CS–Cu at lower pH values was very low. However, the adsorption capacity was dramatically decreased with a further increase in pH. This was probably due to the high concentration of the hydroxide groups within a higher pH range, competing strongly with phosphate for the active sites of CS–Cu, as described in Scheme 1(2). The pH values of the phosphate solutions before and after adsorption by CS–Cu at 25 ◦ C were also detected to illustrate the reaction mechanism in phosphate solutions at a different pH, as shown in Fig. 6. At initial pH values around 5.0, the final adsorption equilibrium pH did not change, at which the phosphate adsorption onto CS–Cu reached the maximum from the aforementioned 12

Before adsorption

10

3.2. Effect of pH on the adsorption of phosphate 8

pH

CS–Cu was applied for the removal of phosphate from the aqueous solution. The pH of the aqueous solution is a very important factor that greatly influences the adsorption of anions and cations at the solid–liquid interfaces, so pH effects on the removal of phosphate were investigated. Fig. 5 shows the adsorption capacities of phosphate by CS and CS–Cu from aqueous solutions at 25 ◦ C with the initial pH varying from 2.0 to 12.0. CS adsorbed very little phosphate, and the adsorption capacity was near zero in all measured pH ranges. After the CS were loaded by the Cu(II) ions, the adsorption capacity of phosphate extensively increased, which indicated that the Cu(II) ions on the surface of the beads played a very important role in phosphate adsorption. As the initial pH value of the solution increased, the adsorption capacity onto CS–Cu initially increased

After adsorption

6

4

2 1

2

3

4

5

6

7

8

Sample Number Fig. 6. The pH of phosphate solutions before and after phosphate adsorption by CS–Cu at 25 ◦ C.

J. Dai et al. / Chemical Engineering Journal 166 (2011) 970–977

adsorption capacity data, as shown in Fig. 5. Thermodynamic calculations revealed that phosphate existed in the solution in the forms of H3 PO4 , H2 PO4 − , HPO4 2− , and PO4 3− at different ratios depending on the pH. For a dilute solution of phosphate, the pH values of transition from the uncharged to the most charged species are 2.2, 7.2, and 12.2, respectively [30]. Therefore, one-charged species of phosphate (H2 PO4 − ) were better adsorbed onto CS–Cu. At the initial pH values of 4.0–6.0, the adsorption equilibrium pHs were all near 5.0, but some differences in the adsorption capacity were observed between pH 4.0 and pH 6.0 according to Fig. 5. From Fig. 6, at initial pH values below 5.0, the final adsorption equilibrium pHs all increased, whereas above 5.0, the final adsorption equilibrium pHs all decreased. At a low initial pH, protons competed for the active sites of –NH2 groups with Cu(II) ions, resulting in a decrease in the number of protons and a further increase in pH, as described in Scheme 1(1). The amount of loaded Cu(II) ions on the adsorbents’ surface decreased at a low pH, which was the active adsorption sites for phosphate, so the phosphate uptake decreased. However, at a high initial pH, excessive hydroxyl groups gradually substituted phosphate on the CS–Cu surface, resulting in the consumption of hydroxyl groups and the decrease of pH values, as described in Scheme 1(2). The change in pH values before and after the adsorption of phosphate was consistent with the effect of pH on the adsorption capacity of CS–Cu, as previously discussed. 3.3. Effect of additional electrolytes on the adsorption of phosphate Salts such as NaCl and Na2 SO4 were imported to study the effect of additional electrolytes on phosphate adsorption at 25 ◦ C, respectively, as shown in Fig. 5. The adsorption capacities both decreased in the presence of the two electrolytes, suggesting that the concomitant anions interfered with the phosphate adsorption process by competing for the adsorption sites. The effect of Na2 SO4 on the decrease of phosphate adsorption was more evident than that of NaCl. Furthermore, based on the experimental results shown in Fig. 5, the adsorption capacity was much higher under the conditions of pH 5.0 and 0.1 mol/L NaCl or Na2 SO4 than those under pH 10 or 11, which suggested that OH− had a higher affinity with the Cu-loaded CS than SO4 2− and Cl− . 3.4. Adsorption equilibrium study The equilibrium isotherm is fundamental in describing the interactive behavior between solutes and adsorbent. The adsorption isotherm data of phosphate onto CS–Cu at 25 ◦ C and around pH 5.0 are presented in Fig. 7. At the beginning, the adsorption capacity of phosphate linearly increased with the increase in the initial concentrations of phosphate, then reached surface saturation at high concentrations around 100 mgP/L. This indicated that at lower initial concentrations, the adsorption sites on the beads were sufficient, and the adsorption capacity relied on the amount of phosphate transported from the bulk solution to the surfaces of the beads. However, at higher initial concentrations, the adsorption sites on the surfaces of the beads reached saturation, and the adsorption of phosphate achieved equilibrium. Based on Fig. 7 and Table 1, the experimental equilibrium phosphate uptake of CS–Cu had an average of 28.86 mgP/g. The correlation of the equilibrium data using either a theoretical or empirical equation is essential for the adsorption interpretation and prediction of the extent of adsorption. Adsorption data are generally interpreted by the Langmuir, Freundlich, Sips, Dubinin–Radushkevich (D–R), and Dubinin–Radushkevich–Kaganer (D–R–K) isotherm models. The Langmuir model is based on the assumption of a structurally homogeneous adsorbent where all adsorption sites are identical

35

30

qe( mgP/g )

974

25

20

15

10 0

30

60

90

120

150

Ce( mgP/L ) Fig. 7. Adsorption isotherm of phosphate onto CS–Cu at 25 ◦ C and around pH 5.0.

and energetically equivalent. The Langmuir model is used for the fitting of a monolayer and/or chemical adsorption. It is represented as follows [31]: 1 Ce Ce = + qe qmax qmax b

(3)

where qe (mgP/g) is the amount of phosphate adsorbed at equilibrium, Ce (mgP/L) is the liquid-phase phosphate concentration at equilibrium, qmax (mgP/g) is the maximum adsorption capacity of the adsorbent, and b (L/mgP) is the Langmuir adsorption constant, respectively. The Freundlich model is applied to describe a heterogeneous system characterized by a heterogeneity factor of 1/n. This model describes reversible adsorption and is not restricted to the formation of the monolayer. The Freundlich model is expressed as follows [32]: log qe =

1 log(Ce ) + log K n

(4)

where Ce (mgP/L) is the liquid-phase phosphate concentration at equilibrium, K is the Freundlich isotherm constant, and 1/n (dimensionless) is the heterogeneity factor, respectively. The Sips isotherm model can be considered as a combination of the Langmuir and Freundlich equations and is represented as follows [33]: qe =

qm (bCe )

1/n

1 + (bCe )

1/n

(5)

where qm (mgP/g) is the amount of phosphate adsorbed at equilibrium, b (L/mg) is the median association constant, and 1/n is the heterogeneity factor. The value for 1/n  1 indicates heterogeneous adsorbent, whereas that close to or is 1.0 indicates a material with relatively homogenous binding sites. In this case, the Sips model is reduced to the Langmuir equation. The equilibrium data is also subjected to the D–R isotherm model to determine the nature of biosorption as a physical or chemical process. The D–R equation is given by the following relationship [34]: ln Qe = ln Qm − Kε2

(6)

where Qe (mgP/g) is the amount of phosphate adsorbed at equilibrium, K (mol2 /kJ2 ) is the constant related to the mean free energy of sorption, Qm (mgP/g) is the theoretical saturation capacity, and ε (kJ/mol) is the Polanyi potential (ε = RT ln(1 + (1/Ce ))). The D–R constant can give valuable information regarding the mean energy of

J. Dai et al. / Chemical Engineering Journal 166 (2011) 970–977

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Table 1 The isotherm and kinetic parameters for phosphate adsorption onto CS–Cu at 25 ◦ C and around pH 5.0. Adsorption isotherm Langmuir Freundlich Sips Dubinin–Radushkevich Dubinin–Radushkevich–Kaganer

Qm,cal (mgP/g) 30.12 K 11.78 qm (mgP/g) 32.38 Qm,cal (mgP/g) 24.84 qm (mgP/g) 27.56

b × 103 (L/mg) 95.13 K × 107 (mol2 /kJ2 ) 1.68

k1 × 103 (min−1 ) 8.24 k2 × 104 (g/(mg min)) 1.81

Second-order kinetics

adsorption (E) using the following equation: (7)

In addition, the D–R–K isotherm model is applied to fit the adsorption isotherms, which is based on the assumption of a structural monolayer adsorbent by the following equation [35]:



Es

ln

 C ∗ n  Ce

R2 0.997 R2 0.994

3.5. Adsorption kinetics

(8)

where qm (mgP/g) is the amount of phosphate adsorbed at equilibrium, n is equal to 4, C* (mgP/L) is the saturation concentration, and Es (kJ/mol) is the characteristic energy of adsorption. The Langmuir, Freundlich, Sips, D–R, and D–R–K models were all tried to fit the experimental data. Their parameters were listed in Table 1. The Langmuir equation had the best fit to the experimental data (R2 = 0.998) among the models. The maximum adsorption capacity calculated by this equation was 30.12 mgP/g, which was quite close to that actually determined at around 28.86 mgP/g. This indicated that the chemical adsorption mechanism might be involved in the adsorption process of phosphate for Cu(II) ions binding with phosphate. Phosphate was adsorbed in the form of a monolayer coverage on the surface of the adsorbents. According to the analysis of the Sips model in Table 1, the calculated values for 1/n were relatively close to 1, which indicated that the Cu(II)-loaded chitosan were homogeneous adsorbents. Furthermore, according to the analysis from the D–R model, the mean adsorption energy (E) can be primarily deduced as shown in Table 1. The E value was far higher than 8.0 kJ/mol, although R2 was less than 0.9, which indicated that the adsorption behavior of the CS–Cu was chemical adsorption [36]. From Table 1, the D–R–K model showed better fitness than the D–R model, which further confirmed the analysis of the Langmuir model that the adsorption mechanism followed monolayer adsorption. For comparison, the phosphate uptakes by other reported adsorbents are summarized in Table 2. Most of the maximum phosphate uptakes by various adsorbents were at a pH near 5.0, which were consistent with CS–Cu, and indicated that most adsorbents preferred to adsorb dihydrogen phosphate. In addition, the phosphate uptake of CS–Cu was only less than that of LIG [19], calcined Mg–AlLDHs [42], thermal-activated natural palygorskite [13], and crab shells [18] but higher than that of most other adsorbents. Moreover, CS–Cu was still a kind of disused adsorbents. These results all indicated that CS–Cu was a good adsorbent for the removal of phosphate from aqueous solutions.

Adsorption kinetics was measured to establish the time course of phosphate uptake on the beads. Examining whether the adsorption behavior of phosphate can be described by a predictive theoretical model was also desirable. The typical experimental results of adsorbed phosphate on CS–Cu versus time are shown in Fig. 8. The adsorption of phosphate onto CS–Cu was found to attain equilibrium within approximately 400 min at 25 ◦ C and around pH 5.0, which was acceptable for application. To investigate the adsorption mechanisms, pseudo first-order and second-order models were applied to study the experimental data. The pseudo first-order and second-order models were given as Eqs. (5) and (6), respectively [45]: log(qe − qt ) = log qe −

k1 t 2.303

(9)

1 t t = + qt qe k2 q2e

(10)

where qe and qt (mgP/g) are the amounts of phosphate adsorbed onto the adsorbents at equilibrium and at time t, respectively. k1 (min−1 ), and k2 (g/(mg min)) is the rate constant of first-order and second-order adsorption, respectively. The pseudo first-order and second-order models were used to fit the experimental data, and their parameters are listed in Table 1. The correlation coefficient R2 for the pseudo first-order and secondorder models had extremely high values at around 0.997 and 0.994, respectively. These showed that both models can describe the adsorption kinetics of phosphate onto CS–Cu, which indicated that 35 30 25

qt(mgP/g)

E = (2K)−1/2

  RT

1/n 0.89 E (kJ/mol) 17.2 Es (kJ/mol) 4.96

Adsorption kinetics First-order kinetics

qe = qm exp

R2 0.998 R2 0.961 R2 0.960 R2 0.752 R2 0.872

b (L/mgP) 0.17 n 5.11

20 15 10 5 0 0

100

200

300

400

500

600

700

t (min) Fig. 8. Adsorption kinetics of phosphate onto CS–Cu at 25 ◦ C and around pH 5.0.

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Table 2 Phosphate uptakes onto various adsorbents. Adsorbent

pH

Initial concentration of phosphate (mgP/L)

Phosphate adsorption capacity (mgP/g)

Data source

Fe-EDA-SAMMS Steel slag Red mud Thermal activated natural palygorskite Fe(III)/Cr(III) hydroxide Ca-ZFA Crab shells LIG MgMn-3-300 Montmorillonite Kaolinite Illite Al-PILC Al-PILC La(III)-chelex resin La(III)-modified zeolite Calcined Mg–Al-LDHs Dry iron oxide tailings LaAl-PILC Zr-ICF Fe-ICF

5.0 5.5 5.5 No data 4.0 6.65–9.54 6.5 4.5 8.5 7.6 7.0–8.0 7.5 3.0–4.0 5.5 5.5 6.0 6.0 3.5 5.0 3.0–6.0 3.0–6.0

1.14 22.79 No data 1000 No data 50 667 No data 0.30 3.5–25 3.4–40 10–30 99.2 No data 155 No data 50 10 No data No data No data

14.26 5.30 0.58 42.00 2.19 19.11 36.07 93.00 7.30 0.75 0.09 2.51 20.46–26.97 7.10 3.04 24.60 44.00 8.00 13.02 28.50 26.10

[5] [12] [13] [14] [15] [17] [18] [19] [20] [37] [37] [37] [38] [39] [40] [41] [42] [43] [44] [16] [16]

Table 3 Adsorption and desorption (recovery) behaviors of phosphate onto CS–Cu. Cycle

Cycle I Cycle II Cycle III Cycle IV Cycle V Cycle VI

CS–Cu Adsorption amount (mg/g)

Recovery (%)

28.86 28.43 28.06 27.86 27.49 26.97

98.98 98.83 98.97 98.89 98.79 98.82

physical and chemical adsorption may both be involved in the adsorption process. 3.6. Reusability study For practical application, the adsorption and desorption processes were repeated to examine the potential application of the CS–Cu for recycling. First, based on the pH dependence of the stability of the phosphate-loaded CS–Cu as shown in Fig. 5, phosphate was released almost fully at a pH higher than 11.0. Furthermore, according to Fig. 4, the CS–Cu were stable enough at alkaline conditions, and no Cu(II) ion was desorbed. Therefore, the phosphate desorption process was carried out in a 0.1 mol/L NaOH aqueous solution. Table 3 showed the experimental results on the amounts of phosphate adsorbed and the percentages of desorption in six consecutive adsorption–desorption cycles. The desorption efficiency was generally high, and the adsorption capacity was almost unaffected. Due to their high recycling efficiency, the beads thus qualified for practical application. 4. Conclusions In this paper, a new and efficient method for the treatment of disused adsorbents was provided. The disused adsorbents can be used as new ones for an altered surface structure. CS after the removal of Cu(II) ions can be applied to adsorb phosphate directly from the aqueous solutions. CS–Cu was proven to be stable enough and suitable to adsorb phosphate at a pH above 4.0. The maximum adsorption capacity of phosphate was achieved at around pH 5.0, and dihydrogen phosphate (H2 PO4 − ) was favored in the adsorption onto CS–Cu from the aqueous solution. The effect of additional

electrolytes on adsorption capacity indicated that the concomitant anions interfered with the phosphate adsorption process by competing for the adsorption sites. Adsorption equilibrium and kinetics study indicated that the adsorption behavior was mainly chemical monolayer adsorption for phosphate facile to bind with Cu(II). The reusability study showed that phosphate can be desorbed easily and effectively at strong alkaline conditions, and the regenerated adsorbent can be reused almost without any loss of adsorption capacity for a few cycles.

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