Water Sanitization by the Elimination of Cd2+ Using Recycled PET

Research Article pubs.acs.org/journal/ascecg

Water Sanitization by the Elimination of Cd2+ Using Recycled PET/ MWNT/LDH Composite: Morphology, Thermal, Kinetic, and Isotherm Studies Shadpour Mallakpour*,†,‡ and Vajiheh Behranvand† †

Organic Polymer Chemistry Research Laboratory, Department of Chemistry, Isfahan University of Technology, Isfahan, 84156-83111, Islamic Republic of Iran ‡ Nanotechnology and Advanced Materials Institute, Isfahan University of Technology, Isfahan, 84156-83111, Islamic Republic of Iran S Supporting Information *

ABSTRACT: The fabrication and characterization of recycled polyethylene terephthalate nanocomposites (R-PET NCs) reinforced with multiwalled carbon nanotubes/Mg−Al mixed metal oxides (MWNT/LDH) is reported in this study. The morphology images and X-ray diffraction indicated that the MWNT/LDH hybrids were mostly exfoliated in the R-PET matrix. The good dispersion of filler in the R-PET caused MWNT/LDH to operate as a gas barrier and slowed down the thermal degradation of the R-PET matrix. The prepared composite was further employed to eliminate Cd2+ from aqueous media. The influence of pH, contact time, coexist ions, temperature, and initial pollutant concentration on the sorption procedure was examined. The high removal efficiency was gained for the Cd2+ sorption by R-PET/MWNT/LDH NC 4 wt %. The sorption equilibria were well explained by both the Freundlich and Langmuir models. The adsorption kinetics of Cd2+ onto R-PET/MWNT/LDH NC 4 wt % could be well described by the pseudo-second-order model. The extracted E value from the Dubinin−Radushkevich equation and thermodynamic data proposed that the sorption was mainly physical. In addition, without the use of magnetic separation, prepared sorbent could be simply collected and reused. KEYWORDS: Multiwalled carbon nanotubes, Mg−Al mixed metal oxides, Recycling, PET composites, Heavy metal removal, Reusability

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INTRODUCTION Layered double hydroxides (LDH)s were recognized as a group of well-ordered two-dimensional (2D) layered compounds. The layered construction of LDHs and their anion-exchange properties make them interesting due to their potential uses in various fields.1 Combinations of one-dimensional (1D) carbon nanotubes (CNT)s and 2D LDH create 3D hierarchical hybrid materials with surprising features for special uses.2 Poly(ethylene terephthalate) (PET) with high performance features does not generate a straight danger to the environment, but due to its considerable fraction by volume in the waste stream and its high resistance to biological and atmospheric agents, it is understood as a harmful material.3,4 One of the best choices for the management of plastic solid waste is recycling. In this process, compared to burning, the waste volume can be diminished and environmental concern decreases.5 Though, there are some key difficulties for recycling of polymers, such as separation and distinguishing of them. In addition, the structure of the polymer is decomposed after numerous processing cycles, presenting inferior mechanical properties compared to the pure polymer. To overwhelm these © 2017 American Chemical Society

restrictions, it seems that the simplest method for waste plastics recycling is a growth of blends and composites.6 One of the global worries is the pollution of water sources due to the unselective removal of many organic and inorganic pollutants from industries and human life.2 Up to now, different physicochemical approaches have been established for the extraction of pollutants from aqueous media such as adsorption, chemical coagulation, membrane separation, photodegradation, and so on. Among them, adsorption is extensively applied as one of the most favorable methods for pollutants removal due to the simplicity of the process, high efficiency, and cost.7,8 The intrinsic structure property permits LDH to get capable sorbent to adsorb a variety of contaminants in aqueous media.9 Additionally, physically, the great specific surface areas of CNTs, their hollow and layered structures, create an ideal sorption solid, as well.10,11 The achieved MWNT/LDH which was employed for pollutant sorption is usually in the form of Received: February 4, 2017 Revised: April 22, 2017 Published: May 16, 2017 5746

DOI: 10.1021/acssuschemeng.7b00344 ACS Sustainable Chem. Eng. 2017, 5, 5746−5757

Research Article

ACS Sustainable Chemistry & Engineering

Joyner, and Halenda method (BJH). Thermogravimetric analysis (TGA) was performed on a PerkinElmer system (Waltham, USA) instrument, model STA 6000. The samples were heated at 20 °C min−1 from 25 to 800 °C in an argon-purged atmosphere (20 mL min−1). Preparation of the samples was accomplished using Topsonics ultrasonic liquid processors (Tehran, I. R. Iran) with a frequency of 25 kHz and power of 80 W. A flame atomic absorption spectrophotometer (AAS; PerkinElmer 2380-Waltham, air-acetylene oxidizing flame) was used for Cd2+ measurements. Coupled plasma optical emission spectrometry (ICP-OES) was used to determine Al content after the adsorption process. A glass electrode pH meter (Model RpH, Raad teb novin, Iran) was employed to adjust the pH. Preparation of Nanostructured MWNT/LDH hybrids. The preparation of MWNT/LDH was based on a rapid and simple approach by an ultrasonic assisted coprecipitation technique according to our previous work.9 Briefly, Mg(NO3)2·6H2O and Al(NO3)3·9H2O were dissolved in 50 mL of deionized (DI) water (solution a). Then, MWNT was spread into an aqueous solution (b) having NaOH and Na2CO3. Afterward, solution a was poured into solution b, and the resulting suspension was aged at 60 °C with stirring for 2 h. Next, the black mixture was ultrasonicated for 60 min. The solid was washed with DI and dried under vacuum conditions at 100 °C for 6 h. Recycling of PET Bottle and Composite Manufacture. First, a PET bottle was cut into small parts. Then, 20 mL of DMSO as a solvent was added to 4 g of chopped PET. It was heated at 180 °C while it was stirring in order for all parts to be dissolved. The obtained solution was cast on a Petri dish and rapidly cooled with distilled water as a nonsolvent to form white powder. The polymer was washed several times, filtered, and dried in an oven at 100 °C overnight.18 R-PET/MWNT/LDH NCs were prepared under ultrasonic irradiations as follows: DMAc (10 mL) was added to R-PET (0.2 g), and the mixture was sonicated for 10 min. Then, 1, 2, and 4 wt % MWNT/LDH relative to polymer were added to this mixture and then ultrasonicated in a sonicator probe for 30 min at room temperature to obtain R-PET/MWNT/LDH NCs of 1, 2, and 4 wt %. Finally, the solution was placed in a Petri dish and dried in a vacuum at 100 °C for 24 h. Batch Kinetic and Equilibrium Investigations. All experiments dealing with Cd2+ were done applying a batch system. Typically, RPET/MWNT/LDH NC 4 wt % (20 mg) was added to the adsorbate solutions of 20−100 mg L−1 in aqueous media (10 mL) at pH = 6 adjusted by NaOH or HCl solutions. The suspended mixtures were shaken at a constant temperature for 24 h. After this period, the particles were removed by centrifugation at 4000 rpm, and the residual concentration of Cd2+ in the solution was determined using AAS. The adsorption capacity of pollutant on R-PET/MWNT/LDH NC 4 wt % was calculated using eq 1:

powder. So, its separation is difficult from a suspension. To improve the separation efficiency, it can be used in magnetic separation technology or coagulant agents. For example, Shan et al.12 discovered that a combination of LDH with Fe3O4 led to easy separation of the powdered sorbent from the water with an external magnetic field. Zhao et al. employed graphene oxide (GO) as sorbents to eliminate Cd2+ and Co2+ from water.13 Also, removal of Cd2+ and arsenic by GO was studied by Ren et al.14 Zou et al.15 concluded that LDH can be a favorable material for the efficient separation of GO using a simple and rapid chemical coagulation. In another study, glycerinummodified LDH, LDHs, and TiO2 were introduced as efficient coagulants for GO coagulation from suspension by Zou and coworkers.16 LDHs have positively charged layers with anionic spaces, and GO has plentiful functional groups including oxygen. Thus, the proposed interaction mechanism of GO coagulation on LDHs may be related to the hydrogen-bonding, electrostatic, and π−π interactions. Other carbon-based materials such as CNTs due to having a structure close to that of GO with conjugated construction and having oxygenincluding functional groups may be coagulated with LDH. However, a precise explanation of coagulating CNTs is still not obtainable and requires more studies. In addition to LDHs, CNTs, and GOs, there are other adsorbents like zeolites, agricultural residues, activated carbon, and so on which have been used for the purification of water. However, these sorbents have key drawbacks such as relatively weak interactions with metallic ions, low sorption capacities, and difficult recycling of some of them from aqueous solution. The combination of these nanostructures with polymer containing a variety of functional groups led to strong binding affinities to pollutants and fairly high adsorption capacities.17 In agreement with the explanations stated, this study is concentrated on the cadmium (Cd2+) adsorption by recycled PET (R-PET) when reinforced with a 1, 2, and 4 wt % carboxylated MWNT/LDH (MWNT/LDH) hybrid.

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MATERIALS AND METHODS

Materials. Mg(NO3)2·6H2O, Al(NO3)3·9H2O, and N,N-dimethylacetamide (DMAc) were purchased from Merck Chemical Co. (Darmstadt, Germany). MWNTs which have an inner and outer diameter of 5−10 and 10−20 nm, respectively, a length of ∼30 μm, a grafted amount of −COOH of 2.00 wt %, and >95 wt % purity were obtained from Nanosabz Co. (Tehran, Iran). Cadmium nitrate tetrahydrate was gained from Riedel-de Haën AG (Seelze, Germany). Sodium chloride (NaCl) and sodium acetate anhydrous (CH3COONa) were obtained from Daejung Chemical Co. (South Korea). In this work, scraps of R-PET were obtained from chopping of a postconsumer bottle. Characterizations. The FT-IR spectra of the samples were recorded in a Jasco-680 (Japan) spectrophotometer in the form of KBr pills of finely ground samples. Field emission scanning electron microscopy (FE-SEM) was accomplished on a HITACHI S-4160 (Japan) instrument, at acceleration voltages of 20 and 30 kV. For the analysis, the surface of the composites was coated with an ultrathin gold layer in a sputter coating method. Transmission electron microscopy (TEM) was done by a Philips CM120 microscope (Germany; accelerating voltage of 150 kV). Nitrogen (N2 ) adsorption−desorption isotherms were determined at 77 K on a BELSORP-mini II (Japan) instrument. One-hundred-milligram samples were degassed at 120 °C before the measurements. The Brunauer, Emmett, and Teller (BET) adsorption isotherms were applied to measure the BET surface area and mean pore size of the samples. The pore-size distribution of pure polymer and R-PET/ MWNT/LDH nanocomposite (NC) was calculated with the Barrett,

qe = (C0 − Ce) × V /m

(1)

where qe is the amount of ion taken up by the adsorbent (mg g−1), C0 is the initial pollutant concentration (mg L−1), Ce is the pollutant concentration after the adsorption procedure (mg L−1), m is the mass of adsorbent (g), and V is the volume of the pollutant solution (L). To study the effect of MWNT/LDH on Cd2+ sorption, elimination percentages of R-PET/MWNT/LDH NCs of 1, 2, and 4 wt % were examined. For this aim, each R-PET/MWNT/LDH NC (20 mg) with various MWNT/LDH loadings was added to the adsorbate solutions of 20 mg L−1 in aqueous media (10 mL) at pH = 6. After 24 h, the particles were removed, and the residual concentrations of Cd2+ in the solution were determined using AAS. The adsorption kinetics of Cd2+ (C0: 50 mg L−1, V = 10 mL) onto the composite (20 mg) were done at pH 6 and at different times (5, 15, 30, 60, 360, 480, 600, 720, and 1440 min) in aqueous media. The mixtures were shaken at 170 rpm. After this time, the Cd2+ solutions were detached by centrifugation. To investigate the thermodynamic features, the experiment was done at 298, 313, 323, and 343 K, (25, 40, 50, and 70 °C) at pH 6, with 20 mg of sorbent in 10 mL Cd2+ solutions (50 mg L−1) for 24 h. 5747


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Scheme 1. (A) Digital Pictures of R-PET/MWNT/LDH NC Suspensions in DMAc and (B) Feasible Interactions of R-PET Matrix and MWNT/LDH as a Filler

For the renewal of sorbent, an adsorption−desorption procedure was done. Since PET can be degraded and LDH can be dissolved under strong acidic conditions such as HNO3, we could not employ this conventional desorption method. For this aim, in each cycle, the separated Cd-adsorbed NC was put into a vessel containing 25 mL of CH3COONa aqueous solution (1 mol L−1). It was shaken for 2 h at 298 K and then was washed with DI water. Finally, the regenerated RPET/MWNT/LDH NC 4 wt % was used for the next cycle. The adsorption−desorption cycles were performed three times, and the removal efficiency of Cd2+ was calculated for each cycle. For consistency, all of the tests were accomplished in duplicate.

coprecipitation technique which was fast and green according to our previous work.9 A schematic structure of R-PET/MWNT/LDH NC 4 wt % is proposed and presented in Scheme 1. There are intermolecular H-bond interactions between carbonyl and −OH groups in the structure of MWNT and R-PET. There are also weak π···π interactions between aromatic rings in the polymer and nanotube structure. FT-IR Analysis. The FT-IR spectra of the MWNT/LDH, RPET, and R-PET/MWNT/LDH NCs 2 and 4 wt % are shown in Figure 1. The characteristics peaks of the MWNT/LDH hybrid were observed at around 1383 (υ3 vibration of the interlayer carbonate) and 780 cm−1 (bending modes of the interlayer carbonate). Additionally, bending and stretching modes of the hydrotalcite-like lattice below 800 cm−1 could be noticed by the metal−oxygen (M−O) modes.9 The FT-IR spectrum of R-PET represents the presence of a conjugated CO group with an aromatic ring which appeared at 1720 cm−1. There are also bands at 2965 cm−1 (C−H asymmetric stretching), 1505 cm−1 (aromatic C−C stretching), and 1451 cm−1 (C−H bending; Figure 1a). A band corresponding to −OH groups due to the terminal hydroxyl groups of the PET chains was seen in the spectrum of the pure R-PET (about 3431 cm−1) and R-PET/MWNT/LDH NCs. While the mentioned band was less intense than the one observed for the composites. This peak was broadening for R-PET/ MWNT/LDH NCs relative to that of the pure polymer because of the increase in H-bonding by introducing a filler. Only slight changes in the band broadening was recorded when the MWNT/LDH was loaded. Likewise, as predicted, all vibrations earlier explained are recognized for R-PET and the R-PET/MWNT/LDH NCs as well, since in all samples, PET is the most plentiful compound. The percent of LDH was very

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RESULTS AND DISCUSSION Ultrasonic Assisted Coprecipitation Technique. Coprecipitation of the selected trivalent and divalent cations from solution is the most generally applied technique for the fabrication of LDHs. Throughout the coprecipitation, by adding constituents with definite cation adsorption capacity into the solution process, precipitation will happen on the surface of these materials, directing toward in situ growth of LDHs.2 In this investigation, the MWNTs in aqueous solution containing both NaOH and Na2CO3 were impregnated by an aqueous solution consisting both Al(NO3)3 and Mg(NO3)2 for the adsorption of Mg2+ and Al3+, and coprecipitation occurred. Then, a hydrothermal process was done on the impregnated materials for crystal evolution, and LDH loadings as high as 29% were gained. Afterward, postsynthesis action was obtained by ultrasonic irradiations. The use of ultrasonication in nanomaterials synthesis has multiple impacts. The most noticeable is that the materials dispersing in liquids reduce particulate aggregation.19,20 In fact, ultrasound treatment produced LDH particles with small size and a narrow particle-size distribution. For these reasons, in this work, MWNT/LDH was synthesized through an ultrasonic assisted 5748


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LDH9 were grown on the nanotubes surfaces which act as supporter. In the case of R-PET/MWNT/LDH NCs, a (003) reflection corresponding to a basal spacing of LDHs was not observed in their spectra and has been disappeared. This result evidently shows the damage of a well-ordered layer construction of LDH during the intercalation of R-PET chains between the interlayer spacing of LDH and formation of exfoliated construction.22,23 It is because of the large gallery height of intercalated layers which causes the layer relationship that it was not distinguished by XRD. Though, a partial image about dispersion of filler is provided by XRD, and disappearance of the peak due to d-spacing does not approve permanently the exfoliated NCs.22 It is worth it to note that this phenomenon was further approved by TEM images. Morphological Surface Analysis by FE-SEM and TEM. Figure 3A shows the FE-SEM images of the dried MWNT/

Figure 1. FT-IR spectra of the (a) MWNT/LDH hybrids, (b) R-PET, (c) R-PET/MWNT/LDH NC 2 wt %, and (d) R-PET/MWNT/LDH NC 4 wt %.

low for making any important variation in the IR spectra. Consequently, the R-PET/MWNT/LDH NCs spectra permitted recognition of mostly one organic composite. XRD Analysis. The XRD patterns of the pure MWNT, MWNT/LDH hybrid, pure R-PET, and R-PET/MWNT/LDH NCs of 2 and 4 wt % are shown in Figure 2. Figure 2b shows

Figure 3. (A) FE-SEM images of MWNT/LDH were taken at two magnifications and (B) TEM images corresponding to (a) pure MWNT, (b) pure LDH, and (c) MWNT/LDH hybrids.

LDH powder. These images show a porous structure with a uniform dispersion of MWNTs. The TEM examination further presented small and crystalline hexagonal platelets of attached LDH to the nanotubes. The size of the sheets was around 50 nm. There are no isolated LDH nanosheets, and all platelets seem to be connected with MWNTs (Figure 3B). FE-SEM images of pure R-PET and composites are shown in Figure 4. As can be observed, the pure R-PET represented spherical shapes with uniform size after ultrasonication in DMAc. The presence of nanotubes was more obvious in composites with increasing percentages of filler. Figure 5 shows the TEM images of the R-PET/MWNT/ LDH NC 4 wt %. As can be seen, the outer surface of the nanotube is not smooth but covered by a thin LDH layer, confirming strong interaction between them. The sizes of formed LDHs on the surface of nanotubes were below 50 nm, which are shown with yellow lines in Figure 5. Furthermore, in this investigation, LDH sheets with different sizes were not

Figure 2. XRD diffraction patterns of (a) pure MWNT, (b) MWNT/ LDH, (c) pure R-PET, (d) R-PET/MWNT/LDH NC 2 wt %, and (e) R-PET/MWNT/LDH NC 4 wt %.

the characteristic reflections due to 2D hydrotalcite-like compounds (JCPDS no. 14-191) which could be indexed according to it.21 Also, in the XRD pattern of MWNT/LDH, the diffraction peak at 26.2° is related to the (002) plane of graphitic carbon (JCPDS no. 41-1487),6 observable in the XRD pattern of pure MWNT (Figure 2a). It can be clearly observed that the LDHs with higher crystallinity compared to the pure 5749


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Figure 4. FE-SEM images corresponding to (a) pure R-PET after sonication in DMAc, (b) R-PET/MWNT/LDH NC 1 wt %, (c) R-PET/MWNT/ LDH NC 2 wt %, and (d) R-PET/MWNT/LDH NC 4 wt %.

Figure 5. TEM images corresponding to R-PET/MWNT/LDH NC 4 wt % at three different magnifications.

prepared. However, it may be expected that when the size of LDH decreases, the surface area increases; therefore the chemical activity and sorption capacity of it for the adsorption of heavy metals on their surface will improve. It has been understood that the larger surface area per mass creates a surface with more sites for adsorption, which increases removal yield and enhances the rates in water treatment.24,25 It would be very interesting for us to examine the effect of different LDH sizes on the pollutant removal in future works. The strong attraction between the LDH nanosheets and the CNTs avoids formation of agglomerates of the filler in the R-PET matrix. The dark parts represent the LDH layers, while the bright areas

represent the R-PET matrix. The dark areas represent that ordered and stacked layered LDH construction was missed mostly, and LDH sheets were exfoliated with the chains of the PET molecules. BET and Porosity Investigation. BET surface area, pore volume, and mean pore diameter values of the pure R-PET and R-PET/MWNT/LDH NC 4 wt % were calculated and listed in Table 1. Also, a related BET graph was shown in Figure 6. As can be observed in Table 1, the BET surface area, pore volume, and mean pore diameter of R-PET/MWNT/LDH NC 4 wt % are 7.98 m2·g−1, 0.089 cm3·g−1, and 44.87 nm, respectively. These values are more than that of pure R-PET. Thus, more 5750


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pollutant adsorption capacity by the obtained composite as adsorbent at different pH values are shown in Figure 7b. As can

Table 1. BET Results for the Pure R-PET and R-PET/ MWNT/LDH NC 4 wt % sample pure R-PET R-PET/MWNT/ LDH NC 4 wt %

BET surface area (m2 g−1)

Mean pore diameter (nm)

Total pore volume (cm3 g−1)

4.85 7.98

37.49 44.87

0.045 0.089

Figure 7. (a) The change in pHfinal as a function of pHinitial in the RPET/MWNT/LDH NC 4 wt % suspensions in water during the determination of the PZC. (b) Influence of initial pH on the adsorption of Cd2+ by R-PET/MWNT/LDH NC 4 wt %.

Figure 6. N2 adsorption−desorption isotherms of pure R-PET and RPET/MWNT/LDH NC 4 wt %.

active sites for Cd2+ removal will be provided due to the higher surface area of R-PET/MWNT/LDH NC 4 wt %. Both isotherms related to pure polymer and R-PET/MWNT/LDH NC 4 wt % indicated type IV hysteresis loops representing the presence of mesoporous construction in the two compounds. Thermal Decomposition Behavior. Figure S1 presents the TGA curves of MWNT and MWNT/LDH. Details of information about the stages of their weight losses are provided in the Supporting Information. The influence of MWNT/LDH hybrids on thermal degradation of PET polymer is shown in Figure S2. The temperature obtained from the TGA curves at which the 5% and 10% weight loss rates occur (T5% and T10%), and the fraction of the residue outstanding at 800 °C (char yield) referred to as the char, are listed in Table 2. The NCs

be observed, the obtained sharp adsorption capacity at pH = 8 is related to two factors: (1) adsorption by sorbent and (2) precipitation of Cd2+ as insoluble Cd(OH)2. The influence of pH on Cd2+ adsorption by NC can be described according to the point of zero charge (PZC), which for the net total particle charge is zero at this pH. The PZC as a qualitative parameter can be applied for the sorbent surface charge balance. As illustrated in Figure 7a, the value of pHPZC was around 7 for RPET/MWNT/LDH NC 4 wt %. For pH < pHPZC, the surface of the sorbent is protonated and thus positively charged, whereas it is deprotonated and gains negative charges at pH > pHPZC. Hence, the effect of pH on Cd2+ adsorption onto NC can be described by an electrostatic attraction mechanism between NC and charged sorbent surfaces. Corresponding to the electrostatic interaction force, for cationic Cd2+, the adsorption quantity reduced with decreasing pH value owing to electrostatic repulsion force.7,27−29 It is interesting to mention that further obtained information from isotherms will approve this proposition. Effects of Ionic Strength and Coexist Ions on Cd2+ Sorption. To assess the ionic strength influence on the sorption of Cd2+ ions, experiments were accomplished by adding NaCl at various concentrations (0, 0.001, 0.01, and 0.1 mol L−1). In the tests, the initial Cd2+ concentration was 0.001 mol L−1, and the pH was 6. The results are demonstrated in Figure 8a. As can be seen, Cd2+ sorption improved with an increase in the ionic strength of the electrolyte solution. The existence of NaCl salt in the solution may have two opposite influences. Since the electrostatic interaction between opposite charges on the metal ions and sorbent surface is screened by the salt, the removal efficiency should be reduced with the increase of salt concentration. On the other hand, the presence of salt led to dissociation promotion of the functional groups such as carboxyl groups on the sorbent, and then the removal efficiency rises.30 The second effect appears to be dominant for Cd2+ sorption on R-PET/MWNT/LDH NC 4 wt % at a lower

Table 2. Thermogravimetric Results for Pure R-PET and RPET/MWNT/LDH NCs sample

T5%

T10%

char yield

pure R-PET R-PET/MWNT/LDH NC 1 wt % R-PET/MWNT/LDH NC 2 wt % R-PET/MWNT/LDH NC 4 wt %

402 401 392 390

412 401 392 407

12 15 13 16

showed lower weight loss and higher char yields compared to the pure polymer. More char yields for NCs might be ascribed to the presence of the nanoconstruction, which can diminish the heat transfer in the burning procedure. Effect of pH on Pollutant Sorption. The pH value of the solution is one of the most significant parameters, which affects the efficiency of a sorbent. The desired pH of the R-PET/ MWNT/LDH NC 4 wt % suspensions in each vessel was adjusted in the range of 4−8 with an initial Cd2+ concentration of 50 mg L−1 by adding negligible volumes of HCl or NaOH (0.1 mol·L−1) at room temperature and a contact time of 24 h. Studies at higher than pH 8 were not tried because of the Cd(OH)2 precipitation.26 The results of pH influence on 5751


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ACS Sustainable Chemistry & Engineering Table 3. Comparison of the Kinetic Models

Cd2+

kinetic models −1

qea

(mg g ) pseudo-second-order qeb (mg g−1) k2 (g mg−1 min−1) υ0 (mg g−1 min−1) R2 pseudo-first-order k1 (min−1) qeb (mg g−1) R2 intraparticle diffusion Ki (mg g−1 min −1/2) Ci R2

a

concentration of NaCl. Though, both effects are equivalent at a higher NaCl concentration (0.1 mol L−1), so the effect of NaCl on Cd2+ sorption was negligible. The effect of Ni2+ and Pb2+ as coexisting ions with similar charge densities and ionic sizes to those of Cd2+ (with Cd2+ to coexist ions ratio 1:1 and 1:2) was studied, and the results were presented in Figure 8b. According to the obtained results, despite the existence of these competitive ions, the prepared RPET/MWNT/LDH NC 4 wt % showed high removal efficiency toward the Cd2+. Effects of Contact Time and Adsorption Kinetics. The influence of contact time on adsorption of Cd2+ onto an adsorbent was measured, and the results showed that the sorption capacity on the adsorbent increased with contact time. The time required to reach equilibrium was 12 h. Pseudo-firstorder, pseudo-second-order, and intraparticle diffusion adsorption kinetics models were used to fit the obtained experimental data and to examine the adsorption properties of the adsorbent: ln(qe − qt) = ln qe − k1t

(2)

Pseudo‐second‐order

t /qt = 1/k 2qe 2 + t /qe

(3)

Intraparticle diffusion

qt = kit 0.5 + C

(4)

20.24 0.001 0.41 0.9916 0.001 5.22 0.9447 0.14 14.24 0.8951

Experimental. bCalculated.

linear plots of the models were illustrated in Figure 9. The kinetic data were fitted with a pseudo-second-order kinetic model with a higher R2 value (0.9916). By comparing the experimental and the calculated adsorption capacity values, it was found that pseudo-second-order adsorption kinetics can present the adsorption results well. According to the pseudosecond-order model, chemical sorption is the rate-limiting step, so the obtained results dependably proposed that the sorption manner may implicate electrons sharin or their exchange between the Cd2+ ions and R-PET/MWNT/LDH NC 4 wt %. Shan et al.12 have reported similar outcomes for the adsorption of Cd2+ by Mg−Al−CO3−LDH and magnetic Mg−Al−CO3− LDH. Thermodynamic Studies. In order to fully realize the adsorption nature, the changes in thermodynamic factors such as free energy (ΔG0), enthalpy (ΔH0), and entropy (ΔS0) were determined by using the following equations:

Figure 8. (a) Effect of ionic strength on the sorption of Cd2+ by RPET/MWNT/LDH NC 4 wt % (initial Cd2+ concentration, 0.001 mol L−1 (112 mg L−1); time, 24 h). (b) Effect of heavy metal ions Pb2+ and Ni2+ on the sorption of Cd2+ by R-PET/MWNT/LDH NC 4 wt % (initial Cd2+ concentration, 9 mol L−1 (10 mg L−1); time, 24 h).

Pseudo‐first‐order

20.13

ΔG 0 = −RT ln Kd

(5)

ln Kd = −ΔH 0/RT + ΔS 0/R −1

−1

(6) −1

R (8.314 J mol K ) and Kd (L g ) are the gas constant and the standard thermodynamic equilibrium constant, respectively. Kd (distribution coefficient (mL g−1)) can be achieved by division of qe into Ce. The ΔH0 and ΔS0 values were calculated from the slope and intercept of the linear Von’t Hoff plot7 of ln Kd versus 1/T (Figure 10). All thermodynamic parameters are together given in Table 4. According to the negative amounts of ΔG0 and positive values of ΔH0, it can be concluded that the adsorption of Cd2+ onto obtained composite is spontaneous and endothermic. Thus, removal efficiency of sorption increased with the increasing of temperature from 25 to 70 °C. This presents the endothermic nature of the procedure. The ΔS0 with positive value showed increasing in randomness at the solid/ solution interface during the adsorption. In general, when the ΔG0 value is in the range of −20 to 0 kJ·mol−1, the sorption procedure could be proposed as physisorption while with ΔG0 value between −400 and −80 kJ·mol−1, a chemisorption process can be expected.12 The obtained values of ΔG0 in this work were >−30 kJ mol−1, which shows that the main mechanism in this process is physical adsorption. Also, examination of ΔH0 values can help to distinguish the kind of adsorption. ΔH0 values lower than 20 kJ mol−1 show

−1

where qt and qe (mg g ) are the respective adsorption capacities of the metal at a time t and equilibrium, respectively; k1, k2, and ki are the adsorption rate constants of first-order, second-order, and intraparticle diffusion kinetic models (min−1, g mg−1 min−1, and mg g−1 min−0.5), respectively. The adsorption rate constants (k1, k2, and ki), the amount of adsorption at equilibrium (qe), and Ci could be determined using the slopes and the intercepts of each linear plot.30,31 Table 3 lists the corresponding kinetic parameters for the Cd2+ removal by adsorbent using different kinetic models. The 5752


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Figure 9. Kinetic curves: (a) pseudo-first-order model, (b) pseudo-second-order model, and (c) intraparticle diffusion model (sorbent dosage: 20 mg, T = 25 °C).

In Figure 11, the impact of initial Cd2+ concentrations (C0) on its sorption capacity and removal percent (R%) by the R-PET/ MWNT/LDH NC 4 wt % can be observed. According to it, with an increase in the C0 of Cd2+, a decrease in R% and increase in the sorption capability in aqueous media were seen (70−94%). The R% decrease and the presence of a plateau in the graph with more of an increase of Cd2+ concentration can be due to the fact that large numbers of porosity and active sites are available for the sorption at a lower initial concentration (20 ppm), and then with an increase of Cd2+ concentration, the sites are saturated, and it is difficult to capture the Cd2+. Also, Figure 11 shows high removal percentages for Cd2+ adsorption by the R-PET/MWNT/LDH NC 4 wt %. Freundlich and Langmuir as empirical and theoretical equilibrium isotherms are the most popular isotherm equations which can be applied to study adsorption results in the aqueous phase. In addition, the Dubinin−Radushkevich (DR) isotherm was employed to calculate the molecular sorption energy and describe the sorption mechanism. The linear forms of these equations can be described as

Figure 10. Liner plot of ln Kd versus 1/T of the Cd2+ sorption onto RPET/MWNT/LDH NC 4 wt %.

Table 4. Thermodynamic Parameters for Cd2+ Adsorption on R-PET/MWNT/LDH NC 4 wt % temperature (K)

ΔG0 (kJ mol−1)

298 310 323 343

−19.37 −22.90 −27.71 −29.91

ΔH0 (kJ mol−1)

ΔS0 (J mol−1 K−1)

52.54

243.37

physisorption processes (such as van der Waals interactions). Another type of physisorption is electrostatic interactions, which differ in the range of 20−80 kJ mol−1, and the chemisorption procedure can differ from 80 to 450 kJ mol−1.32 The resulted ΔH0 value also showed that the probable interaction involved in Cd2+ uptake is the electrostatic interaction. Adsorption Isotherm Models Fitting. The influence of MWNT/LDH loading on Cd2+ elimination was carried out, and the results showed an increase in Cd2+ removal percentages with increasing MWNT/LDH content as follows: 77%, 83%, and 94% for R-PET/MWNT/LDH NCs 1, 2, and 4 wt %, respectively. As was observed, R-PET/MWNT/LDH NC 4 wt % exhibited the highest removal percentages for Cd2+ sorption.

Ce/qe = Ce/qm + 1/KLqm

Langmuir

(7)

ln qe = 1/n ln Ce + ln KF

Freundlich

(8)

ln qe = ln qm − KDR ε 2

Dubinin−Radushkevich

(9)

In these equations, Ce is the equilibrium pollutant concentration (mg L−1), qe represents the adsorption capacity in equilibrium (mg g−1), KL (L mg−1) denotes the Langmuir constant related to adsorption energy, and qm is the maximum monolayer adsorption capacity of sorbent (mg g−1). Freundlich adsorption constants are n and KF corresponding to the intensity of adsorption and capacity of it, respectively. In eq 9, qm (mg g−1) is the theoretical isotherm saturation capacity. KDR 5753


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Figure 11. (A) The Freundlich, (B) Langmuir, and (C) Dubinin−Radushkevich isotherms for the Cd2+ on the NC surface. (D) Effect of initial solution concentration on adsorption capacity and removal percentage of Cd2+ (sorbent dosage: 20 mg, T = 25 °C).

Table 5. Isotherms Values and Constants for the Adsorption of Cd2+ on R-PET/MWCNT/LDH NC 4 wt % Langmuir constants

Freundlich constants

Dubinin−Radushkevich constants

qm (mg g−1)

KL (L mg−1)

R2

n

KF (L g−1)

R2

KDR (mol2/kJ2)

qm (mg g−1)

E (kJ mol−1)

R2

38.91

0.20

0.9821

2.53

9.27

0.9835

5 × 10−0.07

27.28

0.34

0.8473

Table 6. A List of Adsorbents for Comparison of Their qmax for Cd2+ Removal adsorbents pure bentonite Mg−Al−CO3−LDH pure CNTs magnetic Fe3O4/Mg−Al−CO3−LDH amino-functionalized magnetite/kaolin clay recycled PET/MWCNT-ZnO quantum dot grafted PET with 2-hydroxy propyl methacrylate poly(vinyl alcohol)/tetraethyl orthosilicate R-PET R-PET/MWNT/LDH NC 4 wt %

conditions T T T T T T T T T T

= = = = = = = = = =

25 30 30 30 25 25 25 25 25 25

°C; °C; °C; °C; °C; °C; °C; °C; °C; °C;

sorbent sorbent sorbent sorbent sorbent sorbent sorbent sorbent sorbent sorbent

dosage, dosage, dosage, dosage, dosage, dosage, dosage, dosage, dosage, dosage,

is the isotherm constant (mol2 kJ2), and ε represents the Polanyi potential obtained from ε = RT ln(1 + 1/Ce)

100 mg; Ci, 0−70 mg L ; pH = 5 ± 0.2 80 mg; 80−500 mg L−1 0.5 g/L; pH = 6.0 ± 0.1 80 mg; 80−500 mg L−1 80 mg; 20−200 mg L−1 20 mg; Ci, 20−100 mg L−1; pH = 6 20 mg; Ci, 5−60 mg L−1; pH = 6.7 100 mg; Ci, 10−500 mg L−1; pH = 5 20 mg; Ci, 20−100 mg L−1; pH = 6 20 mg; Ci, 20−100 mg L−1; pH = 6

qmax (mg g−1)

ref.

10.47 61.40 14.45 45.60 22.10 56.00 13.65 55.05 11.35 38.91

34 12 35 12 36 37 38 39 this work this work

favorability of the sorption procedure can be presented by RL as a separation factor without dimension, which is defined as RL = 1/(1 + KLC0)

(10)

(12) −1

where C0 is the initial ion concentration (mg L ) and KL is the the Langmuir constant (L mg−1). The obtained values of RL for R-PET/MWNT/LDH NC 4 wt % were varying from 0.05 to 0.20 that were between 0 and 1, which indicated favorable adsorption toward Cd2+. This result is consistent with the n parameter obtained from the Freundlich model. To distinguish the chemical and physical adsorption of Cd2+ ions on adsorbent, the D−R model was used. It has been revealed that if the 8 < E < 16 kJ mol−1 ion-exchange interactions have happened but E < 8 kJ mol−1 shows the physiosorption process.33 In this system, the E value 0.34 kJ mol−1 was obtained, demonstrating that the Cd2+ ions were adsorbed on the R-PET/MWNT/LDH NC 4 wt % surface

R and T are the gas constant (8.314 J mol−1 K−1) and absolute temperature (K), respectively. According to the D−R isotherm, E (alteration in free energy due to transferring of 1 mol of Cd2+ from the bulk of the solution to the solid surface) can be calculated by the following form:

E = 1/√−2KDR

−1

(11)

The data could be extracted from isotherms (slopes and intercepts; Figure 11) and are listed in Table 5. The R2 values showed that both isotherms are appropriate for description of the adsorption equilibrium (Table 5). The achieved value for n was >1 (2.53), which supports promising adsorption on the composite. In addition, the relative 5754


Research Article

ACS Sustainable Chemistry & Engineering Scheme 2. Possible Sorption Mechanisms of Cd2+ onto R-PET/MWNT/LDH NC 4 wt % Surface

wt % may be improved certainly by adding higher amounts of MWNT/LDH. Since the Al is not a biocompatible element, the presence of an Al value that can be released in the adsorption process was studied with an ICP-OES instrument and found to be 0.019 mg L−1. The Environmental Protection Agency (EPA) determined 0.2 mg L−1 as the maximum permissible Al amount in drinking water. Preferably, the Al level in drinking water should be less than 0.05 mg L−1, and the maximum amount permitted is 0.2 mg L−1.40 According to the above-mentioned results, the Cd 2+ adsorption by R-PET/MWNT/LDH NC 4 wt % probably can occur in two pathways,35,41,42 which were illustrated in Scheme 2: (1) The formation of inner complexes (bidentate and monodentate complexes) between −COOH and −OH functional groups on the surface of R-PET/MWNT/LDH NC 4 wt % and Cd2+ ions. (2) The formation of outer complexes (electrostatic attractions) between negatively charged composite groups and Cd2+ cations. Overall, the obtained results proposed that the main pathway in the sorption process between the R-PET/MWNT/LDH NC 4 wt % and Cd2+ is the physical sorption in this study. Reusability of R-PET/MWNT/LDH NC 4 wt %. From an economical point of view, regeneration and reuse of sorbents is an important factor. Thus, batch desorption studies were accomplished with an adsorption−desorption process for three cycles, and desorption capacities were compared. As displayed in Figure 12, the adsorption capacity of Cd2+ on the regenerated R-PET/MWNT/LDH NC 4 wt % reduced fairly compared to the first state. Though, the decrease is not significant after being reused three times. However, it had the potential to be reused for more cycles. The increase in adsorption capacity in the third stage may be related to the fact that in this stage, collected sorbent was shaken in CH3COONa more times. In conclusion, the combination of a small amount of costly CNTs with a cheap PET bottle obtained an attractive opportunity for introducing an efficient and low cost sorbent for pollutants removal from an industrial viewpoint. Furthermore, it is worth noting that with a combination of

through physical adsorption (electrostatic attraction). This result agrees well with the consequences of the pH effect. The comparison of the qmax parameter among previous different sorbents based on LDH and other composite materials and the obtained R-PET/MWNT/LDH NC 4 wt % is briefly mentioned in Table 6. Also, to examine the influence of MWNT/LDH loading on adsorption behavior of R-PET and as a blank test, behavior of pure R-PET on Cd2+ sorption beside R-PET/MWNT/LDH NC 4 wt % was examined, and its qmax was reported in Table 6. Accordingly, as fabricated R-PET/ MWNT/LDH NC 4 wt % seems to be favorable adsorbent for the Cd2+ elimination from aqueous systems. CNTs, are well-known to be one of the most favorable candidates for metal cations and oil sorption due to high porosity and surface area. On the other hand, corresponding to positively charged layers of LDHs, and strange anion-exchange features, they have been extensively discovered as sorbent materials for the elimination of anionic contaminants for aqueous media. Consequently, the MWNT/LDH hybrids are attractive components with outstanding performance for the removal of all the cationic, anionic, neutral, inorganic, and organic pollutants by adsorption.2 Furthermore, according to Table 6, not only was the adsorption capacity of the obtained composite more than that of pure bentonite and pure CNTs and other sorbents based on LDH mentioned, but also it may be said that this composite has low cost compared to all of them. Moreover, this sorption capacity was obtained just by adding of very low amount of MWNT/LDH (0.004 g) in the R-PET matrix. On the other hand, high removal efficiency was gained (about 94%) just by a small amount of sorbent (0.02 g) for 20 ppm of Cd2+. However, in most other works, it was used at higher amounts such as about 0.1 g of sorbent. In addition, the matrix of the polymer used is a recyclable PET bottle, and it changed to a valuable sorbent with high removal efficiency for Cd2+ sorption. In addition, it is worth noting that with a combination of MWNT/LDH with the R-PET matrix, its separation has gotten easy and fast. However, the maximum sorption capacity of the fabricated R-PET/MWNT/LDH NC 4 5755



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Research Article

AUTHOR INFORMATION

Corresponding Author

*Tel.: +98-31-3391-3267. Fax: +98-31-3391-2350. E-mail: [email protected], [email protected], mallakpour84@ alumni.ufl.edu. ORCID

Shadpour Mallakpour: 0000-0003-0304-6613 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank from the Research Affairs Division Isfahan University of Technology (IUT), Isfahan, Iran, for partial financial support. Also, the authors are thankful to Iran Nanotechnology Initiative Council (INIC), National Elite Foundation (NEF) and Center of Excellence in Sensors and Green Chemistry (IUT).

Figure 12. Diagram of reusability test on the regenerated R-PET/ MWNT/LDH NC 4 wt %.

MWNT/LDH with the R-PET matrix, its separation has gotten easy and fast. Hence, it can be found that the potential of simple recycling and reuse for pollutant removal was observed for R-PET/MWNT/LDH 4 wt %.

■

■

CONCLUSIONS Ultrasonication as a fast and green method was applied for LDH intercalation and polymer composite preparation. PET bottle waste was recycled and employed as a composite matrix. A facile method was used to prepare R-PET reinforced with a MWNT/LDH hybrid. Morphological investigation represented that the MWNT/LDH was well distributed in the R-PET, and good interfacial adhesion between the filler and matrix caused improvement in the thermal resistance of pure R-PET. Most of the LDH was exfoliated in the R-PET, which was confirmed by XRD and TEM analyses. The Cd2+ adsorption results showed dependency of the adsorption capacity of the obtained sorbent on the pH, contact time, and pollutant concentrations. Removal percentages were increased with an increase of MWNT/LDH content from 1 wt % to 4 wt %. The adsorption data were well fitted by the Freundlich and Langmuir isotherms. A good relationship with the pseudo-second-order model was represented by the kinetics data, which showed chemisorption throughout the Cd2+ sorption. According to various analyses, it was concluded that Cd2+ adsorption onto R-PET/MWNT/ LDH NC 4 wt % could be explained mainly by electrostatic interaction between composite and charged sorbent surfaces. The obtained qmax from the Langmuir isotherm exposed that 1 g of obtained composite can adsorb 38.91 mg of Cd2+. The results also showed that the sorption on the obtained R-PET/ MWNT/LDH NC 4 wt % follows a reversible procedure due to the possibility of sorbent activation by solvents and its reusage for sequential elimination actions. According to these results and their comparison with other sorbents, fabricated RPET/MWNT/LDH NC 4 wt % is a potential and active reusable sorbent for favorable heavy metals adsorption. The combination of a small amount of expensive CNTs with a cheap PET bottle obtains an attractive chance for presenting an effective and low cost sorbent for contaminant elimination from an industrial viewpoint.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00344. Thermal decomposition behavior and Figures S1 and S2 (PDF) 5756


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Water Sanitization by the Elimination of Cd2+ Using Recycled PET

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