Water treatment with CoZnAl-LDH and its mixed metal oxide

Many studies have been done into the potential use of layered double hydroxides (LDHs) in wastewater treatment. LDHs application is increasing day by day due to their special properties, including large specific surface, the ability to remove organic and inorganic species, easy and inexpensive synthesis, and the possibility of producing diverse LDH species with different metals and interlayer anions for different applications (Mostafa et al. 2016).

Such structures enable the synthesis of LDHs with different metals and interlayer anions suitable for diverse applications as catalysts (Xu et al. 2011), catalyst supports (Kovanda & Jirátová 2011), adsorbents (Rathee et al. 2020), scavengers (Kong et al. 2019), electrocatalysts (Speed 2016) and photocatalysts (Li et al. 2020).

Adsorption has been used to remove various pollutants, especially heavy metals and synthetic dyes, because of properties such as low cost, lack of toxic byproducts, simple design for large-scale use and high removal efficiency (Patra et al. 2016). Materials including zeolites, carbon compounds, resins, biomaterials, and polymers are used to remove dyes. LDHs are also considered as adsorbents, today, because of their special properties.

LDHs remove solutes from water via two mechanisms – adsorption onto their positively charged layers and, alternatively, by anion exchange and intercalation (Ahmed & Mohamed 2017).

Photocatalysis is an advanced oxidation process that works on the basis of producing hydroxyl radicals (^.OH) with high oxidation potential (⁠⁠), which can oxidize a wide range of organic compounds. In recent decades, TiO₂ and ZnO have received attention as non-toxic, stable, low-cost and efficient photocatalysts. Due to their large energy band gaps, they are only active under ultraviolet light (Yu et al. 2016), which limits their widespread use due to high energy consumption.

LDHs, and the mixed metal oxides (MMOs) obtained by their calcination and active as photocatalysts in visible light, have been studied in dye removal, with reports of them acting as adsorbents and photocatalysts simultaneously. Morimoto et al. (2011) surveyed anionic dye removal by ZnAl-LDH and MgAl-LDH, via adsorption and photodegradation mechanisms. LDH composites have also been introduced as adsorbents and photocatalysts (Yu et al. 2016; Li et al. 2017).

In this work, CoZnAl-LDH was prepared by easy and inexpensive co-precipitation, and its adsorption capacity for acid orange 7 (AO7) removal investigated. The adsorption and photocatalytic properties of CoZnAl-MMO, obtained by thermal treatment of the LDH, were also investigated.

ZnCl₂.6H₂O, CoCl₂.6H₂O, and AlCl₃.6H₂O were purchased from the Merck Company. AO7 was obtained from the local market – its characteristics are presented in Table 1. Solution pH values were adjusted using 0.1 M solutions of HCl or NaOH, as appropriate. All reagents were used without further treatment and distilled water was used both in synthesis and to wash the precipitates.

The CoZnAl-LDH was synthesized by co-precipitation. The appropriate amounts of CoCl₂.6H₂O, ZnCl₂.6H₂O, and AlCl₃ (on the basis that Co:Zn:Al should be 1:1:1) were dissolved in 40 ml of distilled water subject to constant stirring with a magnetic stirrer. The correct proportion of a 3 M solution of NaOH was added to bring the pH to 10 and the solution was then maintained at 60 for 24 hours. The precipitate was separated by centrifugation, washed twice and then dried at 40 °C. The MMO was obtained by calcining the LDH at 300 °C for 3 hours.

XRD patterns of the as-prepared samples were obtained using a Bruker AXS (Germany) model D8 advanced diffractometer for CuK_α radiation (⁠ = 1.54 ⁠). Sample morphology was studied by field emission scanning electron microscopy (FESEM) (ULTRA55, Carl Zeiss MST AG, Germany). The Brunauer Emmett Teller (BET) surface area measurements were done using a Quantachrome chemBET device. The samples' UV–vis absorbance was recorded using a T80⁺ UV/VIS Spectrometer (PG Instruments Ltd, UK), and that of the solutions, against water at 484 nm, with a Perkin-Elmer 550 SE spectrophotometer.

The point of zero charge (PZC) of the samples was determined in 0.1 M NaNO₃ solution at room temperature. 20 ml aliquots of the NaNO₃ solution were placed in five titration flasks and their pHs adjusted to 4 to 9 using 0.1 M HNO₃ and NaOH solutions using a pH meter (Eutech pH 510, Malaysia). 0.1 g sample aliquots were poured into each flask and shaken for 24 hours in a shaker-incubator, after which the pH was measured. The difference between the initial and final solution pH (ΔpH) was plotted against the initial pH (pH_i) values. The PZC was estimated from the plot as the pH at which ΔpH was equal to zero (Mahdizadeh & Aber 2015).

Batch adsorption experiments were done in the dark and at room temperature. In order to identify the adsorption isotherm of the process, 10 mg of CoZnAl-LDH or CoZnAl-MMO were agitated in 50 mL AO7 solution for 24 hours with dye concentrations of 40 to 100 mg/L for LDH experiments and 80 to 120 mg/L for MMO.

The adsorption kinetic model of the samples, as synthesized, was determined by putting 10 mg of adsorbent into 50 mL AO7 solution with dye concentrations of 30 to 80 mg/L for LDH experiments and 80 to 120 mg/L for MMO, and taking samples at regular intervals. Solution absorbance was measured by UV-vis spectrophotometer at 484 nm, which is the maximum absorbance wavelength for AO7. An AO7 calibration diagram was used to calculate dye concentration.

To consider the photodegradation efficiency of the MMO, as-prepared, AO7 was degraded in a photoreactor equipped with a 45 W visible-light lamp (CCP, Iran). The contributions of the adsorption and photodegradation mechanisms were determined by conducting the experiments simultaneously. The difference between the diagrams in the dark and under light was taken as being the photodegradation removal. All tests were performed twice and, for all points, error bars were drawn based on the standard deviation.

The XRD analyses of CoZnAl-LDH and CoZnAl-MMO proved their successful synthesis, and the characteristic reflections of the LDH structure are visible (Figure 1). The peaks at could be indexed as (003), (006), (110) and (113), which are typical reflections of the hydrotalcite-like structure (JCPDS No 15-0.087) (Xu et al. 2014). In forming CoZnAl-MMO, the layered structure of CoZnAl-LDH is destroyed so the related peaks disappear. Most peaks in the MMO XRD pattern are related to ZnO (JCPDS card No 36-1451). Because of the low calcination temperature (300 °C) of CoZnAl-LDH, it is not possible to form oxides of cobalt or aluminum (Klemkaite et al. 2011). Increasing the LDH calcination temperature leads to increased MMO particle size, which reduces its adsorption capacity and photocatalytic activity.

The morphology of the LDH and MMO was characterized by SEM coupled with energy-dispersive X-ray spectroscopy (EDS) analysis. As can be seen in Figure 2, the synthesized CoZnAl-LDH and CoZnAl-MMO consist of uniform spherical particles, with average diameters of 10 to 20 and 20 to 30 nm, respectively.

The EDS plot for the LDH is shown in Figure 2(b). The X-ray energies related to Co, Zn, Al and O are distinctive, providing further evidence for successful CoZnAl-LDH synthesis. Figure 3(a) and 3(b) show the samples' nitrogen adsorption-desorption isotherms. Both isotherms belong to type IV with an H₃ hysteresis loop, showing that they have a layered structure, which is a characteristic of LDHs and LDH-derived MMOs. On the other hand, both samples contain mesopores, as a characteristic of type IV isotherm (Thommes et al. 2015).

Table 2 shows the specific surface, average pore size and total pore volume of the samples. The high specific surface of the samples with the positive surface charge leads to the good adsorption of the anionic dye by these materials.

Figure 4(b) shows the relationship between and ⁠. The band gap energy of CoZnAl-MMO, determined by extrapolating the linear part of the against ⁠, was about 2.2 and 3.4 eV. The UV-vis spectrum of CoZnAl-LDH proved that it is not active in the visible range.

AO7 was used as the model pollutant to study the samples' adsorption and photocatalytic properties. The experiments were done over a pH range of 4.5 to 8. Figures 5 and 6 show the effects of initial pH on the proportional removal of AO7 over time. The AO7 removal efficiency by LDH declined with rising pH, showing that AO7 is adsorbed easily on the LDH surface in neutral or acid conditions.

Figure 7 shows that the pH_pzc of LDH and MMO is 5.6 and 5.8, respectively. At pH < ⁠, the surface charge is positive, causing electrostatic attraction between anionic AO7 and the CoZnAl-LDH surface (Wang & Wang 2008). But at pH > ⁠, good adsorption capacity was observed for LDH, while experiments indicated that, contrary to expectation, MMO's proportional removal of AO7 increased with increasing pH. There should, therefore, be another adsorption mechanism at pHs exceeding ⁠. It is possible that AO7's –SO₃⁻ group assisted the creation of a hydrogen bond between adsorbent and dye. At higher pHs, the hydroxyl groups' concentration is higher on the surface, and the possibility of hydrogen bond formation increases. Li et al. (2017) discussed hydrogen bond formation between MoS₂/CoAl-LDH/HCF and Congo red in this regard.

For further verification of the adsorption mechanism, the XRD analyses of the LDHs before and after dye adsorption are presented in Figure 8. After AO7 adsorption, the LDH XRD pattern is almost unchanged, thus, demonstrating that the dye was adsorbed on the external surface of the CoZnAl-LDH rather than between its layers.

A marked increase in AO7 adsorption was observed for MMO compared with LDH. During calcination, the blocked anions (mainly Cl⁻) leave the LDH structure. When MMO is placed in a solution containing anions, it is possible for anions and water molecules to enter between its layers; this phenomenon is called reconstruction (Cheng et al. 2009). On this basis, AO7 can be adsorbed either on MMO's external surface or intercalated between its layers, increasing AO7 adsorption efficiency.

The photocatalytic activity of CoZnAl-MMO falls at higher pHs – see Figure 9. This may be due to the fact that in acidic conditions, anionic AO7 is adsorbed directly onto the positive MMO surface and destroyed. The concentration of OH⁻ ions rises with increasing pH, and these ions compete with AO7 to occupy the LDH surface (Ayoubi-Feiz et al. 2014). Interaction between dye and photocatalyst is thus reduced. The MMO degraded 5.15 mg/L of AO7 at pH = 6 within an hour.

Three adsorption isotherms, Langmuir, Freundlich, and Temkin were studied by fitting the equilibrium data.

Table 3 shows the parameters obtained from the three isotherms. The R² value for the Langmuir isotherm is the closest to one, suggesting that AO7 adsorption onto CoZnAl-LDH and CoZnAl-MMO should be monolayer. The R_L value was between 0 and 1, indicating that adsorption is favorable. It is also noted that AO7's maximum adsorption capacities onto as-prepared LDH and MMO were 243.90 and 526.32 mg/g, respectively – substantial amounts among common adsorbents.

Table 4 clearly shows that the adsorption capacities of CoZnAl-LDH and especially CoZnAl-MMO for AO7 are much higher than those of other adsorbents used to date. In this context, the simplicity and low cost of preparation are major benefits of CoZnAl-LDH and MMO with respect to AO7 removal from water.

In order to further study the adsorption properties of AO7 onto CoZnAl-LDH and MMO, the experimental results were fitted to pseudo-first-order and pseudo-second-order models.

Table 5 shows the parameters obtained from two kinetic models. AO7 adsorption on CoZnAl-LDH and CoZnAl-MMO follows the pseudo-second-order and pseudo-first-order equations, respectively. In both cases R² value is close to one.

CoZnAl-LDH, which has a high specific surface, was synthesized by co-precipitation and showed good AO7 adsorption capacity. CoZnAl-MMO was obtained by calcining the initial LDH and demonstrated excellent adsorption performance. XRD, SEM and EDS analyses showed that sample synthesis was successful. The BET results showed that the samples had high specific surfaces, which makes them function efficiently as adsorbents. Calculation of the band gap energy of CoZnAl-MMO and the experimental results both showed that CoZnAl-MMO can be active in visible light. Kinetic studies confirmed the pseudo-second-order and pseudo-first-order AO7 adsorption kinetics on the CoZnAl-LDH and CoZnAl-MMO, respectively. AO7 adsorption on both LDH and MMO fits the Langmuir isotherm model.

The authors are grateful to the University of Tabriz for financial and other supports.

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