Coagulation-sedimentation Method Using Aluminum Sulfate and Calcium Hydroxide for Removal of Boric Acid from Wastewater —Improvement of Removal Rate of Boric Acid by Addition of Gypsum into Wastewater—

Abstract

In this study, we investigated the change in the removal rate of boric acid relative to the saturation rate (=dissolved amount/saturated amount) for gypsum in solution and its pH dependence, as well as the composition of the compounds found in the precipitate after the reaction. The results showed that almost 100% of boric acid in solution is removed by the addition of gypsum at a saturation rate of >0.75 in the pH range of 10.5–13, which is probably due to the increase of the amount of ettringite formed in the precipitate. This makes it possible to thoroughly process boric acid-containing wastewater using the same amount of coagulants as in the conventional method.