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.