Reduction of bromate formation in electrochemical production of hypobromite for water disinfection

Abstract

Hypohalites are powerful water disinfectants for many applications. Continuous production of hypobromite was studied using an undivided electrochemical cell operating at 20 A cell current (28.9 mA cm−2) and an electrolyte flow of 50 L h−1. With 34 mM NaBr as electrolyte current efficiency is between 25 and 42 %. Anodic oxidation and disproportionation of hypobromite led to formation of substantial concentrations of BrO3− in the electrolyte. BrO3− concentrations up to 1 mM (22.6 mg L−1) were formed during anodic Br− oxidation. Replacement of NaBr by mixtures of NaCl and NaBr as electrolyte led to reduction of BrO3− concentration to 0.5 mM (11.3 mg L−1). By combination of NaCl electrolysis with following dosage of NaBr solution the BrO3− concentration could be reduced below the detection limit of the analytical method which was used to quantify BrO3− and hypobromite. In a freshly prepared 5 mM hypobromite solution the BrO3− concentrations remained below 0.1 mM (2.2 mg L−1). When applied in typical dosage for halogen based disinfectants (10−3 mM and 10−2 mM hypohalite) the BrO3− concentration dilutes to 0.02 μM – 0.2 μM. The optimised process thus leads to BrO3− concentrations below the legal limit for BrO3− in drinking water (10 μg L−1 BrO3−).