Kinetics of element release during combined oxidation and pHstat leaching of anoxic river sediments

Element release from seriously contaminated sediments (especially with Cd, Zn and As) originating from two rivers in northern Belgium was investigated. Oxidation experiments that simulated the deposition of dredged sediments on land or the resuspension of riverbed sediments were performed in the laboratory. The oxidation of anoxic sediments during pHstat tests and the comparison of pHstat leaching in fresh (anoxic) and air-dried (oxic) sediments provided additional information on the changes in heavy metal and As mobility and speciation during oxidation. One of the sediments (GB(rs)) was characterised by a pH-decrease from pH 7.2 to pH 5.9 upon resuspension, while the pH of the other sediment (GL(rs)) did not decrease significantly. Although the release of Cd and Zn was influenced by pH, redox potential was also an important parameter in determining Zn- and Cd-release upon oxidation. In sediment (GB(rs)), Cd and Zn were only released after 48 h of resuspension, when the pH decreased to a value of 5.9. Even when the sediment/water suspension was titrated to pH 4 or 6 during oxidation, it took more than 12 h to have a substantial release of Cd. The apparent correlation in the release of image indicates the association of Zn and Cd with sulphides. From an environmental point of view, especially Cd seems of concern because of the rather elevated concentrations that are released into the porewater and into the suspension upon oxidation. Contrary to other elements such as Zn and As, Cd is not removed from the suspension by coprecipitation and/or readsorption with precipitating Fe-(hydr)oxides within the time span of the observations.