Partitioning of heavy metals between estuarine sediments and dissolved phase as a function of salinity, pH and time (Scheldt estuary)

Abstract

The distribution coefficient (Kd) and kinetics of the heavy metals Cd, Cu, Zn and Pd as a function of salinity and pH are investigated experimentally considering desorption processes and adsorption of 109Cd radiotracer for natural particulate matter from the Scheldt estuary (Belgium/The Nitherlands). The desorption of Cu from polluted estuarine sediments is very low. Only at high salinities minor desorption was detected. At persistently low pH (<4), desorption was completed to 100 %. Also the Pb desorption is very limited and only strong acidification of the solution will efficiently desorb a considerable fraction of the element. The desorption of Cd is significant as a function of all the considered parameters. The desorption process is fast and up to 60 % of the adsorbed Cd can be released into natural water. Both the salinity and the pH can change the distribution between the solid and the dissolved phase significantly. The extent of Zn desorption is intermediate between Cd and Pb. Both salinity and pH are important parameters to be considered for the sorption processes in the natural environment. The adsorption of 109Cd on natural particulate matter from the Scheldt estuary changes as a function of temperature, salinity and composition of the particulate matter. The temperature has only a minor influence on the Kd-salinity relation, while the composition of the suspended matter showed to be very important. Even though the number of samples used was small, preliminary results show that organic content of sediments may be the most important parameter for adsorption of Cd. A similar relation between the Kd of the adsorption process using 109Cd and the desorption process as a function of salinity, indicates that the metal binding character of the adsorbed 109Cd may be identical to that of the naturally bound Cd released during the desorption process. The magnitude of the Kd, however, needs to be determined according to the different environmental parameters.