Journal of Chemistry
Volume 2020 (2020), Article ID 6375749, 17 pages
https://doi.org/10.1155/2020/6375749

Research Article

Design and Synthesis of Reusable Nanoparticles for Reversible Chemisorption of Hexavalent Chromium Anions from Aqueous Media and Catalysis

Solomon Omwoma Lugasi

Department of Physical Sciences, Jaramogi Oginga Odinga University of Science and Technology, P.O. Box 210-40601, Bondo, Kenya

Correspondence should be addressed to Solomon Omwoma Lugasi; solomwoma@yahoo.com

Received 17 April 2020; Revised 30 May 2020; Accepted 15 June 2020; Published 14 July 2020

Guest Editor: Tapan Sarkar

Copyright © 2020 Solomon Omwoma Lugasi. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

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, 15–17]. 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 [18–21]. 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) [26–28]. 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.

Figure 1: Comparisons between the new proposed synthetic pathway of fabricating layered rare-earth hydroxide nanosheets with POMs and literature method.

Figure 2: Proposed synthetic pathway of preparing Fe₃O₄@SiO₂@LEuH@PW₁₀ nanocomposite material; water molecules avoided for clarity.

Figure 3: Proposed adsorption processes of Fe₃O₄@SiO₂@LEuH@PW₁₀ adsorbent with hexavalent chromium anions.

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.

Figure 4: The external UV standard curve used in determining Cr(VI) concentrations in test solutions at 530 nm wavelength.

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].

Figure 5: (a) SEM image of LEuH-Cl; (b) HRTEM of delaminated LEuH nanosheets, inset: a higher magnification at the edge of a single plate lying on a copper grid. (c) SAED pattern taken from an individual plate lying on a copper grid.

Figure 6: XRD diffraction patterns of LEuH-Cl [Eu₂ (OH)₅Cl·H₂O].

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.

(a)

(b)

(c)

Figure 7: XPS spectra of Fe₃O₄@SiO₂@LEuH@PW₁₀ for (a) elemental information and chemical state information for (b) Eu3d and (c) for W4f.

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).

Figure 8: XRD patterns of the synthesized Fe₃O₄@SiO₂@LEuH@PW₁₀ nanocomposite material in comparison with starting materials.

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.

Figure 9: FT-IR spectra of the synthesized Fe₃O₄@SiO₂@LEuH@PW₁₀ nanoparticles in comparison with starting materials.

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.

Figure 10: SEM images of (a) Fe₃O₄@SiO₂ and (b) Fe₃O₄@SiO₂@LEuH@PW₁₀ and (c) EDX metal mapping for W (all the scale bars are 200 nm).

Figure 11: HRTEM, SAED patterns, and EDX images of the as-synthesized core-shell nanostructured PW₁₀O₃₆⁷⁻ material (Si is from the silicon wafer).

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.

(a)

(b)

Figure 12: (a) N₂ sorption isotherms and (b) pore size distribution of Fe₃O₄@SiO₂@LEuH@PW₁₀ nanospheres (type III isotherm): the abrupt rise near  = 1 is due to the pores within the nanomaterial and not on the surface of the nanomaterial.

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.

Figure 13: Thermogravimetric analysis curves for Fe₃O₄@SiO₂@LEuH@PW₁₀.

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.

Figure 14: (a) Magnetic separation of a sample after reaction; (b) sample prepared for UV analysis using 1,5-diphenylcarbazide reagent. (c) A sample reduced from an initial concentration of 100 ppm to 0 ppm Cr(VI) with Fe₃O₄@SiO₂@LEuH@PW₁₀ after 1 hour reaction time at 24°C and pH 6; dilution factor = 20.

Table 1: Test results of 0.4 g Fe₃O₄@SiO₂@LEuH@PW₁₀ adsorbent with different chromate concentrations in both tap water and wastewater at 25°C for 1 hour at pH = 6 for tap water and pH = 4.4 for wastewater.

Test material
Volume (ml)
Initial CrO₄²⁻ (ppm)
Final CrO₄²⁻ (ppm)
Adsorbent reuse

Tap water
100
2
ND
ND

200
2
ND
ND

300
2
ND
ND

500
2
ND
ND

Wastewater
100
2
ND
ND

200
2
ND
ND

300
2
ND
ND

500
2
ND
ND

Tap water†
100
100
ND
ND

200
100
ND
ND

300
100
ND
ND

500
100
ND
ND

Wastewater†
100
100
ND
ND

200
100
ND
ND

300
100
ND
ND

500
100
ND
ND

Note. Below detection limit (ND) was 0.007 ppm. Adsorbent reused ten times giving similar results; †4.0 g of adsorbent used.

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.

Figure 15: Schematic presentation of hexavalent chromium removal from wastewater and reusability mechanisms.

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.

(a)

(b)

Figure 16: (a) Effect of pH on adsorption of Cr(VI) (100 ppm in 50 ml) by 0.2 g of Fe₃O₄@SiO₂@LEuH@PW₁₀; reaction time = 1 hour at 25°C; (b) effect of time on adsorption of Cr(VI) (100 ppm in 50 ml) by 0.2 g of Fe₃O₄@SiO₂@LEuH@PW₁₀ nanocomposite; T = 25°C.

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:

Figure 17: Effect of temperature on adsorption and desorption of Cr(VI) (100 ppm in 50 ml) by 0.2 g Fe₃O₄@SiO₂@LEuH@PW₁₀.

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.

(a)

(b)

Figure 18: (a) Optimal adsorbed amount (qm) and (b) the Langmuir adsorption isotherm of Cr(VI) by Fe₃O₄@SiO₂@ LEuH@PW₁₀ nanocomposite at 25°C and pH = 6.

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].

(a)

(b)

Figure 19: (a) Pseudo-second-order kinetics of Cr(VI) sorption onto 0.02 g of Fe₃O₄@SiO₂@LEuH@PW₁₀ at various temperatures: low concentration of 10 ppm Cr(VI) solution used. (b) Van’t Hoff plot for determination of activation energy in the adsorption mechanisms of Fe₃O₄@SiO₂@LEuH@PW₁₀ with CrO₄²⁻.

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).

Table 2: Catalytic bromination activities of LEuH-Cl, Fe₃O₄, Fe₃O₄@SiO₂, and Fe₃O₄@SiO₂@PW₁₀ catalysts.

Entry
Catalyst
BET surface area (m³/g)
Yield (%)
Time (h)
Rate (mmol·g⁻¹s⁻¹)

1
LEuH-Cl
57.30
22
2
1.2210⁻⁷

2
Fe₃O₄
7.32
23
2
1.2810⁻⁷

3
Fe₃O₄@SiO₂
13.57
21
2
1.1710⁻⁷

4
Fe₃O₄@SiO₂@LEuH@PW₁₀
16.62
99
0.1
5.510⁻³

Reaction conditions are as follows: 10 ml of 1.0 mM phenol red in water + 10 ml of 10.0 mM NH₄Br in water + 0.5 g of catalyst + 2.5 mM·H₂O₂, in a round-bottomed flask. Time = 10 minutes, T = 298 K, and  atm. Rate of conversion = mole Br⁻ oxidized per total weight of catalyst (g) per second. Symbol “” denotes a multiplication sign.

Figure 20: Bromination of phenol red to bromophenol blue using Fe₃O₄@SiO₂@LEuH@PW₁₀ as a POM nanocomposite catalyst. The reaction was monitored at time intervals of 30 seconds (1 = 0 s, 2 = 30 s, …, 12 = 330 s). Reaction conditions are as follows: 10 ml of 1.0 mM phenol red in water + 10 ml of 10.0 mM NH₄Br in water + 0.5 g of catalyst + 2.5 mM·H₂O₂, in a round-bottomed flask. T = 298 K and  atm.

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).

(a)

(b)

Figure 21: Effect of (a) amount of catalyst (Fe₃O₄@SiO₂@LEuH@PW₁₀) and (b) amount of H₂O₂ used in bromination of phenol red to bromophenol blue. Reaction conditions are as follows 10 ml of 1.0 mM phenol red in water + 10 ml of 10.0 mM NH₄Br in water + 2.5 mM H₂O₂ in reaction a + 0.5 g of catalyst in reaction b, in a round-bottomed flask. Time = 10 minutes, T = 298 K, and  atm.

Figure 22: Effect of different concentrations of NH₄Br (1 = 0, 2 = 2, …, 10 = 10 mM) on the rate of formation of bromophenol blue after 10 minutes. Reaction conditions are as follows: 10 ml of 1.0 mM phenol red in water + 10 ml of 10.0 mM NH₄Br in water + 0.5 g of catalyst + 2.5 mM H₂O₂, in a round-bottomed flask. T = 298 K and  atm.

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).

Figure 23: Reuse of Fe₃O₄@SiO₂@LEuH@PW₁₀ catalyst for the conversion of phenol to bromophenol blue. The experimental conditions included 10 ml of 1.0 mM phenol red + 10 ml of 10.0 mM NH₄Br + 0.5 g of catalyst + 2.5 mM H₂O₂, in a round-bottomed flask. Time = 10 minutes, T = 298 K, and  atm.

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:

Figure 24: The proposed mechanism of reaction for the bromination of phenol red to bromophenol blue using Fe₃O₄@SiO₂@LEuH@PW₁₀ as a catalyst.

Figure 25: The proposed full reaction mechanism of the limiting reagent NH₄Br producing molecular oxygen.

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.

Acknowledgments

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.

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