Phytoremediation and enhanced natural attenuation of perchlorate and N-Nitrosodimethylamine as a single and co-contaminants
Yifru, Dawit Desalegn
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The fate of two emerging contaminants, perchlorate (ClO4-) and N-Nitrosodimethylamine (NDMA) (C2H6N2O), in planted systems, was investigated using phreatophytes grown in hydroponic and soil bioreactors under greenhouse conditions. The recent detection of perchlorate in the food chain and growing concern over the recycling of perchlorate in the environment by decaying senescent leaves have been attributed to plant perchlorate uptake and phytoaccumulation. This study found higher perchlorate concentrations in mature and dry vegetation than in the young leaves collected from Longhorn Army Ammunition Plant (TX) and the Las Vegas Wash (NV), implying phytoaccumulation and possible recycling of perchlorate into the ecosystem. The study also investigated the feasibility of enhancement of rhizodegradation of perchlorate by willow trees (Salix nigra) using three electron donor sources of dissolved organic carbon (DOC). In planted hydroponic and soil bioreactors dosed with DOC, 40 mg L-1 perchlorate was completely degraded to below the IC method detection limit of 2 µg L-1 in under 10 days. For planted control bioreactors, more than 70 days were required to remove the same amount of perchlorate. Addition of DOC to the root zone biostimulated rapid degradation of perchlorate and reduced the amount of perchlorate taken up and phytoaccumulated by an order of magnitude. Biostumulation and sustained rapid rhizodegradation will minimize the undesirable uptake and accumulation of perchlorate in agricultural products and possible recycling of perchlorate into the ecosystem. Hybrid poplar (Poplus deltoides × nigra, ND34) and black willow trees were used to remove 1 mg L-1 NDMA to below the GC/MS method detection limit of 10 µg L-1 in less than 60 days in hydroponic system. A linear correlation between the volume of water transpired by the plants and the mass of NDMA removed from the root zone indicated that the phytoremediation mechanism of NDMA is primarily by uptake and phytovolatilization. The calculated transpiration stream concentration factor of 0.28 ± 0.06 suggests passive uptake of NDMA. In experiments conducted with radiolabeled (14C-NDMA), 46.4 ± 1.1% of the total 14C-activity was recovered in the plant tissue while 47.5% was phytovolatilized. This study found no evidence of competitive uptake of perchlorate and NDMA, however rhizodegradation of perchlorate decreases in the presence of NDMA.