Cr(VI) adsorption by montmorillonite nanocomposites

Applied Clay Science 124–125 (2016) 111–118

Contents lists available at ScienceDirect

Applied Clay Science journal homepage: www.elsevier.com/locate/clay

Research paper

Cr(VI) adsorption by montmorillonite nanocomposites Guifang Wang a,b,⁎, Yuyan Hua a, Xin Su c, Sridhar Komarneni b,⁎⁎, Shaojian Ma a, Yujue Wang d a

Guangxi Experiment Centre of Science and Technology, School of Resource and Metallurgy, Guangxi University, Nanning 530004, China Department of Ecosystem Science and Management and Materials Research Institute, 204 Materials Research Laboratory, Pennsylvania State University, University Park, PA 16802, USA c School of Environment, Guangxi University, Nanning 530004, China d School of Environment, Tsinghua University, Beijing 100084, China b

a r t i c l e

i n f o

Article history: Received 23 November 2015 Received in revised form 6 February 2016 Accepted 9 February 2016 Available online xxxx Keywords: Montmorillonite Adsorption Hexavalent chromium Montmorillonite nanocomposites

a b s t r a c t Various montmorillonite (Mt) nanocomposite adsorbents were prepared with Al13 cations, dodecyl trimethyl ammonium chloride (DTAC) or dodecyl amine (DA) and characterized by Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD) and BET surface area and pore size analyses. The adsorption of hexavalent chromium, Cr(VI) onto various Mt nanocomposites as a function of adsorbent dosage, initial Cr(VI) concentration, contact time and solution pH was investigated. The results showed that the obtained nanocomposites had large basal spacing and good porous structure, and the specific surface areas followed the order: OH-AlMt N DA-Al-Mt N DTAC-Al-Mt N Na+-Mt N DTAC-Mt N DA-Mt. The removal efficiency of Cr(VI) ions increased with increasing the adsorbent dosage and contact time, but decreased with increasing initial Cr(VI) concentration, as expected. The adsorption of Cr(VI) was highly pH-dependent and the maximum removal efficiency of Cr(VI) was found in the acid environment. The adsorption equilibrium time was 2 h and the adsorption kinetic data of Cr(VI) on various adsorbents were well described by the pseudo-second-order kinetics model, which indicated that the adsorption reaction of Cr(VI) ions with the adsorbents was mainly due to chemical adsorption. Both the Langmuir model and Freundlich model fitted the equilibrium data well, which suggested that the Cr(VI) adsorption onto various adsorbents was both as monolayer and on heterogeneous surface conditions. The adsorption results indicated that among all the adsorbents used in this experiment, the dodecyl amine and Al13 cations composited with Mt (DA-Al-Mt) was the most effective for removing Cr(VI) from wastewater. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Chromium and its compounds are important chemicals for electroplating, metal processing, metallurgy, leather tanning, pigments and various other industries. However, large volumes of wastewater containing chromium were discharged into soils and water bodies, leading to a serious environmental threat. In industrial wastewater, chromium is mainly present in the form of hexavalent chromium and trivalent chromium. Although both have toxicity, hexavalent chromium is a highly toxic agent that is carcinogenic, mutagenic and teratogenic to living organisms and its toxicity is 100 times that of trivalent chromium (Xi et al., 2002; Dupont and Guillon, 2003). Furthermore, crop growth is inhibited when the concentration of hexavalent chromium reaches 10 mg/L (Chen, 1980). Chromium in aquatic environment has been classified as group A of human carcinogens (Costa, 2003) by the United

⁎ Correspondence to: G. Wang, School of Resource and Metallurgy, Guangxi University, Nanning 530004, China. ⁎⁎ Correspondence to: S. Komarneni, Materials Research Laboratory, Materials Research Institute, Pennsylvania State University, University Park, PA 16802, USA. E-mail addresses: [email protected] (G. Wang), [email protected] (S. Komarneni).

http://dx.doi.org/10.1016/j.clay.2016.02.008 0169-1317/© 2016 Elsevier B.V. All rights reserved.

State Environmental Protection Agency (USEPA). The World Health Organization recommends that the maximum permissible concentration value in drinking water for total Cr should be 0.05 mg/L (WHO, 2001), while the maximum level of chromium in drinking water is regulated by USEPA to be below 0.10 mg/L EPA U.S., 2012). The removal of chromium from wastewater is, therefore, a priority to prevent contamination of drinking water. The conventional methods for removal of chromium from wastewater include chemical precipitation, ionic exchange, filtration, electrochemical treatment, membrane techniques and recovery by evaporation (Tiravanti et al., 1997; Gupta et al., 2005; Kula et al., 2008; Gupta and Rastogi, 2009). However, most of these methods are not widely used because of certain drawbacks, such as the high operational cost including the relatively highenergy cost, and the use of complicated techniques. Therefore, there is an urgent need to develop economically favorable and technically feasible technologies such as adsorbent technology. Although many adsorbents such as clay (Li et al., 2014), sandy soil (Fonseca et al., 2009) and agricultural waste products (Demirbas et al., 2008) of low cost, high efficiency and ease of operation have been reported for the removal of chromium in recent years, further improvement of adsorption technology is feasible. Among all the adsorbent materials proposed for Cr(VI) removal, naturally occurring inorganic adsorbents such as zeolites

112

G. Wang et al. / Applied Clay Science 124–125 (2016) 111–118

(Lv et al., 2014), kaolinite (Matusik and Bajda, 2013) and montmorillonite (Mt) (Bajda and Klapyta, 2013) received a great deal of attention because of their low cost. Mt is the main mineral component in naturally occurring bentonite deposits. It is a 2:1 type layered clay mineral, which is composed of one octahedral sheet sandwiched between two tetrahedral sheets with cations balancing the negative layer charge in the interlayer spaces. The negative charge through isomorphic substitution, high electronegativity of the surfaces, cation exchange property, expansibility and dispersion property are the basic characteristics of Mt (Luan and Li, 1998). Mt has been reported to be one of the most important adsorbents for the treatment of chromium-containing wastewater because of its low cost, ease of operation and easy regeneration. However, the Cr(VI) adsorption capacity of natural Mt without modification is limited. Generally, inorganic pillaring or organic intercalation is used to enhance the adsorptive capacity of Mt. After modification by inorganic or organic compounds, such as polyoxometal cations or surfactant cation, the resulting modified Mt has a larger basal spacing and better porous structure, and these properties are favorable for the adsorption of inorganic or organic pollutants due mainly to the cation, ligand exchange or solution partition mechanism (Aouad et al., 2006; Zhu and Zhu, 2007; Zhou et al., 2008). Therefore, the objective of this study here is to improve its adsorption capacity through modification of Mt by nanocompositing with organic, inorganic or inorganic-organic phases for making more efficient adsorbents for Cr(VI) removal from wastewater. The structure of Mt is highly amenable for modification by different phases. Here, dodecyl trimethyl ammonium chloride (DTAC), dodecyl amine (DA) and the pillaring reagent of hydroxy aluminum polycations (Al13) were used to modify Mt. The resulting nanocomposite adsorbents were thoroughly characterized by different techniques and were evaluated for the uptake of Cr(VI). 2. Materials and methods 2.1. Materials The raw montmorillonite (Mt) used in this study was obtained from Weifang bentonite deposit in Shandong province of China. The raw Mt was first purified by sedimentation technique and then Na+ exchanged by treating with Na2CO3. The Na+ exchanged sample was used in all subsequent experiments for the preparation of Al-pillared Mt and other organic/inorganic nanocomposites. The technical parameters of the Na+-montmorillonite (Na+-Mt) used in these studies are listed in Table 1. All the chemicals used in this study were of analytical grade and procured from different suppliers as follows: dodecylamine (DA) (Shanghai Jingchun Reagent Company, China), dodecyl trimethyl ammonium chloride (DTAC) (Tianjin Kemiou Chemical Reagent Co., Ltd.), AlCl3·6H2O (Tianjin BASF Chemical Company, China), Na2CO3 (Tianjin Reagent Chemical Company, China), AgNO3 (Sinopharm Chemical Reagent Company, China), Potassium dichromate (K2Cr2O7) (Tianjin Kemiou Chemical Reagent Co., Ltd.). Distilled water was used for the preparation of all solutions. 2.2. Synthesis of hydroxy-Al pillared Mt The first step in the pillaring process was the preparation of pillaring agent. A 0.4 mol/L solution of Na2CO3 was added at the rate of 1 mL/min to a 0.2 mol/L solution of AlCl3·6H2O under vigorous stirring while maintaining the temperature at 80 °C to prepare a hydrolyzed solution

of Al. The final OH–/Al3+ mole ratio of Al pillaring solution was equal to 2.4. The hydrolyzed Al solution was stirred for another 2 h at 80 °C followed by aging at room temperature for 24 h. Under this preparation condition, the hydroxy-aluminum is expected to exist primarily as Al13 cations (Wang et al., 2011). The above hydroxyl Al solution was then reacted with a 1% Mt dispersion at 80 °C under vigorous stirring. An Al3+/clay ratio of 10 mmol/g was used to ensure pillaring with hydroxyl Al. The above slurry was stirred for an additional 2 h at 80 °C followed by further aging at 60 °C for 24 h. This clay dispersion was washed several times with distilled water by centrifugation to separate the solid from solution until the wash water was free of chloride (tested with 0.1 mol/L AgNO3 solution). The separated Al-pillared clay was air-dried, then ground and sieved to obtain b74 μm size. The ground and sieved product was denoted as hydroxy-Al pillared Mt (OH-Al-Mt). 2.3. Preparation of organic pillared Mt Mt was modified either by DTAC or DA as follows: An amount of organic modifier equal to the CEC of Na+-Mt (Table 1) was put into 1% of Na+-Mt dispersion under stirring and then the mixture was heated at 80 °C under fast stirring for 2 h to facilitate the intercalation of organic into the interlayer spaces of Na+-Mt. The products were then washed with distilled water several times by centrifugation until the wash water was free of chloride (tested with 0.1 mol/L AgNO3 solution), then dried at 80 °C and ground and sieved to obtain b74 μm. The Mt modified with DTAC and DA was designated as DTAC-Mt and DA-Mt, respectively. 2.4. Preparation of organic-inorganic intercalated Mt The Na+-Mt powder was first dispersed in distilled water to prepare a 1% Mt dispersion in a beaker and then controlled amount of DTAC or DA was added into the dispersion and the mixture was stirred for 2 h at 80 °C to complete the intercalation. The amounts of DTAC or DA used above were equal to one half of the CEC of Na+-Mt (Table 1). Hydroxy-Al pillaring solution was then added to the above organic modified Mt dispersions at the rate of 3 mL/min under vigorous stirring and heating at a temperature of 80 °C for 12 h with continued stirring. Then, the dispersion was centrifuged, washed until no Cl− was detected in supernatant with AgNO3 solution. Finally, the samples were dried at a temperature of 80 °C in a dryer, ground and sieved to obtain b74 μm. The ground products were labelled as DTAC-Al-Mt and DA-Al-Mt for DTAC plus Al hydroxy polymer modified Mt and DA plus Al hydroxy polymer modified Mt, respectively. 2.5. Characterization methods Powder XRD patterns were recorded using a RigaKu D/max-rB diffractometer with Cu kα radiation at 40 kV and 100 mA. A 2θ range of 0.8° to 40° at a scanning rate of 4°/min was used for the Mt samples. FTIR spectra of samples in KBr pellets were obtained using Nicolet 380 FTIR Spectrometer. The FTIR spectra were recorded in the 400–4000 cm−1 region with a resolution of 2 cm−1 and 64 scans. N2 adsorption-desorption isotherms were measured on a SSA-4300 equipment at liquid nitrogen temperature (77 K). The specific surface area (SBET) was calculated by using the multi-point Brunauer-EmmettTeller method (Gregg and Sing, 1982). The pore volume distribution and porous structure of clays was analyzed by BJH method (BarrettJoyner-Halenda) and the classification method as proposed by De

Table 1 Technical parameters of the Na+-Mt. Colloid volume [mL/g]

Swelling volume [mL/g]

Cation exchange capacity (CEC) [mmol/100 g]

Methylene blue adsorption [g/100 g]

d001 [nm]

158.0

74.0

90.0

39.5

1.24

G. Wang et al. / Applied Clay Science 124–125 (2016) 111–118

113

Boer, respectively (De Boer et al., 1965; Zhao, 2005). Thermogravimetric (TG) and differential thermal analysis (DTA) were carried out with a TG/ SDTA851e thermal analyzer from Mettler Toledo Company of Switzerland. The samples were heated at the rate of 20 °C/min under a flow of high purity nitrogen from 30 to 1000 °C. 2.6. Adsorption experiments Adsorption experiments were conducted using a batch method in stoppered conical flasks. 0.05–0.5 g of adsorbent was dispersed in 50 mL conical flasks to which 20 mL Cr(VI) solution with different initial concentrations of 20–100 mg/L were added. The Cr(VI) solution was prepared by dissolving K2Cr2O7 in distilled water. The pH was adjusted by adding a small amount of dilute HCl and NaOH solution using a PHS3C pH meter. All the experiments were performed at a shaking speed of 200 rpm for different times at room temperature to ensure the equilibrium of the adsorption process. The concentrations of Cr(VI) in aqueous solutions was determined by using a SP-756 ultraviolet-visible spectrophotometer at 540 nm. All adsorption experiments were conducted at room temperature (25 °C) unless otherwise noted, and all of the adsorption results were corrected by blank tests in which no adsorbent was added into the Cr(VI) solution. All the experiments were run in duplicate and average values are reported here. The kinetic studies were performed at 25 °C as follows: 0.2 g of samples were equilibrated using an initial concentration of 20 mg/L Cr(VI) at a pH of 2.0. The samples were equilibrated for different time intervals of 5, 10, 20, 30, 45, 60, 120, 240 or 360 min and then the solid and solution were separated by centrifugation to determine Cr(VI) concentration in solution. Adsorption isotherms were obtained by batch technique at 25 °C using 50 mL conical flasks (with sealed caps). A 20 mL each of Cr(VI) solutions with varying initial concentrations (20–100 mg/L) were mixed with 0.2 g of adsorbents at pH 2.0. The dispersions were shaken for 2 h at the stirring speed of 200 rpm, then centrifuged to separate solution from solid followed by determination of Cr(VI) concentration in solution. 3. Results and discussion 3.1. Characterization of nanocomposites of montmorillonite (Mt) Powder XRD patterns of Na+-Mt and nanocomposites of Mt are presented in Fig. 1. The 001 reflection of Na+-Mt is at 1.24 nm resulting from the presence of Na+ cations and one layer of water molecules in the interlayer spaces. The OH-Al-Mt, DTAC-Mt and DTAC-Al-Mt samples showed 001 reflections at 1.94, 1.83, 1.69 nm, respectively because of intercalation of inorganic and/or organic species (Fig. 1). The replacement of hydrated exchangeable cations with hydroxy-Al cations or surfactant cations led to the increased ‘d’ values of the 001 reflections (Rathnayake et al., 2015). The d-value of 001 reflection of DA-Mt sample increased from 1.86 nm to 2.02 nm after treating with hydroxy-Al pillaring agent (Fig. 1). This increase was apparently caused by Al13 cations entering into the interlayer spaces of DA-Mt because DA satisfied only half the charge on the layers as a DA concentration equivalent to 0.5 CEC was used for intercalation. Therefore, further intercalation of Al13 cations into the interlayer spaces of DA-Mt was possible leading to increased basal spacing of about 2.02 nm. By using a DTAC concentration of 1.0 CEC, the DTAC modified Mt without hydroxy-Al pillaring agent treatment showed a reflection at 1.83 nm while hydroxy-Al pillared Mt (OH-Al-Mt) showed a reflection at 1.94 nm (Fig. 1). However, the DTAC-Al-Mt sample with the treatment of both DTAC (0.5 CEC) and Al13 cations showed two reflections at 1.69 nm and 2.10 nm (Fig. 1). This suggests that DTAC cations with long alkyl chains were fixed in the interlayer spaces as they were intercalated first and the remaining cations in the interlayer spaces of the

Fig. 1. XRD characterization results of Na+-Mt, OH-Al-Mt, DA-Al-Mt, DTAC-Al-Mt, DTACMt and DA-Mt samples. The used DA or DTAC amounts were 0.5 CEC of Na+-Mt in the DA-Al-Mt and DTAC-Al-Mt samples and 1 CEC in the DA-Mt and DTAC-Mt samples. The used Al13 amounts were fixed at 10 mmol Al/g Mt.

intercalated Mt could be replaced by Al13. Moreover, since Al13 has a strong electrostatic interaction with the charge sites on Mt because of its high charge, it could also have the chance to replace some preadsorbed DTA+ on the charge sites (Zhu et al., 2009). Thereby, a certain amount of Al13 could be intercalated into DTAC-Mt. Moreover, Al13 and DTA+ simultaneously exist in the interlayer spaces, which may influence their distribution and make their arrangement more complex than that of single agent modified Mt. On the other hand, DA-Mt and DA-Al-Mt samples showed the d-values of 001 reflections at 1.86 and 2.02 nm, respectively, which are more than that of DTAC-Mt and DTAC-Al-Mt. For DTAC-Al-Mt and DA-Al-Mt samples, the (001) reflections significantly broadened and also decreased in intensity. The FTIR spectra of Na+-Mt and nanocomposites of Mt are shown in Fig. 2, and all the band assignments are listed in Table 2. The band at about 3630 cm−1 is assigned to the stretching vibration of octahedral OH groups, which are attached to Al3+ or Mg2+ (Bukka et al., 1992). In the FTIR pattern of Na +-Mt, the sharp peak at 3630 cm− 1 , 3450 cm− 1 , 1040 cm − 1 , 917 cm− 1, 845 cm − 1 , 523 cm− 1 and 469 cm − 1 (Table 2) are all characteristic peaks of Mt. The absorption bands in the 700–1400 cm−1 region are primarily assigned to stretching vibration of the Si\\O\\Si bonds and the deformation modes of OH groups attached to various ions (e.g. Al3+, Mg2+ and Fe3+). The band observed at 917 cm−1 is assigned to OH groups attached to Al3+ ions. A new absorption band occurs at 1470 cm− 1 for the DA-Al-Mt, DTAC-Al-Mt, DTAC-Mt and DA-Mt samples compared to Na+-Mt, which is attributed to the \\CH2 shearing vibration while the new bands at 2855 cm− 1 and 2930 cm− 1 are caused by the symmetric stretching and asymmetric stretching of C\\H. Additionally, a new band at 730 cm−1 in the case of DA-Al-Mt, DTAC-Al-Mt and OH-Al-Mt

114

G. Wang et al. / Applied Clay Science 124–125 (2016) 111–118

Fig. 3. Nitrogen adsorption-desorption isotherm of Na+-Mt, OH-Al-Mt and DTAC-Al-Mt samples.

Fig. 2. FTIR spectra of Na+-Mt and nanocomposite Mt.

samples is assigned to Al\\O stretching vibration of Keggin ions ([Al13O4(OH)24(H2O)12]7+), which indicates that the Keggin ions were intercalated into the interlayer spaces of Mt. The absorption peak at 1635 cm−1 is attributed to\\OH deformation of interlayer water, and the bands at 1635 cm−1 of all organic or organic-inorganic nanocomposite Mt samples became wider compared to Na+-Mt. The broadened and larger bands are more obvious with DA-Al-Mt, DTAC-Al-Mt and OH-Al-Mt than those of DTAC-Mt and DA-Mt (Fig. 2). The latter two samples showed much smaller water bands (Fig. 2), as expected because they are hydrophobic. This suggests that the amount of interlayer water is higher when both the aluminum polyhydroxy cation and organic modifier entered into the interlayer spaces of Mt compared to those samples modified with organic cations only (Brindley and Sempels, 1977; Bottero et al., 1980). The nitrogen adsorption-desorption isotherms of three samples (Fig. 3) were found to be of Type-IV in the Brunauer, Deming, Deming

and Teller (BDDT) classification with H3-type loops according to the classification of International Union of Pure and Applied Chemistry (IUPAC). The hysteresis loops are of B-type in the De Boer classification (De Boer et al., 1965). The characteristic of B-type hysteresis loop is that adsorption isotherms rise abruptly with the pressure close to P0 owing to capillary condensation of the sorbate in the mesopores of sample, while desorption isotherms fall steeply under medium pressure and the great separation between the adsorption and desorption isotherms occurred. This implies that there are different sizes and types of pores in the samples, and these pores are characterized as parallel plate slit or “house-of-cards-like” wedge-shaped pores, which are formed by novel meso-microporous delaminated structure and fragments (Liu and Wang, 2003). The pore size distribution (PSD) curves (Fig. 4) were calculated from the BJH method. The primary peaks at ca. 2 nm in the curves of the Na+Mt, OH-Al-Mt and DTAC-Al-Mt samples correspond to the main population of pores, but with a continuous distribution of pores in the range of 2–20 nm. The BET specific surface areas (SBET), total pore volumes and the average pore radius of the Na+-Mt and various nanocomposites of Mt are listed in Table 3. The specific surface areas of different materials increased in the following order: OH-Al-Mt N DA-Al-Mt N DTACAl-Mt N Na+-Mt N DTAC-Mt N DA-Mt. The specific surface areas of

Table 2 Positions and assignments of the FTIR vibration bands. Position (cm−1)

Assignments

3630 3450 2930 2855 3630 1635 1470 1040 917 845 796 730 625 523

\ \OH stretching of Al\ \OH and Mg\ \OH \ \OH stretching of interlayer water Asymmetric stretching of C\ \H Symmetric stretching of C\ \H \ \OH stretching of Al\ \OH and Mg-OH \ \OH deformation of interlayer water \ \CH2 shearing vibration Stretching of Si\ \O\ \Si \ \OH deformation of AlIV\ \OH \OH \ \OH deformation of MgIV\ Bending vibration of Keggin\ \(AlIV)OOH IV Stretching of Keggin\ \(Al )OOH Bending vibration Mg\ \OH Bending vibration Si\ \O\ \Mg

Fig. 4. Desorption pore volume distribution of Na+-Mt, OH-Al-Mt and DTAC-Al-Mt samples.

G. Wang et al. / Applied Clay Science 124–125 (2016) 111–118

115

Table 3 Porous properties of various samples. Samples

DTAC-Mt

DA-Mt

DTAC-Al-Mt

DA-Al-Mt

OH-Al-Mt

Na+-Mt

BET surface area (m2/g) Total porous volume (cm3/g) Average pore radius (nm)

34.9 0.286 7.7

22.9 0.141 5.1

71.4 0.232 4.8

101.2 0.264 4.7

170.1 0.252 3.6

48.6 0.151 5.1

organoclays, DATC-Mt and DA-Mt are lower than that of Na+-Mt because the organic molecules entered into the Mt interlayer spaces and blocked the access to N2 molecules thereby reducing the specific surface area. In contrast, the samples modified by Al13 cations or both organic cations and Al13 cations showed higher specific surface areas and pore volumes as the Al13 cations led to interlayer porosity. 3.2. Kinetics and isotherms of Cr(VI) adsorption on Mt samples The effect of pH on Cr(VI) adsorption capacity of all the materials in the initial pH range of 2.0–12.0 are shown in Fig. 5. The uptake of Cr(VI) highly depended on the initial solution pH, which affects the surface properties of the adsorbents and the ionic state of chromium(Cr). When the initial solution pH increased from 2.0 to 12.0, the adsorption capacity of Cr(VI) firstly increased and then decreased slightly on the DTAC-Al-Mt, DA-Al-Mt and OH-Al-Mt materials (Fig. 5). The maximum adsorption capacity of Cr(VI) by DTAC-Al-Mt and OH-Al-Mt were found at about pH 6.0 while those of DA-Al-Mt were at about pH 4.0. Moreover, irrespective of Na+-Mt, DTAC-Mt or DA-Mt as adsorbent, the adsorption capacity of Cr(VI) firstly decreased and then increased slightly in the pH range of 2.0–12.0, and the maximum adsorption capacity of Cr(VI) were found at about pH 2.0. In general, the maximum adsorption capacity of Cr(VI) on all absorbents were found under acid conditions. These results are in good agreement with the previously published results (Hu et al., 2004). To better explain the above results of the pH analysis, the equilibrium solution pH values after the Cr(VI) adsorption were measured, and a comparison of pH values of solutions before and after the Cr(VI) adsorption is shown in Table 4. The pH values after Cr(VI) adsorption were always higher than those before the Cr(VI) adsorption when the initial pH values were 2.0 and 4.0, except in the samples of OH-Al-Mt, DA-Al-Mt and DTAC-Al-Mt where the initial pH value was 6.0, which suggested that the Cr(VI) adsorption at a pH lower than 6.0 resulted in the release of OH– to solution. Additionally, irrespective of 4.0 or 6.0 as the initial solution pH value, for the same adsorbent, the solution pH values after the Cr(VI) adsorption were almost the same, but the Cr(VI) adsorption capacities under the similar equilibrium solution pH

value were remarkably different. This result suggested that the effect of pH value and the adsorption mechanisms on removal of Cr(VI) were complicated and hence, the Cr(VI) adsorption mechanism of all the prepared materials was analyzed as follows: It is well known that Cr(VI) forms different stable anions at different pH values. At pH values ranging from 2.0 to 6.0, the predominant species of Cr present in the system is Cr2O27 − with a small quantity of 2− dominates (Perez-candela et al., HCrO− 4 whereas at pH N 7.0 CrO4 1995; Amuda et al., 2009; Sarkar et al., 2010). This shows that little adsorption for Cr(VI) occurs from the surface of the “broken bonds” on the edge of the layers that bind the ambient cations, because Cr(VI) mainly exists in the form of an anion, which produces repulsion force with the negative charge on the adsorbent surface (Li et al., 2014). Hence, the particular adsorption behavior of Cr(VI) on various Mt nanocomposites could not be simply explained completely by electrostatic attraction, and a probable mechanism was proposed to be the synergistic effect of redox reaction and ion exchange. At low pH of 2.0–6.0, the reduction for HCrO− 4 could take place through the following redox reaction (1) (Li et al., 2014): 3þ þ − þ 4H2 O HCrO− 4 þ 7H þ 3e →Cr

ð1Þ

Simultaneously, the ion exchange between chromium and surface \\OH groups or hydroxide groups of Al polycations proceeded in the interlayer spaces may have also occurred during the sorption. At low pH,\\OH groups are protonized to\\OH+ 2 , which facilitates the ligand exchange since\\OH+ 2 is easier to displace from metal binding sites than\\OH. The ion exchange could take place through the following reactions (Ma and Zhu, 2006; Li et al., 2014; Ji et al., 2015): `S\OH– þ Hþ →`S\OHþ 2 –

ð2Þ –

ð3Þ

− − `S\OHþ 2 þ HCrO4 →`S\HCrO4 þ H2 O

ð4Þ

`S\OH þ

− HCrO− 4 →`S\HCrO4

þ OH

where `S represents the available adsorption sites. Additionally, for DTAC-Al-Mt and DTAC-Mt samples, at pH b 6.0, the adsorption of chromium occurs mainly through salt formation 2− [(DTA)HCrO− 4 and (DTA)2 Cr2O7 ] and to some extent by anion exchange where the counter anion (Cl−) of DTAC is displaced by HCrO− 4 or Cr2O2− from the exchange sites forming Mt-(DTA)+-HCrO− 7 4 or Mt(Krishna et al., 2000; Sarkar et al., 2010; Zhang et al., 2(DTA)+-Cr2O2− 7 2015). However, at the pH higher than 6.0, the Mt nanocomposites become more negatively charged and hence leads to electrostatic repulsion of negatively charged Cr(VI) species (Yuan et al., 2010), and the decrease in uptake capacity may be also explained by the dual competition of Table 4 Comparison of pH value in solution between before and after Cr(VI) adsorption. Sample name +

Fig. 5. Effect of initial pH on adsorption amount of Cr(VI).

Na -Mt DA-Mt DTAC-Mt OH-Al-Mt DA-Al-Mt DTAC-Al-Mt

Before

After

Before

After

Before

After

2.0 2.0 2.0 2.0 2.0 2.0

2.9 3.2 2.5 3.6 3.9 3.9

4.0 4.0 4.0 4.0 4.0 4.0

9.5 7.9 7.9 5.5 5.1 4.6

6.0 6.0 6.0 6.0 6.0 6.0

9.4 8.0 8.2 5.6 5.3 4.7

116

G. Wang et al. / Applied Clay Science 124–125 (2016) 111–118 Table 5 Adsorption kinetics parameters of Cr(VI) adsorbed on Na+-Mt and Mt nanocomposites. Adsorbate

DA-Mt DTAC-Mt DA-Al-Mt DTAC-Al-Mt OH-Al-Mt Na+-Mt

Fig. 6. Effect of adsorbent dosage on the removal efficiency of Cr(VI).

qe,exp/ mg∙g−1

1.623 1.420 1.631 1.469 1.120 0.483

Pseudo-first-order model

Pseudo-second-order model

qe,cal/ mg∙g−1

k1/ h−1

R2

qe,cal/ mg∙g−1

k2/ g∙mg−1 ∙h−1

R2

1.544 1.351 1.559 1.428 1.068 0.457

7.154 9.250 11.308 12.777 6.827 4.195

0.927 0.870 0.835 0.933 0.824 0.952

1.660 1.440 1.646 1.494 1.148 0.499

6.624 10.327 11.827 16.039 9.274 12.163

0.983 0.995 0.988 0.981 0.965 0.992

later stage was apparently due to slower diffusion of anions into the interior of the adsorbents (Guo et al., 2011). Maximum adsorption occurred after 2 h and there was almost no further increase in adsorption beyond this time. Therefore, the adsorption time was fixed to 2 h in all the adsorption experiments to make sure that the adsorption reached equilibrium. The kinetics curve of Cr(VI) adsorption was simulated using the pseudo-first-order model and pseudo-second-order model (Ho and McKay, 1999a; Krishnan and Anirudhan, 2003; Shawabkeh and Tutunji, 2003). The linear form of the pseudo-first-order model is given by

– CrO2− 4 and OH for adsorption (Suksabye et al., 2009). Therefore, pH 2.0 was selected as the optimum pH value, and the above conclusion of the Cr(VI) adsorption mechanism was further studied by the following batch adsorption experiments. The effect of the dosage of Na+-Mt and Mt nanocomposites on Cr(VI) removal efficiency (R%) are presented in Fig. 6. The removal efficiency of Cr(VI) in the solution increased gradually with the increase of the amount of Na+-Mt and Mt nanocomposites under the conditions of the same initial Cr(VI) concentration (Fig. 6), as expected. When the dosage was more than 20 g/L, removal efficiency (R%) changed slowly. The removal efficiency of DA-Al-Mt was the highest with 95.73% followed by DTAC-Al-Mt with 94.65% at the highest dosage. However, for Na+-Mt, the highest removal efficiency (R%) was only 26.46%. The adsorption capacity and removal efficiency (R%) of Cr(VI) on the Mt nanocomposites were significantly higher than that of Na+-Mt because Mt nanocomposites have higher BET specific surface area, pore volume and density of the adsorption sites after modification by hydroxyl-Al and/or organic cations. Fig. 7 displays the adsorption capacity of Cr(VI) by the different materials as a function of time. All the materials showed fast adsorption during the first 30 min of the reaction and a much slower subsequent removal followed by a steady state within 2 h. The rapid adsorption process at the initial stage could be attributed to the easily accessible active sites on the surfaces and near the edges and the slow adsorption rate in

where C0 and Ct (mg/L) are the initial concentration and the concentration of the Cr(VI) at time (t), respectively, ms (g/L) is the concentration of adsorbents, qe is the amount of Cr(VI) adsorption per unit mass (mg/g) at equilibrium, are the first and the second order rate constants, respectively and t (h) is the adsorption time. The adsorption of Cr(VI) by Na+-Mt and Mt nanocomposites follow the pseudo-second-order model better than the pseudo-first-order model as per the simulation results (Table 5). This indicates that the rate-limiting step may be chemical adsorption or chemisorption involving valency changes through sharing or exchange of electrons between adsorbent and adsorbate (Ho and Mckay, 1999b). The experimentally determined amounts of Cr(VI) adsorbed/g of adsorbents, qe,exp, is

Fig. 7. Effect of time on adsorption amount of Cr(VI).

Fig. 8. Effect of initial concentration of Cr(VI) on adsorption amount of Cr(VI).

Ct ¼ C0

  m s qe ms qe −k1 t þ 1− e C0 C0

ð5Þ

and the expression of the pseudo-second-order model is given as Ct ms qe qe k2 t ¼ 1− C0 C0 1 þ qe k2 t

ð6Þ

G. Wang et al. / Applied Clay Science 124–125 (2016) 111–118

117

Table 6 Fitting results of adsorption isotherm of Cr(VI) adsorbed on Na+-Mt and Mt nanocomposites. Adsorbent

Model

Equations

Correlation coefficient, R2

DTAC-Mt

Langmuir model Freundlich model Langmuir model Freundlich model Langmuir model Freundlich model Langmuir model Freundlich model Langmuir model Freundlich model Langmuir model Freundlich model

qe = 0.07492Ce/(1 + 0.00178Ce) qe = 0.1045C0.8915 e qe = 0.07674Ce/(1 + 0.023Ce) qe = 0.1178C0.8613 e qe = 0.07033Ce/(1 + 0.00321Ce) qe = 0.12703C0.81008 e qe = 0.08326Ce/(1 + 0.00267Ce) qe = 0.1299Ce0.85139 qe = 0.05609Ce/(1 + 0.00338Ce) qe = 0.10026C0.8098 e qe = 0.0245Ce/(1 + 0.00579Ce) 0.72512 qe = 0.0548Ce

0.9992 0.9988 0.9962 0.9958 0.9977 0.9990 0.9980 0.9964 0.9975 0.9974 0.9949 0.9935

DA-Mt DTAC-Al-Mt DA-Al-Mt OH-Al-Mt Na+-Mt

shown in Fig. 7. while the calculated values, i.e., qe,cal derived from the simulation of adsorption kinetics are shown in Table 5. The experimental values of qe,exp (Fig. 7) are in better agreement with the qe,cal values by the pseudo-second-order model than those of the pseudo-first-order model. Thus the simulation results suggest that the adsorption of Cr(VI) by Na+-Mt and Mt nanocomposites fit the pseudo-second-order model quite well. Moreover, the pseudo-second-order rate constants shown in Table 5 suggest that the rate of Cr(VI) adsorption decreases in the following order: DTAC-Al-M N Na+-Mt N DA-Al-Mt N DTAC-Mt N OHAl-Mt N DA-Mt. The effect of the initial Cr(VI) concentration on the adsorption capacity of Cr(VI) onto Na+-Mt and various Mt nanocomposites are shown in Fig. 8. The adsorption amounts of Cr(VI) onto all the adsorbents increase rapidly and continuously with increasing initial concentration of Cr(VI) but do not reach equilibrium (Fig. 8). To find out the calculated maximum adsorption capacities of various absorbents, the present adsorption data are fitted to two equilibrium adsorption models and the results are shown in Table 6. These results show that both the Langmuir model and Freundlich model fit the equilibrium data well due to the high R2 values (R2 N 0.99), which indicates that the adsorption of Cr(VI) onto various modified Mt is both as monolayer and on heterogeneous surface conditions (Zhang et al., 2015). The adsorption process may be chemisorption based on the above data and also kinetic model analysis. In addition, Cr(VI) adsorption by the different adsorbents decreases in the following order: DA-Al-Mt N DTAC-Mt N DA-Mt N DTAC-Al-Mt N OH-Al-Mt N Na+-Mt when the initial Cr(VI) concentration was kept constant. Among all prepared adsorbents, the organic or inorganic-organic modified Mt have a relatively large Cr(VI) adsorption capacity, and the Cr(VI) adsorption capacities of DA-Al-Mt, DTAC-Mt and DA-Mt samples are similar. This result may be attributable to the large basal spacing and the creation of a great number of cationic adsorption sites for Cr(VI) due to high surfactant loadings (Table S1), and the DA or DTAC loading amounts of Mt nanocomposites were calculated from the thermogravimetric analysis data (Fig. S1). These cationic adsorption sites might be favorable to the Cr(VI) adsorption through anion exchange between 2− the counter anion of surfactant and HCrO − 4 or Cr 2 O 7 . However, the Cr(VI) adsorption on OH-Al-Mt and Na+-Mt mainly depended on electrostatic attraction and the ion exchange between chromium and surface \\OH groups or hydroxide groups of Al polycations proceeded in the interlayer (Li et al., 2014; Ji et al., 2015), which leads to a relatively low Cr(VI) adsorption capacity. 4. Conclusions Various montmorillonite (Mt) nanocomposites were prepared using Al13 cations, organic, and organic-inorganic cations and were characterized by XRD, FTIR spectroscopy and N2 adsorption-desorption isotherm analyses. The d(001) values of all Mt nanocomposites were much larger than that of Na+-Mt. The specific surface areas increased in the following order: OH-Al-Mt N DA-Al-Mt N DTAC-Al-Mt N Na+-Mt N DTAC-Mt N

DA-Mt. Cr(VI) adsorption results showed that the solution pH, initial Cr(VI) concentration, contact time and adsorbent dosage had significant effects, as expected. The adsorption equilibrium time was 2 h and the maximum removal efficiency of Cr(VI) was found in the acid environment. When the adsorption studies were conducted at pH 2.0, the DA-Al-Mt nanocomposite was highly effective for removing Cr(VI) from wastewater followed by DA-Mt. Kinetics studies revealed that Cr(VI) adsorption data followed the pseudo-second-order kinetics model. Isotherm fitting results showed that both the Langmuir model and Freundlich model fitted the equilibrium data well. The mechanism of Cr(VI) adsorption by all adsorbents appears to be chemical adsorption or chemisorption as monolayer and on heterogeneous surface conditions. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 51464004), Natural Science Foundation of Guangxi, China (Grant No. 2012GXNSFBA053146) and Guangxi Experiment Centre of Science and Technology, China (Grant No. YXKT2014021). One of the authors (SK) was supported by the College of Agricultural Sciences under Station Research Project No·PEN04566. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.clay.2016.02.008. References Amuda, O.S., Adelowo, F.E., Ologunde, M.O., 2009. Kinetics and equilibrium studies of adsorption of chromium (VI) ion from industrial wastewater using Chrysophyllum albidum (Sapotaceae) seed shells. Colloids Surf. B: Biointerfaces 68, 184–192. Aouad, A., Pineau, A., Tchoubar, D., Bergaya, F., 2006. Al-pillared montmorillonite obtained in concentrated media. Effect of the anions (nitrate, sulfate and chloride) associated with the Al species. Clay Clay Miner. 54, 626–637. Bajda, T., Klapyta, Z., 2013. Adsorption of chromate from aqueous solutions by HDTMAmodified clinoptilolite, glauconite and montmorillonite. Appl. Clay Sci. 86, 169–173. Bottero, J.Y., Cases, J.M., Fiessinger, F., Poirier, J.E., 1980. Studies of hydrolyzed aluminum chloride solutions. Nature of aluminum species and composition of aqueous solutions. J. Phys. Chem. 84, 2933–2939. Brindley, G.W., Sempels, R.E., 1977. Preparation and properties of some hydroxyl-aluminium beidellites. Clay Miner. 12, 229–237. Bukka, K., Miller, J.D., Shabtai, J., 1992. FTIR study of deuterated montmorillonite: structural features relevant to pillared clay stability. Clay Clay Miner. 40 (1), 92–102. Chen, C.Q., 1980. Research the effects of chromium on cabbage and amaranth growth. Environ. Pollut. Control. J. 4, 21–22. Costa, M., 2003. Potential hazards of hexavalent chromate in our drinking water. Toxicol. Appl. Pharmacol. 188, 1–5. De Boer, J.H., Linsen, B.G., Van Der Plas, Th., Zondervan, G.J., 1965. Studies on pore systems in catalysts: VII. Description of the pore dimensions of carbon blacks by the t method. J. Catal. 4, 649–653. Demirbas, E., Kobya, M., Konukman, A.E.S., 2008. Error analysis of equilibrium studies for the almond shell activated carbon adsorption of Cr(VI) from aqueous solutions. J. Hazard. Mater. 154, 787–794. Dupont, L., Guillon, E., 2003. Removal of hexavalent chromium with a lignocellulosic substrate extracted from wheat bran. Environ. Sci. Technol. 37, 4235–4241.

118

G. Wang et al. / Applied Clay Science 124–125 (2016) 111–118

EPA, U.S., 2012. National Primary Drinking Water Regulations. United States Environmental Protection Agency, Washington D.C. Fonseca, B., Maio, H., Quintelas, C., Teixeira, A., Tavares, T., 2009. Retention of Cr(VI) and Pb(II) on a loamy sand soil kinetics, equilibria and breakthrough. Chem. Eng. J. 152, 212–219. Gregg, S.J., Sing, K.S.W., 1982. Adsorption, Surface Area and Porosity. second ed. Academic Press, London. Guo, Z.Q., Li, Y., Zhang, S.W., Niu, H.H., Chen, Z.S., Xu, J.Z., 2011. Enhnaced sorption of radiocobalt from water by Bi(III) modified montmorillonite: a novel adsorbent. J. Hazard. Mater. 192, 168–175. Gupta, V.K., Rastogi, A., 2009. Biosorption of hexavalent chromium by raw and acid treated green alga Oedogonium hatei from aqueous solutions. J. Hazard. Mater. 163, 396–402. Gupta, V.K., Saini, V.K., Jain, N., 2005. Adsorption of As(III) from aqueous solutions by iron oxide-coated sand. J. Colloid Interface Sci. 288, 55–60. Ho, Y.S., McKay, G., 1999a. Comparative sorption kinetic studies of dye and aromatic compounds onto fly ash. J. Environ. Sci. Health, Part A: Tox. Hazard. Subst. Environ. Eng. 34, 1179–1204. Ho, Y.S., Mckay, G., 1999b. Pseudo-second order model for sorption processes. Process Biochem. 34, 451–465. Hu, J., Lo, I.M.C., Chen, G., 2004. Removal of Cr(VI) by magnetite nanoparticle. Water Sci. Technol. 50, 139–146. Ji, M., Su, X., Zhao, Y.X., Qi, W.F., Wang, Y., Chen, G.Y., Zhang, Z.Y., 2015. Effective adsorption of Cr(VI) on mesoporous Fe-functionalized. Akadama clay: optimization, selectivity, and mechanism. Appl. Surf. Sci. 344, 128–136. Krishna, B.S., Murty, D.S.R., Prakash, B.S.J., 2000. Thermodynamics of chromium (VI) anionic species sorption onto surfactant-modified montmorillonite clay. J. Colloid Interface Sci. 229, 230–236. Krishnan, K.A., Anirudhan, T.S., 2003. Removal of cadmium (II) from aqueous solutions by steam-activated sulphurised carbon prepared from sugar-cane bagasse pith: kinetics and equilibrium studies. Water SA 29, 147–156. Kula, I., Ugurlu, M., Karaoglu, H., Celik, A., 2008. Adsorption of Cd(II) ions from aqueous solutions using activated carbon prepared from olive stone by ZnCl2 activation. Bioresour. Technol. 99, 492–501. Li, T., Shen, J.F., Huang, S.T., Li, N., Ye, M.X., 2014. Hydrohermal carbonization synthesis of a novel montonrillonite supported carbon nanosphere adsorbent for removal of Cr(VI) from waste water. Appl. Clay Sci. 93-94, 48–55. Liu, L.B., Wang, X.H., 2003. Fractal analysis of bentonite porosity by using nitrogen adsorption isotherms. J. Chem. Eng. Chin. Univ. 17, 591–595. Luan, W.L., Li, M.L., 1998. Development and Application of Bentonite. Geological Publishing House, Beijing. Lv, G.C., Li, Z.H., Jiang, W.T., Ackley, C., Fenske, N., Demarco, N., 2014. Removal of Cr(VI) from water using Fe(II)-modified natural zeolite. Chem. Eng. Res. Des. 92, 384–390.

Ma, J.F., Zhu, L.Z., 2006. Simultaneous sorption of phosphate and phenanthrene to inorgano-organo-bentonite from water. J. Hazard. Mater. 136, 982–988. Matusik, J., Bajda, T., 2013. Immobilization and reduction of hexavalent chromium in the interlayer space of positively charged kaolinites. J. Colloid Interface Sci. 398, 74–81. Perez-candela, M., Martin-martinez, J.M., Torregrosa-Macia, R., 1995. Chromium(VI) removal with activated carbons. Water Res. 29, 2174–2180. Rathnayake, S.I., Xi, Y.F., Frost, R.L., Ayoko, G.A., 2015. Structural and thermal properties of inorganic-organic montmorillonite: implications for their potential environmental applications. J. Colloid Interface Sci. 459, 17–28. Sarkar, B., Xi, Y.F., Megharaj, M., Krishnamurti, G.S.R., Rajarathnam, D., Naidu, R., 2010. Remediation of hexavalent chromium through adsorption by bentonite based Arquad ® 2HT-75 organoclays. J. Hazard. Mater. 183, 87–97. Shawabkeh, R.A., Tutunji, M.F., 2003. Experimental study and modeling of basic dye sorption by diatomaceous clay. Appl. Clay Sci. 24, 111–120. Suksabye, P., Nakajima, A., Thiravetyan, P., Baba, Y., Nakbanpote, W., 2009. Mechanism of Cr(VI) adsorption by coir pith studied by ESR and adsorption kinetic. J. Hazard. Mater. 161, 1103–1108. Tiravanti, G., Petruzzelli, D., Passino, R., 1997. Pretreatment of tannery wastewaters by an ion exchange process for Cr(III) removal and recovery. Water Sci. Technol. 36, 197–207. Wang, G.F., Lu, X.J., Zhang, S., Ma, S.J., Qiu, J., Yang, J.L., 2011. Study on microstructure variation laws of Al-pillared montmorillonite. Adv. Mater. Res. 158, 248–255. WHO, 2001. Guidelines for Drinking-water Quality. fourth ed. World Health Organization Geneva, Switzerland. Xi, D.L., Sun, Y.S., Liu, X.Y., 2002. Environmental Monitoring. Higher Education Press, Beijing, pp. 60–61. Yuan, P., Liu, D., Fan, M.D., Yang, D., Zhu, R.L., Ge, F., Zhu, J.X., He, H.P., 2010. Removal of hexavalent chromium [Cr(VI)] from aqueous solutions by the diatomite-supported/ unsupported magnetite nanoparticles. J. Hazard. Mater. 173, 614–621. Zhang, S., Yang, J., Xin, X.D., Yan, L.G., Wei, Q., Du, B., 2015. Adsorptive removal of Cr(VI) from aqueous solution onto different kinds of modified bentonites. Environ. Prog. Sustainable Energy 34, 39–46. Zhao, Z., 2005. Application Principle of Adsorption. Chemical Industry Press, Beijing. Zhou, Q., He, H.P., Zhu, J.X., Shen, W., Frost, R.L., Yuan, P., 2008. Mechanism of p-nitrophenol adsorption from aqueous solution by HDTMA+-pillared montmorillonite—implications for water purification. J. Hazard. Mater. 154, 1025–1032. Zhu, L.Z., Zhu, R.L., 2007. Simultaneous sorption of organic compounds and phosphate to inorganic–organic bentonites from water. Sep. Purif. Technol. 54, 71–76. Zhu, R.L., Wang, T., Ge, F., Chen, W.X., You, Z.M., 2009. Intercalation of both CTMAB and Al-13 into montmorillonite. J. Colloid Interface Sci. 335, 77–83.