Thousands of Department of Defense (DoD) sites require remediation as a result of contaminated soil. Heavy metals are among the most common pollutants and include cadmium (Cd), arsenic (As), chromium (Cr), and lead (Pb). Pollution with these metals at DoD facilities can be extensive, but complete removal is prohibitively expensive. Alternative remediation methods are needed, particularly one that could significantly reduce the bioavailability of the metals simply by adding amendments to the soil.

The overall objective of this project was to identify an amendment or combination of amendments that could be applied to reduce chemical lability and bioavailability in metal-contaminated soils.

The impact of adding phosphorus, sulfur-based compounds, iron-rich composted organic matter, and limestone (individually and combined) on the aqueous solubility and extractability of contaminant metals will be determined. The residual toxicity of amended, metal-contaminated soils will be quantified using standard bioassays, including earthworms, lettuce germination and emergence, nematodes, and soil microorganisms. The effects of soil amendments on plant uptake of contaminant metals will be measured using both metal-sensitive plants that respond negatively to elevated metal concentrations and hyperaccumulating species. It is anticipated that metal bioavailability will be lowered to the point that these sensitive species show no toxicity symptoms and hyperaccumulating species are unable to assimilate elevated concentrations. Chemical bonding to soil surfaces, changes in speciation, and the chemical environment of the stabilized metals will be verified using advanced spectroscopic and x-ray techniques.

Researchers located three soils that were contaminated with Pb, Cd, Cr, and As. The soils were characterized for an array of chemical and physical properties including total metals. All soils had a mixture of metals requiring attention and made the remediation challenge much greater because the chemistry of each metal was quite different from the others. The approach to identifying a remediation solution using in situ amendments was to sequentially address the metals with additives known to target at least one metal. The soils were then examined for chemical lability (concentrations of metals removed from the soil by an extractant), bioaccessibility (metals available for removal from the soil by a sequence of extractants demonstrated to be correlated with availability to a given organism), and biotoxicity.

Orthophosphate is a known successful amendment for Pb. Quite predictably, the addition of orthosphosphate decreased Pb but greatly increased As and sometimes Cr concentrations. Therefore, the challenge was to find additional amendments that could suppress the other metals without impacting the effect of phosphate on Pb. In laboratory studies, combinations of chemical amendments, including rare earth elements manganese (Mn) and phosphorus (P), were added to soil with low redox potential to reduce the bioaccessible fraction of As, Cr, Cd, and Pb. Lanthanum (La) and cerium (Ce) were able to form low solubility precipitates with As, as determined in aqueous solutions. Spectroscopic studies confirmed that LaAsO₄(s) can form under pH conditions as low as 2.2.

Cerium was not affected by the low redox potential or possible interaction with sulfur, and the addition of Ce was able to decrease the bioaccessible As fraction although ratio and time dependent. Combination amendments of Ce, Mn, and P showed promising results. With the addition of 1:5 Pb+Cd:Mn and Pb+Cd:P, bioaccessible Cd was reduced below detection limit and bioaccessible Pb was reduced to 11% compared to 66% in the control. Also, the addition of 1:3 As:Ce and any ratio of Mn and P were able to decrease the Cr bioaccessible fraction significantly compared with the control. The bioaccessible fraction of As increased with the addition of Mn and P, and Ce was unable to offset this decrease. There was a slight offset with the addition of 1:3 As:Ce, but this was not significant compared with 1:1 As:Ce.

The three metal-contaminated soils collected from field sites were amended with combinations of Mn, P, and Ce. A sandy soil from a former Cd paint pigment manufacturing site (New Jersey soil) was amended, but the amendments increased toxicity to earthworms. Amendments had no effect on barley germination, but they did depress root growth. When the same amendments were applied to an organic soil from a former smelter site (smelter site soil), earthworm survival improved, earthworms gained biomass, and earthworms had reduced metal tissue concentration compared to the unamended smelter site soil. A sandy loam soil with slightly elevated metal levels (Utah soil) was amended, and amendment addition caused reduced lettuce root length and significantly elevated Cd earthworm tissue concentration.

Speciation is the key factor in controlling mobility and bioavailability, and information on the mineralogy and geochemistry of contaminant metals is important for developing in situ remediation strategies. Researchers sampled a Histosol that received runoff and seepage water from the site of a former lead smelter. The synchrotron x-ray microprobe on beamline X26A at the National Synchrotron Light Source at Brookhaven National Laboratory was used to obtain micro x-ray diffraction patterns (μ-XRD) and micro x-ray fluorescence patterns (μ-XRF) for soil aggregates approximately 100-200 μm in diameter. Arsenic and chromium x-ray absorption near edge structure (XANES) spectra were then obtained for aggregates with significant concentrations of As or Cr. Results show a clear pattern of metal speciation changes with depth. The oxidized yellow surface layer (0-10 cm depth) is dominated by goethite and poorly crystalline akaganeite. Lead and As are highly associated with these Fe oxides by forming stable innersphere surface complexes. The occurrence of akaganeite in a natural soil is reported for the first time. Gypsum, schwertmannite, and jarosite were identified in the surface layer as well, particularly for samples collected during dry periods. Fe(II)-containing minerals, including magnetite, siderite, and possibly wustite, occur in the intermediate layers (10-30 cm depth). The unusual presence of hematite and wustite in the subsurface horizons is probably the result of a burning event at this site. Iron, Pb, and As sulfide minerals predominate at depths greater than approximately 30 cm, and phases included realgar, greigite, galena, sphalerite, alacranite, and others. Most of these minerals occur as almost pure phases in sub-millimeter aggregates and appear to be secondary phases that have precipitated from solution. Mineralogical and chemical heterogeneity and the presence of phases stable under different redox conditions make this a challenging soil for in situ remediation.

This broadly integrated effort enhances DoD's ability to evaluate the application of soil amendments to measurably reduce the bioavailability and chemical mobility of As, Cr, and Cd in contaminated soils in situ. (Project Completed – 2008)