Abstract:
Various embodiments of contaminant removal systems, compositions, and methods are described herein. In one embodiment, a method for oxidizing a contaminant includes contacting the contaminant with a peroxygen compound and initializing, maintaining, or propagating degradation of the peroxygen compound with an oxygenated organic compound, thereby releasing oxidizing radicals. The method also includes oxidizing the contaminant with the released oxidizing radicals.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
       [0001]    This work was supported in part by Strategic Environmental Research and Development Program # ER-1489. The government has certain rights in this work. 
     
    
     BACKGROUND 
       [0002]    A well-documented problem in many countries is contaminated subsurface soil by volatile organic compounds (VOCs), semi-volatile organic compounds (SVOCs), pesticides, polychlorinated biphenyls (PCBs), polyaromatic hydrocarbons (PAHs), total petroleum hydrocarbons (TPH), and/or other contaminants. Such contaminants can become sources of water contamination. For example, certain toxic VOCs can move through soil by dissolving into water passing through. Examples of such toxic VOCs include trichloroethylene (TCE), vinyl chloride, tetrachloroethylene (PCE), methylene chloride, 1,2-dichloroethane, 1,1,1-trichloroethane (TCA), 1,1-dichloroethane, 1,1-dichloroethene, carbon tetrachloride, benzene, chloroform, chlorobenzenes, ethylene dibromide, and methyl tertiary butyl ether. 
         [0003]    Many techniques have been developed for remediation of contaminated soil, groundwater, or wastewater. Example techniques include dig-and-haul, pump-and-treat, biodegradation, sparging, and vapor extraction. However, using such techniques to meet stringent clean-up standards can be costly, time-consuming, and ineffective for recalcitrant compounds. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]      FIG. 1  is a flowchart illustrating a process for oxidizing a contaminant in accordance with embodiments of the technology. 
           [0005]      FIG. 2   a  is a plot showing degradation of nitrobenzene as an hydroxyl radical probe using various base-persulfate ratios with 5 mM glucose addition in accordance with embodiments of the technology. 
           [0006]      FIG. 2   b  is a plot showing degradation of nitrobenzene as an hydroxyl radical probe using various base-persulfate ratios without glucose addition in accordance with embodiments of the technology. 
           [0007]      FIG. 3  is a plot showing degradation of hexachloroethane (HCA) as a nucleophile/reductant probe using various base-persulfate ratios with 5 mM glucose addition in accordance with embodiments of the technology. 
           [0008]      FIG. 4  is a plot showing degradation of HCA as a nucleophile/reductant probe using various base-persulfate ratios without addition of a base in accordance with embodiments of the technology. 
           [0009]      FIG. 5  is a plot showing persulfate degradation at various base to persulfate ratios with 5 mM glucose addition in accordance with embodiments of the technology. 
           [0010]      FIG. 6  is a plot showing degradation of hexachloroethane as a nucleophile/reductant probe with additions of glucose, fructose, and galactose in accordance with embodiments of the technology. 
           [0011]      FIG. 7  is a plot showing degradation of HCA as a nucleophile/reductant probe by pyruvate-activated persulfate at neutral pH in accordance with embodiments of the technology. 
       
    
    
     DETAILED DESCRIPTION 
       [0012]    Various embodiments of contaminant oxidation systems, compositions, and methods are described below. Particular examples are describe below for illustrating the various techniques of the technology. However, a person skilled in the relevant art will also understand that the technology may have additional embodiments, and that the technology may be practiced without several of the details of the embodiments described below with reference to  FIGS. 1-7 . 
         [0013]    In situ chemical oxidation (ISCO) technology includes a group of chemical processes for treating contaminated soils and groundwater. Permanganate, catalyzed H 2 O 2  propagations (CHP), and activated persulfate (e.g., Na 2 S 2 O 8 ) are oxidants that may be used in ISCO processes. Each of these oxidants has limitations. For example, permanganate has limited reactivity and may be consumed by natural organic matter. CHP is characterized by rapid hydrogen peroxide decomposition in the subsurface, which can limit contact period with contaminants. 
         [0014]    Activated persulfate has a number of advantages over permanganate and 
         [0015]    CHP. Unlike permanganate, persulfate activation generates a suite of reactive oxygen species that can oxidize and/or otherwise degrade many organic contaminants. In addition, persulfate is more stable than hydrogen peroxide in subsurface soil. Persulfate can persist for weeks to months instead of hours to days for hydrogen peroxide to allow its transport down-gradient and increase the potential contact with contaminants. 
         [0016]    To the best knowledge of the inventor, activation mechanisms of persulfate in subsurface soil are not well understood. Common persulfate activators include sodium hydroxide (NaOH) or transition metals, e.g., iron (II). However, both activation techniques have certain drawbacks. Without being bound by theory, it is believed that the iron (II) activation of persulfate is similar to a Fenton initiation reaction in which iron (II) mediates the decomposition of persulfate to sulfate radicals (SO 4   •− ) and sulfate anions (SO 4   2− ) as follows: 
         [0000]        − O 3 S—O—O—SO 3   − +Fe 2+ →SO 4   •− +SO 4   2− +Fe 3+   (1)
 
         [0000]    Sulfate radicals can then react with water to generate hydroxyl radical (OH • ): 
         [0000]      SO 4   •− +H 2 O→OH • +SO 4   2−   (2)
 
         [0000]    In addition to sulfate radicals and hydroxyl radicals, reductants or nucleophiles (e.g., superoxide (O 2   − ) or alkyl radicals) have been detected in activated persulfate systems. 
         [0017]    There are certain limitations of using iron (II) to activate persulfate. First, the iron (III) that forms in reaction (1) precipitates as an iron hydroxide at pH&gt;4. As a result, an acidic medium is needed to start and/or sustain the activation. Secondly, unlike CHP systems in which iron (III) is reduced to iron (II) after formation, iron (III) is stable in persulfate systems, and thus the initiation reaction may stall. 
         [0018]    It is also believed that a base (e.g., sodium hydroxide) can activate persulfate by first promoting base-catalyzed hydrolysis of persulfate to form hydroperoxide ( −O   3 S—O—O—SO 3   − H + ) which then reduces another persulfate molecule to form a sulfate radical and a sulfate anion. Oxidation of hydroperoxide results in the formation of superoxide. Although such a system has the potential to be highly reactive, base-activated persulfate reaction is very slow. Also, base-activated persulfate reaction eventually stalls, resulting in failure of the ISCO system. Though persulfate has potentials as an ISCO oxidant, conventional persulfate activation techniques may not be effective. 
         [0019]    The present technology is directed to activation of a peroxygen compound (e.g., sodium persulfate) or mixtures thereof in an oxidation system containing an oxygenated organic compound. In particular, embodiments of the present technology use an oxygenated organic molecule (e.g., sugar) as an activator to initiate, maintain, and/or propagate degradation or decomposition of the peroxygen compound. As a result, reactive radicals may be formed for oxidation of chemical contaminants such as VOCs, SVOCs, herbicides and pesticides in contaminated soils and water. 
         [0020]    The present technology may be applied in remediation of earth, sediment, clay, rock, and the like (hereinafter collectively referred to as “soil”) and groundwater (i.e., water found underground in cracks and spaces in soil, sand and rocks), process water (i.e., water resulting from various industrial processes), or wastewater (i.e., water containing domestic or industrial waste) contaminated with VOCs, SVOCs, pesticides, herbicides, and/or other contaminants. In addition, the present technology may also be applied to degrade contaminants in sludge, sand, and/or tars. 
         [0021]      FIG. 1  is a flowchart illustrating a process  100  for oxidizing a contaminate In accordance with embodiments of the present technology. As shown in  FIG. 1 , the process  100  includes contacting the contaminant with a oxidation system comprising a peroxygen compound at stage  102 . The contaminant may be present in an environmental medium including soil, groundwater, process water, and/or wastewater. As used herein, a “peroxygen compound” generally refers to a chemical compound having at least one oxygen-oxygen single bond. 
         [0022]    The peroxygen compound can be generally water soluble and include at least one of sodium persulfate, potassium persulfate, ammonium persulfate, other monopersulfates and dipersulfates, and mixtures thereof. The concentration of the peroxygen compound can be about 0.5 mg/L to about 250,000 mg/L, or other suitable values based on particular treatment application. In one particular example, sodium persulfate (Na 2 S 2 O 8 ) can be introduced into contaminated soil or other environmental media. In other embodiments, a mixture containing persulfate (Na 2 S 2 O 8 ) can be introduced into contaminated soil or other environmental media. 
         [0023]    As shown in  FIG. 1 , the process  100  also includes activating the peroxygen compound with an oxygenated organic compound at stage  104 . The phrase “oxygenated organic compound” is used herein to refer to a monomeric or oligomeric carbon containing compound having at least one of an alcohol, ketone, carboxylic acid, ester, anhydride, or other oxygen bearing functional groups. Examples of oxygenated organic compound can include sugars (e.g., glucose, fructose, lactose, and galactose), carbohydrates, acetone, sodium pyruvate, pyruvate acid, citrate, 1-propanol, 2-propanol, t-butyl alcohol, formaldehyde, 2-butanone, 2-pentanone, 2-heptanone, oxalic acid, acetoacetic acid, malic acid, succinic acid, 1-pentanol, 2-pentanol, 3-pentanol, acetaldehyde, propionaldehyde, butyraldehyde, levulinic acid, isobutanol, and mixtures thereof. 
         [0024]    In certain embodiments, a mole ratio of the peroxygen compound to oxygenated organic compound can be about from 1:1000 to about 1000:1. In other embodiments, the mole ratio can be from about 500:1 to about 1:500, about 250:1 to about 1:250, about 100:1 to about 1:100, about 50:1 to about 1:50, about 1:20 to about 20:1, or other suitable values. Optionally, in certain embodiments, a pH modifier may also be introduced at stage  105 . The pH modifier may include an acid, a base, a buffer, and/or other suitable compounds or compound mixtures capable of maintaining a target pH (e.g., greater than about 10) in an environmental medium. In other embodiments, the pH modifier may be omitted. 
         [0025]    The process  100  can then include decomposing the peroxygen compound to generate oxidizing radicals at stage  106 . Based on conducted experiments discussed below, the inventor has recognized that the oxygenated organic compound can activate and/or otherwise facilitate decomposition of the peroxygen compound. In one example, sugar was observed to activate the decomposition of a persulfate salt to generate sulfate radicals as follows: 
         [0000]        −O   3 S—O—O—SO 3   − +sugar→SO 4   •− +SO 4   2−   (3)
 
         [0000]    The generated sulfate radical can then react with water to generate hydroxyl radical (OH • ) as discussed above in reaction (2). In addition, other oxidizing radicals, reductants, or nucleophiles (e.g., superoxide or alkyl radicals) may also be generated. 
         [0026]    The process  100  can then include oxidizing the contaminant with the generated oxidizing radicals. Example contaminants that may be oxidized can include chlorinated solvents such as trichloroethylene (TCE), vinyl chloride, tetrachloroethylene (PCE), methylene chloride, 1,2-dichloroethane, 1,1,1-trichloroethane (TCA), carbon tetrachloride, chloroform, chlorobenzenes. Other example VOCs and SVOCs that may be oxidized with embodiments of the oxidation system can include benzene, toluene, xylene, ethyl benzene, ethylene dibromide, methyl tertiary butyl ether, polyaromatic hydrocarbons, polychlorinated biphenyls, pesticides and/or herbicides phthalates, 1,4-dioxane, nitrosodimethyl amine, chlorophenols, chlorinated dioxins and furans, petroleum distillates (e.g., gasoline, diesel, jet fuels, fuel oils). 
         [0027]    In certain embodiments, oxidizing the contaminant may be carried out in situ, i.e., in the physical environment where the contaminant(s) are found. In other embodiments, oxidizing the contaminant may be carried out ex situ by removing a contaminated medium from an original location and treating the removed contaminated medium at a different location. In any of the foregoing embodiments, contacting the contaminant can include injecting the peroxygen compound and/or the oxygenated organic compound into the contaminated medium. 
         [0028]    In any of the foregoing embodiments, the amount of the introduced peroxygen compound and/or oxygenated organic compound may be adjusted to reduce the concentration of the contaminants in the environmental medium to a desired level. In certain embodiments, oxidizing the contaminant can also include adjusting an injection rate of the peroxygen compound based upon hydrogeological conditions of the contaminated medium, e.g., the ability of the oxidation system to displace, mix, and disperse with existing groundwater and move through the contaminated medium. In other embodiments, the injection rate may also be adjusted to satisfy an oxidant demand and/or chemical oxidant demand of the contaminated medium. In further embodiments, the injection rate may be adjusted based on other suitable conditions. 
         [0029]    Even though the process  100  in  FIG. 1  is shown as having activating decomposition of the peroxygen compound with the oxygenated organic compound subsequent to contacting contaminant with the peroxygen compound, in other embodiments, the oxygenated organic compound may be introduced into the environmental medium to active the peroxygen compound in combination with the peroxygen compound, sequentially before, or in repeated sequential applications to the peroxygen compound introduction. In further embodiments, the peroxygen compound and the oxygenated organic compound may be combined into a stable form (e.g., granule, powder, or other solid form) and prepared before introduction into the medium by adding a solvent (e.g., water) or other suitable compounds. 
       Experiments 
       [0030]    Sodium hydroxide (reagent grade, 98%), sodium bicarbonate, nitrobenzene, potato starch, and hexane (&gt;98%) were obtained from J.T. Baker (Phillipsburg, N.J.). Sodium persulfate (Na 2 S 2 O 8 ) (reagent grade, &gt;98%), magnesium chloride (MgCl 2 ) (99.6%), and hexachloroethane (HCA) (99%) were obtained from Sigma Aldrich (St. Louis, Mo.). A purified solution of sodium hydroxide was prepared by adding 5-10 mM of MgCl 2  to 1 L of 8 M NaOH, which was then stirred for a minimum 8 hours and passed through a 0.45 μM membrane filter. Sodium thiosulfate (99%), potassium iodide, methylene chloride, and mixed hexanes were purchased from Fisher Scientific (Fair Lawn, N.J.). Deionized water was purified to &gt;18 MΩ·cm. Nitrobenzene, which has a high reactivity with hydroxyl radicals (kOH•=3.9×10 9  M −1 s −1 ) and negligible reactivity with sulfate radicals (kSO 4   •− =10 6  M −1 s −1 ), was used to detect hydroxyl radicals. HCA was used as a reductant probe. 
         [0031]    All reactions were conducted in 20 mL borosilicate vials capped with polytetrafluoroethylene (PTFE) lined septa. Each reaction vial contained sodium persulfate, an oxygenated organic compound (e.g., glucose) used as an activator, and the selected probe (1 mM of nitrobenzene or 2 μM of hexachloroethane). Some reactions also contained a base (e.g., NaOH). At selected time points, sodium persulfate was measured using iodometric titrations, and the residual probe concentration was analyzed with gas chromatography (GC) after extracting the contents of the reactor with hexane. 
         [0032]    Hexane extracts were analyzed for nitrobenzene using a Hewlett Packard Series 5890 GC with a 0.53 mm (id)×15 m SPB-5 capillary column and flame ionization detector (FID). Chromatographic parameters included an injector temperature of 200° C., detector temperature of 250° C., initial oven temperature of 60° C., program rate of 30° C./min, and a final temperature of 180° C. Hexane extracts were analyzed for HCA using a Hewlett Packard Series 5890 GC with electron capture detector (ECD) by performing splitless injections onto a 0.53 mm (id)×30 m Equity-5 capillary column. Chromatographic parameters included an injector temperature of 220° C., detector temperature of 270° C., initial oven temperature of 100° C., program rate of 30° C./min, and a final temperature of 240° C. A 6-point calibration curve was developed using known concentrations of nitrobenzene or hexachloroethane solutions respectively. Sodium persulfate concentrations were determined by iodometric titration with 0.01 N sodium thiosulfate. 
         [0033]    The results of  FIGS. 2   a - 7  demonstrate that the reactivity of persulfate can be enhanced (and controlled) by the addition of an oxygenated organic compound as an activator.  FIG. 2   a  shows hydroxyl radical generation (quantified through nitrobenzene degradation) for a range of base to persulfate ratios. As shown in  FIG. 2   a,  persulfate activation increased with increasing basicity; however, glucose activation of persulfate was significant even with minimal base addition.  FIG. 2   b  shows hydroxyl radical generation in systems containing a base and no glucose addition. As shown in  FIG. 2   b,  minimal persulfate activation was observed when no glucose was added. 
         [0034]    The results demonstrated that the addition of glucose resulted in increased degradation of the hydroxyl radical probe nitrobenzene, relative to base-activated persulfate. Even more surprising results were found using the reductant probe hexachloroethane (HCA) as shown in  FIG. 3 . As shown in  FIG. 3 , reductants such as superoxide or alkyl radicals were generated by glucose activation of persulfate. 
         [0035]    Degradation of the nucleophile/reductant probe hexachloroethane with persulfate and glucose addition, but without the addition of base, is shown in  FIG. 4 . The glucose-activated persulfate system is effective without pH adjustment, although some base might be needed to maintain pH neutrality. The decomposition of persulfate in glucose-activated persulfate systems is shown in  FIG. 5 . The results demonstrate that higher glucose amounts may not consume large masses of persulfate. Degradation of the nucleophile/reductant probe hexachloroethane with additions glucose, fructose and galactose is shown in  FIG. 6 . The results demonstrate that glucose, fructose, and galactose are all effective in activating persulfate. 
         [0036]    Pyruvate was also investigated as a keto acid for activation of persulfate at neutral pH. Hexachloroethane was used as a nucleophile/reductant probe in aqueous solutions containing 0.5 M persulfate and 5 mM pyruvate and 0.5 M persulfate and 50 mM pyruvate. Control systems included hexachloroethane in deionized water and in 0.5 M persulfate without the addition of pyruvate. All systems were adjusted to pH 7. The results, shown in  FIG. 7 , demonstrate that pyruvate activates persulfate at neutral pH using both 5 mM and 50 mM pyruvate. Furthermore, it is also believed that a rate of persulfate activation is inversely proportional to the chain length of a keto acid. As such, the rate of persulfate activation can potentially be controlled by selecting the appropriate keto acid as an activator. 
         [0037]    From the foregoing, it will be appreciated that specific embodiments of the technology have been described herein for purposes of illustration, but that various modifications may be made without deviating from the disclosure. In addition, many of the elements of one embodiment may be combined with other embodiments in addition to or in lieu of the elements of the other embodiments.