Synthetic zeolite derived from coal fly ash decorated with magnetic alginate bead: Application to detoxification of arsenic and vanadium

Sodium meta arsenate (NaAsO₃), sodium vanadate (NaVO₃) were obtained from Sigma-Aldrich (St Louis, MO, USA). Poly ethylenimine, PEI M.W., 750 kDa, were procured from Alfa-Aesar (Ward Hill, MD, USA). The alginic acid sodium salt, CaCl₂, NaOH, KOH, NH₄OH HCl, FeCl₂·4H₂O, and FeCl₃·6H₂O were obtained from Chem-pure chemicals (Krakow, Poland). All other reagents were purchased from analytical grade without further purification, and double distilled water was used throughout the experiments.

For hydrothermal synthesis, 10 g of fly ash samples were mixed with 12 g of NaOH, and the resulting mixture was heated up to 823 K for 60 minutes (Kunecki et al. 2021). After cooling to room temperature, sodium aluminate was slowly added into the grinded mixture to control their molar ratio of SiO₂/Al₂O₃. Subsequently, the as obtained mixture was further magnetically agitated at room temperature for 16 hours and then temperature was raisin up to 373 K for another 24 hours (Amoni et al. 2022). After cooling down to room temperature, the suspension was filtered, and then the solid product was repetitively washed with 1 L of deionized water and dried up 378 K for 16 hours. Figure 1 shows detailed synthetic processes of zeolite (NaX-UP) derived from coal fly ash are demonstrated (Izidoro et al. 2013; Kunecki et al. 2021; Amoni et al. 2022). The main chemical compositions are as follows: silica, alumina, sodium, and other components as shown in Table 1, these elemental compositions were confirmed by X-ray fluorescence techniques.

Figure 1.

A known amount of 1.0 g of zeolite (NaX-UP) was immersed in 150 mL of de-ionized water and 10 mL of PEI (10 % V/V, 20 mg mL^-1). The resulting mixture was homogeneously stirred for 10 hours (Mthombeni et al. 2016, 2018; Santhana Krishna Kumar et al. 2022), and the terminal amino group from PEI functionalization through their electrostatic interaction with negatively charged zeolite surface of silanol (Si-OH and Al-OH) groups. After separating their supernatant solution, the as obtained solid compound was denoted to be (PEI modified zeolite NSs).

Initially, PEI-modified zeolite NSs (0.3 g) were dispersed in DD water (120 mL), followed by FeCl₂·4H₂O (7.2 g) and FeCl₃·6H₂O (2.8 g), were gradually mixed with the obtained mixture was stirred at 80°C for 30 minutes, the resulting solution was deoxygenated by bubbling with argon gas for 30 minutes, followed by quick injection an ammonia solution (15 mL, 4 M), the entire solution turned to be a dark black color suspension which confirmation that, Fe₃O₄ NPs were strongly bonded with the surface of PEI modified zeolite NSs, the-as obtained solution was further brought down to ambient temperature and then rinsed multiple times with DD water (Santhana Krishna Kumar et al. 2022). For comparison, a control experiment was subjected into prepare bare Fe₃O₄ NPs. As mentioned above, the synthetic route was followed without adding PEI-zeolite NSs.

To prepare magnetic alginate bead (adsorbent), a known amount of alginic acid 2.0% w/v was individually mixed with 30 mg mL^-1 of different nanomaterials such as PEI-zeolite NSs, bare Fe₃O₄ NPs and PEI-zeolite NSs@ Fe₃O₄ NPs were slowly stirred at 80 °C for 2 hours to get the homogeneous solution, the as-prepared alginic acid modified nanomaterials were gradually dropping into 3.0 % w/v of CaCl₂ solution and kept for crosslinking 24 hours to obtain their homogeneous well-shaped calcium cross-linked PEI-zeolite NSs modified alginate beads and Fe₃O₄ NPs decorated PEI modified zeolite NSs alginate beads are displayed in (Figures 2A and B), which are turned to be great flexibility. Furthermore, the schematic illustration of the functionalization procedure is depicted in Figure 3.

Figure 2.

Figure 3.

Batch adsorption studies were systematically assessed by monitoring various pH, temperature and known amounts of adsorbent (2000 mg). The adsorption isotherm studies were established by increasing their heavy metals initial concentration (C_initial) varied from 10 to 500 mg mL^-1 for As (V) and V (V). Similarly, the kinetics studies were also conducted by varying their contact time between 0 to 240 mins intervals at fixed concentrations of heavy metals, pH and adsorbent doses (Kalidhasan et al. 2012; Santhana Krishna Kumar et al. 2013, 2022). After the metal ions’ successful adsorption, the supernatant solution was free of As(V) and V(V). Moreover, the unabsorbed (C_e) (supernatant) metal ions concentrations were determined by ICP-MS techniques (Santhana Krishna Kumar et al. 2022). The amount of adsorbed contaminated (q_e) was calculated from the mass balance equation: 1 qe=(Cinitial−Ce)VW \[{{q}_{e}}=\frac{({{C}_{initial}}-{{C}_{e}})V}{W}\] where W and V are the weight (g) of the adsorbent and the volume (L) of supernatant solution, respectively. The removal efficacy (%RE) was calculated according to the equation: 2 %RE=(Cinitial−Cfinal)Cinitial×100

Brunauer-Emmett-Teller (BET) theory was used to measure the surface area of NaX-UP zeolites. It needs to be stated here that this method does not allow the measurement of the surface properties of Fe₃O₄ NPs loaded PEI-NaX-UP zeolites and PEI-NaX-UP zeolite. The BET measurement of NaX-UP zeolite displays a curvilinear pattern in relation to the type IV isotherm, with a distinct H3 ring representing the mesoporous structure (Figure 4). The BJH (Barrett, Joyner, and Halenda) method was exploited to estimate surface area and total pore volume values (Table 2). Nitrogen adsorption-desorption isotherm measurements indicate that the surface area, pore volume, and pore diameter of NaX-UP zeolites were 472 m² g^-1, 0.13 cm³ g^-1, and 6.5 nm, in sequence (Figure 4).

Figure 4.

As shown in (Figure 5A, black line), the as synthesized NaX-UP zeolite shows a powder XRD spectra with significant diffraction peaks of 10.1°, 12.0°, 15.5°, 20.3°, 23.4°, 26.8°, 31.2°, 33.7°, 37. 6°, 40.9°, 53.4°, and 57.6°. These result are indicates that NaX-UP zeolites are highly crystalline which have limited amorphous potential, to correlates well with previous studies in the literature (Izidoro et al. 2013; Kunecki et al. 2021; Amoni et al. 2022). The XRD spectra of the as synthesized bare Fe₃O₄ NPs show significant diffraction peaks at 30.3°, 35.7°, 43.2°, 53.7°, 57.2°, and 63.1, corresponding to the (220), (311), (400), (422) and (511) facets of the lattice of cubic Fe₃O₄ NPs (JCPDS No. 019-0629) (Figure 5C, dark blue line) [Santhana Krishna Kumar et al. 2022]. Thus, Fe₃O₄ NPs decorated with PEI-modified NaX-UP zeolites show intense diffraction peaks of 35.8°, 43.5°, 53.7°, 57.4°, and 63.1° corresponding to the cubic Fe₃O₄ NPs (Figure 5D, dark yellow line). This peak position are slightly shifted up and down, indicating more number of Fe₃O₄ NPs decorated on their zeolite surface, which are suggesting that a strong binding interaction between Fe₃O₄ NPs and PEI-modified NaX-UP zeolite [Kunecki et al. 2021; Santhana Krishna Kumar et al. 2022]. In other words, Fe₃O₄ NPs have a great combination on the surface of crystalline behavior of PEI-modified NaX-UP zeolite, and it is noteworthy that the XRD patterns of Fe₃O₄ NPs move somewhat to lower angles after the modification process with the zeolite porous solid support. Moreover, no other contaminating phases were observed from the nano-composites of Fe₃O₄ NPs loaded PEI-modified NaX-UP zeolite, indicating that the combination of PEI-modified NaX-UP zeolites and Fe₃O₄ NPs is well immobilized on their surfaces [Kunecki et al. 2021; Santhana Krishna Kumar et al. 2022].

Figure 5.

Field emission scanning electron microscopy (FE-SEM) coupled with elemental (EDS) mapping was further adapted to demonstrate nano-composite formation, as shown in Figure 6. The as-prepared adsorbent of magnetic alginate bead consists of silicon, alumina, carbon, nitrogen, oxygen and iron, the iron element which arises from the contribution of Fe₃O₄ NPs. These results are strongly recommend that Fe₃O₄ NPs were successfully assembled on the surface of PEI-zeolite NSs, as depicted in (Figure 6D). The above information infers that the as-prepared magnetic alginate bead (adsorbent) provides more defective structure (Figure 6A), which are highly favorable for efficient sorption of As(V) and V(V) are depicted in Figures 6B and 6C. Inspired by this information, we further identified that, after sorption of As(V) and V(V), the existence of these elements from the surface of the adsorbent, as we depicted from EDS spectroscopic studies, were shown in (Figures 6E and F).

Figure 6.

Adsorption equilibrium studies were used to determine the mass of the adsorbent per unit weight and their concentration of As(V) and V(V) at a fixed pH. The q_e value rises sharply while increasing their C_e values correspondingly. This feature indicates that electrostatic attraction dominated for capturing the anionic species of As (V) and V (V) on their surface of adsorbent (Kalidhasan et al. 2012, 2013; Santhana Krishna Kumar et al. 2012, 2013, 2022). Moreover, the plateau portion of the curve signifies that the number of binding sites on the surface of these specified adsorbents is limited for capturing As (V) and V (V) from an aqueous solution as shown in Figures 7A and 7B. Considering that Freundlich and Langmuir’s isotherms are frequently applied to evaluate their type of adsorption isotherms, the resultant equilibrium data of these three different type of adsorbents were fitted by two isotherm models, from that Freundlich isotherm model are well fitted as shown in Table 3 regression coefficient are close to 1, these results are demonstrating that the as proposed adsorbents are sufficiently correlating their heterogeneous surface for multilayer sorption of As (V) and V (V) (Kalidhasan et al. 2012, 2013; Santhana Krishna Kumar et al. 2012, 2013, 2022). Additionally, the maximum adsorption capacity (q_m) value was estimated to be 163 mg g^-1 for As (V), similarly 151 mg g^-1 for V (V) according to the following equation 3 (Santhana Krishna Kumar et al. 2022), 3 qe=qmbCe1+bCe \[{{q}_{e}}=\frac{{{q}_{m}}b{{C}_{e}}}{1+b{{C}_{e}}}\]

Figure 7.

Table 3, displays the calculated values of q_e (mg g^-1), C_e(mg L^-1), q_m (mg g^-1), and b (L mg^-1; adsorption energy). It’s worth emphasizing that the q_e values of PEI-zeolite NSs modified alginate beads was estimated to be [(35 mg g^-1 for As(V), 38 mg g^-1 for V(V)], bare Fe₃O₄NPs modified alginate beads were estimated to be [(88 mg g^-1 for As(V), 53 mg g^-1 for V(V)], similarly the-as prepared Fe₃O₄ NPs decorated PEI-modified zeolite NSs alginate beads were estimated to be [(163 mg g^-1for As(V), 151 mg g^-1 for V(V)] respectively. Thus, the assembly of Fe₃O₄ NPs decorated on their surface of PEI-zeolite NSs modified alginate beads can improve their sorption efficiency for capturing As (V) and V(V (Kalidhasan et al. 2012; Santhana Krishna Kumar et al. 2013; (Kalidhasan et al. 2013; Santhana Krishna Kumar et al. 2012, 2022). However, exhibiting a better q_e value than most of the previously published nano-materials as shown in Tables 4 and 5.

By contrast, the Langmuir isotherm model somewhat offers not the best fit with the Freundlich isotherm model. We recommend that, the as-prepared adsorbents contain an unlimited number of active sites have equal affinities towards to sorption of As(V) and V(V). The Freundlich isotherm parameters were obtained according to the following equation 4: 4 qe=KFCe1n \[{{q}_{e}}={{K}_{F}}C_{e}^{{}^{1}\!\!\diagup\!\!{}_{n}\;}\] where K_F [(mg g^-1) (L mg^-1)^1/n] are Freundlich constant corresponding to their adsorption capacity, and n is denoted to be adsorption intensity. The n value is 2.1 for As (V), and 1.9 for V (V), the as-prepared adsorbent (Fe₃O₄ NPs decorated PEI-modified zeolite NSs alginate beads), which is larger than 1.0, suggesting that their specific interaction with As(V) and V(V) (Santhana Krishna Kumar et al. 2022). 5 KF=qmC01n \[{{K}_{F}}=\frac{{{q}_{m}}}{C_{0}^{{}^{1}\!\!\diagup\!\!{}_{n}\;}}\]

In addition, in each case of three different types of adsorbents, the q_t values gradually increase with contact time further that could achieve a saturation level after 100 min. The as-prepared adsorbents can efficiently capture a large amount of As(V) and V(V). This fast sorption time reflects that the as-prepared magnetic alginate bead (Fe₃O₄ NPs decorated PEI-modified zeolite NSs alginate beads) have a large surface area for interacting with more amount of anionic species are As(V) and V(V) (Kalidhasan et al. 2012, 2013; Santhana Krishna Kumar et al. 2012, 2013, 2022).

Additionally, the adsorption time is relatively shorter than that of previously published nanomaterials as shown in Tables 6 and 7. It’s well assumed that the assembly of Fe₃O₄ NPs decorated on their porous zeolite adsorbent could further shorten their adsorption time for As(V) and V(V) (Santhana Krishna Kumar et al. 2022). The pseudo-first-order rate constant (k₁)and pseudo-second-order rate constant (k₂) were separately calculated by the following expressions 6 and 7: 6 qt=qe(1−e−K1t) \[{{q}_{t}}={{q}_{e}}(1-{{e}^{-{{K}_{1}}t}})\] 7 qt=k2qe21+k2qet \[{{q}_{t}}=\frac{{{k}_{2}}q_{e}^{2}}{1+{{k}_{2}}{{q}_{e}}t}\]

The kinetic data of the three different types of adsorbents, which are following a pseudo-second-order model as shown in Table 8, implies that chemisorption dominates their adsorption process. These findings are consistent with the above data. These information’s are well related to the experimental evidence of effect of solution pH and the removal efficiency of As(V) and V(V) against their adsorbent. It’s recommended that, the chemisorption of As(V) and V(V) on the surface of adsorbent (Fe₃O₄ NPs decorated PEI modified zeolite NSs alginate bead), which involves two main sorption processes: (1) exterior mass transfer whereas anionic species of As(V) and V(V) can diffuse from the bulk solution to the surface of adsorbent; (2) chemisorption of As(V) and V(V) species penetrates intra-particle pores and then strongly binding with their active sites of adsorbent (Santhana Krishna Kumar et al. 2022).

Thermodynamic studies are an important parameter to ascertain the feasibility and nature of the adsorption process. Therefore, the temperature effect was examined in the range of 298-328 K, and the obtained results are depicted in (Figure 8). These thermodynamic parameters, namely, standard free energy (ΔG⁰), standard enthalpy (ΔH⁰), and standard entropy (ΔS⁰), changes, and these changes were determined at various temperatures; thermodynamic studies which gives an equilibrium constants, ΔH⁰ and ΔS⁰, were obtained from the slope and intercept of van’t Hoff plot from lnK against 1/T (see in Figures 8B and 8C), for an exothermic reaction slope is positive, and the equilibrium constants are increases with temperature increases (Kalidhasan et al. 2012, 2013; Santhana Krishna Kumar et al. 2013, 2022). 8 ΔG0=−RTlnK \[\text{ }\!\!\Delta\!\!\text{ }{{G}^{0}}=-RT\ln K\] 9 lnK=−ΔH0RT+ΔS0R \[\ln K=\frac{-\text{ }\!\!\Delta\!\!\text{ }{{H}^{0}}}{RT}+\frac{\text{ }\!\!\Delta\!\!\text{ }{{S}^{0}}}{R}\] Where R is the gas constant (J K^-1 mol^-1), T is the temperature in (Kelvin), and K is obtained from the ratio of the concentration of adsorbate (As(V) and V(V)) in the solid and liquid phases respectively, broadly speaking, K > 1 implies that the adsorption is effective and spontaneous at a given temperature. The adsorption gradually increases at a higher temperature, and this result reveals that adsorbate–adsorbent interaction is feasible at higher temperatures, indicating that higher temperature is more favourable to the adsorption process (PEI-Fe₃O₄ NPs beads and Fe₃O₄ NPs loaded PEI-zeolite NSs beads), the adsorption of As(V) and V(V) gradually increases at a higher temperature. This attraction is more efficient at higher temperatures because their obtained ΔG⁰ values decreases along with temperature (Table 9). The negative free energy (ΔG⁰) values further confirm the effectiveness of electrostatic interaction (ion-pair interaction) between the heavy metals and adsorbent surface (Kalidhasan et al. 2012, 2013; Santhana Krishna Kumar et al. 2013, 2022). Therefore, a higher temperature indicates that more efficiently to adsorb the inorganic pollutants. Here, the bonding between the adsorbent (PEI-Fe₃O₄ NPs beads and Fe₃O₄ NPs loaded PEI-zeolite NSs beads) and adsorbate is mainly due to ion-pair interaction.

Figure 8.

Furthermore, the enthalpy change (ΔH°) was negative, confirming that, the exothermic nature of adsorption. The magnitude of ΔH° gives information about the mechanism of adsorption process; in physical sorption, the process is fast and usually reversible due to the small energy requirement. London requires energies from 4 to 8 kJ mol^-1, and Van der Waals interactions are needed from 8 to 40 kJ mol^-1. In contrast, enthalpy associated with chemical sorption is about 40 kJ mol^-1, a value that has been recognized in the literature as the transition boundary between both types of sorption processes (Kalidhasan et al. 2012, 2013; Santhana Krishna Kumar et al. 2012, 2013). For our current studies, the calculated ΔH⁰ values for As (V) and V (V) sorption were lower than that of 4 kJ mol^-1, indicating that weak interaction between the adsorbate and adsorbents were observed throughout our studies (Table 9).

For an endothermic reaction, the slope is positive (see Figure 8A) and the equilibrium constant (lnK) decreases with increasing temperature, which reflects the free energy values are obtained at lower temperatures, and lower temperatures favor the adsorption process (PEI-zeolite NSs alginate beads), as can be seen from these values (see Table 9). Thus, the bonding process decreases with increasing temperature. In the case of PEI-zeolite NSs alginate beads, the adsorption is influenced by the physisorption. The literature reports that physisorption initially increases as more metal ions gain enough energy for strong bonding. Further temperature increase leads to a break in the binding between the metal ions and the adsorbent, reducing their adsorption behavior at higher temperatures. Thus, higher temperatures are unfavorable for the adsorption process, indicating that adsorption favors 298 K, as shown by the ΔG⁰ values are obtained at different temperatures (see Table 9). In addition, the enthalpy change (ΔH⁰) is positive, confirming the endothermic nature of the adsorption (see Table 9) (Kalidhasan et al. 2012, 2013; Santhana Krishna Kumar et al. 2012, 2013, 2022).