Enhancement of eggshell-kaolin composite for fluoride removal in water
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Date
2026-07
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NM-AIST
Abstract
Fluoride (𝐹⁻) in groundwater is a major public health concern when concentrations exceed the WHO guideline of 1.5 mg/L, particularly in fluoride-prone regions such as Manyara. This study investigated the spatial distribution of 𝐹⁻, its relationship with hydrogeochemical and physicochemical parameters, and the associated non-carcinogenic health risks in the Manyara region. To address this, a novel ternary eggshell-kaolin-MgO composite was synthesized via wet impregnation and co-precipitation methods; its surface and physicochemical properties were characterized, and then assessed for defluoridation in batch and fixed-bed column systems. Results show that the highest percentage of groundwater samples that exceeded the WHO limit was found in Simanjiro (85.71%) and Hanang (79.25%). The dominant hydrogeochemical facies was Ca-Mg-HCO3, while groundwater with high fluoride concentrations (>10 mg/L) was predominantly associated with the Na-HCO3 hydrogeochemical facies. The ionic relationships suggest that fluoride mobility and removal may be influenced not only by pH, alkalinity, and the (Ca2+ + Mg2+)/(Na+ + K+) ratio, but also by evaporation, water-rock interaction, and ion-exchange processes. Batch adsorption studies showed that equilibrium data followed the Freundlich isotherm, indicating multilayer adsorption on heterogeneous surfaces, while kinetics were best described by the pseudo second-order model (R2 = 0.996). The process was spontaneous and endothermic (ΔH° = 52.28 kJ/mol, ΔS° = 180.07 J/mol·K, ΔG° = –1.41 to –5.01 kJ/mol). Combined characterization and adsorption modelling suggested that fluoride removal by the CEKM composite involved both physisorption and chemisorption via electrostatic attraction, hydrogen bonding, and surface complexation. Maximum 𝐹⁻ removal (95%) occurred at 180 min, 0.6 g, pH 7, and 5 mg/L initial 𝐹⁻ concentration. Column experiments demonstrated high adsorption capacity (6.35 mg/g), good model fit (R2 ≥ 0.94), and effective regeneration over four cycles. Co-existing anions inhibited 𝐹⁻ uptake in the order PO₄³⁻ > SO₄²⁻ > NO₃⁻ > Cl⁻. The synthesized composite exhibited good adsorption performance, which may be attributed to its high specific surface area (158.5 m²/g) and the presence of active Ca2+ and Mg2+ sites, suggesting its potential as an efficient and sustainable adsorbent for fluoride remediation in groundwater.
Sustainable Development Goals
SDG 6: Clean Water and Sanitation