X-ray absorption analysis of Zn and Cu speciation in soils around the sediment ponds of chemical plants

Abstract

<p>Study of Zn and Cu accumulation and transformation in highly contaminated technogenically transformed soils near the sediment pond of a chemical plant using a combination of direct nondestructive physical methods, including X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) based on synchrotron radiation. The object of the study was technogenic soils (Technosol) subjected to long-term anthropogenic load. The object of research was a territory of sludge collectors region of the Atamanskoe Lake natural basin, the city of Kamensk-Shakhtinskii, Southern Russia. The contents of Zn and Cu were determined by the X-ray fluorescence method. The EXAFS and XANES experimental data were obtained at the Structural Material Science Station at the 1.3b beamline of the Kurchatov Center of Synchrotron Radiation ''Kurchatov Institute''. Soil samples were analysed with the sequential extraction procedure recommended by BCR. This procedure can be described as follows: first step (exchangeable fraction), second step (reducible fraction) and third step (oxidisable fraction). The studied Technosols are characterized by exceeding of the lithosphere clark for Zn in hundreds of times (26 000-66 000 mg/kg) and for Cu in tens of times (376-577 mg/kg). It has been found that in the oxidisable fraction of Zn is coordinated by four O atoms and only two Zn atoms. The Zn-Zn distance is 3.36 Å, and there are two different short Zn-O bonds (1.95 and 2.04 Å). In the reducible and exchangeable fractions, the main peaks of the EXAFS Fourier transform are shifted to the region of large values, which indicates the possible presence of Zn-S bonds of 2.34 Å. The simulation results have shown a high content of ZnS in the reducible fraction; ZnS with an admixture of ZnSO<sub>4</sub> and ZnO dominates in the exchangeable fraction. The oxidisable fraction is characterized by a high content of ZnSO<sub>4</sub> with the addition of ZnO. The significant difference in the position of the absorption edge and the values of the main features of the spectrum with Cu–S and Cu–O bonds has made it possible to reliably diagnose these types of Cu environments in Technosol. Peaks of the EXAFS Fourier transforms of Cu spectra indicate the predominance of Cu–O bonds in the oxidisable fraction and Cu–S bonds in the reducible and exchangeable fractions. The results of fitting Cu spectra by a linear combination indicate that the spectra of the reducible fraction coincide with high accuracy with the spectra of Cu<sub>2</sub>S. In the exchangeable fraction, the content of Cu<sub>2</sub>S is also high, although there are CuSO<sub>4 </sub>impurities. The oxidisable fraction is characterized by a high content of CuCO<sub>3</sub> and the presence of Cu<sub>2</sub>S and CuSO<sub>4</sub> as impurities as trace amounts. Thus, sequential chemical selective fractionation and subsequent X-ray spectral diagnostics based on synchrotron radiation and molecular calculations have made it possible to identify and evaluate the Zn and Cu phases in Technosol.</p><p>The reported study was funded by RFBR, projects no. 19-34-60041 and 19-05-50097.</p>