Remediation of waste water by Co–Fe layered double hydroxide and its catalytic activity

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Remediation of waste water by Co–Fe layered double hydroxide and its catalytic activity S.A. Abdel Moaty a,∗, A.A. Farghali b, M. Moussa a,c, Rehab Khaled d a

Materials Science Lab, Chemistry Department, Faculty of Science, Beni-Suef University, Beni Suef, Egypt Materials Science and Nanotechnology Department, Faculty of Postgraduate Studies for Advanced Sciences, Beni-Suef University, Egypt c Future Industries Institute and School of Engineering, University of South Australia, Mawson Lakes, SA 5095, Australia d Department of Chemistry, Faculty of Science, Beni-Suef University, Beni-Suef, Egypt b

a r t i c l e

i n f o

a b s t r a c t

Article history: Received 11 June 2016 Revised 20 November 2016 Accepted 1 December 2016 Available online xxx

Layered double hydroxide (LDH) nanoparticles have tremendous anion-intercalating property. Co–Fe LDH has been synthesized using the ball milling method. Characterization of Co–Fe LDH was done by XRD, SEM, TEM and FT-IR. The prepared LDH was used, for the first time, as a catalyst for multi-walled carbon nanotubes (MWCNTs) via chemical vapor deposition (CVD) of acetylene at different temperatures (400– 700 °C). Also, the ability of Co–Fe LDH as an adsorbent was investigated for the removal of Cd2+ ions from aqueous solutions. Various physicoechemical parameters such as pH, initial metal ion concentration, and time were studied. To get the adsorption isotherms, the concentrations of the metal ions ranging from 6 to 18 mg/l were used. The adsorption process follows pseudo-second-order reaction kinetics, as well as Langmuir adsorption isotherms. Interestingly, Co–Fe LDH demonstrated 95% Cd2+ removal at pH 9 and 6 h which could be applied in wastewater treatment characterized by a high efficiency and low cost. © 2016 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

Keywords: Cobalt–iron LDH MWCNTs Adsorption Cadmium ions Wastewater

1. Introduction Layered materials have been extensively studied in recent years due to the wide range of properties that could be tailored by intercalation and functionalization (surface chemical modification) reactions, allowing them to be used in different fields of science and technology. Therefore, the preparation of the layered materials to investigate their properties and applications has been the focus of our research group [1]. Layered double hydroxide (LDH) is a layered inorganic compound that is composed of an ionic lamellar solid that contains infinite layers of brucite-type, positively charged and exchangeable hydrate gallery anions. The ability of LDHs to intercalate anions makes them useful as catalysts, tailor made adsorbents or precursor materials for oxides. The general formula of n− x+ x− II III are LDH is [MII1−x MIII x (OH )] .[ (A x ).mH2 O] , where M and M n

the divalent and trivalent cations, respectively, and An − is the interlayer anion of charge n that leads to the electro-neutrality of the LDH. The coefficient x is equal to the molar ratio [MIII /(MII +MIII )], and m is the number of water molecules located in the interlayer region with the anions [2,3].

∗

Corresponding author. E-mail addresses: [email protected], [email protected] (S.A.A. Moaty).

Layered double hydroxides (LDHs) are the antitypes of clay minerals. Clay minerals found increasing interest as adsorbents by virtue of their properties, which make them attractive materials for adsorbing heavy metal ions. Adsorption reactions at solid– water interfaces decrease solute mobility and often control the fate, bioavailability, and transport of heavy metal ions such as Zn2+ ,Cu2+ ,Pb2+ and Ni2+ in the environment [4,5] The adsorption mechanisms of metal ions on the surfaces of clay minerals and others are significant for understanding their fate in the environment. LDHs could adsorb metal cations from aqueous solution in spite of positive layer charge. A major reaction could be surfaceinduced precipitation that occurs due to localized high pH values and the released carbonate ions available to metal cations. Positive layer charge attracts hydroxide ions around the surfaces of LDH crystals in aqueous solution to induce formation of metal hydroxides. Meanwhile, charge-compensating carbonate ion attached on the surface and edge could also contact with metal cations to form insoluble metal carbonates. There is also another possibility to adsorb metal cations via diodachy, as suggested by Komarneni et al. [6]. These imply that LDHs could be utilized as an adsorbent for heavy metal cations. However, there are only a few studies on the interaction between LDH and heavy metal cations in solution. The layered structure of layered double hydroxides is destroyed and transformed into mixed oxides with large surface area and good dispersion of metal cations after a controlled thermal treatment. This makes them capable of acting as an excellent precursor

http://dx.doi.org/10.1016/j.jtice.2016.12.001 1876-1070/© 2016 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

Please cite this article as: S.A.A. Moaty et al., Remediation of waste water by Co–Fe layered double hydroxide and its catalytic activity, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.12.001

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S.A.A. Moaty et al. / Journal of the Taiwan Institute of Chemical Engineers 000 (2016) 1–13 Table 1 Reported CNT for some layered double hydroxide structures. Catalyst

Type of the tube

Diameters

Reference

Co/Fe/Al layered double hydroxides Fe/Mg/Al LDH flake Fe/Mg/Al LDH flake Zn–Al LDH Co–Al LDH Fe0.1 Mg2 Al0.9 LDH Fe0.1 Zn2 Al0.9 LDH Fe0.1 Cu2 Al0.9 LDH Co–Fe LDH

Multiwalled carbon nanotubes Multiwalled carbon nanotubes Graphene/single-walled carbon nanotube Zn–Al (LDH) and poly acrylic acid-functionalized multiwalled carbon nanotubes Multiwalled carbon nanotubes single-walled carbon nanotubes Multi-walled carbon nanotubes and carbon nanofibers

7–13.5 nm 4–7 nm

[57] [58] [59] [60] [17] [41]

Multiwalled carbon nanotubes

20–30 nm 20–30 nm 1–3 nm 15–50 nm 50–200 nm 3–17 nm

In present work

Table 2 Reported adsorption capacities of some adsorbents for Cd2+ metal ion. Adsorbent

Adsorption capacity (mg/g)

Cd2+ concentration used mg/l−1

Source

Vanadium mine tailing Carbon nanotubes Chelating sponge Mg–Al–CO3 –LDH Magnetic Fe3 O4 /Mg–Al–CO3 –LDH Biogenic selenium Nanoparticles 1∗ Co–Fe (LDH) nanoparticles 2∗ Carbon nanotubes prepared on the Co–Fe LDH surface

3.52–8.83 14.45 90–100 61.40–70.20 45.60–54.70 18 1∗ 65–94 at 6 h 2∗ 70–94 at 3 h

20 mg/l 10–60 mg/l 1.0–5.0 mmol/l 100 mg/l 100 mg/l 30–210 mg/l 12 mg/l

[29] [30] [31] [28] [28] [27] This study

to prepare catalyst-supported materials [7–11] for CNTs production as shown in Table 1. Catalyst mostly containing Fe, Co, Ni components supported over porous materials or high-surface-area oxide matrix in order to enhance the reactivity of catalyst clusters have great effort for the growth of CNTs [12–16], this due to catalytically active site in Co nanoparticles [17]. This may have a distinct effect on the dispersion of Co2+ ions in catalysts and thus the catalytic performance for CNTs production. Carbon nanotubes have attracted worldwide attention because of not only their unique physicochemical properties but also their promising applications in transistors, field-emission tips, sensors, supercapacitors and bio-medical fields [18–22]. Electric-arc discharge and laser ablation synthesis techniques, catalytic pyrolysis of carbon-containing gases via catalytic chemical vapor deposition (CCVD) has been widely investigated in the production of various of carbon nanotubes (CNTs) [23–26]. Particle size and particle dispersion of the catalyst are the key factors for controlling the growth of nanotubes during the CCVD process. Good dispersion and size control of catalyst particles can be expected due to the ordered prearrangement of metal cations in the layers of the LDH precursor at an atomic level. Although previous researchers allocated their attention on the adsorption of Cd2+ ion using different adsorbents, [27–31], our present study demonstrated 95% removal for Cd2+ ion by using Co–Fe LDH as illustrated in Table 2. Co–Fe LDH was prepared different methods as shown in Table 3, Co–Fe LDHs have been synthesized using coprecipitation methods. However, the conventional methods have the following serious drawbacks: the processes are very complicated, accurate pH control and subsequent heating are needed, and the reaction times are long. Another synthesis method such as topochemical technique and laser ablation technique have been developed. However, the above-mentioned drawbacks (complicated processes and long overall treatment times) remain in these methods. Therefore, a simple method for the rapid synthesis of Co–Fe LDHs is required. One-step synthesis method that requires neither pH control nor heating. In current study it was prepared by ball milling method with an advantage that it can easily be operated and produces large amounts of nanostructured powders for a short period of time [32] because the force of the ball impact is unusually great, and it is possible to produce a fine and homogeneous distribution of components in the final solids [33].

Then used as catalyst for MWCNTs synthesis by CVD. The effect of temperature on catalyst crystallite size, CNTs morphology change and yield percent were studied (see Scheme 1). 2. Experimental 2.1. Materials Iron nitrate [Fe (NO3 ).9H2 O] SDFCL, India, cobalt nitrate [Co (NO3 )2 .4H2 O] and cadmium nitrate [Cd (NO3 )2 .4H2 O], Oxford laboratory reagent, India, sodium hydroxide, NaOH, Piochem for laboratory chemicals, EGYPT and hydrochloric acid, HCl. All used chemicals were of analytical reagent grade and were not more purified and all solutions were prepared by using bi-distilled water. 2.2. Synthesis of Co–Fe layered double hydroxide The Co–Fe nitrate LDH was prepared by blending a mixture of cobalt nitrate and the iron nitrate (3:1 molar ratio) (Fig. 1) with 8 M of sodium hydroxide solution, the resulting product was inserted into photon ball milling vessel, see Table 4, for 10 h with a continuous rotational speed of 200 rpm. After ball milling the dark brown precipitate was filtered and washed with bi-distilled water many times. Then, the obtained precipitate was dried at 80 ± 0.5 ◦ C for 24 h. 2.3. Catalytic properties of Co–Fe layered double hydroxide for synthesizing MWCNTs Catalytic reactions were carried out in a continuous-flow fixed bed. 0.5 g of Co–Fe LDH catalyst was packed in a cylindrical alumina cell. Co–Fe LDH catalyst preheated to 40 0–70 0 °C in a flow of nitrogen gas (70 ml/min) for 10 min. Then acetylene gas was allowed to pass over the catalyst bed with a rate of 10 ml/min for 30 min. The acetylene gas flow was stopped and the product on the alumina cell was cooled to room temperature in a flow of nitrogen gas. The weight of the carbon deposited along with the catalyst was noted. The percentage of carbon deposit (C%) obtained in each reaction was determined using the following relationship:

C (% ) =

W 2 − W 1  W1

× 100

(1)

Please cite this article as: S.A.A. Moaty et al., Remediation of waste water by Co–Fe layered double hydroxide and its catalytic activity, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.12.001

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Scheme 1. The application of Co–Fe LDH in purification of wastewater and it is catalytic activity to produce CNT.

Fig. 1. EDAX of Co–Fe layered double hydroxide.

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Table 3 Different method for preparing Co–Fe DH and different charecterization method for different application. Method of preparation

Characterization

Application

Reference

Mechanochemical process

X-ray, FT-IR FESEM, TG-DTA PXRD, HRTEM Co K-edge and Fe K-edge XANES spectra UV–vis reflectance spectra X-ray, HRTEM FESEM, Magnetic properties X-ray, FT-IR EDS, TEM SEM, elemental map AFM-SAED X-ray, FT-IR HRTEM, FESEM Surface area

–

[61]

Water oxidation photocatalysts under visible light

[62]

Magnetic properties

[63]

–

[64]

Co-precipitation method under an inert atmosphere with cobalt and iron ratios of 2.23, 3.33, and 4.16 Separate nucleation and aging steps Topochemical synthetic approach

Ball milling techniques

Desription

Vessel size Balls diameters Materials of vessels Materials of balls Ball/precipitate mass ratio Speed Time

7.5 cm diameter Ranged from 1.11–1.75 cm diameter Stainl steel Porclien 8:1 mass ratio 300 rpm 10 h

2.4. Adsorption of cadmium Cd2+ ions The adsorption of cadmium was studied by a batch operation at 25 ± 0.5 ◦ C temperature [27]. The adsorption was carried out by mixing the Co–Fe LDH and Cd2+ ions solution. The prepared samples were shaken with an orbital shaker at a shaking speed of 200 rpm at room temperature for 24 h. Then solid/liquid phases were separated by filtration. The concentration of the Cd2+ ions before and after adsorption was determined using Atomic Absorption spectrometry (Agilent Technologies 200 Series AA). In this study, we used different factors to optimize Cd2+ ions removal, such as pH (3)–(9), initial concentration of Co–Fe LDH (25– 200 mg/l), contact time (from 1 to 24 h, that even though equilibrium is reached within 400 min, the experiments was continued up to 24 h to ensure maximum absorption). All our experiments were performed triplicates to confirm their reproducibility and the average of concentration was calculated. Using SPSS version 16, means and standard deviation (SD) values were computed and P values less than 0.05 were considered as statistically significant. The adsorbed amounts of metal ions onto the Co–Fe LDH were determined according to the following Eq. (2):

(Co − Ct ) Co

× 100

(2)

where Q is the adsorptivity (%), Co represents the initial concentration of Cd2+ and Ct is the concentration of Cd2+ ions in (mg/l) after adsorption at time t (min). The amount of Cd2+ ions adsorption at equilibrium qe (mg/g) was determined through the Eq. (3):

qe =

V (Co − Ce ) W

∗

Catalytic activity for CNT production

Current study

2.5. Characterization

where W1 is the initial weight of the catalyst (Co–Fe LDH), W2 is the weight of carbon deposited and catalyst.

Q=

Adsorption of Cd2+ from waste water

is the equilibrium concentration of Cd2+ ions in mg/l. The metal ion solution volume in liters represents by V, and W stands for the adsorbent weight in gram.

Table 4 Ball milling conditions for preparing Co–Fe DH. Condition

∗

(3)

In the equation, the equilibrium adsorption capacity of adsorbent in mg (metal)/g (adsorbent) represents by qe , Co stands for the initial concentration of Cd2+ ions before adsorption in mg/l, Ce

X-ray diffraction experiments were conducted on a PANalytical (Empyrean) X-ray diffraction using Cu Kα radiation (wave length 0.154 cm−1 ) at an accelerating voltage 40 KV, current of 35 mA, scan angle range from 20° to 70° and scan step 0:02°. FT-Raman spectra were recorded with a Bruker (Vertex 70 FTIR-FT Raman) spectrometer. High Resolution Transmission Electron microscope images were taken by JEOL-JEM 2100 (Japan) with an acceleration voltage of 200 KV. Scanning Electron Microscope (SEM) images were taken by (Gemini, Zeiss-Ultra 55) field emission high resolution scanning electron microscope). EDAX, Field Emission Scanning Electron Microscope (FESEM) were taken by (Quanta FEG250). BET surface area was determined from adsorption isotherms using a Quantachrome NOVA Automated Gas Sorption System. The stability of the suspensions was monitored in water. Zeta potential measurements were performed on a Malvern (Malvern Instruments Ltd) in 10 mM NaCl which is convenient for zeta potential determination because of its optimal ionic strength. Dynamic light scattering (PCS-Photon Correlation Spectroscopy) was applied to measure the hydrodynamic diameter of the particles in ultrapure water. The scattering was measured at an angle of 173° to the incident He/Ne laser beam. The autocorrelation functions were analyzed with the Contin algorithm to derive intensity weighed particle size distributions. A second-order cumulant analysis of the correlation function and application of the Stokes– Einstein relation, taking the viscosity of the suspension into account, resulted in the intensity-weighted average hydrodynamic diameter. For stability determination, Co–Fe LDH sols were diluted at least 5 times in ultrapurewater in order to obtain reliable particle size measurements. 3. Results and discussion 3.1. Characterization of the Co–Fe LDH The XRD patterns are indexed according to the ICDD cards no. 01-074-3080 for Iron and no. 0 0-0 07-0169 for cobalt. Fig. 2 illustrates the XRD patterns of Co–Fe LDH structure. In particular, characteristic peaks at 2θ = 19.8°, 36.7°, 38.5°, 44.7°, 65.1°, 78.2° which correspond to diffraction from the (003), (101)/(222), (012), (400), (110)/(531) and (021) planes of LDHs, respectively, were observed. The peaks ascribed to the cobalt phase are red marked and those ascribed to iron phase are blue marked. The crystallite size was calculated using Deby–Sherrer’s formula using the corrected

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Table 5 Surface area measurements for Co–Fe LDH. Surface area (m2 /g) Total pore volume (cc/g) Average pore diameter (nm) Micro pore volume (cc/g)

Fig. 2. XRD patterns of Co–Fe LDH, ICDD cards no. 0 0-0 07-0169 for cobalt and no. 01-074-3080 for Iron.

FWHM of the 100% peak of each separate plan namely (012) for cobalt phase and (400) for Iron phase, which indicate that the crystallite size of cobalt layer is 32.9 nm, while the size of iron is 51.9 nm. The morphology and the dimensions of Co–Fe LDH are investigated with SEM, TEM and FESEM. SEM images and FESEM (Fig. 3(a), (c) and (d)) of the prepared LDHs indicates all layers are agglomerated and possessed plate-like morphology [34]. The TEM micrograph (Fig. 3(b)) shows that all prepared LDHs were crystallites, plate like morphology and uniformity in nature. This could be attributed to a homogeneous and slow nucleation process [35]. It is also clear the formation of two layers with different ranges of diameter 61 nm and 43 nm and that confirm the formation of the layers. Zeta potential values reveals information regarding the stability of the synthesized nanoparticles [36]. The pH of the sample should always be reported along with the zeta potential. Sometimes it is useful to adjust the native sample pH to a more relevant pH (e.g., closer to physiological pH for nanoparticles intended for biomedical applications). The zeta potential value for Co–Fe LDH obtained at pH ∼3, 4, 6, 7, 8 and 9 were 27, 5.8, −11.3, −15.1, −14.6 and −17.8, indicating the high stability of synthesized nanoparticles, as shown in Fig. 4(a). The zeta potential of Co–Fe LDH dispersions in 1 mM NaCl solution was negative as expected for LDH around neutral pH. Upon suspending particles in aqueous of solution increase. The reduction of the zeta potential is caused by compensation of the negative charges at the Co–Fe LDH surface by the protons in solution [37].

77 0.0375 19.4 0.0774

Particle sizes distribution determined by DLS were 892 nm. They are expected to be larger than in SEM or TEM as illustrated in Fig. 4(b). The reason is that DLS measures the intensity weighted average particle size, over estimating the relative contribution of the largest particles because these particles are the strongest scatters. In addition, DLS probes the equivalent hydrodynamic diameter of the colloids in suspension which is larger than the diameter observed in SEM and TEM. FT-IR spectra of Co–Fe LDH are shown in Fig. 5(a); the bands around 3400 cm−1 can be ascribed to the stretching mode of OH group with hydrogen bonding and of interlayer water molecules. The peak located at 1383 cm−1 is assigned to the ν 3 stretching vibration of the NO3 groups in the LDH interlayer. The bands at approximately 654, 579 and 459 cm−1 arises from metal–oxygen bonds M–O vibration in the brucite-like [38]. To give a further insight on the specific surface area and porosity of the as-prepared Co–Fe LDH, nitrogen sorption measurement was carried out. Fig. 6(a) displays the N2 adsorption–desorption isotherm and the corresponding pore size distribution curve for the Co–Fe LDH. The Co–Fe LDH material exhibits a typical III isotherm with a H3-type hysteresis loop, in the general features primarily indicated that the pore structures are wedge-shaped pores with open ends [39]. The average pore diameter for sample indicates mesoporous structure (average pore diameter <50 nm). This is in accordance with the SEM observations (Fig. 3(b) and (d)). This result is further confirmed by the corresponding wide distribution of pore size in Fig. 6(b) (5–65 nm, maximum at 48 nm), resulting from the hierarchical LDH structure. A high specific surface area (77 m2 /g) was obtained for the Co–Fe LDHs owing to the hierarchical structure [40] as shown in Table 5. 3.2. Catalytic properties of Co–Fe layered double hydroxide The catalytic nature of the Co–Fe LDH was investigated for synthesizing MWCNTs. It is obvious that, the carbon yield % and morphologies are affected by the operating temperature (400 to 700 °C), the maximum carbon yield % (352%) is achieved at decomposition temperature of 500 °C. Fig. 7(a) shows the XRD patterns of the calcined LDH catalyst at different temperatures (40 0–70 0 °C). The XRD indicate that the LDH layered structure partially collapses after calcination. Two phases of mixed oxides are formed Co3 O4 and Fe2 O3 at 40 0–70 0 °C. The intensity of beaks increase with increasing temperatures this due to increasing the crystallite size of catalyst from 8, 12, 39 and 39.6 nm at 40 0–70 0 °C respectively. The carbon nanostructures are related to the size of the active species in the catalysts [41]. The shape and thickness of carbon nanotubes are affected by operating temperatures [42]. To illustrate, at 400 °C small particle size of the catalyst leads to the formation of narrow carbon nanotubes (3 nm), by increasing the temperatures to 500, 60 0 and 70 0 °C, larger particle size result in the formation of more wide carbon nanotubes from 10, 13 and 17 respectively, with high quality and less amorphous carbon as illustrated in Fig. 8(a)–(d). There are correlation between the size of the catalyst nanoparticles and the CNT diameter, were observed a direct dependence of the two quantities. When the nanoparticles are prepared in holes or pores, the diameter of the former is subjected to the size of the latter and thus the resulting CNTs have a diameter roughly equal to the diameter of hole or pore. Nikolaev et al. stated that the particle

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Fig. 3. (a) SEM of Co–Fe nitrate layered double hydroxide, (b) TEM of Co–Fe nitrate layered double hydroxide, (c) and (d) FESEM of Co–Fe LDH.

size after CNT growth is larger than the CNT diameter, suggesting that particles continue to grow even after nucleating a tube. The relevant size of the nanoparticles for the resulting diameter of the CNTs is thus their size at the time of nucleation [43]. LDH nanoscale particles were encapsulated at the ends of nanotubes, confirming a tip-growth mechanism [44]. The mixed oxides particles seem to be necessary for the growth because they are often found at the tip inside the nanotube or also somewhere in the middle of the tube as shown in Fig. 9(a) and (b) (marked with black spot). It is supposed that acetylene decomposes at different temperatures 40 0–70 0 °C on the top of a supported catalyst as shown in Fig. 9(c). The dissolved carbon diffuses in the catalyst, precipitates on the rear side and forms nanotubes. The carbon diffuses through the catalyst due to a thermal gradient formed by the heat release of the exothermic decomposition of acetylene [42]. FTIR spectra of MWCNTs synthesized at different temperatures are shown in Fig. 7(b) broad peak at ∼3436 cm−1 , which refers to the O–H stretch of the hydroxyl group on the surfaces of MWCNTs could be due to water vapor which may be adsorbed by MWCNTs surface. The absorptions at 2925 cm−1 associated with C–H stretch modes while the peak observed at 1628 cm−1 is the C=C stretch of the MWCNTs. It is clear that with increasing temperatures these led to decrease in intensity of bands at 3867–3400 cm−1 (with a sharp peak at 3436 cm−1 ), these due to disappearance of the most interlayer water bands. While the decreasing at 2925 cm−1 –CH

stretching and gradual decrease of band intensity at 1628 cm −1 was observed and attributed to upon calcination, the crystalline LDH phase progressively changes to amorphous Co–Fe mixed oxide with increasing calcination temperature. The difference in intensity of some adsorption bands in the IR spectra of MWCNTs can be associated with formation of new IR active bonds. Another possibility is a change of molecular symmetry due to reorganization of bonds. Chemical and physical transformations of MWCNTs that reduce the symmetry of C=C bonds resulted in the decrease in intensity of the ∼1628 cm−1 band [45]. 3.3. Adsorption studies 3.3.1. Changing conditions effect on the adsorption of Cd2+ ions As previously illustrated, the target of this study was to make a remediation for waste water. Therefore, different experimental conditions were conducted such as initial solution pH, catalyst concentration and contact time. This was to reach the optimum condition for cadmium ion remediation. Previous studies [1] showed that, the pH of solution is the most important variable governing heavy metal ions adsorption. Also, the medium affects metal speciation and toxicity. To illustrate, at acidic medium, more protons are available to saturate metalbinding sites, while, at the under basic conditions, metal ions replace protons to form other species such as hydroxo-metal

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Fig. 4. (a) Particle size distribution and (b) Zeta potential of Co–Fe LDH nanoparticle suspension.

complexes, especially at very low concentrations. Some of the hydroxo-metal complexes are soluble, like those formed with cadmium, nickel, and zinc, while those formed with chromium and iron are not. Moreover, as the metal toxicity increases with pH, increasing pH led to the increase of the toxicity of cadmium ion. Because the Cd2+ ion produces mono hydroxo complex species that are more toxic soluble in water, cells may take up or adsorb more of the metal ions under these conditions. However, at highly acidic conditions, metals compete with protons for binding sites on the cell surface. Also, various functional groups associated with the membrane would be protonated under acidic conditions reducing the electrostatic attraction between the metal cations and the membrane. A third possibility is when the metals are removed from the cell more efficiently under acidic conditions by efflux

pumps that are driven by the proton motive force. Another possible explanation for increased toxicity at a higher pH is the formation of species that are more toxic, such as the hydroxo-metal species [46]. In the present study, the effect of pH in the range from 3 to 9 was studied by evaluating the adsorption of Cd2+ ions on the Co–Fe LDH. The effect of pH on the adsorption efficiency is shown in Fig. 10(a). From this figure, removal of Cd2+ ions increases linearly with increasing the pH values from 3 to 9 to reach 95% at pH = 9, pH values above 9 were not applied because there was no wastewater pH value more than 9. Similar behavior had been reported by many authors [28]. For the uptake of metal ions on various adsorbents. Low et al reported that at low pH, the surface of the adsorbent would be firmly connected with hydronium

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Fig. 7. (a) XRD patterns for Co–Fe LDH calcined at 40 0–70 0 °C, (b) FT-IR spectra of carbon nanotubes synthesized at 40 0–70 0 °C.

ing the pH. The positive charge on adsorbent surface gradually decreases by increasing pH, reducing the electrical repulsion between sorbing surface and cations. Moreover, lower H+ concentration also favors cation sorption by mass action. For example, the adsorption of bivalent cations such as M2+ on iron oxide can be written as: 2+ FeOH+ → FeOM+ + 2H+ 2 +M

Fig. 5. FT-IR spectra of Co–Fe nitrate layered double hydroxide (a) before (b) after adsorption of Cd2+ metal ions.

particles (H3 O+ ) and hold for the most part protonated locales [47]. In this way, the surface keeps up a net positive charge. So it obstructs the entrance of the metal particles to the surface function groups. Thusly, removal of metal ions decreases by decreas-

Lowering H+ concentration will drive this reaction toward the right-hand side and favor the sorption of M2+ by increasing pH [48]. The adsorption at the LDH surface was dependent on the pH value [49,50]. The metal acted as a Lewis acid and interacted by exchange or bound with H+ , OH− , M2+ and MOH+ . The neutral active sites could protonate or deprotonate as in Eqs. (4)–(6), respectively [51].

LDH − OH + H+  LDH − OH2

(4)

LDH − OH  LDH − O− + H+

(5)

Fig. 6. (a) N2 sorption isotherms and (b) pore size distribution of Co–Fe LDH.

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Table 6 Adsorpitivity % for different catalysts ions = 12 mg/l, catalyst 0.1 g for 3 h. Sample Co–Fe LDH CNT prepared CNT prepared CNT prepared CNT prepared

Fig. 8. HRTEM of CNT on Co–Fe LDH at different temperatures (40 0–70 0 °C).

LDH − OH + M2+  LDH − OM+ + H+

(6)

If the metal ion present in solution as mono hydroxo complex the reaction will be as following Eq. (7).

LDH − OH + MOH+  LDH − OM+ + H2 O

(7)

Also, the present of hydroxyl groups at Co–Fe LDH surface as functional groups that can react with Cd2+ ion by chemical binding reaction to form complexes at the inner-sphere. Some deprotonated hydroxyl groups (LDH–O− ) which might form complexes species at the outer-sphere with Cd2+ metal ions through electrostatic attraction. The effect of the LDH catalyst concentrations from 25 to 200 mg/l on the adsorption of Cd2+ ions was evaluated as a function of the initial Cd2+ ion concentrations (12 mg/l) at a pH value of 9. The adsorption time was fixed at 24 h to achieve an equilibrium state. As illustrated in Fig. 10(b), the Cd2+ ions adsorpitivity increase with the increasing of initial catalysts concentration to reach 95% at 200 mg/l. This may be attributed to the increase in the driving force from the concentration gradient, and increasing

for

(Cd2+

metal

Adsorpitivity % on on on on

the the the the

Co–Fe Co–Fe Co–Fe Co–Fe

LDH LDH LDH LDH

at at at at

400 °C 500 °C 600 °C 700 °C

65 70 94 87 83

in active sites in the catalysts, leading to the distinctively increased adsorpitivity. The effect of contact time on the adsorption of Cd2+ ions at initial metal ion concentration (12 mg/l) is shown in Fig. 10(c). During the experiment, the contact time was varied from 1 to 24 h. The analysis of batch adsorption of metal ions was carried out in 60 min steps and the concentration of each sample was measured by atomic absorption spectroscopy. There is increase in adsorption with the increasing of contact time and maximum adsorption takes place at 6 h after this time there was no further adsorption. This may be because of the way that at first every single adsorbent site were empty and the solute concentration gradient was high later, the lead uptake rate by adsorbent was diminished essentially because of the reduction in number of adsorption sites. Fig. 5(b) showed the decrease of the absorbance intensity of peak at 1383 cm−1 corresponding to the nitrate anion, confirm that interlayer nitrate anion were replaced by Cd2+ ion adsorbed from solution. The decrease of the FT-IR peaks intensities might be attributed to the intermolecular interactions among the metal ion and solid phases of LDH, which might be described as surface complex formation or by electrostatic attraction. By replicating the adsorption experimental on the carbon nanotubes samples prepared on the surface of Co–Fe LDH for 3 hours only and compared with Co–Fe LDH results. It was found that the adsorption process was improved from 65% to reach the maximum values 94% at 500 ◦ C by reaching to maximum adsorpitivity % at less time consuming as shown in Fig. 11 and Table 6. 3.3.2. Adsorption kinetics analysis The study of adsorption kinetics describes the solute removal rate and evidently this rate controls the residence time of adsorbate removal at the solid–solution interface including the diffusion process. The mechanism of adsorption depends on the physical and chemical characteristics of the adsorbent as well as on the mass transfer process [52]. Also kinetic analysis of adsorption process is very important for the design of adsorbents because the

Fig. 9. HRTEM of CNT on Co–Fe LDH at different temperatures (a), (b) showing tip growth mode, (c) growth model of vapour grown carbon nanotubes.

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Fig. 11. The adsorpitivity % of different samples of Cd2+ at 3 h.

kinetic model assumes that the binding is originated from physical adsorption and given by Eq. (8) as:

ln(qe− qt ) = ln (qe ) − k1 t

(8)

Where qe and qt are the amounts of Cd2+ ions adsorbed on the LDH catalysts in mg (adsorbate)/g(adsorbent) at equilibrium and at time t, respectively. k1 is the rate constant of the pseudo first-order model for the adsorption (min−1 ) [53]. The values of qe and k1 can be determined from the intercept and the slope of the linear plot of ln(qe − qt ) versus t. The pseudo second-order model is based on chemical adsorption (chemisorption) and given by Eq. (9) as:

t 1 t = + q qe k2 q2e

(9)

Where qe and qt follow the same definition as the pseudo firstorder model and k2 is the rate constant of the pseudo second-order model for adsorption (g/mg min). The slope and intercept of the linear plot of t/qt against t yielded the values of qe and k2 . In addition, the initial adsorption rate h (mg/g min) can be determined from h = k2 qe 2 . In addition, the kinetic results will be analyzed by the intraparticle diffusion model to elucidate the diffusion mechanism. This model is expressed in Eq. (10) as:

qt = ki t 0.5 + C

Fig. 10. The plots of effect of solution pH (a), adsorbent dosage (b) and contact time on Cd2+ adsorption Co– Fe LDH.

kinetics provide essential information on the adsorption mechanism and the metal ion uptake rate. The process of Cd2+ ions removal from an aqueous phase by adsorbent can be explained by using kinetic models and examining the rate-controlling mechanism of the adsorption process such as chemical reaction, diffusion control and mass transfer. The kinetic parameters are useful in predicting the adsorption rate which can be used in designing and modeling of the adsorption process. The kinetics of metal ions removal is explicitly explained in the literature using pseudofirst-order, pseudo-second-order, and intraparticle diffusion kinetic models. In order to investigate the mechanisms of metal adsorption process, the linear equations of pseudo-first-order, pseudosecond-order and intraparticle diffusion kinetic models were applied and the results were shown in Fig. 12. The pseudo-first-order

(10)

Where C is the intercept and ki is the intraparticle diffusion rate constant (mg/g min1/2 ), which can be evaluated from the slope of the linear plot of q versus t0.5 [54]. The results of Fig. 12(a)–(c) are fitted using pseudo-first- and second-order models and intraparticle diffusion model. The fit of these models was checked by each linear plot of ln (qe − qt ) versus t, (t/qt ) versus t, and qt versus t0.5 , respectively. Table 7 presented the coefficients of the pseudo-first and second-order adsorption kinetic models and the intraparticle diffusion model. By comparing the regression coefficients for each expression, first order rate expression and intraparticle diffusion model is not fully valid for the present system due to low correlation coefficients. A good agreement of the experimental data with the second order kinetic model was observed for the adsorbate which is presented in Fig. 12(b). Correlation coefficients for the linear plots using the pseudo-second order model are superior (in most cases >0.99), and theoretical and experimental qe values show excellent agreement. Therefore, this clarify that the sorption of Cd2+ ions by LDH catalyst is kinetically controlled by the second order reaction rather than the first order process.

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11

Table 7 Coefficients of pseudo-first and second order adsorption kinetic models and intraparticle diffusion model [(Cd2+ metal ions = 12 mg/l, Co–Fe nitrate layered double hydroxide 0.1 g]. Order models

Parameters

Co–Fe nitrate layered double hydroxide

Pseudo first order

qe Cal. (mg/g) qe Exp. (mg/g) K1 (min−1 ) R2 qe Cal. (mg/g) qe Exp. (mg/g) K2 (g/mg min) R2 ki (mg/g min0.5 ) C (mg/g) R2

0.075 5.91 0.01 0.996 6.02 5.91 0.31 0.999 3.4e−3 5.93 0.92

Pseudo second order

Intraparticle diffusion model

3.3.3. Adsorption isotherm studies Adsorption isotherms describe the relationship between the amount of the adsorbed substance per unit mass of adsorbent at constant temperature and its concentration in the equilibrium solution. They are used for describing adsorption equilibrium for waste water treatments. The equilibrium adsorption isotherms are important to determine the adsorption capacity of Cd2+ ions and diagnose the nature of adsorption onto the LDH catalyst. The equilibrium adsorption capacity of adsorbent was calculated by Eq. (2), where Ce was measured for initial concentration ranged from 6 to 18 mg/l after equilibrium time. The adsorption isotherms of Cd2+ ions on Co–Fe LDH are shown in Fig. 13(a). In this figure, the equilibrium uptake increased with Cd2+ ions concentration. This is a result of the increase in the driving force from the concentration gradient. At the same conditions, if the concentration of Cd2+ metal ions is higher, the active sites of Co–Fe nitrate layered double hydroxide are surrounded by many more Cd2+ ions and the process of adsorption would occur. Since the more common models used to investigate the adsorption isotherm are Langmuir and Freundlich equations, the experimental results of this study are fitted with these two models. The Langmuir model quantitatively describes the formation of a monolayer adsorbate on the outer surface of the adsorbent, after which no further adsorption occurs [55]. The model represents the equilibrium distribution of the adsorbate between the solid and liquid phases. The Langmuir adsorption isotherm, the most widely used isotherm for the adsorption of pollutants from a liquid solution, is based on the following hypotheses: (1) monolayer adsorption; (2) adsorption takes place at specific homogeneous sites on the adsorbent; (3) once a pollutant occupies a site, no further adsorption can occur in that site; (4) the adsorption energy is constant and does not depend on the degree of occupation of an adsorbent’s active centers; (5) the strength of the intermolecular attractive forces is believed to decrease rapidly with distance; (6) the adsorbent has a finite capacity for the pollutant; (7) all sites are identical and energetically equivalent; (8) the adsorbent is structurally homogeneous; and (9) there is no interaction between the molecules adsorbed onto the neighboring sites [55]. The Langmuir equation is based on the assumption of a structurally homogeneous adsorbent where all sorption sites are identical and energetically equivalent. The Langmuir adsorption isotherm applied to equilibrium adsorption assuming monolayer adsorption onto a surface with a finite number of identical sites and given by Eq. (11) as follows [55]:

Ce 1 1 = + Ce qe (qoKL ) (qo )

(11)

Where Ce (mg/l) is the equilibrium concentration, qe (mg/g) is the amount of adsorbate adsorbed per unit mass of adsorbate, and qo and KL are the Langmuir constants related to the adsorption capacity and the rate of adsorption, respectively. When Ce /qe was plot-

Table 8 Isotherm parameters for removal of Cd2+ ions by Co–Fe nitrate layered double hydroxide. Isotherms Langmuir

Freundlich

Parameters

Co–Fe nitrate layered double hydroxide

q0 (mg/g) KL (l/mg) RL R2 KF 1/n N R2

13.8 38.4 0.001 0.98 0.0184 0.426 2.347 0.96

ted against Ce , a straight line with a slope of 1/qe was obtained Fig 13(b), The Langmuir constants KL and qo were calculated from this isotherm and their values are listed in Table 8. Another important parameter, RL , called the separation factor or the equilibrium parameter, is evaluated in this study and determined from the following relation RL , can be determined from Eq. (12) [56].

RL =

1 [1 + KLCo]

(12)

Here, KL is the Langmuir constant (l/mg) and Co (mg/l) is the highest Cd+2 metal ions concentration. The value of RL shows whether the type of the isotherm is either unfavorable (RL > 1), linear (RL = 1), favorable (0 < RL < 1) or irreversible (RL = 0). RL values for Cd2+ adsorption onto Co–Fe LDH were calculated to be less than 1 and greater than zero, demonstrating favorable adsorp. The Freundlich equilibrium isotherm equation is an empirical equation used for the description of multilayer adsorption with interaction between the adsorbed moleculesion [37]. Also describe heterogeneous systems, in which it is characterized by the heterogeneity factor, n. The linear form of Freundlich adsorption isotherm takes the following form Eq. (13):

ln qe = ln KF +

1 n

ln Ce

(13)

Where qe is the amount adsorbed at equilibrium (mg/g) and Ce is the equilibrium concentration of Cd2+ ions. KF and n are Freundlich constants, where KF [(mg/g)/(mg/l)1/n] is the adsorption capacity of the adsorbent and n giving an indication of how favorable the adsorption process. The slope of the linear relation in Eq. (9) is 1/n and its value ranging from 0 to 1. This slope is a measure of adsorption intensity or surface heterogeneity, where the surface becomes more heterogeneous as its value gets closer to 0. The plot of ln qe versus ln Ce Fig. 13(c) gives straight lines with slope 1/n. This figure shows that the adsorption of Cd2+ ions follows the Freundlich isotherm. Accordingly, Freundlich constants (KF and n) were calculated and listed in Table 8.

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Fig. 12. Regressions of kinetic plots for Co–Fe LDH: (a) pseudo-first-order model, (b) pseudo second order model and (c) intraparticle diffusion model.

4. Conclusion Co–Fe nitrate LDH catalyst was successfully prepared by ball milling method. The crystallite size of cobalt layer is 32.9 nm while the size of iron is 51.9 nm with high surface area 77 m2 /g and particles size distribution 892 nm. The effect of temperature on the

Fig. 13. (a) Adsorption isotherms of Cd2+ ions on the Co–Fe LDH, (b) Langmuir, (c) Freundlich isotherms for Cd2+ ions on the Co–Fe LDH.

prepared catalyst was studied and it was found that the crystallite size increased as the temperature increased from 400 to 700 °C. The MWCNTs were synthesized by chemical vapor deposition (CVD) of acetylene on the catalyst surface at different temperatures. The growing temperature have great effect on the quality of CNT but the carbon yield % decreased with increasing tempera-

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tures at 60 0, 70 0 °C. The effect of temperature on FTIR spectra of MWCNTs was studied and it was found that the peaks intensity decreased as the temperature increased. The LDH was used for an adsorption study of Cd2+ . The adsorption of Cd2+ ions by Co–Fe LDH is increased linearly with the increasing of the pH values from 3 to 9. The optimum contact time for adsorption of the heavy metal was found to be 6 h and after which there was no further adsorption at the same time the adsorption give the same result with CNT–LDH structures but at 3 h only. The kinetics studies suggest that the adsorption of Cd2+ by Co–Fe LDH followed the secondorder kinetics model, which relies on the hypothesis that chemical adsorption may be the rate-limiting step involving valence forces through sharing or exchange of electrons between adsorbent and adsorbate. The adsorption experimental results of Cd2+ are in a good agreement with the Langmuir adsorption which demonstrating favorable adsorption. Acknowledgment The work was supported by the Faculty of Science, Materials Science Lab, Chemistry Department and Faculty of Postgraduate Studies for Advanced Sciences, Materials Science and Nanotechnology Department Beni-Suef University, Egypt. References [1] Moaty SA, Farghali A, Khaled R. Materials Science and Engineering: C 2016;68:184–93. [2] Cavani F, F Trifirò, Vaccari A. Catalysis today 1991;11:173–301. [3] Fan G, Li F, Evans DG, Duan X. Chemical Society Reviews 2014;43:7040–66. [4] González M, Pavlovic I, Rojas-Delgado R, Barriga C. Chemical Engineering Journal 2014;254:605–11. [5] Yang F, Sun S, Chen X, Chang Y, Zha F, Lei Z. Applied Clay Science 2016;123:134–40. [6] Komarneni S, Kozai N, Roy R. Journal of Materials Chemistry 1998;8:1329–31. [7] Valente JS, Figueras F, Gravelle M, Kumbhar P, Lopez J, Besse J-P. Journal of Catalysis 20 0 0;189:370–81. [8] Carja G, Delahay G. Applied Catalysis B: Environmental 2004;47:59–66. [9] Fornasari G, Glöckler R, Livi M, Vaccari A. Applied Clay Science 2005;29:258–66. [10] Li F, Duan X. Layered double hydroxides:193-223. Springer; 2006. Number of p. 193–223. [11] Shiraga M, Kawabata T, Li D, Shishido T, Komaguchi K, et al. Applied Clay Science 2006;33:247–59. [12] Chen Y, Ciuparu D, Lim S, Yang Y, Haller GL, Pfefferle L. Journal of Catalysis 2004;225:453–65. [13] Chen Y, Ciuparu D, Lim S, Yang Y, Haller GL, Pfefferle L. Journal of Catalysis 2004;226:351–62. [14] Gulino G, Vieira R, Amadou J, Nguyen P, Ledoux MJ, et al. Applied Catalysis A: General 2005;279:89–97. [15] Chai S-P, Zein SHS, Mohamed AR. Applied Catalysis A: General 2007;326:173–9. [16] Tran KY, Heinrichs B, Colomer J-F, Pirard J-P, Lambert S. Applied Catalysis A: General 2007;318:63–9. [17] Li F, Tan Q, Evans DG, Duan X. Catalysis Letters 2005;99:151–6. [18] Iijima S. Nature 1991;354:56–8. [19] An KH, Kim WS, Park YS, Choi YC, Lee SM, et al. Advanced Materials 2001;13:497–500. [20] Bachtold A, Hadley P, Nakanishi T, Dekker C. Science 2001;294:1317–20. [21] Zhu H, Xu C, Wu D, Wei B, Vajtai R, Ajayan P. Science 2002;296:884–6. [22] Zanello LP, Zhao B, Hu H, Haddon RC. Nano letters 2006;6:562–7.

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