In situ measurements of heavy metal distributions in soil columns during miscible displacement experiments using portable X-ray fluorescence

Authors: Padilla, Joshua; ELBANA, TAMER - National Research Centre; SUN, WENGUANG - Colorado State University; SELIM, MAGDI - Louisiana State University

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/22/2025
Publication Date: 11/29/2025
Citation: Padilla, J.T., Elbana, T.A., Sun, W., Selim, M. 2025. In situ measurements of heavy metal distributions in soil columns during miscible displacement experiments using portable X-ray fluorescence. Journal of Environmental Quality. 55. Article e70103. https://doi.org/10.1002/jeq2.70103.
DOI: https://doi.org/10.1002/jeq2.70103

Interpretive Summary: The movement of chemicals in soil is often investigated by leaching contaminated soil with water and tracking chemical concentrations in the leachate over time. However, it is difficult to measure chemical concentrations within the soil itself, and therefore, such measurements are typically ignored. To address this, we developed a new method that allows for measuring concentrations of nickel (Ni) and zinc (Zn) within soils over time. We used a column made from Kapton film, along with a portable X-ray fluorescence (pXRF) device that can measure metal concentrations without disturbing the soil. To improve the accuracy of pXRF measurements, we developed custom calibrations for each combination of soil and metal. These calibrations were based on the relationship between known and pXRF-measured Zn and Ni. During experiments, we evaluated the accuracy of pXRF measurements using mass balance (comparing measured Zn or Ni to that added). Our custom calibrations significantly reduced mass balance errors in three of four datasets. We also compared pXRF-measured Zn and Ni concentrations to those predicted by a computer model. The model correctly predicted when the highest concentrations of metals would appear at different depths in the soil, although its accuracy depended on the metal and soil material. Overall, our results show that a Kapton film column and pXRF device can provide reasonably accurate measurements of Zn and Ni distributions within soil columns over time; we expect that our method will also work for other heavy metals. Such information on the distribution of heavy metals in soil, and how the distribution changes with time, can guide remediation efforts and provide a metric for validating environmental models.

Technical Abstract: Miscible displacement experiments traditionally rely on measured effluent concentrations of a given chemical with time to characterize its transport through soils. Time-dependent distributions of heavy metals with depth are difficult to obtain and are typically ignored. We developed a methodology for measuring time-dependent distributions of zinc (Zn) and nickel (Ni) within soil using a Kapton film (KF) column and portable X-ray fluorescence device (pXRF). Matrix-matched calibrations were developed for each matrix/solute combination based on linear regressions between known and pXRF-measured Zn and Ni concentrations within the KF column. We assessed the accuracy of pXRF-measurements using mass balance calculations; our matrix-matched calibration significantly reduced cumulative mass balance errors for three of the four experimental datasets and is therefore recommended. We compared pXRF-measured Zn or Ni concentrations at various depths to those predicted by a single-site, nonlinear kinetic model. The model accurately predicted the timing of peak Zn or Ni concentrations at each depth in reference sand and Wolfpen soil, however, overall model performance was element- and matrix-dependent. Our results demonstrate that use of a KF column and pXRF device enables the acquisition of time-dependent distributions of Zn or Ni distributions in soil columns. We expect that this approach will be appropriate for other heavy metals, particularly those with higher energy fluorescent X-rays. Accurate descriptions of heavy metal distributions with depth and time will provide an additional metric to validate reactive transport models.