DOC determination. The total organic carbon (TOC) was measured based on
the combustion catalytic oxidation/nondispersive infrared analysis (NDIR)
method (TOC-V CPH; Shimadzu, Duisburg, Germany). Before analysis, samples
were filtered (prewashed 0.2-m Millex syringe filter [Millipore, Billerica, MA]),
and the TOC was then referred to as DOC.