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Adsorptive remediation of Cr (VI) from aqueous solution using cobalt ferrite: Kinetics and isotherm studies
Materials Today: Proceedings xxx (xxxx) xxx
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Adsorptive remediation of Cr (VI) from aqueous solution using cobalt ferrite: Kinetics and isotherm studies Suraj Prakash Tripathy a, Raghunath Acharya b,c, Mira Das d, Rashmi Acharya a,⇑, Kulamani Parida a,⇑ a
Centre for Nanoscience and Nanotechnology, ITER, S‘O’A Deemed to be University, Bhubaneswar 751030, India Radiochemistry Division, Bhabha Atomic Research Centre (BARC), Mumbai 400094, India c Homi Bhabha National Institute, Department of Atomic Energy, Mumbai 400094, India d Department of Chemistry, ITER, S‘O’A Deemed to be University, Bhubaneswar 751030, India b
a r t i c l e
i n f o
Article history: Received 23 December 2019 Received in revised form 25 January 2020 Accepted 28 January 2020 Available online xxxx Keywords: Cobalt ferrite (CF) Sol–gel Adsorption capacity Cr (VI) Pseudo-second-order Langmuir isotherm
a b s t r a c t Facile fabrication of cobalt ferrite (CF) through sol–gel approach followed by calcination at 600 °C was carried out for the adsorptive remediation of a well-known carcinogenic heavy metal pollutant; Cr (VI). The formation of CF was confirmed from PXRD and FT-IR analysis. Further the adsorptive behavior of CF towards Cr (VI) was explored with respect to pH, CF dose, sorption kinetics and isotherm studies. Adsorption process followed pseudo-second order kinetics and Langmuir isotherm. The maximum adsorption capacity (Qm) obtained from Langmuir isotherm was found to be 10.35 mg g1 at pH 3 and CF dose of 1 g L1 in 2 h. Ó 2020 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of the scientific committee of the National Conference on Trends in Minerals & Materials Technology.
1. Introduction Deterring water quality caused by accelerating population and improper industrial waste management leads to catastrophic impact on living organisms. Among a spectrum of aqueous mediated pollutants; heavy metals are prioritized as most toxic ones. Chromium and its compounds are extensively used in various industrially essential processes like tanning of leather, electroplating, chrome plating, alloy production, dye and pigment synthesis. The wastes generated from these anthropogenic sources enrich the level of Cr (VI) in water bodies and pose threat not only to aquatic animals but also to the downstream users. Cr (VI) exposure can cause liver damage, respiratory cancer, upper abdominal pain, pulmonary congestion [1,2]. As a consequence, World Health Organization (WHO) has mandated Cr (VI) toxic limit as 0.005 ppm in wastewater [3]. Therefore, remediation of Cr (VI) from aqueous environment has now been a global concern. Nanomaterials find great attention in solving current energy crisis and environmental issues [4–9]. Magnetic iron oxides, layered double hydroxides, titanate nanotubes and metal organic ⇑ Corresponding authors.
frameworks have been used as adsorbents for removal of Cr (VI) [10–12]. UiO-66-NH2 (UNH), a Zr-based metal organic framework exhibited maximum Cr (VI) adsorption capacity of 25.9 mg g1 at pH 2.0 [13]. However, use of this adsorbent may be restricted due to difficulties in separation process. Ferrites are considered as adsorbents due to their chemical stability, low cost, and super paramagnetic properties [14]. Jia et al. synthesized mesoporous NiFe2O4 by twice pore-forming method and studied its adsorption behaviour towards Cr (VI). This magnetic iron oxide exhibited maximum adsorption capacity of 43.68 mg g1 and could easily be recovered from the treated solution by employing an external magnetic field [15]. Among different ferrites, cobalt ferrite (CoFe2O4) is extensively used for its high chemical stability, strong mechanical strength, large coercivity and moderate saturation magnetization [16]. In this paper, we have fabricated cobalt ferrite (CF) by the sol–gel process followed by calcination at 600 °C. X-ray diffraction (XRD) and Fourier Transform Infrared spectroscopy (FTIR) techniques were employed to characterize CF. Then it was utilized for sorption of Cr (VI) ions from aqueous media. The effect of initial pH, adsorbent dose, and time on adsorption of Cr (VI) was investigated. The kinetics and isotherm studies were performed to get an insight into the Cr (VI) sorption by CF.
E-mail addresses: [email protected] (R. Acharya), kulamaniparida@soa. ac.in (K. Parida). https://doi.org/10.1016/j.matpr.2020.01.534 2214-7853/Ó 2020 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of the scientific committee of the National Conference on Trends in Minerals & Materials Technology.
Please cite this article as: S. Prakash Tripathy, R. Acharya, M. Das et al., Adsorptive remediation of Cr (VI) from aqueous solution using cobalt ferrite: Kinetics and isotherm studies, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.01.534
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2. Materials & methods 2.1. Chemicals used Cobalt (II) nitrate, Iron (III) nitrate, D-glucose, Hydrochloric acid, Potassium bromide, Sodium hydroxide, Potassium dichromate, Acetone, 1, 5-Diphenylcarbazide and sulphuric acid were obtained from Merck Chemicals. Analytical grade chemicals are used in this study. All the experiments were done using de-ionized water.
where, Qt is the adsorbed Cr (VI) in mg g1 at time t; K1 and K2 are the rate constants for pseudo-first-order and pseudo-second-order reactions respectively. Adsorption isotherm was studied using simultaneous varying of temperature (298–313 K) and initial Cr (VI) concentration (5– 25 mg L1) at pH 3 for a time period of 2 h. Experimental results obtained were fitted using Langmuir and Freundlich isotherm model as depicted in Eqs. (3) and (4) respectively [19,20].
Q e ¼ Q m bCe =ð1 þ bCe Þ
ð3Þ
2.2. Adsorbent preparation
Q e ¼ Kf Cne
Adsorbent was synthesised in a facile sol–gel synthesis approach. In the first step 1:2 M ratios of the nitrate salts of Cobalt (II) and Iron (III) were dissolved in deionized water using a magnetic stirrer at room temperature for 1 h respectively. To this solution excess of D-Glucose was added slowly with a constant stirring at 60 °C. Then the solution formed was slowly evaporated at 60 °C. The residue obtained was grinded using mortar pestle and then, calcined in air at 600 °C to form the calcined product. The calcined product was grinded to fine powder and named as cobalt ferrite (CF) which was stored for further studies.
where, Qm represents the maximum loading capacity for monolayer adsorption (mg g1); the Langmuir adsorption constant (L mg1) is represented by b; Kf represents a constant for Freundlich isotherm (L mg1)n, n is the intensity of adsorption. Cr (VI) concentration following sorption is determined with the aid of UV–Visible spectrophotometer (JASCO-V750) using Diphenylcarbazide method [21]. The adsorption capacity (Qe) and removal efficiency of CF for Cr (VI) adsorption was calculated by using the equations given below;
1
ð4Þ
Adsorption capacityðQ eÞ ¼ 2.3. Adsorbent characterization Crystalline phase identification of prepared adsorbent (CF) was explored by powder X-ray diffraction (PXRD) technique using Rigaku-Ultima-IV diffractometer powered with a Cu Ka source having k = 0.1541 nm in the 2h range of 20–70°. The functional group of the synthesized adsorbent was extensively studied using JASCO FT/IR 4600LE spectrometer in the wave number range of 400–4000 cm1 with reference to Potassium Bromide.
Removal efficiency ¼
ðCo Ce ÞV M
Co Ce X100 Co
ð5Þ ð6Þ
Here, Qe is adsorption capacity expressed in mg g1, Co is initial Cr (VI) concentration in mg L1 and Ce represents the equilibrium Cr (VI) concentrations in mg L1. V and M are the solution volume (L) and CF mass used (g) respectively [18]. 3. Result & discussion
2.4. Stock solution 3.1. Synthesized adsorbent characterization Potassium dichromate was taken as a synthetic Cr (VI) source for lab-scale sorption experiments using synthesized Cobalt ferrite as adsorbent. 2.83 g of solid K2Cr2O7 (Purity 99.9%) was dissolved in one liter de-ionized water to prepare stock solution of Cr (VI) with concentration of 1000 mg L1. Cr (VI) solutions of desired concentration were prepared by diluting this stock solution with deionized water. 2.5. Adsorption studies Cr (VI) sorption by the prepared ferrite was carried out using batch sorption technique in a temperature controlled water bath shaker (REMI-R12) at constant shaking. Solution pH was measured with the help of Labman-LMPH-10 digital pH meter and was adjusted to a desired pH value by adding solutions of 0.01 M HCl or 0.01 M NaOH. The effect of pH on Cr (VI) sorption was studied by shaking constantly 5 mg L1 of Cr (VI) solution with 1 g L1 CF for 2 h at 298 K. The effect of CF dose on Cr (VI) adsorption was carried out using 5 mg L1 Cr (VI) solution by varying 1–4 g L1 CF at pH 3 for 2 h at room temperature. The contact time was varied from 5 min to 4 h by using Cr (VI) solution of 5 mg L1 with 1 g L1 CF dose at pH 3. Kinetic equations of pseudo first order and pseudo second order presented in Eqs. (1) and (2) respectively, were used for non-linear fitting of experimental data obtained from variation of contact time [17,18].
Q t ¼ Q e ð1 eK1 t Þ
ð1Þ
Q t ¼ K2 Q 2e t=ð1 þ K2 Q e tÞ
ð2Þ
PXRD diffractogram of CF is depicted in Fig. 1a. The PXRD pattern clearly shows that the synthesized adsorbent resembles the crystal structure of cubic spinel structure of CoFe2O4. The PXRD pattern of synthesized material corresponds to that of the characteristic peaks of Cobalt Ferrite (JCPDS card no. 22-1086) [22]. The sharp and intense PXRD corresponds to high crystallinity of the prepared CF. Further the functional group analysis by FT-IR technique as depicted in Fig. 1b reveals a sharp peak at 580 cm1 and a broad peak around 3300 cm1 which corresponds to the stretching vibrations of Fe3+–O2 in the tetrahedral sites which is a characteristic of ferrite materials and the stretching vibration of O–H bond of the surface adsorbed water molecules respectively [23]. Results of both PXRD and FT-IR analysis supported the successful formation cobalt ferrite (CF). 3.2. Adsorption studies 3.2.1. Effect of pH Cr (VI) removal efficiency of an adsorbent is greatly affected by solution pH; hence the effect of pH was studied in a wide pH range of 3–11 and was presented in Fig. 2a. The synthesized adsorbent, CF showed heightened Cr (VI) sorption tendency at pH 3 (76%) and sorption performance eventually decreased with the rise in solution basicity i.e. 26% at pH 11. Such pH controlled sorption behavior of CF is attributed to the surface charge of the adsorbent, speciation of Cr (VI) and competition from OH– ions [24]. Cr (VI) ions occur in the aqueous media in form of oxy anio2 2 nic species like HCrO etc. [25]. These oxyanions 4 , CrO4 , Cr2O7 are attracted by positively charged surfaces of adsorbents by elec-
Please cite this article as: S. Prakash Tripathy, R. Acharya, M. Das et al., Adsorptive remediation of Cr (VI) from aqueous solution using cobalt ferrite: Kinetics and isotherm studies, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.01.534
S. Prakash Tripathy et al. / Materials Today: Proceedings xxx (xxxx) xxx
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Fig. 1. (a) PXRD pattern (b) FT-IR spectra of synthesized adsorbent CF.
Fig. 2. (a) Initial pH variation study [Cr (VI) = 5 mg L1, CF dose = 1 g L1, pH = 3–11, time = 2 h, Temp. = 298 K]. (b) Effect of adsorbent dose study [Cr (VI) = 5 mg L1, CF dose = 1–4 g L1, pH = 3, time = 2 h, Temp. = 298 K].
trostatic force of attractions. As cobalt ferrites have a pHpzc around 7.2, they tend to be positive surface charge distribution in acidic medium which enhances their Cr (VI) oxy anion separation efficacy at lower pH as seen for CF at pH 3. However with rising pH the surface charge becomes less positive and also at higher pH the oxy anionic Cr (VI) ions face severe competition from hydroxide ion for adsorption which is reflected in poor removal efficiency of CF at higher pH values [26]. This suggested that electrostatic attraction is the plausible mechanism for Cr (VI) adsorption onto CF. 3.2.2. Effect of adsorbent dose Optimum adsorbent dose for the removal of Cr (VI) by CF determined with variation of adsorbent dosage (1, 2, 3 and 4 g L1) at room temperature which is plotted in Fig. 2b. From this it is clearly seen that the adsorption capacity of CF decreases with rising adsorbent dosage. Such behavior can be attributed to availability of enough active surface sites at lower adsorbent (CF) dose. These sites are exposed for adsorption of Cr (VI), leading to enhanced adsorption capacity. On increasing the adsorbent dose, CF active sites are available for same concentration of Cr (VI). As a result, adsorption capacity decreases at higher CF dosage [27]. Maximum sorption capacity was obtained when CF dose was 1 g L1 and hence further adsorption experiments were carried out with this adsorbent dose.
3.2.3. Sorption kinetics Contact time plays a vital role in adsorption studies. It suggests the time period during which the adsorbate gets diffused from solution to the adsorbent surface. From the time variation experimental curve as shown in Fig. 3a, it is clearly established that sorption equilibrium (Qe = 3.54 mg g1) is attained after 2 h and the removal efficiency was not appreciably changed with further increase in contact time. In order to contemplate the sorption kinetics of CF for Cr (VI) removal, the experimental time variation data were fitted non-linearly with the pseudo-first-order and pseudo-second-order kinetics equations as shown in Fig. 3a. The fitting results were tabularized in Table 1 and it was revealed that pseudo-second-order kinetics was followed for the adsorption of Cr (VI) onto CF because it showed higher correlation coefficient (R2 = 0.96). Hence the sorption of Cr (VI) on CF caused through bimolecular interaction [28]. 3.2.4. Sorption isotherm studies The experimental data obtained by varying initial Cr (VI) concentration and temperature was presented in Fig. 3b. These experimental results were non-linearly fitted with two parameters; Freundlich and Langmuir isotherm models as plotted in Fig. 3b. Isotherm model’s acceptability was considered on the basis of higher correlation coefficient (R2) [29]. The sorption isotherm parameters calculated from fitted models are mentioned in Table 2.
Please cite this article as: S. Prakash Tripathy, R. Acharya, M. Das et al., Adsorptive remediation of Cr (VI) from aqueous solution using cobalt ferrite: Kinetics and isotherm studies, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.01.534
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Fig. 3. (a) Adsorption Kinetics Study [Cr (VI) = 5 mg L1, CF dose = 1 g L1, pH = 3, Temp. = 298 K]. (b) Adsorption isotherm study [Cr (VI) = 5–25 mg L1, CF dose = 1 g L1, pH = 3, time = 2 h].
Table 1 Kinetics fitting parameters obtained for Cr (VI) sorption by CF. Models?
Pseudo-first-order Kinetics
Parameters?
Qe
K1
R2
Qe
Pseudo-second-order Kinetics K2
R2
Values ?
3.61
1.47
0.94
4.30
0.38
0.96
Table 2 Isotherm model fitted results obtained for Cr (VI) adsorption by CF. Models?
Langmuir Isotherm
Temperature;
Qm
b
R2
Freundlich Isotherm Kf
n
R2
298 K 303 K 313 K
10.35 10.32 10.15
0.12 0.10 0.09
0.92 0.91 0.91
2.03 1.83 1.76
2.41 2.26 2.22
0.88 0.86 0.85
Isotherm fitting curves indicates that the Langmuir isotherm model is best suited for the adsorptive remediation of Cr (VI) by CF. The maximum adsorption capacity (Qm) calculated from Langmuir isotherm model is found to be 10.35 mg g1 at 298 K suggesting monolayer type adsorption [30]. From the Fig. 3b it’s observed that with rising temperature the removal efficiency of CF slightly decreased, suggesting the exothermic nature of adsorption [31]. Further to ascertain a favorable adsorption of Cr (VI) onto CF, Separation factor (RL) as given below was explored. 1 Where, ‘b’ and Co represent Langmuir constant and RL ¼ 1þbC o
10.35 mg g1 at 293 K. Therefore, CF can be considered as an effective adsorbent for removal of Cr (VI) from aqueous medium.
initial Cr (VI) concentration respectively. If we evaluate RL using the ‘b’ values of Table 2 it can be clearly seen that RL lies in the range of 0 to 1, suggesting the sorption of Cr (VI) onto CF is a favorable process [32].
Acknowledgements
4. Conclusion In a conclusive summary, the adsorbent CF was successfully prepared by the combination of sol–gel and calcination at 600 °C. The PXRD and FT-IR studies revealed that prepared CF is highly crystalline and pure. The optimization of Cr (VI) adsorption onto CF was carried out by considering various sorption parameters such as pH, CF dosage and contact time. The optimal pH and CF dose were found to be 3 and 1 g L1 respectively. The experimental Cr (VI) sorption data was fitted with pseudo-second-order kinetic model. The fitting of experimental data with Langmuir model indicated monolayer adsorption process. The maximum Cr (VI) adsorption capacity (Qm) calculated from Langmuir isotherm model was
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Authors are very much grateful to DAE-BRNS, Government of India for financial support under the research project number 37 (2)/14/02/2018/37002 to carry out the current work. Authors also thankfully acknowledge to the management of S‘O’A (Deemed to be University), Bhubaneswar for providing adequate infrastructure in pursuing this work. Dr. P.K. Pujari, Director, RC&I Group and Head, RCD, BARC is thankfully acknowledged for his motivation and encouragement. References [1] R. Dubey, J. Bajpai, A.K. Bajpai, Green synthesis of graphene sand composite (GSC) as novel adsorbent for efficient removal of Cr (VI) ions from aqueous solution, J. Water Process Eng. 5 (2015) 83–94. [2] R. Acharya, S. Martha, K.M. Parida, Remediation of Cr (VI) using clay minerals, biomasses and industrial wastes as adsorbents, Adv. Mater. Waste Water Treat. (2017) 129–170.
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Please cite this article as: S. Prakash Tripathy, R. Acharya, M. Das et al., Adsorptive remediation of Cr (VI) from aqueous solution using cobalt ferrite: Kinetics and isotherm studies, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.01.534
Contents lists available at ScienceDirect
Materials Today: Proceedings journal homepage: www.elsevier.com/locate/matpr
Adsorptive remediation of Cr (VI) from aqueous solution using cobalt ferrite: Kinetics and isotherm studies Suraj Prakash Tripathy a, Raghunath Acharya b,c, Mira Das d, Rashmi Acharya a,⇑, Kulamani Parida a,⇑ a
Centre for Nanoscience and Nanotechnology, ITER, S‘O’A Deemed to be University, Bhubaneswar 751030, India Radiochemistry Division, Bhabha Atomic Research Centre (BARC), Mumbai 400094, India c Homi Bhabha National Institute, Department of Atomic Energy, Mumbai 400094, India d Department of Chemistry, ITER, S‘O’A Deemed to be University, Bhubaneswar 751030, India b
a r t i c l e
i n f o
Article history: Received 23 December 2019 Received in revised form 25 January 2020 Accepted 28 January 2020 Available online xxxx Keywords: Cobalt ferrite (CF) Sol–gel Adsorption capacity Cr (VI) Pseudo-second-order Langmuir isotherm
a b s t r a c t Facile fabrication of cobalt ferrite (CF) through sol–gel approach followed by calcination at 600 °C was carried out for the adsorptive remediation of a well-known carcinogenic heavy metal pollutant; Cr (VI). The formation of CF was confirmed from PXRD and FT-IR analysis. Further the adsorptive behavior of CF towards Cr (VI) was explored with respect to pH, CF dose, sorption kinetics and isotherm studies. Adsorption process followed pseudo-second order kinetics and Langmuir isotherm. The maximum adsorption capacity (Qm) obtained from Langmuir isotherm was found to be 10.35 mg g1 at pH 3 and CF dose of 1 g L1 in 2 h. Ó 2020 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of the scientific committee of the National Conference on Trends in Minerals & Materials Technology.
1. Introduction Deterring water quality caused by accelerating population and improper industrial waste management leads to catastrophic impact on living organisms. Among a spectrum of aqueous mediated pollutants; heavy metals are prioritized as most toxic ones. Chromium and its compounds are extensively used in various industrially essential processes like tanning of leather, electroplating, chrome plating, alloy production, dye and pigment synthesis. The wastes generated from these anthropogenic sources enrich the level of Cr (VI) in water bodies and pose threat not only to aquatic animals but also to the downstream users. Cr (VI) exposure can cause liver damage, respiratory cancer, upper abdominal pain, pulmonary congestion [1,2]. As a consequence, World Health Organization (WHO) has mandated Cr (VI) toxic limit as 0.005 ppm in wastewater [3]. Therefore, remediation of Cr (VI) from aqueous environment has now been a global concern. Nanomaterials find great attention in solving current energy crisis and environmental issues [4–9]. Magnetic iron oxides, layered double hydroxides, titanate nanotubes and metal organic ⇑ Corresponding authors.
frameworks have been used as adsorbents for removal of Cr (VI) [10–12]. UiO-66-NH2 (UNH), a Zr-based metal organic framework exhibited maximum Cr (VI) adsorption capacity of 25.9 mg g1 at pH 2.0 [13]. However, use of this adsorbent may be restricted due to difficulties in separation process. Ferrites are considered as adsorbents due to their chemical stability, low cost, and super paramagnetic properties [14]. Jia et al. synthesized mesoporous NiFe2O4 by twice pore-forming method and studied its adsorption behaviour towards Cr (VI). This magnetic iron oxide exhibited maximum adsorption capacity of 43.68 mg g1 and could easily be recovered from the treated solution by employing an external magnetic field [15]. Among different ferrites, cobalt ferrite (CoFe2O4) is extensively used for its high chemical stability, strong mechanical strength, large coercivity and moderate saturation magnetization [16]. In this paper, we have fabricated cobalt ferrite (CF) by the sol–gel process followed by calcination at 600 °C. X-ray diffraction (XRD) and Fourier Transform Infrared spectroscopy (FTIR) techniques were employed to characterize CF. Then it was utilized for sorption of Cr (VI) ions from aqueous media. The effect of initial pH, adsorbent dose, and time on adsorption of Cr (VI) was investigated. The kinetics and isotherm studies were performed to get an insight into the Cr (VI) sorption by CF.
E-mail addresses: [email protected] (R. Acharya), kulamaniparida@soa. ac.in (K. Parida). https://doi.org/10.1016/j.matpr.2020.01.534 2214-7853/Ó 2020 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of the scientific committee of the National Conference on Trends in Minerals & Materials Technology.
Please cite this article as: S. Prakash Tripathy, R. Acharya, M. Das et al., Adsorptive remediation of Cr (VI) from aqueous solution using cobalt ferrite: Kinetics and isotherm studies, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.01.534
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2. Materials & methods 2.1. Chemicals used Cobalt (II) nitrate, Iron (III) nitrate, D-glucose, Hydrochloric acid, Potassium bromide, Sodium hydroxide, Potassium dichromate, Acetone, 1, 5-Diphenylcarbazide and sulphuric acid were obtained from Merck Chemicals. Analytical grade chemicals are used in this study. All the experiments were done using de-ionized water.
where, Qt is the adsorbed Cr (VI) in mg g1 at time t; K1 and K2 are the rate constants for pseudo-first-order and pseudo-second-order reactions respectively. Adsorption isotherm was studied using simultaneous varying of temperature (298–313 K) and initial Cr (VI) concentration (5– 25 mg L1) at pH 3 for a time period of 2 h. Experimental results obtained were fitted using Langmuir and Freundlich isotherm model as depicted in Eqs. (3) and (4) respectively [19,20].
Q e ¼ Q m bCe =ð1 þ bCe Þ
ð3Þ
2.2. Adsorbent preparation
Q e ¼ Kf Cne
Adsorbent was synthesised in a facile sol–gel synthesis approach. In the first step 1:2 M ratios of the nitrate salts of Cobalt (II) and Iron (III) were dissolved in deionized water using a magnetic stirrer at room temperature for 1 h respectively. To this solution excess of D-Glucose was added slowly with a constant stirring at 60 °C. Then the solution formed was slowly evaporated at 60 °C. The residue obtained was grinded using mortar pestle and then, calcined in air at 600 °C to form the calcined product. The calcined product was grinded to fine powder and named as cobalt ferrite (CF) which was stored for further studies.
where, Qm represents the maximum loading capacity for monolayer adsorption (mg g1); the Langmuir adsorption constant (L mg1) is represented by b; Kf represents a constant for Freundlich isotherm (L mg1)n, n is the intensity of adsorption. Cr (VI) concentration following sorption is determined with the aid of UV–Visible spectrophotometer (JASCO-V750) using Diphenylcarbazide method [21]. The adsorption capacity (Qe) and removal efficiency of CF for Cr (VI) adsorption was calculated by using the equations given below;
1
ð4Þ
Adsorption capacityðQ eÞ ¼ 2.3. Adsorbent characterization Crystalline phase identification of prepared adsorbent (CF) was explored by powder X-ray diffraction (PXRD) technique using Rigaku-Ultima-IV diffractometer powered with a Cu Ka source having k = 0.1541 nm in the 2h range of 20–70°. The functional group of the synthesized adsorbent was extensively studied using JASCO FT/IR 4600LE spectrometer in the wave number range of 400–4000 cm1 with reference to Potassium Bromide.
Removal efficiency ¼
ðCo Ce ÞV M
Co Ce X100 Co
ð5Þ ð6Þ
Here, Qe is adsorption capacity expressed in mg g1, Co is initial Cr (VI) concentration in mg L1 and Ce represents the equilibrium Cr (VI) concentrations in mg L1. V and M are the solution volume (L) and CF mass used (g) respectively [18]. 3. Result & discussion
2.4. Stock solution 3.1. Synthesized adsorbent characterization Potassium dichromate was taken as a synthetic Cr (VI) source for lab-scale sorption experiments using synthesized Cobalt ferrite as adsorbent. 2.83 g of solid K2Cr2O7 (Purity 99.9%) was dissolved in one liter de-ionized water to prepare stock solution of Cr (VI) with concentration of 1000 mg L1. Cr (VI) solutions of desired concentration were prepared by diluting this stock solution with deionized water. 2.5. Adsorption studies Cr (VI) sorption by the prepared ferrite was carried out using batch sorption technique in a temperature controlled water bath shaker (REMI-R12) at constant shaking. Solution pH was measured with the help of Labman-LMPH-10 digital pH meter and was adjusted to a desired pH value by adding solutions of 0.01 M HCl or 0.01 M NaOH. The effect of pH on Cr (VI) sorption was studied by shaking constantly 5 mg L1 of Cr (VI) solution with 1 g L1 CF for 2 h at 298 K. The effect of CF dose on Cr (VI) adsorption was carried out using 5 mg L1 Cr (VI) solution by varying 1–4 g L1 CF at pH 3 for 2 h at room temperature. The contact time was varied from 5 min to 4 h by using Cr (VI) solution of 5 mg L1 with 1 g L1 CF dose at pH 3. Kinetic equations of pseudo first order and pseudo second order presented in Eqs. (1) and (2) respectively, were used for non-linear fitting of experimental data obtained from variation of contact time [17,18].
Q t ¼ Q e ð1 eK1 t Þ
ð1Þ
Q t ¼ K2 Q 2e t=ð1 þ K2 Q e tÞ
ð2Þ
PXRD diffractogram of CF is depicted in Fig. 1a. The PXRD pattern clearly shows that the synthesized adsorbent resembles the crystal structure of cubic spinel structure of CoFe2O4. The PXRD pattern of synthesized material corresponds to that of the characteristic peaks of Cobalt Ferrite (JCPDS card no. 22-1086) [22]. The sharp and intense PXRD corresponds to high crystallinity of the prepared CF. Further the functional group analysis by FT-IR technique as depicted in Fig. 1b reveals a sharp peak at 580 cm1 and a broad peak around 3300 cm1 which corresponds to the stretching vibrations of Fe3+–O2 in the tetrahedral sites which is a characteristic of ferrite materials and the stretching vibration of O–H bond of the surface adsorbed water molecules respectively [23]. Results of both PXRD and FT-IR analysis supported the successful formation cobalt ferrite (CF). 3.2. Adsorption studies 3.2.1. Effect of pH Cr (VI) removal efficiency of an adsorbent is greatly affected by solution pH; hence the effect of pH was studied in a wide pH range of 3–11 and was presented in Fig. 2a. The synthesized adsorbent, CF showed heightened Cr (VI) sorption tendency at pH 3 (76%) and sorption performance eventually decreased with the rise in solution basicity i.e. 26% at pH 11. Such pH controlled sorption behavior of CF is attributed to the surface charge of the adsorbent, speciation of Cr (VI) and competition from OH– ions [24]. Cr (VI) ions occur in the aqueous media in form of oxy anio2 2 nic species like HCrO etc. [25]. These oxyanions 4 , CrO4 , Cr2O7 are attracted by positively charged surfaces of adsorbents by elec-
Please cite this article as: S. Prakash Tripathy, R. Acharya, M. Das et al., Adsorptive remediation of Cr (VI) from aqueous solution using cobalt ferrite: Kinetics and isotherm studies, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.01.534
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Fig. 1. (a) PXRD pattern (b) FT-IR spectra of synthesized adsorbent CF.
Fig. 2. (a) Initial pH variation study [Cr (VI) = 5 mg L1, CF dose = 1 g L1, pH = 3–11, time = 2 h, Temp. = 298 K]. (b) Effect of adsorbent dose study [Cr (VI) = 5 mg L1, CF dose = 1–4 g L1, pH = 3, time = 2 h, Temp. = 298 K].
trostatic force of attractions. As cobalt ferrites have a pHpzc around 7.2, they tend to be positive surface charge distribution in acidic medium which enhances their Cr (VI) oxy anion separation efficacy at lower pH as seen for CF at pH 3. However with rising pH the surface charge becomes less positive and also at higher pH the oxy anionic Cr (VI) ions face severe competition from hydroxide ion for adsorption which is reflected in poor removal efficiency of CF at higher pH values [26]. This suggested that electrostatic attraction is the plausible mechanism for Cr (VI) adsorption onto CF. 3.2.2. Effect of adsorbent dose Optimum adsorbent dose for the removal of Cr (VI) by CF determined with variation of adsorbent dosage (1, 2, 3 and 4 g L1) at room temperature which is plotted in Fig. 2b. From this it is clearly seen that the adsorption capacity of CF decreases with rising adsorbent dosage. Such behavior can be attributed to availability of enough active surface sites at lower adsorbent (CF) dose. These sites are exposed for adsorption of Cr (VI), leading to enhanced adsorption capacity. On increasing the adsorbent dose, CF active sites are available for same concentration of Cr (VI). As a result, adsorption capacity decreases at higher CF dosage [27]. Maximum sorption capacity was obtained when CF dose was 1 g L1 and hence further adsorption experiments were carried out with this adsorbent dose.
3.2.3. Sorption kinetics Contact time plays a vital role in adsorption studies. It suggests the time period during which the adsorbate gets diffused from solution to the adsorbent surface. From the time variation experimental curve as shown in Fig. 3a, it is clearly established that sorption equilibrium (Qe = 3.54 mg g1) is attained after 2 h and the removal efficiency was not appreciably changed with further increase in contact time. In order to contemplate the sorption kinetics of CF for Cr (VI) removal, the experimental time variation data were fitted non-linearly with the pseudo-first-order and pseudo-second-order kinetics equations as shown in Fig. 3a. The fitting results were tabularized in Table 1 and it was revealed that pseudo-second-order kinetics was followed for the adsorption of Cr (VI) onto CF because it showed higher correlation coefficient (R2 = 0.96). Hence the sorption of Cr (VI) on CF caused through bimolecular interaction [28]. 3.2.4. Sorption isotherm studies The experimental data obtained by varying initial Cr (VI) concentration and temperature was presented in Fig. 3b. These experimental results were non-linearly fitted with two parameters; Freundlich and Langmuir isotherm models as plotted in Fig. 3b. Isotherm model’s acceptability was considered on the basis of higher correlation coefficient (R2) [29]. The sorption isotherm parameters calculated from fitted models are mentioned in Table 2.
Please cite this article as: S. Prakash Tripathy, R. Acharya, M. Das et al., Adsorptive remediation of Cr (VI) from aqueous solution using cobalt ferrite: Kinetics and isotherm studies, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.01.534
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Fig. 3. (a) Adsorption Kinetics Study [Cr (VI) = 5 mg L1, CF dose = 1 g L1, pH = 3, Temp. = 298 K]. (b) Adsorption isotherm study [Cr (VI) = 5–25 mg L1, CF dose = 1 g L1, pH = 3, time = 2 h].
Table 1 Kinetics fitting parameters obtained for Cr (VI) sorption by CF. Models?
Pseudo-first-order Kinetics
Parameters?
Qe
K1
R2
Qe
Pseudo-second-order Kinetics K2
R2
Values ?
3.61
1.47
0.94
4.30
0.38
0.96
Table 2 Isotherm model fitted results obtained for Cr (VI) adsorption by CF. Models?
Langmuir Isotherm
Temperature;
Qm
b
R2
Freundlich Isotherm Kf
n
R2
298 K 303 K 313 K
10.35 10.32 10.15
0.12 0.10 0.09
0.92 0.91 0.91
2.03 1.83 1.76
2.41 2.26 2.22
0.88 0.86 0.85
Isotherm fitting curves indicates that the Langmuir isotherm model is best suited for the adsorptive remediation of Cr (VI) by CF. The maximum adsorption capacity (Qm) calculated from Langmuir isotherm model is found to be 10.35 mg g1 at 298 K suggesting monolayer type adsorption [30]. From the Fig. 3b it’s observed that with rising temperature the removal efficiency of CF slightly decreased, suggesting the exothermic nature of adsorption [31]. Further to ascertain a favorable adsorption of Cr (VI) onto CF, Separation factor (RL) as given below was explored. 1 Where, ‘b’ and Co represent Langmuir constant and RL ¼ 1þbC o
10.35 mg g1 at 293 K. Therefore, CF can be considered as an effective adsorbent for removal of Cr (VI) from aqueous medium.
initial Cr (VI) concentration respectively. If we evaluate RL using the ‘b’ values of Table 2 it can be clearly seen that RL lies in the range of 0 to 1, suggesting the sorption of Cr (VI) onto CF is a favorable process [32].
Acknowledgements
4. Conclusion In a conclusive summary, the adsorbent CF was successfully prepared by the combination of sol–gel and calcination at 600 °C. The PXRD and FT-IR studies revealed that prepared CF is highly crystalline and pure. The optimization of Cr (VI) adsorption onto CF was carried out by considering various sorption parameters such as pH, CF dosage and contact time. The optimal pH and CF dose were found to be 3 and 1 g L1 respectively. The experimental Cr (VI) sorption data was fitted with pseudo-second-order kinetic model. The fitting of experimental data with Langmuir model indicated monolayer adsorption process. The maximum Cr (VI) adsorption capacity (Qm) calculated from Langmuir isotherm model was
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Authors are very much grateful to DAE-BRNS, Government of India for financial support under the research project number 37 (2)/14/02/2018/37002 to carry out the current work. Authors also thankfully acknowledge to the management of S‘O’A (Deemed to be University), Bhubaneswar for providing adequate infrastructure in pursuing this work. Dr. P.K. Pujari, Director, RC&I Group and Head, RCD, BARC is thankfully acknowledged for his motivation and encouragement. References [1] R. Dubey, J. Bajpai, A.K. Bajpai, Green synthesis of graphene sand composite (GSC) as novel adsorbent for efficient removal of Cr (VI) ions from aqueous solution, J. Water Process Eng. 5 (2015) 83–94. [2] R. Acharya, S. Martha, K.M. Parida, Remediation of Cr (VI) using clay minerals, biomasses and industrial wastes as adsorbents, Adv. Mater. Waste Water Treat. (2017) 129–170.
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Please cite this article as: S. Prakash Tripathy, R. Acharya, M. Das et al., Adsorptive remediation of Cr (VI) from aqueous solution using cobalt ferrite: Kinetics and isotherm studies, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.01.534