Lead and Antimony Speciation in Shooting Range Soils: Molecular Scale Analysis, Temporal Trends, and Mobility
Dr. Thomas Trainor | University of Alaska Fairbanks
The objective of this project was to provide a detailed analysis of the changes in lead (Pb) and antimony (Sb) speciation that occur over time as bullet fragments weather in shooting range impact berm soils. This project was motivated by the fact that most bullet alloys are composed of about 90% Pb with up to 5% Sb, and lesser amounts of copper (Cu), nickel (Ni), Zinc (Zn) and other metal(loid)s. Bullet fragments found in berm soils associated with training activities are highly susceptible to oxidation and weathering processes that can generate mobile and bio-available forms of these metal(loids). In order to understand the potential for metal(loid) migration, as well as evaluate remediation scenarios, a detailed understanding of how the speciation of the key metal(loids) change under typical soil/geochemical conditions is required. This project set out to investigate three topics related to Pb and Sb in shooting range soils: (i) how soil physical/chemical properties will influence on how metal-fragments weather over time, (ii) understand the significance of sorption processes controlling the mobility of Pb and Sb in range soils, and (iii) determine how the mobility of the oxidized Pb and Sb metals depends on soil solution properties such as pH. Finally, to test the use of iron-amendments as a means of limiting the mobility of both Pb and Sb range soils.
These questions were approached with a combination of field and laboratory experiments. A new test site at the Cold Regions Test Center (CRTC) Ft. Greely Donnelly Training Area (DTA), Alaska was constructed. At this site, fresh shooting range berms were made using four soil types: a local silt-loam end member, a sand end-member and soils of intermediate composition made by mixing the end member. These berms were loaded with bullets in a single controlled firing event and monitored from 2010 to 2016. A series of batch reactor studies was performed with a focus on characterizing oxidation rates and products, particularly for Sb metal. Column experiments, using the range soils, were used to investigate weathering rates and metal(loid) speciation under controlled conditions, including the degree of soil saturation. These studies were also used to investigate the amendment procedure and to test the degree to which metal(loids) may be mobilized via colloid facilitated transport. These measurements were complimented by a series of surface specific measurements used to investigate the speciation/structure of Pb and Sb sorbed to iron-oxide surfaces. These surface measurements also formed the basis for understanding the efficacy of using well characterized mineral substrates as a passive sensor, useful for providing a qualitative indication of metal(loid) mobility. These studies were carried out using a range of analytical tools; traditional of soil and solution composition analysis, aqueous speciation methods for distinguishing between Sb(III) and Sb(V), colloid characterization/separation methods, solid phase analysis using electron probe and synchrotron x-ray methods, and surface specific diffraction techniques. These experimental methods and analytical tools generated a unique data set of trace element speciation and temporal/spatial variation with physicochemical conditions.
Solution samples from berm runoff and column experiments were monitored for total “dissolved” metal(loid) concentrations. An important observation from the solution data was that the Pb and Sb were highly mobile, with concentrations on the order of 10’s to 100’s of μg/L sustained over the course of the experiments in all soil types. Furthermore, it was observed that while Sb only makes up roughly 1wt% of the bullets used (99wt% Pb), the dissolved concentrations of Sb typically exceeded the Pb, showing that Sb was highly susceptible to oxidation and mobilization in the test soils. Speciation analysis confirmed that the mobile Sb was dominated by Sb(V), with only a trace of Sb(III) ever being observed in solution. Field flow fractionation measurements of solutions from soils and column experiments showed that the majority of Sb was truly dissolved, while a significant fraction of “dissolved” (operationally defined by 0.45 μm filter) was actually associated with Fe-bearing colloids, and to a lesser extent organic colloids. The measurements suggested that Pb concentrations did not strongly vary with soil type, while the Sb concentrations showed a trend toward greater mobility in higher sand content soils. The observation of rapid oxidation agreed with the batch studies of oxidation rates of metallic Sb, where initial oxidation to aqueous Sb(III) was rapid in a range of solution conditions. The further oxidation to Sb(V) was also rapid, but was strongly influenced by the solution composition, as was the ultimate weathering (solid phase) products in highly concentrated systems. Characterization of the solid phase speciation in test soils and in historic range soils showed that Pb forms PbO, and increasingly over PbCO3 as the primary weathering phases – which were in turn likely the predominant sources of dissolved Pb(II). Little evidence of secondary minerals containing oxidized Sb in the test soils was found. Both Pb and Sb were found to sorb to minerals in the soil matrix, with iron-oxides serving as the predominant sorbent. The studies of Pb(II) and Sb(V) binding to model iron-oxide substrates showed that both species form innersphere multi-dentate coordination with the mineral surfaces – suggesting strong sorbate-sorbent interaction. The studies of the use of iron as a potential soil amendment showed that it may be highly effective for reducing dissolved Sb concentrations. However, the difficulty in controlling pH indicated that such treatments may result in increases in Pb concentrations, emphasizing the difficulty in controlling both the cation and oxyanion mobilities with a single treatment.
This research provided a highly detailed analysis of the speciation changes observed for Pb and Sb during the weathering of bullets in a range soil environments. Results of this work will contribute to building a model of the weathering and mobilization pathways, that is essential to assessing risk and predicting the efficacy of remediation scenarios. This work also highlights how some solution and solid phase speciation tools may used in compliment to traditional analytical methods in providing a detailed characterization of weathering processes.