Comparison of physical and chemical methods to characterize trace metals and PAH fractionation in contaminated sediments from runoff water infiltration basins

Trace metals and organic pollutants from urban runoff waters are partly accumulated in soils of retention-infiltration basins. These basins are widely used in urban stormwater management to provide protection from flooding, enhancement of stormwater quality and recharge of aquifers. Within the context of sustainable development, taking sludge management into account would appear necessary in order to maintain both the hydraulic and maintenance functions of these ponds. The recycling of basin sediments is sometimes considered in engineering applications, yet economic suitability and environmental acceptability must still be taken into account. One prerequisite to possible sediment reuse is a strong knowledge of the inherent characteristics of the materials. Among these characteristics, trace metals release availability requires studying trace metals speciation. However, a direct determination of the speciation of an element is often not attainable. In many cases, fractionation can provide useful information on release availability. The aim of this study was therefore to compare various methods of fractionation of both trace metals and organic pollutants in sediments, in view of improving their environmental impact assessment. The methodology was applied to three sediments with various mineralogical and chemical properties. Trace metals (Cd, Cr, Cu, Ni, Pb, Zn) fractionation was determined using sequential and kinetic extractions and a density fractionation. The sequential extractions were conducted using the three-steps BCR protocol. EDTA was used as reagent for kinetic extractions. A chemical extraction using methyl isobutyl ketone (MIBK) was applied to study the speciation of PAH residues in the three sediments. This method was recommended in the literature to obtain a better division of the mineral fraction. The traditional alkali-extraction method of IHSS was also carried out as a reference method on the same materials. Trace metals and PAHs were analyzed in each fraction respectively by ICP-OES and GC-MS. The results of sequential extractions showed quite similar fractionation of trace metals in the three sediments. Cadmium is mainly bound to the exchangeable fraction. Chromium and nickel are mostly present in the residual fraction. Lead lies in the reducible and oxidisable fractions and zinc is concentrated in the exchangeable and reducible fractions. Copper is either mainly in the residual fraction for one specific sediment, or equally distributed in the exchangeable, oxidisable and reducible fractions. The comparison with kinetic extractions let to associate the lability and non-extractability of trace metals to the fractions defined in sequential extractions. SEM observations highlighted that the presence of numerous bulky aggregates in sediments has an impact on the efficiency of extractions and that high energy sonification is required to obtain single particles and small aggregates. It was shown that the MIBK method wasn't a relevant method for the sediment. No additional information was gained by comparison with the IHSS protocol. Finally PAH residues were mainly found in the light fractions and lipidic fractions. Both the strong aggregation state and the origin of pollutants were proving to govern the reactivity of the materials and mainly control the fractionation