Modeling the photochemical degradation of ethylenediaminetetraacetate in the River Glatt

Laboratory experiments, field measurements, and mathematical modeling were used to evaluate the processes responsible for the fate of ethylenediaminetetraacetate (EDTA) in the River Glatt, a tributary to the River Rhine. Direct photolysis was identified as the most important process for the degradation of EDTA. With respect to environmental conditions, direct photolysis acts only on the Fe(III)-EDTA complex. Thus, the extent to which EDTA is degraded in rivers is directly related to its speciation. Approximately 50% of EDTA present in the River Glatt is discharged in the form of Fe(III)-EDTA from sewage treatment plants. Available Fe(III)-EDTA in the River Glatt is photolyzed within approximately 1 d during summer daylight conditions (global irradiation of about 800 W h m-2 at noon). The results of a spike experiment were used for the calibration of the light conditions within the water column. It showed that a great deal of sunlight was absorbed by water plants, which led to a decrease of the photolysis rate of Fe(III)-EDTA in the River Glatt of 85%. The predictions of the chosen model showed good general agreement with data from field investigations.