From: Public Services and Procurement Canada
The chemical oxidation of contaminated sediments is a technology based on creating contact between an oxidizer and the contaminants present in the sediment in order to transform the contaminants into chemical forms less toxic to human health and the environment. It can be used to treat oxidizable contaminants, including volatile organic compounds (VOCs) such as dichloroethene (DCE), trichloroethene (TCE), tetrachloroethene (PCE), benzene, toluene, ethylbenzene, xylene (BTEX) and semi-volatile organic compounds (SVOCs) such as polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs). In most cases, the degradation products generated are water and carbon dioxide, and in the case of chlorinated compounds, inorganic chlorine. Some reactions also generate residual oxygen that may promote biodegradation of residual contaminants after chemical oxidation.
There are several different oxidizers that can be used. The most common are permanganate (MnO4-), hydrogen peroxide (H2O2) with or without ferrous iron (Fe2+) as catalyst (Fenton’s reagent), sodium persulfate (Na2S2O8), persulfate (S2O82-) and ozone (O3). A table discussing a variety of oxidants and their effectiveness on a variety of contaminants can be found in Huling and Pivetz, 2006. The type and form of the oxidizer dictate the mode of handling and injection. Of the oxidizers mentioned above, only ozone is in a gaseous state, although it too can be dissolved in a liquid. All other oxidizers are present in liquid form or as a salt. The persistence of the oxidizer in the sediment matrix is important since its persistence directly affects the ability to reach contaminants. For example, MnO4- is relatively persistent (> 3 months), allowing a better diffusion through the sediment. Conversely, H2O2 is very labile (minutes to hours) and diffusion will be limited to a small area around the injection site. The effectiveness of oxidizers also depends on several factors including the pH of sediment, organic matter content, the presence of carbonates and sulphides, particle size, temperature, sediment conductivity, and dosage of reagents.
This technology is currently available only at the laboratory stage, but has been demonstrated to be effective under well-controlled conditions matching the specific characteristics of the sediments to be treated. One of the biggest challenges is the effective injection of oxidizing agents into sediments in order to achieve the distribution necessary for permitting efficient oxidation of target contaminants. This challenge is made greater due to the dynamic nature of contaminated sediments, which are generally spread over very large areas and the high proportion of organic matter in sediments. Hydrophobic organic compounds have a high affinity to organic matter and tend to adsorb to the latter, making them less available for reactions with oxidizing agents. Organic matter also reduces the effectiveness of oxidizing agents since it competes with contaminants in the consumption of reagents. Higher doses of oxidizing agents may be necessary to achieve the desired rate of degradation. It is therefore necessary to conduct laboratory-based tests with sediments from the target site to determine the optimal oxidizing agent, the dosage required, the mode of injection and, if required, the need for a catalyst, acid, or stabilizer. The benthic community has little capacity to withstand large applications of amendments, and therefore multiple sequential applications and long-term monitoring may be needed.
Currently chemical oxidation remains in the laboratory phase for sediment remediation, as of 2016 no field studies exist, and application processes have not been adequately developed. However, information may be adapted from the application of this remedial technology for use in contaminated aquifers and soil, as the state of technology is more advanced for these media. Projects that move forward with a full-scale or on-site pilot scale remediation may involve the following actions:
The most common oxidants used in situ chemical oxidation are permanganate, persulfate, hydrogen peroxide, and ozone. In general, oxidizers are not themselves flammable, however, in the event of a fire, they may provide the oxygen source to allow continued burning. Oxidizing agents may be highly reactive, and should be stored and handled according to the manufacturer’s recommendations. They should always be stored in a secondary containment, away from flammable or combustible products.
Workers should wear protective clothing, which may include respirators, when handling oxidants. They should continue to wear protective equipment on the site until the reaction time between the oxidants and the contaminated sediments has elapsed.
As sediments are treated in situ, there is little waste generated on-site. Site waste may consist of typical construction litter, including plastic containers, but in addition may contain used/spent sorbent pads. Surplus oxidants and reagents may be returned to the chemical purchaser, or disposed of in a manner acceptable to the local regulatory authority.
As chemical oxidants are hydrophilic, they may be absorbed into the water column during the initial injection, and as oxidants migrate throughout the sediment. Care should be taken to prevent loss of oxidants into the overlaying water body. In addition to oxidant loss, the sediment disruption that may occur during injections may result in resuspension and loss of sediment tot the water body. Potential mitigation efforts may include slow rates of injection, to allow permeation through deep sediments and prevent channelling. Deep injections of oxidants may also reduce the likelihood of surface-sediment disruption and oxidant channelling perforating the sediment surface. Chemical oxidation may be combined with sediment capping to reduce oxidant loss in sites with surface contamination (where deep injection is not sufficient to remediate the sediment).
Some oxidation reactions may cause contaminants to disassociate from the solid particles and become dissolved in the interstitial water. This may create a pathway for contaminants to become suspended into the water column, particularly for contaminants at or near the sediment surface.
Chemical oxidation pilot tests within soils have occasionally resulted in gas generation. Gas generation during sediment treatment is typically caused by ebullition, which may disrupt the sediment surface, causing resuspension of sediment into the water body. Laboratory tests should observe instances of gas generation, and mitigation efforts should be considered.
Notes:
On-site treatment trials will establish the efficiency of the technology and the parameters that influence the treatment time and cost (e.g. residence time, pump flow rate, requirements for pre-treatment, etc.).
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.
Chemical oxidation applies to a wide range of contaminants including organic contaminants, and any contaminant that can be oxidized. Soil trials have shown success in the treatment of trichloroethylene (TCE), and BTEX. Laboratory testing indicates similar success for the treatment of dichloroethene (DCE), and tetrachloroethene (PCE), polycyclic aromatic hydrocarbons (PAHs), and polychlorinated biphenyls (PCBs).
Chemical oxidation applies to:
It applies to locations:
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 (such as, cold temperatures and ice conditions) and short seasonal windows to conduct work may limit remediation options. The local populations often rely on aquatic animals (such as, seals and whales) and fish as an important source of food. Because of this, restrictions and limitations placed on consumption of native organisms, often required when contaminated sediment is present, may have a significant impact on communities. All of these factors may change the approach and remediation options for remote regions.
As chemical oxidation techniques are still in the testing phase, there have been no full-scale trials in a northern setting to date. The cost and logistical difficulty in mobilizing the equipment and chemical oxidants necessary to implement the technology may limit its application to northern environments.
Timelines listed above are theoretical. The timeline for treatment is highly dependent on the number of applications, time required between applications, and potential for contaminant rebound. In addition, any limitations placed around timing of application (for example avoiding applications during sensitive periods for aquatic biota) will increase the overall treatment time required for remediation.
Comments: Chemical oxidation has been demonstrated as a viable technology for the remediation of soil and groundwater contamination. Sediment remediation through chemical oxidation remains an unproven technology outside of laboratory experimentation.
Long-term monitoring may be required when homogenous distribution of chemical oxidants is difficult. Pilot testing of chemical oxidation in soils has indicated a potential for contaminant “rebound.” Causation has been linked to heterogeneous mixing of the oxidant into the matrix, leaving pockets of low-permeability (such as silt or clay) untreated. As contaminants remain at the site, they may continue to permeate and may re-contaminate treated zones. This phenomenon is expected to occur in sediments with similar structures.
In most cases, the degradation products generated are water, carbon dioxide and, in the case of chlorinated compounds, inorganic chlorine. Oxidation with O3 and H2O2 also increases the oxygen content of the matrix, which can accelerate the biodegradation of residual compounds. Some oxidative reactions are exothermic (such as H2O2), so they can increase desorption and dissolution of contaminants adsorbed onto the solid phase. Exothermic reactions can have a positive effect on treatment efficacy, but may also lead to contaminant migration into the water column. Moreover, if oxidation of the contaminants is incomplete, toxic intermediates can be generated. For example, the incomplete degradation of PAHs can produce phenols, aldehydes, carboxylic acids, ketones and quinones (Ukiwe et al., 2013) whose toxicity is high for certain animals that live in the benthic zone, such as fish and water fowl (Grung et al., 2016).
Reagents have been shown to improve treatment effectiveness of various oxidizers. Reagents include catalysts, which increase the speed of a chemical reaction, acids which promote reaction by lowering the pH, and stabilizers which prevent early reaction with the organic content. For example, Feo et al. (2014) paired hydrogen peroxide with a UV catalyst for the improved remediation of decabromodiphenyl ether (deca-BDE) contaminated sediments, and Yang et al. (2014) showed improved degradation of phthalate esters in river sediment with the combined oxidizers Fe3O4 and S2O82- in tandem with electrokinetic remediation processes, to aid the reaction in occurring. The stability of amendments used may be increased through modifications to their physical characteristics, such as the addition of binders or weighted agents, in an effort to reduce occurrences of resuspension and loss of amendments to water movement off-site. Alternatively, chemical oxidation can be combined with other methods such as in situ capping, biodegradation, sequestration or monitored natural recovery (MNR), in an effort to improve the treatment effectiveness.
No secondary treatment is required if the target contaminant concentration is reached.
Following treatment, it is possible for contaminant concentrations to increase again (called rebounding). In these circumstances, a second or third injection of oxidants may be required. Rebounding occurs when the oxidants do not reach all of the contamination, or when the oxidants are used up before all of the contamination has been treated.
No examples of successful pilot scale tests are available. However, examples of laboratory tests can be found in Ferrarese et al., 2008, Hong et al., 2008 and Brown et al., 2009.
Performance reported in the literature during laboratory testing of contaminated sediments or reconstituted spiked natural sediments in the laboratory demonstrated a more than 90% reduction in concentrations of PAHs and PCBs. The only known pilot test conducted in the field (Thomas et al., 2008) reported no significant degradation of organic contaminants (petroleum hydrocarbons) despite achieving concentrations of 20 g of peroxide/kg of sediment per location. Moreover, mobilization of metals contained in sediments was observed in the interstitial water. Performance of this technique at a field scale level is therefore uncertain at this stage.
Full-scale field testing has not been conducted to date using chemical oxidation to remediate contaminated sediments. If this technology moves forward with full-scale field testing, sustainability of the technology may be improved through thorough testing and design of the oxidant types, implementing mitigation measures to offset the impact of the oxidant on the benthic community, and optimizing the number of oxidants and injection locations necessary to meet remediation objectives.
At very low doses, most chemical oxidizers have minimal impact on aquatic and benthic life. However, at high doses, oxidizing agents may negatively affect the aquatic and benthic communities, potentially increasing mortality rates. Smaller benthic organisms have less ability to withstand the addition of oxidants, and are more susceptible to early mortality, as compared to larger organisms and fish. Dosing quantities should therefore be dictated by both contaminant volumes and by the capacity of native benthic organisms to withstand the application of oxidants. When the required oxidant dose is greater than the amount tolerable by the native benthic community, oxidants may be applied in consecutive thin layers, allowing time for benthic recovery between each application.
Exposure Pathway Triggers (Remediation stages)
Residency or Transport Media
Public Exposure Routes (On-site and Off-site)
Monitoring, Action Levels & Mitigation Approaches
In situ treatment
Chemical oxidizers (permanganate [MnO4-], hydrogen peroxide [H2O2], sodium persulfate [Na2S2O8], persulfate [S2O82-] and ozone [O3])
Skin contact, inhalation of particulates, incidental ingestion
Educate staff on safety and provide appropriate personal protective equipment (PPE) and reactionary materials (such as sorbent pads), as necessary. Follow measures for safe storage and handling to minimize exposure, as outlined in MSDS sheets.
Reagents (Fenton’s reagent [Fe2+])
Educate staff on safety and provide appropriate PPE. 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 8, 2016
