Variation in photoreactivity of iron hydroxides taken from an acidic mountain stream

The photoreduction of iron hydroxides is known to exert significant influence over many biogeochemical processes in streams impacted by acid mine drainage. Using laboratory and in-stream measurements, the variation in reactivity of iron hydroxides taken from a stream receiving acid mine drainage (AMD) was studied. The reactivity decreased for material collected at sites progressively downstream from the AMD inflow. In the presence of two simple organic ligands, photoreduction increased for the fresher iron hydroxides but remained unchanged for the older hydroxides. The importance of ligand coordination to the enhancement of photoreduction in natural waters was further demonstrated in experiments using two types of fulvic acids. In-stream measurements of hydrogen peroxide concentration are consistent with the conclusions drawn from the batch experiments. Iron hydroxides were observed to age over time, becoming less photoreactive. This aging was accompanied by an increase in crystallinity. The loss of photoreactivty for the older material can be explained by a decrease in the number of active surface sites, a change in the nature of the surface sites, or a combination of both.