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Final Phytoremediation Treatability Study

Section 4 - Treatability Study Results and Data Evaluation

The purpose of this section is to summarize the sampling results and provide an evaluation of the data collected during the treatability study.

4.1 - Soil Analysis

A single soil fertility analysis was performed by Soil Testing Laboratory in New Brunswick, New Jersey, using standard soil fertility analytical procedures. The B-20 soil sample is a clay loam and contains a moderate level of organic matter.

Results from the soil fertility tests indicate that SWMU B-20 soils will support vegetation for phytoextraction. The slightly alkaline soil (7.7 pH) necessitates the use of soil additives, such as elemental sulfur to reduce soil pH. Soil fertility results are presented below:

Table 4.1 - CSSA Soil Fertility Results

4.2 - Metal Content

The average of the four subsamples, 1 gram replicate samples, analyzed using USEPA SW846 6010B method indicated that the total average lead concentration in the soil is approximately 833 mg/kg. Copper had an average concentration of 108 mg/kg and zinc 160 mg/kg. Cadmium, chromium, arsenic, and nickel were not detected above the method detection limits. Results for water-soluble metals were all at or near detection limits, indicating insignificant levels of water-soluble metals. Barium and mercury analyses were performed by ITS laboratory and as such are considered screening data. ITS reported total barium and mercury concentration in the soil as 256 mg/kg and 0.384 mg/kg, respectively.

4.3 - Phytoextractable Metals

Analyses were conducted to evaluate the availability of the metals for plant uptake (phytoextractable metals) and also to provide information for the application of soil amendments to enhance metal uptake by the plants.

Analysis indicates that the majority (64%) of the lead contamination can be found in the silt fraction. In general, contaminants with smaller particle sizes are more difficult to phytoextract from soils due to the increased potential for metal sorption and retention. However, the effectiveness of soil amendments to increase metal solubility may increase in the final fractions due to an increase in surface area.

Sequential extractions were also performed to measure lead in various chemical fractions. Each step provided the concentration of a different chemical fraction of lead, including exchangeable lead, lead carbonate, lead oxide, organic lead, and finally, acid-extractable lead. For this analysis, the sequential extractions consisted of five steps. The sequential extraction process uses selected chemical extractants to remove particular soil fractions based on solubility relationships. The resulting target constituent speciation, therefore, is operationally defined based on the solubility or extractability of the constituent with each defined fraction (exchangeable, carbonate, oxide, organic, and residual). For example, exchangeable lead is defined as that lead solubilized using 1 M MgCl2, with the magnesium serving to extract adsorbed lead through ion exchange mechanisms. However, water soluble lead forms are also removed in this extraction as well.

In the next step, the carbonate fraction is defined using sodium acetate buffered at pH 5 to remove carbonates from the soil. Any other mineral phase soluble under those conditions are also extracted. The lead that is extracted is defined as carbonate-bound lead, but may include lead oxides, hydroxides, phosphates, and other forms soluble at pH 5.

The remaining fractions are extracted using hydroxyhlanine hydrochloride at pH 0 which includes lead associated with iron and manganese oxides; 0.1 M nitric acid, and 30 percent hydrogen peroxide to extract lead associated with organic matter; and concentrated nitric acid and 30 percent hydrogen peroxide to remove lead from the residual soil minerals.

Each additional step taken represents a less bioavailable form of soil metals. Generally, the exchangeable and carbonate fractions are the most bioavailable, with the acid-extractable lead considered non-phytoextractable. The results of this analysis are presented in Table 4.2.

Table 4.2 - Phytoextractable Metals

Results of the chemical fractions study concluded that a large portion (54 percent) is in the lead carbonate fraction. In the soils at CSSA, metallic lead particulates undergo oxidation and subsequent reaction with common soil anions to form lead carbonate, lead hydrocarbonate, lead sulfate, and lead oxide. These oxidation products are typically found as crusts, tightly bound to the metallic lead fragments. Lead carbonate is generally not taken up to any appreciable extent (typically <5 percent) in the shoots of Indian mustard plants (VanCantfort, 1998). However, the use of soil amendments enhances phytoremediation. With the use of soil amendments (chelating agents), lead carbonate becomes much more bioavailable.

4.4 - Plant Growth and Uptake

Soil amendments were tested to determine their effects on lead uptake in the Indian mustard plant shoots during the growth chamber pot studies. Plants were grown under three conditions: untreated SWMU B-20 soils, �treated� (i.e., soil amended with chelating agents) soils, and �treated� soils with sulfur. As shown in Figure 4.1, soil amendments significantly increased the lead and zinc soil extractions. Plants in �treated� soils increased their lead shoot concentration by approximately 1,116 mg of lead/kg (dry weight basis) (850%) and zinc concentration by 196 mg/kg (125%) over plants grown in untreated soils. Zinc was included as a study parameter by Phytotech for an additional metal evaluation result.

Sulfur was also included with the soil amendment in an attempt to lower the soil pH level. Although higher metal extraction concentrations were achieved with the addition of sulfur, the pH was not effectively lowered due to the short duration of the test. CSSA soils are present under basic conditions (i.e., pH>7) as were the test site soils (pH 7.7) Soils with basic conditions present, the available metal anions are mobilized and useful cations are adsorbed to mineral surfaces or precipitate, decreasing the metal bioavailability for plant uptake. The capacity of the soil to absorb lead increases with increasing pH, cation exchange capacity, organic carbon content, and phosphate levels. The sulfur amendment to the �treated� soils may have slightly lowered the pH thus allowing the more mobile lead species to be extracted by the indian mustard plants. However, results from a previous treatability study (Electrokinetics, Volume 4) indicated that CSSA soils have a high buffering capacity. Thus, the ability of sulfur to reduce the pH, even over a longer duration, is doubtful.

Multiple Crop Evaluation

The multiple crop evaluation, which included two harvests, was used to evaluate the effect of phytoremediation on the total average lead concentration. Initial lead concentrations varied greatly among the ten soil samples used for this study. Concentrations ranged from 487 to 6,215 mg/kg with an initial average concentration of 1,155 � 1,040 mg/kg.

The first harvest yielded plants with a biomass lead concentration of 913 mg/kg. The second harvest yielded higher concentrations, averaging 3,261 mg/kg. While both harvests� soils included amendments, the second harvest�s amendments added a component to acidify soils, a process that yielded higher extractions in previous testing. This acidification may, in part, explain the increase of lead extraction during the simulated second harvest over the first.

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