Fluoride Removal From Drinking Water Using Alumina Adsorbent: Batch And Column Experiments And Working Efficiency Of Engineered Pilot Plant

Groundwater of the Thar Desert in Sindh province of Pakistan is known to have high fluoride concentration whose use for drinking purpose has led to a catastrophic prevalence of dental and skeletal fluorosis. Therefore, as a remedial measure to this problem, with an aim to bring fluoride levels in drinking water below the WHO limit of 1.5 mg/L, adsorption experiments have been carried out in this study in the batch and column modes and extended to the pilot scale investigations using alumina as the adsorbent. The batch mode experiments helped optimize the adsorption parameters such as adsorption time, adsorbent dose, and pH and the values were found to be 50 min, 2.0 g, and 6-7, respectively. Batch adsorption data further showed greater compatibility with Freundlich adsorption isotherm as well as pseudo-second-order model. The thermodynamic experiments revealed that the process is endothermic and spontaneous with Gibb's free energy ranging from -7.56 to -8.13 kJ/mol. A percentage removal of almost 95% in the batch mode was encouraging to carry the study to the column mode which represents a more pragmatic approach. Column experiments were carried out in a column of 33 cm length and 3.4 cm of the internal diameter which resulted in the breakthrough adsorption capacity of 4.98 mg/g at an initial fluoride concentration of 10 mg/L. Thomas and Yoon-Nelson adsorption models fitted well with the data with high values of determination coefficients. The maximum adsorption capacity Q(m) and the rate constant K-TH obtained via Thomas model were found to be 8.82 mg/g and 0.0012 L/min mg, respectively, and the values of KYN and tau from Yoon-Nelson model were found to be 0.0128 min(-1) and 426.83 min, respectively. The column containing 5.0 g of alumina reached exhaustion after passing 7.1 L of 10.3 mg/L fluoride solution. It was subsequently regenerated with 1% NaOH with a desorption efficiency of 92%. Based on these results, a pilot plant was designed that contained 60 kg bed of alumina in a vessel of 13 inch internal diameter and 60 inch height. Experiments with pilot plant revealed that the plant can serve around 20,000 L of fluoride-safe drinking water if the initial fluoride concentration of the water is approximately 10 mg/L. The plant was successfully used up to the third cycle with decreasing efficiency of fluoride removal. These results are highly encouraging and pragmatic which will be helpful in engineering a real-time plant for the people of Thar desert to provide them with a fluoride-safe drinking water and overcome the menace of the fluorosis completely from the area.