Possibilities of chemical fractionation and X-ray spectral analysis in estimating the speciation of Cu2+ with soil solid-phase components

Abstract Soil phases that are likely to scavenge ‘free’ metals include amorphous Mn and Fe oxides, organic matter, and clays. This paper reviews free forms of Cu and the methods in current use to quantify them. The information value of sequential extraction for assessing the speciation of Cu 2+ in artificially contaminated Haplic Chernozem (up to 2000 Cu mg/kg) was shown in the selective removal of the main solid-phase components from the soil. It was revealed that carbonates, Fe (hydr)oxides, organic matter, and layered silicates affect the uptake of Cu by the soil. The speciation of Cu in the soil was studied by the Tessier sequential extraction method. To assess the selectivity of the extractants used and to estimate the role of different soil components in Сu partitioning, carbonates, Fe-Mn sesquioxides, and organic matter were removed from soil samples prior to the application of each sequential extraction scheme. No metal was revealed in the fraction bound to the removed soil component, which indicated the selectivity of the used extractants with respect to the soil components. It was found that, in the absence of a soil component, the role of other components in the retention of metal ions increases. When organic matter was removed from the soil, nonsilicate Fe compounds become the most active components in metal sorption, and the role of organic matter in metal retention increases in the absence of Fe oxides. Upon the removal of carbonates, the accumulation of metal in the exchangeable form increases significantly. Mechanisms controlling the fixation of metal by soil solid-phase components on the molecular level were determined using synchrotron radiation X-ray absorption near edge structure (XANES) spectroscopy. It was found that the surface structure and the composition of functional groups of the adsorbent have the leading importance in metal sorption. The combined use of chemical and physical analytical techniques fully characterizes the interaction of Cu with soil solid-phase components, which reduces its availability to adjacent environments and mitigates environmental risks.