Occurrence, formation and persistence of halobenzoquinones: A case study on 2, 6 -dichloro-1, 4 -benzoquinone
David A. Reckhow
Halobenzoquinones (HBQs) constitute an emerging class of potentially carcinogenic disinfection by-products (DBPs). Given that HBQs are not measured in routine analysis of drinking water, there is little data on their occurrence in the USA. The presence of 2, 6-dichloro-1, 4-benzoquinone (2, 6-DCBQ) in US drinking water facilities was investigated in 8 utilities to provide an initial assessment of occurrence and fate in relation to the regulated DBPs. Point of entry (POE) and distribution system (DS) samples with DCBQ concentrations greater than the 90th percentile values were from treatment plants that used free chlorine. Across distribution systems, DCBQ exhibited decreasing concentrations with water age whereas trihalomethane (THM) concentrations increased with water age. In an effort to better understand the source of DBP precursors, controlled laboratory experiments were conducted to examine the formation of DCBQ from chlorination and chloramination of specific classes of lignin model compounds namely p-hydroxyl phenols, vanillyn, syringyl and cinnamyl phenols, poly phenols and alkoxy groups. DCBQ yields from chlorination depended on the type and position of the substituents and potential intermediates and varied between n.d-0.8 percent while chloramination did not result in DCBQ formation. Chlorination of p-hydroxyl phenols produced the highest DCBQ yield for the reaction period considered (6 h). To address the apparent loss of DCBQ in full-scale systems, the impact of different pHs on DCBQ degradation at ambient temperature was investigated and modeled. DCBQ remained relatively stable below pH 7 while the degradation rate above this pH was determined to be first order in [OH-] with a second order rate constant of 156 M-1 s-1.