Chemical Degradation Methods for Wastes and Pollutants - Chapter 8 pot

8 Solvated Electron Reductions: A Versatile Alternative for Waste Remediation Gerry D. Getman Commodore Solution Technologies, Inc., Marengo, Ohio, U.S.A. Charles U. Pittman, Jr. Mississippi State University, Mississippi State, Mississippi, U.S.A. I. INTRODUCTION Polychlorinated biphenyls (PCBs) and other chlorinated aromatic com- pounds are distributed in soils, sludges, estuaries, etc., at over 400 sites in the United States alone. Chlorinated aliphatic hydrocarbons (CAHs), widely used for degreasing and cleaning of engines, auto parts, and electronic com- ponents, are serious contaminants at 358 major hazardous wastes sites in the United States. CAHs migrate vertically through soils to form dense non- aqueous phase liquids (DNAPLs) on aquifer bottoms. Chlorinated organics are also frequently found in mixed wastes (those containing radioactive con- taminants). The Environmental Protection Agency (EPA)’s Emergency Re- sponse Notification System recorded almost 3600 accidents involving PCBs between 1988 and 1992. These facts highlight the need to develop methods to decontaminate soils, sludges, and aggregates containing chloroorganic compounds to include both ex situ and in situ methods [1–6]. Portable remediation methods that can be located at the contaminated site are needed to reduce the costs of transporting large volumes of soil to an off-site treat- ment location. TM Copyright © 2003 by Marcel Dekker, Inc. All Rights Reserved. The remediation of soils and DNAPLs has been a high-priority research area at the EPA, Department of Energy (DOE), and Department of Defense (DOD). To give just one example, the DOE’s Hanford site has massive soil and groundwater contamination from a carbon tetrachloride subsurface plume extending for over 70 square miles. More generally, PCBs, CAHs, dioxins, furans, halogenated pesticides, benzenes–toluenes–xylenes (BTX), explosives, chemical warfare agents , and chlorofluorocarbons (CFCs), which are all widely distributed in the environment, must be remediated to meet today’s stringent standards. Vast quantities of soil, sludge, job equipment, adsorbents, process liquids, and building materials must be treated to remove these species, which may be present in parts-per- million (ppm) quantities. Work at the Commodore Solution Technologies, Inc. (Commodore) and the Mississippi State University has now demonstrated a generalized solvated electron technology (SET) to decontaminate (in situ and ex situ) soils or sludges contaminated with PCBs, CAHs, CFCs, explosive wastes, and chemical warfare agents. Furthermore, bulk samples of these chemicals can also be degraded. The early patents of Weinberg et al. [7,8] and the reports by Pittman and Tabaei [9] and Pittman and Mohammed [10] proved that neat PCBs and PCB-contaminated soils containing up to 30% water could be decontaminated in liquid ammonia slurries when treated with Ca/NH 3 or Na/NH 3 at room temperature. PCB destruction efficiencies of >99.9% were achieved in only 30 sec. The products were biphenyl or reduced biphenyls and CaCl 2 or NaCl. The Commodore has developed a total systems approach to such remediation, called Solvated Electron Technology (SETk), and has received a nationwide EPA operating permit for the nonthermal destruction of PCBs in soils, oils, surfaces, and solid ma- terials. The SoLVk process is Commodore’s total remediation process incorporating pretreatments and posttreatments applicable to liquids, so- lids, soils, protective equipment, and job materials. After discussing some basic chemical considerations, this chapter will provide an overview of the technology and specific examples of solvated electron remediations. II. SOLVATED ELECTRON CHEMISTRY FOR ENVIRONMENTAL REMEDIATION: BACKGROUND AND FUNDAMENTALS A. General Description Deep blue solutions of solvated electrons are formed when Li, Na, K, Ca, or other group I and group II metals are dissolved into liquid ammonia (Eq. (1)). These media have long been used to reduce organic compounds. The widely used Birch reduction [11–18], known for 80 years, is employed Getman and Pittman344 TM Copyright © 2003 by Marcel Dekker, Inc. All Rights Reserved. routinely on a commercial scale to accomplish a variety of reductions. Among the many functional groups reduced by this process, chloroorganic compounds are the ones reduced at the highest rates (Eq. (2)). However, this chemistry was never applied to environmental (soil, sludge, DNAPLs) cleanups in the past because of the widely accepted belief that the solvated electron would react rapidly with water. Thus, it was thought that water in environmental samples would consume the solvated electrons, leading to prohibitive costs. This perception was put to rest in early studies that dealt with using solvated electrons for reducing contaminants in environmental samples [7–10]. Indeed, the reduction of water is much slower than de- chlorination, allowing wet soils to be dechlorinated rapidly without undue consumption of metals (Na) through a reaction with water. How can the use of solvated electron solutions to decontaminate soils, sludges, and DNAPLs be feasible in the presence of excess water? The reaction of solvated electrons with water (i.e., e (s) À +H 2 O! 1/2H 2 + À OH) has a far higher kinetic barrier than electron transfer to chlorinated or nitrated organic molecules. Furthermore, when ammonia is present with water, the half-life of the solvated electron dramatically increases. In pure water, the half-life of the solvated electron is short (t 1/2 = f 100 Asec) [19]. However, the transfer of solvated electrons to chlo- rinated organic compounds is much faster. For a 20% solution of water in ammonia, the half-life of the solvated electron increases to about 100 sec [20,21]. In pure ammonia, t 1/2 = f 300 hr [21]. Thus, the desired detoxifica- tion reductions of chlorinated organic molecules will occur much faster than side reactions with water when ammonia is used as the solvent. The transfer of an electron to RCl occurs in f 1 Asec vs. the transfer to H 2 Oto give 1/2H 2 (in 20% H 2 O/80% NH 3 )in f 100 sec. One can estimate that chloroorganics are reduced f 10 7 times faster than water even when the medium contains 20% water. However, significant barriers to the application of SET might still exist. Typically, oxygen and iron will be present in soils and other hazardous wastes. Both Fe 3+ and O 2 catalyze the reaction of solvated electrons with NH 3 to produce hydrogen and amide [22,23], as shown in Eq. (3) [13,22,23]. Also, if solvated electrons must diffuse into soil particles, these electrons could be consumed in a variety of reactions in competition with diffusion and mass transfer. However, extensive work has shown that this need not be the case. For example, slurrying the soil in NH 3 first has several benefits. It reduces particle size, swells clay layers, and preextracts contaminants. Thus, (1) (2) Solvated Electron Reductions 345 TM Copyright © 2003 by Marcel Dekker, Inc. All Rights Reserved. mass transport limitations and diffusional barriers into solid particles may be avoided or reduced greatly. B. Detailed Description of Method Pittman et al. [24] have demonstrated the minimum sodium stoichiometry required to completely dechlorinate a variety of aliphatic, aromatic, and phenolic chlorocarbons in dry NH 3 (l) and NH 3 (l) containing 5-, 20-, and 50- fold molar excesses of water (relative to chlorine atoms). Even in the presence of a 50-mol excess of water, the incremental amount of Na required was modest (see Table 1). The dechlorination of CAHs and chloroaromatics appeared to be diffusion-controlled in reactions where substoichiometric amounts of Na were used [24]. For example, when CCl 4 was reacted with 2EqofNainNH 3 (l), only CH 4 (45%) and CCl 4 (54%) were observed. No monochloromethanes, dichloromethanes, or trichloromethanes were formed, suggesting that the CCl 4 in the vicinity of dissolving Na particles was completely dechlorinated before more CCl 4 could diffuse into the region of the particle (despite rapid stirring). Similarly, treating 3,4-dichlorotoluene with 2 Eq of Na in NH 3 or NH 3 /H 2 O gave 40% toluene and 60% of the starting material, but no monochloro product was observed [24]. Separate studies have observed that metal consumption efficiencies differ depending on the mode of reaction, stirring rate, and metal particle size [24]. Usually, higher efficiencies are observed when sodium is added to preformed sol- Table 1 Sodium Consumption Per Chlorine Removed Required to Completely Dechlorinate Model Compounds in Liquid NH 3 at Room Temperature Na consumed per Cl removed at complete dechlorination Substrate No H 2 O 50 mol H 2 O a 4-Chlorotoluene 1.5 2.5 1,2-Dichlorobenzene 1.4 2.5 1,2,3,4-Tetrachlorobenzene 1.3 2.2 2,4,6-Trichloroethane 1.5 3.3 1,1,1-Trichloroethane 1.2 1.7 Carbon tetrachloride 1.1 1.6 a Moles H 2 O per mole of chlorine. (3) Getman and Pittman346 TM Copyright © 2003 by Marcel Dekker, Inc. All Rights Reserved. utions of chlorinated compounds. When soils are remediated, higher metal efficiencies are often found if the soil is first slurried in NH 3 (l) and then metal is added. Metal efficiencies were similar at 25jC and À55jC, except when substrate solubility limitations occurred at À55jC [24]. Which metal is best suited for remediation work? Both Ca and Na have been examined extensively. Commodore has built recently the SETk and SoLV processes technology around Na, based on extensive commercial experience. Pittman et al. [24] demonstrated recently that the order of metal efficiencies for the dechlorination of aliphatic and aromatic model com- pounds at 25jC was Na>K>Ca>Li in dry ammonia and Na>K,Ca,Li in the presence of a 50-mol excess of water. These laboratory studies were carried out in the absence of soil [24]. The presence of water significantly reduced the efficiency of both Ca and Li, whereas Na and K efficiencies were only modestly reduced. Solvated electrons are extremely powerful reducing agents. In NH 3 (l), they cleave C–N, C–O, C–S, N–N, N–O, P–halogen, S–S, C–halogen (C– X), aromatic rings, and other functions [11–18]. Aromatic halide reactions with solvated electrons were described in 1914 by Chablay [25] and in 1963 in a thesis by Hudson [26]. Kennedy [27] demonstrated that all the halogens were completely stripped from 19 different pesticides (such as atrazine, DDT, paraquat, trifluralin) in Na/NH 3 . The strength of CX bonds in- creases in the order of C–I<C–Br<C–Cl<C–F. MacKenzie et al. [28] found that the reactivities of a-halogenated naphthalenes (x=F, Cl, Br) exhibited no obvious differences in the selectivity to Na/NH 3 . Pittman et al. [24] showed that 3-fluoro-o-xylene required 2 Eq of Na to be completely defluorinated (vs. 1.5 Eq for the chloro derivative) in dry NH 3 . Four equiv- alents of Na were required for a complete defluorination in NH 3 with 50 equivalents of H 2 O. Thus, whereas defluorinations were very fast, competi- tive side reactions required the use of more Na, indicating that defluorination is slower than dechlorination. Phenols are present as phenoxide ions in Na/NH 3 solutions. Therefore, the transfer of an electron into a k-antibonding orbital of a phenoxide ion should be slower than the transfer to benzene. Both chlorophenols and fluorophenols are dehalogenated more slowly than chlorobenzenes or fluo- robenzenes [24]. Fluorophenoxide ions were more difficult to defluorinate. Thus, 4-fluoro-2-chlorophenol could be converted selectively to 4-fluorophe- nol in Na/NH 3 [24]. In contrast, chlorofluorobenzenes lost both Cl and F at close to diffusion-controlled rates [24], indicating that using a stoichiometric deficiency of Na would not give any selectivity to fluorobenzene. Chlorinated aromatic hydrocarbons can be reduced directly to the par- ent aromatic hydrocarbon [13,24,29,30]. The parent aromatic molecule can reduce further to give dihydroaromatics or tetrahydroaromatics [11,12,15], Solvated Electron Reductions 347 TM Copyright © 2003 by Marcel Dekker, Inc. All Rights Reserved. or can react to form dimers and higher molecular weight oligomers [16,31,32] in solvated electron solutions. Aromatic compounds are dechlorinated by the general mechanism shown in Sch. 1. Electron transfer to a k-antibonding orbital forms an aro- matic radical anion, which then ejects Cl À to give an aromatic radical. This radical picks up a second electron to give a very basic j-anion, which abstracts a proton either from NH 3 or from a more acidic source like water, when water is present. If water is not present, then an À NH 2 anion can be formed. The presence of À NH 2 can lead to the formation of aminated products via the benzyne mechanism. Aminated products were formed in dry NH 3 but not when water was present [24]. A further reduction via radical anion formation and proton abstraction can give dihydroaromatics or tetrahydroaromatics, or dimerization may occur. In soils, both water and Scheme 1 Getman and Pittman348 TM Copyright © 2003 by Marcel Dekker, Inc. All Rights Reserved. other functions that are more acidic than NH 3 are present. Thus, halogen- ated aromatics will not produce aniline derivatives during Na/NH 3 treat- ments in these systems. Aliphatic halides are reduced by dissociative electron transfer. A solvated electron is transferred into an antibonding j-orbital, causing the simultaneous loss of Cl À . This is a single-step, concerted process as illustrated by the secondary deuterium isotope effect studies of Holm [33]. He and Pittman [29] showed that 1-fluorononane was defluorinated sig- nificantly slower than 1-chlorononane in M/NH 3 (M=Na, Ca, Sr, Ba), but this defluorination was accelerated remarkably in the presence of TiCl 4 . Pittman and He [30,31] have studied the Na/NH 3 remediation of organic soils contaminated with 3000–5000 ppm of such CAHs as CH 3 CCl 3 . With excess Na, the remediation to a level of 1 ppm is readily possible. The effi- ciency (moles Na consumed/moles Cl removed) was high at high CH 3 CCl 3 concentrations. However, this efficiency drops as the amount of CH 3 CCl 3 remaining in the soil decreases. As CH 3 CCl 3 decreases to below 20 ppm, competitive reactions require substantial excesses of Na to lower the residual CH 3 CCl 3 to 1 ppm or below. III. TREATMENT OF ENVIRONMENTAL SAMPLES A. Laboratory-Scale and Commercial-Scale Setkkk Experiments In early works, contaminated soils were slurried in NH 3 (l) at ambient temperature and, after premixing, a weighed quantity of solid Na or Ca was dropped directly into the stirring slurry. The metal quickly dissolved. Conductivity and calorimetry showed that the reactions were completed within a few seconds. Reactions of neat PCBs and CAHs are exothermic, but NH 3 reflux can be used to control the exotherm. In soil, sludge, and related decontaminations, the pollutants are diluted in the matrix and are then more highly diluted in the NH 3 slurry. Thus, exotherms are not a problem. Typically, the volume of NH 3 used is twice that of the soil volume. Table 2 demonstrates the remediation of f 100 g of PCB-contami- nated soil samples by Ca/NH 3 (excess Ca). In clay, sandy, or organic soils, the destruction efficiencies were >99.9%. Similar studies were done with sodium. Calcium can be used effectively but becomes far less efficient than sodium as the amount of water in NH 3 increases [24]. Commodore has scaled up these treatments and has developed several process variations depending on the nature of the material being remedi- ated. Modules are tailored to each particular remediation site to achieve the highest cost-effectiveness. Mobile equipment is available at the site in the SoLV process, which eliminates the expense of transporting the hazardous Solvated Electron Reductions 349 TM Copyright © 2003 by Marcel Dekker, Inc. All Rights Reserved. substrates. Front-end modules to remove water or extract contaminants can be used. A solvated electron treatment module (SETk ) is the centerpiece of each process. Back-end modules are available to recycle NH 3 , to adjust pH, and to concentrate or fix the reaction products, depending on client needs. A commercial 1200-L unit is available to treat PCBs, CAHs, etc., in oil, liquid pesticides, or contaminants, which have been extracted from soil, sludges, or other matrices. Most commercial soil decontaminations operate via extrac- tion first, followed by decontamination with Na. A flow diagram of the process is shown in Fig. 1. A sodium transfer station heats sodium (in shipping drums) to a liquid state and pumps the liquid to the solvator tank. This tank is filled with anhydrous ammonia, which dissolves the sodium. The resulting solvated electron solution is dischar ged to a reactor vessel, where a volume of approximately 65 gal of the solvated solution is maintained. Contaminated liquid (soil extracts, oil, etc.) is pumped to the reactor vessel, where organics are rapidly destroyed. The solution conductivity is monitored continuously. When the conductivity drops to 200 Mho, the Na/NH 3 feed is stopped. The destruction reaction is essentially diffusion-controlled. Removing ammonia vapor controls the temperature and pressure of the vessel. The feed rate is approximately 1600 lb of soil per day. After the reaction, the solution is transferred to a separator using the natural vapor pressur e of the ammonia as the motive force. Ammonia is heated to approximately 125jF and pumped, as a vapor, to a condenser for recycling by a commercial refrigeration subsystem. The treated material is discharged to a storage vessel. After pH adjustment, the product is suitable for on-site or nonhazardous disposal. A more detailed schematic diagram of a multimedia remediation unit is shown in Fig. 2. Table 2 Treatment of PCB-Contaminated Soils with Ca/NH 3 at Room Temperature a Soil matrix Pretreatment PCB level (ppm) Posttreatment PCB level (ppm) Destruction efficiency (%) Clay 290 0.05 >99.9 Clay 29 <0.06 >99.9 Sand 6200 1.6 >99.9 Organic 660 0.16 >99.9 Organic 83 <0.04 >99.9 a Experiments were carried out in a stirred 1.3-L reactor made of steel. Preweighed soil samples were slurried for 10–20 min in liquid NH 3 at ambient temperature. Then a calcium bar was dropped in. The calcium dissolved in a few seconds. The reduction reactions were completed as fast as the calcium dissolved. Getman and Pittman350 TM Copyright © 2003 by Marcel Dekker, Inc. All Rights Reserved. Solids are treated as-is in a 10 ton/day screw reactor in which the contaminated solid and NH 3 (l) are mixed. In most cases, the contaminants are extracted into the ammonia. When sodium dissolves in liquid ammonia, a sodium cation is formed together with a solvated electron. The solutions are deep blue and conducting. Each solvated electron exists over a volume of solution. The solvated electron is best described as a quantum mechanical particle which, due to its tiny mass, is perhaps better thought of in terms of its wave properties. Hence, solvated electrons in NH 3 migrate exceptionally rapidly, penetrating clay layers and other obstructions to reach pollutant molecules that may be otherwise unavailable to less mobile reactants. However, NH 3 also swells soils and sludges and, aided by stirring, effectively extracts pollutants into the solvent medium. Upon the addition of sodium, the extracted pollutant molecules are often reduced at diffusion-controlled rates. Thus, reaction times are very short. Less sodium is consumed by side reactions when pollutants are extracted, leading to higher sodium utility than when the solvated electrons are transported into a solid matrix. Sodium metal is added in both molten or solid form to the NH 3 /solids slurry, and the solvated electrons formed proceed to destroy the contaminants. The NH 3 is recycled and the treated solid is returned to the environment. Wet Figure 1 A SETk process flow diagram. Solvated Electron Reductions 351 TM Copyright © 2003 by Marcel Dekker, Inc. All Rights Reserved. sludges often require water removal by drying, or by using a prewash module. The sludge is subsequently remediated. A back-end module removes ammonia. Following pH adjustment, the material can be disposed of in a nonhazardous waste landfill. B. Scale-Up Remediations of Contaminated Substrates 1. Contaminated Soils and Sludges A wide variety of soils and sludge have now been treated. Soil characteristics that can impact the SETk chemistry include the general soil type, which is treated (loam, sand, silt, and clay), the presence of humic material, the pH value, the soil’s cation exchange capacity, its particle size, the amount of water present, and the iron content. Processes have been engineered to accommodate this wide range of variables [7,8,34]. Some soils can be treated Figure 2 A detailed schematic diagram of a multimedia remediation unit. Getman and Pittman352 TM Copyright © 2003 by Marcel Dekker, Inc. All Rights Reserved. [...]... HQ96GR0267 9-1 and HQGR0 088 ) and by the U.S EPA (grant nos GAD# R826 180 and R -8 2 94210 1-0 ) REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 TM Subsurface Contaminants Focus Area: Technology Summary, Rainbow Series, U.S Department of Energy, Office of Science and Technology, August 1996 (Report No DOE/EM-0296, NTIS Order No DOE/EM-0296; available through EM Helpline: Tel.: + 1 -8 0 0-7 3 6-3 282 ) Dense Non-Aqueous Phase... Total percent extraction Ba 182 183 1 . explosive wastes, and chemical warfare agents. Furthermore, bulk samples of these chemicals can also be degraded. The early patents of Weinberg et al. [7 ,8] and the reports by Pittman and Tabaei [9] and. Formula Destruction efficiency (%) CFC-11 CCl 3 F 99.99 CFC-12 CCl 2 F 2 99.99 HCFC-22 CHCIF 2 99.99 HFC-32 CH 2 F 2 99.99 CFC-113 CCl 2 FCCIF 2 99.99 CFC-114 CClF 2 CClF 2 99.99 CFC-115 CClF 2 CF 3 99.99 HFC-134a CH 2 FCF 3 99.99 HFC-152a. CH 2 FCF 3 99.99 HFC-152a CH 3 CHF 2 99.99 R-500 CFC-12+HFC-152a 99.99 R-502 CFC-115+HCFC-22 99.99 Halon 1211 CBrClF 2 99.99 Halon 1301 CBrF 3 99.99 Halon 2402 CBrF 2 CBrF 2 99.99 a Reductions conducted at 18jC