Zakia D. Parrish

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.

Influence of Citric Acid Amendments on the Availability of Weathered PCBs to Plant and Earthworm Species

A series of small and large pot trials were conducted to assess the phytoextraction potential of several plant species for weathered polychlorinated biphenyls (PCBs) in soil (105 μ g/g Arochlor 1268). In addition, the effect of citric acid on PCB bioavailability to both plants and earthworms was assessed. Under small pot conditions (one plant, 400 g soil), three cucurbits (Cucurbita pepo ssp pepo [zucchini] and ssp ovifera [nonzucchini summer squash], Cucumis sativus, cucumber) accumulated up to 270 μg PCB/g in the roots and 14 μg/g in the stems, resulting in 0.10% contaminant removal from soil. Periodic 1 mM subsurface amendments of citric acid increased the stem and leaf PCB concentration by 330 and 600%, respectively, and resulted in up to a 65% increase in the total amount of contaminant removed from soil. Although citric acid at 10 mM more than doubled the amount of PCB desorbed in abiotic batch slurries, contaminant accumulation by two earthworm species (Eisenia foetida and Lumbricus terrestris) was unaffected by citric acid at 1 and 10 mM and ranged from 11–15 μg/g. Two large pot trials were conducted in which cucurbits (C. pepo ssp pepo and ssp ovifera, C. sativus) and white lupin (Lupinus albus) were grown in 70 kg of PCB-contaminated soil. White lupin was the poorest accumulator of PCBs, with approximately 20 μ g/g in the roots and 1 μ g/g in the stems. Both C. pepo ssp ovifera (summer squash) and C. sativus (cucumber) accumulated approximately 65–100 μ g/g in the roots and 6–10 μ g/g in the stems. C. pepo ssp pepo (zucchini) accumulated significantly greater levels of PCB than all other species, with 430 μg/g in the roots and 22 μ g/g in the stems. The mechanism by which C. pepo spp pepo extracts and translocates weathered PCBs is unknown, but confirms earlier findings on the phytoextraction of other weathered persistent organic pollutants such as chlordane, p,p′-DDE, and polycyclic aromatic hydrocarbons.

Phyto-Extraction of Field-Weathered DDE by Subspecies of Cucurbita and Exudation of Citric Acid from Roots

Two subspecies of Cucurbita pepo L. (summer squash) differ in phyto-extraction of weathered DDE when grown in the field. Three cultivars were selected from each of the two subspecies; Cucurbita pepo ssp pepo (zucchini) with a greater ability to take up DDE, and Cucurbita pepo ssp ovifera (summer squash) with a lesser ability to take up DDE. When grown in the field, ssp pepo extracted 0.4 to 1.0 milligrams of DDE per plant, while ssp ovifera removed from the soil only 0.02 to 0.1 milligram per plant. These cultivars were grown in hydroponics to evaluate whether exudation of organic acids from the roots was involved in uptake of weathered DDE. Phosphorus nutrition played a significant role in exudation of organic acids into the hydroponics solution. For both subspecies, the better the phosphorus nutrition, the more tartaric and less citric acid was exuded. Subspecies pepo showed a greater increase in citric acid exuded under phosphorus depletion than ovifera. However, when solutions of root exudates were used to batch extract DDE from soil, effect of subspecies was opposite to that seen in the field. This suggests there are factors other than disruption of the soil matrix that play a role in phyto-extraction by plants. Nevertheless, among subspecies of Cucurbita pepo, the exudation of citric acid was related to phyto-extraction of more weathered organic contaminants in soil.