CATALYZED PERSULFATE: Advancing in Situ Chemical Oxidation (ISCO) Technology
Total Page：16
File Type：pdf, Size：1020Kb
CATALYZED PERSULFATE: Advancing In Situ Chemical Oxidation (ISCO) Technology Scott Wilsona; William Farone, PhDb; Gareth Leonardc; Jeremy Birnstingl, PhDc; Alberto Leombruni, PhDd aREGENESIS, San Clemente, CA, USA; bApplied Power Concepts, Anaheim, CA, USA; cREGENESIS UK, Bath, UK; dREGENESIS Ltd, Milano, Italia 1.0 Introduction For over a decade persulfate has been used to oxidize contamination in the field of environmental remediation. Most project applications have involved the use of persulfate in conjunction with traditional activation chemistries. While these activation technologies can be used successfully to degrade contamination in the field, each has its drawbacks. Over the past decade little was accomplished toward improving the efficacy, cost effectiveness or occupational safety related the use of persulfate oxidation chemistry for environmental remediation. Recently, however, a significant advancement has emerged in the form of a new all-in-one oxidant product that employs advanced catalyst-based activation chemistry. The focus of this paper is to: 1) outline for the reader oxidation technologies employed in environmental remediation, 2) discuss the traditional technologies employed to activate persulfate, and 3) introduce a new catalyzed persulfate chemistry that has been demonstrated to be effective at degrading contaminants in situ, while reducing the need for activation chemicals. 2.0 In Situ Chemical Oxidation (ISCO) In situ chemical oxidation (ISCO) of groundwater and soil contaminants is a remediation approach widely practiced throughout the world. The technique generally involves the use of a chemical oxidant applied into the environmental media such that direct contact is made with the target contaminant. In the case of subsurface soil or groundwater treatment, this usually involves the injection of the oxidants into the subsurface in the form an aqueous solution or slurry. In the case of treating soils on the ground surface, mixing of oxidant powder or slurries into the soil is not uncommon. Commercial use of chemical oxidation for subsurface remediation began in earnest in the early 1990s and focused primarily on the use of Fenton’s reagent (hydrogen peroxide activated with iron under low pH). Over time this broadened into a suite of closely related approaches based upon catalyzing hydrogen peroxide in what is often today referred to as catalyzed hydrogen peroxide (CHP). Throughout the 1990s this was a popular remediation approach due to its low unit cost and aggressive performance in degrading a wide range of contaminants. While this approach is still used today for some specific remediation projects types, use of CHP-type oxidants has fallen out of favor. This has been due primarily Page 1 © 2013 REGENESIS Bioremediation Products, San Clemente, CA to rapid decomposition of the oxidant in the subsurface (affecting limited area) and safety concerns associated with the often highly exothermic reactions, off-gassing and pressure buildup (US EPA, 2013). Permanganate use as an oxidant for remediation (typically employing either the potassium or sodium salt) has gained popularity over the past decade and is widely practiced. Use of these compounds is limited primarily to the destruction of substituted alkenes such as the chlorinated ethene series as the oxidation reactivity of permanganates is insufficient to oxidize key groundwater and soil contaminants such as petroleum hydrocarbons, chlorinated ethanes, etc. Application of permanganates in high concentration can lead to formation of solid manganese dioxide within aquifer pore space reducing reagent distribution and restricting aquifer hydraulic conductivity (Schroth et al., 2001). Manganese itself is also a regulated groundwater contaminant under certain environmental regulatory regimes (e.g. Decreto Legislativo 152/2006-Italia). In recent years, the use of a sodium percarbonate-based oxidation chemistry has become a widely used remediation technology. This particular chemistry, which employs a proprietary catalyst system applied with a soluble percarbonate (RegenOx®1), has shown significant performance in degrading a broad array of environmental contaminants. Previously, thoughts on the use of percarbonate as an oxidant for remediation were hampered by theories that the carbonate ion would scavenge desired radicals and render the chemistry ineffective. However, this has been largely dispelled by excellent field performance of the oxidant and recent research showing catalyzed percarbonate propagates the formation of the superoxide radical in significant concentration (Watts, 2012). 3.0 Persulfate as an ISCO Reagent For over a decade, persulfate has been used to oxidize contaminants in situ. While various forms of persulfate are used as chemical oxidants in different industries, it is sodium persulfate that is the predominant form of the compound employed in ISCO processes. Technically this compound is the sodium salt of peroxydisulfate (Na2S2O8) and is a solid white powder at standard temperature and pressure. Figure 1 depicts the persulfate molecular structure. Figure 1: Molecular Structure of Sodium Persulfate 1 RegenOx® is a registered trademark of REGENESIS, San Clemente, CA Page 2 3.1 Oxidation Reactions Oxidation of organic compounds by persulfate is very complex and is the topic of a considerable amount of recent research. A range of oxidation reactions are known to be involved and each of these is influenced by various factors in the subsurface including site specific geochemistry, pH, temperature, contaminant type, native organic matter, etc. Known oxidation reactions related to persulfate include both direct oxidation reactions and radical oxidations. 3.1.1 Direct Oxidation Persulfate reacts directly with many organic materials, exchanging electrons in a process known as direct oxidation. Persulfate is considered a powerful direct oxidant as it has a theoretical standard 2- - 2- oxidation potential of 2.0V (for the reaction S 2O8 + 2e → 2SO4 ). This is similar to that of ozone (2.1V) and greater than that of hydrogen peroxide (1.8V), sodium percarbonate (1.8V), and permanganate (1.7V) (ITRC, 2005). This strong oxidizing potential of persulfate makes oxidation of many organic contaminants thermodynamically favorable. It should be noted, however, that a thermodynamically favorable reaction means only that the energy of the products is lower than the energy of the reactants. The reaction may or may not proceed at rates that are favorable. In fact the direct oxidation of organic pollutants by persulfate, where the two electrons are transferred simultaneously as above, has long been shown to proceed at kinetically slow rates (House, 1962). 3.1.2 Radical Oxidation In addition to the direct oxidation pathway, persulfate has the propensity to generate radicals. Radicals are generated when the persulfate anion reacts with another compound to form atoms with unpaired electrons (radicals). Radicals generally are very reactive, rapidly oxidizing other compounds (increased kinetic rates compared to direct oxidation) and often forming a series of radicals in a chain reaction referred to as “propagation”. Radical species within a propagation series can be inorganic species or organic radicals generated by electron exchange. It should be noted, however, that radicals by their very nature are highly reactive and ephemeral. As a consequence, they are notoriously difficult to detect. Much of the understanding of radical formation, propagation and behavior throughout the ISCO field remains theoretical and inferred through a combination of thermodynamic probability and the nature of transitional oxidation intermediates formed. The most common radicals generated by persulfate under conditions employed in ISCO are thought to be the sulfate radical and the hydroxyl radical (Peyton, 1993; Furman et al., 2012). Eq.1: Formation of sulfate radical: 2- - 2- •- S 2O8 + 2e → SO4 + SO4 Page 3 Eq. 2: Propagations forming hydroxyl radical: •- - 2- • SO4 + OH → SO4 + OH Both the sulfate radical and hydroxyl radical are very strong oxidizers with standard oxidation potentials of 2.43 (Huie et al., 1991) and 2.59V (Bosmann et al., 1998), respectively. These primary radicals react with other compounds very rapidly through what is thought to be a variety of reaction mechanisms including direct electron transfer, addition to double bonds and hydrogen abstraction (removal of hydrogen atom, often generating another radical). Most of the research conducted to date on persulfate radicals generated during persulfate ISCO has focused upon the impact of these two species. Other radical species, however, are also thought to play an important role including the perhydroxl • •- radical (HO2 ) and superoxide anion (O2 ) (Watts, 2012). 3.2 Activation As stated above, sodium persulfate has the ability to transfer electrons directly in the process of direct oxidation. It is generally accepted, however, that the kinetics of contaminant destruction through direct oxidation alone is far less suitable for ISCO than the kinetics achieved by activating the formation of radicals (Petri et al., 2011). The radical oxidation of organic compounds with persulfate can be viewed as having three distinct phases. Activation is generally defined as the initiation of radical formation. This is followed by a period of Propagation, where the first radicals oxidize the target contaminant, but may also produce other inorganic radicals and perhaps radicals of organic compounds (including target pollutant fragments).