Gillow, J & Horst, J 2008, 'Stabilization of Pyritic Solid Phases to Mitigate Acid Generation Potential and Metals Leaching', in AB Fourie, M Tibbett, I Weiersbye & P Dye (eds), Mine Closure 2008: Proceedings of the Third International Seminar on Mine Closure
, Australian Centre for Geomechanics, Perth, pp. 531-538, https://doi.org/10.36487/ACG_repo/852_48
This paper will review the concept of halting and/or controlling the oxidation of pyritic solids through
successful bench testing of a process developed to geochemically-stabilize lead and arsenic-bearing pyritic
solids. The solids tested have been contributing to low pH and elevated arsenic concentrations in the
groundwater underlying a site in the eastern U.S. The goal of the stabilization was to provide sufficient long-
term control to allow the materials to be safely consolidated in a vadose-zone environment, while still
protecting groundwater and the environment.
A conceptual geochemical model was developed based on an in-depth analysis of the source material
mineralogy and natural sequestration mechanisms affecting the arsenic and lead. This model provided the
basis for selecting the appropriate stabilization chemistry. The solids that were the focus of the study were
primarily comprised of iron oxide minerals (up to 33% iron), with pyritic sulphur in excess of 1% by weight
in some areas. The solids had a corresponding net acid generation potential of over 40 tons of calcium
carbonate per 1000 tons of material (tons/k). In addition to the trace amounts of lead and arsenic in the
pyrite, solid anglesite presented a second source of lead through the intrinsic solubility of that mineral
phase. The natural oxidation of the sulphide minerals was releasing acidity, iron, arsenic, and lead, with the
acidity increasing the mobility of the metals. The iron oxides also provided surfaces to which the arsenic and
lead could partition, but still be available to desorb into water percolating through the material.
The primary components of the stabilization chemistry were designed to address the net acid generation
potential, the leachable lead and arsenic separately. When combined with the materials, the amendments
worked in synergy to enhance pH buffering and decrease metal leachability. In the testing, homogenates
parameters requiring treatment ranged up to >15 tons/k of acid generation potential, 24,000 milligrams per
kilogram (mg/kg) of total lead, and 600 mg/kg of total arsenic. The initial phase of testing involved a single
homogenate, which was then followed by additional testing of the most promising amendment admixture.
This last step applied the selected amendment admixture to multiple additional samples of the original
treatment homogenate, as well as to several new homogenates representing a range of total inorganic
contaminant and mineral characteristics that might be encountered in the material. Testing of multiple
batches showed that the amendments were capable of successfully moderating pH to neutral/slightly alkaline
conditions, dropping arsenic leachate concentrations from as much as 0.3 mg/L to <0.02 mg/L, and
dropping lead leachate concentrations from as much as 34 mg/L to 0.2 mg/L in the most highly-impacted
material. The long-term stability of the treated material is also discussed in this paper.
EPA (1996) Method 3052, microwave assisted acid digestion of siliceous and organically based matrices in Test
Methods for Evaluating Solid Wastes: Physical/Chemical Methods. EPA SW-846, U.S. Environmental
Protection Agency, Office of Solid Waste and Emergency Response, Washington, D.C. Available at:
Jackson, M.L., Lim, C.H. and Zelazny, L.W. (1986) Oxides, Hydroxides and Aluminosilicates in Methods of Soil
Analysis, Part 1, Physical and Mineralogical Methods, A Klute (ed.). Agronomy Monograph No. 9. Soil Science
Society of America, Madison, Wisconsin.
Keon, N.E., Swartz, C.H., Brabander, D.J., Harvey, C. and Hemond, H.F. (2001) Validation of an arsenic sequential
extraction method for evaluating mobility in sediments. Environmental Science and Technology, 35,
Tessier, A., Campbell, P.G.C., and Bisson, M. (1979) Sequential extraction procedure for the speciation of particulate
trace metals. Analytical Chemistry, 51, pp. 844-851.
Wright, M.T., Parker, D.R. and Amrhein, C. (2003) Critical evaluation of the ability of sequential extraction procedures
to quantify discrete forms of selenium in sediments and soils. Environmental Science and Technology, 37,