Metal nanoparticles transport in the subsurface: a review

The increasing use of metal nanoparticles in industrial products has induced a global pollution, yet their fate in ecosystems is not fully understood. Here, we review the transport and transformation of metal nanoparticles in the subsurface with emphasis on mechanisms, research techniques, and numerical modeling. Transport can be explained by the Derjaguin–Landau–Verwey–Overbeek theory, the colloid filtration theory, advection–dispersion, Brownian diffusion, interception, gravitational sedimentation, attachment–detachment, straining, blocking and ripening, colloid-facilitated transport, hetero-aggregation, and competitive blocking. Transport research techniques include columns, lysimeters, quartz crystal microbalance, and parallel plate and atomic force microscopy. Metal nanoparticle transformation is characterized by diffuse gradient in thin films and combined microscopy and spectrometry. There are transformation, transport, co-transport, and coupling models. Metal nanoparticles are commonly retained in porous media with small grain sizes, rough surfaces, and surface charges opposite to that of the metal nanoparticles. Transport of metal nanoparticles is favored by colloids and competitive blocking during co-transport. The transformation of metal nanoparticles significantly changes the properties and transport characteristics, which limits the predictions of models.