PNNL

Abstract

The interaction of contaminants and nutrients with soil constituents is controlled by processes in intergranular and intragranular pore spaces of organic matter or/and common secondary minerals such as ferrihydrite, Fe3+10O14(OH)2. This contribution shows that the distribution and speciation of the contaminants As, P, Pb, Si, Sn and Zn in a porous ferrihydrite grain is greatly affected by the heterogeneous pore size distribution as well as the ability of the elements to polymerize. TEM and APT studies are conducted on Focused Ion Beam (FIB) sections extracted from a porous ferrihydrite grain from the smelter-impacted top soils in Sudbury, Ontario, Canada. The ferrihydrite grain has pore spaces ranging in diameter from tenths to hundreds of nanometers. TEM and Scanning-TEM studies indicate that the surfaces of the pore walls are enriched in Si and that in contrast to P, the elements As, Pb, Sn and Zn are heterogeneously distributed within or on the surface of the pore walls. APT confirms the clustering of As, Pb, Sn and Zn and suggests the occurrence of polymerized (As3+Ox) and (Pb2+Ox) polyhedra. Speciation and distribution of Si, As, Pb, Sn and Zn implies that sequestration processes of these elements in porous constituents within a soil environment is affected by adsorption, desorption, diffusion, polymerization and nucleation processes. Understanding these processes in soil environments requires multi-analytical approaches and in this regard advantages and disadvantages of the combination TEM and APT to characterize complex environmental samples at the atomic- to nanometer scale are discussed.