Adsorption of cobalt(II) ions from aqueous solutions by palygorskite

Applied Clay Science 54 (2011) 292–296

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Adsorption of cobalt(II) ions from aqueous solutions by palygorskite Miaoying He a, 1, Yi Zhu a,⁎, 1, Yang Yang b, Boping Han b, Yuanming Zhang a,⁎ a b

Department of Chemistry, Jinan University, Guangzhou, 510632, PR China Institute of Hydrobiology, Jinan University, Guangzhou, 510632, PR China

a r t i c l e

i n f o

Article history: Received 18 December 2010 Received in revised form 28 September 2011 Accepted 30 September 2011 Available online 26 October 2011 Keywords: Adsorption Cobalt Palygorskite Desorption

a b s t r a c t A study on the removal of cobalt ions from aqueous solutions by palygorskite was conducted under batch conditions. The effect of time, pH of the dispersion, ionic strength, temperature and initial metal concentration on the adsorption of Co 2+ onto palygorskite was investigated. The adsorption of Co2+ was a fast process that followed the pseudo-second-order kinetics. This process could be described by the Langmuir model and gave a maximum Co 2+ adsorption capacity of 8.88 mg/g at 35 °C. The adsorption was dependent on pH and was influenced by the ionic strength. The adsorption mechanism primarily involved inner-sphere complexation, with minor contributions of the exchange of structural Mg2+ ions. Palygorskite could be completely regenerated with Na2CO3 and continued to exhibit excellent Co 2+ adsorption even after three consecutive cycles. © 2011 Elsevier B.V. All rights reserved.

1. Introduction

2. Experimental

Palygorskite is a hydrated magnesium aluminium silicate present in nature with a fibrous morphology. There are large reserves in South China (Wu et al., 2007). In recent years, palygorskite was intensively investigated as an adsorbent for removing heavy metal ions from wastewater. In spite of similar and high adsorption capacities of palygorskite for various heavy metal ions, the adsorption mechanisms were observed to be quite different. For example, during the adsorption of Pb 2+, exchange reactions dominated at low pH, and surface complexation reactions dominated at neutral or alkaline conditions (Fan et al., 2009a). However, cation exchange was the primary mechanism for the removal of Cd2+ (Álvarez-Ayuso and GarcíaSańchez, 2007). Cobalt is a relatively rare element that is essential for human health since it is a part of vitamin B12 (Smićiklas et al., 2006). However, higher concentrations of Co 2+ are detrimental to human health, resulting in paralysis, diarrhoea, lung irritations and bone defects (Kudesia, 1990). Hence, a large variety of materials were explored to remove Co 2+ from solution: vermiculite (da Fonseca et al., 2005), zeolites (Erdem et al., 2004), sepiolite (Kara et al., 2003) and kaolinite (Yavuz et al., 2003). To the best of our knowledge, there has not been much study on the adsorption behaviour and mechanism of Co 2+ on palygorskite. In this study, the application of palygorskite for the removal of Co 2+ from aqueous solutions was investigated.

2.1. Materials

⁎ Corresponding authors. Tel.: + 86 20 85225036; fax: + 86 20 85226262. E-mail addresses: [email protected] (Y. Zhu), [email protected] (Y. Zhang). 1 Both authors contributed equally to this work. 0169-1317/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2011.09.013

All chemicals were of analytical grade and used without further purification. The sample of palygorskite was obtained from the Minghui Mining Co. (Anhui, China) and used without any pretreatment. The chemical composition of the palygorskite was determined by the PANalytical Axios (PW4400) XRF Spectrometer (Chen and Wang, 2007) as follows: SiO2 54.47%, Al2O3 8.61%, Fe2O3 4.42%, MgO 10.28%, K2O 1.07% and CaO 1.00%. The specific surface area of the sample was 143 m 2/g, which was measured via the Micromertics TriStar 3000 Analyzer using the Brunauer–Emmett–Teller (BET) method (Zhang et al., 2011). SEM images, taken on a JEOL JSM6330F scanning electron microscopy, showed that the size of the powdered samples was about 10 μm. The cation exchange capacity (CEC) of the sample was 8.2 meq/100 g, which was determined by the copper ethylediamine complexation method (Bergaya and Vayer, 1997). A cobalt stock solution was prepared by dissolving Co(NO3)2·10H2O in ultrapure water. 2.2. Adsorption and desorption procedures The dispersions of palygorskite were mixed with NaNO3 and cobalt stock solutions to achieve the desired concentrations of the different components. The pH of the solutions was adjusted by adding negligible volumes of 0.1 or 0.01 M HNO3 or NaOH. After the dispersions were shaken for 24 h in a thermostatic shaker bath at the desired temperatures, the solid and liquid phases were separated by filtration. The collected filtrates were used to determine Co2+ and Mg2+ concentrations by inductively coupled plasma atomic emission (ICP-AES)

M. He et al. / Applied Clay Science 54 (2011) 292–296

spectrophotometry. The detection limits for Co2+ and Mg2+ were 0.001 mg/L and 0.003 mg/L respectively. The relative standard deviation of every test result was less than 3%. For the blank experiment to determine the amount of Mg 2+ due to palygorskite dissolution, palygorskite was added into the deionised water without Co 2+. Mg 2+ was released into solution due to dissolution. When the release of Mg 2+ reached equilibrium, the amount of released Mg 2+ due to dissolution was measured. For the desorption experiments, palygorskite was added to seven conical flasks with the solid–liquid ratio of 5 g/L, which contained 100 mL 30 mol/L, 35 mol/L, 40 mol/L, 45 mol/L, 50 mol/L, 55 mol/L and 60 mol/L of Co 2+ at the desired initial pH. The desired initial pH was adjusted by adding negligible volumes of 0.1 or 0.01 M HNO3 or NaOH. Then conical flasks were shaken for 24 h in a thermostatic shaker bath at 35 °C to reach adsorption equilibrium. Half of the supernatant was pipetted out, and an equal volume of electrolyte solution NaNO3 with the same pH was added. The dispersion was then shaken for 24 h to reach desorption equilibrium and filtered under the same conditions as in the adsorption experiments.

2.3. Regeneration The Co2+-loaded palygorskites which underwent adsorption were left in contact with 0.01 mol/L solutions of inorganic salts (CaCl2, NaCl, NaNO3), acids (H2SO4, HNO3, HAc, HCl), alkalies (NaHCO3, NaOH, Na2CO3) and EDTA in a solid–liquid ratio of 8 g/L. After being shaken for 1.5 h, the sample was separated from the dispersion by filtration, washed with distilled water three times and dried for reuse.

(Ho and Mckay, 1999) qt ¼ ki t

The adsorption of Co2+ onto palygorskite was fast, and equilibrium was achieved after only 3 h (Fig. S1). Generally, ion exchange or physical adsorption of metal ions onto solid particles is slow compared to chemisorption or inner-sphere complexation. The rapid adsorption and high adsorption capacity indicated that chemisorption or innersphere complexation primarily contributed to the adsorption of Co2+ onto palygorskite (Wang et al., 2005a, 2005b, 2007). Three different kinetic models were used to fit the experimental data (Table 1): pseudo-first-order (Eq. (1)), pseudo-second-order (Eq. (2)) and intraparticle diffusion (Eq. (3)). qt and qe are the adsorption capacities at time t and equilibrium, while k1, k2 and ki are the rate constants, and C is a constant. logðqe −qt Þ ¼ logqe −

k1 t 2:303

ð1Þ

(Lagergren, 1898)

t 1 1 ¼ þ qt k2 qe 2 qe

ð2Þ

1=2

þC

ð3Þ

(Weber and Morriss, 1963) A good correlation coefficient r 2 was obtained for the pseudosecond-order kinetic model, indicating that the adsorption rate of Co 2+ depended on the concentration of ions on the adsorbent surface. The observed behaviour over the entire adsorption range also indicated that chemical adsorption was the rate-controlling step (Baskaralingam et al., 2006; Guibal et al., 1998). 3.2. Adsorption isotherms The adsorption of Co2+ onto palygorskite at different temperatures was determined as a function of equilibrium Co2+ concentration Ce (Fig. S2). Langmuir (Eq. (4)) and Freundlich (Eq. (5)) models (Seader and Herley, 1998) were used to evaluate the experimental data. The corresponding parameters (i.e., maximum adsorption capacity qm, Langmuir isotherm constant b, Freundlich isotherm constants Kf and n) and correlation coefficients are listed in Table 2. Comparison of the r 2 values suggested that the Langmuir model yielded a closer fit to the data. This may be due to the homogeneous distribution of active sites on the palygorskite surface because the Langmuir adsorption isotherm assumed that adsorption occurred at specific, homogeneous sites on the adsorbent (Bekta et al., 2004). Ce 1 C ¼ þ e qe bqm qm

ð4Þ

logqe ¼ logKf þ n logCe

ð5Þ

3. Results and discussion 3.1. Kinetics studies

293

Values of qm were estimated to be 8.88 and 9.02 mg/g at 35 °C and 55 °C. The fact that the adsorption of Co 2+ was enhanced at higher temperatures, suggested that the adsorption was endothermic. 3.3. Effects of pH and ionic strength The dependence of the Co 2+ adsorption on the ionic strength was studied using the initial pH (pHi) range 2–12 (Fig. S3). The initial pH represented the pH of the dispersion when palygorskite was just added. The adsorption of Co 2+ was dependent on ionic strength at pHi b 8.5 and independent of the ionic strength at pHi > 8.5 (Fig. S3). As well known, ion exchange is influenced by the ionic strength, whereas the inner-sphere complexation is not affected. Hence, it was concluded that ion exchange contributed to the adsorption process at pHi b 8.5. The equilibrium pH (pHf), which represented the pH of the dispersion when adsorption equilibrium was reached after palygorskite was added into the dispersion and kept for 24 h, reflected the interaction between adsorbent and adsorbate (Chen and Wang, 2007). The relationship between pHf and pHi was plotted (Fig. S4). As was mentioned in the literature, the solution was buffered to its pHf value after the reaction with palygorskite. Specific adsorption would decrease the pHf of the dispersion (Chen et al., 2007; Lazarević et al., 2007; Smićiklas et al., 2006). The adsorption of Co2+ onto palygorskite decreased the pHf

Table 1 Pseudo-first-order, pseudo-second-order and intraparticle rate constants. Adsorbent

Pseudo-first-order qe (mg g

Attapulgite

2.50

Pseudo-second-order k1× 10

−1

)

(h

4

r

2

−1

9.21

)

qe (mg g

0.800

4.35

Intraparticle diffusion

k2 −1

)

(g mg 0.016

r −1

h

2

−1

)

ki (mg g

0.998

0.031

r2

C −1

h

−1/2

)

(mg g 3.62

−1

) 0.862

294

M. He et al. / Applied Clay Science 54 (2011) 292–296

Table 2 Langmuir and Freundlich constants and correlation coefficients for the adsorption isotherms of Co2+ onto palygorskite. Temperature

Langmuir equation

35 °C 55 °C

Freundlich equation

qm (mg g−1)

b (L mg−1)

r2

Kf (L g−1)

n

r2

8.88 9.02

0.044 0.26

0.993 0.999

2.11 5.84

3.94 11.85

0.952 0.896

from 7.8 to 7.3, suggesting that specific adsorption occurred and innersphere complexation was involved (Eqs. (6), (7)) (Chen et al., 2007): 2þ

þ

þ

S\OH þ Co ↔S\OCo þ H ;

2þ

ð6Þ

þ

2S\OH þ Co ↔ð2S\OÞ2 Co þ 2H ;

ð7Þ

where S represents Si, Al and Mg. Adsorption increased relatively fast within the pHi range of 2.5–4.5 because fewer H+ ions were competing with Co2+ as pHi increased (Chen et al., 2007). However, adsorption reached a plateau between pHi 4.5 and 8.5 as the palygorskite surface buffered the dispersions to the same final pHf value of 7.3 (Fig. S3) (Smićiklas et al., 2006). Additionally, the distribution of ionic cobalt species as a function of pH, calculated by visual MINTEQ software (Fig. S5), showed that Co(OH)2 was formed at pH 8 and becomes the dominant species as pH increases up to 12, which led to the sharp increase at pH> 8.5. The palygorskite surface, which was negatively charged at pH> 6.04 (Fan et al., 2009b), may additionally contribute to the overall adsorption process through the adsorption of positively charged hydrolyzed species such as, Co(OH)+ and Co4(OH)44+.

Mg 2+ at the edges of octahedral sheets. The amount of Mg 2+ due to palygorskite dissolution was determined in the blank experiment. By subtracting the concentration of released Mg 2+ due to dissolution from the total concentration of released Mg 2+ over the pH range of 4.0–8.0, the concentration of Mg 2+ due to ion exchange and replacement was calculated to be approximately 3.5 mg/L (Fig. 1). The CEC indicated that the concentration of Mg 2+ due to the ion exchange was 2.1 mg/L. The remaining 1.4 mg/L Mg 2+ in solution were, therefore, caused by the replacement of structural Mg 2+, providing the evidence that the replacement of structural Mg 2+ also contributed to the adsorption process. 3.5. Role of inner-sphere complexation Since two monovalent cations or one divalent cation in the channels could potentially be exchanged by one Co 2+ ion, and one structural Mg 2+ ion at the edges of the octahedral sheets could be replaced with one Co 2+ ion due to the similar radius of the hydrated ions, the increment of two monovalent cations or one divalent cation in solution corresponds to the adsorption of one Co 2+ cation onto palygorskite. Hence, the adsorption of the Co 2+ ions due to the ion exchange and substitution can be calculated from the increment of the ion (i.e., Na +, K +, Ca 2+ and Mg 2+) concentration before and after adsorption. The amounts of Co 2+ inner-sphere complexes can then be calculated by the difference to the total amounts adsorbed. It was obvious that the inner-sphere complexation contributed significantly to the Co 2+ adsorption, and was enhanced as pH increased (Fig. 2). 3.6. Desorption isotherms

Considering the suggestion that replacement of structural Mg 2+ at the edges of the octahedral sheets was a mechanism involved in heavy metal adsorption on palygorskite (Shirvani et al., 2006), we investigated the role that replacement of structural Mg 2+ played in the adsorption process. Hydrated Mg ions have a radius very similar to that of Co (rhMg: 4.28 Å; rhCo: 4.23 Å, Nightingale, 1959), making feasible its substitution at the edges of the octahedral sheets of palygorskite. The concentration of Mg 2+ ions released into the solution is related to the dissolution of palygorskite particles, the ion exchange of Mg 2+ in the channels and the replacement of structural

Cations adsorbed by ion exchange would be more easily be desorbed than those bound by inner-sphere complexation and substitution (Appel and Ma, 2002; Ma et al., 1993; McBride, 2000). Hence, desorption reflects the stability and irreversibility of the adsorption process and provided clues to its mechanism (Shirvani et al., 2006). Desorption isotherms were lying above the adsorption isotherms at pH 2.6, 4.0, 6.0 and 8.5 (Fig. 3), indicating an adsorption–desorption hysteresis. The k dðdesorbÞ (average distribution coefficient for adsorption) values were almost two times the k dðadsorbÞ (average distribution coefficient for desorption) values, and HC (adsorption– desorption hysteresis coefficient) values were close to 50 (Table 3). This indicated that the adsorbed Co 2+ ions were difficult to be displaced by other cations, and palygorskite can be used as an effective adsorbent for Co 2+ over a wide pH range. We also conclude that

Fig. 1. Concentrations of Mg2+ in solution caused by the different adsorption mechanisms.

Fig. 2. Co2+ ions adsorbed by ion exchange, substitution and inner-sphere complexation.

3.4. Role of structural Mg 2+ exchange

M. He et al. / Applied Clay Science 54 (2011) 292–296

295

Fig. 3. Adsorption (■) and first (●) and second (▲) desorption isotherms of Co2+ onto palygorskite at pH 2.6, 4.0, 6.0 and 8.5 (T = 35 ± 1 °C, m/v = 5 g/L, I = 0.01 M NaNO3, t = 24 h).

chemisorption rather than physical adsorption contributed to the Co2 + adsorption, and inner-sphere complexation and structural Mg2+ substitution predominated over outer-sphere complexation (Fan et al., 2008). Furthermore, HC values at pH 4.0, 6.0 and 8.5 were higher than at pH 2.0 because inner-sphere complexation was strengthened with increasing pH. 3.7. Regeneration Compared with inorganic salts, acids and EDTA, alkalies were more effective in regenerating palygorskite. After alkali regeneration, the adsorption was close to that of unused palygorskite (Fig. S6). Na2CO3 was the most effective, yielding adsorption capacities of the regenerated palygorskite over three consecutive cycles of 101%, 87% and 85% compared to unused palygorskite. Inorganic salts were also effective, while low adsorption capacities were obtained with acids (Fig. S6). 4. Conclusions Palygorskite was an effective adsorbent for removing Co 2+ from aqueous solution. The adsorption process was fast, with a maximum Co 2+ adsorption of 8.88 mg/g at 35 °C. Only minimal amounts of adsorbed Co 2+ ions were desorbed in aqueous solution. The adsorption of the Co 2+ ions was attributed to inner-sphere complexation, replacement of structural Mg 2+ (substitution) and ion exchange. Inner-sphere complexation was the primary adsorption process was strengthened with increasing pH. Complete regeneration of Co 2+Table 3 Values of k d and HC for the Co2+ adsorption onto palygorskite. pH

2.6 4.0 6.0 8.5 a

HC% ¼

k dðadsorbÞ

k dðdesorb1Þ

k dðdesorb2Þ

HC(desorb1)a

HC(desorb2)a

(L/g)

(L/g)

(L/g)

(%)

(%)

0.019 0.036 0.030 0.041

0.033 0.079 0.068 0.092

0.064 0.177 0.144 0.212

42.4 54.4 55.9 55.4

48.4 55.4 52.8 56.6

k dðdesorbÞ −k dðadsorbÞ k dðdesorbÞ

 100%:

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