DEGRADATION OF HUMIC SUBSTANCES BY UV IRRADIATION

Aqueous solutions of humic substances (humic and fulvic acids) were ir radiated with light at lambda = 254 nm followed by analyses of total o rganic carbon (TOC), ultraviolet (UV) absorbance and molecular weight distribution. A degradation of the dissolved humic compounds by up to 50% after 6 h was observed. The average molecular weight decreased (fr om 1800 to approx. 300) as well as the UV absorbance of the remaining organic fraction. Low molecular weight organic acids were formed durin g the irradiation and tentatively identified by capillary electrophore sis (formic, acetic, succinic, and oxalic acids). The low molecular or ganic acids continued to degrade by further irradiation; the loss of T OC is assumed to be due to CO2 formation. Indications of a large-scale photo degradation process in nature was observed as a seasonal variat ion of the ratio humic substances/TOC related to the length of the day . Weekly sampling and analysis of UV absorbance and TOC in the Kalix R iver in northern Sweden indicate a higher humic fraction of the TOC du ring winter (dark period) than during summer.