Determination of the affinity of heavy metals to carrier phases in soils

Abstract

A new index is proposed to determine the affinity of heavy metals (HM) to carrier phases (A_HM-fraction), which, in contrast to the traditional index pC_HM = 100 C_HM-fraction/C_HM-soil (Fedotov et al., Environ Chem 16:323-332, 2019; Land et al., Chem Geol 160:121-138, 1999), considers the sum of all metals in the phase as a share of the sum of the bulk content of all HM in the soil. Then, index of the affinity of HM for the given phase (A_Ln-phase) is determined by formula: A_HM = (C_HM-phase/ΣC_HM-phase)/(C_HM-soil/ΣC_HM-soil), where C_Ln-phase is the content of the given HM in fraction under consideration, ΣC_Ln-phase is the sum of all HM in this fraction, C_Ln-soil is the content of the given HM in the initial soil, and ΣC_Ln-soil is the sum of all metals in the initial soil. All values of HM are taken after rationing on the European Clark of chemical elements according to (Kabata-Pendias, Trace elements in soils and plants N.Y. CRC Press, Boca Raton, 2011). At A_HM-fraction > 1, the metal has affinity for the given phase; vice versa, at A_HM-fraction < 1, the affinity is absent. Comparison of the affinity series of metals for phases based on two indices revealed their discrepancy in most cases. The discrepancy of affinity series of metals for phases is provoked by insufficient substantiation of index pC_HM, which depends on the "strength" of reagent but does not influence index A_HM-fraction. Therefore, we propose a new index that can take into consideration the discrepancy in affinity of the given metal for phases extracted by different strength reagents. Siderophility of metals depends on the properties of iron oxides: Cu and Ni show affinity for amorphous iron oxides (ferrihydrite); Cu and Pb display affinity for crystalline oxides (magnetite and goethite). In uncontaminated and contaminated soils, Cu, Pb and La show affinity for iron minerals that are dissolved in hydroxylamine and Tamm's reagent. Organophility of all three metals (Cu, Pb and La) is only realized in the uncontaminated alluvial soil (hereafter, Fluvisols) in the Kama River floodplain. Contaminated soils, however, are affected by additional factors that weaken the fixation of classic organophilic metals. Reasons for such distortion in the organophility of metals in the contaminated soils are likely related to a specific mineral composition of HM pollutants and the specificity of organic matter composition.

Keywords: Affinity of heavy metals for mineral and organic colloids; Clarke values of heavy metals; Normalization; Sequential chemical extraction.