multiwalled carbon nanotubes magnetic nanocomposite for toxic hexavalent chromium removal from solution

Accepted Manuscript Preparation and characterization of chitin/magnetite/multiwalled carbon nanotubes magnetic nanocomposite for toxic hexavalent chromium removal from solution

Mohamed Abdel Salam PII: DOI: Reference:

S0167-7322(16)34294-5 doi: 10.1016/j.molliq.2017.03.023 MOLLIQ 7058

To appear in:

Journal of Molecular Liquids

Received date: Revised date: Accepted date:

17 January 2017 23 February 2017 6 March 2017

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ACCEPTED MANUSCRIPT Preparation and Characterization of Chitin/Magnetite/Multiwalled Carbon nanotubes Magnetic Nanocomposite for Toxic Hexavalent Chromium Removal from Solution Mohamed Abdel Salam

Chemistry Department, Faculty of Science, King Abdulaziz University, P.O Box 80200-

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Jeddah 21589, Kingdom of Saudi Arabia

*Corresponding author Tel.: +966-541886660; fax: +966-2-6952292 [email protected]

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ACCEPTED MANUSCRIPT Abstract Natural organic polymer; chitin, was physically mixed with magnetite nanoparticles and the well-known adsorbent; multiwalled carbon nanotubes (MWCNTs), in order to enhance its adsorption ability and obtain magnetic nanocomposite. Morphological, chemical and physical properties of the prepared chitin/magnetite/MWCNTs (CMM) were investigated

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using various characterization techniques. The TEM revealed that MWCNTs and magnetite

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nanoparticles were homogenously dispersed over the chitin surface. Also, the prepared CMM

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magnetic nanocomposite showed higher surface area and significant magnetic properties compared with the natural chitin. Removal of Cr(VI) by CMM magnetic nanocomposite was

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explored, and it was found that significant enhancement in the %Cr(VI) removed by increasing the removal time, CMM mass, and by decreasing the temperature of solution,

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were observed. Moreover, Cr(VI) removal was explored kinetically and thermodynamically, and the results showed that pseudo-second-order kinetic model was the most appropriate

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model for describing the removal process, and the removal was spontaneous, exothermic, and

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accompanied with a decrease in the degree of randomness. Finally, the results showed the significant enhancement of the removal ability of natural chitin upon its modification and the

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preparation of the CMM magnetic nanocomposite, which is a potential and promising

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adsorbent for environmental remediation.

Keywords:

Chitin;

Carbon

nanotubes;

magnetite;

Cr(VI);

removal;

kinetics;

thermodynamics 2

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Introduction One of the major problems that threatens the existence of living organism on the earth is pollution. Pollution mainly occurs due to the extensive anthropogenic activities to maintain the living standard of mankind world-wide. Water pollution is considered to be the most

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severe form of environmental pollution and has mainly originated from the introduction of different classes of pollutants into the aquatic environment that limits the amount of water

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available for different human activities. Water pollution by heavy metals such as chromium,

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has varying effects on living organism due to their proven toxicity and bioaccumulation.

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Nowadays, many industrial activities depend on the application of chromium and its compounds in their processes, such as plastics, inks, semiconductor, dyes, leather tanning,

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and paints. During these industrial processes, unused chromium salts are usually discharged in the final effluents, which usually cause serious problems to the environment [1]. It is well

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known that chromium exits in two oxidation states in aqueous phase; chromium (III), and

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chromium (VI). Ironically, chromium (III) is essential for human nutrition, and chromium (VI) is very toxic to living organisms [2]. Chromium (VI) has strong mutagenic and

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teratogenic properties [3], and accordingly was labeled by the International Agency for Research on Cancer as a carcinogen. Nowadays, the USEPA regulations set 0.1 mg/L or

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100 ppb as the maximum total content of chromium in drinking water [4]. Industrial waste water containing chromium is usually treated in different methods in order to remove or decrease the total chromium content before being discharged. These methods including electrocoagulation [5], microfiltration [6], membrane [7], photocatalytic reduction [8], catalytic reduction [9], and adsorption [10-12]. Among these method, efficient treatment of wastewater via adsorption is the most promising one because it characterized with the low cost, and ease of application, in addition to the regeneration ability of both adsorbent, and pollutants [10-16]. The quest for new type of adsorbents which could be used for efficient 3

ACCEPTED MANUSCRIPT treatment of wastewater is the main concern of the research scientists worldwide. Carbon nanotubes and the natural polymer such as chitin and chitosan are considered among the potential adsorbents used for the removal of many pollutants from water [17-24]. Many research studies showed that the modification of chitin with other adsorbents greatly enhanced their selectivity and adsorption capacity. For example, mixing graphene oxide with

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chitin significantly enhanced the removal ability of the chitin for the effective removal of

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organic dyes from aqueous solution [25]. Also, modification of chitin with bentonite and the

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formation of the chitin/bentonite biocomposite used for removal of Cr(VI) from aqueous solution [26]. Moreover, removal capacity of Ni(II), and Cd(II) by chitin from water was

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significantly enhanced upon their modification with Kraft lignin [27], and doping chitin with MnO2 greatly enhanced their methylene blue dye adsorption ability from water [28]. At the

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same time, modification of natural adsorbents such as chitosan with MWCNTs significantly enhanced their adsorption behavior for the efficient removal of methyl orange [29],

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chromium ion [30], and other heavy metals from water [31].

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Furthermore, regeneration of the solid adsorbent such as MWCNTs or chitin after the adsorption process consider as a serious problem in environmental remediation. After the

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removal process solid adsorbents could be regenerated and separated from aqueous solution either by centrifugation or filtration. This usually adds extra steps and costs to the

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environmental remediation. One of the most promising ways out to such a problem is the application of the magnetic-separation technology. Separation of the solid adsorbents based on their magnetic properties has been the focus of many researchers world-wide. MWCNTs were decorated with the magnetic ferrite to form magnetic nanocomposites that was used effectively for uranium ions and organic pollutants removal from water [32-34]. In other studies, MWCNTs were decorated with magnetic iron oxide nanoparticles and applied for the removal of organic dyes [35], antibiotics [36], and cadmium ions [37]. The modification of natural chitin with the carbon nanotubes, and magnetite, and their application for the removal 4

ACCEPTED MANUSCRIPT of the toxic hexavalent chromium is limited in literature, as a MWCNTs/magentite nanoparticles/chitin magnetic nanocomposite was used for Rose Bengal dye removal from water [38]. In this study, the modification of the biopolymer chitin by the well-known nanoadsorbent MWCNTs and the magnetic magnetite to form chitin/magnetite/MWCNTs

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(CMM) magnetic nanopcomposite was studied. Mixing chitin with MWCNTs would

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enhance its adsorption, and adding magnetite should facilitate the separation process. The

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morphological structure of the produced CMM magnetic nanopcomposite using different chemical and physical techniques, then was used for toxic Cr(VI) removal from water.

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Different kinetic models were applied to explore the Cr(VI) removal from water by the CMM magnetic nanocomposite. Finally, the thermodynamic parameters were estimated for better

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2.1 Chemical

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2. Materials and Methods

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understanding of the removal process.

Coarse natural chitin, magnetite nanoparticles (iron (II,III) oxide), and analytical

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grades potassium dichromate (K2Cr2O7) were obtained from Sigma-Aldrich. Multi-carbon nanotubes (> 80% purity) were obtained from Sun Nanotech (China).

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2.2. Chitin/Magnetite/MWCNTs Magnetic Nanocomposite Preparation By mixing of chitin, magnetite nanoparticles, and MWCNTs (6:1:3 weight percent) using agate mortar. 2.3. Characterization Techniques JEOL JEM-1230 TEM was used to investigate the morphological surface structure of the CMM magnetic nanocomposite. The texture properties and surface area analysis were performed using N2-adsorption/desorption isotherms using automated gas sorption system; 5

ACCEPTED MANUSCRIPT NOVA 3200e Quantachrome, USA. Vibrating sample magnetometer (VSM) were used to determine the magnetic properties at room temperature with an applied field: −5 kOe ≤ H ≤ 5 kOe, 2.3. Removal Procedure The removal procedure was carried out as the following: various solution with

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different concentrations of Cr(VI) were prepared, then the solution pH was adjusted to pH 2,

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and finally certain amount of the CMM magnetic nanocomposite was mixed with the

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solution. The solution was then mixed for a period of time, and at a certain time, and using ordinary magnet collecting the CMM magnetic nanocomposite, 5.0 ml of the solution was concentration in the supernatant was determined using

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collected. The Cr(VI)

spectrophotometer; Perkin Elmer Lambda 25 UV-Vis USA at a 355 nm wavelength. The % the removal capacity of the CMM magnetic

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Cr(VI) removed from solution and

(Co  Ct )V m

,

(1)

(2)

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qt 

(Co  Ct ) X 100 Co

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% Cr (VI ) removed 

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nanocomposite; qt (mg g−1), were calculated according to the following equations:

where C0 and Ct are the initial and final Cr(VI) concentration in solution (mg/L); V is the

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solution volume (L); and m is the CMM magnetic nanocomposite mass (g). 3. Results and discussion 3.1. CMM magnetic nanocomposite Characterization The TEM image of the CMM magnetic nanocomposite was presented in Fig. 1. It is clear from the image that MWCNTs and the spherical magnetite nanoparticles were homogenously distributed at the top of the chitin within the nanocomposite. The chitin, MWCNTs, magnetite, and their magnetic nanocomposite BET-specific surface areas were measured from the nitrogen adsorption/desorption isotherms at 77 K, and were 5.2, 258, 6

ACCEPTED MANUSCRIPT 65.7, and 69.1 m2.g–1, respectively, indicating the great enhancement of the chitin surface area upon mixing with MWCNTs and magnetite nanoparticles. Fig. 2 presents the CMM magnetic nanocomposite magnetic properties using VSM. The magnetic properties for the natural chitin and CMM magnetic nanocomposite were measured and a significant enhancement of chitin magnetic properties was observed, as their saturation magnetization

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(Ms), coercivity (Hc), and remanent magnetization (Mr), increased from 0.0695 emu.g-1, 80.2

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G, and 0.006 emu.g-1 for the natural chitin to 5.775 emu.g-1, 44.92 G, 0.196 emu.g-1 for the

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CMM magnetic nanocomposite; respectively. These enhancement of the magnetic properties of the CMM magnetic nanocomposite enable facile collection of the CMM magnetic

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nanocomposite using ordinary magnet. It is noteworthy to mention that chitin is a nonmagnetic biopolymers, and the change in its magnetic behavior is due to the presence of

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the magnetite nanoparticles only due to the formation of CMM nanocomposite and may not improve but reduce the magnetic character due to displacement of magnetic centers in the

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3.2 Removal study

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nanocomposite [39,40].

For the sake of comparison, the % Cr(VI) removed by the natural chitin , MWCNTs,

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magnetite nanoparticles, and the CMM magnetic nanocomposite was measured using same experimental conditions; 10.0 ml solution, pH 2.0, 0.01 mg solid adsorbent, 298 K, and

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Cr(VI) concentration of 50.0 mg L-1), and the results showed % Cr(VI) removed were 9.1%, 71.6%, 4.2%, 30.9%; respectively. This demonstrated that removal ability of the natural chitin was enhanced 3.4 times upon their modification with the MWCNTs and the magnetite nanoparticles. This significant enhancement may be due to the intensive increase in the specific surface area of the natural chitin from 5.2 m2.g–1 to 69.1 m2.g–1 upon their modification, and the presence of the MWCNTs; which considered as a very strong adsorbent to Cr(VI), at the chitin surface.

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ACCEPTED MANUSCRIPT Remediation of water from certain pollutant using solid adsorbent is very crucial process and mainly depends on many operational factors such as the mass, contact time, and the solution temperature. Therefore, the effect of these operational factors on Cr(VI) removal by the CMM magnetic nanocomposite from water was explored. Fig. 3 shows the variation of % Cr(VI) removed from solution with the CMM magnetic nanocomposite mass. It is clear

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that a gradual increase in the % removal of Cr(VI) from solution associated with increasing

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the CMM magnetic nanocomposite mass, 50.0 mg of the CMM magnetic nanocomposite was

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able to remove completely the Cr(VI) from solution. The gradual increase of % Cr(VI) removed from solution with increasing nanocomposite mass mainly due to the increase of

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active sites number available for the Cr(VI) binding. The effect of removal contact time between the Cr(VI) and CMM magnetic nanocomposite was studied and the results were

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presented in Fig. 4, which showed a fast increase in the % Cr(VI) removed from solution within the first 30 minutes; 80.4% of the Cr(VI) was removed. Further increase in the

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contacted time to 45 minutes, increased % Cr(VI) removed to 84.5%, and this percent

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removed did not changed significantly with more contact time. Accordingly, it could conclude that 45 minutes was enough to attain equilibrium between Cr(VI) and CMM

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magnetic nanocomposite. The effect of temperature of solution on the % Cr(VI) removed by CMM magnetic nanocomposite investigated, as it is well known that solution temperature is

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one of the most important factor could affect the removal process. As it is presented in Fig. 5, a gradual rise in solution temperature from 283 K to 323 K, led to a drastic decrease in the removal of Cr(VI) from solution; from 90.3% to 55.5%, indicating that Cr(VI) removal is exothermic process; which will be evaluated and discussed at the thermodynamics study part. Kinetics of the removal process using solid adsorbent such as CMM magnetic nanocomposite is very important in order to explore the interaction between the adsorbate; Cr(VI), and solid adsorbent; CMM magnetic nanocomposite, which usually involves the study of the removal rate to determine the environmental conditions effect on the Cr(VI) 8

ACCEPTED MANUSCRIPT removal. Cr(VI) removal from solution by CMM magnetic nanocomposite at different solution temperature was studied kinetically at 283 K, 295 K, 308 K, and 323 K, and the experimental results were presented in Fig. 6 in terms of % Cr(VI) removed and amount Cr(VI) adsorbed per g of CMM magnetic nanocomposite. It is obvious from the figure that rising temperature drastically decreased the Cr(VI) removal from solution, indicating the

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exothermic nature of the process. The experimental data at Fig. 6 were fitted kinetically to

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the most well-known and used kinetic models to understand the dynamics of the removal

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process; Lagergren pseudo-first-order [41], and pseudo-second-order [42] models. Lagergren pseudo-first-order (Equation 3), and pseudo-second-order (Equation 4) kinetic

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models commonly used to study the removal of pollutants by solid adsorbent from an aqueous solution and their linear forms as the following: (3)

(4)

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t 1 t   2 qt k 2 qe qe

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ln qe  qt   ln qe  k1t

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where k1 (min−1), k2 (min.g.mg-1), qe, and qt are the pseudo-first-order, and pseudo-secondorder rate coefficients, the amount Cr(VI) removed per unit mass of CMM magnetic

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nanocomposite at equilibrium and at any time t, respectively. Pseudo-first-order kinetic model was applied to the experimental using ln (qe− qt) vs. t plot at different temperatures

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for the Cr(VI) removal by CMM magnetic nanocomposite, and the results did not converge well, and low correlation coefficient (R2) were obtained, as is shown in Table 1. Furthermore, the amounts Cr(VI) removed at equilibrium were calculated from the intercept (qe,calc), and their values did not agreed with the experimental values (qe,exp), indicating the unsuitability of the Lagergren pseudo-first-order kinetic model for the description of the Cr(VI) removal from solution by CMM magnetic nanocomposite.

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ACCEPTED MANUSCRIPT The experimental data for removal of Cr(VI) using CMM magnetic nanocomposite from aqueous solution was applied to the pseudo-second-order kinetic model by plotting t/qt vs. t according to Eq. 4, qe and k2 can be estimated from the plot’s slope and intercept, respectively. As it is clear from Fig. 7, the experimental data converged very well and an excellent correlation coefficients were obtained (R2 > 0.99) with straight lines, indicating the

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suitability of the pseudo-second-order kinetic model for describing the removal of Cr(VI) by

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CMM magnetic nanocomposite from solution. The pseudo-second-order rate equation

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parameters, qe, and k2, were calculated from the slope and intercept of the plot of t/qt vs. t, and their values were presented in Table 1. The table showed good agreement between the

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qe,calc and qe,exp values of Cr(VI) removed, which confirm more the suitability of the pseudosecond-order kinetic model for describing the removal of Cr(VI) by the CMM magnetic

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nanocomposite from aqueous solution. Accordingly, based on the correlation coefficient values for both kinetic models, and the agreement between the (qe,calc) and (qe,exp) values, it

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could be determined that pseudo-second-order kinetic model is the suitable model to describe

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the Cr(VI) removal from solution by CMM magnetic nanocomposite, which agreed well with other studies showed that Cr(VI) removal by solid adsorbents from solution usually best

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described by pseudo-second-order kinetic model. For example, the removal of Cr(VI) by green synthesized iron based nanoparticles [43], 3D hierarchical α-Fe2O3 structures [44],

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CoFe layered double hydroxides [45], amino functionalized graphene oxide decorated with Fe3O4 nanoparticles [46], Lagerstroemia speciosa embedded magnetic nanoparticle [47], and Magnetite graphene oxide encapsulated in alginate beads [48]. 3.2.3 Thermodynamic studies In order to investigate thermodynamic spontaneity of Cr(VI) removal by CMM magnetic nanocomposite from solution, the thermodynamic parameters; enthalpy change (ΔH), entropy change (ΔS), and Gibbs free-energy change (ΔG), were calculated based on

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ACCEPTED MANUSCRIPT the variation of the thermodynamic distribution coefficient D with a change in temperature according to the equation [49]:

q D e Ce

(7)

where Ce is the equilibrium concentration of Cr(VI) in solution (mg/L). The ΔH and ΔS can

S H  R RT

(8)

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ln D 

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be calculated according to the following equation [50]:

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The ΔH and ΔS values were calculated from the slope and the intercept of ln D vs. 1/T plot,

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which presented at Fig. 8, respectively. The calculated ΔH and ΔS values were -32.3 kJ.mole, and -106.7 J.mole-1.K-1, respectively, indicating the exothermic nature of the removal

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process as well as the decrease in the degrees of freedom; negative change entropy value, due to the adsorption of Cr(VI) by CMM magnetic nanocomposite. The ΔG value was calculated

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at 298K from the relation:

(9)

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ΔG= ΔH - TΔS

and was -0.503 kJ.mole-1, indicating the spontaneity of the removal process of Cr(VI) by the

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CMM magnetic nanocomposite from the aqueous solution. Accordingly, the negative values of ∆G, ∆H and, ∆S suggests that the removal of Cr(VI) by the CMM magnetic

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nanocomposite is an enthalpy-driven process. Moreover, the small value of ΔG and ΔH may indicating that Cr(VI) interaction with CMM magnetic nanocomposite was physical in nature, as the ΔG value was in the range of − 20 to 0 kJ/mol, and ΔH value was less than 84 kJ/mol [51-52].

3.4. Reusability and Environmental applications Economic visibility and cost effectiveness is one of the crucial factor for determining the suitability of solid adsorbents for environmental remediation application. Hence, the 11

ACCEPTED MANUSCRIPT regeneration, and reusability of CMM magnetic nanocomposite for the removal of Cr(VI) from the aqueous solution was investigated. The results showed that CMM magnetic nanocomposite could be used three consecutive times for the efficient removal of Cr(VI) from the aqueous solution with % Cr(VI) removed around 99.1% using 50 mg of the CMM magnetic nanocomposite. Also, the CMM magnetic nanocomposite used for Cr(VI) removal

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from real waste water sample collected from the King Abdulaziz University Wastewater

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treatment plant, after spiking it with Cr(VI) to reach final concentration of 50 mg.L-1. The

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results showed that CMM magnetic nanocomposite could remove 90.7% of the Cr(VI) from the spiked waste water using 50 mg. The %Cr(VI) obtained for the spiked waste water

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sample was lower than that obtained using model solution due to the fact that the waste water sample contains other pollutants which indeed compete with Cr(VI) to bind with the CMM

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magnetic nanocomposite. These results significantly indicate the suitability of the CMM magnetic nanocomposite for the efficient removal of Cr(VI) from real polluted aquatic

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environment. Finally, these results showed the significant enhancement of the removal ability

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of natural chitin upon its modification and the preparation of the CMM magnetic

Conclusions

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nanocomposite, which is a potential and promising adsorbent for environmental remediation.

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Chitin/magnetite/MWCNTs (CMM) magnetic nanocomposite was prepared, and characterized using different techniques; TEM, surface area analysis and magnetometer. The TEM image showed MWCNTs and the spherical magnetite nanoparticles were homogenously distributed at the top of the chitin within the magnetic nanocomposite, and mixing chitin with magnetite nanoparticles and MWCNTs greatly enhanced the specific surface area of the chitin. The magnetometer measurements showed significant enhancement of magnetic properties of the chitin upon their mixing with MWCNTs and the spherical magnetite nanoparticles. Cr(VI) removal from solution by CMM magnetic nanocomposite 12

ACCEPTED MANUSCRIPT was studied and the results showed that the enhancement in the %Cr(VI) removed by increasing the CMM mass, contact time, and by lowering the solution temperature, and the optimum operation conditions were 50.0 mg, 45 minutes, and 295 K. Also, the removal of Cr(VI) was studied kinetically and the results showed that pseudo-second-order kinetic model was appropriate for describing the removal process. The thermodynamics study

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showed the spontaneity, exothermic, physical nature of the removal process as well as the

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decrease in the degrees of freedom due to the bonding of Cr(VI) on the CMM surface. Also,

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the results revealed that CMM magnetic nanocomposite could be used efficiently for many times for the removal of Cr(VI) from environmental water. Finally, the results showed the

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significant enhancement of the removal ability of natural chitin upon its modification and the preparation of the CMM magnetic nanocomposite, which is a potential and promising

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adsorbent for environmental remediation.

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Acknowledgment

This work was funded by the Deanship of Scientific Research (DSR) at King Abdulaziz

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University, Jeddah, under grant No. (D-061-130-34). The author, therefore, acknowledges

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with thanks DSR for technical and financial support.

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ACCEPTED MANUSCRIPT Figure Captions Fig. 1. Transmission electron microscope image of chitin/magnetite/MWCNTs magnetic nanocomposite. Fig. 2. Vibrating sample magnetometer measurement of chitin/magnetite/MWCNTs magnetic nanocomposite.

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Fig. 3. Effect of CMM magnetic nanocomposite mass on the removal of Cr(VI) from an

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Fig. 7. Pseudo-second-order kinetic model plots for the adsorption of Cr(VI) by CMM magnetic nanocomposite at different temperatures. Fig. 8. Variation of the adsorption distribution coefficient (D) with temperature for Cr(VI) adsorbed by CMM nanocomposite.

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Temperature 382 K 295 K 308 K 333 K

Pseudo-second-order kinetic model qe, exp (mg/g) qe, calc (mg/g) 11.3 11.4 10.7 11.1 9.34 9.83 7.00 7.42

k1 0.063 0.067 0.066 0.062

k2 0.083 0.083 0.020 0.021

R2 0.758 0.882 0.884 0.864

R2 0.999 0.998 0.996 0.998

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Temperature 382 K 295 K 308 K 333 K

Pseudo-first-order kinetic model qe, exp (mg/g) qe, calc (mg/g) 11.3 11.3 10.7 10.7 9.34 9.34 7.00 7.00

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Fig. 3. Effect of CMM magnetic nanocomposite mass on the removal of Cr(VI) from an aqueous solution. (Experimental conditions: 10.0 ml solution, pH 2.0, 60 minutes, 298 K,

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Fig. 5. Effect of solution temperature on the removal of Cr(VI) from an aqueous solution by CMM magnetic nanocomposite. (Experimental conditions: 10.0 ml solution, pH 2.0, 40.0 mg

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Fig. 6. Effect of solution temperature on the adsorption kinetics of Cr(VI) by CMM magnetic nanocomposite. (Experimental conditions: 10.0 ml solution, pH 8.0, 2.5 mg CMM magnetic nanocomposite, and Cr(VI) concentration 5.0 mg L-1)

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Graphical Abstract

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Research Highlights



Chitin/magnetite nanoparticles/MWCNTs (CMM) magnetic nanocomposite was

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prepared. CMM magnetic nanocomposite was characterized with TEM, magnetometer, and

CMM magnetic nanocomposite used for the efficient removal of the toxic Cr(VI) from

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surface area analyzer.

water.

The removal ability of chitin was greatly enhanced upon mixing with MWCNTs, and

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CMM magnetic nanocomposite is a potential and promising adsorbent for

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environmental remediation.

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magnetite.

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