Effect of quenching time and quenching agent dose on total organic halogen measurement

Total organic halogen (TOX) is a collective parameter and a toxicity indicator for all the halogenated organic disinfection byproducts (DBPs) in a water sample. TOX measurement involves concentration of halogenated organic DBPs by adsorption onto activated carbon. Prior to activated carbon adsorption, quenching the chlorine residual in a water sample is an indispensable step to eliminate the positive interference resulting from continued chlorination with organic compounds adsorbed on the activated carbon surface or with the activated carbon surface itself. Arsenite is generally applied as the quenching agent. In this study, the arsenite quenching agent dose that was 100% of the stoichiometric amount of the chlorine residual in a water sample was demonstrated to be most appropriate. In practice, to ensure complete reduction of chlorine residual, slight overdose of arsenite is often recommended, but this was found to cause negative interferences in the TOX measurement due to two reasons. First, the competitive adsorption existed between halogenated organic DBPs and the excessive arsenite on the activated carbon. This competitive adsorption effect was pronounced within a quenching time of 15min. After 15min of quenching, a large fraction of arsenite might have reacted with some easily-reduced halogenated DBPs to become arsenate, which is partially ionized at pH 2 and less likely adsorbed on the activated carbon. In this case, more halogenated organic DBPs could be adsorbed on the activated carbon, resulting in the measured TOX concentrations to be much closer to the actual one contained in the sample. Second, excessive arsenite might lead to reduction or decomposition of halogenated organic DBPs with a prolonged quenching time (>60min). Thus, to avoid the negative interferences, the appropriate quenching time should be 15-60min.