Adsorption Isotherm, Kinetics, Column Dynamics and Techno-Economic Assessment of Activated Carbon–ZnO Nanocomposite for Defluoridation of Groundwater in Fluorosis

Fluoride contamination of groundwater above the WHO guideline value of 1.5 mg/L affects an estimated 66 million people across 19 Indian states, with Rajasthan, Andhra Pradesh, Telangana, and Gujarat recording the highest endemic fluorosis prevalence. The scale of this public health crisis — manifesting as dental fluorosis in children and skeletal fluorosis in adults with prolonged high-fluoride exposure — demands decentralised, low-cost, and operationally robust treatment solutions appropriate for rural villages where piped water infrastructure is absent and chemical dosing-based defluoridation technologies that require trained operators are impractical. This paper presents a comprehensive characterisation of an Activated Carbon-ZnO nanocomposite (AC-ZnO) synthesised from agricultural waste Prosopis juliflora wood as the carbon precursor and ZnO nanoparticles precipitated in situ, targeting fluoride removal from groundwater in the concentration range 2-15 mg/L. Batch adsorption studies establish isotherm parameters (Langmuir qmax=145 mg/g, KL=1.8 L/mg; Freundlich KF=22, n=0.41), kinetic parameters (pseudo-second-order model best fit, k₂=0.009 g/mg·min), thermodynamic parameters (ΔG°=−18.4 kJ/mol confirming spontaneous adsorption, ΔH°=+32.6 kJ/mol confirming endothermic process), and the effect of pH, co-ions, and initial concentration. Fixed-bed column studies at three bed heights (5, 10, 15 cm) are modelled using Thomas, Adams-Bohart, and Yan models. A techno-economic comparison against five competing defluoridation technologies positions AC-ZnO as the optimal choice for village-scale groundwater treatm