Abstract :
[en] Dredging of waterways is essential in order to improve and promote fluvial transport. However, according to the Walloon legislation (AGW 30/11/1995), dredged sediments are classified as waste materials (class A or B depending on their pollutant content and release) and must eventually be treated before valorization or disposal. Increased costs resulted in slowing down dredging operation during several years and 6 million of m³ of sediments have been accumulated, whose about 65% are polluted by heavy metals or organic compounds.
In the framework of an ERDF funded project (SOLINDUS) an experimental treatment platform was developed to treat sediments by a mineralurgical process in order to reduce contamination and enable their beneficial reuse. Considering that contaminants are generally concentrated in fine fractions, the treatment relies on a granulometric separation. Silt (15-63µm) and fine fraction (<15µm) represents the major volume of the raw sediments (20-40% and 35-55% respectively).
The aim of this research is the study of the behavior of residual trace metals contained in the silt fraction. Indeed, despite an additional flotation treatment, this fraction still contain heavy metals (sometimes above the legal limit) and in order to reuse it (e.g. in landscape management), the treatment has to be improved. For this purpose, the comprehension of retention phenomena of heavy metals in the material is of importance, as well as to evaluate environmental risks linked to further sediment reuse.
In order to evaluate the retention mechanisms of heavy metals, geochemical modelling with PHREEQC v.3.0 [1] is conducted in parallel of laboratory experiments (characterization of samples, measurement of metals release in function of pH, acquisition of input data for the model). The model take into account sorption of heavy metals on amorphous iron and aluminum hydroxides [2], on organic matter [3], on clays and mineral precipitation.
For the first studied sample, mainly contaminated by Zn, Cd and Pb, the shape of the leaching curves in function of pH for heavy metal of interest is pretty well represented. The model allows to calculate the repartition of elements between the different sorption phases; at natural pH sorption onto hydroxides controls retention of heavy metals.
In order to evaluate model sensitivity to input parameters and prioritize further experimental work, a screening analysis has been conducted by the mean of design of experiments. The results show that surface area of hydroxides is the most influent factor on the model results, except for the quantities of heavy metal sorbed on organic matter.
[1] D. L. Parkhurst and C. A. J. Appelo, 'Description of Input and Examples for PHREEQC Version 3 - A Computer Program for Speciation , Batch-Reaction , One-Dimensional Transport , and Inverse Geochemical Calculations,' 2013.
[2] D. A. Dzombak and F. M. M. Morel, Surface Complexation Modeling: Hydrous Ferric Oxide. New York: John Wiley and Sons, 1990.
[3] E. Tipping, 'Humic Ion-Binding Model VI : An Improved Description of the Interactions of Protons and Metal Ions with Humic Substances,' Aquat. Geochemistry, vol. 4, no. 1976, pp. 3-48, 1998.
