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Uptake of heavy metal cations by chitosan-modified montmorillonite: Kinetics and equilibrium studies
Materials Chemistry and Physics 236 (2019) 121784
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Uptake of heavy metal cations by chitosan-modified montmorillonite: Kinetics and equilibrium studies Tomohito Kameda *, Reina Honda, Shogo Kumagai, Yuko Saito, Toshiaki Yoshioka Graduate School of Environmental Studies, Tohoku University, 6-6-07 Aoba, Aramaki, Aoba-ku, Sendai, 980-8579, Japan
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� Cu2þ and Ni2þ adsorption by CTSmodified MMT was investigated. � Adsorption follows a pseudo-secondorder kinetic equation. � Adsorption isotherms show that adsorption follows the Langmuir equation. � Adsorption is attributable to MMT cation adsorption and CTS interaction with ions. � Reused CTS-modified MMT achieves a desorption rate of approximately 90%. A R T I C L E I N F O
A B S T R A C T
Keywords: Montmorillonite Chitosan Modification Uptake Heavy metal ions
The adsorption of Cu2þ and Ni2þ by chitosan-modified montmorillonite (CTS-modified MMT) was investigated. Kinetic analyses suggested that the adsorption of Cu2þ and Ni2þ by MMT, CTS, and CTS-modified MMT can be modeled using a pseudo-second-order kinetic equation. The adsorption isotherms clearly indicate that the adsorption follows the Langmuir equation. The adsorption of metal cations by CTS-modified MMT is thought to be attributable to the cation adsorption capacity of MMT and the ability of CTS to form complexes with metal ions. Desorption and re-adsorption experiments were also conducted to determine the possibility of reusing the synthesized adsorbent. Using a 100-mmol/L HNO3 solution, a desorption rate of approximately 90% was ach ieved for the CTS-modified MMT. The CTS-modified MMT obtained after desorption could achieve the same level of adsorption as fresh the CTS-modified MMT, indicating that this adsorbent can be reused.
1. Introduction Montmorillonite (MMT) is a 2-octahedral-type 2:1 layered silicate and a subset of smectite. Its general chemical formula is (Mþ,M2þ 1/2)xþy 2þ (Y3þ 2-y,Yy ) (Si4-x,Alx)O10(OH)2・nH2O, where M and Y indicate inter layer and octahedral cations, respectively [1,2]. The isomorphous sub stitution of cations in its 2:1 layered structure generates a permanent negative charge that is independent of pH on the surface of the tetra hedral sheet. MMT has a structure in which this negative charge is
compensated by the interlayer cations; the thickness of its host layer is 0.96 nm. The ease of intercalation of different guest inorganic cations between the MMT layers follows the order: Liþ < Naþ < Kþ < Mg2þ < Ca2þ < Ba2þ < Al3þ < Fe3þ < Hþ. The higher the valency of the ion (or the larger the ion radius, if the valency is the same), the more selectively do the exchangeable cations between the layers intercalate via cation exchange. Hydrogen ions form hydrogen bonds with the oxygen on the MMT crystal layer and intercalate on the crystal planes, preferentially over polyvalent cations. The exchanged
* Corresponding author. E-mail address: [email protected] (T. Kameda). https://doi.org/10.1016/j.matchemphys.2019.121784 Received 27 March 2019; Received in revised form 20 June 2019; Accepted 24 June 2019 Available online 25 June 2019 0254-0584/© 2019 Elsevier B.V. All rights reserved.
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cations can intercalate organic molecules as well as inorganic cations. MMT has swelling potential, which is believed to be attributable to water permeating between its layers. Swelling may occur if the hydra tion energy generated when water molecules form hydrogen bonds with the oxygen at the interlayer oxygen surface or if the hydration of exchangeable cations between the layers is stronger than the interlayer bond. Chitin is a polysaccharide of N-acetyl-D-glucosamine (2-acetamido-2deoxy-D-glucose) residues linked by β-(1,4) bonds; its deacetylated form is chitosan (CTS) [3]. Cellulose, which has structure similar to that of chitin, is the most abundant biomass resource on the Earth, with an estimated annual production of 1 � 1011 t. The estimated annual pro duction of chitin is almost equal to that of cellulose at 1 � 109 t to 1 � 1011 t. Cellulose is versatile and widely used in lumber, fiber, and paper production; however, chitin is primarily converted to CTS and used as a flocculant in water treatment; most of its potential applications are still in the research stage. Therefore, the effective utilization of chitin/CTS through conversion to high value-added components will allow the use of abundant biological resources. Chitosan is considered a promising resource because of the free primary amino groups (NH2) present in its glucosamine residues and the hydroxyl groups of its pri mary and secondary alcohols. Furthermore, it is an excellent ligand for metal ions, forming CTS-metal ion composites and coordinate bonds with transition metal ions both selectively and efficiently. Previous studies have successfully demonstrated the adsorption of metal cations, such as cadmium, cobalt, copper, lead, manganese, nickel, and zinc by MMT [2]. Heavy metal cations are adsorbed through intercalation between the layers of MMT owing to its cation exchange capacity. Other studies have reported that the selectivity of metal cation adsorption by CTS is in the order: Co2þ < Ni2þ < Cd2þ < Zn2þ < Hg2þ < Cu2þ [4]. In addition, Fen et al. developed a surface plasmon resonance optical sensor for sensitive and selective detection of Pb2þ based on p-tert-butylcalix [4]arene-tetrakis (BCAT) immobilized in CTS thin film as an active layer [5]. The adsorption selectivity of CTS is defined by the radius of the metal cation and stability constant of the complex formed. Metal removal using CTS and MMT composites as a single system has been explored. One study, reported cobalt adsorption of approximately 2.5 mmol/g using CTS-modified MMT [6]. This study applied a Temkin-type adsorption model and the adsorption of cobalt followed a pseudo-second-order ki netic model. A number of other studies have also considered the adsorption of metal ions and cationic dyes by CTS-modified MMT [7–11]. However, no previous study has varied the CTS/MMT mass ratio to increase the metal adsorption amount or calculated the activation en ergy of adsorption to determine the adsorption mechanism. As mentioned above, MMT and CTS can adsorb metal ions selectively. Furthermore, as MMT is swellable and CTS is acid-soluble, they are seldom used separately. We believe that by using a composite of MMT and CTS, their individual limitations as adsorbents can be overcome, thereby increasing the metal adsorption capacity. Additionally, the adsorption selectivity of CTS and MMT composites for metal ions needs to be clarified. It is likely that the combination of the different adsorp tion selectivities of both components will create a highly selective metal adsorbent. This study investigated the possibility of recovering heavy metals using MMT intercalated with CTS, which forms complexes with heavy metals. CTS-modified MMT is adaptable to solutions of varied concentrations, has high separability and selectivity for heavy metals in solution, and can be concentrated from liquid to solid phase. Therefore, we focused on capturing Cu2þ and Ni2þ using CTS-modified MMT. In this study, CTS-modified MMT was synthesized by intercalating CTS between MMT layers through cation exchange reactions. By varying the quantity of MMT and CTS, CTS-modified MMT with different CTS/ MMT mass ratios was prepared. The equilibrium and adsorption kinetics of Cu2þ and Ni2þ by the synthesized CTS-modified MMT were studied. Furthermore, in equilibrium analysis, the effect of different CTS/MMT
mass ratios on the metal adsorption capacity of the adsorbent was examined. Additionally, to determine reusability of the synthesized CTSmodified MMT, Cu2þ and Ni2þ were desorbed from used CTS-modified MMT and the adsorbent was reused for another round of Cu2þ and Ni2þ adsorption. 2. Experimental 2.1. Synthesis of CTS-modified MMT MMT (Product name: Kunipia F) with a cation-exchange capacity (CEC) of 115 cmol/kg was supplied by Kunimine Industries Co., Ltd., Japan, with the only interlayer cation being Na. The structural formula of MMT (formula weight: 738) when cation substitution (Al→Mg) is occurring at the octahedral layer is Na0.85Si8(Al3.15Mg0.85)O20(OH)4. To prepare the CTS-modified MMT, 50 mL of 10(V/V)% acetate so lution and 0.5 g of CTS were added to a 200 mL beaker in order to make –NHþ 3 in the structure of CTS. The mixture was then agitated for 3 h at 60 � C. After cooling to 30 � C, 0.5 g, 1.0 g, or 5.0 g of MMT were added, resulting in CTS/MMT mass ratios of 1.0, 0.5, and 0.1, respectively, and agitated for 24 h. The suspension was then filtered under suction using a glass filter paper and the residue was washed with acetate and ion ex change water. The product was then dried under vacuum for 24 h at 40 � C and pulverized. X-ray diffraction (XRD) and elemental analyses of the synthesized composite were carried out to determine interlayer distance and CTS content. 2.2. Adsorption of Cu2þ and Ni2þ using CTS-modified MMT To study the adsorption kinetics, 20 mL each of Cu(NO3)2 and Ni (NO3)2 solutions were taken in separate 50 mL Erlenmeyer flasks, to which 0.1 g each of MMT, CTS, and CTS-modified MMT with a CTS/ MMT ratio of 0.1 were added. After shaking the suspension at 10 � C, 30 � C, and 60 � C for 2, 4, 6, 8, 10, 20, 30, and 60 min, 10 mL aliquots of the suspension were collected and filtered through a 0.45 μm membrane filter. To make adsorption isotherms, Cu(NO3)2 and Ni(NO3)2 solutions (20 mL each) were added to 50-mL Erlenmeyer flasks, followed by 0.1 g of MMT, CTS, and CTS-modified MMT with CTS/MMT mass ratios of 0.1, 0.5, and 1.0. After shaking the suspension at 30 � C for 24 h, it was filtered under suction through a 0.45 μm membrane filter and the res idue was washed with ion exchange water. Furthermore, we used MMT alone as well as the CTS-modified MMT (CTS/MMT ¼ 0.1) as the adsorbent in our study. The individual concentrations of Cu2þ and Ni2þ in the mixture of Cu2þ-Ni2þ ions in solution was 20 mmol/l. The heavy metal ion concentration in the filtrate was determined via inductively coupled-atomic emission spectroscopy (ICP-AES). Furthermore, the residue on the filter was dried at 40 � C for 24 h and analyzed via XRD. 2.3. Desorption of Cu2þ and Ni2þ from the adsorbent For the desorption study, 20-mmol/L Cu(NO3)2 and Ni(NO3)2 solu tions (500 mL each) were poured in five-neck round-bottom flasks, which were agitated at 30 � C. Subsequently, 2.5 g of CTS-modified MMT (CTS/MMT ¼ 0.1) was added and the suspension was agitated for 24 h. After this, suspensions were filtered under suction and the residue was washed and vacuum-dried for 24 h at 40 � C. Following pulverization of the residue, the interlayer distance and CTS content were determined via XRD and elemental analysis, respectively. For desorption of Cu2þ and Ni2þ from the used adsorbent, 20 mL each of HNO3 solutions of varying concentrations (1, 2, 5, 10, 20, 50, and 100 mmol/L) were added to 50mL Erlenmeyer flasks, followed by CTS-modified MMT (CTS/ MMT ¼ 0.1) that have adsorbed 0.1 g each of Cu2þ and Ni2þ. After shaking at 30 � C for 24 h, the suspensions were filtered under suction through 0.45 μm membrane filters. The metal ion concentrations in the filtrates were analyzed via ICP-AES; the solid residue was vacuum-dried 2
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at 40 � C for 24 h and pulverized, after which it was subjected to XRD and elemental analyses to determine the interlayer distance and CTS content. 2.4. Adsorption of Cu2þ and Ni2þ using generated CTS-modified MMT The CTS-modified MMT obtained after desorption (2.5 g) was introduced to 20-mmol/L Cu(NO3)2 and Ni(NO3)2 solutions (500 mL) and agitated for 24 h at 30 � C. A total of 0.2 g of CTS-modified MMT with adsorbed Cu2þ and Ni2þ was added to 40 mL of a 100-mmol/L HNO3 solution and agitated for 24 h at 30 � C. After desorption, 0.1 g of this CTS-modified MMT was added to 20-mmol/L Cu(NO3)2 and Ni(NO3)2 solutions and agitated for 24 h at 30 � C. The suspension was filtered under suction and the metal concentration in the filtrate was measured via ICP-AES.
Fig. 2. CTS content with CTS-modified MMT with CTS/MMT ¼ 0.1, 0.5, and 1.0, as well as the percentage of the cations (-NHþ 3 ) of CTS to the CEC of MMT.
3. Results and discussion
deacetylation degree of the CTS used in this study ranged between 75% and 85%, it was assumed that approximately 20% of the N exists not as NHþ 3 but as NHCO3. The elemental analysis revealed the presence of N at all CTS/MMT mass ratios, thereby confirming the intercalation of CTS in the composites. With an increase in the CTS/MMT ratio between 0.1 and 0.5, the CTS content in the CTS-modified MMT increased; however, for CTS/MMT ratios of 0.5 and 1.0, the CTS content was practically the same. These results suggest that the CTS content becomes saturated at a CTS/MMT ratio of 0.5. Furthermore, NHþ 3 /CEC is below 100%, and the NHþ 3 group of CTS does not exceed the CEC of MMT. It is assumed that these factors resulted in the intercalation of CTS between the MMT layers by cation exchange, leading to the synthesis of CTS-modified MMT. We believe that in this structure, the negative charge of the MMT host layer is compensated by the NHþ 3 group of CTS. For kinetics evaluation, CTS-modified MMT with a CTS/MMT mass ratio of 0.1 was used. For adsorption isotherms, CTS-modified MMT with CTS/MMT mass ratios of 0.1, 0.5, and 1.0 was used.
3.1. Synthesis of CTS-modified MMT Fig. 1 shows the XRD patterns of the raw MMT and CTS-modified MMT with CTS/MMT ratios of 0.1, 0.5, and 1.0. A peak characteristic to MMT was observed in all CTS-modified MMT with different mass ratios. The d001 value for MMT, which corresponds to the interlayer distance, was found to be 1.23 nm. For CTS-modified MMT with CTS/ MMT ratios of 0.1, 0.5, and 1.0, this value was 1.73, 1.73, and 1.72 nm, respectively, indicating an increase in interlayer distance of approxi mately 0.50 nm after modification with CTS. It has been reported that, in the synthesis of CTS-modified MMT, when CTS is intercalated between MMT layers, the interlayer distance increases by approximately 0.18–0.49 nm [6,12]. Therefore, the increase in the d001 value observed in our study is valid and confirms the intercalation of CTS between the MMT layers. Fig. 2 shows the CTS contents of CTS-modified MMT with CTS/MMT ratios of 0.1, 0.5, and 1.0, as well as the percentages of the þ cations of CTS (NHþ 3 , referred to as NH3 /CEC hereafter) to the CEC of MMT. These percentages were determined by assuming that all the NH2 groups of CTS dissolved in acetate to form NHþ 3 , and calculating the amount NHþ 3 groups of CTS in the CTS-modified MMT from the N con tent obtained through the elemental analysis. However, as the
3.2. Adsorption of Cu2þ and Ni2þ using CTS-modified MMT Figs. 3–5 and S1–S3 show temporal changes in the amounts of Cu2þ and Ni2þ adsorbed by MMT, CTS, and CTS-modified MMT (CTS/ MMT ¼ 0.1) at 10 � C, 30 � C, and 60 � C. The amounts of adsorbed Cu2þ and Ni2þ increased immediately after the addition of MMT (Fig. 3), suggesting that adsorption by MMT was a rapid process. Furthermore, with MMT, there was no significant difference in the adsorption amount with change in temperature. However, when CTS and CTS-modified MMT were used (Figs. 4 and 5), the adsorption amount increased with time and with increasing temperature. Figs. S4–S6 show temporal changes in pH during the adsorption of Cu2þ and Ni2þ by MMT, CTS, and CTS-modified MMT (CTS/MMT ¼ 0.1) at 10 � C, 30 � C, and 60 � C. The pH decreased with increasing temperature in all cases. Therefore, it can be concluded that the final pH of the suspension did not cause hydroxide precipitation in any of the adsorption cases. We analyzed the reaction rate rates, shown in Figs. S3–S5, using pseudo-first-order and pseudo second-order kinetic equations and intraparticle diffusion model equations. First, the reaction rates were fitted to a pseudo-first-order kinetic equation (Eq. (1)) that is widely used to model adsorption of adsorbates from aqueous solutions: lnð1
xÞ ¼ kt
(1)
where t is time [min], k is the apparent reaction rate constant [min 1], and x represents the adsorption rate [ ]. Plots of t against ln (1-x) at each temperature, calculated using Eq. (1), are shown in Figs. S4–S6. There is no clear linear relationship be tween time t and ln (1-x), indicating that the adsorption reaction does not follow the pseudo-first-order kinetic equation. Second, the reaction
Fig. 1. XRD patterns of (a) MMT and CTS-modified MMT with CTS/MMT ¼ (b) 0.1, (c) 0.5, and (d) 1.0. 3
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Fig. 3. Change in the adsorption amount of (a) Cu2þ and (b) Ni2þ with MMT at 10, 30, and 60 � C.
Fig. 4. Change in the adsorption amount of (a) Cu2þ and (b) Ni2þ with CTS at 10, 30, and 60 � C.
Fig. 5. Change in the adsorption amount of (a) Cu2þ and (b) Ni2þ with CTS-modified MMT (CTS/MMT ¼ 0.1) at 10, 30, and 60 � C.
rate data were fitted to the pseudo-second-order kinetic equation: t 1 1 ¼ þ t qt k2 q2e qe
kinetic equation [13]. Plots of t/qt against t, calculated using Eq. (2), are shown in Figs. S7–S9. Time t and t/qt have a clear linear relationship, and therefore we can conclude that the adsorption of Cu2þ and Ni2þ by MMT, CTS, and CTS-modified MMT follows the pseudo-second-order kinetic equation. The adsorption of Pb2þ, Zn2þ, and Cd2þ by mustard waste biomass like CTS also follows the pseudo-second-order kinetic equation [14]. Finally, the reaction rates were fitted into the intraparticle diffusion model [15]. This model assumes that the adsorption amount depends on the particle size of the adsorbent (r) and the intraparticle diffusion rate
(2)
where k2 is the apparent reaction rate constant [g/(mmol.min)], and qe and qt [mmol/g] are the adsorption amounts at equilibrium and time t [min], respectively. Assuming that chemical adsorption via sharing of electrons between the adsorbent and adsorbate is the rate-determining step, it is possible to conclude that the adsorption behavior follows the pseudo-second-order 4
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of the adsorbate (D) and that the adsorption amount changes in pro portion to (Dt/r2)1/2; that is:
Table 2 Value of calculated k2 and qe .
� �1=2 qt ∝ Dt r2
Adsorbent
Therefore, the intraparticle diffusion model equation can be expressed as Eq. (3):
MMT
Ni2þ
(3)
qt ¼ kp t1=2
where qt is the adsorption amount [mmol/g], kp is the reaction rate constant [mmol/g・min1/2], and t is time [min]. This means that if adsorption is represented by Eq. (3), it only progresses with diffusion within the particles of the adsorbate. Plots of qt against t1/2, calculated using Eq. (3), are shown in Figs. S10–S12. No linear relationship was observed between qt and t1/2 and therefore we can conclude that the adsorption of Cu2þ and Ni2þ by MMT, CTS, and CTS-modified MMT does not follow the intraparticle diffusion model equation. The correlation coefficients (R2) obtained by fitting the adsorption rate data to the pseudo-first-order kinetic, pseudo-second-order kinetic, and intraparticle diffusion model equations are listed in Table 1. Using Eq. (2), the reaction rate constant k2 ½g=mmolg=min� and equilibrium adsorption amount qe ½g=mmolg� for the adsorption of Cu2þ and Ni2þ by MMT, CTS, and CTS-modified MMT (CTS/MMT ¼ 0.1) were calculated (Table 2) from the slope and intercept of the approximate curves shown in Figs. S7–S9. In all adsorption cases, the reaction rate constant increased with increasing temperature. The reaction rate constant k2 is given by Arrhenius’ equation: � � Ea k2 ¼ Aexp (4) RT
CTS
Ea RT
CTS-modified MMT (CTS/ MMT ¼ 0.1)
MMT CTS CTS-modified MMT (CTS/MMT ¼ 0.1)
MMT
Cu2þ Ni2þ
CTS
Cu2þ Ni2þ
CTS-modified MMT (CTS/ MMT ¼ 0.1)
Cu2þ Ni2þ
Pseudosecond order
Intraparticle diffusion
10 30 60 10 30 60 10 30 60 10 30 60 10 30 60 10 30 60
0.874 0.696 0.0875 0.131 0.0004 0.200 0.918 0.0932 0.000470 0.868 0.0932 0.333 0.543 0.116 0.0682 0.680 0.476 0.522
1.00 1.00 1.00 1.00 1.00 1.00 0.966 0.993 0.987 0.997 0.920 0.993 1.00 0.996 1.00 1.00 1.00 1.00
0.936 0.647 0.215 0.178 0.590 0.0441 0.934 0.812 0.00365 0.817 0.802 0.403 0.632 0.215 0.059 0.825 0.631 0.663
10 30 60 10 30 60 10 30 60 10 30 60 10 30 60 10 30 60
5.19 9.80 28.0 7.82 9.04 25.7 3.04 4.98 83.0 10.5 22.4 30.0 17.4 33.7 155 10.5 22.4 30.0
0.479 0.487 0.488 0.428 0.439 0.457 0.0775 0.0747 0.0704 0.149 0.162 0.175 0.144 0.160 0.185 0.149 0.162 0.175
Activation energy [kJ/mol] 2þ
Cu Ni2þ Cu2þ Ni2þ Cu2þ Ni2þ
26.5 19.1 53.0 55.5 34.7 19.1
these findings, it is possible to conclude that the adsorption of Cu2þ and Ni2þ by MMT, CTS, and CTS-modified MMT follows a pseudo-second-order kinetic equation. Furthermore, the activation en ergy suggests that the rate-determining step in the chemical adsorption of Cu2þ and Ni2þ by MMT, CTS, and CTS-modified MMT is the inter change of electrons between the adsorbent and adsorbate. The results, shown in Figs. 3–5, indicate that the adsorption of Cu2þ and Ni2þ reaches the equilibrium state at approximately 60 min for all three adsorbents. Hence, we carried out a 24 h adsorption experiment under equilibrium conditions at 30 � C. The adsorption isotherms of Cu2þ and Ni2þ by MMT and CTS are shown in Fig. 6, and those of Cu2þ and Ni2þ by CTS-modified MMT (CTS/MMT ratios of 0.1, 0.5, and 1.0) are shown in Fig. 7. The adsorption isotherms obtained were applied and fitted into the Langmuir equation:
Table 1 Correlation efficient of the various kinetic models. Pseudofirst order
qe [mmol/ g]
Adsorbent
Figs. S13–S15 show the Arrhenius plots of the adsorption of Cu2þ and Ni by MMT, CTS, and CTS-modified MMT (CTS/MMT ¼ 0.1) using the calculated reaction rate constant given by Eq. (5). Activation energies determined from the slope of the Arrhenius plots are shown in Table 3. The activation energy for chemical adsorption is reported to be in the range of 8.4–83.7 kJ/mol [16,17]. Therefore, the calculated activation energy is indicative of the occurrence of chemical adsorption. Based on
R2 [-]
k2 [g/mmol/ min]
Table 3 Activation energy calculated based on the Pseudo-second order kinetics.
2þ
Temp. [� C]
Cu2þ Ni2þ
(5)
Adsorbent
Cu2þ Ni2þ
where A is the frequency factor [mmol/(g・min1/2)], T is the tempera ture [K], and R is the gas constant [kJ/(K・mol)]. If the logarithm of Eq. (4) is taken, Eq. (5) is obtained as: lnk2 ¼ ln A
Cu2þ
Temp. [� C]
Ce 1 1 ¼ þ Ce qe KL qm qm
(6)
where qe is the equilibrium adsorption amount [mmol/g], Ce is the equilibrium concentration [mmol/L], qm is the maximum adsorption amount [mmol/g], and KL is the adsorption equilibrium constant [L/ mol]. Plots of Ce/qe against Ce, calculated using Eq. (6), for adsorption of Cu2þ and Ni2þ by MMT and CTS are shown in Fig. S16, and those for adsorption by CTS-modified MMT are shown in Fig. S17. There is a clear linear relationship between Ce/qe and Ce for all three adsorbents, thereby suggesting that the adsorption of Cu2þ and Ni2þ by MMT, CTS, and CTS-modified MMT satisfies the Langmuir equation and is a chemical process. The adsorption of Pb2þ by CTS is known to obey the Langmuir equation [18]. Furthermore, the adsorption of Pb2þ, Zn2þ, and Cd2þ by mustard waste biomass like CTS is also known to obey the Langmuir equation [14]. Next, the adsorption isotherms were fitted to the Freundlich equation:
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Fig. 6. Adsorption isotherms on the adsorption of (a) Cu2þ and (b) Ni2þ by MMT and CTS.
Fig. 7. Adsorption isotherms on the adsorption of (a) Cu2þ and (b) Ni2þ by CTS-modified MMT.
1 logqe ¼ logKF þ logCe n
(7)
Table 5 Parameter calculated based on Langmuir equation.
where KF is the adsorption distribution coefficient and n is a constant. Plots of log qe against log Ce, calculated using Eq. (7), for the adsorption of Cu2þ and Ni2þ by MMT and CTS are shown in Fig. S18, and those for adsorption by CTS-modified MMT are shown in Fig. S19. There was no clear linear relationship between log qe and log Ce for any of the three adsorbents; therefore, the Freundlich equation is not applicable for any of the adsorption cases. The correlation coefficients R2 obtained by fitting the adsorption isotherms to the Langmuir and Freundlich equations are listed in Table 4. The Langmuir equation was applicable to all adsorption cases. Table 5 shows the parameters calculated using the Langmuir equation.
Adsorbent MMT CTS CTS-modified MMT (CTS/MMT ¼ 0.1) CTS-modified MMT (CTS/MMT ¼ 0.5) CTS-modified MMT (CTS/MMT ¼ 1.0)
MMT
R2 [-]
CTS CTS-modified MMT (CTS/MMT ¼ 0.1) CTS-modified MMT (CTS/MMT ¼ 0.5) CTS-modified MMT (CTS/MMT ¼ 1.0)
Cu2þ Ni2þ Cu2þ Ni2þ Cu2þ Ni2þ Cu2þ Ni2þ Cu2þ Ni2þ
Langmuir
Freundlich
0.99 0.96 0.98 0.91 0.98 1.0 0.97 0.96 1.0 0.97
0.79 0.78 0.90 0.92 0.97 0.93 0.81 0.98 0.99 0.93
qm [mmol/g]
KL [L/mol]
0.50 0.61 1.6 1.3 0.33 0.30 0.21 0.29 0.18 0.23
0.52 7.3 0.19 0.058 1.8 2.4 0.40 0.31 0.53 0.32
The maximum adsorption amounts calculated from the Langmuir equation are shown in Fig. 8. The maximum adsorption amount with CTS-modified MMT was lower than that with either MMT or CTS. We believe that this is because the modification of MMT with CTS hindered the introduction of heavy metal cations between the MMT layers. Furthermore, no significant change in metal adsorption amount was observed with changes in the CTS/MMT ratio. This indicates that the CTS-modified MMT did not have the strong metal adsorption capacity of CTS. The maximum adsorption of Cu2þ by CTS was almost same as that by carbon nanofiber [19]. The maximum adsorption of Ni2þ by CTS is slightly lower than that by vermiculite-based nanoscale hydrated zir conium oxides [20]. Fig. 9 shows the amount of Cu2þ and Ni2þ adsorbed by MMT alone and by CTS-modified MMT (CTS/MMT ¼ 0.1) from a solution containing a mixture of Cu2þ and Ni2þ. In both cases, the
Table 4 Correlation efficient of the Langmuir and Freundlich models. Adsorbent
Cu2þ Ni2þ Cu2þ Ni2þ Cu2þ Ni2þ Cu2þ Ni2þ Cu2þ Ni2þ
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space of MMT. This suggests that the CTS detached from between the MMT layers. This adsorption is likely attributable to the cation adsorp tion capacity of MMT and the ability of CTS to form complexes with heavy metal ions [21]. In the case of tetracoordinate metal ions, the formation of such complexes is coordinated by an amino group and three electron-donating groups of CTS. Two possible electron-donating groups are the hydroxyl group of CTS and water molecules. 3.3. Desorption of Cu2þ and Ni2þ from the adsorbents The Cu2þ and Ni2þ amounts adsorbed by the CTS-modified MMT (CTS/MMT ¼ 0.1) were 0.258 and 0.313 mmol/g, respectively. Fig. S21 show the Cu2þ and Ni2þ desorption rates from the CTS-modified MMT (CTS/MMT ¼ 0.1) and Fig. S22 shows the pH of the suspensions after desorption. The desorption rates increased with increases in the con centration of the HNO3 solution; at a concentration of 100 mmol/L, desorption rates for both Cu2þ and Ni2þ were approximately 90%. Fig. S23(A) shows the X-ray diffraction pattern of CTS-modified MMT (CTS/MMT ¼ 0.1) before adsorption and before and after desorption of Cu2þ with a 20-mmol/L HNO3 solution. After desorption, the peak that indicates the 001 face broadened and became less intense. Fig. S23(B) shows the X-ray diffraction pattern of the CTS-modified MMT (CTS/MMT ¼ 0.1) before adsorption and before and after desorption of Ni2þ with a 20-mmol/L HNO3 solution. As with Cu2þ, after desorption, the peak for the 001 face broadened and reduced in in tensity. Fig. S24 shows the CTS contents of the CTS-modified MMT (CTS/MMT ¼ 0.1) before and after desorption of Cu2þ and Ni2þ with a 20-mmol/L HNO3 solution. CTS contents before and after desorption were almost equal, suggesting that desorption did not cause detachment of CTS. We believe that the adsorption of heavy metal cations by CTSmodified MMT is attributable to the cation exchange capacity of MMT and the ability of CTS to form complexes with heavy metal ions. Therefore, the desorption of heavy metal cations from different adsorption sites was analyzed. The ease with which different inorganic cations intercalate between MMT layers is in the order Liþ < Naþ < Kþ < Mg2þ < Ca2þ < Ba2þ < Al3þ < Fe3þ < Hþ. This in dicates that the heavy metal cations adsorbed via cation exchange were replaced by Hþ, which can intercalate between the MMT layers more easily than heavy metal cations during desorption. Furthermore, it is possible that the heavy metal cations were adsorbed by CTS-modified MMT via the formation of complexes with CTS.
Fig. 8. Comparison of maximum adsorption amount of heavy metal ions by various adsorbents.
Fig. 9. Adsorption amount of Cu2þ and Ni2þ adsorbed by MMT and by CTSmodified MMT (CTS/MMT ¼ 0.1) from a solution containing a mixture of Cu2þ Ni2þ.
amount of Ni2þ adsorbed was greater than that of Cu2þ. This is consis tent with the adsorption selectivity of MMT, but not CTS (Ni2þ
3.4. Adsorption of Cu2þ and Ni2þ using generated CTS-modified MMT Fig. 10 shows the amounts of Cu2þ and Ni2þ adsorbed by the CTS-
Fig. 10. Adsorption amount of Cu2þ and Ni2þ (a) before and (b) after the desorption of them by the CTS-modified MMT (CTS/MMT ¼ 0.1). 7
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modified MMT (CTS/MMT ¼ 0.1) during the first adsorption (using fresh CTS-modified MMT) and second adsorption (using generated CTSmodified MMT). The Cu2þ and Ni2þ desorption rates achieved using a 100-mmol/L HNO3 solution were 82% and 86%, respectively. The adsorption amounts during the first and second adsorption were the same for both Cu2þ and Ni2þ, indicating that the synthesized adsorbent can be reused.
[2] K.G. Bhattacharyya, S.S. Gupta, Adsorption of a few heavy metals on natural and modified kaolinite and montmorillonite: a review, Adv. Colloid Interface Sci. 140 (2008) 114–131. [3] C.J. Brine, P.A. Sandford, J.P. Zikakis, Advances in Chitin and Chitosan, Elsevier Applied Science, London, 1991. [4] C. Gerente, V.K.C. Lee, P.L. Cloirec, G. McKay, Application of chitosan for the removal of metals from wastewaters by adsorption—mechanisms and models review, Crit. Rev. Environ. Sci. Technol. 37 (2007) 41–127. [5] Y.W. Fen, W.M.M. Yunus, N.A. Yusof, Surface plasmon resonance optical sensor for detection of Pb2þ based on immobilized p-tert-butylcalix[4]arene-tetrakis in chitosan thin film as an active layer, Sensor. Actuator. B 171–172 (2012) 287–293. [6] H. Wang, H. Tang, Z. Liu, X. Zhang, Z. Hao, Z. Liu, Removal of cobalt(II) ion from aqueous solution by chitosan-montmorillonite, J. Environ. Sci. (China) 26 (2014) 1879–1884. [7] D. Fan, X. Zhu, M. Xu, J. Yan, Adsorption properties of chromium (VI) by chitosan coated monmorillonite, J. Biol. Sci. 6 (2006) 941–945. [8] P. Monvisade, P. Siriphannon, Chitosan intercalated montmorillonite: preparation, characterization and cationic dye adsorption, Appl. Clay Sci. 42 (2009) 427–431. [9] A.R. Nesica, S.J. Velickovic, D.G. Antonovic, Characterization of chitosan/ montmorillonite membranes as adsorbents for Bezactiv Orange V-3R dye, J. Hazard Mater. 209 210 (2012) 256–263. [10] T. Jintakosol, W. Nitayaphat, Adsorption of silver (I) from aqueous solution using chitosan/montmorillonite composite beads, Mater. Res. 19 (5) (2016) 1114–1121. [11] J. Lia, J. Cai, L. Zhong, H. Cheng, H. Wang, Q. Ma, Adsorption of reactive red 136 onto chitosan/montmorillonite intercalated composite from aqueous solution, Appl. Clay Sci. 167 (2019) 9–22. [12] M. Darder, M. Colilla, E. Ruiz-Hitzky, Biopolymer-clay nanocomposites based on chitosan intercalated in montmorillonite, Chem. Mater. 15 (2003) 3774–3780. [13] Z.P. Liang, Y.Q. Feng, Z.Y. Liang, S.X. Meng, Adsorption of urea nitrogen onto chitosan coated dialdehyde cellulose under biocatalysis of immobilized urease: equilibrium and kinetic, Biochem. Eng. J. 24 (2005) 65–72. [14] L. Nemes¸, L. Bulgariu, Optimization of process parameters for heavy metals biosorption onto mustard waste biomass, Open Chem. 14 (2016) 175–187. [15] A. Rodriguez, J. Garcia, G. Ovejero, M. Mestanza, Adsorption of anionic and cationic dyes on activated carbon from aqueous solutions: equilibrium and kinetics, J. Hazard Mater. 172 (2009) 1311–1320. [16] W. Zou, R. Han, Z. Chen, Z. Jinghua, J. Shi, Kinetic study of adsorption of Cu(II) and Pb(II) from aqueous solutions using manganese oxide coated zeolite in batch mode, Colloid Surface A Physicochem. Eng. Aspect. 279 (2006) 238–246. [17] M. Kragovi�c, A. Dakovi�c, M. Markovi�c, J. Krsti�c, G.D. Gatta, N. Rotiroti, Characterization of lead sorption by the natural and Fe(III)-modified zeolite, Appl. Surf. Sci. 283 (2013) 764–774. [18] D. Asandei, L. Bulgariu, E. Bobu, Lead (II) removal from aqueous solutions by adsorption onto chitosan, Cellul. Chem. Technol. 43 (2009) 211–216. [19] S. Yu, Y. Liu, Y. Ai, X. Wang, R. Zhang, Z. Chen, Z. Chen, G. Zhao, X. Wang, Rational design of carbonaceous nanofiber/Ni-Al layered double hydroxide nanocomposites for high-efficiency removal of heavy metals from aqueous solutions, Environ. Pollut. 242 (2018) 1–11. [20] D. Liu, S. Deng, M. Vakili, R. Du, L. Tao, J. Sun, B. Wang, J. Huang, Y. Wang, G. Yu, Fast and high adsorption of Ni(II) on vermiculite-based nanoscale hydrated zirconium oxides, Chem. Eng. J. 360 (2019) 1150–1157. [21] Y.W. Fen, W.M.M. Yunus, Z.A. Talib, N.A. Yusof, Development of surface plasmon resonance sensor for determining zinc ion using novel active nanolayers as probe, Spectrochim. Acta A Mol. Biomolecul. Spect. 134 (2015) 48–52.
4. Conclusions We synthesized CTS-modified MMT via intercalation of CTS between MMT layers. The adsorption of Cu2þ and Ni2þ by MMT, CTS, and CTSmodified MMT were found to follow a pseudo-second-order kinetic equation, and the activation energy of adsorption revealed that it is a chemical process. Furthermore, all adsorption cases follow the Langmuir adsorption isotherm. The adsorption of metal cations by CTS-modified MMT is likely due to the cation adsorption capacity of MMT and the formation of CTS and metal cation complexes. The maximum adsorption amounts for CTS-modified MMT prepared with different CTS/MMT ra tios were the same (0.18–0.33 mmol/g for Cu2þ, 0.23–0.30 mmol/g for Ni2þ); however, these values were lower than those for MMT (0.50 mmol/g for Cu2þ, 0.61 mmol/g for Ni2þ) and CTS (1.6 mmol/g for Cu2þ, 1.3 mmol/g for Ni2þ). This indicates that CTS-modified MMT does not possess the strong metal adsorption capacity of CTS. Furthermore, desorption and re-adsorption experiments were conducted to determine whether the synthesized adsorbent can be reused. The generated CTSmodified MMT achieved the same level of adsorption as the fresh CTSmodified MMT. Acknowledgments This work was supported by JSPS KAKENHI [grant number 15KK0021]. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.matchemphys.2019.121784. References [1] F. Uddin, Clays, nanoclays, and montmorillonite minerals, Metall. Mater. Trans. A 39A (2008) 2804–2814.
8
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Uptake of heavy metal cations by chitosan-modified montmorillonite: Kinetics and equilibrium studies Tomohito Kameda *, Reina Honda, Shogo Kumagai, Yuko Saito, Toshiaki Yoshioka Graduate School of Environmental Studies, Tohoku University, 6-6-07 Aoba, Aramaki, Aoba-ku, Sendai, 980-8579, Japan
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� Cu2þ and Ni2þ adsorption by CTSmodified MMT was investigated. � Adsorption follows a pseudo-secondorder kinetic equation. � Adsorption isotherms show that adsorption follows the Langmuir equation. � Adsorption is attributable to MMT cation adsorption and CTS interaction with ions. � Reused CTS-modified MMT achieves a desorption rate of approximately 90%. A R T I C L E I N F O
A B S T R A C T
Keywords: Montmorillonite Chitosan Modification Uptake Heavy metal ions
The adsorption of Cu2þ and Ni2þ by chitosan-modified montmorillonite (CTS-modified MMT) was investigated. Kinetic analyses suggested that the adsorption of Cu2þ and Ni2þ by MMT, CTS, and CTS-modified MMT can be modeled using a pseudo-second-order kinetic equation. The adsorption isotherms clearly indicate that the adsorption follows the Langmuir equation. The adsorption of metal cations by CTS-modified MMT is thought to be attributable to the cation adsorption capacity of MMT and the ability of CTS to form complexes with metal ions. Desorption and re-adsorption experiments were also conducted to determine the possibility of reusing the synthesized adsorbent. Using a 100-mmol/L HNO3 solution, a desorption rate of approximately 90% was ach ieved for the CTS-modified MMT. The CTS-modified MMT obtained after desorption could achieve the same level of adsorption as fresh the CTS-modified MMT, indicating that this adsorbent can be reused.
1. Introduction Montmorillonite (MMT) is a 2-octahedral-type 2:1 layered silicate and a subset of smectite. Its general chemical formula is (Mþ,M2þ 1/2)xþy 2þ (Y3þ 2-y,Yy ) (Si4-x,Alx)O10(OH)2・nH2O, where M and Y indicate inter layer and octahedral cations, respectively [1,2]. The isomorphous sub stitution of cations in its 2:1 layered structure generates a permanent negative charge that is independent of pH on the surface of the tetra hedral sheet. MMT has a structure in which this negative charge is
compensated by the interlayer cations; the thickness of its host layer is 0.96 nm. The ease of intercalation of different guest inorganic cations between the MMT layers follows the order: Liþ < Naþ < Kþ < Mg2þ < Ca2þ < Ba2þ < Al3þ < Fe3þ < Hþ. The higher the valency of the ion (or the larger the ion radius, if the valency is the same), the more selectively do the exchangeable cations between the layers intercalate via cation exchange. Hydrogen ions form hydrogen bonds with the oxygen on the MMT crystal layer and intercalate on the crystal planes, preferentially over polyvalent cations. The exchanged
* Corresponding author. E-mail address: [email protected] (T. Kameda). https://doi.org/10.1016/j.matchemphys.2019.121784 Received 27 March 2019; Received in revised form 20 June 2019; Accepted 24 June 2019 Available online 25 June 2019 0254-0584/© 2019 Elsevier B.V. All rights reserved.
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cations can intercalate organic molecules as well as inorganic cations. MMT has swelling potential, which is believed to be attributable to water permeating between its layers. Swelling may occur if the hydra tion energy generated when water molecules form hydrogen bonds with the oxygen at the interlayer oxygen surface or if the hydration of exchangeable cations between the layers is stronger than the interlayer bond. Chitin is a polysaccharide of N-acetyl-D-glucosamine (2-acetamido-2deoxy-D-glucose) residues linked by β-(1,4) bonds; its deacetylated form is chitosan (CTS) [3]. Cellulose, which has structure similar to that of chitin, is the most abundant biomass resource on the Earth, with an estimated annual production of 1 � 1011 t. The estimated annual pro duction of chitin is almost equal to that of cellulose at 1 � 109 t to 1 � 1011 t. Cellulose is versatile and widely used in lumber, fiber, and paper production; however, chitin is primarily converted to CTS and used as a flocculant in water treatment; most of its potential applications are still in the research stage. Therefore, the effective utilization of chitin/CTS through conversion to high value-added components will allow the use of abundant biological resources. Chitosan is considered a promising resource because of the free primary amino groups (NH2) present in its glucosamine residues and the hydroxyl groups of its pri mary and secondary alcohols. Furthermore, it is an excellent ligand for metal ions, forming CTS-metal ion composites and coordinate bonds with transition metal ions both selectively and efficiently. Previous studies have successfully demonstrated the adsorption of metal cations, such as cadmium, cobalt, copper, lead, manganese, nickel, and zinc by MMT [2]. Heavy metal cations are adsorbed through intercalation between the layers of MMT owing to its cation exchange capacity. Other studies have reported that the selectivity of metal cation adsorption by CTS is in the order: Co2þ < Ni2þ < Cd2þ < Zn2þ < Hg2þ < Cu2þ [4]. In addition, Fen et al. developed a surface plasmon resonance optical sensor for sensitive and selective detection of Pb2þ based on p-tert-butylcalix [4]arene-tetrakis (BCAT) immobilized in CTS thin film as an active layer [5]. The adsorption selectivity of CTS is defined by the radius of the metal cation and stability constant of the complex formed. Metal removal using CTS and MMT composites as a single system has been explored. One study, reported cobalt adsorption of approximately 2.5 mmol/g using CTS-modified MMT [6]. This study applied a Temkin-type adsorption model and the adsorption of cobalt followed a pseudo-second-order ki netic model. A number of other studies have also considered the adsorption of metal ions and cationic dyes by CTS-modified MMT [7–11]. However, no previous study has varied the CTS/MMT mass ratio to increase the metal adsorption amount or calculated the activation en ergy of adsorption to determine the adsorption mechanism. As mentioned above, MMT and CTS can adsorb metal ions selectively. Furthermore, as MMT is swellable and CTS is acid-soluble, they are seldom used separately. We believe that by using a composite of MMT and CTS, their individual limitations as adsorbents can be overcome, thereby increasing the metal adsorption capacity. Additionally, the adsorption selectivity of CTS and MMT composites for metal ions needs to be clarified. It is likely that the combination of the different adsorp tion selectivities of both components will create a highly selective metal adsorbent. This study investigated the possibility of recovering heavy metals using MMT intercalated with CTS, which forms complexes with heavy metals. CTS-modified MMT is adaptable to solutions of varied concentrations, has high separability and selectivity for heavy metals in solution, and can be concentrated from liquid to solid phase. Therefore, we focused on capturing Cu2þ and Ni2þ using CTS-modified MMT. In this study, CTS-modified MMT was synthesized by intercalating CTS between MMT layers through cation exchange reactions. By varying the quantity of MMT and CTS, CTS-modified MMT with different CTS/ MMT mass ratios was prepared. The equilibrium and adsorption kinetics of Cu2þ and Ni2þ by the synthesized CTS-modified MMT were studied. Furthermore, in equilibrium analysis, the effect of different CTS/MMT
mass ratios on the metal adsorption capacity of the adsorbent was examined. Additionally, to determine reusability of the synthesized CTSmodified MMT, Cu2þ and Ni2þ were desorbed from used CTS-modified MMT and the adsorbent was reused for another round of Cu2þ and Ni2þ adsorption. 2. Experimental 2.1. Synthesis of CTS-modified MMT MMT (Product name: Kunipia F) with a cation-exchange capacity (CEC) of 115 cmol/kg was supplied by Kunimine Industries Co., Ltd., Japan, with the only interlayer cation being Na. The structural formula of MMT (formula weight: 738) when cation substitution (Al→Mg) is occurring at the octahedral layer is Na0.85Si8(Al3.15Mg0.85)O20(OH)4. To prepare the CTS-modified MMT, 50 mL of 10(V/V)% acetate so lution and 0.5 g of CTS were added to a 200 mL beaker in order to make –NHþ 3 in the structure of CTS. The mixture was then agitated for 3 h at 60 � C. After cooling to 30 � C, 0.5 g, 1.0 g, or 5.0 g of MMT were added, resulting in CTS/MMT mass ratios of 1.0, 0.5, and 0.1, respectively, and agitated for 24 h. The suspension was then filtered under suction using a glass filter paper and the residue was washed with acetate and ion ex change water. The product was then dried under vacuum for 24 h at 40 � C and pulverized. X-ray diffraction (XRD) and elemental analyses of the synthesized composite were carried out to determine interlayer distance and CTS content. 2.2. Adsorption of Cu2þ and Ni2þ using CTS-modified MMT To study the adsorption kinetics, 20 mL each of Cu(NO3)2 and Ni (NO3)2 solutions were taken in separate 50 mL Erlenmeyer flasks, to which 0.1 g each of MMT, CTS, and CTS-modified MMT with a CTS/ MMT ratio of 0.1 were added. After shaking the suspension at 10 � C, 30 � C, and 60 � C for 2, 4, 6, 8, 10, 20, 30, and 60 min, 10 mL aliquots of the suspension were collected and filtered through a 0.45 μm membrane filter. To make adsorption isotherms, Cu(NO3)2 and Ni(NO3)2 solutions (20 mL each) were added to 50-mL Erlenmeyer flasks, followed by 0.1 g of MMT, CTS, and CTS-modified MMT with CTS/MMT mass ratios of 0.1, 0.5, and 1.0. After shaking the suspension at 30 � C for 24 h, it was filtered under suction through a 0.45 μm membrane filter and the res idue was washed with ion exchange water. Furthermore, we used MMT alone as well as the CTS-modified MMT (CTS/MMT ¼ 0.1) as the adsorbent in our study. The individual concentrations of Cu2þ and Ni2þ in the mixture of Cu2þ-Ni2þ ions in solution was 20 mmol/l. The heavy metal ion concentration in the filtrate was determined via inductively coupled-atomic emission spectroscopy (ICP-AES). Furthermore, the residue on the filter was dried at 40 � C for 24 h and analyzed via XRD. 2.3. Desorption of Cu2þ and Ni2þ from the adsorbent For the desorption study, 20-mmol/L Cu(NO3)2 and Ni(NO3)2 solu tions (500 mL each) were poured in five-neck round-bottom flasks, which were agitated at 30 � C. Subsequently, 2.5 g of CTS-modified MMT (CTS/MMT ¼ 0.1) was added and the suspension was agitated for 24 h. After this, suspensions were filtered under suction and the residue was washed and vacuum-dried for 24 h at 40 � C. Following pulverization of the residue, the interlayer distance and CTS content were determined via XRD and elemental analysis, respectively. For desorption of Cu2þ and Ni2þ from the used adsorbent, 20 mL each of HNO3 solutions of varying concentrations (1, 2, 5, 10, 20, 50, and 100 mmol/L) were added to 50mL Erlenmeyer flasks, followed by CTS-modified MMT (CTS/ MMT ¼ 0.1) that have adsorbed 0.1 g each of Cu2þ and Ni2þ. After shaking at 30 � C for 24 h, the suspensions were filtered under suction through 0.45 μm membrane filters. The metal ion concentrations in the filtrates were analyzed via ICP-AES; the solid residue was vacuum-dried 2
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at 40 � C for 24 h and pulverized, after which it was subjected to XRD and elemental analyses to determine the interlayer distance and CTS content. 2.4. Adsorption of Cu2þ and Ni2þ using generated CTS-modified MMT The CTS-modified MMT obtained after desorption (2.5 g) was introduced to 20-mmol/L Cu(NO3)2 and Ni(NO3)2 solutions (500 mL) and agitated for 24 h at 30 � C. A total of 0.2 g of CTS-modified MMT with adsorbed Cu2þ and Ni2þ was added to 40 mL of a 100-mmol/L HNO3 solution and agitated for 24 h at 30 � C. After desorption, 0.1 g of this CTS-modified MMT was added to 20-mmol/L Cu(NO3)2 and Ni(NO3)2 solutions and agitated for 24 h at 30 � C. The suspension was filtered under suction and the metal concentration in the filtrate was measured via ICP-AES.
Fig. 2. CTS content with CTS-modified MMT with CTS/MMT ¼ 0.1, 0.5, and 1.0, as well as the percentage of the cations (-NHþ 3 ) of CTS to the CEC of MMT.
3. Results and discussion
deacetylation degree of the CTS used in this study ranged between 75% and 85%, it was assumed that approximately 20% of the N exists not as NHþ 3 but as NHCO3. The elemental analysis revealed the presence of N at all CTS/MMT mass ratios, thereby confirming the intercalation of CTS in the composites. With an increase in the CTS/MMT ratio between 0.1 and 0.5, the CTS content in the CTS-modified MMT increased; however, for CTS/MMT ratios of 0.5 and 1.0, the CTS content was practically the same. These results suggest that the CTS content becomes saturated at a CTS/MMT ratio of 0.5. Furthermore, NHþ 3 /CEC is below 100%, and the NHþ 3 group of CTS does not exceed the CEC of MMT. It is assumed that these factors resulted in the intercalation of CTS between the MMT layers by cation exchange, leading to the synthesis of CTS-modified MMT. We believe that in this structure, the negative charge of the MMT host layer is compensated by the NHþ 3 group of CTS. For kinetics evaluation, CTS-modified MMT with a CTS/MMT mass ratio of 0.1 was used. For adsorption isotherms, CTS-modified MMT with CTS/MMT mass ratios of 0.1, 0.5, and 1.0 was used.
3.1. Synthesis of CTS-modified MMT Fig. 1 shows the XRD patterns of the raw MMT and CTS-modified MMT with CTS/MMT ratios of 0.1, 0.5, and 1.0. A peak characteristic to MMT was observed in all CTS-modified MMT with different mass ratios. The d001 value for MMT, which corresponds to the interlayer distance, was found to be 1.23 nm. For CTS-modified MMT with CTS/ MMT ratios of 0.1, 0.5, and 1.0, this value was 1.73, 1.73, and 1.72 nm, respectively, indicating an increase in interlayer distance of approxi mately 0.50 nm after modification with CTS. It has been reported that, in the synthesis of CTS-modified MMT, when CTS is intercalated between MMT layers, the interlayer distance increases by approximately 0.18–0.49 nm [6,12]. Therefore, the increase in the d001 value observed in our study is valid and confirms the intercalation of CTS between the MMT layers. Fig. 2 shows the CTS contents of CTS-modified MMT with CTS/MMT ratios of 0.1, 0.5, and 1.0, as well as the percentages of the þ cations of CTS (NHþ 3 , referred to as NH3 /CEC hereafter) to the CEC of MMT. These percentages were determined by assuming that all the NH2 groups of CTS dissolved in acetate to form NHþ 3 , and calculating the amount NHþ 3 groups of CTS in the CTS-modified MMT from the N con tent obtained through the elemental analysis. However, as the
3.2. Adsorption of Cu2þ and Ni2þ using CTS-modified MMT Figs. 3–5 and S1–S3 show temporal changes in the amounts of Cu2þ and Ni2þ adsorbed by MMT, CTS, and CTS-modified MMT (CTS/ MMT ¼ 0.1) at 10 � C, 30 � C, and 60 � C. The amounts of adsorbed Cu2þ and Ni2þ increased immediately after the addition of MMT (Fig. 3), suggesting that adsorption by MMT was a rapid process. Furthermore, with MMT, there was no significant difference in the adsorption amount with change in temperature. However, when CTS and CTS-modified MMT were used (Figs. 4 and 5), the adsorption amount increased with time and with increasing temperature. Figs. S4–S6 show temporal changes in pH during the adsorption of Cu2þ and Ni2þ by MMT, CTS, and CTS-modified MMT (CTS/MMT ¼ 0.1) at 10 � C, 30 � C, and 60 � C. The pH decreased with increasing temperature in all cases. Therefore, it can be concluded that the final pH of the suspension did not cause hydroxide precipitation in any of the adsorption cases. We analyzed the reaction rate rates, shown in Figs. S3–S5, using pseudo-first-order and pseudo second-order kinetic equations and intraparticle diffusion model equations. First, the reaction rates were fitted to a pseudo-first-order kinetic equation (Eq. (1)) that is widely used to model adsorption of adsorbates from aqueous solutions: lnð1
xÞ ¼ kt
(1)
where t is time [min], k is the apparent reaction rate constant [min 1], and x represents the adsorption rate [ ]. Plots of t against ln (1-x) at each temperature, calculated using Eq. (1), are shown in Figs. S4–S6. There is no clear linear relationship be tween time t and ln (1-x), indicating that the adsorption reaction does not follow the pseudo-first-order kinetic equation. Second, the reaction
Fig. 1. XRD patterns of (a) MMT and CTS-modified MMT with CTS/MMT ¼ (b) 0.1, (c) 0.5, and (d) 1.0. 3
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Fig. 3. Change in the adsorption amount of (a) Cu2þ and (b) Ni2þ with MMT at 10, 30, and 60 � C.
Fig. 4. Change in the adsorption amount of (a) Cu2þ and (b) Ni2þ with CTS at 10, 30, and 60 � C.
Fig. 5. Change in the adsorption amount of (a) Cu2þ and (b) Ni2þ with CTS-modified MMT (CTS/MMT ¼ 0.1) at 10, 30, and 60 � C.
rate data were fitted to the pseudo-second-order kinetic equation: t 1 1 ¼ þ t qt k2 q2e qe
kinetic equation [13]. Plots of t/qt against t, calculated using Eq. (2), are shown in Figs. S7–S9. Time t and t/qt have a clear linear relationship, and therefore we can conclude that the adsorption of Cu2þ and Ni2þ by MMT, CTS, and CTS-modified MMT follows the pseudo-second-order kinetic equation. The adsorption of Pb2þ, Zn2þ, and Cd2þ by mustard waste biomass like CTS also follows the pseudo-second-order kinetic equation [14]. Finally, the reaction rates were fitted into the intraparticle diffusion model [15]. This model assumes that the adsorption amount depends on the particle size of the adsorbent (r) and the intraparticle diffusion rate
(2)
where k2 is the apparent reaction rate constant [g/(mmol.min)], and qe and qt [mmol/g] are the adsorption amounts at equilibrium and time t [min], respectively. Assuming that chemical adsorption via sharing of electrons between the adsorbent and adsorbate is the rate-determining step, it is possible to conclude that the adsorption behavior follows the pseudo-second-order 4
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of the adsorbate (D) and that the adsorption amount changes in pro portion to (Dt/r2)1/2; that is:
Table 2 Value of calculated k2 and qe .
� �1=2 qt ∝ Dt r2
Adsorbent
Therefore, the intraparticle diffusion model equation can be expressed as Eq. (3):
MMT
Ni2þ
(3)
qt ¼ kp t1=2
where qt is the adsorption amount [mmol/g], kp is the reaction rate constant [mmol/g・min1/2], and t is time [min]. This means that if adsorption is represented by Eq. (3), it only progresses with diffusion within the particles of the adsorbate. Plots of qt against t1/2, calculated using Eq. (3), are shown in Figs. S10–S12. No linear relationship was observed between qt and t1/2 and therefore we can conclude that the adsorption of Cu2þ and Ni2þ by MMT, CTS, and CTS-modified MMT does not follow the intraparticle diffusion model equation. The correlation coefficients (R2) obtained by fitting the adsorption rate data to the pseudo-first-order kinetic, pseudo-second-order kinetic, and intraparticle diffusion model equations are listed in Table 1. Using Eq. (2), the reaction rate constant k2 ½g=mmolg=min� and equilibrium adsorption amount qe ½g=mmolg� for the adsorption of Cu2þ and Ni2þ by MMT, CTS, and CTS-modified MMT (CTS/MMT ¼ 0.1) were calculated (Table 2) from the slope and intercept of the approximate curves shown in Figs. S7–S9. In all adsorption cases, the reaction rate constant increased with increasing temperature. The reaction rate constant k2 is given by Arrhenius’ equation: � � Ea k2 ¼ Aexp (4) RT
CTS
Ea RT
CTS-modified MMT (CTS/ MMT ¼ 0.1)
MMT CTS CTS-modified MMT (CTS/MMT ¼ 0.1)
MMT
Cu2þ Ni2þ
CTS
Cu2þ Ni2þ
CTS-modified MMT (CTS/ MMT ¼ 0.1)
Cu2þ Ni2þ
Pseudosecond order
Intraparticle diffusion
10 30 60 10 30 60 10 30 60 10 30 60 10 30 60 10 30 60
0.874 0.696 0.0875 0.131 0.0004 0.200 0.918 0.0932 0.000470 0.868 0.0932 0.333 0.543 0.116 0.0682 0.680 0.476 0.522
1.00 1.00 1.00 1.00 1.00 1.00 0.966 0.993 0.987 0.997 0.920 0.993 1.00 0.996 1.00 1.00 1.00 1.00
0.936 0.647 0.215 0.178 0.590 0.0441 0.934 0.812 0.00365 0.817 0.802 0.403 0.632 0.215 0.059 0.825 0.631 0.663
10 30 60 10 30 60 10 30 60 10 30 60 10 30 60 10 30 60
5.19 9.80 28.0 7.82 9.04 25.7 3.04 4.98 83.0 10.5 22.4 30.0 17.4 33.7 155 10.5 22.4 30.0
0.479 0.487 0.488 0.428 0.439 0.457 0.0775 0.0747 0.0704 0.149 0.162 0.175 0.144 0.160 0.185 0.149 0.162 0.175
Activation energy [kJ/mol] 2þ
Cu Ni2þ Cu2þ Ni2þ Cu2þ Ni2þ
26.5 19.1 53.0 55.5 34.7 19.1
these findings, it is possible to conclude that the adsorption of Cu2þ and Ni2þ by MMT, CTS, and CTS-modified MMT follows a pseudo-second-order kinetic equation. Furthermore, the activation en ergy suggests that the rate-determining step in the chemical adsorption of Cu2þ and Ni2þ by MMT, CTS, and CTS-modified MMT is the inter change of electrons between the adsorbent and adsorbate. The results, shown in Figs. 3–5, indicate that the adsorption of Cu2þ and Ni2þ reaches the equilibrium state at approximately 60 min for all three adsorbents. Hence, we carried out a 24 h adsorption experiment under equilibrium conditions at 30 � C. The adsorption isotherms of Cu2þ and Ni2þ by MMT and CTS are shown in Fig. 6, and those of Cu2þ and Ni2þ by CTS-modified MMT (CTS/MMT ratios of 0.1, 0.5, and 1.0) are shown in Fig. 7. The adsorption isotherms obtained were applied and fitted into the Langmuir equation:
Table 1 Correlation efficient of the various kinetic models. Pseudofirst order
qe [mmol/ g]
Adsorbent
Figs. S13–S15 show the Arrhenius plots of the adsorption of Cu2þ and Ni by MMT, CTS, and CTS-modified MMT (CTS/MMT ¼ 0.1) using the calculated reaction rate constant given by Eq. (5). Activation energies determined from the slope of the Arrhenius plots are shown in Table 3. The activation energy for chemical adsorption is reported to be in the range of 8.4–83.7 kJ/mol [16,17]. Therefore, the calculated activation energy is indicative of the occurrence of chemical adsorption. Based on
R2 [-]
k2 [g/mmol/ min]
Table 3 Activation energy calculated based on the Pseudo-second order kinetics.
2þ
Temp. [� C]
Cu2þ Ni2þ
(5)
Adsorbent
Cu2þ Ni2þ
where A is the frequency factor [mmol/(g・min1/2)], T is the tempera ture [K], and R is the gas constant [kJ/(K・mol)]. If the logarithm of Eq. (4) is taken, Eq. (5) is obtained as: lnk2 ¼ ln A
Cu2þ
Temp. [� C]
Ce 1 1 ¼ þ Ce qe KL qm qm
(6)
where qe is the equilibrium adsorption amount [mmol/g], Ce is the equilibrium concentration [mmol/L], qm is the maximum adsorption amount [mmol/g], and KL is the adsorption equilibrium constant [L/ mol]. Plots of Ce/qe against Ce, calculated using Eq. (6), for adsorption of Cu2þ and Ni2þ by MMT and CTS are shown in Fig. S16, and those for adsorption by CTS-modified MMT are shown in Fig. S17. There is a clear linear relationship between Ce/qe and Ce for all three adsorbents, thereby suggesting that the adsorption of Cu2þ and Ni2þ by MMT, CTS, and CTS-modified MMT satisfies the Langmuir equation and is a chemical process. The adsorption of Pb2þ by CTS is known to obey the Langmuir equation [18]. Furthermore, the adsorption of Pb2þ, Zn2þ, and Cd2þ by mustard waste biomass like CTS is also known to obey the Langmuir equation [14]. Next, the adsorption isotherms were fitted to the Freundlich equation:
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Materials Chemistry and Physics 236 (2019) 121784
Fig. 6. Adsorption isotherms on the adsorption of (a) Cu2þ and (b) Ni2þ by MMT and CTS.
Fig. 7. Adsorption isotherms on the adsorption of (a) Cu2þ and (b) Ni2þ by CTS-modified MMT.
1 logqe ¼ logKF þ logCe n
(7)
Table 5 Parameter calculated based on Langmuir equation.
where KF is the adsorption distribution coefficient and n is a constant. Plots of log qe against log Ce, calculated using Eq. (7), for the adsorption of Cu2þ and Ni2þ by MMT and CTS are shown in Fig. S18, and those for adsorption by CTS-modified MMT are shown in Fig. S19. There was no clear linear relationship between log qe and log Ce for any of the three adsorbents; therefore, the Freundlich equation is not applicable for any of the adsorption cases. The correlation coefficients R2 obtained by fitting the adsorption isotherms to the Langmuir and Freundlich equations are listed in Table 4. The Langmuir equation was applicable to all adsorption cases. Table 5 shows the parameters calculated using the Langmuir equation.
Adsorbent MMT CTS CTS-modified MMT (CTS/MMT ¼ 0.1) CTS-modified MMT (CTS/MMT ¼ 0.5) CTS-modified MMT (CTS/MMT ¼ 1.0)
MMT
R2 [-]
CTS CTS-modified MMT (CTS/MMT ¼ 0.1) CTS-modified MMT (CTS/MMT ¼ 0.5) CTS-modified MMT (CTS/MMT ¼ 1.0)
Cu2þ Ni2þ Cu2þ Ni2þ Cu2þ Ni2þ Cu2þ Ni2þ Cu2þ Ni2þ
Langmuir
Freundlich
0.99 0.96 0.98 0.91 0.98 1.0 0.97 0.96 1.0 0.97
0.79 0.78 0.90 0.92 0.97 0.93 0.81 0.98 0.99 0.93
qm [mmol/g]
KL [L/mol]
0.50 0.61 1.6 1.3 0.33 0.30 0.21 0.29 0.18 0.23
0.52 7.3 0.19 0.058 1.8 2.4 0.40 0.31 0.53 0.32
The maximum adsorption amounts calculated from the Langmuir equation are shown in Fig. 8. The maximum adsorption amount with CTS-modified MMT was lower than that with either MMT or CTS. We believe that this is because the modification of MMT with CTS hindered the introduction of heavy metal cations between the MMT layers. Furthermore, no significant change in metal adsorption amount was observed with changes in the CTS/MMT ratio. This indicates that the CTS-modified MMT did not have the strong metal adsorption capacity of CTS. The maximum adsorption of Cu2þ by CTS was almost same as that by carbon nanofiber [19]. The maximum adsorption of Ni2þ by CTS is slightly lower than that by vermiculite-based nanoscale hydrated zir conium oxides [20]. Fig. 9 shows the amount of Cu2þ and Ni2þ adsorbed by MMT alone and by CTS-modified MMT (CTS/MMT ¼ 0.1) from a solution containing a mixture of Cu2þ and Ni2þ. In both cases, the
Table 4 Correlation efficient of the Langmuir and Freundlich models. Adsorbent
Cu2þ Ni2þ Cu2þ Ni2þ Cu2þ Ni2þ Cu2þ Ni2þ Cu2þ Ni2þ
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space of MMT. This suggests that the CTS detached from between the MMT layers. This adsorption is likely attributable to the cation adsorp tion capacity of MMT and the ability of CTS to form complexes with heavy metal ions [21]. In the case of tetracoordinate metal ions, the formation of such complexes is coordinated by an amino group and three electron-donating groups of CTS. Two possible electron-donating groups are the hydroxyl group of CTS and water molecules. 3.3. Desorption of Cu2þ and Ni2þ from the adsorbents The Cu2þ and Ni2þ amounts adsorbed by the CTS-modified MMT (CTS/MMT ¼ 0.1) were 0.258 and 0.313 mmol/g, respectively. Fig. S21 show the Cu2þ and Ni2þ desorption rates from the CTS-modified MMT (CTS/MMT ¼ 0.1) and Fig. S22 shows the pH of the suspensions after desorption. The desorption rates increased with increases in the con centration of the HNO3 solution; at a concentration of 100 mmol/L, desorption rates for both Cu2þ and Ni2þ were approximately 90%. Fig. S23(A) shows the X-ray diffraction pattern of CTS-modified MMT (CTS/MMT ¼ 0.1) before adsorption and before and after desorption of Cu2þ with a 20-mmol/L HNO3 solution. After desorption, the peak that indicates the 001 face broadened and became less intense. Fig. S23(B) shows the X-ray diffraction pattern of the CTS-modified MMT (CTS/MMT ¼ 0.1) before adsorption and before and after desorption of Ni2þ with a 20-mmol/L HNO3 solution. As with Cu2þ, after desorption, the peak for the 001 face broadened and reduced in in tensity. Fig. S24 shows the CTS contents of the CTS-modified MMT (CTS/MMT ¼ 0.1) before and after desorption of Cu2þ and Ni2þ with a 20-mmol/L HNO3 solution. CTS contents before and after desorption were almost equal, suggesting that desorption did not cause detachment of CTS. We believe that the adsorption of heavy metal cations by CTSmodified MMT is attributable to the cation exchange capacity of MMT and the ability of CTS to form complexes with heavy metal ions. Therefore, the desorption of heavy metal cations from different adsorption sites was analyzed. The ease with which different inorganic cations intercalate between MMT layers is in the order Liþ < Naþ < Kþ < Mg2þ < Ca2þ < Ba2þ < Al3þ < Fe3þ < Hþ. This in dicates that the heavy metal cations adsorbed via cation exchange were replaced by Hþ, which can intercalate between the MMT layers more easily than heavy metal cations during desorption. Furthermore, it is possible that the heavy metal cations were adsorbed by CTS-modified MMT via the formation of complexes with CTS.
Fig. 8. Comparison of maximum adsorption amount of heavy metal ions by various adsorbents.
Fig. 9. Adsorption amount of Cu2þ and Ni2þ adsorbed by MMT and by CTSmodified MMT (CTS/MMT ¼ 0.1) from a solution containing a mixture of Cu2þ Ni2þ.
amount of Ni2þ adsorbed was greater than that of Cu2þ. This is consis tent with the adsorption selectivity of MMT, but not CTS (Ni2þ
3.4. Adsorption of Cu2þ and Ni2þ using generated CTS-modified MMT Fig. 10 shows the amounts of Cu2þ and Ni2þ adsorbed by the CTS-
Fig. 10. Adsorption amount of Cu2þ and Ni2þ (a) before and (b) after the desorption of them by the CTS-modified MMT (CTS/MMT ¼ 0.1). 7
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modified MMT (CTS/MMT ¼ 0.1) during the first adsorption (using fresh CTS-modified MMT) and second adsorption (using generated CTSmodified MMT). The Cu2þ and Ni2þ desorption rates achieved using a 100-mmol/L HNO3 solution were 82% and 86%, respectively. The adsorption amounts during the first and second adsorption were the same for both Cu2þ and Ni2þ, indicating that the synthesized adsorbent can be reused.
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4. Conclusions We synthesized CTS-modified MMT via intercalation of CTS between MMT layers. The adsorption of Cu2þ and Ni2þ by MMT, CTS, and CTSmodified MMT were found to follow a pseudo-second-order kinetic equation, and the activation energy of adsorption revealed that it is a chemical process. Furthermore, all adsorption cases follow the Langmuir adsorption isotherm. The adsorption of metal cations by CTS-modified MMT is likely due to the cation adsorption capacity of MMT and the formation of CTS and metal cation complexes. The maximum adsorption amounts for CTS-modified MMT prepared with different CTS/MMT ra tios were the same (0.18–0.33 mmol/g for Cu2þ, 0.23–0.30 mmol/g for Ni2þ); however, these values were lower than those for MMT (0.50 mmol/g for Cu2þ, 0.61 mmol/g for Ni2þ) and CTS (1.6 mmol/g for Cu2þ, 1.3 mmol/g for Ni2þ). This indicates that CTS-modified MMT does not possess the strong metal adsorption capacity of CTS. Furthermore, desorption and re-adsorption experiments were conducted to determine whether the synthesized adsorbent can be reused. The generated CTSmodified MMT achieved the same level of adsorption as the fresh CTSmodified MMT. Acknowledgments This work was supported by JSPS KAKENHI [grant number 15KK0021]. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.matchemphys.2019.121784. References [1] F. Uddin, Clays, nanoclays, and montmorillonite minerals, Metall. Mater. Trans. A 39A (2008) 2804–2814.
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