Pilot-scale treatment of RDX-contaminated soil with zerovalent iron

Soils in Technical Area 16 at Los Alamos National Laboratory (LANL) are severely contaminated from past explosives testing and research. Our objective was to conduct laboratory and pilot-scale experiments to determine if zerovalent iron (Fe⁰) could effectively transform RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine) in two LANL soils that differed in physicochemical properties (Soils A and B). Laboratory tests indicated that Soil A was highly alkaline and needed to be acidified [with H₂SO ₄, Al₂(SO₄)₃, or CH₃COOH] before Fe⁰ could transform RDX. Pilot-scale experiments were performed by mixing Fe⁰ and contaminated soil (70 kg), and acidifying amendments with a high-speed mixer that was a one-sixth replica of a field-scale unit. Soils were kept unsaturated (soil water content = 0.30-0.34 kg kg^-1) and sampled with time (0-120 d). While adding CH ₃COOH improved the effectiveness of Fe⁰ to remove RDX in Soil A (98% destruction), CH₃COOH had a negative effect in Soil B. We believe that this difference is a result of high concentrations of organic matter and Ba. Adding CH₃COOH to Soil B lowered pH and facilitated Ba release from BaSO₄ or BaCO₃, which decreased Fe ⁰ performance by promoting flocculation of humic material on the iron. Despite problems encountered with CH₃COOH, pilot-scale treatment of Soil B (12 100 mg RDX kg^-1) with Fe⁰ or Fe⁰ + Al₂(SO₄)₃ showed high RDX destruction (96-98%). This indicates that RDX-contaminated soil can be remediated at the field scale with Fe⁰ and soil-specific problems (i.e., alkalinity, high organic matter or Ba) can be overcome by adjustments to the Fe⁰ treatment.