Experimental Investigation of Boron Removal Mechanism from Wastewater by Calcined Ettringite

Abstract

The development of efficient and inexpensive boron removal techniques is required to treat wastewater from many industrial processes. We investigated the removal mechanism of boron from wastewater by calcinated ettringite, a concrete sludge, and compared the advantage of this method to coprecipitation with ettringite. The amount of boron remaining in solution decreased slightly from an initial concentration of 2.3 mmol/dm3 to 1.3–1.4 mmol/dm3 after reaction for 60 min with ettringites calcinated at 55 and 65 °C. The boron removal efficiency increased greatly, with only 0.30 mmol/dm3 remaining in solution after treatment for 60 min with samples calcinated at 75 °C, but samples calcinated at 95 and 175 °C removed slightly less boron than those calcinated at 75 °C. This enhancement of boron removal performance is due to the amorphization of the ettringite structure by calcination. However, excess heating inhibits ion exchange between the sulfate ions in the ettringite and the borate ions in the solution. The coprecipitation method also achieved efficient boron removal at high Al/B molar ratio (0.44 mmol/dm3 boron remaining after 60 min). However, uptake by a different amorphous precipitant became the dominant removal mechanism. Large amounts of sludge are generated by this method. Adsorption using calcinated ettringite is therefore the optimal method for the efficient and inexpensive removal of boron from wastewater.