Drinking water treatment residuals (DWTRs) are multi-phase material byproducts from the coagulation, flocculation, and sedimentation processes used in conventional drinking water treatment. The reported high sorption capacity of DWTRs makes them promising candidates as capping materials for sediments contaminated with metals including copper. To understand the sorption processes of copper (Cu) to these materials, x-ray absorption spectroscopy (XAS) was used to probe the average local environment of this metal when sorbed to six different DWTR samples. These DWTRs were formed using aluminum salts (e.g. alum, Al2(SO4)3) as coagulants and a range of chemical additives including flocculant aids (e.g. polyacrylamide polymers), oxidants (e.g. KMnO4), and water softeners (CaO). DWTRs were suspended in 10 mM KNO3/10 mM buffered solutions (6.9&lt;pH&lt;8.5), and small aliquots of Cu(NO3)2 were added to avoid potential competing precipitation reactions; DWTRs were collected after one day on 0.45 μm nylon filters for XAS analysis. Data were collected at the Advanced Photon Source and processed using Larch to perform spectral decompositions by linear combination fitting (LCF), cluster analysis, and structural modeling. Principal component analysis was used to identify representative reference materials. X-ray absorption near-edge spectroscopy (XANES) cluster analysis suggests chemical additives used at drinking water treatment plants drive subtle differences in the coordination environment of sorbed Cu. At the mass loadings used in these experiments, LCF of first-derivative XANES data for all DWTRs supports that most Cu is sorbed to Al- or Fe-oxides. Extended x-ray absorption fine structure (EXAFS) analysis shows that Cu is bound to 4.7-6.8 oxygen atoms at 1.90-1.94 Å, indicating an octahedral coordination geometry. Some DWTRs lead to the emergence of a well-defined second-shell feature at 3.1-3.2 Å due to the presence of light (C/N/O) elements, which may have implications on the stability or the driving factors of the sorption processes.

Probing Cu sorption to drinking water treatment residuals (DWTRs) by XAS

ACS Spring Meeting 2021

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