For large-scale field remediation of impacted sediments, conventional methods include in-place strategies, such as in situ capping, and relocation actions, like landfill disposal and transfer to sea. For in situ remediation using active capping substrates, many studies have identified sorbents with high sorption capacities, particularly when targeting metals in impacted environments. While such knowledge can inform laboratory production/tailoring of sorbents, manufacturing approaches significantly increase the cost of the remediation process, making them economically less attractive for large-scale remediation efforts. Cost-effective and efficient remedial approaches that avoid adverse effects on treated systems and human health are of interest. 

In this study, the project team investigated the efficiency of a heterogeneous and readily available waste material, known as drinking water treatment residuals (DWTR), as an effective sorbent for in situ remediation of metal-impacted sediments. Additionally, the project team sought to investigate the potential limitations associated with the use of DWTR as capping material for metal-impacted sediments. 

The project team hypothesized first, that metal cations would form highly stable surface complexes with DWTR through a combination of different sorption mechanisms; and second, that if used as an in situ capping substrate, DWTR would significantly reduce metal effluxes from impacted sediments toward the water column, decreasing therefore their bioavailability. 

The specific objectives of this study were as follows: 

• Determine the characteristics of a wide variety of DWTR and develop a screening approach for identifying DWTR suitable for introduction into U.S. aquatic systems. 
• Conduct laboratory studies to assess the efficacy of DWTR in immobilizing metals from aqueous solutions and controlling the release of metals from impacted sediments into the overlying water, with a focus on how these processes are affected by changes in key water parameters. 
• Investigate the effects of extreme environmental changes on the ability of DWTR to sorb and immobilize metals when used as in situ capping material for impacted sediments.

Leptocheirus plumulosus on DWTR

Laboratory studies were conducted to comprehensively evaluate the characteristics of DWTR. Characterization studies included chemical analyses, focusing primarily on the determination of total metal concentrations on (i) acid-digested DWTR, (ii) elutriates from toxicity characteristic leaching procedure, and (iii) elutriates from DWTR leaching with synthetic fresh or marine waters. 

Ecotoxicological bioassays were used to assess the potential impact of DWTR to aquatic organisms. Solid phase analytical techniques were used to elucidate the importance of the various chemical moieties that control the association of metals with different functional groups present in DWTR. 

Furthermore, the capacity of these substrates to immobilize dissolved metals was investigated through batch and flow-through column studies. Gust chamber studies were also conducted using metal-impacted sediment cores to assess the efficacy of DWTR in reducing or eliminating the efflux of metals from impacted sediments to the overlying water. Additionally, 96-hour exposure studies with fathead minnow juveniles were conducted to evaluate the potential of DWTR to mitigate metal impact to water column organisms. 

Lastly, diffusive gradients in thin films were employed to probe the effects of capping metal-impacted sediments with DWTR on metal concentrations in both pore and overlying waters, providing further insights into the remediation efficiency of DWTR as a capping material.

The main findings of this research can be summarized as follows:

• Differences in the chemical composition of both treated waters and additives used in the water treatment processes led to highly heterogeneous DWTR, and based on State and Federal regulations, not all these substrates will be adequate for introduction to U.S. waters. Results led to the development of a screening method for selection of potentially safe DWTR for use as capping substrate for metal-impacted sediments.
• Batch studies on the determination of the ability of DWTR to sequester and immobilize metals from aqueous effluents showed that the affinity of metal cations for these substrates was significantly higher for borderline and type-B metals than for type-A metals.
• The results of fixed bed column studies showed that DWTR were able to significantly delay the breakthrough of metals when compared to an inert substrate (sand).
• The use of metal-impacted sediment cores in laboratory experiments, performed with Gust chambers showed that sediment capping with DWTR does reduce both sediment erosion and the upward flux of metals from sediments to the overlying water. Results showed that the use of DWTR as a sediment capping layer could reduce the metal impact of the overlying water by up to 95%.
• Fish exposure studies using waters equilibrated with metal-impacted sediments in either the absence (control) or presence (treatment) of a cap layer made of either sand or DWTR showed the ability of DWTR to reduce/eliminate impact to pelagic organisms.
• The use of passive porewater samplers showed that the presence of DWTR as capping layer at the water-sediment interface impacted the vertical profiles of soluble metal concentrations in both the overlying and pore waters by reducing the total dissolved metal concentrations through adsorption and precipitation processes.

This research program consisted of laboratory studies conducted primarily to determine if DWTR could be used as in situ capping substrates for metal-impacted sediments. Based on the results of the screening component of the study, some DWTR can qualify for introduction to U.S. waters. This initial research shows clearly that the use of DWTR for capping metal-impacted sediments is promising. The Gust chamber studies showed that DWTR can act as both physical barrier and active sorbents for metal chemicals of concern. As a result, DWTR can reduce, and in some cases eliminate, the impact of metals to pelagic organisms. Application of the results of this research will ultimately bolster operational capabilities and warfighter preparedness by mitigating the impacts of these chemicals. (Project Completion - 2023)

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.

Wallace, S. M., Y. Zhang, L. Zhou, Q. Ma, W.E. Guise, N.D. Denslow, and J.F. Gaillard. 2023. The Diversity of Aluminum-Based Drinking Water Treatment Residuals for Use in Environmental Remediation.  Environmental Science: Water Research & Technology, 9(3):935-947. doi.org/10.1039/D2EW00387B.