Organic-rich sediments are known to be effective accumulators for uranium and arsenic. Much is known about the capacity for metal or metalloid fixation by microbes and organic compounds as well as inorganic sediment particles. Experiments investigating the effect of microbes on the process of metal fixation in sediments require sterilized sediments as control treatment which is often realized by gamma-sterilization. Only few studies show that gamma-sterilization has an effect on the remobilization of metal and metalloids and on their physico-chemical properties. These studies deal with sediments with negligible organic content whereas almost nothing is known about organic-rich sediments including a probably high microbial activity. In view of this, we investigated the effect of gamma-sterilization of organic-rich sediments on uranium and arsenic fixation and release. After ten days within an exposure experiment we found a significant higher remobilization of uranium and arsenic in sterile compared to unsterile treatments. In line with these findings the content of dissolved organic carbon (DOC), manganese, and iron increased to even significantly higher concentration in the sterile compared to unsterile treatment. Gamma-sterilization seems to change the physico-chemical properties of organic-rich sediments. Microbial activity is effectively eliminated. From increased DOC concentrations in overlaying water it is concluded that microbes are eventually killed with leaching of cellular compounds in the overlaying water. This decreases the adsorption capacity of the sediment and leads to enhanced uranium and arsenic remobilization.
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