Perchlorate retention and mobility in soils

Abstract

Adsorption and release of perchlorate in a variety of soils, minerals, and other media were studied when the solid media were exposed to low and high aqueous solutions of perchlorate salts. Low level ClO₄⁻ exposure was investigated by subjecting triplicate 5.0 g portions of a solid medium (38 different soils, minerals, or dusts) to 25 mL of an aqueous ammonium perchlorate (NH₄ClO₄) solution containing 670 ng mL⁻¹ (6.8 µM) perchlorate. This corresponds to a perchlorate-to-soil ratio of 3.4 µg g⁻¹ (34 nmol g⁻¹). At this level of exposure, more than 90% of the perchlorate was recovered in the aqueous phase, as determined by ion chromatography. In some cases, more than 99% of the perchlorate remained in the aqueous phase. In some cases, the apparent loss of aqueous perchlorate was not clearly distinguishable from the variation due to experimental error. The forced perchlorate anion exchange capacities (PAECs) were studied by soaking triplicate 5.0 g portions of the solid media in 250 mL of 0.20 M sodium perchlorate (NaClO₄) followed by repeated deionized water rinses (overnight soaks with mixing) until perchlorate concentrations fell below 20 ng mL⁻¹ in the rinse solutions. The dried residua were leached with 15.0 mL of 0.10 M sodium hydroxide. The leachates were analyzed by ion chromatography and the perchlorate concentrations thus found were subsequently used to calculate the PAECs. The measurable PAECs of the insoluble and settleable residua ranged from 4 to 150 nmol g⁻¹ (µmol kg⁻¹), with most in the 20–50 nmol g⁻¹ range. In some soils or minerals, no sorption was detectable. The mineral bentonite was problematic, however. Overall, the findings support the widely accepted idea that perchlorate does not appreciably sorb to soils and that its mobility and fate are largely influenced by hydrologic and biologic factors. They also generally support the idea that intrasoil perchlorate content is depositional rather than sorptive. On the other hand, sorption (anion replacement) of perchlorate appears to occur in some soils. Therefore, the measurement of perchlorate in soils requires accounting for ion exchange phenomena; leaching with water alone may give inaccurate results. If perchlorate anion exchange is confirmed to be negligible, then leaching procedures may be simplified accordingly.