Redox Oscillation Impact on Natural and Engineered Biogeochemical Systems: Chemical Resilience and Implications for Contaminant Mobility

Many geochemical systems fluctuate regularly from oxic to anoxic conditions (flooded soils and nuclear waste surface repositories, for instance). In these conditions many inorganic contaminants including Sb, Se, Cr, As, and U are highly sensitive to changes in redox conditions. These oscillations may result in changes to their speciation, toxicity, and mobility. We demonstrate through the combination of redox-stat batch-reactor experiments that periodic and cumulative changes to matrix mineralogy, contaminant speciation, and mineral surface properties occur following periodic cycles of reduction and oxidation. These changes result in both short-term (intra-cycle) and long- term (inter-cycle) changes to Kd values for a range of redox sensitive contaminants. These results demonstrate that naturally occurring redox oscillations may result in long-term immobilization of contaminants in the solid phase in addition to short-term variations in mobility.