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

A common assumption in aquatic biogeochemistry is that photochemical reacti vity of dissolved organic carbon (DOC) in natural waters is mainly due to t he abundant humic- and fulvic acids. The high aromaticity and resulting lig ht- and UV-absorbing properties of these compounds have been stressed as im portant features promoting this activity. We exposed aqueous solutions of o rganic matter isolated from a humic lake to artificial UV radiation. We sho w that in addition to humic substances (XAD-8 acids), the less coloured and more hydrophilic fraction (XAD-4 acids) have a similar photochemical react ivity-potential, measured as DOC-normalized production of inorganic carbon and some low molecular weight carboxylic acids upon exposure of this materi al 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. Co nsequently, depth-integrated, photochemical degradation of dissolved organi c matter in surface waters could be governed by the ratio between the hydro philic- and hydrophobic acid fraction, at least in systems where these comp ounds are major DOC fractions and influence from scattering particles is ne gligible. (C) 1999 Elsevier Science Ltd. All rights reserved.