Continuous Fixed-Bed Column Defluoridation of Groundwater Using Activated bark of Azadirachta indica and Prosopis juliflora

This study evaluates the defluoridation efficiency of calcined and acid activated bark of Azadirachta indica and Prosopis juliflora Sw. (DC.) using a laboratory-scale fixed-bed column and fluoride contaminated groundwater (initial fluoride concentration~4.7 mg L^-1) from Shavantgera village, Karnataka. Experiments were conducted as a continuous flow study, with flow rates adjusted to achieve flow duration of 250 min for Azadirachta indica (25 g, pH~8.0) and 200 min for Prosopis juliflora (20 g, pH~7.0). Under these conditions, Azadirachta indica achieved nearly complete fluoride removal (final concentration~0.04 mg L^-1), while Prosopis juliflora achieved 86.4% removal (final fluoride concentration~0.64 mg L^-1). Kinetic analysis indicated a strong fit to the pseudo second-order model (coefficient of determination (R²) = 0.995-0.996), suggesting that chemisorption governs fluoride uptake. Equilibrium modelling showed better correlation with the Langmuir isotherm, consistent with monolayer adsorption behaviour. Scanning electron microscope (SEM) analysis revealed increased surface roughness and porosity after activation, while Fourier transform infrared-spectroscopy (FTIR) confirmed the involvement of hydroxyl and carbonyl functional groups in fluoride binding. Based on endpoint column measurements, the adsorption capacities were found to be 0.093 mg g^-1 for Azadirachta indica and 0.102 mg g^-1 for Prosopis juliflora. Although continuous breakthrough curves were not recorded, the study demonstrated the practical potential of the abundantly available and low-cost biosorbents for rural defluoridation applications.