Adsorption behavior of MnO2 functionalized multi-walled carbon nanotubes for the removal of cadmium from aqueous solutions

Accepted Manuscript Adsorption behavior of MnO2 functionalized multi-walled carbon nanotubes for the removal of cadmium from aqueous solutions Chao Luo, Rongyan Wei, Dan Guo, Shengfang Zhang, Shiqiang Yan PII: DOI: Reference:

S1385-8947(13)00467-1 http://dx.doi.org/10.1016/j.cej.2013.03.128 CEJ 10613

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

7 February 2013 26 March 2013 27 March 2013

Please cite this article as: C. Luo, R. Wei, D. Guo, S. Zhang, S. Yan, Adsorption behavior of MnO2 functionalized multi-walled carbon nanotubes for the removal of cadmium from aqueous solutions, Chemical Engineering Journal (2013), doi: http://dx.doi.org/10.1016/j.cej.2013.03.128

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Adsorption behavior of MnO2 functionalized multi-walled carbon nanotubes for the removal of cadmium from aqueous solutions Chao Luo, Rongyan Wei, Dan Guo, Shengfang Zhang, Shiqiang Yan* College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, P. R. China

*

Corresponding author. Tel.:+86 931 8912582; Fax: +86 931 8912582 E-mail address:[email protected] 1

Abstract Manganese dioxide (MnO2) was coated on acid oxidized multi-walled carbon nanotubes (MnO2/o-MWCNTs) by immersing o-MWCNTs into a KMnO4 aqueous solution. The materials were characterized by different techniques, such as Fourier-transform infrared spectroscopy, X-ray diffraction, thermogravimetric analysis, and BET surface area measurement. MnO2/o-MWCNTs were the most effective materials for cadmium ion adsorption among other adsorption materials synthesized at different conditions. Cadmium ion adsorption by MnO2/o-MWCNTs was strongly pH dependent. The Langmuir isotherm model is consistent with the experimental data at different temperatures; the maximum adsorption capacity was determined (qmax = 41.6 mg·g-1). A contact time of at least 150 min was required to attain adsorption equilibrium, and kinetic adsorption can be described by the pseudo-second-order rate equation. The overall rate process was apparently influenced by external mass transfer, intraparticle diffusion, and chemical adsorption. Moreover, the thermodynamic parameters indicated that the adsorption process was spontaneous and endothermic, and that the adsorption mechanism included both physical and chemical adsorption mechanisms. Keywords:

Cadmium;

Multi-walled

carbon

Adsorption behavior

2

nanotubes;

Manganese

dioxide;

1. Introduction Carbon nanotubes (CNTs), which include single-walled CNTs and multi-walled CNTs (MWCNTs), are relatively new materials that have recently gained considerable attention because of their mechanical, electrical, optical, physical, and chemical properties [1, 2]. The excellent properties of CNTs offer unprecedented opportunities for the novel applications of these materials in many fields, CNTs can be utilized as electrochemical capacitors [3], conductive polymer composites [4], hydrogen storage [5], catalyst support [6], and even as tools for daily use [7]. Considering their large surface areas and  -

electrostatic interactions, CNTs exhibit exceptional

adsorption capabilities for gas and liquid phases, such as organic vapors [8], inorganic pollutants [9], perfluorinated compounds [10], fulvic acid [11], and several heavy metal ions [12]. However, the application of CNTs is significantly hindered by their poor dispersion into solvents because of the strong intermolecular van der Waals interaction between tubes, which can lead to the formation of aggregates [13]. Strong acid oxidation, such as that involving HNO3, H2SO4 acid, and/or the mixture of these two oxides, can improve dispersion and maximize the use of CNTs because of the resulting change in the surface characteristics of CNTs [14]. The introduction of new functional groups via strong acid oxidation enhances the adsorption capacity of CNTs [14, 15]. Nevertheless, the adsorption capacity of oxidized CNTs remains limited. Manganese dioxide (MnO2) is considered as the most important scavenger of aqueous trace metals in soil, sediments, and rocks because of its seemingly dominant 3

adsorptive behavior [14]. Compared with iron or aluminum oxides, manganese oxides have higher affinities and provide an efficient scavenging pathway for many heavy metals [16]. However, using pure manganese oxide as adsorption material is unfavorable because of economic factors as well as the poor physical and chemical characteristics of such material [14]. Nevertheless, manganese oxide coated on media surfaces using low-cost and simple methods may provide effective surfaces, thereby becoming promising commercial materials for the removal of heavy metals from wastewater. The presence of a high amount of various heavy metals in effluents is known to be detrimental to human health and the environment [17]. In fact, such heavy metals are generally non-biodegradable, highly toxic, and are usually considered as carcinogenic. Therefore, the removal of heavy metal compounds from natural waters and wastewater streams is of great environmental relevance. Adsorption is considered as an attractive method for the removal of heavy metals. Among the heavy metal ions, cadmium ions (Cd2+) are one of the most dangerous ones; they are generally more difficult to remove through adsorption compared with others, such as lead, zinc, and copper [18]. In some cases, Cd2+ at low concentrations cannot be removed at all [19, 20]. In this work, MnO2 was spontaneously coated onto oxidized MWCNTs (MnO2/o-MWCNTs) by a simple method [21]. Hence, this material has two kinds of adsorption groups, namely, oxygen-containing functional groups and manganese oxide.

The

adsorption

capacities

of 4

raw-MWCNTs,

o-MWCNTs,

and

MnO2/o-MWCNTs synthesized at different conditions were compared to determine the most effective adsorbent material. To evaluate these adsorption behaviors further, solution pH, contact time, and temperature were examined. The adsorption kinetics, rate-controlling mechanisms, and thermodynamics of the adsorbents were also investigated. Moreover, the effects of different adsorption groups on adsorption behavior and mechanisms were discussed. 2. Materials and methods 2.1. Materials MWCNTs were purchased from Alpha Nano Technology Co., Ltd (China). The specific surface area was over 200 m2·g-1. The outer diameter was 20 nm to 30 nm, and the length was 5 µm to 20 µm. The purity of MWCNTs, which are referred to as ―raw-MWCNTs,‖ was over 95%. 2.2. Preparation of material The raw-MWCNTs were immersed in concentrated nitric acid for 24 h. Then, they were oxidized with nitric acid solution (68%) at 130 °C for 4 h. The resulting MWCNTs were rinsed with deionized water and dried at 105 °C for 24 h. These materials were denoted as ―o-MWCNTs.‖ 2.3. Preparation of MnO2/o-MWCNTs MnO2 was spontaneously deposited onto the o-MWCNTs through a direct redox reaction between the o-MWCNTs and MnO4- [21]. First, 200 ml of 0.1 M KMnO4 solutions were heated to 70 °C using a circulator. Then, 1.0 g of o-MWCNTs

5

were added to the solutions with different pH values. The pH of the solution was controlled by HCl. During synthesis, the temperature of the solution was maintained at 70 °C for 3 h using the circulator. Both protons and electrons are needed to reduce MnO4- ions into MnO2, as shown in the following reaction: MnO4  4H   3e  MnO2  2H 2O .

(1)

The suspension was filtered and washed several times using deionized water and then dried at 105 °C for 12 h. These materials were denoted as ―MnO2/o-MWCNTs.‖ 2.4. Characterization of MWCNTs The

functional

groups

of

the

raw-MWCNTs,

o-MWCNTs,

and

MnO2/o-MWCNTs were examined using a NEXUS 670 Fourier-transform infrared (FTIR) spectrometer (Nicolet Instrument Corporation, USA). Thermogravimetric analysis (TGA; DuPont 1090B, DuPont, USA) for temperatures ranging from 15 °C to 800 °C was performed at a heating rate of 10 °C·min-1 and a nitrogen flow of 100 mL·min-1. The BET surface area, pore volume, and pore size were characterized using nitrogen adsorption at liquid nitrogen temperatures (Sorptomatic 1990, Thermo, USA). The inorganic phases of the o-MWCNTs and MnO2/o-MWCNTs were examined using X-ray diffraction (XRD; XRD-6000, Shimadzu, Japan). 2.5. Determination of Cd2+ concentration Cd2+ concentrations were measured using the spectrophotometric method (721G visible spectrophotometer, Shanghai Precision & Scientific Instrument Co., Ltd., China). In each Cd2+ measurement experiment, a 1 ml solution containing Cd2+ (10×

6

dilution of each sample solution) was transferred into a 250 ml separating funnel. Then, 100 ml deionized water, 0.25 ml hydroxylammonium chloride solution (20%), 15 ml dithizone chloroform solution (0.0002%), and 5 ml sodium hydroxide (40%)-potassium cyanide (0.05%) solution were successively added to the Cd2+ solution. The mixture was forcefully shaken for 2 min and collected in a glass cuvette after being left standing for a short period. The resultant sample solution was transferred to the spectrophotometer for analysis at 518 nm. 2.6. Adsorption experiments Analytical grade cadmium nitrate was used to prepare a 1000 mg·L-1 stock solution, which was further diluted to the required concentration before use. The raw-MWCNTs, o-MWCNTs, and MnO2/o-MWCNTs synthesized at different initial solution pH values (pH = 7, pH = 5, pH = 3, and pH = 1) were prepared (denoted as ―MnO2/o-MWCNTs (pH=7)‖, ―MnO2/o-MWCNTs (pH=5)‖, ―MnO2/o-MWCNTs (pH=3)‖ and ―MnO2/o-MWCNTs (pH=1)‖, respectively) as adsorbents for Cd2+ removal. To determine the adsorption capacities of the different materials, 0.05 g of the different adsorbents were added into a 100 ml solution with initial Cd2+ concentrations from 5 mg·L-1 to 30 mg·L-1 (5 mg·L-1 intervals) at a pH of about 5.0. The solution was stirred for 150 min at a fixed temperature (25 °C). The two phases were separated by filtration using a 0.45 µm microporous membrane filter. The finial Cd2+ concentrations remaining in the solution were analyzed by spectrophotometric method with dithizone. The equilibrium metal adsorption capacity was calculated for each Cd2+ sample 7

using the following expression: qe 

V (C0  Ce ) , m

(2)

where qe is the adsorbent adsorption capacity (mg·g-1), V is the sample volume (L), C0 is the Cd2+ concentration (mg·L-1), Ce is the equilibrium Cd2+ concentration (mg·L-1), and m is the weight of the adsorbents (g). The most effective material with the maximum adsorption capacity was chosen as the adsorbent of the following experiments and was kept constant. The pH values of 2.0, 4.0, and 7.0 at initial concentrations from 5 mg·L-1 to 30 mg·L-1 were chosen to evaluate the effect of pH on the Cd2+ adsorption process. In studying the adsorption isotherms, weighed amounts of the adsorbent (0.05 g) were added to a fixed volume of a sample solution containing varying concentrations ranging from 5 mg·L-1 to 30 mg·L-1 at 295, 315, and 325 K. The effect of contact time ranging from 1 min to 300 min was investigated. The samples were prepared by adding 1 g of the adsorbent into a 2000 ml solution with a Cd2+ concentration of 15 mg·L-1 at 298 K. At predetermined time intervals, samples were collected using a 0.45 µm membrane filter. Kinetic experiments were performed using a series of 100 ml flasks containing 0.05 g adsorbent and 100 ml of 15 mg·L-1 Cd2+ solution within a temperature range of 293 K to 323 K from 1 min to 300 min. The adsorption thermodynamic experiments were performed at 295, 315, and 325 K. Various solutions with initial Cd2+ concentrations ranging from 5 mg·L-1 to

8

30 mg·L-1 were prepared, and 0.05 g of the adsorbents were then added to each solution. 3. Results and discussion 3.1. Characterization of different materials The FTIR spectra of raw-MWCNTs, o-MWCNTs, and MnO2/o-MWCNTs are compared in Fig. 1. The HNO3 treatment produced carboxyl groups on the external surface of the MWCNTs because of oxidation, as indicated by the characteristic peaks at 3443 and 1727 cm-1 of the stretching vibrations of v(OH) and v(C=O) of the carboxylic groups (COOH), respectively. The bands within the 1650 cm-1 to 1550 cm-1 range are associated with the C=O groups under different environments (ketone/quinone) and with the C=C in aromatic rings, whereas the bands within the 1300 cm-1 to 950 cm-1 range prove the presence of C–O bonds in various chemical surroundings [22]. Detectable quantities of oxygen-containing functional groups, such as hydroxyl, carboxyl, and carbonyl groups, were found to be present on the surface of o-MWCNTs. These functional groups act as the initiation points for adsorbing external materials. In Fig. 1, a broad band is observed in the composite at the low-frequency region around 518 cm-1. The presence of this band is attributed to the Mn–O and Mn–O–Mn vibrations [23]. Insert Fig. 1 here Fig. 2 shows the typical XRD patterns of o-MWCNTs and MnO2/o-MWCNTs. The XRD pattern of o-MWCNTs has a sharp peak and a broad weak peak. The XRD pattern of MnO2/o-MWCNTs shows the diffraction pattern of MWCNTs, but exhibits 9

few well-defined peaks involving crystal MnO2. These results suggest that the MnO2 is at a non-stoichiometric and amorphous phase [24]. Insert Fig. 2 here TGA provided useful information on o-MWCNTs and MnO2/o-MWCNTs because most of the functional groups in MWCNTs were thermally unstable (Fig. 3). The sample of raw-MWCNTs was stable and hardly decomposed below 800 °C, whereas the o-MWCNTs exhibited a weight loss of about 11%. The thermal degradation of o-MWCNTs and MnO2/o-MWCNTs is a multistage process because of the different adsorption groups introduced onto the surface of CNTs. The first weight loss interval of all the samples below 200 °C is attributed mainly to the evaporation of saturated water [13]. At higher temperatures (≥200 °C), the weight loss of o-MWCNTs can be attributed to the thermal decomposition of side-groups, such as the hydroxyl, carboxyl, and carbonyl groups. And the weight loss of MnO2/o-MWCNTs at temperatures between 200 °C and 800 °C corresponds to the loss of oxygen atoms from the octahedral layer framework associated with the partial reduction of Mn4+ to Mn3+ and organic functional groups [13, 21]. Insert Fig. 3 here The surface area of o-MWCNTs (177 m2·g-1) is smaller than that of raw-MWCNTs (≥200 m2·g-1). This difference can be attributed to the shortened length of o-MWCNTs and to the confined space among isolated MWCNTs, which appeared narrower after the oxidation of the raw-MWCNTs. The surface area, pore volume, and pore size of MnO2/o-MWCNTs (96 m2·g-1, 0.318 cm3·g-1 and 13.3 nm) 10

are evidently much lower than those of the o-MWCNTs (177 m2·g-1, 0.770 cm3·g-1 and 17.4 nm). The formation of oxygen-containing functional groups and MnO2 blocks the pore entrances, thereby decreasing the surface area, pore volume, and pore size of MWCNTs. Nevertheless, various groups introduced on the surface of raw-MWCNTs provide numerous sorption sites, thereby increasing the sorption capacities of o-MWCNTs and MnO2/o-MWCNTs. The dispersibility of raw-MWCNTs in water remarkably changed after modification (Fig. 4a–c). The raw-MWCNTs had strong tendencies to agglomerate because of their nano-size and high surface energy, leading to their poor dispersion in water (Fig. 4a). However, oxidation introduced polar (hydrophilic) groups on the surface of MWCNTs (Fig. 4b), creating the electrostatic stability required for a long-term stable dispersion in water [13]. The dispersibility of MnO2/o-MWCNTs (Fig. 4c) is better than that of o-MWCNTs because of the possible greater electrostatic repulsion provided by MnO2, which stabilizes the MWCNTs against van der Waals attractions [25]. Good dispersibility of materials in water is beneficial to the adsorption process and allows the facile manipulation and processing of these materials in analytical chemistry and environmental protection. Insert Fig. 4 here 3.2. Study on spectrophotometric method with dithizone Cd2+ concentrations were measured using the spectrophotometric method. Dithizone was used as the chelating agent for Cd2+, and chloroform was used as the solvent for dithizone. Cd2+ formed red complexes with the dithizone chloroform 11

solution in the alkaline solution and were extracted by chloroform. The maximum adsorption

wavelength

was

518 nm,

and

the

molar

absorptivity

was

8.56 × 104 L·mol-1·cm-1. Determining the Cd2+ calibration curve necessitated the measurement of absorbance at 518 nm as a function of concentration and the fitting of data to the Lambert–Beer law (r = 0.999). This calibration curve was used to convert absorbance data into concentrations. 3.3. Adsorption capacity of different materials Spontaneously depositing MnO2 onto the MWCNTs by a direct redox reaction is the simplest method for synthesizing MnO2 on MWCNTs [21]. However, in this study, the organic functional groups may influence the synthesis process; the Mn7+ ions in the solution are adsorbed onto the surface of o-MWCNTs by carboxyl groups through electrostatic attraction, and these ions are in situ deoxidized to MnO2. The reaction can be written as follows [26]: 4MnO4  4H   4MnO2  2H 2O  3O2  .

(3)

The initial solution pH is evidently important during the synthesis process. Note that in the current work, the weight of MnO2/o-MWCNTs increased with the decrease of the initial synthesis solution pH (1.876 g, pH = 7.0; 2.357 g, pH = 5.0; 2.497 g, pH = 3.0; 2.581 g, pH = 1.0). Such changes indicate that the MnO2 loading level on the surface of o-MWCNTs increased with the decrease in the initial synthesis solution pH from 7.0 to 1.0. The adsorption capacities of different materials were investigated, and the results are presented in Fig. 5. The adsorption capacity of o-MWCNTs was much higher than 12

that of raw-MWCNTs, suggesting that the organic functional groups on the o-MWCNTs, such as hydroxyl, carboxyl, and carbonyl groups, are expected to be the active centers for Cd2+. In Fig. 5, the adsorption capacities of most MnO2/o-MWCNTs are shown to be greater than those of the o-MWCNTs, indicating that MnO2/o-MWCNTs provided more binding sites for Cd2+ compared with o-MWCNTs. The adsorption capacity of Cd2+ increased from 5.68 mg·g-1 to 35.97 mg·g-1 with the increase in the initial synthesis solution pH from 1.0 to 5.0. This change is probably due to the MnO2 morphology and the MnO2 loading level on the surface of o-MWCNTs. As cited in previous literature [21], the roughness of MnO2 deposits on the surface of o-MWCNTs increases with the decrease in the initial synthesis solution pH because the reduction reaction rate increases with decreasing initial solution pH. Another reason is the reduction rate of MnO4- ions to MnO2 by o-MWCNTs, which affects the morphology of MnO2 deposits on o-MWCNTs. The morphology of MnO2 on the surface of o-MWCNTs is more uniform, more continuous, and thinner with increasing initial synthesis solution pH. This characteristic is beneficial to the effective removal of Cd2+. However, the excess MnO2 load hinders the adsorption of organic functional groups, effectively decreasing the volume. In addition, the low loading level of MnO2 (at an initial synthesis solution pH = 7.0) causes the adsorption capacities of MnO2/o-MWCNTs to worsen compared with those at pH = 3.0. Therefore, the adsorption capacities of different materials mainly depend on the functional groups as well as on the morphology and loading level of MnO2 on the adsorbents surface. In this work, the most effective removal of Cd2+ occurred with

13

MnO2/o-MWCNTs at an initial synthesis solution of pH = 5.0. Insert Fig. 5 here 3.4. Effect of solution pH The pH value is known as one of the parameters with the greatest effect on the adsorption of metallic ions [27]. This characteristic is attributed to the surface electric charge density of the adsorbent, which facilitates or reduces electrostatic interaction and ion exchange [28, 29]. Notably, deposition plays a main role in the adsorption process in a basic region. At a pH of above 7.5, only Cd(OH)+ and Cd(OH)02 significantly contribute to the total cadmium in a solution. Therefore, the main species in a solution at a pH of above 7.5 are predominantly Cd(OH)+ and Cd(OH)02, as shown in the following possible reactions: Cd 2  OH   Cd (OH ) ,

(4)

Cd (OH )  OH   Cd (OH )02 .

(5)

Moreover, metal hydroxides in the pores or spaces around particles are improbable [30]. Thus, the percentage removal of Cd2+ by precipitation is much greater than that by adsorption at a pH of above 7.5. Fig. 6 shows that the Cd2+ adsorption capacity increases with the increase in pH value from 2.0 to 7.0. At a pH of 2.0, the adsorption effect is very weak because of an effective competition between high concentrations of H+ and H3O+ [24]. Adsorption capacity increases sharply at pH 4.0, which can be attributed to an increase in affinity of the adsorbent surfaces toward Cd2+. Thus, with an increase in pH, the competition

14

for adsorption sites between protons and metal species decreases. Moreover, surface positive charge decreases, which leads to less coulombic repulsion of metals . At a pH of 7.0, the hydrolysis of Cd2+ is greater than that at pH values of 2.0 and 4.0, and the higher adsorption capacity of MnO2/o-MWCNTs may also be the result of the relationship between adsorption and precipitation. Insert Fig. 6 here 3.5. Adsorption isotherms Isotherms are the equilibrium relations between the adsorbate concentrations in the solid phase and that in the liquid phase. Maximum adsorption capacity can be obtained from isotherms. Equilibrium adsorption isotherms are often described by the Langmuir and Freundlich isotherms. These two models are the most common isotherms used to describe the solid–liquid adsorption system [31]. The Langmuir and Freundlich isotherm parameters for Cd2+ adsorption onto MnO2/o-MWCNTs at different temperatures (295, 315, and 325 K) are given in Table 1. 3.5.1. Langmuir isotherm Langmuir isotherm is often used to describe the solute adsorption from liquid solutions. This model assumes a monolayer adsorption onto a homogeneous surface with a finite number of identical sites; it is calculated using the following equation [32]: Ce C 1   e, qe bqm qm

(6)

15

where qe (mg·g-1) is the amount of solution adsorbed per unit mass of adsorbent, Ce (mg·L-1) is the solute equilibrium concentration, qm (mg·g-1) is the maximum adsorbate amount that forms a complete monolayer on the surface, and b (L·mg-1) is the Langmuir constant related to adsorption heat. When Ce/qe is plotted against Ce and the data are regressed linearly (Fig. S1), qm and b constants can be calculated from the slope and the intercept. The shape of the isotherm has been discussed with the aim of predicting the favorableness of an adsorption system. The essential features of the Langmuir model can be expressed in terms of the separation factor or the equilibrium parameter RL, which is defined as [33]

RL 

1 , 1  bC0

(7)

where b (L·mg-1) is the Langmuir constant, and C0 is the initial Cd2+ concentration (mg·L-1). The values of RL indicate the isotherm shapes, which can be unfavorable (RL  1) or favorable (0  RL  1) [34]. Table 1 presents the calculated RL values versus the initial Cd2+ concentration at 295, 315, and 325 K. The RL values were found to be in the 0–1 range, which indicates the suitability of MnO2/o-MWCNTs as a Cd2+ adsorbent. 3.5.2. Freundlich isotherm The Freundlich isotherm model considers the multilayer and heterogeneous adsorption . Its linearized form can be given as follows [35]: 16

1 ln qe  ln K F  ln Ce , n

(8)

where qe and Ce have the same definitions as those in the Langmuir equation cited above. KF and n are Freundlich constants related to adsorption capacity and adsorption intensity, respectively. When ln qe is plotted against ln Ce and the data are regressed linearly (Fig S2), 1/n and KF constants are determined from the slope and the intercept. Table 1 show that the Langmuir isotherm model better fits the experimental results over the experimental range with good correlation coefficients. The experimental data fitted well with the Langmuir isotherm, which suggests the metal ion adsorption from the monolayer coverage on the adsorbent surface [36]. The maximum Cd2+ adsorption capacity increased from 38.0 mg·g-1 to 41.6 mg·g-1 when the temperature increased from 295 K to 325 K, which suggests the possibility that Cd2+ sorption by MnO2/o-MWCNTs is an endothermic process. This phenomenon will be further discussed in relation to thermodynamic studies in Section 3.7. Insert Table 1 here 3.6. Adsorption kinetics and rate-controlling mechanism 3.6.1. Effect of contact time The effect of contact time is shown in Fig. 7. At up to 45 min of initial contact time, the Cd2+ adsorption rate was relatively rapid, and a slow increase of Cd2+ adsorption was observed between 45 and 120 min of contact time. The equilibrium of

17

the adsorption process was observed at nearly 150 min. Increasing the contact time obviously increased Cd2+ adsorption. The rapid adsorption at the initial contact time is related to the availability of a large number of active sites on the adsorbent surface, which improves Cd2+ diffusion to the adsorbent surface. The contact was set to 150 min in each experiment. From an initial concentration of 15 mg·L-1 after 150 min of equilibration time, the amount of adsorbed Cd2+ by MnO2/o-MWCNTs was 22.3·mg L-1. Insert Fig. 7 here 3.6.2. Kinetic models The kinetics of Cd2+ adsorption can be modeled using the pseudo-first-order equation, the second-order rate equation, and the pseudo-second-order rate equation, which are written as log(qe  qt )  log qe 

k1 t, 2.303

(9)

1 1   k2t , and qe  qt qe

(10)

t 1 t   , 2 qt k3qe qe

(11)

where qe and qt (mg·g-1) are the adsorption capacities at equilibrium and at time t (min), respectively; k1 (min

-1

), k2 (g·mg-1·min-1), and k3 (g·mg-1·min-1) are the

pseudo-first-order, second-order, and pseudo-second-order rate adsorption constants, respectively. The straight line plots of log (qe-qt) versus t are 1/(qe-qt) versus t and t/qt versus t 18

for the three kinetic models. The pseudo-second-order rate constants were used to calculate the initial adsorption rate h (mg·g-1·min-1) [37], which is calculated as follows: h  k3qe2 .

(12)

The kinetic parameters and correlation coefficients of Cd2+ at different temperatures are listed in Table 2. The qe is the theoretical value, which is calculated using the kinetic models. The value of r3 (pseudo-second-order kinetic model) is extremely high (  0.997), followed by those of the pseudo-first-order and the second-order

kinetic

models,

respectively.

The

qe

calculated

from

the

pseudo-first-order kinetic model were 18.4 mg·g-1 (293 K), 17.8 mg·g-1 (313 K), and 16.4 mg·g-1 (323 K), respectively, which were slightly lower than those from the experimental. The qe values of the second-order kinetic model (33.1 mg·g-1 at 293 K, 38.9 mg·g-1 at 313 K, and 79.2 mg·g-1 at 323 K) were much higher than the experimental qe,exp values. These results suggest that adsorption process could be optimum under the pseudo-second-order kinetic model. The rate constants of the pseudo second-order kinetic model were small (2.28 × 10-3 g·mg-1·min-1 to 4.68×10-3 g mg-1 min-1), indicating that the adsorption process was slow. The initial adsorption rate increased correspondingly with the temperature within a given adsorption system. Insert Table 2 here 3.6.3. Weber–Morris kinetic model If the adsorbate movement from the bulk liquid to the liquid film surrounding the adsorbent is ignored, then the adsorption process in porous solids can be separated 19

into three stages as follows [38]: I. The first region is the external mass transfer of adsorbate across the liquid film to the adsorbent exterior surface, which is called film diffusion. II. The second region is the transport of adsorbate from the adsorbent exterior surface to the pores or capillaries of the adsorbent internal structure. The process is called intraparticle diffusion (or inner diffusion). III. The last region is the adsorption of the adsorbate ions on the interior surface of the adsorbent. The third step is very fast and cannot therefore be treated as a rate-limiting step [39]. To predict the actual rate-controlling step involved in the adsorption process of Cd2+, the Weber–Morris equation was used. This equation is calculated as follows [40]: qt  Kd t1/2  L ,

(13)

where Kd is the intraparticle diffusion rate constant (mg·g-1·min-1/2), and the values of L (mg·g-1) depict the thickness of the boundary layer. The values of Kd and L can be obtained by plotting qt against t1/2. According to this model, if the plot of qt versus t1/2 is linear and passes through the origin, then inner diffusion controls the mass transfer rate. If the plot is non-linear or linear but does not pass through the origin, then film diffusion or chemical reaction controls the adsorption rate [41]. In instances when qt versus t1/2 is multi-linear, the overall adsorption may be controlled by two or more steps such as film diffusion,

20

intraparticle diffusion, and chemical reaction [41]. As shown in Fig. 8, the plots did not pass through the origin, suggesting that intraparticle diffusion is not the unique rate-controlling step [42]. The external mass transfer is also important in the rate-controlling step because the intercepts of the second linear portion is larger than those of the first. By contrast, the plots for the Weber–Morris kinetic model for the MnO2/o-MWCNTs at three different temperatures had three distinct regions. The sharp first linear portion was from 0 min to 45 min, which represented external mass transfer. The second linear portion included the sorption period from 45 min to 150 min, which represented intraparticle diffusion. The third linear portion indicated adsorption–desorption equilibrium, which included the period from 150 min to 300 min [36]. The ratio of external mass transfer duration to intraparticle diffusion was about 1:2. Intraparticle diffusion clearly prevailed over the external mass transfer. Insert Fig. 8 here 3.6.4. Initial adsorption factor To determine the initial adsorption behavior of the initial adsorption factor (Ri), the Weber–Morris model is calculated from the following equation [43]:

qt t  1  Ri [1  ( )1/2 ] , qref tref

(14)

where tref is the longest time in the adsorption process (min), and qref is the amount of Cd2+ adsorbed on the MnO2/o-MWCNTs at time t = tref and Ri = Kdt1/2/qref. The Ri value is divided into four zones: 1) 1  Ri  0.9 is called weak initial 21

adsorption (zone 1), 2) 0.9  Ri  0.5 is the intermediate initial adsorption (zone 2), 3) 0.5  Ri  0.1 is the strong initial adsorption (zone 3), and 4) 0.1  Ri approaches a complete initial adsorption (zone 4) [43]. The percentage indicating that the initial adsorption has already reached (Ini.%) can be calculated as follows [42]: Ini.%  (1  Ri ) 100 .

(15)

The values of Ini.% were 12.90% (Ri = 0.871, r2 = 0.931), 31.6% (Ri = 0.684, r2 = 0.914), and 45.9% (Ri = 0.541, r2 = 0.918) at 293, 313, and 323 K, respectively; the Ri values belonged to zone 2 ( 0.9  Ri  0.5 ). The above results demonstrate that the initial adsorption behavior occurred within an intermediate time period. The values of Ini.% correspondingly increased with the temperature, which agrees well with the obtained initial sorption rate h (Table 2). 3.6.5. Arrhenius equation The results in Fig. 8 are multi-linear, which indicates that the rate-limiting step may include chemical adsorption. These results are similar to previous ones. To gain insight into the rate-controlling steps affecting the kinetic of adsorption, the activation energy for Cd2+ adsorption was calculated using the Arrhenius equation [44]: k3  k0 exp[

 Ea ], RT

(16)

where k3 (g·mg-1·min-1) is the pseudo-second-order rate adsorption constant, k0 (g·mmol-1·min-1) is the temperature-independent factor. Ea (kJ·mol-1) is the activation energy of the adsorption reaction, R is the gas constant (8.314 J·mol-1·K-1), and T is

22

the adsorption absolute temperature (K). The linear form is as follows: ln k3 

 Ea  ln k0 . RT

(17)

When ln k3 is plotted versus 1/T, a straight line with slope –Ea/R is obtained. The activation energy (Ea) was found to be 18.9 kJ·mol-1, indicating that the overall adsorption process was controlled by several processes, including external mass transfer, intraparticle diffusion, and chemical adsorption. 3.7. Thermodynamic studies Thermodynamic parameters were calculated from the variation of the thermodynamic equilibrium constant K 0 with the change in temperature [45]. For adsorption reactions, K0 is defined as follows [46]:

K0 

as vs Cs  , ae ve Ce

(18)

where as is the activity of the adsorbed Cd2+, ae is the activity of the Cd2+ in solution at equilibrium, Cs (mmol·g-1) is the amount of Cd2+ adsorbed per MnO2/o-MWCNT mass, Ce (mmol·ml-1) is the Cd2+ concentration in solution at equilibrium, vs is the activity coefficient of the adsorbed Cd2+, and ve is the activity coefficient of Cd2+ in solution. As the Cd2+ concentration in the solution decreases and approaches zero, K0 can be obtained by plotting ln (Cs/Ce) versus Cs and by extrapolating Cs to zero [45]. The adsorption standard Gibbs free energy changes ( G 0 ) can be calculated as follows: G 0 =-RT ln K0 ,

(19)

23

where R is the gas constant (8.314 J·mol-1·K-1), and T is the temperature in Kelvin. The average standard enthalpy change ( H 0 ) is obtained from the Van’t Hoff equation:

H 0 (T1  T3 ) 1 1 ln K 0 (T3 )  ln K 0 (T1 )  (  ), R T3 T1

(20)

where T3 and T1 are two different temperatures. The standard entropy change ( S 0 ) can be obtained by G0 =H 0 -T S 0

(21)

The thermodynamic parameters are listed in Table 3. The thermodynamic values provide information on the adsorption process of the MnO2/o-MWCNTs under study. The

negative

G 0

values

suggest

that

the

adsorption

of

Cd2+

onto

MnO2/o-MWCNTs is a spontaneous process. The G 0 values decrease when the temperature increases. Hence, the process is efficient at higher temperatures. The enhancement of the adsorption capacity of adsorbent at higher temperatures may be attributed to the increase in pore size and/or to the activation of the adsorbent surface [47]. At higher temperatures, solvated Cd2+ are readily desolvated, the pores increase quickly, and adsorption processes become highly favorable [13]. The change in Gibbs free energy for the physical adsorption, together with chemical adsorption, is between -20 kJ·mol-1 to -80 kJ·mol-1 [48]. The calculated G 0 values indicate that the adsorption processes can be considered as contributions of both physical and chemical adsorption processes. The

positive

H 0

values

suggest 24

that

Cd2+

adsorption

onto

the

MnO2/o-MWCNTs is an endothermic process. One possible interpretation for this endothermicity is that the ion [Cd(H2O)6]2+ needs energy to break off the hydration shell and to be accessible for interaction with the surface of MnO2/o-MWCNTs. The removal of water molecules from the [Cd(H2O)6]2+ ions is an endothermic process. Physical sorption is known to involve an enthalpy change between 2 kJ·mol-1 and 21 kJ·mol-1. Thus, Cd2+ adsorption onto MnO2/o-MWCNTs is mainly a physical process, whereas both physical and chemical adsorptions contribute to the adsorption process [49]. The positive S 0 values suggest a tendency toward a higher randomness of the studied system at equilibrium or structural changes at the surface interface of Cd2+ loaded onto MnO2/o-MWCNTs, which can contribute to positive entropy change. Other processes, such as ion exchange, can also contribute to positive entropy change [50]. Insert Table 3 here 3.8. Cd2+ adsorption mechanisms by MnO2/o-MWCNTs The maximum adsorption capacity of the MnO2/o-MWCNTs increased at temperatures ranging from 295 K to 325 K, indicating a complex adsorption process and changes in the contributions of individual adsorption mechanisms to the overall process: the co-existence of physical adsorption, ion exchange, electrostatic attraction and chemical adsorption, as well as surface complexation [13]. Cd2+ adsorption onto MnO2/o-MWCNTs may consist of three parts. First, Cd2+ could be attracted by the  electron densities of the graphene structure and by the Cd2+/hydrogen exchange occurring on MnO2/o-MWCNTs (MWCNTs behave as weak 25

proton acceptors), which is shown in the following reaction [13]: 2(MWCNT  H  )  Cd 2

(MWCNT )2  Cd 2  2H  .

(22)

Second, acidic oxygen-containing groups behave as ion-exchange sites for Cd2+ retention, creating a metal ligand surface of MnO2/o-MWCNTs with polar functional groups (p) (COOH, C=O, and OH), which can be presented as follows [48]: Cd 2  2(MWCNT  P )

Pb(MWCNT  P)2 ,

(23)

Cd 2  2(MWCNT  HP)

Pb(MWCNT  P)2  2H  ,

(24)

or via hydrogen bonding between the surface functional groups and cadmium cations. This process is considered chemical adsorption rather than physical adsorption [13]. Finally, the manganese oxide surface groups can function as a base (Lewis base), and this condition shows a high coordination capacity with Cd2+ adsorbed onto MnO2/o-MWCNTs and possibly involves an exchange reaction of Cd2+ with manganese oxide, which can be written as follows [18]:

 MnOH  Cd 2

 MnOCd 2  2H  ,

(25)

 MnO  Cd 2

 MnOCd 2 ,

(26)

2( MnOH )  Cd 2

( MnO )2 Cd 2  2H  ,

(27)

2( MnO )  Cd 2

( MnO )2 Cd 2 .

(28)

The pH of the solution was found to decrease after adsorption by MnO2/o-MWCNTs at the end of the experiments. Based on the findings, more metal ions are adsorbed onto MnO2/o-MWCNTs, and more hydrogen ions are released. 3.9. Comparison of adsorbent performance with literature data 26

A comparison of qmax values for Cd2+ on MnO2/o-MWCNTs with those reported previously using different absorbents as shown in Table 4. It can be seen that MnO2/o-MWCNTs have the excellent maximum adsorption capacity of all the other absorbents. The cadmium removal capacities of other adsorbents such as biosorbent, fly ash, commercial activated carbon, HEU-type zeolite, raw and modified MWCNTs were lower than MnO2/o-MWCNTs. Thus, the comparison suggests that MnO2/o-MWCNTs have great potential for use as heavy metal ion adsorbents in wastewater treatment. The cost of adsorbents is also an important parameter for MWCNTs employment in adsorption process. The current cost of MWCNTs (20 $/g) [19] is higher than of the other traditional adsorbents, such as activated carbon (0.08 $/g), synthetic resin (3-25 $/kg) [13], agricultural waste (100 $/t) [48]. But the encouraging news is that improved manufacture and large-scale production have already caused the price of chemical vapor deposition (CVD) produced CNTs to fall substantially, from around 200 $/g in 1999 to 2-20 $/g today [19]. CVD is deemed to be a promising route to reduce the cost of CNTs in the future, which would increase the use of CNTs in environmental protection applications [48]. Insert Table 4 here 4. Conclusions MnO2 was spontaneously deposited on o-MWCNTs by simply immersing the o-MWCNTs into an aqueous KMnO4 solution. FTIR, TGA, and XRD results of the samples at different stages of the functionalized process confirmed that 27

MnO2/o-MWCNTs have two kinds of adsorption groups (oxygen-containing functional groups and manganese oxide). This study is the first one to use MnO2/o-MWCNT as adsorbent for cadmium removal. The adsorption capacity of raw-MWCNTs was improved by oxidation but remained lower than that of MnO2/o-MWCNTs. The MnO2/o-MWCNTs synthesized at pH = 5.0 were the most effective materials for removing Cd2+ ions. The Langmuir model provided a better correlation coefficient, and the separation factor (RL) indicated that MnO2/o-MWCNTs are suitable adsorbents for Cd2+. Kinetic studies suggest that equilibrium is achieved within 150 min. The kinetic data of the adsorption

under

all

investigated

temperatures

fitted

well

with

the

pseudo-second-order kinetic model, and the initial adsorption rate (h) correspondingly increased with the temperature. The overall process appeared to be influenced by external mass transfer, intraparticle diffusion, and chemical adsorption. Cadmium adsorption onto the MnO2/o-MWCNTs is a complex process, the mechanism of which may include both physical and chemical adsorptions. The metal ion adsorption capacity of MnO2/o-MWCNTs strongly depended more on their surface groups, pH, and temperature than on surface area, pore volume, and pore diameter. The results obtained in this work suggest that MnO2/o-MWCNTs with an initial solution of pH = 5 are efficient adsorbents for the removal of cadmium from water.

28

Acknowledgments The authors thank the financial support from the Xinjiang Blue Ridge TunHe Polyester Co.,Ltd.

29

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32

Tables.doc

Table 1 The parameters of Langmuir and Freundlich isotherms for Cd2+ adsorption onto MnO2/o-MWCNTs Langmuir isotherm

Freundlich isotherm

T(K)

qm(mg g-1)

b

r1 2

RL

1/n

KF

r2 2

295

38.0

0.3093

0.995

0.097-0.393

0.489

9.616

0.908

315

41.6

0.2903

0.990

0.103-0.408

0.498

10.120

0.924

325

41.6

0.2880

0.992

0.107-0.418

0.496

10.509

0.924

1

Table 2 Kinetic parameters for Cd2+ adsorption onto MnO2/o-MWCNTs at various temperatures T(K)

qe,exp

Pseudo-first-order kinetic model Second-order kinetic model k1

qe

r1

k2

qe

r2

293

19.1

0.0228

18.4

0.990

0.00347

33.1

0.978

313

22.3

0.0259

17.8

0.986

0.00452

38.9

0.977

323

24.6

0.0273

16.4

0.982

0.00563

79.2

0.945

T(K)

Pseudo-second-order kinetic model k3×10-3

qe

h

r3

293

2.28

20.7

0.984

0.997

313

3.57

23.3

1.940

0.999

323

4.68

25.4

3.021

0.999

2

Table 3 Thermodynamic parameters for Cd2+ adsorption onto MnO2/o-MWCNTs T(K)

ln K0

G 0 (kJ·mol-1)

H 0 (kJ·mol-1)

S 0 (kJ·mol-1 )

r

295

9.035

-22.209

4.242

8.96  10-2

0.990

315

9.054

-23.931

4.242

8.97  10-2

0.984

325

9.147

-24.899

4.242

8.99  10-2

0.987

3

Table 4 The maximum adsorption capacities qmax (mg·g-1) of Cd2+ on MnO2/o-MWCNTs and other adsorbents Adsorbates

Adsorbents

qmax (mg·g-1)

References

Cd2+

MWCNT (HNO3)

10.9

[19]

Biosorbent

24.3

Fly ash

8.0

Oxidized nitrogen-doped MWCNTs

10.0

[15]

Ethylenediamine-functionalized

25.7

[13]

MWCNTs Commercial activated carbon

4.3

HEU-type zeolite

12.2

CNTs (H2O2)

2.6

CNTs (KMnO4)

11.0

MnO2/o-MWCNTs

41.6

4

[20]

Present work

Figures.doc

Fig. 1.

FTIR transmission spectra of raw-MWCNTs, o-MWCNTs, and

MnO2/o-MWCNTs

1

Fig. 2.

XRD patterns of o-MWCNTs and MnO2/o-MWCNTs

2

Fig. 3. TGA curves of raw-MWCNTs, o-MWCNTs, and MnO2/o-MWCNTs

3

a

b

c

Fig. 4. Dispersion of (a) raw-MWCNTs, (b) o-MWCNTs, and (c) MnO2/MWCNTs in water

4

Fig. 5. Adsorption capacities of different materials with regard to Cd2+ adsorption

5

Fig. 6. Effect of solution pH on cadmium ions adsorbed onto MnO2/o-MWCNTs

6

Fig. 7. Effect of contact time on cadmium ions adsorbed onto MnO2/o-MWCNTs

7

Fig. 8. Weber–Morris plots for the kinetic modeling of Cd2+ adsorbed onto MnO2/o-MWCNTs at different temperatures

8

Highlights > The removal of cadmium using MnO2/o-MWCNTs was studied for the first time. > The MnO2/o-MWCNTs which synthesized at pH=5 is the most effective adsorbent. > A detailed study for the adsorption process on cadmium removal was obtained. > The maximum adsorption capacity was 41.6 mg g-1. > Adsorption mechanisms were revealed and elucidated in detail.

1