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Adsorption of heavy metal ions by mesoporous ZnO and TiO2@ZnO monoliths: Adsorption and kinetic studies
Accepted Manuscript Adsorption of heavy metal ions by mesoporous ZnO and TiO2@ZnO monoliths: Adsorption and kinetic studies
Manisha Sharma, Jasminder Singh, Satyajit Hazra, Soumen Basu PII: DOI: Reference:
S0026-265X(18)31022-1 doi:10.1016/j.microc.2018.10.026 MICROC 3409
To appear in:
Microchemical Journal
Received date: Revised date: Accepted date:
17 August 2018 11 October 2018 11 October 2018
Please cite this article as: Manisha Sharma, Jasminder Singh, Satyajit Hazra, Soumen Basu , Adsorption of heavy metal ions by mesoporous ZnO and TiO2@ZnO monoliths: Adsorption and kinetic studies. Microc (2018), doi:10.1016/j.microc.2018.10.026
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ACCEPTED MANUSCRIPT Adsorption of Heavy Metal Ions by Mesoporous ZnO and TiO2@ZnO Monoliths: Adsorption and Kinetic Studies Manisha Sharma§, Jasminder Singh⸸, Satyajit Hazraǂ and Soumen Basu⸸* §
Division of Chemistry, Kyoto University, Kitashirakawa, Sakyo-ku, Kyoto 606-8502, Japan.
⸸
Institute of Nuclear Physics, Kolkata 700064, India.
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ǂSaha
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School of Chemistry and Biochemistry, Thapar University, Patiala, Punjab-147004, India.
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Corresponding E-mail: [email protected]
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Abstract: Up till now, the excessive and unconstrained release of Pb2+ and Cd2+ ions in water
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becomes a foremost concern by way of threatening human health seriously. Therefore, for removal of heavy metal ions from waste water, adsorbent with high efficacy and low-cost technologies is
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most needed to alleviate the situation. In this work, robust, mesoporous ZnO & TiO2@ZnO
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monoliths with the valuable surface area (120-332 m2/g) were synthesized via nanocasting process and their performance as an adsorbent for removal of heavy metal (Pb2+ and Cd2+) ions from
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aqueous water was successfully evaluated. The adsorption data shows better fit at pH 6 to
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Freundlich isotherm model. Applicability of pseudo-second order (PSO) kinetic model specifies the chemisorption process. Thermodynamic parameters confirmed that removal of Pb2+ and Cd2+
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ions was an endothermic process. The maximum adsorption efficacy of Pb2+ ions based on monolayer adsorption was found to be 790 and 978 mg/L for ZnO and TiO2@ZnO monoliths, respectively whereas the maximum adsorption efficacy of Cd2+ ions based on monolayer adsorption was found to be 643 and 786 mg/L for ZnO and TiO2@ZnO monoliths respectively. Besides, used mesoporous ZnO & TiO2@ZnO monoliths could be efficiently reused for at least three times after treatment with NaOH. The high uptake capacity for Pb2+ and Cd2+ ions with good reusability of ZnO & TiO2@ZnO monoliths made them a potentially attractive adsorbent.
ACCEPTED MANUSCRIPT Keywords: Metal oxide monoliths; Mesoporous; Heavy metals; Adsorption; Equilibrium and kinetic study; Thermodynamic parameters. Introduction:
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Shortage of drinking water became a serious matter worldwide in consequence of a change
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in a global climate model. Raw sewage waste containing dyes, heavy metal ions and other toxic
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compounds from industries get directly released into the environment without any treatment or extraction of the harmful compounds due to which the main source of drinking water e.g. lakes,
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river and ground water get contaminated [1, 2]. Heavy metals (like lead and cadmium), are counted
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as most toxic elements among the priority pollutants because of their non-degradable and accumulative nature [3]. To overcome the ongoing drinking water shortage, researchers are
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working on to find a conventional method for waste water treatment prior to discharge. Till now,
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a few of conventional methods like reverse osmosis, ion exchange, ultra-filtration and precipitation have been encountered for metal ions removal with specific disadvantages such as high cost of
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treatment, incomplete removal or production of high concentration of waste, the complexity of
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treatment and high energy requirements [4-6]. Adsorption method is tested as one alternative effective process without chemical
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degradation of solid adsorbents used for metal ion removal from water. Some advantages like high efficiency, most effective, sludge free, insensitivity to toxic material made adsorption techniques most apt to work [7]. Numerous adsorbents like zeolites, metal oxides, clays, functionalized polymers, activated carbons, etc. have been well-known and verified for the adsorption of heavy metal ions [8]. Nowadays, metal-oxide based nanomaterials are considered as suitable material for water treatment. Despite huge demand, most of the available nanomaterials are not user-friendly
ACCEPTED MANUSCRIPT and eco-friendly. Also, separation of the metal-oxides from aqueous solution is very challenging because of nanoscale particle size. Moreover, metal-ions at the time of absorption occupied most of the surface of nanomaterials, due to the large size of metal ions, adsorption efficacy gets decreased. In recent years, substantial research has been focused on to encounter mesoporous
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adsorbents with high adsorption capacity and low cost. For adsorption of heavy metal ions, mesoporous metal oxides with the high specific surface
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area, great density, and uniform pore size distribution can act as unprecedented adsorbents. In past
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few years, worldwide research gets focused on structures with hierarchically ordered porous material that combine mesopores with macropores like silica, carbon, and metal-oxides. The
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hierarchically ordered porous monolithic system will act as an advantage due to large size, high
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porosity in consequence of bimodal porous nature [9]. Even though numerous papers have been published on heavy metal ions adsorption by using mesoporous structure, but only a few are
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focused on the synthesis of mesoporous metal-oxides in monolithic form like titanium oxide (TiO2)
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and zinc oxide (ZnO) as adsorbents. TiO2 and ZnO, both metal-oxide individually has advantages of chemical and physical stability, ease of availability, low cost, high surface area and porosity
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[10]. The coupling of these two metal oxides enhanced individual and combined properties. TiO2
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and ZnO hetero-junctions have been well reported in the literature but most are focused on photocatalytic reactions and photovoltaic processes [11-13] but not a single has reported the adsorption efficacy. Therefore, for the first time, we have reported ZnO core - TiO2 shell monolithic system having both mesoporous and microporous nature, synthesized via nanocasting method. The adsorbents prepared in this work are eco-friendly with low cost and high surface area. The purpose of this finding was to analyze the adsorption efficacy of both ZnO and TiO2@ZnO
ACCEPTED MANUSCRIPT for removal of Pb2+ and Cd2+ from aqueous solution. Both kinetic and equilibrium features were discussed in order to explain the adsorption process for Pb2+ and Cd2+ in detail. Material and methods:
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All the purchased material and chemicals were of analytical grade and were used as obtained
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without further purification. Here, synthetic wastewater was prepared by mixing of metal ions
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(lead and cadmium).
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Characterization of monoliths:
PANALYTICAL X’Pert PRO X-ray diffractometer was used for the analysis of crystal
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phase detection of synthesized monoliths. Surface morphology of the monolith was by scanning
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electron microscopy and Energy Dispersive X-ray Spectroscopy (JEOL- 7000 SEM). The surface
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properties of bimetal oxide monolith were explored via Brunauer-Emmett-Teller (BET) surface area analyzer using Microtec Belsorp Mini–II (Microtrac BEL Corp. Pvt. Ltd, Japan). The
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oxidation state of the monolith was verified by X-ray photoelectron spectroscopy (XPS) system
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from PHI 5200 mode. The kinetic parameters for the dye degradation were stately studied using a UV-Vis spectrophotometer (Champion UV- 500). For adjustment and analysis of pH, pH meter
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from cyber scan pH 1100 is used.
Experimental procedures: Synthesis of ZnO monolith: The detailed procedure for the synthesis of silica monolith via sol-gel method has been discussed in our previous paper [14]. Zinc nitrate hexahydrate solution (3 M) was used as a precursor for ZnO monolith synthesis. Zinc nitrate solution was impregnated into as prepared silica
ACCEPTED MANUSCRIPT monoliths after degassing them under vacuum. The wet monoliths were dried at 150 °C for 10 h (1 °C/min). Impregnation process was repeated for at least five times to get uniform ZnO-SiO2 monoliths. Later, synthesized composites were calcined at 400 °C for 5 h (1 °C /min). Finally, silica part was leached out from a composite of ZnO-SiO2 to get pure ZnO monoliths by using HF
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(10%).
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Synthesis of TiO2@ZnO monolith:
Above synthesized ZnO monoliths were impregnated with a TiCl4 solution (2.5 M) after
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degassing them under vacuum. The wet monoliths were dried at 100 °C for 10 h (1 °C/min) and at
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least 5 impregnation cycles were repeated to get uniform ZnO core-TiO2 shell monolith.
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Adsorption study:
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Heavy metal ions (Pb2+ and Cd2+) adsorption on ZnO and TiO2@ZnO monoliths was performed in the batch arrangement at room temperature. ZnO and TiO2@ZnO monoliths (0.02 g)
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were added into 50 ml of aqueous solution of metal ions (10-50 mg L-1 for Pb2+ and 5-20 mgL-1
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for Cd2+) and stirred at a speed of 200 rpm for different time period. After completion of adsorption phase, synthetic water with metal ions was centrifuged for 15 min at 6000 rpm and the supernatant
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solution was taken out to analyze the concentration of remaining metal ion concentration through AAS. Standard solution of lead nitrate and cadmium carbonate were used to analyze the amount of dissolved Pb2+ and Cd2+ ions in water using calibration curve. The adsorption competence (Qe) of monoliths for metal ion adsorption was calculated by the formula: Qe
( Co Ce ) V w
(1)
ACCEPTED MANUSCRIPT where Co is the initial and Ce is the equilibrium concentration (mg/L), while V is the volume of the solution (L) and w is the weight of the adsorbent (g). To control correctness, all adsorption experiments in the batch were executed in triplicates and the average value was reported.
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Result and discussion:
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The porosity of synthesized solid ZnO and TiO2@ZnO monoliths was evaluated by Brunauer-
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Emmett-Teller analyzer along with Barrett–Joyner–Halenda (BJH) plot. As per IUPAC classification, synthesized ZnO and TiO2@ZnO monoliths exhibits N2 sorption isotherms adjacent
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to typical Langmuir type – IV isotherm and categorized as H1 type hysteresis loop (with relative
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pressure, P/P0 = 0.30 to 0.90) which confirms the mesoporous characteristics (Fig. 1a). Textural and porous properties of synthesized monoliths are discussed in Table 1 and the data from BET
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equation shows that specific surface area of ZnO gets an increase after coating of TiO2. According
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to pore size distribution (Fig. 1b), synthesized monoliths holds mesoporous nature along with a small number of micropores (inset of Fig. 1b) and such kind of structures are favorable for water
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treatment.
Monoliths
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Table 1: Textural properties of ZnO and TiO2@ZnO monoliths. Me. D. (nm)
Mi. D. (nm)
Me. V. (cm3 g-1)
Mi. V. (cm3 g-1)
120
9.49
1.9
0.28
0.14
332
3.06
0.6
0.25
0.10
S.A.
ZnO
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(m2 g-1)
TiO2@ZnO
S.A. = Surface area; Me. D. =Mesopore diameter; Mi. D.= Micropore diameter; Me. V.= Mesoporous volume; Mi. V.=Micropore volume
Fig. 1c, display XRD patterns for the phase and purity of synthesized ZnO and TiO 2@ZnO monoliths. XRD confirms the formation of the hexagonal structure of ZnO. ZnO monolith shows dominant peaks for ZnO, 2Ө at 31.8°, 34.4°, 36.23, 47.5, 56.6 and 62.8° corresponding to lattice
ACCEPTED MANUSCRIPT planes of (100), (002), (101), (102), (110) and (103) respectively, which is confirmed by JCPDS card no. 36-1451 [15]. Where as in the case of TiO2@ZnO monoliths, a few peaks in the XRD patterns corresponding to anatase TiO2 were observed (JCPDF no. 21-1272). The morphological structures of synthesized monoliths determined by FESEM are shown in Fig. 2. Similar kind of
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interconnected macroporous structures having mesopores and micropores (a little amount) with in
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wall are obtained, but higher magnification images are difficult to obtain due to the charging of
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the sample. The clarity about the concentration of the synthesized metal oxide monoliths was confirmed by EDS analysis. Uniform distribution of the precursor TiO2 salt throughout the core of
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Adsorption studies for Pd(II) and Cd(II) ions:
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the ZnO monoliths was responsible for chemical homogeneity.
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Numerous factors like pH, contact time, concentration of adsorbate and adsorbent, play a
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significant part in the adsorption process of toxic pollutants like chemical nature of pollutant, chemical properties of the adsorbent and types of interaction occur between the adsorbent and
Effect of pH:
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pollutants.
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pH plays an important role in concluding the reaction rate of the surface reactions between adsorbate and adsorbents. Here, the influence of pH on adsorption competence was studied over the range from pH 2.0 to pH 7.0 and the best adsorption capacity was observed at pH 6 (Fig. 3 (ab)). Below pH 6, excessive protonation takes place on the surface of adsorbent which results in weak interaction between adsorbate and adsorbents. The increase of pH of solution follows the increase in quantitative adsorption efficacy [16, 17].
ACCEPTED MANUSCRIPT Effect of contact time: In this study, the impact of contact time with respect to varied amount of metal ions was studied between 10-15 min (Fig. 3 (c-d)). At pH 6, the initial concentration of metal ions used was 10 mg/L for both Pb2+ and Cd2+, respectively. Sorption process for metal ions was started even in
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small span of time due to fast initial stage adsorption which is followed by a second stage slow
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adsorption process until equilibrium was attained [18]. No considerable change in adsorption was
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observed after 80 min, so the reaction time for the supporting experiments was kept 150 min. The obtained results proved that the adsorption efficacy was directly proportional to the initial
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concentration of metal ions.
Adsorption kinetics:
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The type of adsorption mechanism preceding and potential rate controlling steps of metals
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ions adsorption process can be determined by kinetic studies. Also, chemical and physical properties of adsorbents along with the mass transfer rate of toxic ions from adsorbate onto
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adsorbent surface defines the adsorption mechanism [19]. Therefore, to extrapolate the type of
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adsorption mechanism, different kinetic models were used like pseudo first order (PFO), pseudo second order (PSO) and intraparticle diffusion (IPD) model. In this study, batch experiments were
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performed at pH 6 and 30 °C with 0.2 g/ L adsorbent amount. Pseudo first order (PFO) model: To explin the kinetic processes on solid surfaces at the time of adsorption, Lagergren reported the PFO equation in 1898 and was the first reaction formed to explain the adsorption process in solid and liquid state. The mathematical representation of PFO equation is expressed as [20]:
log(Qe Qt ) log Qe k1t
(2)
ACCEPTED MANUSCRIPT Where k1 is the PFO rate constant (min-1), Qe and Qt (mg/g) are a number of metal ions adsorbed at equilibrium and at time t. Pseudo second order (PSO) model: PSO was projected by Y.S. Ho in an effort to explain adsorption process of divalent metal ions at the surface of sphagnum moss [21]. He explained that
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Where k2 is the rate constant for PSO (g/mg min).
(3)
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t 1 t 2 Qt k2Qe Qe
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The mathematical representation of PSO equation is expressed as [20]:
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PSO is based upon sorption efficiency of the adsorbent that is linked to the existing active sites.
Plots showing PFO and PSO model showing adsorption of Pb2+ and Cd2+ ions on solid ZnO
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and TiO2@ZnO monoliths are presented in Fig. S1 (a-d), respectively and the correlation
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coefficients for kinetic models are evaluated from these linear plots (Table 2). From Fig. S1, we can conclude that both PFO and PSO can be applied to describe the kinetic mechanism but PSO
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shows the best fitting which states that the sorption process is typically monitored by
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chemisorption.
Table 2: PFO and PSO kinetic parameters for the adsorption of metal ions on ZnO and
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TiO2@ZnO. Model PFO
PSO
Pd2+
ZnO Cd2+
Qe (cal)
33.3
31.4
TiO2@ZnO Cd2+ 47.6 45.4
K1
0.083
0.10
0.092
0.13
r2
0.942
0.975
0.984
0.974
Qe (cal)
142.8
90.9
200
110.4
K2
0.0011
0.0008
0.002
0.0011
Parameters
Pd2+
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0.9816
0.989
0.993
0.990
Intraparticle diffusion (IPD) model: The mechanism of the adsorption process can be estimated through IPD model. In 1962, Webber Morris explained that Interaparticle diffusion model can be
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applied in 3 different forms [22]:
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1. Straight line plot for qt Vs t1/2 which is mandatory to forward through origin.
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2. Multi linear plot for qt Vs t1/2 which states that two or more diffusion steps are included in this
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process. In this procedure, at first step immediate adsorption occurs; at second step steady adsorption take place which mainly controls the intraparticle diffusion and equilibrium is known
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as final step where solute passes from macro/ mesopore to micro pores.
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3. Straight line plot for qt Vs t1/2 not mandatory to forward through the origin point.
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IPD model is known to review the ability and rate of diffusion can be calculated using
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mention equation:
Qt kit 0.5 I
(4)
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Where ki is the diffusion constant and I is the intercept. Figure 4 a-b shows a plot of Qt and t0.5, in
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which first linear line depicts the macropore and mesopore diffusion while the second linear line represents the micropore diffusion. Table 3 describes the intercept values for the intraparticle diffusion model.
Table 3: IPD parameter and regression coefficients for metal ion adsorption on ZnO and TiO2@ZnO monoliths. Model
Parameters ki
ZnO Pd2+
Cd2+
21.7
7.93
TiO2@ZnO Cd2+ 24.5 10.02
Pd2+
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I
1.63
1.79
2.06
3.45
R2
0.950
0.944
0.971
0.961
Equilibrium studies and modeling:
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The adsorption isotherms are explored to explain the steadiness of adsorption process in
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liquid- solid and gas liquid system [23]. In liquid- solid system, the connection among adsorbate
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and adsorbent is vibrant process and it offers suitable quantitation of adsorption efficiency in precise environment like pH, temperature, time. Here, to explain the connection among adsorbate
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and adsorbent using ZnO and TiO2@ZnO monoliths Langmuir and Freundlich isotherm model
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knowledge about the adsorption process.
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were used. All auxiliary experiments were performed at pH 6 and at 25◦C for 2h to get a better
Langmuir isotherm: Langmuir model is one of the prominent model used for the description of
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monolayer formation at the specific sites onto adsorbent surface. This model is focused on the idea
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that all the specific sites onto adsorbent surface has constant binding energy due which no development of strong bonds are possible on the surface of adsorbate. The mathematical
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representation of Langmuir model is expressed as [24]: Ce C 1 e Qe Qo b Qo
(5)
where Qe (mg/g) is the equilibrium concentration of metal ion on the adsorbent, Ce (mg/L) is the equilibrium concentration of metal ion, Q0 (mg/L) is the monolayer sorption ability and b is the Langmuir constant. Freundlich isotherm: It defines the heterogeneous adsorption process on surface and at specific active sites through discrete energy formed on multilayer adsorption. This model provides
ACCEPTED MANUSCRIPT estimation of sorption efficiency of adsorbate on an adsorbent. The mathematical representation of Langmuir model is expressed as [25]: log Qe log K f
log Ce n
(6)
where Ce (mg/L) is the equilibrium of metal ion concentration, Kf (mg1-1/nL1/n g-1) is
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adsorption capacity, and 1/n is the empirical parameter linked to adsorption intensity.
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The linear plots for Langmuir (Ce/Qe vs. Ce) and Freundlich isotherms (log Qe vs. log Ce) are shown in Fig. S2 (a-d). Values of slope and parameter of equilibrium isotherms were calculated
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through linear plots for Pb2+ and Cd2+ ion and are given in Table 4. By comparing the experimental data of isotherms based on the parameters obtained, it was realized that both the model were fitted.
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Freundlich isotherm describes the more fit than the other due to higher r2 values. A comparative
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of monolayer adsorption efficacy for Pb2+ and Cd2+ ions from literature are listed in Table 5 and
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thus it can be concluded that ZnO and TiO2@ZnO monoliths synthesized in this work were effective and satisfactory adsorbent for metal removal from waste water.
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Langmuir Model
Parameters
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Model
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Table 4: Isotherm parameters for metal ion adsorption on ZnO and TiO2@ZnO monoliths
Freundlich Model
Q0 b R2 Kf n R2
ZnO Pd2+ 790 1.25 0.965 17.9 3.25 0.975
Cd2+ 643 0.28 0.926 5.64 1.02 0.957
TiO2@ZnO Pd2+ Cd2+ 978 786 2.34 1.1 0.984 0.954 20.78 10.34 4.53 1.83 0.991 0.973
Table 5: A comparative account of Langmuir constants of heavy metal ions from reported literature. Adsorbents
Maximum adsorption
Conditions
References
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Fe2O3
79.35
68.9
--
TiO2
220
857
770
CCM 8000
469.8
71.95
--
Commercial activated carbon PAN-oxime fabrics Fe2O3/SiO2 monolith
4273
16.84
17.23
--
263.45
--
562
690
Carbon monoliths ZnO
1103
1128
120
790
TiO2@ZnO
332
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850
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989
786
[8] [26] [27] [28] [29]
t- 96 h
[30]
pH-6 T-30 °C t-120 min pH-6; T-30 °C; t-150 min pH-6 T-30°C t-150 min
[31]
[32] Present study
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978
643
pH -7 t-120 min pH -5 T-45°C pH-6; T-30 °C; t-80 min t- 34 h; T- 30 °C pH- 7.8; t- 180 min
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446.4
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369
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Cd2+
Effect of temperature:
(mg/g)
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Pb2+
AF-Fe3O4
Surface area (m2/g) 25.94
The effect of temperature like change in entropy (∆ S), enthalpy (∆ H) and Gibbs free energy
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(∆ G) on adsorption process were attained by the following mathematical expression [33]: G RT ln b
ln b
S H R RT
(7) (8)
Where R is the ideal gas constant (8.314 Jmol-1K-1) and T is the temperature. The adsorption experiments were performed at different temperature (20, 25, 30 and 35 ℃) onto the surface of ZnO and TiO2@ZnO monolith. The change in enthalpy and entropy were evaluated from plot
ACCEPTED MANUSCRIPT between ln b (obtained from Langmuir isotherm) and 1/T (Fig. 5 a-b). Here, the adsorption process is identified as spontaneous and endothermic in support of positive enthalpy and negative Gibb’s energy. The outcomes of the thermodynamic parameters are displayed in Table 6. The positive values of entropy are due to the change of energy between metal ions and monoliths.
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Table 6: Thermodynamic parameters for adsorption of metal ion adsorption at different
Temperature
Pd2+
20
∆H (kJ mol−1) 6.1
ZnO ∆S (J mol K−1) 21.4
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25 30
9.2
20
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25
34.4
TiO2@ZnO ∆H (kJ ∆S (J mol ∆G (kJ mol−1) K−1) mol−1) 6.4 25.5 -0.92
-1.28
-1.36
-1.53
-1.61
-1.78
-1.87
-0.95
15.5
45.6
-0.97
-1.36
-1.55
-1.69
-2.06
-2.09
-2.67
CE
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30 35
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35 Cd2+
∆G (kJ mol−1) -0.93
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Metalions
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temperatures.
Reusability of adsorbent:
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The efficiency of porous ZnO and TiO2@ZnO monolith is further evaluated for their repeated regeneration for metal ions removal experiments using 1 M NaOH [34] and found that the synthesized porous ZnO and TiO2@ZnO monolith shows good reusability up to 3 consecutive cycles as shown in Fig. 5 c-d. The obtained results show that adsorption efficacy of porous ZnO and TiO2@ZnO monolith for metal ion adsorption gradually decreases with repeated use of monoliths because a maximum number of adsorption sites get engaged due to the interactions between adsorbent and adsorbate. The good reusability of mesoporous ZnO and TiO2@ZnO
ACCEPTED MANUSCRIPT monoliths is due to the micrometer-sized structure with high surface area, which has a benefit of reducing the overall cost for adsorption process. Furthermore, desorption conditions are more simple and accessible as no specific filtration techniques are required for adsorbent extraction due to its large size.
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Conclusion:
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In this work, Mesoporous ZnO and TiO2@ZnO monoliths synthesized via nanocasting method were utilized as an adsorbent for adsorption of Pb2+ and Cd2+ ions from aqueous solution.
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For adsorption process of metal ions, all the auxilary experiments were executed in different batches in precise environment like pH, temperature, time. Kinetic and thermodynamic
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experiments verified that metal ion adsorption process was chemisorption, spontaneous and
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endothermic in nature. The original outline of this work is utilization of synthesized mesoporous
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ZnO and TiO2@ZnO monoliths for removing heavy metal ions from aqueous wastewater to
Acknowledgments:
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generate clean water.
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The financial assistance for the validation of this research work was offered by DAE/ BRNS (34/14/63/2014), Mumbai, India and other instrument/infrastructure facilities were
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provided by TIET, Patiala, Punjab and Sprint Testing Solutions-Mumbai.
Conflicts of interest: The authors declare that there is no conflict of interests regarding the publication of this paper.
References:
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enhanced
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photoelectrochemical water splitting under solar light illumination, ACS Appl Mater Interfaces, 6
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(2014) 12153-12167.
[12] M.M. Momeni, Y. Ghayeb, Visible light-driven photoelectrochemical water splitting on
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ZnO–TiO2 heterogeneous nanotube photoanodes, Journal of Applied Electrochemistry, 45 (2015)
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557-566.
[13] Y. Lei, G. Zhao, M. Liu, Z. Zhang, X. Tong, T. Cao, Fabrication, Characterization, and Application of
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Photoelectrocatalytic
ZnO
Nanorods
Grafted
on
Vertically Aligned
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TiO2Nanotubes, The Journal of Physical Chemistry C, 113 (2009) 19067-19076. [14] M. Sharma, P. Jain, A. Mishra, A. Mehta, D. Choudhury, S. Hazra, S. Basu, Variation of
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surface area of silica monoliths by controlling ionic character/chain length of surfactants and polymers, Materials Letters, 194 (2017) 213-216. [15] A. Gomes, T. Frade, K. Lobato, M.E.M. Jorge, M.I. da Silva Pereira, L. Ciriaco, A. Lopes, Annealed Ti/Zn-TiO2 nanocomposites tested as photoanodes for the degradation of Ibuprofen, Journal of Solid State Electrochemistry, 16 (2011) 2061-2069.
ACCEPTED MANUSCRIPT [16] M.R. Awual, M. Khraisheh, N.H. Alharthi, M. Luqman, A. Islam, M. Rezaul Karim, M.M. Rahman, M.A. Khaleque, Efficient detection and adsorption of cadmium(II) ions using innovative nano-composite materials, Chemical Engineering Journal, 343 (2018) 118-127. [17] M. Sharma, A. Mishra, A. Mehta, D. Choudhury, S. Basu, Effect of Surfactants on the
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Structure and Adsorption Efficiency of Hydroxyapatite Nanorods, Journal of Nanoscience and
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Nanotechnology, 17 (2017) 1-11.
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[18] N. Feng, X. Guo, S. Liang, Adsorption study of copper (II) by chemically modified orange peel, Journal of hazardous materials, 164 (2009) 1286-1292.
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[19] M. Sharma, A. Mishra, A. Mehta, D. Choudhury, S. Basu, Effect of Surfactants on the
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Structure and Adsorption Efficiency of Hydroxyapatite Nanorods, Journal of nanoscience and nanotechnology, 18 (2018) 623-633.
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[20] C. P. Bergmann, F. M. Machado, (Ed.), Carbon Nanomaterials as adsorbents for
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enviornmental & biological applications, Springer, 2015. [21] Y.S. Ho, Review of second-order models for adsorption systems, Journal of hazardous
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materials, 136 (2006) 681-689.
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[22] R.S. Azarudeen, M.A. Riswan Ahamed, R. Subha, A.R. Burkanudeen, Heavy and toxic metal ion removal by a novel polymeric ion-exchanger: synthesis, characterization, kinetics and
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equilibrium studies, Journal of Chemical Technology & Biotechnology, 90 (2015) 2170-2179. [23] M. Athar, U. Farooq, M. Aslam, M. Salman, Adsorption of Pb(II) ions onto biomass from Trifolium resupinatum: equilibrium and kinetic studies, Applied Water Science, 3 (2013) 665-672. [24] H.-J. butt, Karlheinz Graf, M. Kappl, Physics and Chemistry interfaces, WILEY-VCH GmbH & Co. KGaA, Germany, 2003.
ACCEPTED MANUSCRIPT [25] J. Watson, Separation Methods for Waste and Enviornmental Applications, Marcel Dekker, Inc., United States of America, 1999. [26] S. Rajput, L.P. Singh, C.U. Pittman, Jr., D. Mohan, Lead (Pb2+) and copper (Cu2+) remediation from water using superparamagnetic maghemite (gamma-Fe2O3) nanoparticles
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synthesized by Flame Spray Pyrolysis (FSP), J Colloid Interface Sci, 492 (2017) 176-190.
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[27] M. Sharma, D. Choudhury, S. Hazra, S. Basu, Effective removal of metal ions from aqueous
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solution by mesoporous MnO 2 and TiO 2 monoliths: Kinetic and equilibrium modelling, Journal of Alloys and Compounds, 720 (2017) 221-229.
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[28] Y.P. Teoh, M.A. Khan, T.S.Y. Choong, Kinetic and isotherm studies for lead adsorption from
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aqueous phase on carbon coated monolith, Chemical Engineering Journal, 217 (2013) 248-255. [29] E. Asuquo, A. Martin, P. Nzerem, F. Siperstein, X. Fan, Adsorption of Cd(II) and Pb(II) ions
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from aqueous solutions using mesoporous activated carbon adsorbent: Equilibrium, kinetics and
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characterisation studies, Journal of Environmental Chemical Engineering, 5 (2017) 679-698. [30] K. Saeed, S. Haider, T.-J. Oh, S.-Y. Park, Preparation of amidoxime-modified
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polyacrylonitrile (PAN-oxime) nanofibers and their applications to metal ions adsorption, Journal
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of Membrane Science, 322 (2008) 400-405. [31] J. Singh, M. Sharma, S. Basu, Heavy metal ions adsorption and photodegradation of remazol
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black XP by iron oxide/silica monoliths: Kinetic and equilibrium modelling, Advanced Powder Technology, 29 (2018) 2268-2279. [32] M. Sharma, J. Singh, S. Hazra, S. Basu, Remediation of heavy metal ions using hierarchically porous carbon monolith synthesized via nanocasting method, Journal of Environmental Chemical Engineering, 6 (2018) 2829-2836.
ACCEPTED MANUSCRIPT [33] C. Lei, X. Zhu, B. Zhu, C. Jiang, Y. Le, J. Yu, Superb adsorption capacity of hierarchical calcined Ni/Mg/Al layered double hydroxides for Congo red and Cr(VI) ions, Journal of hazardous materials, 321 (2017) 801-811. [34] J.-S. Hu, L.-S. Zhong, W.-G. Song, L.-J. Wan, Synthesis of Hierarchically Structured Metal
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Oxides and their Application in Heavy Metal Ion Removal, Advanced Materials, 20 (2008) 2977-
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ACCEPTED MANUSCRIPT Adsorption of Heavy Metal Ions by Mesoporous ZnO and TiO2@ZnO Monoliths: Adsorption and Kinetic Studies Manisha Sharma§, Jasminder Singh⸸, Satyajit Hazraǂ and Soumen Basu⸸* §
Division of Chemistry, Kyoto University, Kitashirakawa, Sakyo-ku, Kyoto 606-8502, Japan.
⸸
School of Chemistry and Biochemistry, Thapar University, Patiala, Punjab-147004, India.
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Institute of Nuclear Physics, Kolkata 700064, India.
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ǂSaha
Corresponding E-mail: [email protected]
150 100
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50 0 0.2
TiO2@ ZnO
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ZnO;
0.0
0.4
0.6
0.8
0.4
0.00
ZnO;
25
1.2
1.6
30
2.0
TiO2@ ZnO
35
40
45
50
Pore diameter (nm)
Z (103)
T (004)
1600
T (211) Z (110)
TiO2@Zno Z (102)
T (101)
20
15
10
5
1.0
Z (100) Z (002)
CE Intensity (a.u.)
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2400
0.8
Pore diameter (nm)
Z (101)
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3200
ZnO
0.01
Relative Pressure (P/P0) 4000
TiO2@ ZnO
3
0.02
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TiO2@ZnO SA: 332 m2/g
(b)
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3
ZnO SA: 120 m2/g
200
Vol. adsorbed (cm /g)
250
3
Vol. adsorbed (cm /g)
0.03
(a)
Vol. adsorbed (cm /g)
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Figures
800 0
ZnO 20
25
30
35
40
45
50
55
60
65
2 (degree)
Fig. 1: a) N2 sorption curve, b) BJH plot showing pore size distribution (inset contains micropore distribution curves through MP plot) and c) XRD pattern for ZnO and TiO2@ZnO monoliths.
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(a)
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1 µm
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(b)
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1 µm
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Fig. 2: SEM and EDS spectra for a) ZnO and b) TiO2@ ZnO monoliths.
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(b) Cd(II) adsorption
80
60
ZnO;
60
40
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80
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% Adsorption
% Adsorption
100
(a) Pb(II) adsorption
100
20
TiO2@ZnO
3
4
5
6
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40 7
3
pH 100
(c) Pb(II) adsorption
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TiO2@ZnO 6
7
pH
(d) Cd(II) adsorption
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100
4
ZnO;
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% Adsorption
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60
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40
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ZnO; 0
20
40
60
80
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% Adsorption
80
100
120
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Time (min)
TiO2@ZnO 140
160
60
40
20
ZnO;
TiO2@ZnO
80
120
0 0
20
40
60
100
140
160
Time (min)
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Fig. 3: Plot for effect of pH (a-Pb2+and b-Cd2+) [adsorbent amount: 0.02 g/L]; effect of time (cPb2+and d-Cd2+) for ZnO and TiO2@ZnO monoliths. [agitation speed: 200 rpm, temperature: 25 °C].
ACCEPTED MANUSCRIPT
240
(a) Pb(II) adsorption
80
(b) Cd(II) adsorption
60
120 80
40
20
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160
Qt(mg/g)
Qt(mg/g)
200
0
2
4
0
TiO2@ ZnO 6
8
ZnO;
0
10
0.5
t
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ZnO;
0
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40
2
4
TiO2@ ZnO 6
8
10
0.5
t
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Fig. 4: Plots showing intra-particle diffusion model (a- Pb2+ and b- Cd2+) for ZnO and TiO2@ZnO monoliths
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5.2 6.8
(a) Pb2+ adsorption
5.0
6.4
4.8
6.0
4.4 4.2
5.6 5.2 4.8
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4.6
4.4
4.0
0.00320
4.0
TiO2@ZnO 0.00325
0.00330
0.00320
0.00335
-1
1/T(K ) ZnO;
80
0.00325
0.00330
0.00335
-1
ZnO;
(d)
TiO2@ZnO
Cd2+ adsorption
60
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% Removal
60
40
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% Removal
TiO2@ZnO
(c) Pb2+ adsorption
80
TiO2@ZnO
1/T(K )
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100
ZnO;
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ZnO; 3.8
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ln(Qe/Ce)
ln(Qe/Ce)
(b) Cd2+ adsorption
0 1
2
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20
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No. of cycle
3
40
20
0 1
2
3
No. of cycle
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Fig. 5: Plot showing effect of temperature for adsorption studies on ZnO and TiO2@ZnO monoliths (a- Pb2+ and b- Cd2+) and recyclability of monoliths (c- Pb2+ and d- Cd2+)
ACCEPTED MANUSCRIPT Highlights
Mesoporous ZnO & TiO2@ZnO monoliths were synthesized by nanocasting method.
The synthesized monoliths were superior in Pb2+ and Cd2+ adsorption from aqueous solution. Freundlich model was found to be more fitted for the adsorption process.
Thermodynamic experiments confirmed the spontaneous & endothermic nature of
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adsorption.
Manisha Sharma, Jasminder Singh, Satyajit Hazra, Soumen Basu PII: DOI: Reference:
S0026-265X(18)31022-1 doi:10.1016/j.microc.2018.10.026 MICROC 3409
To appear in:
Microchemical Journal
Received date: Revised date: Accepted date:
17 August 2018 11 October 2018 11 October 2018
Please cite this article as: Manisha Sharma, Jasminder Singh, Satyajit Hazra, Soumen Basu , Adsorption of heavy metal ions by mesoporous ZnO and TiO2@ZnO monoliths: Adsorption and kinetic studies. Microc (2018), doi:10.1016/j.microc.2018.10.026
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ACCEPTED MANUSCRIPT Adsorption of Heavy Metal Ions by Mesoporous ZnO and TiO2@ZnO Monoliths: Adsorption and Kinetic Studies Manisha Sharma§, Jasminder Singh⸸, Satyajit Hazraǂ and Soumen Basu⸸* §
Division of Chemistry, Kyoto University, Kitashirakawa, Sakyo-ku, Kyoto 606-8502, Japan.
⸸
Institute of Nuclear Physics, Kolkata 700064, India.
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ǂSaha
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School of Chemistry and Biochemistry, Thapar University, Patiala, Punjab-147004, India.
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Corresponding E-mail: [email protected]
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Abstract: Up till now, the excessive and unconstrained release of Pb2+ and Cd2+ ions in water
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becomes a foremost concern by way of threatening human health seriously. Therefore, for removal of heavy metal ions from waste water, adsorbent with high efficacy and low-cost technologies is
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most needed to alleviate the situation. In this work, robust, mesoporous ZnO & TiO2@ZnO
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monoliths with the valuable surface area (120-332 m2/g) were synthesized via nanocasting process and their performance as an adsorbent for removal of heavy metal (Pb2+ and Cd2+) ions from
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aqueous water was successfully evaluated. The adsorption data shows better fit at pH 6 to
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Freundlich isotherm model. Applicability of pseudo-second order (PSO) kinetic model specifies the chemisorption process. Thermodynamic parameters confirmed that removal of Pb2+ and Cd2+
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ions was an endothermic process. The maximum adsorption efficacy of Pb2+ ions based on monolayer adsorption was found to be 790 and 978 mg/L for ZnO and TiO2@ZnO monoliths, respectively whereas the maximum adsorption efficacy of Cd2+ ions based on monolayer adsorption was found to be 643 and 786 mg/L for ZnO and TiO2@ZnO monoliths respectively. Besides, used mesoporous ZnO & TiO2@ZnO monoliths could be efficiently reused for at least three times after treatment with NaOH. The high uptake capacity for Pb2+ and Cd2+ ions with good reusability of ZnO & TiO2@ZnO monoliths made them a potentially attractive adsorbent.
ACCEPTED MANUSCRIPT Keywords: Metal oxide monoliths; Mesoporous; Heavy metals; Adsorption; Equilibrium and kinetic study; Thermodynamic parameters. Introduction:
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Shortage of drinking water became a serious matter worldwide in consequence of a change
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in a global climate model. Raw sewage waste containing dyes, heavy metal ions and other toxic
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compounds from industries get directly released into the environment without any treatment or extraction of the harmful compounds due to which the main source of drinking water e.g. lakes,
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river and ground water get contaminated [1, 2]. Heavy metals (like lead and cadmium), are counted
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as most toxic elements among the priority pollutants because of their non-degradable and accumulative nature [3]. To overcome the ongoing drinking water shortage, researchers are
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working on to find a conventional method for waste water treatment prior to discharge. Till now,
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a few of conventional methods like reverse osmosis, ion exchange, ultra-filtration and precipitation have been encountered for metal ions removal with specific disadvantages such as high cost of
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treatment, incomplete removal or production of high concentration of waste, the complexity of
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treatment and high energy requirements [4-6]. Adsorption method is tested as one alternative effective process without chemical
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degradation of solid adsorbents used for metal ion removal from water. Some advantages like high efficiency, most effective, sludge free, insensitivity to toxic material made adsorption techniques most apt to work [7]. Numerous adsorbents like zeolites, metal oxides, clays, functionalized polymers, activated carbons, etc. have been well-known and verified for the adsorption of heavy metal ions [8]. Nowadays, metal-oxide based nanomaterials are considered as suitable material for water treatment. Despite huge demand, most of the available nanomaterials are not user-friendly
ACCEPTED MANUSCRIPT and eco-friendly. Also, separation of the metal-oxides from aqueous solution is very challenging because of nanoscale particle size. Moreover, metal-ions at the time of absorption occupied most of the surface of nanomaterials, due to the large size of metal ions, adsorption efficacy gets decreased. In recent years, substantial research has been focused on to encounter mesoporous
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adsorbents with high adsorption capacity and low cost. For adsorption of heavy metal ions, mesoporous metal oxides with the high specific surface
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area, great density, and uniform pore size distribution can act as unprecedented adsorbents. In past
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few years, worldwide research gets focused on structures with hierarchically ordered porous material that combine mesopores with macropores like silica, carbon, and metal-oxides. The
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hierarchically ordered porous monolithic system will act as an advantage due to large size, high
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porosity in consequence of bimodal porous nature [9]. Even though numerous papers have been published on heavy metal ions adsorption by using mesoporous structure, but only a few are
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focused on the synthesis of mesoporous metal-oxides in monolithic form like titanium oxide (TiO2)
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and zinc oxide (ZnO) as adsorbents. TiO2 and ZnO, both metal-oxide individually has advantages of chemical and physical stability, ease of availability, low cost, high surface area and porosity
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[10]. The coupling of these two metal oxides enhanced individual and combined properties. TiO2
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and ZnO hetero-junctions have been well reported in the literature but most are focused on photocatalytic reactions and photovoltaic processes [11-13] but not a single has reported the adsorption efficacy. Therefore, for the first time, we have reported ZnO core - TiO2 shell monolithic system having both mesoporous and microporous nature, synthesized via nanocasting method. The adsorbents prepared in this work are eco-friendly with low cost and high surface area. The purpose of this finding was to analyze the adsorption efficacy of both ZnO and TiO2@ZnO
ACCEPTED MANUSCRIPT for removal of Pb2+ and Cd2+ from aqueous solution. Both kinetic and equilibrium features were discussed in order to explain the adsorption process for Pb2+ and Cd2+ in detail. Material and methods:
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All the purchased material and chemicals were of analytical grade and were used as obtained
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without further purification. Here, synthetic wastewater was prepared by mixing of metal ions
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(lead and cadmium).
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Characterization of monoliths:
PANALYTICAL X’Pert PRO X-ray diffractometer was used for the analysis of crystal
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phase detection of synthesized monoliths. Surface morphology of the monolith was by scanning
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electron microscopy and Energy Dispersive X-ray Spectroscopy (JEOL- 7000 SEM). The surface
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properties of bimetal oxide monolith were explored via Brunauer-Emmett-Teller (BET) surface area analyzer using Microtec Belsorp Mini–II (Microtrac BEL Corp. Pvt. Ltd, Japan). The
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oxidation state of the monolith was verified by X-ray photoelectron spectroscopy (XPS) system
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from PHI 5200 mode. The kinetic parameters for the dye degradation were stately studied using a UV-Vis spectrophotometer (Champion UV- 500). For adjustment and analysis of pH, pH meter
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from cyber scan pH 1100 is used.
Experimental procedures: Synthesis of ZnO monolith: The detailed procedure for the synthesis of silica monolith via sol-gel method has been discussed in our previous paper [14]. Zinc nitrate hexahydrate solution (3 M) was used as a precursor for ZnO monolith synthesis. Zinc nitrate solution was impregnated into as prepared silica
ACCEPTED MANUSCRIPT monoliths after degassing them under vacuum. The wet monoliths were dried at 150 °C for 10 h (1 °C/min). Impregnation process was repeated for at least five times to get uniform ZnO-SiO2 monoliths. Later, synthesized composites were calcined at 400 °C for 5 h (1 °C /min). Finally, silica part was leached out from a composite of ZnO-SiO2 to get pure ZnO monoliths by using HF
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(10%).
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Synthesis of TiO2@ZnO monolith:
Above synthesized ZnO monoliths were impregnated with a TiCl4 solution (2.5 M) after
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degassing them under vacuum. The wet monoliths were dried at 100 °C for 10 h (1 °C/min) and at
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least 5 impregnation cycles were repeated to get uniform ZnO core-TiO2 shell monolith.
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Adsorption study:
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Heavy metal ions (Pb2+ and Cd2+) adsorption on ZnO and TiO2@ZnO monoliths was performed in the batch arrangement at room temperature. ZnO and TiO2@ZnO monoliths (0.02 g)
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were added into 50 ml of aqueous solution of metal ions (10-50 mg L-1 for Pb2+ and 5-20 mgL-1
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for Cd2+) and stirred at a speed of 200 rpm for different time period. After completion of adsorption phase, synthetic water with metal ions was centrifuged for 15 min at 6000 rpm and the supernatant
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solution was taken out to analyze the concentration of remaining metal ion concentration through AAS. Standard solution of lead nitrate and cadmium carbonate were used to analyze the amount of dissolved Pb2+ and Cd2+ ions in water using calibration curve. The adsorption competence (Qe) of monoliths for metal ion adsorption was calculated by the formula: Qe
( Co Ce ) V w
(1)
ACCEPTED MANUSCRIPT where Co is the initial and Ce is the equilibrium concentration (mg/L), while V is the volume of the solution (L) and w is the weight of the adsorbent (g). To control correctness, all adsorption experiments in the batch were executed in triplicates and the average value was reported.
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Result and discussion:
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The porosity of synthesized solid ZnO and TiO2@ZnO monoliths was evaluated by Brunauer-
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Emmett-Teller analyzer along with Barrett–Joyner–Halenda (BJH) plot. As per IUPAC classification, synthesized ZnO and TiO2@ZnO monoliths exhibits N2 sorption isotherms adjacent
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to typical Langmuir type – IV isotherm and categorized as H1 type hysteresis loop (with relative
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pressure, P/P0 = 0.30 to 0.90) which confirms the mesoporous characteristics (Fig. 1a). Textural and porous properties of synthesized monoliths are discussed in Table 1 and the data from BET
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equation shows that specific surface area of ZnO gets an increase after coating of TiO2. According
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to pore size distribution (Fig. 1b), synthesized monoliths holds mesoporous nature along with a small number of micropores (inset of Fig. 1b) and such kind of structures are favorable for water
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treatment.
Monoliths
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Table 1: Textural properties of ZnO and TiO2@ZnO monoliths. Me. D. (nm)
Mi. D. (nm)
Me. V. (cm3 g-1)
Mi. V. (cm3 g-1)
120
9.49
1.9
0.28
0.14
332
3.06
0.6
0.25
0.10
S.A.
ZnO
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(m2 g-1)
TiO2@ZnO
S.A. = Surface area; Me. D. =Mesopore diameter; Mi. D.= Micropore diameter; Me. V.= Mesoporous volume; Mi. V.=Micropore volume
Fig. 1c, display XRD patterns for the phase and purity of synthesized ZnO and TiO 2@ZnO monoliths. XRD confirms the formation of the hexagonal structure of ZnO. ZnO monolith shows dominant peaks for ZnO, 2Ө at 31.8°, 34.4°, 36.23, 47.5, 56.6 and 62.8° corresponding to lattice
ACCEPTED MANUSCRIPT planes of (100), (002), (101), (102), (110) and (103) respectively, which is confirmed by JCPDS card no. 36-1451 [15]. Where as in the case of TiO2@ZnO monoliths, a few peaks in the XRD patterns corresponding to anatase TiO2 were observed (JCPDF no. 21-1272). The morphological structures of synthesized monoliths determined by FESEM are shown in Fig. 2. Similar kind of
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interconnected macroporous structures having mesopores and micropores (a little amount) with in
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wall are obtained, but higher magnification images are difficult to obtain due to the charging of
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the sample. The clarity about the concentration of the synthesized metal oxide monoliths was confirmed by EDS analysis. Uniform distribution of the precursor TiO2 salt throughout the core of
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Adsorption studies for Pd(II) and Cd(II) ions:
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the ZnO monoliths was responsible for chemical homogeneity.
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Numerous factors like pH, contact time, concentration of adsorbate and adsorbent, play a
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significant part in the adsorption process of toxic pollutants like chemical nature of pollutant, chemical properties of the adsorbent and types of interaction occur between the adsorbent and
Effect of pH:
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pollutants.
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pH plays an important role in concluding the reaction rate of the surface reactions between adsorbate and adsorbents. Here, the influence of pH on adsorption competence was studied over the range from pH 2.0 to pH 7.0 and the best adsorption capacity was observed at pH 6 (Fig. 3 (ab)). Below pH 6, excessive protonation takes place on the surface of adsorbent which results in weak interaction between adsorbate and adsorbents. The increase of pH of solution follows the increase in quantitative adsorption efficacy [16, 17].
ACCEPTED MANUSCRIPT Effect of contact time: In this study, the impact of contact time with respect to varied amount of metal ions was studied between 10-15 min (Fig. 3 (c-d)). At pH 6, the initial concentration of metal ions used was 10 mg/L for both Pb2+ and Cd2+, respectively. Sorption process for metal ions was started even in
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small span of time due to fast initial stage adsorption which is followed by a second stage slow
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adsorption process until equilibrium was attained [18]. No considerable change in adsorption was
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observed after 80 min, so the reaction time for the supporting experiments was kept 150 min. The obtained results proved that the adsorption efficacy was directly proportional to the initial
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concentration of metal ions.
Adsorption kinetics:
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The type of adsorption mechanism preceding and potential rate controlling steps of metals
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ions adsorption process can be determined by kinetic studies. Also, chemical and physical properties of adsorbents along with the mass transfer rate of toxic ions from adsorbate onto
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adsorbent surface defines the adsorption mechanism [19]. Therefore, to extrapolate the type of
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adsorption mechanism, different kinetic models were used like pseudo first order (PFO), pseudo second order (PSO) and intraparticle diffusion (IPD) model. In this study, batch experiments were
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performed at pH 6 and 30 °C with 0.2 g/ L adsorbent amount. Pseudo first order (PFO) model: To explin the kinetic processes on solid surfaces at the time of adsorption, Lagergren reported the PFO equation in 1898 and was the first reaction formed to explain the adsorption process in solid and liquid state. The mathematical representation of PFO equation is expressed as [20]:
log(Qe Qt ) log Qe k1t
(2)
ACCEPTED MANUSCRIPT Where k1 is the PFO rate constant (min-1), Qe and Qt (mg/g) are a number of metal ions adsorbed at equilibrium and at time t. Pseudo second order (PSO) model: PSO was projected by Y.S. Ho in an effort to explain adsorption process of divalent metal ions at the surface of sphagnum moss [21]. He explained that
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Where k2 is the rate constant for PSO (g/mg min).
(3)
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t 1 t 2 Qt k2Qe Qe
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The mathematical representation of PSO equation is expressed as [20]:
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PSO is based upon sorption efficiency of the adsorbent that is linked to the existing active sites.
Plots showing PFO and PSO model showing adsorption of Pb2+ and Cd2+ ions on solid ZnO
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and TiO2@ZnO monoliths are presented in Fig. S1 (a-d), respectively and the correlation
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coefficients for kinetic models are evaluated from these linear plots (Table 2). From Fig. S1, we can conclude that both PFO and PSO can be applied to describe the kinetic mechanism but PSO
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shows the best fitting which states that the sorption process is typically monitored by
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chemisorption.
Table 2: PFO and PSO kinetic parameters for the adsorption of metal ions on ZnO and
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TiO2@ZnO. Model PFO
PSO
Pd2+
ZnO Cd2+
Qe (cal)
33.3
31.4
TiO2@ZnO Cd2+ 47.6 45.4
K1
0.083
0.10
0.092
0.13
r2
0.942
0.975
0.984
0.974
Qe (cal)
142.8
90.9
200
110.4
K2
0.0011
0.0008
0.002
0.0011
Parameters
Pd2+
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0.9816
0.989
0.993
0.990
Intraparticle diffusion (IPD) model: The mechanism of the adsorption process can be estimated through IPD model. In 1962, Webber Morris explained that Interaparticle diffusion model can be
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applied in 3 different forms [22]:
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1. Straight line plot for qt Vs t1/2 which is mandatory to forward through origin.
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2. Multi linear plot for qt Vs t1/2 which states that two or more diffusion steps are included in this
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process. In this procedure, at first step immediate adsorption occurs; at second step steady adsorption take place which mainly controls the intraparticle diffusion and equilibrium is known
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as final step where solute passes from macro/ mesopore to micro pores.
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3. Straight line plot for qt Vs t1/2 not mandatory to forward through the origin point.
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IPD model is known to review the ability and rate of diffusion can be calculated using
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mention equation:
Qt kit 0.5 I
(4)
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Where ki is the diffusion constant and I is the intercept. Figure 4 a-b shows a plot of Qt and t0.5, in
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which first linear line depicts the macropore and mesopore diffusion while the second linear line represents the micropore diffusion. Table 3 describes the intercept values for the intraparticle diffusion model.
Table 3: IPD parameter and regression coefficients for metal ion adsorption on ZnO and TiO2@ZnO monoliths. Model
Parameters ki
ZnO Pd2+
Cd2+
21.7
7.93
TiO2@ZnO Cd2+ 24.5 10.02
Pd2+
ACCEPTED MANUSCRIPT Intra particle diffusion model
I
1.63
1.79
2.06
3.45
R2
0.950
0.944
0.971
0.961
Equilibrium studies and modeling:
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The adsorption isotherms are explored to explain the steadiness of adsorption process in
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liquid- solid and gas liquid system [23]. In liquid- solid system, the connection among adsorbate
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and adsorbent is vibrant process and it offers suitable quantitation of adsorption efficiency in precise environment like pH, temperature, time. Here, to explain the connection among adsorbate
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and adsorbent using ZnO and TiO2@ZnO monoliths Langmuir and Freundlich isotherm model
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knowledge about the adsorption process.
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were used. All auxiliary experiments were performed at pH 6 and at 25◦C for 2h to get a better
Langmuir isotherm: Langmuir model is one of the prominent model used for the description of
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monolayer formation at the specific sites onto adsorbent surface. This model is focused on the idea
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that all the specific sites onto adsorbent surface has constant binding energy due which no development of strong bonds are possible on the surface of adsorbate. The mathematical
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representation of Langmuir model is expressed as [24]: Ce C 1 e Qe Qo b Qo
(5)
where Qe (mg/g) is the equilibrium concentration of metal ion on the adsorbent, Ce (mg/L) is the equilibrium concentration of metal ion, Q0 (mg/L) is the monolayer sorption ability and b is the Langmuir constant. Freundlich isotherm: It defines the heterogeneous adsorption process on surface and at specific active sites through discrete energy formed on multilayer adsorption. This model provides
ACCEPTED MANUSCRIPT estimation of sorption efficiency of adsorbate on an adsorbent. The mathematical representation of Langmuir model is expressed as [25]: log Qe log K f
log Ce n
(6)
where Ce (mg/L) is the equilibrium of metal ion concentration, Kf (mg1-1/nL1/n g-1) is
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adsorption capacity, and 1/n is the empirical parameter linked to adsorption intensity.
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The linear plots for Langmuir (Ce/Qe vs. Ce) and Freundlich isotherms (log Qe vs. log Ce) are shown in Fig. S2 (a-d). Values of slope and parameter of equilibrium isotherms were calculated
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through linear plots for Pb2+ and Cd2+ ion and are given in Table 4. By comparing the experimental data of isotherms based on the parameters obtained, it was realized that both the model were fitted.
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Freundlich isotherm describes the more fit than the other due to higher r2 values. A comparative
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of monolayer adsorption efficacy for Pb2+ and Cd2+ ions from literature are listed in Table 5 and
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thus it can be concluded that ZnO and TiO2@ZnO monoliths synthesized in this work were effective and satisfactory adsorbent for metal removal from waste water.
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Langmuir Model
Parameters
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Model
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Table 4: Isotherm parameters for metal ion adsorption on ZnO and TiO2@ZnO monoliths
Freundlich Model
Q0 b R2 Kf n R2
ZnO Pd2+ 790 1.25 0.965 17.9 3.25 0.975
Cd2+ 643 0.28 0.926 5.64 1.02 0.957
TiO2@ZnO Pd2+ Cd2+ 978 786 2.34 1.1 0.984 0.954 20.78 10.34 4.53 1.83 0.991 0.973
Table 5: A comparative account of Langmuir constants of heavy metal ions from reported literature. Adsorbents
Maximum adsorption
Conditions
References
ACCEPTED MANUSCRIPT
Fe2O3
79.35
68.9
--
TiO2
220
857
770
CCM 8000
469.8
71.95
--
Commercial activated carbon PAN-oxime fabrics Fe2O3/SiO2 monolith
4273
16.84
17.23
--
263.45
--
562
690
Carbon monoliths ZnO
1103
1128
120
790
TiO2@ZnO
332
AN
850
ED
M
989
786
[8] [26] [27] [28] [29]
t- 96 h
[30]
pH-6 T-30 °C t-120 min pH-6; T-30 °C; t-150 min pH-6 T-30°C t-150 min
[31]
[32] Present study
CE
PT
978
643
pH -7 t-120 min pH -5 T-45°C pH-6; T-30 °C; t-80 min t- 34 h; T- 30 °C pH- 7.8; t- 180 min
T
446.4
IP
369
CR
Cd2+
Effect of temperature:
(mg/g)
US
Pb2+
AF-Fe3O4
Surface area (m2/g) 25.94
The effect of temperature like change in entropy (∆ S), enthalpy (∆ H) and Gibbs free energy
AC
(∆ G) on adsorption process were attained by the following mathematical expression [33]: G RT ln b
ln b
S H R RT
(7) (8)
Where R is the ideal gas constant (8.314 Jmol-1K-1) and T is the temperature. The adsorption experiments were performed at different temperature (20, 25, 30 and 35 ℃) onto the surface of ZnO and TiO2@ZnO monolith. The change in enthalpy and entropy were evaluated from plot
ACCEPTED MANUSCRIPT between ln b (obtained from Langmuir isotherm) and 1/T (Fig. 5 a-b). Here, the adsorption process is identified as spontaneous and endothermic in support of positive enthalpy and negative Gibb’s energy. The outcomes of the thermodynamic parameters are displayed in Table 6. The positive values of entropy are due to the change of energy between metal ions and monoliths.
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T
Table 6: Thermodynamic parameters for adsorption of metal ion adsorption at different
Temperature
Pd2+
20
∆H (kJ mol−1) 6.1
ZnO ∆S (J mol K−1) 21.4
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25 30
9.2
20
ED
25
34.4
TiO2@ZnO ∆H (kJ ∆S (J mol ∆G (kJ mol−1) K−1) mol−1) 6.4 25.5 -0.92
-1.28
-1.36
-1.53
-1.61
-1.78
-1.87
-0.95
15.5
45.6
-0.97
-1.36
-1.55
-1.69
-2.06
-2.09
-2.67
CE
PT
30 35
M
35 Cd2+
∆G (kJ mol−1) -0.93
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Metalions
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temperatures.
Reusability of adsorbent:
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The efficiency of porous ZnO and TiO2@ZnO monolith is further evaluated for their repeated regeneration for metal ions removal experiments using 1 M NaOH [34] and found that the synthesized porous ZnO and TiO2@ZnO monolith shows good reusability up to 3 consecutive cycles as shown in Fig. 5 c-d. The obtained results show that adsorption efficacy of porous ZnO and TiO2@ZnO monolith for metal ion adsorption gradually decreases with repeated use of monoliths because a maximum number of adsorption sites get engaged due to the interactions between adsorbent and adsorbate. The good reusability of mesoporous ZnO and TiO2@ZnO
ACCEPTED MANUSCRIPT monoliths is due to the micrometer-sized structure with high surface area, which has a benefit of reducing the overall cost for adsorption process. Furthermore, desorption conditions are more simple and accessible as no specific filtration techniques are required for adsorbent extraction due to its large size.
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Conclusion:
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In this work, Mesoporous ZnO and TiO2@ZnO monoliths synthesized via nanocasting method were utilized as an adsorbent for adsorption of Pb2+ and Cd2+ ions from aqueous solution.
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For adsorption process of metal ions, all the auxilary experiments were executed in different batches in precise environment like pH, temperature, time. Kinetic and thermodynamic
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experiments verified that metal ion adsorption process was chemisorption, spontaneous and
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endothermic in nature. The original outline of this work is utilization of synthesized mesoporous
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ZnO and TiO2@ZnO monoliths for removing heavy metal ions from aqueous wastewater to
Acknowledgments:
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generate clean water.
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The financial assistance for the validation of this research work was offered by DAE/ BRNS (34/14/63/2014), Mumbai, India and other instrument/infrastructure facilities were
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provided by TIET, Patiala, Punjab and Sprint Testing Solutions-Mumbai.
Conflicts of interest: The authors declare that there is no conflict of interests regarding the publication of this paper.
References:
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surface area of silica monoliths by controlling ionic character/chain length of surfactants and polymers, Materials Letters, 194 (2017) 213-216. [15] A. Gomes, T. Frade, K. Lobato, M.E.M. Jorge, M.I. da Silva Pereira, L. Ciriaco, A. Lopes, Annealed Ti/Zn-TiO2 nanocomposites tested as photoanodes for the degradation of Ibuprofen, Journal of Solid State Electrochemistry, 16 (2011) 2061-2069.
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Structure and Adsorption Efficiency of Hydroxyapatite Nanorods, Journal of Nanoscience and
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[19] M. Sharma, A. Mishra, A. Mehta, D. Choudhury, S. Basu, Effect of Surfactants on the
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Structure and Adsorption Efficiency of Hydroxyapatite Nanorods, Journal of nanoscience and nanotechnology, 18 (2018) 623-633.
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[20] C. P. Bergmann, F. M. Machado, (Ed.), Carbon Nanomaterials as adsorbents for
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equilibrium studies, Journal of Chemical Technology & Biotechnology, 90 (2015) 2170-2179. [23] M. Athar, U. Farooq, M. Aslam, M. Salman, Adsorption of Pb(II) ions onto biomass from Trifolium resupinatum: equilibrium and kinetic studies, Applied Water Science, 3 (2013) 665-672. [24] H.-J. butt, Karlheinz Graf, M. Kappl, Physics and Chemistry interfaces, WILEY-VCH GmbH & Co. KGaA, Germany, 2003.
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[28] Y.P. Teoh, M.A. Khan, T.S.Y. Choong, Kinetic and isotherm studies for lead adsorption from
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Oxides and their Application in Heavy Metal Ion Removal, Advanced Materials, 20 (2008) 2977-
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ACCEPTED MANUSCRIPT Adsorption of Heavy Metal Ions by Mesoporous ZnO and TiO2@ZnO Monoliths: Adsorption and Kinetic Studies Manisha Sharma§, Jasminder Singh⸸, Satyajit Hazraǂ and Soumen Basu⸸* §
Division of Chemistry, Kyoto University, Kitashirakawa, Sakyo-ku, Kyoto 606-8502, Japan.
⸸
School of Chemistry and Biochemistry, Thapar University, Patiala, Punjab-147004, India.
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Institute of Nuclear Physics, Kolkata 700064, India.
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ǂSaha
Corresponding E-mail: [email protected]
150 100
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50 0 0.2
TiO2@ ZnO
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ZnO;
0.0
0.4
0.6
0.8
0.4
0.00
ZnO;
25
1.2
1.6
30
2.0
TiO2@ ZnO
35
40
45
50
Pore diameter (nm)
Z (103)
T (004)
1600
T (211) Z (110)
TiO2@Zno Z (102)
T (101)
20
15
10
5
1.0
Z (100) Z (002)
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2400
0.8
Pore diameter (nm)
Z (101)
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3200
ZnO
0.01
Relative Pressure (P/P0) 4000
TiO2@ ZnO
3
0.02
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TiO2@ZnO SA: 332 m2/g
(b)
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3
ZnO SA: 120 m2/g
200
Vol. adsorbed (cm /g)
250
3
Vol. adsorbed (cm /g)
0.03
(a)
Vol. adsorbed (cm /g)
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Figures
800 0
ZnO 20
25
30
35
40
45
50
55
60
65
2 (degree)
Fig. 1: a) N2 sorption curve, b) BJH plot showing pore size distribution (inset contains micropore distribution curves through MP plot) and c) XRD pattern for ZnO and TiO2@ZnO monoliths.
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(a)
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1 µm
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(b)
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1 µm
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Fig. 2: SEM and EDS spectra for a) ZnO and b) TiO2@ ZnO monoliths.
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(b) Cd(II) adsorption
80
60
ZnO;
60
40
T
80
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% Adsorption
% Adsorption
100
(a) Pb(II) adsorption
100
20
TiO2@ZnO
3
4
5
6
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40 7
3
pH 100
(c) Pb(II) adsorption
5
TiO2@ZnO 6
7
pH
(d) Cd(II) adsorption
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100
4
ZnO;
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% Adsorption
80
60
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40
20
ZnO; 0
20
40
60
80
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% Adsorption
80
100
120
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Time (min)
TiO2@ZnO 140
160
60
40
20
ZnO;
TiO2@ZnO
80
120
0 0
20
40
60
100
140
160
Time (min)
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Fig. 3: Plot for effect of pH (a-Pb2+and b-Cd2+) [adsorbent amount: 0.02 g/L]; effect of time (cPb2+and d-Cd2+) for ZnO and TiO2@ZnO monoliths. [agitation speed: 200 rpm, temperature: 25 °C].
ACCEPTED MANUSCRIPT
240
(a) Pb(II) adsorption
80
(b) Cd(II) adsorption
60
120 80
40
20
T
160
Qt(mg/g)
Qt(mg/g)
200
0
2
4
0
TiO2@ ZnO 6
8
ZnO;
0
10
0.5
t
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ZnO;
0
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40
2
4
TiO2@ ZnO 6
8
10
0.5
t
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Fig. 4: Plots showing intra-particle diffusion model (a- Pb2+ and b- Cd2+) for ZnO and TiO2@ZnO monoliths
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5.2 6.8
(a) Pb2+ adsorption
5.0
6.4
4.8
6.0
4.4 4.2
5.6 5.2 4.8
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4.6
4.4
4.0
0.00320
4.0
TiO2@ZnO 0.00325
0.00330
0.00320
0.00335
-1
1/T(K ) ZnO;
80
0.00325
0.00330
0.00335
-1
ZnO;
(d)
TiO2@ZnO
Cd2+ adsorption
60
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% Removal
60
40
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% Removal
TiO2@ZnO
(c) Pb2+ adsorption
80
TiO2@ZnO
1/T(K )
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100
ZnO;
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ZnO; 3.8
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ln(Qe/Ce)
ln(Qe/Ce)
(b) Cd2+ adsorption
0 1
2
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20
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No. of cycle
3
40
20
0 1
2
3
No. of cycle
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Fig. 5: Plot showing effect of temperature for adsorption studies on ZnO and TiO2@ZnO monoliths (a- Pb2+ and b- Cd2+) and recyclability of monoliths (c- Pb2+ and d- Cd2+)
ACCEPTED MANUSCRIPT Highlights
Mesoporous ZnO & TiO2@ZnO monoliths were synthesized by nanocasting method.
The synthesized monoliths were superior in Pb2+ and Cd2+ adsorption from aqueous solution. Freundlich model was found to be more fitted for the adsorption process.
Thermodynamic experiments confirmed the spontaneous & endothermic nature of
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adsorption.