THE EFFECTS OF (BICARBONATE ALKALINITY ON BROMATE FORMATION DURING OZONATION (WITH AND WITHOUT AMMONIA)

摘要

Ammonia has been shown to minimize bromate formation during ozonation by blocking two of three potential bromate formation pathways. It can be hypothesized that (bi)carbonate alkalinity in the presence of ammonia would further inhibit bromate formation since the pathway that ammonia does not block (the I/D pathway)requires the hydroxyl radical (OH~·), and (bi)carbonate alkalinity is an OH~· scavenger (Figure 1). Experiments where (bi)carbonate alkalinity was increased from 50 mg/L to 357mg/L NaHCO_3 in the presence of excess ammonia resulted in a 40% reduction in bromate formation, providing evidence in support of this theory. In addition to bromate minimization, the OH~· scavenging ability of (bi)carbonate alkalinity also results in an increased ozone residual half-life and the formation of carbonate radicals (CO_3~-) which react with free bromine (OBr~*) to form OBr. This promotes bromate formation in the D and D/I pathways in the absence of ammonia (Figure 1). In the presence of a constant ozone residual, experiments where (bi)carbonate alkalinity was increased from 50 mg/L to 357mg/L NaHCO_3 (with no ammonia) resulted in an increasing bromate formation trend. This demonstrates that the principal effect of (bi)carbonate alkalinity is the promotion of bromate formation via the carbonate radical.