Bromate Formation during Ozonation of Bromide in the Presence of Organic Matter
The objective of our work was to evaluate the importance of various parameters (natural organic matter, pH, ammonia, bicarbonate, hydrogen peroxide) affecting the formation of bromate during the ozonation of natural waters containing bromide. Bench scale experiments were carried out on synthetic solutions prepared in phosphate buffer (10-2 - 10-3 M). Bromate concentrations were determined by ion chromatography after the complete consumption of the added ozone (24 h contact time); the ion chromatograph was equipped with a suppressed conductivity detection system (DIONEX) with a detection limit of ca. 2 μg BrO3-/L. The impact of organic matter was evaluated with model organic compounds: fulvic acid and hydrophilic acids (FA and HyA) isolated from a river water with XAD8 and XAD4 resins, and a tripeptide (Tyr-Gly-Gly). These different organic compounds were studied individually or as a mixture (60% FA, 30% HyA, 10% tripeptide). All solutions were spiked with 200 mg/L of bromide and the applied ozone dose was 5 mg O3/L; the DOC content varied from 0 to 5 mg C/L. For given experimental conditions, bromate formation varies depending on the origin and nature of the organic matrix (natural organic matter or tripeptide), probably due to their respective ozone consumption. For a particular ozone dose, an increase in DOC results in decreasing bromate production. In fact, ozone consumption by the natural organic matter reduces the available ozone and somewhat inhibits the efficiency of bromate formation reactions. More bromate is formed with increasing pH, for the relative proportion of hypobromite increases in relation to hypobromous acid. In the presence of ammonia and natural organic matter, bromate formation is lowered because ammonia diverts part of the HOBr to form organobrominated compounds, and thus participates indirectly in ozone consumption. Bicarbonate addition favors molecular ozone type reactions by scavenging free radicals. Ozone is stabilized in the medium and thus more bromate is formed than in the absence of any scavenger. The reaction of OH radicals leading to bromate formation needs the presence of BrO-. Thus the use of hydrogen peroxide with an excess of ozone, or ozonation prior to hydrogen peroxide addition, may lead to the production of high bromate concentrations. This experiments shows that the higher the available ozone, the higher the bromate production. The use of ozone for disinfection purposes involves maintaining a residual ozone concentration during a due time in order to achieve the required C. t for bacterias and viruses inactivation. This increasing C. t may result in high bromate formation.