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. 2020 Feb 12;5(7):3181–3193. doi: 10.1021/acsomega.9b03153

Synthesis and Characterization of Magnetic Superadsorbent Fe3O4-PEG-Mg-Al-LDH Nanocomposites for Ultrahigh Removal of Organic Dyes

Subramanian Natarajan †, Venkatesan Anitha †, Gnana Prakash Gajula ‡, Viruthachalam Thiagarajan †,*
PMCID: PMC7045307  PMID: 32118134

Abstract

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Considering the huge demands for economical and reliable eco-remediation applications, the goal of the present work is to synthesize cost-effective and functionally efficient magnetic layered nanocomposite adsorbents for the effective adsorption of dyes followed by easy separation from wastewater. This would ensure good reusability of adsorbents without altering its adsorption capacity in a relatively short time manner. To achieve this, different molecular weights of polyethylene glycol (PEG)-modified Fe3O4 combined with Mg–Al-layered double hydroxides (MAN-LDH) were synthesized and characterized using powder X-ray diffraction, Fourier transform infrared, scanning electron microscopy, transmission electron microscopy, thermogravimetric analysis, differential thermal analysis, energy-dispersive X-ray, and inductively coupled plasma optical emission spectroscopy. The efficacy of various adsorption parameters for the removal of methyl orange (MO) from water using Fe3O4-PEG-Mg-Al-LDH (FPL) adsorbents with different molecular weights of PEG (2FPL, 4FPL, and 6FPL) were investigated, and the results were compared. The maximum adsorption capacities of 2FPL, 4FPL, and 6FPL for MO were found to be 775.19, 826.44, and 833.33 mg/g, respectively. Detailed adsorption studies confirm that the higher adsorption capacity of 6FPL is due to the fast exchange of anions (NO3–) by MO in the interlayers of MAN-LDH, larger surface area, hydrogen bonding, and electrostatic interaction between adsorbate and adsorbent. The thermodynamic data indicate that the adsorption behavior is spontaneous and endothermic in nature. The reusability of all FPL adsorbents is observed to be excellent. The MAN-LDH recoated after the 31st-cycle nanocomposites show a recovery of 100% adsorption efficiency, similar to the freshly prepared 6FPL. Such systematic studies greatly help in advancing the applications of newly functionalized nanomaterials toward eco-remediation approaches.

1. Introduction

Water is now becoming a scarce resource globally and no longer treated as a free commodity.1 In many countries, clean water is a paid and taxable amenity.2 Improper management of industrial effluent, mainly organic dyes, is one of the main reasons for environmental damage because of their complex structure and low bio-degradability.3−5 Among various dyes, azo dyes such as methyl orange (MO), which are frequently used in many industries, can produce roughly 20 categories of carcinogenic aromatic amines that can alter the human DNA construction.6−9 Researchers from different areas have come together to find ultramodern, efficient, recyclable adsorbents to remove dyes from the effluent.10−14 However, the major problems are low adsorption capacity, difficulty in separating the adsorbed dyes, and their less-efficient reusability. Among the various methods, adsorption method shows promising results due to its advantages of being reversible and reusable, and it removes dyes in a large scale.15−17

In recent times, research on the magnetic adsorbent materials gains vital importance because of their unique physical and chemical properties. Especially, magnetite (Fe3O4) nanoparticles are gaining importance in the area of wastewater management.18 Due to their superparamagnetic property after reaching saturation in the adsorption process, the magnetic adsorbents can be easily separated from a solution with the support of an external magnetic field without using tedious filtration or centrifugation, and thus they exhibit high efficiency in wastewater treatment.18 In fact, the alteration of morphology and dimension is crucial in planning and developing adsorbents with novel adsorption properties.

The distinctive multiple positively charged metal cationic two-dimensional laminate with an interlayer balancing anion and huge interlayer spaces is obtainable by precise chemical composition and an appropriate number of exchangeable anions. The large surface area of layered double hydroxides (LDH) offers high potential for adsorption and catalysis in biological and pharmaceutical applications.19 LDHs are a member of two-dimensional anionic clays or hydrotalcite-like compounds with the general formula [MII1 – xMIIIx(OH)2]x+(An–)x/n.yH2O, where MII denotes a divalent cation, MIII represents a trivalent cation, and An– is an interlayer anion; x is defined as the molar ratio of MII and MIII metal ions.20 The structure of LDH is constructed as brucite-like layers (Mg(OH)2)− in which the cations are surrounded by octahedral of OH– groups.21 These octahedral units form infinite layers by edge-sharing, and these layers are stacked on top of one another to form the 3D structure.22

In addition to increasing the dye adsorption capacity of adsorbents, a new research area has emerged that aims to design Fe3O4 surface modified with LDH-based adsorbents for dye removal.23 Fe3O4-based LDH hierarchical structures have evoked considerable interest due to their excellent characteristics such as large surface area, enlarged interlayer free space, combination of numerous functionalities good separation efficiency with better successive reusability and a simple isolation with a magnet.24 Magnetic layered nanocomposites with a core–shell structural design have notable attention due to their distinctive morphology, stability and specifically the magnetic properties. They have the ability to remove dyes from an aqueous solution.25

The intention of this current work is to produce effective magnetic layered nanocomposite adsorbents to ease the separation of dyes after adsorption from wastewater to ensure the reusability along with high adsorption capacity within a short time. For that purpose, we synthesized a new magnetic nanocomposite material containing Mg–Al LDH (MAN-LDH), different molecular weights of polyethylene glycol (PEG), and magnetic iron oxide nanoparticles (Fe3O4). The main objective is to amplify the ability of adsorption of a dye at the surface of MAN-LDH deposited over Fe3O4-PEG (FP) and at the same time the reusability of nanocomposites by magnetic separation. Here, we report the synthesis of Fe3O4-PEG-MAN-LDH (FPL) nanocomposites and their characterization and adsorption activity toward MO dyes in an aqueous medium. The impact of several parameters such as adsorbent dose, initial dye concentration, adsorption rate, temperature, pH, and reusability of adsorbents on MO adsorption of FPL was studied. The adsorption kinetics, isotherms, and thermodynamic parameters have also been investigated. To our knowledge, this is one of the important reports with considerably higher adsorption capacity along with rapid adsorption of MO using FPL adsorbents compared to existing reported adsorbents.

2. Results and Discussion

The different molecular weight of PEG-modified Fe3O4 and its respective MAN-LDH-deposited nanocomposites were successfully synthesized. Figure 1a reveals the diffraction patterns of Fe3O4, 1.5FP, 2FP, 4FP, and 6FP and the diffraction peaks at 2θ were 30.23, 35.69, 43.23, 57.19, and 62.79°, correlating with the (220), (311), (400), (511), and (400) planes, respectively,26 and it confirms the presence of standard magnetite (Fe3O4) X-ray diffraction (XRD) patterns of a cubic spinel structure. No notable variation in the basic crystal structure of the 1.5FP, 2FP, 4FP, and 6FP was observed. Figure 1b displays the MAN-LDH diffraction peaks at 2θ were 11.42, 22.97, 31.86, 38.86, and 48.02°, correlating with the (003), (006), (012), (015), and (018) diffraction planes obtained in the LDH structure, respectively.23−25The XRD spectrum of FPL containing the peaks of both Fe3O4-PEG and MAN-LDH proves that both phases are present in the structure of magnetic layered nanocomposites.27,28 The entire XRD patterns reveal the presence of both Fe3O4 and MAN-LDH in the newly formed magnetic layered nanocomposites (FPL).

Figure 1.

Figure 1

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XRD Patterns of (a) Fe3O4, 1.5FP, 2FP, 4FP, and 6FP and (b) 1.5FPL, 2FPL, 4FPL, and 6FPL.

FT-IR studies were carried out to analyze the surface modification of Fe3O4 by PEG molecules. Figure 2a displays the FT-IR spectra of Fe3O4, free 6kPEG, and 6FP. The spectrum of Fe3O4 showed characteristic bands correlated to the Fe–O vibrations close to 628 (shoulder) and 583 cm–1. A broad −O–H stretch around 3416 cm–1 was noticed in 6FP. The spectrum of free 6kPEG showed that the characteristic bands correlated to the −O–H, −C–H, and −C–O vibrations are 3433, 2919, and 1101 cm–1, respectively. Compared with the spectrum of Fe3O4, there are some new peaks in the spectrum of 6FP. The bands at 2925 and 1083 cm–1 are related with the −C–H stretch vibration and −C–O vibration, respectively; these two vibrations were noticed in both the free 6kPEG and 6FP.26 The band at 628 (shoulder) and 583 cm–1 are correlated to the Fe–O vibrations, and these peaks are not detected in the spectrum of 6kPEG. Thus, it is evident that the PEG-modified Fe3O4 was obtained during hydrothermal reaction. Figure 2b displays the FT-IR spectra of 6FP, MAN-LDH, and 6FPL. The spectrum of MAN-LDH showed the characteristic bands correlated to the −O–H, −NO3– anions, and metal oxygen vibrations are 3495, 1381, and 416 cm–1, respectively. The spectrum of 6FPL indicates a mixture of 6FP and MAN-LDH, with a very small shift observed in the O–H and C–H stretching bands. The bands at 3450, 2925, 1381, and 394 cm–1 are related to the −O–H, −C–H, and −NO3– anions and metal oxygen vibrations, respectively. A shoulder peak is seen at around 2984 cm–1 for MAN-LDH as well as for 6FPL and is attributed to the hydrogen bonding between water molecules and nitrates in the interlayers of MAN-LDH sheets.29−31

Figure 2.

Figure 2

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FT-IR spectra of (a) Fe3O4, 6kPEG, and 6FP and of (b) 6FP, MAN-LDH, and 6FPL.

Surface morphology of the as-developed magnetic nanocomposites adsorbent is clearly displayed through SEM images. Figure 3 and Figure S1a show the surface morphologies of 6FP, 6FPL, 2FPL, and MAN-LDH adsorbents. The SEM image of 6FP shows that the surfaces of Fe3O4 were heterogeneous and spherical with an average diameter of around 16–30 nm. The MAN-LDH image shows a layered platelet placed one over the other. In the case of FPL nanocomposites, the MAN-LDH layer associates around the surface of the spherical FP nanoparticles. These results reveal that the MAN-LDH sheets are closely anchored around the 6FP.

Figure 3.

Figure 3

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Morphological evaluation of (a) 6FP and (b) 6FPL by SEM.

The EDS spectra of corresponding SEM images are presented in Figure S1b. Fe, Mg, Al, C, H, O, and Na element signals were identified in the spectra of 6FP, 6FPL, 2FPL, and MAN-LDH. The presence of Na confirms the deposition of MAN-LDH over FP. Compared to 6FPL, 2FPL has less percentage of carbon, due to the presence of a lower molecular weight of PEG. This can be an additional evidence to confirm that the different molecular weights of PEGs are incorporated inside the nanocomposites.

Elemental compositions of 6FPL, 2FPL, 6FP, and MAN-LDH in atomic percentage are given in Table S1. The carbon content in magnetic layered nanocomposite adsorbents is increasing in the sequence from 2FPL to 6FPL. This reveals that the presence of PEG in the 2FPL and 6FPL is on the rise in proportion with the molecular weight of PEG. The existence of nitrogen in the 6FPL, 2FPL, and MAN-LDH are due to the availability of interstitial nitrate ions in the LDH interlayer. It may precede from the precursor salts of Mg and Al. The presence of the minimum quantity of Na identified in the 6FPL, 2FPL, and MAN-LDH is possibly attributable to the use of NaOH during the deposition of MAN-LDH over FP. 6FPL, 2FPL, and MAN-LDH adsorbents reveal a Mg:Al molar ratio of 3:1, which is used during the synthesis of nanocomposites. This is a strong proof for the formation of MAN-LDH over different molecular weights of FP.

The Fe, Mg, and Al metals in 2FPL, 4FPL, 6FPL, and MAN-LDH nanocomposites were investigated by using inductively coupled plasma optical emission spectroscopy (ICP-OES), as these elements were existing at various concentration levels. Table S2 clearly displays the concentration of the metal content in 2FPL, 4FPL, 6FPL, and MAN-LDH nanocomposites. This result further confirms the presence of Mg:Al with a molar ratio of 3:1 in newly formed magnetic nanocomposites.

TEM analysis was performed on both the 6FP and 6FPL to study the particulars of the structures and obtain the additional confirmation. Figure 4a shows the existence of uniformly well-dispersed nanometer-ranged magnetic particles of an average diameter around 16–30 nm. Figure 4b exhibits the multiple layers of MAN-LDH around FP. The dark well-dispersed FP nanoparticles were interred consistently in the light grey color of FPL.

Figure 4.

Figure 4

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TEM images of (a) 6FP and (b) 6FPL.

The TGA curves for 6FP, MAN-LDH, and 6FPLnanocomposites are shown in Figure S2a. The TGA curve of 6FP nanoparticles displayed two stages of weight loss. Nearly 3.1% of weight loss was found from 25 to 360 °C, which was ascribed mostly to the decomposition of PEG26,32,33 and removal of adsorbed water molecules. Above 360 °C, around 15.69% of the huge weight loss observed was attributed to the phase transition of Fe3O4 and decomposition of impurities. The TGA curve of MAN-LDH nanocomposites also exhibited two stages of weight loss. The first weight loss is due to the loss of physically adsorbed water in the MAN-LDH. It was close to 1.64% at a temperature range of 25 to 150 °C. The second huge weight loss is due to the dehydroxylation, interlayer water molecules, and disappearance of intercalated ions and, furthermore, the LDH-layered structure collapse by 42.38% at the temperature range of 250 to 615 °C.34,35

The TGA curves of 6FPL magnetic layered nanocomposites resulted in two stages of weight loss from room temperature to 800 °C. The first weight loss of 13.5% in the variation from 175 to 335 °C is associated with the removal of water molecules, decomposition of PEG, and the dehydroxylation of the interlayer water molecules and the fact of intercalated ions. The second major weight loss occurring (42.32%) in the range of 450 to 700 °C is related to the loss of LDH structures, formed metal oxides, and the phase transition of Fe3O4. These weight losses are in good agreement with 6FP and MAN-LDH, and thus confirming that MAN-LDH deposited over FP. The TGA data shows the comparatively lesser thermal stability for 6FP compared to that of the MAN-LDH and 6FPL. The maximum weight losses as noted at 800 °C for the 6FP, MAN-LDH, and 6FPL are 17.77, 45.13, and 45.95% respectively.35,36 As shown in Figure S3a, the TGA data show comparatively lesser thermal stability of 6FP than that of LDH, 6FPL, 4FPL, 2FPL, and 1.5FPL. The total weight loss recorded at 800 °C for 6FP, LDH, 6FPL, 4FPL, 2FPL, and 1.5FPL are 17.77, 45.13, 45.95, 43.63, 47.14, and 45.65%, respectively.

Figure S2b clearly displays the DTA curves of 6FP, MAN-LDH and 6FPL, 2FPL, 4FPL, and 1.5FPL. Two stages of weight loss were obtained in 6FP, and it was noticed as endothermic peaks. The first decomposition temperature in the range of around 308 °C is coherent to the decomposing of PEG molecules. The second decomposition temperature at 462.85 °C is attributed to the phase transition of Fe3O4.26 Two stages of weight loss were found in MAN-LDH in the range of room temperature to 515 °C, and they were seen as exothermic peaks. The first decomposition temperature was in the range of room temperature to 275 °C and is attributed to the removal of surface water molecules. The second decomposition in the range of 275 to 515 °C is due to dehydroxylation, removal of intercalated anions, and collapse of the LDH structures.

As indicated by the DTA curve, 6FPL magnetic layered nanocomposites show two stages of decomposition temperature within 800 °C as exothermic peaks. The first decomposition temperature ranges from room temperature to 250 °C, and it is attributable to the disappearance of surface-adsorbed water molecules. The second decomposition temperature of 6FPL shows the temperature range of 250–480 °C. This is due to the removal of intercalated anions, collapse of the LDH structures, decomposition of PEG molecules, and dehydroxylation (Figure S3b).

The hysteresis loops was achieved when magnetic moment (emu/g) is plotted against field strength (G), as presented in Figure S4. 6FP exhibits strong magnetism, and it can be revealed as the saturation magnetization value of 70.00 emu/g.26 6FPL also exhibits magnetism, but it is relatively less due to the extra layer of shielding by nonmagnetic MAN-LDH. They have a saturation magnetization value of 10.98 emu/g.37 The saturation magnetism value of 6FPL enables to separate the adsorbents using an external magnet during adsorption studies.

The nitrogen adsorption–desorption isotherms and the correlated pore size dispersals of 6FPL are presented in Figure S5. It reveals a type-IV isotherm with a perfect hysteresis loop, indicating mesoporous characteristics. The specific surface area of 6FPL exhibited a value of 18 m2/g. This might be more desirable for the adsorbate to contact the active sites and achieve good adsorption capacity. The total pore volume of 6FPL is 0.04 cm3/g. This pore volume of 6FPL could afford more capable conveyance pathways to their internal voids, which in turn shows increased adsorption capacity of MO.29,38

2.1. Impact of the Molecular Weight of PEG on MO Dye Removal

The MO adsorption capacities of various nanocomposites with and without MAN-LDH and PEG have been studied and displayed in Figure 5 and Figure S6. MAN-LDH modified FP adsorbents show an increasing adsorption capacity compared to those that of Fe3O4, FP, MAN-LDH, and Fe3O4-MAN-LDH. Among all the adsorbents, 6FPL has a significant impact on their adsorption capacity, owing to the large adsorption sites and exchange of anion capability, and the higher molecular weight of PEG also plays a major role.39

Figure 5.

Figure 5

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Adsorption capacity of various adsorbents toward MO adsorption (initial MO concentration, 100 mg/L; adsorbent dose, 0.05 g; adsorption time, 24 min; temperature, 30 °C).

However, the increase in adsorption capacity of 1.5FPL, 2FPL, 4FPL, and 6FPL are related to the increase in the molecular weight of PEG. Adsorption capacity of 1.5FPL was found to be low when compared to those of 2FPL, 4FPL, and 6FPL. Hence, a detailed study was performed only with 2FPL, 4FPL, and 6FPL. In order to understand the adsorption ability of 6FPL toward other anionic and cationic dyes, adsorption studies were carried out with 0.05 g of 6FPL magnetic layered nanocomposites in 25 mL of a 100 mg/L solution of MO, acid blue 113 (AB-113), Nile blue A (NB-A), and acid blue 147 (AB-147). The solution was shaken at 200 rpm up to a predefined contact time at 30 °C until the equilibrium was reached. The percentage adsorption of MO, AB-113, NB-A, and AB-147 are exhibited in Figure S7. The percentage adsorption for MO is significantly higher than AB-113, AB-147, and NB-A, which may be due to influence of their size and charge of dyes.

2.2. Impact of Adsorbent Dose

The effects of different adsorbent dosages were studied to examine cost effectiveness. As displayed in Figure 6, it is obvious that the adsorption percentage of MO increases from 54.22 to 96.79, 71.47 to 97.84, and 77.31 to 98.89% with respect to 2FPL, 4FPL, and 6FPL doses respectively, increasing from 0.01 to 0.06 g on a 100 mg/L MO solution with a short contact time of 24 min. However, there is only a minor change of MO adsorption percentage when the 2FPL, 4FPL, and 6FPL dosage was above 0.05 g. This may be due to low adsorbent dosage, leading to the dispersal of 2FPL, 4FPL, and 6FPL into the aqueous solution; therefore, all adsorbent surface sites were completely exposed, which eases the adsorption capability of MO. In this process, the adsorbent surface sites are saturated quickly, leading to high adsorption capacity. Therefore, the optimum adsorbent dose is kept at 0.05 g for further detailed adsorption studies.

Figure 6.

Figure 6

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Impact of adsorbent dose on the MO adsorption for various FPL adsorbents in an aqueous solution (initial MO concentration, 100 mg/L; adsorbent dose, 0.01 to 0.06 g; adsorption time, 24 min; temperature, 30 °C).

2.3. Impact of Contact Time and Initial Dye Concentration

Two initial MO concentrations of 100 and 1000 mg/L were chosen to study the impact of contact time and initial concentration for adsorption of MO on 2FPL, 4FPL, and 6FPL. As shown in Figure 7, at a lower MO concentration, the adsorption rate is dependent on time along with very fast kinetics with an adsorption percentage of 87.54 (5 min), 91.07 (4 min), and 95.01% (1 min) for 2FPL, 4FPL, and 6FPL, respectively. Subsequently, in the extended contact time, the adsorption rate became moderate, and the equilibrium was reached quickly at around 9, 7, and 2 min for 2FPL, 4FPL, and 6FPL, respectively. At a higher initial concentration of MO dye (1000 mg/L), the adsorption rate was also based on time and moderately fast at initial interval with adsorption percentages of 75.58 (7 min), 81.27 (5 min), and 83.00% (4 min) for 2FPL, 4FPL, and 6FPL, respectively (Figure 7b). Thereafter, during the prolonged contact time, the adsorption rate became slow, and equilibrium was reached at about 16, 14, and 10 min for 2FPL, 4FPL, and 6FPL, respectively.

Figure 7.

Figure 7

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Impact of contact time on MO adsorption of 2FPL, 4FPL, and 6FPL at two different adsorbate concentrations of (a) 100 mg/L (inset figure, before (I) and after (II) adsorption of MO) and (b) 1000 mg/L. (c) Effect of the initial MO concentration on 2FPL, 4FPL, and 6FPL (adsorbent dose, 0.05 g; contact time, 24 min; temperature, 30 °C).

In lower and higher concentrations, the faster adsorption rate of MO on 2FPL, 4FPL, and 6FPL is attributed to the interactions between MO with active sites of FPL, fast exchange of anions (NO3–) by MO in interlayers of MAN-LDH, larger surface area, and strong electrostatic interaction between MO and FPL. Moreover, the adsorption rate of 6FPL is much quicker than 2FPL and 4FPL, which is due to the higher molecular weight of PEG that plays a vital part in achieving a higher adsorption capacity and rate. As the molecular weight of PEG increases, the distance between the PEG and surface of the MAN-LDH layer also increases, which in turn results in more active sites in 6FPL. Thus, the equilibrium of adsorption capacity was reached within a short time interval. This is the most excellent parameter satisfying the application of 6FPL on large scale wastewater management units. The short adsorption rate suggests that additional treatment batches can be managed in every time interval and also the number of wastewater management units may be reduced.

The initial dye concentration of MO ranging from 100 to 2000 mg/L was used for an adsorption batch investigation, and the results are displayed in Figure 7c. The increase in the initial MO concentration leads to the decrease in adsorption percentage. At high concentrations, the quantity of MO is more than that of available adsorption sites. In contrast, at low concentrations, the quantity of MO is less than that of the available adsorption sites, which leads to a higher adsorption percentage of MO. The percentage adsorption of MO was decreased from 96.07 to 69.89, 98.70 to 78.02, and 99.63 to 78.36% with an increase in the initial MO concentration from 100 to 2000 mg/L for 2FPL, 4FPL, and 6FPL, respectively at a 24 min contact time.

2.4. Adsorption Kinetics

A second-order kinetic model40 was used to investigate the adsorption rate and mechanism of 2FPL, 4FPL, and 6FPL for MO. The linearized format of the rate equation, eq SE1, and its details are presented in the Supporting Information.

The kinetic constants are determined from the linear plot of eq SE1. The calculated values of the pseudo-second-order kinetic parameters and correlation coefficients (R2) are given in Table S3, and the linear plots of pseudo-second-order kinetic model obtained are shown in Figure S8. As stated in Table S3, all values of correlation coefficients (R2) achieved by the pseudo-second-order kinetic model were closer to one (0.99) for 2FPL, 4FPL, and 6FPL at different initial dye concentrations. The determined qcal values by the pseudo-second-order kinetic model are very close to the experimental values (qexp). The rate constant value of k2 decreases with an increase in the initial dye concentration from 100 to 2000 mg/L. These observations explain a strong interaction between FPL and MO similar to the earlier literature.41,42

2.5. Effect of pH

Effect of pH on MO adsorption of 2FPL, 4FPL, and 6FPL was investigated in the pH range of 3 to 10. As displayed in Figure S9, adsorption performances of all three adsorbents are in the same range of pH. The percentage of MO adsorption by 2FPL, 4FPL, and 6FPL altered from 87.65 to 88.19, 90.82 to 94.04 and 92.37 to 96.67%, respectively with respect to pH ranges from 3 to 10. There were only minor changes of adsorption percentage in the pH range of 4 to 8. All three adsorbents show a higher percentage of MO adsorption at the natural pH (pH = 6.8). In less than pH 4, there were moderate changes in the MO adsorption percentage. In acidic pH, dissolution occurs in the magnetic layered nanocomposite adsorbents. However, in the alkaline medium, there is an increase in the amount of hydroxide ions, which in turn leads to a decrease in the percentage of MO adsorption. The electrostatic attraction between the positively charged adsorbent surface layer and the anionic adsorbate leads to higher adsorption of MO on 2FPL, 4FPL, and 6FPL. Investigation of pH impact on adsorption percentage values revealed the electrostatic interaction between the adsorbent and adsorbate.25,43

2.6. Effect of Temperature

Temperature is an essential physicochemical study factor, as the adsorption capacity of the adsorbent may vary with changes in temperature. Generally, thermodynamic investigations are carried out at different temperatures to analyze the data. Adsorption studies were carried out at five different temperatures (288.15, 298.15, 308.15, 318.15, and 328.15 K) to determine the thermodynamics parameters such as standard enthalpy (ΔH), standard entropy (ΔS), and standard free energy (ΔG) using eqs SE2 and SE3.

The linear plots of lnKo versus 1/T are shown in Figure S10. All thermodynamics parameters are calculated and presented in Table 1. The positive ΔS value confirms the greater attraction of MO toward FPL during the adsorption process. The positive ΔH value indicates that this adsorption system was endothermic. The negative ΔG value confirms that the adsorption of the adsorbate on the adsorbent was spontaneous and thermodynamically feasible at higher temperatures. The thermodynamic parameters confirm that the FPL could be a potential adsorbent for the adsorption of MO from aqueous solutions.42

Table 1. Values of Thermodynamic Parameters for MO Adsorption on 2FPL, 4FPL, and 6FPL at a 2 min Contact Time.

adsorbents T (K) –ΔG (kJ/mol) ΔS (J/mol/K) ΔH (kJ/mol)
2FPL 288.15 –59.7592843 97.51 29.60
298.15 –96.75705295
308.15 81.89276181
318.15 185.9127934
328.15 424.7144652
4FPL 288.15 –59.24516949 128.71 37.95
298.15 18.54912766
308.15 232.9304793
318.15 518.3760204
328.15 765.843375
6FPL 288.15 2.799804451 176.55 51.03
298.15 120.7632626
308.15 308.3261326
318.15 628.6217777
328.15 1252.513822
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Figure S11 shows the adsorption behavior of 2FPL, 4FPL, and 6FPL with a 1000 mg/L MO solution and a 24 min time interval. The adsorption capacity increases with the increase in temperature from 15 to 55 °C, and it represents the endothermic nature of MO adsorption. The adsorption capacity of MO at 2 min on 2FPL, 4FPL, and 6FPL are found to be 414.73, 454.48, and 481.49 mg/g, respectively (55 °C). The results exhibit that the temperature aids for strong interaction between adsorbate and adsorbent. The synthesized magnetic layered nanocomposites could be a favorable material for the excellent removal of MO from aqueous solutions.42

2.7. Impact of Ionic Strength

The ionic strength is one of the parameters that have an effect on the hydrophobic and electrostatic interaction between anionic dyes and adsorbent surface sites. Figure 8 displays the effect of NaCl concentration on the adsorption percentage and capacity of MO. As the NaCl concentration increases from 0.1 to 0.8 M, the adsorption capacity and percentage adsorption for 6FPL decreases from 47.61 to 20.69 mg/g and 94.91 to 31.69%, respectively at a 2 min contact time. These results confirm the electrostatic interactions operative between MO and FPL. The other adsorbents also followed the same pattern on increasing the ionic strength.

Figure 8.

Figure 8

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Effect of salt concentration on MO removal by 6FPL (initial MO concentration, 100 mg/L; adsorbent dose = 0.05 g; contact time, 2 min; temperature, 30 °C).

2.8. Adsorption Isotherm

The adsorption isotherm reveals the adsorption performance of MO on the surface of 2FPL, 4FPL, and 6FPL magnetic layered nanocomposites. The experimental data were fitted by two isotherm models Langmuir44 and Freundlich.45 These isotherm equations are conveyed as eqs SE4 and SE5 in the Supporting Information.

The adsorption isotherm of MO onto the 2FPL, 4FPL, and 6FPL adsorbents was achieved by varying the initial dye concentration from 100 to 2000 mg/L, as shown in Figure 9. The equilibrium adsorption capacity increases with the increase in the initial MO concentration for 2FPL, 4FPL, and 6FPL. The adsorption capacity of 6FPL is considerably higher than 4FPL and 2FPL. The equilibrium adsorption capacity values are 682.09, 761.38, and 764.70 mg/g at the equilibrium concentrations of 588.67, 429.57, and 428.98 mg/L, respectively.

Figure 9.

Figure 9

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MO uptake per gram of 6FPL, 4FPL, and 2FPL adsorbents (initial MO concentration, 0–2000 mg/L; adsorbent dose, 0.05 g; contact time, 24 min; temperature, 30 °C).

The values of isotherm constants for the two isotherm models were calculated from the slope and intercept of the linear plot of the two isotherm equations, eqs SE4 and SE5. The values of adsorption isotherm constants and correlation coefficients are given in Table 2. The linearized plots of the Langmuir and Freundlich isotherm models are presented in Figure S12. As indicated in Table 2, the acquired values of R2 of 2FPL, 4FPL, and 6FPL for the Langmuir isotherm were close to 1 and show a better fit than that of Freundlich isotherms. The data of the Langmuir constant Vm demonstrates the monolayer adsorption capacity for 2FPL, 4FPL, and 6FPL that are close to the experimental values. The calculated monolayer adsorption capacities of MO on the 2FPL, 4FPL, and 6FPL are 775.19, 826.44, and 833.33 mg/g, respectively.37,43

Table 2. Isotherm Parameters of MO Adsorption on 2FPL, 4FPL, and 6FPL.

    isotherm constants
isotherm model constant 2FPL 4FPL 6FPL
Langmuir bL 0.0124 0.0254 0.0652
Vm 775.19 826.45 833.33
R2 0.994 0.990 0.998
Freundlich n 1.767 2.043 2.513
kf 26.24 51.52 93.95
R2 0.917 0.935 0.880
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The lower values of the Langmuir constant bL, expressive of the adsorption bond energy, indicate the physical characteristics of the adsorption of MO on 2FPL, 4FPL, and 6FPL. The Freundlich constant values of n greater than one denote the better adsorption characteristics of the 6FPL, 4FPL, and 2FPL in the adsorption of MO. The values of correlation coefficients (R2) show fitness of the Langmuir isotherm model for the existing adsorbent–adsorbate model. Therefore, the Langmuir isotherm model can be regarded as the best model for MO adsorption on 6FPL, 4FPL, and 2FPL.

The adsorption capacity values of 2FPL, 4FPL, and 6FPL magnetic layered nanocomposites are compared with other adsorbents as reported in the literature and presented in Table S4. In this table, the experimental adsorption capacity, calculated adsorption capacity from Langmuir adsorption model, adsorption contact time, adsorbent dose, reusability cycle, or number of reused cycles are compared. From this data, it is very clear that the adsorbents used in this study are found to have greater adsorption capacity in the removal of effluents from wastewater within a short interval of time.42,46−56

2.9. Mechanism of MO Uptake by the 6FPL Adsorbent

To know the adsorption mechanism for removal of MO using 6FPL magnetic layered nanocomposites, XRD and FTIR studies were carried out for 6FPL, 4FPL, and 2FPL before and after adsorption of MO and are shown in Figure 10.

Figure 10.

Figure 10

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(a) XRD patterns of 6FPL, 4FPL, and 2FPL before and after adsorption (b) FT-IR spectra of 6FPL before and after MO adsorption.

The XRD patterns of the MO-adsorbed 6FPL, 4FPL, and 2FPL were recorded and compared with that of the free 6FPL, 4FPL, and 2FPL (Figure 10a). The harmonic peak observed at 11.5° (0.769 nm, d003 space) in all three FPL adsorbents corresponds to characteristics of MAN-LDH with the nitrate interlayer ions. The XRD patterns of MO-adsorbed 6FPL, 4FPL, and 2FPL show new peaks due to the 003 diffraction plane that corresponds to nitrate ion replacement by MO molecules at 2θ positions 10.08° (0.880 nm), 11.16° (0.792 nm), and 11.21° (0.785 nm), respectively.42 The enlargement in the d-space value after adsorption shows an intercalation of MO within the magnetic layered nanocomposites of MAN-LDH. The increased d003 spacing is in the order of 6FPL > 4FPL > 2FPL, which confirms that more MO molecules are adsorbed into the interlayers of LDH in 6FPL.42

The FT-IR spectra of MO-adsorbed 6FPL, free 6FPL, and MO are compared in Figure 10b. The intense and broad absorption band between 3800 and 2900 cm–1 in the spectra of 6FPL and MO-adsorbed 6FPL are due to stretching vibrations of hydrogen-bonded hydroxyl groups in the magnetic layered nanocomposites. The FT-IR spectrum of 6FPL shows a broad peak at 1382.72 cm–1 due to antisymmetric stretching vibration of intercalated nitrate ions in the interlayer space. In contrast, the spectrum of MO-adsorbed 6FPL shows a sharp peak at 1381.46 cm–1. This difference is due to the replacement of nitrate ions by MO. The moderate sharp peak between 1609–1630 cm–1 is due to the O–H bending vibration of the interlayer water molecules. The 6FPL displays peak at 2889, 1040, and 1220 cm–1 that are characteristic bending vibration peaks of C–O–H and symmetric stretching vibration of the C–H group of PEG. The sharp broad peak at 650 cm–1 is due to the metal oxides of Fe, Mg, and Al. The MO spectra shows characteristic absorption bands of MO at 1612 and 1514 cm–1 for the aromatic C=C and the azo group (−N=N−), respectively. The absorption bands due to the benzene group can be seen at 1125 cm–1. The above mentioned MO peaks are present in MO-adsorbed 6FPL. This observation confirms the MO adsorption uptake process.42

2.10. Reusability and Regenerative Studies

Reusability of the adsorbents is a significant factor in continuous batch adsorption studies. The reusability studies of adsorbents are shown in Figure 11. For the reusability cycle, 0.05 g of every adsorbent was shaken for 24 min in 25 mL of 100 mg/L of the MO solution. After each adsorption cycle, the adsorbent was regenerated by washing three times with DD water and ethanol. The amount of MO adsorbed by the magnetic layered nanocomposites was determined by measuring the absorbance of unabsorbed MO at 463 nm using a UV–vis spectrophotometer. It is clearly observed from Figure 11 that the recycled adsorbent after desorption shows gradual decrease in efficiency in further cycles. The MO adsorption decreases from 100 to 80% for all three adsorbents in cycles 0–6. After that, the percentage adsorption was gradually decreased for further cycles.

Figure 11.

Figure 11

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Reusability studies of 2FPL, 4FPL, and 6FPL toward MO adsorption in an aqueous solution (initial MO concentration, 100 mg/L; adsorbents dose = 0.05 g; adsorption time, 24 min; temperature, 30 °C).

Compared to 2FPL and 4FPL, 6FPL has more reusable capacity (Figure 12a) due to the presence of the high molecular weight PEG. 6FPL gives moderate results up to 30 cycles of reusability, after which it attains the saturation level. The recycled nanocomposites after the 31st cycle were separated into four parts. The first part of the 6FPL was recoated with MAN-LDH, the second part of the 6FPL was washed with 0.1 N HCl, the third part of the 6FPL was washed with 0.1 N NaOH, and the fourth part of the 6FPL was washed with both 0.1 N HCl and 0.1 N NaOH. Further, the adsorption studies were carried out with four separated parts of recycled 6FPL in 100 mg/L MO, and their results are presented in Figure 12b. The 0.1 N HCl, NaOH, and both HCl and NaOH washed 6FPL showed 61.79, 51.63, and 39.38% adsorption of MO. The MAN-LDH recoated 6FPL resulted in 99.37% of adsorption as similar to the freshly prepared 6FPL. Thus, an innovative technique to reuse FPL has been found out. It can be concluded that 6FPL can be used repeatedly with negligible loss in their adsorption capacity for MO.

Figure 12.

Figure 12

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(a) Results of reusing 6FPL on MO. (b) Percentage adsorption of recoated MAN-LDH after HCl and NaOH washing of the adsorbent (6FPL) (initial MO concentration, 100 mg/L; adsorbents dose = 0.05 g; contact time, 24 min; temperature, 30 °C). (c) Schematic representation of the regeneration protocol.

3. Conclusions

In this study, magnetic adsorbent FPL’s with different molecular weights of PEG were synthesized and characterized. The morphology of the synthesized magnetic layered nanocomposites and their adsorption performance with MO were investigated. It was confirmed that the morphologies of nanocomposites significantly influenced their adsorption capacity. The results show that all three adsorbents exhibit excellent adsorption performance within a short time interval of 5 min. There is a remarkable high adsorption capacity and easy separation of adsorbents with a magnet after the adsorption process was achieved. The kinetics of 2FPL, 4FPL, and 6FPL were well defined by the pseudo-second-order kinetic model. Isotherm investigations elucidated that the adsorption of MO on 2FPL, 4FPL, and 6FPL was better described by the Langmuir model of monolayer adsorption. Among the three adsorbents, 6FPL shows a higher adsorption capacity with a value of 833.33 mg/g. The thermodynamic data suggests that the adsorption of MO on FPL is endothermic in nature. The adsorption process has been found to be spontaneous and thermodynamically feasible for FPL and MO. The reusability of FPL is quite excellent for MO adsorption. The high adsorption capacity of the 6FPL revealed its potential as an effective adsorbent in wastewater treatment plants.

4. Experimental Section

4.1. Reagents

The chemicals were of analytical reagent grade and used without further purification. FeCl3·6H2O, FeCl2·4H2O, and urea were purchased from Sigma-Aldrich. Al(NO3)3·6H2O, Mg(NO3)2·6H2O, NaOH, and ethylene glycol were purchased from Merck. Polyethylene glycols (PEG) with different molecular weights (1.5, 2.0, 4.0, and 6.0 k) were purchased from Loba Chemie. Methyl orange, acid blue 113, acid blue 147, and Nile blue A were purchased from Alfa Aesar. The stock solution for all the experiments was prepared using double distilled (DD) water.

4.2. Synthesis of the Adsorbents

4.2.1. Synthesis of PEG Modified Fe3O4 (FP)

The PEG-modified Fe3O4 nanoparticles were prepared via the hydrothermal method. Synthesis of PEG-modified Fe3O4 was carried out by the following procedure. A total of 0.06 N of FeCl3·6H2O was entirely dissolved in 60 mL of ethylene glycol for the formation of a homogeneous solution under active stirring. Subsequently, 1 mL of 1.5 k PEG, 0.6 N of urea, and 10 mL of DD water were added to the above solution. The mixture was kept under continuous stirring until the solution become clear, and after that, it was transferred into a 100 mL Teflon-lined stainless-steel autoclave. It was heated at 205 °C for 10 h and then cooled to room temperature. The magnetic nanoparticles were separated using magnets, washed with DD water and ethanol to remove organic and inorganic impurities, and then dried in a hot air oven at 80 °C for 10 h. The dried magnetic nanoparticles were thereafter denoted as 1.5FP. A similar protocol was followed for synthesizing 2FP, 4FP, and 6FP with respective molecular weight PEG.

4.2.2. Deposit of MAN-LDH over PEG-Modified Fe3O4 (FPL)

Fe3O4-PEG-MAN-LDH (FPL) magnetic layered nanocomposites were developed via a simple chemical precipitation method. A total of 1.0 g of 1.5FP was dispelled into 30 mL DD water for 15 min and later transferred into a 500 mL beaker at 60 °C in a magnetic thermostat with constant stirring. The molar ratio of 3:1 Mg(NO3)2·6H2O and Al(NO3)3·6H2O mixed metal precursor solution was co-precipitated over the 1.5FP dispersed medium. Subsequently, a 2 N NaOH solution was added drop-wise to the above solution with constant stirring until the solution pH of 10 ± 0.2 at 60 °C. The above mixture was aged for 15 h under continuous stirring at room temperature. The resultant grey magnetic layered nanocomposites precipitate was easily separated by magnet. It was systematically washed with DD water to take away any extra alkalinity and impurities. Washed magnetic layered nanocomposites were dried at 80 °C for 10 h and then powdered. The magnetic layered nanocomposites are thereafter represented as 1.5FPL. The same protocol was followed for synthesizing 2FPL, 4FPL, and 6FPL. A brief schematic illustration is shown in Scheme 1.

Scheme 1. Schematic Illustration of the Fabrication of Magnetic Layered Nanocomposites (Fe3O4-PEG-MAN-LDH).

Scheme 1

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4.3. Characterization Techniques

Powder X-ray diffraction (XRD) was performed with a Phillips powder diffractometer, X’Pert MPD, with Cu Kα (λ = 1.540589° A) radiation in the 2θ range of 2–80°. The Fourier transform infrared (FT-IR) spectral analysis of the adsorbents was performed in an Equinox 55 Bruker with the ATR method over a wavenumber region of 400–4000 cm–1. The thermal analysis (TA) of the adsorbents was executed with a Q50 from TA Instruments of thermogravimetric analyzer under air in the temperature range of 25–800 °C and the heating rate of 20 °C/min. The morphological investigation of the adsorbents were performed with a field emission scanning electron microscopy (FESEM) using JEOL, JSM-6701F field emission instrument at the electron source of cold field emission (<310> W crystal), with an accelerating voltage variable from 0.5 to 30 kV, magnification variable from ×25 to ×650,000, and working distance of 10 mm. Transmission electron microscopy (TEM) images were obtained using Tecnai F20 (FEI Company) at 200 kV.

The amount of iron, magnesium, and aluminum metal ions present in MAN-LDH, 2FPL, 4FPL, and 6FPL were quantified by using ICP-OES (Perkin-Elmer Optima 5300 DV spectrometer). About 5 mg of the sample is digested in 5 mL of concentrated hydrochloric acid (HCl) overnight. Then, 45 mL of deionized water is added to the solution for dilution. The resulting solutions were filtered and directly used for measurement. During the analysis, the HCl mixture was used as the reagent blank, and a calibration plot was made using high-purity ICP standards. The specific surface areas of the adsorbents were measured with a Quantachrome Instruments (Version 5.0) by the Brunauer–Emmett–Teller (BET) method using the nitrogen adsorption–desorption technique at 77 K. The magnetic properties of the adsorbents were assessed by JDM-13 vibrating sample magnetometer (VSM).

4.4. Adsorption Studies

The MO dye was used for FPL adsorption investigation. In the batch adsorption study, a 25 mL dye solution of diverse concentrations and the needed quantity of adsorbents were taken in a 100 mL stopper glass conical flask shaken at 200 rpm up to a predefined contact time at 30 °C. At the end of the adsorption test, the adsorbent was isolated from the dye solution using an external magnet, and the remaining dye concentrations were found out from the absorbance value measured at 483 nm using a Cary 100 UV–vis spectrophotometer.

The percentage adsorption was calculated using the formula

4.4.

The adsorption capacity was calculated using the formula

4.4.

where C0 is the initial MO concentration, Ce is the equilibrium MO concentration (mg/L), qe is the amount of MO adsorbed per unit gram of the adsorbent at equilibrium, v is the volume of the MO solution (L), and w is the weight of the adsorbent (g).

The impacts of the adsorption efficiency of three adsorbents were studied with an MO concentration of 100 mg/L in a 25 mL solution. To investigate the impact of the quantity of adsorbent on the adsorption capacity, a sequence of adsorption studies were performed by varying the mass of adsorbent from 0.01 to 0.06 g in a 25 mL dye solution, which were kept in a shaker for 24 min at 30 °C. For investigations, the adsorption rates were tested on a time interval of 0–24 min with an initial MO concentration of 100–2000 mg/L at predefined time intervals. The adsorption isotherms were tested with a series of initial MO concentrations from 100 to 2000 mg/L at 30 °C. The impact of temperature was studied at five different temperatures (283.15, 298.15, 308.15, 318.15, and 328.15 K) with a 1000 mg/L initial MO concentration and a time interval of 0–2 min contact time. The impact of ionic strength was studied at five different NaCl concentrations from 0.1 to 0.8 N, and the initial MO concentration was 100 mg/L with a contact time of 2 min at 30 °C. To investigate the impact of pH on the adsorption competency, adsorption tests were performed with 25 mL of a 100 ppm MO dye solution at various pH values (3–12) with 0.05 g of the adsorbent. To ascertain reusability of 2FPL, 4FPL, and 6FPL, the utilized FPL’s were cleaned with DD water and ethanol for three times, and then dried at 80 °C in a hot air oven for 2 h before further use.

Acknowledgments

V.T. thanks SERB, New Delhi, India [grant no. YSS/2014/00026], DST Nanomission [grant no. DST/NM/NB-2018/10(G)], for financial support and UGC, New Delhi, for a start-up grant and UGC FRP faculty award [F. 4-5(24-FRP)/2013(BSR)]. S.N. acknowledges UGC-RFSMS for fellowship. We thank DST, New Delhi for providing NMR, HRMS and TCSPC facilities under FIST program.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.9b03153.

  • Elemental compositions of 6FPL, 2FPL, 6FP, and MAN-LDH; concentration of metal contents in 2FPL, 4FPL, 6FPL, and MAN-LDH; SEM images; TGA and DTA curves; magnetic hysteresis loops; N2 adsorption–desorption isotherms; adsorption capacity of FP and FPL adsorbents with different molecular weights of PEG toward MO; percentage adsorption of 6FPL for various anionic and cationic dyes; pseudo-second-order model for the adsorption of MO on FPL adsorbents; effect of pH and temperature on Mo adsorption; Langmuir and Freundlich isotherm models of MO adsorption; comparison of MO adsorption capacities of various adsorbents reported in the literature; characteristics of MO before adsorption and after desorption using absorption and HR-MS spectra (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao9b03153_si_001.pdf (571.9KB, pdf)

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