*Article* **Adsorption/Desorption Capability of Potassium-Type Zeolite Prepared from Coal Fly Ash for Removing of Hg2+**

**Yuhei Kobayashi <sup>1</sup> , Fumihiko Ogata <sup>1</sup> , Chalermpong Saenjum 2,3 , Takehiro Nakamura <sup>1</sup> and Naohito Kawasaki 1,4,\***


**Abstract:** The feasibility of using potassium-type zeolite (K-type zeolite) prepared from coal fly ash (CFA) for the removal of Hg2+ from aqueous media and the adsorption/desorption capabilities of various potassium-type zeolites were assessed in this study. Potassium-type zeolite samples were synthesized by hydrothermal treatment of CFA at different intervals (designated CFA, FA1, FA3, FA6, FA12, FA24, and FA48, based on the hours of treatment) using potassium hydroxide solution, and their physicochemical characteristics were evaluated. Additionally, the quantity of Hg2+ adsorbed was in the order CFA, FA1 < FA3 < FA6 < FA12 < FA24 < FA48, in the current experimental design. Therefore, the hydrothermal treatment time is important to enhance the adsorption capability of K-type zeolite. Moreover, the effects of pH, temperature, contact time, and coexistence on the adsorption of Hg2+ were elucidated. In addition, Hg2+ adsorption mechanism using FA48 was demonstrated. Our results indicated that Hg2+ was exchanged with K<sup>+</sup> in the interlayer of FA48 (correlation coefficient = 0.946). Finally, adsorbed Hg2+ onto FA48 could be desorbed using a sodium hydroxide solution (desorption percentage was approximately 70%). Our results revealed that FA48 could be a potential adsorbent for the removal of Hg2+ from aqueous media.

**Keywords:** hydrothermal activation treatment; recycling technology; heavy metal; ion exchange

### **1. Introduction**

The 2030 agenda for sustainable development, such as clean water and sanitation (Goal 6) and life below water (Goal 14), were adopted by all member states of the United Nations in 2015 [1], to establish a sustainable society, which is a matter of global concern. In particular, heavy metal pollution has become a severe global environmental issue, including in the developing countries. Among them, mercury (Hg2+), lead (Pb2+), and cadmium (Cd2+) are referred to as the "big three" heavy metals with the greatest potential risk to human health and water environment [2–4]. They are highly toxic to organisms [5]. Mercury (Hg) and its compounds can cause serious threats to organisms, including humans, because of their bioaccumulative properties, damaging the bones, liver, kidney, and nervous system [6–8]. The Minamata Convention on Mercury was adopted by the Intergovernmental Negotiating Committee in 2017. The International Agency for Research on Cancer categorizes methylmercury compounds as group 2B (possibly carcinogenic to humans), and metallic Hg and inorganic Hg compounds as group 3 (unclassifiable as to carcinogenicity in humans) [9]. In addition, the maximum permissible limit of Hg in drinking water as recommended by the U.S. Environmental Protection Agency and many

**Citation:** Kobayashi, Y.; Ogata, F.; Saenjum, C.; Nakamura, T.; Kawasaki, N. Adsorption/Desorption Capability of Potassium-Type Zeolite Prepared from Coal Fly Ash for Removing of Hg2+ . *Sustainability* **2021**, *13*, 4269. https://doi.org/10.3390/su13084269

Academic Editors: Mohd Rafatullah and Masoom Raza Siddiqui

Received: 10 March 2021 Accepted: 9 April 2021 Published: 12 April 2021

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countries are 2 µg/L and 1 µg/L, respectively [7,10,11]. Therefore, removal of Hg2+ from aqueous media is crucial for human health and conservation of the water environment.

Coal is one of the most abundant energy sources worldwide [12]. A previous study reported that the global trend of increasing energy production continued in 2018 [13]. In addition, it is desirable to increase coal-fired power generation by up to 46% of the total electricity production by 2030 [14]. Additionally, the demand for coal-fired power plants has increased after the Fukushima Daiichi Nuclear Power Station disaster (2017) in Japan. Accordingly, approximately 800–900 million ton per year of coal fly ash (CFA), a by-product from the combustion of coal, is generated worldwide. Although the CFA has been recycled as supplements for cement, concrete, soil conditioners, and fertilizer materials [15–17], a major portion has been disposed of in landfills. Thus, from the perspective of a sustainable society, it is necessary to develop a recycling technology for CFA.

In this study, we focused on the preparation and production of zeolites from CFA. The CFA, characterized by aluminosilicate and silicon phases, is a superior material for zeolite synthesis [18]. Zeolite is a microporous crystalline hydrated aluminosilicate characterized by a three-dimensional network of tetrahedral (aluminum and silicon) O4 units that form a system of interconnected pores [18]. The applications of CFA derived zeolite are well-known. They have been used for heavy metal removal from aqueous media [19,20], as well as for the remediation of acid mine drainage [21,22]. Thus, this conversion of CFA to zeolite is useful for the development of a sustainable society as it decreases the waste generated from coal-fired power plants. Many conversion technologies, namely fusion-assisted hydrothermal treatment [14], multi-step treatment [14], sonication [14], conventional hydrothermal treatment [23,24], and microwave irritation [25,26] have been reported in previous studies. Zeolite synthesis involves three steps: dissolution, condensation, crystallization [27]. Among the technologies, conventional hydrothermal treatment is comparatively simple and inexpensive [23].

Previously, we reported that potassium-type zeolite (K-type zeolite) prepared from CFA had characteristic physicochemical properties and showed potential in heavy metal adsorption from aqueous media [28]. In addition, previous studies have assessed the adsorption capacity and mechanism of Hg2+ removal using CFA [29]. However, there are no reports on the adsorption of Hg2+ using potassium-type zeolites prepared from CFA using conventional hydrothermal treatment. Thus, if potassium-type zeolite could be explored for the removal of Hg2+ from aqueous media, this alternative would contribute considerably to the waste reduction from coal-fired power plants or water conservation.

This study aimed to investigate the possibility of Hg2+ removal from aqueous media using K-type zeolite prepared from CFA. The effects of pH, temperature, contact time, coexistence, and selectivity on the adsorption of Hg2+ were assessed.

### **2. Materials and Methods**

### *2.1. Materials*

The standard solution of Hg2+ (HgCl<sup>2</sup> in 0.1 mol/L HNO3) was purchased from FUJIFILM Wako Pure Chemical Co., Osaka, Japan. Coal fly ash (CFA) was obtained from the Tachibana-wan Power Station (Shikoku Electric Power, Inc., Tokushima, Japan). Additionally, the standard solutions of Na<sup>+</sup> (NaCl in water), Mg2+ (Mg(NO3)<sup>2</sup> in 0.1 mol/L HNO3), K<sup>+</sup> (KCl in water), Ca2+ (CaCO<sup>3</sup> in 0.1 mol/L HNO3), Ni2+ (Ni(NO3)<sup>2</sup> in 0.1 mol/L HNO3), Cu2+ (Cu(NO3)<sup>2</sup> in 0.1 mol/L HNO3), Zn2+ (Zn(NO3)<sup>2</sup> in 0.1 mol/L HNO3), Sr2+ (SrCO<sup>3</sup> in 0.1 mol/L HNO3), and Cd2+ (Cd(NO3)<sup>2</sup> in 0.1 mol/L HNO3) were also obtained from FUJIFILM Wako Pure Chemical Co., Osaka, Japan. Potassium-type zeolite (Ktype zeolite) was prepared by hydrothermal activation treatment using CFA in potassium hydroxide solution [28]. Three grams of CFA was mixed with 3 mol/L potassium hydroxide solution (240 mL). The mixture solution was heated at 93 ◦C for 1(FA1), 3(FA3), 6(FA6), 12(FA12), 24(FA24), and 48(FA48) h, followed by filtering through a 0.45 µm membrane filter (Advantec MFS, Inc., Tokyo, Japan) [30]. The residue was washed with distilled water and dried at 50 ◦C for 24 h. Potassium hydroxide, nitric acid, and sodium hydroxide were

purchased from FUJIFILM Wako Pure Chemical Co. (Osaka, Japan). All reagents were of special grade.

We had previously reported the physicochemical characteristics of K-type zeolites [28]. X-ray diffraction (XRD) and morphology analyses were performed using MiniFlex II (Rigaku, Osaka, Japan) and SU1510 (Hitachi High-Technologies Co., Tokyo, Japan), respectively. The cation exchange capacity (CEC) and pHpzc were measured using the Japanese Industrial Standard Method (JIS K 1478) and the method previously reported by Faria et al. [31]. Additionally, the specific surface area and pore volume were measured using NOVA4200*e* (Quantachrome Instruments Japan G.K., Tokyo, Japan). The binding energy was measured using a JXA-8530F (JEOL Ltd., Tokyo, Japan). Finally, the solution pH was measured using an F-73S digital pH meter (HORIBA, Ltd., Kyoto, Japan).
