4626
Ind. Eng. Chem. Res. 2007, 46, 4626-4632
Optimizing a Washing Procedure To Mobilize Polycyclic Aromatic Hydrocarbons (PAHs) from a Field-Contaminated Soil Tao Yuan and William D. Marshall* Department of Food Science and Agricultural Chemistry, Macdonald Campus of McGill UniVersity, 21,111 Lakeshore Road, Ste-Anne-de-BelleVue, QC, Canada H9X 3V9
A laboratory study was conducted to assess the feasibility of a washing process with nonionic/anionic surfactant for the mobilization of PAH compounds from a field-contaminated soil. Soil washing was combined with surfactant regeneration and detoxification steps to generate innocuous products. Ultrasonication of fieldcontaminated soil with a 3% (w/v) surfactant suspension for 5 min mobilized appreciable quantities of all polycyclic aromatic hydrocarbon (PAH) compounds. Of the three surfactants, the Brij 98 formulation proved to be slightly more efficient for three successive extractions, mobilizing 88% of the soil PAH burden, whereas companion extractions using fresh reagents each time mobilized 89% of the soil PAH content. Formulating the Brij 98 surfactant in 0.1 M phosphate buffer (pH 8.0) increased the recovery of all PAHs as well as the recovery of surfactant (>90%), but soil residues exceeded permissible maxima for five- and six-ringed analytes. On the basis of the cumulative recoveries of PAH compounds in the aqueous fraction, five successive washes were predicted to reduce the residual soil burdens to legislatively permissible levels. 1. Introduction 1.1. Soil Washing. Washing is a treatment process that can be used to remediate both organic and inorganic chemical constituents in contaminated soils, sludges, and sediments.1,2 This process involves high-energy contact between the contaminated soil and an aqueous washing solution. Soil washing can be a physical and/or chemical process resulting in the separation, segregation, and volume reduction of hazardous materials with/without the chemical transformation of contaminants to nonhazardous, unregulated substances. There are several advantages to soil washing as a remediation technique. First, the actual process takes place in a closed system, which permits control of the ambient environmental conditions. Second, the process can result in a significant volume reduction of the contaminated mass. Third, soil washing has extensive applications for varied waste groups, and the hazardous waste can remain on site because of mobile technology. A well-designed treatment can represent a permanent solution. The time to complete the cleanup is relatively short, and the cost of soil washing is relatively small compared to other technologies designed to treat a variety of contaminants concurrently (hydrophilic organic contaminants and heavy metals)3,4 and appreciably less than the cost of land filling. Finally, regulatory and public acceptance is generally high. Soil washing also has disadvantages as a remediation procedure. When the soil-washing treatment is only a physical process, there is little reduction in the toxicity of the contaminants. If chemical processes are involved, potentially hazardous chemicals that are used in the remediation process might then be difficult to remove from the treated soil. The effectiveness of soil washing is also limited by the following factors: (i) complex waste mixtures can make formulating the washing fluid difficult; (ii) high humic content of the soil may require pretreatment; (iii) it may be necessary to treat the aqueous stream after it is equilibrated with the contaminated soil; (iv) additional treatment steps may be required to address hazardous levels of * To whom correspondence should be addressed. Tel.: (514) 3987921. Fax: (514) 398-797. E-mail:
[email protected].
washing solvent remaining in the treated soil; and (v) high finegrained clay content can compromise the efficiency of removal of the toxicants. Surfactants are particularly attractive for such applications because they potentially have low toxicity and favorable biodegradability in the environment relative to organic-solvent based systems. Yet guidance in selecting surfactants for ex situ soil washing remains somewhat fragmentary.5 The success of soil washing with surfactants can be attributed to the capacity of these compounds, at concentrations above the critical micelle concentration (CMC), to appreciably enhance the aqueous solubility of lipophilic organic compounds. Surfactant-enhanced soil washing can result from three main detergency mechanisms that are active when a deposit is mobilized from the solid surface: solubilization, snap-off, and roll-up. In the solubilization mechanism, the hydrophobic contaminant is dissolved in the hydrophobic core of micelles that are formed from the self-assembly of surfactant molecules in concentrations above the CMC. Researchers6,7 have evaluated surfactants for their ability to act as soil-washing agents and as facilitators of subsurface remediation of hydrocarbon spills. The extent to which surfactants influence the distribution of hydrophobic organic compounds (HOCs) depends principally on the HOC’s sorption to the solid phases.8 Surfactant washing can be ineffective for soils that contain more than 20-30% silt/clay9 or appreciable quantities of organic matter. The quantity of HOCs solubilized by micelles was observed to be in the order of nonionic surfactant, cationic surfactant, then anionic surfactant for similar nonpolar chain lengths.10 It has been suggested11 that nonionic surfactants are better choices than anionic surfactants in washing performance to decrease the portion of HOCs sorbed to soil particles. It has also been suggested12 that surfactant effectiveness in washing out hydrophobic contaminants does not depend strongly on ionic characteristics of the surfactant because the surfactants that increased contaminant removal from soil were characterized by identical properties such as low surface tension, soil dispersion, good detergency, and solubilization. The effectiveness of the surfactants in removing contaminants from soil is also dependent on the hydrophilic/hydrophobic structure [hydrophile/lipophile bal-
10.1021/ie070119h CCC: $37.00 © 2007 American Chemical Society Published on Web 05/19/2007
Ind. Eng. Chem. Res., Vol. 46, No. 13, 2007 4627 Table 1. Selected Physicochemical Characteristics of the Analyte PAH Compounds
a
PAH
molecular weight
no. of rings
aqueous solubilitya (mmol L-1)
vapor pressurea (Pa)
Cdn soil quality standardsb (µg g-1)
naphthalene 1-methylnaphthalene 2-methylnaphthalene acenaphthylene acenaphthene fluorene phenanthrene anthracene fluoranthene pyrene benz[a]anthracene chrysene benzo[b]fluoranthene benzo[a]pyrene benzo[k]fluoranthene dibenz[ah]anthracene indeno[1,2,3-cd]pyrene benzo[ghi]perylene
128.17 142.20 142.20 152.20 154.21 166.22 178.23 178.23 202.26 202.26 228.29 228.29 252.32 252.32 252.32 278.35 276.34 276.34
2 2 2 3 3 3 3 3 4 4 4 4 5 5 5 5 6 6
2.4 × 10-1 2.0 × 10-2 1.8 × 10-2 1.0 × 10-1 2.9 × 10-2 1.2 × 10-2 7.2 × 10-3 3.7 × 10-4 1.3 × 10-3 7.2 × 10-4 4.8 × 10-5 2.6 × 10-5 1.8 × 10-5 1.5 × 10-5 3.2 × 10-6 2.2 × 10-6 7.0 × 10-7 9.0 × 10-7
1.0 × 10-2
22.0
9.0 × 10-1 3.0 × 10-1 9.0 × 10-2 2.0 × 10-2 1.0 × 10-3 1.2 × 10-3 6.0 × 10-4 2.8 × 10-5 5.7 × 10-7
50.0 100.0 10.0 10.0 0.7 10.0 10.0 10.0
7.0 × 10-7 5.2 × 10-8 3.7 × 10-10 1.0 × 10-10 1.4 × 10-8
Data taken from ref 30. b Data taken from ref 31.
Table 2. Mean PAH Content in Soil (µg g-1 ( 1 SDa) and Mean Percent Mobilized ((1 SD)b from This Matrix (3 g) with Various Surfactants [3% (w/v), 30 mL] Induced by Sonication for 5 min in an Ice-Water Bath or Shaking for 24 h at Room Temperature (25 °C)a PAH
soil contentb (µg g-1)
Brij 98c
Triton X-301c
Tween 20c
mixedc
Brij 98 shakingc
naphthalene 2-methylnaphthalene 1-methylnaphthalene acenaphthylene acenaphthene fluorene anthracene phenanthrene fluoranthene pyrene benzo[b]fluoranthene benzo[k]fluoranthene benz[a]anthracene chrysene benzo[a]pyrene indeno[1,2,3-cd]pyrene benzo[ghi]perylene dibenz[ah]anthracene
83 ( 6 15 ( 1 9 ( 0.4 15 ( 0.3 23 ( 2 24 ( 0.5 88 ( 4 63 ( 4 423 ( 17 260 ( 16 106 ( 10 102 ( 13 111 ( 9 84 ( 18 80 ( 14 40 ( 11 32 ( 8 9 ( 0.4
31 ( 2 a 34 ( 3 a 38 ( 2 a 19 ( 1 a,b 61 ( 4 a 70 ( 4 a 58 ( 4 a 39 ( 4 b 51 ( 3 a 64 ( 4 a,b 30 ( 3 b 34 ( 3 a,b 44 ( 3 a b 55 ( 6 a 20 ( 2 b 15 ( 3 a,b 14 ( 2 a 25 ( 11 a
19 ( 0.8 b 20 ( 0.4 a,b 24 ( 0.5 a,b 17 ( 0.2 b 63 ( 2 a 55 ( 2 b 48 ( 3 b 59 ( 6 a 43 ( 13 a 73 ( 9 a 44 ( 2 a 37 ( 1 a,b 36 ( 0.7 b 44 ( 4 a,b 21 ( 12 a,b 16 ( 4 a 9(1a 28 ( 2 a
20 ( 6 b,c 19 ( 2 b,c 24 ( 3 b 20 ( 3 a,b 52 ( 3 a 79 ( 62 a 47 ( 9 b 46 ( 9 a,b 46 ( 2 a 62 ( 4 b 34 ( 19 b 41 ( 3 a 48 ( 7 a 55 ( 1 a 28 ( 1 a 23 ( 4 a 16 ( 6 a 38 ( 10 a
14 ( 3 c 15 ( 3 c 19 ( 3 b,c 15 ( 3 a,b 44 ( 9 a 45 ( 4 b,c 38 ( 7 b 37 ( 15 a,b,c 40 ( 9 a 66 ( 12 a,b 32 ( 9 a,b 39 ( 10 a,c 44 ( 7 a b 62 ( 25 a 23 ( 10 a,b 21 ( 3 a 16 ( 6 a 46 ( 9 a
16 ( 0.2 c 15 ( 0.5 c 19 ( 0.6 c 22 ( 0.9 a 52 ( 3 a 71 ( 19 a,b 43 ( 2 b 28 ( 2 c 36 ( 1 b 46 ( 0.5 b 14 ( 1 c 24 ( 5 a,c 33 ( 3 b 41 ( 4 b 13 ( 2 b 13 ( 2 b 6(2a 5(1b
46 ( 1 a
44 ( 5 a,b
45 ( 2 a
41 ( 9 a,b
32 ( 2 b
Sum/Mean
1568 ( 64
a,b,c: Numbers in the same row, followed by the same letter, are not significantly different (p ) 0.05). One standard deviation based on three replicate trials. c Mean percent mobilized ( one standard deviation based on three replicate trials. a
ance (HLB)] of the surfactant molecule and the CMC. Yet mixtures of two different surfactants often show a “synergistic” interaction.13,14 This has been attributed to a decrease in the CMC for the surfactant mixture and an increase in the partition coefficient between the mixed surfactant micelles and the aqueous phase. The addition of a water-miscible organic solvent (often triethylamine, acetone, or n-butyl alcohol) to the washing solution has also been reported to increase the mobilization of hydrophobic contaminants.15,16 1.2. PAH Compounds. The polycyclic aromatic hydrocarbons (PAHs) represent a class of organic compounds that consist of two or more fused aromatic rings. Representatives of this class of toxicant can be detected in almost all components of our environment.17 They are components of coal tar, creosote, and crude oil, and can also be formed during the incomplete combustion of organic materials. Because of their carcinogenic and mutagenic properties, PAHs have long been regarded as environmental priority pollutants that require metabolic activation to electrophilic intermediates18-20 and subsequent covalent adduct formation with cellular DNA to elicit their adverse biological activity.21 Although several different activation pathways have been identified, strong evidence points to the
b
prominent role of bay- and fjord-region dihydrodiol epoxides as ultimate mutagenic and carcinogenic metabolites22 of PAH compounds. The chemical properties and, hence, the environmental fate of a PAH molecule are dependent in part upon both molecular size (the number of aromatic rings) and molecule topology, or the pattern of ring linkage. Generally, an increase in the size and angularity of a PAH molecule results in a concomitant increase in hydrophobicity and electrochemical stability.23 PAH molecule stability and hydrophobicity are two primary factors that contribute to the persistence of high molecular weight PAHs in the environment. In addition to increases in environmental persistence with increasing PAH molecule size, evidence suggests that, in some cases, PAH genotoxicity also increases with size, up to at least four or five fused benzene rings.24 The U.S. EPA has classified seven PAHs (benzo[a]pyrene, benz[a]anthracene, chrysene, benzo[b]fluoranthene, benzo[k]fluoranthene, dibenz[a,h]anthracene, and indeno[1,2,3-cd]pyrene) as Group B2, probable human carcinogens.25 It has been demonstrated recently that the mutagenic activity of selected PAH toxicants can be circumvented by catalytic hydrogenation.26 Whereas benzo[a]pyrene and chrysene were potently mutagenic, their perhydrogenation products were
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Table 3. Effects of (A) Temperature during Sonication (3 min) or (B) Concentration of Surfactant (5 min Sonication) on the Mean Percent Recovery (ug g-1 ( 1 SDa) of Individual PAH Compounds from Soil (2 g) by Washing with 20 mL of Aqueous Tween 20 Surfactant (A) temperature (°C)
a
4
25
50
3
6
9
naphthalene 2-methylnaphthalene 1-methylnaphthalene acenaphthene acenaphthylene fluorene anthracene phenanthrene fluoranthene pyrene benz[a]anthracene chrysene benzo[b]fluoranthene benzo[k]fluoranthene benzo[a]pyrene indeno[1,2,3-cd]pyrene benzo[ghi]perylene dibenz[ah]anthracene
19 ( 3 20 ( 1 22 ( 5 57 ( 5 27 ( 0.9 54 ( 4 48 ( 4 44 ( 2 38 ( 4 46 ( 2 34 ( 5 38 ( 4 25 ( 77 31 ( 12 25 ( 4 12 ( 6 12 ( 7 33 ( 5
18 ( 8 20 ( 9 22 ( 6 61 ( 7 27 ( 3 58 ( 4 45 ( 6 52 ( 10 39 ( 5 50 ( 11 46 ( 7 51 ( 8 25 ( 51 39 ( 5 19 ( 78 5 ( 23 9 ( 20 44 ( 87
21 ( 3 20 ( 0.4 22 ( 0.5 65 ( 3 40 ( 2 62 ( 1 52 ( 0.3 51 ( 16 42 ( 0.5 50 ( 5 45 ( 12 49 ( 14 34 ( 9 43 ( 21 31 ( 22 8 ( 87 3 ( 54 33 ( 14
20 ( 29 20 ( 11 22 ( 13 52 ( 15 20 ( 5 88 ( 5 48 ( 19 46 ( 20 46 ( 4 62 ( 6 49 ( 14 55 ( 2 34 ( 7 41 ( 2 27 ( 4 28 ( 4 22 ( 2 55 ( 10
14 ( 83 20 ( 32 22 ( 30 65 ( 7 27 ( 29 75 ( 28 46 ( 15 59 ( 20 44 ( 33 67 ( 22 48 ( 26 62 ( 3 25 ( 6 50 ( 2 37 ( 2 8(8 6 ( 44 22 ( 40
23 ( 11 27 ( 12 33 ( 9 61 ( 7 27 ( 6 67 ( 5 53 ( 8 54 ( 12 50 ( 15 60 ( 6 51 ( 11 57 ( 10 23 ( 8 46 ( 3 27 ( 32 20 ( 33 16 ( 8 33 ( 29
One standard deviation based on three replicate trials.
Table 4. Variations, with Sonication Time, in the Mean Percent Recovery (ug g-1 ( 1 SDa) of PAH Compounds from Soil (2 g) by Washing with 20 mL of Aqueous Tween 20 (6%, w/v) Surfactant PAH
3 min
5 min
10 min
15 min
naphthalene 2-methylnaphthalene 1-methylnaphthalene acenaphthylene acenaphthene fluorine anthracene phenanthrene fluoranthene pyrene benz[a]anthracene chrysene benzo[k]fluoranthene benzo[b]fluoranthene benzo[a]pyrene indeno[1,2,3-cd]pyrene benzo[ghi]perylene dibenz[ah]anthracene
17 ( 11 20 ( 7 22 ( 8 27 ( 12 48 ( 9 50 ( 13 43 ( 11 43 ( 5 39 ( 6 46 ( 5 40 ( 2 48 ( 2 19 ( 110 5(1 14 ( 73 2(7 2(7 89 ( 5
14 ( 82 20 ( 33 22 ( 30 27 ( 7 65 ( 13 75 ( 29 45 ( 15 59 ( 20 44 ( 34 67 ( 22 48 ( 26 62 ( 4 50 ( 6 25 ( 2 39 ( 2 8(8 6 ( 45 22 ( 40
25 ( 5 27 ( 4 33 ( 4 33 ( 6 74 ( 2 71 ( 3 65 ( 5 68 ( 5 42( 5 76 ( 3 61 ( 4 65 ( 3 53 ( 14 25 ( 26 40 ( 26 5 ( 17 12( 24 67 ( 9
39 ( 17 33 ( 15 44 ( 16 33 ( 23 87 ( 22 50 ( 51 65 ( 27 76 ( 8 41 ( 8 80 ( 34 69 ( 38 59 ( 19 32 ( 63 25 ( 115 25 ( 74 32 ( 43 25 ( 68 78 ( 11
mean PAH extraction efficiency
32
39
47
49
a
(B) surfactant concentration (% w/v)
PAH
One standard deviation based on three replicate trials.
without detectable activity in the reverse mutation assay with five strains of bacteria. Moreover, partial hydrogenation (that resulted in one aromatic ring of the substrate remaining intact) provided products that were nonmutagenic in these assays. 1.3. Site. A decommissioned steel plant and ancillary coke ovens in Sydney, Nova Scotia, Canada, operated from 1901 through 1988. The oven area contained 400 coke ovens, 4 blast furnaces, and 10 open-hearth furnaces.27 The coal tar produced in making coke was released into the Tar Ponds, a wetland situated at tidewater within the harbor. This site covers an area of about 400 ha and is adjacent to the Muggah Creek estuary.28 These tar ponds have acted as a settling basin for the steel-mill effluent and for coking-oven wastes trapping particles contaminated with PAHs and other compounds and resulting in PAH burdens29 that are among the highest in Canada. The Muggah creek system (including the tar ponds) has been estimated to contain 715 000 m3 of contaminated sediment, including 2-4 million kg of PAH material as well as other organic materials. The coke ovens and steel plant deposited several million tons of particulate matter on the industrial site and the surrounding community.
The objectives of the current studies were to evaluate the efficacy of a combination of unit operations that would partition PAH compounds from soil into a limited volume of aqueous surfactant suspension. The PAH fraction was then to be recovered by back-extraction into hexane and subsequently detoxified by hydrogenation, whereas the cleaned surfactant solution was to be returned to the soil to mobilize more toxicants. 2. Materials and Methods 2.1. Chemicals. Test surfactants, nonionic [Tween 20 (CAS #: 9005-64-5), Tween 40 (CAS #: 9005-66-7), Tween 80 (CAS #: 9005-65-6), Tween 85 (CAS #: 9005-70-3), Brij 98 (CAS #: 9004-98-2), or Triton 405 (CAS #: 9002-93-1)] or anionic [Triton X-301 (CAS #: 12627-38-2), Triton XQS 20 (CAS #: 65256-19-1) or sodium dodecylsulfate (SDS, CAS #: 151-213)] formulations, were purchased from Sigma-Aldrich (Oakville, ON). Cobaltous nitrate [Co(NO3)2‚6H2O], ammonium thiocynate (NH4SCN), methylene blue, disodium ethylenediamine tetraacetate (EDTA), sodium dihydrogen phosphate monohydrate, and disodium hydrogen phosphate heptahydrate were obtained from Fisher Chemical (Fair Lawn, NJ). Randomly methylated β-cyclodextrin [RAMEB, degree of substitution (DS) ) 12.6 (CAVASOL W7M)] was generously provided by Dr. Mark Harrison, Wacker Specialties, Adrian, MI. High-performance liquid chromatography (HPLC) grade ethanol, chloroform, and hexane were obtained from Fisher Scientific (Ottawa, ON). All chemicals, solvents, and reagents were of ACS Reagent grade or better and were used as received. 2.2. Gas Chromatographic Analysis. Gas chromatographymass spectrometry (GC-MS) was performed on a Varian model 3900 gas chromatograph fitted with a model 8400 autosampler and a model 2100T MS detector. The DB-5 capillary column (30 m × 0.25 mm inner diameter; 0.25 µm film thickness) was eluted with helium at 1.0 mL min-1. After an initial hold of 1 min at 50 °C, the column was ramped, at 10 °C min-1, to 300 °C and held for a further 3 min prior to cool down. The temperatures of the injector, the transfer line, and the detector were maintained at 250, 250, and 150 °C, respectively. Preliminary identification of the eluting components was performed by comparing the experimental mass spectra with spectra catalogued in the National Institute of Standards and Technology (NIST) or the Saturn mass spectral libraries.
Ind. Eng. Chem. Res., Vol. 46, No. 13, 2007 4629 Table 5. Cumulative Mean Percent Recovery ((1 SDa) of PAH Compounds Post Three Successive Sonication Washes (20 mL) of Soil (1 or 2 g) with the Same Charge of Various Surfactant Formulations [3% (w/v) at Room Temperature for 10 min] PAH
Triton X-301 (1 g)
Tween 20 (2 g)
Tween 20 (1 g)
Brij 98 20 (2 g)
Brij 98 (1 g)
naphthalene 2-methylnaphthalene R-methylnaphthalene acenaphthylene acenaphthene fluorene anthracene phenanthrene fluoranthene pyrene benz[a]anthracene chrysene benzo[k]fluoranthene benzo[b]fluoranthene benzo[a]pyrene indeno[1,2,3-cd]pyrene benzo[ghi]perylene dibenz[ah]anthracene
93 ( 2 111 ( 27 102 ( 4 83 ( 5 107 ( 8 109 + 3 114 ( 2 114 ( 6 80 ( 3 89 ( 2 69 ( 0.6 85 ( 21 70 ( 4 66 ( 2 59 ( 5 46 ( 5 33 ( 1 69 ( 12
77 ( 9 82 ( 9 87 ( 9 59 ( 4 111 ( 6 121 ( 2 97 ( 11 99 ( 8 71 ( 13 88 ( 11 78 ( 17 82 ( 8 73 ( 12 58 ( 8 44 ( 13 38 ( 4 34 ( 5 48 ( 2
105 ( 5 104 ( 5 106 ( 10 99 ( 17 117 ( 0.4 104 ( 4 111 ( 7 104 ( 6 75 ( 8 83 ( 8 82 ( 7 91 ( 10 67 ( 7 61 ( 3 64 ( 3 52 ( 6 49 ( 2 62 ( 20
83 ( 22 93 ( 20 99 ( 20 63 ( 4 110 ( 15 111 ( 8 107 ( 4 109 ( 9 76 ( 4 99 ( 5 89 ( 9 100 ( 8 82 ( 7 50 ( 3 60 ( 10 41 ( 2 41 ( 3 44 ( 10
108 ( 2 110 ( 10 110 ( 7 99 ( 10 112 ( 7 111 ( 10 95 ( 2 116 ( 3 83 ( 2 100 ( 4 92 ( 3 105 ( 4 77 ( 0.7 66 ( 3 63 ( 4 52 ( 5 49 ( 1 56 ( 6
82 ( 3
75 ( 10
81 ( 6
sum a
Table 6. Mean Percent Recovery ((1 SDa) of Separate PAH Classes in the Hexane Extract from Three Successive Washes of Soil (1 g) with the Same Charge of Brij 98 (3% w/v) for 10 min at Room Temperature
1st sonication/equilibration 2nd sonication/equilibration 3rd sonication/equilibration a
83 ( 2
88 ( 2
One standard deviation based on three replicate trials.
sum of 2-, 3-, and 4-ringed analytes
sum of 5- and 6-ringed analytes
53 ( 2 32 ( 2 15 ( 0.7
33 ( 2 18 ( 0.9 9 ( 0.4
One standard deviation based on three replicate trials.
Quantification of the sample was performed on a HP 5890 gas chromatograph with flame ionization detector (FID) under similar chromatographic conditions as described previously. 2.3. Contaminant Mobilization/Extraction. In a typical trial, soil (1-3 g) was equilibrated with 20 mL of 3% (w/v) surfactant emulsion in 50 mL centrifuge tubes immersed in an ice bath. Equilibrations were achieved by sonicating the soil suspension for 5 min with an ultrasonic homogenizer (XL 2020 Sonic dismembrator, Misonix Inc., NY). An extended horn of 25 cm length × 1.2 cm width, tuned at 20 kHz frequency, delivered ultrasonic energy (240 W) in a pulsed mode with a fixed vibration amplitude setting of 6-7. The equilibration consisted of pulsed surges of power delivered for 2 s followed by a 3 s cooling phase. Post sonication, the suspension was centrifuged at 1789 x g. The PAH burdens, in the soil, were determined by exhaustive Soxhlet extraction. Soil (10 g) was subjected to Soxhlet extraction with acetone-hexane (1:1, v/v, 300 mL) for 18 or 24 h. PAH compounds in the extract were determined by gas chromatography-mass spectrometry (GC-MS). 2.4. PAH Removal from Soil Extracts. PAHs in the supernatant fraction were back-extracted three times with hexane (3-5 mL) to partition PAHs from the aqueous soil extract. The cumulative hexane fraction was centrifuged (1789 x g) to remove aqueous surfactant and then diluted with 1 mL ethanol to disrupt the hexane-surfactant emulsion induced by agitation. PAHs that had been extracted with hexane were determined by GC-MS. 2.5. Recycle of Mobilizing Reagents. Post PAH removal, the cleaned mobilizing reagent emulsion was re-equilibrated with the particulate fraction to mobilize more PAHs. Soil particulates were equilibrated again by sonication with 20 mL of cleaned mobilizing reagent. The resulting aqueous supernatant fraction
was again back-extracted to remove PAHs as described previously, and a portion of the aqueous wash (60 µL) was assayed as described in the next section. 2.6. Surfactant Analysis. Surfactant concentrations were determined spectrophotometrically (DR 4000; Hach Co., Loveland, CO, single-beam spectrophotometer). The standard analysis methods (APHA 5540 C and 5540 D)30 were followed (with minor modifications) to determine the concentrations of anionic or nonionic surfactants, respectively, in both solid and supernatant fractions. For nonionic surfactant, Co(SCN)2 (3 mL, 0.2 M) was added to a small test tube [inner diameter 1.0 cm (o.d. 1.2 cm) × 10.0 cm] prior to the addition of nonionic micelle suspension (20, 40, 60, 80, or 100 µL aliquots of 30 g L-1) followed by 5 mL of CHCl3 to obtain calibration standards that were permitted to stand for 4 h prior to 3-5 vigorous agitations with a Vortex mixer. After phase separation by centrifugation, the absorbance was recorded at 620 nm vs a blank of CHCl3. For anionic surfactants, a mixture of methylene blue (0.1 mL, 0.25 mg L-1) and phosphate buffer (0.4 mL, pH 7) was used as the complexing reagent.31 CHCl3 (6 mL) was added, followed by surfactant solution, and the mixture was treated as described previously. The absorbance at 655 nm was recorded vs a blank of CHCl3. 3. Results and Discussion 3.1. PAH Mobilization. Selected physiochemical parameters32 that suggest increased environmental persistence with increasing molecular weight of the PAH compounds monitored in these studies and maxima,33 permitted by legislation, for soil destined for commercial/industrial activities are recorded in Table 1. Previous studies had demonstrated that high-energy ultrasonic blending, for short periods, appreciably increased the mobilization of lipophilic toxicants (PCB compounds) from soil,4 compared to reciprocal shaking for 24 h. Nine commercial surfactants were selected for this study: six nonionic and three anionic formulations. Amphoteric and cationic surfactants were avoided because of their stronger tendencies to sorb to soil particles.9 The nine surfactants included at least one from each of the four common groups of commercial surfactants: (i) ethoxylated alcohols (nonionic), (ii) ethoxylated alkylphenol (nonionic), (iii) sulfate (anionic), and (iv) sulfonate (anionic)
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Table 7. Mean Cumulative Percent Recovery of PAH Compounds from Soil (1 g) with Three or Five Successive Washes with the Same Charge (20 mL) of 3% (w/v) Brij 98 Surfactant Sonicated during 10 min in the Presence/Absence of Phosphate Buffer or Using Fresh Reagents for Each Wash
PAH benzo[k]fluoranthene benzo[b]fluoranthene benzo[a]pyrene indeno[1,2,3-cd]pyrene dibenz[a,h]anthracene benzo[ghi]perylene
predicted mean Σ (1st-5th residue (µg g-1) in soil after Σ (1st + 2nd + Σ (1st + 2nd + 3rd ) equilibrations) 5 washes with Σ (1st + 2nd + Σ (1st + 2nd + 3rd equilibrations) (same charge in pH 8, (same charge in same charge 3rd equilibrations) 3rd equilibrations) (same charge in 0.1 M phosphate buffer + pH 8, phosphate of phosphate buffer (same charge) (fresh reagent) phosphate buffer, pH 8) 0.1 M EDTA) buffer) 77 66 63 53 56 49
77 67 58 52 56 47
functional groups. On the basis of PAH extraction efficiency from an agricultural soil (a sandy loam) that had been spiked (50 µg g-1) with phenanthrene, chrysene, and benzo[a]pyrene, three formulations, Triton X-301, Tween 20, and Brij 98, were selected for further study. Each of the nine aqueous test surfactants (3% w/v) proved to be superior to a wash with randomly methylated β-cyclodextrin (RAMEB, 10% w/v) under similar conditions. In subsequent experiments, a “tar pond” soil,27,28 which had been field-contaminated over many years, was used to assess efficiency of washing to remove PAH compounds. Once airdried, homogenized, and sieved through a 2 mm screen, the soil, a sandy loam (60% sand and 40% silt/clay), possessed a high organic matter content (24.1%), intermediate water content (7.1%), and a low pH (3.1). Somewhat arbitrarily, 18 PAH analytes were monitored (including 16 that had been identified as priority pollutants by the U.S. EPA). PAH levels in the soil indicated high levels of contamination. Moreover, the assemblage of individual compounds within the mixture was characteristic of combusted fossil fuels (dominance of four- and five-ringed species). The extraction efficiencies for individual analytes with a single blending, for 5 min, with Triton X-301, Tween 20, or Brij 98 are summarized in Table 2. Whereas the Brij 98 formulation was more efficient at mobilizing the low molecular weight analytes (2- to 4-rings), significant differences among surfactant washes were not detected for the high molecular weight PAHs (5- and 6-ringed compounds). As had been observed previously, ultrasonic blending for 5 min in an ice bath (column 3, 4, or 5) mobilized more toxicants than did 24 h of reciprocal shaking (column 7), and interactions among the surfactants were not evident. A 1:1:1 mixture of the three surfactants (Table 2, column 6) extracted less analyte or was not significantly different from extraction efficiencies observed for an individual surfactant (columns 3, 4 or 5). Subsequent experiments consisted of identifying an optimal sonication temperature and surfactant concentration for efficient PAH mobilization from the test soil. The Tween 20 formulation was selected arbitrarily to define suitable extraction conditions. Although somewhat increased at the higher extraction temperature, the PAH contents in the surfactant supernatant fraction for equilibrations conducted at 4, 25, or 50 °C, (Table 3A) were not different from each other. For these trials, with 5 min sonication with Tween 20 (6% w/v), the sum of the quantities of PAHs (615 µg g-1 ( 6%) mobilized at 50 °C was apparently not appreciably different from the quantity mobilized at 25 °C (582 µg g-1 ( 6%) or at 4 °C (541 µg g-1 ( 1%). Previous studies with polychlorinated biphenyl (PCB) compounds had indicated that extractions from soil with a 3% (w/v) concentration were comparable to a 5% suspension and more efficient than washing PCBs from soil with a 1% suspension.32 No significant differences were observed in the quantities of
88 76 84 65 68 58
70 64 87 60 76 63
102 91 99 77 79 71
10 0.8 9 2 9
extracted PAH compounds with 3, 6, or 9% (w/v) surfactant suspensions (Table 3B). It is considered that these surfactant concentrations were approximately an order of magnitude greater than the apparent critical micelle concentration (aCMC) in the presence of soil particles. Time trials with the Tween 20 formulation followed a similar trend (Table 4). The percent of PAHs mobilized by 15 min sonication (51%) was not different from that of 10 min (50%) or 5 min of sonication (46%) but frequently was increased significantly over recoveries observed after 3 min of sonication (34%). Subsequent extractions were performed with a 3% surfactant suspension and 10 min of sonication at room temperature. Subsequently, the influence of surfactant recycle was evaluated. The addition of ethanol (to hasten disruption of the ensuing emulsion), while convenient for the scale of the experiments, is probably not practical on a field scale. The cumulative percent recoveries of PAH analytes after three successive washes with the same charge of surfactant (from either 2 or 1 g of soil) are summarized in Table 5. For 2 g of soil, recoveries of the PAH analytes were quantitative for the 3-ringed analytes but not for any of the 2-, 4-, 5-, or 6-ringed analytes. Of the three surfactants, the Brij 98 formulation proved to the most efficient. Mobilization efficiencies were increased when working with 1 g of soil so that (based on the mean sum of analyte PAH in the three washes) the mean recoveries of 2-, 3-, and two of 4-ringed analytes were virtually quantitative. However the mean removal of benz[a]anthracene and fluoranthene remained efficient (92 and 83%, respectively, with the Brij formulation) but not quantitative. Mean recoveries of 5-ringed or 6-ringed analytes were less efficient and ranged from 49 to 77%. The mobilization efficiencies with the Brij 98 formulation are explored in greater detail in Table 6, which compares mean PAH recoveries in each of the three successive washes. Whereas the mean mobilization for 2-, 3-, and 4-ringed analytes was 53% after one equilibration (and a further 32 and 15% were mobilized by the second and third equilibrations), the removal efficiencies of these PAH classes were reduced appreciably for 5- and 6-ringed analytes. Three successive washes mobilized means of 33, 18, and 9% of the higher molecular weight PAH compounds (those with 5or 6-rings). Given the loading within the soil, rather than performing more washes, a more efficient mobilization strategy was sought. The use of fresh Brij 98 surfactant (3% w/v) for each of three successive washes of the soil (1 g) did not change the recovery of PAH analytes perceptibly relative to the recycle mode in which the same charge of surfactant was used for all three washes of the soil (Table 7). Whereas recovery of each of the 2-, 3-, or 4-ringed analytes remained virtually quantitative (data not shown in this table), the extraction efficiency of 5- and 6-ringed analytes was not improved by the use of fresh reagents
Ind. Eng. Chem. Res., Vol. 46, No. 13, 2007 4631 Table 8. Mean Net Surfactant Recovery (percent ( 1 SDa) at Each Experimental Stage of Processing for Three Successive Washes of the Soil (3 g) with the Same Charge of Aqueous Surfactant (3% w/v) and Sonication at Room Temperature for 10 min surfactant recovery (% ( 1 SD )
a
post
Brij 98
Brij 98 (fresh reagents)
1st equilibration 1st hexane extraction 2nd hexane extraction 3rd hexane extraction
91 ( 2 84 ( 3 82 ( 5 76 ( 5
91 ( 0.2 82 ( 3 89 ( 1 86 ( 1
Brij 98 + (phosphate buffer)
Triton X-301
Tween 20
97 ( 1 94 ( 1 91 ( 2 89 ( 0.5
70 ( 8 62 ( 7 67 ( 3 75 ( 6
90 ( 2 82 ( 3 88 ( 2 83 ( 3
One standard deviation based on three replicate trials.
(column 3 vs column 2). Hydrophobic organic contaminants can be sorbed strongly to the organic matter in soil. As a possible strategy to mobilize HOCs, solutions that can release the organic matter fraction from soil might also release more PAHs from the particulates fraction. Estimates of the organic matter (SOM) in soil have frequently employed an alkaline EDTA extracting solution. The PAH recovery from the soil was increased by formulating the Brij surfactant in 0.1 M (pH 8.0) phosphate buffer (Table 7, column 4). Yet the residual levels of 5- and 6-ringed PAHs in the soil exceeded permitted maxima. The inclusion of EDTA in the extracting formulation did not increase the mobilization of penta- and hexacyclic analytes (column 5 vs column 4) for three of the analytes. In consequence, further washing became necessary. On the basis of the cumulative recovery of analytes and the uncertainties associated with measurements, five successive washes (column 6) mobilized sufficient toxicant to account for the soil PAH burdens. However, the observed mobilization efficiencies are specific for the test soil (high SOM) and might not be directly applicable to other soils. The levels of surfactant (Brij 98, Tween 20, or Triton X-301) that remained in the aqueous wash after sonication of the soil and back-extraction to recover the PAHs are summarized in Table 8. The loss of the anionic surfactant to the soil (∼30%) was appreciably greater than losses of either the Brij (∼9%) or the Tween (∼10%) formulation. After the initial sonication stage, additional losses were observed for subsequent equilibration and back-washing steps, so that the net losses of surfactant were 25, 24, and 17%, respectively. Although differences were not great, the presence of the phosphate buffer seemed to increase the recovery of the Brij surfactant so that, in this case, the net loss after three washes of the soil amounted to ∼11% (Table 8). 3.2. Extract Detoxification. Detoxification of the hexane extract was accomplished at line by catalytic hydrogenation in a slightly hydrogen-rich atmosphere.26 The hexane back-extracts of the soil washes were merged with a stream of supercritical carbon dioxide-molecular hydrogen mixture (scCO2-H2, 5% w/v) that transferred the PAH containing liquid to a reactor column maintained at 90 °C. Passage through the column filled with palladium supported on alumina (17 g, 5% Pd0/γ-Al2O3) quantitatively reduced the PAHs to their perhydrogenated analogs. A capillary silica restrictor (0.05 mm x 25 cm) maintained sufficient back-pressure (10.37 MPa) to maintain a supercritical condition. The reactor column, when fed, at 0.1 mL min-1, with hexane extract of the soil washes, produced totally hydrogenated species without any evidence of substrate rearrangement or cracking. Specifically, neither chrysene nor benzo[a]pyrene nor partially hydrogenated products of these substrates were detected in cumulative fractions of reactor eluate collected during 15 min. However, an envelope of peaks with molecular weight consistent with total hydrogen was readily quantified. The sum of the peak areas corresponding to prehydrogenated products accounted for the levels of influent
benzo[a]pyrene and chrysene to the reactor. Moreover, the column was operated continuously (with hexane extract from the nonionic soil wash) for 4 h without any perceptible loss of hydrogenating activity. 4. Conclusions Surfactant washing was evaluated for the removal of PAH compounds that had been accumulated in a soil over many years. Five successive washes of the soil with the same charge of a nonionic surfactant, Brij 98, [formulated at 3% (w/v) in phosphate buffer] reduced the concentrations of PAH compounds in the soil to limits permitted for industrial/commercial use. Moreover, once mobilized, the contaminants were readily detoxified at line in a slightly hydrogen-rich atmosphere of supercritical carbon dioxide. These techniques recycle the surfactant and remove and detoxify the PAH contaminants from the soil. However, because the soil remains contaminated with surfactant residues, microbial activity might be affected adversely. Acknowledgment The kind donation of the soil sample by the Sydney Tar Ponds Agency and financial support from NSERC (Natural Science and Engineering Research Council of Canada) are gratefully acknowledged. Literature Cited (1) FRTR. Section 4.19, Soil Washing. Remediation technologies screening matrix and reference guide, version 4.0; FRTR: APG, MD, 2002 (http://www.frtr.gov/ matrix2/section4/4-19.html). (2) NRC, Commission on Geosciences, Environment and Resources (CGER). Groundwater and Soil Cleanup: ImproVing Management of Persistent Contaminants; National Academy Press: Washington, DC, 1999; p 285. (3) Ehsan, S.; Prasher, S. O.; Marshall, W. D. A washing procedure to mobilize mixed contaminants from soil. II. Heavy metals. J. EnViron. Qual. 2006, 35 (6), 2084-2091. (4) Ehsan, S.; Prasher, S. O.; Marshall, W. D. A washing procedure to mobilize mixed contaminants from soil. I. Polychlorinated biphenyl compounds. J. EnViron. Qual. 2006, 35 (6), 2146-2153. (5) Deshpande, S.; Shiau, B. J.; Wade, D.; Sabatini, D. A.; Harwell. J. H. Surfactant selection for enhancing ex situ soil washing. Water Res. 1999, 33 (2), 351-360. (6) Abdul, S. A.; Gibson, T. L. Laboratory studies of surfactant-enhanced washing of polychlorinated biphenyl from sandy material. EnViron. Sci. Technol. 1991, 25 (4), 665-671. (7) Edwards, D. A.; Liu, Z.; Luthy, R. G. Surfactant solubilization of organic compounds in soil/aqueous systems. J. EnViron. Eng. 1994, 120 (1), 5-22. (8) Chiou, C. T.; McGroddy, S. E.; Kile, D. E. Partition Characteristics of Polycyclic Aromatic Hydrocarbons on Soils and Sediments. EnViron. Sci. Technol. 1998, 32 (2), 264-269. (9) Mulligan, C. N.; Yong, R. N.; Gibbs, B. F. Surfactant-enhanced remediation of contaminated soil: A review. Eng. Geol. 2001, 60 (1-4), 371-380.
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(25) U.S. Environmental Protection Agency. Integrated Risk Information System (IRIS) on Polycyclic Organic Matter; National Center for Environmental Assessment, Office of Research and Development: Washington, DC, 1999. (26) Yuan, T.; Fournier, A. R.; Proudlock, R.; Marshall, W. D. Continuous catalytic hydrogenation of polyaromatic hydrocarbon (PAH) compounds in hydrogen-supercritical carbon dioxide (scCO2). EnViron. Sci. Technol. 2007, 41 (6), 1983-1988. (27) Lambert, T. W.; Lane, S. Lead, Arsenic, and Polycyclic Aromatic Hydrocarbons in Soil and House Dust in the Communities Surrounding the Sydney, Nova Scotia, Tar Ponds. EnViron. Health Perspect. 2004, 112 (1), 35-41. (28) Furimsky, E. Sydney tar ponds: Some problems in quantifying toxic waste. EnViron. Manage. 2002, 30 (6), 872-879. (29) Vandermeulen, J. H. PAH and heavy metal pollution of the Sydney Estuary: Summary and review of studies to 1987. Can. Tech. Rep. Hydrogr. Ocean Sci. 1989, 108, 48. (30) APHA/AWWA/WEF (American Public Health Association/ American Water Works Association/Water Environment Federation). Part 5540. Surfactants. In Standard methods for the examination of water and waste water, 15th ed.; Greenberg, A. E. et al., Eds.; American Public Health Association: Washington, DC, 1995; pp 5-33-5-40. (31) George, A. L.; White, G. F. Optimization of the methylene blue assay for anionic surfactants added estuarine and marine water. EnViron. Toxicol. Chem. 1999, 18 (10), 2232-2236. (32) Mackay, D.; Shiu, W. Y.; Ma, K. C. Illustrated Handbook of Physical-Chemical Properties and EnVironmental Fate for Organic Chemicals. Vol. 2. Polynuclear Aromatic Hydrocarbons Polychlorinated Dibenzodioxins and Dibenzofurans; Lewis Publishers: Chelsea, MI, 1992. (33) Canadian Soil Quality Guidelines for the Protection of EnVironmental and Human Health. Summary Tables; CCME: 2004 (http:// www.ccme.ca/assets/ pdf/sqg_summary_table.pdf).
ReceiVed for reView January 18, 2007 ReVised manuscript receiVed April 20, 2007 Accepted April 23, 2007 IE070119H
