THE ROLE OF FERRIHYDRITE IN THE TRANSPORT AND BIOACCESSIBILITY OF NI, CR, PB, AS AND CD IN WATER, SOIL AND AIR PARTICULATES IN A VARIETY OF METAL CONTAMINATED ENVIRONMENTS

Chemically similar nano-crystalline to colloidal size siliceous-ferrihydrite has been confirmed to be a ubiquitous phase in water, soil and air samples from areas contaminated with a variety of metals. In addition to Fe₂O₃ the ferrihydrite includes Si (up to 20% SiO₂), Al( ~10% Al₂0₃) and P₂O₅ (~2%) with minor and trace elements of environmental concern including Cr, Ni, Pb, As, Cd and other metals including Ti, V, Mn, Zn, and Ag. Semi-quantitative major, minor and trace element analysis using a 200kV Transmission Electron Microscope (TEM) standardized with Standard Reference Materials from the Smithsonian, USGS and NIST was applied to analyze soil and water samples from below the Coeur D’Alene (CDA) mining district, serpentine soil from the Coast Range of California and air samples from California’s Central Valley which routinely contain up to 10% ferrihydrite in the less than 2µm fraction. In all of the samples the ferrihydrite occurs as a continuous range of sizes from 3-5nm nano-crystal clusters, typically immersed in a phyllosilicate matrix, up to 100nm crystals with coherent and consistent lattice across the entire grain. The more coherent grains have selected area electron diffraction (SAED) patterns indicating d-spacings of 2.52, 1.97 and 1.71Å while nano-crystals display more diffuse rings/spots but have the same general d-spacings. The chemistry of the ferrihydrite does not vary in any consistent manner as a function of crystallinity or grain size. Large surface areas, especially for the nano-crystalline ferrihydrite, can lead to higher metal availability from an environmental and human health perspective. Ferrihydrite in the serpentine soils routinely contain up to 1.13 wt% Cr, 1.29% Ni; lake samples downriver from the CDA mining district contain up to 5 wt% Pb and 0.38 wt% As, and Central Valley air samples up to 1.42 wt% Cr. The exact metal content of the ferrihydrite is a function of the metals available in any certain environment rather than a general preference of ferrihydrite to incorporate the different metals. The wide range of transition metals found in substantial concentrations in the ferrihydrite imply that this phase could be very important in determining the mobility and bioaccessibility for metals that pose health concerns.