Fluoride in geothermal water: Occurrence, origin, migration and environmental impact

Geothermal water, a vital renewable energy source extensively harnessed for heating and power generation, is marred by a prevalent issue – high fluoride content. The environmental impact of geothermal fluoride has been recognized globally. The natural discharge of geothermal water, coupled with its widespread exploitation, instigates the translocation of geothermal fluoride toward shallow and surface water ecosystems, culminating in escalating fluoride concentrations, thereby posing potential threats to both ecosystems and human health. Nevertheless, despite the pivotal significance of fluoride in geothermal water, a comprehensive understanding of its origins, migratory dynamics, ecological consequences, and ameliorative methodologies remains to be studied. This review provides a comprehensive examination of fluoride's global occurrence and distribution in geothermal waters, emphasizing the contrast between volcanic and non-volcanic geothermal systems. It analyzes the various sources of fluoride in these waters and elucidates the mechanisms driving its mobilization. In volcanic geothermal systems, fluoride primarily derives from magmatic volatiles, while in non-volcanic systems, it mainly results from the dissolution of minerals. Temperature is a key factor influencing fluoride concentration in geothermal waters, with alkaline conditions and low calcium levels contributing to higher fluoride enrichment. The review details how fluoride concentrations change as geothermal fluids migrate from deeper to shallower layers. Based on the enrichment characteristics of fluorides, this paper explores the potential applications of geothermal fluorides. It also examines the environmental impacts of geothermal fluoride, presents various treatment methods, and provides a summary of current research both domestically and internationally, while proposing directions for future studies. This study is instrumental in formulating judicious fluoride management policies and establishing sustainable strategies for the development of geothermal resources.