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

Hindawi

Journal of Chemistry

Volume 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

License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is

properly cited.

A magnetically active nanocomposite material has been synthesized from the reaction mixture of magnetite core iron nano-

particles electrostatically coated with SiO₂, hydrotalcite nanosheets ([Eu₈(OH)₂₀(H₂O)_n]⁴⁺), and decatungstophosphate anion

([α-PW₁₀O₃₆⁷⁻]). ,e resulting nanocomposite material, denoted as Fe₃O₄@SiO₂@LEuH@PW₁₀, is demonstrated to eﬀectively

adsorb chromate anions from aqueous solutions. ,e adsorption isotherms ﬁt the Langmuir model with a capacity of 23 mmol·g⁻¹

after 42 minutes at 25^°C. ,e reaction is spontaneous at room temperature with 44.22 kJ·mol⁻¹of 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@PW₁₀is reused for chromate removal in aqueous

solutions for at least ten times without obvious loss of activity. ,is spontaneous reversible chemisorption mechanism for

chromate adsorption provides a new pathway for separation and cleaning of industrial wastewater contaminated with chromate

ions. ,e robust catalytic activity of the nanocomposite is also demonstrated.

hydrolyzed forms are chromic acid, hydrogen chromate ion,

chromate ion, and dichromate ion [5, 7].

1. Introduction

Chromium is used in various industrial activities such as

electroplating, wood preservations, metallurgy, tanning,

refractory materials, catalysis, dye, and pigment [1]. Chro-

mium 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 contami-

nated 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]. ,e 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

,e maximum set standard level for total chromium in

drinking water is 0.1 ppm and Cr(VI) is 0.05 ppm [8]. In-

deed, the set limit is high, and as a result, it is being

reconsidered for revision based on the emerging toxico-

logical eﬀects of Cr(VI) [9]. Trivalent chromium is an es-

sential 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)₃by in-

creasing pH to 9∼10 using lime [12, 13]. ,e disadvantages of

such technology lie in the fact that (1) the reduction process

produces large amounts of sludge. For example, 32 kg of

2

Journal of Chemistry

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 mes-

oporous zirconium titanium oxides have been utilized for

adsorption of the chromium species [5, 12, 15–17]. Nev-

ertheless, similar problems of adsorbent disposal and cost

implications still dominate. In addition, most Cr(VI) ad-

sorbents reported so far use permanent physisorption

mechanisms that do not allow reusability of adsorbents nor

recovery of the Cr(VI) anions.

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. ,erefore, such reversible chemisorp-

tion process provides a new pathway for the Cr(VI) removal

from industrial wastewater.

In order to prove that the PW₁₀anions do not dissociate

into its individual elements during the nanoparticle syn-

thesis, a catalytic experiment synonymous with POMs is

done. Under which phenol red is catalysed to bromophenol

blue using Fe₃O₄@SiO₂@LEuH@PW₁₀nanocomposite

material as a catalyst.

In terms of bacteria, fungi, algae, and diﬀerent plants as

adsorbents, they do show good adsorption capability in the

laboratory [12]. ,e scale-up experiments by the application

of these biological adsorbents into water treatments are not

successful due to their relatively poor natural abundance

[12]. ,erefore, removal of Cr(VI) from wastewater still

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. ,e

syntheses of the decatungstophosphate anion ([α-PW₁₀O₃₆]

⁷⁻) [23], layered europium hydroxide (Eu₂(OH)₅Cl·nH₂O)

[26] (LEuH-Cl), and silicon-coated magnetic iron nano-

particles of Fe₃O₄@SiO₂nanospheres [24] were synthesized

and characterized according to literature methods.

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, remark-

able 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 H₃PW₁₂O₄₀to an

aqueous solution of K₂CrO₄at room temperature

(pH � 6.6 0.2) led to the formation of the ﬁnal product K₁₄

[P₂W₂₀O₇₂]·24H₂O (denoted as P₂W₂₀) [22]. It was pro-

posed that a Cr(VI)-stabilized [PW₁₀O₃₆(CrO₄)₂]⁹⁻inter-

mediate was formed, which played a signiﬁcant role in the

formation of the P₂W₂₀cluster.

2.2. Characterization. Powder X-ray diﬀraction (XRD)

patterns were recorded on a Rigaku XRD-6000 diﬀrac-

tometer 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 elec-

tron 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. Induc-

tively 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. ,e ﬂuorescence emission spectra were recorded

with a Hitachi F-7000 ﬂuorospectrometer with a Xe lamp as

the excitation source.

,e UV absorption measurements were performed

with a TU-1901 UV/Vis spectrophotometer with 1 nm

optical resolution over the range of 190–900 nm. ,e

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. ,e OH

content was obtained by neutralization back-titration

after the sample had been dissolved in 0.1 N standard

H₂SO_4.,ermogravimetric analysis was carried out on a

locally produced HCT thermal analysis system in ﬂowing

N₂with a heating rate of 10^°C·min⁻¹.

In other words, without the addition of K₂CrO₄in 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 tempera-

ture 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. ,erefore, 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 immobiliza-

tion of [PW₁₀O₃₆]⁷⁻anions onto the surface of Fe₃O₄@

SiO₂@LEuH (LEuH represents [Eu₈(OH)₅(H₂O)_n]⁴⁺) is

presented (Figure 1). ,e 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]. ,e resulting nanocomposite material of

Fe₃O₄@SiO₂@LEuH@PW₁₀is 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

Journal of Chemistry

3

Immersion in POM

anions

LDH-CO₃

adsorption

LDH-CO₃

adsorption (n)

Calcination

@480°C

Fe₃O₄@SiO₂@LDO

Fe₃O₄@SiO₂

@LDH-POM

Fe₃O₄@SiO₂@LDH-CO₃

Fe₃O₄@SiO₂@(LDH-CO₃)_n

SiO₂

encapsulation

Angew. Chem. Int. Ed., 2009, 48, 5888 Intercalated POM

Fe₃O₄

Fe₃O₄@SiO₂

Delaminated LEuH

nanosheets

Immersion in

POM anions

Fe₃O₄@SiO₂@LEuH

This work

Fe₃O₄@SiO₂@LEuH@POM

Surface-immobilized POM

LDH = layered double hydroxides

LDO = layered double oxides

LEuH = layered europium hydroxide

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

and literature method.

Sol-gel

Delamination

H₂O

+

LEuH-Cl

Fe₃O₄@SiO₂

nanosphere

LEuH

nanosheets

PW

10

H₂O

Eu

OH

Fe₃O₄@SiO₂@LEuH@PW₁₀

[Eu₈(OH)₂₀(H₂O)_n]⁴⁺

CI^–

7–

[PW₁₂O₄₀

]

Figure 2: Proposed synthetic pathway of preparing Fe₃O₄@SiO₂@LEuH@PW₁₀nanocomposite material; water molecules avoided for

clarity.

2.3. Preparation of Fe₃O₄@SiO₂@LEuH@PW₁₀Nanocomposite.

Fe₃O₄@SiO₂(0.1 g) was dispersed in deionized water under

sonication. LEuH nanosheets were prepared by ultra-

sonicating 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 undela-

minated material. ,e resultant delaminated nanosheets of

LEuH (0.05 g) were added dropwise to the above-prepared

Fe₃O₄@SiO₂.

repeated 5 times to get the Fe₃O₄@SiO₂@LEuH brown

material.

,e wet sample of Fe₃O₄@SiO₂@LEuH (0.11 g) was

dispersed in deionized water (50 ml) under sonication for

10 min. ,e sonicated Fe₃O₄@SiO₂@LEuH was transferred

into a beaker and Cs₇[PW₁₀O₃₆] (5 g), prepared by dis-

persion in deionized water (40 ml) under sonication for

10 min, was added dropwise as the contents were stirred at

RT overnight. ,e 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).

,e 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

4

Journal of Chemistry

equilibrium. ,e magnetic nanocomposite was separated

and the resulting aqueous solutions tested for Cr(VI) con-

centration. ,e amount of Cr(VI) up taken per gram of the

nanoparticles (q) 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:

@25 degrees

a

b

C − C V

ꢀ

ꢁ

0

(1)

q �

.

m

@40 degrees

,e optimized reaction temperature and pH for the

nanocomposite with Cr(VI) anions were determined by

reacting the Fe₃O₄@SiO₂@LEuH@PW₁₀nanocomposite

(0.2 g) with 50 ml of Cr(VI) solution (100 ppm) at diﬀerent

temperatures and pH. ,e pH was adjusted by 0.1 N HCl or

diluted by NaOH solutions, respectively.

Cr (VI)

PW₁₀

Figure 3: Proposed adsorption processes of Fe₃O₄@SiO₂@LEuH@

PW₁₀adsorbent with hexavalent chromium anions.

2.6. Determination of Cr(VI) Concentration

2.6.1. Preparation of Reagents. ,e 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.

,en, Solution A was added to Solution B and it was diluted

to 1 L. ,e solution was used for the determination of the

absorption amount of Cr(VI) anions.

2.4. Adsorption Experiments. ,e Fe₃O₄@SiO₂@LEuH@

PW₁₀nanocomposite 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.

,en, 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. ,e separated nanoparticles after the ex-

periments 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. ,e

chromate anions were recovered by adding stoichiometric

amounts of KOH solution to the solution to yield potassium

chromate powder after solvent evaporation.

2.6.2. Preparation of Standard Cr(VI) Solutions and Mea-

surement of Unknown Sample Concentration. K₂CrO₄was

dried at 100^°C to a constant weight for 12 hours. ,en,

0.283 g of K₂CrO₄was dissolved in deionized water and

diluted to 100 mL. ,e 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. ,e 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.

2.5. 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@PW₁₀nano-

composite for 0, 10, 20, 30, 40, 50, 60, 80, 100, and 150

minutes. After each experiment, the magnetic nano-

composite was separated and the deionized water tested for

Cr(VI) concentration.

2.7. Method Detection Limit Determination. ,e standard

deviations of seven samples containing 0.005 ppm

K₂CrO₄were determined. ,e half range for the pre-

diction 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. ,e upper and lower limits for the prediction

2.5.2. Adsorption Isotherms. ,e Fe₃O₄@SiO₂@LEuH@

PW₁₀nanocomposite (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

Journal of Chemistry

5

Hence, the delaminated nanosheets have been assumed to

have the formula [Eu₈(OH)₂₀·nH₂O]Cl₄(LEuH) in ac-

cordance with the intensive characterization of these ma-

terials in [28].

0.6

0.4

0.2

0.0

,e delaminated LEuH nanosheets have been applied to

immobilize [PW₁₀O₃₆]⁷⁻anions onto Fe₃O₄@SiO₂magne-

tite 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 non-

immobilized PW₁₀anions are washed away. We did not

perform any experiments that would show if the immobi-

lization 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. ,e 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]. ,e binding

energies of Eu_3dare recorded at 1135 eV, while the W_4f

doublet (W4f_7/2and W4f_5/2) can be observed at 35.7 and

37.8 eV, corresponding to the hexavalent state of W⁶⁺[32].

,us, the binding energies of Eu³⁺and W⁶⁺do not change

after the fabrication of the nanocomposite material of

Fe₃O₄@SiO₂@LEuH@PW₁₀.

y = 0.02 + 0.53x

R²= 0.988

0.0

0.2

0.4

C (ppm)

0.6

0.8

1.0

Figure 4: ,e external UV standard curve used in determining

Cr(VI) concentrations in test solutions at 530 nm wavelength.

interval of the results were tested using the following

equation:

mean + HRPIR

(2)

× 100 ≤ 150%,

fortified concentration

and the lower prediction interval results were calculated as

follows:

mean + HRPIR

(3)

× 100 ≥ 50%.

,e XRD diﬀraction patterns clearly show the exis-

tence of peaks from LEuH and Fe₃O₄@SiO₂starting

materials in addition to other peaks that might arise out

of the immobilized POM anions (Figure 8).

fortified concentration

,e 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:

In the FT-IR spectra of Fe₃O₄@SiO₂@LEuH@PW₁₀

(Figure 9), the two stretching bands at 1093 cm⁻¹and

1045 cm⁻¹can be due to the P-O stretching, while the bands

at 943, 875, and 793 cm⁻¹are attributed to asymmetric

stretching of the W–O_dbond (O_dis the terminal oxygen),

W–O_b–W bridge (O_bis the bridging oxygen that links two

corner-sharing octahedron), and asymmetric stretching of

the W–O_c–W (O_cis the bridging oxygen that links two edge-

sharing octahedron), respectively [23]. Compared with the

vibration bands of Fe₃O₄@SiO₂and Cs₇PW₁₀O₃₆(Figure 9),

most of the stretching bands in the FT-IR spectrum of

Fe₃O₄@SiO₂@LEuH@PW₁₀overlap.

DL � S × t(n − 1), 1 − α � 0.99,

(4)

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]. ,e 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. ,e 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. ,e brighter spots correspond to a

SEM images of the Fe₃O₄@SiO₂@LEuH@PW₁₀

nanocomposite exhibit well-dispersed spherical mor-

phology with energy-dispersive X-ray spectroscopy ele-

mental mapping (EDX) results for tungstate shown

(Figure 10). Although HRTEM images of the Fe₃O₄@

SiO₂@LEuH@PW₁₀nanocomposite show core-shell

structures, the SAED pattern of the Fe₃O₄@SiO₂@LEuH@

PW₁₀nanocomposite 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.

fundamental cell in

a

pseudohexagonal symmetry

˚

(a_f� 3.7 A). ,erefore, we can calculate d₁₀₀� 2√3a_f�

˚

12.81 A and d₀₁₀� 2a_f� 7.4 A. ,e fundamental cell indicates

a hexagonal arrangement of Eu-Eu atoms closely related to a

Other characterization done includes BET (Figure 12). It

is important to note that the abrupt rise near P/P_o� 1 in the

BET experiments is due to the pores within the nano-

composite material and not on the surface of the

˚

LDH-like host layer (Eu-Eu: 3.7 A) (Figure 5(c)). ,ese

results have been further conﬁrmed by XRD measurements

(Figure 6), and they are in agreement with the literature [28].

6

Journal of Chemistry

(a)

400nm

1nm

(b)

(c)

a_f

51(nm)

50nm

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.

d₀₀₁= 0.86nm

d₀₁₀= 0.752nm

d₁₀₀= 1.29nm

10

20

30

2 theta (degree)

40

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

nanomaterial. ,is is particularly true as such pores would

otherwise be visible in the SEM images shown in Figure 10.

,ermogravimetric curves (Figure 13) reveal three dis-

tinct weight losses. ,e 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. ,e second weight loss from

around 150^°C to 300^°C is attributed to the loss of water from

the condensation of hydroxyl groups. ,e next step above

400^°C should involve the loss of Cl anions.

3.2. Hexavalent Chromium Adsorption Results. ,e ad-

sorption of Cr(VI) anions in aqueous media using the

nanocomposite material of Fe₃O₄@SiO₂@LEuH@PW₁₀has

been investigated. After the adsorption experiments, the

Journal of Chemistry

7

O_1s

O_KLL

Eu_3d

Fe_2p

Si_2sSi_2p

C_1s

W_4f

1200

1000

800

600

400

200

0

Binding energy (eV)

(a)

W 4f_5/2

Eu 3d_5/2

W 4f_7/2

42

39

36

33

1125

1130

1135

1140

1145

Binding energy (eV)

(b)

(c)

Figure 7: XPS spectra of Fe₃O₄@SiO₂@LEuH@PW₁₀for (a) elemental information and chemical state information for (b) Eu_3dand (c) for

W_4f.

LEuH

Fe

₃

O

₄@SiO₂@LEuH@PW₁₀

Fe

₃

O

₄@SiO₂

10

20

30

40

50

60

70

2 theta (degree)

Figure 8: XRD patterns of the synthesized Fe₃O₄@SiO₂@LEuH@PW₁₀nanocomposite material in comparison with starting materials.

8

Journal of Chemistry

Fe₃O₄@SiO₂

Cs₇

PW

₁₀

O

₃₆

allows the recycling of adsorbent (Table 1). ,e chromate

anions are recovered by adding stoichiometric amounts

of KOH solution to the Cr(VI) solution to yield potas-

sium chromate powder after solvent evaporation (Fig-

ure 15). ,is is in agreement with the proposed reaction

pathway of PW₁₀Cr_{2 (aq)}species [22]. Most importantly,

we are able to isolate this intermediate species.

,e adsorption of hexavalent chromium anions by

Fe₃O₄@SiO₂@LEuH@PW₁₀is 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. ,e maximum

time required for complete adsorption of 100 ppm chromate

anions by 0.2 g of the Fe₃O₄@SiO₂@LEuH@PW₁₀nano-

composite is 42 minutes at 25^°C (Figure 16(b)). ,is ad-

sorption time might be acceptable for industrial wastewater

treatment. ,erefore, it can be concluded that the nano-

composite material is eﬀective in treating wastewater and tap

water in a relatively wide pH range.

Fe

₃

O

₄@SiO₂@LEuH@PW₁₀

4000

3000

2000

Wavenumber (cm^–1

1000

)

To show the temperature eﬀect on adsorption of Cr(VI)

by the Fe₃O₄@SiO₂@LEuH@PW₁₀nanocomposite, the ex-

periments 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. ,e optimized adsorption tem-

perature of chromate anions is 25^°C. It is noted that the

active sites (PW₁₀anions) of Fe₃O₄@SiO₂@LEuH@PW₁₀Cr₂

start to lose the adsorbed chromate anions at T ≥ 40^°C as

shown in Figure 17. ,e Gibbs free energies of the ab-

sorption system of the Fe₃O₄@SiO₂@LEuH@PW₁₀and

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, K_c, for

the abovementioned two systems are evaluated at diﬀerent T

according to equation (1), where q_eis the equilibrium

concentration of chromate concentration on the adsorbent

and C_eis the equilibrium concentration of chromate in

solution:

Figure 9: FT-IR spectra of the synthesized Fe₃O₄@SiO₂@LEuH@

PW₁₀nanoparticles in comparison with starting materials.

(a)

(b)

(c)

Figure 10: SEM images of (a) Fe₃O₄@SiO₂and (b) Fe₃O₄@SiO₂@

LEuH@PW₁₀and (c) EDX metal mapping for W (all the scale bars

are 200 nm).

q_e

K_c�

,

(5)

(6)

(7)

C_e

ΔG � −RT ln K_c,

resultant ﬁltrate is derivatized with 1,5-diphenylcarbazide in

2−

order to detect CrO₄at a wavelength of 530 nm (Fig-

ΔH ΔS

ln K_c�

ΔS �

+

,

ure 14). ,e 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 ex-

periments 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@PW₁₀material.

RT

R

ΔH − ΔG

(8)

(9)

,

T

C

�

1

+

C,

Q

q_mK_Lq_m

t

1

�

+

t,

(10)

(11)

q_tk₂q²_eq_e

Heating the adsorbed composite material, Fe₃O₄@

SiO₂@LEuH@PW₁₀Cr₂, at 40^°C in aqueous solution

yields a concentrated chromate anion solution, which

k₂

PW₁₀+ CrO²⁻

⟶ PW₁₀Cr_{2(solid phase)}

.

4(soln)

Journal of Chemistry

9

Si

2

O

Fe

keV

0

6

Fe₃O₄@SiO₂

SAED pattern

EDX

Delaminated LEuH nanosheets

+

[PW₁₀O₃₆]^7–

O

Si/W

Eu/Fe

W

P

2

Eu

keV

0

6

Fe₃O₄@SiO₂@LEuH@PW₁₀

SAED pattern

EDX

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

silicon wafer).

120

0.0004

80

0.0002

40

0.0000

0

10

20

30

40

0.0

0.2

0.4

0.6

0.8

1.0

Pore radius Dv (r) (nm)

Relative pressure P/P_o

Absorbed

Desorbed

(a)

(b)

Figure 12: (a) N₂sorption isotherms and (b) pore size distribution of Fe₃O₄@SiO₂@LEuH@PW₁₀nanospheres (type III isotherm): the

abrupt rise near P/P_o� 1 is due to the pores within the nanomaterial and not on the surface of the nanomaterial.

Plots of lnK_cversus 1/T 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⁻¹to be −3.151 × 10⁶J, indicating the reaction

to be favorable. ,e values of ∆H and ∆S for the adsorption

can be calculated to be −2.83 ×10⁵J·mol⁻¹and

9.81 × 10²J·K⁻¹, respectively.

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 ×10⁵J·mol⁻¹and −1.13 ×10³JK⁻¹, respectively

(curve a). ,e positive ∆H value indicates the endo-

thermic character of Fe₃O₄@SiO₂@LEuH@PW₁₀Cr₂

losing the chromate anions. And the negative ∆S value

suggests the decrease of the randomness.

,e desorption could also result from the antiferro-

magnetism of chromium. Before 38^°C, chromium possesses

antiferromagnetic properties. ,e t � 38^°C is the Neel

,e negative ∆H value suggests the exothermic

character of the reaction between Fe₃O₄@SiO₂@LEuH@

PW₁₀nanocomposite material and the Cr(VI) anions in

10

Journal of Chemistry

100

96

Fe₃O₄@SiO₂@LEuH@PW₁₀

%

92

200

400

T (ºC)

600

800

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

(a)

1.5

1.0

i

0.5

0.0

(b)

ii

400

500

600

Wavelength (nm)

i ^∗100ppm Cr⁶⁺

ii 0ppm Cr⁶⁺

(c)

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@PW₁₀after 1 hour reaction time at

24^°C and pH 6; ^∗dilution factor � 20.

Table 1: Test results of 0.4 g Fe₃O₄@SiO₂@LEuH@PW₁₀adsorbent 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.

2−

Test material

Volume (ml)

Initial CrO₄(ppm)

Final CrO₄(ppm)

Adsorbent reuse^∗

100

200

300

500

100

200

300

500

100

200

300

500

100

200

300

500

2

ND

Tap water

2

Wastewater

Tap water^†

Wastewater^†

2

100

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

Journal of Chemistry

11

K₂CrO₄

Industry

Products

Wastewater

Distillation

K₂CrO₄

KOH

CrO₄

40°C

CrO₄

Fe₃O₄@SiO₂@LEuH@PW₁₀

Redissolve

Magnetic separation

Fe₃O₄@SiO₂@LEuH@PW₁₀Cr₂

in small amount of water

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

100

80

y = 100 – 81/(1 + exp((x – 16.64)/6))

60

x at dy/dx = 0 is 42 mins

60

40

20

0

20

40

60

80

100

120

4

6

8

10

12

Time (mins)

pH

(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@PW₁₀nanocomposite; T � 25^°C.

temperature for chromium [33]. ,at is why it easily des-

orbed from adsorbent’s surface under temperature higher

than 40^°C.

the same binding energy and can only form monolayer. ,e

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.

,e BET isotherm also rules out the possibility of phys-

isorption mechanisms. As a result, chemisorption dominates

the adsorption process.

However, it is important to point out that the vertical rise

at P/P₀region � 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.

,is is particularly due to the size of the macropores,

As shown in Figure 18, the adsorption proﬁles ﬁt per-

fectly with the Langmuir adsorption model. ,e maximum

uptake capacity of the Fe₃O₄@SiO₂@LEuH@PW₁₀nano-

composite for chromate anions, q_m, can be determined from

the reciprocal of the slopes of the straight lines in

Figure 18(b). ,is corresponds well to the determined q_m

from the Langmuir isotherm curve in Figure 18(a). ,e

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

12

Journal of Chemistry

a

4

2

Desorption curve

y = 41537.6x – 135.7

R²= 0.983

0

–2

–4

–6

–8

–10

b

Absorption curve

y = 118 – 34050x

R²= 0.952

0.0030

0.0032

1/T (K^–1

0.0034

0.0036

)

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.

q_m

0.3

0.2

0.1

0.0

1200

800

400

0

y = –3.1E – 4 + 8.3E – 4x

R²= 0.999

0

100

200

300

400

0

100

200

300

C (ppm)

(a)

400

500

C (ppm)

(b)

Figure 18: (a) Optimal adsorbed amount (q_m) and (b) the Langmuir adsorption isotherm of Cr(VI) by Fe₃O₄@SiO₂@ LEuH@PW₁₀

nanocomposite at 25^°C and pH � 6.

diameter 60.7 nm determined by BET experiments, which

would otherwise be visible in SEM and TEM images (Fig-

ures 10 and 11).

,e Langmuir isotherm is valid for monolayer sorption

onto a surface with a ﬁnite number of identical sites and

uniform adsorption energies [34, 35]. ,e 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@PW₁₀nanocomposite

(Figure 2).

k₂q_e²(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@PW₁₀nano-

composite (equation (11)). ,e 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]. ,is result again

supports the chemisorption process. Other models can be

applied to test the chemisorption behavior including Elovich

kinetic model [38, 39].

To further verify the chemisorption process, pseudo-sec-

ond-order rate expression, commonly used for analyzing

chemisorption kinetics [36], is applied at low initial concen-

trations of Cr(VI) anions. ,e pseudo-second-order model

constant, k₂, can be determined experimentally by plotting t/q_t

against t from equation (10) to obtain the graph in Figure 19.

,e activation energy E_acan be determined from plots of lnk₂

versus 1/T to yield a straight line, with slope −Ea/R (Figure 19)

according to the Arrhenius equation [37]. In equation (10),

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

using NH₄Br and H₂O₂as feedstock, phenol red was suc-

cessfully catalysed to bromophenol blue using Fe₃O₄@

SiO₂@LEuH@PW₁₀nanocomposite material as a catalyst

(Table 2). ,e bromination reaction catalysed by Fe₃O₄@

SiO₂@LEuH@PW₁₀nanocomposite material was monitored

at time intervals of 30 seconds (Figure 20). ,e reaction was

Journal of Chemistry

13

0.12

0.8

0.6

0.4

0.2

y = 8.3E – 1 + 3.5E5x

R²= 0.98

0.08

0.04

0.00

0.0

0.2

0.4

0.6

0.8

1.0

0.00324

0.00330

1/T (K^–1

0.00336

)

0.00342

Time (hours)

y = 0.00595 + 0.10294x; R²= 0.99

y = 0.01093 + 0.11647x; R²= 0.98

y = 0.00699 + 0.09147x; R²= 0.99

y = 0.00491 + 0.10447x; R²= 0.99

y = 0.00565 + 0.09382x; R²= 0.99

298K

293K

303K

308K

313K

(a)

(b)

Figure 19: (a) Pseudo-second-order kinetics of Cr(VI) sorption onto 0.02 g of Fe₃O₄@SiO₂@LEuH@PW₁₀at 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

2−

Fe₃O₄@SiO₂@LEuH@PW₁₀with CrO₄

.

Table 2: Catalytic bromination activities of LEuH-Cl, Fe₃O₄, Fe₃O₄@SiO₂, and Fe₃O₄@SiO₂@PW₁₀catalysts.

Entry

Catalyst

BET surface area (m³/g)

Yield (%)

Time (h)

Rate (mmol·g^{−1 −1}

s )

1

2

3

4

LEuH-Cl

Fe₃O₄

Fe₃O₄@SiO₂

57.30

7.32

13.57

16.62

22

23

21

99

2

1.22^∗10⁻₋⁷₇

1.28^∗10₋₇

1.17^∗10

Fe₃O₄@SiO₂@LEuH@PW₁₀

0.1

5.5^∗10⁻³

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 P � 1 atm. Rate of conversion � mole Br⁻oxidized per total weight of catalyst (g) per second. Symbol “^∗”

denotes a multiplication sign.

100

80

R²= 0.998

60

40

20

0

2

4

6

8

10

Time (minutes)

Figure 20: Bromination of phenol red to bromophenol blue using Fe₃O₄@SiO₂@LEuH@PW₁₀as a POM nanocomposite catalyst. ,e

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

P � 1 atm.

14

Journal of Chemistry

1.0

0.8

0.6

0.4

0.2

0.0

1.0

0.8

0.6

0.4

0.2

0.0

0.2

0.4

0.6

0.8

1.0

0

1

2

3

4

5

H₂O₂conc. (mM)

Catalyst amount (g)

(a)

(b)

Figure 21: Eﬀect of (a) amount of catalyst (Fe₃O₄@SiO₂@LEuH@PW₁₀) and (b) amount of H₂O₂used 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

H₂O₂in reaction a + 0.5 g of catalyst in reaction b, in a round-bottomed ﬂask. Time � 10 minutes, T � 298 K, and P � 1 atm.

100

80

R²= 0.978

60

40

20

0

2

4

6

8

10

Conc. of NH₄Br (mM)

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 cata-

lyst + 2.5 mM H₂O₂, in a round-bottomed ﬂask. T � 298 K and P � 1 atm.

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@SiO₂exhibit 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 re-

action mixture raises steadily and reaches its maximum after

10 minutes (Figure 20).

intermediate compound that does not absorb UV light in the

tested region. Optimal conditions of the bromination re-

action were experimentally determined (Figures 21 and 22).

For instance, the maximum catalyst amount was determined

as 0.5 g for the Fe₃O₄@SiO₂@LEuH@PW₁₀nanocomposite

material (Figure 21(a)), while the maximum H₂O₂con-

centration 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 inter-

calated catalyst took 32 minutes to complete [40], while the

,e reduction in the concentration of phenol red in the

ﬁrst 1.5 minutes is probably due to the formation of an

Journal of Chemistry

15

100

80

60

40

20

0

1

2

3

4

5

6

7

8

9

10

Number of cycles

Figure 23: Reuse of Fe₃O₄@SiO₂@LEuH@PW₁₀catalyst for the conversion of phenol to bromophenol blue. ,e 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 P � 1 atm.

Phenol red

Bromophenol blue

OH

+

Br

HO

OH

Br

OH

–

OBr^–

SO₃

O

S

O

Br^–

[PW₁₀O_36–n(O₂)_n]^7–

[PW₁₀O₃₆]^7–

H₂O₂

H₂O

Fe₃O₄@SiO₂@LEuH@PW₁₀

Figure 24: ,e proposed mechanism of reaction for the bromination of phenol red to bromophenol blue using Fe₃O₄@SiO₂@LEuH@PW₁₀

as a catalyst.

OH

Br

[PW₁₀O_36–n(O₂)_n]^7–

Br

O

nH₂O

Br

+ Phenol

+ H₂O₂

OBr^–

OH

O

S

O

HOBr,

Br₂

Bromophenol

¹O₂

+

Br^–

nH₂O₂

Br^–

7–

[PW₁₀O₃₆

]

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

16

Journal of Chemistry

new Fe₃O₄@SiO₂@LEuH@PW₁₀nanocomposite material

takes less than 10 minutes to complete a similar reaction

(Figure 20). Moreover, in addition to ease of catalyst sep-

aration 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).

,e 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@

PW₁₀nanocomposite. ,e 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. ,ese OBr⁻radicals are then involved

in an electrophilic reaction with phenol red to get the

bromophenol blue. ,ese 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. ,ere are

other side reactions in the system as demonstrated in

Figure 25:

nanocomposite can be eﬀective too. ,e chromate ad-

sorption capacity of the synthesized Fe₃O₄@SiO₂@

LEuH@PW₁₀nanocomposite 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. ,e 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

,e 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 Tech-

nology is acknowledged.

7−

n H O + PW O

⟶ PW O

O

2

+ nH₂O

(12)

ꢂ

ꢃ

ꢂ

ꢀ

ꢁ ꢃ

2

10 36

10 36− n

n

7−

+ nBr⁻⟶ PW O

+ n OBr⁻

(13)

References

PW O

O

ꢂ

ꢀ

ꢁ ꢃ

ꢂ

ꢃ

10 36− n

2

10 36

n

[1] J. A. Jacobs and S. M. Testa, “Overview of chromium (VI) in

the environment: background and history,” in Chromium (VI)

Handbook, CRC Press, New York City, NY, USA, 2005.

[2] A. H. Stern, “A quantitative assessment of the carcinogenicity

of hexavalent chromium by the oral route and its relevance to

human exposure,” Environmental Research, vol. 110, no. 8,

pp. 798–807, 2010.

C₁₉H₁₄O₅S + 4OBr⁻⟶ C₁₉H₁₀Br₄O₅S + 2H₂O₂

(14)

In summary, we present a new approach of eﬀectively

heterogenizing PW₁₀anions on a magnetically active

nanospherical support material. ,e catalytic versatility of

the as-synthesized Fe₃O₄@SiO₂@LEuH@PW₁₀nano-

composite material has been demonstrated by the conver-

sion of phenol red to bromophenol blue. In this reaction,

Fe₃O₄@SiO₂@LEuH@PW₁₀takes 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@PW₁₀can be reused

(minimum 10 times tested) with almost similar results

obtained. In addition, it can be easily be separated using an

external magnet.

[3] S. Tahir and R. Naseem, “Removal of Cr (III) from tannery

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Article Doi

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

DOI: 10.1155/2020/6375749

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