Miljøprojekt,1025 – Undersøgelse af reaktiviteten af reducerende stoffer i nogle danske sedimenter – et pilotstudie

Undersøgelse af reaktiviteten af reducerende stoffer i nogle danske sedimenter – et pilotstudie

Summary and conclusions

The present pilot-study was conducted to assess the reactivity of reduced compound in sediments during oxidation caused by oxygen.

A suitable lab set up was developed and tested on sediment samples with different inherited amounts of reduced compounds collected from the oxidized and reduced sediment environments at 7 locations within Denmark.

During the supply of oxygen to a suspension of sediment (< 2 mm) and deionized water with a sediment:water ratio of 1:4 the development in pH and redox potention were monitored in the suspension over time. The reaction time varied from 175 hours to approximately 400 hours.

The pH and redox conditions showed up to be fairly constant in suspension prepared with oxidized sediments. In suspensions prepared with reduced sediments the introduction of oxygen lead to an increase in the pH within the first 24-48 hours, most likely due to mineralization of bioavailable organic matter in the sediments. After some time the increase in pH vas followed by a drop in pH due to the oxidation of pyrite. In a suspension made of low buffered sediment the pH changed from 7 to about 4 after 195 hours supply of oxygen.

On the basis of the information obtained during the present study a separation of the contributions from the different reduced compounds to the development of the quality of the suspension has not been possible.

In future studies on oxidation of reduced compounds in sediments it may be valuable to increase the number of parameters to be monitored and include e.g., measurements on alkalinity, dissolved organic carbon, dissolved nitrogen, sulphate, ferrous iron and heavy metals in the solute phase of the suspension. It is also recommended to continue the experiments over longer time than used in the present study. A longer reaction time may bring important information on the oxidation of pools of reduced compounds characterised by different reactivity.