A Sno2/Ceo2 Nano-Composite Catalyst for Alizarin Dye Removal from Aqueous Solutions

nanomaterials

Article A SnO2/CeO2 Nano-Composite Catalyst for Alizarin Dye Removal from Aqueous Solutions

Saad S. M. Hassan 1,*, Ayman H. Kamel 1,* , Amr A. Hassan 1,2 , Abd El-Galil E. Amr 3,4,* , Heba Abd El-Naby 1 and Elsayed A. Elsayed 5,6

1 Chemistry Department, Faculty of Science, Ain Shams University, Abbasia 11566, Cairo, Egypt; [email protected] (A.A.H.); [email protected] (H.A.E.-N.) 2 Department of Chemistry, Virginia Commonwealth University, Richmond, VA 23284, USA 3 Pharmaceutical Chemistry Department, Drug Exploration & Development Chair (DEDC), College of Pharmacy, King Saud University, Riyadh 11451, Saudi Arabia 4 Applied Organic Chemistry Department, National Research Center, Dokki 12622, Giza, Egypt 5 Bioproducts Research Department, Zoology Department, Faculty of Science, King Saud University, Riyadh 11451, Saudi Arabia; [email protected] 6 Chemistry of Natural and Microbial Products Department, National Research Centre, Dokki 12622, Cairo, Egypt * Correspondence: [email protected] (S.S.M.H.); [email protected] (A.H.K.); [email protected] (A.E.-G.E.A.); Tel.: +20-1222162766 (S.S.M.H.); +20-1000743328 (A.H.K.); +966-565-148-750 (A.E.-G.E.A.) Received: 10 January 2020; Accepted: 27 January 2020; Published: 1 February 2020

Abstract: A new SnO2/CeO2 nano-composite catalyst was synthesized, characterized and used for the removal of alizarin dyes from aqueous solutions. The composite material was prepared using a precipitation method. X-ray powder diffractometry (XRD), high resolution transmission electron microscopy (HR-TEM), Brunauer–Emmett–Teller methodology (BET) and Fourier Transform Infrared Spectrometry (ATR-FTIR) were utilized for the characterization of the prepared composite. The prepared nano-composite revealed high affinity for the adsorption and decomposition of alizarin dyes. The adsorption capacity under different experimental conditions (adsorbate concentration, contact time, adsorbent dose and pH) was examined. Under optimized experimental conditions, the removal of alizarin yellow, alizarin red and alizarin-3-methylimino-diacetic acid dyes from aqueous solutions was about 96.4%,87.8% and 97.3%, respectively. The adsorption isotherms agreed with the models of Langmuir, Freundlich and Temkin isotherms.

Keywords: tin oxide/cerium oxide; nano-composite; adsorption of dyes; alizarin dyes removal

1. Introduction The lack of water resources requires humanity to save each drop of it, but unfortunately there are plenty of pollutants that may affect the quality of the water resources. The contamination of water resources with dyes is an important source of pollution. Many industries use dyes during processing of their products such as textiles, dyestuff, distilleries, tanneries, paper, rubber, plastics, leather, cosmetics, food and pharmaceuticals for the coloration of their products. Hence, effluents from these industries commonly contain dye residue. Millions of tons of dye effluent are dumped into water bodies and cause environmental problems [1]. Alizarin dye (1,2-dihydroxy-9,10 anthraquinone sulfonic acid sodium salt) is a pollutant, released chiefly by textile industries. Suspected carcinogenic effects of this dye make it important to remove it from water [2–4]. Membrane separation, ion-exchange methods [5,6], photocatalysis [7–10],

Nanomaterials 2020, 10, 254; doi:10.3390/nano10020254 www.mdpi.com/journal/nanomaterials Nanomaterials 2020, 10, 254 2 of 14 the electro-Fenton process [11] and adsorption methods [12–21] are commonly used techniques for the removal of this group of dyes. Nano-technology offers an opportunity to develop adsorbents with large surface area [22], high adsorption capacity, high removal rate and large number of active surface sites [23]. In recent years, metal oxide nano-composites have attracted considerable attention for energy and environmental applications because of their ease of fabrication, low processing cost and, particularly, that the presence of nano-grains with various sizes and various kinds of nano-composite materials have been investigated for efficient removal of different organic pollutants. For example, either SnO2or CeO2nano-particles (NPs) coupled with some metal oxides such as TiO2 [24], ZnO [25], CeO2 [26], MnO2 [27] and Fe3O4 [28] are used as for the removal of dyes. In the present work, SnO2 doped with CeO2nano-particles was prepared using a simple and cost effective co-precipitation method at room temperature and utilized for the removal of some alizarin dyes. The composite was characterized by different spectrochemical techniques. Physical parameters affecting maximum dye removal were examined.

2. Materials and Methods

2.1. Materials All chemicals used were of analytical reagent grade. Alizarin red S was purchased from Riedel-Dehaën (Delhi, India), and alizarin-3-methylimino-diacetic acid was from Fluka (Ronkonoma, NY, USA). Ceric ammonium nitrate (NH4)2Ce(NO3)6 was from SDFCL (Delhi, India). Ammonium hydroxide and tin (IV) chloride pentahydrate SnCl4.5H2O were from ADWIC (Cairo, Egypt). Polyethylene glycol 6000 was from Merck (St. Lois, MO, USA). All chemicals were used as received without any further puriﬁcation.

2.2. Preparation of SnO2/CeO2 Nano-Composite Catalyst Three and one-half grams of SnCl 5H O and 0.274 g of (NH ) Ce (NO ) were dissolved in 4· 2 4 2 3 6 100 mL distilled water followed by addition of 1.0 g polyethylene glycol (PEG) as a stabilizing agent. The mixture was kept under magnetic stirring for one hour. To the above mixture, ammonia solution (35.04 v/v%) was added drop-wise with vigorous stirring until the solution reached pH 9. The mixture was kept under magnetic stirring for another two hours and then left overnight. The precipitate was ﬁltered oﬀ and washed with distilled water until the washings were free from chloride ion. The obtained yellowish precipitate was then dried at 70 ◦C for two hours and calcined at 900 ◦C in air for three hours.

2.3. Characterization of Nano-Composite Catalyst The prepared composite was characterized using high-resolution transmission electron microscopy (HR-TEM) using Jeol 2100 (Osaka, Japan), X-ray powder diﬀractometry (XRD) using X’Pert PRO, PANalytical (Amsterdam, The Netherlands) with CuKα radiation (λ = 0.154060 nm) in the angular region of 2θ = 4–80◦ operated at 40 KV and 40 mA. The spectra were recorded at a scanning 1 speed of 8◦ min− . A Nicolet™ iS50 (ATR-FTIR) spectrometer was used in a spectral range of 1 4000–500 cm− . The Brunauer–Emmett–Teller (BET) surface area measurements were carried out by nitrogen adsorption–desorption at 77 K using NOVA 3200s (Florida, CA, USA), at the relative pressure (P/Po) of 0.95104.

2.4. Dye Uptake Study

Fifteen milligrams of the adsorbent (SnO2/CeO2 nano-composite) was added to 30 mL aliquots of alizarin dye solutions (25, 30, 40, 50, 60, 70, 80 and 90 µg/mL). The solutions were stirred for 30 min at room temperature and ﬁltered. Dye concentrations, before and after the treatment, were Nanomaterials 2020, 10, 254 3 of 14

measured spectrophotometrically at λmax (422 or 515 nm) in an acidic and alkaline medium, respectively. The removal percentage of the dye was calculated using the following equation:

Removal % = [(C Ct)/C ] 100 (1) 0 − 0 × where C0 and Ct are the dye concentration in mg/L at initial and after time t, respectively.

2.5. Adsorption Studies Aliquots (30 mL) of varying concentrations (25–90 mg/L) of the dye solution were treated with different adsorbent doses (0.01–0.2 gm), under different pH values (2–9) for varying contact times (5–180 min). The dye solutions were stirred and filtered, and the dye concentration was spectrophotometrically measured at either λmax 422 or 515 nm in the pH range 2–5 and pH range 6–9, respectively.

3. Results and Discussions

3.1. Characterization of the Nano-Composite Catalyst

3.1.1. X-ray Diﬀraction Pattern of SnO2/CeO2 Nano-Composite

Tin oxide/cerium oxide (SnO2/CeO2) nano-composite materials consisting of 93–97% SnO2 and 7–3% CeO2 were prepared and characterized. The XRD pattern (Figure1) of the pure SnO 2 and CeO2 showed peaks matched with the diﬀraction data of the tetragonal structure of tin oxide (JCPDS 04-008-8131) and of the cubic structure of CeO2 (JCPDS 00-033–0334). On the other hand, the composites showed main diﬀraction patterns at 26.4◦, 34.3◦ and 52.4◦ corresponding to (110), (101) and (211) of SnO2. No characteristic peaks of the CeO2 composite appeared in the XRD spectrum due to the small amount of this oxide in the composite. The mean crystallite size (D) of the nano-particles was calculated using the Debye–Scherrer formula [29] (Equation (2)).

D = Kλ/βcosθ (2) where K and λ are the Scherrer constant and the X-ray wavelength of radiation used (Kα Cu = − 0.154060 nm), respectively. The constants β and θ are the full width at half maximum (FWHM) of diﬀraction peak and the Bragg diﬀraction angle, respectively. Since the position of the main peak is (2θ = 26.42) and the width of the peak is 0.3149 nm, the crystallite size (D) is about 27 nm SnO2 doped with 5% CeO2. The mean crystallite size (D) of SnO2 doped with 3% and 7% CeO2 nano-particles was estimated to be 18.06 nm.

3.1.2. High-Resolution Transmission Electron Microscopy (HR-TEM) of SnO2/CeO2 Nano-Composite

The morphology and average particle size of the prepared SnO2/CeO2 (95%:5%) nano-composites were examined by high-resolution transmission electron microscopy (HR-TEM). As shown in Figure2, it indicates that SnO2/CeO2 NPs have spherical morphologies with particle sizes varying from 11.5 to 30 nm, which is in agreement with the crystallite size obtained from XRD data. Nanomaterials 2020, 10, 254 3 of 14 where C0 and Ct are the dye concentration in mg/L at initial and after time t, respectively.

2.5. Adsorption Studies Aliquots (30 mL) of varying concentrations (25–90 mg/L) of the dye solution were treated with different adsorbent doses (0.01–0.2 gm), under different pH values (2–9) for varying contact times (5– 180 min). The dye solutions were stirred and filtered, and the dye concentration was spectrophotometrically measured at either λmax 422 or 515 nm in the pH range 2–5 and pH range 6–9, respectively.

3. Results and Discussions

3.1. Characterization of the Nano-Composite Catalyst

3.1.1. X-ray Diffraction Pattern of SnO2/CeO2 Nano-Composite

Tin oxide/cerium oxide (SnO2/CeO2) nano-composite materials consisting of 93%–97% SnO2 and 7–3% CeO2 were prepared and characterized. The XRD pattern (Figure 1) of the pure SnO2 and CeO2 showed peaks matched with the diffraction data of the tetragonal structure of tin oxide (JCPDS 04- 008-8131) and of the cubic structure of CeO2 (JCPDS 00-033–0334). On the other hand, the composites showed main diffraction patterns at 26.4°, 34.3° and 52.4° corresponding to (110), (101) and (211) of SnO2. No characteristic peaks of the CeO2 composite appeared in the XRD spectrum due to the small amount of this oxide in the composite. The mean crystallite size (D) of the nano-particles was calculated using the Debye–Scherrer formula [29] (Equation 2).

D = Kλ/βcosθ (2) where K and λ are the Scherrer constant and the X-ray wavelength of radiation used (Kα − Cu = 0.154060 nm), respectively. The constants β and θ are the full width at half maximum (FWHM) of diffraction peak and the Bragg diffraction angle, respectively. Since the position of the main peak is (2θ = 26.42) and the width of the peak is 0.3149 nm, the crystallite size (D) is about 27 nm SnO2 doped with 5% CeO2. The mean crystallite size (D) of SnO2 doped with 3% and 7% CeO2 nano-particles was Nanomaterials 2020, 10, 254 4 of 14 estimated to be 18.06 nm.

Figure 1. XRD pattern of SnO2/CeO2 nano-particles (NPs).

(A) (B)

Figure 2. TEM images of SnO /CeO nano-composite (A): magniﬁcation of 75,000 and 2 2 × (B): magniﬁcation of 120,000. ×

3.1.3. Fourier Transform Infrared Spectroscopy of SnO2/CeO2 Nano-Composite

The FTIR spectrum of SnO2/CeO2 nano-particles is shown in Figure3. The characteristic peak at 1 3405.29 cm− was due to the stretching vibration of the O–H bond of the physically adsorbed water 1 molecule on the SnO2/CeO2 surface [24]. The peak appearing presented at 1636.20 cm− was due to 1 the bending vibration of the adsorbed water molecule, and the displayed peak at 617.63 cm− was due to the stretching modes of the M–O bond.

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3.1.2. High-Resolution Transmission Electron Microscopy (HR-TEM) of SnO2/CeO2 Nano-Composite

The morphology and average particle size of the prepared SnO2/CeO2 (95%:5%) nano- composites were examined by high-resolution transmission electron microscopy (HR-TEM). As shown in Figure 2, it indicates that SnO2/CeO2 NPs have spherical morphologies with particle sizes varying from 11.5 to 30 nm, which is in agreement with the crystallite size obtained from XRD data.

(A) (B)

Figure 2. TEM images of SnO2/CeO2 nano-composite (A): magnification of ×75,000 and (B): magnification of ×120,000.

3.1.3. Fourier Transform Infrared Spectroscopy of SnO2/CeO2 Nano-Composite

The FTIR spectrum of SnO2/CeO2 nano-particles is shown in Figure 3. The characteristic peak at 3405.29 cm−1 was due to the stretching vibration of the O–H bond of the physically adsorbed water molecule on the SnO2/CeO2 surface [24]. The peak appearing presented at 1636.20 cm−1 was due to theNanomaterials bending2020 vibration, 10, 254 of the adsorbed water molecule, and the displayed peak at 617.63 cm−1 was5 due of 14 to the stretching modes of the M–O bond.

100

1456.36 80 1636.20

2923.38 1030.86 3405.29 60

Transmittance, % Transmittance, 40

20 617.63 4000 3000 2000 1000

-1 wavenumbers, cm Nanomaterials 2020, 10, 254 5 of 14 FigureFigure 3. 3. FTIRFTIR spectrum spectrum of of SnO SnO22/CeO/CeO22 NPs.NPs. 3.1.4. Porous Structure 3.1.4. Porous Structure The Brunauer–Emmett–Teller (BET) method was used for investigating the pore structure of the The Brunauer–Emmett–Teller (BET) method was used for investigating the pore structure of the prepared nano-composite material. The nitrogen adsorption–desorption experiment of SnO2/CeO2 prepared nano-composite material. The nitrogen adsorption–desorption experiment of SnO2/CeO2 nano-composite gives the isotherms depicted in Figure 4, which is of the type IV [30] with nano-composite gives the isotherms depicted in Figure4, which is of the type IV [ 30] with mesoporous mesoporous structure. The BET surface area and pore volume of the nano-composite are shown in structure. The BET surface area and pore volume of the nano-composite are shown in Table1. Table 1.

10 volume (cc/gm) 5

00.0 0.2 0.4 0.6 0.8 1.0 Relative pressure (p/po)

FigureFigure 4.4. NN22 adsorption–desorptionadsorption–desorption isothermsisotherms ofof coupledcoupled SnOSnO22//CeOCeO22 NPs.

Table 1. General surface characteristics of SnO2/CeO2 nano-composite obtained byN2 adsorption at Table 1. General surface characteristics of SnO2/CeO2 nano-composite obtained byN2 adsorption at 77 K. 77 K. Surface Area Average Pore Volume, Average Pore Diameter SampleSample Surface Area (m2 g 1) Average Pore Volume, (cm3 g 1) Average Pore Diameter (nm) (m2 g−−1) (cm3 g−1) − (nm) 2 SnO2/CeO2 NPs 18.970 3.399 10− 7.164 SnO2/CeO2 NPs 18.970 × 3.399 × 10−2 7.164

3.1.5.3.1.5. Electrocatalytic Behavior of SnOSnO22/CeO/CeO2 Nano-CompositeNano-Composite VoltammetricVoltammetric measurements measurements were were performed performed to toprove prove the theelectrocatalytic electrocatalytic effect e ﬀofect the of nano- the nano-compositecomposite SnO2/CeO SnO22/ CeOusing2 using a potentiostat a potentiostat (Model (Model 273A 273A Princeton Princeton Applied Applied Research Research (PAR), Princeton, Oak Ridge, TN, USA). Three electrode cell was used for voltammetric measurements containing an Ag/AgCl/KCl(s) reference electrode (BAS Model MF-2063, BASi, West Lafayette, OH, USA), and a platinum wire (BAS Model MW-1032) was used as a counter electrode. The working electrode was a Teflon rod with an end cavity (3 mm diameter and 5 mm deep) bored at one end for paste filling (BASi-MF-2010, West Lafayette, OH, USA) and connected with a copper wire through the center of the rod [31]. The carbon paste (CP) was prepared by thoroughly hand mixing 0.05 g SnO2/CeO2 nano-composite and 0.95 g of graphite powder with 360 µL of nujol oil in an agate mortar with pestle to give a homogenous 5% (w/w) SnO2/CeO2/CP. The cyclic voltammograms of SnO2/CeO2/CP electrode is shown in Figure 5 in a scan range of −0.5 to 1.5 V at a scanning rate of 300 mV/s. The peaks obtained correspond to the surface reactions, where Sn4+ and Ce4+ were reduced to Sn2+ and Ce3+, respectively. Table 2 shows the redox potentials (E) of cerium, tin and alizarin. As shown in Figure 5, the reduction peaks that appeared at Esn = −0.03 V and Ece = 1.0 V reflect the electrocatalytic effect of the catalyst and its capability to oxidize alizarin dye in aqueous solutions (EALZ = −0.59).

Nanomaterials 2020, 10, 254 6 of 14

Princeton, Oak Ridge, TN, USA). Three electrode cell was used for voltammetric measurements containing an Ag/AgCl/KCl(s) reference electrode (BAS Model MF-2063, BASi, West Lafayette, OH, USA), and a platinum wire (BAS Model MW-1032) was used as a counter electrode. The working electrode was a Teflon rod with an end cavity (3 mm diameter and 5 mm deep) bored at one end for paste filling (BASi-MF-2010, West Lafayette, OH, USA) and connected with a copper wire through the center of the rod [31]. The carbon paste (CP) was prepared by thoroughly hand mixing 0.05 g SnO2/CeO2 nano-composite and 0.95 g of graphite powder with 360 µL of nujol oil in an agate mortar with pestle to give a homogenous 5% (w/w) SnO2/CeO2/CP. The cyclic voltammograms of SnO2/CeO2/CP electrode is shown in Figure5 in a scan range of 0.5 to 1.5 V at a scanning rate of 300 mV/s. The peaks obtained − correspond to the surface reactions, where Sn4+ and Ce4+ were reduced to Sn2+ and Ce3+, respectively. Table2 shows the redox potentials (E) of cerium, tin and alizarin. As shown in Figure5, the reduction Nanomaterials 2020, 10, 254 6 of 14 peaks that appeared at Esn = 0.03 V and Ece = 1.0 V reflect the electrocatalytic effect of the catalyst and − itsNanomaterials capability 2020 to, oxidize10, 254 alizarin dye in aqueous solutions (EALZ = 0.59). 6 of 14 Table 2. Redox potentials (E) of cerium, tin and− alizarin.

System TableTable 2.2. RedoxRedox potentialspotentials ((EE)) ofofStandard cerium,cerium, tintin redox andand alizarin.alizarin. potential, E° (V) Ref. 4+ – 3+ CeSystemSystem(aq) + e Ce (aq) StandardStandard Redox redox Potential,+1.61 potential, E◦ (V) E° (V) Ref.[32]Ref. Sn4+4+4(aq)+ + 2 e– – Sn3+3+2+(aq) CeCe (aq)(aq) + ee− Ce Ce (aq)(aq) +1.61+0.15+1.61 [32] 2+4+ – 2+ [32] SnSnSn4+(aq)(aq)(aq) ++ 222 ee –− Sn Sn(s) Sn2+(aq)(aq) +0.15−0.14 2+ +0.15 Sn (aq) + 2 e− Sn(s) 0.14 Alizarin2+ – Alizarin oxidation product − −0.59 [33] SnAlizarin(aq) + 2 e Alizarin oxidation Sn(s) product 0.59−0.14 [33] − Alizarin Alizarin oxidation product −0.59 [33]

bar CPE SnO /CeO 30 2 2 bar CPE SnO /CeO 30 2 2 0

0 -30 I/µA -30 I/µA -60

-60 -90 -0.5 0.0 0.5 1.0 1.5 -90 -0.5E/V 0.0vs. (Ag/AgCl/KCl) 0.5 1.0 1.5 E/V vs. (Ag/AgCl/KCl) Figure 5. Cyclic voltammograms of carbon past electrode (CPE) and SnO2/CeO 2/CP electrode at a Figure 5. Cyclic voltammograms of carbon past electrode (CPE) and SnO2/CeO2/CP electrode at a scanning rate of 300 mV/s. scanning rate of 300 mV/s. Figure 5. Cyclic voltammograms of carbon past electrode (CPE) and SnO2/CeO2/CP electrode at a 3.2. Usescanning of SnO rate2/CeO of 3002 Nano-Composite mV/s. for Alizarin Dye Removal 3.2. Use of SnO2/CeO2 Nano-Composite for Alizarin Dye Removal The effect of CeO2 amount doped with SnO2 on the removal efficiency was tested. Three different 3.2. UseThe of effect SnO of2/CeO CeO2 Nano-Composite2 amount doped forwith Alizarin SnO2 on Dye the Removal removal efficiency was tested. Three different nano-composites with a percentage amount of CeO2of 3%, 5% and 7% (w/w) were chosen for alizarin nano-composites with a percentage amount of CeO2of 3%, 5% and 7% (w/w) were chosen for alizarin removal.The effect It was of found CeO2 amount that as thedoped amount with ofSnO CeO2 on2 thedoped removal increased efficiency in the was composite, tested. Three the removaldifferent removal. It was found that as the amount of CeO2 doped increased in the composite, the removal percentagenano-composites increased with to a percentage be 91.3%, 96.4%amount and of CeO 98.3%2of for3%, 3%, 5% 5%and and7% (w/w) 7% CeO were2, dopedchosen withfor alizarin SnO2, percentage increased to be 91.3%, 96.4% and 98.3% for 3%, 5% and 7% CeO2, doped with SnO2, respectively.removal. It was This found implies that that as the CeO amount2 enhanced of CeO the2 doped adsorption increased and electrocatalyticin the composite, powers the removal of the respectively. This implies that CeO2 enhanced the adsorption and electrocatalytic powers of the synthesizedpercentage increased nano-composite. to be 91.3%, As seen 96.4% in Figure and 698.3%, pure for SnO 3%,2 and 5% CeO and2 had7% CeO no catalytic2, doped degradation with SnO2, synthesized nano-composite. As seen in Figure 6, pure SnO2 and CeO2 had no catalytic degradation onrespectively. alizarin dyes. This This implies reflects that the CeO enhanced2 enhanced catalytic the poweradsorption of the and synthesized electrocatalytic nano-composites. powers of the on alizarin dyes. This reflects the enhanced catalytic power of the synthesized nano-composites. synthesized nano-composite. As seen in Figure 6, pure SnO2 and CeO2 had no catalytic degradation on alizarin dyes. This reflects the enhanced catalytic power of the synthesized nano-composites. 0.4 25 ppm AY 3% CeO 0.4 2 5%25 ppmCeO AY 0.3 3% CeO2 7% CeO 2 5% CeO2 100% SnO2 0.3 7% CeO 2 100% CeO2 100% SnO2 0.2 2 100% CeO 2 0.2 0.1 Absorbance 0.1 Absorbance 0.0

0.0 300 350 400 450 500 550 600 300 350wavelength(nm) 400 450 500 550 600 wavelength(nm) Figure 6. UV spectra of alizarin dye subjected to removal with different CeO2 % at λ max = 422 nm.

Figure 6. UV spectra of alizarin dye subjected to removal with different CeO2 % at λmax = 422 nm.

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Table 2. Redox potentials (E) of cerium, tin and alizarin.

System Standard redox potential, E° (V) Ref. Ce4+(aq) + e– Ce3+(aq) +1.61 [32] Sn4+(aq) + 2 e– Sn2+(aq) +0.15 Sn2+(aq) + 2 e– Sn(s) −0.14 Alizarin Alizarin oxidation product −0.59 [33]

bar CPE SnO /CeO 30 2 2

-30 I/µA

-60

-90 -0.5 0.0 0.5 1.0 1.5 E/V vs. (Ag/AgCl/KCl)

Figure 5. Cyclic voltammograms of carbon past electrode (CPE) and SnO2/CeO2/CP electrode at a scanning rate of 300 mV/s.

3.2. Use of SnO2/CeO2 Nano-Composite for Alizarin Dye Removal

The effect of CeO2 amount doped with SnO2 on the removal efficiency was tested. Three different nano-composites with a percentage amount of CeO2of 3%, 5% and 7% (w/w) were chosen for alizarin removal. It was found that as the amount of CeO2 doped increased in the composite, the removal percentage increased to be 91.3%, 96.4% and 98.3% for 3%, 5% and 7% CeO2, doped with SnO2, respectively. This implies that CeO2 enhanced the adsorption and electrocatalytic powers of the synthesized nano-composite. As seen in Figure 6, pure SnO2 and CeO2 had no catalytic degradation Nanomaterials 2020, 10, 254 7 of 14 on alizarin dyes. This reflects the enhanced catalytic power of the synthesized nano-composites.

0.4 25 ppm AY 3% CeO 2 5% CeO 2 0.3 7% CeO 2 100% SnO 2 100% CeO 2 0.2

0.1 Absorbance

0.0

300 350 400 450 500 550 600 wavelength(nm)

NanomaterialsFigureFigure 2020 6. 6.UV ,UV 10, spectra 254spectra of of alizarin alizarin dye dye subjected subjected to to removal removal with with di differentﬀerent CeO CeO2 2% % at atλ λmaxmax == 422 nm. 7 of 14

3.2.1. Eﬀect of pH and Catalytic Activity of SnO2/CeO2 Nano-Composite 3.2.1. Effect of pH and Catalytic Activity of SnO2/CeO2 Nano-Composite

Zeta potential was measured to interpret the behavior of SnO2/CeO2 nano-composite at diﬀerent Zeta potential was measured to interpret the behavior of SnO2/CeO2 nano-composite at different pH values. Figure7 shows that in an acidic medium (below pH 5), the catalyst surface became pH values. Figure 7 shows that in an acidic medium (below pH 5), the catalyst surface became positively charged. This could enhance the adsorption via electrostatic attraction between alizarin positively charged. This could enhance the adsorption via electrostatic attraction between alizarin dye and the catalyst. The zero point charge of the catalyst at pH 5–6 led to no or very slight alizarin dye and the catalyst. The zero point charge of the catalyst at pH 5–6 led to no or very slight alizarin removal. In alkaline media, the catalyst surface became negatively charged, and a gradual decrease in removal. In alkaline media, the catalyst surface became negatively charged, and a gradual decrease alizarin removal was noticed. in alizarin removal was noticed.

0 3456789 -10 pH -20

zeta pot. (mV) -30

-40

Figure 7. Zeta potential of SnO2/CeO2 nano-catalyst at different pHs. Figure 7. Zeta potential of SnO2/CeO2 nano-catalyst at different pHs. To optimize the pH under which maximum removal of alizarin took place, a 30 mL aliquot of To optimize the pH under which maximum1 removal of alizarin took place, a 30 mL aliquot of alizarin dye solution containing 25mg L− was treated with 0.15 g of the adsorbent at pH values −1 rangingalizarin fromdye 2solution to 9 and containing was stirred 25mg for 30 L min. was A treated maximum with removal 0.15 g ofof alizarin the adsorbent with the at value pH values 95.3% wasranging detected from at2 to pH 9 and 3.0 andwas thenstirred decreased for 30 min. to A 68.0% maximum at pH removal 6.0. A further of alizar increasein with inthe the value removal 95.3% ewasfficiency detected 87.8% at waspH detected3.0 and then at pH decreased 6.5–7.5, and to 68.0% then it at began pH 6.0. to decline A further after increase further increasein the removal in the pHefficiency values (Figure87.8% was8). It detected has been at reported pH 6.5–7.5, that and tin (IV) then oxide it began is predominately to decline after a Lewis further acid, increase with weak in the BronstedpH values acidity (Figure evolving 8). It has in the been presence reported of waterthat tin vapor. (IV) Strongoxide is Bronsted-acid predominately sites a canLewis be producedacid, with byweak protonation Bronsted in acidity acidic media.evolving Additionally, in the presence more adsorptionof water vapor. at acidic Strong pH indicatesBronsted-acid that the sites lower can pH be resultsproduced in an by increase protonation in H+ inions acidic on the media. adsorbent Additionally, surface that more result adsorption in significantly at acidic strong pH indicates electrostatic that + attractionthe lower between pH results alizarin in an molecules increase in and H the ions adsorbent on the adsorbent surface. surface that result in significantly strong electrostatic attraction between alizarin molecules and the adsorbent surface.

75 Removal % Removal 70

65 12345678910 pH

Figure 8. Effect of pH on alizarin removal (V = 30 mL, AY conc. = 25 µg/mL, contact time = 30 min and catalyst amount = 0.15 g).

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3.2.1. Effect of pH and Catalytic Activity of SnO2/CeO2 Nano-Composite

Zeta potential was measured to interpret the behavior of SnO2/CeO2 nano-composite at different pH values. Figure 7 shows that in an acidic medium (below pH 5), the catalyst surface became positively charged. This could enhance the adsorption via electrostatic attraction between alizarin dye and the catalyst. The zero point charge of the catalyst at pH 5–6 led to no or very slight alizarin removal. In alkaline media, the catalyst surface became negatively charged, and a gradual decrease in alizarin removal was noticed.

0 3456789 -10 pH -20

zeta pot. (mV) -30

-40

Figure 7. Zeta potential of SnO2/CeO2 nano-catalyst at different pHs.

To optimize the pH under which maximum removal of alizarin took place, a 30 mL aliquot of alizarin dye solution containing 25mg L−1 was treated with 0.15 g of the adsorbent at pH values ranging from 2 to 9 and was stirred for 30 min. A maximum removal of alizarin with the value 95.3% was detected at pH 3.0 and then decreased to 68.0% at pH 6.0. A further increase in the removal efficiency 87.8% was detected at pH 6.5–7.5, and then it began to decline after further increase in the pH values (Figure 8). It has been reported that tin (IV) oxide is predominately a Lewis acid, with weak Bronsted acidity evolving in the presence of water vapor. Strong Bronsted-acid sites can be produced by protonation in acidic media. Additionally, more adsorption at acidic pH indicates that the lower pH results in an increase in H+ ions on the adsorbent surface that result in significantly Nanomaterials 2020, 10, 254 8 of 14 strong electrostatic attraction between alizarin molecules and the adsorbent surface.

75 Removal % Removal 70

65 12345678910 pH µ FigureFigure 8. Effect 8. Effect of ofpH pH on onalizarin alizarin removal removal (V (V = 30= 30mL, mL, AY AY conc. conc. = 25= 25µg/mL,g/mL, contact contact time time = 30= 30min min and and catalystcatalyst amount amount = 0.15= 0.15 g). g). Nanomaterials 2020, 10, 254 8 of 14 In addition, when the catalyst was irradiated by the solar light, electrons from the conduction band and holesIn from addition, the valance when the band catalyst were was formed irradiated [25], generatingby the solar thelight, primary electrons oxidant, from the hydroxyl conduction radicals band and holes from the valance band were formed [25], generating the primary oxidant, hydroxyl (OH ), due to the reaction between hydroxide ions and positive holes. The hydroxyl radicals are •radicals (OH•), due to the reaction between hydroxide ions and positive holes. The hydroxyl radicals consideredare considered the major the oxidation major oxidation species sp inecies an in alkaline an alkaline pH medium.pH medium. At At pH pH levels levels greater greater than than 7.0,7.0, there was athere decrease was a indecrease alizarin in removalalizarin removal due to thedue electrostatic to the electrostatic repulsion repulsion of the of negatively the negatively charged charged surface of thesurface nano-composite of the nano-composite and hydroxide and hydroxide ions, which ions, in which turn in prevented turn prevented the formation the formation of OH of •OHradicals• (primaryradicals oxidant). (primary So, oxidant). it can be So, concluded it can be thatconclu theded high that removal the high percentage removal percentage implies the implies occurrence the of physicaloccurrence and chemical of physical adsorption and chemical in acidic adsorption solutions in andacidic chemical solutions degradation and chemical in degradation alkaline solutions. in alkaline solutions. 3.2.2. Effect of Contact Time 3.2.2. Effect of Contact Time The optimal time required for maximum dye removal was tested by fixing all other parameters (30 mL ofThe 25 mgoptimal/L alizarin time required dye (AY), for pH maximum 3, 7 and dye 0.15 remova g of thel was nano-composite) tested by fixing all and other varying parameters the contact (30 mL of 25 mg/L alizarin dye (AY), pH 3, 7 and 0.15 g of the nano-composite) and varying the time from 5 to 180 min. At pH 3, the dye removal increased from 85.5% after a 5 min contact time contact time from 5 to 180 min. At pH 3, the dye removal increased from 85.5% after a 5 min contact to 94%time after to 94% 30 min after (Figure 30 min9 A).(Figure Further 9A). increaseFurther in ofcrease the contact of the timecontact over time 30 over min 30 had min no had noticeable no effectnoticeable for the removal effect for of the more removal dye of concentration. more dye concen Attration. pH 7, theAt pH removal 7, the removal of thedye of the increased dye increased from 60% to 91.1%from for 60% the to above91.1% for mentioned the above contactmentioned time contact (Figure time9B). (Figure Further 9B). increaseFurther increase of the contactof the contact time over 30 mintime led over to a30 rapid min led decrease to a rapid in the decrease removal in the effi removalciency. Thisefficiency. can beThis attributed can be attributed to the electrostatic to the repulsionelectrostatic between repulsion the negatively between the charged negatively surface charged present surface on present the adsorpant on the adsorpant at this pH at andthis pH alizarin molecules.and alizarin A contact molecules. time A of contact 30 min time was of chosen 30 min forwas all chosen subsequent for all subsequent measurements. measurements.

100 (A) 95 (B)

90 40 Removal % Removal % Removal 20

85 0 0 30 60 90 120 150 180 0 306090120150180 Time,min Time,min.

Figure 9. Eﬀect of contact time on alizarin removal at (A) pH 3.0, (B) pH 7.0 (V = 30 mL, AY conc. = 25 µgFigure/mL and 9. Effect catalyst of contact amount time= 0.15on alizarin g). removal at (A) pH 3.0, (B) pH 7.0 (V = 30 mL, AY conc. = 25 µg/mL and catalyst amount = 0.15 g).

3.2.3. Effect of the Catalyst Dose Different quantities of NP material (0.01 to 0.2 g) were mixed with 30 mL (25 mg/L) alizarin dye solution at pH 3.0 and stirred for 30 min. Figure 10A shows that the removal of the dye increased from 28% with 0.01 g adsorbent to 94.8% for 0.15 g of adsorbent. Under similar conditions, but at pH 7, 46.2% and 88.8% removal efficiency were obtained (Figure 10B). An adsorbent quantity of 0.15 g was used for further study.

Nanomaterials 2020, 10, 254 9 of 14

3.2.3. Effect of the Catalyst Dose Different quantities of NP material (0.01 to 0.2 g) were mixed with 30 mL (25 mg/L) alizarin dye Nanomaterials 2020, 10, 254 9 of 14 solution at pH 3.0 and stirred for 30 min. Figure 10A shows that the removal of the dye increased from 28% with 0.01 g adsorbent to 94.8% for 0.15 g of adsorbent. Under similar conditions, but at pH 7, 46.2%Nanomaterials and 88.8%2020, 10 removal, 254 efficiency were obtained (Figure 10B). An adsorbent quantity of 0.15 g9 wasof 14 100 used for further(A) study. 90 (B)

80 10080 (A) 90 (B) 70 6080 80 60 70

40 Removal % Removal % 60 50 60

20 40 40 Removal % Removal % 0.00 0.05 0.10 0.15 0.20 50 0.05 0.10 0.15 0.20 Amount, g Amount, g 20 40 0.00 0.05 0.10 0.15 0.20 0.05 0.10 0.15 0.20 Amount, g Amount, g Figure 10. Effect of adsorbent amount on alizarin removal at (A) pH 3.0, (B) pH 7.0 (V = 30 mL, AY

conc.Figure = 10.25 µg/mLEffect of and adsorbent contact amounttime = 30 on min). alizarin removal at (A) pH 3.0, (B) pH 7.0 (V = 30 mL, AY conc. = 25 µg/mL and contact time = 30 min). 3.2.4.Figure Effect 10.of AlizarinEffect of adsorbentDye Concentration amount on alizarin removal at (A) pH 3.0, (B) pH 7.0 (V = 30 mL, AY 3.2.4.conc. Effect = 25 of µg/mL Alizarin and Dye contact Concentration time = 30 min). The effect of dye concentration in the range from 25 to 90 mg/L at pH 3 and 7 was investigated under3.2.4.The Effect the e ffoptimized ectof Alizarin of dye concentration experimentalDye Concentration inparameters. the range fromThirty 25 tomilliliter 90 mg/ Laliquots at pH 3 of and alizarin 7 was investigatedyellow dye solutionunder the at optimized both pH 3.0 experimental and 7.0 were parameters. allowed to Thirty interact milliliter with 0.15 aliquots gm SnO of alizarin2/CeO2 nano-composite yellow dye solution for The effect of dye concentration in the range from 25 to 90 mg/L at pH 3 and 7 was investigated 30at bothmin. pHIt was 3.0 noticed and 7.0 that were the allowed removal to percentage interact with decreased 0.15 gm as SnO the2 /concentrationCeO2 nano-composite of the dye for increased 30 min. under the optimized experimental parameters. Thirty milliliter aliquots of alizarin yellow dye fromIt was 25 noticed to 90 thatmg/L. the At removal pH 3, 94.8% percentage and 81% decreased alizarin as theremoval concentration was obtained of the dyewith increased 25 mg/L fromand 80 25 solution at both pH 3.0 and 7.0 were allowed to interact with 0.15 gm SnO2/CeO2 nano-composite for mg/Lto 90 mgalizarin/L. At concentration, pH 3, 94.8% and respectively 81% alizarin (Figure removal 11A). was At obtained pH 7, withthe removal 25 mg/L efficiency and 80 mg decreased/L alizarin 30 min. It was noticed that the removal percentage decreased as the concentration of the dye increased fromconcentration, 87.8% to respectively11.4% when (Figure the concentration 11A). At pH of 7, the removaldye increased efficiency from decreased 25 mg/L from to 80 87.8% mg/L, to from 25 to 90 mg/L. At pH 3, 94.8% and 81% alizarin removal was obtained with 25 mg/L and 80 11.4%respectively when the(Figure concentration 11B). of the dye increased from 25 mg/L to 80 mg/L, respectively (Figure 11B). mg/L alizarin concentration, respectively (Figure 11A). At pH 7, the removal efficiency decreased from 87.8% to 11.4% when the concentration of the dye increased from 25 mg/L to 80 mg/L, 100 100 respectively (Figure(A) 11B). (B)

80 95 100 100 (A) (B) 60 90 80 95 40 60 Removal % Removal 85 90 Removal % 20 40 80

Removal % Removal 85 20 30 40 50 60 70 80 90 100 Removal % 20 30 40 50 60 70 80 90 [ALZ.], ppm 20 [AIZ.], ppm 80

20 30 40 50 60 70 80 90 100 20 30 40 50 60 70 80 90 Figure 11. Effect of alizarin dye concentration on the removal of the dye at (A) pH 3.0, (B) pH 7.0 (V = [ALZ.], ppm [AIZ.], ppm Figure30 mL, 11. contact Effect time of alizarin= 30 min dye and concentration catalyst amount on the= 0.15g). removal of the dye at (A) pH 3.0, (B) pH 7.0 (V = 30 mL, contact time = 30 min and catalyst amount = 0.15g). 3.3. Removal of Alizarin-3-Methylimino-Diacetic Acid (AMA) Figure 11. Effect of alizarin dye concentration on the removal of the dye at (A) pH 3.0, (B) pH 7.0 (V 3.3. RemovalUnder theof Alizarin-3-Meth previously optimalylimino-Diacetic experimental Acid conditions, (AMA) 95.0% of alizarin-3-methylimino-diacetic = 30 mL, contact time = 30 min and catalyst amount = 0.15g). acidUnder dye was the removed, previously confirming optimal experimental the efficiency conditions, of SnO2/CeO 95.0%2 nano-composite of alizarin-3-methylimino-diacetic for the removal of different alizarin group dyes. acid3.3. Removal dye was of removed, Alizarin-3-Meth confirmingylimino-Diacetic the efficiency Acid of(AMA) SnO2 /CeO2 nano-composite for the removal of different alizarin group dyes. Under the previously optimal experimental conditions, 95.0% of alizarin-3-methylimino-diacetic acid dye was removed, confirming the efficiency of SnO2/CeO2 nano-composite for the removal of 3.4. Modeling of Adsorption different alizarin group dyes. Langmuir (Equation 3), Freundlich (Equation 4) and Temkin (Equation 5) models were applied to3.4. calculate Modeling the of sorptionAdsorption of alizarin dye. Langmuir (Equation 3), Freundlich (Equation 4) and Temkin (Equation 5) models were applied to calculate the sorption of alizarin dye.

Nanomaterials 2020, 10, 254 10 of 14

1/Qt = 1/XmbCt + 1/Xm (3)

LogQt = (1/n) log Ct + log kF (4)

Nanomaterials 2020, 10, 254 Qt = (RT/BT) ln Ct + (RT/BT) lnKT 10(5) of 14 where Qt is the adsorption capacity at equilibrium (mg/g); Ct is the equilibrium concentration of the AY3.4. solution Modeling (mg/L); of Adsorption t (min) is the contact time; Xm (mg/g) is the maximum monolayer adsorption capacity and b (L/mg) is the adsorption equilibrium constant. A linear relationship was obtained by Langmuir (Equation (3)), Freundlich (Equation (4)) and Temkin (Equation (5)) models were plotting 1/Qtversus1/Ct (Figure 12a). The slope (Xm) and the intercept (b) values were 18.52 and 0.16, applied to calculate the sorption of alizarin dye. respectively, suggesting monolayer adsorption with correlation coefficient (R2 = 0.983). From the Freundlich model, the relative adsorption capacities (n) and sorption intensities (Kf) 1/Qt = 1/XmbCt + 1/Xm (3) (mg/gm) were calculated from the slope and intercept of plotting logQt vs. log Ct (Figure 12b). The values were n = 2.08, indicating a favorableLogQt = sorption(1/n) log process Ct + log as kF n > 1 [34], and Kf = 3.55 mg/g with(4) a correlation coefficient (R2 = 0.966). Temkin constants, BT (k J/mol) and KT (L/mg), which are constants of heat of sorption and the equilibriumQt = binding(RT/BT) lnconstant Ct + (RT corresponding/BT) lnKT to maximum binding energy,(5) werewhere estimatedQt is the from adsorption the slope capacity and intercept at equilibrium of Qt vs. (mg ln /Cg);t (FigureCt is the 12c). equilibrium Values of concentration0.533 and 1.44 ofwere the obtained,AY solution respectively. (mg/L); t (min) This isrefers the contactto the time;heat ofX msorption(mg/g) isand the confirms maximum the monolayer physical adsorption processcapacity with and ba(L correlation/mg) is the adsorptioncoefficient equilibrium(R2 = 0.970). constant. All of these A linear values relationship confirmed was a obtainedreasonable by adsorptionplotting 1/Qtversus1 capacity/ Candt (Figure suggested 12a). Thethe slopeoccurrence (Xm) and of theboth intercept physical (b )(mul valuestilayer) were 18.52and chemical and 0.16, (monolayer)respectively, adsorption suggesting monolayer[35] between adsorption the prepared with nano-composite correlation coeﬃ cientand alizarin (R2 = 0.983). dye that confirms the results obtained from the pH effect.

0.25 (a) 1.2 (b)

0.20 1.1

1.0 t t 0.15 0.9 1/Q

log Q 0.8 0.10 0.7

0.6 0.05 0.2 0.4 0.6 0.8 1.0 1.2 1.4 0.0 0.1 0.2 0.3 0.4 0.5 log C 1/C t t

16 (c) 14

10 Q 8

0.5 1.0 1.5 2.0 2.5 3.0 ln C t

FigureFigure 12. 12. (a(a)) Langmuir, Langmuir, ( (bb)) Freundlich Freundlich and and ( (cc)) Temkin Temkin isotherms isotherms for for alizarin removal.

3.5. RegenerationFrom the Freundlich of SnO2/CeO model,2 Nano-Composite the relative adsorption capacities (n) and sorption intensities (Kf) (mg/gm) were calculated from the slope and intercept of plotting logQt vs. log Ct (Figure 12b). The adsorbent was regenerated after each adsorption cycle of alizarin by heating at 600 °C for The values were n = 2.08, indicating a favorable sorption process as n > 1 [34], and Kf = 3.55 mg/g 60 min. After five cycles of regeneration,2 the efficiency of the nano-composite for the removal of with a correlation coefficient (R = 0.966). Temkin constants, BT (k J/mol) and KT (L/mg), which alizarin dye became 77.8%. are constants of heat of sorption and the equilibrium binding constant corresponding to maximum binding energy, were estimated from the slope and intercept of Qt vs. ln Ct (Figure 12c). Values of 0.533 and 1.44 were obtained, respectively. This refers to the heat of sorption and confirms the physical adsorption process with a correlation coefficient (R2 = 0.970). All of these values confirmed a reasonable adsorption capacity and suggested the occurrence of both physical (multilayer) and Nanomaterials 2020, 10, 254 11 of 14 chemical (monolayer) adsorption [35] between the prepared nano-composite and alizarin dye that confirms the results obtained from the pH effect.

3.5. Regeneration of SnO2/CeO2 Nano-Composite

The adsorbent was regenerated after each adsorption cycle of alizarin by heating at 600 ◦C for 60 min. After ﬁve cycles of regeneration, the eﬃciency of the nano-composite for the removal of alizarin dye became 77.8%.

3.6. Comparison with Other Sorbents for Removal of Alizarin Table3 represents a comparison of the performance of the present suggested sorbent with some of those previously described nano-materials for the removal of alizarin veiled. It can be seen from Table3 that high maximum adsorption capacity [ 7], reasonable contact time [7,13,14,18,19,36–41], high removal percentage [7,8,10,14,15,18,36,38,39,41] and low amount of adsorbent dosage [7,8,10,15,38] were oﬀered by the present suggested nano-composite material in this work.

Table 3. Comparison of various nano-adsorbents for alizarin dyes removal.

Maximum Contact Optimum Best Fit Adsorbent Removal, Adsorbent Type Adsorption Ref. Time pH Isotherm Dosage, gm % Capacity, mg/g

ZnO/TiO2 12.5 120 min 8 Langmuir 5.0 84.4–92.9 [7] Zinc doped WO3 catalyst NR 10 min NR NR 0.4 80 [8] α-Fe2O3/NiS NR NR 5 NR 1.0 88.3 [10] Activated carbon/γ-Fe2O3 108.6 60 min 2 Langmuir 0.01 99.4 [13] nano-composite Poly METAC/Fe O 3 4 NR 2 days NR NR NR 80–96 [14] magnetic nanoparticles Nanocrystalline NR 5 min NR Freundlich 0.2 83% [15] Cu0.5Zn0.5Ce3O5 PPy-coated Fe O 3 4 116.3 60 min 4–5.4 Langmuir 0.1–0.12 78.7 [18] nanoparticles Chitosan-coated Fe O 3 4 40.12 50 min 3 Langmuir 0.1 NR [19] nanoparticles 2,4-dinitrophenyl hydrazine/Nano 47.8 60 min 4 Langmuir 0.05 95.6 [36] γ-Alumina MWCNTs/PANI 884.8 50 min 8.5 Langmuir 0.02 N.R [37] Fe3O4 nanoparticles 45.8 5 min 5 Langmuir 0.02 99 [42] CuFe O @graphene 2 4 145 40 min 3 Langmuir 0.5 95 [38] nanocomposite Chitosan/ZnO nanorod 36.4 27 h 2 Freundlich 0.1 85 [39] composite Fe3O4@MCM@Cu–Fe–LDH 121.9 10 min 9 Langmuir 0.03 N.R [43] Biosorbent from 46.5 N.R 2 Freundlich 0.1 N.R [40] Mikaniamicrantha Green carbon composite-derived 104 1day 3 Freundlich NR N.R [44] polymer resin and waste cotton ﬁbers NiFe O /Polyaniline 2 4 186 90 min 4-8.6 Langmuir 0.03 96 [41] Magnetic Composite Freundlich SnO /CeO 2 2 18.5 30 min 3 and 0.15 96.4 This work nano-composite Langmuir * NR (not reported), (METAC) (methacryloyloxy)ethyl trimethyl-ammonium chloride, DNPH (2,4-dinitrophenyl hydrazine), MCM (Mobil composition of matter), LDH (layered double hydroxides), multi-walled carbon nanotubes (MWCNTs), polyaniline (PANI), poly pyrrol (PPy).

4. Conclusions A facile method was presented for the removal of alizarin dyes from aqueous industrial eﬄuents. The method was based on the use of SnO2/CeO2 nano-composite catalyst. The catalyst was synthesized by the so-called “co-precipitation method” and characterized by X-ray powder diﬀractometry (XRD), high-resolution transmission electron microscopy (HR-TEM), Brunauer–Emmett–Teller methodology Nanomaterials 2020, 10, 254 12 of 14

(BET) and Fourier transform infrared spectrometry (ATR-FTIR). Polycrystalline and spherical structure were confirmed with a mean average grain size of 27 nm. The prepared nano-composite revealed a high affinity for the adsorption and decomposition of alizarin dyes. Alizarin-3-methylimino-diacetic acid, alizarin yellow and alizarin red S dyes showed removal efficiencies of 95.0%, 95.3% and 87.8%, respectively. The optimal conditions for the adsorption capacity were pH 3.0, 25 mg/L dye, 30 min contact time and 0.15 gm catalyst. The adsorption isotherms agreed with Langmuir, Freundlich and Temkin isotherms. A comparison of the performance of the present suggested sorbent with some of those previously described nano-materials for the removal of alizarin is shownin Table3. The presented method offered high maximum adsorption capacity, reasonable contact time, high removal percentage and low amount of adsorbent dosage compared to those presented by the previously reported methods.

Author Contributions: The listed authors contributed to this work as described in the following: H.A.E.-N., A.A.H., S.S.M.H. and A.H.K. gave the concepts of the work, interpretation of the results, the experimental part and prepared the manuscript, A.H.K., S.S.M.H. and A.E.-G.E.A. cooperated in the preparation of the manuscript and A.H.K. and S.S.M.H. performed the revision before submission. A.E.-G.E.A. and E.A.E. revealed the financial support for the work. All authors have read and agreed to the published version of the manuscript. Funding: King Saud University, Project (Project No. RSP-2019/66). Acknowledgments: Authors are grateful to King Saud University for funding the work through Researchers Supporting Project (Project No. RSP-2019/66). Conflicts of Interest: The authors declare no conflict of interest.

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