The remediation of metal contaminated sediments remains one of the most intractable problems of environmental restoration, and cost effective and efficient remedial approaches that avoid adverse effects on treated systems and human health are still needed. This project will investigate the efficacy of a heterogeneous and readily available waste sorbent, drinking water treatment residuals (DWTR), as capping material for in situ remediation of metal contaminated sediments.
DWTR produced worldwide on a daily basis are discarded in landfills or simply discharged either on dry land, lagoons or into waterways. One way to add value to this free and readily available waste material is to take advantage of its physicochemical reactivity with—and high sorption capacity for—certain inorganic pollutants in the immobilization of metals. Since DWTR exhibit large specific surface areas and contain electron donor atoms that are ideal for the sequestration of metal cations, it is hypothesized that: "metal cations do form highly stable surface complexes with DWTR through a combination of different sorption mechanisms. Therefore, if used as in situ sediment capping material, it would significantly reduce metal bioavailability to exposed sediment dwelling organisms and eliminate the release of dissolved metals from contaminated sediments to the overlying water, over a range of pH, dissolved organic matter concentrations, and redox conditions typical to aquatic sediments."
This project is sub-divided into four main tasks. Following the collection of DWTR samples produced from raw waters of different chemical compositions, the first task will consist of (i) physicochemical characterization studies, (ii) the determination of toxicity characteristics of DWTR, including its potential effects on sediment dwelling organisms, and (iii) the identification of the main binding sites as well as the binding mechanisms of metals onto DWTRs through molecular scale investigations using synchrotron radiation.
The second task will consist of "gust chamber" experiments investigating the efficiency of DWTRs to control the release of metals from contaminated sediments to the overlying water as a function of pH, redox, dissolved organic carbon (DOC) concentrations, and the thickness of the DWTR cap layer.
In the third task, flow-through column studies will be used to study the potential adverse impacts of extreme change events on the ability of DWTR to sorb and immobilize metals if used as in-situ capping material for contaminated sediments.
Finally, in the fourth task, intact metal-contaminated sediment cores will be used in pilot studies to investigate metal concentration profiles and metal bioavailability using diffusive gradient thin film (DGT) techniques and detection by ICP-MS. Experiments in this last task will also include benthic organism based bioassays to determine the combined toxic effects of "DWTR-capped metal-contaminated sediments". Task 4 experiments are suggested as transition step toward in situ pilot studies.
This research program revolves around the notion of value-added to a readily available waste material, DWTR. A successful completion of this project will be of great significance as it would pave the way for the reuse of a readily available and abundant waste material in water pollution control. As a sorbent, DWTRs would prevent the contamination of water resources by toxic metals, thus protecting both the environment and human health. Additionally, the experiments will help shed light on the mechanisms by which different metal types can be immobilized on a highly heterogeneous sorbent as a function of changing key water/sediment parameters. These pilot studies will lay the groundwork for the use of DWTR for in situ remediation of metal-contaminated sediments. However, while in situ pilot studies constitute the logical next step to the research outlined in this project, it will be conducted only if the different milestones identified as points of "Go/No go" decisions favor such in situ testing of DWTR.
Lang, Z., S.M. Wallace, N.D. Denslow, J. Gaillard, P. Meyer, and J.J. Bonzongo. 2021. A Screening Approach for the Selection of Drinking Water Treatment Residuals for Their Introduction to Marine Systems. Environmental Toxicology and Chemistry, 40(4):1194-1203. doi.org/10.1002/etc.4950.