BIOSTIMULATION OF RHIZODEGRADATION OF PERCHLORATE

Ammonium perchlorate is an oxygen-adding component in propellants for rockets, missiles and fireworks. Perchlorate salts are also used extensively in many commercial and industrial processes. Perchlorate (ClO₄^-) poses potential environmental concerns because its ionic radius and charge are similar to that of iodine, which allows perchlorate to competitively block thyroid iodine uptake.

Previous hydroponics and field studies show that phytodegradation and rhizodegradation are the two predominant mechanisms by which plants metabolize perchlorate. Phytodegradation is the slower process and poses ecological risk because it involves uptake and temporal accumulation of perchlorate in plant leaves. Rhizodegradation uses anaerobic microbes and plant exudates/enzymes in the root zone to rapidly degrade ClO₄^- to Cl^- with ClO₃^- and ClO₂^- as intermediate metabolites. In this study, rhizodegradation of perchlorate by willow trees (Salix nigra) was enhanced by stimulation of the root zone perchlorate degrading microbes with dissolved organic carbon (DOC). Sterilized and unsterilized mushroom compost, chicken litter extract, and acetate were used as DOC sources. The concentration of DOC in solutions was about 500 mg/L. In reactors amended with DOC, perchlorate was completely degraded from an initial concentration of 25 mg/L to below the IC method detection limit of 2 μg/L in under 10 days. The average rate of perchlorate removal from the root zone was estimated as 3.1 mg/L.day. In contrast, very slow rate of rhizodegradation (0.25 mg/L.day) were observed for control plants that received no DOC. The lag-times and rhizodegradation rates for plants supplied with DOC from mushroom compost and chicken litter sources were similar to acetate. No significant difference was observed between sterilized and unsterilized nutrient amendments added to the planted bioreactors, suggesting that ubiquitous perchlorate degrading bacteria were present in the tree roots. This study has provided some of the first evidence that the addition of DOC to the root zone reduces the lag-time that precedes rhizodegradation of perchlorate by root zone microbes. The ecological significance of this research is that enhancing rhizodegradation minimizes the undesirable uptake and accumulation of perchlorate in plant leaves and possible recycling of perchlorate into the ecosystem.