From: Public Services and Procurement Canada
Chemical oxidation of contaminated sediments is a technology based on creating contact between an oxidizer and the contaminants present in the sediments to transform the contaminants into chemical forms that are less toxic or non-toxic to human health and the environment.
It can be used to treat oxidizable contaminants, including Volatile Organic Compounds (VOCs) such as dichloroethane (DCE), trichloroethylene (TCE), tetrachloroethylene (PCE), Benzene, Toluene, Ethylbenzene, and Xylenes (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 the 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).
This technology is currently available only at the laboratory stage and in certain pilot projects, but it has been demonstrated to be effective under well-controlled conditions that match the specific characteristics of the sediments to be treated.
Internet links:
Implementation of this technology may include:
On-site storage may include fuels, lubricants and other site materials required for operating the machinery and equipment for the implementation of the technology.
Depending on the type and state of the oxidant, storage structures, handling and injection elements may be required. Oxidizers are usually present in liquid or salt form. Hazardous materials stored on-site may include strong oxidants, acids or bases. Separation and containment of these products are very important.
In general, oxidants are not flammable, but in the event of a fire, they can provide the source of oxygen required for combustion. Therefore, oxidants should be stored in secondary containment, away from flammable or combustible products, and handled according to the manufacturer’s recommendations.
Workers should wear personal protective equipment such as full-face respirators and protective clothing when handling and injecting/applying oxidants.
Important health and safety measures must be implemented at the site where oxidants are stored and prepared prior to distribution to the injection area. These measures may include clean water tanks, eyewash stations, liquid retention and containment tanks, etc. The oxidant storage and handling area must also be clearly marked and controlled.
As sediments are treated in situ, little waste is generated on-site. Site waste may consist of typical construction litter, including plastic containers, but it may also contain used/spent sorbent pads. Surplus oxidants may be returned to the chemical supplier 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 injection and/or application, and as oxidants migrate throughout sediments. Care should be taken to prevent oxidant loss into the overlying waterbody. In addition to oxidant loss, the sediment disruption that may occur during injections may result in resuspension and loss of sediments in the waterbody.
Deep injections of oxidants can also reduce the probability of surface sediment disturbance and perforation of the sediment surface due to the preferential path of oxidants. 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 sediments).
Some oxidation reactions may cause contaminants to disassociate from solid particles and dissolve in interstitial water. This may create a pathway for contaminants to become suspended in 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 or oxidation reactions, which may disrupt the sediment surface, causing the resuspension of sediments into the waterbody. Laboratory tests should investigate instances of gas generation, and mitigation efforts should be considered.
Notes:
To date, no large-scale implementation for in situ sediments has been completed. 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 treating TCE and BTEX . Laboratory testing indicates similar success for the treatment of DCE , PCE , PAH s, and PCB s. Chemical oxidation applies to contamination adsorbed onto the solid phase and/or dissolved in interstitial water and to remediation of free-phase contamination, as indicated by laboratory testing.
Following the injection of the oxidant into sediments, the persistence of the oxidant in the sediment matrix is important, as it directly impacts its ability to reach the contaminants. For example, MnO4- is relatively persistent (> 3 months), allowing a better diffusion through sediments. Conversely, H2O2 is very labile (minutes to hours) and its diffusion will be limited to a small area around the injection site.
The technology can be applied in areas with sensitive environments, such as wetlands and areas with sensitive benthos, where removal or encapsulation would be harmful. However, chemical oxidation is still a technology that can be toxic to certain organisms; therefore, laboratory tests should be carried out beforehand on the absorption and reaction of benthos. This is even more important when oxidants are injected at or near the surface.
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Chemical oxidation has proven to be a viable technology for the remediation of soil and groundwater contamination. Sediment remediation using chemical oxidation has been extensively studied and covered in the scientific literature. However, few technical reviews and case studies are available.
The treatment times proposed above are theoretical. The treatment schedule largely depends on the number of applications, the time required between applications and the chances of contaminants experiencing a rebound effect. In addition, any limitations imposed on the timing of applications (such as avoiding applications during sensitive periods for aquatic biota) will increase the overall treatment time required for remediation.
Long-term monitoring may be required when the homogenous distribution of chemical oxidants is difficult. Pilot testing of chemical oxidation in soils has indicated 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. A behaviour similar to the rebound effect observed in soil treatment is anticipated in sediment treatment with this technology. In addition, oxidation reactions can lead to significant desorption of contaminants from sediments into the aqueous medium, with a risk of migration of these contaminants transferred to the dissolved phase. These phenomena also increase the risk of recontamination of treated areas.
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 partial degradation of PAH s 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 waterfowl (Grung et al., 2016).
Reagents have been shown to improve the 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. (2015) showed improved degradation of phthalate esters in river sediment with the combined oxidizers Fe3O4 and S2O82- in parallel with electrokinetic remediation processes, to aid the occurrence of the reaction. 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, in an effort to improve the treatment effectiveness.
No secondary treatment is required if the target contaminant concentration is reached. However, following treatment, it is possible for contaminant concentrations to increase again (identified as “rebound effect”). In these circumstances, additional injection phases 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.
Application examples are available at these links:
Performance reported in the literature during laboratory testing of contaminated sediments or reconstituted spiked natural sediments in the laboratory demonstrated a reduction in concentrations of PAH s and PCB s of more than 90%. A 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 sediments per location. Moreover, mobilization of metals contained in sediments was observed in the interstitial water. The performance of this technique at a large-scale level is therefore uncertain at this stage.
The effectiveness of oxidants also depends on several factors including the pH of sediments, organic matter content, the presence of carbonates and sulphides, particle size, temperature, sediment conductivity, and dosage of reagents. The technology is still in the testing phase, with large-scale performance still to be demonstrated. Laboratory tests are also required to determine whether remediation objectives are achievable with this technology.
The minor and major potential human health exposure pathways are presented in the following table.
Exposure Pathway Triggers (Remediation Stages)
Residency or Transport Media
Public Exposure Routes (On-Site & Off-Site)
Monitoring
Mitigation Measures according to Residency or Transport Media
Site Preparation
Injection/Application of Amendments
Dust
Inhalation of Dust
Dust Monitoring
Vapours
Surface Water
Groundwater
Animals and Plants
Sediments
Inhalation of Vapours
Air Quality Monitoring
Chemical and Geochemical Processes
(runoff of oxidants)
Ingestion of Drinking Water
Country Foods Consumption
Surface Water Monitoring
(migration/lixiviation of oxidants)
Groundwater Monitoring
(including fish, shellfish and wildlife)
Sediment Quality Monitoring
Animal and Plant Tissue Monitoring
Sediment
At very low doses, most chemical oxidants have minimal impact on aquatic and benthic life. However, agents may negatively affect the aquatic and benthic communities at high doses, 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 the capacity of native benthic organisms to withstand the application of oxidants. When the required oxidant dose is greater than the amount acceptable for the native benthic community, oxidants may be applied in consecutive thin layers, allowing time for benthic recovery between each application.
Composed by : Bruno Vallée M.Sc, LVM Inc.
Updated by : Ashley Hosier, P.Eng. Royal Military College of Canada
Updated Date : December 8, 2016
Latest update provided by : Juliette Primard, Frédéric Gagnon and Sylvain Hains. WSP Canada Inc.
Latest update date :March 31, 2024
