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Final Phytoremediation Treatability Study

Section 3 - Treatability Study Methods

3.1 - Summary of Activities

This section summarizes activities performed by Parsons ES and Phytotech during the treatability study. A detailed description of methods and techniques used for the phytoextraction benchscale treatability study is presented in Appendix B.

3.2 - Background

Phytoremediation is the term used to describe the remediation of soil and groundwater contaminants through use of whole plants and processes taking place in the rhizosphere, the area directly around the root system of plants. This remediation technology has grown out of the emphasis to use natural processes to resolve contamination of man-made and natural chemicals.

In general, remediation through use of plants can occur through three mechanisms: 1) direct uptake of the contaminant by the plant, 2) metabolism or cometabolism of the contaminant, and 3) stimulation of microbial biotransformation of the contaminant in the plant rhizosphere. Phytoextraction, the phytoremediation method implemented at CSSA, removes contaminants from surface soils through plant uptake and stores them in the plant shoots. Of all the phytoremediation processes, phytoextraction appears to have the greatest effect on remediation of heavy metals and radionuclides. The advantage of phytoextraction is that it is �designed to concentrate metals in plant tissues, thus minimizing the amount of solid or liquid hazardous waste which require treatment, with the ultimate goal of developing an economical method of reclaiming metals from plant residue� (Salt, et al, 1995). There are four factors that influence or determine the ability of phytoextraction to effectively remediate a metal-contaminated site:

1. Site arability and plant biomass yields;

2. Metal solubility and availability for uptake;

3. The ability of the plant to accumulate metals in the harvestable plant tissue; and

4. Regulatory criteria.

The optimal plant for phytoextraction is one that transfers contaminants from the root system to the harvestable part of the plant, one that grows rapidly given the site conditions, thus maximizing the number of crops per year, and one that may tolerate high levels of contaminants. The Brassicaceae family was one of the first plant families identified for their ability to extract metals from soils and transport them above ground to the parts of the plants that can be harvested. During extensive testing, the Indian mustard plant, Brassica juncea, was identified as being a hyperaccumulator of lead, cadmium, chromium, nickel, zinc, and copper. EdenSpace Systems Corporation, the technology vendor, is continually expanding applications of phytoextraction systems to address a variety of contaminants (including those listed previously) and site conditions. EdenSpace's proprietary and patented technology can address other contaminants of concern, depending on the specific site conditions, concentrations, and regulatory goals. However, this report and the associated benchscale tests provide results and discussions on the efficiency of phytoextraction techniques of lead in CSSA soils.

3.3 - Technical Approach

3.2.1   Soil Sample Collection

Parsons ES personnel collected 5-gallon soil samples from SWMU B-20/21 soil piles in October 1997. Samples were taken from sifted soil piles created during UXO and scrap metal removal activities. More than 40 kg of soil were collected from various soil sifting piles in an attempt to obtain representative samples. Soil samples were tumbled in a mixer for 24 hours before being shipped to Phytotech to ensure a well-mixed sample.

Sifted soils were used for the treatability study because site characterization results indicated significantly higher lead levels than those observed within the 10-acre area in the central portion of the site.

3.2.2   Phytotech Soil Remediation Design

Soils received by Phytotech were mixed and saturated with water to obtain a homogenous mixture free of soil aggregates. Soils were then air dried and sieved to 2 millimeter (mm) in preparation for the growth chamber pot studies.

Plant growth studies were conducted in a controlled environment growth chamber to evaluate metals uptake and physiological development of the Indian mustard plant, Brassica juncea, in the SWMU B-20 soil. Soils were amended with 3 percent (by weight) peat moss to improve the growth rate of the plants for the benchscale treatability study. Sulfur was added in an attempt to acidify the soils which potentially would increase the mobility of the lead. After 30 days, plants were treated with proprietary soil amendments to stimulate growth and metal extraction.

To simulate real life application of this technology, a multiple crop evaluation was also performed. During this test, plants were grown under similar conditions as during the single growth studies. After simulated crop maturation, both the plant biomass and the root system were removed, dried and analyzed. The soil was then mixed and replanted, representing a second growing season.

3.2.3   Treatability Study Activities

Several types of treatability tests were performed to collect data to determine the efficacy of phytoremediation of SWMU B-20 soils. The tests include: