A magnetically active nanocomposite material has been synthesized from the reaction mixture of magnetite core iron nanoparticles electrostatically coated with SiO2, hydrotalcite nanosheets ([Eu8 (OH)20 (H2O)n]4+), and decatungstophosphate anion ([α-PW10O367−]). The resulting nanocomposite material, denoted as Fe3O4@SiO2@LEuH@PW10, is demonstrated to effectively adsorb chromate anions from aqueous solutions. The adsorption isotherms fit the Langmuir model with a capacity of 23 mmol·g−1 after 42 minutes at 25°C. The reaction is spontaneous at room temperature with 44.22 kJ·mol−1 of activation energy required. In addition, heating the chromate-adsorbed nanocomposite material at 40°C results in dissociation of the chromate anions from the nanocomposite material. As such, the recycled adsorbent Fe3O4@SiO2@LEuH@PW10 is reused for chromate removal in aqueous solutions for at least ten times without obvious loss of activity. This spontaneous reversible chemisorption mechanism for chromate adsorption provides a new pathway for separation and cleaning of industrial wastewater contaminated with chromate ions. The robust catalytic activity of the nanocomposite is also demonstrated.

1. Introduction

Chromium is used in various industrial activities such as electroplating, wood preservations, metallurgy, tanning, refractory materials, catalysis, dye, and pigment [1]. Chromium has unique properties such as anticorrosion, color, toxicity, heat resistivity, and high melting point that make it very difficult to replace in industry [2]. As such, the related industries generate large amounts of wastewater contaminated with chromium [3, 4].

Once in an aquatic environment, chromium exists in the form of trivalent or hexavalent species depending on the prevailing redox conditions [5]. The trivalent form (Cr3+) adsorbs or precipitates as a solid (oxy)-hydroxide phase [Cr(OH)]2+ over the pH range of most natural waters [6]. In contrast, the hexavalent form, (Cr(VI)), exists as oxyacids in the pH range of most surface waters, where the primary hydrolyzed forms are chromic acid, hydrogen chromate ion, chromate ion, and dichromate ion [5, 7].

The maximum set standard level for total chromium in drinking water is 0.1 ppm and Cr(VI) is 0.05 ppm [8]. Indeed, the set limit is high, and as a result, it is being reconsidered for revision based on the emerging toxicological effects of Cr(VI) [9]. Trivalent chromium is an essential micronutrient to plant and animal metabolism whereas Cr(VI) is carcinogenic and highly toxic to animals and plants at levels above 0.1 ppm, and it causes allergic dermatitis [10, 11].

To date, the most adopted technology of Cr(VI) removal from wastewater involves its reduction to the trivalent form at pH 2 and subsequent precipitation of Cr(OH)3 by increasing pH to 9∼10 using lime [12, 13]. The disadvantages of such technology lie in the fact that (1) the reduction process produces large amounts of sludge. For example, 32 kg of sludge can be generated in order to remove 1 kg of Cr(VI) [14]; (2) the use of lime to precipitate the trivalent chromium makes the process complicated and more expensive.

Under such circumstances, a number of adsorbents such as activated carbon, lignocellulose, hydrotalcite, and mesoporous zirconium titanium oxides have been utilized for adsorption of the chromium species [5, 12, 1517]. Nevertheless, similar problems of adsorbent disposal and cost implications still dominate. In addition, most Cr(VI) adsorbents reported so far use permanent physisorption mechanisms that do not allow reusability of adsorbents nor recovery of the Cr(VI) anions.

In terms of bacteria, fungi, algae, and different plants as adsorbents, they do show good adsorption capability in the laboratory [12]. The scale-up experiments by the application of these biological adsorbents into water treatments are not successful due to their relatively poor natural abundance [12]. Therefore, removal of Cr(VI) from wastewater still presents a great challenge in wastewater treatment technology.

Polyoxometalates (POMs) are a class of discrete anionic metal oxides of groups 5 and 6 and POMs exhibit attractive properties such as thermal and oxidative stability, remarkable electronic and magnetic properties, and Brønsted acidity, which result in intriguing applications ranging from medicine, catalysis to material science [1821]. Recently, it was reported that the slow addition of H3PW12O40 to an aqueous solution of K2CrO4 at room temperature (pH = 6.6 ± 0.2) led to the formation of the final product K14 [P2W20O72]·24H2O (denoted as P2W20) [22]. It was proposed that a Cr(VI)-stabilized [PW10O36 (CrO4)2]9− intermediate was formed, which played a significant role in the formation of the P2W20 cluster.

In other words, without the addition of K2CrO4 in the reaction mixture, the final product could not be obtained. Moreover, the Cr(VI)-stabilized species of [PW10O36 (CrO4)2]9− could dissociate upon the increase of temperature above 40°C. Unfortunately, this intermediate was not isolated. However, the proposed intermediate shows the capability of Cr(VI) anions to chemically bond with PW10 anions. Therefore, a better understanding of the mechanism of reaction herein can be very beneficial for Cr(VI) removal from aqueous solutions provided that the [PW10O36]7− (denoted as PW10) [23] cluster is heterogenized.

In this contribution, a report on the isolation of the abovementioned intermediate by electrostatic immobilization of [PW10O36]7− anions onto the surface of Fe3O4@SiO2@LEuH (LEuH represents [Eu8 (OH)5 (H2O)n]4+) is presented (Figure 1). The core magnetite iron nanoparticles coated with silica [24, 25] are electrostatically interacted with the layered rare-earth hydroxide nanosheets of LEuH (Figure 2) [2628]. The resulting nanocomposite material of Fe3O4@SiO2@LEuH@PW10 is able to sorb chromate anions efficiently from aqueous solution (Figure 3). And the chromate anions can further dissociate from the absorbed composite material by applying heat above 40°C, leading to the release of the original Fe3O4@SiO2@LEuH@PW10. As a result, the magnetically active nanocomposite material can be recycled and reused for Cr(VI) removal for at least ten times without obvious loss of activity. Scale-up experimental results indicate that the magnetically active nanocomposite material of Fe3O4@SiO2@LEuH@PW10 (Figure 1) is efficient for Cr(VI) removal. Therefore, such reversible chemisorption process provides a new pathway for the Cr(VI) removal from industrial wastewater.

In order to prove that the PW10 anions do not dissociate into its individual elements during the nanoparticle synthesis, a catalytic experiment synonymous with POMs is done. Under which phenol red is catalysed to bromophenol blue using Fe3O4@SiO2@LEuH@PW10 nanocomposite material as a catalyst.

2. Materials and Methods

2.1. Chemical Materials

Analytically pure KOH, NaOH, Na2WO4, CH3COONa, H2WO3, Eu2O3, FeCl3, H3PO4, HCl, H2SO4, CsOH, ethanol, methanol, ammonium solution, ethylene glycol, and tetraethyl orthosilicate were purchased from Alfa Aesar and used without further purification. The syntheses of the decatungstophosphate anion ([α-PW10O36]7−) [23], layered europium hydroxide (Eu2 (OH)5Cl·nH2O) [26] (LEuH-Cl), and silicon-coated magnetic iron nanoparticles of Fe3O4@SiO2 nanospheres [24] were synthesized and characterized according to literature methods.

2.2. Characterization

Powder X-ray diffraction (XRD) patterns were recorded on a Rigaku XRD-6000 diffractometer under the following conditions: 40 kV, 30 mA, Cu-Ka radiation (λ = 0.154 nm), scan step of 0.01°, and scan range between 3° and 80°. Fourier transform infrared (FT-IR) spectra were recorded on a Bruker Vector 22 infrared spectrometer, using the KBr pellet method. Scanning electron microscopy (SEM) images and energy-dispersive X-ray spectroscopy (EDX) analytical data were obtained using a Zeiss Supra 55 SEM equipped with an EDX detector.

Transmission electron microscopy (TEM) micrographs were recorded using a Hitachi H-800 instrument. Inductively coupled plasma-atomic emission spectroscopy (ICP-AES) analysis was performed using a Shimadzu ICPS-7500 spectrometer after a weighed amount was dissolved in HCl solution. The fluorescence emission spectra were recorded with a Hitachi F-7000 fluorospectrometer with a Xe lamp as the excitation source.

The UV absorption measurements were performed with a TU-1901 UV/Vis spectrophotometer with 1 nm optical resolution over the range of 190–900 nm. The specific surface area determination and pore volume and size analysis were performed by Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) methods, respectively, by use of a Quantachrome Autosorb-1C-VP analyzer. Prior to the measurements, the samples were degassed at 100°C for 6 h. The OH content was obtained by neutralization back-titration after the sample had been dissolved in 0.1 N standard H2SO4. Thermogravimetric analysis was carried out on a locally produced HCT thermal analysis system in flowing N2 with a heating rate of 10°C·min−1.

2.3. Preparation of Fe3O4@SiO2@LEuH@PW10 Nanocomposite

Fe3O4@SiO2 (0.1 g) was dispersed in deionized water under sonication. LEuH nanosheets were prepared by ultrasonicating freshly prepared LEuH (0.2 g) in aqueous media for 10 min to delaminate the positively charged nanosheets, followed by centrifugation at 2000 rpm to remove undelaminated material. The resultant delaminated nanosheets of LEuH (0.05 g) were added dropwise to the above-prepared Fe3O4@SiO2.

The mixture was stirred at room temperature (RT) for 12 hours, separated by a magnet, and washed several times with deionized water and this procedure was repeated 5 times to get the Fe3O4@SiO2@LEuH brown material.

The wet sample of Fe3O4@SiO2@LEuH (0.11 g) was dispersed in deionized water (50 ml) under sonication for 10 min. The sonicated Fe3O4@SiO2@LEuH was transferred into a beaker and Cs7[PW10O36] (5 g), prepared by dispersion in deionized water (40 ml) under sonication for 10 min, was added dropwise as the contents were stirred at RT overnight. The desired Fe3O4@SiO2@LEuH@PW10 material was washed several times with deionized water before being dried at 40°C for 12 hours to obtain a brown powder (0.018 g).

2.4. Adsorption Experiments

The Fe3O4@SiO2@LEuH@PW10 nanocomposite material was used to remove Cr(VI) anions from deionized water, laboratory tap water, and municipal wastewater. For deionized water and tap water, a known concentration of Cr(VI) anions was introduced into the water sample and the nanocomposite (0.4 g) was added. Then, the test solution was kept stirring for 1 hour. After that, the nanocomposite was separated by a magnet and the water was tested for Cr(VI) using EPA method 218.7.

For the municipal wastewater, a known concentration of Cr(VI) was introduced into the sampled water in Teflon bottles and left to acclimatize to the sample collection ambient environment. After two days, the wastewater was filtered and treated with the nanoparticles as indicated above and the concentration of Cr(VI) determined using EPA method 218.7. The separated nanoparticles after the experiments were heated at 40°C in a small amount of aqueous solution for 30 minutes to yield a concentrated chromate anion solution and the adsorbent was reused ten times. The chromate anions were recovered by adding stoichiometric amounts of KOH solution to the solution to yield potassium chromate powder after solvent evaporation.

2.5. Kinetic Studies
2.5.1. Determination of Adsorption Rate and Equilibrium

Ten samples of deionized water (100 ppm) solutions were treated with 0.2 g of Fe3O4@SiO2@LEuH@PW10 nanocomposite for 0, 10, 20, 30, 40, 50, 60, 80, 100, and 150 minutes. After each experiment, the magnetic nanocomposite was separated and the deionized water tested for Cr(VI) concentration.

2.5.2. Adsorption Isotherms

The Fe3O4@SiO2@LEuH@PW10 nanocomposite (0.2 g) was stirred with 50 ml solutions having different concentrations (1, 5, 10, 20, 50, 100, 200, 300, 400, 500, and 1000 ppm) for one hour to insure equilibrium. The magnetic nanocomposite was separated and the resulting aqueous solutions tested for Cr(VI) concentration. The amount of Cr(VI) up taken per gram of the nanoparticles () was determined according to equation (1), in which Co is the initial Cr(VI) concentration, C is the concentration of the equilibrated final solution, V is the volume of the aqueous phase, and m is the mass of the nanoparticles in the system:

The optimized reaction temperature and pH for the nanocomposite with Cr(VI) anions were determined by reacting the Fe3O4@SiO2@LEuH@PW10 nanocomposite (0.2 g) with 50 ml of Cr(VI) solution (100 ppm) at different temperatures and pH. The pH was adjusted by 0.1 N HCl or diluted by NaOH solutions, respectively.

2.6. Determination of Cr(VI) Concentration
2.6.1. Preparation of Reagents

The DPC reagent, 2 mM of 1,5-diphenylcarbazide in 10% methanol and 0.5 M (1 N) sulfuric acid, was prepared from the following solutions [29]. Solution A: 100 mL of methanol was added to 0.50 gram of 1,5-diphenylcarbazide, and the mixture was sonicated for five minutes to dissolve the solid. Solution B: 28 mL of sulfuric acid was added to 500 mL of deionized water in 1 L beaker, and the solution was cooled to room temperature. Then, Solution A was added to Solution B and it was diluted to 1 L. The solution was used for the determination of the absorption amount of Cr(VI) anions.

2.6.2. Preparation of Standard Cr(VI) Solutions and Measurement of Unknown Sample Concentration

K2CrO4 was dried at 100°C to a constant weight for 12 hours. Then, 0.283 g of K2CrO4 was dissolved in deionized water and diluted to 100 mL. The stock solution containing 1000 μg/mL or 1000 ppm was stored at room temperature. A series of calibration standards were prepared by diluting the stock solution using deionized water. The calibration standards were used to generate a standard curve (Figure 4) and unknown sample concentrations determined by the UV/Vis spectrophotometer (TU-1901 UV/Vis spectrophotometer) from the standard curve at 530 nm. Both standard solutions and samples were treated by DPC before analysis. In a typical procedure, 10 mL of the test solution was treated with 1 mL of DPC before UV analysis at 530 nm using the external standard method.

2.7. Method Detection Limit Determination

The standard deviations of seven samples containing 0.005 ppm K2CrO4 were determined. The half range for the prediction interval of results (HRPIR) was determined using the following equation: HRPIR = 3.963S, where S is the standard deviation and 3.963 is a constant value for seven replicates. The upper and lower limits for the prediction interval of the results were tested using the following equation:and the lower prediction interval results were calculated as follows:

The results of 0.0148 (148%) for upper limit and 0.0047 (47%) for lower limit allowed the acceptance of the chosen detection limit. Subsequently, the detection limit was determined using the following equation:where DL is the detection limit, t is Student’s t-test value, n is the number of replicates, S is the standard deviation of replicate analyses, and α is the replicated samples that have been done. 1 − α is the degree of freedom [30]. The detection limit was determined to be 0.002 × 3.963 = 0.007 ppm.

3. Results and Discussion

3.1. Adsorbent Characterization

As shown in Figure 5(a), SEM images of LEuH-Cl exhibit plate-like morphology with 200 nm (length) × 100 nm (width). HRTEM images of the delaminated LEuH nanosheets (Figure 5(b)) clearly show the platelet morphology. The selected area electron diffraction pattern (SAED) taken from an individual delaminated nanoplate lying on a copper grid shows the well-ordered diffraction (Figure 5(c)), indicating the formation of the superlattice structure. The brighter spots correspond to a fundamental cell in a pseudohexagonal symmetry (af = 3.7 Å). Therefore, we can calculate d100 = 2√3af = 12.81 Å and d010 = 2af = 7.4 Å. The fundamental cell indicates a hexagonal arrangement of Eu-Eu atoms closely related to a LDH-like host layer (Eu-Eu: 3.7 Å) (Figure 5(c)). These results have been further confirmed by XRD measurements (Figure 6), and they are in agreement with the literature [28]. Hence, the delaminated nanosheets have been assumed to have the formula [Eu8 (OH)20·nH2O]Cl4 (LEuH) in accordance with the intensive characterization of these materials in [28].

The delaminated LEuH nanosheets have been applied to immobilize [PW10O36]7− anions onto Fe3O4@SiO2 magnetite nanospheres to obtain a new nanocomposite material of Fe3O4@SiO2@LEuH@PW10. Note that the C7PW10O36 POM salt used is insoluble in water, and hence, by washing the desired product several times with water, all the nonimmobilized PW10 anions are washed away. We did not perform any experiments that would show if the immobilization is achieved through ionic exchange of POMs and Cl on the LEuH nanosheets or by electrostatic interactions. However, XPS spectroscopy of Fe3O4@SiO2@LEuH@PW10 (Figure 7) shows the existence of the starting materials in their original oxidation states. The peaks were fitted to specific oxidation status of individual elements using the mixed Gaussian–Lorentzian function, which is a nonlinear squares fitting algorithm, and Shirley-type background subtraction that uses XPS peak fit software [31]. The binding energies of Eu3d are recorded at 1135 eV, while the W4f doublet (W4f7/2 and W4f5/2) can be observed at 35.7 and 37.8 eV, corresponding to the hexavalent state of W6+ [32]. Thus, the binding energies of Eu3+ and W6+ do not change after the fabrication of the nanocomposite material of Fe3O4@SiO2@LEuH@PW10.

The XRD diffraction patterns clearly show the existence of peaks from LEuH and Fe3O4@SiO2 starting materials in addition to other peaks that might arise out of the immobilized POM anions (Figure 8).

In the FT-IR spectra of Fe3O4@SiO2@LEuH@PW10 (Figure 9), the two stretching bands at 1093 cm−1 and 1045 cm−1 can be due to the P-O stretching, while the bands at 943, 875, and 793 cm−1 are attributed to asymmetric stretching of the W–Od bond (Od is the terminal oxygen), W–Ob–W bridge (Ob is the bridging oxygen that links two corner-sharing octahedron), and asymmetric stretching of the W–Oc–W (Oc is the bridging oxygen that links two edge-sharing octahedron), respectively [23]. Compared with the vibration bands of Fe3O4@SiO2 and Cs7PW10O36 (Figure 9), most of the stretching bands in the FT-IR spectrum of Fe3O4@SiO2@LEuH@PW10 overlap.

SEM images of the Fe3O4@SiO2@LEuH@PW10 nanocomposite exhibit well-dispersed spherical morphology with energy-dispersive X-ray spectroscopy elemental mapping (EDX) results for tungstate shown (Figure 10). Although HRTEM images of the Fe3O4@SiO2@LEuH@PW10 nanocomposite show core-shell structures, the SAED pattern of the Fe3O4@SiO2@LEuH@PW10 nanocomposite exhibits the atoms organized in a tetragonal symmetry, which is in good contrast to that by Fe3O4@SiO2 (Figure 11). Moreover, the EDX results clearly indicate the presence of the desired elements in the as-synthesized materials.

Other characterization done includes BET (Figure 12). It is important to note that the abrupt rise near  = 1 in the BET experiments is due to the pores within the nanocomposite material and not on the surface of the nanomaterial. This is particularly true as such pores would otherwise be visible in the SEM images shown in Figure 10.

Thermogravimetric curves (Figure 13) reveal three distinct weight losses. The initial weight loss below 150°C is due to the removal of interlayer water, and the amount can be determined from the weight loss below 150°C to be 3 for Fe3O4@SiO2@LEuH@PW10. The second weight loss from around 150°C to 300°C is attributed to the loss of water from the condensation of hydroxyl groups. The next step above 400°C should involve the loss of Cl anions.

3.2. Hexavalent Chromium Adsorption Results

The adsorption of Cr(VI) anions in aqueous media using the nanocomposite material of Fe3O4@SiO2@LEuH@PW10 has been investigated. After the adsorption experiments, the resultant filtrate is derivatized with 1,5-diphenylcarbazide in order to detect CrO42− at a wavelength of 530 nm (Figure 14). The results show a complete adsorption of the chromate anions from the deionized water (Figure 14(c)). For tap water and municipal wastewater samples, the nanocomposite material adsorbs the chromate anions to undetectable limits (denoted as ND, Table 1). Scale-up experiments using the nanocomposite material for Cr(VI) removal in 0.5 L of water show complete adsorption of Cr(VI) to a nondetectable limit, indicating a great potential for the application of such magnetically active Fe3O4@SiO2@LEuH@PW10 material.

Heating the adsorbed composite material, Fe3O4@SiO2@LEuH@PW10Cr2, at 40°C in aqueous solution yields a concentrated chromate anion solution, which allows the recycling of adsorbent (Table 1). The chromate anions are recovered by adding stoichiometric amounts of KOH solution to the Cr(VI) solution to yield potassium chromate powder after solvent evaporation (Figure 15). This is in agreement with the proposed reaction pathway of PW10Cr2 (aq) species [22]. Most importantly, we are able to isolate this intermediate species.

The adsorption of hexavalent chromium anions by Fe3O4@SiO2@LEuH@PW10 is stable in the pH range of 4∼12 (Figure 16(a)). It is noted that other mechanisms other than adsorption could have contributed to chromium removal from the studied water due to pH variations. The maximum time required for complete adsorption of 100 ppm chromate anions by 0.2 g of the Fe3O4@SiO2@LEuH@PW10 nanocomposite is 42 minutes at 25°C (Figure 16(b)). This adsorption time might be acceptable for industrial wastewater treatment. Therefore, it can be concluded that the nanocomposite material is effective in treating wastewater and tap water in a relatively wide pH range.

To show the temperature effect on adsorption of Cr(VI) by the Fe3O4@SiO2@LEuH@PW10 nanocomposite, the experiments were investigated and the results are presented in Figure 17, in which curve a is the desorption curve and curve b is the adsorption curve. The optimized adsorption temperature of chromate anions is 25°C. It is noted that the active sites (PW10 anions) of Fe3O4@SiO2@LEuH@PW10Cr2 start to lose the adsorbed chromate anions at T ≥ 40°C as shown in Figure 17. The Gibbs free energies of the absorption system of the Fe3O4@SiO2@LEuH@PW10 and desorption system of Fe3O4@SiO2@LEuH@PW10Cr2 nanocomposites have been determined based on equations (5)–(9), where R is the gas constant (8.314 J·k−1·mol−1), T is the absolute temperature, ∆H is the enthalpy change, and ∆S is the entropy change. Equilibrium constant values, Kc, for the abovementioned two systems are evaluated at different T according to equation (1), where qe is the equilibrium concentration of chromate concentration on the adsorbent and Ce is the equilibrium concentration of chromate in solution:

Plots of lnKc versus give straight lines with positive slope for temperature between 25°C and 40°C whereas negative slope at temperatures between 25°C and 10°C (Figure 17). ∆G for chromate adsorption is calculated from curve b at 298 K−1 to be −3.151 × 106 J, indicating the reaction to be favorable. The values of ∆H and ∆S for the adsorption can be calculated to be −2.83 × 105 J·mol−1 and 9.81 × 102 J·K−1, respectively.

The negative ∆H value suggests the exothermic character of the reaction between Fe3O4@SiO2@LEuH@PW10 nanocomposite material and the Cr(VI) anions in the adsorption process, whereas the positive ∆S value indicates the increase of the randomness. In contrast, for desorption of chromate anions, the values of ∆H and ∆S are 3.45 × 105 J·mol−1 and −1.13 × 103 JK−1, respectively (curve a). The positive ∆H value indicates the endothermic character of Fe3O4@SiO2@LEuH@PW10Cr2 losing the chromate anions. And the negative ∆S value suggests the decrease of the randomness.

The desorption could also result from the antiferromagnetism of chromium. Before 38°C, chromium possesses antiferromagnetic properties. The t = 38°C is the Neel temperature for chromium [33]. That is why it easily desorbed from adsorbent’s surface under temperature higher than 40°C.

As shown in Figure 18, the adsorption profiles fit perfectly with the Langmuir adsorption model. The maximum uptake capacity of the Fe3O4@SiO2@LEuH@PW10 nanocomposite for chromate anions, qm, can be determined from the reciprocal of the slopes of the straight lines in Figure 18(b). This corresponds well to the determined qm from the Langmuir isotherm curve in Figure 18(a). The ppm/g value is converted to mmol/g by dividing it with molar mass of chromium to give 23 mmol/g. It is, therefore, assumed that the adsorption sites are equivalent and have the same binding energy and can only form monolayer. The BET isotherm (Figure 12) provides further evidence for the above assumptions with a type III isotherm graph, indicating the absence of pores in the adsorption process and the evidence for the monolayer adsorption-desorption behavior. The BET isotherm also rules out the possibility of physisorption mechanisms. As a result, chemisorption dominates the adsorption process.

However, it is important to point out that the vertical rise at region = 1 (Figure 12), indicating the presence of macropores, is due to the spaces found in between the nanoparticles and not on the surface of the nanoparticles. This is particularly due to the size of the macropores, diameter 60.7 nm determined by BET experiments, which would otherwise be visible in SEM and TEM images (Figures 10 and 11).

The Langmuir isotherm is valid for monolayer sorption onto a surface with a finite number of identical sites and uniform adsorption energies [34, 35]. The model postulates a homogenous surface for the adsorbent. Accordingly, the fitting of our results to this model supports the proposed structure of the Fe3O4@SiO2@LEuH@PW10 nanocomposite (Figure 2).

To further verify the chemisorption process, pseudo-second-order rate expression, commonly used for analyzing chemisorption kinetics [36], is applied at low initial concentrations of Cr(VI) anions. The pseudo-second-order model constant, k2, can be determined experimentally by plotting against t from equation (10) to obtain the graph in Figure 19. The activation energy can be determined from plots of lnk2 versus to yield a straight line, with slope (Figure 19) according to the Arrhenius equation [37]. In equation (10), k2qe2 (mg·g−1·h−1) is the initial adsorption rate. In pseudo-second-order rate expression, it is assumed that two chromate anions (CrO42−) can be adsorbed onto one sorption site (PW10) on the surface of the Fe3O4@SiO2@LEuH@PW10 nanocomposite (equation (11)). The Van’t Hoff plot shows the reaction requires an activation energy of 44.22 kJ/mol, which falls in the range of 40∼800 kJ/mol [37]. This result again supports the chemisorption process. Other models can be applied to test the chemisorption behavior including Elovich kinetic model [38, 39].

3.3. Catalytic Activity of Fe3O4@SiO2@LEuH@PW10

By using NH4Br and H2O2 as feedstock, phenol red was successfully catalysed to bromophenol blue using Fe3O4@SiO2@LEuH@PW10 nanocomposite material as a catalyst (Table 2). The bromination reaction catalysed by Fe3O4@SiO2@LEuH@PW10 nanocomposite material was monitored at time intervals of 30 seconds (Figure 20). The reaction was complete in 10 minutes with a 99% conversion of phenol red to bromophenol blue (Figure 20). Contrast experiments suggest that LEuH-Cl, Fe3O4, and Fe3O4@SiO2 exhibit 22%, 23%, and 21% yield in 2 hours, respectively (Table 2). It is noted that there was no formation of bromophenol blue in the first 1.5 minutes (Figure 20), despite a reduction in the concentration of the starting material, phenol red. However, after 2 minutes, the level of bromophenol blue in the reaction mixture raises steadily and reaches its maximum after 10 minutes (Figure 20).

The reduction in the concentration of phenol red in the first 1.5 minutes is probably due to the formation of an intermediate compound that does not absorb UV light in the tested region. Optimal conditions of the bromination reaction were experimentally determined (Figures 21 and 22). For instance, the maximum catalyst amount was determined as 0.5 g for the Fe3O4@SiO2@LEuH@PW10 nanocomposite material (Figure 21(a)), while the maximum H2O2 concentration was 2.5 mM (Figure 21(b)). NH4Br is the rate-determining reagent in the bromination of phenol red to bromophenol blue (Figure 22).

Under similar experimental conditions, bromination of phenol red to bromophenol blue using LDH/POM intercalated catalyst took 32 minutes to complete [40], while the new Fe3O4@SiO2@LEuH@PW10 nanocomposite material takes less than 10 minutes to complete a similar reaction (Figure 20). Moreover, in addition to ease of catalyst separation from the products using an external magnet, the nanocomposite can be reusable for at least 10 catalytic circles with no obvious loss of activity observed (Figure 23).

The mechanism of the reaction is proposed in Figure 24. In the presence of H2O2, the active W-peroxo species can be formed on the surface of the spherical Fe3O4@SiO2@LEuH@PW10 nanocomposite. The W-peroxo species are readily reduced by the bromide ions from NH4Br (aq), in which the activated oxygens from peroxotungstate can be transferred to Br to get a 2-electron oxidation in solution, which results into OBr formation. These OBr radicals are then involved in an electrophilic reaction with phenol red to get the bromophenol blue. These processes can be summarized in equations (12)–(14). Note that n can be different according to the number of W=O units exposed to H2O2. There are other side reactions in the system as demonstrated in Figure 25:

In summary, we present a new approach of effectively heterogenizing PW10 anions on a magnetically active nanospherical support material. The catalytic versatility of the as-synthesized Fe3O4@SiO2@LEuH@PW10 nanocomposite material has been demonstrated by the conversion of phenol red to bromophenol blue. In this reaction, Fe3O4@SiO2@LEuH@PW10 takes less than 10 minutes to complete the conversion under the experimental conditions as compared to 32 minutes for LDHs. Moreover, the nanocomposite of Fe3O4@SiO2@LEuH@PW10 can be reused (minimum 10 times tested) with almost similar results obtained. In addition, it can be easily be separated using an external magnet.

4. Conclusion

The immobilization of PW10 onto Fe3O4@SiO2 using LEuH nanosheets results in the formation of a new nanocomposite material of Fe3O4@SiO2@LEuH@PW10. This nanocomposite material has been demonstrated to be able to chemically adsorb chromate anions from aqueous media. The adsorption reaction yields a Cr(VI)-absorbed nanocomposite material (Fe3O4@SiO2@LEuH@PW10Cr2), which loses the Cr(VI) anions at ≥ 40 C, resulting in the restoration of the original Fe3O4@SiO2@LEuH@PW10. The adsorption-desorption process can be repeated at least ten times without obvious loss of activity. Scale-up experimental results indicate that the magnetically active Fe3O4@SiO2@LEuH@PW10 nanocomposite can be effective too. The chromate adsorption capacity of the synthesized Fe3O4@SiO2@LEuH@PW10 nanocomposite is 23 mmol/g from aqueous solutions at 25°C for 42 minutes in the pH range of 4∼12 with an activation energy of 44.22 kJ/mol. The adsorption isotherms fit very well with the Langmuir model.

Data Availability

All the necessary information required for replication of this work and/or conducting secondary analysis are included within the article.

Conflicts of Interest

The authors declare that they have no conflicts of interest.


Financial support from National Research Fund of Kenya (2016/17), the RSC Allan Ure Bursary Fund of 2017, and Jaramogi Oginga Odinga University of Science and Technology is acknowledged.