Improvement in adsorption of Hg²⁺ from aqueous media using sodium-type fine zeolite grains Open Access

Recently, mercury-containing wastewater discharge in the environment has surged. Paper, paint and battery industries, metallurgy and mining processes, and volcanic eruptions are considered sources of mercury ions (Hg²⁺) (Gupta et al. 2021). Mercury ions are very toxic and pose a significant risk to humans and nature (Crini 2005; Gupta et al. 2021; Liakos et al. 2021). Tremors, gingivitis, dermatitis, acrodynia and pulmonary, renal and neurologic functions impairment are all caused by Hg²⁺ uptake in humans (Gupta et al. 2021). The Intergovernmental Negotiating Committee adopted the Minamata Convention on mercury in 2017. It aims to reduce mercury emissions from anthropogenic activities including those mentioned above (Fu et al. 2020). Organic and inorganic Hg compounds are divided in two groups: group 2B (probably carcinogenic to humans) and group 3 (unclassifiable for human carcinogenicity), respectively, by The International Agency for Research on Cancer (IARC 2021). The permissible limits of mercury were determined to be 0.002 and 0.006 mg/L by the U.S. Environmental Protection Agency and the World Health Organization, respectively (Gupta et al. 2021). Therefore, effective removal of Hg²⁺ from wastewater is a key issue in solving mercury-induced health hazards and water pollution.

Coal fly ash, the waste biomass from coal combustion in coal-fired power plants produced annually will grow by 2030 (Yao et al. 2015; Czarna-Juszkiewicz et al. 2020). Thus, attention and research are required on possible recycling technologies because of serious human health hazards and environmental pollution (Blissett & Rowson 2012; Franus et al. 2015; Tauanov et al. 2018). Additionally, Sustainable Development Goals have been adopted by the United Nations to establish an ecologically balanced society. Therefore, from a sustainable perspective, evaluating technologies for recycling coal fly ash for waste reduction is necessary. Several studies have shown that coal fly ash can be converted from waste into value-added products such as synthetic zeolites, which can remove heavy metals and other contaminants from wastewater or purify gas in several fields such as agriculture, medicine, industry and environmental engineering (Franus 2012; Jha & Singh 2014; Wdowin et al. 2014; Tauanov et al. 2017, 2018).

Currently, fine grains such as nanomaterial with engineered properties such as wide dispersion of reaction sites, high surface area, adsorptive removal capacity, small radius and large porosity play a leading role in emerging applications in diverse areas (Piao et al. 2008; Jin et al. 2009; Rosenholm et al. 2009; He et al. 2015; Ma et al. 2016; Yang et al. 2021). Characteristic zeolites were produced by a hydrothermal treatment using sodium hydroxide or potassium hydroxide solution in our related studies. Subsequently, their adsorption capacity for heavy metals was assessed in detail (Kobayashi et al. 2020a, 2020b, 2021). However, to our knowledge, few reports have evaluated the physicochemical characteristics of fine zeolite grains prepared from coal fly ash using hydrothermal treatment and their adsorptive removal of heavy metals such as mercury from aqueous media. Hence, when fine zeolite grains can be tested for the adsorptive removal of Hg²⁺ from aqueous media, this recycling technology will contribute significantly to reducing coal fly ash waste from coal-fired power plants or wastewater conservation including the removal of heavy metals.

In this study, we used a hydrothermal treatment to prepare fine zeolite grains from coal fly ash and tested their physicochemical properties and adsorptive removal ability for Hg²⁺ from aqueous media.

The standardized mercury solution (HgCl₂ in 0.1 mol/L HNO₃) and each standard solution of cations such as Na⁺ (NaCl) and K⁺ (KCl) in water were purchased from FUJIFILM Wako Pure Chemical Co., Japan. Additionally, Mg²⁺ (Mg(NO₃)₂), Ca²⁺ (CaCO₃), Ni²⁺ (Ni(NO₃)₂), Cu²⁺ (Cu(NO₃)₂), Zn²⁺ (Zn(NO₃)₂), Sr²⁺ (SrCO₃) and Cd²⁺ (Cd(NO₃)₂ in 0.1 mol/L HNO₃ were also obtained from FUJIFILM Wako Pure Chemical Co., Japan. The Tachibana-wan Power Station (Shikoku Electric Power, Inc., Tokushima, Japan) provided fly ash (FA).

Sodium-type zeolite was prepared using a previously reported hydrothermal activation treatment (Kobayashi et al. 2020a). Briefly, the reaction mixture, containing 3 g FA and 3 mol/L sodium hydroxide solution in the volume of 240 mL, was heated for 24 hours at 93 °C (Okada 1991). After that, the residue obtained from 0.45 μm membrane filters was rinsed with distilled water and dried for 24 hours at 50 °C (denoted as ZE1). Next, ZE1 and zirconium beads of 0.5 mm diameter were mixed and crushed using a Shake Master NEO (Bio-Medical Science Co., Ltd, Tokyo, Japan) at 1,500 rpm for 3 h. The obtained ZE from a milling process was denoted as ZE2. Additionally, ZE3 was prepared by rotation/revolution mixing using a Nano Pulverizer NP-100 (THINKY CORPORATION, Tokyo, Japan). Briefly, ZE1 was triturated using a pestle and mortar for 30 min. Then the triturated ZE1 was added to distilled water and milled using a Nano Pulverizer NP-100 (2,000 rpm for 5 min × 3 times, 4 °C). The wet-milled sample was denoted as ZE3 in this study (Figure 1).

The morphology of ZE1, ZE2 and ZE3 was monitored using scanning electron microscopy SU1510 (SEM, Hitachi High-Technologies Co., Tokyo, Japan) and X-ray diffraction (XRD) analysis was performed using a MiniFlex II (Rigaku, Osaka, Japan). Moreover, Fourier transform infrared (FT-IR) spectra were recorded using a FT-IR-460Plus spectrometer (JASCO, Co., Japan). Additionally, the cation exchange capacity (CEC) was analyzed using the Japanese Industrial Standard Method (JIS K 1478: 2009) and the solution pH of prepared sample was determined using an F-73 digital pH meter (HORIBA, Ltd, Japan). Finally, the pore volume and specific surface area were measured using a NOVA 4200e (Yuasa Ionics, Japan).

Firstly, the reaction mixtures composed of 10 mg/L Hg²⁺ solution and 0.01 g of each prepared samples in the total volume of 50 mL were shaken at 100 rpm and 25 °C for 24 h. Subsequently, the obtained solutions before and after adsorption from 0.45 μm membrane filters were measured to determine concentration of Hg²⁺ using an inductively coupled plasma–optical emission spectrometer (ICP-OES, iCAP-7600 Duo, Thermo Fisher Scientific Inc., Osaka, Japan). The different concentrations of Hg²⁺ in the solution before and after experiment were calculated for the quantity of adsorbed Hg²⁺.

Secondly, to evaluate the effect of contact time, the reaction mixtures composed of 10 mg/L Hg²⁺ solution and 0.01 g of each prepared sample in the total volume of 50 mL were shaken at 100 rpm and 25 °C for 1, 3, 6, 9, 12, 21 and 24 h. Thereafter, the solution obtained from 0.45 μm membrane filters was collected. Thirdly, to evaluate effects of temperature and initial concentration, the reaction mixtures composed of 1, 3, 5, 7 and 10 mg/L Hg²⁺ solution and 0.01 g of each prepared sample in the total volume of 50 mL were shaken at 100 rpm and 7, 25 and 45 °C for 24 h. Subsequently, the solution obtained from 0.45 μm membrane filters was collected. To determine the adsorption mechanism, the concentration of Na⁺ released from each sample was measured, and elemental analysis was conducted using the ICP-OES and JXA-8530F instruments (JEOL, Tokyo, Japan), respectively. Finally, to evaluate the effect of pH, reaction mixtures were prepared containing 10 mg/L Hg²⁺ solution and 0.01 g of each sample in the total volume of 50 mL in the different pH conditions, i.e., 2, 3, 5, 7, 9 and 11. Then the reaction mixture was shaken at 100 rpm and 25 °C for 24 h. The quantity of Hg²⁺ adsorbed was calculated using the aforementioned method.

To investigate the Hg²⁺ adsorption selectivity, 50 mL of a binary solution was added to 0.01 g of each sample. Hg²⁺ at the concentration of 10 mg/L in the binary solution was combined individually with Na⁺, K⁺, Ni⁺, Mg²⁺, Ca²⁺, Cd²⁺, Cu²⁺, Zn²⁺ or Sr²⁺. The reaction mixture was shaken at 100 rpm for 24 h at 25 °C. Subsequently, the obtained solutions before and after adsorption from 0.45 μm membrane filter were measured to determine concentration of Hg²⁺ using an ICP-OES instrument. The different concentrations of Hg²⁺ in the solution before and after experiment were calculated for the quantity of adsorbed Hg²⁺.

The particle size distribution of each sample is shown in Table S1. The particle sizes of ZE1, ZE2 and ZE3 were 16.363 ± 0.365, 1.454 ± 0.357 and 0.607 ± 0.377 μm, respectively. In general, the specific surface area becomes larger with decreasing particle size. These phenomena increase the contact frequency between adsorbent and adsorbate under experimental conditions. Therefore, this treatment would positively affect the adsorption capacity in this study. Scanning electron microscopy images of ZE1, ZE2 and ZE3 are shown in Figure S1. The particle size of ZE2 was smaller than that of ZE1, indicating that the dry-grinding treatment of ZE1 with zirconium beads successfully led to decreased particle size under our experimental conditions. The ZE3 particles were aggregated after the wet-grinding treatment. Because ZE3 was originally obtained as a suspension, the scanning electron microscopy image of ZE3 was recorded after drying. Fine grains such as nanomaterials are usually more reactive than bulk materials. The van der Waals and electric double layer interactions between each particle explain that all types of fine grains such as nanomaterials tend to agglomerate in large particles (Petosa et al. 2010; Subana et al. 2020). Therefore, it could be determined whether the net interaction phenomenon between the particles was repulsive or attractive (Zhang et al. 2008). The FT-IR spectra of the prepared samples as shown in Figure S2 demonstrated that H–O–H bending at 1,653 cm⁻¹, Si–O–Si or Al–O–Si asymmetric stretching at 977–982 cm⁻¹, and Al–O–Al stretching vibrations at 566 and 462 cm⁻¹ were found in ZE1, ZE2 and ZE3. No significant differences were observed between each sample. The main components of zeolite are typically aluminum (Al) and silicon (Si) (Shirono & Daiguji 2002; Ayuso et al. 2008; He et al. 2016). The Al (or Si)–O–Al (or Si) asymmetric stretching was observed in this study, consistent with related study observations (Kobayashi et al. 2020a). Next, we recorded XRD patterns of the prepared samples (Figure 2). The structures of ZE1, ZE2 and ZE3 comprised mullite, quartz, hydrosodalite and zeolite P under our experimental conditions. These results indicated that the dry-grinding treatment of ZE1 with zirconium beads successfully maintained the aluminosilicate gel structure. The CEC, specific surface area and pore volume of each sample are shown in Table 1. All values (except those of the mean pore diameter) increased in the order ZE1 < ZE2 < ZE3. Particularly, the CEC and specific surface area of ZE3 were approximately 3.2 and 7.6 times greater than those of ZE1. Therefore, using our experimental conditions, a novel adsorbent could be created from coal fly ash combining hydrothermal activation treatment and fine crystallization treatment.

The amounts of Hg²⁺ adsorbed onto ZE samples are shown in Figure S3. The amount of Hg²⁺ adsorbed was in the order ZE1 (5.0 mg/g) < ZE2 (9.4 mg/g) <ZE3 (20.4 mg/g), indicating that fine zeolite grains such as ZE2 and ZE3 improved the adsorption of Hg²⁺ from aqueous media. In addition, the relationship between the adsorption capacity for Hg²⁺ and the physicochemical characteristics of the ZEs were also evaluated. With coefficient values of 0.999, 1.000, 0.956, 0.985 and 0.943, we found positive correlations between the amount of Hg²⁺ adsorbed and CEC, specific surface area and micropore (d ≤ 20 Å), mesopore (20< d ≤ 500 Å) and macropore diameters (d > 500 Å), respectively. The current data demonstrated that physicochemical properties such as CEC, specific surface area and pore volume strongly affected the adsorption of Hg²⁺ from aqueous media.

Table 2 shows the results of kinetics data fitting using PFOM and PSOM (the linear fitting of the pseudo-secondo-order model is shown in Figure S4). According to the obtained results, the value of the correlation coefficient (R²= 0.785–0.884) of PFOM was smaller than that of PSOM (R²= 0.994–0.998) under our experimental conditions. Therefore, PSOM should be applied to describe adsorption kinetics better. Additionally, the calculated value of quantity of adsorbed Hg²⁺ at equilibrium for PSOM was closer to the experimental result than that for PFOM, indicating the applicability of the pseudo-second-order model. Moreover, the obtained results probably correspond to the summation of some mechanisms: ion exchange (described below in detail), which is fast, and other adsorption mechanisms which are slower. Adsorption kinetics of Hg²⁺ using ZE samples are ultimately defined by the slower mechanisms, which are better described by the PSOM than by PFOM under our experimental conditions. A similar phenomenon was observed in a related study (Lopes et al. 2007). Moreover, to avoid including error in kinetic models, the chi-square analysis (χ²) was evaluated in this study (Ho 2004). If obtained data from kinetic models were similar to the experimental data, χ² would be a small number and vice versa. The χ² values of kinetic models are shown in Table 2. The χ² value of the pseudo-second-order model (0.03–0.13) was smaller than that of the pseudo-first-order model (0.20–9.6 × 10²). Therefore, the obtained data were considered to be a better much for the pseudo-second-order model.

Secondly, the effects of initial concentration and temperature on the adsorption of Hg²⁺ were evaluated (Figure 4). The amount adsorbed was in the order ZE1 <ZE2 < ZE3 and 7 °C < 25 °C < 45 °C. These results indicated that the adsorption temperature strongly affected the adsorption of Hg²⁺ onto the ZE samples. Adsorption isotherms are presented graphically or by an equation and relate to the equilibrium quantity of adsorbed material on a solid adsorbent at a given temperature (Malamis & Katsos 2013). Two classical adsorption isotherm models, the Freundlich and Langmuir isotherms, are used to evaluate the relationship between adsorbate and adsorbent in detail (Yang et al. 2021). The Freundlich and Langmuir models indicate the heterogeneity of the surface of an adsorbent and monolayer adsorption of active centers on the adsorbent surface, respectively (Borandegi & Nezamzadeh-Ejhieh 2015). Therefore, these two isotherm models were used to fit the data.

The fitting results are shown in Table 3 (the linear fitting of the Langmuir model is shown in Figure S5). It became clear that the Langmuir isotherm model (R²= 0.962–0.998) correlated better with the adsorption process of Hg²⁺ compared with the Freundlich isotherm model (R²= 0.806–0.976). The maximum adsorption capacity (q_max) increased with the adsorption temperature (except for ZE2 at 45 °C). These phenomena agreed with the adsorption isotherms shown in Figure 4. When the strength of adsorption (1/n) values were 0.1–0.5, the adsorption of Hg²⁺ occurred easily; in contrast, when the 1/n values were above 2, adsorption of Hg²⁺ was difficult. In this study, the 1/n value ranged from 0.19 to 0.61, indicating that removing Hg²⁺ from aqueous media using ZE samples was effective under our experimental conditions (Abe et al. 1976). The chi-square analysis (χ²) was also conducted for the evaluation of isotherm models to avoid including error (Ho 2004). From Table 3, the χ² values of the Langmuir model (0.05–1.37) were smaller than that of the Freundlich model (0.44–5.90). These results suggest that the data were considered to be a better match for the Langmuir model under our experimental conditions. These findings confirmed that the adsorption of Hg²⁺ on the ZE samples occurred via the monolayer adsorption process.

Moreover, the adsorption mechanism of Hg²⁺ in detail using the ZE samples was also determined; the relationship between the amount of Hg²⁺ adsorbed and the amount of Na⁺ released was investigated and is demonstrated in Figure 5. The amount of Hg²⁺ adsorbed increased with the amount of Na⁺ released. The correlation coefficient (r) was positive (0.966–0.979), indicating that ion exchange with Na⁺ in interlayers of the ZE samples was one of the adsorption mechanisms of Hg²⁺ from the aqueous media. Similar trends have been reported previously (Kobayashi et al. 2020a, 2020b, 2021). In addition, as mentioned in ‘Quantity of Hg²⁺ adsorbed using ZE samples’, the coefficient for the positive correlation between the quantity of Hg²⁺ adsorbed and CEC was 0.999. These results supported this finding. In this study, the fit for the data for the lowest Hg²⁺ concentration was poor in the Langmuir model under our experiment. These phenomena might be caused by different adsorption mechanisms. Ion exchange with Na⁺ in interlayers of the ZE samples mainly controlled the adsorption mechanisms of Hg²⁺ in low concentration conditions. Therefore, the fit for data in low concentration was not completely satisfactory in the Langmuir model.

Moreover, the elemental distribution of Hg on the adsorbent surface before and after adsorption was analyzed using a JXA-8530F instrument (Figure 6). The intensity of Hg after adsorption increased by approximately 20% compared with that before adsorption. These results revealed that adsorbed Hg²⁺ partially existed on the adsorbent surface. The results, as mentioned earlier in ‘Quantity of Hg²⁺ adsorbed using ZE samples’, also corroborated this phenomenon.

Table 4 lists the thermodynamic parameters for the adsorption of Hg²⁺. The increased trend of negative ΔG values with increasing adsorption temperature indicates that the adsorption process becomes spontaneous and feasible under our experimental conditions. In addition, the adsorption of Hg²⁺ onto ZE samples was evaluated to be an endothermic reaction due to the positive value of ΔH (Liakos et al. 2021). A related study reported that the value of ΔH at 80–200 kJ/mol indicated chemical adsorption and that the value of ΔH at 2.1 to 20.9 kJ/mol indicated physical adsorption (Roghanizad et al. 2020). Therefore, physical adsorption was also related to the adsorption mechanism of Hg²⁺ using ZE samples under our experimental conditions (the value of ΔH was from 4.6 to 12.8 kJ/mol). Moreover, the positive value of ΔS indicated an increase in disorder at the solid to solution interface during the Hg²⁺ adsorption process (Yan et al. 2010).

Finally, the effect of pH on the adsorption of Hg²⁺ using ZE samples is demonstrated in Figure 7. One of the most important factors in the adsorption of Hg²⁺ is pH from aqueous media. The amount adsorbed increased with a pH from 2 to 3, but decreased with a rise in pH above 5.0 under our experimental conditions. Related studies have shown that the dominant Hg species is Hg²⁺ below pH 2.9 and HgOH⁺ in the pH range from 1.5 to 4.5, whereas Hg(OH)₃⁻ appears below pH 13.2 (Powell et al. 2005; Ugrina et al. 2020). Moreover, precipitation of Hg²⁺ begins at pH 2.4, and the proportion of Hg(OH)₂ increases with a further increment in pH. At pH below approximately 3–4, competing protons (in both ion exchange and complexation processes) and the repulsive force between the protonated ZE sample surface led to a decrease in the amount of Hg²⁺ adsorbed from the aqueous media (Ugrina et al. 2020). Therefore, the optimal pH condition was approximately 3.0 under our experimental conditions.

To confirm the applicability of ZE in the real samples, their competitive adsorption characteristics for Na⁺, K⁺, Ni⁺, Mg²⁺, Ca²⁺, Cd²⁺ Cu²⁺, Zn²⁺ and Sr²⁺ cations were determined (Table S2). The adsorption capacity of the ZE samples for Hg²⁺ did not change significantly, indicating that samples were not influenced and had good selectivity for Hg²⁺. Related studies have already reported that the radius of hydrated ions and the electronegativity of Hg²⁺ affected the adsorption capacity during the liquid phase (Wang et al. 2005; Yadanaparthi et al. 2009). Therefore, these factors would be related to the adsorption mechanism for this selectivity. However, further study is necessary to elucidate the reason for this selectivity. Finally, this confirms that ZE samples are potential adsorbents for Hg²⁺ from aqueous media.

Table S3 shows the adsorption capacity of ZE3 and other reported adsorbents for Hg²⁺ (Lopes et al. 2007; Shang et al. 2010; Sheela et al. 2012; He et al. 2015; Sun et al. 2017; Song et al. 2018; Kobayashi et al. 2020a, 2020b, 2021). The adsorption capacity of ZE3 was greater than that of other reported adsorbents (except FeS, Defective-MoS₂/Fe₃O₄, and ZnO). In addition, initial concentration and/or amount of adsorbent used in this study was lower compared with other reported studies. Therefore, the cost–effectiveness would be high. These results proposed that ZE3 would be a potential candidate agent for the adsorption of Hg²⁺ from aqueous media. In this study, ZE samples could be prepared from waste material (coal fly ash in this study) as an adsorbent for the purification of wastewater. Therefore, the cost of preparing the adsorbent is lower compared with other adsorbents. Moreover, this conversion technique can contribute to the recycling of waste material such as coal fly ash. Our related study has already reported that the synthesis of zeolite prepared from coal fly ash (potassium-type zeolite) could show the repetition of the adsorption/desorption capability of Hg²⁺ from aqueous media (Kobayashi et al. 2021). However, these obtained results provided preliminary knowledge regarding the adsorption of Hg²⁺ using ZE3. Therefore, further studies are necessary to evaluate the adsorption capacity and/or adsorption mechanism for Hg²⁺ from aqueous media.

In this study, we improved the adsorption of Hg²⁺ using fine zeolite grains such as ZE3 and investigated the physicochemical characteristics of prepared ZE samples and the adsorption mechanism in detail. The particle size of ZE3 was approximately 27 times lower than that of ZE1, hinting that ZE3 would act as a more efficient adsorbent for removing Hg²⁺ from aqueous media. Hg²⁺ adsorption was strongly dependent on the adsorption time, initial concentration, temperature and pH under our experimental conditions. The adsorption kinetics and adsorption isotherm data fitted the pseudo-second-order and Langmuir models, respectively. In addition, the efficiency of adsorption of Hg²⁺ using ZE samples was highly influenced by pH; an increase in pH from 2 to approximately 4 increased the efficiency of adsorption. Hg²⁺ was selectively adsorbed from a binary solution system containing Na⁺, K⁺, Ni⁺, Mg²⁺, Ca²⁺, Cd²⁺ Cu²⁺, Zn²⁺ and Sr²⁺ cations. Moreover, we obtained information on the mechanism of adsorption of Hg²⁺ using ZE samples. The results implied that the adsorption mechanism was related to ion exchange with Na⁺ in interlayers of the ZE samples and/or physical properties such as specific surface area and pore volume. In conclusion, the prepared fine zeolite grains (ZE3) could be considered a promising agent for removing Hg²⁺ from aqueous media in terms of efficiency and feasibility.

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