Author(s)

L.T. NGUYEN, T. LUNDGREN, K. HÅKANSSON & B.H. SVENSSON

Abstract

This study aimed to examine the consequent release of the metals Fe, Zn, Cu and Cd from the sediments under simulated redox changes. Two contaminated sediments, ‘A’ representing the top layer and ‘B’ representing the deeper layers taken from Lake Håcklasjön downstream of a hydropower plant in southern Sweden, were incubated in the lake water in flow-cells under both anoxic and aerated conditions. Under anoxic conditions, Fe was rapidly released from both sediments (A and B) into the solution, which was likely a result of diffusion from porewater along with the reductive dissolution of hydroxides, whereas the concentrations of dissolved Zn, Cu and Cd remained low. The opposite results occurred for all the studied metals during aeration: i.e. precipitation of Fe and a progressive release of Zn, Cu and Cd. The oxidation of ferrous to ferric Fe was the most likely process responsible for the removal of Fe from the water column from both sediments. Meanwhile the release of the trace metals was probably a result of the oxidation of sulphides, degradation of particulate organic matter or diffusion/advection. The water flow applied during the aeration period likely induced resuspension, which would be a contributing factor to the metal release by enhancing both porewater diffusion/advection and interactions between the resuspended sediments and the overlying water. The decrease in pH in overlying water of sediment A could be the reason for the faster increase in trace metal concentrations compared to that of sediment B. Copper was probably not affected by pH change, as Cu was complexed with dissolved organic carbon and carbonates using the Visual MINTEQ model. The results from the experiment show that aeration of the sediment samples is an important factor for the release of Zn, Cu and Cd into the water column. The low flow of water applied in the experiment compared to that generated by operation periods of the hydropower plant suggests that probably more metals will be released under field conditions.

Keywords

anoxic, aeration, chemical modelling, diffusion, flow-cell, hydropower plant, redox potential, resuspension, sediment, water flow