An Examination of Drinking Water Treatment Residuals as Metal Sorbents

Wallace, Samuel McDonald

doi:https://doi.org/10.21985/n2-8z7c-n371

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An Examination of Drinking Water Treatment Residuals as Metal Sorbents

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Drinking water treatment residuals (DWTRs) are waste materials generated during the clarification process of drinking water treatment. The application of DWTRs as sorbent materials offers an opportunity to reuse and add value to these materials. For instance, as metal sorbents, DWTRs could be used as active capping materials to remediate contaminated sediments. This thesis focuses on characterizing the sorption of three metals – copper (Cu), mercury (Hg), and lead (Pb) – to a selection of aluminum-based DWTR samples. With the application of sediment capping in mind, total and labile metal contents of DWTRs are measured, and toxicity bioassay experiments with benthic organisms are conducted. The DWTR samples undergo intensive characterization with particular attention to potential sorbent phases. X-ray absorption spectroscopy (XAS) is applied to sorbed metals (Hg, Cu, Pb) and component metals of DWTRs (Mn and Fe). To process this data, a novel method for the identification and removal of aberrant points ("glitches") is developed. A screening process for DWTRs, based on their total and elutriated metals, is applied to narrow the scope of subsequent research. Based on these results, along with bioassays, it is found that at least several of the DWTRs meet the standards for application into marine environments. Results for freshwater bioassays are mixed, though in the absence of a definitive etiology, further exploration is warranted. The characterization of DWTRs reveals the importance of non-coagulant additives in determining features of these materials. DWTRs with a lime (Ca(OH)2) additive are predominantly calcium carbonate, limiting the quantity of more active sorptive phases. In DWTRs without lime, there is ample organic matter; despite its importance in dictating the fate of trace metals in the environment, organic matter has generally not been identified as the most important factor driving metal sorption to DWTRs. Activated carbon, occasionally used in drinking water treatment, can provide abundant surface area and microporosity, suggesting it can continue to hold value even as a spent waste material. Finally, DWTRs that include a potassium permanganate (KMnO4) additive could eventually transform to include manganese oxide minerals highly active in sorption processes, but at least initially, manganese is predominantly found in the +2 oxidation state as a carbonate mineral (MnCO3). Sorption and XAS studies of mercury bound to DWTRs show the influence of organic matter on the association of mercury with DWTRs. For samples shown to have high organic matter content, mercury-sulfur bonds are found to be important at low mass loading values of mercury through extended X-ray absorption fine structure (EXAFS). This suggests that the mercury is coordinating with thiol groups in the organic matter of these samples. At higher mass loadings and in lime-based samples with limited organic matter, mercury associates primarily with oxygen. Regardless, at environmental concentrations of mercury, this research is indicative of the critical nature of organic matter in influencing sorption. Copper and lead sorption to DWTRs are examined through scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS) and XAS. SEM-EDS shows broad associations of Cu and Pb throughout the DWTR samples. For copper, no associations with iron or manganese are observed, though one sample suggests a correlation between copper and carbon content; on the other hand, lead distinctly concentrates on several phases within the DWTRs, including transition metal oxides and carbon-rich areas. EXAFS of copper sorbed to DWTRs suggest the importance of manganese oxides and activated carbon in copper sorption processes for some samples. Others show a well-defined second-shell feature which may be modeled using either aluminum, which would indicate association with the coagulant, or oxygen, which is suggestive of coordination with organic matter. Lead is much less distinct, both in the X-ray absorption near-edge structure (XANES) and EXAFS, showing highly-disordered and less-discriminate associations with oxygen atoms on DWTRs.

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