Low-cost fluoride adsorbents prepared from a renewable biowaste: Syntheses, characterization and modeling studies

Abstract

The preparation of composite adsorbents for fluoride anion was performed from natural cow dung and cow dung impregnated by a solution of calcium and iron salts. These starting materials were dried and carbonized at 300, 500 or 700°C for 2h leading to Cow Dung Carbon (CDC) or Ca–Fe Impregnated Cow Dung Carbon ICDC. All these carbons were used as adsorbent in fluoride removal studies. Batch mode experiments using CDC and ICDC were conducted in which the fluoride removal varied linearly as a function of contact time, pH, adsorbent dose, initial fluoride concentration, temperature and interfering anionic species. Both kinetic and isotherm based models were checked for the applicability of the present fluoride sorption system onto CDC and ICDC. For carbonization at 300°C, the doping with Ca and Fe increased the defluoridation activity. The most efficient carbon was CDC which was prepared at 500°C. It showed a defluoridation activity of 15mgg−1. This carbon was made from renewable cow dung without the need of a chemical activation. However, the recycling of F-loaded adsorbents may be carried out by washing in alkaline solution. CDC and ICDC were analyzed by scanning electron microscopy and X-ray diffraction in order to understand the reasons of their excellent defluoridation capacity. The growing of plants requires the consumption of essential inorganic nutrients such as Ca, Mg, and P which are known to be fluorophilic elements in fluoride adsorbents. After ingestion and digestion of plants by ruminants, some of these nutrients remained in cow dung and then in the corresponding carbonized materials where they acted as fluoride scavenger. This exceptional defluoridation capacity of CDC allows their use in rural countries.