Efficient radon removal using fluorine-functionalized natural zeolite

Highlights

• Redundancy analysis of radon was conducted in groundwater.

• Natural zeolite was modified with fluorine for effective radon removal.

• FFNZ was found to be more efficient for radon removal from groundwater.

• The results suggest that radon interacts with fluorine through van der Waals force.

Abstract

Radon (Rn) can easily leak into the environment through groundwater owing to its high water solubility. Therefore, studying the chemical factors influencing the content and removal of Rn from groundwater is crucial for the evaluation and mitigation of its radiological risks to public health. In this study, we conducted a redundancy analysis (RDA) of Rn in groundwater and performed batch sorption experiments for efficient Rn removal from the groundwater collected from Daejeon using natural zeolite (NZ) and fluorine-functionalized natural zeolite (FFNZ) sorbents. The redundancy analysis revealed a positive correlation between the concentrations of Rn and fluorine (F) in groundwater, indicating that F can support the long-term retention of Rn in groundwater. NZ and FFNZ achieved ~40% and ~70% removal of Rn, respectively, following 24 h of treatment, indicating a significant impact of F (in FFNZ) toward Rn removal from groundwater. Based on the results, Rn is considered to interact with F through the van der Waals force, which limits the volatilization of Rn from the solution. Similarly, the fluorine-functionalized sorbent would interact preferentially with Rn, thereby enhancing its sorption and removal from groundwater.

Introduction

Radon is a radioactive inert gas classified as a Group I carcinogen radionuclide (IARC, 2012). The fission of naturally occurring radioactive materials such as ²³⁸U and ²³²Th in crustal rocks and minerals generates gaseous radionuclides, such as ²²²Rn, ²¹⁹Rn, and ²²⁰Rn. Among these products, ²²²Rn, commonly known as Radon (Rn), is the most stable and possesses the longest half-life (3.82 days). Rn is colorless, tasteless, odorless, and heavier than air. Rn accumulates in the atmosphere where air circulation is insufficient because it is heavier than air, and it directly causes cancer by emitting alpha rays or decaying into daughter nuclides following inhalation (Samet, 1989). The actual risk posed by Rn is primarily ascribed to its daughter species, such as polonium, rather than to Rn itself.

Although Rn is an inert gas, it is also highly soluble in water (i.e., 230 cm³/L at 25 °C) at atmospheric pressure (Table 1) (Greenwood et al., 1997), and so can easily leak into the environment through groundwater. Rn, which is present in groundwater, affects the human body through two different routes of exposure, namely breathing exposure due to the volatilization of Rn in groundwater, and ingestion caused by drinking the water originating from Rn-contaminated groundwater Rn (Yu et al., 2001). Exposure to Rn through the respiratory pathway causes lung cancer, while drinking water contaminated with Rn exposes the gastrointestinal tract to Rn and its decay products.

Recently, the proportion of houses that take in groundwater and use drinking water and domestic water has increased, and so it is necessary to evaluate the health of the human body due to indoor contamination generated by the indoor volatilization of radon dissolved in the groundwater (Council, 1999). The globally averaged Rn concentration in groundwater is 4946 pCi/L (183 Bq/L) (NCRP, 1984). Since a Rn concentration of 10,000 pCi/L (370 Bq/L) in groundwater increases the indoor Rn concentration by 1 pCi/L (0.037 Bq/L), the recommended standard for the indoor Rn concentration in Korea has been set at 148 Bq/m³, while that for groundwater has been set at 148 Bq/L, as outlined in the Drinking Water Management Act of 2016. Unless appropriate action is taken to reduce the levels of Rn in groundwater, it will infiltrate into habitable areas and exhibit an adverse impact on the environment and society.

Due to their crystalline porous structures and flexible frameworks (suitable for controlled chemistry), zeolites have been widely employed in catalysis, sorption, and separation applications, among others (Davis, 2002; Breck et al., 1974; Guo et al., 2015; Yang et al., 1999; Cha et al., 2015; Fang et al., 2017). More specifically, zeolites have been applied for the sorption and removal of various hazardous species, including radionuclides, toxic metals, and gases, such as Rn (Yang et al., 2016; Ibrahim et al., 2010; Hedstrom et al., 2012). While many studies have been conducted into the removal of gaseous Rn, the extent of fundamental research into the removal of Rn from aqueous media remains limited.

However, recent reports have suggested that the leakage of Rn into the environment through groundwater is becoming a more serious concern, and research into its sorption and removal is gaining interest once again. The aim of this study is therefore to improve our understanding of the chemical factors that affect the Rn distribution in groundwater, and in particular, the presence of fluorine (F). Thus, we herein report a novel strategy for the sorption and efficient removal of Rn from groundwater (obtained from Daejeon, South Korea) using natural zeolite (NZ) and fluorine-functionalized natural zeolite (FFNZ) sorbents. Our findings can serve as a basis for the design and development of effective sorbents for Rn removal from natural groundwater in the future.