Assessment of multi-ion competition on fluoride release from agricultural soils using fractional factorial design

Fluoride contamination in Punjab, India, is an emerging groundwater quality concern, driven by agricultural soils under intensive cultivation and sustained groundwater abstraction. Despite its significance, the mechanisms controlling depth-dependent F⁻ release from these soils and the role of background ionic chemistry remain poorly understood. This study quantifies F⁻ mobilization using batch experiments with and without representative background ions, providing a mechanistic evaluation of soil–water interactions governing fluoride desorption in agricultural subsurface environments. Results show that F⁻ release is highest in intermediate subsurface layers compared to surface and deeper layers, indicating depth-dependent geochemical controls on mobilization. Variations in soil pH and organic matter content were identified as key drivers governing this vertical heterogeneity. Experiments without background ions exhibited higher F⁻ release rates, highlighting the strong regulatory role of coexisting ions on sorption–desorption dynamics. The effects of six ions (SiO₃²⁻, PO₄³⁻, HCO₃⁻, SO₄²⁻, Ca²⁺, and Cl⁻) were systematically evaluated in single- and multi-ion systems using a \({2}_{IV}^{6-2}\) fractional factorial design (FFD). Silicate and calcium emerged as dominant controls, with silicate, phosphate, and bicarbonate enhancing F⁻ release, while calcium consistently suppressed it. Sulfate and chloride exerted minor but contrasting effects between single- and multi-ion conditions. Interaction analysis revealed that the combined effect of silicate and bicarbonate (SiO₃²⁻ × HCO₃⁻) was the most influential driver of F⁻ mobilization, particularly under low-calcium conditions. Despite being site-specific, the findings offer transferable insight into fluoride-affected subsurface and agro-environmental systems, providing a framework for understanding persistent groundwater fluoride enrichment.