International Forum for Green Asia 2020

Electronic structure of ferric doped montmorillonite and its application tophotocatalytic reduction of chromate
Li Zhang1, Chitiphon Chuaicham1, Keiko Sasaki1
1

Department of Earth Resources Engineering, Faculty of Engineering, Kyushu University, Fukuoka
819-0395, Japan

Abstract
Clay based photocatalysts have been reported to be highly active photocatalysts for the environmental andenergy applications because of unique 2D structures and optical properties. In this work, Fe(III)-modified
montmorillonite was prepared by ion-exchange for the photocatalytic reduction of toxic Cr(VI)2O72-. The
best performance was observed with the Fe(III) modified montmorillonite with 0.377 mmol-Fe(III)/g. The
enhancement of photocatalytic activity of Fe(III)-modified montmorillonite might be derived from not only
the reduction of energy band gap but also the generation of new electronic levels from Fe(III) between thevalence band and the conduction band of montmorillonite. The new electronic state from Fe(III) could accept
the electrons from valence band of montmorillonite to avoid the recombination of the electron-hole pairs.
Moreover, the reusability of Fe(III)-modified montmorillonite was evaluated. The photocatalytic activity
decreased after the repeated cycles, which is due to the releasing of Fe(III) from the modified
montmorillonite. The finding can be used to develop a various clay based photocatalysts for wastewater
treatment and beyond.
1. Introduction
Montmorillonite (Mt) is a representative 2:1-type ofclay mineral with one octahedral Al–O(H) sheet
sandwiched between two tetrahedral Si–O sheets in
one layer. It has a high cation exchange capacity(~1.14 meq/g). The clay minerals have been often
applied as supporting materials to make a better
dispersion of the main material on them andimprove the separation efficiency of the material
from the liquid, for example environmental
remediation materials [1], [2].
Although clay minerals were generally regarded tohave no photocatalytic property [3], acid activatedMt endued the photocatalytic activities bygenerating the Brønsted acid sites [4]. So it has
been found that the acid activated Mt can be utilized
as a component of photocatalyst composites.
However, the treatment of clay minerals requiresvery strongly acidic conditions, where theconcentration of proton is usually > 5 M, and longtime, usually for longer than 2 h [3], [4].
Iron (Fe) is one of widely distributed elements in thenatural environments. The role of iron in thephotocatalytic reaction arouses the researchesinterest. It seems like the Mt not only acts as asupport material in the composite with the main
photocatalyst, the clay minerals itself with iron

doping can act as a photocatalyst independently. AZn/Fe-layer double hydroxide (LDH) has been
observed to photodegrade various azo dyes more
efficiently than bare Fe2O3 or ZnO under visible
light [5]. The authors proposed that this may be dueto the important role that the Fe(III) ions in the FeO6
octahedrons of the Zn/Fe LDH structure have in theabsorbance of visible light. Also, the hydroxyl
groups in the LDH capture the photo-induced holes(h+) thus preventing the recombination of the h+ andelectron(e-) leading to enhanced photoactivity [5].
Other researchers have attributed the prevention ofrecombination to the oxo-bridges between the twodifferent metals in the brucite layer [6]. However,these assumptions explaining the mechanisms ofCr(VI) photocatalytic reduction are still
controversial in the literature.
Up to our best knowledge, iron doped Mt has notbeen used for the photocatalytic reduction of Cr(VI).
In the present work, Fe(III)-doped Mt with varied
stoichiometry of Fe(Ⅲ) dosage was synthesized at
pH 2 within 1 h to use this for Cr(VI) photocatalytic
reduction under acidic condition avoiding theprecipitation of Fe(III) source, and elucidate the roleof Fe(III) in the photocatalytic mechanisms ofCr(VI) reduction.

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2. Materials and methods
2.1. Preparation of Fe(III)-modified Mt
Na-montmorillonite (Kunipia-F) was obtained from
Kunimine Industries Co. Ltd. (Tokyo, Japan). To
synthesize the Fe-Mts, Mt was dispersed into thesolution where Fe(III) stoichiometrically equivalent
to the 0.25, 0.5, 1, 2, and 4 times of CEC for Mt.
Then Fe-Mts were collected and named dependingon the added Fe(III). The remaining concentrationof Fe(III) in supernatant was determined by an
inductively coupled plasma-optical emission
spectroscopy (ICP-OES; Perkin Elmer 8500,
Waltham, MA, USA) and listed in Table 1.

interlayer space in H-Mt is attributed to themigration of the structural Fe(II)/Fe(III) from thealuminosilicate layers to the interlayer. As for other
Fe-Mt, the expansion of interlayer distance wasdominated by both the intercalation of Fe(III) andthe migration of the structural iron.
In the preparation of Fe-Mt, the immersion of theMt into The Fe(III) solution at low pH generated
some defects on the Mt structure, which confirmedby the release of Fe in H-Mt. Not only the addedFe(III) from Fe(III) solution can intercalate into theinterlayer space of Mt, but also the Fe(III) baring in
the original Mt structure can diffuse/migrate to thechannel of Mt layers. The dynamic of the structuraliron and added Fe(III) in the interlayer space was
not detectable, but the dynamics of iron species inthe solution was detected by ICP-OES and listed inTable 1. With the increase of the initial Fe(III)addition, the doped amount of Fe(III) increased in
H-Mt, 0.25Fe-Mt, 0.5Fe-Mt, and 1Fe-Mt; andtended to be stable when it was over 1 time CEC ofMt. Based on the chemical formula of the Kunipia
F, the Na(Ⅰ) engages ~0.858 meq/g charges on Mt
layers. As the adsorbed Fe(III) reached a subsaturated state of 0.720 meq/g in 1Fe-Mt, most ofthe Na(Ⅰ) sites were replaced by Fe(III). The
additional Fe(III) intercalated should start to occupy
the Ca(II) exchangeable sites which require morechange in entropy.

2.2. Solid characterization
Powder X-ray diffraction (XRD) patterns of theoriginal Mt and Fe-Mts were recorded using an
Ultima IV X-ray diffractometer (Rigaku, Akishima,Japan). X-ray photoelectron spectroscopy (XPS)
was conducted on an ESCA 5800 system (UlvacPHI, Kanagawa, Japan. The N2- adsorption /desorption isotherms were measured on aBELSORP-max instrument (BEL Inc., Osaka,Japan). Diffuse reflectance spectra (DRS) weremeasured on a UV-2450 spectrophotometer(Shimadzu Co. Ltd., Kyoto, Japan). Scanningelectron microscopy (SEM) images were observed
using a VE-9800 SEM (Keyence, Osaka, Japan).
2.3. Photocatalytic activity test
The photocatalytic reduction of Cr(VI) were tested
under a 300 W Xe lamp irradiation in 10 mg/L
Cr(VI) solution with S/L=1:1 at pH 2.0 in aphotoreactor with a cooling water jacket outside.
After the adsorption/desorption equilibrium within
10 min, ~ 1 mL of the suspensions was sampledfrom the reaction cell at appropriate. The remaining
Cr(VI) was determined by the carbazide methodwith a UV–Vis spectrophotometer (UV-2450,Shimadzu, Kyoto, Japan) at 540 nm.

Table 1 Fe(III) loading amount, d001-value of Mt, SBET,
and Eg of Fe(III) modified Mts
adsorbed Fe(III)Sample3. Results and discussion
3.1. Characterizations
The 001 reflection in XRD of the original Mt
emerged at 7.25° in 2θ is correspondent to theinterlayer distance to be 1.21 nm including thethickness of one Mt layer (0.96 nm). Even without
any Fe(III) addition, the H-Mt still shows
correspondingly to the similar interlayer distance(~1.39 nm) with other Fe-Mt. The expanded

11d0012Eg

3(eV)

Q (mmol/g)

meq/g

H-Mt

-0.001-0.003

1.38

3.50

0.25Fe-Mt

0.091

0.238

1.38

3.10

0.5Fe-Mt

0.175

0.462

1.38

2.70

1Fe-Mt

0.274

0.720

1.382.65

2Fe-Mt

0.299

0.786

1.38

2.60

4Fe-Mt

0.312

0.822

1.40

2.60

(nm)

Calculated based on ICP-OES measurement.
Estimated by XRD.
3
Evaluated by Tauc plotting Kubelka–Munk function
versus the energy.
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The SEM images for the H-Mt were similar to theoriginal Mt (Fig. 1). As the addition of Fe(III)increased, the stacked layers tend to be non-parallel
lead to the increase of face to edge cross and the Mt
layers on the surface broke down into smaller layers.
From the N2 adsorption/desorption isotherm, thetype IV curve implied the mesoporous features ofsamples.

Fig. 2 DRS-UV spectra of Fe-Mt.

3.3. Photocatalytic activity
Fig. 3 shows the Cr(VI) removal in the presence ofFe-Mts. Under the dark condition, slight adsorptionof Cr(VI) with ~2- 6% was observed. Bichromate(Cr2O72-) is unlikely to be adsorbed on Mt in itself,
but the adsorbed Fe(III) might be electrostaticallybound with it. Under light irradiation, the H-Mt
showed only around 20% reduction of Cr(VI) after
60 min. As is suggested in XRD result (Table 1),Fe(III)/Fe(II) in the octahedral sheet diffused to theinterlayer which has an improved effect on theoptical property and photocatalytic activity of H-Mt.
With further Fe(III) loading in 0.25Fe-Mt and0.5Fe-Mt, the improvement of Cr(VI) reduction wasrelated to the enhanced effect of newly addedinterlayer Fe(III) and the removal of Cr(VI) reached
22% and 35%. When the interlayer doped Fe(III)amount increased to 0.274 mmol/g-Mt in 1Fe-Mt,
the photocatalytic reduction of Cr(VI) reached to themaximum with 47%. A slight increase of Fe(III)loading in 2Fe-Mt and 4Fe-Mt slightly reduced theperformance, with 44% and 43% efficiency. The
kinetics of Cr(VI) photocatalytic reduction fitted tothe pseudo-first-order kinetics (Eq. 2):−ln (./.$ ) = %0(2)where . and .$ (mmol/g) are the remaining andinitial concentrations of Cr(VI), the % (min−1) is
the rate constants of the pseudo-first order model.
The % are 2.97 × 10-3, 5.30 × 10-3, 6.31 × 10-3,
10.52 × 10-3, 9.22 × 10-3, and 9.08 × 10-3 min-1.
The inconspicuous difference of the surface
morphology (Fig. 1) depending on doped amounts
Fe(III) on Mt indicated to have a negligible effecton the photocatalytic reaction.

Fig. 1 SEM images of (a) H-Mt, (b) 0.25Fe-Mt, (c)0.5Fe-Mt, (d) 1Fe-Mt, (e) 2Fe-Mt and (f) 4Fe-Mt.

3.2. Optical and charge separation properties
The UV–vis diffuse reflectance spectra (UV-DRS)of Fe-Mt showed a significant red shift of theabsorption edge as compared to H-Mt (Fig. 2) whichsignificantly improved the optical property of thesamples. The band gap energy (Eg) of Fe-Mt can beevaluated according to Eq. (1):!ℎ# = %(ℎ# − ())!/#(1)
where ! , ℎ , # , % , and () represents theabsorption coefficient, Planck constant, lightfrequency, band gap energy, and the proportionality
constant, respectively.
The value of n is dependent on the type of opticaltransition of the measured semiconductor (directtransition at n = 1 and indirect transition at n = 4).
The calculated band gap values of the samples were
estimated to be approximately 3.50, 3.10, 2.70, 2.40,2.40, and 2.38 eV for H-Mt, 0.25Fe-Mt, 0.5Fe-Mt,
1Fe-Mt, 2Fe-Mt, and 4Fe-Mt (Table 1). The XPS
revealed the valence band (VB) energy was around
2.0 eV. Therefore, the conduction band energy (CB)of 1Fe-Mt is −0.65 eV.
The correlated Fe(III)-doping (Table 1) and the ()from H-Mt to 1Fe-Mt enhanced the photogeneration of charge carriers (e- and h+) which may
was caused by the formation of new middle gaplevel(s) driven from the doped Fe(III). AdditionalFe(III) trapped the e- from the Cr(VI) or overlapped
to compete converting the light.

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decreased the Cr(VI) photocatalytic reduction,
probably because the extra of Fe(III) in the Fe-Mt
can not only act as recombination center of e-,leading to inhibition of e- transfer in the Cr(VI)reduction, but also some Fe(III) cover on the surfaceof Mt, resulting to decrease in surface areas of Mt toreceive the light. The Fe(III) might decide thegeneration of a new electronic level.
Acknowledgements
This work was supported by Kyushu University
Progress 100 Strategic Partnership Acceleration,and Cooperative Research Programs at Hokkaido
University Institute for Catalysis (Nos. 19B1002
and 20A1001), which were provided to KS. LZ
sincerely appreciates the Advanced Graduate
Program in Global Strategy for Green Asia.

Fig. 3 Adsorption and photocatalytic reduction ofCr(VI) over Fe-Mt under irradiation time at pH 2, and(inset) the pseudo-first-order kinetic plot.

3.4. Mechanism of Cr(VI) reduction
The photocatalytic activity of Fe-Mt photocatalystfor Cr(VI) reduction can be described by theproduction of the e--h+ pairs induced through thetransition of e- under UV light [7]. The Fe-Mts
showed significant increase in photocatalytic
reduction of Cr(VI) compared with H-Mt. Also fromthe DRS and () (Fig. 2), the H-Mt sample showedlarger () than Fe-Mts, indicating that the presenceof iron in the structure of Mt might can reduce ()of Mt by formation of new electronic level between
VB and CB of Mt. Photo-generated charge carriers(e- and h+) of Fe-Mt were generated and separated
at different electronic states: the photo-generated ecan move to new middle gap state, and leaving
photo-generated h+ in the VB. For photocatalytic
reduction of Cr(VI), the dissolved Cr(VI) in thesolution could be reacted with an e- in middle gapstates and reduced it to Cr(III), while the h+remained in the VB or reacted with H2O molecules
to enhance the photocatalytic performance in Cr(VI)reduction. Reduced Cr(III) species should bepartially adsorbed on the surface of Mt anddeceasing its release into the solutions.

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4. Conclusions
Fe(III)-doping
significantlyimproved
thephotocatalytic reduction of Cr(VI) by Mt. The
optimal doping amount of Fe(III) was 0.720 meqFe(III)/g-Mt showing 47% reduction of Cr(VI)while it was only 20% without doping Fe(III).
However, the excess doping amounts of Fe(III)Email: zhang@mine.kyushu-u.ac.jp36