Organic carbon biostimulates rapid rhizodegradation of perchlorate.

Previous hydroponics and field studies identified phytodegradation and rhizodegradation as the two main mechanisms by which plants metabolize perchlorate. Plant uptake and phytodegradation of perchlorate is a slower and undesired process that poses ecological risks resulting from phytoaccumulation of some fraction of the perchlorate. Meanwhile, rhizodegradation is a more rapid and favored process involving perchlorate-degrading bacteria utilizing dissolved organic carbon (DOC) as a carbon and energy (electron) source to rapidly degrade perchlorate to innocuous chloride. In the present study, rhizodegradation of perchlorate by willow trees (Salix nigra) was biostimulated using electron sources obtained from natural and artificial carbon sources. In bioreactors provided with carbon sources as 500 mg/L DOC, 25 to 40 mg/L of initial perchlorate concentrations were removed to below the ion chromatography method detection limit of 2 microg/L in approximately 9 d. For planted controls provided with no electron donors, the time required for the complete removal of the same doses of perchlorate was up to 70 d. Enhancement of rhizodegradation by organic carbon reduced the phytoaccumulated fraction of perchlorate by an order of magnitude from approximately 430 to 20 mg/kg. The implication of the present study is that the high fraction uptake and phytoaccumulation of perchlorate in agricultural products and the recycling of perchlorate into the ecosystem can be significantly curtailed by supplying electron donors derived from organic carbon sources to the root zone of plants.

Uptake of N-nitrosodimethylamine (NDMA) from water by phreatophytes in the absence and presence of perchlorate as a co-contaminant.

The uptake and fate of the emerging contaminants N-nitrosodimethylamine (NDMA) and perchlorate in phreatophytes was studied in a hydroponics system under greenhouse conditions. NDMA is a potent carcinogen, and perchlorate disrupts the functioning ofthe human thyroid gland. The rate of removal of NDMA from solution by rooted cuttings of black willow (Salix nigra) and hybrid poplar (Populus deltoides x nigra, DN34) trees varied seasonally, with faster removal in summer months when transpiration rates were highest. A linear correlation between the volume of water transpired and mass of NDMA removed from the root zone was observed, especially at higher NDMA concentrations. In bioreactors dosed with both NDMA (0.7-1.0 mg L(-1)) and perchlorate (27 mg L(-1)), no competitive uptake of NDMA and perchlorate was observed. While NDMA was primarily removed from solution by plant uptake, perchlorate was predominantly removed by rhizodegradation. In the presence of NDMA, a slower rate of rhizodegradation of perchlorate was observed, but still significantly faster than the rate of NDMA uptake. For experiments conducted with radiolabeled NDMA, 46.4 +/- 1.1% of the total 14C-activity was recovered in the plant tissues and 47.5% was phytovolatilized. The 46.4 +/- 1.1% recovered in the plants was distributed as follows: 18.8 +/- 1.4% in leaves, 15.9 +/- 5.9% in stems, 7.6 +/- 3.2% in branches, and 3.5 +/- 3.3% in roots. The poor extractability of NDMA with methanol-water (1:1 v/v) from stem and leaf tissues suggested that some fraction of NDMA was assimilated. The calculated transpiration stream concentration factor (TSCF) of 0.28 +/- 0.06 suggests that NDMA is passively taken up by phreatophytes, and mainly phytovolatilized.

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.