In aquatic environments that are impacted by contaminated sediments, risk management strategies focus on interrupting potential exposure pathways by which contaminants might pose an ecological or human health risk. Conventional approaches such as dredging and capping tend to be costly, energy-intensive, and disruptive to the environment. Emerging research has shown that contaminant transport pathways can be interrupted by modifying and enhancing the binding and contaminant assimilation capacity of natural sediments. This enhancement is achieved by adding amendments such as activated carbon for persistent organic pollutants; minerals such as apatite, zeolites, bauxite, and alumina for metals or metalloids; ion exchange resins for metals or other inorganic contaminants; or lime for pH control or nitroaromatics degradation. Although individual contaminant and sorbent types have been studied in recent work, there has been no organized effort to develop an in situ stabilization technology that targets mixtures of contaminants commonly found in the environment.

The objective of this project was to develop a rational selection process for use of multifunctional amendments based on established and emerging knowledge in the field of in situ remediation of contaminated sediments.

Sample images of produced agglomerates: a) 55% powdered activated carbon, 25% bentonite, 15% sand, 5% cellulose; b) 60% powdered activated carbon, 20% bentonite, and 20% sand

Research focused on comparing different sorbents either alone or in formulated combinations of new amendments, improving scientific understanding of how multiple amendments function together and reduce contaminant bioavailability, and developing efficient delivery methods for applying amendments to impacted sediments.

Results indicated that sorption of metals and organics can take place simultaneously with a suitable combination of sorbents. A list of 75 potential sediment amendments was identified from those currently under development for remediation of individual sediment contaminants and from existing industrial sorbents used for water- and gas-phase treatments. Initial screening was based on literature-derived information on sorptive/degradative capacity, potential ecological effects in the sediment environment, and potential complications during deployment such as material density, erodability, decay, or transformation in freshwater and saltwater environments. The screening yielded a list of 11 sorbents used for further sorption testing of metals and organics.

Based on pH-edge sorption tests, natural sorbents were eliminated because of inferior performance. Sorption properties of engineered amendments were compared in freshwater and saltwater matrices. A thiol-amended mesoporous silica (Th-SAMMS) and a titanosilicate mineral (ATS) demonstrated the highest sorption capacity for cadmium (Cd) and lead (Pb) respectively. Sequential extraction tests conducted after mixing engineered sorbents with contaminated sediment demonstrate transfer of metal contaminants from a weakly bound state to a more strongly bound state. Biouptake of Cd and Pb into a freshwater oligochaete was evaluated after the sediment was amended with Th-SAMMS and ATS, respectively. Biouptake of Cd was reduced by 98% after amendment of Thiol-SAMMS. The speciation of Cd was also altered with large reductions in the easily extractable fractions. The reduction of biouptake of native Pb was insignificant, while the treatment reduced the easily extractable portion of Pb in the sediment.

Th-SAMMS demonstrated the highest sorption capacity for mercury (Hg) in both freshwater and saltwater matrices. Treatment of Peninsula Harbor freshwater sediment with sorbent amendments resulted in lower methylmercury (MeHg) bioaccumulation factors in benthic organisms (P < 0.05). The treatments reduced MeHg worm bioaccumulation factors by 54% (GAC), 58% (HGR), and 83% (Thiol-SAMMS). Biouptake reduction of Hg and MeHg was achieved for both artificially spiked and native mercury present in sediments. Thiol-SAMMS added to sediment along with activated carbon reduced bioaccumulation of native Hg, Me-Hg, and polychlorinated biphenyls (PCB) present simultaneously in sediments. With the individual and mixture placement of the amendments, the reductions were more than 90% for MeHg, which indicated the reductions were not as sensitive to co-existence of the amendments.

Most amendments were nontoxic in chronic exposures. Organism survival in sediment amended with ATC, ATS, ConSepC, and HGR were not statistically different from the control for all endpoints evaluated. The toxicity observed for Thiol-SAMMS was moderate at a dose of 5% by dry weight with a reduction in H. azteca survival from 98% to 73%. Although Thiol-SAMMS reduced survival in clean control sediments, it may increase survival of organisms in Hg-contaminated sediments to levels that may be acceptable for meeting remediation goals. For example, L. plumulosus survival in diluted Augusta Bay sediment was 24% compared to a survival of 69% in the same sediment after amendment with Th-SAMMS.

Sorbent agglomerates were developed with the following properties: dense enough to sink through the water column and provide a light non-suffocating layer on the sediment, dense enough to be resistant to resuspension over the period it takes to be worked into the sediments, and able to break down to release active agents over the period of days to weeks. In addition, the binders used for the agglomerate were nontoxic. The most promising formulation was made of powdered activated carbon, clay, and sand and was produced in larger quantities for use in biological tests. In biouptake experiments, the carbon incorporated as a pellet was as effective in reducing the bioaccumulation of PCBs as powdered activated carbon applied directly, indicating that the pellets can break down and be mixed effectively into the active layer by bioturbation.

Through a partnership with the U.S. Environmental Protection Agency and Menzie Cura and Associates, an amendment delivery platform was developed in the form of engineered pellets referred to as SediMite™. Development of the agglomerate involved testing a variety of materials for the ability to form the agglomerate using scalable laboratory techniques, testing the physical characteristics of the resulting agglomerates, and testing the interaction of the activated carbon and other agglomerate additives. The most promising formulation was made of powdered activated carbon, bentonite clay, and sand. Evaluation of SediMite™ effectiveness is ongoing with support from the Environmental Security Technology Certification Program (ESTCP).

Most Department of Defense (DoD) aquatic environments are burdened by multiple contaminants, commonly consisting of both organic and inorganic compounds. This project identified, screened, and compared potential amendment mixtures for their optimal performance, both alone and in combination, to address such complex mixtures. It also addressed the feasibility of multifunctional amendment fabrication for easy deployment in the field. This comprehensive approach provides DoD with cost-effective alternatives for in situ management and also screens incompatible amendment mixtures. (Project Completed – 2009)