114-8 Impacts of iron on lead (Pb) capture in urban stormwater catchment ponds and implications of sediment iron speciation

Session: Environmental Geochemistry and Health (Part I)

Presenting Author:

Christof Zweifel

Authors:

Zweifel, Christof¹, McBride, Sara², Pollack, Lea³, Janke, Ben⁴, Finlay, Jacques⁵, Snell-Rood, Emilie⁶, Santelli, Cara⁷

(1) University of Minnesota, , (2) University of Minnesota, , (3) University of Minnesota, , (4) University of Minnesota, , (5) University of Minnesota, , (6) University of Minnesota, , (7) University of Minnesota, ,

Abstract:

Many cities contain lead (Pb) in air, soil, and water from ongoing industrial emissions alongside legacy leaded paint and gasoline. Throughout the breakdown and movement of Pb-containing material, iron (Fe) is an important mediator, controlling reactivity and mobility. However, the effects of changing redox conditions on Pb associations with common soil Fe minerals as sediment is buried in stormwater ponds must be investigated to understand the fate of urban Pb pollution. Fe enters pond sediments from soil, infrastructure, vehicle wear, and iron enhanced sand treatments used to adsorb reactive phosphate. Pb flows through stormwater infrastructure via erosion and surface runoff, where Pb is captured in pond sediments as sulfides and adsorbed species. Sediments are elevated for Pb (200-2,000 ppm) in several ponds from the Minneapolis-St. Paul metro area MN, USA. This study combines laboratory experiments and synchrotron based techniques to investigate the importance of Fe-Pb interactions for retaining Pb within sediments. Sediments contained 3-5 wt% Fe and a mixture of galena, organic-bound Pb, and Fe(III) oxide-bound Pb. Sediment Fe(III) oxide-bound Pb may be unstable, yet seemed to persist in reduced porewaters that contained an average of 15 ppm dissolved Fe, low sulfate, and high organic matter. Using synchrotron based X-ray fluorescence (XRF) mapping and µ-X-ray absorption spectroscopy (XAS), we investigated the spatial co-occurance of Fe and Pb species. Pb was present in isolated hotspots, far above background levels, while Fe was distributed more uniformly. Fe and Pb spots were mutually exclusive in some sediment sections, but highly correlated in others. Bulk XAS and µXAS spectra will determine the bulk Fe speciation and micro-scale Fe speciation at Pb hotspots. The high spatial heterogeneity shows the need for synchrotron based XRF mapping and µXAS to gather information on the role of Fe within Pb capture. Experimentally, all of the relevant Fe species lowered dissolved Pb from synthetic stormwater. Interactions between synthetic stormwater and local sediments revealed a strong pH dependence for Pb dissolution, the possibility of Pb rich colloids > 0.22 µm, and significant impacts of Fe on Pb dissolution. Better understanding the local speciation of Fe and its sorption capabilities will inform how we understand stormwater Pb pollution, and has implications for Pb capture when Fe is applied for phosphate remediation. 

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