Defluoridation of Groundwater by Activated Carbon Derived From Water Hyacinth (E. Crassipes) by Phosphoric Acid Activation

Abstract

Fluoride contamination of groundwater poses serious health concerns in Kenya where 39% of rural households depend on groundwater for drinking water. Efforts to develop appropriate defluoridation techniques have experienced challenges such as low efficiency, unaffordability, lack of skills, and cultural perceptions. This study evaluated the use of water hyacinth-derived activated carbon as a locally available, environment-friendly, and socio-economically acceptable alternative defluoridation technique. Water hyacinth stems were impregnated with concentrated H3PO4 to three times their weight, followed by calcination at a rate of 5°C /min up to 600°C to produce water hyacinth activated carbon (WHAC). The WHAC was characterized by proximate analysis, Fourier transform infrared (FT-IR) spectroscopy and scanning electron microscopy (SEM). Batch experiments determined effects of pH, contact time and adsorbent dosage on defluoridation efficiency. The results revealed a fixed carbon content of 66.7%, ash content of 8.9% and bulk density of 0.123 g/cm3. The elevated fixed carbon content, low ash and bulk density inferred WHAC’s potential for high surface area and porosity, parameters that are indicative of good quality activated carbon. These characteristics were similar to those of selected commercial activated carbons (CACs) of ash content less than 10% and fixed carbon content greater than 65%. The SEM revealed a rough and irregular texture that indicated high porosity, a desirable characteristic in activated carbon material. The WHAC achieved a fluoride removal efficiency of 82.6%, at pH 3, contact time of 120 min and an adsorbent dose of 1.2 g per 100 mL of groundwater. Adsorption of fluoride by WHAC was best described by Freundlich isotherm model with a correlation factor (R2) of 0.952 and an adsorption intensity (n) of 0.285 indicating that the WHAC was heterogeneous. The adsorption was described by the pseudo-second order kinetic model with a correlation factor (R2) of 0.999 and a close match between the experimental and theoretical adsorption capacities of 0.4608 and 0.4656 respectively, suggesting chemisorption adsorption of fluoride onto WHAC.