*2.5. Adsorption/Desorption of Hg2+ Using Sodium Hydroxide Solution*

To evaluate the recycling of FA48 in Hg2+ adsorption/desorption, FA48 (0.15 g) was added to a 150 mL Hg2+ solution at 250 mg/L. The suspension was shaken at 100 rpm, 25 ◦C for 24 h, and filtered through a 0.45 µm membrane filter. The concentration of Hg2+ was measured using an iCAP-7600 Duo. The amount of Hg2+ adsorbed was calculated as described in Section 2.2. After adsorption, FA48 was collected, dried, and used for the desorption experiment. The collected FA48 (0.05 g) was added to 50 mL sodium hydroxide solution at 10, 100, 1000 mmol/L. The suspension was shaken at 100 rpm, 25 ◦C for 24 h, and filtered through a 0.45 µm membrane filter. The concentration of Hg2+ released from FA48 was also measured using an iCAP-7600 Duo. The amount of Hg2+ desorbed was calculated using the levels before and after desorption. All results in this study are expressed as mean ± standard error (*n* = 2–3, Sections 2.2–2.5). In addition, each Figure was prepared using Microsoft Excel.

### **3. Results and Discussion**

### *3.1. Properties of Potassium-Type Zeolite*

Zeolites are characterized by physicochemical properties, such as specific surface area, pore volume, and CEC (Table 1). These characteristics are related to the parameters of hydrothermal treatment, such as heat temperature, pressure, solution alkalinity, activation solution to CFA ratio, and formation process [18]. In this study, six types of potassium-type zeolites were prepared using the above-mentioned method [28]. In addition, our previous study reported the physicochemical properties of potassium-type zeolites in detail [28]. The XRD patterns indicate that CFA was mainly composed of mullite and quartz. The XRD patterns of FA1, FA3, FA6 and FA12 were similar to those of CFA under our experimental conditions. Zeolite F appeared in FA24 and FA48 structures. We observed changes in the surface of FA series with the treatment time. Aluminosilicate gels were clearly produced on FA24 and FA48 surfaces. These processes were in the following order: Al and Si dissolution, geopolymer formation, crystalline structure nucleation, finally zeolite crystal growth [18]. The CEC of FA48 (8.98–11.77 mmol/g) was the highest compared to other FA series. This value of FA48 was 26–69 times higher than that of CFA. The pHpzc of FA was 9.8, and that of FA24 and FA48 were 10.4. Finally, specific surface area and pore volume (*d* 5 20 Å) of FA48 (47.3 m2/g and 10 Å) was 34 and 100 times higher than CFA. Additionally, the value of FA48 (potassium-type zeolite) was greater than that of sodium-type zeolite [32]. Thus, these results indicate that potassium-type zeolite (FA48) can be prepared from coal fly ash by conventional hydrothermal treatment using potassium hydroxide.


