Processes dealing with the EarthÕs elemental partitioning and distribution, which includes, e.g., heterogeneous catalysis, reaction pathways, mineral growth, transformation, and weathering, and in a broader sense biotic-abiotic Earth interactions, are all related to phenomena at the nanoscale, often involving naturally-occurring nano-sized geomaterials, as well as a minority but very important anthropogenic component. What happens in this dimensional realm has no equivalent at either smaller or larger scales. Therein lies the essence and future scientific capital (and complexity) of nano-geo and -environmental science.
Considering everything that passes through a 0.22-mm or even a 2-nm filter as ÒdissolvedÓ is not appropriate, as advanced analytical technology and various (ultra)filtration methods now make it possible to identify and study the tremendous intricacies of nanoparticles. Aqueous reactions involving, e.g., a particular transition metal present as a hydrated aqueous ion, vs. an aqueous molecular cluster, vs. a 2-nm mineral nanoparticle with the same composition as the cluster, vs. a 50-nm or larger mineral grain with the same composition, will all show significantly different reaction pathways and kinetics.
Examples that will be discussed include: 7-nm hematite (α-Fe2O3) nanocrystals increase the surface-area normalized oxidation rate of aqueous Mn2+ by one to two orders of magnitude compared to 37-nm crystals, resulting in the rapid formation of Mn (oxy)hydroxide minerals that are important heavy metal sorbents in water and soils; the dissolution rate of nano- and micro-galena (PbS), important in the release of dissolved aqueous Pb into the environment, varies by more than one order of magnitude simply by varying the size and the aggregation state of the particles; phytoplankton in the worldÕs oceans, performing more than half of the photosynthesis on Earth, depend on Fe as a limiting nutrient supplied not only by dissolved (in)organic complexes, but 2 to 20 nm Fe oxide nanoparticles that phytoplankton process in completely different ways.