Superfund Research Program

Phytoremediation of Contaminated Soils

Project Leader: Jodi R. Shann
Grant Number: P42ES004908
Funding Period: 2001 - 2006

Project-Specific Links

Final Progress Reports

Year:   2005 

Dr.Shann’s initial premise was that soil contaminated with metals and PAHs would be remediated by two separate plant-based processes, metal uptake and PAH degradation in the rhizosphere.  Rhizosphere degradation was hypothesized to be a function of root exudation and the microbial community under its influence.  Study results showed that plant metal uptake is influenced by the presence of other metals, but not PAHs.  Further, when plants and microorganisms were exposed to metals in hydroponic solutions or freshly spiked into soil (at levels commonly found in contaminated environments), toxicity was observed.  The degree of response was highly correlated to concentrations of the individual metals found in plant tissues, and to levels extracted from the soil by some solvents and procedures.  Although Dr. Shann’s lab proposed to look at the relative significance of uptake and degradation for remediation, more than half of the investigations conducted were designed to elucidate the role that plant exudates play in the rhizosphere.  A significant finding of this project was that root exudates increased PAH degradation in soil, and they did it by altering the composition of the microbial community.  Collectively, these studies clarified the potential for these plant-driven mechanisms to contribute to soil remediation. 

 

It also became clearer, however, that these mechanisms could only operate when contaminants were available.  In addition to hydroponic, sterile system, and spiked soil investigations, Dr. Shann’s studies included soil taken from actual RCRA and Brownfield sites where historical contamination had aged into the substrate.  Working with these real-world materials and field sites led the group to conclude that bioavailability is likely the primary controlling factor in remediation.  Specifically, metal bioavailability determines the degree of phytoextraction and PAH degradation in soil.  Although not as well evaluated, PAH availability in aged soil was not as significant a deterrent to microbial degradation of organics as was metal toxicity.  This was observed in controlled studies, and on an independently studied RCRA field site, where PAH levels were reduced significantly over a three-year period while metals remained unchanged (stabilized).  In addition, the onsite plant community established and progressed as would be locally expected with no appreciable levels of metal detected in shoot tissues or symptoms of toxicity.  This occurred in spite of the high soil metal levels present in the root zone (e.g. 650, 85, and 1045 µg/g of Cr, Ni, and Pb, respectively).  The lack of response of and the overall health of the vegetative community, suggests that they were not exposed to these phytotoxic metals. 

 

The use of highly aged soils – as well as results from the 20-year old RCRA site – has led the researchers to believe that phytoextraction of metals is technically possible, but not likely to occur on older sites.  PAH degradation may, but the rates are determined - not primarily by plant factors - but rather by PAH availability and by the sequestration of toxic metals.