Why comparison between different chemical extraction procedures is necessary to better assess the metals availability in sediments

Abstract

Single and sequential extractions are current and useful tools for estimating the availability of metals in soils or sediments. Many chemical extraction procedures have been proposed in the literature, making the comparison difficult. This study compares the data consistency of the potential availability given by four chemical extractions commonly found in the literature: ➢ two single procedures (using dilute HCl or EDTA as reactant) ➢ two sequential procedures (the modified standardized 3-steps procedure of the Standards, Measurements and Testing Programme (SMTP), namely BCR-extraction, and the 7-steps procedure of Leleyter and Probst (1999), namely LP-extraction).The leaching procedures are all performed on the reference lake sediment CRM BCR-701, used as a reference to assess the accuracy of the modified standardized procedure of Rauret et al. (1999). The results show similar availabilities between HCl and EDTA single leaches for Zn, Cu, Cr and Pb and between HCl and the BCR procedures for Zn, Cu, Cr and Ni. The LP-extraction is the most aggressive of the four tested procedures (except for Pb extraction), mainly due to a better extraction of the exchangeable fraction, by a better dissolution of the acidosoluble phase and particularly to a better dissolution of the amorphous and crystalline Fe-oxides. The comparison between the four procedures reveals a major problem of Pb extraction, potentially due to the formation of lead precipitates during HCl and the LP-extractions, even if an overestimation of the lead availability by the BCR procedure cannot be excluded.

Integrated Assessment of Affinity to Chemical Fractions and Environmental Pollution with Heavy Metals: A New Approach Based on Sequential Extraction Results

“…Thus, the alkaline urban soils of Moscow located in the southern taiga are characterized by a high content of Zn and Cd in the carbonate fraction, while the presence of HMs in the carbonate fraction is not typical for the background acidic soddy-podzolic soils [82,83]. When calculating EF for chemical fractions of HMs, the use of Al, Ti, Sc, Zr, or Fe as reference elements can distort the results of pollution assessment since these elements are usually found in the most stable phases-in silicates and residual fraction [6,84,85], while HMs, actively emitted by anthropogenic sources, sharply increase their affinity to more mobile and less stable phases-exchangeable, carbonate, reducible, and organic [6,43].…”

“…The main limitation of ICF, RAC, MF, BF, MC, SC, and PC is non-selectivity, that is, the impossibility of defining the chemical affinity of HMs to a specific geochemical phase. ASI uses Al as a normalizing element, which has a strong affinity to silicate and residual fractions [6,85]. ASI and BMI do not allow assessing the chemical affinity of HMs to a specific geochemical phase.…”

“…An effective method for assessing the distribution of HMs between different geochemical phases with similar chemical speciation, their level of mobility, and reactivity is chemical fractionation. This approach is based on ion-exchange reactions, the selectivity, and specificity of the chemical reagents used for fractionation, as well as the dissolution of solid-phase compounds [6]. Detailed information on HM ratios in the various geochemical phases in soils, bottom sediments, road dust, and other solid environmental media can be obtained by the multi-step sequential extraction procedure.…”

“…Each next reactant has greater chemical activity than the previous one, allowing the stepwise dissolution of more stable compounds [7], which are considered less mobile, bioaccessible, and environmentally hazardous of HM fractions [8]. Thus, a sequential extraction procedure provides essential information about the strength of HMs binding to particulates and the degree of their "anthropogenicity" or "geogenicity" [6], which is crucial in understanding the relationship between geochemical processes and HMs mobilization as well as the availability of HM forms for organisms and, accordingly, potential environmental risks [5,9].…”

Evaluation of Element Mobility in River Sediment Using Different Single Extraction Procedures and Assessment of Probabilistic Ecological Risk

“…Mobility and availability depend on the reactivity and on the binding behavior of toxic elements with the components of the matrix, and cannot be assessed only from the values of the total concentrations [8]. In order to assess their distribution between residual and non-residual fractions and their environmental availability i.e., their ability to be available to living organisms in case of changes in environmental parameters, many authors use chemical extractions [9]. Extraction is one of the most common methods for isolating chemical elements and compounds.…”