Within the complex architecture of soils, rate limiting processes controlling the partitioning and redox behavior of metal ions can be either (bio)geochemical or physical.  Over the past 35 years, Professor D. L. Sparks has illuminated how transport processes coupled with chemical kinetics dictate the adsorption/desorption, dissolution/precipitation, and oxidation/reduction of metals and metalloids within soils.  He has further helped to pioneer means to both measure reaction rates and decipher rate controlling phenomena.  Herein, we examine chemical and biological mediated reactions controlling the redox state, dissolved concentrations, and transport of arsenic and chromium within simulated or native structure of soils.  Further, we illustrate where (bio)geochemical or physical limitations in reaction rate provide the overriding influence on ion fate. <br/> <br/>              Owing to soil structure, even seemingly aerobic soils may have a substantial anaerobic volume; microbial oxygen demand may exceed supply within the interior of soil aggregates or peds, leading to anaerobic conditions despite aerated or oxygenated exteriors.  The outcome of heterogenous redox conditions common to soils is steep concentration gradients of redox sensitive elements such as iron and manganese over micron-scales, which further controls the distribution and redox state of trace elements such as arsenic and chromium.  We observe that Fe(III) and Mn(III/IV) are concentrated in the exterior of soil aggregates as a result of oxygen gradients, with aggregate interiors being anaerobic and thus leading to Fe(II) and Mn(II) production.  Arsenic similarly follows iron and manganese, being microbially reduced in aggregate interiors but oxidized by microbially mediated processes (either enzymatically or by biogenic Mn-oxides) and partitioning in aggregate exteriors.  Although Mn-oxides serve as facile oxidants of As(III), the resulting As(V) strongly partitions to the Fe(III)-oxides.  Chromium, in contrast to arsenic, is easily reduced under anaerobic conditions but often resistant to oxidation even under well aerated conditions.  We illustrate that the oxidation rate of Cr(III) solids is depend on (i) their solubility and (ii) proximity to higher valent Mn-oxides.  In sum, our results illustrate the dynamic interplay of biogeochemical transformation, physical heterogeneity, and solid-phase distribution on ion fate in soils.

Combined impact of (bio)chemical and physical processes in defining metal dynamics within soils

249th National Meeting (2015)

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