## 2 Materials and Methods
### Analytical Procedures
The obtained pellets (32 mm diameter) were analyzed in vacuum atmosphere. The ZSX software 200 was used for spectra recording and data processing. Spectrum interpretation was performed using the calibration curve method, based on CRMs with similar matrix (soil, sediment). A monotonous dependency of signal intensities versus the content of elements was characterized by correlation factors between 0.9987 and 0.9994. Each measurement was performed three times, resulting in a relative error of analysis ranging from 0.2% to 5%. The study of soil morphology and qualitative analysis was conducted in the Electron Microscopy Laboratory at Dunarea de Jos University of Galati using a Scanning Electron Microscope coupled with Energy Dispersive X-ray Analysis (SEM-EDX) system [33]. Morphological analyses were performed on a FEI Quanta 200 SEM microscope (Thermo Fischer Scientific, Hillsboro, OR, USA). Secondary electron signals were detected to provide high-resolution images of the texture, shape and size of the soil particles, at different magnifications. Prior to imaging, the specimen preparation is a very important step. The powders with discontinuous grading were mounted on a metal support using carbon double-adhesive tape. To avoid charging effects on the electron images, a metal thin layer coating was necessary to ensure the electrical conductivity of the samples (SPI Supplies sputtering equipment, West Chester, PA, USA). An ultra-thin window Ametek EDX detector (silicon crystal) (Gatan, Pleasanton, CA, USA) along with an SEM apparatus was used for the elemental composition measuring. The semi-quantitative assay was performed using the ZAF algorithm (corrections for Z--atomic number; A--absorption; F--fluorescence) and controlled by GENESIS software 5.10. The following settings were used for the analysis: an accelerating voltage (EHT) of 15 kV, a working distance (WD) ranging between 8.9 and 10.3 mm and a probe current of less than 100 nA. The EDX system utilized a Silicon Drift Detector (SDD). All samples were analyzed in high-vacuum (HiVac) mode with a chamber pressure of 60 Pa, at magnifications ranging from 250\(\times\) to 500\(\times\). An accelerating voltage of 25 kV was set to maximize the quality of the microstructural results. SEM-EDX was used in five scanning fields, randomly selected on the surface, for mineral characterization. After the elemental compounds' identification, they were spatially mapped using the EDX analyzer. Soil characterization in terms of organic and inorganic compounds was carried out using Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy (ATR-FTIR), which is the second most widely used method for studying soil organic matter, including its constituents, water content and mineral and artificial soil components [39]. We used a BRUKER TENSOR 27 FTIR spectrometer (Bruker Optik GmbH, Ettlingen, Germany) coupled with a A225/Q diamond Bruker Platinum-ATR accessory (Bruker Optik GmbH, Ettlingen, Germany), at the INPOLDE research centre from Dunarea de Jos University of Galati. Soil samples were air dried and sieved at 200 \(\upmu\)m, then pressed in a thin layer directly onto the ATR crystal. This simple and non-destructive method of analysis offers significant advantages; however, a potential drawback lies in the possibility of imperfect contact between the soil and the ATR crystal. This imperfection can lead to a loss of sensitivity and precision in the final results [39]. Each sample was measured with 32 scans between 4000 and 400 cm\({}^{-1}\), with a spectral resolution of 4 cm\({}^{-1}\). The spectra were corrected for H\({}_{2}\)O and CO\({}_{2}\) using OriginPro 2016 software (version 9.3.226) [33; 40].