Advanced oxidation of rhodamine B with hydrogen peroxide over Zn-Cr layered double hydroxide catalysts

Degradation efﬁciency (a) of RhB over ZneCr layered hydroxide catalysts ([RhB] ¼ 20 mg/L, 0.3 g of catalyst, pH ¼ 6.5, room temperature) and UVevis spectra (b) of the reaction solution i[r]

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Original Article

Advanced oxidation of rhodamine B with hydrogen peroxide over

a Faculty of Chemistry, Vietnam National University Hanoi, 19 Le Thanh Tong Street, Hanoi, 100000, Viet Nam

b School of Chemical Engineering, Hanoi University of Science and Technology, 1 Dai Co Viet Street, Hanoi, Viet Nam

a r t i c l e i n f o

Article history:

Received 10 July 2017

Received in revised form

11 July 2017

Accepted 14 July 2017

Available online xxx

Keywords:

Degradation

Rhodamine B

Layered hydroxides

AOP

Zn/Cr materials

Cr 2 O 3

a b s t r a c t

Zn/Cr layered zinc hydroxide materials with different molar ratios of Cr/Zn have been synthesized through the coprecipitation method at pH of 9.0e9.5 At high Cr/Zn molar ratios of 0.5/1e1/3, the ma-terials possess some layered structure with carbonate anions between the interlayer galleries The cat-alysts present uniform particle sizes and quite high surface area An isomorphous substitution of Zn2þby

Cr3þin the brucite-like sheets makes the layered Cr-doped zinc hydroxides acting as catalysts for efﬁcient oxidation of rhodamine B with H2O2solution The experimental results indicated that the intra-lattice

Cr3þ ions are more active than Cr2O3 components in the oxidative removal of rhodamine B The degradation efﬁciency is dependent on the intra lattice Cr3þcontents and reaction variables The Cr/Zn LDH gave a high decolorization (99%) of rhodamine B at near neutral pH and room temperature

© 2017 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)

1 Introduction

The organic dyes have been widely used in the textile industry for

last decades Nowadays, they have also known as toxic and polluted

compounds because of their high persistence in soil sediments and

water resources These chemicals may be degraded to the

carcino-gens and toxic products under sunlight irradiation, affecting aquatic

life and ecosystem Therefore, in recent years, there has been an

increasing demand for the destruction of organic dyes dissolved in

water [1e3] Traditionally, biological, physical, and chemical

methods have been applied for the removal of dyes[2e7] While the

biological degradation of these dyes still faces challenges, chemical

process and physical adsorption are more attractive in the treatment

of wastewater[5,6] However, the chemical method in many cases

generates a huge amount of sludge disposal that requires the

post-treatment[35,7] In context, traditional physical methods such

adsorption, ﬁltration… are easy to operate, but they are

nondestructive processes and certainly transfer the organic

pollut-ants in water into the solid waste only[4e6]

Recently, advanced oxidation processes have paid more atten-tion as efﬁcient routes approaching a complete destruction of organic dyes Particularly, semiconductor based photocatalysts are

of great interest for hazardous wastewater removal Numerous materials such as iron oxide [7,8], Nb-based oxides [9,10], W-containing materials[11,12], titanium dioxides and Ti-related cat-alysts[13e20]are known as good photocatalysts for the oxidation

of organic pollutants, however, these solids can only be activated under UV-light irradiation because of their large band gap There-fore, these catalysts need to either modify for the utilization under visible light or undergo the advanced oxidation reactions in the degradation of organic pollutants[11,16,18,20]

Layered double hydroxides (LDHs) are classiﬁed into a class of anionic mineral clays with the general formula expressed as [M12þxMx þ(OH)

2]xþ(An)x/n$mH2O, where the M2þ (Mg2þ, Ni2þ,

Co2þ, Cu2þor Zn2þ) and M3þ(Al3þ, Cr3þ, Fe3þ) cations are divalent and trivalent metal ions positioned in the center of an octahedron and the Ananions are occupied in the spaces between two octa-hedral layers[5,21e24] LDHs have been widely synthesized for the applications in basic catalysts[21,22], oxidation-reduction catalysts

[24e27], adsorbents [28], anion exchangers [21,29], and so on Recently, LDHs have been shown a good photocatalytic activity for the degradation of organic compounds [23,25,27,30e32] For

* Corresponding author Fax: þ84 04 3824 1140.

E-mail address: ntthao@vnu.edu.vn (N.T Thao).

Peer review under responsibility of Vietnam National University, Hanoi.

Contents lists available atScienceDirect Journal of Science: Advanced Materials and Devices

j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / j s a m d

http://dx.doi.org/10.1016/j.jsamd.2017.07.005

2468-2179/© 2017 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license ( http://creativecommons.org/licenses/by/4.0/ ).

Journal of Science: Advanced Materials and Devices xxx (2017) 1e9

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example, Zn/Cr LDHs was reported to be good reactivity for the

degradation of methylene blue under visible-light irradiation[32]

Zn/Ti LDH catalysts showed a good activity of in the degradation

of rhodamine B in the presence of air[31] Thus, layered hydroxide

materials are expected to be potential catalysts for the applicability

to oxidation of dyes in water

In the present work, we report on the synthesis of layered

double hydroxide containing both Cr and Zn in the brucite-like

layers Indeed, a substitution of Zn2þby Cr3þin the zinc

hydrox-ide layers leads to a variation in the solid composition formulated

as [(Zn1yCry(OH)2)þy$(X2y/nn )2y ]$mH2O and creates more OH

deﬁciencies in the hydroxide layers This modiﬁcation allows

pre-paring some modiﬁed Zn-based catalysts with some desired

cata-lytic properties due to the ability of hydroxide layers to

accommodate some cations of various sizes and valences The

synthesized ZneCr LDHs are expected to be effective catalysts used

in the oxidation of rhodamine B in water

2 Experimental

2.1 Catalyst preparation

ZneCr layered double hydroxides were prepared by

coprecipi-tation of the Zn(NO3)2$6H2O salt (Wako) with the desired amount

of Cr(NO3)3$xH2O (Wako) in a beaker containing 300 mL of solution

of urea (1.45 M) heated at 90C The mixture was then stirred under

reﬂux at the same temperature for 48 h Under reported

experi-mental conditions, the hydrolysis of urea results in the formation of

ammonium cyanate and the resultant is further hydrolyzed into

ammonium carbonate[31] As a consequence, the solution reaches

a constant pH of 9e9.5 due to the hydrolysis of ammonium and

carbonate ions Then, the resultant slurry was then cooled to room

temperature and separated byﬁltration, washed with hot distilled

water several times until obtaining the neutralﬁltrate The

ﬁlter-cake was then dried at 80 C for 24 h in oven For the sake of

brevity, the prepared catalysts are denoted as CrZnx in which x is a

molar ratio of Cr/Zn as reported inTable 1

For a mixed oxide (reference) sample, two salts, Zn(NO3)2$6H2O,

Cr(NO3)3$9H2O were separately calcined at 450C in air for 3 h,

then two powder oxides were blended with Cr/Zn molar ratio of 1/

2 The reference sample is designated as MiOx

2.2 Characterization

Powder X-ray diffraction (XRD) patterns were recorded on a D8

Advance-Bruker instrument using CuKaradiation (l¼ 0.1549 nm)

The scanning electron microscopy (SEM) images were obtained

with a JEOS JSM-5410 LV TEM images were collected on a Japan

Jeol Jem.1010 The speciﬁc surface area was calculated by the

BrunauereEmmetteTeller (BET) method, and the pore size

distri-bution and total pore volume were determined by the

Bru-nauereJoynereHallenda (BJH) method using an Autochem II 2920

(USA) Fourier transform infrared (FT-IR) spectra were obtained in

4000e400 cm1range on a FT/IR spectrometer (DX-Perkin Elmer,

USA) Energy-dispersive spectroscopy (EDS) data were obtained from Varian Vista Ax X-ray energy-dispersive spectroscope 2.3 Degradation of rhodamine B aqueous solution

Rhodamine B (RhB) has been chosen as the degrading pollutions

to test the photocatalytic activities of the as-prepared samples The catalyst of 0.30 g dispersed in Rh B aqueous solution (100 mL,

20 ppm) was dropped by H2O2solution (30%) at theflow rate of about 1.0 mL/min The reaction was magnetically stirred under ambient condition (under 40 W-fluorescent-lamp in the laboratory, room temperature of 28C) The degradation efficiency of RhB was monitored using the UVevis absorption spectra by measuring the peak value of a maximum absorption of rhodamine RhB solution During the irradiation, about 2 mL of suspension was continually taken from the reaction solution at given time intervals for the determination of rhodamine B concentration (Ct) The degradation

efﬁciency (%) can be calculated as decoloration ¼ C0 Ct/C0 100% where C0is the initial concentration of dye and Ctis the measured concentration of rhodamine B in aqueous solution at a given time (t) The concentration of the target dye is calculated by a calibration curve The maximum absorption of RhB was at a wavelength of

553 nm The UVevis spectrophotometer (Shimazdu, Cary 100 UV-VIS spectrophotometer) with quartz cuvettes was used for the determination of color intensity in the range of 300e600 nm Calibration based on the BeereLambert law was used to quantify the dye concentration

3 Results and discussion 3.1 Characteristics of catalysts Some typical characteristics of the catalysts are summarized in

Table 1 X-ray diffraction patterns of all samples are reported in

Fig 1 The diffraction peaks of starting Zn(OH)2 matched with a standard pattern (JPCDS 00-048-1066) [33] After adding chro-mium(III) ions to the zinc hydroxide lattice, some new diffraction peaks indexed into the (003), (006), (012), and (110) planes typi-cally characterize for the layered structure of the Cr-rich materials (x> 1/3) This result clearly demonstrated that the Zn(OH)2lattices areﬁrmly adopted a certain amount of Cr3þto form a zinc chro-mium carbonate layered hydroxide (JPCDS Card No 00-052-0010) The CreZn layered hydroxide phase was detected as a dominant component at high concentration of Cr ions in the solids (sample CrZn1eCrZn2) while a mixture of zinc hydroxides and Cr-doped LDHs are present in the Cr-poor samples (x 1/3) [27,34] Furthermore, the diffraction peaks of layered hydroxides gradually become broader when the Cr content increases from sample CrZn5

to CrZn1, reﬂecting the formation of nanometric CreZn layered hydroxide crystals Such morphology is conﬁrmed by microscopy techniques

As seen inFig 2, the fresh Cr-doped samples are composed of irregular-shaped nanoparticles with the mean size of 50e70 nm Meanwhile, the two Cr-poorer samples (CrZn3 and CrZn5) possess

Table 1

Some typical characteristics of all catalyst samples.

Batch # Theoretical formula of CreZn LDH samples Cr:Zn ratio Cr (wt.%) Zn (wt.%) S BET (m 2 /g) CrZn1 Zn 1/2 Cr 1/2 (OH) 2 (CO 3 ) 1/4 ,xH 2 O 1:1 20.78 29.45 148 CrZn2 Zn 2/3 Cr 1/3 (OH) 2 (CO 3 ) 1/6 ,xH 2 O 1:2 13.17 38.34 154 CrZn3 Zn 3/4 Cr 1/4 (OH) 2 (CO 3 ) 1/8 ,xH 2 O 1:3 10.17 42.34 139 CrZn5 Zn 5/6 Cr 1/6 (OH) 2 (CO 3 ) 1/12 ,xH 2 O 1:5 6.48 54.21 134

N.T Thao et al / Journal of Science: Advanced Materials and Devices xxx (2017) 1e9 2

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some large nanoplates, in addition to numerous grains of layered

hydroxide crystals Each individual plate has an average diameter of

300e400 nm and a length of microns, orients outward from center

More plates are observed on the Zn-richer samples (Fig 2), which

are described as the Zn(OH)2component This observation is in good harmony with the TEM observation As seen inFig 3, sample CrZn2 presents mainly thin sheet-shaped morphologies and the border lengths of these sheets are a few dozens of nanometers

Fig 1 Powder-XRD patterns of CreZn layered hydroxide samples.

Fig 2 SEM micrographs of CreZn layered hydroxides.

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Moreover, the TEM image also reveals planar structure which is

typically characteristic for the layered hydroxide materials

[21,27,31] As the Cr content decreases, the inhomogeneous

shaped-morphologies are observed: a mixture of sheet-shaped and

plate-shaped crystals are clearly present in the samples (CrZn5) The

mean crystal domain of the nano-sized palettes is around 80 nm

while that of the large plates is in the scope of several microns

[35,36] In addition, the aggregation of uniform nanoparticles leads

to the formation of voids between grains, resulting in a large

external surface area while the close-folded agglomeration of the

zinc hydroxide plates leading to decrease of speciﬁc surface area of

the solids (Table 1)

The surface chemical composition of the synthesized CreZn

LDH materials was ascertained using EDS technique.Fig 3indicates

that Zn, Cr, Cr, O, C all can be detected The molar ratio of Zn and Cr

is close to the theoretic stoichiometric atomic ratio A small

devi-ation in molar composition is attributed to the coprecipitdevi-ation at

pH constant and the inhomogeneous composition of the catalyst

surface (Figs 2 and 3)[30,36,37] Indeed, as seen inFig 4, analysis

of nitrogen physic sorption indicates the BrunauereEmmetteTeller

(BET) surface area of all samples in the range of 100e154 m2/g The

BET surface area of layered hydroxide samples is likely related to

the composition of the solids Furthermore, Fig 4 represents

nitrogen adsorption/desorption isothermal curves of the catalysts belonging to the type IV according to the IUPAC classiﬁcation The hysteresis loops with the sloping adsorption branch and nearly vertical desorption branch correspond to type H2, which is essen-tially characteristic for pore-like-shapes with empty spaces[35,38] The chemical bonding behavior of the Cr-doped layered zinc hydroxides is investigated by FT-IR spectroscopy.Fig 5displays the FT-IR spectra for the fresh and some spent CreZn LDH samples No major difference in spectra between the fresh and spent samples is observed In both cases, it is observed that a very intense and broad absorption band around 3560 cm1is assigned the stretching mode

of layer hydroxyl groups and the stretching mode of interlayer water molecules This broadening band in the lower wavenumbers

is due to the hydrogen bonding between the water molecules and the interlayer anions in the CreZn LDH [21,35] Furthermore, a weak band around 1650 cm1is attributed to the bending mode of water molecules[26,35,39] A sharp band at 1380 cm1 is solely assigned to the antisymmetrical stretching mode of the carbonate anions in interlamellar spaces [20,24,26,35,39] The absorption bands below 1000 cm1 can be attributed to the MO vibration modes of the ZneCr patterns Thus, the presence of Cr3 þions in the

layered zinc hydroxides is expected to be active sites for the destructive oxidation of rhodamine B in water

Fig 3 TEM images (left) and EDS spectra (right) of CreZn layered hydroxide samples.

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3.2 Oxidative removal of rhodamine B aqueous solution

The catalytic oxidation of rhodamine dye aqueous solution has

been carried out using H2O2aqueous solution at room temperature

and atmospheric pressure All the experiments were performed at

the pH around 6.5 with a catalyst amount of 0.30 g After a given

time interval, the corresponding concentration of the dye (as

measured by UV spectrophotometer and the concentration

evalu-ated using BeereLambert's law) was taken as the measured

con-centration of the dye for all the catalyzed reactions

3.2.1 Catalytic degradation of rhodamine B with H2O2

The rhodamine B aqueous solution was stirred under white light

over the CreZn LDH catalysts in the presence of H2O2for catalytic

reaction at room temperature Firstly, the control experiments were

performed in the dark without H2O2 oxidant to determine the

adsorption ability of RhB over the catalysts In the UVevis spectra of

RhB solution, the absorbance of dye solution at 553 nm (n/p*

transition of C]N, C]O groups) is used to monitor the

decoloni-zation level of the dye between time intervals[6,8,10,31,39] It is

noted that when reaching equilibrium adsorption state under the

dark conditions, the samples CrZn1eCrZn5 shows about 3.8e5.6% adsorption of RhB molecules Another experiment (blank test) was carried out under (laboratory) visible light and H2O2in the absence

of the catalysts for comparison.Fig 6demonstrates that the blank test has no conversion of rhodamine B (seeFig 1S in Supplemen-tary Materials) although a remarkable amount of H2O2was added

to the reaction mixture In other context, Zn(OH)2and MiOx sam-ples both exhibit a negligible amount of RhB removed from the solution (Figs 6b and2S) Meanwhile, all ZneCr LDH catalysts show very high activity in the degradation of rhodamine B (Fig 6a), substantiating a crucial role of Cr-doped layered zinc hydroxides in the oxidation of the organic dye with H2O2 In a series of Cr-containing hydroxide catalysts, the catalytic activity decreases in the order of CrZn2> CrZn5 > CrZn1 ~ CrZn3, which are believed to

be correlated with the intra-lattice Cr3þcontent and the catalyst texture As indicated in Fig 1, the sample CrZn2 exhibits mainly

CreZn LDH phase while the others possess both CreZn LDHs and Zn(OH)2constituents The catalytic activity of Cr3þin the oxidation

of rhodamine was corroborated by the observable changes in

UVevis spectra of RhB in different reaction periods (Fig 2S-c, Supplementary Materials) It is clear to observe a rapidly decreased

Fig 4 Nitrogen adsorption/desorption isothermal curves of CreZn layered hydrotalcite samples.

Fig 5 FT-IR spectra of fresh and reacted ZneCr layered hydroxide catalysts: (a): CrZn2-Fresh; (b): CrZn2-Spent; (c): CrZn3-Fresh; (d): CrZn3-Spent; (e): CrZn5-Fresh (spent samples: catalysts were recorded FT-IR spectra after reaction).

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absorption intensity of RhB at 553 nm after 30 min and then

absorbance of RhB solution likely approaches the baseline after

210 min of reaction These results re-afﬁrm the promotional effects

of Cr3þin the zinc hydroxide lattice on the advanced oxidation of

rhodamine B in the presence of H2O2(Fig 2S)[31,40] Furthermore,

no shift of adsorption wavelength in UVevis spectra (Fig 2S)

sug-gests the simultaneous degradation of both the dye chromophores

and the aromatic rings under these reaction conditions

[12,20,31,40] A lower activity of the Zn-richer catalysts (CrZn3 and

CrZn5) attributed to the existence of a remarkable amount of the

zinc hydroxides in the catalysts (Fig 2)[25,30,31,33] The activity of

CrZn1 is on a par with that of CrZn3, interpreting that the zinc

hydroxides would only accommodate a certain amount of Cr3þions

in the layered double hydroxide framework[30,35,37]

All experiments have been carried out under the laboratory light

exposure (40 W-ﬂuorescent lamps); the photocatalytic reaction

may be presumably activated In order to shred in light of the

na-ture of catalytic reaction, an additional set of reactions was

performed in dark under controlled conditions.Fig 7depicts the catalytic activity versus reaction time and the temporal UVevis spectral changes of RhB aqueous solution during the catalytic degradation reaction (Fig 7b and c) It could be noted that no

sig-nificant difference in catalytic activity tested in dark or in visible lighteexposure for 120 min-reaction-course, and all experiments show a high decoloration percent of RhB after 210 min Further-more, the UVevis spectra of these RhB solutions are almost similar (Fig 7b and c), referring that catalytic reaction was insignificantly affected by thefluorescent light source in the present work[2] Moreover, the temporal UVevis spectra of RhB aqueous solution (Fig 7) shows a gradual reduction on the absorbance at 553 nm only, confirming that the degradation of RhB proceeds via chro-mophore cleave, followed by some other reactions including hy-droxylation, aromatic ring opening and mineralization [39e42] Therefore, it is suggested that the organic dye was eliminated by the advanced chemical oxidation process in the presence of CreZn LDH catalysts[30,40]

Fig 6 Degradation efﬁciency of RhB (a) during visible light irradiation (20 mg/L of rhodamine B, 0.30 grams of catalyst, room temperature, pH ¼ 6.5, H 2 O 2 solution, laboratory light) after 210 min and (b) UVevis absorption spectra of rhodamine B aqueous solution over three samples.

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3.2.2 Effect of the oxidant nature

Since the CreZn LDHs exhibit a high activity in the oxidation of

rhodamine B with H2O2aqueous solution, the catalysts have been

performed in the presence of oxygen/air under similar reaction

conditions for comparison Fig 8 displays the variation of the

decoloration percent versus reaction times under two different

oxidant agents

Under similar reaction conditions, H2O2can decolorize about

99% of rhodamine B after 1.5 h while air can only give 10% of

rhodamine B after 14 h-on-time The experimental results also

indicate that H2O2is much reactive oxidant for the oxidation of RhB

over ZneCr LDH catalysts, which may be related to the great ability

of H2O2 to form HOradicals in the Cr-doped LDH catalyst bulk

[8,20,31] Meanwhile, oxygen molecule has likely no impact on the

oxidative removal of rhodamine B[7,31,41,42] The RhB

decolor-ation level in the latter case is close to that of the blank testing

reaction (Figs 6a and 7a), reﬂecting the no reactivity of oxygen

oxidant in the degradation process of RhB on the catalysts

3.2.3 Effect of oxidant amounts Because H2O2oxidant exhibits an excellent ability to decolorate rhodamine B aqueous solution over CreZn LDH catalysts, different amounts of H2O2are added into the reaction mixture in order to optimize the reaction conditions.Fig 9illustrates the degradation level of rhodamine B varying with the concentration of H2O2in the range of 3e12 g/L

The degradation level of rhodamine B changes from 10.2 to 99.1% after 120 min, depending on the amount of H2O2present in the reaction mixture (Fig 9) At a low initial concentration of H2O2, the catalytic activity trend likely approaches a plateau at the decoloration of RhB around 18e22% because of limiting reagent of

H2O2in these experiments No doubt, the decoloration of RhB is proportional to the contribution of additional HOradicals pro-duced from H2O2dissociation as the H2O2concentration increases

[37,42] An increased H2O2 amount in the reaction solution, the decoloration sharply goes up and reaches approximately to 100% after 90 min However, no observable differentiation of removal

efﬁciency of RhB in the H2O2initial concentration between 9 and

12 g/L This phenomenon is explained by the fact possibility that the formation of HO2radicals is resulted from an interaction be-tween H2O2and HOat high initial amount of hydrogen peroxide solution[12,37] Another possibility is that the reaction between

HO2and HOradicals to form H2O and O2also leads to a major decreased concentration of HOin the reaction mixture on the

CreZn based catalysts[10,12,31] As a consequence, a higher initial concentration of H2O2would not facilitate the removal oxidative reactions of RhB on ZneCr LDH catalysts

3.2.4 Effect of the catalyst dosages Since the amount of H2O2 in the reaction solution has some remarkable effects on the removal efficiency of rhodamine B, the catalyst dose may be an important factor influencing on the degradation of organic dyes [31,37] In the present study, the quantity of catalyst in the range of 0e3 g/L is examined at reaction conditions reported inFig 10 Obviously, the decoloration trend of RhB varies proportionally with the catalyst doses Indeed, the cat-alytic activity is related to the availability of more active sites on catalyst surface and the amount of Cr-doped layered zinc hydroxide phase present in the catalyst sample As other transition metal ions located in the material framework[26,31,35,43], these results mean that Cr3þions adopted in the layered hydroxide sheets could be stabilized and efficiently proceed the decoloration of rhodamine B The highest decolorization was observed with 3.0 g/L and there-after increase in material dosage had significant effect on degra-dation of RhB in the examined catalyst dosages[10,12,30,37]

4 Conclusion The structure and morphology of the synthesized ZneCr layered hydroxides depend on the preparation conditions and Cr/Zn molar ratios At a Cr/Zn molar ratio of½, the CreZn LHD gave layered structure with carbonate anions in the interlayer regions and uni-form particle sizes and high external surface areas while other Cr/

Zn ratios result in a mixture of oxides and layered hydroxides The

CreZn layered hydroxide catalyst showed a good activity in the advanced oxidation of rhodamine B with H2O2at a near neutral pH The intra-layer Cr3þ ions are active sites for the complete destruction of rhodamine B while the extra-hydroxide framework chromium oxide exhibited a negligible decoloration level of RhB The degradation efﬁciency depends on the intra-layered hydroxide

Cr3þcontents and reaction variables The highest degradation ef-ﬁciency of rhodamine of 99.8% was observed on sample ZnCr2after

90 min Our ﬁndings indicate that CreZn layered hydroxide

Fig 7 Degradation efﬁciency (a) of RhB over ZneCr layered hydroxide catalysts

([RhB] ¼ 20 mg/L, 0.3 g of catalyst, pH ¼ 6.5, room temperature) and UVevis spectra

(b) of the reaction solution in dark and (c) in visible light irradiation.

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Fig 8 Effect of oxidants on the decoloration of rhodamine B, [RhB] ¼ 20 mg/L, 0.30 g of CrZn2 layered hydroxide catalyst, room temperature, air ﬂowrate of 5 mL/min for 14 h.

Fig 9 Effect of H 2 O 2 amounts on the decoloration of rhodamine B, [RhB] ¼ 20 mg/L, 0.30 g of CrZn2 layered hydroxide catalyst, pH ¼ 6.5, room temperature.

Fig 10 Effect of catalyst dosage on the decoloration of rhodamine B, (sample CrZn3, 20 mg/L of rhodamine B, 0.30 g of catalyst, room temperature, pH of 6.5, room temperature).

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catalysts are promising catalysts for degradation of organic dyes in

water under an ambient condition

Acknowledgment

This research is funded by Vietnam National Foundation for

Science and Technology Development (NAFOSTED) under grant

number 104.05e2017.04

Appendix A Supplementary data

Supplementary data related to this article can be found athttp://

dx.doi.org/10.1016/j.jsamd.2017.07.005

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