Hexavalent chromium removal from aqueous solution by adsorption on aluminum magnesium mixed hydroxide

Hexavalent chromium removal from aqueous solution by adsorption on aluminum magnesium mixed hydroxide

water research 43 (2009) 3067–3075 Available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/watres Hexavalent chromium removal ...

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water research 43 (2009) 3067–3075

Available at www.sciencedirect.com

journal homepage: www.elsevier.com/locate/watres

Hexavalent chromium removal from aqueous solution by adsorption on aluminum magnesium mixed hydroxide Yujiang Lia,*, Baoyu Gaoa,**, Tao Wub, Dejun Sunb, Xia Lia, Biao Wanga, Fengjuan Lua a

School of Environmental Science & Engineering, Shandong University, Jinan 250100, PR China Key Laboratory of Colloid & Interface Science of Education Ministry, Shandong University, Jinan 250100, PR China

b

article info

abstract

Article history:

A series of sols consisting of aluminum magnesium mixed hydroxide (AMH) nanoparticles

Received 29 December 2008

with various Mg/Al molar ratios were prepared by coprecipitation. The use of AMH as

Received in revised form

adsorbent to remove Cr(VI) from aqueous solution was investigated. Adsorption experi-

3 April 2009

ments were carried out as a function of the Mg/Al molar ratio, pH, contact time, concen-

Accepted 7 April 2009

tration of Cr(VI) and temperature. It was found that AMH with Mg/Al molar ratio 3 has the

Published online 17 April 2009

largest adsorption efficiency due to the smallest average particle diameter and the highest zeta potential; AMH was particularly effective for the Cr(VI) removal in a pH range from

Keywords:

acid to slightly alkaline, even though the most effective pH range was between 2.5 and 5.0.

Adsorption

The adsorption of Cr(VI) on AMH reached equilibrium within 150 min. The saturated

Kinetics

adsorption capacities of AMH for Cr(VI) were 105.3–112.0 mg/g at 20–40  C. The interaction

Hexavalent chromium

between the surface sites of AMH and the Cr(VI) ions may be a combination of both anion

Aluminum magnesium mixed

exchange and surface complexation. The pseudo-second-order model best described the

hydroxide

adsorption kinetics of Cr(VI) onto AMH. The results showed that AMH can be used as a new adsorbent for Cr(VI) removal which has higher adsorption capacity and faster adsorption rate at pH values close to that at which pollutants are usually found in the environment. ª 2009 Elsevier Ltd. All rights reserved.

1.

Introduction

Hexavalent chromium, Cr(VI), is present in the effluents of electroplating, leather tanning, chromite beneficiation, fertilizer and several other industries (Terry, 2004). It is highly toxic and harmful to living organisms due to its carcinogenic and mutagenic properties. According to the Chinese standard, the permissible limit of Cr(VI) for industrial effluents to be discharged to surface water is 0.05 mg/l. The Cr(VI) species may be in the form of dichromate (Cr2O2 7 ), hydrogen chromate 2 (HCrO 4 ), or chromate (CrO4 ) in solutions of different pH

values (Weckhuysen et al., 1996). Due to the repulsive electrostatic interactions, these Cr(VI) anion species are generally poorly adsorbed by the negatively charged soil particles in the environment and, hence, they can transfer freely in the aqueous environments. It is, therefore, worthwhile to study the removal of chromium from industrial effluents and aqueous environmental systems. There are various methods to treat Cr(VI) contaminated water, such as electrochemical precipitation, ion exchange, adsorption, coprecipitation and membrane filtration. Among all those processes, adsorption is one of the more popular and

* Corresponding author. Tel./fax: þ86 531 88363358. ** Corresponding author. Tel./fax: þ86 531 88364832. E-mail addresses: [email protected] (Y. Li), [email protected] (B. Gao), [email protected] (T. Wu), [email protected] (D. Sun), [email protected] (X. Li), [email protected] (B. Wang), [email protected] (F. Lu). 0043-1354/$ – see front matter ª 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2009.04.008

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economically feasible alternatives (Lazaridis and Asouhidou, 2003). Although nowadays activated carbon is the best well known and most efficient adsorbent, its high cost restricts its comprehensive use. In recent years, research into different adsorbents, such as clay minerals, metal oxides, organic polymers, etc., has grown (Zhao et al., 1998; Deng and Bai, 2004; Mor et al., 2007). Among the new adsorbents, the layered double hydroxides (LDHs) and their calcined products have received deserved interest, due to their large ionic exchange capacities (Carriazo et al., 2007). The layered double hydroxides (LDHs), also called hydrotalcite-like compounds (HLCs), with the general formula [Mg2þ1xAl3þx(OH)2]xþ[Anx/n$mH2O]x, where An is an n-valent anion, are important innovative materials for the adsorption of contaminants from aqueous solutions and wastewater (Goswamee et al., 1998). These layered materials contain positively charged metal hydroxide sheets and require anions   (An) such as CO2 3 , Cl or NO3 , and water molecules, which are present on the surface and/or in the interlayer spaces to maintain an overall neutral charge. The surface and interlayer anions and water molecules can be exchanged with other inorganic and organic anions from contaminated waters (Constantino and Pinnavaia, 1995; Orthman et al., 2003). The carbonate-containing LDHs can be transformed into the mixed oxide type undergoing dehydroxylation and decarbonation by calcinatoion, which increase their exchange capacity and surface area. The calcined LDHs can rehydrate and incorporate anions in order to rebuild the hydrotalcite structure. Therefore, LDHs as well as their calcined products have potential use as adsorbents for removal of toxic anions. So far, most tests of Cr(VI) adsorption in which LDHs are involved have been performed using the carbonate-containing LDHs and their calcined products. Terry (2004) reported that hydrotalcite ion exchange could reduce aqueous Cr(VI) concentration, and the process is highly pH dependent, only yielding significant removals at pH range between 2.0 and 2.1. But the Cr(VI) adsorption capacity of carbonate-containing LDHs is low compared to other adsorbents, because LDHs have a special affinity for carbonate ions (Prasanna et al., 2006; Goh et al., 2008). Calcining carbonate-containing LDHs can enhance their adsorption efficiency for anions in aqueous solution (Lazaridis and Asouhidou, 2003). But large amount of energy is consumed during the calcining process. Researchers have reported that the largest adsorption capacity for Cr(VI) is achieved with samples calcined at 450  C (Goswamee et al., 1998; Lazaridis et al., 2004). Significant efficiency has been also achieved for Cr(VI) separation from tannery wastes and

finishing wastewaters using calcined hydrotalcite (MartinezGallegos et al., 2004; Alvarez-Ayuso and Nugteren, 2005). Generally, Cl and NO3-containing LDHs can be used for Cr(VI) adsorption (Prasanna et al., 2006). However, NO 3 ions which are released into water during the adsorption process are considered as environmental hazard. Recently, Carriazo et al. (2007) reported that chloride Cl-containing LDHs (NaOH is used as the precipitating agent in the synthesis) are better adsorbents than calcined carbonate-containing LDHs, but the time required to reach adsorption equilibrium is very long except under ultrasound. Therefore, it is important to develop new system with similar structure for Cr(VI) removal which have higher adsorption capacity, faster adsorption rate and can be used as adsorbents at pH values of actual environment of existing pollutants. When An is Cl and ammonia is used as precipitating agent in the synthesis, the product is a sol of aluminum magnesium mixed hydroxide (Han et al., 1996) (essentially a sol solution consists of aluminum magnesium layered double hydroxide in which the surface and interlayer anions are chloride anions, denoted hereafter as AMH). Not only do AMH sols contain chloride anions which can be easily replaced by other anions, but AMH colloidal nanoparticles have a small average particle diameter and can diffuse easily in water solution, by which the adsorption efficiency of other anions is enhanced. Therefore, AMH has attracted considerable attention in different areas due to its unique structure and anion exchange ability. However, research on adsorption of hexavalent chromium anions in aqueous solution by AMH is limited. In this study, AMH was synthesized and characterized in relation to its physicochemical structure. The aim of the present work is to examine the possibility of using AMH as an adsorbent toward the removal of hexavalent chromium from aqueous solution.

2.

Materials and methods

2.1.

Materials

The AMH sol used was synthesized by coprecipitation. A mixed aqueous solution of AlCl3 and MgCl2 was prepared with a definite Mg/Al molar ratio (total cation concentration of 0.5 mol/L), and then diluted ammonia (5:1 (v/v)) was slowly pumped into the mixed solution with stirring. Table 1 lists the molar ratios of the samples prepared. The final pH (about 9.5) of the suspension was adjusted with ammonia. The

Table 1 – Effect of the Al/Mg molar ratio on properties of the AMH products and their adsorption capability. Sample

Mg/Al molar ratio

Zeta potential (mV)

Average particle diameter (nm)

Appearance

Chromium removal efficiency (%)a

AMH0.5 AMH1.0 AMH2.0 AMH3.0 AMH4.0

0.5 1.0 2.0 3.0 4.0

þ41 þ43 þ47 þ48 þ46

– 367.1 78.9 68.6 73.7

Gel Sol Sol Sol Sol

78.6 90.2 97.0 98.4 98.2

a The initial Cr(VI) concentration 100 mg/L, the initial pH of the Cr(VI) solution 4.0 and the AMH dosage at 2 g/L.

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2.2.

Characterization of AMH

AMH samples were characterized by XRD. The X-ray diffraction patterns were recorded on an X-ray diffractometer (D/ max rA model, Hitachi), using CuK radiation. Morphological characterization of AMH was carried out using a Scanning Probe Microscope (SPM) (NanoScope IIIA model, Digital Instruments). The average particle diameter of AMH was measured with a Zetasizer (3000 model, Marvern). The zeta potential of AMH particles was measured by a micro-electrophoretic mobility detector (DXD-II model, Jiangsu Optical Industrial Co.).

2.3.

Adsorption methods

A stock solution of hexavalent chromium (1000 mg/L) was prepared in deionized double distilled water using potassium dichromate. All working solutions of varying concentrations were obtained by successive dilution. The pH of the solution was adjusted to the required value using HNO3 and NaOH. Adsorption experiments for AMH were undertaken by a batch equilibrium technique. The adsorption of chromium was performed by shaking a predetermined amount of AMH in a 50 ml synthetic chromium solution (with known initial chromium concentration and pH) at 200 rpm in a Jintan SHZ82 type thermostated shaker at different temperatures. After a given contact time for adsorption, the solid material was separated by centrifugation (using a LG10-2.4A centrifuge at 7000 rpm). The residual chromium concentration in the solution was determined spectrophotometrically at 540 nm using a UV–vis spectrophotometer (UV-1601, Shimadzu), following the 1,5-diphenylcarbazide method (APHA, 1998). The adsorption capacity, qt (mg/g), was determined from the difference between the initial chromate concentration (C0) and the concentration at time t or at equilibrium (Ct) per gram of solid adsorbent: qt ¼ (C0  Ct) V/W, where W is the mass of adsorbent (g). The percent chromium removal efficiency, R, was calculated by the following equation: R(%) ¼ (C0  Ct)/ C0  100%.

110  C are shown in Fig. 1. As compared with the diffraction patterns of hydrotalcite ASTM 22-700 and aluminum hydroxide ASTM 12-460, when Mg/Al  2.0 the XRD patterns of the samples (products AMH2.0, AMH3.0 and AMH4.0) show typical sharp and symmetric peaks which are characteristic of hydrotalcite-like compounds and the material consists of only one phase. Therefore the synthesized AMH has a well-crystallized hydrotalcite-type structure. But when Mg/Al  1, as in samples AMH0.5 and AMH1.0, the XRD results show not only the peaks of hydrotalcite-like compounds, but also the typical Al(OH)3 peaks (a and b in Fig. 1) which indicate that products AMH0.5 and AMH1.0 were mixtures of AMH and Al(OH)3. The effect of the Mg/Al ratio on the properties of the products and their Cr(VI) removal results is shown in Table 1. All products except product AMH0.5 are sols. The average particle diameter of the AMH decreases and the zeta potential increases as the Mg/Al molar ratio increases from 0.5 to 3.0. When the Mg/Al molar ratio further increases, the average particle diameter of the AMH increases, but the zeta potential decreases (see product AMH4.0). The Cr(VI) removal efficiency increased with the Mg/Al molar ratio until the ratio reached 3.0; but further increases in the Mg/Al molar ratio, on the contrary, decreased the Cr(VI) removal efficiency. We can conclude from Table 1 that the average particle diameter and the zeta potential of the AMH play important roles in Cr(VI) adsorption. Small particle diameter and high zeta potential are favorable factors for Cr(VI) removal. Small particle diameter implies high surface area, resulting in the higher diffusion

4.0

3.0

Intensity

precipitate was aged for 2 h in the parent solution at room temperature. The product was filtered and washed in the filter with deionized water to remove excess ammonia. The filter cake was peptized at about 80  C in an oven to convert it into AMH sol. In order to examine the influence of the Mg/Al ratio on Cr(VI) removal, samples with Mg/Al ratio from 0.5:1 to 4:1 were prepared. The AMH sol was dried at 110  C overnight for the XRD test. All chemical reagents used were of AR grade.

2.0

1.0

0.5

α

3.

Results and discussion

β Al(OH)3

3.1. The effect of the Mg/Al molar ratio on AMH characteristics and Cr(VI) removal A series of AMH products with various Mg/Al molar ratios (Table 1) were prepared in order to examine the influence of this ratio on the characteristics of AMH and its ability to remove Cr(VI). The XRD patterns of the products are dried at

0

10

20

30

40

50

60

70

2Theta (degree) Fig. 1 – X-ray powder diffraction patterns of compounds (AMH0.5–AMH4.0 see Table 1) and Al(OH)3.

80

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Effect of pH on adsorption

Solution pH is one of the most important variables affecting the adsorption characteristics. The effect of pH on Cr(VI) removal was studied within the range of 1.0–10.0 at an initial Cr(VI) concentration of 100 mg/l and with adsorbent doses of 2.0 g AMH/L. In Fig. 3a pH is seen to have a significant effect on the adsorption of chromium on AMH. The adsorption of Cr(VI) was favored (95.8–98.6%) when the initial solution pH was between 2.5 and 5.0. With the increase of the solution pH from pH 5.0 the adsorption efficiency decreased slightly. However, a sharp decline in Cr(VI) adsorption occurred when the pH was lower than 2.5. The pH dependence of metal adsorption is largely related to the metal chemistry in the solution and the properties of the adsorbent (Mor et al., 2007). Cr(VI) exists in different ionic forms in solution. The most important Cr(VI) states in solution 2 are chromate (CrO2 4 ), dichromate (Cr2O7 ) and hydrogen ), depending on the solution pH and total chromate (HCrO 4 chromate concentration (Weckhuysen et al., 1996; Park and

Fig. 2 – The morphology of AMH3.0 sol particles measured by SPM.

Removal efficiency (%)

100

10

90 80

8

70 6

60 50

4

Removal efficiency Equilibrium pH

40 30

2 0

2

4

6

8

10

12

Initial pH

b

2 1

IogC (in g/L as Cr)

3.2.

a

Equilibrium pH

coefficients of Cr(VI) in AMH. And high zeta potential is expected to enhance the interaction between Cr(VI) and AMH. Cr(VI) removal efficiencies are lower for products in which the Mg/Al molar ratios are lower (samples AMH0.5 and AMH1.0). This is probably due to the existence of Al(OH)3 in these products, as the Al(OH)3 precipitate has a greater average particle diameter and a lower zeta potential than AMH (Hou and Xia, 1998; Sposito, 1998). Therefore, in the following study only AMH with Mg/Al molar ratio 3, which has the largest adsorption efficiency, was used as adsorbent for Cr(VI) removal. The chemical formula of AMH3.0 can be expressed as: [Mg0.68Al0.32(OH)2](Cl)0.30(CO3)0.010.66 H2O. The morphological characteristics of AMH were evaluated using a scanning probe microscope (SPM) which showed that the AMH3.0 sol particles are hexagonal (Fig. 2). The zeta potential was þ48 mV for AMH3.0 in suspension.

Cr2O72-

0 ( 300 mg/L)

-1 -2

HCrO4-

H2CrO4

CrO42-

-3 -4

( 0.05 mg/L) -2

-1

0

1

2

3

4

5

6

7

8

9

10

pH Fig. 3 – Effect of initial pH on adsorption and equilibrium pH (AMH dosage 2.0 g/L, initial Cr(VI) concentration 100 mg/L, T [ 30 8C, t [ 150 min) (a) and the predominance diagram showing the relative distribution of different Cr(VI) species in water as a function of pH and total Cr(VI) concentration (b).

Jang, 2002). The following are the important equilibrium reactions (Butler, 1967; Stumm and Morgan, 1996). H2 CrO4 #Hþ þ HCrO 4

(1)

2 þ HCrO 4 #H þ CrO4

(2)

2 2HCrO 4 #Cr2 O7 þ H2 O

(3)

Since the distribution of Cr(VI) species is dependent on both pH and total Cr(VI) concentration, based on thermodynamic database listed in Butler (1967) and Stumm and Morgan (1996), a predominance diagram (Fig. 3b) is presented using both pH and total Cr(VI) as variables. The area between the two horizontal dashed lines indicates the range of Cr(VI) concentration between 300 mg/L and 0.05 mg/L as Cr(VI). From Fig. 3b, HCrO 4 and CrO2 4 are the most predominant species at the experimental total Cr(VI) concentration. For pH lower than 6.8, HCrO 4 is the dominant species of hexavalent chromium, and above pH 6.8, only CrO2 4 is stable. Considering the properties of the adsorbent, AMH is a hydrotalcite-like compound, and its colloidal particles possess not only permanent positive charges because of isomorphous replacement, but also variable charges due to

water research 43 (2009) 3067–3075

the adsorption of Hþ or OH in solution (Hou and Xia, 1998; Sposito, 1998). At low pH, the hydrated surface of AMH is protonated and therefore has an acquired positive charge. Sur-OH þ H3 Oþ /Sur-OHþ 2 þ H2 O

(4)

But at high pH, the hydrated surface of AMH is deprotonated, thereby acquiring a negative charge. Sur-OH þ OH /Sur-O þ H2 O

(5)

where Sur represent the surface sites of AMH. The pH at which the zeta potential equals zero is called the isoelectric point (IEP) and it can be used to qualitatively assess the adsorbent surface charge and characterize the protonation and deprotonation of the amphoteric surface functional groups. At values below the IEP, the hydrated surface of AMH is protonated and therefore has an acquired positive charge. At pH values higher than the IEP, the hydrated surface of AMH is deprotonated, thereby acquiring a negative charge. The IEP of AMH is pH 10.9 as measured by a micro-electrophoretic mobility detector. Therefore, at values below the IEP, there may be two possible mechanisms for Cr(VI) adsorption onto AMH. The main mechanism in LDHs is anion exchange, where the Cl ions that are associated with the surface or interlayer of AMH exchange with the anionic Cr(VI) molecules in solution (Carriazo et al., 2007), which may be written as:    þ  Sur-OHþ 2 =Cl þ HCrO4 /Sur-OH2 =HCrO4 þ Cl

(6)

Another mechanism may be surface complexation, which is suggested for anion adsorption onto hydrous solids such as metal oxides, hydroxides and clay minerals. The formation of surface complexes in anion/hydrous solid systems, such as 2 2 CrO2 4 /g-Al2O3, CrO4 /a-FeOOH and CrO4 /MgO, was verified by previous studies (Mikami et al., 1983; Weckhuysen et al., 1996; Fendorf et al., 1997). The association between the positively charged surface of AMH and the HCrO 4 anions may be written as:  þ  Sur-OHþ 2 þ HCrO4 /Sur-OH2 =HCrO4

(7)

Thus, acidic conditions (pH 2.5–5.0) are favored for Cr(VI) adsorption. As the pH increases from pH 5.0, the Cr(VI) removal efficiency gradually decreases due to the decrease of the positive charge on the surface of AMH. At alkaline pH, CrO2 4 is now the dominant species, and not only is the positive charge on the surface of AMH decreased due to the deprotonation, but also the CrO2 4 must compete with OH in the solution for anion exchange sites that are associated with the surface or interlayer spaces of AMH. Consequently, despite the fact that the adsorption affinity of   CrO2 4 is higher than OH and it may exchange with OH ion (Hu et al., 2005), the amount adsorbed decreases when the initial solution pH is above 7.0. This result is in agreement with previous studies which found that the adsorption capacity of these sorts of materials is increased under lower pH conditions (Lazaridis and Asouhidou, 2003). But under strongly acidic conditions (pH < 2.5), a significant decline in Cr(VI) adsorption occurred. The decrease in Cr(VI) adsorption may be caused by the dissolution of AMH at too low solution pHs.

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The relationship between the initial and equilibrium pHs for Cr(VI) adsorption (Fig. 3) indicated that the equilibrium pH is increased due to the protonation and/or the dissolution of AMH. When the initial pH was between 2.5 and 5.0, the equilibrium pH would be in the range of 4.5–7.2, thus, the maximum adsorption for AMH was obtained. When the initial pH was above 5.0, the equilibrium pH would gradually change into the alkaline region during the adsorption process and result in a slight decline in Cr(VI) adsorption. Previous studies have clearly demonstrated that at pH less than 6.0, the monovalent hydrogen chromate, i.e., HCrO 4 , can be removed effectively using different sorts of adsorbents and commercially available anion-exchange resins. However, at neutral to alkaline pH, the chromate removal capacity is drastically reduced for these adsorbents and commercial anion exchangers (Sengupta and Clifford, 1986; Zhao et al., 1998; Deng and Bai, 2004; Weng et al., 2008). Nevertheless, many contaminated wastestreams exist at neutral to slightly alkaline pH. Due to the high buffer capacities of these waters resulting from high alkalinity and other limitations as governed by the intended applications of the treated water, it is not often viable to adjust the pH through addition of acid. Therefore, an adsorbent with high chromate capacity at neutral to alkaline pH would be very effective in such cases. Although the adsorption efficiencies of AMH for Cr(VI) at neutral to slightly alkaline pH (adsorption efficiency 82.7– 88.2% when pH 7.0–9.0) are less than that at acidic pHs, AMH is shown to be an acceptable adsorbent at neutral to slightly alkaline conditions. This is an important advantage when using AMH instead of other adsorbents, since AMH can be used at pH values close to that at which pollutants are usually found in the environment, while for other adsorbents the maximum adsorption capacity is achieved at low pH values. The adsorption capacity of AMH for dissolved hexavalent chromium is optimal in an initial pH range between 2.5 and 5.0. Considering the adsorption capacity and the feasibility of actual operation, initial pH 4.0 was used in the following experiments.

3.3. Effect of contact time and initial Cr(VI) concentration The effect of adsorption time on Cr(VI) removal at various concentrations of Cr(VI) is shown in Fig. 4. As above, the AMH dose was 2 g/L. The amount of adsorbed Cr(VI) increased with contact time for any initial Cr(VI) concentration and attained equilibrium within 90 min, showing that the adsorption occurred quickly. But the time required to reach equilibrium on AMH clearly depends on the initial chromium concentration. The equilibrium times were 30, 50, 60 and 90 min, respectively, for initial Cr(VI) concentrations of 20, 50, 100 and 200 mg/L. In order to ensure complete adsorption equilibrium, 150 min was chosen as the contact time in each batch equilibrium adsorption experiment. The reason AMH rapidly adsorbs Cr(VI) might be that the AMH colloidal nanoparticles have a small average particle diameter and little internal diffusion resistance. Kinetic modeling not only allows estimation of adsorption rates but also leads to suitable rate expressions characteristic of possible reaction mechanisms. In this respect, several

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Amount adsorbed (mg/g)

100

qt (mg/g)

120

20 mg/L 50 mg/L 100 mg/L 200 mg/L

120

80 60 40 20

100 80 60 40 303K 313K 293K

20 0

0 0

20

40

60

80

100

120

140

160

0

180

Time (min)

60

80

100

120

Fig. 5 – Adsorption isotherms of Cr(VI) onto AMH (AMH dosage 2.0 g/L, initial pH 4.0).

3.4. Adsorption isotherm and thermodynamic evaluation

kinetic models including the pseudo-first-order equation (Eq. (10)) (Lagergren, 1898; McKay, 1984), pseudo-second-order equation (Eq. (11)) (McKay, 1984; Ho and McKay, 2000) and intraparticle diffusion model (Eq. (12)) (Weber and Morris, 1963) were tested.   ln qe  qt ¼ ln qe  k1 t

(8)

t=qt ¼ 1=k2 q2e þ t=qe

(9)

qt ¼ ki t

40

Equilibrum concentration (mg/L)

Fig. 4 – Effect of contact time on the uptake of Cr(VI) by AMH at different initial concentrations (T [ 30 8C, AMH dosage 2.0 g/L, initial pH 4.0).

0:5

20

þC

The adsorption isotherms of Cr(VI) onto AMH at different temperatures are illustrated in Fig. 5. The batch experimental data were analyzed by the Langmuir and Freundlich isotherm models, using a least squares method based on an optimization algorithm. The models are represented mathematically as follows: q ¼ qm KL Ce =ð1 þ KL Ce Þ

(11)

q ¼ KF Ce1=n

(12)

(10)

where qe and qt are the adsorption capacity of the adsorbate (mg/g) at equilibrium and at time t (min); k1 and k2 are the pseudo-first-order and pseudo-second-order rate constants, ki is the intraparticle diffusion rate constant and C is the intercept. The calculated kinetic parameters for Cr(VI) adsorbed by AMH are listed in Table 2. Of the three kinetic equations tested, the pseudo-second-order model is the most suitable in describing the adsorption kinetics of Cr(VI) on AMH, based on the correlation coefficient (R2). It can also be seen from Table 2 that as the initial Cr(VI) concentration increased the pseudosecond-order rate constant k2 decreased. These results are consistent with a previous study of Cr(VI) adsorption by calcined Mg–Al–CO3 hydrotalcite (Lazaridis and Asouhidou, 2003).

where q is the amount of Cr(VI) adsorbed by AMH; qm is the saturated adsorption capacity of the Cr(VI) by AMH; KL, a constant of the Langmuir isotherm and Ce, the concentration of the Cr(VI) solution; and KF and n are parameters of the Freundlich isotherm. Table 3 presents the parameters of the Langmuir and Freudlich isotherm. It shows that the isotherm data has been fitted to the Langmuir model, which provides the best results for these sorts of curves based on the correlation coefficients (>0.98). The results are in agreement with those previously reported by other authors for adsorption tests on this type of solid (Goswamee et al., 1998; Alvarez-Ayuso and Nugteren, 2005; Carriazo et al., 2007). The saturated adsorption capacities (qm) of the Cr(VI) by AMH at 20–40  C are 105.3–112.0 mg/g.

Table 2 – Comparison of the pseudo first-order, second-order and intraparticle diffusion adsorption constants at different initial concentrations. Run no.

1 2 3 4

C0 (mg/L)

20 50 100 200

First-order

Pseudo-second-order

Intraparticle diffusion

qe,cal

k1  102

R2

qe,cal

k2  104

R2

ki

C

R2

10.7 20.9 68.8 118.0

17.90 11.62 10.35 0.73

0.9778 0.975 0.9814 0.9801

11.3 28.2 59.9 111.1

235.24 63.23 14.46 4.88

0.9992 0.9947 0.9962 0.9977

1.82 3.59 6.65 10.68

1.27 3.92 4.65 12.16

0.9153 0.8771 0.9414 0.9129

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T ( C)

Langmuir isotherm qm (mg/g) K (L/mg) K (L/mol)

20 30 40

105.3 109.6 112.0

0.379 0.501 0.603

19.71 26.02 31.36

Freudlich isotherm R2

k

n

R2

0.9973 39.75 4.34 0.9410 0.9913 45.76 4.66 0.9399 0.9859 48.96 4.79 0.9428

The higher adsorption capacity of AMH means it has great potential for application in Cr(VI) removal from aqueous solution or wastewater. The correlation coefficients for the Freundlich model in all cases exceeded 0.93, although they are less than that for the Langmuir model. The best-fit Freundlich parameters n at 20, 30 and 40  C are 4.34, 4.66 and 4.79, respectively. The n value in the range of 2–10 indicates a favorable adsorption process (Ho et al., 2000). The thermodynamic parameters of the adsorption process such as change in standard free energy (DG), enthalpy (DH ), and entropy (DS ) were obtained from experiments at various temperatures using the following equations (Wang and Li, 2005): DG ¼ RTlnKL

(13)

lnKL ¼ DS=R  DH=RT

(14)

where KL are the Langmuir constants (L/mol), R is the ideal gas constant (8.31 J/mol K), and T is the absolute temperature. DH and DS were calculated from the slope and intercept of van’t Hoff plots of ln KL versus 1/T. The DG, DH, and DS values are listed in Table 4. The negative values of the Gibbs free energy change (6.26, 8.21 and 8.96 kJ/mol at 20, 30 and 40  C, respectively) indicate that the adsorption process is spontaneous; the positive enthalpy (17.74 kJ/mol) reveals that energy is absorbed as adsorption proceeds, and the extent of the endothermic reaction will increase at higher temperatures (Sandler, 1999). The entropy changes in this study were found to be positive; meaning that increased disorder appeared on the AMH–solution interface during the adsorption process. The positive entropy change (85.41 J/mol) may be due to the release of water molecules and the chloride anions which are present on the surface or in the interlayer spaces of AMH after the ion exchange reaction with the Cr(VI) adsorbate.

3.5.

Regeneration of chromium-adsorbed AMH

Desorption tests were conducted to regenerate chromiumadsorbed AMH using NaCl and FeCl2 solutions for desorption

of Cr(VI). The desorption of the Cr(VI)-adsorbed AMH in the NaCl solution was slow, and the desorption efficiency of Cr(VI) from AMH was only about 87% after 60 min of desorption (regeneration) in the 2.0 M NaCl solution. In contrast, the desorption of the Cr(VI)-adsorbed AMH was very fast in the FeCl2 solution and it took only about 2 min to reach 97% of the desorption in the 0.2 M FeCl2 solution at pH 2.0. After the desorption or regeneration was completed, the AMH was used again in a subsequent adsorption experiment. The amounts of chromium adsorption on the regenerated AMH decreased only 10.6% in three consecutive adsorption– regeneration cycles, indicating that the regeneration of the AMH by FeCl2 solution was quite effective. The decrease of Cr(VI) adsorption may be attributed to the surface decrease of the regenerated AMH because it no longer consists of colloidal nanoparticles.

3.6.

Characterization of chromium-AMH products

Houri et al. (1999) proposed that chromate adsorption takes place exclusively on the external surface of LDHs crystallites through electrostatic interaction between the oxoanion and the positively charged brucite-like layers. Alternatively, other authors (Ulibarri et al., 1995; Goswamee et al., 1998) suggested that the intercalation of chromate, via anionic exchange, may also take place. In order to check if chromate is actually located in the interlayer space after the adsorption tests, XRD patterns of the solids used in the experiments were recorded. The XRD patterns of AMH and AMH after adsorption of Cr(VI) (AMH-Cr) are compared in Fig. 6. Both samples show a hydrotalcite structure, but the difference in crystallinity between AMH and AMH-Cr is significant. While AMH exhibits fine and intense peaks characteristic of a well-crystallized material, the AMH-Cr displays a poorly crystalline state with wide and ill-defined peaks. AMH sol particles belonged to the hexagonal system. The characteristic parameters of XRD patterns for AMH and AMH-Cr were compared in Table 5. It can be seen from Table 5 that AMH-Cr shows a broad (003)

(003) (006)

Intensity

Table 3 – Langmuir and Freundlich isotherm parameters for adsorption of Cr(VI) on AMH.

(009) AMH

(110)

AMH–Cr

Table 4 – Values of thermodynamic parameters for Cr(VI) removal with AMH. T ( C) 20 30 40

ln KL 2.9811 3.2589 3.4455

DG (kJ/mol) 6.26 8.21 8.96

DH (kJ/mol) 17.74

DS (J/mol.K) 85.41

0

10

20

30

40

50

60

2Theta (deg) Fig. 6 – Powder XRD patterns for AMH and AMH after adsorption of Cr(VI) (AMH-Cr).

70

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Table 5 – Characteristic parameters of XRD patterns for AMH and AMH-Cr. d003 (nm)

AMH AMH-Cr

0.760 0.861

d110 (nm)

Lattice constants (nm)

0.152 0.152

a

c

0.304 0.304

2.280 2.583

4.

diffraction peak centered at 0.86 nm, similar to the value reported by Del Arco et al. (2005). If the width of the brucitelike layer (0.48 nm) is subtracted, the resulting gallery height is 0.38 nm. The change in the d-spacing of AMH from 0.76 nm to 0.86 nm after adsorption of chromium, indicating an increase in the interlamellar distance as the ionic radius of HCrO 4 , is larger than that of Cl (Malherbe and Besse, 2000). This implies that chromate is actually located in the interlayer space via anion exchange. The lower crystallinity of AMH-Cr is probably related to the surface complexation of Cr(VI) on AMH. Some authors have assigned the poorly crystalline state of anionexchanged LDHs to a disturbance in the stacking sequence of the layers caused essentially by anions adsorbed on the surface (Miyata and Okada, 1977; Malherbe and Besse, 2000). The FTIR spectra of AMH and AMH-Cr are shown in Fig. 7. The AMH spectrum exhibited characteristic bands related to the H-bonding stretching vibrations of the OH group in the brucite-like layer at 3386 cm1 and the H2O bending vibration at 1627 cm1. The bands between 400 and 800 cm1 can be attributed to the superposition of the characteristic vibrations of magnesium and aluminum oxides. A band at 1369 cm1 may be related to contamination by carbonate ions in the AMH synthesis. The characteristic infrared band of chromate, due to mode nd(Cr–O), recorded at 890 cm1 for free chromate (Nakamoto, 1986), appeared at 884 cm1 for the AMH sample after adsorption of chromium. This indicates that the interlayer Cl ions were exchanged with HCrO 4 ions in solution. And the slight shift of the band toward the lower frequency (from 890 cm1 to 884 cm1) indicates that the Cr–O bond for AMH–Cr is weaker than for free chromate, which may be

Transmittance/(%)

AMH 1627 1369

682 448

3386 1632

AMH-Cr

1371

3402

4000

3000

related to hydrogen bonding of the HCrO 4 with interlayer water molecules or layer hydroxyl groups.

2000

884 679 447 1000

Wavenumbers/(cm-1) Fig. 7 – FTIR spectra for AMH, and AMH-Cr.

Conclusion

Aluminum magnesium mixed metal hydroxide (AMH) is a promising adsorbent for the removal of Cr(VI) from aqueous systems. It exhibited adsorption properties towards Cr(VI) species present in aqueous solution. The above studies show that the effective range of pH for adsorption of Cr(VI) was between 2.5 and 5.0. With the increase of the solution pH from pH 5.0 the adsorption efficiency decreased slightly. However, a significant decline in Cr(VI) adsorption occurred when the pH was lower than 2.5 due to the dissolution of AMH. This result indicates that the interaction between the surface sites of the AMH and Cr(VI) is a combined effect of both anion exchange and surface complexation. The experimental equilibrium data can be interpreted by Langmuir and Freundlich equations. The saturated adsorption capacities of AMH for Cr(VI) are 105.3–112.0 mg/g at 20–40  C. The adsorption kinetics was successfully fitted by pseudo-second-order kinetics and the adsorption process is endothermic in nature.

Acknowledgements This work was supported by the National Hi-Tech Research and Development Program of China (2006AA06Z217) and the Natural Science Foundation of Shandong province in China (Y2007B10). We thank Dr. Pamela Holt and Prof. Rutao Liu for proofreading the manuscript and express our thanks to Mrs. Weijie Lu and Mr. Ji Huang, Shandong University for supporting the SPM and XRD analysis. We also thank the reviewers for their helpful comments and valuable suggestions.

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