The Mobility and Interaction of Poly(ethylene glycol) in Column Experiments with Cambisol

The transport of colloidal organic matter (OM) in soil is governed by colloidal hydrodynamics and frequently features strong interactions at the biogeochemical interfaces provided in the soil pore space. Conventional reactive tracers used to study solute transport usually fail to cover the hydrodynamics of small-sized colloidal OM. This impedes a clear observation of transport phenomena that are characteristic for these OM fractions. Tailor-made poly(ethylene glycol) (PEG) is available in a molar mass range featuring similar hydrodynamic sizes as colloidal OM. Thus, characterizing the transport of PEG could help to decipher the transport behavior of colloidal OM in soil.
We studied the transport of PEG in soil columns filled with homogenized Cambisol material. PEG was labelled with fluorophores to enable a highly resolved and sensitive detection via fluorescence spectroscopy. Parallel factor analysis (PARAFAC) was applied to the measured excitation-emission matrices to estimate the concentration of PEG in the column effluent. Additionally, batch experiments were conducted to determine the adsorption isotherms of PEG with the column substrate and typical soil minerals.
The resulting breakthrough of PEG was retarded by about an order of magnitude and with a pronounced tailing when compared to the breakthrough of non-reactive NaCl. The retardation points towards organo-mineral associations. This was corroborated by the adsorption observed in batch experiments with high maximum adsorption capacity with homogenized soil and clay minerals. The observed tailing may be due to the varying molecular dimensions of PEG contributing to kinetic interactions with soil minerals.
With representative hydrodynamics, varying molecular dimensions as colloidal OM and the possibility of forming organo-mineral associations with soil minerals, tailor-made PEGs are promising candidates to also follow the transport of other colloidal OM.