Simple Aminated Modified Zeolite 4A Synthesized Using Fly Ash and Its Remediation of Mercury Contamination: Characteristics and Mechanism

Abstract

In this study, through simple ammonia impregnation, more amine functional groups could be introduced into the zeolite 4A synthesized using fly ash, which efficiently improved the mercury ion removal capacity of modified zeolites. The impregnation-modification mechanism of NH₃·H₂O, ammonium chloride, and silane coupling agent (KH792) for zeolite 4A, and the Hg²⁺ ion removal-efficiency by aminated zeolites, were studied and compared. Through ion exchange and hydroxyl reactions, NH₃·H₂O impregnation introduced the same kinds of nitrogen-containing groups into zeolite as KH792 grafting, which was more than the NH₄Cl modification. The Hg²⁺ ion adsorption capacity of NH₃·H₂O-zeolite was higher than those of KH792-zeolite and NH₄Cl-zeolite through ion exchange and the complexation of nitrogen-containing groups. When coexisting with Pb²⁺, Cu²⁺, and Zn²⁺ ions, the Hg²⁺ ion removal rate of NH₃·H₂O-zeolite was still higher than 99%. After five adsorption and desorption cycles, the Hg²⁺ ion removal rate of NH₃·H₂O-zeolite was 72.03%. When NH₃·H₂O-zeolite was added to the leaching of mercury-contaminated soil, the content of soluble mercury significantly decreased. Therefore, we synthesized a potential cheap and safe adsorbent using fly ash as the main raw material through the simple NH₃·H₂O impregnation modification for the treatment of mercury-contaminated water and soil.

1. Introduction

Mercury (Hg) has strong toxicity. After entering the human body, even a small amount of Hg will lead to multiple human diseases and neurological and renal dysfunction [1]. Therefore, mercury-contaminated water and soil may have adverse effects on human health. Among mercury-removal methods, the adsorption method is commonly used because of its low cost, fast treatment speed, simple operation, and renewable advantages [2,3]. Zeolite is a conventional adsorbent that can remove heavy metal ions from a solution through ion exchange. It is often used to remove Hg²⁺ ions in water [4].

Due to the fact that the main elements of zeolite are silicon and aluminum, industrial waste fly ash with silicon and aluminum as the main components is often used to synthesize zeolite to reduce the material cost and realize waste utilization [5,6]. However, the removal ability of zeolite for Hg²⁺ ions needs to be improved, so it is often modified to increase its adsorption capacity [7]. Due to the fact that amino groups can chelate with Hg²⁺ ions through the free-electron bimodal characteristics of nitrogen and have a strong adsorption ability for Hg²⁺ ions in solution [8], zeolites are often aminated to improve their mercury-removal capacities [9].

There are two main methods for the amination modification of a zeolite: one is high-temperature activation, the other is impregnation. High-temperature activation modification is to introduce an amino group from ammonia to the zeolite structure. NH₃ can only be decomposed with the removal of a H atom to form an amino group after being heated to above 600 °C, which is then connected with Si on the zeolite surface [10,11]. Therefore, NH₃ modification has a high energy consumption; in addition, the nitrogen content introduced into microporous zeolites is low, generally 1 wt%–2 wt% [12]. Impregnation modification involves impregnating the zeolite into inorganic ammonium salt or an organic nitrogen-containing substance solution, then introducing an ammonium ion or amine functional group in the zeolite channel and surface through ion exchange, loading, and grafting [13,14]. The impregnation is a commonly used method for the zeolite amination modification because of its simple operation and low energy consumption [15].

When inorganic ammonium salt is used to modify the zeolite by impregnation, ammonium ions can enter the zeolite channel by exchange with cations in zeolite [16], but due to the influence of the cation site of the zeolite, they can only exchange with cations in the quaternary ring and hexagonal ring on the zeolite surface [17], and the nitrogen content introduced is low. When organic nitrogen-containing substances are used to impregnate zeolites, the nitrogen-containing functional groups are easily grafted onto the zeolite surface [18]. Due to the fact that organic substances can contain more nitrogen-containing functional groups, more amino groups can be introduced in the zeolite; thus, zeolites modified by organic amines have a high adsorption efficiency for pollutants [19]. However, when a zeolite modified by organic amines is used to treat mercury-contaminated water and soil, due to the weak interaction between the introduced organic nitrogen-containing groups and the zeolite, the organic amines may leach into the environment and bring risks [20]. Thus, although organic impregnation modification can introduce many nitrogen-containing groups into a zeolite, the organic products may bring environmental risks, while the previous inorganic impregnation modification will not bring environment risks, but the ammonium content introduced into the zeolite is limited, and the adsorption capacity of modified zeolites for Hg²⁺ ions cannot be effectively improved. Therefore, it is necessary to study how to improve the content of nitrogen-containing groups introduced into zeolite by using the inorganic impregnation method.

In this study, a simple inorganic-ammonia-impregnation method was used to modify zeolite 4A synthesized using fly ash. The modification mechanism, the contents of the introduced amino groups, and the Hg²⁺ ions’ removal efficiency were analyzed and compared with organic grafting modification and inorganic ammonium chloride modification. On this basis, the Hg²⁺ ion adsorption mechanism, the removal efficiency of multiple coexisting ions, and the reusability of the modified zeolite were further determined. Finally, the modified zeolite was added to the leaching solution of the mercury-contaminated soil and the Hg²⁺ ion content was analyzed.

This study attempts to supply an inorganic amination modification method of a zeolite with a low-energy operation and efficient removal of Hg²⁺ ions using fly ash as a raw material to synthesize a potential cheap and safe adsorbent for the treatment of mercury-contaminated water and soil.

2. Materials and Methods

2.1. Materials and Reagents

The raw materials used in the synthesis of zeolite 4A were fly ash (43.98% Al₂O₃, 43.89% SiO₂) obtained from Ordos (Inner Mongolia, China) and diatomite (2.49% Al₂O₃, 89.16% SiO₂) purchased from Fengsheng Mining of China as a supplementary Si source. The reagents used in this study included sodium hydroxide (NaOH, 96%, assay), ammonia monohydrate (NH₃·H₂O, 25%, assay), silane coupling agent (KH792, 99%, assay), hydrochloric acid (HCl, 38%, assay), sodium carbonate (Na₂CO₃, 99.80%, assay), ammonium chloride (NH₄Cl, 99%, assay), and potassium iodide (KI, 99%, assay). The Hg-contaminated solution was prepared using mercury chloride (HgCl₂, 99.50%, assay), and ZnSO₄·7H₂O (99.00%, assay), Pb(NO₃)₂ (99.00%, assay), and Cu(NO₃)₂·3H₂O (99.50%, assay) were used to prepare multiple ion coexistence solutions.

2.2. Instruments

An X-ray diffractometer (XRD, Ultima IV, Rigaku Corporation, Tokyo, Japan) was used to analyze the diffraction patterns of the modified zeolite and the zeolite before and after the adsorption of the Hg²⁺ ions. A scanning electron microscope (SEM, SU8010, Hitachi Ltd., Tokyo, Japan) was used to observe the surface morphology of the zeolite. A surface area and porosity analyzer (ASAP 2460, Micromeritics Instrument Corporation, Norcross, GA, USA) was used to measure the BET surface area and N₂ adsorption–desorption isotherm of the adsorbents before and after modification. The contents of the organic materials before and after zeolite grafting were measured by an organic-element analyzer EA (UNICUBE, Elemental Analysis Systems Co., Ltd., Hanau, Germany). The functional groups before and after modification and the adsorption of the zeolite were measured by Fourier transform infrared spectroscopy (FTIR, Perkin Elmer Frontier, Perkin-Elmer Ltd., Waltham, MA, USA). An inductively coupled plasma optical emission spectrometer (ICP-OES, ICPE-9800, Shimadzu Company, Kyoto, Japan) was used to measure the Na⁺ ion concentration before and after zeolite modification. The Hg²⁺ ion concentration in the solution was measured by flame atomic absorption spectrometry with a hydride generator (FAAS-HVG-100, Shimadzu Company, Kyoto, Japan).

2.3. Synthesis and Amination Modification of Zeolite 4A

2.3.1. Zeolite 4A Synthesis

The zeolite 4A used in the experiment was hydrothermally synthesized with fly ash as the raw material. The specific synthesis steps were described in our previous study [21]. Fly ash, diatomite supplemented with a silicon source, and the alkali activator NaOH were mixed in a proportion of 9.1:0.9:16, and zeolite 4A was obtained after alkali melting, stirring, aging, crystallization, washing, and drying.

2.3.2. Inorganic Amination Modification

First, zeolite 4A, synthesized using fly ash, was dried, ground and passed through 200 mesh sieves, and then 3 g was added into NH₃·H₂O with different concentrations for 48 h. Through batch experiments, we determined that the best concentration of NH₃·H₂O was 10 wt%. After impregnation, the zeolite was vacuum filtered, washed, dried at 50 °C for 12 h, and then sealed and stored. The zeolite modified by NH₃·H₂O impregnation was named NH₃·H₂O-zeolite 4A.

The NH₄Cl modification mainly refers to Ikeda et al.’s method [22], but was further improved. The preparation steps were as follows: 3 g of zeolite 4A was slowly added into 0.5 M NH₄Cl, and then the mixture was stirred at 80 °C for 4 h, washed with deionized water, and filtered. Finally, the mixture was dried at 50 °C overnight and a modified zeolite was obtained, named NH₄Cl-zeolite 4A.

2.3.3. Organic Graft Modification

The organic graft modification mainly refers to the method put forth by Han et al. [23]. First, 50 mL of 95 wt% ethanol solution was added to a flask and stirred at 25 °C for 5 min. Then, 5 g of zeolite was put into the ethanol solution and continuously stirred. Then, 50 mL of 90 wt% ethanol was added to another flask and stirred at 25 °C for 5 min, and 5 wt% silane coupling agent (KH792) was slowly dropped with a disposable syringe and then stirred with ultrasonication for 0.5 h to ensure reagent hydrolysis. The obtained hydrolysis solution was quickly added to the mixture of the zeolite and ethanol and stirred and reacted at 60 °C for 4 h. Then, the obtained product was washed repeatedly with absolute ethanol and filtered in vacuum to remove the unreacted KH792. The filtered product was dried in a vacuum oven at 80 °C for 2 h and then ground and passed through a 200 mesh sieve to obtain the modified zeolite named KH792-zeolite 4A.

2.4. Adsorption Experiment

HgCl₂ (0.1354 g) was dissolved in 100 mL of deionized water to obtain a 1 g/L standard solution. Then, the standard solution was diluted, and Hg²⁺ ion solutions with different concentrations were prepared. The pH of the solution was adjusted with 0.1 M HCl and 0.1 M NaOH. Without special instruction, the initial concentration of Hg²⁺ ions was 10 mg/L, the amount of adsorbent was 0.1 g, the adsorption temperature was 25 °C, and the adsorption time was 240 min.

To determine the adsorption efficiency and the mechanism of the adsorbent for Hg²⁺ ions, batch experiments were carried out on the dosage of adsorbent, pH of the initial solution, concentration of the initial solution, adsorption time, and adsorption temperature. According to our previous research [24], when the solution’s pH was lower than 2, the zeolite structure could be destroyed under strong acid conditions. Moreover, due to the existence of Cl⁻ ions in the solution, when the solution pH was low, the Cl⁻ and Hg²⁺ ions could form anionic complexes, such as HgCl³⁻ [25,26,27], while when the pH was high, the Hg²⁺ ions would form negatively charged metal complexes with OH⁻ ions in alkaline conditions [28]. Thus, the pH range of the Hg²⁺ initial solution in this study was set from 3 to 7. The Hg²⁺ ion concentration gradients of isothermal adsorption were set to 5, 10, 20, 30, 50, 80, 100, 200, 300, 400, 450, and 500 mg∙L⁻¹. The time gradients of adsorption kinetics were 2, 7, 12, 30, 60, 120, 180, and 240 min, respectively. The thermodynamic temperature gradients were set at 25 °C, 35 °C, and 45 °C. All experiments were carried out in an oscillator at 180 rpm. After adsorption, the solution was filtered using a disposable syringe with a 0.45 μm filter membrane. Each adsorption experiment was repeated twice. The adsorption rate Qc (%) and adsorption capacity Qe (mg/g) of the adsorbent for the Hg²⁺ ions were calculated by the following equations:

(1)$Qc = \frac{\left(C_0-C_e\right)}{C_0}\times 100\%$

(2)$Qe = \frac{\left(C_0-C_e\right)V}{m}$

where C₀ and C_e are the Hg²⁺ ion concentrations in the initial and adsorption equilibrium solutions, respectively, mg/L; V is the volume of solution, L; and m is the adsorbent amount, g.

2.5. Cation Effect on Hg²⁺ Ion Adsorption

The Hg²⁺ ion adsorption rate of NH₃·H₂O-zeolite 4A with the coexistence of Pb²⁺, Cu²⁺, and Zn²⁺ ions at the same concentration in the solution was studied. The initial ion concentration was 10 mg/L, the pH was 6, and 0.1 g of NH₃·H₂O-zeolite 4A was added. After shaking for 4 h at 25 °C to reach adsorption equilibrium, the concentration of each cation was measured. The experiments were repeated twice, and the average value was taken as the final measured value.

2.6. Zeolite Reuse Experiment

A KI solution was used as the adsorbent eluent. After adsorption saturation with 1 g/L Hg²⁺ ions, 0.5 g of NH₃·H₂O-zeolite 4A was put into 50 mL of the 0.1 M KI solution, and the solution was shaken at 25 °C for 60 min with a frequency of 180 rpm for desorption. Then, the sample was filtered and washed with deionized water, and this process was repeated three times to fully desorb the adsorbed Hg²⁺ ions. Finally, the washed sample was dried for the next adsorption experiment. Five adsorption–desorption experiments were conducted. Each group of experiments was repeated twice.

2.7. Removal of Hg²⁺ Ions in Soil Leaching by the Modified Zeolite

According to our previous study, the soil around the Tangshan steel plant was seriously contaminated by mercury [29]. A 1 g soil sample with the highest mercury content around the steel plant was added to 20 mL of deionized water, and then 0.01 g of NH₃·H₂O-zeolite 4A was added. At the same time, a blank control experiment was set. The samples were shaken for 6, 18, 24, 36, and 48 h to measure the Hg²⁺ ion concentration in the soil-leaching solutions. Each experiment was repeated twice.

3. Results

3.1. Amination Modification of Zeolite

3.1.1. Characterization

Zeolite 4A was successfully synthesized using fly ash. The XRD pattern showed that the characteristic peaks of the synthesized zeolite 4A were 7.2°, 10.3°, 12.6°, 16.2°, 21.8°, 24.0°, 27.2°, 29.9°, and 34.2° (Figure 1a), which were consistent with the standard peaks of zeolite 4A [30]. The characteristic peaks of NH₃·H₂O-zeolite 4A, KH792-zeolite 4A, and NH₄Cl-zeolite were the same as zeolite 4A, indicating that the three modifications did not affect the skeleton structure of zeolite (Figure 1b–d).

Zeolite 4A synthesized using fly ash was a typical small cube (Figure 2a). The surface morphologies of NH₃·H₂O-zeolite 4A and NH₄Cl-zeolite 4A showed no significant changes compared with the original zeolite (Figure 2b,c). However, compared with the original zeolite, the surface of KH792-zeolite 4A became rough (Figure 2d), which may have been because the silane group of KH792 reacted with the free hydroxyl group on the zeolite surface to form a covalent bond and reduce the surface tension of the modified zeolite, and the adhesion between the grafted amine functional groups of KH792 increased the trend of large agglomerate formation [31].

3.1.2. Synthesis Mechanism

In the FTIR spectrum, the peak at 465 cm⁻¹ of zeolite 4A corresponded to the bending–stretching vibration of T-O (where T is Al or Si), the peak at 558 cm⁻¹ was the characteristic peak of the double-ring vibration in the tetrahedron, the peak at 993 cm⁻¹ corresponded to the characteristic peak of the tetragonal stretching vibration of TOT (where T is Al or Si), and 3430 cm⁻¹ was related to the stretching vibration of Si-OH. These characteristic peaks were the characteristic peaks of typical zeolite 4A [32] (Figure 3a). The peak at 1401 cm⁻¹ of NH₃·H₂O-zeolite 4A was assigned to the deformation vibration of the NH₄⁺ ions [33], indicating that the partial Na⁺ ions of zeolite 4A might have exchanged with NH₄⁺ in the solution. The peak intensity at 993 cm⁻¹ decreased, indicating that ion exchange might have had an impact on the ring structure. The peak at 3421 cm⁻¹ was attributed to the Si-NH₂ vibration [34], indicating that amino functional groups were formed on NH₃·H₂O-zeolite 4A (Figure 3b). In the FTIR spectrum of KH792-zeolite 4A, the characteristic peak at 1635 cm⁻¹ was attributed to -N-H bonds [35], and the peaks at 2850 and 2920 cm⁻¹ were attributed to -CH- asymmetric and symmetric stretching vibrations [36], indicating that KH792 was successfully grafted onto the zeolite. The peak at 3430 cm⁻¹ of KH792-zeolite 4A was significantly lower than that of zeolite 4A, indicating that KH792 consumed some silanol groups on the surface of the zeolite after grafting (Figure 3c). The peak of NH₄Cl-zeolite 4A at 1401 cm⁻¹ was assigned to the deformation vibration of NH₄⁺ [33], and the obvious peak height indicated that more NH₄⁺ ions had exchanged with Na⁺ ions (Figure 3d). However, no vibration peak of Si-NH₂ was found in the FTIR spectrum of NH₄Cl-zeolite 4A.

Through experimental detection, it was found that the concentration of Na⁺ ions in the solution after impregnating zeolite 4A with NH₃·H₂O was higher than that with deionized-water impregnation (Figure 4a), which shows that Na⁺ ions in the zeolite exchanged with NH₄⁺ ions in NH₃·H₂O, and the cation exchange capacity reached 41.28%. Both this and the FTIR results prove that NH₄⁺ ions successfully entered the structure of zeolite 4A after ammonia impregnation. The determination results of the element analyzer show that the amount of N and C in KH792-zeolite 4A increased compared with that of zeolite 4A (Figure 4b). Thus, the experimental detection and FTIR results prove that amine groups were successfully grafted onto zeolite 4A.

According to the above analysis, the inorganic and organic modification mechanisms of zeolite could be determined. In the NH₃·H₂O and NH₄Cl modifications, the Na⁺ ions in zeolite 4A could exchange with the NH₄⁺ ions in the solution to form nitrogen-containing groups and enter the zeolite structure (Equation (3)). In addition, a large number of hydroxyl groups on the surface of zeolite 4A could react with NH₃·H₂O and introduce -NH₂ groups into the zeolite [37] (Figure 3b, Equation (4)). Therefore, NH₃·H₂O modification could introduce more nitrogen-containing groups than NH₄Cl modification, which might improve the Hg²⁺ ion-removal efficiency of the modified zeolite in the solution.

(3)${\mathrm{Na}}^{+}-\mathrm{zeolite}+{\mathrm{NH}}_4{}^{+}\to {\mathrm{NH}}_4{}^{+}-\mathrm{zeolite}+{\mathrm{Na}}^{+}$

(4)$\mathrm{Zeolite}-\mathrm{OH}+{\mathrm{NH}}_3·{\mathrm{H}}_2\mathrm{O}\stackrel{-2{\mathrm{H}}_2\mathrm{O}}{\to }\mathrm{Zeolite}-{\mathrm{NH}}_2$

During organic modification, the silicon atom in KH792 could react with the hydroxyl group on the zeolite surface [38] and introduce amino functional groups onto zeolite 4A (Figure 5). Due to the fact that KH792 contains two amino functional groups, the amount of amino functional groups introduced by the organic grafting modification was the same as those introduced by inorganic NH₃·H₂O impregnation.

3.1.3. Pore Size Distribution

Through the N₂ adsorption–desorption isotherms (Figure 6), the specific surface areas of zeolite 4A, NH₃·H₂O-zeolite 4A, and KH792-zeolite 4A were 20.9 m²/g, 28.4 m²/g, and 31.33 m²/g, respectively, indicating that the specific surface areas of the modified zeolites were improved. The isotherms of all of the adsorbents were type I [39], which belonged to a typical microporous structure, indicating that aminated modification could not change the microporous structure of zeolite 4A.

3.2. Hg²⁺ Ion Adsorption by Modified Zeolite

3.2.1. Adsorption Efficiency

The Hg²⁺ ion-adsorption rate of zeolite 4A was only 75.5%, while the Hg²⁺ ion-adsorption rates of NH₃·H₂O-zeolite 4A and KH792-zeolite 4A were 99.25% and 98.31%, respectively, indicating that the Hg²⁺ ion-removal abilities of these two kinds of modified zeolites were significantly improved (p < 0.05) (Figure 7). The Hg²⁺ ion-adsorption rate of NH₄Cl-zeolite 4A was 85.05%, which was significantly improved compared with that of the original zeolite 4A, but was significantly lower than those of the other two modified zeolites (p < 0.05), which might be due to the small amount of nitrogen-containing groups introduced into NH₄Cl-zeolite 4A. Thus, the adsorptions of Hg²⁺ ions by NH₃·H₂O-zeolite 4A and KH792-zeolite 4A were mainly studied and compared.

3.2.2. Effect of Initial Solution pH

The Hg²⁺ ion removal rates of NH₃·H₂O-zeolite 4A and KH792-zeolite 4A gradually increased with an increasing initial solution pH (Figure 8). When the initial solution pH was three, the Hg²⁺ ion removal rates of NH₃·H₂O-zeolite 4A and KH792-zeolite 4A were 85.83% and 81.51%, respectively. When the initial solution pH was six, the Hg²⁺ ion removal rates of NH₃·H₂O-zeolite 4A and KH792-zeolite 4A were the highest, reaching 99.25% and 98.31%, respectively. Due to the fact that the amino functional groups might have been protonated to form -NH₃⁺ when the pH value was low [40], which would have reduced the adsorption ability of the modified zeolite for Hg²⁺ ions, the initial solution pH was set to six.

3.2.3. Adsorption Model

With an increase in Hg²⁺ concentration, the adsorption capacity of the modified zeolite gradually reached saturation, and the maximum Hg²⁺ ion-adsorption capacities of NH₃·H₂O-zeolite 4A and KH792-zeolite 4A were 49.45 mg/g and 40.64 mg/g, respectively (Figure 9a). Through isotherm adsorption model fitting (Figure 9b,c and Table 1), the Langmuir model had the highest correlation coefficient for the modified zeolite, indicating that the removal of Hg²⁺ ions by NH₃·H₂O-zeolite 4A and KH792-zeolite 4A was the single-layer surface adsorption and irreversible [41]. According to the isothermal adsorption model fitting, the maximum Hg²⁺ ion-adsorption capacities of NH₃·H₂O-zeolite 4A and KH792-zeolite 4A were 53.55 mg/g and 42.93 mg/g, respectively, which were basically consistent with the experimental data. Therefore, the maximum Hg²⁺ ion-adsorption capacity of NH₃·H₂O-zeolite 4A was slightly higher than that of KH792-zeolite 4A.

Through adsorption kinetic model fitting, it was found that NH₃·H₂O-zeolite 4A and KH792-zeolite 4A showed similar adsorption kinetic characteristics, and the Hg²⁺ ion desorption kinetics models of NH₃·H₂O-zeolite 4A and KH792-zeolite 4A were better fits with the pseudo second-order adsorption model (Figure 10 and Table 2), indicating that chemisorption was the reaction-rate control step and chemisorption was the main reaction mechanism of Hg²⁺ ion-removal by the two kinds of modified zeolite [42]. The kinetic parameter K₂ of NH₃·H₂O-zeolite 4A was significantly greater than that of KH792-zeolite 4A, indicating that NH₃·H₂O-zeolite 4A adsorbed Hg²⁺ ions in solution faster than KH792-zeolite 4A, which might be due to the large molecular polymer of KH792 grafted on the zeolite surface filling or blocking the micropores of the zeolite [43] and affecting the diffusion of Hg²⁺ ions.

The adsorption abilities of NH₃·H₂O-zeolite 4A and KH792-zeolite 4A for Hg²⁺ ions in the solution increased with increasing temperatures (Figure 11). From the thermodynamic parameter calculation (Table 3), the ΔH° of NH₃·H₂O-zeolite 4A and KH792-zeolite 4A were less than 0, indicating that the adsorption processes of NH₃·H₂O-zeolite 4A and KH792-zeolite 4A for Hg²⁺ ions was exothermic; the ΔS° was greater than 0, showing that the Hg²⁺ ion adsorptions by NH₃·H₂O-zeolite 4A and KH792-zeolite 4A were entropy-increasing processes, which might be because the generated chemical bonds increased the chaos of the system [44]; and the ΔG° of NH₃·H₂O-zeolite 4A and KH792-zeolite 4A were less than 0, indicating that the adsorptions of Hg²⁺ ions by these kinds of modified zeolites were spontaneous without energy supply until the zeolites reached adsorption saturation. Usually, when the ΔH° value was in the range of 80–200 kJ/mol, the adsorption method was mainly chemical adsorption [45]. Therefore, the Hg²⁺ ion-removals by NH₃·H₂O-zeolite 4A and KH792-zeolite 4A were mainly conducted through chemical adsorption.

Therefore, because the content of nitrogen-containing groups introduced by simple NH₃·H₂O impregnation was the same as that introduced by the grafting method, NH₃·H₂O-zeolite 4A could effectively remove Hg²⁺ ions in the solution by chemical adsorption. Its Hg²⁺ ion adsorption rate was faster than that of KH792-zeolite 4A due to the fact that the micropores of the zeolite were not blocked, unlike in organic grafting.

3.2.4. Adsorption Mechanism

The XRD pattern shows that the peak value had no obvious change before and after Hg²⁺ ion adsorption by NH₃·H₂O-zeolite 4A (Figure 12a), indicating that no crystal phase formed. However, the diffraction intensity of the characteristic peak after adsorption was significantly lower than that before adsorption, indicating that the crystallinity of the zeolite after adsorption was reduced and that the amorphous phase might have been increased [46]. According to the FTIR spectrum (Figure 12(b1)), the peak at 1401 cm⁻¹ of NH₃·H₂O-zeolite 4A before adsorption was the NH₄⁺ characteristic peak, and this peak was not found after adsorption (Figure 12(b2)), indicating that the ion-exchange reaction between NH₄⁺ ions and Hg²⁺ ions had taken place. In addition, the peak at 1640 cm⁻¹ was characteristic of the Si-OH group peak and at 3421 cm⁻¹ was characteristic of the Si-NH₂ group peak in NH₃·H₂O-zeolite 4A. After the adsorption of the Hg²⁺ ions, the peak value was obviously lower than that before adsorption, indicating that these groups complexed with Hg²⁺ ions (Figure 12c).

Therefore, through the thermodynamic analysis, the Hg²⁺ ion removal by NH₃·H₂O-zeolite was demonstrated to have mainly taken place through chemical adsorption, and the FTIR analysis shows that the main Hg²⁺ ions-removal mechanism was ion exchange with NH₄⁺ introduced and a complexation reaction with the NH₂ group introduced. Thus, the zeolite 4A modified by simple NH₃·H₂O impregnation has a higher adsorption capacity for Hg²⁺ ions than that of other products (Table 4) due to the introduction of more inorganic nitrogen-containing functional groups.

3.3. Coexisting Multiple Ion Effect and Reusability

When Hg²⁺ was mixed with Cu²⁺, Pb²⁺, and Zn²⁺ ions, the Hg²⁺ removal rate of NH₃·H₂O-zeolite 4A in the solution was basically unchanged, and remained at 99.325%. When Hg²⁺ coexisted with Cu²⁺, Pb²⁺, and Zn²⁺ ions, the removal rate of NH₃·H₂O-zeolite 4A for Hg²⁺ ions remained 99.325%, for Cu²⁺ ions was 26.05%, for Zn²⁺ ions was 5.95%, and for Pb²⁺ ions reached 99.05% (Figure 13). Thus, NH₃·H₂O-zeolite 4A had a high adsorption capacity for Hg²⁺ and Pb²⁺ ions, while exhibiting weak removal rates for Cu²⁺ and Zn²⁺ ions, which could be explained by RG Pearson’s soft-and-hard acid-base theory. Hg²⁺ and Pb²⁺ ions are soft acids, and the amino groups of soft bases in the modified zeolite have the ability of preferential adsorption [51]. Although the Cu²⁺ ion is also a soft acid, the Pb²⁺ ion has more orbital overlap with the lone pair electrons of nitrogen atoms, and the adsorption energy of the Pb²⁺ ion is less than that of the Cu²⁺ ion, so the Pb²⁺ ion would be adsorbed preferentially, and the Cu²⁺ ion has difficulty occupying the Pb²⁺ ion-adsorption site [52]. Therefore, when there were a variety of heavy metal ions in the solution, NH₃·H₂O-zeolite 4A maintained a high adsorption ability for Hg²⁺ ions.

The reusability of the adsorbent is very important to save costs in practical applications. The results of the recycling experiment show that the Hg²⁺ ion-adsorption efficiency of NH₃·H₂O-zeolite 4A was 87.22% after four cycles and 72.03% after five cycles (Figure 14a), which indicates that NH₃·H₂O-zeolite 4A had good reusability. FTIR spectra showed that the surface functional groups of NH₃·H₂O-zeolite 4A had almost no change (Figure 14b), which indicates that the nitrogen-containing functional groups introduced into the zeolite after NH₃·H₂O impregnation would not be affected by multiple elution. The gradual decline of the removal rate might be due to the loss of adsorbents during elution.

3.4. Hg²⁺ Ion Adsorption in Soil Leaching

After adding deionized water to mercury-contaminated soil, the concentration of Hg²⁺ ions in the leaching solution reached equilibrium after 24 h (Table 5). The maximum content of Hg²⁺ ions in the soil leaching was 5.82 mg/kg, while the content of Hg²⁺ ions in the soil leaching was only 0.17 mg/kg after adding 1% (0.01 g) NH₃·H₂O-zeolite 4A. Therefore, in mercury-contaminated soil leaching, NH₃·H₂O-zeolite 4A could efficiently remove Hg²⁺ ions and reduce additional soil-pollution risks.

4. Conclusions

In this study, zeolite 4A synthesized using fly ash was modified by simple inorganic ammonia impregnation, which could introduce more kinds of inorganic amine functional groups into the zeolite, thus improving the removal capacity of the modified zeolite for Hg²⁺ ions. The mechanism of impregnation modifications with NH₃·H₂O, KH792, and NH₄Cl for zeolite 4A, and the removal capacity of Hg²⁺ ions by modified zeolites, were studied and compared. It was found that NH₃·H₂O modification could introduce nitrogen-containing groups through ion exchange and a reaction with hydroxyl groups on the zeolite surface, while NH₄Cl modification could introduce NH₄⁺ into the zeolite through ion exchange, and organic modification could introduce KH792 containing two kinds of nitrogen-containing groups into the zeolite through grafting. Therefore, NH₃·H₂O modification and organic grafting introduced more nitrogen-containing groups than NH₄Cl modification.

The Hg²⁺ ion-removal rates of NH₃·H₂O-zeolite 4A were significantly higher than those of KH792-zeolite 4A and NH₄Cl-zeolite 4A. According to the isothermal adsorption model fitting, the maximum Hg²⁺ ion-adsorption capacity of NH₃·H₂O-zeolite 4A achieved was 53.55 mg/g. The thermodynamic analysis showed that the Hg²⁺ ion adsorption of NH₃·H₂O-zeolite 4A in the solution was mainly achieved through chemical adsorption. The XRD and FTIR analyses showed that the adsorption of Hg²⁺ ions by NH₃·H₂O-zeolite 4A was mainly achieved through ion exchange between NH₄⁺ and Hg²⁺ ions and the complexation of nitrogen-containing functional groups of the modified zeolite with Hg²⁺ ions.

When Cu²⁺, Pb²⁺, Zn²⁺, and Hg²⁺ ions coexisted, the Hg²⁺ ion adsorption ability of NH₃·H₂O-zeolite 4A was not affected. After five adsorption and desorption cycles, the Hg²⁺ ion-adsorption rate of NH₃·H₂O-zeolite 4A was 72.03%. Therefore, NH₃·H₂O-zeolite 4A had good reusability. When NH₃·H₂O-zeolite 4A was added to the leaching of mercury-contaminated soil, the content of soluble mercury in the leaching could be reduced from 5.82 mg/kg to 0.17 mg/kg. Thus, NH₃·H₂O-zeolite 4A exhibited an efficient Hg²⁺ ion-removal ability

In this study, we used cheap industrial solid-waste fly ash as a raw material to synthesize zeolite 4A, then applied simple NH₃·H₂O impregnation with low energy consumption to obtain a modified zeolite with an effective adsorption capacity for Hg²⁺ ions. The modified zeolite was not only low in synthesis cost and simple in modification method, but also had environmental safety due to inorganic modification. Therefore, our synthesis product has broad potential application prospects in the treatment of mercury-contaminated water and soil. However, to avoid environmental risks, further research is needed on the recovery of mercury in eluate during the reuse of modified zeolites. In addition, our study was only based on a laboratory experiment, so small-scale research on the practical applications of modified products needs to be further carried out to provide a feasible technical basis for the practical applications of modified products.

Author Contributions

Conceptualization, L.Y., T.N. and M.G.; methodology, L.Y. and M.G.; validation, T.N.; analysis, M.G., S.Y., T.J. and F.W.; experiment, M.G., T.J. and F.W.; writing—original draft preparation, M.G. and L.Y.; writing—review and editing, L.Y. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Figures and Tables

Figure 1. XRD patterns of zeolite 4A (a), NH3·H2O-zeolite 4A (b), KH792-zeolite 4A (c), and NH4Cl-zeolite 4A (d).

Figure 2. Morphological characteristics of zeolite 4A (a), NH3·H2O-zeolite 4A (b), NH4Cl-zeolite 4A (c), and KH792-zeolite 4A (d).

Figure 3. FTIR spectra of zeolite 4A (a), NH3·H2O-zeolite 4A (b), KH792-zeolite 4A (c), and NH4Cl-zeolite 4A (d).

Figure 4. Na+ ion concentration in H2O and NH3·H2O (a) and the contents of N and C elements in zeolite 4A and KH792-zeolite 4A (b).

Figure 5. Chemical-grafting process between KH792 and zeolite 4A.

Figure 6. N2 adsorption–desorption isotherms of zeolite 4A (a), NH3·H2O-zeolite 4A (b), and KH792-zeolite 4A (c).

Figure 7. Hg2+ ion-adsorption rates of zeolite 4A and amination-modified zeolites (adsorbent dosage of 0.1 g, pH 6, temperature 25 °C, and Hg2+ ion concentration of 10 mg/L). (The same letter on the column shows no significant difference, while different letters indicate a significant difference p < 0.05).

Figure 8. Effect of initial solution pH on the Hg2+ ion-adsorption efficiency of modified zeolites (adsorbent dosage of 0.1 g, temperature 25 °C, and Hg2+ ions concentration of 10 mg/L).

Figure 9. Effect of Hg2+ ion concentration on the adsorption capacities of modified zeolites (a) and the isothermal Hg2+ ion-adsorption model fitting of NH3·H2O-zeolite 4A (b) and KH792-zeolite 4A (c).

Figure 10. The Hg2+ ion-adsorption kinetic model fitting of NH3·H2O-zeolite 4A (a) and KH792-zeolite 4A (b).

Figure 11. Langmuir isothermal adsorption models of NH3·H2O-zeolite 4A (a) and KH792-zeolite 4A (b) at different temperatures.

Figure 12. XRD patterns before (1) and after (2) the adsorption of Hg2+ ions by NH3·H2O-zeolite 4A (a); FTIR spectra before (1) and after (2) the adsorption of Hg2+ ions by NH3·H2O-zeolite 4A (b); and the reaction mechanism of Hg2+ ion adsorption by NH3·H2O-zeolite 4A (c).

Figure 12. XRD patterns before (1) and after (2) the adsorption of Hg2+ ions by NH3·H2O-zeolite 4A (a); FTIR spectra before (1) and after (2) the adsorption of Hg2+ ions by NH3·H2O-zeolite 4A (b); and the reaction mechanism of Hg2+ ion adsorption by NH3·H2O-zeolite 4A (c).

Figure 13. Adsorption rate of NH3·H2O-zeolite 4A for Hg2+, Cu2+, Pb2+, and Zn2+ ions when these ions coexist (adsorbent dosage of 0.1 g, pH 6, temperature of 25 °C, and coexisting ion concentration of 10 mg/L).

Figure 14. The reusability of NH3·H2O-zeolite 4A for the adsorption of Hg2+ ions (a) and FTIR spectra before (1) and after 5 cycles (2) of adsorption of Hg2+ ions by NH3·H2O-zeolite 4A (b).

Table 1

Isothermal adsorption parameters for the adsorption of Hg²⁺ ions by modified zeolites.

Isothermal Adsorption Model	Parameter	NH3·H2O-Zeolite 4A	KH792-Zeolite 4A
Langmuir model	Qmax (mg/g)	53.55	42.93
	KL (L/mg)	0.003	0.0014
	RL	0.625	0.78
	R2	0.9997	0.9993
Freundlich model	Kf (mg/g)	25.36	0.29
	R2	0.9958	0.9977

Explanation of model parameters in Supplementary Materials.

Table 2

The adsorption kinetic model fitting parameters.

	Pseudo First-Order Dynamics			Pseudo Second-Order Dynamics
	Qe (mg/g)	K1	R2	Qe (mg/g)	K2	R2
NH3·H2O-zeolite 4A	1.79	0.27	0.4194	1.97	0.21	0.9063
KH792-zeolite 4A	1.74	0.28	0.4293	1.87	0.16	0.9617

Explanation of model parameters in Supplementary Materials.

Table 3

Thermodynamic parameters of Hg²⁺ ion-adsorption by NH₃·H₂O-zeolite 4A and KH792-zeolite 4A.

	ΔH° (kJ/mol)	ΔS° (kJ/mol·K)	ΔG° (kJ/mol)
			25 °C	35 °C	45 °C
NH3·H2O-zeolite 4A	−89.22	0.13	−127.96	−130.56	−133.16
KH792-zeolite 4A	−93.08	0.17	−143.74	−147.14	−150.54

Explanation of model parameters in Supplementary Materials.

Table 4

Comparison of Hg²⁺ ion-adsorption capacities of various adsorbents.

Sample	Adsorption Condition	Maximum Adsorption Capacity (mg/g)	References
Thiol- and hydroxyl-containing bifunctional vermiculite	pH 4 and dosage 2 g/L	8.57	[47]
Adsorption columns using activated carbon and zeolite	pH from 0 to 7 and dosage 1 g/L	5.5	[48]
Opal-palygorskite clay zeolite 4A	pH 3 and dosage 1 g/L	41.99	[49]
13X zeolite	pH from 5 to 6 and dosage 100 g/L	16.35	[50]
KH792-zeolite 4A	pH 6 and 5 g/L	42.93	This work
NH3·H2O-zeolite 4A	pH 6 and 5 g/L	53.55	This work

Table 5

Concentration of Hg²⁺ ions (mg/kg) in the soil leaching at different times.

	Washing Time (h)
	6	12	24	36	48
Adding NH3·H2O-zeolite 4A	0.013	0.112	0.157	0.142	0.170
No adding	1.065	3.355	5.76	5.74	5.82

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/su142315924/s1. References [53,54,55,56,57] are cited in the Supplementary Materials.

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