The mobility and interaction of colloidal-sized poly(ethylene glycol) in column experiments with carbonate rock

Soil releases a significant proportion of organic colloids such as humic substances, proteins, and polysaccharides that are mobile and reactive within subsurface fluids. The mobility of such colloids is governed by colloidal hydrodynamics and frequently features strong interactions at biogeochemical interfaces in porous media. Yet, the compositional and functional diversity of organic colloids makes it difficult to trace the mobility of specific colloidal fractions and identify the alteration of surfaces following local interactions. Additionally, conventional reactive tracers used to study solute transport in the subsurface usually fail to cover hydrodynamics of small-sized organic colloids. Hence, transport principles governing the mobility of organic colloids in the subsurface are not comprehensively explored. In this study, we applied tailor-made poly(ethylene glycol) (PEG) as a reactive tracer in column and batch experiments with naturally occurring calcium carbonate as the substrate. We demonstrate that PEG transport features strong interactions at carbonate biogeochemical interfaces as known for humic substances, proteins, and polysaccharides. Such interaction can be facilitated by electrostatic interactions between PEG and the surfaces of the carbonate substrate. With the tendency to alter mineral surfaces, scanning electron microscopy (SEM) images of substrates after transport experiments showed a characteristic modification of surface morphology. Besides sharing similar reactivity with organic colloids, PEG breakthrough was reconstructed using a continuum scale model with high accuracy. With PEG being available in similar hydrodynamic sizes as small-sized organic colloids, it can be a promising tracer to follow mobility of other organic colloids in the subsurface.