Column evaluation of a water defluoridation technique based on phosphoric acid-enhanced limestone adsorption

Abstract

Fluoride removal from water by phosphoric acid-enhanced limestone defluoridation has been studied in a continuous-flow mode using crushed limestone column by adding dilute phosphoric acid (PA) to the influent water. The flow rate of the gravitationally fed water in an up-flow mode is controlled with a flow controller. Fluoride has been removed from initial 5 mg/L to 0.1 mg/L using 0.01-0.05 M initial PA. The pH of the treated water was between 7.4 and 7.7. The present continuous-flow mode show almost four times higher fluoride removal capacity of the limestone than that of the batch mode reported earlier. The continuous-flow mode also shows better regeneration of the exhausted limestone and lower recurring cost compared to that of the batch mode. The kinetic and mechanistic behaviour of fluoride removal in the continuous-flow mode has been found to be somewhat different from that of the batch mode. Three stages of fluoride removal observed as a function of throughput volume have been attributed to gradually diminishing precipitation and sorption of fluoride as the limestone surface is gradually covered by adsorbed fluoride, and precipitate of calcium carbonate and fluoroapatite.