Investigating contact precipitation as a viable water defluoridation technique

Abstract

Fluorosis is a condition caused by excessive intake of fluoride, and is characterized mainly by mottling of teeth. A number of defluoridation techniques have been proposed, studied and even implemented in a number of countries, to varying degrees of success. Among these techniques, contact precipitation, a hybrid technique involving the use of calcium and phosphate compounds together with bone char to effect defluoridation of water has been extensively used. The mechanism of fluoride ion removal by this technique was investigated in this study. The objective was achieved by carrying out four sets of experiments, each done in triplicates. The first set of experiment was aimed at determining the saturation point of a known amount of bone char. The second set was aimed at determining the role and fate of calcium and phosphate compounds in contact precipitation, while the third and fourth sets were aimed at determining the efficacy of natural rock phosphate either as a defluoridation medium or as an alternative additive in contact precipitation respectively. These experiments were set up in similar glass columns. The column effluent in the first three experiments was analysed for calcium, phosphate, fluoride, pH and electrical conductivity. For the fourth set of experiment, only the fluoride levels were determined in the column effluent. The calcium concentrations were obtained volumetrically using EDTA and the phosphate concentrations obtained using the UV spectrophotometer. The amount of fluoride was estimated colorimetrically using acid zirconium – alizarin complex, whereas the pH and conductivity were obtained using the pH meter and conductivity meter respectively. The results obtained were presented in form of tables and graphs and their respective trends studied and discussed. It was found that the defluoridation capacity of bone char was limited to a maximum of 1.99 mg/g, an efficiency of 75.72%, while that of contact precipitation was limited to 2.59 mg/g, an efficiency of 94%. Plain Minjingu rock recorded a maximum defluoridation capacity of 1.0 mg/g, an efficiency of 33.33% while the combined media of Minjingu rock phosphate and saturated bone char recorded a defluoridation capacity of 2.12 mg/g, an efficiency of 70.6%. From the results, it was concluded that fluoride immobilization in contact precipitation involved a series of reactions namely Adsorption, Ion exchange and Co- precipitation.