Persulfate

Klozur™ activated sodium (FMC Chemicals) has recently emerged as a robust chemical oxidant for treatment of soil and groundwater contamination. Persulfate is safe to handle and has a solubility of approximately 35% under field conditions. It also treats a wide range of organic compounds including: chlorinated alkenes, chlorinated alkanes, BTEX, PAHs, Freon, PCBs, di/tri-chlorobenzene, MTBE, etc. Sodium persulfate can be catalyzed with iron, acid, base and heat. This allows sodium persulfate to be effective over a wide range of subsurface conditions. A total organic carbon test is completed with sodium persulfate at 60C. Therefore, when thermally catalyzed persulfate is used, it is equivalent to completing an in situ TOC test.

Persulfate is a strong oxidant that has been widely used for initiating emulsion polymerization reactions, clarifying swimming pools, hair bleaching, micro-etching of copper printed circuit boards, and TOC analysis. Among all the persulfate salts typically manufactured (sodium, potassium, and ammonium salts), the sodium form is the most commonly used for environmental applications in the last few years. Sodium persulfate has the potential to in-situ destruct chlorinated and non-chlorinated organic compounds commonly encountered in contaminated soil and groundwater.

The persulfate anion is one of the strongest oxidants used in remediation. The standard oxidation reduction potential for the reaction

S₂O₈^2- + 2H⁺ + 2e^– => 2 HSO₄^–

is 2.1 V, as compared to 1.8 V for hydrogen peroxide (H2O2). This potential is higher than the redox potential for the permanganate anion (MnO₄^– ) at 1.7 V, but slightly lower than that of ozone at 2.2 V.

Another persulfate oxidation mechanism is through free radicals. With the presence of some catalyst, like heat, persulfate can be induced to form sulfate radicals.

S₂O₈^2- + initiator => SO₄^•?+ SO₄^•?

The sulfate radical has a similar reaction mechanism to the hydroxyl radical generated by the Fenton’s chemistry. The sulfate radical is one of the strongest oxidizing species with a redox potential estimated to be 2.6 V, similar to that of the hydroxyl radical, 2.7 V.

Persulfate and sulfate radical oxidation has several advantages over other oxidant systems. In addition to its robust treatment capabilities, the sulfate radical is more stable than the hydroxyl radical stable and therefore able to transport to greater distances in the sub-surface. Comparing to permanganate, persulfate has less affinity for natural soil organics (Brown 2003) and is thus more efficient in high organic soils. These attributes make persulfate a viable option for the chemical oxidation of a broad range of contaminants.

Although the persulfate anion by itself was a strong oxidizer, its reaction kinetics is slow for the more recalcitrant contaminants, such as trichloroethylene (TCE). However, generation of sulfate radicals can significantly enhance the kinetics of persulfate oxidation. Some catalysts, such as heat, transition metals, and UV radiation, were found to be able to initiate sulfate radical generation. With the reaction mechanism yet to be further studied, alkaline was also found to have the capacity to enhance the persulfate oxidation efficiency (Block et al., 2004). Activation of persulfate yields a very potent tool for the remediation of a wide variety of contaminants, including chlorinated solvents (ethenes, ethanes and methanes), BTEX, MTBE, 1,4-dioxane, PCBs and polyaromatic hydrocarbons. These activation processes are discussed as following.

Heat Activation

In a laboratory study, heat-activated persulfate has been demonstrated to be able to decompose a wide range of contaminants in aqueous systems (Block et al., 2004). Table 1 lists the oxidation of various compounds as a function of temperature. Although some compounds require higher temperature that others, all compounds tested were oxidized at 45°C and above.

In-situ heat-activated persulfate oxidation in the field scale requires installation of a parallel heating system to heat the aquifer matrix to a certain temperature, which may lead to a higher capital cost and operating expense. In situ heating can be achieved through steam or hot air injection, electrical resistance (joule) heating, or radio frequency heating. In situ heating, with an external heating source, is probably not a wise option for treating large groundwater plumes, but more suitable for source treatment where the target area is relatively small.

Table 1. List of contaminants decomposed with heat catalyzed persulfate
at different temperatures (Block et al., 2004)

Contaminants With > 90% Decomposition Treated with Persulfate @ 20°C	Xylene, 1,1-DCE, 1,2-Dichlorobenzene, 1,3-Dichlorobenzene, 1,2,4-Trichlorobenzene
Additional Contaminants With > 90% Decomposition Treated with Persulfate @ 35°C	1,2-DCE, PCE, TCE, Vinyl Chloride, Carbon Tetrachloride, 1,1-DCA; 1,2-DCA, Benzene, Chlorobenzene, MTBE
Additional Contaminants With > 90% Decomposition Treated with Persulfate @ 45°C	Methylene Chloride, Chloroform, 1,1,1-TCA

Transition Metal Activation

Ferrous iron (Fe²⁺) is the most commonly used and readily available activator for transition metal catalysis. It’s generally applied in the form of ferrous sulfate (FeSO4) and ferrous chloride (FeCl2). To effectively activate persulfate 100 to 250 mg/L of iron is generally required. Excessive ferrous iron (greater than 750 mg/L) can result in the rapid decomposition of persulfate and therefore a loss in oxidation performance. The naturally occurring reduced metals in the subsurface can often serve as metal catalyst for persulfate oxidation and thus save the effort of injecting ferrous iron.

Many of the compounds that can be oxidized by the heat-activated persulfate are also susceptible to ferrous iron activated persulfate. These compounds include BTEX, chlorobenzene, dichlorobenze, DCE, TCE, and PCE. However, some chlorinated ethanes, such as TCA, and chlorinated methanes, such as chloroform, are relatively recalcitrant to both heat-activated and iron II- activated persulfate oxidation.

The application of iron II as a persulfate activator is limited by its transportability. Iron II is eventually oxidized by the persulfate to iron III, which is insoluble at a pH above 4. The net reaction is:

2Fe²⁺ + S₂O₈^2- => 2Fe³⁺ + 2 SO₄^2-

Fe³⁺ + 3 H₂O => Fe(OH)³⁺ + 3 H⁺

Recent research on activated persulfate has been focused on several novel activation technologies which are transportable in a groundwater system and has the capacity to react with a broad range of contaminants (FMC-ERM, 2002; FMC-Orin, 2003). The following is a discussion of these novel activators.

Chelated Metal Catalysts

With solubility and availability of the transition metal catalysts being critical factors in the activation of persulfate, chelation is an effective way of maintaining metal activity at neutral and alkaline ground water conditions. Chelating the transition metal catalyst provides protection from hydration and subsequent precipitation under the neutral pH conditions that may be found in the field. Chelated metal catalysts are complexes of transition metals bound to strong chelating agents. Examples of chelating agents include: ethylenediaminetetraacetic acid (EDTA), citrate, polyphosphate, glycolic acid, catechol, nitrotriacetic acid (NTA), Tetrahydroquinine (THQ) and others in this category.

Chelated iron complexes have been shown to activate hydrogen peroxide for the destruction of complex pesticides (Pignatello, 1992). Chelated ferric iron (Fe³⁺), in addition to ferrous iron (Fe²⁺), was found to have excellent oxidation performance.

The Fe(III)-EDTA was found to be the best performing complex to activate persulfate oxidation of volatile organic compounds (VOCs) (Block et al., 2004). However, it is not effective with the chloroethanes or chloromethanes.

Peroxide-Persulfate Dual Oxidant System

Fenton’s reagent, which is hydrogen peroxide catalyzed by ferrous iron, has been widely utilized in treating soil and groundwater contaminants. It is highly reactive and is able to oxidize a wide range of contaminants. The effectiveness of Fenton’s reagent is limited on the stability of peroxide in some soil matrices, where it rapidly decomposes. A dual oxidant system (FMC-Orin, 2003) utilizing hydrogen peroxide and sodium persulfate has been developed. It combines the reactivity of peroxide in the destruction of compounds of concern with the enhanced stability of persulfate. Combination of hydrogen peroxide and persulfate may have several synergistic effects. First, hydroxyl radicals generated from peroxide can initiate persulfate radical formation, and vice versa. Secondly, hydrogen peroxide may destruct a significant portion of the more susceptible contaminants, allowing the sulfate radicals to destroy the more recalcitrant compounds. Finally, a combination of hydroxyl and sulfate radicals may result in a multi-radical attack mechanism, yielding either a higher efficiency in destroying contaminants, or allowing for recalcitrant compounds to be more readily degraded.

The combined peroxide-persulfate reaction system appears to have a broad range of applicability. It not only oxidizes compounds generally amenable to persulfate oxidation, but also oxidizes compounds not readily oxidized by conventional persulfate technology. A laboratory study showed that peroxide-persulfate dual oxidant system achieved significant destruction not only for chlorinated ethenes, but the more recalcitrant chlorinated ethanes as well (FMC-Orin, 2003).

Alkaline Activation

Persulfate is known to be highly reactive at acidic conditions, but it is also highly reactive at pH values greater than 10. It should thus be possible to “activate” persulfate creating an alkaline condition.

A laboratory study showed that alkaline activated persulfate has a broad reactivity, and that it is effective even on some historically difficult to destroy compounds, such as chlorinated ethanes and methanes. The alkaline activation of persulfate appears to be possible with a number of different bases, including potassium hydroxide, sodium hydroxide, and lime. The alkaline-activated persulfate oxidation of contaminants is not just a matter of high pH, but also of the buffering capacity (mole ratio of pH modifier to persulfate). It is essential to have sufficient base supply (excess buffering capacity) in application of the alkaline-persulfate activator technology. The acidity in the soil needs to be taken into account when deciding the quantity of base needed, some time a caustic demand test is recommended.