De-fluoridation of drinking water by co-precipitation with magnesium hydroxide in electrolysis

Abstract

A diagnostic of fluorosis, such as dental and skeletal fluorosis caused by excess fluoride in drinking and cooking water, is endemic in at least 25 countries around the globe; the number of patients would be in the tens of millions. Other reports have estimated that more than 70 million people in the world suffer from dental or skeletal fluorosis. Since many people in developing countries rely on groundwater for their drinking water and are unable to access any other water resources, the removal of fluoride from drinking water is urgently needed. In the current research, fluoride (F−) was removed from well water using a novel batch electrolysis system composed of carbon electrodes and two cells separated by a membrane diaphragm with the goal of its application in developing countries. The removal of F− was investigated using synthesized well water with various levels of F− and magnesium (Mg2+) at a constant current. As the electrolysis progressed, Mg(OH)2 precipitation appeared in the cathode cell, which co-precipitated with F− by forming a complex. No chemicals are needed, since the Mg2+ already in the well water is utilized for the precipitation of magnesium hydroxide. The F− removal rate gradually increased with the increase of Mg2+ concentration up to 80% when the Mg2+ concentration was 200 mg/L. The F− concentration slightly affected the F− removal rate above 50 mg/L of Mg2+ in the solution. The system’s capability to achieve F− concentration of less than 1 mg/L, which is the Sri Lankan standard for drinking water, was examined using Sri Lankan groundwater with typical F− and Mg2+ concentrations. Well-water samples—183 from the Anuradhapura and 56 from the Polonnaruwa regions—were used for the estimation. The system showed high capability as 84% and 72% for Anuradhapura well water and for Polonnaruwa well water, respectively. It was confirmed that a high percentage of well water having F− concentrations of more than 1 mg/L could be treated by the electrolysis system without the addition of any chemicals.