Factors affecting formation of haloacetonitriles, haloketones, chloropicrin and cyanogen halides during chloramination

Abstract

Effects of contact time, monochloramine doses, monochloramine application modes, pH, temperature and bromide ion concentrations on formation of disinfection by-products (DBPs), including haloacetonitriles, haloketones, chloropicrin, cyanogen halides and trihalomethanes, during chloramination were investigated using model solutions containing 5 mg/L (as DOC) Suwannee River natural organic matter (NOM). Chloramine speciation and some DBPs were measured using membrane introduction mass Spectrometer (MIMS). Longer reaction times led to continued formation over time for dichloroacetonitrile (DCAN), 1,1-dichloro-2-propanone (1,1-DCP) and chloroform. Cyanogen chloride (CNCl) formation occurred over time, but after reaching a peak concentration CNCl concentrations decreased over longer time periods. Linear relationships were observed between the formation of DCAN, 1,1-DCP, CNCl or chloroform and the dosage of monochloramine. Chloramination modes (addition of preformed monochloramine or variable sequential additions of free chlorine and ammonium salts) exhibited the largest impact on chloroform formation but displayed little effect on the formation of DCAN, 1,1-DCP and CNCl. Over the range in pH from 4 to 9 profound differences in DBP formation were observed; pH values between 5 and 6 resulted in the highest DBP concentrations. An increase in temperature enhanced the formation of chloroform but did not affect DCAN, 1,1-DCP and CNCl formation. Chloropicrin concentrations were always low (around detection limits) under all conditions. Increasing the concentrations of bromide ions enhanced the formation of bromine-substituted DBPs.