Experimental techniques are described for conducting greenhouse tests to examine the pathways for phytoremediation of volatile organic compounds (VOCs), distinguishing between uptake of the VOC along with water transpired by living plants and the passive, physical process of volatilization. The experimental techniques were applied to assess the potential for phytoremediation of methyl-tertiary-butyl-ether (MTBE). Experimental data indicate that MTBE was readily taken up from water by hybrid poplar saplings, yielding 25% reduction in aqueous MTBE concentration and 30% reduction in MTBE mass over a 1-week period. These reductions in plant systems were significantly greater than in controls, indicating great potential for MTBE phytoremediation in the field. Engineering parameters for phytoremediation were determined from the experiments, yielding an MTBE transpiration stream concentration factor of 1 and a root concentration factor of 0.7–1.4. Mass balance studies showed good closure on MTBE mass balance, indicating no significant degradation of MTBE in the young poplar saplings used in this study. These results suggest that phytovolatilization may be the primary pathway for MTBE phytoremediation.

Phytoremediation of uranium (U) contaminated soil has been hampered by a lack of information relating U speciation to plant uptake. The goals of the present study are to (1) provide fundamental information regarding uptake of U by plants; and (2) improve the phytoextraction of U from contaminated soil. The first was achieved through speciation modeling and hydroponic experiments that demonstrated that the uranyl $({\mathrm{UO}}^{\mathit{2}{\mathit{+}}_{\mathit{2}})}$ cation is the chemical species of U most readily accumulated in plant shoots. A subsequent soil incubation experiment examined the solubilization of U from contaminated soil by synthetic chelates and organic acids. The results indicated that citric acid solubilized >100 times more U than the other amendments. The results of the hydroponic and soil experiments were then integrated in a study that grew red beets in U-contaminated soils amended with citric acid or N-hydroxylethylene diamine triacetic acid. Citric acid was again a highly effective amendment, increasing shoot U content by 14-fold compared to controls. In addition to providing fundamental information regarding the uptake of U by plants, this study illustrates the importance of basic research to phytoremediation.

Methyl tert-butyl ether (MTBE) is an important ground-water contaminant. Gasoline spills and leaks from pipelines, underground and aboveground storage tanks, and other transport modes are major sources of MTBE contamination. A six-channel system was set up and used to examine the effect of alfalfa plants on the fate and transport of MTBE. Contamination by MTBE was established by continuously feeding a solution of 0.84 mM MTBE into each channel at 1 L/day. The concentrations of MTBE in the channel ground-water effluent, at the channel soil surface, and within alfalfa plant stems were monitored. By incorporating plant water use with the concentrations of MTBE in plant water, the authors obtained the mass fluxes of MTBE transported by alfalfa plants grown in soil channels. Even with the same contaminant concentration in the growth channel, alfalfa plants took up MTBE nonuniformly. The more vigorous the growth of plants, the higher the MTBE concentration in the plant water, and the more MTBE removed by the plants.

To ensure passenger safety during flight operations, the Federal Aviation Administration demands the use of glycol-based aircraft deicing fluids (ADFs) at airports. However, from the deicing process a significant amount of ADF runoff enters storm-water collection systems and/or finds its way to soil and ground-water ecosystems. The environmental impact of the ADF mixture is a combination of the extremely high biological oxygen demand from glycols and the toxicity to bacteria, aquatic organisms, and plants of the corrosion inhibitor tolyltriazole. The present paper discusses some alternatives that have been proposed to manage ADF waste and the need for an environmental management technology able to deal with both kinds of contaminants. Phytoremediation has been explored because a rhizosphere effect improves the efficiency of land treatment for many substances and only fungi and vegetation have been reported to be able to degrade the corrosion inhibitor (Wu et al. 1998). Below a toxic threshold of about 100 mg of methyl benzotriazole/L in the aqueous phase, plants can degrade approximately about 2 mg of methyl benzotriazole/(g dry root⋅day). Some of the advantages, difficulties, and conditions to apply the technology are outlined.

This paper reports the performance of two horizontal subsurface flow constructed wetland systems in the removal of pathogen and pathogen indicators under variable loading and operating conditions. The two constructed wetland systems evaluated under this project are located near Lincoln, Nebr. Firethorn is located in a small housing community and Rogers Farm at a rural single-family dwelling. Samples at Firethorn were collected at the wetland cell influent, effluent, and sand filter effluent (system effluent) from June 1996 through April 2000 and at the influent and effluent points from Rogers Farm from June 1999 through April 2000. The main monitoring parameters were fecal coliform at both sites and Salmonella spp. (from August 1999 through January 2000) at Firethorn. Firethorn demonstrated effective removal of fecal coliforms with 96.3% removal in the wetland cell and 98% by the wetland system, which included a sand filter. Rogers Farm had an average fecal coliform removal of 99.3%. Although Salmonella spp. was not detected at Firethorn, reduction of other bacteria was observed through the wetland system. In general, fecal coliform removal was excellent through the wetland system and no distinct difference in removal efficiency was observed with changes in season and temperature at either facility.

Hybrid poplar cuttings were shown to impact the fate and transport of subsurface benzene, while toxicity to the poplars was not observed. Laboratory experiments investigated the toxicity response of poplar cuttings to benzene exposure, contaminant distribution in plant tissues, contaminant degradation in the soil profile, and contaminant volatilization from the soil and plant tissues. Two separate studies were conducted to evaluate these parameters. The first study examined the toxicity of benzene to hybrid poplar cuttings in batch reactors. Poplar cuttings were exposed to various concentrations of benzene contaminated water in two different types of soil. Transpiration rates were measured as an indicator of acute toxicity. No acute toxicity was noted for dose concentrations up to 1,000 ppm. The second study evaluated benzene fate and transport. Live poplar cuttings and excised controls were planted in flow-through reactors and supplied with an influent benzene stream to mimic plume conditions. The presence of live poplar cuttings enhanced benzene degradation and decreased the effluent mass of benzene. A small amount of benzene was also volatilized from the plant tissues, providing evidence of plant-enhanced volatilization. Causes for the higher degradation rates appeared to be greater microbial populations of benzene degraders and a more oxygen-rich environment. The higher redox potential observed may be an artifact of the laboratory reactor design. The results obtained in this research combined with previous studies provide evidence that phytoremediation has the potential for effective, efficient, and environmentally friendly application at sites highly contaminated with benzene and potentially for other sites contaminated with biodegradable organics or volatile organic compounds.