Efficient decolorization of Methylene Blue catalyzed by MgFe-layered double hydroxides in the presence of hydrogen peroxide

Organic dyes are widely used in modern industries (Martínez-Huitle & Brillas 2009), leading to the discharge of dyeing wastewaters on a large scale. Dyeing wastewaters are characterized by high chemical oxygen demand (COD), deep color, non-biodegradability and toxicity (Natarajan et al. 2011). Traditional methods including adsorption, coagulation, precipitation and so on (Rfn & Fajardo 2017) were occupied for the treatment of dyeing wastewaters, but limited by disadvantages such as low efficiency, secondary pollution, high cost and so on (Gong et al. 2017). Advanced oxidation processes (AOPs) characterized by generation of hydroxyl radicals (·OH) are promising in degradation of refractory contaminants in aqueous solutions or air. Fenton, which consists of the ferrous ion (Fe²⁺) and hydrogen peroxide (H₂O₂), is efficient and environmentally friendly, but only available in narrow pH window. Fe²⁺ involved is hard to separate, and it transforms to iron sludge as a secondary pollutant. So, there is an urgent need to fabricate novel solid catalysts for Fenton processes. Fida et al. 2017 synthesized a La-Fe montmorillonite (La-Fe MMT) composite and applied it for the degradation of organic dyes. The as-prepared La-Fe MMT is of high stability and can be recycled. Li et al. (Zhu et al. 2016; Li et al. 2017) fabricated Fe₃O₄/C/Cu as a catalyst for the degradation of Methylene Blue (MB). Fe₃O₄/C/Cu was reported to be of good stability and can be separated easily for recycling. These catalysts are of high cost and hard to prepare. Layered double hydroxides (LDHs) as typical lamellar materials have drawn a lot of attention in recent years. LDHs are usually formulated as [M(II)_1−xM(III)_x(OH)₂]^x+[A_x/nⁿ⁻]^x−·mH₂O, in which M(II) and M(III) are divalent and trivalent cations (Shao et al. 2013), and Aⁿ⁻ is the interlayered anion. LDHs with large surface area and porous structure have already been used as adsorbent and catalyst in photo-assisted Fenton systems (Tang & Liu 2016; Gong et al. 2017). Iron-based LDHs with easy preparation methods and low costs have drawn increasing attention, and are actively considered for the catalysis of the Fenton process. In our previous study (Wang et al. 2014), FeFe-LDH was prepared and used as a catalyst for the degradation of MB, but FeFe-LDH was hard to prepare under an air atmosphere. The samples obtained were of low purity for the oxidation of ferrous in the preparation process under an air atmosphere. In this study, Mg²⁺ was occupied to complex with Fe³⁺ to fabricate MgFe-LDH for its stability in air. MgFe-LDHs synthesized by co-precipitation method were used as the catalyst to establish a heterogeneous Fenton system. The morphology and chemical properties of MgFe-LDHs are investigated. Influences of parameters including initial solution pH, concentration of H₂O₂, dosage of catalyst and temperature on the degradation of MB were investigated. Active species were identified to propose the reaction mechanism. The stability and reusability of MgFe-LDHs were also investigated.

All chemicals bought from Jingchun Chemical Regent Co. were analytical grade. All solutions were freshly prepared with ultrapure water from Milli-Q device (18.2 MΩ·cm at 25 °C).

The MgFe-LDH materials were synthesized by co-precipitation method. At first, mixed solutions A (NaOH and Na₂CO₃) and B (Fe(NO₃)₃·9H₂O and Mg(NO₃)₃6H₂O) were slowly dropped into a glass reactor containing 100 mL de-ionized water with stirring at 260 r/min at room temperature. The LDH was denoted as LDHx as the ratio of [Mg²⁺]/[Fe³⁺] varied in the range of 2:1–6:1, with concentrations of NaOH ([OH⁻] = 1.6([Mg²⁺] + [Fe³⁺])) and Na₂CO₃ ([CO₃²⁻] = 2[Fe³⁺]) pre-determined (Table S1). The obtained suspensions were aged at 80 °C for 3 h, and then centrifuged and washed several times with deionized water until neutral. Finally, solid materials obtained were dried at 80 °C to get the final product, MgFe-LDHs, which were kept sealed after grinding.

The catalytic oxidation experiments were conducted in a 500 mL glass reactor with constant stirring at room temperature (20 ± 2 °C). Initial solution pH was adjusted by NaOH (0.5 mol/L) and H₂SO₄ (1 mol/L), H₂O₂ were added into 200 mL 10 mg/L MB solutions containing given amounts of MgFe-LDHs to start the reaction. Samples were withdrawn at predetermined intervals and filtered immediately for analysis. The used catalysts were recovered and washed for the next cycling use.

The crystalline phases of MgFe-LDHs were identified by X-ray diffraction (XRD) patterns on a XRD-6000 diffractometer (Shimadzu, Japan) with Cu-Kα radiation (40 kV, 30 mA) at a scanning speed of 4°/min in the range of 4–70°. Fourier transform infrared spectroscopy (FTIR) was recorded in the range of 500–4,000 cm⁻¹ using a Nexus spectrometer (Thermo Nicolet, America). Thermo-stability of as-prepared materials was detected by a TGA/DSC1 thermogravimetric analyzer (Mettler Toledo, Switzerland) from 30 to 600 °C at a heating rate of 15 °C/min under air atmosphere. The scanning electron microscopy (SEM) images were recorded by field emission scanning electron microscopy (Rigaku S4800). X-band ESR spectra were recorded by a Bruker EMX-10/12 microspectrometer at 90 K. The operating frequency and power are 9.43 GHz and 19.83 mW, respectively.

Concentration of MB in solution was determined at 660 nm using a UV–vis spectrophotometer (UV755B, Youke, Shanghai). Concentration of H₂O₂ was measured using the potassium titanyl oxalate method at 410 nm (Chen et al. 2012). The apparent generation efficiency of ·OH was detected by Fe(phen)₃²⁺ method at 508 nm (Zhou et al. 2018a, 2018b), and concentrations of ferrous ion and total iron ions were determined by 1,10-phenanthroline method at 510 nm (Zhou et al. 2012).

Figure 1(a) shows XRD patterns of the as-prepared MgFe-LDH materials. All LDHx materials have a crystalline structure showing prominent and sharp diffraction peaks centered at 2θ = 11.46°, 23.12° and 34.12°, corresponding to the reflections of (003), (006), (009). Only LDH2 and LDH3 show clear symmetrical diffraction peaks at 2θ = 58° to 63° (Wang et al. 2015), indicating the single phase of Rhombohedral structure (Thomas & Kamath 2006). The peak at 2θ = 52.43 presented in the pattern of LDH2 was determined to be Fe(OH)₃ (JCPDS#46-1436), and LDH3 is in good agreement with standard Pyroaurite diffraction peak (JCPDS#70-2150, Mg₆Fe₂(OH)₁₆CO₃· (H₂O)_4.5)_0.375). Lattice parameters a and d(110) reflect laminate and (110) crystal atoms density, and all samples were analyzed similarly, as well as d(003). The influences of the ratio of [Mg²⁺]/[Fe³⁺] on the density of crystal atoms are not obvious (Thomas & Kamath 2006).

FT-IR spectra of LDHx were recorded in the range of 4,000–400 cm⁻¹ as shown in Figure 1(b). The absorption peak at around 3,500 cm⁻¹ is attributed to the stretching vibration of hydroxyl groups in brucite-like layers (Wei et al. 2011; Gong et al. 2017). The absorption peak at about 1,600 cm⁻¹ can be designated to the hydroxyl bending vibration of crystallization water interlayered (Yang et al. 2015; Gong et al. 2017). The characteristic absorption peak at about 1,360 cm⁻¹ is attributed to the vibration of CO₃²⁻ (Yang et al. 2015; Gong et al. 2017). The group bands characteristic for LDHs in the range of 500–1,000 cm⁻¹ are interpreted as the vibration of metal-oxygen (M-O) or metal-hydroxyl (M-OH) in the host layer. Note that absorption peaks of LDHx shifted to high wavenumber as x increased from 2 to 6, indicated that the bonding interaction between interlayered water molecules and CO₃²⁻ or hydroxyl are weakened (Wei et al. 2011). Characteristic bands in the range of 500–1,000 cm⁻¹ in LDH4 and LDH6 were fuzzy.

Thermogravimetric analyses were performed to reveal the thermostability of LDHx. As shown in Figure S1(a) and S1(b) (Supporting Information), two major endothermic peaks and the corresponding weight loss stages can be observed in LDH2 and LDH3 materials. Approximately 12.56% and 14.55% weight-losses ascribed to crystal water were observed at around 193 °C respectively. Weak endothermic peaks at around 99 °C were due to the existence of xFe(OH)₃·yFe₂(CO₃)₃. Weight losses ascribed to dehydroxylation at around 365 °C were calculated to be 14.2% and 18.4% (Bai et al. 2017). The endothermic peaks at around 60 °C in LDH4 and LDH6 (Figure S1(c) and S1(d)) may be caused by the thermal decomposition of Magnesium carbonate basic (Chaturvedi et al. 2019; Erlund & Zevenhoven 2018), and endothermic peaks occurred at higher temperatures as x increased. This was in accordance with XRD analysis, indicating that MgFe-LDH 3 was of high purity.

As LDH 3 was verified of high purity, the morphologies of fresh LDH 3 and that after Fenton reaction were examined by SEM (Figure S2). The SEM image of fresh LDH 3 shows that as-prepared samples present a rock-like morphology in much larger scale with none uniform size. LDH 3 that after Fenton reaction was much smaller, its diameter was about one-tenth of fresh LDH 3.

Catalysis capacities of LDHx were compared at pH 1 (Figure 2(a)); the results indicated that almost 100% removal of MB can be obtained in Fenton-like systems catalyzed by LDHx. MB was removed faster when catalyzed by LDH 2 and LDH 3, but LDH 3 was more favored for its high purity. To clarify the active radicals involved in Fenton-like system catalyzed by LDH 3, ESR with 5, 5-dimethyl-1-pyrroline N-oxide (DMPO) was acquired to probe the active free radicals generated on the surface of LDH 3 (Figure 2(b)). Four characteristic peaks with an intensity ratio of 1:2:2:1 were observed, indicating that ·OH can be generated and plays an important role in Fenton system catalyzed by LDH3 (Liu et al. 2016).

The generation of ·OH and the degradation of target pollutants in Fenton reactions can be influenced by initial solution pH, dosages of H₂O₂ and catalyst, reaction temperature and so on; thus, influences of the factors mentioned above in an LDH-catalyzed heterogeneous Fenton system were investigated.

The degradation of MB in heterogeneous Fenton-like reaction catalyzed by LDH3 was performed in pH ranging from 1 to 4 and the apparent generation of ·OH is shown in Figure 3(a) and 3(b). Degradation efficiency of MB increased immediately as the reaction initiated, without any retention time. The degradation of MB was more favored under strong acidic condition, the degradation efficiency of MB decreased as pH increased, and 97.07% degradation efficiency can be obtained in 15 min at pH 1. This result was not consistent with that optimal pH window of 2–4 as reported, for surface alkalinity of LDHs. Small declines of degradation efficiencies can be seen at around 5–10 min at pH 2–4, and this phenomenon maybe due to the influence of LDH particle suspensions and dissolved iron in solution (Chen et al. 2012). Higher concentrations of ferrous ion and total iron were detected as initial solution pH decreased (Figure 3(c)). Variation of ·OH under the same conditions (Figure 3(b)) indicated that the generation of ·OH is favored under acidic conditions and ·OH performed with excellent oxidation potential (2.80 eV) under strong acidic conditions. But H₂O₂ was reported to be stable under strong acidic conditions (pH < 2) and reacted with H⁺ to form H₃O₂⁺ (Daud & Hameed 2010), and ferrous ion in solution may transformed to [Fe(H₂O)₆]²⁺and [Fe(H₂O)₆]³⁺ at pH 1 (Bahmani et al. 2013), but precipitated as Fe(OH)₂ and Fe(OH)₃ under conditions of pH ≥3 (Bahmani et al. 2013; Ersöz 2014), thus the apparent generation of ·OH was detected at higher at pH 2.

Additionally, the degradation of MB and generation of ·OH were found to be temperature-dependent and higher efficiencies were performed at higher temperatures (Figure 6(a)), for the generation rate of ·OH is accelerated (Dutta et al. 2001; Elshafei et al. 2010). The tendency of the degradation of MB is in good agreement with the production of ·OH (Figure 6(b)), inferring that ·OH played a predominant role in the MB removal process.

Four recycling uses of LDH3 were performed (Figure 8(a)) and the XRD pattern of LDH after four cycles of use are shown in Figure 8(b) to investigate its stability. LDH exhibited good catalytic activity in the fourth cycle and 88.93% degradation of MB was obtained. The XRD pattern of LDH after four times use still followed JCPDS#70-2150, without impurities, indicating excellent stability of LDH.

In this study, LDHx were prepared using the co-precipitation method with the ratio of [Mg²⁺]/[Fe³⁺] varied in the range of 2:1–6:1. LDHx as-prepared were characterized with XRD, FTIR, TGA/DSC methods, and occupied as catalysts to establish heterogeneous Fenton systems for the degradation of MB. The results indicated that LDH3 is of high purity, and more than 97% degradation of MB can be obtained in 20 min under optimal condition. ·OH was catalytically generated, and a reaction mechanism was proposed. LDH3 was still of excellent catalytic activity and good stability after four cycling use.

This work was financially supported by the national nature science foundation of China (No. 51608412), Young Talent fund of University Association for Science and Technology in Shaanxi, China (No. 20190607), the Natural Science Basic Research Plan in Shaanxi Province of China (No. 2019JQ-428), the Special Fund Project of Education Department in Shaanxi Province of China (No. 19JK0409) and Chongqing Key Laboratory of Environmental Materials and Remediation Technology (Chongqing University of Arts and Sciences) (No. CEK1804).

The Supplementary Material for this paper is available online at https://dx.doi.org/10.2166/wst.2020.161.