Defluoridation efficiency of animal bone char and coconut shell charcoal: a comparative adsorption isotherm and kinetic study

ABSTRACT Groundwater in the East African Rift Valley is highly contaminated with fluoride, leading to severe public health challenges, necessitating the exploration of cost-effective removal methods. This study evaluates the efficacy of activated carbon derived from cattle bones (CBs) and coconut shells (CSs) for fluoride removal through adsorption techniques. A comparative study was performed using a batch adsorption reactor with varied adsorbent doses (0.5–10 g), treatment time (10–120 min), pH (3–10), and fluoride concentration (2–10 mg/L). The Box–Behnken Design of Response Surface Methodology was applied, achieving correlation coefficients of 0.99 for CBs and 0.96 for CSs, validating the predictive models. The results showed that CB has a higher fluoride removal efficiency (96%) compared to CS (90%) under optimal conditions. CB had a maximum adsorption capacity of 9.09 mg/g, while CS reached 4.55 mg/g. Kinetic studies confirmed that fluoride adsorption followed pseudo-second-order kinetics, suggesting chemisorption as the rate-limiting step. XRF and XRD analyses revealed fluoride removal through ion exchange with hydroxyapatite (CB) and surface complexation with amorphous carbon (CS). The findings show CB is ideal for higher fluoride concentrations, while CS excels at lower levels, demonstrating these locally available materials as affordable, practical defluoridation solutions for rural, resource-limited communities.