Reaction of chlorine with NOM adsorbed on powdered activated carbon

In this study, first, the polluted raw water of a municipal water treatment plant was subjected to direct chlorination or chlorination preceded by PAC adsorption. The results show that, with PAC, the aqueous phase NPDOC was found to increase with increasing chlorine dosage after the dosage exceeded certain value. The concentration of the halogenated organics in the aqueous phase of both cases was found to increase with increasing chlorine dosage. Furthermore, the concentration from the system involved PAC may be higher than that without. Those phenomena are supposed to be due to the release of dissolved organics into aqueous phase, as a result of reaction between chlorine and the adsorbed organics. Next, synthetic waters, each containing only one organic fraction extracted from the raw water, were subjected to similar experiments. A similar response to chlorination as to that of raw water, i.e. increase in NPDOC value with increasing chlorine dosage and the concentration of halogenated organics from the system involved PAC can be higher than that without, can only be found in the system with low molecular weight hydrophilic neutral organics, but not the system with high molecular weight humic acid. It is speculated that the adsorbed low molecular weight organics were desorbed from carbon surface more easily, compared to the adsorbed high molecular weight organics. Therefore, the organic contents in raw water may affect its behavior during the adsorption–chlorination process.