Degradation Kinetics of Perchlorate in Sediments and Soils

This study investigated the intrinsic perchlorate (ClO₄ ^-)degradation kinetics of sediments and soils from multiple sites in microcosm studies, including the influence of varying nitrate concentration (NO₃ ^--N from 1 to 22.8 ppm) and up to 300 ppm sulfate. The first-order degradation rates and lag times of both ClO₄ ^- and NO₃ ^- degradation were site-specific and dependent on environmental conditions such as organic substrate availability, nitrate, initial ClO₄ ^- concentration, and prior ClO₄ ^- exposure. At an initial ClO₄ ^- concentration of 5 ppm, ClO₄ ^- degradation rates ranged from 0.13 to 0.46 day^-1, and lag times of ClO₄ ^- degradation ranged from 0 to 60.0 days; while NO₃ ^- degradation occurred at rates ranging from 0.03 to 1.42 day^-1, with lag times ranging from 0 to 29.7 days. Under the same treatment conditions, NO₃ ^- degradation rates were relatively higher than that of ClO₄ ^-. Perchlorate degradation rates remained constant at both lower (0.5 ppm) and higher (5 ppm) ClO₄ ^- concentrations. Generally, ClO₄ ^- rates were affected by the availability of organic substrate, which was represented here by Total Volatile Solids (TVS) of sediments and soils, and not by NO₃ ^-. Nitrate did increase the lag time of ClO₄ ^- degradation, which may account for the persistence of ClO₄ ^- in the environment, especially when ClO₄ ^- is typically ppb levels in the environment compared to ppm levels of NO₃ ^-. This study showed rapid intrinsic ClO₄ ^- degradation in sediments and soils of contaminated sites, and highlighted the potential for natural attenuation of ClO₄ ^- in the environment.