Mechanistic changes during phytoremediation of perchlorate under different root-zone conditions

Two types of hydroponic bioreactors were used to investigate the mechanisnistic changes during phytoremediation of perchlorate under different root-zone conditions. The bioreactors included: (1) an aerobic ebb-and-flow system planted with six willow trees, and (2) individual willow trees grown in sealed root-zone bioreactors. Rhizosphere probes were used to monitor for the first time during phytoremediation of perchlorate, diurnal swings in oxidation-reduction potential (E(H)), dissolved oxygen (DO), and pH. Radiolabeled (36Cl-labeled) perchlorate was used as a tracer in a subset of the sealed bioreactor experiments to quantify the contribution of phytodegradation and rhizodegradation mechanisms. Rhizodegradation accounted for the removal of 96.1 +/- 4.5% (+/-95% CI) of the initial perchlorate dose in experiments conducted in sealed hydroponic bioreactors with low DO and little or no nitrate N. Meanwhile, the contribution of rhizodegradation decreased to 76 +/- 14% (+/-95% CI) when nitrate (a competing terminal electron acceptor) was provided as the nitrogen source. Slower rates of phytoremediation by uptake and phytodegradation were observed under high nitrate concentrations and aerobic conditions, which allowed perchlorate to persist in solution and resulted in a higher fraction uptake by the plant. Specifically, the rate of removal of perchlorate from bulk solution ranged from 5.4 +/- 0.54 to 37.1 +/- 2.25 mg/L/d (+/-SE) in the absence of nitrate to 1.78 +/- 0.27 to 0.46 +/- 0.02 mg/L/d (+/-SE) at high nitrate concentration. The results of this study indicate that the root-zone environment of plants can be manipulated to optimize rhizodegradation and to minimize undesirable processes such as uptake, temporal phytoaccumulation, and slow phytodegradation during phytoremediation of perchlorate. Rhizodegradation is desired because contaminants resident in plant tissue may remain an ecological risk until completely phytodegraded.