ZnCr-LDHs with dual adsorption and photocatalysis capability for the removal of acid orange 7 dye in aqueous solution

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This study aims to explore the potential of layered double hydroxides (LDHs) materials for the treatment of contaminated waters by organic pollutants, using a combined dual process, namely adsorptionphotocatalysis. A series of LDHs of ZnCr-X where (X ¼ Cl, SO4 and CO3) were synthesized and characterized by several techniques such as DRX, FTIR, BET, TEM, and UV-Visible spectrophotometry.

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Nội dung Text: ZnCr-LDHs with dual adsorption and photocatalysis capability for the removal of acid orange 7 dye in aqueous solution

Journal of Science: Advanced Materials and Devices 6 (2021) 118e126 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Original Article ZnCr-LDHs with dual adsorption and photocatalysis capability for the removal of acid orange 7 dye in aqueous solution Lekbira EL Mersly a, El Mountassir El Mouchtari a, El Mostafa Moujahid b, Claude Forano c, Mohammadine El Haddad a, Samir Briche d, Abdelaaziz Alaoui Tahiri a, Salah Rafqah a, * a Laboratoire de Chimie Analytique et Moleculaire, Facult e Polydisciplinaire de Saﬁ, Universit e Cadi Ayyad, Morocco b Laboratoire Physico-Chimie des Materiaux, Faculte des Sciences, Universit e Chouaib Doukkali, EL Jadida, Morocco c Universit e Clermont Auvergne, CNRS, SIGMA Clermont, ICCF, F-63000, Clermont-Ferrand, France d Departement Stockage de l'Energie et Rev^etements Multifonctionnels (SERM), MAScIR Foundation, Rabat, Morocco a r t i c l e i n f o a b s t r a c t Article history: This study aims to explore the potential of layered double hydroxides (LDHs) materials for the treatment Received 27 April 2020 of contaminated waters by organic pollutants, using a combined dual process, namely adsorption- Received in revised form photocatalysis. A series of LDHs of ZnCr-X where (X ¼ Cl, SO4 and CO3) were synthesized and charac- 4 August 2020 terized by several techniques such as DRX, FTIR, BET, TEM, and UV-Visible spectrophotometry. The Accepted 10 August 2020 Available online 25 August 2020 pollutant acid orange 7 (AO7) is efﬁciently adsorbed on the surface of the three LDHs. The adsorption depends on the nature of the intercalated anion. The adsorption percentage of AO7 achieved for the intercalated anions SO4 2 , CO3 2 and Cl, was 25, 32 and 49.5% respectively. We ﬁnd that ZnCreSO4 LHD Keywords: Layered double hydroxides is the most effective photocatalyst for AO7 dye decomposition in the UV and visible range and its Anions exchange photocatalytic activity is preserved during three cycles of photocatalytic tests. Such remarkable prop- Adsorption erties allow considering ZnCrSO4-LDH as a promising photocatalyst with high activity, long-term dura- Photocatalysis bility and excellent applicability. Acid orange 7 dye © 2020 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 R represents the MII/MIII molar ratio). Their 2D-structures based on the stacking of positive hydroxylated layers ([MII1-xMIIIx(OH)2]xþ) In recent years, several new processes have been developed for separated by interlayer domains that contain intercalated anions the treatment of water contaminated by persistent products and water molecules (Xm-x/m.nH2O) provide unique surface (pharmaceuticals, pesticides, dyes …). These processes have adsorption and/or intercalation properties into the gallery space of focused on developing new advanced oxidation processes (AOPs) anionic dye molecules [6,7]. [1] based on multifunctional materials with both, efﬁcient selective In view of such relevant properties, LDHs have increasingly been adsorption and photodegradation properties of the targeted pol- applied in environmental concerns that target the removal of toxic lutants [2,3]. products from wastewaters that could endanger natural ecosys- Among the various 2D materials investigated in environmental tems and human health. Combining the adsorption with photo- remediation, the layered double hydroxides (LDHs) have attracted degradation of organic pollutants using these LDHs materials considerable attention as adsorbent, and catalysts in recent de- seems to be an interesting way that deserves to be explored and cades [4,5]. Their general formula can be represented as [MII1- optimized. Because LDHs can be prepared using low cost and efﬁ- III xþ m- II III xM x(OH)2] [X x/m.nH2O] hereafter noted M RM -X (where cient elaboration processes such as coprecipitation, they present a II III m M : divalent cation, M : trivalent cation, X : interlayer anion and good alternative for the treatment process in wastewater treatment plants (WWTP). Their use in WWTP could then be a very promising solution to completely remove many bio-recalcitrant organic * Corresponding author. Laboratoire de Chimie Analytique et Mole culaire (LCAM), compounds. Many researchers have extensively investigated these Departement de Chimie, Faculte Polydisciplinaire de Saﬁ, Universite Cadi Ayyad, LDH materials to assess their potential as adsorbents for various Sidi Bouzid, B.P. 4162, 46000, Saﬁ, Morocco. Fax: þ00 212 524669516. contaminants from domestic wastewaters and industrial dis- E-mail address: rafqah@gmail.com (S. Rafqah). charges. The large surface area and the exchange capacity of LDHs Peer review under responsibility of Vietnam National University, Hanoi. https://doi.org/10.1016/j.jsamd.2020.08.002 2468-2179/© 2020 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/).
L. EL Mersly, E.M. El Mouchtari, E.M. Moujahid et al. Journal of Science: Advanced Materials and Devices 6 (2021) 118e126 have offered multiple opportunities to remove several kinds of Typically, 2 g of ZnCreCl LDH precursor was dispersed in 500 mL organic pollutants in aqueous medium [4,8]. distilled water (under nitrogen atmosphere in the case of Several studies have also been devoted to the photocatalytic ZnCreSO4. A solution of sodium bicarbonate (or sodium sulphate) efﬁciency of LDHs in order to remove organic pollutants from is then introduced into the medium with a CO3 2 =Cr3þ and SO4 2 = aqueous solution. Baliarsingh et al. [9], studied the effects of diva- Cr3þ molar ratio equal respectively to 5 and 10). The exchanges lent metal ions on the photocatalytic properties of layered double were conducted under magnetic stirring for 24 h. The resulting hydroxides MIICrIII-CO3, with a MII/CrIII ratio equal to 2, by varying products were then washed and dried under the same conditions the MII metal cation (Co, Ni, Cu and Zn). employed for the ZnCreCl precursor. The photocatalytic performance of these catalysts was exam- Elemental analysis of the obtained LDHs samples was per- ined for the degradation of methyl orange dye (MO) under sunlight formed using inductive conduction plasma coupled to atomic irradiation. The results show that the cobalt-based LDH presents emission spectroscopy (ICP/AES). the highest photoactivity with the elimination of 90% of MO after The chemical compositions of the as-prepared LHDs are 3 h irradiation. Seftel et al., studied the photocatalytic decomposi- [Zn0.67Cr0.33(OH)2]Cl0.32,0.86H2O, [Zn0.67Cr0.33(OH)2](CO3)0.17,0.75H2O tion of the same dye molecule by ZnRAl-CO3 with different Zn/Al and [Zn0.67Cr0.33(OH)2](SO4)0.16, 0.98 H2O. molar ratios (1 ¼ R 4) calcined at different temperatures. Ac- cording to this study, the photocatalytic activity increases by 2.2.2. Adsorption and photocatalytic studies increasing both the cationic ratio R and the calcination tempera- The photocatalytic and adsorption experiments were conducted ture. The dye could be almost completely (93%) photodegraded by simultaneously in a jacketed reactor maintained at a temperature of Zn4AleCO3 calcined at 500 C [10]. In the present study, we are 20 C. In parallel with this experiment, the same solution is kept combining the dual adsorption/photocatalysis process for the stirring in the dark of light (Subtraction operation is used to mea- treatment of organic pollutants using ZnCr-X LDH synthesized by sure the photocatalytic degradation percentage of AO7). The soft chemistry routes. The adsorption and photocatalysis tests were adsorption studies evaluate the ability of LDHs to adsorb the AO7 carried out using the acid orange 7 (AO7) dye as a model waste- dye. Different doses of LDHs (5e80 mg) were contacted with 50 mL water organic pollutant. Effects of parameters (nature of interca- of AO7 dye solution (50 ppm) and then shaken by a “2 Mag Mag- lated anions, pH, reactive species, irradiation zone …) on these two netic Motion” magnetic shaker at 400 rpm between (0e3 h) in the processes were adopted to optimize degradation efﬁciency. dark. The adsorption capacity (q in mg g1) was calculated using 2. Experimental equation (1): 2.1. Materials ðC0 Ct ÞV q¼ (1) m All chemicals were of analytical grade and were used without puriﬁcation. Chromium (III) chloride hexahydrate (CrCl3), where C0 and Ct are the initial and ﬁnal concentration (mg/L) of the perchloric acid (HClO4) and sodium hydroxide were obtained from dye in solution, respectively, V is the solution volume (L), and m is Loba Chimie. Sodium carbonate (Na2CO3) and orange acid 7 were the mass (g) of LDH. obtained from Acros Organics. Zinc (II) hexahydrate chloride We use the irradiation system equipped with an Ultra-Vitalux (ZnCl2) is from Sigma Aldrich. Sodium sulﬁde (Na2SO4) from VWR lamp at 300 W and high-pressure tungsten ﬁlament. A double- chemicals, hydrochloric acid (HCl) is the VWR prolabo, isopropanol jacketed glass reactor allows maintaining the temperature of so- was obtained from Prolabo chromanorm, and triethanolamine was lution at 25 C during the irradiation time. The visible light tests purchased from Carlo Erba. were performed with the same lamp using an Edmund Optics high- pass interference ﬁlter, completely absorbing radiation below 2.2. Materials elaboration and methods 400 nm. Small solution aliquots (0.2 mm) were periodically with- drawn using a syringe and ﬁltered through 0.2 mm Millipore syringe 2.2.1. Synthesis ﬁlter to measure the concentration variations of AO7 as a function The phase ZnCreCl was synthesized via a coprecipitation of irradiation or contact time. The photocatalytic degradation of method at a constant pH of 5.5. Under inert atmosphere (N2), an AO7 dye was measured after subtracting the amount adsorbed for aqueous solution (500 mL) containing 27.26 g (0.4M) of ZnCl2 and each time. It was determined by plotting (Ct/C0) as a function of the 15.83 g (0.2 M) of CrCl3 (with a Zn2þ/Cr3þ molar ratio of 2) was irradiation time, where C0 and Ct are the concentrations of AO7 added dropwise to a solution of NaOH (1 M). After 24 h of aging before and after exposure, respectively. time, the anionic clay was washed several times with the decar- bonated water and dried in an oven at 50 C for 72 h. 2.3. Materials characterization The equation for coprecipitation reaction is as follow: X-ray diffraction (XRD) patterns were recorded on a PANalytical 2 ZnCl2(aq) þ CrCl3(aq) þ 6 NaOH(aq) þ n H2O / [Zn2Cr (OH)6Cl, n X0 Pert Pro Powder Diffractometer equipped with a Cu anticathode H2O] þ 6 NaCl (lKa1 ¼ 1.540598 Å, lKa2 ¼ 1.544426) Å and an X'celerator detector under an acceleration voltage of 40 kV and a ﬁlament current of The formation of ZnCreCl LDH is governed by the direct 30 mA. The diffractograms were recorded in the range of 5 e70 in condensation between hexaeaquazinc complexes [Zn(OH2)6]2þ 2 theta with a step size of 0.038 at a scanning rate of 2 min1. and Cr(III) aqua oligomers [23]. The infrared FTIR spectra of the different samples were taken The ZnCreCO3 and ZnCreSO4 LDH phases are obtained using a Thermo Scientiﬁc IR spectrophotometer over a the wave- following a topotactic anionic exchange reaction from the ZnCreCl number range of 400e4000 cm1. The various samples of acid or- precursor having easily exchangeable Cl anions [11]. The exchange ange 7 (AO7) taken during adsorption or photocatalysis have been reactions can be described by the following equilibrium: analysed using a UV-6300PC spectrophotometer equipped with UV-Vis Analyst software for storing and processing spectra. Quartz [ZnCreCl] þ (1/2) X2 / [ZnCr-X0.5] þ Cl (X ¼ CO3 or SO4) cells with a 1 cm optical path were used. 119
L. EL Mersly, E.M. El Mouchtari, E.M. Moujahid et al. Journal of Science: Advanced Materials and Devices 6 (2021) 118e126 Transmission Electron Microscopy (TEM) observations were the intercalated anion. The particle size was evaluated at 13, 62 and performed using a BRUKER-QUANTAX FEI instrument operated at 70 nm for ZnCr LDH intercalated with Cl, SO4 2 and CO3 2 , 200 keV. The speciﬁc surface area of samples was measured using a respectively. The particle size increases for the exchanged Micromeritics 3-FLEX and the BrunauereEmmetteTeller (BET) ZnCreCO3 and ZnCreSO4 LDH probably due to the platelet aggre- method at 77K with nitrogen adsorption and desorption. gation favored by the highly charged CO3 2 and SO4 2 surface anions. 3. Results and discussion 3.1. Characterization of the LDH phases 3.2. Study of AO7 adsorption by ZnCr LDH phases Figure 1(A) shows the XRD patterns of the three prepared The evolution of the adsorbed amounts of AO7 dye as a function phases ZnCreCl, ZnCreCO3 and ZnCreSO4. All X-ray diffractograms of contact time (0e3 h) for the three ZnCr LDH phases are repre- display a series of (hkl) diffraction lines characteristic of a LDH sented in Fig. 1S. Experimental data for all LDH phases shows that single-phase with hexagonal lattice and R-3m symmetry. Typically, the adsorption equilibrium was reached after 2 h. The maximum the sharp and symmetric (003) and (006) reﬂections of the basal adsorption (%) are in the order SO4 2 (25.0%) < CO3 2 (32.0%) < Cl planes appear at low angle beside the broad and asymmetric re- (49.5%) anions. As expected, the amount of AO7 adsorbed is ﬂections of the non-basal (012), (015), and (018) planes. The unit inversely proportional to the anion selectivity of the corresponding cell parameters a and c were estimated from the positions of the ZnCr LDH phases. (110) (a ¼ 2d110) and (003) (c ¼ 3d003) reﬂections. From these results and for the following study, a contact time of Calculated a and c values were consistent with those reported in 2 h was selected to ensure the sorption equilibrium of AO7 dye by the literature [12]. Since the Zn/Cr layer composition is similar, the the LDH adsorbents. three LDH phases have almost the same parameter a (3.10(9) Å). The BET surface areas of ZnCr LDH materials intercalated by Cl, The calculated lattice parameters c were found to be 23.124, 22.821 CO3 2 and SO4 2 were found to be 71, 22, and 22 m2/g, respectively. and 33.06 Å for ZnCreCl, ZnCreCO3 and ZnCreSO4, respectively. With very close pore sizes (5.4e5.6 nm), these materials can be This is in good agreement with the expected increase of the basal considered as mesoporous [14]. In addition, the calculated pore spacing ZnCreSO4 > ZnCreCl > ZnCreCO3 as already explained by volume (0.03 cm3/g) of the ZnCreCO3 and ZnCreSO4 LDH phases both size and charge of the corresponding anions. are lower than that of the ZnCreCl LDH (0.097 cm3/g). Conse- FT-IR spectra of the three obtained compounds shown in quently, a good correlation between the adsorption capacity and Fig. 1(B) exhibit characteristic bands of ZnCr LDH phases with Cl, BET surface area of the studied LDH materials was established. CO3 2 and SO4 2 anions. The broad absorption band around These results and those of TEM are in perfect agreement with the 3440 cm1 is assigned to the stretching vibration of hydroxyl absorption capacity for the three ZnCr LDH. groups. The absorption at 1620 cm1 is representative of the The kinetic data of the adsorption of acid orange 7 on the three bending vibration of water. In the region of low energy LDHs was analysed using the pseudo-ﬁrst-order model [15] and the (
L. EL Mersly, E.M. El Mouchtari, E.M. Moujahid et al. Journal of Science: Advanced Materials and Devices 6 (2021) 118e126 Fig. 2. TEM pictures of synthesized ZnCr- X LDHs (X: Cl, CO3 2 , SO4 2 ). Table 1 % is 62.2%, 42.4% and 43.4% for ZnCreCl, ZnCreCO3 and ZnCreSO4, Kinetic parameters according to the pseudo second order and pseudo-ﬁrst order respectively. On the other hand, the obtained results (Table 2) show models for the AO7 adsorption. a signiﬁcant difference in the sorption ability of these materials, Pseudo-ﬁrst-order Pseudo-second-order which was found to decrease in the following order: Qe K1,ads R2 Qe K2,ads R2 ZnCreCl > ZnCreCO3> ZnCreSO4. This could be explained by the (mg.g1) (min1) (mg.g1) (g.mg1.mn1) high afﬁnity of the carbonate and sulphate ions for LDHs materials ZnCreCl 36.63 0.03 0.898 163.9 0,0008 0.969 vs. the chloride anions [17]. ZnCreCO3 22.17 0.028 0.924 33.31 0.0056 0.993 ZnCreSO4 18.05 0.026 0.875 32.13 0.0057 0.991 3.3. Photocatalytic study 3.3.1. Effect of the interlayer anion Ce Ce 1 Since some organic pollutants may undergo direct photolysis ¼ þ ðLangmuirÞ (2) Qe Qmax Qmax KL when irradiated with solar light [18], it is necessary to conﬁrm this hypothesis for AO7 dye. In fact, an aqueous solution of AO7 was 1 irradiated using the same condition in absence of the LDHs. From LnQe ¼ LnKF þ LnCe ðFreundlichÞ (3) n the curve shown in Fig. 4(a) no photodegradation of AO7 is observed during 120 min of irradiation time. This indicates that where Qmax and KL represent respectively the Langmuir maximum under our experimental conditions, AO7 dye is photochemically capacity, and the adsorption constant and KF and n1 are the pa- stable. rameters of the Freundlich isotherm. The experimental data of AO7 We also carried out the irradiation of the pollutant AO7 in the adsorption by ZnCr LDHs and the Langmuir and Freundlich ﬁtted presence of the each of the prepared LDHs working under the same values are shown in Fig. 3. All parameter values and correlation conditions (CAO7 ¼ 50ppm; [LHD] ¼ 0.4 mg/L; pH ¼ 8.1). It turned coefﬁcients for both isotherms are presented in Table 2. out, from the results obtained (Fig. 4-b-c-d) that the three materials According to comparison of the correlation coefﬁcients of both behave as photocatalysts. Indeed, they have a remarkable capability models (between 0.999 and 0.987), the Langmuir model is best to induce photochemical degradation of the AO7 in aqueous solu- suited to describe the AO7/ZnCr adsorption process indicating the tion. However, the degradation of AO7 is very important in the case homogeneous nature of adsorption sites of the ZnCr LDHs and the of ZnCreSO4 as it reaches a reduction rate of 66% during 2 h. formation of AO7 molecule monolayer on the surface of these ZnCreCl LDH displays also a high photocatalytic efﬁciency with a solids. This is in agreement with an adsorption mechanism via percentage of decomposition evaluated at 54% in the meantime, surface X⟶AO7 anion exchange reactions. It can be noted that while ZnCreCO3 appears much less reactive, with only 21% of AO7 under these experimental conditions, total exchange was not ob- degradation. Not only adsorption properties of AO7 by the various tained since maximum adsorption capacities calculated from the LDHs seem to facilitate photodegradation but also the ZnCr layers a.e.c. (Table 2). and measured in AO7/ZnCr LDHs weight per weight play a key role in adsorption and transfer of light to the organic Fig. 3. Non-linear Langmuir and Freundlich isotherms obtained for AO7 adsorption on ZnCr LDH phases. [AO7] ¼ 20e100 mg.L1 [LDHs] ¼ 0.4g.L1. 121
L. EL Mersly, E.M. El Mouchtari, E.M. Moujahid et al. Journal of Science: Advanced Materials and Devices 6 (2021) 118e126 Table 2 Langmuir and Freundlich isotherm constants for AO7 dye adsorption on ZnCr-LDH. LDH sorbents a.e.c (mg/g) Langmuir model Freundlich model Qm K R2 KF 1/n R2 (mg/g) (L/mg) mg/g (L/mg)1/n ZnCreCl 327.8 204.08 0.24 0.999 48.91 0,6 0.950 ZnCreCO3 331.9 140.84 0.28 0.999 43.81 0.7 0.940 ZnCreSO4 315.7 136.98 0.34 0.987 10.23 0.55 0.970 and found to be 2.13 eV, 1.97 eV and 1.90 eV for ZnCreCl, ZnCreCO3 and ZnCreSO4, respectively. These differences in ZnCr-X band-gap inﬂuence their photocatalytic efﬁciency as reported earlier by Pausov a et al. [21]. Our results conﬁrm that the ZnCr-X LDH can be photoactive the UV and visible range, especially in the case of ZnCreSO4 that absorbs more light in this range than the other two LDHs. This absorption zone is largely untapped in the case of several conventional photocatalysts (TiO2, ZnO …) well known for their wide application in the ﬁeld of photocatalysis. Interestingly, the photocatalytic activity can be tuned by the nature of the interlayer anion. Generally, the photodegradation of single substrate follows a ﬁrst order kinetic law [22]. Experimental data were ﬁtted using the ln (C0/ C) ¼ f (t) equation. According to the graphical representation of this equation, it is clear that the photocatalytic degradation of AO7 fol- lows a linear behaviour during the ﬁrst minutes of irradiation time. From the evolution of the concentration, the half-life times of the dye were determined for each reaction. The values of the apparent rate constants of and half-life times are given in Table 3. Fig. 4. Kinetics of photocatalytic degradation of AO7 (50 ppm), pH ¼ 8.1. From the above results, the apparent rate constant of the pho- [LDH] ¼ 0.40 g/L, Irradiation: Osram Ultra Vitalux lamp (300 W). tocatalytic degradation of the dye in presence of the ZnCreSO4 is highest. This shows that the AO7 degrades faster in the presence of substrate. This can be explained by the absorption properties of LDH intercalated by sulphate anions. The photocatalytic efﬁciency ZnCr LDH materials that display two absorption bands in the visible of ZnCreSO4 was evaluated by comparing the degradation per- range (at 405 and 560 nm) (Fig. 5) due to the t2g-eg electronic centage with some previously reported materials. The results pre- transitions of the d3 Cr3þ. As previously described by Bauclair et al. sented in Table 4 indicates that ZnCreSO4 has a signiﬁcant activity [19], these bands correspond to n1(4A2g/4T2g) and n2(4A2g/4T1g) by comparison with other well-known photocatalysts. transitions for ZnCr LDH. This explains the large photocatalytic ef- This photocatalytic activity can only be conﬁrmed by the ﬁciency of LHD phases after light excitation within this wavelength responsiveness of the AO7 with HO and/or hþ species formed range. under light excitation of LDH (vide infra). The band gap of the LDH samples were calculated from the UV diffuse-reﬂectance spectra using the KubelkaeMunk function [20] 3.3.2. Effect of pH on photocatalytic activity of ZnCreSO4 The pH of the solution is one of the parameters, which affects the photocatalytic efﬁciency of many materials, as the reactive species responsible for the degradation of organic pollutants are formed in most cases from the hydroxide ions [25]. Moreover, it can affect the surface charge of the adsorbent and the active sites, and the adsorption capacity that can enhanced the photocatalytic pro- cess [26]. To highlight the inﬂuence of pH on the kinetic adsorption and degradation of AO7 in the presence of the most photoactive LDH, ZnCreSO4, irradiation experiments were conducted at three different pH (4.0, 6.7, 8.1). First, adsorption capacity of AO7 (50 mg/L) by ZnCreSO4 LDH (0.4 g/L) was determined at the three selected pH values and at Table 3 Constants of apparent rate and half-life times of photocatalytic degradation of Acid Orang 7. ZnCreSO4 ZnCreCl ZnCreCO3 2 1 Constant rate x 10 , mn 1.65 1.14 0.27 half-life (t1/2), min 42 61 257 Fig. 5. UV-Vis spectra in mode reﬂectance of the three ZnCr-X LDHs (X ¼ Cl, CO3 2 , R2 0.99 0.99 1.00 SO4 2 ). 122
L. EL Mersly, E.M. El Mouchtari, E.M. Moujahid et al. Journal of Science: Advanced Materials and Devices 6 (2021) 118e126 Table 4 Comparison of the photocatalytic degradation efﬁciency for AO7 using different catalysts. Catalyst Adsorption % Light source Degradation (%) Time Ref (hours) Calcinated LDH 6 365 nm 65 5 [21] NT-TiO2 2 UVA 88 40 [23] TiO2 15 UV 67 4 [24] ZnCrSeO4 25 UVA-Vis 66 2 This study adsorption equilibrium which was reached after 2h. As shown on forming a radical cation (pollutantþ) [30]. To elucidate the nature Fig. 6(a), pH conditions have a signiﬁcant inﬂuence on the per- of the photogenerated reactive species responsible for the degra- centage retention of the pollutant. Indeed, the maximum of dye dation AO7, we made use of traps to hydroxyl radicals and hþ holes. retention was obtained at pH 4.0 whatever the contact time, and it Indeed, several studies indicate that alcohols such as methanol, decreases according to this order pH 4.0 > pH 6.7 > pH 8.1. This is ethanol, isopropanol can be used as inhibitors of HO because of probably due to a strong interaction between the dye and sheets their high reactivity and very high rate constants. The effectiveness ZnCreSO4, which become cationic in acid medium since most of of these alcohols to capture HO radicals increases in the order of LDH have a higher point of zero charge (pzc) [27]. This phenome- Methanol < Ethanol < Isopropanol [31]. Isopropanol is described as non can also be explained by the decrease in solubility in an acid the best inhibitor of these radicals [32] with a rate constants medium AO7 (pKa ¼ 6.8). In an acid medium, there is the pre- k ¼ 1.9 109 M1 s1. dominant molecular form that can be deposited on the surface of In order to study the participation of HO radicals in the pho- LDH form of very ﬁne particles. tocatalytic process, a solution of AO7 (50 ppm) in the presence of As previously pointed out, the photocatalytic degradation per- ZnCreSO4 (0.4 g L1) was irradiated, under the same conditions centage was calculated without taking into account the adsorption described previously, in the presence of an amount of isopropanol rate. Indeed, the curves illustrated in Fig. 6(b) present just the (2%). After 2 h of irradiation, the AO7 disappears with a percentage photodegradation kinetics. These results also support the conclu- equal to 7.6%, while in the absence of alcohol, the disappearance of sion that the pH effect on the ZnCreSO4 photocatalytic efﬁciency is the AO7 is of 66% for the same irradiation time. far from negligible. Clearly, at pH ¼ 4, no photodegradation occurs This expected result conﬁrms the participation of the HO rad- and C/C0 variation arises only from adsorption/desorption phe- icals to the photocatalytic degradation of the dye in the medium. nomena. The best activity was obtained for the highest pH value, i.e. Since hydroxyl radicals are not the only species responsible for 8.1. These results can be explained by: (i) an increase in the number the elimination of AO7. We investigate the effect of triethanolamine of HO hydroxyl radicals generated favourably at basic pH, and on the photocatalytic degradation AO7. This product is well known which are capable of oxidizing organic compounds causing subse- for its selective capacity to trap holes hþ, which leads by electron quent disposal into the medium [28], (ii) a screening effect of the transfer to the formation of the triethanolamine cation radical photocatalytic reduction in acidic medium. This last effect is due to TEOAþ [33]. Indeed, inhibition of the activity of the positive hole the high mobility of AO7 molecules into the ZnCreSO4 active sites hþ was carried out by introducing 1 mmol L1 triethanolamine in characterized by their high retention rate at pH ¼ 4, which leads to the reaction medium containing AO7 and ZnCreSO4. It is noted that the formation of a dye layer on LDH particles in suspension, triethanolamine has also a remarkable inﬂuence on the photo- inhibiting their light excitation. catalytic degradation of Acid Orange 7 in the medium as conﬁrmed by the decrease in the degradation rate. The percentage values for 3.3.3. Highlight of reactive species the degradation of AO7 after 2 h of irradiation decreases from 66% In photocatalysis, several studies show that hydroxyl radicals to 10.8% in the absence and in the presence of triethanolamine, are the main oxidizing species leading to the degradation of organic respectively. pollutants in aqueous solution [29]. Other studies reveal the All these results suggest that the hydroxyl radicals and holes hþ intervention of the holes (hþ) in this process via oxidation reactions play a major role in the degradation of the AO7 dye. The Fig. 6. Percentage of AO7 adsorbed (a) and Kinetics of Photocatalytic degradation of AO7 (b) as a function of pH (C0 ¼ 50 ppm, [ZnCreSO4] ¼ 0.4 g/L). 123
L. EL Mersly, E.M. El Mouchtari, E.M. Moujahid et al. Journal of Science: Advanced Materials and Devices 6 (2021) 118e126 photocatalytic degradation of AO7 in presence of ZnCr LDH inter- 3.3.5. Decolorization of AO7 solutions calated by chloride and carbonate anions do not behave in the same From the point view of mechanistic study, the adsorption pro- way as for LDH intercalated with sulphates. The Cl and CO3 2 cess initially involves transferring orange 7 acid from the liquid anions are likely slowing down the degradation process, as proved phase to a solid phase by surface reaction. After that, the irradiation in several studies [34,35], due to the inhibition of the activity of the of the reactive species are formed (HO, hþ …) on this surface, hydroxyl radicals and the holes hþ formed during the process as allowing for their reaction with the adsorbed AO7, and subse- follow [36]: quently leading to its degradation in the medium. Fig. 8 shows the disappearance of AO7 as a function of irradiation time in presence Cl þ HO #ClOH (4) of the ZnCreSO4 LDH, measured by UV-Visible absorption spec- troscopy. The characteristic band at 485 nm and a shoulder band at 425 nm, which can be attributed to the hydrazone-and azo forms of CO3 2 þ HO #CO 3 (5) AO7 decreases as a function of irradiation time until the total decolorization of solution. In the meantime, absorbance of the two CO3 2 þ hþ #CO 3 (6) bands at 255 nm and 308 nm, characterized by the benzene and - naphthalene cycles, respectively, decrease simultaneously until These two species (ClOH ; CO3 ) remain blocked inside the their disappearance. This indicates a total mineralisation of the LDH internal galleries, since the interlamellar distances of the organic dye through the photocatalytic process activate by related ZnCr LDH are small d003 x 7.6 Å, while in the case of sul- ZnCreSO4 LDH. phates, given the higher interlamellar distance d003 x 11 Å of ZnCreSO4, the radicals SO4 (Eq. (7)) formed have signiﬁcant mobility. Moreover, they are strong oxidizing agents for which a 3.4. Analysis of the LDH material after adsorption/photocatalysis reduction potential in the range of 2.5e3.1 V, which allows easy reaction oxidation of the dye in the medium [37,38]. After the adsorption process and photocatalytic treatment of SO4 2 þ HO #SO 4 $$$ (7) AO7, the ZnCreSO4 LDH material were collected by ﬁltration without washing, and then dried for 8 h under vacuum at 50 C. The collected solids were analyzed using IR spectroscopy. The FTIR 3.3.4. Effect of irradiation ﬁeld spectra of the AO7, ZnCreSO4 LDH after adsorption of AO7, and As shown above, the ZnCreSO4 LDH spectrum in the absorption ZnCreSO4 LDH after photocatalytic degradation of AO7 are showed mode extends to the visible range. It seems very promising to in Fig. 9. The spectra of ZnCreSO4 LDH after adsorption of AO7 implement photocatalysis reaction under visible light, energy presents the different characteristic peaks of AO7: the band at domain is very little exploited although it presents 46% of the 1447 cm1 corresponded to the azo bond vibration. The band spectrum of solar emission. The visible light application test was located at 1512 cm1 is assigned to NeH bond vibrations, the band performed at wavelengths over than 400 nm by using a ﬁlter that at 1625 cm1 was a combination of C¼N stretching group and the removes all the UV radiation from emission lamp. phenyl ring vibrations. The bands in the region 1450e1600 cm1 From Fig. 7, we found that AO7 degrades even under the exci- were linked to characteristic aromatic C¼C vibrations. The bands in tation of ZnCreSO4 in the visible range. It is remarkable to note that 1000e1250 cm1 region were associated to the SeO stretching and its elimination percentage in this energy range is higher and rea- aromatic ¼ CeH bending vibrations. The peaks at 1127 and ches a value of 56.8% after 2 h compared to that obtained by irra- 1036 cm1 corresponded to the coupling between the benzene diation in the UV domain, which does not exceed 44% under the mode and ns(-SO3). These AO7 bands are absent in the LDH curve same experimental conditions. This is due to the absorption ca- after photocatalytic reaction, which indicates that the AO7 dye pacity of ZnCreSO4 in the visible range, since it is a dark purple adsorbed on the LDH catalyst is photodegraded in situ and clearly powder and has the low band gab value (1.9 eV). shows that the process involved is not just a simple adsorption Fig. 7. Effect of the light excitation domain on the degradation photocatalytic of AO7 in Fig. 8. Absorption spectra of AO7 as a function of irradiation time, in the presence of the presence of ZnCreSO4. (pH ¼ 8.1, [LDH] ¼ 0.40 g/L). the ZnCrSO4. 124
L. EL Mersly, E.M. El Mouchtari, E.M. Moujahid et al. Journal of Science: Advanced Materials and Devices 6 (2021) 118e126 adsorption phenomenon is a key step for the subsequent photo- catalytic process. The photocatalysis efﬁciency is correlated with the structural properties, essentially the interlayer spacing of ZnCr LDH phases. The ZnCreSO4 LDH exhibited exceptional capacity for removal of AO7 in the solution. This photocatlytic activity is favoured in basic medium, being OH hydroxyl radicals and hþ holes the species involved in this process. The regeneration of ZnCreSO4 LDH and its activity were preserved during three cycles, which allows us to consider this new LDH material as a promising photocatalyst with high activity, long-term durability and excellent applicability. Declaration of competing interest The authors declare that they have no known competing Fig. 9. FT-IR spectra of fresh ZnCreSO4 (a), irradiated AO7/ZnCreSO4 (b), AO7 adsorbed ﬁnancial interests or personal relationships that could have on ZnCreSO4- (c), AO7 (d). appeared to inﬂuence the work reported in this paper. Acknowledgments The authors gratefully acknowledge Center for Analysis and Characterization (CAC) afﬁliated to Cadi Ayyad University- Marrakech, for providing some sample characterizations and Na- tional Center for Scientiﬁc and Technical Research (CNRST- Morocco) for his ﬁnancial support (Merit scholarship. No: 8UCA2019). The authors would also like to thank the reviewers for their fruitful comments and suggestions. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jsamd.2020.08.002. References Fig. 10. Recycling and reuse of ZnCreSO4 LDH for the photocatalytic degradation of [1] M.A. Oturan, J.-J. 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