Conditions of formation of organoiodinated compounds during chlorination of aqueous solutions containing iodide ions
This work consisted of specifying the conditions of iodoform formation during chlorination of a raw water containing iodides. To reach this objective, there was need to spike the studied natural water with potassium iodide (200 μg.L-1) in order to increase the low natural iodide content. Free and combined chlorine, chlorinated and brominated trihalomethanes (THMs) and iodoform were analyzed. It was shown that : - iodoform is formed for chlorine doses prior to the breakpoint, in a region where the formation of the most classical chlorinated and brominated THMs is usually disfavored (Figures 1-4); - in the presence of chloramines the rate of production of iodoform increases with increasing I- or I2 (Figure 5); - the direct reaction of I2 with THM precursors to produce iodoform is slow and independent of the presence of ammonia (Table 1). - Nitrogenated compounds such as amines and amino acids would also take part in the production of organoiodinated compounds during chlorination (Figure 7). However, under water treatment conditions, taking into account the amine and amino acid content of natural waters, this class of compounds will only take a small part in the mechanism of iodoform formation. Among the possible routes that could account for the observations made in this research, the formation of iodamines or chloroiodamines as intermediates is suggested (Figure 8). From a practical point of view, the removal of ammonia from water by a biological process (nitrification step) would inhibit the iodoform formation potential and allow the application of the final chlorination step. Another alternative would involve replacing the chlorination step by oxidation with chlorine dioxide.