Solid–Water Extraction Ratio on Apparent Dissolved Trace Element Mobility and Nanoparticle Carriers in Soil Solutions

Solid–water extractions are widely used to collect soil solutions to evaluate trace element (TE) mobility and nanoparticle (NP) transport, and they also simulate changes in soil moisture associated with changing climate. However, differences in extraction ratios used to isolate dissolved (<0.45 μm) TEs may introduce methodological bias and limit comparability among studies. Here, we investigated how solid–water extraction ratios influence dissolved TE concentrations and their distribution among colloidal fractions. Three contrasting soils were examined: an agricultural podzol and two mining-related reclamation soils, overburden and a peat–mineral mixture. Soil solutions were extracted at ratios of 1:20, 1:10, and 1:5 and analyzed for dissolved Al, Mn, Fe, Cu, Pb, and U using inductively coupled plasma mass spectrometry (ICP-MS), while colloidal distributions were characterized using asymmetric flow field-flow fractionation coupled to a UV detector and ICP-MS. The 1:5 solid–water extraction resulted in higher apparent dissolved TE concentrations in the soil extracts and enhanced AF4 fractogram peak intensities. However, after correcting for dilution and normalizing to soil mass, Al and Fe mobilization was significantly lower at the 1:5 extraction ratio across all soils, whereas other elements showed more variable soil-dependent responses. These results demonstrate that the solid–water extraction ratio influences apparent TE mobility and the detectability of colloidal phases in soil solutions.