Application of Carbonized MetalâOrganic Framework as Efficient

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Materials and Interfaces

Application of carbonized metalorganic framework as highly efficient adsorbent of cationic dye Martyna Barylak, Krzysztof Cendrowski, and Ewa Borowiak-Palen Mijowska Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b03790 • Publication Date (Web): 21 Mar 2018 Downloaded from http://pubs.acs.org on March 22, 2018

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Industrial & Engineering Chemistry Research

Title: Application of carbonized metalorganic framework as highly efficient adsorbent of cationic dye. Authors: Martyna Barylak, Krzysztof Cendrowski*, Ewa Mijowska Nanomaterials Physicochemistry Department, Faculty of Chemical Technology and Engineering West Pomeranian University of Technology, Piastow avenue 45, 70-311 Szczecin, Poland

Abstract Herein, we report study about adsorption kinetics of rhodamine B onto metal organic framework containing cobalt (CoOF). The CoOF can be successfully produced from the reused waste organic ligands and solvents. The as-prepared material was carbonized and purified prior testing. The adsorbent was characterized with scanning electron microscopy (SEM), Fourier transform infrared spectrophotometer (FT-IR), X-ray diffractometer (XRD), thermogravimetric analysis (TGA), Raman spectroscope and N2 adsorption/desorption isotherms. The kinetics of dye adsorption onto structure was examined with the UV–vis spectrophotometer. Various physiochemical parameters such as initial dye concentration, pH of dye solution and temperature were investigated in an adsorption technique. The adsorption uptake was found to increase with increase in initial dye concentration. An increase in adsorption capacity was noticed when the solution was changed to basic, optimum conditions obtained at pH 7. The results indicate that along increase in the temperature the adsorption capacities decrease. The maximum monolayer adsorption capacity of RhB onto CoOF was 72.150 mg/g. Kinetic adsorption data were analyzed using the pseudo-first-order kinetic model, the pseudo-second-order kinetic model and the intraparticle diffusion model. The adsorption kinetics well fitted using a pseudo-secondorder kinetic model. The experimental data were analyzed by the Langmuir and Freundlich adsorption isotherms. Equilibrium data fitted well with Freundlich isotherm. Thermodynamic parameters such as Gibbs free energy, enthalpy and entropy were determined. It was found that the adsorptions of RhB onto CoOF was a spontaneous and exothermic physisorption 1. Keywords: metalorganic frameworks; cobalt; rhodamine B; kinetics; adsorption

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Introduction Colored compounds comprising pigments and dyes are widely used in textile industry, food, dyeing, paper printing, cosmetic and pharmaceutical industries. Some of these dyes can penetrate into groundwater and hence affecting aquatic life. Many of them are toxic or even carcinogenic [1]. The numerous chemical and physical methods that have been explored for the dealing of such colored waste water include reverse osmosis, ion exchange, coagulation, precipitation. The common studied methods of organic pollutants degradation are not very successful because most synthetic dyes are stable to heat and UV light irradiation. Among the available physical methods, adsorption seems to be the most effective treatment for the removal of color dyes from waste water [2]. One of the most common colored compound is rhodamine B (RhB). Rhodamine B (RhB) is widely used as a colorant in textiles and food stuffs, and it is also well-known as a staining fluorescent dye [3]. It is harmful to human beings and animals, and causes skin, eyes and respiratory tract damage. It has been proved that rhodamine B is carcinogenic, reproductive and developmental toxic, neurotoxic toward humans and animals [4]. Therefore it is crucial to find efficient and cost effective rout to deal with RhB. For this purpose metal–organic frameworks (MOFs) can be interesting adsorbent of RhB. MOFs are crystalline materials which are well known for their various applications [5-12]. The particular interest in MOF materials is due to the easy tunability of their pore size and shape from a micro to a meso scale by changing the connectivity of the inorganic moiety and the nature of organic linkers [13]. Metal–organic frameworks have been studied in a broad range of potential application such as adsorption [14], separation [15], gas storage [16] or catalysis [17]. All of them receive a considerable amount of attention, but there are only a few reports on wastewater treatment. Ling Li et.al concerned the adsorption of methylene blue onto MOF:HKUST-1 and HKUST-1/GO. The results indicate that the MOFs can potentially be used for the adsorption of dyes [18]. Xiaoli Zhao et al. were investigated a magnetic metal-organic framework (MOF) as sorbent of methylene blue from simulated water samples. They synthesized iron(II,III) oxide nanoparticles and encapsulated them by a shell of Cu3(BTC)2. They show that this material indicates a very good sorption of methylene blue [19]. However, relatively low interest

in the application of MOFs for the wastewater treatment can be caused by their low


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structure stability in water environment. Therefore, it is crucial to be aware that MOFs can be a great precursor of highly porous carbons with high

specific surface area upon their

carbonization. During the carbonization process the organic bridging linkers tend to form fine porous carbon frameworks, while the metal centers can be transformed into metal/metal oxide nanoparticles [20]. Obtained nanocomposites after carbonization are capable to retain their shape and similar or even enhanced surface area [21]. Depending on the type of used metal for MOF structure and carbonization parameters, obtained nanocomposites can exhibit wide range of properties – photocatalytic [22], luminescence [23] and magnetic [24]. R. Jothirani et al. showed that agricultural waste like corn pith can be used as a precursor for the preparation of an effective adsorbent of malachite green [25]. Similar approach of agricultural waste management was reported by the P. Senthil Kumar et al. using cashew nut shell for adsorption of congo red from waste water [26]. The second approach in recycling and reusing organic waste as a adsorbents is it carbonization. The example of the was pyrolysis of the cashew nut shell [27]. Pyrolysised cashew nut shell shows high efficiency in methylene blue adsorption from aqueous solution [28, 29], that was previously studied by P. Senthil Kumar and his team [26] Carbonize cashew nut shell also shows high adsorption capacity of the congo red dye from aqueous solution [30]. S. Suganya et al. also showed that sawdust can be successful pyrolysised in microwave cavity oven, producing effective adsorbent of methylene blue [31]. P. Senthil Kumar also showed that carbon materials prepared from the pyrolysis of sawdust for adsorption congo red [32] has higher dye adsorption capacity then non carbonized sawdust [31]. Except of cashew nut shell [33] and sawdust, other agricultural (organic) waste are studied as absorbent for removal dye in the form of dehydrate dust [34-37] or in the pyrolysised form [30]. The adsorbent used for the removal of the organic dyes should not only characterised with high efficiency but also should contribute in the waste management by recycling other waste like polymer or agricultural waste. In this study, a cobalt based metalorganic framework (CoOF) synthesized by an alternative method – using cobalt (II) chloride and benzene-1,4-dicarboxylic acid – was analyzed as absorbent of colored dye – Rhodamine B (RhB). The prepared structure exhibited flake-like morphology which is preserved after the carbonization and purification procedures. Proposed MOF can be easily prepared using organic solvent from the previous synthesis and organic ligand



obtained from the decomposition of the polymer waste. Here, we reveal the effects of pH, temperature and initial RhB concentration on the adsorption performance of CoOF. Adsorption kinetics, thermodynamics and adsorption isotherm were used to describe the specific adsorption mechanism. In this study we used pseudo-first order, pseudo-second order and intraparticle diffusion model to describe kinetics of adsorption. pseudo-first-order and pseudo-second-order ones based on the solution concentration. When the initial concentration of solute is low, then the adsorption process obeys the pseudo-second-order model. Conversely pseudo-first-order models can be applied to higher initial concentrations. The rate constant of the pseudo-second-order model is a complex function of the initial concentration of the solute. On the basis of models, the mechanism of dye adsorption onto CoOF can be better understood.

2. Experimental section 2.1 Materials Hydrochloric acid (36 %), N, N – dimethyloformamide (DMF) and Rodamine B were purchased from Chempur (Poland). Cobalt chloride (CoCl2) and terephthalic acid (C6H4 (COOH)2) were bought from Sigma Aldrich.

2.2 Synthesis, carbonization and purification of cobalt organic framework [CoOF] Carbon flake-like structure were prepared according to previously described report [38]. To synthesize the cobalt based metal organic framework (CoOF), 574 mg terephthalic acid and 740 mg cobalt (II) chloride (CoCl2) were dispersed in 183 ml of DMF solution. The resulting mixture was vigorously stirred and refluxed in 125 oC for 24 h. CoOF were recovered from the mixture by centrifugation. The concentrated CoOF suspension was dried at 150 oC in oven, under the inert gas atmosphere. Prepared CoOF in above described route was carbonized in a horizontal oven at 600 °C for 5 minutes in inert gas atmosphere. To purify CoOF it was placed in a flask with chloric acid (36%) with vigorously stirring and refluxing in 50 oC for 72 h.

2.3 Characterization techniques


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The composition of the sample and its structure was analyzed by transmission electron microscopy using an FEI Tecnai G2 F20 S Twin with an accelerating voltage of 200 kV, and Xray dispersion spectroscopy (EDS). The morphology of the samples was investigated by scanning electron microscopy (TESCAN, VEGA SBU3), acquired in the 30 kV acceleration voltage. The N2 adsorption/desorption isotherms were acquired at liquid nitrogen temperature (77 K) using a Quadrosorb SI (Quantachrome Instruments) and the specific surface area and pore size distribution

was

calculated

by

the

Brunauer–Emmett–Teller

(BET)

method.

The

crystallographic phase identification was performed using X,Pert Philips PROX-ray diffractometer (X,Pert PRO Philips diffractometer, CoKa radiation). Thermogravimetric analysis was performed using a DTA-Q600 SDT TA instrument with a heating rate of 10 °C min−1. The vibronic properties measurements were carried out using a Renishaw InVia Raman spectroscope (excitation k = 785 nm). The kinetics of dye adsorption onto structure was examined with the UV–vis spectrophotometer Helios alpha (Thermo Scientific)

2.4 Adsorption experiments Dye solution (10 ml) with initial concentration was placed in a conical flask. The initial concentrations of dye were: 10, 12,5, 15, 20, 25 mg/L, respectively. The flask with dye solution was placed with magnetic stirrer at constant temperature bath on a hotplate. Before mixing with CoOF various pH levels of the dye solution was adjusted by adding a few drops of hydrochloric acid (0.1 M) or sodium hydroxide (0.1 M). To observe the effect of temperature the experiments were carried out in the range of 20-60 °C. When the temperature was reached a 10 mg of CoOF was added into the flask. 2 ml of aqueous sample was taken from the solution after setting the time. The liquid was separated from the adsorbent by magnetic separation. Using the calibration plot and maximal absorbance (for rhodamine B at 553 nm), changes in the dye concentration were determined. The adsorption capacity at equilibrium qe (mg/g) was calculated by the following equation [39]:

qe =

(C 0 − C e )⋅V m


(1)


where C0 (mg L−1) is the initial dye concentration, Ce (mg L−1) the dye concentration at equilibrium, V (L) the volume of the solution and m (g) is the mass of the adsorbent.

3. Results and discussion 3.1

Characterization of the adsorbent

The flakes of MOF crystals ([Co3(BDC)3(DMF)4]n) were synthesized from a DMF solution of Co(NO3)2·6H2O and terephthalic acid (H2BDC) ligands, via a solvothermal reaction. The model and morphology of the sample are presented in Figure 1 a-c. Most of the species exhibited hexagonal and flake-like structure. However, part of the sample underwent deformation already during the synthesis. This can be explained by the low stability of MOF structure. In order to prevent further crystals deformations, MOFs structures were carbonized at 600 ºC for 5 min under a N2 flow. During CoOF carbonization, the metal organic framework decomposed to the carbon derived structures and cobalt oxide nanoparticles. Carbonized CoOF products maintained the original hexagonal-like shape of CoOF (Figure 1d-e). The elemental mapping (Figure 2) of the pristine CoOF structure revealed that Co species are distributed all over the hexagonal-like structure. Cobalt signal in the carbonized metal organic framework is attributed to the spherical nanoparticles, visible on the flakes as a brighter spots on the SEM images (Figure 2b and 2b'). As presented the carbonized and acid purified CoOF (Figure 2c and 2c') flakes still contain bright spots, but less intense. The presence of cobalt in the purified CoOF have been confirmed by TGA analysis (Figure S1 in Supporting Information). According to the presented analysis, the cobalt elements are located inside the carbon structure (around 50 wt%). They resisted acid etching. The detailed COF carbonization mechanism was reported previously [38]. Confined cobalt in between graphene flakes has magnetic properties that was confirmed with EPR spectroscopy (Figure S2 in Supporting Information) and with the photographs of the magnetically separated nanoparticles after dye adsorption (Figure 4 and Figure S3 in Supporting Information). According to the Brunauer–Emmett–Teller method (BET) surface area of the carbonized CoOF before (Figure 3a) and after (Figure 3b) purification was determined to be 93 m2/g and 199 m2/g, respectively. After the CoOF carbonization pore volume reached 0.385 cm3/g with average pore radius at 7.34 nm. The acid purification of the carbonized MOF increased the pore volume up to 0.812 cm3/g and the average pore radius up to 8.142 nm (after purification). The


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modification of the surface area and pore volume are probably related to cobalt removal. The nitrogen adsorption isotherms of the carbonized and carbonized/purified CoOF samples show a characteristic Type IV isotherm, with hysteresis loop in the higher partial pressure. This confirms that both samples has mesoporous structure. The isotherm loop has small intensity that corresponds to the pores volume. Pores volume determines the surface area. The XRD diffraction patterns (Figure 3) of the carbonized cobalt metalorganic frameworks correspond to the mixture of cobalt (NO. 05-0727) and cobalt oxide – CoO (NO. 43-1004) nanoparticles. The XRD analysis presented in Figure 3c is agreement with the literature data reported by the Liu et al. on the isostructural coordination polymers of the cobalt(II) and 2,6-naphthalenedicarboxylate [39]. The Raman spectra (Figure 3d) shows that organic ligands in the CoOF, during heat-treatment transform into graphitic structure with the typical modes related to the G band, D band, and 2D band [40]. During the MOF pyrolysis, the decomposition of CoOF led to the generation of Co based nanoparticles and graphitic flakes [38].

3.2 Adsorption kinetics

The adsorption studies have been conducted with different initial dye concentrations (1025 mg/L) at pH of 7 at 20 °C (Figure 4). There is a rapid increase in adsorption capacity for the first 10 minutes of contact time for all concentrations and thereafter it proceeded at slower rate and finally achieved equilibrium point. Figure 4 shows that increasing initial dye concentration led to an increase in the adsorption capacity at equilibrium of rhodamine B. The equilibrium adsorption capacity increase from 9.871 to 24.573 mg/g. This allows to determine that the initial concentration of the dye has a significant influence on the adsorption capacity of dye onto CoOF. The same trend was also observed by K. Kadirvelu et. al. [41] who studied the adsorption of rhodamine onto activated carbon with different particle sizes. However, the equilibration of the process in their case, took more time for each initial concentrations. Faster adsorption on our material can be due to a higher pore volume (0.812 cm3/g) and pores size despite lower specific surface area. The average pore radius is 7.34 nm while rhodamine particles is ~ 1.79 nm. Therefore, rhodamine B molecules can be easily accumulated in the pores of CoOF [42].



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In the present investigation, three kinetic models have been considered to interpret the rate of adsorption: the pseudo-first-order, pseudo-second-order and intraparticle diffusion kinetic model. The pseudo-first-order kinetic model is represented by the following equation [43]:

dq = k1 dt

(qe − qt )

(2)

or in the linear form of this equation:

(

)

ln q e − q r = ln q e − k 1 t

(3 )

where k1 (min−1) is the first-order rate constant adsorption. Values of k1 and qe can be calculated from the plots of ln(qe − qt) versus t. The pseudo-second-order kinetic model can be expressed as follows [44]:

dq dt

= k2

(q e − q t )

2

(4)

or in the linear form:

1 1 t t = + qt qe k 2 q e2

(5)

where k2 (g mg−1 min−1) is the rate constant for the pseudo-second-order adsorption kinetics. Values of k2 and qe can be calculated from the slope and intercept of the linear plots of t/qt versus t (Figure 5). The results determined from the pseudo-first-order and the pseudo-second-order kinetic model along with the corresponding correlation coefficients R2 are given in Table 1. Basing on the correlation coefficient the adsorption of rhodamine B onto CoOF is well represented by pseudo-second order kinetic model. The correlation coefficient of the pseudo-second-order kinetic model (R2=1 for most concentrations) was higher than that of the pseudo-first order (R2≤0.994). Also differences in values between experimental and calculated values of qt for the pseudo-second-order kinetic model are insignificant. There was also a good agreement between experimental and calculated qe values at different initial concentrations. Kinetic data were also calculated with Webber and Moris the intraparticle diffusion model, which is described by the following equation [45] :


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q t = k p t 0 .5 + C

(6 )

where C (mg g−1) is the intercept and kp (mg g−1 min−0.5) is the intraparticle diffusion rate constant, which can be calculated from the slope of the linear plots of qt versus t0.5 (Figure 6). The theory proposed by Weber and Morris states, that adsorption process is controlled only by the intraparticle diffusion if the regression of qt versus t0.5 is linear and passes through the origin. The regression was linear, but the plot did not pass-through the origin. This indicates that although intraparticle diffusion was involved in the adsorption process, it was not the sole rate-controlling step. In Figure 6 there are two linear sections with different slopes. It means that diffusion of dye onto CoOF occurs in two stages. The first linear section (dashed line) is faster and it is equivalent for the boundary diffusion effect. The second linear section (solid line) is attributed to the intraparticle diffusion as the limiting step of adsorption. The two phases in the intraparticle diffusion plot suggest that the adsorption process proceeds by the surface adsorption and the intraparticle diffusion. A similar type of plot was reported by Wang et al. [46] and Dogan et al. [47]. The initial part of the plot indicates the boundary layer effect while the second linear part of the plot is due to intraparticle or pore diffusion. The values of kp and C were obtained from second linear part of the plot. The values of the intercept C indicates the thickness of the boundary layer: larger C value corresponds to a greater boundary layer diffusion effect [48] The C values (9.835- 24.560 mg/g) increased with the initial dye concentrations. Therefore, it can be concluded that increase in the initial dye concentrations promoted the diffusion effect of the boundary layer [49]. The correlation coefficients for the intraparticle diffusion kinetic model (R2 ⩽ 0.828) are lower than that of the pseudo-second-order kinetic model for the adsorption of rhodamine B onto CoOF. This suggests that the pseudo-second-order adsorption mechanism is predominant [50]. The process fitted better to the pseudo-secondary kinetic model suggests that the process is chemisorption, but only after determining the thermodynamic parameters can be determined whether it is chemisorption or physisorption.



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3.3 Adsorption isotherms

In the present investigation the adsorption data were analyzed using Langmuir and Freundlich adsorption isotherms. The Langmuir isotherm is based on an assumption that the adsorption occurs at specific homogeneous sites within the adsorbent. The Freundlich isotherm assumes heterogeneous adsorption due to the diversity of the active sites on the surface [45]. A linear form of Freundlich expression is represented by following equation [48]:

 1  ln q e = ln K F +   ln C e  n 

(7)

where KF (mg g−1(L mg−1)1/n) and n are Freundlich constants, which represent adsorption capacity and adsorption strength, respectively. The values of KF and n can be calculated from the intercept and slop of the linear plot ln qe versus ln Ce. The values of n ranging from 1.0 to 10.0 indicated that the adsorption process under this conditions is favorable. The linear form of Langmuir isotherm model given by following equation [51]:

C e C e 1 = + q e Q 0 b Q 0

(8)

where Q0 (mg g−1) is the monolayer adsorption capacity and b (L mg−1) is a constant related to the energy of adsorption. Langmuir isotherm can be expressed in terms of an equilibrium parameter (RL) (dimensionless), which is defined by the following equation:

1 R L = 1+ b C 0

(9)

where b (L mg−1) is Langmuir constant and C0 (mg L−1) is the highest initial concentration of the adsorbate. The value of RL indicates the type of the isotherm to be either unfavorable (RL > 1), linear (RL = 1), favorable (0 < RL < 1) or irreversible (RL = 0) [50]. The RL values in range of 0-1 representing favorable adsorption conditions.


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Langmuir and Freundlich isotherm plots are given in Figure 7. The data of the fitted models and correlation coefficients R2 are presented in Table 2. The resulting values of RL and n indicate that the adsorption process was favorable. However, the correlation coefficient for Freundlich isotherm (R2=0.981) was greater than for Langmuir isotherm. It means that Freundlich model suits better for the description of the adsorption of RhB onto CoOF. The Freundlich isotherm is suitable for non-ideal adsorption on heterogeneous surfaces. The heterogeneity is caused by the presence of different functional groups on the surface, and several adsorbent–adsorbate interactions. The values of 1/n are smaller than 1 (0.761) suggesting that CoOF possesses heterogeneous surface. However, it is also reported that the adsorption of rhodamine B follows Langmuir isotherm [52], what is depended on the nature of adsorbent.

3.4 Effect of pH and mechanism of adsorption

pH is one of the most important parameter controlling the adsorption process. The effect of pH of the solution on the adsorption of Rhodamine B onto CoOF was determined. The results are shown in Figure 8. The pH of the solution was controlled by the addition of 0.1 M HCl or NaOH. The solutions were adjusted to obtain an initial pH value in the range of 3–11. Figure 8 shows that pH of solution has great influence on adsorption capacity RhB onto CoOF. An increase in adsorption capacity was noticed when the solution was changed to basic, optimum conditions obtained at pH 7. Above this pH value, the adsorption capacity decreases slightly and stabilizes. The results show that with the increase in pH the adsorption capacity also increases. With increase in pH from 2.0 to 10.0 the interactions between RhB and the surface groups on the adsorbents increased, with optimum conditions obtained at pH 7. At this pH, RhB has the quinonoid structure (i.e. it exists in the zwitterion form) [53]. Additionally, with increase in pH the dissociation of the aromatic carboxylic acid groups and deprotonation of the nitrogen groups in RhB will be increased, thereby increasing electrostatic repulsion between RhB and the adsorbent. This therefore suggests that adsorption is primarily influenced by the electrostatic interaction between the RhB zwitterion and the adsorbents. However, other factors such as the molecular structure of the adsorbate might also influence adsorption. All further adsorption experiments were carried out at pH 7, since this value is in the optimum range.



FTIR spectrum of carbonized/purified sample and sample after adsorption of RhB onto CoOF (Figure 9) show the band at 570 cm−1 and 669 cm−1 corresponding to the stretching vibrations of the cobalt–oxygen bond. It represents cobalt cations in an octahedral position. The band at 570 cm−1 is also attributed to the vibrations in the spinel lattice [54] The broad band ranging from 3000 to 3600 cm−1 with peak centered at 3423 cm−1 is attributed to the O–H stretching vibrations of adsorbed water molecules [55-56] Adsorption peak at 3745 cm−1 is assigned to the isolated hydroxyl groups [57]. In most cases a peak (with different intensity) around 2349 cm−1 originate from CO2 in the beam (poor background correction) [58]. Also, bands from carbonyl group were found at 1720v and 1644 cm−1 1910 cm−1 [59] [60]. The range from 2850-3000 cm−1 belongs to

the saturated systems (alkanes) [61]. Peak at 1375 cm−1 comes

from the deformation of CH3 groups [62]. The band corresponding to 1585 cm−1 appears due to C–C stretching of the aromatic ring [63-64]. On the spectrum after adsorption, no new peaks are visible compared to the pre-adsorption spectrum. This testifies that there are no reaction occur during the adsorption of Rhodamine B onto CoOF. Adsorption on the carbon nanomaterials can occurs through the π-π interactions, hydrogen bonding and Van der Waals interactions [65-66]. The dominant adsorption mechanism on the reduced graphene oxide was reported as a π-π interactions that are immune to the influenced by environmental factors including pH, ionic concentration, temperature and the concentration.[6768] Since presented CoOF adsorption capacities are depended of the environmental conditions the hydrophobic interactions (π–π stacking and hydrophobic effects) between CoOF and RhB were precluded and dye hydrogen bonding and Van der Waals interactions were considered [69]. The RhB adsorption can be explained by electrostatic interactions between the charged surface of CoOF and the positively charged cationic dyes and the hydrogen binding. The zeta potential of the absorbent allows to determined surface charge of the adsorbent. The surface charge decreases from the positive value to the negative, reaching zero net charge at pH = 6.5. Since RhB as the cationic dyes dissociate to chloride ions and amino cations in aqueous solution, negative charges at surface CoOF (in pH above 6.5) benefits the adsorption of cationic dyes through electrostatic interaction. At pH 7, the surface charge stabilizes at negative value. Therefore, the adsorption rates decelerates. This can be interpreted as the hydrogen bond interaction between cationic dyes and CoOF, which is stronger than that of an electrostatic interaction.


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3.5 Effect of temperature

The effect of temperature on equilibrium adsorption of Rhodamine B onto CoOF was investigated for three temperatures: 20, 40, 60°C, initial dye concentration at 25 mg/L and pH=7. The results indicate that along increase in the temperature the adsorption capacities decrease (Figure 10), which is compatible with the exothermic nature of the adsorption reaction. In order to study the thermodynamics of adsorption of RhB onto CoOF, three basic thermodynamic parameters, enthalpy (∆H°), entropy (∆S°) and Gibbs free energy (∆G°), were calculated following [70]: ln K a =

∆ S O R

−

∆ H O R ⋅T

q Ka = e Ce

(10)

(11)

∆ G Ο = − R T ln K a (1 2 ) where T is the solution temperature (K), Ka is the adsorption equilibrium constant, and R is the gas constant. Enthalpy (∆H°) and entropy (∆S°) were calculated from the slope and intercept from the plot of lnqe/Ce versus 1/T . The obtained plot fits well to linearity (correlation coefficient R2=0.923) (Figure 11). The value of Gibbs free energy (∆G°) was calculated using Equations 12. These thermodynamic parameters are listed in Table 3. Calculated ∆H° value for RhB is -39.60 kJ/mol. The negative values of this parameter indicate that adsorption process is an exothermic. The value of ∆H° under 40 kJ/mol suggested that physisorption took place [71]. The value of entropy (∆S°) was -100.13 J/mol*K. The negative value of this parameter indicates that the degrees of freedom decreases at the solid/solution interface during the adsorption RhB onto CoOF. Negative value of ∆S is not encountered for rhodamine B, but it is characteristic for metalorganic frameworks. There are no significant changes in the internal structure during the adsorption process [72].



Negative values ∆H and ∆G are characteristic for adsorption of Rhodamine B. It was reported previously e.g. in [73, 74]. Negative value of ∆H indicates that adsorption process is exothermic, and negative values of ∆G indicates that the adsorption process is spontaneous and thermodynamically favorable. Also these negative values within the range of -20 kJ/mol to 0 kJ/mol confirm that physisorption took place in the studied process.

3.6 Comparison

Table 4 lists the comparison of maximum monolayer adsorption capacity of RhB dye onto various adsorbents. The values of maximum adsorption capacity Q0 was 267 mg/g onto Indium-based metal-organic framework/graphite oxide composite at 25°C. The minimum adsorption capacity was 1.05 mg/g for zeolite MCM-22. Therefore, CoOF is effective and promising adsorbent for removal of Rhodamine B from aqueous solution.

3.7 Nanomaterials stability

The structure stability of the adsorbents is an essential aspect of their industrial applications. Figure 12 a,b and 12 c-f exhibits the SEM and TEM images of the bare CoOF and the Rhodamine B dye-loaded CoOF nanoadsorbent, respectively. The same hexagonal and flakelike structure with the spherical nano particles on their surface are observed. It is obvious that the dye molecules did not influence the morphology of the CoOF nanostructures. Additionally analysis, shows differences in the CoOF chemical compositions. XRD determination showed cobalt (II) oxide phase transformation to the mixture of cobalt (II) and (III) oxide after the rhodamine B adsorption (Figure 12g). The presented phase transformation is related to the cobalt oxide oxidation in the aqueous solutions [83, 84]. The oxidation effect of the cobalt nanoparticles is more related on the water molecules ability to penetrate graphene layers [85] then physical destruction carbon structure or extraction of the cobalt spices. The peaks corresponding to the cobalt metallic phase are after the rhodamine adsorption still present in the sample. As presented in the TGA plot (Figure 12h), the amount of the inflammable particles is constant in the CoOF


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before and after rhodamine B adsorption (14 and 15 wt% respectively). The results demonstrate the stability of the structure (shape and chemical composition and cobalt particles binding to the carbon structure) and potential reusability of the as-prepared CoOF nanostructures as rhodmaine B adsorbent.

3.8 Waste reduction in the synthesis of CoOF by recycling organic solvent

Previous studies on the CoOF carbonization provided evidence that it is possible to control the structure and chemical composition of the carbonized CoOF by controlling the carbonization parameters. The advantage in applying the CoOF is the possibility to reuse the DMF after the reaction. In order to successfully synthetize CoOF from the reused DMF, the organic solvent should be separated from the unreacted substrates and by-products such as cobalt nanoparticles, by distillation. DMF is regarded toxic for natural environment and can cause birth defects in humans [86]. Therefore, the DMF was recycled during the synthesis of CoOF. Such approach will reduce the amount of toxic waste if applied in the industry. SEM images (Figure 1b-c) show the morphology of CoOF and the structure received from the synthesis with the unreacted DMF. Observed nanocomposites showed characteristic features of CoOF such as flat, and the hexagonal shape of the nanocomposite, with an average diameter of 13 µm. SEM images (Figure 13) indicate that CoOF structures synthesised from the distilled DMF were comparable to the nanocomposites from previous synthesis. These CoOF nanocomposites exhibited similar shape, although with the diameter lower by 2.5 µm. The reported size of the CoOF from the unreacted and distilled DMF was in the range of the previously reported data (showed in the supporting information) [38]. CoOF structure showed a tendency to deform (break) during the synthesis which was a result of the physical interaction and collisions in the environment. The deviations from the ideal shape depending on synthesis parameters are known and have been previously reported [38]. The essential aspect of the CoOF synthesis is it application to reduce the polymer waste like polyethylene terephthalate. PET structure are composed from terephthalic acid that can be released under hydrothermal conditions in a microwave oven. As obtained terephthalic acid from chemical recovery of waste PET, was successful used for synthesis of MIL-53(based on aluminium) and MIL-47(based on vanadium) [87]. Yu-Ting Huang et al. also reported that from



the recovered acid linker NTHU-2, NTHU-3, MOF-5, MIL-53 and MIL-101 structures were also successfully prepared [88]. Jianwei Ren et al. showed that PET hydrolysis and Cr-MOF synthesis can occur simultaneously. [89]

4

Conclusion

This study indicates that carbonized metalorganic frameworks based on cobalt can be efficient and effective adsorbent for the removal of Rhodamine B from aqueous solutions. Proposed synthesis rout allowed to obtain the graphitic structure with the cobalt nanoparticles localized in side carbon structure. Therefore cobalt nanoparticles are protected from the acid corrosion simultaneously functionalizing carbon structure with magnetic properties. This adsorbent can be easily separated from the aqueous solution by an external magnet field. The proposed structure can be synthesized reusing organic solvents (DMF after distillation) and from the terephthalic acid recovered from the polymer waste. The effects of various operating conditions, like initial dye concentration, pH and temperature, were investigated and following conclusion can

be presented:

• Adsorption data fitted well for Freundlich isotherm • Maximum capacity was 24.57 mg/g • A comparison of kinetic models on the adsorption rate of Rhodamine B onto CoOF showed that the process was best described by pseudo-second-order kinetics. • Thermodynamics parameters ∆H°, ∆S°, ∆G° were evaluated and it was found that it was spontaneous and exothermic physisorption.

Acknowledgements The authors are grateful for the financial support of National Science Centre, Poland, within SONATA BIS 2012/07/E/ST8/01702

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Table 1 Comparison of the pseudo-first-order, pseudo-second-order and the intraparticle diffusion models for different initial concentrations of rhodamine B at 20°C Pseudo-first-order C0 [mg/L]

DYE

RHODAMINE B

qe, exp (mg/g)

Pseudo-second-order

k1 (1/min)

qe,cal (mg/g)

R2

k2 (g mg-1min-

Intraparticle diffusion model

qe, cal (mg/g)

R2

kp (mg g-1 min -0,5)

C (mg/g)

R2

1)

10

9.871

0.305

16.352

0.763

0.336

9.917

0.999

0.005

9.835

0.788

12,5

12.332

0.206

9.406

0.994

0.125

12.449

0.999

0.010

12.258

0.739

15

14.753

0.102

6.121

0.990

0.052

14.950

0.999

0.010

14.650

0.828

20

19.701

0.091

5.423

0.946

0.042

19.952

0.999

0.012

19.576

0.702

25

24.573

0.228

23.171

0.946

0.463

24.540

0.999

0.023

24.560

0.369

Table 2 Langmuir and Freudlich parameters for the adsorption of rhodamine B onto CoOF. Experimental conditions: T=20 °C Dye

Langmuir isotherm Q0 (mg/g) b (L/mg) RL

RHODAMINE B

72.150

1.187

Freundlich isotherm KF n ((mg/g)(L/mg)1/n) 46.569 1.314

2

R

0.033

0.755

R2 0.981

Table 3 Thermodynamic parameters for the adsorption of the RhB onto CoOF Dye

Dye concentration

∆H°

∆S°

(mg/L)

(kJ/mol)

(J/mol K)

25

-39.60

-100.13

Rhodamine B

∆G° at temperature (°C) (kJ/mol) 20

40

60

-9.88

-9.07

-5.77

Table 4. Comparison of the maximum monolayer adsorption capacity (Q0) of RhB onto CoOF with others reported in the literature Adsorbent

Adsorbate

Qe(mg/g)

Conditions


Referece


Perlite RGO TA-G Sodium montmorillonite MCM-22 DNc CCDNc In-MOF@GO-2 nano-NiO CNSs CoOF

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RhB RhB RhB

8.72 13.15 201.207

T=30°C T=25°C T=25°C

73 75 76

RhB

42.19

T=30°C

77

RhB RhB RhB RhB RhB RhB RhB

1.05 52.90 217.39 267 111 1.95 72.15

T=30°C T=26°C T=26°C T=25°C T=20°C T=25°C T=20°C

78


79 80 81 82 This article

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Graphical abstract 84x62mm (300 x 300 DPI)



Figure 1. Models (a) and SEM images of the cobalt-based metal organic frameworks (b-c), carbonized cobalt organic frameworks before (d-e) and after hydrochloric acid purification (f-g). 82x82mm (300 x 300 DPI)


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Figure 2. Isotherms of the nitrogen uptake by the carbonized CoOF before (a) and after (b) acid purification, X-ray diffraction spectra of the as produced and carbonized CoOF (c) and Raman spectra of carbonized CoOF 65x52mm (300 x 300 DPI)



Figure 3. Electron diffraction spectroscopy and SEM of the cobalt-based metal organic frameworks (a, a') and carbonized cobalt-based metal organic frameworks before (b, b') and after carbonization (c, c') with marked signal from the cobalt (magenta). 114x160mm (300 x 300 DPI)


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Figure 4. The effect of initial dye concentration on adsorption capacity of the RhB onto CoOF (A) and photographs of the dye solution with the nanoparticles before (B) and after (C) adsorption. Experimental conditions: T=20 °C, pH=7 1733x586mm (96 x 96 DPI)



Figure 5. Pseudo-second-order kinetics of adsorption RhB onto CoOF. Experimental conditions: T=20 °C, pH=7 238x196mm (300 x 300 DPI)


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Figure 6. Intraparticle diffusion model of the adsorption RhB onto CoOF.Experimental conditions: T=20 °C, pH=7 117x94mm (299 x 299 DPI)



Figure 7. Freundlich (a) and Langmuir (b) adsorption isotherm of the RhB onto CoOF at 20°C 248x100mm (300 x 300 DPI)


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Figure 8. The effect of initial pH of dye solution on adsorption capacity of the RhB onto CoOF and zeta potential changes. Experimental conditions: T=20 °C, pH=7 96x72mm (300 x 300 DPI)



Figure 10. FTIR spectrum of carbonized and acid purified CoOF before and after dye adsorption 287x200mm (300 x 300 DPI)


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Figure 11. Van't Hoff plot for the adsorption of the RhB onto CoOF 118x97mm (300 x 300 DPI)



Figure 12. SEM and TEM images of the carbonized COF before (a-b) and after (c-f) Rhodamine B (RhB) adsorption. XRD (g) of the CoOF, carbonized CoOF and carbonized CoOF after RhB adsorption and TGA (h) of the carbonized CoOF before and after RhB adsorption with the reference plot of the RhB 90x100mm (300 x 300 DPI)


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Figure 13. SEM images CoOF after synthesis from fresh (A-B) and recycled (C-D) DMF 79x78mm (300 x 300 DPI)


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