Assessment of acid neutralizing capacity and potential mobilisation of trace metals from land-disposed dredged sediments

In the past decades, large amounts of contaminated sediments have been dredged and spread out along rivers. The understanding of the fate and the behaviour of contaminants in dredged sediments is essential to deal with the management of contaminated sediments. Heavy metal leaching behaviour in dredged sediments of different ages since disposal, varying from 10 to 70 years, was assessed by pHstat leaching test and potentiometric titrations. Based on the pHstat titration results, two operationally defined pools of heavy metals, i.e., a labile and a slowly labile pool of heavy metals, were distinguished and quantified by mathematical description of leaching behaviour. The mathematical description of leaching behaviour during pHstat and potentiometric titrations allowed deduction of four groups of elements with a different type of leaching behaviour. Considering heavy metals, Zn, Cd and Ni displayed a leaching behaviour similar to acid neutralizing capacity (ANC), whereas the release of Cu, Pb and Cr was slower than ANC. However, no significant differences with regard to heavy metal leaching behaviour were found between sediments with different time since disposal on land. The elevated acid neutralizing capacity of the dredged sediments analysed in this study strongly decrease the risk of soil acidification with associated heavy metal leaching. The prediction of soil acidification over an extended period of time, together with the potential release of trace metals, was considered an important advantage of pHstat leaching tests compared to conventional leaching procedures.