Reversing the tide of trouble: harnessing invasive water hyacinth biochar to mitigate groundwater fluoride contamination

Abstract

Fluoride, a geogenic contaminant, has gained significant global attention due to its adverse health effects. Currently available technologies for its removal are often expensive and inaccessible to many communities. Biochar, a cost-efficient and sustainable adsorbent with proven pollutant removal capabilities, offers a promising alternative. This investigation explores the adsorption capacity of exfoliated biochar (EB) derived from water hyacinth (Pontederia crassipes). Batch adsorption experiments were conducted to analyze the effects of contact time, initial fluoride concentration, biochar dose, reaction temperature and pH. Response surface methodology identified the optimized reaction condition (pH 6, 30 °C, 20 mg L⁻¹ initial F⁻ concentration and 6.5 g L⁻¹ EB dose) which resulted in 86.08% removal when replicated in the laboratory. Adsorption kinetics followed a pseudo-second-order model (R² = 0.997), while the Langmuir isotherm (R² = 0.995) best-explained monolayer adsorption and chemisorption, with a maximum adsorption capacity of 4.24 mg g⁻¹. Brunauer-Emmet-Teller and Barett-Joyner-Halendar analysis supported the exfoliation of native biochar using 2(M) HNO₃, by showing an increase in pore volume (0.037 cc g⁻¹) and surface area of EB (22.291 m² g⁻¹). Field emission scanning electron microscopy and energy-dispersive X-ray spectroscopy analysis confirmed the presence of F⁻ in EB. Additionally, Fourier transform infrared spectroscopy highlighted shifts in functional groups, confirming fluoride interaction with biochar. These findings establish biochar as a viable, eco-friendly solution for groundwater fluoride remediation.