The emergence of nanotechnology has not only made it possible to purposefully manipulate material structures at nanometer scales but has also greatly advanced our understanding of how nanometer-scale structures give rise to novel physical and chemical properties of materials. Nanophases and nanostructures are widely present in natural environments and expected to have a direct impact on the interactions of solid earth materials with geologic fluids and biota.
The study of mineral-water interface chemistry as controlled by nanostructures is a necessary step to bridge the existing gap between the molecular level understanding of a geochemical process and the macro-scale laboratory and field observations. In this presentation, I will review the recent progresses in nanoscience and provide a perspective on how these progresses can potentially impact geochemical studies. My presentation will be focused the following areas:(1) characterization of nanophases and nanostructures in natural systems,(2) size- and shape-dependent chemical reactivity and stability of mineral particles, (3) behavior of water and aqueous species in nanoconfinement, (4) effects of nanopores on geochemical reaction and mass transfers, and (5) biomineralization and nano-scale interactions between microbes and mineral surfaces.
Specifically I will show that nanopores are ubiquitous in porous geologic media and may account for >90% of total mineral surface areas. I will demonstrate that the space confinement within nanopores can significantly modify geochemical reactions in porous geologic media.
As the pore size is reduced to a few nanometers, the difference between surface acidity constants (pK2 - pK1) decreases, giving rise to a higher surface charge density on a nanopore surface than that on an unconfined mineral-water interface. The change in surface acidity constants results in a shift of ion sorption edges and enhances ion sorption on nanopore surfaces. Also, the water activity in a nanopore is greatly reduced, thus increasing the tendency for inner sphere complexation and mineral precipitation. All these effects combine to preferentially enrich trace elements in nanopores, as observed in both field and laboratory studies. This work sheds new light on such fundamental geochemical issues as the irreversibility of ion sorption and desorption, the bioavailability of subsurface contaminants, and the enrichment of trace metals in ore deposits, as well as the kinetics of mineral dissolution/precipitation.
This work was performed at Sandia National Laboratories, which is a multiprogram laboratory operated by Sandia Corporation, a Lockheed-Martin Company, for the DOE under contract DE-AC04-94AL8500.