Reactive Dechlorination of PCE Using Zero Valent Iron Plus

Chapter 10

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 10, 2016 | http://pubs.acs.org Publication Date: November 10, 2002 | doi: 10.1021/bk-2002-0837.ch010

Reactive Dechlorination of PCE Using Zero Valent Iron Plus Surfactants Hyun-Hee

Cho

and Jae-Woo Park

National SubsurfaceEnvironmentalResearch Laboratory (NSERL), Ewha Womans University, 11-1 Daehyon Dong Sedaemun-Gu, Seoul 120-750, South Korea

Zero valent iron (ZVI) is particularly useful as a reductant of chlorinated hydrocarbons because of its low cost and lack of toxicity. Surfactants also have been used widely for decontamination of subsurface soil and groundwater. The objective of this study is to examine the effect of various surfactants on enhancing the dechlorination of perchloroethene(PCE)byZVI.Three surfactants—anionic, nonionic, andcationic—werechosen:sodium dodecylbenzene sulfonate (SDDBS), Triton X-100, and cetylpyridinum chloride (CPC). The rate of dechlorination of PCE using ZVI withCPCand TritonX-100wasmuch higher than withoutout surfactant. The combination of nonionic and cationic surfactants with ZVI increased the degradation of PCE, because PCE that bound with the surfactant in aqueous phase was readily available for dechlorination. Using SDDBS resulted in no observed dechlorination of PCE with ZVI, because no trichloroethene (TCE) was detected. However, PCEwasimmobilized on the sorbed SDDBS phase.

© 2003 American Chemical Society Henry and Warner; Chlorinated Solvent and DNAPL Remediation ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

141

142


Introduction The common occurrence in groundwater of chlorinated compounds, such as perchloroethene (PCE) and trichloroethene (TCE), is due largely to their extensive use by industry and their resistance to degradation under natural conditions (1-3). The release of these compounds into subsurface environments has contaminatednumerous groundwater resources. Permeable reactive barriers, which are gaining increased use in remediating groundwater, most commonly utilize zero-valent iron (ZVI) as the primary treatment material. ZVI is particularly attractive as a reductant of chlorinated hydrocarbons because of its low cost and lack of toxicity. When ZVI is used to support in situ remediation of contaminants, the chemical reactions mediated by ZVI convert toxic compounds into non-toxic forms (1-5). Previous studies have shown that the variability in first-order rates of disappearance for chlorinated compounds primarily reflects differences in reactivity of individual chemical contaminants and in available iron surface area (6). Factors such as pH, contaminant concentration, and flow or mixing rate also affect degradation kinetics to varying degrees (7). Studies of reductive dechlorination of PCE and TCE have shown that dechlorination proceeds in a stepwise fashion, with the degradation rate slowing substantially with each dechlorination step. Hydrogenolysis and reductive βelimination are primary pathways for degradation by ZVI (5, 8, 9). The reaction rate was found to be highest for saturated and perhalogenated organic oxidants, with most systems exhibiting accelerated reaction in the presence of water (10). In aerobic waters, oxygen is consumed and Fe° oxidizes to Fe , Fe , and OH" ions, as shown by the flowing reactions: 2+

2H 0 + 0 + 4e" -> 40H" 2Fe° + 0 + 2H 0 -> 2Fe + 40H" 2

2

2+

2

2

3+

0) (2) +

In anaerobic systems, water is the oxidizing agent, oxidizing Fe to Fe , OH", and H ions, as shown by the reactions below: +

2H 0 + 2e -> H + 40H Fe° + 2H 0 -> Fe + H 4- 20H 2

2

2+

2

2

(3) (4)

Although iron oxidation gives off electrons, the organic pollutants can be reduced if the electrons are received, as shown below in the equation for the reaction of iron and the chlorinated compound designated by RX: +

2+

Fe° + RX + H -» Fe + RH + X

(5)

Henry and Warner; Chlorinated Solvent and DNAPL Remediation ACS Symposium Series; American Chemical Society: Washington, DC, 2002.


143

Surfactants have been used widely for decontamination of the subsurface through soil-washing or contaminant immobilization (11-15). Surfactant molecules, which consist of hydrophilic and hydrophobic moieties, form micelles up to a certain aqueous concentration, called the critical micelle concentration (CMC) (16). The central part of a micelle is hydrophobic, so aqueous solubility is enhanced greatly for poorly water soluble compounds at concentrations higher than their CMCs (/ 7). Bizzigotti et al. (1997) studied the reduction of PCE under the influence of hydroxypropyl-y^cyclodextrin (HP-/2-CD), which enhances the solubility of PCE in both static and flowing water systems. They found that the rate of PCE dechlorination decreased because the PCP molecules were partitioned into the hydrophobic interiors of ΗΨ-β-CD molecules (18). Sayles et al. (1997), who studied the influence of the nonionic surfactant Triton X - l 14 on the rate of 1,1,1 -trichloro-2,2-bis(p-chlorophenyl)ethane (DDT), 1 -dichloro-2,2-bis(pchlorophenyl) ethane (DDD), and l,l-dichloro-2,2-bis(p-chlorophenyl) ethylene (DDE) dechlorination by ZVI (19), found that dechlorination rates for DDT and DDE were independent of the amount of iron, with or without Triton X - l 14, but were higher with Triton X - l 14 than without. The objective of the present study was to investigate the effects of various surfactants at low concentrations, below or about their CMCs, on the dechlorination of PCE by ZVI. Because both immobilization with surfactants and reductive dechlorination with ZVI are vaiable technologies for remediating chlorinated organics in the subsurface, we can imagine a situation in which both these technologies are applied as a combined treatment barrier or zone, in series or mixed. In such a case, the inflow of surfactant, desorbed into groundwater from the sorbed phase, is unavoidable and, even at low concentrations, could affect the reaction by ZVI. Therefore, low concentrations of surfactant were used in this study, compared to previous studies, which focused on flushing with surfactant solutions for washing rather than immobilization (8, 9).

Experimental Methods Iron powder was obtained from Junsei (Japan), and 95% of the particles passed through a 100-mesh sieve. The surface area was 1.96 m /g based on BET adsorption using an ASAP 2000 device. PCE purchased from Fisher was 99% pure. The surfactants used were sodium dodecylbenzene sulfonate (SDDBS) from Aldrich, Triton X-100 from Sigma, and cetylpyridium chloride (CPC) from Aldrich. Table I lists the characteristics of the three surfactants. The buffer solution was 20 mM of 3- (N-morpholino) propanesulfonic acid (MOPS) with pH adjusted to 7. 2



144

To determine the rate of PCE removal, 1.0 g of ZVI (cleaned by washing in IN HC1), 20 mL of buffer solution, and 20 mL of surfactant solution at concentrations below and above CMC were put into 40-mL vials capped with Teflon septa, then PCE was added. The concentrations of Triton X-100 were 10, 50, 100, and 150 mg/L; those of CPC were 5, 10, and 20 mg/L; and those of SDDBS were 10, 100, 500, and 1300 mg/L. The vials were mixed at 250 rpm and 25±1 °C on a shaker table. All duplicate samples were filtered through 0.1 μιη inorganic membrane filter (Whatman). The concentration of PCE was measured with an HPLC [high performance liquid chromatography] (Waters 515). The analyses were conducted at a UV wavelength of 220 nm and with a flow rate of 1.8 mL/min, a mobile phase of 50% water and 50% acetonitrile (Fisher, optima grade), and a 3.9X300 mm β -bondapak CI 8 reverse phase column (Waters). To evaluate the sorption isotherm for each surfactant, 1.0 g of ZVI, 15 mL of surfactant solution at initial concentrations ranging from 50 to 30,000 mg/L, and 15 mL of the buffer solution were put into centrifuge bottles having nominal volumes of 25 mL. The bottles were shaken for 48 hr at 250 rpm and 25±1 °C on a shaker table. After centrifugation, a TOC [ total organic carbon] analyzer (Shimadzu, TOC-5000A) was used to analyze the supernatant for the equilibrium surfactant concentration. The sorbed mass of surfactant was calculated from the difference between the initial and equilibrium aqueous concentrations. All experiments were duplicated.

Table I. Properties of Surfactants Used Surfactant Ionic type M.W.

CMC (mg/L)

Structure CH

Triton X-100

Nonionic

628.0

130

3

CH -C-CII -C-(QV(0CH CH ) . 0H 3

2

CH , R

SDDBS

Anionic

348.5

2

CH

3

1000 Ύο 3

CPC

Cationic

339.5

15

+

C H N (CH ) CH Cr 6

5

2

15

3


2

9

5

145

Results and Discussion


Reductive dechlorination o f P C E using Z V I w i t h surfactants

The removal of PCE using ZVI with Triton X-100 in time series experiments is shown in Figure 1(a). Whereas the removal of PCE rarely was observed in the control test without ZVI, the concentration of PCE decreased in the presence of ZVI both with and without surfactant. Half the initial concentration of PCE remained after 24 hours for all ZVI samples. The decrease in aqueous PCE concentration was greater when ZVI and Triton X-100 were combined than without the surfactant. In the presence of ZVI with Triton X-100, the formation rate of TCE was higher than without Triton X-100 (Figure 1(b)). The amount of TCE formation was less than the amount of PCE removed because the transformation rate of TCE to DCEs was very fast. Figure 2(a) shows the removal of PCE, and Figure 2(b) shows the formation of TCE, using ZVI with CPC. The removal rate of PCE using ZVI with CPC was higher than with ZVI alone. TCE formation from the degradation of PCE by ZVI was observed after 120 hours, whereas TCE formation started after 24 hours when ZVI with CPC was used. The yield of TCE was higher using ZVI with CPC than without CPC. The higher concentration of surfactant further increased PCE removal, but did not result in a corresponding increase in the formation of TCE. Figure 3 shows the removal rate of PCE using ZVI with SDDBS. Ninety percent of the initial concentration of PCE was removed after 24 hours. The rate of PCE removal by ZVI with SDDBS was much higher than without SDDBS. With SDDBS, the anionic surfactant, the dechlorination of PCE by ZVI was not observed, because TCE was scarcely detected. Therefore, the removal of PCE might be explained by sorption of PCE on SDDBS-modified ZVI, rather than by dechlorination. S o r p t i o n isotherms o f surfactant on Z V I

Sorption isotherms of three surfactants on ZVI are shown in Figure 4. The sorption isotherms of Triton X-100 and CPC conformed to the L-type isotherm (Figure 4(a)), whereas the isotherm of SDDBS conformed to the S-type isotherm (Figure 4(b)). The sorption data for Triton X-100 and CPC fitted to the Langmuir isotherm model (Figure 5). A Langmuir isotherm reflects a relatively high affinity between the adsorbate and adsorbent, and usually is indicative of chemisorption. An Stype isotherm suggests "cooperative adsorption," which operates if the adsorbate-adsorbate interaction is stronger than the adsorbate-adsorbent interaction. That is to say, ZVI (Fe°) was converted to ferrous iron (Fe ) in aqueous solution, which greatly supported the interaction between iron and SDDBS. The sequence comparison of the amount of surfactant sorbed to the iron was SDDBS > Triton X-100 > CPC. 2+



146

Figure L Dechlorination of PCE using ZVI with Triton X-l00. - #- control O- Fe only; - V- Fe with 10 mg/L Triton X-100; - V- Fe with 50 mg/L X-100; -ÊÊ- Fe with 100 mg/L Triton X-100; -Π- Fe with 150 mg/L Trito 100.



147

(b)

Time (hr)

Figure 2. Dechlorination of PCE using ZVI with CPC. - Φ- control; - O- F only; - Ψ- Fe with 5 mg/L CPC; - V- Fe with 10 mg/L CPC; -Et- Fe with mg/L CPC.



148

Figure 3. Removal of PCE (a) and TCE formation (b) using ZVI with SDD #- control; - O- Fe only; - Ψ- Fe with 10 mg/L SDDBS; - V- Fe with 1 SDDBS; ÊS- Fe with 500 mg/L SDDBS; -Π- Fe with 1300 mg/L SDDBS


149


35

E q u i l i b r i u m concentration (mg/L) 200

0

2000 4000 6000 8000 10000120001400016000 E q u i l i b r i u m concentration (mg/L)

Figure 4. Sorption isotherms of Triton X-100, CPC (a), and SDDBS (b) on ZVI.


150


12 (a) T r i t o n X I 0 0

10 ^

X

8

W) ^

r = 0.99 2

4 2 0 r. 0.00

ι „, 0.02

ι __ 0.04

i, 0.06

1/Ce(mg/L)

I

1

0.08

0.10

0.12

-i

(b) C P C

ε ι

< ^ r

= 0.89

2

\

- 0.00

j

ι

0.02

0.04

i,

0.06

0.08

- ,

1

0.10

1/Ce ( m g / L )

-1

0.12

0.14

0.16

1

Figure 5. Plot of experimental data of 1/q versus 1/Ce. Symbols are experimental, and lines represent data predicted using the Langmuir m



151

Table II shows the distribution of surfactants between sorbed and aqueous phase. Given 10 mg/L of surfactant concentration, representing less than the CMC, 78.8% of SDDBS was sorbed on the iron surface, while 17.04% of Triton X-100 and 7.64% of CPC were sorbed. The same trend can be seen with concentrations greater than the CMCs: more Triton X-100 and CPC were in aqueous phase and more SDDBS was in sorbed phase. These results show that PCE in aqueous phase was more easily available for dechlorination than was PCE in sorbed surfactant on ZVI. Therefore, PCE was simply immobilized on the sorbed SDDBS phase, although the removal rate of PCE with SDDBS was the highest of the three.

T a b l e II. Content o f Surfactants i n S o l i d a n d Aqueous Phases

Surfactant Surfactant Content in solid Content in aqueous concentration phase (%) phase (%) type Below CMC Above CMC

10 10 10 150 20 1300

Triton X-100 CPC SDDBS Triton X-100 CPC SDDBS

17.04 7.64 78.80 24.51 8.91 60.19

82.96 92.36 21.20 75.49 91.09 39.91

Figure 6 shows the removal of PCE by ZVI with three surfactants at concentrations below and above the CMCs (Table II). The removal rate of PCE by ZVI with the three surfactants was higher than without them, although with SDDBS most PCE was just sorbed rather than dechlorinated, as it was with Triton X-100 and CPC.

Conclusions The present research was performed in order to assess the potential benefit of surfactants desorbed from the immobization layers, usually at low concentrations below CMCs, on the rate of PCE dechlorination. The results of the research suggests that immobilization with surfactants and dechlorination by ZVI can be combined into one or a series of reactive zones or areas to treat a groundwater plume contaminated with chlorinated organics. The rate of PCE dechlorination was found to be faster using ZVI with Triton X-100 or CPC than without, because the TCE formation rate was higher for ZVI with Triton X-100 or CPC than for ZVI without. The combination of the nonionic or cationic surfactant with ZVI strongly increased the dechlorination of PCE because the



152

Figure 6. Removal of PCE by ZVI with three surfactants. - Φ- control, only, - V- Fe with SDDBS, - V- Fe with Triton X-100, -M- Fe with C



153

PCE that bounded with surfactant in aqueous phase was available for dechlorination. With SDDBS, PCE primarily was immobilized on the sorbed SDDBS phase, rather than dechlorinated, and thus TCE was scarcely detected. Because most soil and clay particles are negatively charged, cationic surfactants would be most useful for immobilization with surfactants, although anionic and nonionic surfactants also could be used depending on the application (20). According to the results of this research, cationic and nonionic surfactants, which had been sorbed in the immobilization layers and desorbed by the natural groundwater flow, would not hinder the dechlorination reaction in ZVI layers. Even with the anionic surfactant, the removal of PCE from the aqueous phase was enhanced because more PCE could be sorbed on the iron particles, even though the dechlorination under this condition could not be positively confirmed.

References

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Reactive Dechlorination of PCE Using Zero Valent Iron Plus

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