Carrying nanoparticles through porous media using bubbles

Abstract:

Bubbles dispersed into a liquid phase create a wet foam, which is a complex fluid with a volume fraction of the gas phase up to 97 vol.%. Nanoparticles have been used to stabilize bubbles. They can irreversibly adsorb at the gas-liquid interface providing a remarkable stability to the foam. This irreversible process opens the opportunity of carrying nanoparticles through porous media using bubbles. Application is, e.g., in the remediation of contaminated sites. Nanoparticle adsorption onto bubbles helps overcoming the major challenges of reactive nanoparticle transport, i.e., mobility control, aggregation, and retention. There are few worked in the literature focused on reactive nanoparticle transport in porous media with bubbles and at the current stage transport models cannot describe the system as the mechanisms of particle-particle and particle-bubble interaction during transport are not well understood, yet.

Here, an experimental and modeling work is presented. Nanoparticles made of silica and nanoscale zero valent iron (nZVI) in combination with a cationic surfactant were used to stabilize air-bubbles. Experiments were carried out using a column-flood system made of sand at conditions of optimal foam stability. Constitutive equations for degradation, attachment/detachment, straining, and agglomeration were accounted for in the model in addition to foam kinetics. Simulations of the calibrated model agree well with the measurements. However, it was observed that stability of the bubbles could be significantly enhanced at typical concentration used in remediation of contaminated sites, which are much lower than the optimal concentrations from stability tests, without compromising the permeability of the medium. Moreover, the presence of bubbles helps reaching low permeability zones. From the prospective of an environmental engineering application in nano-remediation, this work suggests that foam can be a valuable alternative to conventional methods to deliver nanoparticles in the subsurface in an efficient and sustainable manner, given the negligible amount of water required.