Size fractionation of dissolved (

Abstract

Dissolved (<0.45 µm) trace elements (TEs) represent the sum of free ions, simple complexes and colloid-associated forms which have different mobility and bioavailability in soils. The distribution of TEs amongst these chemical forms was directly quantified in soil extracts using asymmetric flow field-flow fractionation (AF4) coupled to ultraviolet–visible absorbance spectrophotometry (UV) and inductively coupled plasma mass spectrometry (ICP-MS). The soil extracts were obtained using single extraction method with water and 0.01 M CaCl2, respectively. The yields of dissolved TEs extracted from the soils were profoundly impacted by extractants. Using AF4-UV-ICPMS, we show that dissolved species of Ba, Cr, Li, Mn and Mo were primarily present as “truly dissolved”/mainly ionic species (<1 kDa), e.g., hydrated cations, simple complexes or oxyanions, and therefore, likely represented the most bioavailable fraction. The distribution of these TEs amongst dissolved forms was unaffected by the different extractants. However, their dissolved concentrations were profoundly affected. Distributions of Al, As, Co, Cu, Fe, Ni, Pb, Th, Tl, U, V and Zn among the various chemical forms significantly differed with water and CaCl2 extractants. In water extracts, a greater proportion of these elements was associated with colloidal forms having sizes from 1 kDa to 0.45 µm, i.e., dissolved organic matter (DOM) or/and inorganic colloids. Water not only released greater colloid-complexed concentrations of TEs, like Al, As, Fe, Pb, Th, Tl, U and V, but also liberated greater amounts associated with ionic and small forms. Extractants like water and CaCl2 are useful for recovering bioavailable TEs from soils. However, the dissolved TEs extracted using water or CaCl2 represented TE concentrations and forms with different bioavailability. The AF4-UV-ICPMS technique is useful for directly quantifying TEs existing as mainly ionic species and those bound with DOM and inorganic colloids, and thus offers clear insight into their bioavailability in soils. This method also facilitates a better understanding of the effects of extractants on estimating TE bioavailability.