Zn2+âSilica Modified Cobalt Ferrite Magnetic Nanostructured

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Zn –silica modified cobalt ferrite magnetic nanostructured composite for efficient adsorption of cationic pollutants from water Arundhati Sengupta, Rohan R Rao, and Dhirendra Bahadur ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b01186 • Publication Date (Web): 09 Jan 2017 Downloaded from http://pubs.acs.org on January 19, 2017

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ACS Sustainable Chemistry & Engineering

Zn2+–silica modified cobalt ferrite magnetic nanostructured composite for efficient adsorption of cationic pollutants from water Arundhati Sengupta, Rohan Rao and D. Bahadur* Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology Bombay, Powai, Mumbai - 400 076, India

* Corresponding author. Tel.: +91 22 2576 7632; Fax: +91 22 2572 3480. E-mail address: [email protected] (D. Bahadur).

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ABSTRACT Zn2+–silica modified CoFe2O4 (CZFS) nanostructured composite, useful for adsorbing cationic pollutants from water, was prepared by a wet-chemical method. The composite comprises cubic spinel crystallites (average 18 nm size) with amorphous silica clusters decorated on the crystallites-surface. Improved surface area 59.8 m2/g of CZFS over those of Zn2+ modified CoFe2O4, CZF (32.6 m2/g), and CoFe2O4, CF (42.8 m2/g), together with its high negative zeta potential −35.4 mV (from surface SiO−) provides CZFS with improved adsorption capacity for Methylene blue (MB) over that of CZF and CF. MB adsorption (initial adsorbate concentration C0 = 5–25 mg/L) conforms to the Langmuir isotherm model, with maximum monolayer adsorption capacity Qm = 25.6 mg/g. CZFS exhibits adsorption efficiency Ae ≥ 98% for removal of heavy metal ions Cr3+, Cu2+ and Pb2+ (C0 = 5 mg/L). High Ae = 99.9% for Pb2+ dropped only to Ae = 98.8% for higher C0 = 20 mg/L (Qm = 19.8 mg/g). Saturation magnetization of 39 emu/g enables easy magnetic separation of CZFS from water. Good reusability of CZFS adsorbent was observed for up to three cycles. In summary, CZFS efficiently removes MB as well as heavy metal ions (especially Pb2+) from contaminated water.

KEYWORDS: Magnetic nanostructure, Surface properties, Adsorption, Dye, Heavy metal ions, Water purification

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INTRODUCTION Cobalt ferrite (CoFe2O4) has a partially inverse spinel structure with Co2+ and Fe3+ ions distributed among the tetrahedral (T) and octahedral (O) sites as expressed by the formula [CoxFe1−x]T [Co1−xFe1+x]OO4.1,2 Apart from being a ferrimagnetic material with high saturation magnetization (Ms) value,1,3 CoFe2O4 in nanodimension is known for good surface properties useful for adsorbing foreign molecules or ions.1,4,5 Magnetic nanostructured composite materials have been widely exploited as adsorbents in waste-water remediation as these can be easily separated from water by using an external magnet.6–9 Magnetic separation is faster and more efficient than gravity separation and it is better than separation by centrifugation which may cause desorption of loosely bound adsorbates if the rpm is not optimal. Simultaneous removal of organic and inorganic contaminants from waste-water by an efficient as well as low-cost method is still a global challenge. While conventional methods like ion-exchange, oxidation, coprecipitation, membrane based filtration and foam floatation have drawbacks in terms of costeffectiveness, performance efficiency, applicability for a wide range of pollutants, or reusability,10,11 adsorptive removal of pollutants is advantageous since a proper selection of the adsorbent (hybrid/ composite material) can address the drawbacks well. Research is on-going to find an effective replacement for activated carbon adsorbent which is costly to produce and regenerate/ reuse.12,13 Efforts have been made to differently modify CoFe2O4 nanoparticles and composites to achieve desired adsorption properties for removal of toxic dye molecules14,15 and heavy metal ions16–18 such as As3+, Pb2+ and Hg2+ from waste-water. Porous CoFe2O4 based nanocomposite synthesized using egg albumen14 and Al3+/ In3+/ Cu2+ modified CoFe2O41 have been explored for adsorption of Brilliant blue-R and Congo red (both anionic) dyes, respectively. CoFe2O4 composited to multiwalled carbon nanotube (MWCNT) functionalized with –COOH has been used to adsorb Rhodamine B (cationic dye).15 CoFe2O4 nanoparticles aggregated during formation of schwertmannite is found effective for removal of As3+,17 while thiol functionalized CoFe2O4 has shown good removal efficiency for Pb2+ and Hg2+.16,18 Modifying CoFe2O4 with Zn2+, or a partial Zn2+→ Fe3+/Co2+ substitution in CoFe2O4, is known to improve chemical stability,

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CoyZnxFe3−(x+y)O4.19,20 Compositing CoFe2O4 with SiO2 provides better conjugation with various functional groups (useful for adsorption of varied toxic molecules or ions) along with improved thermal and chemical stability of the composite.20,21 A layer of SiO2 on CoFe2O4 can provide a non-toxic surface protecting the ferrite core from oxidation.21,22 In this paper, we report surface and adsorption properties of Zn2+–silica modified CoFe2O4 nanocomposite synthesized by a wet-chemical route. Methylene blue (MB) dye, a commonly found cationic organic pollutant in waste-water from textile industries, and heavy metal ions (Mn+ = Cr3+, Ni2+, Cu2+, Cd2+, Hg2+, and Pb2+) have been used as model adsorbates for studying their adsorptive removal from water using the magnetic nanocomposite adsorbent. While MB is known to cause eye damage, heavy metal ions are known for toxic effects upon consumption above respective permissible limits (0.003–2 mg/L23 as per the Mn+). Studies described here help us characterize Zn2+–silica modified CoFe2O4 and assess its suitability for waste-water remediation.

EXPERIMENTAL DETAILS Materials. Cobalt nitrate hexahydrate [Co(NO3)2·6H2O], ferric nitrate nonahydrate [Fe(NO3)3·9H2O], tetraethyl orthosilicate [(C2H5O)4Si] (all ≥98%) and NaOH pellets were purchased from Merck. Anhydrous Zinc chloride (ZnCl2) (99%) was purchased from Sigma Aldrich. Ethanol (99%), NH3 solution (28%), HCl (35%) and Milli-Q water were used for various experiments. MB dye (analytical grade) and various metal salts Cu(NO3)2·3H2O, PbCl2, HgCl2, CdCl2·H2O, CrCl3·6H2O and Ni(NO3)2·6H2O (all ≥98%) from Fluka were used for adsorption studies.

Synthesis of CoFe2O4 (CF), Zn2+ modified CoFe2O4 (CZF) and Zn2+–silica modified CoFe2O4 (CZFS). Typically, to obtain 2.0 g CoFe2O4 (CF), at first Co(NO3)2·6H2O (0.124 g/mL) and Fe(NO3)3·9H2O (0.344 g/mL) aqueous solutions were mixed in equal volumes in a total of 40 mL. This was then added to 40 mL 2 M NaOH solution taken in a flask with continuous stirring for 15 min at room temperature to enable the reactants to mix homogeneously and allow complete precipitation of the product (black color). The flask was then heated in an 4


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oil-bath at 65 °C for 1 h. Thereafter, the product was cooled to room temperature and aged for 10 h. It was then magnetically separated from solution, washed thoroughly and dried in air at 65 °C to obtain a fine black powder (FBP) which was divided into three parts. While one part was annealed at 600 °C for 2 h to obtain CF, the other two parts were modified with Zn2+ and Zn2+– silica. To obtain Zn2+ modified CoFe2O4 (CZF), 160 mL water–ethanol (1:1.3) and 21 mL NH3 solutions were added to 1.499 g FBP with stirring for 15 min. Then, 20 mL aqueous ZnCl2 solution (0.119 g/mL) was added with stirring continued for 4 h allowing the reactants to mix homogeneously and ZnCl2 to hydrolyse completely. A black product thus obtained was separated from solution, washed, dried and annealed at 600 °C (2 h) to obtain CZF. To obtain Zn2+–silica modified CoFe2O4 (CZFS), a mixture comprising 1.499 g FBP, 195 mL water–ethanol (1:2) and 15 mL NH3 solution was sonicated for 15 min to obtain a homogeneous dispersion. To this, 0.492 mL (C2H5O)4Si was added and the mixture was stirred for 6 h allowing complete hydrolysis of (C2H5O)4Si. This was magnetically separated from solution and washed with water. Then, 160 mL water–ethanol (1:1.3) and 21 mL NH3 solutions were added with stirring for 15 min. Subsequently, 20 mL of aqueous ZnCl2 solution (0.119 g/mL) was added with stirring continued for 4 h. A dark-brown product thus obtained was separated from solution, washed, dried and annealed at 600 °C (2 h) to obtain CZFS.

Characterization techniques. X-ray diffraction (XRD) patterns of CF, CZF, and CZFS powders were recorded on a diffractometer (XPERT-PRO PANalytical) using CuKα radiation (wavelength λ = 0.15406 nm) over 5–80° diffraction angle (2θ) at 0.05°/s scanning rate. Microstructure was studied using a field emission gun scanning electron microscope (FEG-SEM) at 10 kV (JEOL, JSM-7600F) and a field emission gun transmission electron microscope (FEGTEM) at 200 kV (JEOL, JEM-2100F). Sample compositions were confirmed with energy dispersive X-ray spectroscopy (EDS) performed in combination with FEG-SEM imaging. Inductively coupled plasma - atomic emission spectroscopy (ICP-AES) using ARCOS, Simultaneous ICP Spectrometer (SPECTRO Analytical Instruments GmbH) and X-ray photoelectron spectroscopy (XPS) recorded on a Kratos Analytical (AXIS Supra) X-ray photoelectron spectrometer (using Al-Kα excitation source) were used to further confirm sample compositions. Vibrational spectra were recorded on a Fourier transform infrared (FTIR) 5



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spectrometer (3000 Hyperion Microscope, Vertex 80). Surface features were studied with a surface area and porosity analyzer (Micromeritics ASAP 2020), using N2 as the adsorptive at −195.6 °C. The samples were degassed at 200 °C for 4 h before measurement. Zeta potential (ζ) of the samples in aqueous dispersion (at pH = 6.5) were studied with an instrument Zetasizer nano series (Malvern Instruments). Magnetic properties were measured with a vibrating sample magnetometer (Lakeshore). Adsorption of model cationic pollutants (MB and Mn+) by the samples were studied at room temperature (at pH = 6.5). Residual MB concentration in water after MB adsorption by CF, CZF or CZFS adsorbent was determined from the optical density (OD) value at λ = 665 nm (corresponding to maximum OD for MB) using a UV-visible spectrophotometer (CECIL, CE3021) and a calibration plot (OD versus concentration) for known MB concentrations. Concentration of residual Mn+ in water after Mn+ adsorption by CF, CZF or CZFS was determined from ICP-AES.

Adsorption studies. To study MB adsorption by CF, CZF, and CZFS, 50 mg sample (adsorbent) was added to 80 mL aqueous MB solution (initial adsorbate concentration C0 = 5–25 mg/L). The mixture was sonicated for 5 min and then shaken continuously for 24 h. A 1 mL aliquot collected from the dispersion at time t = 24 h was centrifuged (2000 rpm; 5 min). UVvisible spectrum of the supernatant was analyzed to determine the final adsorbate concentration (Ce) in the solution at adsorption–desorption equilibrium. Adsorption efficiency (Ae) and adsorption capacity (Qe) of the adsorbent were obtained from relations: Ae = (C0 – Ce)/C0 and Qe = (C0 – Ce) V/M, respectively, where V = initial volume of adsorbate solution, M = mass of adsorbent. To study time dependence of MB adsorption capacity, 1 mL aliquots were taken at different times over t = 5–150 min and the concentration Ct of adsorbate at time t was determined from UV-visible spectroscopy. The quantity of adsorbate adsorbed at time t was obtained from: Qt = (C0 − Ct) V/M. To test the reusability of CZFS for MB adsorption, CZFS was recovered using a magnet from MB solution (C0 = 10 mg/L) after an adsorption experiment, then washed with acetone, 0.01 M HCl, and finally water (several times) before reuse in subsequent experiments (up to three cycles). Mn+ adsorption was studied by adding 50 mg adsorbent to 40 mL aqueous Mn+ solution (C0 = 5–25 mg/L, confirmed with ICP-AES) and shaking continuously for 24 h. The dispersions 6


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obtained (with different Mn+) were then placed on a magnet to separate the adsorbent from solution. The supernatants were collected for each Mn+–adsorbent system and analyzed in terms of the Mn+ concentration using ICP-AES. The Ae and Qe values of the adsorbent for different Mn+ were obtained. Reusability of CZFS for Mn+ adsorption was tested by recovering CZFS with a magnet from Mn+ solution (C0 = 5 mg/L) after an adsorption experiment, washing it with 0.02 M HCl and water before reuse in subsequent experiments (up to three cycles). Kinetics studies were performed on Pb2+ solutions (C0 = 5–25 mg/L) by adding 50 mg CZFS to 80 mL Pb2+ solution, shaking for 24 h, and obtaining the Qt-values at different t-values.

RESULTS AND DISCUSSION Structure and physical properties. Structure and physical properties of CZFS powder were studied in comparison to those of CZF and CF powders. XRD pattern of CF (Figure S1a; Supporting Information) shows a single phase of CoFe2O4 nanocrystallites. In CZF (Figure S1b) and CZFS (Figure S1c), the XRD peaks are seen to shift towards lower 2θ-values with respect to those of CF, confirming formation of Zn2+ substituted CoFe2O4 [CoyZnxFe3−(x+y)O4] nanocrystallites (structural parameters summarized in Table S1). CZFS exhibits an amorphous silica phase accompanying CoyZnxFe3−(x+y)O4 crystallites. XPS analyses of the samples (Figure S2, a-e) support the XRD results (detailed in Supporting Information). Microstructure (FEGSEM and FEG-TEM images) of CF (Figure S3a,b) and CZF (Figure 1a) show particles (cuboids) with average cross-width W ~ 25 nm (fairly close to the average crystallite size of CF and CZF as per XRD line broadening in Figure S1a,b). Lattice images of CF and CZF (magnified region ‘A’ from inset of Figure 1a for CZF) in Figure S3c and Figure 1b, respectively, show (111) oriented crystallites. Electron diffraction patterns from CF (Figure S3d) and CZF (Figure 1c) support the findings from XRD data. On silica-modification, the microstructure of CZFS particles (Figure 1d) show decreased W ~ 18 nm. The FEG-TEM image in Figure 1d (inset) shows cuboids decorated with irregular-shaped clusters (presumably amorphous silica). Magnified region ‘B’ in Figure 1e shows a (222) oriented CZFS crystallite attached to amorphous clusters (silica). Electron diffraction pattern from CZFS (Figure 1f) conforms to CoyZnxFe3−(x+y)O4 crystallites with coexistant amorphous silica (evident from the halo near (111) 7



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ring, consistent with the XRD in Figure S1c). Figure 2 shows a schematic representation of (a) CZF cuboids and (b) CZF cuboids decorated with amorphous silica clusters in a hybrid CZF– silica composite structure which suggests that the surface properties of this composite would arise from both CZF and silica as per the exposed surface of each phase. Elemental composition/ distribution in CZF and CZFS are given in the Supporting Information (Figures S4a-f and S5ag). FTIR spectra of CF, CZF, and CZFS, given in the Supporting Information (Figure S6a-c), further confirm their structure and phases. Surface area and pore nature of the samples were studied with the help of N2 adsorption-desorption isotherms and the Barrett-Joyner-Halenda24 (BJH) pore size distributions (Supporting Information; Figures S7 and S8). The BrunauerEmmett-Teller (BET) surface area (SBET) is highest for CZFS (59.8 m2/g), followed by CF (42.8 m2/g) and CZF (32.6 m2/g). The BJH average pore diameter Da = 8.5 nm for CZFS stands lower than that of CF (14.8 nm) and CZF (26.4 nm). Study of ζ-values of the samples show a high negative ζ = −48.4 mV in CF, consistent with the fact that it has abundant adsorbed CO32− along with surface OH− (refer FTIR analysis; Figure S6a). A lowered value ζ = −27.9 mV in CZF (with significantly lower adsorbed CO32− content; refer Figure S6b) improves to ζ = −35.4 mV in CZFS owing to SiO− groups (from deprotonation of Si–OH in water)25,26 of amorphous silica clusters decorated on CoyZnxFe3−(x+y)O4 crystallites (refer Figure S6c). The acidic nature of CZFS surface is further established from a ζ versus pH plot for CZFS aqueous dispersion (pH over 2 to 11) in Figure S9, with point of zero charge obtained at pH = 2.3. The SBET, Da, and ζ values govern the adsorption characteristics of the samples. Magnetic properties of the samples are described in the Supporting Information (Figure S10a-c). A net Ms = 39 emu/g for CZFS is good enough to allow it to be easily separated from water using a magnet.

Adsorption of MB. To understand effectiveness of CZFS with mesopores and negative ζ-value for adsorbing cationic pollutants from water, we first observed its Ae-value for different C0-values of MB as given in Figure 3a. An Ae–value = 99.3% for C0 = 5 mg/L drops only slightly to Ae = 96.0% for C0 = 15 mg/L, but then drops sharply to nearly Ae ~ 70% for C0 = 20–25 mg/L. This can be explained considering how the MB molecules distribute between the phases — CZFS adsorbent and water, at equilibrium. The relation between the two phases at equilibrium 8


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can be described with two well-known isotherm models by Langmuir and Freundlich,27–29 as shown in Figure 3b and Figure 3c, respectively. The Langmuir model, which assumes a homogeneous adsorbent surface with a monolayer of adsorbate molecules, can be expressed linearly as: Ce/Qe = 1/(KLQm) + Ce/Qm, where KL is the Langmuir adsorption constant (related to energy of adsorption) and Qm refers to the maximum monolayer adsorption capacity of the adsorbent. The separation factor RL, related to the nature of adsorption and shape of the Langmuir isotherm, is given by: RL = 1 / (1 + KLC0). A linear form of the Freundlich model, which assumes a heterogeneous adsorbent surface, is given by: log Qe = log KF + 1/f (log Ce), where KF is the Freundlich adsorption constant (related to adsorption capacity) and 1/f is the heterogeneity factor (related to adsorption intensity). From a linear fitted plot of Ce/Qe versus Ce for the CZFS–MB adsorbent–adsorbate system in Figure 3b, the Qm-value obtained is 25.6 mg/L, with KL = 6.8 L/mg and RL = 0.028– 0.006 (for C0 = 5–25 mg/L). A comparison of the Qm-value obtained here with those of other magnetic adsorbents9,30–34 (considering adsorbent concentration and initial pH of adsorbate solution) is given in Table 1. It shows that the Qm-value for CZFS is much better than those of magnetic-MWCNT,30 sponge-like porous MnFe2O4,31 and CoFe2O4/MWCNT.34 Low RL-value obtained here (0 < RL < 1) indicates a highly favorable adsorption of MB molecules onto CZFS surface. A linear fitted plot of log Qe versus log Ce for CZFS–MB system in Figure 3c shows a poor correlation coefficient R2 = 0.709, with KF = 18.7 (mg/g)(L/mg)1/f and f = 5.0. A high fvalue (> 2) indicates a very good adsorption process (consistent with findings from Figure 3b). A poor R2 value indicates that MB adsorption onto CZFS does not follow the Freundlich model, but rather follows the Langmuir model for C0 = 5–25 mg/L. The results essentially support a homogeneous CZFS surface which adsorbs a monolayer of MB molecules. Furthermore, it indicates that once the available surface sites on CZFS are filled-up with MB monolayer, the MB-removal efficiency (Ae-value) of CZFS would drop sharply. Thus, above a certain optimal MB concentration, i.e., close to 15 mg/L, the Ae-value drops from ≥ 96% to ~70% for 20–25 mg/L MB (Figure 3a). UV-visible spectra of supernatent MB solutions taken after different times (t = 0–55 min) of contact of CZFS with MB solution (C0 = 10 mg/L) in Figure 4a shows that the OD-value at 9



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665 nm drops sharply by 84% in just 5 min, before gradually dropping to nearly 0 at t = 55 min. Reusability test of CZFS with MB solution (C0 = 10 mg/L) in Figure 4b shows fairly good results up to three cycles, with the Qe-value dropping by only ~10% of the initial value after the third cycle. Time dependence (t = 0–150 min) of MB adsorption capacity (Qt) of CZFS for C0 = 5–25 mg/L is given in Figure 4c. Adsorption capacity of CZFS is seen to improve upon increasing the C0-value from 5 mg/L (Qe = 7.45 mg/g) to 15 mg/L (Qe = 24.35 mg/g). This is because an increase in the number of available MB molecules provides a higher driving force for overcoming the diffusion resistance between the adsorbate and adsorbent phases. For C0-values over 15–25 mg/L, the Qt versus t plots nearly merge (Qe = 27.04 mg/g for C0 = 25 mg/L), indicating adsorption saturation of MB molecules on CZFS (consistent with findings from Figure 3a). This phenomenon of rising Qe-value with increasing C0-value until Qe-value saturates for an optimal C0-value is a well-known trend for monolayer adsorption process.29 The trend of the Qevalues conforms to the pseudo-second order kinetic model,35 described in the Supporting Information (Figure S11 and Table S2). Figure 4d compares Qt versus t plots for CF, CZF and CZFS (C0 = 15 mg/L). It shows that the adsorption capacity of CZFS for MB (Qe = 24.35 mg/g) is far better than that of CF (Qe = 3.79 mg/g) and CZF (Qe = 0.63 mg/g). For CF, the Qt-value initially rises above that of CZFS for up to 15 min (due to higher negative ζ-value of CF than that of CZFS), but soon drops progressively below that of CZFS with increasing time. This is because while a high negative ζvalue of CF causes cationic MB molecules to quickly approach and attach to CF surface (via mainly CO32− functionalities), the attachment 2MB+…CO32− is not strong enough to persist stably in the solution with shaking for prolonged time. An attachment MB+…SiO− on the surface of CZFS is retained and is stable (stronger electrostatic interaction), depicting the role of amorphous silica clusters on CZFS surface in improving the adsorption performance of CZFS for MB over that of CF and CZF. Silica nanosheets36 have shown to exhibit Qe = 6.88 mg/g (for C0 ~16 mg/L), nearly 3.5 times lower than that of CZFS. Thus, the Qe-value of CZFS nanocomposite seems to be synergistically improved over those of CF / CZF and silica components. A comparison of FTIR spectra of bare CZFS with that of MB adsorbed CZFS is given in Figure S12 (a, b). 10


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Adsorption of Mn+. Comparison of Ae-values of CF, CZF, and CZFS for different Mn+ with C0 = 5 mg/L (Figure 5a) shows that for CF sample, Ae ≥ 90% for each Mn+, consistent with its high negative ζ-value. For CZF having lower ζ-value, the Ae-values decrease to between 49– 72% for different Mn+ with the exception of Pb2+ for which Ae = 99%. For CZFS with ζ-value inbetween that of CF and CZF, Ae = 38–69% for Ni2+, Cd2+, Hg2+ and Ae ≥ 98% for Cr3+, Cu2+, Pb2+. Adsorption behavior of Mn+ onto different adsorbents can be explained considering several factors which influence the diffusion resistance of adsorbate and the adsorbent–adsorbate interaction. Some of these are: electronegativity (EN), ionic radius (ri) of Mn+, surface charge/ functionality, surface area and pore nature of the adsorbent. For Cr3+ adsorption by CZFS, a relatively low ri = 0.062 nm (ri = 0.069–0.119 nm for other Mn+ used here) possibly plays a dominant role in reducing the diffusion resistance, thereby improving the Ae-value [although its EN = 1.66 is lower than those of Ni2+ (1.91), Cd2+ (1.69) and Hg2+ (2.00)]. Higher EN-values of Cu2+ (1.90) and Pb2+ (2.33) than that of Cd2+ (1.69) explain a better Ae-value for Cu2+ and Pb2+ (from stronger electrostatic interaction between Mn+ and CZFS surface) as compared to that for Cd2+. Improved Ae-value for Cu2+ and Pb2+ over Ni2+ and Hg2+ is possibly influenced by other factors such as: hydrated radii of Mn+ and the exact nature of interaction between CZFS surface and Mn+, i.e., an electrostatic attraction as SiO−…Mn+ or formation of a (SiO)nM surface complex. While a strong adsorbent–adsorbate electrostatic interaction should improve the Aevalue in general, formation of complexes at the adsorbent surface may decrease the Ae-value.7 Larger hydrated radius of Mn+ can reduce the strength of SiO−…Mn+ electrostatic interaction and decrease the Ae-value. It is promising that Ae > 99% for Pb2+ irrespective of the adsorbent CF, CZF or CZFS (99.9% using CZFS). Comparison of Ae-values of CZFS for different Mn+ with C0-values varied as 5, 10 and 20 mg/L (Figure 5b) shows that the Ae-value decreases in general with increasing C0-value of Mn+ owing to increased diffusion resistance at higher adsorbate concentration. However for Pb2+, the Ae-value does not drop much and reaches Ae = 98.8% at C0 = 20 mg/L (from Ae = 99.9% at C0 = 5 mg/L) indicating highly favorable diffusion path for Pb2+ ions and CZFS–Pb2+ (SiO−…Pb2+) interaction. FTIR spectra of Pb2+ adsorbed CZFS is given in Figure S12c. Reusability of CZFS for different Mn+ (C0 = 5 mg/L) for up to three cycles (Figure 5c) shows encouraging results, 11



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with the Ae-value dropping by only 2–4% of the initial value after the third cycle (reusability for C0 = 10 mg/L given in Figure S13). A test of leaching of Co, Fe, or Zn from CZFS (compared with CZF or CF) during acid washing of CZFS (for reusability) given in Table S3 shows improved resistance to acid leaching in Zn2+ modified samples. Since the highest Ae-value (99.9%) is obtained for Pb2+ adsorption by CZFS among other Mn+ used here, we studied Pb2+–CZFS adsorbate–adsorbent system in terms of the adsorption isotherms and time dependence of the adsorption. A linear fitted Ce/Qe versus Ce plot (Langmuir model) in Figure 6a exhibits R2 = 0.960, with Qm = 19.8 mg/g, KL = 23.9 L/mg, and RL = 0.002– 0.009 for C0 = 5–25 mg/L. A linear fitted log Qe versus log Ce plot (Freundlich model) in Figure 5b shows improved R2 = 0.986, with KF = 24.1 (mg/g)(L/mg)1/f and f = 3.1. The RL and f-values obtained from the two models support a highly favorable adsorption of Pb2+ by CZFS. Better R2 value for the Freundlich model (Figure 6b) suggests an inhomogeneous adsorption of Pb2+ on CZFS surface. Table 2 compares the Qm-value for Pb2+ adsorption by CZFS with respect to other magnetic adsorbents.37–42 It shows that the present Qm-value is improved over those of carbon coated Ni37 and Fe3O4/SiO2-NH238 nanocomposites, and Fe3O4 nanospheres.40 Variation of Pb2+ adsorption capacity (Qt) of CZFS with time (t = 0–24 h) for C0 = 5–25 mg/L in Figure 7 shows that the adsorption capacity improves upon increasing the C0-value from 5 mg/L (Qe = 5.77 mg/g) to 25 mg/L (Qe = 39.44 mg/g). This is expected owing to an increase in the number of Pb2+ ions (upon increasing C0) which in turn increases the driving force for overcoming the diffusion resistance between the Pb2+ solution and CZFS phases. It is promising that for C0 = 5– 15 mg/L, the Qe-value is almost achieved by t = 1 h. Pb2+ adsorption by CZFS conforms to the pseudo-second order kinetic model, as described in the Supporting Information (Figure S14 and Table S4).

CONCLUSIONS Zn2+ substituted CoFe2O4 crystallites decorated with surface-bonded amorphous silica clusters in a nanocomposite mesoporous structure (CZFS) was prepared by a wet chemical method. The CZFS composite has been studied in terms of its adsorption properties (with respect to model cationic adsorbates MB and Mn+) in comparison to those of samples CZF and CF, prepared without silica and Zn2+–silica modifications, respectively. Higher SBET = 59.8 m2/g of 12


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CZFS (mesopores with Da = 8.5 nm) with respect to that of CZF and CF together with high ζ = −35.4 mV leads to its improved MB adsorption capacity over CZF and CF. MB adsorption by CZFS (for C0 = 5–25 mg/L) obeys the Langmuir model well, supporting an almost homogeneous CZFS surface with mostly equivalent sites for MB adsorption. Qm = 25.6 mg/g achieved for CZFS for adsorption of MB is found promising. CZFS exhibits improved Ae ≥ 98% for Cr3+, Cu2+ and Pb2+ as compared to Ae = 38–69% for Ni2+, Cd2+, Hg2+ for C0 = 5 mg/L. High Ae = 99.9% obtained for Pb2+ drops only to Ae = 98.8% for increased C0 = 20 mg/L. Inhomogeneous Pb2+ adsorption on CZFS surface is implied from better conformity to the Freundlich model. CZFS can be efficiently reused as an adsorbent of MB and Mn+ for up three cycles. A good Msvalue = 39 emu/g aids CZFS to be easily separated from water with a magnet. In summary, CZFS is effective for removal of MB as well as Pb2+ from contaminated water.

ACKNOWLEDGEMENTS We are thankful to DST Nanomission, Govt. of India for the financial support. A. Sengupta gratefully acknowledges the financial support by IIT, Bombay. Central characterization facilities at IIT, Bombay are gratefully acknowledged.

ASSOCIATED CONTENT Supporting Information XRD, XPS, microstructure (of CF), composition (of CZF and CZFS), FTIR, surface and magnetic properties analyses of the samples, pseudo-first and pseudo-second order kinetics for MB and Pb2+ adsorption by CZFS, CZFS reusability for Mn+ (C0 = 10 mg/L), leaching test, analysis of MB and Pb2+ adsorbed CZFS.

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Tables Table 1. Comparison of Qm-value of CZFS with respect to other magnetic adsorbents for adsorption of MB dye Adsorbent material

Adsorbent concentration (g/L)

pH

Qm (mg/g)

References

ZnO/ZnFe2O4

0.80

7.0

37.3

[9]

Magnetic-MWCNT

0.50

7.0

15.7

[30]

Sponge-like porous MnFe2O4

0.80

3.5

20.7

[31]

Magnetite loaded MWCNT

0.40

~7

48.1

[32]

3D porous NiFe2O4

0.30

9.0

138.5

[33]

CoFe2O4/MWCNT

1.0

~7

11.1

[34]

CZFS

0.62

6.5

25.6

Present work

Table 2. Comparison of Qm-value of CZFS with respect to other magnetic adsorbents for adsorption of Pb2+ Adsorbent material

Adsorbent concentration (g/L)

pH

Qm (mg/g)

References

Carbon coated Ni

0.80

4.0

13.8

[37]

Fe3O4/SiO2-NH2

2.0

4.0

14.7

[38]

Fe3O4/SiO2-xanthan gum

2.5

6.0

21.3

[39]

Fe3O4 nanospheres

2.0

5.0

13.4

[40]

Graphene oxide/NiFe2O4

0.75

5.5

25.0

[41]

Magnetic chitosan/graphene oxide imprinted Pb2+

5.0

5.0

79.8

[42]

CZFS

0.62

6.5

19.8

Present work

18


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Figure captions Figure 1. (a) FEG-SEM (inset: FEG-TEM) image, with (b) lattice image (magnified region ‘A’) and (c) electron diffraction pattern from CZF. (d) FEG-SEM (inset: FEG-TEM) image, with (e) lattice image (magnified region ‘B’) and (f) electron diffraction pattern from CZFS nanocomposite. Figure 2. Schematic representing (a) CZF cuboids and (b) CZF cuboids decorated with amorphous silica in sample CZFS. Figure 3. (a) Absorption efficiency (Ae), (b) Langmuir isotherm and (c) Freundlich isotherm for adsorption of MB (C0 = 5–25 mg/L) by CZFS nanoadsorbent. Figure 4. (a) UV-visible spectra of supernatent MB solutions taken after different times (t = 0– 55 min) of contact of CZFS (50 mg) with MB solution (C0 = 10 mg/L), with the (b) reusability test of CZFS for MB-removal up to three cycles; (c) Time dependence of MB adsorption capacity of CZFS (C0 = 5–25 mg/L) and (d) a comparison of the time dependent MB adsorption capacities of CF, CZF and CZFS (C0 = 15 mg/L). Figure 5. Comparisons of adsorption efficiencies of (a) CF, CZF, and CZFS for different metal ions Mn+ (C0 = 5 mg/L), (b) CZFS for different Mn+ with C0 = 5, 10 and 20 mg/L, and (c) CZFS for up to three cycles of reuse for different Mn+ (C0 = 5 mg/L). Figure 6. (a) Langmuir and (b) Freundlich isotherms for adsorption of Pb2+ ions (C0 = 5–25 mg/L) by CZFS. Figure 7. Time dependence of Pb2+ adsorption capacity of CZFS (C0 = 5–25 mg/L).

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Table of Contents (TOC) graphic

Zn2+–silica modified cobalt ferrite magnetic nanostructured composite for efficient adsorption of cationic pollutants from water Arundhati Sengupta, Rohan Rao and D. Bahadur

Synopsis: Zn2+–silica modified cobalt ferrite nanocomposite serves as an efficient adsorbent material for removal of pollutants from waste-water

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Abstract / TOC graphic Abstract 45x28mm (300 x 300 DPI)



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Figure 1. (a) FEG-SEM (inset: FEG-TEM) image, with (b) lattice image (magnified region ‘A’) and (c) electron diffraction pattern from CZF. (d) FEG-SEM (inset: FEG-TEM) image, with (e) lattice image (magnified region ‘B’) and (f) electron diffraction pattern from CZFS nanocomposite. Figure 1 99x56mm (300 x 300 DPI)


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Figure 2. Schematic representing (a) CZF cuboids and (b) CZF cuboids decorated with amorphous silica in sample CZFS. Figure 2 83x51mm (300 x 300 DPI)



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Figure 3. (a) Absorption efficiency (Ae), (b) Langmuir isotherm and (c) Freundlich isotherm for adsorption of MB (C0 = 5–25 mg/L) by CZFS nanoadsorbent. Figure 3 85x84mm (600 x 600 DPI)


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Figure 4. (a) UV-visible spectra of supernatent MB solutions taken after different times (t = 0–55 min) of contact of CZFS (50 mg) with MB solution (C0 = 10 mg/L), with the (b) reusability test of CZFS for MBremoval up to three cycles; (c) Time dependence of MB adsorption capacity of CZFS (C0 = 5–25 mg/L) and (d) a comparison of the time dependent MB adsorption capacities of CF, CZF and CZFS (C0 = 15 mg/L). Figure 4 118x96mm (300 x 300 DPI)



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Figure 5. Comparisons of adsorption efficiencies of (a) CF, CZF, and CZFS for different metal ions Mn+ (C0 = 5 mg/L), (b) CZFS for different Mn+ with C0 = 5, 10 and 20 mg/L, and (c) CZFS for up to three cycles of reuse for different Mn+ (C0 = 5 mg/L). Figure 5 190x482mm (300 x 300 DPI)


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Figure 6. (a) Langmuir and (b) Freundlich isotherms for adsorption of Pb2+ ions (C0 = 5–25 mg/L) by CZFS. Figure 6 35x15mm (600 x 600 DPI)



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Figure 7. Time dependence of Pb2+ adsorption capacity of CZFS (C0 = 5–25 mg/L). Figure 7 185x154mm (300 x 300 DPI)


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Zn2+âSilica Modified Cobalt Ferrite Magnetic Nanostructured

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