Visualization of pore water colloids in intact soil using a new diffusive gradients in thin films (DGT)-based approach

Abstract

Mobile colloids, most notably natural nanoparticles (NP) within the <100 nm range, enhance the mobility of nutrients and contaminants in soil; however, unbiased sampling methods are lacking to properly quantify colloid-facilitated transport. We advanced the imaging diffusive gradients in thin films (DGT) method to sample not only solutes but also pore water colloids in intact soils. The DGT setup consisted of a zirconium oxide–Chelex binding layer separated from soil by a 9 μm thick membrane with a pore size of 1 μm, providing a short diffusion length to increase colloid sensitivity. The method was tested in a vertical 15 cm section near the plough pan at about 30 cm depth in an arable soil (Luvisol) with silt-loam texture. Intact soil cores were sampled, sliced along the longitudinal axis, and the DGT was deployed on the exposed surface. Two-dimensional mapping of the DGT by LA-Time-of-Flight (TOF)-MS identified striking co-localisation of Al, Si, Rb, and Cs, indicating clay colloids. Pore water extracted from the same soil cores and analysed using Flow Field Flow Fractionation confirmed 2 : 1 clay minerals as the dominant colloids. Laboratory studies confirmed the potential of the DGT binding layer to concentrate clay colloids from suspensions, likely due to cation bridging between the negatively charged clay surfaces and the zirconium oxide in the binding layer. Colloids were present on the soil-deployed gels primarily at locations where pore water Ca²⁺ concentrations were lower, likely corresponding to larger pores that were mostly drained at the time of sampling. This study presents the first 2D map of pore water colloids in soil, future work will focus on converting DGT data into resident colloid concentrations.