Fe-based layered double hydroxides for removing arsenic from water: sorption–desorption–regeneration

Arsenic is among the group 1 cancer-causing compounds by the International Agency for Research on Cancer. Arsenic is naturally present at high levels in the groundwater of countries such as India, Argentina, China, Mexico, Chile, and the USA. Groundwater is the cheaper and abundant source of drinking water in most living areas in the world. Drinking water poses the greatest threat to public health from arsenic (Türk 2017). The source of arsenic for humans is not only drinking water. Especially in areas with high arsenic concentrations, irrigation waters and contaminated soil represent the primary cause of arsenic toxicity (Smedley & Kinniburgh 2002). Prolonged exposure to drinking water containing arsenic causes diabetes, high blood pressure, discoloration of the skin, skin thickening (hyperkeratosis), neurological disorders, muscle weakness, anorexia, and nausea (Zhang et al. 2017).

To prevent drinking water pollution with arsenic in many countries around the world and the health effects caused by it, the maximum pollutant level adopted by the World Health Organization (WHO 1993) for arsenic in drinking water has been reduced to 0.01 mg/L in 1993 (Lee et al. 2018). The removal of arsenic from waters was achieved with different techniques. The arsenic removal method from underground and surface waters is selected according to the arsenic concentration in the water, its place and purpose, cost, and toxic waste generation. Conventional methods, such as precipitation, coagulation, adsorption and ion exchangers, membrane filtration, and polymer ligand exchangers, are used for arsenic removal (Babaeivelni & Khodadoust 2016; Türk 2016). Among these technologies, adsorption is considered to be one of the low-cost and easily feasible techniques (Yadav et al. 2019). Many kinds of synthetic and natural adsorbents have been developed for the removal of arsenic from drinking water. Layered double hydroxides (LDHs) have high arsenic exchange capacities, which are common features with other clays. The main reason for ion exchange in terms of thermodynamics is the electrostatic interaction of positively charged brucite layers and negatively charged anions and a little hydration-free energy (Israëli et al. 2000). LDHs are also called anionic clays. Naturally, occurring hydrotalcite is one of the most representative minerals of the group. The structure of an LDH is basically formed from the main structure brucite Mg(OH)₂ with the isomorphic substitution of divalent cations. To ensure the electroneutrality of the positively charged layers, CO₃²⁻ ions were arrested between the layers. The structure of hydrotalcite is shown as Mg_0.75Al_0.25(OH)_2.5H₂O. Different anions can be placed in the area between the layers of the LDHs. The prominent features of LDHs are that they are abundant, nature-friendly, and cheap (Chetia et al. 2012; Nhat Ha et al. 2016). The synthesis of LDHs is relatively easy and inexpensive on laboratory and industrial scales. There are a number of methods used in the synthesis of LDHs, and these include precipitation, restructuring, hydrothermal, and urea hydrolysis methods (Goh et al. 2008). Within the scope of the study, modified Fe hydrotalcite (MgFeHT), known as iron-based layered double hydroxide, was used in adsorption/desorption process.

In practice, to remove arsenic in remediation systems, it is desirable to renew and reuse layered double hydroxides. In a successful desorption/regeneration process, the first properties of the sorbent should be recovered. Parameters that have an effect on the desorption efficiency (DE) of arsenic include the surface properties of the adsorbent, the concentration of the ion and desorption solution, the pH, and the duration of the arsenate contact with the desorption solution (Pigna et al. 2006).

Although several studies have been studied on the desorption of layered double hydroxides with arsenic, almost all studies on these adsorbents have focused on arsenic removal capacities (Türk & Alp 2010, 2012). Oxyanion-loaded LDHs can be desorbed using various alkaline salt solutions. Caporale et al. (2013) performed desorption with LDHs using PO₄ in arsenite desorption. After 48 h of reaction with PO₄, the Mg–Fe LDH released 25.9% (PO₄/As(III) molar ratio). In one study, arsenate desorption was performed with NaOH from akaganéite-type nanocrystalline, and DE was 75% (Deliyanni et al. 2003). The desorption of arsenate from maghemite has not been achieved (Yean et al. 2005).

Adsorption is often used to remove arsenic in small drinking-water systems. Adsorbent replacement in adsorption technology accounts for about 80% of the total operating cost. Regeneration is a potential method to reduce the cost of the adsorptive media if the adsorbent can be regenerated and reused without little or no loss of arsenic removal capacity (Sorg et al. 2018). In the present study, we evaluated the capacity of MgFeHT to remove As(V) from polluted aqueous solutions. Specifically, we examined the effect of the desorption by NaOH, KHPO₄, and regeneration of MgFeHT. Thus, the MgFeHT product can be renewed, and an option for cost reduction can be offered.

In this study, the five different synthesis methods for the production of Fe-Hydrotalcite (MgFeHT) were studied. The syntheses were carried out in five different ways by co-precipitation (Table 1). Since adsorption is a surface treatment, adsorption power is an important function of surface properties. Among the surface properties of the adsorbent, the most important parameter affecting the adsorption process is the surface area value (Beyhan 2003). In this study, it is aimed to have a maximum surface area of the adsorbent to be produced. The selection of the material to be used as an adsorbent was made as a result of the characterization studies.

Each synthesis product was characterized by XRD, BET, and grain size analysis. XRD patterns (XRD) were measured by the Rigaku Smart model diffractometer. XRD was recorded between 4° and 65°. The surface area was measured by an ASiQwin™ instrument. In the analysis process, the degassing temperature was determined as 105 °C, and the degassing time was 8 h. To determine the particle size distribution of the materials, grain size distribution analysis was performed in the Master Sizer device.

The experiments were carried out in a Wise Shake SHO-2D brand orbital mixer, 100 mL flask, at 25 °C and 200 rpm for a predetermined contact time with MgFeHT. The solution was centrifuged at 4,000 rpm. Arsenic analyses were performed at 193 nm wavelength with the Perkin Elmer-AAS 100 model Hydride Atomic Absorption Spectrometer (HG-AAS). The effect of adsorbent dose was examined with (0.05–0.1–0.5–1–2–3–4–5) g/L MgFeHT (2,000 μg/L As, pH 9, 120 min). pH was studied at (3–5–7–9–12) (2,000 μg/L As and 2 g/L MgFeHT doses). For adsorption isotherm studies (500–5,000 μg/L As), the solution was used at a 2 g/L MgFeHT.

XRD was used to determine the mineralogical composition of the adsorbents. The peaks (003) and (006) belong to the basal reflections in relation to the cluster of the brucite layer. The XRD peaks of the syntheses performed in this study are shown in Figure 2. Other peaks belong to non-basal reflections. The sharpness of the peaks (003) and (006) indicates that the sample has a high degree of crystallinity and a stratified structure. Nishida et al. (2004), in their study, carried out XRD studies for MgFeHT, which they synthesized by the precipitation method. The peaks they obtain are compatible with the peaks obtained in this study.

Grain size is one of the parameters affecting adsorption. The adsorption rate increases, as the grain sizes decrease. However, it has no effect in terms of adsorption capacity (Yang et al. 2000). The values obtained as a result of the analysis are shown in Table 2.

The maximum surface area value was evaluated by testing with specific surface area measurements (Table 3). There is an important relationship between the adsorption process and the surface properties of the adsorbent. Among the adsorbent surface properties, the most important parameter affecting adsorption is the surface area (Orozco et al. 2014). When the surface areas of the synthesis products are examined, it is seen that the production method has an important effect. MgFeHT-3 is the material with the highest specific surface area with a surface area of 97.210 m²/g. In addition, when we look at the particle size analysis values of MgFeHT-3, the particle size is lower than the others. The lower particle size will enable faster adsorption (Table 2). Therefore, it is considered to use MgFeHT-3 in future adsorption/desorption/regeneration process.

pH (3–5–7–9–12) studies with MgFeHT-3 are shown in Figure 3(a). When the results were examined, it was shown that arsenic intake was not affected by the initial pH. Final concentration of As(V) in solution increased from 2.02 to 9.10 μg/L when its pH rises from 3 to 9, possibly due to changes in arsenic types and changes in LDH surface charge (Figure 3(a)). However, when the initial pH was 12, the arsenic concentration in the solution significantly increased to 53.2 μg/L. The reason that there is no change between pH 3 and 9 is due to the strong buffer effect seen in layered double hydroxides. This situation has been also observed by many researchers. The value of pH 9 was chosen optimally, and all experiments were carried out at this pH.

In dosage studies with MgFeHT-3, the arsenic concentration in the solution decreased with increasing dosage. However, there was no change after a certain dosage value. Yang et al. (2005) determined that as the adsorbent dose increased, the loading capacity increased and there was no change after a certain point. In dosage studies conducted with MgFeHT-3 samples (Figure 3(b)), it was observed that the removal of arsenic increased as the dose increased. For MgFeHT-3, the optimum adsorbent dosage for As(V) removal was chosen as 2 g/L.

Adsorption/desorption/regeneration study was carried out with MgFeHT-3 as described in the experimental section to determine the desorption properties of the adsorbent and to examine its reusability. NaOH and KH₂PO₄ were tested as desorbents. Some researchers have previously conducted studies on the effect of NaOH on the desorption of anionic pollutants through proton removal, electrostatic repulsion, and ion exchange (Lee et al. 2018). KH₂PO₄ was used in previous studies to remove arsenic from soil aggregates (Min et al. 2008; Zhao et al. 2016). To select the appropriate desorbent solution, different NaOH and KH₂PO₄ concentrations were evaluated depending on the desorption rate of arsenic from As-MgFeHT-3. As-MgFeHT-3 was shaken at different NaOH (0.5–2–4 M) and KH₂PO₄ (0.1–0.2–0.3 M) concentrations (Figure 5). There was little increase in DE with increasing NaOH concentration. While 69.7% of the DE was obtained with 0.5 M NaOH, 74.1% of the DE was obtained with 4 M NaOH (Figure 5). Increasing DE with increasing NaOH concentration is due to the electrostatic attraction force and the deprotonation surface of LDH between negative charged layered double hydroxides and anions (Zhang et al. 2010). In total, 82.9% of the DE was obtained with 0.3 M KH₂PO₄ (Figure 5). DE increased when the KH₂PO₄ concentration increased. The maximum desorption of As(V) was achieved 74.1 and 82.9% with 0.5 M NaOH and 0.3 M KH₂PO₄, respectively. However, desorption with KH₂PO₄ is not effective. The reason for this; KH₂PO₄ is due to its tendency to resolve MgFeHT-3. This has been observed by other researchers (Zhao et al. 2016). In their 2016 study, they found that KH₂PO₄ dissolved metal hydroxy oxides such as Fe and Al.

In Figure 6(a), the effect of adsorbent dosages (1–2–3g/L) on DE is examined. When 1 and 3 g/L of MgFeHT-3 were used, 74.1 and 52.9% of the DEs were obtained, respectively. It is seen that DE decreases as adsorbent dosage increases (Figure 6). The solid parts obtained after desorption tests were used in regeneration experiments after washing with ultrapure water and drying at 60 °C for 2 h. Regeneration tests were carried out in four cycles. As seen in Figure 7, the regeneration efficiency (RE) of NaOH has decreased slightly from the second regeneration cycle (99.3%) to the fourth regeneration cycle (97.4%). However, the RE of KH₂PO₄ has decreased continuously since the second cycle. It decreased by 54% at the end of the fourth cycle. The reduction observed after the second regeneration stage may be attributed to the partial dissolution of the LDH sorbent. Therefore, it can be said that the regeneration and reuse of MgFeHT-3 with KH₂PO₄ is negative.

In this article, a high surface area of (62–97 m²/g) MgFeHT was produced by the co-precipitation method. In addition, it has been observed that applying the same synthesis method in different ways affects the surface area of MgFeHT. In adsorption studies with MgFeHT-3, arsenic ion was removed from waters with an initial arsenic concentration of 2,000 μg/L in a 99.5% yield. The arsenic concentration has been reduced below the maximum allowed arsenic concentration in drinking water by the World Health Organization. The desorption of arsenic ions adsorbed by MgFeHT-3 was studied by batch experiments. DE was found to increase with increasing concentration of desorption reagents (NaOH and KH₂PO₄). DE was 74.1 and 82.9%, respectively, when using NaOH and KH₂PO₄. As seen from the results, it was determined that KH₂PO₄ was more effective in removing arsenic from the As-MgFeHT-3 structure. Regeneration studies were conducted on the adsorptive media used in the arsenic removal system. Adsorptive media replacement, which accounts for about 80% of the total operating cost, can be important for small systems. The aim of this research was to determine the renewability of Fe layered double hydroxide products instead of discharge and thus to reduce the operating cost. In regeneration tests conducted to investigate the reusability of the adsorbent, it was found that repeated use of the adsorbent is possible if NaOH is used as the desorption reagent. It was observed that KH₂PO₄ disrupted the FeHT structure and reduced its adsorbent properties and negatively affected arsenic removal performance in repeated use. As a result, arsenic removal can be achieved at a lower cost with MgFeHT-3, which has a high arsenic removal potential by regenerating an iron-based medium and re-uses the medium.

The authors thank the Karadeniz Technical University Scientific Research Fund (Project No. FBA-2017-5682) for the financial support of this work.

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