Perchlorate Fate in Soils.

Ammonium and potassium perchlorate are oxidizers used in military and non-military applications.  While negative charge of perchlorate ion would result in ion exclusion in predominantly negatively charged soils, soils where minerals with pH-dependent charge predominate can develop substantial anion exchange capacity (AEC) (ability to retain and exchange anions) and as a result retain perchlorate.  Soils expected to adsorb perchlorate are Oxisols (highly weathered soils) that develop net positive charge at low pH values and Andisols (volcanic soils) with amorphous aluminosilicates that have high surface area and high point of zero charge (net negative charge over a wide pH range).  The objective of this study was to determine the potential for natural attenuation of perchlorate in soil through adsorption and degradation, linking obtained transport descriptors to soil properties. Batch perchlorate sorption experiments were conducted using a range of soils including volcanic-derived, weathered and organic soils.  Adsorption kinetics and soil to solution ratios were determined in preliminary studies.  Samples were analyzed for perchlorate by ion chromatography with a conductivity detector (US EPA Method 314.0) and by ion selective electrode.  Multiple point adsorption isotherms were obtained for each soil.  Adsorption coefficients were determined from the isotherms.  Soil physical and chemical properties (organic carbon, pH, CEC, AEC, clay content, iron content) were determined and used to develop correlations with adsorption coefficients and degradation rates determined from the study.  For several soils effect of pH on adsorption was also evaluated.  It was observed that adsorption of perchlorate is positively correlated with the andic parameter and increases with decrease in pH.