From: Public Services and Procurement Canada
Sequestration of contaminants in sediments occurs when amendments are incorporated into the sediment and act as a sorbent to the contaminants, altering the physical and chemical characteristics of contaminants to reduce their release into the biologically active zone. The goal is to minimize the bioavailability and mobility of contaminants and, therefore, their uptake by benthic and aquatic organisms. The processes involved in contaminant sequestration are adsorption onto amendment surfaces, the formation of stable complexes, and precipitation to a solid state. Currently, most sequestration techniques remain in the pilot stage.
The most common approach to sequestration uses activated carbon to bind hydrophobic chemicals capable of bioaccumulating and biomagnifying within the environment. Experimental studies and field applications adding activated carbons to sediments have shown that this technique reduces bioavailability and contaminant concentrations within the interstitial pore water of the sediment, and results in an associated reduction in contaminant mobility. In the case of contamination of metals, reduction in bioavailability and mobility occurs by precipitation or sorption, thereby decreasing their solubility. Because of these sorption processes, the immobilization in sediments of many ions, such as Pb, Ba, Cd, Co, Cu, Mg, Mn, Th, U, and Zn, can be accomplished using compounds such as the mineral apatite.
Pilot studies have shown that the application of activated carbon onto sediments contaminated with hydrophobic organic compounds has little effect on the stability and sheer strength of surface sediments, and causes little reduction in mobility and activity of the local biota. These findings suggest that the technology appears to have no negative effects on the health of the benthic community in the short term. As full-scale applications of remediation through sequestration have not been conducted, long-term impacts are not known. Several factors influence the success of sequestration amendments, including pH, redox potential (the potential for a chemical to be reduced), the type of contaminant, the cation exchange capacity (the number of cations a matrix is capable of holding), sediment type, and others.
Solidification and stabilization are an alternate sequestration technology, involving the use of agents such as cement, to bind the sediments together and hold them in place. This differs from sequestration, as contaminants remain in their original state (i.e., they are not precipitated or made more stable) and mobility and bioavailability are reduced through alteration of their surrounding environment. Solidification/stabilization in situ has currently only been applied to soils (see fact sheet for Solidification/Stabilization–in situ), but it has been used ex situ to treat sediments following removal through excavation or dredging (see fact sheet for Solidification/Stabilization–ex situ).
Integrated Environmental Assessment and Management. 2015. In situ treatment using activated carbon.
http://onlinelibrary.wiley.com/doi/10.1002/ieam.1589/full
US EPA. 2013. Use of Amendments for In Situ Remediation at Superfund Sediment Sites.
https://clu-in.org/download/contaminantfocus/sediments/In_situ_AmendmentReportandAppendix_FinalApril2013.pdf
The process of treating contaminated sediment through sequestration has been demonstrated using activated carbon to bind chemicals and using concrete for solidification and stabilization. Both options have only been demonstrated in the pilot stage, and further research is required to develop a comprehensive process of contaminant sequestration in sediment. For organic contaminants, the amendments most commonly used are activated carbon and organic clays (bentonite clays modified with amines to achieve 30 to 35% organic carbon). For metals, the amendments most frequently used are substances with a high cation exchange capacity, such as apatite (a mineral composed of calcium phosphate), zeolite (alumino-silicate minerals), and sepiolite (a mineral composed of magnesium silicate). Organic clays can also effectively sequester certain metals. Activated carbon and minerals used for the immobilization of metals are relatively inexpensive materials.
As sequestration techniques are relatively new, studies continue to look into alternate amendment options and site optimizations. For example, Wällstedt et al. (2008) found increased sequestration of metals, specifically Al, As, Cd, Co, Fe, Mn, Ni, and Zn, after addition of lime, and Gibney and Nüsslein (2007) found that some arsenic compounds could be sequestered through the respiration of microbial communities.
The steps for treating contaminated sediment through sequestration include:
Isolate the contaminated sediments by removing or redirecting the overlaying surface water. This must be done in advance where in situ solidification/stabilization and mechanical mixing are necessary. Equipment such as sheet piling, cofferdams, and pumps may be necessary. See fact sheet for Excavation (in the Dry) and Off-Site Disposal–Sediments, for a detailed description of surface water removal.
Place appropriate amendments on the surface of contaminated sediments; application methods that have been studied include injection, mechanical mixing, and bulk placement.
Mix in the amendments, as necessary. Mixing may be accomplished through mechanical equipment, or naturally through bioturbation. In situ solidification/stabilization may require large mixers/augers to drill vertically into the sediment, inject additives, and mix the sediment and additives together. Dozens of drill holes may be required to cover the entirety of a particular area.
Locations with significant erosive forces, or amendments with low bulk density, may require covering with clean fill to prevent loss or resuspension of the amendments.
Monitor the treated area immediately after the amendment application to determine and verify the reduction in contaminant bioavailability and mobility.
Reapply amendments as necessary.
When amendment application occurs in the dry, removal of water barriers and re-introduction of surface water is required after treatment. This should be done slowly to prevent resuspension of material and loss of amendment into the water column.
Materials stored on-site consist of desired amendments, which may include activated carbon, cement, kiln dust, lime, organic clays, and nutrients, such as apatite. Amendments should be stored in a sealed container, according to the manufacturer’s recommendations. Care should be taken to prevent loss due to dust and contact with precipitation.
Little waste is involved in sequestration projects, and may be limited to typical construction waste (small amounts of lubricant and used containers), amendment containment, and spent sorbent pads. Discharge may include suspended amendments in the water column or release and resuspension of residual contaminants during mixing-in of amendments. To prevent discharge and dusting, all amendment stockpiles should be covered, and to the extent possible, amendments should be added to sediments in the dry.
Notes:
Tests examining the effect of temperature change on hydraulic conductivity and establishing the zone of freezing with a pilot scale tubing system are recommended to properly design the full-scale containment system.
Laboratory tests using sediments from the study site are recommended to determine:
Impact assessment of groundwater advection is recommended if groundwater treatment is required.
Remediation of sites in northern environments poses unique challenges. Sites are inherently remote and may be difficult to access. Much of the equipment required for site remediation must be transported by boat or plane, typically from hundreds of kilometres away and at great cost. Climate restrictions (e.g., cold temperatures and ice conditions) and short seasonal windows to conduct work may limit remediation options.
The treatment of contaminated sediment may require placing restrictions and limitations on the human consumption of native organisms. These restrictions may negatively affect local communities that rely on aquatic animals (e.g., seals and whales) and fish as important sources of food. All of these factors may change the approach and remediation options for remote regions.
The equipment required to spread and mix amendments into sediments is similar to the equipment used to spread and mix amendments onto agricultural lands. In addition, a large auger may be needed to incorporate amendments into deep sediments. Obtaining specialized spreading and mixing equipment may be difficult in northern regions, and may preclude the use of sequestration.
As sequestration as a remediation technology has been exclusively used in laboratory and pilot scale models, the long-term effects at the site level are largely unknown. Site managers can improve predictions for long-term success by ensuring that the contamination is thoroughly delineated at surface and depth, and by regularly monitoring the site and making timely adjustments.
Sequestration renders contaminants unavailable for interaction and uptake with benthic organisms and aquatic vegetation, which precludes the ability to biodegrade or biotransform contaminants into less-toxic forms. This leaves sites vulnerable to re-contamination when untreated deeper contaminated sediments are uncovered, or when site conditions change (e.g., a change in pH may reduce the effectiveness of the chemical sorbents). Long-term monitoring of the site is required to ensure that re-contamination doesn’t occur and that sequestration and containment of contaminants are ongoing.
In the case of solidification/stabilization, contaminants are made immobile. As such, they are not susceptible to leaching out with environmental fluctuations, such as bioturbation (the mixing of the upper-sediment layer by benthic organisms) and erosion caused by wave action. Ongoing visual monitoring should be conducted to ensure that adequate cover is maintained on the solidified sediments, in an effort to prevent contaminant loss due to erosive forces. Long-term monitoring may also be conducted to identify instances of re-contamination via sedimentation.
There are no by-products generated during in situ treatment of contaminants by sequestration in sediments. Contaminants are still present in the treated matrix, but they are immobilized and their availability is significantly reduced. If the project design ensures that the stability of sediment from erosion is maintained or improved, the migration of contaminants sequestered by the reagents should be marginal and their availability should remain minimal. It is possible that the reactions generated by the amendment may alter sediment pH and redox potential, which could affect the bioavailability of other contaminants.
Solidification/stabilization uses alkaline amendments, and acts to increase the overall pH within the sediments. Contaminants may react to the change in pH; however, they are quickly rendered immobile, significantly reducing their availability for environmental interaction and uptake.
Difficulty in incorporating and evenly distributing reagents to reach all contamination
Restricted ability to treat deeper contaminants that may be difficult to reach through physical mixing
Uniformity and complete mixing of amendments into the sediment is essential to promote mass transfer and immobilization of contaminants. Mixing is difficult to accomplish in situ, while minimizing the amount of resuspension and release
Decreased availability of contaminants to microorganisms, resulting in decreased natural biodegradation of organic contaminants
Parameters such as pH, organic matter content, type of contaminants, the cation exchange capacity, and sediment type can limit the effectiveness of sequestration
Most amendments have a finite capacity to sequester contaminants. Depending on contaminant volumes, large quantities of amendments may be required to achieve remediation objectives
It is possible for sequestered contaminants to be released or transported to another site, due to physical and chemical conditions of the environment. For example, sorption of some metals may be reversed in the presence of other metals (due to cation exchange or a change in pH and redox conditions)
Exposure of benthic organisms to contaminated sediments after sequestration treatment, by ingestion, remains possible. Continued monitoring of sediments (including bioassays) is required to determine the success of the method
Technology is still in development stage, and so field examples, amendment volumes, and application parameters have not been thoroughly developed. Long-term studies are required to develop standards of use as a remediation technology, as well as accurate processes and considerations for use
There is little known about the long-term efficacy of this technology as a remediation option
No degradation of the contaminant
Not particularly effective for volatile contaminants, particularly VOCs
Requires significant mixing, which is only feasible when the sediment bed is exposed (in the dry)
Results in the complete loss of the benthic community
Mechanical mixing of the amendments, when required, results in destruction of the existing benthic organisms and habitats
When mixing of amendments is required, sediment resuspension may occur
Sequestration Possibility of contaminant mobilization (although sequestered) by the currents towards less contaminated areas.
Physical disturbance of top sediment may release any untreated, deeper contamination
Application of amendments may impact the growth and diversity of the aquatic plants and benthos within the site
Erosion has the potential to re-release contamination
Sequestration can be used in conjunction with other in situ technologies to further reduce the availability of contaminants. For example, Choi et al. (2009) have demonstrated the feasibility of combining adsorption, using activated carbon, with dechlorination, using iron/palladium (Fe/Pd) as reagents to degrade PCBs. The activated carbon facilitates the desorption of PCBs, making them available for reduction with the dechlorinizing reagents. Sequestration has been used to treat residual contamination after dredging activities. It may also be used in conjunction with other in situ technologies, for example biodegradation and capping, where the alternate technology is intended to treat the unsequestered fraction of contamination.
No secondary treatment is required if the remediation objectives have been achieved. Otherwise, sequestration can be combined with in situ capping (before capping or as an active process in the overlaid material), biodegradation, chemical oxidation, or natural attenuation (optimized or not). However, the interaction between these methods/processes must be well controlled to ensure project success.
Field Testing of Activated Carbon Mixing and In Situ Stabilization of PCBs in Sediment, Final Report—US DTIC pdf
2006 Activated Carbon Pilot Study—The Grasse River Project, Massena, New York
2005 In Situ Solidification of Soft River Sediments, New York/New Jersey Harbor
2008 Sydney Tar Ponds Agency, Solidification/Stabilization of Sydney Tar Ponds, Sydney, N.S., Canada
Performance of pilot tests in the literature reported more than 90% reduction in PCB concentrations in water and 80% in the tissues of organisms tested when using activated carbon (Patmont et al., 2009). In laboratory tests, apatite and organoclays sequestered more than 80% of heavy metals in solution (Knox et al., 2007). No full-scale trials have been conducted to determine the effects that environmental factors may have on the overall efficacy in sequestering contaminants; however, US EPA (2013) gives details about the amendments that have shown the most promise as sequestering agents.
Solidification/stabilization using cement-based solidifiers is subject to the same erosion processes as cement-based construction materials. Performance evaluations of solidification treatment on soil have shown that after 10 years, performance standards have exceeded expectations (US EPA, 2009). However, little information exists outlining the long-term performance of solidification at sediment sites. Monitoring is required to ensure contaminants are not re-mobilized.
Work is currently being done to test a variety of activated carbon sources, including sustainable re-use options such as biomass waste, biochar, and organic waste fractions from industry processes. Choosing existing and sustainable options for sequestration amendment will help to reduce the overall carbon footprint of the site remediation. For example, Gustaffson et al. (2016) found that renewable activated carbons derived from kraft pulp-mill processes yielded similar sorption properties as traditional anthracite in water and sediments spiked with PAHs.
Potential contaminant release and resuspension may be minimized through methods of application, such as using a direction injection method, isolating the sediment through use of caissons, or impregnating solid material with necessary reagents. These techniques have all shown reduced levels of resuspension of amendments as compared to mechanical mixing.
The use of sorbents such as activated carbon to sequester contamination has shown little effects on the surrounding benthic community, although some studies have identified a reduction in plant growth. Application methods may play a larger role in impacting the aquatic ecosystem, as mechanical mixing often has severe effects on the overlaying benthic community. Prior to sediment disruption through mechanical agitation, the area should be studied for the presence of sensitive organisms and habitats, which may require removal, or preclude the use of mechanical agitation at the sediment surface.
Solidification/stabilization results in the complete destruction of the benthic community. Treated sediments do not provide necessities for habitat redevelopment, and must be covered with clean fill to promote benthic recovery.
Major Human Health Exposure Pathways
Exposure Pathway Triggers (Remediation Stages)
Residency or Transport Media
Public Exposure Routes (On-Site and Off-Site)
Monitoring, Action Levels, and Mitigation Approaches
In situ treatment
Sorbents (e.g. activated carbon, organoclay, minerals) and solidifiers/stabilizers (Portland cement, lime, kiln dust)
Skin contact, inhalation of particulates, incidental ingestion
Educate staff on safety and provide appropriate personal protective equipment (PPE) and reactionary materials (e.g., sorbent pads), as necessary. Follow measures for safe storage and handling to minimize exposure, as outlined in MSDS sheets.
Composed by : Bruno Vallée M.Sc, LVM Inc.
Latest update provided by : Bruno Vallée, M.SC., LVM inc. and Ashley Hosier, Ing., Royal military college
Updated Date : December 12, 2016
