Toward the design of Zn–Al and Zn–Cr LDH wrapped in activated carbon for the solar assisted de-coloration of organic dyes

Abstract

Designing of materials for the extraction of organic toxins are the critical factors for environmental remediation. In the present study, a layered double hydroxide (LDH) of the binary compounds Zn–Al and Zn–Cr wrapped in activated carbon were synthesized through the co-precipitation method. These catalysts were characterized by FESEM, XRD, EDS, XPS, FTIR, PL and DRS. These catalyst were evaluated for the adsorption-assisted photodegradation of acridine orange (AO), malachite green (MG), crystal violet (CV), congo red (CR) and methyl orange (MO) dyes. The adsorption-assisted photodegradation was studied in four different parameters: dark, visible, ultraviolet and sunlight exposure conditions. Under various light effects, AO was selectively removed and Zn–Al/C-LDH showed stronger activity then Zn–Cr/C-LDH. Both catalysts showed approximately 90% removal efficiency of AO in sunlight. PL and DRS data showed prominent bands for the respective catalysts in the visible region. Zn–Al/C-LDH and Zn–Cr/C-LDH showed band gaps of 2.97 eV and 2.91 eV, respectively. Zn–Al/C-LDH showed very good recyclability and durability until the fourth cycle and still remained active after the fourth cycle as compared to Zn–Cr/C-LDH, which also showed good recyclability until the fourth cycle, but remained inactive after the fourth cycle. It was also observed that the specific light intensity and the substrate-catalyst specificity required equal consideration during the development of these new catalysts.

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1. Introduction

In ancient times, people tried to make their environment and surroundings gorgeous and attractive using dyes. However, these dyes are carcinogenic and mutagenic to aquatic life and human health. The removal of organic pollutants from waste water is certainly related to the quality of life. In the last few decades, there has been a growing demand for the removal of mutagenic and carcinogenic compounds from industrial and domestic effluents.^1,2 These organic pollutants have several disorders on human health and other organisms. For instance, a minute quantity of organic toxins in the water were reported to cause endocrine disorders, chronic injuries and develop injurious pathogen resistivities.³ Nevertheless, besides these injurious worries to organisms and environmental threats, dye industries have contributed major roles in the progress and prosperity of a country. Synthetic dyes are used for various purposes in various industries worldwide. The highest amount of dyes were used by the textile industries, which play a significant role in the economy and progress of a country.^4,5 The use of large amounts of synthetic dyes in various industries significantly increases environmental pollution. Dumping of the untreated dyes, or other waste materials, in water is cheaper compared to the treatment of these water materials prior to sending into water sources. It seems that, on one side, textile industries contributed to the growth of a country, but on the other side, it uses a large amount of chemical substances that finally pour into water resources and causes severe health issues. Due to these untreated chemicals, our water resources have become contaminated and therefore, affect the ecosystem and aqueous life. It was estimated that about 2–50% of various dyes that are applied in textile processing are eventually dumped into water resources.⁶ Similarly, certain other industries, such as plastic, paper, food, textile and cosmetic industries, also use high amounts of dyes.¹ These dye-stuffs makes the water colorful at a very small concentration, which make a foam-like layer on the surface, and thus prevent the passage of sunlight and oxygen into the water; and therefore, disturbs the biological phenomena of organisms, which eventually causes the death of aquatic flora and fauna.⁴ Literature surveys have revealed that trace amounts of dyes in the water is a severe threat for the environment, animals, plants and human health.¹ Furthermore, most of these dyes are non-biodegradable and carcinogenic, which comes into the water cycle, and thereby affects human health directly or indirectly.⁷

Dye removal and water purification are considered as some of the fundamental concerns for researchers.^4,8–10 Dyes should be removed before discharge into the water in order to protect living organisms and human health. Various technologies have been administered by the scientific community for water purification. For instance, activated carbon, chitosan, biodegradation, adsorption, coagulation and membrane technologies have been used; however, adsorption and degradation are the most effective, cheap and robust methods.^11–13

In this study, we synthesized layered double hydroxides (LDHs) of Zn–Al/C-LDH and Zn–Cr/C-LDH for dye removal. Both catalysts showed a prominent response for acridine orange (AO) removal from the effluent under solar, visible and ultraviolet light contact, as well as in dark conditions. LDH has received much attention in the scientific field due to its varied catalytic application. For instance, various Zn-based LDHs have been used for dye degradation in the literature. For instance, copper phthalocyanine-immobilized Zn/Al-LDH was used for the degradation of methylene blue (MB) dye under solar light irradiation¹⁴ and Zn–Cr-LDH under visible light irradiation has been used for the degradation of organic pollutants.¹⁵ Similarly, several metals like Co, Ni, Cu and Zn/Cr-LDH have been used for the photocatalytic degradation of methyl orange (MO).¹⁶ LDHs belong to inorganic nanostructured materials. LDH has a brucite-like structure with positively charged cations at the top of the layered structure and negatively charged anions are present in the interlayers. The interlayer anions mostly contain CO₃²⁻, NO₃¹⁻, SO₄²⁻ and Cl¹⁻ along with water molecules.¹⁷ The interlayers of LDH are attached to each other through hydrogen bonding. These are present in the literature that these H-bonding are due to the presence of H₂O molecules inside the brucite interlayers. It is represented through the general formula [M^II_1−x M^III_x(OH)₂]^z+(Aⁿ⁻)_z/n·yH₂O where M^II and M^III are di- and trivalent cations, while Aⁿ⁻ is the counter anion present in the interlayer of these brucite-like sheets.^18–22 LDH is a novel nanosorbent and has been applied largely in catalysis, adsorption, degradation, extraction and many other fields.²³ A wide range of tunable metal ions can be used in LDH without changing the structural characteristics and the exchangeable anions between the interlayers, which makes LDH a suitable candidate for catalysis.

2. Experimental

2.1 Chemicals and reagents

Chemicals such as NaOH, NH₄OH and nitrate and chloride salts of Al, Zn and Cr in the form of Al(NO₃)₃·9H₂O, Zn(NO₃)₂·6H₂O, ZnCl₂ and CrCl₂ were purchased from Sigma-Aldrich, Ireland. The double distilled water was purified through a Millipore-Q machine present in the department of chemistry at the King Abdulaziz University, Saudi Arabia.

2.2 Synthesis of Zn–Al/C-LDH

The LDH nanostructures were wrapped in activated carbon through a co-precipitation method, as reported in our previous papers.^24,25 Salts of Al(NO₃)₃·9H₂O and Zn(NO₃)₂·6H₂O were mixed in 1 : 3 molar ratio. Prior to mixing, each salt was well dispersed in double distilled water and with respect to molar ratio of salt, 10 wt% of activated carbon was added to the solution mixture and was dispersed by magnetic stirring. After mixing, NaOH solution (0.1 M) was added to basify the solution in a dropwise fashion to obtain pH 9 by continuously scrutinizing the pH of the solution. The stated reaction mixture was heated on a hot plate at 60 °C and left overnight with continuous stirring. The supernatant was drawn off and the precipitate was separated and washed with a C₂H₅OH : H₂O mixture (8 : 2) thrice. The product was then put overnight in an oven at 50 °C for drying and stored in clean tubes. The Zn–Cr/C-LDH was prepared by a similar method but the chloride salt of Cr and Zn was used instead of the nitrate salts.

2.3 Adsorption-assisted photodegradation procedure

Initially, the ratio of the catalyst to dye was fixed under visible light irradiation at 400 watt. The optimized amount, 10 mg of the respective catalyst, was added in 100 mL of 0.025 mmol solution of acridine orange (AO), malachite green (MG), crystal violet (CV), methyl orange (MO) and congo red (CR) dyes. By the addition of the catalyst, the decrease in dye concentration was determined through a UV-vis spectrophotometer, while the dye removal efficiency (R. E.) (%) was determined through the following equation:

R. E. (%) = (C₀ − C_t)/C₀ × 100 = (A₀ − A_t)/A₀ × 100 (1)

where C₀ is the original concentration of the dye solution at time = 0, C_t is the concentration of dye solution containing catalyst after time = t. Similarly, A₀ is the absorbance of the original concentration of the dye solution at time = 0; whereas, A_t is the absorbance of dye solution containing catalyst after time = t. 3 mL of the aliquots were taken periodically and monitored for the decrease in concentration through a UV-vis spectrophotometer. AO showed an absorbance maxima at λ_max = 490 nm while MG at 620 nm, CV at 590 nm, CR at 500 nm and MO at 464 nm.

2.4 Instrumental analysis and characterization

Crystal morphology and crystallinity of the synthesized catalysts were examined through powder X-ray diffractometry (PXRD) with a Kα radiation (λ = 0.154 nm) source (Thermo Scientific). FT-IR was used for the determination of functional groups in the synthesized catalyst (Thermo Scientific). Field emission-scanning electron microscopy (FESEM) on a JEOL (JSM-7600F, Japan) was used for the morphology and average size while elemental analysis was scrutinized through energy dispersive X-rays spectrometry (EDS) from an Oxford-EDS system. The X-ray photo electron spectroscopy (XPS) was recorded on Thermo Scientific K-Alpha KA1066 spectrometer (Germany) in the range of 0 to 1350 eV. Photoluminescence (PL) emission spectra were verified at 320 nm excitation wavelength with a fluorescence spectrofluorophotometer (RF-5301 PC, Shimadzu, Japan). The solid state UV-vis diffuse reflectance spectroscopy (DRS) was documented by a PerkinElmer UV-vis diffuse reflectance spectrophotometer. The photocatalytic reaction was monitored through an Evolution 300 UV-vis spectrophotometer (Thermo scientific). The effect of visible and ultraviolet light on the adsorption–degradation process of the dyes, with respective catalyst, were observed under a visible light lamp (400 watt, OSRAM), an ultraviolet lamp (Smiec, Shanghai China, 230 V and 11 watt), normal-day sunlight and under completely dark conditions.

3. Results and discussion

3.1 Structural characterization of catalyst

The average size and morphology of the catalysts were scrutinized through FESEM. The FESEM images show that both catalysts were grown in the form of sheets. In case of Zn–Al/C-LDH, the sheets were made by the aggregation of approximately 14 nm particles, as shown in Fig. 1a–d, while Zn–Cr/C-LDH sheet contains mixed particles and blocks which on aggregation makes Zn–Al/C-LDH nanostructure. On average, the particle size in Zn–Cr/C-LDH sheets were 10 nm while blocks were approximately 50 nm, as indicated in Fig. 1e–h.

Fig. 1 FESEM images of Zn–Al/C-LDH (a–d) and Zn–Cr/C-LDH (e–h) nanosheets.

The EDS spectra show C, O, Al and Zn elements in Zn–Al/C-LDH (Fig. 2a and b) and also confirmed the C, O, Cr and Zn elements in Zn–Cr/C-LDH (Fig. 2c and d). The EDS data reflects that the synthesized catalysts contain C, Al, Cr, O and Zn elements in the respective catalysts. C, O, Al and Zn are present 9.08, 41.29, 13.85 and 27.47 weight% in Zn–Al/C-LDH; while C, O, Cr and Zn are present 9.18, 25.52, 21.42 and 42.88 weight% in Zn–Cr/C-LDH.

Fig. 2 EDS spectra and plot images of Zn–Al/C-LDH (a–c) and Zn–Cr/C-LDH (d–f) nanosheets.

FTIR analysis was recorded for functional group confirmation in the synthesized catalysts. Absorption peaks at 423 to 830 cm⁻¹ confirmed the presence of O–M–O and M = O bonds in Zn–Al/C-LDH and Zn–Cr/C-LDH catalysts.²⁶ A sharp peak at 1355 cm⁻¹ corresponded to the presence of NO₃¹⁻ in Zn–Al/C-LDH and Cl¹⁻ in Zn–Cr/C-LDH. These sharp peaks suggested that the synthesized catalyst have LDH morphology. A broad peak for OH¹⁻ at 3425 cm⁻¹ was exhibited in both catalysts, suggesting the presence of H₂O in the interlayer of the LDH.²⁶ Furthermore, the broadening of this peak suggested the presence of hydrogen bonding between the water molecules in the interlayer morphology of LDH. This peak is recognized as the O–H stretching vibration in the brucite-like layers.^27,28 The absorption at 1637 cm⁻¹ was attributed to the OH bending vibration in Zn–Al/C-LDH and Zn–Cr/C-LDH. The FTIR analysis confirmed the LDH nature of the synthesized catalysts, as shown in Fig. 3a.

Fig. 3 FT-IR spectrum (a) and powder XRD spectrum (b) of Zn–Al/C-LDH and Zn–Cr/C-LDH nanosheets.

The PXRD pattern displayed peaks for metal oxide and LDH in Zn–Al/C-LDH and Zn–Cr/C-LDH. The diffraction peaks at 2θ = 10.8 (003), 23.0 (006) and 33.4 (012) appeared in Zn–Al/C-LDH. Similarly, the peaks at 2θ = 11.4 (003), 23.4 (006) and 34.4 (012) suggested the formation of Zn–Cr/C-LDH. The XRD spectra displayed the characteristic peaks at 10.8 and 11.5 for Zn–Al/C-LDH and Zn–Cr/C-LDH, respectively, confirming the LDH nature of these catalysts. Similarly, a doublet at 2θ = 60–62 (110 and 113) in both catalysts further supported the LDH nature of the Zn–Al/C-LDH and Zn–Cr/C-LDH catalysts. The XRD patterns propose the presence of NO₃¹⁻ and Cl¹⁻ anion intercalation in the inner layer of the LDH in the respective catalysts. The remaining XRD peaks in Zn–Al/C-LDH and Zn–Cr/C-LDH correspond to the presence of Al and Zn doped oxides and Cr and Zn doped oxides, respectively.^29,30 It was concluded from the FTIR and XRD data that the respective catalysts were grown in mixed metal oxide and LDH morphologies. Fig. 3b presents the XRD data for both catalysts.

The binding energy of both catalysts were determined through XPS, as indicated in Fig. 4a and b. The peak appeared at 288 eV confirm the presence of carbon (C, 1s) while peaks for Zn (2p_3/2, 2p_1/2) were appeared at 1000 and 1047 eV and Al (2p, 2s) appeared at 91.8 and 141.7 eV in Zn–Al/C-LDH.^31,32 Similarly, Zn–Cr/C-LDH catalyst showed Zn (2p_3/2, 2p_1/2) at 1000 and 1047 eV and Cr 2p_3/2 at 586 eV while C 1s was appeared at 285 eV. This confirms that the respective catalysts were composed of C, Al, O, Cr and Zn elements.

Fig. 4 XPS spectrum of Zn–Al/C-LDH (a) and Zn–Cr/C-LDH (b) nanosheets.

The band gap was determined through solid state UV-vis DRS. Zn–Al/C-LDH (Fig. 5a and b) and Zn–Cr/C-LDH (Fig. 5c and d) showed a band gap of 2.97 and 2.91 eV, respectively. The low band gap suggested that both catalysts would work in the visible light range.

Fig. 5 Diffuse reflectance spectra of Zn–Al/C-LDH (a) and Zn–Cr/C-LDH (c). Band gap of Zn–Al/C-LDH (b) and Zn–Cr/C-LDH (d).

The PL spectra were obtained by excitation at 320 nm for both catalysts and recorded from 350–800 nm. Both catalysts showed intense peaks in the visible range at 625 nm, as indicated in Fig. 6. Again the PL also suggested that both catalysts would work in the visible light range.

Fig. 6 Comparison of PL spectra of Zn–Al/C-LDH and Zn–Cr/C-LDH.

3.2 Adsorption-assisted photodegradation

Adsorption-assisted photodegradation phenomena of two anionic dyes (CR and MO) and three cationic dyes (AO, MG and CV) were studied (Fig. 7–19, Tables 1 and 2).

Fig. 7 Adsorption-assisted photodegradation of AO under visible light (400 watt) with A1, A3 and without catalyst, concentration decrease of AO (a) and % removal of AO (b). AO, acridine orange; A1, Zn–Al/C-LDH; A3, Zn–Cr/C-LDH; WOC, without catalyst and VL, visible light exposure.

Fig. 8 Adsorption-assisted photodegradation of CR under visible light (400 watt) with A1 and A3: concentration decrease of CR (a) and % removal of CR (b). CR, congo red dye; A1, Zn–Al/C-LDH; A3, Zn–Cr/C-LDH; WOC, without catalyst and VL, visible light exposure.

Fig. 9 Adsorption-assisted photodegradation of MO in visible light (400 watt) with A1 and A3: concentration decrease of MO (a) and % removal of MO (b). MO, methyl orange dye; A1, Zn–Al/C-LDH; A3, Zn–Cr/C-LDH; WOC, without catalyst and VL, visible light exposure.

Fig. 10 % removal of AO, CR and MO by Zn–Al/C-LDH (A1) and Zn–Cr/C-LDH (A3) catalysts.

Fig. 11 Selective removal of AO under visible light using Zn–Al/C-LDH (a) and Zn–Cr/C-LDH (b).

Fig. 12 Adsorption-assisted photodegradation UV-vis spectra of AO under sunlight irradiation with A1 (a) and A3 (b). Concentration decrease of AO (c) and % removal of AO (d). AO, acridine orange; A1, Zn–Al/C-LDH; A3, Zn–Cr/C-LDH and S, normal day sunlight.

Fig. 13 % removal of CV and MG under sunlight irradiation with A1 and A3. A1, Zn–Al/C-LDH; A3, Zn–Cr/C-LDH and S, normal day sunlight.

Fig. 14 Adsorption of AO under ultraviolet light by A1 and A3: concentration decrease of AO (a) and % removal of AO (b). AO, acridine orange dye; A1, Zn–Al/C-LDH; A3, Zn–Cr/C-LDH and U, ultraviolet light.

Fig. 15 Adsorption of AO under dark conditions by A1 and A3: concentration decrease of AO (a) and % removal of AO (b). AO, acridine orange dye; A1, Zn–Al/C-LDH; A3, Zn–Cr/C-LDH and D, dark conditions.

Fig. 16 Langmuir adsorption isotherm of AO with catalyst A1 and A3 under solar (S) and visible light (VL).

Fig. 17 Plausible mechanism for adsorption-assisted photodegradation of dyes using Zn–Al/C-LDH.

Fig. 18 Catalytic activity and no. of cycles of A1 (a) and A3 (b) against AO. AO, acridine orange; A1, Zn–Al/C-LDHand A3, Zn–Cr/C-LDH in visible light. Each cycle lasted for 1 h.

Fig. 19 Color change of AO dye with Zn–Al/C-LDH (a) and Zn–Cr/C-LDH (b) under visible light illumination.

Table 1 Rates of reaction for both catalysts derived from the pseudo-first order Langmuir adsorption isotherm model for adsorption-assisted photodegradation of AO^a

S. no.	Catalyst	Rate (mol L^−1 min^−1)
a AO, acridine dye; A1, Zn–Al/C-LDH; A3, Zn–Cr/C-LDH; S, solar light; VL, visible light; U, ultraviolet light and D, dark.
1	AO–A1–S	1.1 × 10^−2
2	AO–A3–S	1.0 × 10^−2
3	AO–A1–VL	2.8 × 10^−3
4	AO–A3–VL	1.4 × 10^−3
5	AO–A1–U	1.9 × 10^−3
6	AO–A3–U	1.2 × 10^−3
7	AO–A1–D	1.1 × 10^−3
8	AO–A3–D	8.1 × 10^−4

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Table 2 Structure of dyes used in the experiment

Entry	Structure	Dyes	Nature	Mol. mass (g mol^−1)
1		Acridine orange (AO)	Cationic	265.36
2		Malachite green (MG)	Cationic	364.911
3		Crystal violet (CV)	Cationic	407.979
4		Congo red (CR)	Anionic	696.66
5		Methyl orange (MO)	Anionic	327.33

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3.2.1 Effect of visible light. Prior to sunlight exposure, the effects of visible light on the degradation of CR, MO and AO were studied. Solar light intensity is variable from day to day, and even on the same day. Therefore, initially these three dyes were checked under a fixed intensity of 400 watt visible light exposure. Initially, 100 mg of Zn–Al/C-LDH and Zn–Cr/C-LDH were added to 100 mL of 0.025 mmol AO dye and the dye degraded approximately to 80% and 59% in one hour, as shown in Fig. 18a and b for Zn–Al/C-LDH and Zn–Cr/C-LDH, respectively. The amount of catalyst was decreased and an optimized amount (10 mg) of the respective catalyst was used against the degradation of 100 mL of 0.025 CR, MO and AO dyes. As the reaction proceeded, 3 mL aliquots of each dye were taken and the change in original concentration was periodically monitored through UV-vis. Both catalysts showed superior activity for the degradation of AO as compared to CR and MO, as shown in Fig. 10. Both catalysts were selective for AO removal. Interestingly, after 15 min, the removal efficiency of Zn–Al/C-LDH and Zn–Cr/C-LDH against AO was 42.3% and 15.5% while CR was removed only 12.8% and 5.3% and MO 1.9% and 1.0%, respectively. These results showed the selective degradation of AO by both catalysts compared to CR and MO. The % removal efficiency (R. E.) increases directly with the increase in contact time. For instance, after 200 min the R. E. of Zn–Al/C-LDH against AO and MO were 61.1% and 10.7%, respectively, compared to Zn–Cr/C-LDH, which were 36.5% and 6.6%, respectively. But at the same time, Zn–Cr/C-LDH degraded the CR dye a little higher then Zn–Al/C-LDH with a R. E. of 23.0% and 19.7%, respectively. The decrease in concentration by applying the formula C₀ − C_t/C₀ and R. E. of AO is depicted in Fig. 7a and b, CR (Fig. 8a and b) and MO (Fig. 9a and b). The progressive decrease in color change of AO with both catalysts is depicted in Fig. 19. It was concluded that, under visible light exposure, the Zn–Al/C-LDH showed a dominant catalytic performance for AO and MO while Zn–Cr/C-LDH showed a little higher degradation efficiency for CR (Fig. 10).

3.2.2 Dye selectivity. Under visible light exposure (400 watt), AO was selectively removed by both catalysts compared to CR and MO (Fig. 11a and b). This showed that both catalysts were selective for cationic dyes. The better selectivity of AO is due to the carboxylic group (COOH) present in activated carbon, which facilitated the interaction of AO (cationic nature) to the negatively charged COOH group. Among the three dyes, AO showed better selectivity and was further selected for solar light effects.

3.2.3 Effect of solar light. The effect of solar light was studied in normal day sunlight exposure. The normal day sunlight exposure means “the intensity of sunlight at that specific day”. Primarily, adjusted doses of 10 mg of the respective catalysts were used against 100 mL of 0.025 AO, as in the visible light exposure tests. As the reaction proceeded, 3 mL aliquots were taken periodically from the reaction mixtures and the decrease in concentration of dye solutions was observed by UV-vis. After the first 15 min of the experiment, the R. E. of AO by both catalysts were 27.9%. This indicated that, at the start of the experiment, both catalysts show the same trend and the same catalytic activity. However, by increasing the exposure time, the degradation process was increased and after 200 min, the R. E. of AO by Zn–Al/C-LDH and Zn–Cr/C-LDH were 89.4% and 87.4%, respectively. Again, both catalysts indicated approximately the same activity in solar light against AO. The UV-vis graph, decrease in dye concentration and percent R. E. of AO in solar light is depicted in Fig. 12a–d.

We observed that the R. E.s of both catalysts against AO (cationic dye) were higher in solar light compared to visible light. So, we decided to check the catalytic activity of both catalysts against other cationic dyes, such as MG and crystal violet CV, to evaluate the solar light effect on the nature of cationic dyes. Similarly, 10 mg of the adjusted dose of both catalysts were used against the degradation of 100 mL of 0.025 mmol MG and CV. The Zn–Al/C-LDH catalyst showed 52% degradation in 200 min compared to 22% R. E. of Zn–Cr/C-LDH for CV. Similarly, Zn–Al/C-LDH removed 70% MG in 200 min compared to Zn–Cr/C-LDH, which showed only 20% R. E. under the same conditions. Both catalysts showed stronger performance for AO and predominantly degraded AO over MG and CV in solar light. From this experiment, it was concluded that, among the cationic dyes, AO is again selectively removed over CV and MG. The % R. E. of CV and MG with the respective catalysts are indicated in Fig. 13a and b.

3.2.4 Effect of ultraviolet light. As we observed from the above experiment, AO was removed preferentially with both catalysts over the other stated dyes; therefore, we selected AO for further study under ultraviolet light and dark conditions.

The effect of ultraviolet light was studied against AO removal with both catalysts under 230 volts (11 watt) and keeping the same conditions as for solar and visible light. The adjusted dose of 10 mg of the respective catalysts were used against the removal of 100 mL of 0.025 mmol AO. Under ultraviolet light exposure, Zn–Al/C-LDH removed AO after the first 15 mint to 8.5% while Zn–Cr/C-LDH to 7.5%. However, after 200 min, the R. E. of AO by Zn–Al/C-LDH and Zn–Cr/C-LDH was 36.3% and 29.1%, respectively. Again Zn–Al/C-LDH showed dominant catalytic performance over Zn–Cr/C-LDH under ultraviolet light exposure. The decrease in concentration and R. E. of AO by both catalysts are presented in Fig. 14a and b.

3.2.5 Effect of dark conditions. Prior to the effect of sunlight and ultraviolet light, the effect of dark conditions was studied to know if the adsorption or degradation, or both phenomena, occurs simultaneously. The experiment was performed carefully by providing completely dark conditions. The same procedure was followed for the dark as described above for visible light. 10 mg of the respective catalysts were used against the removal of 100 mL 0.025 mmol solution of AO. During dark conditions, the adsorption phenomenon was observed with both catalysts. After the first 15 min, the adsorption efficiency of Zn–Al/C-LDH was 11.3% as compared to 10.0% in Zn–Cr/C-LDH against AO. However, with increasing contact time, the adsorption was increased for both catalysts. For instance after 200 min, the adsorption of Zn–Al/C-LDH was 35.1% and Zn–Cr/C-LDH was 26.6%. The results favor again the strongest adsorption capacity of Zn–Al/C-LDH as compared to Zn–Cr/C-LDH for AO in dark conditions. The decrease in concentration and R. E. of AO by both catalysts are presented in Fig. 15a and b.

Here, we conclude the above experiments: the adsorption phenomenon was observed in dark conditions and adsorption-assisted photodegradation occurs under visible, ultraviolet, and sunlight exposure conditions. Although the % R. E. in the dark and ultraviolet light is approximately same; however, in the dark conditions the dyes are removed through adsorption, and in the ultraviolet light conditions, by adsorption-assisted photodegradation. The metal oxide influenced the degradation phenomenon as it worked efficiently in ultraviolet light.³³ Similarly, both catalysts removed the dye through adsorption-assisted photodegradation in visible and solar light conditions. The adsorption-assisted photodegradation proceeded first by adsorbing the dyes onto the surface of the catalyst and then degradation occurs. It was also found that, for all the dyes in all condition except the CR dye, Zn–Al/C-LDH showed a stronger activity compared to Zn–Cr/C-LDH. However, Zn–Cr/C-LDH showed a little higher efficiency for CR dye then Zn–Al/C-LDH under visible light conditions.

3.2.6 Kinetics of the above experiments. The decrease in reactant concentration, or increase in product concentration with time, is called the rate of reaction. Here, we determined the rate of reaction from the decrease in concentration by applying a pseudo-first order reaction (ln C_t/C₀) rate law as depicted in Fig. 16. As AO is selectively extracted, the equation is applied on the removal of AO. It was found that Zn–Al/C-LDH showed the highest rate in all experiments, as discussed above for AO, and this data is shown in Table 1. Zn–Al/C-LDH and Zn–Cr/C-LDH showed approximately the same rate in solar light exposure, which were 1.1 × 10⁻² and 1.0 × 10⁻² mol L⁻¹ min⁻¹, respectively, as shown in Table 1. Under visible light conditions, the rate of AO removal by Zn–Al/C-LDH (2.8 × 10⁻³ mol L⁻¹ min⁻¹) was higher compared to Zn–Cr/C-LDH (1.4 × 10⁻³ mol L⁻¹ min⁻¹). Similarly, the rate of Zn–Al/C-LDH was also higher in the dark and ultraviolet light conditions compared to Zn–Cr/C-LDH.

3.3 Plausible mechanism of adsorption-assisted photodegradation of dyes

During photodegradation of the dyes, the electrons in the valance band of Zn (Zn–Al/C-LDH) absorbed visible light of appropriate frequencies and were promoted to the conduction, band leaving an electron deficiency (h⁺) in the valance band.³⁴ The electron in the conduction band is taken up by the Al atom, thus preventing it from recombination. At the same time, the dyes are also excited to a higher energy state by the promoting of an electron from the HOMO to the LUMO after receiving an appropriate amount of energy.⁹ Some of these dyes are adsorbed while the remaining are degraded by reactive oxygen species (O₂˙⁻, ˙OH) generated in the system during the redox reaction. Probably, the superoxide oxygen and hydroxide free radical species (O₂˙⁻ and ˙OH) are generated from O₂ and H₂O respectively.³⁵ The O₂ is converted to O₂˙⁻ by receiving an electron, available from the conduction band of Al, while ˙OH is generated from H₂O reacting in the positive hole (h⁺) of the Zn valance band. It might be possible that both h⁺ in the valence band and e⁻ in the conduction band move to the surface defects of the catalyst where they produce the O₂˙⁻ and ˙OH, which are the possible species for the mineralization of dyes. The tentative mechanism for dye degradation is shown in Fig. 17.

3.4 Recyclability of the catalyst

The catalytic recyclability of both catalysts were checked under visible light rather than solar light because the intensity of solar light was variable day to day, and even on the same day. Therefore, we carried out the catalytic recyclability tests under visible light of 400 watt. For the recyclability of the catalyst, 100 mg of the respective catalyst were added in 100 mL 0.025 mmol solutions of AO and checked under the influence of visible light (400 watt). After one hour, a 3 mL aliquot was taken and monitored by UV-vis after carefully separating the catalyst. The recovered catalyst was washed thrice with acetone and then quickly used in the next cycle without any physical or chemical treatment. The recovered catalyst was again added in 100 mL 0.025 mmol solution of AO and kept under visible light for 1 hour exposure. The same procedure was repeated four times for both catalysts. During each cycle, the reaction lasted for 1 hour. The Zn–Al/C-LDH indicated 78% removal efficiency in the first cycle and that decreased to 76%, 47% and 23% in second, third and fourth cycles, respectively. However, Zn–Al/C-LDH remained active after the fourth cycle. Similarly, Zn–Cr/C-LDH indicated 59% R. E. in the first cycle and that decreased to 45%, 23% and 15% in second, third and fourth cycles, respectively. The result shows the strongest catalytic performance of both the catalyst for dye removal as well as best catalytic recyclability and durability. The catalytic recyclability of Zn–Al/C-LDH and Zn–Cr/C-LDH are shown in Fig. 18a and b, respectively.

4. Conclusion

This study unveiled the removal of five dyes (AO, CR, MO, MG and CV) under visible, solar, ultraviolet light and dark conditions through the synthesized Zn–Al/C-LDH and Zn–Cr/C-LDH. The Zn–Al/C-LDH and Zn–Cr/C-LDH were grown in the form of LDH and mixed oxides. The Zn–Al/C-LDH and Zn–Cr/C-LDH had band gaps of 2.97 eV and 2.91 eV, respectively, which reflects their better performance in the presence of visible light and solar light. Dyes were removed through adsorption and adsorption-assisted photodegradation. Complete adsorption occurred in dark conditions and adsorption-assisted photodegradation in ultraviolet, visible and solar light conditions. During adsorption-assisted photodegradation, the dyes were first adsorbed onto the surface of the catalyst and then degraded. It was also observed that the light intensity and the substrate-catalyst specificity both required equal consideration during the development of these new catalysts.

Conflict of interest

The authors confirms that the content of this manuscript have conflict of interest.

Acknowledgements

The authors highly acknowledge center of excellence for advance material research king Abdulaziz University for providing research facilities.

References

1. C. Zubieta, M. B. Sierra, M. A. Morini, P. C. Schulz, L. Albertengo and M. S. Rodríguez, Colloid Polym. Sci., 2008, 286, 377–384.
2. M. S. Ahmed, T. Kamal, S. A. Khan, Y. Anwar, M. T. Saeed, A. M. Asiri and S. B. Khan, Curr. Nanosci., 2016, 12 DOI:10.2174/1573413712666160204000517.
3. M. Shao, J. Han, M. Wei, D. G. Evans and X. Duan, Chem. Eng. J., 2011, 168, 519–524.
4. S. A. Khan, S. B. Khan, T. Kamal, M. Yasir and A. M. Asiri, Int. J. Biol. Macromol., 2016, 91, 744–751.
5. H. Yan, H. Li, H. Yang, A. Li and R. Cheng, Chem. Eng. J., 2013, 223, 402–411.
6. Č. Novotný, N. Dias, A. Kapanen, K. Malachová, M. Vándrovcová, M. Itävaara and N. Lima, Chemosphere, 2006, 63, 1436–1442.
7. B. Hameed, A. M. Din and A. Ahmad, J. Hazard. Mater., 2007, 141, 819–825.
8. F. Chen, Z. Liu, Y. Liu, P. Fang and Y. Dai, Chem. Eng. J., 2013, 221, 283–291.
9. M. Rauf and S. S. Ashraf, Chem. Eng. J., 2009, 151, 10–18.
10. A. Khataee and M. B. Kasiri, J. Mol. Catal. A: Chem., 2010, 328, 8–26.
11. M. Derudi, G. Venturini, G. Lombardi, G. Nano and R. Rota, Eur. J. Soil Biol., 2007, 43, 297–303.
12. J. H. Mo, Y. H. Lee, J. Kim, J. Y. Jeong and J. Jegal, Dyes Pigm., 2008, 76, 429–434.
13. S. A. Khan, S. B. Khan, A. M. Asiri and K. Akhtar, Recent Pat. Nanotechnol., 2016, 10, 27136929.
14. K. Parida, N. Baliarsingh, B. S. Patra and J. Das, J. Mol. Catal. A: Chem., 2007, 267, 202–208.
15. L. Mohapatra and K. Parida, Sep. Purif. Technol., 2012, 91, 73–80.
16. N. Baliarsingh, K. Parida and G. Pradhan, Ind. Eng. Chem. Res., 2014, 53, 3834–3841.
17. H. Abdolmohammad-Zadeh and S. Kohansal, J. Braz. Chem. Soc., 2012, 23, 473–481.
18. J. A. Gursky, S. D. Blough, C. Luna, C. Gomez, A. N. Luevano and E. A. Gardner, J. Am. Chem. Soc., 2006, 128, 8376–8377.
19. F. Leroux and J.-P. Besse, Chem. Mater., 2001, 13, 3507–3515.
20. M. Darder, M. López-Blanco, P. Aranda, F. Leroux and E. Ruiz-Hitzky, Chem. Mater., 2005, 17, 1969–1977.
21. E. Gardner, K. M. Huntoon and T. J. Pinnavaia, Adv. Mater., 2001, 13, 1263.
22. M. Veronica, B. Graciela, A. Norma and L. Miguel, Chem. Eng. J., 2008, 138, 602–607.
23. I. Quesada-Penate, C. Julcour-Lebigue, U. J. Jauregui-Haza, A.-M. Wilhelm and H. Delmas, J. Hazard. Mater., 2012, 221, 131–138.
24. S. B. Khan, S. A. Khan and A. M. Asiri, Appl. Surf. Sci., 2016, 370, 445–451.
25. S. B. Khan, A. M. Asiri, K. Akhtar and M. A. Rub, J. Ind. Eng. Chem., 2015, 30, 234–238.
26. R.-C. Zeng, X.-T. Li, Z.-G. Liu, F. Zhang, S.-Q. Li and H.-Z. Cui, Front. Mater. Sci., 2015, 9, 355–365.
27. S. Babakhani, Z. A. Talib, M. Z. Hussein and A. A. A. Ahmed, J. Spectrosc., 2014, 1–10.
28. Y. Feng, D. Li, Y. Wang, D. G. Evans and X. Duan, Polym. Degrad. Stab., 2006, 91, 789–794.
29. K. Vijayalakshmi and D. Sivaraj, RSC Adv., 2015, 5, 68461–68469.
30. H. Zhou, H. Wang, K. Zheng, Z. Gu, Z. Wu and X. Tian, RSC Adv., 2014, 4, 42758–42763.
31. S. Bai, T. Guo, Y. Zhao, R. Luo, D. Li, A. Chen and C. C. Liu, J. Mater. Chem., 2013, 1, 11335–11342.
32. M. Hafeez, A. Ali, S. Manzoor and A. S. Bhatti, Nanoscale, 2014, 6, 14845–14855.
33. V. Eskizeybek, F. Sarı, H. Gülce, A. Gülce and A. Avcı, Appl. Catal., B, 2012, 119, 197–206.
34. U. I. Gaya and A. H. Abdullah, J. Photochem. Photobiol., C, 2008, 9, 1–12.
35. K. Sahel, N. Perol, H. Chermette, C. Bordes, Z. Derriche and C. Guillard, Appl. Catal., B, 2007, 77, 100–109.

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