Automatic photocatalytic method for the determination of dissolved organic carbon (DOC) in natural waters

Abstract

An automatic method for the determination of dissolved organic carbon in natural waters is described. The method relies on the catalytic oxidation of dissolved organic matter to CO 2 by UV radiation and using a titanium dioxide semiconductor as a catalyst. Several compounds including urea and fulvic acid were mineralised completely. Compared with direct photolysis, the semiconductor catalytic photolysis gave consistently higher mineralisation yields for natural organic matter in seawater. The fraction resistant to direct UV photolysis (i.e. UV-refractory) was found to be in the range of 2%–5% of total DOC and it is suggested that this consists of organic macromolecules that fragment to non-UV absorbing species and escape further oxidation in direct UV photolysis. Organic macromolecules concentrated by molecular filtration contained 10%–50% such material, the higher proportion was found during the productive season in the sea, suggesting their primary biological origin (rather than degraded organic matter). The throughput of the method can be up to 30 samples per h and it is capable of online real-time analysis of DOC. The method gives linear calibration of up to 1000 μM C and blanks (instrument, reagent and distilled water blanks) can be readily and easily evaluated. Attempted validation of the method against CHN elemental analysis, highlighted several inherent basic problems. Initial validation showed that within the analytical errors, the two methods were in close agreement.