A. Paul Schwab

Effectiveness of Phytoremediation as a Secondary Treatment for Polycyclic Aromatic Hydrocarbons (PAHs) in Composted Soil

ABSTRACT A greenhouse study was conducted over a 12-month period to investigate the fate of polycyclic aromatic hydrocarbons (PAHs) in soil using phytoremediation as a secondary treatment. The soil was pretreated by composting for 12 weeks, then planted with tall fescue (Festuca arundinacea), annual ryegrass (Lolium multiflorum), and yellow sweet clover (Melilotus officinalis). Two sets of unvegetated controls also were evaluated, one fertilized and one unfertilized. Total PAH concentrations decreased in the tall fescue, annual ryegrass, and yellow sweet clover treatments by 23.9%, 15.3%, and 9.1%, respectively, whereas the control was reduced by less than 5%. The smaller two- and most of the three-ringed compounds—naphthalene, acenaphthylene, acenaphthene, fluorene, and anthracene—were not found in detectable concentrations in any of the treatments. The most probable number analysis for microbial PAH degraders did not show any statistically significant differences among treatments. There were significant differences among treatments (p < 0.05) for the residual concentrations of five of the target PAHs. Root surface area measurements indicated that tall fescue and annual ryegrass both had significantly higher root surface area than yellow sweet clover, although the two species were not significantly different from each other. The tall fescue treatment resulted in the highest root and shoot biomass, followed by annual ryegrass and yellow sweet clover, and also had the highest percent of contaminant removal after 12 months. These results imply a positive relationship between plant biomass development and PAH biodegradation.

Phytoremediation of polycyclic hydrocarbon contaminated soil: part II. Impact on ecotoxicity.

Several biological assays were used to evaluate the toxic effects of contaminants in soil after phytoremediation. During the treatment process, significant decreases in overall toxicity were observed. Specifically, earthworm survivability and lettuce germination increased over the study period. Microbial respiration improved, but only in planted treatments. Toxicity and total polycyclic aromatic hydrocarbon concentrations showed some correlation, but the relationships generally were not significant. Soil moisture was less of a predictor for biological responses. The presence of plants did not provide a clear advantage for improving toxicity compared to unplanted treatments.

Adsorption of iron cyanide complexes onto clay minerals, manganese oxide, and soil.

The adsorption characteristics of an iron cyanide complex, soluble Prussian blue KFe(III)[Fe(II)(CN)6], were evaluated for representative soil minerals and soil at pH 3.7, 6.4 and 9.7. Three specimen clay minerals (kaolinite, montmorillonite, and illite), two synthesized manganese oxides (birnessite and cryptomelane), and a Drummer soil from Indiana were used as the adsorbents. Surface protonation of variable charge sites increased with decreasing pH yielding positively charged sites on crystal edges and enhancing the attractive force between minerals and iron cyanide complexes. Anion adsorption on clays often is correlated to the metal content of the adsorbent, and a positive relationship was observed between iron or aluminum content and Prussian blue adsorption. Illite had high extractable iron and adsorbed more ferro-ferricyande anion, while kaolinite and montmorillonite had lower extractable iron and adsorbed less. However, less pH effect was observed on the adsorption of iron cyanide to manganese oxides. This may due to the manganese oxide mediated oxidation of ferrocyanide [FeII(CN)6 4−], to ferricyanide [FeIII(CN)6 3−], which has a low affinity for manganese oxides.

Plant germination and growth after exposure to iron cyanide complexes.

Phytoremediation has been proposed for treatment of cyanide-contaminated soil. This study was conducted to identify plants with the highest potential for phytoremediation of iron cyanide contaminated soil. Multiple cultivars of two cyanogenic species, sorghum (Sorghum bicolor) and flax (Linum usitatissimum), and one non-cyanogenic species, switchgrass (Panicum virgatum L), were selected for evaluation. The cultivars were screened by quantifying germination and root elongation. Differences in germination emerged among the cultivars (P < 0.05), but these differences appeared to be unrelated to cyanide concentration. The presence of 1000 mg/kg Prussian blue tended to suppress root growth parameters of flax and switchgrass but did not affect sorghum similarly.

Evaluation of Toxicity Analysis for Foundry Sand Specifications

Byproducts from many industries have the potential to be used as construction materials, but some means is required to determine if the material is environmentally benign. Foundry sands are produced in many states and can be useful as in transportation projects. However, INDOT currently requires the use of the MICROTOX test to assess the potential toxicity of the sands, and this requirement is viewed as an unnecessary impediment by the producers of foundry sands and is a requirement not encountered in other states. Therefore, the goal of this project was to review current requirements for testing of recycled materials, determine the availability of MICROTOX testing, and to make recommendations concerning the continued use of MICROTOX as an assessment tool. Strictly from viewpoint of environmental protection, the inclusion of Microtox makes sense. The test has the sensitivity to detect potentially toxic agents in recycled sand that might escape chemical analysis. The test, therefore, provides a layer of assurance that otherwise would be absent. From the perspective of the foundry industry, the Microtox test is an unneeded hurdle that could potentially block the beneficial use of spent foundry sand. Cost is one consideration, but the lack of local analytical facilities for the Microtox is particularly troublesome. The authors recommend that the Microtox test be retained by INDOT, but they suggest the following: a) Minimize the number of samples of foundry sand that must be tested possibly by reducing the frequency of sampling and testing; b) A consistent, readily available laboratory needs to be established to ensure rapid turn around of analyses and reduced costs. Currently, the demand is low and some dedicated equipment is needed to perform the test. One of the potential outcomes of this project discussed during the negotiations for this project was a possible follow-up project in which the authors would investigate the modifications to the bioassay. This might include exploring alternatives to Microtox or simplifications of the Microtox test. The authors remain open to this possibility, but from the scientific point of view, such a follow-up may not be necessary. Of all the bioassays reviewed, Microtox seemed to be the most widely used (though not for foundry sands), and the authors found no evidence that other bioassays were being offered routinely at commercial labs.

Remediation and Stabilization of Soils Contaminated by Lead Resulting from the Removal of Paint from Bridges

Lead (Pb)-based paints are commonly used for painting steel bridge structures. Soils in the immediate vicinity of older bridges have been contaminated with Pb as a result of normal weathering and peeling of the paint coupled with removal prior to repainting. The objectives of this project were to assess the extent of Pb contamination near highway bridges and to evaluate phytoremediation and immobilization as means of remediation. The authors examined soils in the vicinity of approximately 20 bridges in Indiana that had been repainted recently and were known to have been painted originally with Pb-based paint. They found only three bridges in which the extensive areas of soil were contaminated by Pb in concentrations greater than 400 mg/kg. Two of these sites were used for this project. In the field study, soluble phosphate was added to the soil and transformations of Pb were monitored using chemical extractants. Simultaneously, sunflowers were grown in the field to test the feasibility of phytoextraction for these conditions. Phosphate additions were effective in significantly reducing bioaccessible Pb, but the sunflowers were unable to remove Pb from the soils. A laboratory study using columns of contaminated soils demonstrated the very low mobility of Pb in soils. The recommendation from this study is that the immediate vicinity of recently painted bridges be tested for Pb in the soils. Elevated concentrations can be treated with soluble phosphate to reduce bioavailability of Pb.