Because industrialization and urbanization in developed countries have caused severe environmental pollution, concerns regarding this topic and awareness of the importance of environmental preservation have grown in the international community. The severity of soil pollution and remediation methods have been discussed since the late 20th century; unfortunately, less attention has been paid to this topic compared to other pollution sources that directly affect human lives (such as water or air pollution). However, progress in science has deepened knowledge about pollutants and their toxicities, which resulted in the establishment of stricter regulations regarding the treatment of soil on a global (e.g., WHO and EU) and national level (Oh et al., 2008). Regarding soil pollution, pollutants (mostly from anthropogenic sources) usually move through another medium (such as water or air) rather than entering the soil matrix by themselves. It is difficult to identify the exact direction of the movement, distance, and scale of the influence as soil water and groundwater are mainly distributed by the flow of soil water and groundwater in the subsurface environment (Zhang, 2020). Through numerical modeling, the movement and diffusion of pollutants during a short or long period can be predicted; however, these simulations can mostly be performed for aquifers composed of sand and gravel; the actual transport of pollutants in heterogeneous fractured rocks is difficult to predict (Fetter et al., 2017). Therefore, contaminants must be managed to prevent soil pollution; appropriate action must be taken to prevent leakage of contaminants from waste and by-product storage sites into the ground.
In South Korea, the Soil Environment Conservation Act was enacted in 1995; through several revisions, regulation levels and standard methods for chemical analysis were established. The following 11 categories were designated as soil pollutants for the first time in 1995: six metals (cadmium, copper, arsenic, mercury, lead, and hexavalent chromium), BTEX (benzene, toluene, ethylbenzene, and xylene), TPH (total petroleum hydrocarbons), PCB (polychlorinated biphenyls), organic phosphorus, phenol, and cyanide. Subsequently, through a public hearing and opinion gathering, 16 other types were designated as soil pollutants: nickel, zinc, fluorine, TCE (trichloroethylene), PCE (tetrachloroethylene), and benzo(a)pyrene. Moreover, the Korean Ministry of Environment has operated a soil monitoring network system to provide basic data for establishing strategies for soil conservation, such as preventing soil pollution and remediating contaminated soils by identifying the trend of pollutants in soils nationwide and investigating contaminated lands. According to the survey results from the soil monitoring network system, the Ni, Zn, and F concentrations were relatively high; therefore, they have been designated as pollutants in 2001 (Yang and Lee, 2008).
In general, fluorine is assumed to prevent tooth decay; thus, a project on the control of fluorine in tap water was implemented in South Korea (Kwon et al., 2015). However, excessive exposure can cause fluorine accumulations in teeth and bones (i.e., fluorosis; Czerwinski et al., 1988); in general, less than 1 mg/L fluorine is injected into drinking water to prevent tooth decay. The accumulation of fluorine in the human body can also result in osteoporosis when the exposure quantity reaches 20 to 80 mg/day (Grandjean, 1982). Long-term exposure of plants to fluorine causes yellowing, necrosis, and growth inhibition (Pushnik and Miller, 1990). Fluorine accumulated in mammals such as sheep and cattle through the ingestion of feed, soil, and water causes specific symptoms such as weakness, weaker joint strength, and abnormal growth of bones and teeth (Patra et al., 2000). Therefore, fluorine has been classified as a soil pollutant. The Korean Ministry of Environment (KMOE) has set a standard of 400 mg/kg for residential, agricultural, and forest areas and 800 mg/kg for industrial areas in accordance with the Soil Environment Conservation Act (KMOE, 2013).
Soil becomes polluted by fluorine through different anthropogenic routes: coal combustion, smelting of non-ferrous metals, land use of different sludges from wastewater treatment facilities, landfilling phosphate-gypsum wastes, and the application of phosphate fertilizers (Mikkonen et al., 2018; Fuge, 2019). Fluorine pollution can also occur naturally through weathering or dissolution of minerals such as fluorite (CaF2), fluorapatite (Ca5(PO4)3F), and cryolite (Na3AlF6) (Fuge and Andrews, 1988). The stabilization of fluorine-contaminated soil is considered necessary for preventing damage caused by natural and anthropogenic sources as industrial development continues. Fluorine in soil generally forms complexes in the form of fluoride (F−1) with Al and Fe in an environment with a pH below 6; the formation of complexes becomes less prominent with increasing pH. As the pH increases, which increases the OH−1 concentration, fewer complexes are formed owing to a competition effect, and Al and Fe form insoluble hydroxides in an alkaline state (Xie et al., 2001). In soils with high Ca contents (e.g., calcareous soil), insoluble CaF2 (i.e., Ksp = 3.9 × 10−11) can be formed (An et al., 2013). The granite and gneiss areas in South Korea contain high fluorite (CaF2) contents; as a result, the fluorine contents in the soils and groundwater in the vicinity of the granite and gneiss areas are high (Choo et al., 2008; Lee et al., 2019). According to Oh and Lee who investigated the fluorine concentration in soils in the southern part of Seoul (Oh and Lee, 2003), the concentration of fluoride in the forest area exceeds the regulation level of South Korea (400 mg/kg); the cause is probably of natural origin, i.e., the formation and weathering of granite and gneiss. Other research reports confirmed that the high fluorine contents (549–1,750 mg/kg) in the soils are related to the distribution of fluorite in granitic bedrocks (Chin et al., 1996; Lee et al., 2018). It was assumed that owing to the weathering of granite and gneiss, fluorine contents from natural sources, which exceed the regulation level in South Korea, may exist in the soils (Chin et al., 1996). In a chemical fertilizer manufacturing site in Ulsan, the fluorine concentration in the soil reached 22,617 mg/kg because phospho-gypsum is produced as a by-product in the manufacturing process (Lee, 2007). In Pennsylvania, USA, the fluorine concentration reached 136–990 mg/kg within a depth of 10 cm in some areas of farmland soil (Gilpin and Johnson, 1980). Furthermore, 500–1,000 mg/kg fluoride had been accumulated in the soils of the coastal plains of China owing to the continuous use of fluorine-containing phosphoric acid fertilizer (Zhang et al., 2010). In Bijie city, China, the soils of tobacco plantations contained up to 5979 mg/kg fluorine, which is greatly related to coal combustion (Wang et al., 2021). Loganathan discovered up to 2,700 mg/kg fluorine in the soil near an aluminum smelter (Loganathan et al., 2006) due to the electrolysis of cryolite (Na3AlF6) in the smelting process of aluminum and the resulting formation of fluorine by-products (Lee, 2010). When mineral processing of fluorine-containing ore minerals is performed for further purification and concentration, the separated wastes may include high fluorine contents (Arnesen et al., 1995), thereby causing secondary soil contamination near storage sites. Therefore, evaluating the form and mobility of fluorine in by-products is essential.
In Ulsan, which is the most industrialized metropolitan city at the southeastern shore in South Korea (Figure 1), a phosphate-gypsum waste landfill site in the chemical fertilizer manufacturing plant at a national industrial complex has been contaminated with fluorine. Previous feasibility studies have revealed that fluorine concentrations in some soils exceed the regulation level by far (Lee, 2007). However, the distribution, mobility, and possible mechanisms have not thoroughly been investigated. This study was conducted as a follow-up investigation before taking action for site remediation. It was hypothesized that some areas may be contaminated with fluorine released from phosphate-gypsum wastes and that the mobility of fluorine may be related to the chemical forms in the wastes and soils. The objectives of this study are to determine the distribution of fluorine in the phosphate-gypsum waste landfill site, to examine the mobility of fluorine in soils through sequential extraction and spectroscopic analysis, and to compare different types of analytical protocols for the determination of fluorine concentrations in wastes and soils. The analytical results were evaluated according to the regulation level of fluorine; this paper presents the legal framework, background level, and remediation actions.