Evaluation of fluoride emissions and pollution from an electrolytic aluminum plant located in Yunnan province.

Due to its excellent corrosion resistance, electrical conductivity, and hardness, aluminum is widely used in aerospace, automotive, power transmission, construction, and other industries [1], [2], [3], [4]. According to the Global Aluminum Industry Association, global primary aluminum production in 2023 totaled 70.581 million tons, with China accounting for 41.666 million tons. Due to its rich hydropower resources, Yunnan Province has recently undertaken numerous hydropower aluminum projects in eastern provinces. Its electrolytic aluminum production capacity has increased from 1.585 million tons in 2017 to 6.38 million tons in 2023, nearly one-fifth of the national production capacity. However, the substantial demand for primary aluminum has imposed a heavy burden on the global environmental load. Electrolytic aluminum is a high-energy-consuming and highly polluting industry. It requires 12,650 to 13,350 kWh of electricity and emits 0.2 to 0.6 kg of fluoride for every 1 ton of aluminum produced [5], [6]. Therefore, studying the fluoride emission characteristics of electrolytic aluminum plants is of great practical significance.

Emissions of fluorine from electrolytic aluminum plant constitute a significant source of fluorine pollution in the environment. During primary aluminum production, various forms of fluorinated substances are generated due to the use of cryolite fused salt (NaFAlF₃), which is derived from alumina and fluorinated salts [7], [8], [9]. Raw materials or fluorine salts react with water in the environment ((1), (2), (3)) to produce gases such as HF [10], [11]. Meanwhile, the unreacted fluoride remains in spent electrolytes, aluminum dross, and workshop dust [12], [13], [14], [15]. These fluorides can enter the human body directly or indirectly through the food chain, ultimately affecting human health (Fig. 1).

The fluoride levels are crucial to ecological stability and human health [16], [17], [18], [19]. Trace amounts of fluoride are necessary to maintain human health and fulfill standard requirements. However, highly fluoridated environments can lead to physiological changes such as stunted plant growth and leaf necrosis [20], [21], [22]. High doses of fluoride ingested by humans can cause damage to human bones, the heart, the immune system, and the nervous system [23], [24], [25]. Previous studies have systematically analyzed the effects of fluoride on human health [26], [27]. Urine is the most important metabolic pathway for eliminating fluoride in the body; it is considered a valuable biomarker of fluoride exposure in contemporary populations (rather than individuals) and an essential indicator for monitoring human fluoride intake [28]. The researchers found that industrial processes are the largest source of fluoride-contaminated air, and ingestion of fluoride-contaminated food is the leading cause of high human exposure to fluoride [29]. When the human body ingests excessive fluoride, it can cause enamel fluorosis, which affects the mineralization and final structure of tooth enamel [30]. Therefore, the environmental problems caused by fluoride emissions have has attracted globally significant attention.

Although numerous studies were reported on fluoride environmental impacts, most papers just focused on the effects of fluoride on urban groundwater and crops. For example, researchers have monitored and evaluated fluoride spatial and temporal variations in groundwater, agricultural soils, and crops in the Unnao region, and established models to analyze the relationship between F^– changes in groundwater and soil pollution according to soil’s physical and chemical properties in different seasons [31]. They have investigated the impact of changes in groundwater fluoride levels on the economy and health problems in the lower Ganges plains of India, and the potential health risks were analyzed [32]. Additionally, the global fluoride concentration in groundwater has been analyzed and predicted, and the global hydrochemical and environmental factors related to fluoride accumulation are discussed [16]. However, there are few reports on environmental pollution and health impacts around electrolytic aluminum plant, particularly on the source analysis of fluoride emissions from these facilities [33]. Therefore, it is essential to analyze fluoride emissions from the electrolytic aluminum industry from a multidimensional perspective, which provides further insights into the transport and transformation of fluorine pollutants.

In this study, we selected a typical electrolytic aluminum plant in Yunnan Province as the research object and set up sampling points within the sanitary protection area. The objectives of this study are therefore to (1) identify the different pathways fluoride enters the environment based on material balances and characterization tools, (2) study fluoride contamination in the study area and investigate fluoride spatial and temporal distribution, (3) assess the risk of fluoride impacts on crops and human health. This study should be helpful for identifing the primary emission sources of fluoride pollutants in the aluminum production process at a later stage, which is of great significance in controling fluoride emissions from the aluminum industry.