Identification of oxidative coupling products of xylenols arising from laboratory-scale phytoremediation

• Oxidative coupling products of xylenols in constructed wetlands were determined.
• Biphenyl diols and diphenylether monools proved the most abundant coupling products.
• Structural assignment of the analytes by Zenkevich’s increment method was carried out.
• Oxidative coupling occurs in both aerobic and anerobic systems.

Oxidative coupling reactions take place during the passage of xylenols through a laboratory-scale helophyte-based constructed wetland system. Typical coupling product groups including tetramethyl-[1,1′-biphenyl] diols and tetramethyl diphenylether monools as stable organic intermediates could be identified by a combination of pre-chromatographic derivatization and GC/MS analysis. Structural assignment of individual analytes was performed by an increment system developed by Zenkevich to pre-calculate retention sequences. The most abundant analyte turned out to be 3,3′,5,5′-tetramethyl-[1,1′-biphenyl]-4,4′-diol, which can be formed by a combination of radicals based on 2,6-xylenol or by an attack of a 2,6-xylenol-based radical on 2,6-xylenol. Organic intermediates originating from oxidative coupling could also be identified in anaerobic constructed wetland systems. This finding suggested the presence of (at least partly) oxic conditions in the rhizosphere.