Removal of methylene blue from aqueous solution using Mg-Fe, Zn-Fe, Mn-Fe layered double hydroxide

With rapid economic development, the water environment is facing a more and more serious situation, the water pollution problems have become one of the most difficult environmental problems (Yang et al. 2019). One of the important reasons for water pollution is the discharge of dyeing wastewater, which will pollute groundwater sources, and thus cause serious harm to human health (Pan et al. 2019; Liang et al. 2020). Dyes in wastewater are normally the result of inefficient dyeing processes, which cause as much as 10–15% unused dyestuff entering the watercourse directly (Selvam et al. 2003). Dye effluent normally contains about 10–50 mg/L (Zollinger 1989; Lazaridis et al. 2003), and thus may be perceived as being contaminated and unacceptable. Methylene blue (MB) is a representative phenothiazine type cationic dye, which is usually used in the dyeing industry (Li et al. 2018). MB is toxic to humans and animals, especially aquatic organisms (Thi et al. 2020), and needs to be effectively removed from dyeing wastewater. This poses a challenge to remove MB methods from dyehouse effluent.

Several methods have been employed to deal with dyeing wastewater, such as coagulation (Khorram & Fallah 2018), chemical precipitation (Hua et al. 2014), ozonation (Hu et al. 2016), membrane filtration (Rashidi et al. 2014) and reverse osmosis (Lee et al. 2020). Besides, the adsorption technique is regarded as the most effective technique for MB removal from dyeing wastewater as a result of its simplicity in design, relatively low cost and high efficiency (Altintig et al. 2017; Islam et al. 2017). The use of the synthetic anionic clays such as hydrotalcite (HT) or hydrotalcite-like compounds (HTlc) or layered double hydroxide (LDH) for adsorptive removal of dyes present in aqueous effluents has attracted great attention recently (Zhou et al. 2011; Lan et al. 2014; Chowdhury & Bhattacharyya 2015). In recent years, there have been significant amounts of work on the application of LDH to remove MB (Chuang et al. 2008; Wang et al. 2013; Kalalie et al. 2016). For example, Zhao et al. (2017) have reported Mg-Al-LDH, which was synthesized by using a hydrothermal method and acted as a sorbent for the removal of MB from aqueous solution; the adsorption capacity of Mg-Al-LDH at 298.15 K could reach up to about 20 mg/g. Bi & his co-workers (2011) developed heteropoly blue-intercalated layered double hydroxide (HB-LDH) by the ion-exchange method as an adsorbent for removal of MB and the maximum adsorption capacity of MB was 30.87 mg/g. Xia et al. (2014) observed the removal efficiency for MB by three Ti-based layered double hydroxides under visible light, which followed the order: CeO₂/Zn-Ti LDHs > Zn/Al-Ti/SB LDHs > Zn/Ti LDHs. In this study, the M-Fe LDH was selected as the material of adsorbent to remove MB. This reason was that M-Fe LDH had layered structure, a large specific surface area, strong ion exchange performance and low cost.

In this paper, the double titration co-precipitation method was used to synthesize the Mg-Fe LDH, Zn-Fe LDH, and Mn-Fe LDH, that method provided better crystallinity and better control of the particle size (Yu et al. 2015; Bukhtiyarova 2019). The microscopic morphological characteristics and structural information of M(Mg, Zn, Mn)-Fe LDH were investigated by using the BET method, FT-IR, SEM, and XRD technology. Effect of M (Mg, Zn, Mn) : Fe molar ratio, pH, and M-Fe LDH dosage on the removal of MB were studied. Moreover, adsorption kinetics, adsorption thermodynamics, and adsorption isotherms were also conducted to fit the theory of M-Fe LDH for removal of MB.

All the chemicals used in the study are of analytical grade. All the solutions in the study were prepared using de-ionized water.

The preparation of the adsorbents was carried out by the method of co-precipitation. The 0.675 mol of NaOH and 0.225 mol of Na₂CO₃ were dissolved in 250 mL of de-ionized water to make a base solution. Then 0.045 mol of MgCl₂ (ZnCl₂ or MnCl₂) and 0.015 mol of FeCl₃·4H₂O are added into 100 mL of de-ionized water to make the metal salt solution. The base solution and metal salt solution were added dropwise to 10 mL of de-ionized water, and the pH of this process maintained at 8–10. The filtrate was filtered and washed with excess water, then dried at 353.15 K for 24 h. This sample was denoted as Zn-Fe LDH, and Mg-Fe LDH or Mn-Fe LDH.

In order to investigate the effect of M (Mg, Zn, Mn) : Fe molar ratio on the removal of MB, a series of parallel experiments were carried out at different M (Mg, Zn, Mn) : Fe molar ratios (1:1, 2:1, 3:1, 4:1, and 5:1) for the removal of 10 mg/L MB (25 mL) at 298.15 K for 30 min. To investigate the effect of M-Fe LDH dosage on the removal of MB, a series of parallel experiments was carried out at different M-Fe LDH dosage (0.4, 0.8, 1.2, 1.6, 2.0, 2.4 g/L) for the removed of 10 mg/L MB (25 mL) at 298.15 K for 30 min.

The effect of pH on MB removal was also studied on the 10 mg/L of MB solutions (pH 6.3) at the pH range from 2 to 12 with 2.0 g/L g of adsorbent at 298.15 K for 30 min. The pH values were determined by using a pH-meter and adjusted 0.1 mol/L HCl and 0.1 mol/L NaOH solutions.

The adsorption kinetics experiments were carried out by using 2.0 g/L of M-Fe LDH for removal of 10 mg/L MB (25 mL) at 298.15 K for different time (2, 5, 10, 15, 20, 30, 45, 60, 90, 120 min). The adsorption isotherms experiments were executed by using 2.0 g/L of M-Fe LDH for removal of 25 mL of different concentrations of MB (6, 8, 10, 15, 20, 25, 40, 60, 80, 100 mg/L) at 298.15 K, 308.15 K and 318.15 K, respectively. All experiments were done in duplicate and the average value was used in the evaluation. A blank sample containing no dye and another containing dye solution only were run together with every batch of the sample run.

To conduct the adsorption experiments, a certain amount of M-Fe LDH was added to a 25 mL of MB (10 mg/L) solution at a given pH. The mixture was stirred at a given temperature for 30 min. Then the samples were centrifuged at 7,000 r/min for 30 min. The supernatant was sampled and diluted in the linear range. The sample was analyzed by using an ultraviolet and visible spectrophotometer (TU 1810 pc, Beijing Purkinje General Instrument Co.) at 665 nm (λ_max of MB) to determine the residual MB concentration. The pH of solution was measured with a pH meter using a combined glass electrode.

The Fourier transform infrared spectroscopy (FT-IR) of the samples were collected on NICOLET 6700 equipment and scanned at 0.05 cm⁻¹ resolution between 400 and 4,000 cm⁻¹. The X-ray diffraction (XRD) patterns were obtained on the Bruker AXS D8 Advance with CuKα radiation (λ = 1.5418 Å). The surface morphology of the adsorbent was observed by used a scanning electron microscope (SEM) on a JSM-6360LV. The Brunauer-Emmett-Teller (BET) surface area, the total pore volume, and pore diameters were analyzed by used a micromeritics SSA-6000 volumetric adsorption analyzer.

The effect of M (Mg, Zn, Mn) : Fe molar ratio on the MB removal efficiency at pH 6.3 is shown in Figure 1. The removal efficiency of MB increased with increased M (Mg, Zn, Mn) : Fe molar ratio from 1:1 to 3:1, but decreased with increased M (Mg, Zn, Mn) : Fe molar ratio from 3:1 to 5:1. When the M/Fe molar ratio is less than 3, the adsorption of MB on M-Fe LDH is limited to the surface electrostatic interaction (Chitrakar et al. 2011). Chitrakar et al. (2008) proposed that stronger interactions between the charged layers exist in LDH with high charge density (2:1). These interactions reduce the ability of the LDH to expand and accommodate large anions in the interlayer region. In the case where the M/Fe molar ratio is 3, the adsorption of MB on M-Fe LDH includes the electrostatic adsorption on the outer layer cation and the exchange adsorption of the elastic interlayer anions. This is related to the fact that M-Fe LDH (M/Al = 3) allows the formation of the best crystalline LDH phase (Goh et al. 2008) for uptake of additional guest species within its interlayers, thus resulting in the highest affinity for MB (Li & Duan 2005). M-Fe LDH synthesized at the M/Fe molar ratio rise 4 and 5 may contain certain amounts of amorphous metal hydroxides, which could potentially results in underestimating the sorption capabilities of M-Fe LDH (Wang & Gao 2006). When the M (Mg, Zn, Mn)/Fe molar ratio was 3:1, the removal efficiency of MB was 82% for Mg-Fe LDH, 58% for Zn-Fe LDH, and 70% for Mn-Fe LDH, respectively. Thus, the optimum molar ratio of M/Fe was 3:1 to wipe off MB.

The microstructure characteristics of the M-Fe LDH at the M (Mg, Zn, Mn)/Fe molar ratio of 3:1 were then evaluated via XRD, SEM, FT-IR, and BET method. Figure 2 shows that the microscopic morphological characteristics of M-Fe LDH adsorbent confirmed by SEM. M-Fe LDH adsorbent was spherical. The particle size of Mg-Fe LDH was bigger than that of Zn-Fe LDH and Mn-Fe LDH. At the same time, the surface of Mg-Fe LDH (Figure 2(a)) was floc distribution and rough, and the pores were staggered, which were favorable for adsorption. However, the surface of Mn-Fe LDH was smooth, which resulted in the smaller specific surface area of the adsorbent. N₂ adsorption-desorption technology was further employed to investigate that the specific surface areas of Mg-Fe LDH, Zn-Fe LDH and Mn-Fe LDH were 104.86, 20.29, 50.49 m²/g, respectively. N₂ adsorption-desorption results indicated Mg-Fe LDH with the greatest specific surface area. Then SEM results show that the surface of Mg-Fe LDH is flocculent and rough, which can provide a large specific surface area. These results indicate that Mg-Fe LDH will have a greater adsorption capacity for MB.

The FT-IR spectrum and XRD of Mg-Fe LDH, Zn-Fe LDH and Mn-Fe LDH are shown in Figure 3. The results show in the FT-IR spectrum that a broad peak around 3,450 cm⁻¹ was associated with stretching vibration of hydrogen-bonded hydroxyl groups from both the hydroxyl group of the layers and interlayer water (Fernandez et al. 1998). Sharp peaks around 1,400 cm⁻¹ and 1,600 cm⁻¹ represented symmetric and asymmetric stretching absorption of the C = O group. Those were typical characteristics for layered double hydroxide materials (Cavani et al. 1991). The peaks below 1,000 cm⁻¹ correspond to the metal hydrogen bond, and the metal-oxygen stretching mode was represented by peaks between 500 and 800 cm⁻¹. XRD results showed the crystallography and phase information on M-Fe LDH adsorbent. The XRD patterns showed the Mn-Fe LDH was more crystalline than Mg-Fe LDH and Zn-Fe LDH synthesized under the same conditions.

Figure 4(a) shows the influence of M-Fe LDH (M:Fe = 3:1) dosage on the removal efficiency of 10 mg/L of MB. These results indicated the removal efficiency of MB increased with an increased M-Fe LDH dosage from 0.4 g/L to 2.0 g/L. The reason is that the number of active sites on the surface of the adsorbent increases as the adsorbent dosage increases, which causes more MB to be absorbed on the adsorbent, resulting in the removal efficiency increases. However, the removal efficiency of MB at M-Fe LDH dosages between 2.0 g/L and 2.8 g/L maintained the constant of 57% for Zn-Fe LDH, 70% for Mn-Fe LDH, and 81% for Mg-Fe LDH, respectively. At this time, the increasing dosage of absorbents increases the collision rate between particles, and particles aggregate to become the larger particle resulted in decrease of the specific surface area. The change trend between the removal efficiency of MB and the adsorbent dosage is consistent with Abdelkader et al.'s research. Abdelkader et al. (2011) researched the Orange G amount adsorbed by Mg-Fe LDH increased with increasing adsorbent dose until 1.4 g/L, from this dose the quantities of dye adsorbed on this materials remained unchanged. Hence, the optimum adsorbent dosage of M-Fe LDH on the removal of MB was determined at 2.0 g/L, which were used as the dosage of the following experiments.

The effect of initial pH on the removal efficiency of MB (10 mg/L) was investigated by using 2.0 g/L M-Fe LDH at 298.15 K for 30 min. The results are shown in Figure 4(b) that the maximum efficiency of MB was observed at pH 6.0, which was close to the natural pH of MB solution (6.3). In the solution with lower pH (pH <6.0), H⁺ reacted with the metal hydroxyl compound to make the surface positively charged, H⁺ and the positively charged MB produced competitive adsorption, so the removal rate was low. OH^- and positively charged MB produce a dark purple precipitate in higher pH (pH > 6.0). Thus, all subsequent experiments on the effect of adsorption variables on MB removal was carried out at pH 6.0.

where k is the adsorption rate constant (g·mg⁻¹·min⁻¹), t (min) is time, q_t (mg/g) and q_e (mg/g) are the adsorption capacities at t (min) and at equilibrium.

In order to further study the effect of temperature on the adsorptive capacity of MB onto M-Fe LDH, the adsorption of 10 mg/L MB by using M-Fe LDH adsorbent was carried out at 298.15 K, 308.15 K, and 318.15 K (Figure 6), respectively. The effect of temperature on the pseudo-second-order constants is shown in Figure 6. It is seen from Figure 6 that the pseudo-second-order constant increased with increasing temperature for the same adsorbent. The reason was that the collision probability of the activated sites on the surface of the adsorbent particles increased with increasing temperature resulting in promotion of the adsorption reaction (Salleh et al. 2011).

Figure 8 shows the adsorption isotherms of M-Fe LDH adsorbent for MB removal at different concentrations and temperatures. It was shown that the adsorption capacity of M-Fe LDH adsorbent for MB decreased with the increase of temperature, which indicated that the reaction was exothermic. The MB equilibrium adsorption increased with an increase in initial MB concentration at the same temperature. Table 3 shows the parameters of the Langmuir and Freundlich isotherm model. The correlation coefficient of the Langmuir model was higher than the correlation coefficient of the Freundlich model fitting, which indicated the Langmuir adsorption isotherm equation was more suitable to describe the process of the adsorption on MB by using M-Fe LDH adsorbent. The Langmuir adsorption isotherm model assumes that the adsorbent surface is uniform, the adsorption process is monolayer adsorption. The magnitude of activation energy can indicate whether the adsorption mechanism is physical adsorption (the activation energy of 5–40 kJ·mol⁻¹) or chemical adsorption (the activation energy of 40–800 kJ·mol⁻¹) (Konicki et al. 2012). The activation energy value of 73.5 kJ·mol⁻¹ indicates that the adsorption of MB onto Mg-Fe LDH is mainly a chemical adsorption process. The value of 29.8 and 39.5 kJ·mol⁻¹ for the activation energy indicates that the adsorption of MB onto Zn-Fe LDH and Mn-Fe LDH is mainly a physical adsorption process. The comparison of q_m for MB of Mg-Fe LDH, Zn-Fe LDH, and Mn-Fe LDH adsorbents found that Mg-Fe LDH had a remarkably high adsorption capacity of 71.94 mg/g at 298.15 K.

In this paper, Mg-Fe, Zn-Fe, and Mn-Fe layered double hydroxide were prepared by double titration co-precipitation method for the removal of MB. The optimal molar ratio of M (Mg, Zn, Mn) : Fe is 3:1 for the removal of MB. The adsorption results exhibited that 2.0 g/L of M-Fe LDH for the removal efficiency of MB (10 mg/L) was best at pH 6.0. The adsorption data of M-Fe LDH samples were well fitted by the Langmuir adsorption isotherm and, moreover, its maximum adsorption capacity of Mg-Fe LDH was 71.94 mg/g at 298.15 K. Besides, the adsorption kinetic study showed that the adsorption of MB onto M-Fe LDH followed the pseudo-second-order kinetic model. Adsorption thermodynamics results showed Zn-Fe LDH, with the smallest activation energy, resulting in its adsorption of MB being the fastest. Then, Mg-Fe LDH could be a cost-effective and promising material for MB removal from the dyeing wastewater.

This project was supported by the National Key Research and Development Program of China (2017YFD0800301) and the Liaoning Province Natural Science Foundation of China (2020-MS-232).

All relevant data are included in the paper or its Supplementary Information.