*3.7. Adsorption/Desorption Capability of Hg2+ Using FA48*

Finally, the adsorption/desorption capability of Hg2+ using FA48 was demonstrated in this study (Figure 6). The quantity of Hg2+ desorbed increased with increasing concentration of sodium hydroxide solution from 10 to 1000 mmol/L (the quantity of Hg2+ adsorbed was approximately 40 mg/g). The desorption percentages using 10, 100, and 1000 mmol/L sodium hydroxide solutions were 37.5%, 41.6%, and 68.3%, respectively. Therefore, adsorbed Hg2+ onto FA48 could be easily desorbed using a sodium hydroxide solution under our experimental conditions. Further investigations are needed to elucidate the application of FA48 in these fields.

**Figure 6.** Adsorption/Desorption capability of Hg2+ using FA48. Adsorption condition; initial concentration: 250 mg/L, sample volume: 150 mL, adsorbent: 0.15 g, temperature: 25 ◦C, contact time: 24 h, agitation speed: 100 rpm, Desorption condition; initial concentration: 10, 100, and 1000 mmol/L, sample volume: 50 mL, adsorbent: 0.05 g, temperature: 25 ◦C, contact time: 24 h, agitation speed: 100 rpm.

### **4. Conclusions**

Six types of potassium zeolites (FA1, FA3, FA6, FA12, FA24, FA48) were synthesized by hydrothermal treatment using a potassium hydroxide solution. The values of CEC, specific surface area, and pore volume (*d* 5 20 Å) of FA48 were 26–29, 34, and 100 times higher than that of CFA, indicating that FA48 has a high potential for the removal of Hg2+ from aqueous media. The quantity of Hg2+ was in the order CFA and FA1 (0–0.48 mg/g) < FA3 (2.2 mg/g) < FA6 (3.5 mg/g) < FA12 (4.0 mg/g) < FA24 (7.5 mg/g) < FA48 (11.6 mg/g) under our experiment conditions. These adsorption behaviors were affected by the CEC and/or pore volume (*d* ≤ 20 Å). In addition, the effects of pH, temperature, contact time, and coexistences on the adsorption of Hg2+ using FA48 were demonstrated. The optimal pH was approximately 5.0. The adsorption isotherm data or kinetics data were described by the Freundlich and Langmuir models or the pseudo-second-order model, respectively. Moreover, one of the adsorption mechanisms determined was the ion exchange with K<sup>+</sup> in the interlayer of FA48 (correlation coefficient = 0.946). FA48 showed selectivity for the adsorption of Hg2+ from a binary solution system containing Na<sup>+</sup> , Mg2+, K<sup>+</sup> , Ca2+, Ni2+ , Cu2+, Zn2+, Sr2+, or Cd2+. Finally, adsorbed Hg2+ onto FA48 was easily desorbed using a sodium hydroxide solution. It is evident that FA48 is a useful adsorbent for Hg2+ removal from aqueous media. These techniques may potentially aid in mitigating heavy metal pollution and thus contribute to the establishment of a sustainable society.

**Author Contributions:** Conceptualization, F.O. and N.K.; investigation, Y.K., C.S., and T.N.; writing original draft preparation, Y.K. and F.O.; writing—review and editing, F.O. and N.K.; project administration, N.K. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**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.

### **References**


MDPI St. Alban-Anlage 66 4052 Basel Switzerland Tel. +41 61 683 77 34 Fax +41 61 302 89 18 www.mdpi.com

*Sustainability* Editorial Office E-mail: sustainability@mdpi.com www.mdpi.com/journal/sustainability

MDPI St. Alban-Anlage 66 4052 Basel Switzerland

Tel: +41 61 683 77 34

www.mdpi.com ISBN 978-3-0365-6602-3