Detoxification and Destruction of PCBs, CAHs, CFCs, and

Chapter 14

Detoxification and Destruction of PCBs, CAHs, CFCs, and Halogenated Biocides in Soils, Sludges, and Other Matrices Using Na/NH Downloaded by PENNSYLVANIA STATE UNIV on July 17, 2012 | http://pubs.acs.org Publication Date: October 10, 2003 | doi: 10.1021/bk-2004-0863.ch014

3

Charles U . Pittman, J r . Department of Chemistry, Mississippi State University, Mississippi State, MS 39762

Many toxic or hazardous organic compounds contain one or more chlorine atoms which are central to their biological effects. Examples include pesticides, such as lindane, 1, and Mirex, 2, polychlorinated biphenyls (PCBs), 3 and 4, chlorinated aliphatic hydrocarbons (CAHs), 5, tetrachloroethylene, 6, dioxins, 7, and chlorinated aromatic compounds such as 1,2-dichlorobenzene, 8, or pentachlorophenol, 9. Although not toxic, chlorofluorocarbons such as, 10, are environmental hazards since they play an important role in distraction of ozone in the stratusphere. Since many of these compounds are widely dispersed in the environment, methods to remediate locations where they are pollutants and destroying them in cost efficient ways is an important goal. Processes to destroy chemical warfare agents, non-halogenated pesticides, polynuclear aromatic hydrocarbons, toxic metal compounds and germicides are also needed.

CI

CI

1

2

3

© 2004 American Chemical Society In Pesticide Decontamination and Detoxification; Gan, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

181

Downloaded by PENNSYLVANIA STATE UNIV on July 17, 2012 | http://pubs.acs.org Publication Date: October 10, 2003 | doi: 10.1021/bk-2004-0863.ch014

182

Many methods to destroy these compounds are available including incineration, wet air oxidation, catalytic dehydrochlorination, reaction with superoxide, photolysis in the presence of hydrogen donors, transition metalpromoted reductive dechlorinations using sodium borohydride or alkoxyborohydrides, electrolytic reductions, hydrogenolyses, iron-pranoted dechlorination and thermolysis over solid bases such as CaO/Ca(OH) . All these methods have drawbacks, especially when the toxic pollutants are already distributed in the soil, sludges, ground or surface waters, etc. When concentrated samples are available, even simple methods like combustion require special treatments to remove the HCI generated. Incineration of PCBs and other chlorinated organic compounds can produce small amounts of highly toxic polychlorinated dibenzofurans and dibenzodioxins (e.g. 7). Therefore, methods which mineralize the chlorine as simple chloride salts could have a distinct advantage. High temperature reductions with NaBH4 will reduce PCBs. Lower temperature sodium-based reductions in hydrocarbon media will generate NaCl and they were examined extensively. Unfortuately the kinetics of reduction were slow. Direct thermolysis of PCBs over CaO/Ca(OH) or MgO/Mg(OH) will mineralize the chlorine but high temperatures are needed for efficiency and, when applied in soils, migration of the PCBs may occur. Alkali or alkaline metals dissolve readily in liquid ammonia to give blue solutions of the metal cation and solvated electrons. Independently,our group " and Commodor Solution Technologies " reasoned that solvated electron solutions would rapidly dechlorinate chlorinated organics and this 1,2

3

4

5

6

7,8

9

10

11

12

2

13

14

15,16

2

2

12

30

17

23

24

29

In Pesticide Decontamination and Detoxification; Gan, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

183 concept was then demonstrated using both calcium and sodium. These dechlorinations are very fast at room temperature and even -60 °C! Evidence was presented they often occur at diffusion controlled rates for chlorinated aromatic compounds and CAHs. While it has long been known that dechlorination of organic compounds will occur readily in solvated electron ammonia solutions, the potential application of this reaction to remediation of wet soils or sludges was simply never considered. Instead, it was assumed solvated electrons (or Na) would react rapidly with water. Thus, it was thought that large amounts of sodium (or Ca etc) would be consumed, making such reductions impractical. This assumption has now been shown to be wrong. " Table 1 shows that PCB-contaminated soils can be readily remediated. Simply slurrying the soil in N H followed by adding Na or Ca at ambient temperature results in excellent remediation. Furthermore, after adding Na or Ca, the reduction is completed in seconds or a few minutes. All the soils in Table 1 were wet. The last three examples contained more than 20% wt. water. The amount of sodium required to reach the same degree of decontamination of polluted soils was frequently higher when Na was first dissolved in N H / followed by slurrying the soil. During the time it takes for the NH /solvated electron solution diffuse into the soil (or for intercalated toxic compounds to extract into the NH ), competitive reactions of solvated electrons with H 0 or N H (catalyzed by Fe or dissolved 0 ) occur. 19

30,31

30,31


17

25

3

3

s

3

3+

2

3

2

17

Table 1. Destruction of PCBs in Soils ' Treatment

Na/NH Na/NH Na/NH Ca/NH Ca/NH Ca/NH

Soil Type

Sand, silt, clay Sand, silt Sandy, clay Clay Sandy Organic

3 b 3

3 Cld 3 d 3 d 3

2021

23

' Using Na/NH or Ca/NH 3

a 3

Pre-treatment Post-treatment Destruction Efficiency PCB Level PCB Level (ppm) (ppm) (%) >98.4 99.8 99.8 99.8 2.0 2140 >99.9 1.6 6200 >99.9 0.16 660

a

Pre-weighed soil samples (75-100g) were slurried 10-20 min. at room temperature in liquid NH in a 1 liter reactor. About 300cc of liquid NH containing 1.27 to 3.3% wt. Na was used. Single treatment. No more Na or Ca was added. 3

b

3

Reduction conducted at -33 °C.

c

Montmorillinite clay-rich soilfromStarkville, MS. Each soil was partially dried and then 20% wt.% water was added in addition to the water left after partial drying.

d


184

How do These Dechlorinations Work? Aromatic dechlorinations in solvated electron solutions proceed (see Scheme 1) by rapid transfer of the solvated electron (e ') to the chlorinated aromatic ring to give a radical anion, 11, where the added electron goes into the LUMO (a π orbital) of the aryl ring. This is followed by loss of chloride. The resulting aromatic radical, 12, acquires a second electron to generate anion 13, which is then protonated to give reduction product 14. This process continues successively until all the chlorines are replaced to give biphenyl, IS. Continued, though slower, reduction to phenyl cyclohexene, 16, can occur depending on the conditions. Moisture in the soil and acidic soil functional groups provide ^NKU irons which efficiently transfer protons to intermediate anions.


s

Scheme 1

The transfer of a solvated electron to a chlorinated aliphatic hydrocarbon occurs with the simultaneous loss of chloride because there are no available π* orbitals available. Therefore, the electron must add to a carbon-chlorine antibonding sigma (σ) orbital, thereby breaking the C-Cl bond. This process is called dissociative electron transfer (see Scheme 2). The resulting radical can add a second solvated electron followed by loss of another chloride, as long as a chlorine is still present on that carbon. This is illustrated in Scheme 2 for the dechlorination of C C I 4 in Na/NH . The reduction of was shown to proceed at diffusion controlled rates. For example, when C C I 4 is treated with 2 or 3 equivalents of Na in N H only C H and CC1 were detected as products. No CHC1 , CH C1 or CH C1 were 32

CCI4

3

19

3

4

4

3

2

2


3

185 Scheme 2

R-Cl

β

1^

[κ*----α ^"

^


RH

CCI4

±*L*

.

+

c

r

- R °

·οα + Clι+ I — - C C I 3 3

Na/NH 4NaCl + C H

R

3

e

• :cci

2

+ c r

Further reductive

4

dechlorination found. Thus, four reductions take place on the CCU molecules in the vacinity of sodium particles (added to C C I 4 / N H 3 solutions) before the partially dechlorinated intermediates can diffuse away. This was observed even when solutions were vigorously stirred. Similar experiments were carried out with 3,4-dichlorotoluene. Again, no monochlorotoluene was found. This was true whether or not water was present in the reductions (equation 1). By adding more sodium, complete reduction to toluene was achieved. 19

19,22

5 C 1

ÇH3

+

I

II I

(i) recovered

40%

Why Do These Dechlorinations Work In The Presence of Excess Water? Solvated electrons are destroyed at exceptionally fast rates in water. In pure water, the half-life of the solvated electron is only about lOOpsec! Furthermore, in a soil containing 20 wt.% H 0 and contaminated with 1000 ppm of PCB (using an average of 4 chlorines per PCB molecule) there are only 3 X 10" moles of PCB present for every mole of water. At 1.0 ppm of PCB there are 33,34

2

4


186 7

only 3 X 10* moles of PCB per mole of water. In other words, water molecules outnumber PCB molecules by 3.3 X 10 /1 and 3.3 X 10 /1 at contamination levels of 1000 ppm and 1.0 ppm, respectively. At first glance, it seems impossible that solvated electrons could dechlorinate PCBs, or other chlorinated aromatics, without an enormous consumption of Na (or Ca or other alkali or alkaline earths). So how is it possible these remediations are feasible? The answer is based on two facts. First, in pure liquid N H the half-life of a solvated electron is about 300h. A portion of this stabilizing effect remains present even in 20% H O/80% NH , where the solvated electron's half-life is about 100 sec. The second fact is that electron transfer to chlorinated organic compounds occurs very fast, hence dechlorinations are very fast. Therefore, the relative kinetics greatly favor dechlorination even though the mole ratios greatly favor water. The net effect is modest loss of the metal (Na, Ca —) to side reactions with water. The data summarized in Table 2 shows clearly that both aliphatic and aromatic model compounds can be reduced by Na/NH without excessive consumption of Na when excess water is present. The minimum amount of Na required for complete dechlorination is express in two ways: (1) moles of Na/mole of chlorinated compound and (2) moles of Na/mole of chlorines present. This is shown for no water and for both 20 and 50 mole excesses of water. This data illustrates that only a modest increase in Na consumption occurred. Soils and sludges with very high water contents can be partially dewatered before treatment. Also, if the contaminated matrix is first extracted by preslurrying in liquid NH , most of the chlorinated pollutants move into the N H which is an excellent extractant. N H nicely swells clays to allow intercalated PCBs, etc. to move into the solvent phase. Thus, upon addition of Na, the solvated electrons formed do not need to diffuse into clay layers or other soil aggregates prior to encountering the chlorinated pollutant. This reduces the time frame in which the reaction with water (equation 2) competes with reductive dechlorination. In over ten years of experience with soil and sludge remediations we have found remarkably little hydrogen was generated. 3

6

3

34

2

3


34,35

19,21,22

3

3

3

3

H 0 + ef 2

^

H * +"OH

( 2

17

)

23

Extensive studies at Mississippi State University " and Commodor Solution Technologies " have indicated the efficiencies of Na and Ca are about equal when water is absent. However, the efficiency of Ca decreases relative to that of Na as the amount of water is increased. Neither Li nor Κ was as efficient as Na. ' Laboratory reactions showed that CI and Br are readily removed. ' Aromatic fluorines are also readily removed unless the substrate is a phenol, 24

19

29

20


19 20

187 Table 2. Minimum amount of Na required to completely dechlorinate model compounds at 25 °C. Effect of added water. Substrate

Να/Substrate Mole Ratio and (Na consumed per CI Removed)Requiredfor Complete Dechlorination. Effect of Water H Q/Substrate Mole Ratio 2

NoH 0 1.5(1.5) 2.8(1.4) 5.0(1.25) 4.6(1.53) 3.6(1.2) 4.6(1.15)


2

2-Chloro-/?-xylene 1,2-Dichlorobenzene 1,2,3,4-Tetrachlorobenzene 2,4,6-Trichlorophenol 1,1,1 -Trichloroethane Carbon tetrachloride

20/1 2.4 (2.4) 4.5 (2.3) 7.0(1.75) 8.0 (2.66) 4.5(1.5) 5.5 (1.38)

50/1 2.5 (2.5) 5.0 (2.5) 8.6(2.15) 10.0 (3.33) 5.1 (1.7) 6.4(1.6)

20

where it was found that fluorine in the para position is removed very slowly. Aliphatic fluoro compounds are not readily defluorinated relative to side reactions with water. Table 3 summarizes some representative results.

Example Remediation Studies Many different soil contaminates have been treated including PCBs, CAHs, dioxins, furans, pesticides, chlorinated solvents, hexachlorobenzene etc. The New Bedford Harbor Sawyer Street site in Massachusetts is a superfund site due to PCB contamination of river sediements. Commodore Solution Technologies conducted a demonstration study where river sediment was washed with diisopropylamine (using the RCC B. E. S. T.™ process) which produced an oil concentrate containing PCB levels of 32,800 ppm. Dioxins/furans (TEFs) were also present at 47,000 ppt. This concentrate was treated with Na/NH (Table 4). After treatment, the PCB level was 1.3 ppm and the dioxin/furans were also remediated. Thus, after treatment, the residue was well below regulatory requirements for disposal in non-hazardous waste landfills. Na/NH treatment also removed lead, arsenic and selenium from the concentrate and these were recoveredfromthe ammonia recycle unit. 3

3


188

Table 3. Activity of different metals and halides during solvated electron dehalogenations in liquid NH>


Substrate

H 0/Substrate (mole ratio) 2

2-Chloro-/?xylene

Metal/Substrate (Mole Ratio) Required For Complete Dehalogenation Ca Κ Na Li 2.0

50/1

6.0 (76%)

2

a

F 2-X-/>-xylene

4-X-phenol

b

1.0 a

5.7 2.5 (23%) 2.5 Halogen Substituent Br α

NoH 0 2

2.0

1.5

1.5

50/1

4.0

2.5

2.5

NoH 0

2.0(18%)

2.0

2.2

4.5

6.0

2

b

50/1 a

1.8

1.5

NoH 0

Amount of substrate dechlorinated after rxn. was completed. Only 18% defluorination occurred in 20h.

Table 4. Na/NH Treatment of PCB- and Dioxin-Contaminated Sludge from New Bedford Harbor, M A . 3

Contaminant PCB Dioxin/Furan Mercury Lead Selenium Arsenic

Pre-treatment (ppm) 32,800 47 0.93 73 2.5 2.8

Post-treatment (ppm) 1.3 0.012 0.02 0.2 0.2 0.1


189 Na/NH was effective for destroying hexachlorobenzene in soils. Sandy soil containing 67.6 ppm of hexachlorobenzene from a site near Las Vegas, NV was treated with Na (4% wt.)/NH . The remediated soil contained less than 1 ppm of hexachlorobenzene. GC/MS analysis could not detect chlorinated products in the treated soil. Contaminated transformer oils and cutting fluids have been remediated using Na/NH (See Table 5). Oils containing >20,000 ppm of PCBs were detoxified to levels below 0.5 ppm using N H containing 2 to 4% wt, of Na. Just as impressive, Na/NH successfully remediated dioxins present in waste oil from the McCormick and Baxter superfund site in California. These dioxin levels were reduced from 418,500 parts per trillion (ppt) to only 2.3 ppt and furansfrom14,120 ppt to 1.3 ppt by a single treatment. 3

3

3

3


3

Table 5. Destruction of PCBs in Oils Using Na/NH Oil

Temperature (°C) 16 40 40 40

Motor Oil Transformer Oil Mineral Oil Hexane a

Pre-treatment (ppm) 23,339 509,000 5,000 100,000

3

Post-treatment (ppm)

Detoxification and Destruction of PCBs, CAHs, CFCs, and

Recommend Documents