Atom Exchange Between Fe(II) and Nanocrystalline Iron Phases in Tropical Soils.

The exchange of aqueous iron (Fe) atoms with solid phase Fe atoms has recently been demonstrated for many Fe (oxyhydr)oxide minerals. However, it remains unclear if this process is significant in environmentally complex systems such as soils or sediments. Here, we demonstrate for the first time that this process can occur with natural Fe solid phases, present in a soil from the Bisley Site in the Luquillo Experimental Forest, Puerto Rico. In batch experiments, we introduced an aqueous 57Fe(II) spike to soil slurries under anoxic conditions and traced the isotopic composition of the aqueous phase and in 0.5M and 7M HCl extractions of solid phase over 56 days. By coupling this data with a numerical model, we calculate aqueous Fe atoms can be exchanged with both the labile (0.5 M HCl-extractable) and bulk (7M HCl-extractable) Fe pools, with turnover times on the order of hours and months, respectively. Biologically active treatments exhibited Fe reduction concurrently with Fe atom exchange which diminished the progression of our labeled Fe into the more recalcitrant Fe pools.