Pathway fraction of bromate formation during O₃ and O₃/H₂O₂ processes in drinking water treatment

Ozone process has been widely used for drinking water treatment recently. In the oxidation process, bromate is formed by three pathways, i.e., the direct pathway, the direct-indirect pathway and the indirect-direct pathway. This study developed a method to calculate the percentage of these three pathways for bromate formation during O₃ process and O₃/H₂O₂ process. Two kinds of water, distilled water containing bromide (DW) and surface water from the Yellow River (SW) were selected as raw rater. The result showed that in natural water systems, the direct-indirect pathway was dominant for bromate formation during the oxidation process. When 3 mg L^-1 O₃ was used as the only oxidant, nearly 26% of bromide ion was transferred into bromate in two kinds of water after 80 min. The dominant pathway in DW was the direct pathway (48.5%) and the direct-indirect pathway (46.5%), while that was the direct-indirect pathway (68.9%) in SW. When O₃/H₂O₂ were used as oxidants, as the H₂O₂ dosage increased, the fractions of bromate formation by direct pathway and direct-indirect pathway decreased, while that by indirect-direct pathway increased. The conversion ratio from bromide to bromate first kept stable or increased, then decreased and reached its minimum when [H₂O₂]/[O₃] ratio was 1.0 in DW and 1.5 in SW. Under this condition the indirect-direct pathway took the largest fraction of 70.7% in DW and 64.0% in SW, respectively.