Adsorption of phenanthrene and 1,3-dinitrobenzene on cation-modified clay minerals

Colloids and Surfaces A: Physicochem. Eng. Aspects 377 (2011) 278–283

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Adsorption of phenanthrene and 1,3-dinitrobenzene on cation-modified clay minerals Lichao Zhang, Lei Luo, Shuzhen Zhang ∗ State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for Eco-Environmental Sciences, the Chinese Academy of Sciences, Beijing 100085, PR China

a r t i c l e

i n f o

Article history: Received 18 October 2010 Received in revised form 17 December 2010 Accepted 6 January 2011 Available online 18 January 2011 Keywords: Clay minerals Cation-modification Adsorption Phenanthrene 1,3-Dinitrobenzene

a b s t r a c t Nonpolar nonionic compound (phenanthrene, PHE) and polar nonionic compound (1,3-dinitrobenzene, DNB) were used as probes to explore the dominant mechanism responsible for adsorption of nonionic organic compounds on different cation-modified clay minerals (i.e. smectite, kaolinite, and vermiculite). Batch experiments were conducted, and possible adsorption mechanisms were inferred from adsorption isotherms and characteristics of the modified clay minerals. The results demonstrate that cation-modified clay minerals can adsorb a larger amount of DNB than PHE. Smectite and vermiculite, 2:1 type layered silicate minerals, have a higher adsorption capacity for DNB than the 1:1 type layered kaolinite. K+ modified clay minerals have greater adsorption capacities for DNB than Na+ - and Ca2+ -modified clay minerals; while Ca2+ -modified clay minerals with the exception of vermiculate have greater adsorption capacities of PHE than K+ - and Na+ -modified clay minerals. The results of this study suggest that hydrophobic interaction and inter-layer accommodation are likely to be the dominant mechanisms of PHE adsorption by clay minerals, whereas electrostatic interactions through hydrogen bond and formation of electron donor–acceptor complexes are responsible for DNB adsorption by clay minerals. This study will benefit understanding the adsorption mechanisms of nonionic organic compounds on minerals in the environments. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Organic carbon and clay minerals are typically two most important components in controlling adsorption of organic chemicals that enter the soil environment. It is well established that nonionic organic compounds (NOCs) in soils tend to be taken up by soil organic matter [1,2]. On the other hand, it is a fact that various clay minerals have been employed to remove organic contaminants from aqueous solutions [3,4], and the important role of clay minerals in the adsorption of NOCs has recently received increasing attention [5–7]. Nevertheless, there is a lack of information on the adsorption of NOCs on clay minerals with different structures [8]. Such knowledge is critical to accurately understand the fate of NOCs in the environment [9]. Adsorption of NOCs on clay minerals is greatly governed by structural properties of the sorbents, for example, d-spacing, surface area, charge density and location, and the type of exchangeable cations [8,10–13]. Smectite has attracted much attention in NOC adsorption due to its typical 2:1 type crystal structure and its abundance in soil environment. Hundal et al. [8] observed that cations exerted significant effects on PHE adsorption in smectite and pro-

∗ Corresponding author. Tel.: +86 10 62849683; fax: +86 10 62923563. E-mail address: [email protected] (S. Zhang). 0927-7757/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2011.01.017

posed that capillary condensation of PHE into micropores was a likely adsorption mechanism. Li et al. [14] also examined the effects of cations on the adsorption of nitroaromatic compounds with different polarities on smectite and indicated that modulating the cation type and composition on clay mineral surfaces could manipulate the adsorption behaviors of the compounds. Other minerals with different crystal structure such as kaolinite with 1:1 type [15] and bentonite with 2:1type [3] also received attention and they exhibited contrasting behaviors of NOC adsorption. However, it should be pointed out that most of the current studies focus on NOC adsorption by a single clay mineral, and very few studies have been reported on adsorption of NOCs on clay minerals which have different crystal structures. To thoroughly examine adsorption of NOCs on multi-type minerals will no doubt facilitate our understanding of the adsorption mechanisms of NOCs on clay minerals. Studies have shown that adsorption characters of NOCs on clay minerals such as sorption affinity and nonlinearity also significantly depend on physicochemical properties such as polarity of the sorbates [14,16,17]. It was reported that nitroaromatic compounds adsorbed strongly on expandable clay minerals especially when they were exchanged with weakly hydrated cations (K+ and Cs+ ) [10,11]. It is therefore expected that adsorption of NOCs on clay minerals will be greatly affected by the presence of cations via changes in physical configuration and chemical characteristics of the clay minerals, and such effects may heavily depend on

L. Zhang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 377 (2011) 278–283

physicochemical properties of the NOCs and crystal structure of the minerals. However, comprehensive studies on adsorption of NOCs to clay minerals are still very limited in the literature so far. In the present study, we systematically investigated the adsorption of two typical nonionic aromatic compounds with contrasting polarity, namely, PHE and DNB, on three clay minerals (smectite, kaolinite and vermiculite) modified with different cations (Ca2+ , K+ and Na+ ). The three minerals are abundance in soils and have contrasting structures and expandabilities which are hypothesized to exhibit different sorption behaviors for the NOCs. The objectives of this study were therefore (i) to compare the adsorption of nonpolar and polar nonionic compounds between cation-modified clay minerals using PHE and DNB as the model compounds and further (ii) to explore the influence of structural and surface properties of clay minerals on their sorption of nonpolar and polar nonionic compounds. The results of this study will benefit understanding the mechanisms of NOCs adsorption on minerals in the environments.

279

Table 1 Surface areas, average pore diameters, ␨ potentials and CEC of the cation-modified clay minerals. Clay minerals Smectite Na+ -modified K+ -modified Ca2+ -modified Kaolinite Na+ -modified K+ -modified Ca2+ -modified Vermiculite Na+ -modified K+ -modified Ca2+ -modified

N2 BET SA (m2 g−1 )

Average pore diameter (nm)

 Potential (mV)

CEC (cmol kg−1 )

7.6a 18.4 12.9

23.0 11.0 18.1

−32.9 −29.8 −4.0

89 121 92

1.9 1.8 1.6

22.0 20.8 22.4

−18.4 −14.6 −14.2

29 31 32

33.6 37.3 35.2

13.7 13.1 14.2

−8.0 −7.9 −4.7

107 100 105

a N2 BET SA is the cumulative surface area of pores between 1.7 nm and 300 nm in diameter determined with N2 at 77 K using the Brunauer-Emmentt-Teller (BET) equation.

2. Materials and methods 2.1. Preparation and characterization of the clay minerals Smectite, kaolinite and vermiculite were obtained from Beijing Ruizhx Tech Co., Ltd (Beijing, China). The <2 ␮m fraction of each clay mineral was separated by sedimentation, and then was saturated with 0.5 mol L−1 Na, K and Ca in chloride salts, respectively. After saturated, the excess salts were removed by washing with deionized water by repeated centrifugation and resuspension until a negative chloride test was obtained with AgNO3 . The cation-modified clay minerals were then freeze-dried and kept in a desiccator for later use. The structure and microtopography of the clay minerals were identified by a Hitachi S-3000N scanning electron microscope (SEM) operated at 5 kV (Japan). Surface area and pore size distribution were measured by N2 BET method at 77 K with an ASAP 2020 surface area analyzer (Micromeritics Co., USA). Zeta potential analysis was performed with a Mastersizer 2000 particle size analyzer (Malvern Instruments, UK). The clay mineral suspensions contained 0.1% solid in deionized water and they were exposed to supersonic dispersion for 10 min before determination. Cation exchange capacity (CEC) was determined using barium chloride method [18]. The cation-modified clay mineral samples before and after interacting with PHE and DNB were freeze-dried and prepared on glass slides for X-ray diffraction (XRD) analysis. The XRD patterns were recorded on a PANanalytical X’Pert Pro X-ray diffractometer equipped with a Cu source (Cu K␣). The scanning angle (2) ranged from 4◦ to 15◦ at step of 0.01. 2.2. Preparation of PHE and DNB PHE (>98%, Acros Organics) solutions were prepared in 0.01 M electrolyte solutions (NaCl, KCl, or CaCl2 ) by diluting the PHE stock solution made in HPLC-grade methanol. The concentration of methanol in the final solutions was always kept below 0.1% (v/v). DNB (>98%, Fluka Chemika) stock solution was prepared by dissolving 100 mg of DNB in 1 L of 0.01 M electrolyte solutions (NaCl, KCl, or CaCl2 ) directly. Stock solutions were stored in the dark at 4 ◦ C, and working solutions were prepared freshly within 1 h of use. 2.3. Adsorption experiments Triplicate sorbent samples (0.6000 g) were placed into 30 mL glass centrifuge tubes with Teflon liners and a 20 mL aliquot of PHE or DNB solution of varying concentrations (0.025–1.0 mg L−1 and 2.5–100 mg L−1 , respectively) was added. Controls without clay

were prepared in the same way to account for possible loss of PHE or DNB other than adsorption by the clays. The samples and controls were equilibrated on a reciprocating shaker at 105 rpm in the dark at 25 ± 1 ◦ C for 48 h. Kinetic measurements showed that 48 h was sufficient time for the sorption of PHE or DNB by all the minerals to attain equilibrium (data not shown). After equilibration, the samples and controls were centrifuged for 20 min at 4000 g and the supernatants were served for HPLC analysis. 2.4. Chemical analysis PHE or DNB in the supernatant solution was determined by reversed-phase HPLC using an Agilent eclipse XDB-C18 column with an ultraviolet detector at 254 nm for PHE and 262 nm for DNB, respectively. The amount of PHE or DNB retained by the clay minerals was calculated as the difference in concentration between control and sample. Analytical conditions included an isocratic methanol: water (90:10) mobile phase and a flow rate of 1.0 mL min−1 for PHE and an isocratic methanol: water (78:22) mobile phase and a flow rate of 0.8 mL min−1 for DNB, respectively. 2.5. Data analysis Adsorption isotherms of PHE and DNB were described by the Freundlich model, that is, Cs = Kf Ce n , where Cs is mg PHE or DNB sorbed per kg clay, Ce is equilibrium solution concentration (mg L−1 ) of PHE or DNB, Kf is the Freundlich coefficient [(mg kg−1 )(L mg−1 )n ], and n, an empirical constant, is a measure of isotherm nonlinearity. Kf and n for all adsorption isotherms were calculated by the Freundlich equation fitted using Origin 7.5 software. Statistical analysis was performed using SPSS for Windows (version 17.0, SPSS Inc.) using T-test at the 95% confidence level (p < 0.05). 3. Results and discussion 3.1. Physicochemical characterization of cation-modified clay minerals Fig. 1 shows the SEM images of the three clay minerals. Smectite and kaolinite showed stratified structure, while vermiculite exhibited loosely porous structure. Both smectite and vermiculite were not compact, and kaolinite with relatively flat and smooth surface was more condensed probably due to its 1:1 type structure. As presented in Table 1, in the cation-modified smectite and vermiculite, K+ -modified minerals had the largest surface area,

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d=1.295nm

Ca-type after adsorption

Relative Intensity

d=1.231nm

Ca-type before adsorption

d=1.100nm

K-type after adsorption

d=1.027nm

d=1.197nm d=1.100nm 4

6

K-type before adsorption

Na-type after adsorption Na-type before adsorption

8 10 12 Degrees 2-Theta

14

Fig. 2. XRD patterns of cation-modified smectite before and after interacting with 1 mg kg−1 PHE.

Zeta potential measurements can provide valuable information on surface chemistry of adsorbents. The zeta potentials of all the cation-modified clay minerals showed the same variation trend and tended to be neutral in the order of Na+ -, K+ - and then Ca2+ modified minerals (Table 1). CEC values of the cation-modified minerals were listed in Table 1. The CEC value of K+ -modified smectite was significantly higher than Na+ - and Ca2+ -modified smectite. No obvious difference was observed for kaolinite and vermiculite modified by different cations. The CEC values of cation-modified kaolinite were much lower than cation-modified smectite and vermiculite. The peak positions of the cation-modified minerals from XRD analysis are summarized in Table 2, and the XRD patterns of cationmodified smectite are presented in Fig. 2 as an example. The d001 spacings (the first-order reflections used to characterize the layer distance) of all the modified minerals increased after interacting with PHE or DNB. The diffraction peaks of Na+ -modified smectite and Ca2+ -modified vermiculite decreased in intensity and became broader, and a new peak appeared at 1.459 nm in the XRD pattern of Ca2+ -modified vermiculite. The largest d001 -spacings was observed for cation-modified smectite, followed by cation-modified vermiculate and then kaolinite. 3.2. Adsorption isotherms

Fig. 1. SEM (5 kV) of clay minerals: (A) smectite; (B) kaolinite; (C) vermiculite.

followed by Ca2+ -modified minerals and then Na+ -modified minerals, while no obvious difference in the surface area was observed between kaolinite modified by different cations since it is a less expandable mineral. Cation-modified vermiculite had larger surface area than other minerals probably because of the loosely porous structure shown in Fig. 1. In all the modified minerals, K+ modified minerals had the smallest average pore diameter.

Adsorption isotherms for PHE on all the cation-modified clay minerals and the values of the Freundlich parameters (Kf and n) are shown in Fig. 3. The adsorption isotherms presented a good fit to the Freundlich model (r > 0.99). PHE is mostly sorbed to smectite, followed by vermiculite and then kaolinite. Ca2+ -modified smectite had a significantly higher adsorption capacity than K+ or Na+ -modified smectite (p < 0.05). A same order was obtained for PHE sorption on kaolinite modified by different cations, however the difference in adsorption was not significant between cation modifications (p > 0.05). It is interesting to note that cationmodified vermiculite showed different adsorption characteristic with K+ -modified vermiculite which exhibited the highest adsorption capacity, followed by Ca2+ - and then Na+ -modified vermiculite. Adsorption isotherms and the values of the Freundlich parameters (Kf and n) for DNB adsorption are shown in Fig. 4. The adsorption data also fit well with the Freundlich model (r > 0.99). Among the three clay minerals, DNB was mostly sorbed to smectite, followed by vermiculite and then kaolinite. It is remarkable that K+ -modified minerals exhibited significantly higher adsorption

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281

Table 2 Dominant d-spacing (nm) of the freeze-dried cation-modified clay minerals before and after interacting with 1 mg L−1 PHE and 100 mg L−1 DNB. Smectite

Na-type K-type Ca-type

Vermiculite

Original

With PHE

With DNB

Original

With PHE

With DNB

Original

With PHE

With DNB

1.100 1.027 1.231

1.197 1.100 1.295

1.332 1.201 1.525

0.988 0.988 0.988

0.993 0.990 0.997

0.991 0.995 0.994

0.995 1.002 1.063

1.006 1.005 1.081; 1.459a

1.009 1.004 1.078

A new peak appeared at 1.459 nm.

15 Smectite

n =0.89 Kf =20.72

n =0.76 Kf =13.72

6

2000

0

0.2

0.4

0.6

n =0.65 Kf =1483.34

0.8

0

20

n =0.81 Kf =6.61

2

0 0.0

qe(mg/kg)

qe(mg/kg)

4

Na K Ca 0.2

0.4 Ce(mg/L)

0.6

Kf =5.03

0 0

20

40

60

80

100

40 60 Ce(mg/L)

80

100

Kaolinite

30

n =0.88 Kf =7.39

n =0.51

20

40

n =0.82 Kf =6.83

Kaolinite

Kf =3.81

Na K Ca

Ce (mg/L)

6

n =0.62

1000

Na K Ca

3 0 0.0

60 40

qe(mg/kg)

qe (mg/kg)

9

80

Smectite

3000

n =0.79 Kf =14.65

12

n =0.63 Kf =1.79

10

0

0.8

n =0.87 Kf =0.3

20

n =0.88 Kf =0.31 0

20

40 60 Ce(mg/L)

80

Na K Ca 100

150

4

Vermiculite

n =0.79 Kf =7.22

Vermiculite

n =0.8 Kf =7.8

n =0.48 Kf =14.27

100 qe(mg/kg)

6

qe(mg/kg)

a

Kaolinite

n =0.83 Kf =6.94 2

50

n =0.81 Kf =0.4

Na K Ca 0 0.0

0.2

0.4

0.6

0.8

Ce(mg/L) Fig. 3. PHE adsorption on the cation-modified clay minerals.

0

0

20

n =0.69 Kf =0.46

40 60 Ce(mg/L)

80

Na K Ca

100

Fig. 4. DNB adsorption on the cation-modified clay minerals.

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affinities to DNB than Ca2+ - or Na+ -modified minerals (p < 0.05), which is particularly true for the K+ -modified smectite (Fig. 4). In addition, obvious difference also occurred between Ca2+ - and Na+ -modified clay minerals (p < 0.05).

3.3. Discussion Zeta potential gradually approached neutral, exhibiting the following negative order: Na+ -modified > K+ -modified > Ca2+ modified minerals, indicating the enhancement of the surface hydrophobicity, which would result in a facilitating hydrophobic adsorption of PHE on the solid surfaces [19]. Therefore, Ca2+ modified clay minerals generally showed the highest adsorption capacity, followed by K+ -modified and then Na+ -modified. Lack of correlation between PHE adsorption and zeta potential of the cation-modified vermiculite may result from the less ordered and/or size-reduced quasicrystals referred from the XRD patterns [20]. It was found that Kf values for DNB adsorption isotherms and N2 BET SA of the modified minerals had the same variation trend with the exception for cation-modified kaolinite since kaolinite is relatively stable in its stratified structure and less affected by cation modification. Lack of correlation between DNB adsorption and N2 BET SA of cation-modified kaolinite was mainly due to its less expandable structure as stated above and smaller surface areas. While no relationship was observed between sorption capacity and N2 BET SA of the modified minerals in PHE adsorption, which may result from the failure of the N2 BET SA reflecting PHE adsorption in minerals and soils [8,19]. Information on PHE adsorption mechanisms may be obtained from XRD analysis because the adsorption capacity is significantly influenced by the d001 spacings of the cation-modified clay minerals. The Ca-modified smectite showed a 0 0 1 peak at 1.231 nm, and adsorption of PHE caused the peak broadening and shift (from 1.231 nm to 1.295 nm), implying the presence of interlayer PHE [21–23]. Similar behaviors were also observed for K+ - and Na+ modified smectite. Compared to Na+ - and K+ -modified smectite, the d001 spacing of Ca2+ -modified smectite was the largest no matter before or after interacting with PHE, indicating its higher adsorption capacity than Na+ - and K+ -modified smectite. The diffraction peak of Na+ -modified smectite decreased in intensity and became broader, which means the Na-quasicrystals became somewhat less ordered and/or size-reduced [20]. Therefore, the amount of PHE accommodated in the double-layer structure was diminished. Kaolinite is a 1:1 type mineral with low expandability and the structure characteristics were less affected by cation modification. Consequently, there was no marked difference in adsorption capacity between the different cation-modified kaolinites. The diffraction peak of Ca2+ -modified vermiculite decreased in intensity and became broader, and a new peak appeared at 1.459 nm, indicating the Ca2+ -quasicrystals became somewhat less ordered and/or size-reduced. This would decrease PHE adsorption on the Ca2+ -modified vermiculite. There was no considerable variability in the increases of the spacings of K+ - and Na+ -modified vermiculite, and no obvious difference in PHE adsorption capacity occurred during the treatments. The difference in the adsorption capacity of the modified minerals can be attributed to their d001 spacings. As shown in Table 2, the cation-modified smectite had the largest d001 -spacings, followed by cation-modified vermiculate and then kaolinite, which is consistent with the adsorption capacities of the three modified minerals. Significantly higher adsorption of DNB than PHE by the cation-modified clay minerals was probably related to multiple simultaneous interactions including electrostatic interactions between functional groups of DNB and surface charge sites on the

clay minerals, the hydrogen bond between NO2 groups and the OH groups of the minerals and the strong interactions by electron donor–acceptor, of which the aromatic rings of DNB function as electron acceptors and clay siloxane oxygens behave as electron donators [24–26]. Mineral surfaces are dominated by negatively charged sites arising from isomorphous substitution but also have a few variable charge sites which could be positive or negative [21]. NO2 groups are electron-withdrawing groups with negative, which may interact with negatively charge sites via cation bridging [27]. Hydrogen bonding between polar NO2 groups and OH groups on clay may also contribute to the adsorption of DNB. Weakly hydrated exchangeable cation of K+ retains relatively smaller hydration sphere than Na+ and Ca2+ , thereby increasing the effective size of adsorptive domains between K+ on clay surfaces and enhancing interactions between exchangeable cations and NO2 groups of DNB. As a result, higher adsorption was observed for K+ -modified clay minerals, especially K+ -modified smectite having the highest CEC value, which is consistent with previous studies [10,11,14,16]. As a divalent cation, Ca2+ was able to bridge between negatively charged NO2 groups of DNB and negative charge sites on the clay surfaces. In contrast, monovalent Na+ can only satisfy one negative charge site at a time. Therefore, DNB was adsorbed to a relatively greater extent on Ca2+ -modified clay minerals than on Na+ -modified clay minerals. Relative to cation-modified kaolinite and vermiculite, cation-modified smectite showed a high adsorption capacity for DNB, which might be attributed to the dense adsorption sites on solid surface. 4. Conclusion SEM and XRD analyses revealed that smectite and kaolinite had stratified structure, while vermiculite exhibited loosely porous structure, and the d001 spacings of all the modified minerals increased after interacting with PHE or DNB. At the same time, PHE adsorption capacity commonly increased in the cation-modified minerals when zeta potential of the modified minerals approached neutral. The results indicated that hydrophobic interaction and inter-layer accommodation are likely to be the dominant mechanisms of PHE adsorption by clay minerals. Whereas the adsorption amount of DNB increased when the surface area of the modified minerals increased, and the electrostatic interactions through hydrogen bond and formation of electron donor–acceptor complexes are likely to be the dominant mechanisms of DNB adsorption by clay minerals. Acknowledgements This work was funded by the National Natural Science Foundation of China (grant no. 40730740, 20890110 and 20921063) and the National Basic Research Program of China (project 2009CB421603). References [1] X.L. Wang, T. Sato, B.S. Xing, Sorption and displacement of pyrene in soils and sediments, Environ. Sci. Technol. 39 (2005) 8712–8718. [2] B.S. Xing, J.J. Pignatello, Dual-mode sorption of low-polarity compounds in glassy poly(vinylchloride) and soil organic matter, Environ. Sci. Technol. 31 (1997) 792–799. [3] B.L. Chen, W.H. Huang, J.F. Mao, S.F. Lv, Enhanced sorption of naphthalene and nitroaromatic compounds to bentonite by potassium and cetyltrimethylammonium cations, J. Hazard. Mater. 158 (2008) 116–123. [4] G. Sheng, C.T. Johnston, B.J. Teppen, S.A. Boyd, Adsorption of dinitrophenol herbicides from water by montmorillonites, Clays Clay Miner. 50 (2002) 25–34. [5] S.M. Charles, B.J. Teppen, H. Li, D.A. Laird, S.A. Boyd, Exchangeable cation hydration properties strongly influence soil sorption of nitroaromatic compounds, Soil Sci. Soc. Am. J. 70 (2006) 1470–1479. [6] H. Li, G. Sheng, B.J. Teppen, C.T. Johnston, S.A. Boyd, Sorption and desorption of pesticides by clay minerals and humic acid–clay complexes, Soil Sci. Soc. Am. J. 67 (2003) 122–131.

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