Das Projekt "Biologische Verfuegbarkeit fester Eisenoxide" wird vom Umweltbundesamt gefördert und von Eidgenössische Anstalt für Wasserversorgung, Abwasserreinigung und Gewässerschutz durchgeführt. Laboratory experiments were performed to elucidate the chemical and microbial reduction (dissolution) rates of iron(hydr)oxides in the neutral (natural) pH range. Results revealed that cysteine, a model reductant for bacterial Fe reducing enzymes, applied in the millimolar concentration range, reduced 2-line ferrihydrite much faster in the first 24 hours than did 3 108 cells/mI of the bacterium Shewanella alga, Geobacter sulfurreducens and Geobacter metamreducens (GS-15). The rates measured were 600 and 0.6-4 nmol /min m2, respectively. For goethite, the initial abiotic and biotic dissolution rates stayed in the same range, 4 and 2 - 3 nmol /min m2, respectively. lnitial reduction rates varied little between the different bacteria and iron(hydr)oxides, if calculated per surface area of the oxides. They also depended relatively weakly on the number of bacteria present. The chemical reductive dissolution rate was controlled by the concentration of the surface complex of cysteine at the iron(hydr)oxide. In microbial growth experiments, lasting 7 to 15 weeks, initial dissolution rates decreased to 0.1 to 0.7 nmol /min m2 only, although the culture media contained 4 mM phosphate. After 5 to 10 weeks reduction rates diminished drastically since magnetite was formed that is known to be not available for iron reducers and in addition the iron(hydr)oxide coagulated to larger particles. Phosphate in the millimolar concentration range lowered the chemical reduction rate with cysteine more than one order of magnitude.