A Review of In Situ Chemical Oxidation and Heterogeneity
Chemical oxidants are increasingly being used for the in situ destruction of organic contaminants in groundwater. The most common implementation involves using an injection/withdrawal system to circulate oxidants (e.g., potassium permanganate, hydrogen peroxide, and Fenton’s reagent) through a source zone containing a dense non-aqueous phase liquid (DNAPL). Because the efficiency of chemical oxidation is highly dependent on geological heterogeneities, effective delivery schemes are essential for successful remediation. This article reviews the impact of heterogeneities on the success of in situ chemical oxidation. Physical heterogeneities are primarily concerned with the permeable pathways along which oxidants are transported to the zone of contamination. Chemical heterogeneities refer generally to variability in geochemical properties that also bear on the efficiency of oxidant flooding. Both types of heterogeneities work against bringing the oxidant to zones of high contaminant saturations. The highly heterogeneous distribution of contaminants and difficulties in characterization make it difficult to target specific zones for treatment. As a result, large volumes of sediments could be treated whether they are contaminated or not. Heterogeneities in hydraulic conductivity at most sites provide an intensive dose of chemical reagents along permeable pathways and little treatment of low-conductivity zones. Large quantities of oxidizable materials in geologic units are capable of consuming the oxidant during delivery. Reaction products [e.g., CO2, MnO2, and Fe(OH)3] tend to plug the porous medium, especially in zones with large contaminant saturations. The oxidant flood is diverted away from these zones, making the flooding inefficient.
Comparison of Fenton’s Reagent and Ozone Oxidation of Polycyclic Aromatic Hydrocarbons in Aged Contaminated Soils
Polycyclic aromatic hydrocarbons (PAHs) are formed as a result of incomplete combustion and are among the most frequently occurring contaminants in soils and sediments. PAHs are of great environmental concern due to their ubiquitous nature and toxicological properties. Consequently, extensive research has been conducted into the development of methods to remediate soils contaminated with PAHs. Fenton’s reagent or ozone is the most commonly studied chemical oxidation methods. However, the majority of remediation studies use soils that have been artificially contaminated with either one or a limited number of PAH compounds in the laboratory. Hence, it is essential to extend such studies to soils contaminated with multiple PAHs under field conditions.
Objectives. The objective of this study is to investigate the capacity of Fenton’s reagent and ozone to degrade PAHs in soils. The soils have been collected from a number of different industrial sites and, therefore, will have been exposed to different PAH compounds in varying concentrations over a range of time periods. The capacity of Fenton’s reagent and ozone to degrade PAHs in industrially contaminated soils is compared to results obtained in studies using soils artificially contaminated with PAHs in the laboratory.
Materials and Methods:
Nine soil samples, contaminated with PAHs, were collected from five different industrial sites in Sweden. For the Fenton’s reagent procedure, the pH of the soil slurry samples was adjusted to pH 3 and they were kept at a constant temperature of 70ºC whilst H2O2 was added. For the ozone procedure, soil samples were mixed with 50% water and 50% ethanol and kept at a constant temperature of 45 ºC. Ozone was then continually introduced to each soil sample over a period of four hours. Following the Fenton’s reagent and ozone oxidation procedures, the samples were filtered to isolate the solid phase, which was then extracted using pressurized liquid extraction (PLE). The sample extracts were cleaned up using open columns and then analysed by gas chromatography-mass spectrometry (GC-MS).
Results:
The relative abundance of the detected PAHs varied between soils, associated with different industries. For example, low molecular weight (LMW) PAHs were more abundant in soil samples collected from wood impregnation sites and high overall PAH degradation efficiencies were observed in soils originating from these sites. In the contaminated soils studied, PAHs were more effectively degraded using Fenton’s reagent (PAH degradation efficiency of 40-86%) as opposed to ozone (PAH degradation efficiency of 10-70%). LMW PAHs were more efficiently degraded, using ozone as the oxidizing agent, whereas the use of Fenton’s reagent resulted in a more even degradation pattern for PAHs with two through six fused aromatic rings.
Discussion:
The degradation efficiency for both methods was largely dependent on the initial PAH concentration in the soil sample, with higher degradation observed in highly polluted soils. LMW PAHs are more susceptible to degradation than high molecular weight (HMW) PAHs. As a result of this the relative abundance of large (often carcinogenic) PAHs increased after chemical oxidation treatment, particularly after ozone treatment. Repeated Fenton’s reagent treatment did not result in any further degradation of soil PAHs, indicating that residual soil PAHs are strongly sorbed. The effectiveness of the two oxidation treatment approaches differed between industrial sites, thus highlighting the importance of further research into the influence of soil properties on the sorption capacity of PAHs.
Conclusions:
This study demonstrates that the degree to which chemical oxidation techniques can degrade soil bound PAHs chemical degradation is highly dependent on both the concentration of PAHs in the soils and the compounds present, i.e. the various PAH profiles. Therefore, similarities in the PAH degradation efficiencies in the nine soil samples studied were observed with the two chemical oxidation methods used. However, the degradation performance of Fenton’s reagent and ozone differed between the two methods. Overall, Fenton’s reagent achieved the highest total PAH degradation due to stronger oxidation conditions. LMW PAHs showed higher susceptibility to oxidation, whereas high molecular weight (HMW) PAHs appear to be strongly sorbed to the soils and therefore less chemically available for oxidation. This study highlights the importance of including soils collected from a range of contaminated sites in remediation studies. Such soil samples will contain PAH contaminants of varying concentrations, chemical and physical properties, and have been aged under field conditions. In addition to the chemical and physical properties of the soils, these factors will all influence the chemical availability of PAHs to oxidation.
Recommendations and
Perspectives:
We recommend including aged contaminated soils in chemical degradation studies. In future chemical remediation work, we intend to investigate the potential influence of the chemical and physical properties of PAHs and soil parameters potential influence on the chemical oxidation efficiency in aged contaminated soils.
Due to the vast number of contaminated sites there is a great need of efficient remediation methods throughout the world. This study shows the difficulties which may be experienced when applying remediation methods to a variation of contaminated sites.
Radiation-Chemical Oxidation of Peptides in the Solid State
Gamma-ray irradiation of polypeptides as highly dispersed fluffs under oxygen leads to chemical degradation of the peptide bond with the remarkably high oxygen consumption of about one molecule per 2 ev of absorbed energy. A radical chain mechanism appears to be involved, and there is evidence that excited states of the polypeptide aggregate undergo chemical quenching by molecular oxygen.
Idaho Labs Test Chemical Oxidation As Alternative to MSW Incineration
A technique being tested by the Department of Energy’s Idaho laboratory may promise an alternative to waste incineration that produces no dioxins or low-emission volumes containing polyaromatic hydrocarbons.
The Silver II method, borrowed from AEA Technology Engineering in Virginia, is designed to treat mostly liquid organic waste streams and small solids such as tissues. However, larger solids that are pretreated for size …
Cascades Uses Chlorine Dioxide for Chemical Oxidation of TRS in NCG
As a result of the triai at Cascades’ Fjordcell mill in Jonquière, Que., no organochlorides or other objectionable compounds were measured at significant levels
The total reduced sulfides (TRS) present in non-condensable gases (NCG) are partly responsible for the characteristic odor of kraft pulp mills. These odors originate mostly from the vents associated with pulp production equipment such as digesters, blow tanks and washers, or from black liquor recovery equipment. Since the early 1990s, the Canadian provinces1 and the U.S.2 have implemented regulations that force kraft pulp producers to collect and treat major vents containing TRS.
The traditional approach for treating these gases is incineration, either in a lime kiln, in the plant boilers, or in a dedicated incinerator. However, thermal incineration of NCG has several drawbacks, such as the risk of toxic gas inhalation (leakage points on existing boilers due to age), explosion risks, reluctance of the operating personnel, complexity of the security devices necessary to ensure safe injection of those gases into kilns or process boilers, high operating costs and high boiler modification costs.
Due to the costs involved with NCG incineration, alternate methods were developed in the beginning of the seventies by some mills3, some equipment manufacturers4.6′7 specializing in industrial emission treatment, and by research centres such as PAPRICAN5. Such methods chemically oxidize the contaminants present in the NCG using powerful oxidizers, such as sodium hypochlorite or chlorine dioxide, used to bleach the pulp. These oxidizers are therefore available to bleaching plants on site.
Chemical Oxidation of La2CuO4 Epitaxial Thin Films Grown by Pulsed Laser Deposition
Chemical oxidation is used to induce superconductivity in La2CuO4 expitaxial thin films fabricated by pulsed laser deposition technique. Details about the influence of oxidation time on structural, surface morphology, Raman spectra, and electrical properties have been investigated. The results convince that successful uptake of oxygen occurs in the oxidized films, and the content of the inserted oxygen increases with increasing oxidation interval. The possible mechanism for the excess oxygen insertion into the film is also discussed.
Chemical Oxidation Treats Soil and Groundwater
Chemical oxidation is a rapidly growing remediation technology that has offered a range of practical field results over the past 15 years. Chemical oxidation technologies have involved a variety of oxidants, such as hydrogen peroxide, permanganate, persulfate and percarbonate in combination with a range of application techniques. The end result of this activity has been, by and large, the successful removal of contaminant mass from soils and groundwater at numerous sites.Hydrogen peroxide has been shown to be particularly effective for petroleum-based compounds of concern (COCs), while permanganate has historically been the oxidant of choice for chlorinated solvents. Persulfate is the new kid on the block, and may not have any specified advantage on particular class of compounds, although time will tell. Nevertheless, across the array of COCs, most researchers and practitioners believe that chemical oxidation has the ability to rapidly reduce large masses of contamination. The reason it is characterized as a belief, per se and not a given, is that the good works in higher source areas are sometimes masked between monitoring events with rebound. Still, the overall track record for mass removal has been successful, which sets the stage for natural or enhanced attenuation by biological processes to complete the final steps.
With thousands of applications completed, chemical oxidation is poised at an interesting juncture; it is no longer innovative, but not yet established. Further, it defies a standard methodology, because mastery of the complexities of the site-specific conditions is required for a successful application. Thus, in one sense and under certain conditions, the process can be thought of as more art than science. Although chemical oxidation is a good technology for both ex-situ soil treatment and in-situ soil and groundwater approaches, the primary use of chemical oxidation has been in the subsurface, with delivery achieved via direct push techniques.
Chemical oxidation is not a silver bullet and can only be effective in proportion to the foundation it is built upon. In other words, in-situ chemical oxidation performance is tied to the quality of a site assessment, which can lead to problematic applications, disappointing performance and no real contingency plan when performance goals are not met. Site owners or responsible parties have been disappointed as chemical oxidation has failed to meet their expectations of reaching low (ppb) contaminant concentrations within a short period of time. However, when done right, in-situ chemical oxidation is often the most cost effective and efficient technology for source remediation – but expectations must be managed.
Rebound should be expected in source area remediation. All chemical oxidation reactions occur in the groundwater and as the contaminant is removed from that phase, the sorbed mass – chemicals adhering to the soil – re-equilibrates with the groundwater. Depending on the persistent nature of the oxidant and the treatment level needed, multiple applications of the oxidant are often required.
While conducting a SERDP/ESTCP funded research project[1] C.J. Newell found that many case studies and literature reports documented decreases in contaminant concentrations in the groundwater following source depletion activities. However, the data presented was typically of short duration and did not allow a complete evaluation of whether or not the reduction achieved was permanent. He concluded that the majority of reported chemical oxidation applications had significant rebound in contaminant levels within two to four years after the initial application.
Of great interest to practical researchers is that some of this rebound can be classified as so-called “phantom rebound.†This is a reference to the fact that, depending on the contaminant and the method of analysis, the intermediates of chemical oxidation as well as the oxidation products of background organic carbon can appear as unmanaged native source mass. Further, on a stoichiometric basis, because one is transferring oxygen to both extant and new molecules, there can be an effective mass increase, which is not a bad thing. It should also be briefly noted that oxidation intermediates are just what the microbes ordered. Chemical oxidation effectively sets the table for bioremedial polishing steps. Clearly, more studies need to be done in this area by those in pure research, and by practitioners with the necessary curiosity and budget to help define these all-important aspects of performance analysis.
Chemical oxidation is a contact sport
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The key to successful in-situ chemical oxidation is persistent contact between the oxidant and the contaminant. These must also be in the right proportions so that a first order rate of reaction is achieved. To insure good contact with contaminants, one must have a good understanding of where those contaminants are located in the subsurface. Then an injection scheme must be designed that accounts for site-specific characteristics. Finally, there must be a performance assessment schedule that allows for one to determine the effectiveness of a given injection design.The first important design considerations for in-situ chemical oxidation would be to pick a suitable chemical oxidant for the contaminant of interest and the site-specific conditions. Second, the oxidant loading rate needs to be determined. The amount of oxidant will be calculated based on the mass of the contaminant, the amount of naturally occurring oxidizable material that will consume the oxidant, and a safety factor that will account for uncertainties at all levels (site conditions, mass estimates, etc.). The volume and rate of oxidant application is another important design factor that is a balance of remediation design needs and hydrodynamic limitations of the aquifer. Finally, contingency plans, for additional applications or transition to bioremediation if the desired treatment goals are not met, should be part of the plan. It should also be noted that both economic and molecular biological decision-making tools, which are in the offering as the basis for making a transition from chemical to biological methods, can be counterintuitive.
The future
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Most practitioners now realize that in order to reach low contaminant concentrations with in-situ chemical oxidation, multiple injections may be required. A logical strategy to achieve low contaminant concentrations on project sites with high contaminant mass would be the use of chemical oxidation technology to achieve initial mass reduction, followed by longer term stimulation of in situ bioremediation using cost-effective, injectable compounds. It was thought by many that microbes indigenous to the subsurface would simply be wiped out by the application of harsh chemical oxidants. This notion has since been debunked by an increasing amount of research and field experience.Today, there is little doubt that indigenous microbes present prior to a chemical oxidation application will indeed rapidly re-colonize the treated area and will actually flourish in the soup of pre-digested carbon, i.e. the partially oxidized intermediates. What a counterintuitive development this has been; the logical supposition was that chemical oxidation would simply destroy everything when in fact it can promote dramatic growth and recovery for the reasons cited. Further, the advent of new molecular biological investigation methods seems to be demonstrating that the first organisms to be affected and the last to rebound are the more complex eukaryotes that also function as predators to the contaminant degrading prokaryotic bacterial organisms. PE
SIDEBAR: New Developments in Chemical Oxidation Technology
One of the earliest compounds used in chemical oxidation was hydrogen peroxide where the reaction is more powerful when catalyzed by iron; this is known as Fenton’s reagent. This technology must be managed very carefully as it is extremely exothermic and potentially hazardous. An alternative to this process is to control the violence of the process by using a carrier of hydrogen peroxide – specifically sodium percarbonate (2 Na2CO3 · 3 H2O2). Sodium peroxide is an article of commerce, however, a new development in the form of RegenOx improves on sodium percarbonate with a special activator complex (a composition of ferrous salt embedded in a micro-scale catalyst gel). RegenOx, with this dual catalytic system, has very high activity, and is capable of treating a very broad range of soil and groundwater contaminants including both petroleum hydrocarbons and chlorinated solvents.As a further improvement, RegenOx operates generates alkaline conditions (high pH), and not the acidic conditions (low pH) that are required when operating with Fenton’s chemistry. Under basic conditions, carbonate scavenging of the free radicals that do the actual oxidation is not a concern. It should be noted that the RegenOx chemical oxidation system is now significantly different from Fenton’s for all these reasons such that it should be referred to as a catalyzed hydrogen peroxide reaction. Further, based on its significant longevity (up to 30 days) in the subsurface it can be considered a time-release catalyzed hydrogen peroxide reaction. The extended time of activity can therefore allow for both the initial contaminant degradation and the continued treatment of contaminants desorbing from the matrix.
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