Abstract
Groundwater sources in certain regions of Ukraine are characterized by elevated concentrations of fluoride, which may reach up to 14 or even 18 mg/dm³. The intake of excessive amounts of fluoride ions through drinking water can seriously harm human health. Adsorption technology is a promising method for water defluoridation due to its simplicity, efficiency, and cost-effectiveness. However, the use of adsorption for fluoride removal from water faces several challenges, one of which is the reduced removal efficiency in the presence of foreign (competing) ions. Therefore, the effect of foreign ions on fluoride removal was investigated using iron-based adsorbent samples. Foreign ions compete with fluoride ions for adsorption sites on the surface of the adsorbent material, thereby reducing the overall efficiency of fluoride removal. The results obtained indicate that all studied ions adversely affect the process efficiency to varying degrees. The phosphate ion exhibited the most significant negative impact, as it is specifically adsorbed onto high-valent metal oxides and hydroxides via inner-sphere complexation. Therefore, it can easily compete with fluoride ions, which are also specifically adsorbed. Nitrate and chloride ions are non-specifically adsorbed (via outer-sphere complexation) onto metal oxides and thus cannot effectively compete with fluoride ions at equal concentrations. Sulfate ions may be adsorbed both specifically and non-specifically, and therefore showed a greater competitive effect than chloride and nitrate, but less than phosphate. The concentration ratios of anions in natural and wastewater vary significantly and should be taken into account when assessing the degree of competition.