## 2 Materials and Methods
### Analytical Procedures  
The soil samples were analyzed in order to obtain their physical and chemical properties, as well as the total content of potential toxic elements. All the analyses were performed at Dunarea de Jos University of Galati and Valahia University of Targoviste, Romania. The analytical samples were subjected to pH measurements and electrical conductivity in soil/water according to a standard method with a 1:2.5 soil/water suspension using a Hanna Hl98195 multiparameter meter (pH/ORP and EC/TDS/NaCl/Resistivity) (Hanna Instruments Service SRL, Cluj-Napoca, Romania) with a HI9828-0 calibration standard solution (Hanna Instruments, Woonsocket, RI, USA). These analyses were conducted at the Laboratory of Molecular Spectroscopy, Multiparameter Analysis, and Experimental Data Processing of the INPOLDE research center, Dunarea de Jos University of Galati. X-ray fluorescence spectrometry (XRF) was used for the determination of element content in soils, by employing both energy-dispersive (EDXRF) and wavelength-dispersive (WDXRF) techniques. It is a powerful analytical method employed for the qualitative and quantitative analysis of the chemical composition of materials. XRF is widely used in environmental monitoring, geology, life sciences, materials science, archeology, and many other fields [36]. The method is based on the interaction between X-rays and the atoms of a sample, which leads to the emission of characteristic secondary (or fluorescent) X-rays. This allows for the identification and quantification of chemical elements. Sample analyses were performed using EDXRF at Dunarea de Jos University of Galati with a portable INNOV-X Systems ALPHA SERIES 4000 spectrometer (INNOV-X Systems Inc., Woburn, MA, USA), specifically calibrated for the analysis of soil samples [37], equipped with an X-ray tube featuring an Ag anode and a Be window, and a silica PIN diode for radiation detection. The device performs analyses using the fundamental parameters method. The most notable limitations include poor detection of light elements (Z \(<\) 11) and surface sensitivity, as the penetration depth is only a few microns, which may not always represent the bulk composition. Additionally, the matrix effect was observed, where the presence of other chemical elements in the sample can affect the detection accuracy due to absorption and enhancement effects [38]. For EDXRF analysis, the fine and homogenized soil samples were encapsulated using special capsules, and irradiated three times for 120 s.  
The analytical control was carried out using several Certified Reference Materials (CRMs), such as NIST 2709a (San Joaquin Soil), NIST 2710a (Montana I Soil), NIST 2711a (Montana II Soil), and NIST 1676a (Estuarine Sediment) (National Institute of Standards and Technology (NIST), Gaithersburg, MD, USA), IAEA-356 (Polluted Marine Sediment) (International Atomic Energy Agency (IAEA), Vienna, Austria) and Sigma-Aldrich SQC001-30G (Metals in Soil) (lot LRAC3749) (Sigma Aldrich, Laramie, WY, USA). The obtained analytical errors ranged between 2% and 15%. In order to obtain a complete set of concentration values, multi-element analysis of soil samples was also performed using the Rigaku Supermini200 WDXRF spectrometer (Rigaku, Tokyo, Japan) at the Institute of Multidisciplinary Research for Science and Technology of Valahia University of Targoviste. The system is equipped with three-position crystal changes: LiF (200) for heavy elements (Ti-U), PET and RX 25 for light elements (O-Mg and Al-Sc) and an X-ray tube, 50 kV, 4 mA, 200 W, Pd anode. The detection limit was found to be between 1 ppm and 10 ppb and the precision of the analysis was 0.1-0.5%. For this analysis, samples were prepared by mixing 2 g of fine-ground (200 um) soil sample with 2 g of Fluxana Boreox (C\({}_{12}\)H\({}_{22}\)O\({}_{11}\)) binder (FLUXANA(r) GmbH & Co.KG, Bedburg-Hau, Germany). The pellets used for WDXRF analysis were prepared from the sample-binder mixture using the TestChem LPR-250 laboratory press (Testchem Sp. z o.o., Radlin, Poland).