Photochemical reactivity of XAD-4 and XAD-8 adsorbable dissolved organic compounds from humic waters

A common assumption in aquatic biogeochemistry is that photochemical reactivity of dissolved organic carbon (DOC) in natural waters is mainly due to the abundant humic- and fulvic acids. The high aromaticity and resulting light- and UV-absorbing properties of these compounds have been stressed as important features promoting this activity. We exposed aqueous solutions of organic matter isolated from a humic lake to artificial UV radiation. We show that in addition to humic substances (XAD-8 acids), the less coloured and more hydrophilic fraction (XAD-4 acids) have a similar photochemical reactivity-potential, measured as DOC-normalized production of inorganic carbon and some low molecular weight carboxylic acids upon exposure of this material to UV-A and UV-B radiation. Furthermore, the lower absorbance in the XAD-4 fraction compared to the XAD-8 fraction indicate that hydrophilic acids are more easily degraded by absorbed UV-radiation, i.e. these acids have a higher quantum efficiency for this production than the humic substances. Consequently, depth-integrated, photochemical degradation of dissolved organic matter in surface waters could be governed by the ratio between the hydrophilic- and hydrophobic acid fraction, at least in systems where these compounds are major DOC fractions and influence from scattering particles is negligible.