Portable X-ray fluorescence (pXRF) analysis of heavy metals in contaminated soils: a case study from Varta Insel, Hagen

Urban soils contaminated with persistent and toxic heavy metals are found worldwide. Their remediation is essential to prevent harm to humans and the environment. The integration of portable X-ray fluorescence spectroscopy (pXRF) as a field analytical instrument in remediation projects offers the possibility to rapidly assess soils at low costs. In this study, we test the potential of pXRF in contaminated soil assessment at a former battery factory "Varta Insel" in Hagen, Germany. Soil samples were analyzed (1) by pXRF on site, (2) by pXRF under laboratory conditions, and (3) by microwave assisted aqua regia digestion with inductively coupled plasma optical emission spectroscopy (ICP-OES). Direct on-site measurements were useful for supporting field decisions and selecting samples for further analysis, but results were strongly influenced by matrix effects, grain size, and moisture content, limiting their reliability. Dried and sieved samples were measured by pXRF under laboratory conditions through low-density polyethylene sample bags. The laboratory pXRF results agreed with the more expensive ICP-OES analysis in 94% of cases when classifying samples above or below German regulatory limits. However, pXRF systematically underestimated lead concentrations and overestimated arsenic and chromium. Hence, we conclude that pXRF is not a stand-alone solution for heavy metal analysis of contaminated soil samples. However, it can effectively guide the selection of samples requiring detailed ICP-OES analysis and contribute to higher-density data collection, ultimately enhancing spatial resolution.