I understand that pH is a factor in metal speciation but does the grain size of soil particles have any influence in speciation of metal like Cr into Cr(III) and Cr(VI) because this dictate stheir level of toxicity.
Grain size is definitly a very important factor to take into account while adressing metal speciation, since the soil grain size will limit the amount of organic matter, water content and oxidation of the matrix and thus metal speciation.
Grai size or particle size is important in speciaton. When you speciate, we have available, organic, crystal,amorphous; all these forms are related to particle size.
The binding affinity of metals to silt and clay is high compared to that with sand. primarily this is one reason grain size determination is important. With the variations in grain size the other physico-chemical properties will also change.
The grain size does not necessarily impact the speciation of redox sensitive metals since the speciation is related to the type and concentration of reductants or oxidants in the particles.However, the particle size does influence the number of reactive sites available for redox reactions. Greater surface area often results in more reactive sites. For more information see e.g., Borch, T., Kretzschmar, R., Kappler, A., Cappellen, P. V., Ginder-Vogel, M., Voegelin, A., and Campbell, K., 2010. Biogeochemical Redox Processes and their Impact on Contaminant Dynamics. Environ. Sci. Technol. 44, 15-23.
As Thomas has correctly highlighted, grain size does not necessarily affect speciation of metals in soil, it only provides a surface area for metals to adsorb thus affecting the metal retention capacity.
Also not only does pH affect speciation, salinity and Cation exchange capacity of the soil or sediment also affect speciation.
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