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Soil Remediation Techniques: Examination of In Situ Chemical Oxidation

 

3. Chemical Process

The previously mentioned ISCO technologies are developed through specific chemical processes. Following, this paper will discuss such processes as well as their respective advantages and limitations.

3.1 Catalyzed H2O2 Propagations (CHP)

The CHP process is based on the catalyzed decomposition of hydrogen peroxide by soluble iron, iron chelates, or iron minerals in order to produce strong oxidant hydroxyl radical (OH•) (through Fenton’s initiation reaction) and other reactive oxygen species  as seen in the following reactions (Watts & Teel, 2006):

            H2O2 + Fe2+→OH• + OH + Fe3+                                                                 (1)

                                  OH• + H2O2 HO2• + H2O                                        (2)                    

                                          HO2O2•+ H+                                                                               (3)                                                      

                                         + H2O2 ROH + OH•                                         (4)                               

                                      HO2 + Fe2+HO2 + Fe3+                                                                 (5)                                            

The hydroxyl radical is one of the strongest oxidants found in nature and it is the oxidant of interest in CHP (Watts & Teel, 2006). However, in addition to hydroxyl radical, CHP uses other species produced by subsequent chemical reactions of hydroxyl radical with hydrogen peroxide. Some of these species include superoxide anion (O2), perhydroxyl radical (HO2•), and hydroperoxide anion (HO2) (Watts & Teel, 2006). As further discussed in site compatibility, CHP reactions producing these four species can degrade almost any organic contaminant. 

The nature of catalyst, pH, and hydrogen peroxide concentration greatly impact the CHP process. Added soluble iron and iron chelates, as well as naturally present iron oxide minerals, usually act as catalyst in CHP reactions. Their effectiveness is directly related to pH as soluble iron and iron oxide minerals are most effective in acidic pH environments and iron chelates at neutral pH (Watts & Teel, 2006). It is common practice in the field to increase the hydrogen peroxide concentration in cases where treatment is unsuccessful. Such increment will lead to further propagation reactions (equations 1 through 5) which will produce more reactive oxygen species and increase the treatment effectiveness (Watts & Teel, 2006).  

One of the advantages of CHP technology with respect to others is its capability to treat sorbed contaminants and DNAPLs (Watts & Teel, 2006). On the other hand, a significant limitation is the high rates of hydrogen peroxide decomposition in surface soils and subsurface which means a low oxidant stability (Watts & Teel, 2006).

3.2 Permanganate

Permanganate (MnO4-) is a highly oxidized form of manganese that serves as a selective chemical oxidant in ISCO treatments. Potassium permanganate and sodium permanganate are the two most common sources of the chemical used for ISCO. The water solubility of permanganate depends on the medium temperature, size of crystal, extent of agitation, and concentration (Watts & Teel, 2006). The following equation illustrates the chemical reaction involved in the use of permanganate as an ISCO technique (Hueling & Pivetz, 2006)

                         MnO4- + 4H+ + 3 e- → MnO2 + 2H2O                                  (6)

Permanganate is highly reactive with alkanes; hence it is most commonly used to oxidize chlorinated ethenes (e.g. TCE and PCE). It is also effective in the treatment of dichloroethylene isomers, vinyl chloride, phenols, and some polyaromatic hydrocarbons (Watts & Teel, 2006). However, it is ineffective in oxidizing chloroalkanes, benzene, and other aromatic compounds with low degrees of ring activation. As Watts et al. (2006) share, the cation associated with the permanganate (sodium or potassium) does not have a big influence on its oxidizing capability, but the permanganate concentration does.

Potassium Permanganate is rapid and cost-effective when treating DNAPL source zones or zones of high residual contamination. It is also appropriate for sites where the physical disruption of soils is desirable. 

 

Figure 3

Figure 3: Permanganate in Situ Oxidation Application (Hueling & Pivetz, 2006)

3.3 Ozone

Ozone (O3)is one of the strongest oxidants available for remediation and is unique to other ISCO processes in that it involves the application of a gas (ozone) and therefore a different design and operation than other oxidants. It is composed of dry air or O2 which is inputted into an ozone generator and charged with a high voltage or UV irradiation where O2 molecules can be split and react quickly to form O3. Due to the instability of ozone, this must be generated on site.

Ozone oxidation chemical reactions can be divided into two categories: direct oxidation and indirect oxidation as shown in Equations 7-16. Direct oxidation involves the oxidation of the targeted chemical by ozone in one reaction. Unlike indirect oxidation, it does not rely on the hydroxyl radical (OH•) for achieving targeted results. Indirect oxidation follows a pathway that includes chain-initiating reactions, chain-propagating reactions, and chain-terminating reactions. Indirect oxidation is a faster reaction than direct oxidation due to the formation of the hydroxyl radical which rapidly attack organic contaminants and breaks down their organic carbon-to-carbon bonds (Interstate Technology and Regulatory Council, 2005).

Direct Ozone Reaction

O3 + RC = CR → RCOCR + O2                                    (7)

Chain-Initiating Reactions

O3 + OH- → O2 + OH•                                           (8)

                       O3 + H2O → O2 + 2OH• (in the presence of ultraviolet light)                (9)

Chain-Propagating Reactions

OH• + 2H2O → HO2 • + OH- + 3H+                                  (10)

                                     HO2 • → O2-• + H+                                                                                    (11)                                                                                           

OH• + RH → R• + OH-                                                    (12)

R• + O3 + H2O → ROH + O2 + OH•                                       (13)

Chain-Terminating Reactions

HO2 • + Fe2 + → O2 + H+ + Fe3+                                       (14)

                                HO2 • + Fe2 + → HO2- +Fe3+                                                                   (15)                                                

Fe3 + + O2-• → Fe2 + + O2                                             (16)

 

There are two forms of in situ ozone application: vadose zone injection of ozone gas and ozone sparging below the water table (Interstate Technology and Regulatory Council, 2005). In situ ozone oxidation involves the injection of mixture of air and ozone gas directly into the unsaturated and/or saturated zones. Depending on the reactivity and concentration of reactants, temperature, and pH, the longevity of ozone in the environment and extent of contaminant oxidation will vary significantly. Introduction of the ozone to the contaminant is done via ozone sparging, a similar mechanism to air sparging.

Ozone sparging however, does not result in a uniform distribution, rather it results in the formation of a limited number of air channels in which the majority of the injected air is transported (Huling & Pivetz, 2006). For best results, air channels during sparging must be as close as possible to where mass transfer zones overlap each other for successful transport. Ozone sparging can be advantageous due to its ability to be easily transported in water with high diffusive transport rate, and can be given in large concentrations in the unsaturated zone versus the saturated zone (Huling & Pivetz, 2006). 

Ozone is low in solubility, dependent on temperature, and does not leave a residual. Depending on the properties of the site being remediated, these characteristics should be highly considered when deciding if ozone is the proper oxidant (Derby, 2009).The transport of ozone in unsaturated porous media is impacted by the water content, organic matter in the soil, and metal oxides in the soil. The higher the water content, the quicker the transport. An increase in content of organics and metal oxides decrease the transport. It should be noted that it may volatilize organics and generate high levels of dissolved oxygen which can be problematic with groundwater. There is no safe breathing concentration for ozone with a permissible exposure limit of 0.1 ppmv and immediately dangerous to life or health value of 5 ppmv. Heavy safety measures therefore must be used (Derby, 2009). Figure 4 is provided for a generalized schematic of the application of ozone. 

Figure 4

Figure 4: General conceptual model of in-situ ozonation in the saturated zone with soil vacuum extraction to capture volatile emissions and O3(g) (Huling & Pivetz, 2006).

3.4 Persulfate

The three persulfate salts most commonly used are ammonium persulfate, sodium persulfate, and potassium sulfate. Due to its solubility and content, sodium persulfate is the most widely used oxidant among them. Oxidation can occur via electron transfer or free-radical pathways. It is most common practice to activate persulfate to generate free radicals. 

Na2S2O82- → S2O8 2- + 2Na+                                                (17)

         S2O82- + activator→ SO4- + SO4(or) SO42-                                 (18)

Persulfate along with an activator to form sulfate radical for oxidation, SO42- , is favorable because the sulfate anion has a greater oxidation potential than the persulfate anion produced from Equation 17. The solubility of persulfate indicates that once injected, its transport mechanism will be dominated by density-driven and diffusive transport into low-permeability materials. It is more stable in the subsurface than other oxidants with a half-life of 100 to 500 days; this gives it the ability to be prominent in the system to further decontaminate an area for a longer duration. It has less affinity for natural organic matter than permanganate or peroxide and therefore less of it is needed. Unlike permanganate, it can oxidize benzene so it can be used in the remediation of fuels spills and benzene, toluene, ethylbenzene, and xylene contaminated groundwater. It is usually applied through direct injection or through horizontal or vertical recirculation wells. The powder must be mixed on site. Hydraulic fracturing can also be used if immediate response is required and there is low permeability layer. The density of persulfate increases with increasing concentration and therefore lower concentrations are needed for recirculation approach and higher for direct injection. Sodium Persulfate is more expensive than catalyzed hydrogen and permanganate. Safety considerations include that in powder form it is an irritant, corrosive and sulfate is a drinking water contaminant (Derby, 2009).

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