Thermodynamic study of Cr⁺³ ions removal by "MnO2/MWCNT" nanocomposite

Introduction

Main reason causes environmental problems are toxic metal ions. Chromium is one of the most hazardous heavy metals [1]. It affects on human health, some problems that caused by chromium (III) are as fallow; lung cancer, kidney and liver damage, skin rashes and also it affects human physiology [2]. Although chromium (III) has use in so many industries like leather tanning, paints, glass manufacture and etc [3]. But the most significant point about chromium (III) is, it can be oxidized to Chromium (VI) that cause serious health risks [4,5]. Also admissible amount of chromium (III) in wastewater is 5mg/l [6]. So the elimination of chromium (III) from wastewater actuate tremendous attempt. There are prevalent methods for chromium (III) removal from the industrial wastewater consist of coagulation [7] and adsorption [8], but among these physicochemical [9] wastewater treatment; adsorption is a promising way for water reuse requirements. There are divers materials used as adsorbents but activated carbon is the most effective adsorbent for this application [10]. The amount of Chromium is very rarely in environmental water samples, so very sensitive techniques are used for the determination of chromium in water samples such as flame atomic absorption spectrometry (FAAS) [11], inductively coupled plasma-atomic emission spectrometry (ICP-AES) [12], graphite furnace atomic absorption spectrometry (GFAAS) [13] and inductively coupled plasma-mass spectrometry (ICP-MS) [14]. The purpose of this research is to extend advantages over other wastewater purification processes, and characterizing major factors.

Materials and Methods

Multi-wall carbon nanotube (MWCNT, Outer diameter: 8-15 nm, Length: 50 µm) were purchased from Neutrino, the initial solution and after adsorption were analyzed by Perkin Elmer Analyst 200, and pH meter (ABTGP353). A stock solution was prepared by dissolving 0.256 g CrCl₃.6H₂O in 50 cc distilled water. The pH values of the solution were adjusted to the desired values with either 0.1 mol/L HCl or 0.1 mol/L NaOH solutions. All chemicals were of analytical grade (AR) provided from Merck.

Preparation of MnO₂/MWCNT Nanocomposite

The preparation process for the MnO₂/MWCNT nanocomposite was a co-precipitation method that before described by Zheng et al [15].

We synthesis MnO₂/MWCNT nanocomposite. First of all we modify pure MWCNT by dispersing MWCNT in HNO₃ 65 wt% for 24 h under stirring then rinsed with deionized water and dried in the air at 100° C[16]. Afterwards, about 0.1 g of the treated MWCNT was dispersed into 160 ml of 0.1m KMnO₄ solution under vigorous stirring at 40 °C for 2 h and 20 ml MnSO₄ (0.5 M) solution was added dropwise on the above suspension with intensive stirring and then the reaction mixture was kept at 40 C^° for 24 h. Then the suspension was filtered washed several times with deionized water and alcohol, and the precipitation were collected and dried in an oven under 100 C ^° about 12 h.

Characterization of the Treated MWCNT and MnO₂/MWCNT Nanocomposite

In order to determine crystallization and surface morphology, we used XRD diffraction and SEM techniques.

Figure 1: XRD patterns of (a) activated MWCNT, (b) MnO2/MWCNT nanocomposites.  Click here to View figure

 

Figure 2: SEM photographs of (a) activated MWCNT and (b) MnO2/MWCNT nanocomposite.  Click here to View figure

 

It can be describe the XRD patterns of the treated MWCNT and the synthesized nanocomposite show the obvious peaks that refer to the formation of nano MnO₂ onto MWCNT. The treated MWCNT shows a sharp peak at around 26° and a broad weak peaks at around 43° and 53° (Fig. 1a), which can be well assigned to the (0 0 2), (1 0 0) and (0 0 4) planes of graphite carbon [17]. Fig. 1b showed the two typical broadening patterns of the MnO₂/MWCNT samples of 2  at around 65.5 that can be ascribed the crystal planes of  (0 2 0) in γ-MnO₂ [18,19]. And it is obvious after deposition of MnO₂, all surface of treated MWCNT completely covered by the nanoflakes of MnO₂ (Fig. 2)_. The deposited MnO₂ onto treated MWCNT exhibits a core-shell structure composed of many nanoflakes with a highly porous structure. [15,20]

The adsorbed amount of chromium (III) onto the MnO₂/MWCNT nanocomposite were calculated by the following equations [21,22]:

Where C₀ is the initial concentration (mg/L),C_t is the amount of concentration (mg/L) at any time t and C_e the amount of concentration (mg/L) in equilibrium, V is solution volume (L); and M is MnO₂/MWCNT nanocomposite mass (g).

Results and Discussion

pH Optimization

In this research, the pH of solution influences the distribution of active sites on the surface of MnO₂/MWCNT, to find out the effects of solution pH on adsorption capacity; Cr(III) adsorption was examined by adding different amounts of 0.1 M HCl or 0.1 M NaOH to obtain different pHs. In Fig. 3, it can be described that the amount of Cr(III) adsorbed on MnO₂/MWCNT increased by increasing pH at 5, so we carry out the rest of experiment in this condition.

Figure 3: Effect of pH on Cr (III) adsorption onto MnO2/MWCNT nanocomposite (adsorbent: 0.01 g/10 cc, initial Cr(III) conc.: 10 mg/L, temp.: 25°C, contact time: 40 min).  Click here to View figure

 

Dose of Adsorbent

In the next step, we identify the best adsorbent dosage, as we declare the less amount of adsorbent is also much profitable. Then we were measuring this parameter in 5 different dosage. We choose the adsorbent doses in 0.001g, 0.005 g, 0.008 g, 0.01 g, 0.015 g and we realize in a very low amount of adsorbent can reach to high level of removal too. Therefore we continue the rest of the reaction by using about 0.005 g of adsorbent. These results indicate that MnO₂/MWCNT nanocomposite is promising candidate as highly effective adsorbent (Fig. 4).

Figure 4: Effect of MnO2/MWCNT nanocomposit on Cr(III) adsorption (pH: 5, initial Cr(III) conc.: 10 mg/L, temp.: 25°C, contact time: 40 min).   Click here to View figure

 

Contact time

To demonstrate the optimum contact time 0.005 g of MnO₂/MWCNT nanocomposite was added to 10 ml of Cr(III) solution with concentration of 10 mg/L at 25^° C. the pH of solution was 5 and we carried out this parameter in four different period time, samples were taken in 10 min, 25 min, 40 min (Fig. 5). So we find out even at the initial period contact time, the removal of Cr(III) was very fast, but by increasing contact time we reach to the much more removal of Cr(III).

Figure 5: Effect of contact time on Cr(III) adsorption onto MnO2/MWCNT nanocomposite (adsorbent dose: 0.005 g/10 cc, pH: 5, initial Cr(III) conc.: 10 mg/L, temp.: 25°C).   Click here to View figure

 

Effect of Temperature

To identify the effect of temperature, 0.005 g of MnO₂/MWCNT nanocomposite was added to 10 ml of Cr(III) solution with concentration of 10 mg/L at PH 5 and the experiments were carried out at temperatures of 35, 45, 55 °C. according to our investigations, the adsorbent amount of Cr(III) on MnO₂/MWCNT nanocomposite decreases with the increasing temperature. Fig. 6 shows the effect of temperature on removal of Cr(III).

Figure 6: Effect of temperature on contact Cr(III) adsorption onto MnO2/MWCNT nanocomposite (adsorbent dose: 0.005 g/10 cc, pH: 5, initial Cr(III) conc.: 10 mg/L, contact time: 40 min).   Click here to View figure

 

Influence of Initial ion Concentration on Cr(III) Adsorption

The initial ion concentration provide significant driving force in adsorption process, it can noted that the adsorption percent of Cr(III) decrease by increasing  chromium concentration (Fig. 7, Fig. 8). We carried out our experiment in 10 to 50 mg/L.

Adsorption Isotherm Study

In order to investigate the mechanism of Cr(III) adsorption on adsorbent and for declaring the chemical equilibrium between Cr(III) and adsorbent, Langmuir and Fruendlich and also Temkin isotherms were studied to describe equilibrium. Adsorption isotherm studies were carried out with various initial Cr(III) concentrations ranging from 10 to 50 mg/L.

Figure 7: Effect of Cr(III) initial conc. On its adsorption onto MnO2/MWCNT nanocomposite (adsorbent dose: 0.005 g/10 cc, pH: 5, temp.: 25 °C, contact time: 40 min). Click here to View figure

 

Figure 8: Effect of Cr(III) initial conc. On its adsorption onto MnO2/MWCNT nanocomposite (adsorbent dose: 0.005 g/10 cc, pH: 5, temp.: 25 °C, contact time: 40 min).  Click here to View figure

 

The linearized form of Langmuir adsorption isotherm equation is as follow:

The linearized form of Freundlich adsorption isotherm equation:

The linearized form of Temkin adsorption isotherm equation:

Where C_e (mg/L) is the equilibrium concentration of Cr(III), q_e (mg/L) is the amount of  Cr(III) adsorbed at equilibrium, q_m (mg/g) is the maximum adsorption at monolayer and K_L (L/mg) is the Langmuir constant including the affinity of binding sites. K_F[(mg/g)(L/mg)^1/n] and n are Freundlich constants indicating adsorption capacity and intensity, respectively.

K_T (L/g) and B₁ are the Temkin constants (K_T is the equilibrium binding constant and B₁ is related to the heat of adsorption). The amounts of Langmuir, Freundlich and Temkin parameters were calculated from the slope and intercept of linear plots of 1/q_e versus 1/C_e, Ln q_e versus Ln C_e and q_e versus Ln C_e, respectively [23,24,25].

Further we display the adsorption isotherms plots (Fig. 9).

Figure 9: Plots of linearzed Langmuir (a), Freundlich (b), and Temkin (c) adsorption isotherms.  Click here to View figure

 

It can be concluded from isotherm plots that the reaction process is much more compatible by  Freundlich isotherm than others.

The isotherm constants also listed in table 1.

Table 1: Isotherm parameters for Cr(III) adsorption onto MnO₂/MWCNT nanocomposite (adsorbent dose: 0.005 g/10 mL, pH: 5, temp: 25°C, contact time: 40 min)

Langmuir model	Freundlich model
qm (mg g-1)                       R2	KF                   n                 R2
99.01           2.73         0.9727	78.15              1.74           0.9958

 

In this work, also the thermodynamic parameters were calculated to evaluate the nature of adsorption process [26,27]. Thermodynamic parameters such as Gibss free energy change (ΔG⁰, KJ/mol), enthalpy change (ΔH⁰, KJ/mol) and entropy change (ΔS⁰, J/mol.k) can be estimated as following:

Where K_c is the equilibrium constant of the adsorption process, C₀ and C_e are the initial and equilibrium concentration of Cr(III). In table 2 we also demonstrate thermodynamic parameters.

Table 2: Thermodynamic parameters for adsorption of Cr(III) on MnO₂/MWCNT

t (°C)	C0 (mg/L)	Ce (mg/L)	K	lnK	1/T (K-1)	∆G0 (kJ/mol)	∆H0 (kJ/mol)	∆S0 (J/mol K)
35	10	0.487	19.53	2.97	0.0032	-7.61	-33.26	-81.59
45	10	0.679	13.72	2.62	0.0031	-6.93	-33.26	-81.59
55	10	1.022	8.78	2.17	0.0030	-5.92	-33.26	-81.59

 

We can conclude this reaction is exothermic and spontaneous because Cr(III) adsorption decrease by increasing temperature. The value of ∆S⁰ (J/mol K) indicates Cr(III) molecules reach to the arrangement in this adsorption.

Figure 10: Comparing experimental plot by calculation equation.  Click here to View figure

 

Conclusion

In this work we investigated the optimization conditions for removal of Cr(III) ions by MnO₂/MWCNT nanocomposite that we reach to a fairly good results. We investigated the adsorption in a wide range of pH to identify the best range. So we found that the pH = 5 is better. Therefore we did our measurements in this pH. We also investigated the best dosage of adsorbent, contact time, and also various concentrations have been studied. In lower concentration we got better results, and then we estimated thermodynamic parameters. As we found the studied adsorption was exothermic and spontaneously. Also we compare our experimental results with our calculation results. We concluded, the our results are better comparable with Freundlich isotherm.

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