Compatibility of Surfactants and Thermally Activated Persulfate for

Article pubs.acs.org/est

Compatibility of Surfactants and Thermally Activated Persulfate for Enhanced Subsurface Remediation Li Wang, Libin Peng, Liling Xie, Peiyan Deng, and Dayi Deng* School of Chemistry and Environment, South China Normal University, Guangzhou, Guangdong 510006, China S Supporting Information *

ABSTRACT: Limited aqueous availability of hydrophobic organic contaminants and nonaqueous phase liquids in subsurface environment may seriously impair the effectiveness of traditional in situ chemical oxidation (ISCO). To tackle the issue, a combination of surfactants and thermally activated persulfate was proposed to enhance the aqueous availability and consequent oxidation of organic contaminants. The compatibility of eight representative nonionic, monovalent anionic, and divalent anionic surfactants with persulfate at various temperatures was first studied, to identify suitable surfactants that have high aqueous stability and low oxidant demands to couple with thermally activated persulfate. C12-MADS (sodium dodecyl diphenyl ether disulfonate, a representative divalent anionic surfactant) stands out as the most compatible surfactant. Batch treatability study with coal tar, an example of challenging scenarios for traditional ISCO, was then conducted. The results show that C12-MADS can significantly enhance not only the oxidation of polyaromatic hydrocarbons contained in coal tar but also oxidant utilization efficiency, indicating the potential of the proposed coupling process for the treatment of organic contaminants with low aqueous availability.

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INTRODUCTION Activated persulfate has been increasingly applied to remediate organic contaminated groundwater and soils in the past decade.1−3 However, soil-sorbed hydrophobic organic contaminants (HOCs) and contaminants present as nonaqueous phase liquids (NAPLs) often present great challenges to in situ chemical oxidation (ISCO). The oxidation of these contaminants is generally limited by low aqueous availability of the contaminants,1,4 as ISCO oxidation of contaminants is expected to occur in the aqueous phase.3 Several approaches have been tested to improve the effectiveness of activated persulfate treatment by increasing the aqueous availability of contaminants. Surfactants have been frequently used to enhance the aqueous availability of sorbedHOCs and contaminants from NAPLs.5 Coupling surfactants with activated persulfate processes at ambient temperature has been tentatively investigated in a few lab studies and field applications in recent years.6−11 Heating is another approach often applied to increase the aqueous availability of contaminants and activate persulfate as well, and thermally activated persulfate has often been applied in field applications to improve the treatment effectiveness of soil-sorbed HOCs and NAPLs.12−14 Considering that surfactant addition and heating have synergistic effects on the solubilization of sorbedHOCs and contaminants present in NAPLs,15,16 a combination of surfactant and thermally activated persulfate may increase the treatment effectiveness of HOCs and NAPLs, which remains yet to be investigated. © XXXX American Chemical Society

Surfactant selection is critical to achieve cost-effective oxidation of target contaminants by avoiding potential undesirable surfactant interferences, especially considering that high concentrations of surfactants (e.g., 1.0 wt%) are often applied in surfactant-enhanced subsurface remediation.5 Surfactants are susceptible to loss from aqueous solutions due to clouding phenomena, precipitation, hydrolysis, oxidation, etc.15 Loss of surfactants would reduce aqueous surfactant concentrations and thus their solubilization potential, while increased oxidant demands by surfactants would reduce the amount of oxidant available for contaminant oxidation. Thus, this study first conducted a systematic compatibility study of eight commercial biodegradable representative nonionic and anionic surfactants with persulfate at both ambient and elevated temperatures, to identify compatible surfactants that have high aqueous solubility and low oxidant demands in the presence of persulfate. Additional characteristics of these surfactants, including molecular structures, can be found in Figure S1 and Table S1 of the Supporting Information (SI). Cationic surfactants were not considered due to their general high toxicity and high sorption by the soil solids.16−18 During the test, visual observation of surfactant clouding or precipitation, measurement of oxidant demands by surfactants, monitoring of Received: October 28, 2016 Revised: May 12, 2017 Accepted: May 16, 2017

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DOI: 10.1021/acs.est.6b05477 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Environmental Science & Technology

induced clouding of some surfactants, especially nonionic surfactants, could be observed at all three test temperatures, only samples at 60 °C showed a clear phase separation between the aqueous phase and the oily phase. Thus, only samples at 60 °C were tested. At designated intervals, the samples at 60 °C were taken out of the shaker, and the two phases, if present, were separated by centrifugation.25 HPLC analyses were then conducted to monitor residual surfactants in the aqueous phase and the oily phase, if present. The analytical procedures are provided in Section S2. Surfactant-Enhanced Solubilization of Coal Tar PAHs. Results from the screening study indicate that C12-MADS is the most compatible surfactant among the eight surfactants herein. Thus, experiments thereafter were conducted with C12-MADS, except where noted. The effects of C12-MADS addition and heating on the solubilization and mass transfer of individual PAHs were investigated by varying C12-MADS concentrations at 0 g/L, 1 g/L, 5 g/L, and 10 g/L, and varying the reaction temperatures at 20 °C, 40 °C, 60 °C, and 75 °C. Detailed experimental procedures, results and discussions can be found in Section S3. Oxidation of Coal Tar in the Biphasic System. Approximately 100 mg of coal tar, except where noted, were put as one droplet onto the bottom center of 40 mL EPA VOA vials. Twenty mL of solutions with SPS and C12-MADS at designed concentrations were then added. The resulting samples were then capped and put into temperature-controlled shakers. The effect of reaction temperature on the oxidation of coal tar PAHs was first studied at varied temperatures (20 °C, 40 °C, 60 °C, and 75 °C) in temperature-controlled shakers ([C12MADS]0 = 10 g/L; [SPS]0 = 50 g/L; 150 rpm). Comparative experiment with only SPS and blank control with only water were also run at the designated temperature. The effect of oxidant dosage on the oxidation of coal tar PAHs was then studied at 60 °C, the optimized reaction temperature out of the four reaction temperatures tested, by varying initial oxidant dosage at 0, 25, 50, and 100 g/L ([C12MADS]0 = 10 g/L). However, the effect of surfactant dosage on the oxidation of coal tar PAHs was investigated by varying [C12-MADS]0 at 5 g/L, 10 g/L, and 20 g/L (T = 60 °C; [SPS]0 = 50 g/L). In addition, the impact of surfactant compatibility to the oxidation of coal tar PAHs was investigated by comparing the results of PAHs oxidation in the presence of Triton X-100, SDBS and C12-MADS ([surfactant] = 10 g/L; T = 60 °C; [SPS]0 = 50 g/L). At designated intervals, the samples were taken out of the shakers and chilled in an ice−water bath to stop persulfate oxidation. The samples were then centrifuged to separate the aqueous layer and residual coal tar, followed by pH measurement of the aqueous solution and quantification of residual persulfate with an automatic potentiometric titrator. The aqueous solution was first diluted with deionized water to lower the surfactant concentration to 0.5 g/L and then extracted with hexane, with CaCl2 added as the emulsion breaker for anionic surfactant solutions. Detailed extraction procedures can be found in Section S3. Residual coal tar remained in the original vial was ultrasound-extracted with hexane/acetone (1:1), and the concentrations of 20 2−6 ring PAHs in the organic samples were quantified by GC-FID (Agilent 7890A) and GC-MS (Agilent 7890A-5975C). The sample workup and analytical procedures, except where noted, were already documented in our previous study.23

residual surfactants, and surface tensions measurement were conducted to measure the compatibility of surfactants with thermally activated persulfate. To demonstrate the potential of the coupling process for challenging subsurface remediation scenarios, thermally activated persulfate oxidation of coal tars in the presence of a compatible surfactant identified by the above screening study was carried out. Coal tars are a major environmental concern for thousands of former manufactured gas plants.11,19,20 The highly viscous nature of coal tars and the limited aqueous availability of polyaromatic hydrocarbons (PAHs), primary contaminants of concern (COCs) for coal tars, render coal tars a unique challenge for conventional ISCO.19,21 In this study, the positive contributions of a compatible surfactant posed to thermally activated persulfate oxidation of coal tar PAHs were investigated and established, including enhanced degradation of PAHs and enhanced oxidant utilization efficiency.

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MATERIALS AND METHODS Materials. Sodium dodecyl benzenesulfonate (SDBS), sodium dodecyl sulfonate (SDS), sodium lauryl sulfate (SLS), sodium dodecyl diphenyl ether disulfonate (C12-MADS), sodium dioctyl sulfosuccinate (AOT), polyoxyethylene lauryl ether (Brij 35), polyoxyethylene octyl phenyl ether (Triton X100), and polyoxyethylene (20) sorbitan monooleate (Tween 80), were analytical grade and used as received. The oxidant was sodium persulfate (98%, analytical grade; abbreviated as SPS thereafter). Coal tar (99%) from Alfa Aesar was characterized according to literature procedures.20 All organic solvents (acetone, acetonitrile, hexane, and methanol) were HPLC grade. Deionized water (∼18MΩ·cm) was used to prepare all aqueous solutions. Information about all other chemicals used can be found in Section S1. Surfactant Compatibility with Thermally Activated Persulfate. Tests were conducted in 40 mL EPA VOA vials with open-top screw caps and silicone/PTFE septa. Twenty mL of freshly prepared solutions containing 10 g/L of individual surfactants and 50 g/L of SPS were put into the vials at room temperature (∼20 °C). The surfactant concentration and oxidant dosage were within the ranges of surfactant and oxidant concentrations frequently used in soil washing5,10,22 and ISCO field applications,3,10 respectively. The samples were then agitated (∼150 rpm) in three temperature-controlled shakers, which were maintained at 20, 40, and 60 °C, respectively. Blank controls with only SPS and blank controls with only surfactants were also run at 20, 40, and 60 °C. At designated intervals, the agitation was turned off temporarily. When samples were rested, any appearance of clouding or precipitation in the samples was noted through visual observation. One set of samples were taken out and chilled in ice−water baths to stop potential surfactant oxidation by SPS. Thereafter, the pH of the samples was measured using a pH meter, and residual persulfate was quantified with an automatic potentiometric titrator according to literature procedures.23 Another set of samples were taken out from the shakers, and surface tensions of the samples were measured right away at the original temperatures with an automatic tensiometer using the duNouy Ring method.24 All tests herein and thereafter were run at least in triplicate. One set of samples maintained at 60 °C were used to examine potential impact of SPS to the degradation of surfactants and the distribution of residual surfactants in the aqueous phase and the oily phase, if present. Although SPSB


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Figure 1. Persulfate decomposition profiles in the presence of representative surfactants at ambient and elevated temperatures: (a-1) nonionic surfactants at 20 °C; (b-1) nonionic surfactants at 40 °C; (c-1) nonionic surfactants at 60 °C; (a-2) anionic surfactants at 20 °C; (b-2) anionic surfactants at 40 °C; and (c-2) anionic surfactants at 60 °C ([SPS]0 = 50.0 g/L; [surfactants]0 = 0 or 10.0 g/L; ∼150 rpm).

nonionic surfactants induced by SPS at 20 °C was very mild, but SPS induced rapid and serve clouding of nonionic surfactants at elevated temperature, with clouding developed in 1 d at 40 °C and only in 1 h at 60 °C. These surfactants have poly(oxyethylene) chains as their hydrophilic tails, which are very susceptible to oxidation,26−29 including oxidation by sulfate radicals.27 Literature studies also indicate that the oxidation of nonionic surfactants can substantially lower their cloud points (Brij 35, > 100 °C; Triton X-100, 64 °C; Tween 80, 93 °C)30 by cleaving the hydrophilic poly(oxyethylene) chains and generating intermediates that are more hydrophobic and less soluble than the original surfactants.28,29 All monovalent anionic surfactants are susceptible to precipitation or clouding in the presence of SPS, especially at ambient temperature, as summarized in Table S2. By contrast, C12-MADS, a divalent anionic surfactant, experienced no

Kinetic Experiments. The oxidation kinetics of individual PAHs in the biphasic tar/water system were also studied at 60 °C. Approximately 100 mg of coal and 20 mL of solutions with SPS at 50 g/L and C12-MADS at 10 g/L were capped in 40 mL EPA VOA vials, and agitated at 60 °C (∼150 rpm). Sample workup and analytical procedures are the same as just described above, but much more frequent sampling was conducted in 3 d as illustrated in Figure S9.

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RESULTS AND DISCUSSION Clouding and Precipitation of Surfactants. As summarized in Table S2, nonionic surfactants are susceptible to clouding in the presence of SPS. However, aqueous solutions of these three surfactants remained clear in the absence of SPS at 20 °C, 40 °C, and 60 °C during the test, which indicates that SPS can induce clouding of these surfactants. The clouding of C


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Figure 2. Residual surfactant percentage profiles in the presence of SPS at 60 °C. ([SPS]0 = 50.0 g/L; [surfactants]0 = 10.0 g/L ; ∼150 rpm).

Anionic surfactants generally slowed down the decomposition of persulfate at 20 °C (Figure 1a). The oxidant demands by SDS, SLS, and AOT increased significantly at 40 and 60 °C, with oxidant consumption more than that in the SPS-only control. The potential of SDBS to slow down oxidant decomposition also reduces significantly at elevated temperatures, especially at 60 °C. Only C12-MADS shows high potential to stabilize persulfate consistently at both ambient and elevated temperatures, which makes it an attractive choice for the coupling process. A survey of literature found only a few reports of using organic stabilizers, including sulfonic acid salts, to stabilize persulfate.37,38 However, how these organic stabilizers function to stabilize persulfate has been rarely studied and not well understood. In contrast, organic stabilizers, e.g., phosphonates, have been frequently used to slow down the decomposition of hydrogen peroxide, by acting as metal-sequesters or inhibitors of radical-chain reactions.39,40 Considering the similarity between hydrogen peroxide and persulfate, we speculate that C12-MADS may act similarly to stabilize persulfate, which requires further studies to examine. Surfactant Degradation. Figure 2 highlights that nonionic surfactants are very susceptible to degradation in the presence of SPS at 60 °C. When exposed to SPS at 60 °C, significant loss of nonionic surfactants from the aqueous phase concurred with rapid degradation of nonionic surfactants, with almost 100% loss of surfactants in the aqueous phase at 1d. At the same time, apparent redistribution of surfactants into the oily phase was observed, which also contributed to surfactant loss from the aqueous phase. Such rapid degradation and significant loss of nonionic surfactants from the aqueous phase can significantly reduce their compatibility with thermally activated persulfate. As illustrated in Figure 2, monovalent anionic surfactants are also susceptible to degradation when exposed to SPS at 60 °C. The loss of monovalent anionic surfactants ranged from 64.7% to 85.0% at 1 d. By contrast, C12-MADS is less susceptible to degradation in the presence of SPS than monovalent anionic

precipitation or clouding in the presence of SPS at both ambient and elevated temperatures. Although the precipitation of monovalent anionic surfactant became less severe as temperature increased, SDS and SDBS still experienced precipitation phenomena at 40 °C, whereas AOT solutions continued to develop clouding even at 60 °C. In the absence of SPS, most anionic surfactants dissolved well in water at 10 g/L at 20, 40, and 60 °C, except for SDS and AOT at 20 °C (aqueous solubility at 20 °C: SDS,31 2.27 g/L; AOT,32 8.17 g/ L); SDS and AOT also dissolved very well at 10 g/L at 40 and 60 °C. Thus, SPS can induce precipitation of these monovalent anionic surfactants. Anionic surfactants may be susceptible to precipitate from aqueous solutions by the addition of couterions, e.g., Na+ herein.15,33 This is confirmed by the observation that addition of an equal molar amount of Na2SO4 instead of SPS to the solutions of these monovalent anionic surfactants also induced similar precipitation/clouding phenomena as observed with SPS addition. Clouding or precipitation of surfactants reduces their aqueous concentrations and thus their solubilization potential.15 Another concern is that the resulting oily phase due to surfactant clouding or surfactant precipitate may extract and immobile organic contaminants from the aqueous solution.30,34 In this regard, the resistance of C12-MADS to SPS-induced clouding or precipitation increases the chance of C12-MADS as a compatible surfactant for the proposed coupling process. Oxidant Demands by Surfactants. As illustrated in Figure 1, nonionic surfactants generally accelerated the consumption of persulfate at both ambient and elevated temperatures, compared with the SPS-only control. During the test, nonionic surfactants induced severe loss of oxidant at elevated temperatures, especially at 60 °C. The high oxidant demands by these nonionic surfactants are consistent with their susceptibility to oxidation,35,36 which reduces their chance of being compatible surfactants with thermally activated persulfate. D


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Figure 3. Impact of C12-MADS and reaction temperature on PAHs oxidation: (a) representative ΣPAHs degradation and persulfate consumption time profiles, (b) representative 2−6 ring PAHs degradation profiles with a reaction time of 3 days (coal tar, ∼100 mg; [SPS]0 = 50.0 g/L + [C12MADS]0 = 0 or 10.0 g/L, 20.0 mL; T = 20, 40, 60, or 75 °C; ∼150 rpm).

°C at the end of the study, 0.9−1.1 for samples at 40 °C and 0.9−1.0 for samples at 60 °C. Among these eight surfactants, AOT,41 SLS,42 and Tween 8043 are prone to acid-catalyzed hydrolysis at elevated temperatures. To evaluate potential contribution of acid-catalyzed hydrolysis to the degradation of these three surfactants when exposed to SPS at 60 °C, comparative experiments with surfactant solutions adjust to pH ≈ 1 with sulfuric acid in the absence of SPS were conducted at 60 °C. The results (Figure S2) indicate that AOT and Tween 80 are highly susceptible to acid-catalyzed hydrolysis at elevated temperature. SLS is also susceptible to acid-catalyzed hydrolysis, but to less extent than AOT and Tween 80. Acidcatalyzed hydrolysis of these three surfactants are expected to expedite their degradation to some extent when exposed to SPS

surfactants and nonionic surfactants. As previously discussed, C12-MADS can stabilize persulfate to some extent, which may reduce the production rate of sulfate radicals and thus reduce C12-MADS oxidation by sulfate radicals to some extent. In addition, the charge repulsion between the divalent C12-MADS anions and sulfate radicals is expected to be greater than that between monovalent surfactant anions and sulfate radicals, which may also contribute to the observed less susceptibility of C12-MADS to SPS-induced degradation than monovalent anionic surfactants. During the compatibility study, acidification of the surfactant solutions was observed, due to persulfate decomposition. Initial pH of the surfactant/SPS solutions ranged from 3.1 to 5.6 at the beginning, and was reduced to to 2.0−2.6 for samples at 20 E


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Figure 4. Comparison of PAHs oxidation in the presence of Triton X-100, SDBS and C12-MADS: (a) representative ΣPAHs degradation and persulfate consumption time profiles, (b) representative 2−6 ring PAHs degradation profiles with a reaction time of 3 days (coal tar, ∼100 mg; [SPS]0 = 50.0 g/L + [surfactant]0 = 0 or 10.0 g/L, 20.0 mL; T = 60 °C; 150 rpm).

at elevated temperature. In this regard, alkyl sulfonate surfactants, including C12-MADS, generally have high hydrolytic stability,15 which is desired for the proposed coupling process. In addition, the degradation of surfactants may also interference with the surface tensions of surfactant solutions. Thus, monitoring of surface tensions of the samples was conducted in the screening study, and the results can be found in Table S3. During the whole test period, all surfactant solutions retained their low surface tensions at all three test temperatures when exposed to SPS, even for nonionic surfactants that experienced ∼100% loss of original surfactants from the aqueous solution when exposed to SPS at 60 °C. This implies that the degradation of surfactants may generate some

organic byproducts that also have surface active properties, which has been documented in literature before.29 Oxidation of Coal Tar PAHs in the Biphasic Tar/Water System. The screening study indicates that C12-MADS shows high compatibility with thermally activated persulfate by demonstrating high aqueous stability and low oxidant demand when exposed to SPS at both ambient and elevated temperatures. Thus, further study was mainly carried out with C12-MADS to evaluate the potential of the coupling process for challenging remediation scenarios, with coal tar DNAPLs as an example in this study. The coal tar tested in this study contains 20 2−6 ring PAHs as its primary semivolatile organic contaminants (Table S4), and these PAHs account for 40.6% of the total coal tar weight. The oxidation of PAHs contained in F


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Environmental Science & Technology Table 1. Individual PAHs Oxidation Kinetic Constants in Tar/Water Biphasic Systema phase I (0−4 h) PAHi

C0 (mmol/g tar)

NAP 2MN 1MN ACP DBF FLR PHE ANT FLT PYR BaA CHR B[b,j,k]F BeP BaP DaA ICP BgP

0.4996 0.1650 0.0965 0.2610 0.2810 0.2057 0.3974 0.0716 0.1822 0.1426 0.0406 0.0410 0.0283 0.0117 0.0124 0.0013 0.0053 0.0037

phase II (4−72 h)

kobs,i (h−1)

R2

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.9933 0.9398 0.9969 0.9730 0.9939 0.9703 0.9936 0.9356 0.9709 0.9497 0.9754 0.9791 0.9219 0.9922 0.9791 0.9925 0.9878 0.9796

0.0562 0.0734 0.0848 0.0591 0.0330 0.0431 0.0286 0.0422 0.0166 0.0276 0.0591 0.0477 0.0459 0.0105 0.0439 0.0122 0.0136 0.0183

0.0009 0.0026 0.0004 0.0019 0.0008 0.0010 0.0005 0.0022 0.0013 0.0011 0.0015 0.0006 0.0026 0.0004 0.0008 0.0001 0.0004 0.0006

kobs,i (h−1)

R2

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.9966 0.9899 0.9971 0.9674 0.9802 0.9955 0.9966 0.9861 0.9748 0.9780 0.9684 0.9850 0.9677 0.9922 0.9843 0.9206 0.9893 0.9086

0.0136 0.0099 0.0093 0.0089 0.0034 0.0051 0.0038 0.0058 0.0026 0.0042 0.0040 0.0028 0.0025 0.0029 0.0029 0.0033 0.0050 0.0019

0.0002 0.0008 0.0001 0.0003 0.0001 0.0002 0.0001 0.0001 0.0001 0.0002 0.0003 0.0001 0.0001 0.0001 0.0001 0.0002 0.0002 0.0002

Coal tar, ∼100 mg; [SPS]0 = 50.0 g/L + [C12-MADS]0 = 10.0 g/L, 20 mL; T = 60 °C; 150 rpm; and representative kinetic fitting data were included in Figure S9.

a

enhanced solubilization of PAHs apparently contributes to effective oxidation of coal tar PAHs with the coupling process at 60 °C, as observed in Figure 3. Although increasing the reaction temperature is crucial to enhance the mass transfer and consequent oxidation of coal tar PAHs, further raising the reaction temperature from 60 to 75 °C showed a negative impact to PAH oxidation and oxidant utilization, as shown in Figure S5. The contribution of increasing temperature to the enhancement of solubilization of individual PAHs levels off at high temperature, e.g., 75 °C herein, as illustrated in Figure S3. By contrast, raising the reaction temperature from 60 to 75 °C accelerated oxidant consumption, with rapid depletion of oxidant in 3 d at 75 °C (Figure 3a). The effects of C12-MADS concentration and oxidant dosage on PAHs oxidation were then studied at 60 °C. As illustrated in Figure S6, increasing C12-MADS concentration enhances both PAHs oxidation and oxidant utilization efficiency, which are consistent with the contributions of C12-MADS to the solubilization of PAHs and the stabilization of persulfate. This is highly desired for potential field applications, where addition of high concentrations of surfactants may be required to expedite the remediation. On the other hand, increasing oxidant dosage generally improves PAHs oxidation but lowers oxidant utilization efficiencies, as illustrated in Figure S7. If applied in field applications, then the cons and pros of using high oxidant dosage should be carefully weighed to achieve cost-effective treatment of contaminants. For comparison, persulfate oxidation of coal tar PAHs in the presence of Triton X-100 and SDBS was also carried out at 60 °C. The results are summarized in Figure 4. Compared with persulfate alone, all three surfactants enhanced PAH oxidation, which illustrates the important contribution of surfactantenhanced solubilization to effective PAHs oxidation. Among these three surfactants, SDBS and C12-MADS enhanced PAH oxidation to a greater extent than Triton X-100, but with less persulfate consumption and consequently markedly greater oxidant utilization efficiency, as shown in Figure S8b. This

coal tars is primarily limited by low aqueous availability of PAHs.23 Figure 3 highlights that both heating and C12-MADS are required to achieve effective and efficient oxidation of coal tar PAHs. Without C12-MADS, heating alone could improve PAHs oxidation to some extent. But the oxidation of PAHs, especially 4−6 ring PAHs, was still very limited at 60 °C, whereas further increasing the reaction temperature to 75 °C showed negative impact on PAHs oxidation. By contrast, a combination of heating and C12-MADS significantly improved PAHs oxidation. Especially the oxidation of high molecular PAHs, e.g., 4−6 ring PAHs, could be substantially enhanced with the coupling process. When C12-MADS was present, less oxidant was consumed as well, compared with oxidant consumption in the absence of C12-MADS. Thus, the coupling process can markedly improve oxidant utilization efficiencies ( ΔΣPAHs , ΔPS

total oxidized amount of PAHs divided by the amount of SPS consumed), as illustrated in Figure S5b. The impact of C12-MADS and heating to the oxidation of individual PAHs coincides with the effect of C12-MADS and heating to the solubilzation of individual PAHs from coal tar, which is detailed in Figures S3 and S4. This coincidence is consistent with the assumption that the oxidation PAHs is primarily limited by their mass transfer across tar/water interface, which is confirmed by observation of negligible accumulation of coal tar PAHs in the aqueous phase with/ without C12-MADS in the oxidation experiments when persulfate was not fully consumed. The enhancement levels of individual PAHs solubilization increase with temperature, C12-MADS concentration, and the size and molecular weight of PAHs, as illustrated in Figures S3 and S4. A combination of C12-MADS and heating can signficantly accelerate coal tar PAHs solubilization. In the solubilization experiment, C12MADS at 10 g/L could enhance the apparent solubilities of naphthalene and phenanthrene by a factor of ∼12 and ∼163 at 60 °C, respectively, compared with corresponding aqueous solubilities in the absence of C12-MADS at 20 °C. The G


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oxidation of contaminants, surfactants compatible with thermally activated persulfate is a prerequisite. Proper surfactants should have high aqueous stability and low oxidant demands. In practice, a screening study should be conducted to select surfactants with proper compatibility under the intended treatment conditions, including reaction temperature, surfactant concentration, and oxidant concentrations, etc. The aqueous stability of surfactants can be preliminary measured by surfactant clouding and precipitation in the presence of persulfate, followed by further measurement of the chemical stability of surfactants by monitoring of residual surfactants through HPLC analyses or other proper methods. In addition, surface tensions monitoring of the surfactant solutions is necessary during the test to check whether low surface tensions can be maintained or not. It is also critical to measure the oxidant demands by surfactants through titration of residual persulfate to select surfactants with low oxidant demands. A compatible surfactant, C12-MADS, was identified through the screening study. This surfactant even shows potential to stabilize persulfate at both ambient and elevated temperatures, which also contributes to its relatively low susceptibility to degradation when exposed to persulfate at elevated temperature. A combination of C12-MADS and thermally activated persulfate can significantly improve not only the oxidation of PAHs contained in coal tar but also the oxidant utilization efficiency, showing great potential for challenging subsurface remediation scenarios, such as source areas or hot spots containing soil-sorbed HOCs and NAPLs. Guided by the observations made here, further study is currently being pursued to explore the fundamental mechanisms behind the stabilization of persulfate by C12-MADS and its relatively low susceptibility to degradation by persulfate at elevated temperature, aiming to provide insightful guidance on selecting compatible surfactants with activated persulfate.

correlates with the much higher oxidant demand by Triton X100 than the other two as shown in Figure 1, which highlights the critical impact of oxidant demands by surfactants to effective oxidation of PAHs. Compared with SDBS, C12-MADS achieved further enhanced PAH oxidation and higher oxidant utilization efficiency, consistent with the higher compatibility of C12-MADS with thermally activated persulfate than that of SDBS. PAHs Oxidation Kinetics in the Biphasic Tar/Water System. Further study was conducted at 60 °C to explore the kinetics of the oxidation of individual PAHs in the biphasic tar/ Ci

water system. As illustrated in Figure S9, fitting ln C itar versus t tar,0

shows that the oxidation of individual PAHs can be described by two-phase first-order kinetics, where Citar is the residual concentration of each individual PAHs in coal tar at time t. The kinetic results were summarized in Table 1. As shown Table 1, the apparent rate constant, kobs,i, of the first phase is much higher than that in the second phase. The two-phase first-order kinetic behavior of the oxidation of individual PAHs with the coupling process coincides with the kinetic behavior of surfactant-enhanced solubilization of individual PAHs from coal tar.44 Previous literature studies show that PAHs originally close to the tar/water interface are more readily available for solubilization,44 which accounts for the initial rapid solubilization and consequent oxidation of individual PAHs. After rapid depletion of this portion of individual PAHs, further mass transfer and oxidation of PAHs is expected to be primarily limited by the slow diffusion of individual PAHs inside coal tar instead.45 As illustrated in Table 1, individual kobs,i for each phase generally decreases with the increase of molecular size. The aqueous availability of individual PAHs generally drops with the increase of molecular size,46 which is consistent with the observation (Figure 3b) that low-molecular-weight PAHs are generally oxidized to greater extent that than high-molecularweight PAHs. Although the aqueous solubilities of 4−6 ring PAHs are normally several orders less than those of 2-ring PAHs,46 kobs,i of 4−6 ring PAHs for each phase generally have the same order of magnitude as those of 2-ring PAHs. First, the enhanced levels to the apparent solubilities of individual PAHs by surfactants generally increase with the size and hydrophobicity of individual PAHs,47 as illustrated in Figure S3. Thus, C12MADS enhances the solubilization and consequent oxidation of high-molecular-weight to a greater extent than those of lowmolecular-weight PAHs. Second, the reactivity of individual PAHs generally increases with their size.46 The high reactivity of high-molecular-weight PAHs can render much more rapid oxidation of these PAHs in the aqueous phase,46 which, in return, can enhance mass their transfer across the tar/water interface to some extent.48 The effect of the reactivity of individual PAHs on their kobs,i for each phase is also illustrated by greater kobs,i of linear anthracene than its angular isomer phenanthrene, as anthracene has higher reactivity than phenanthrene.46 Thus, the kinetic results indicate that the overall oxidation rate of individual PAHs can be understood as a function of the molecular-weight dependent phase partitioning and reactivity. Environmental Implications. A combination of surfactants and thermally activated persulfate was proposed to enhance the aqueous availability and consequent oxidation of organic contaminants. In order to achieve cost-effective

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.6b05477. Section S1, chemicals; Section S2, HPLC analyses of residual surfactant; Section S3, C12-MADS assisted coal tar solubilization; Figure S1, structures of representative nonionic and anionic surfactants; Figure S2, acidcatalyzed hydrolysis of SLS, AOT, and Tween 80; Figure S3, impact of C12-MADS and temperature on PAHs solubilization; Figure S4, impact of C12-MADS concentration on PAHs solubilization; Figure S5, impact of reaction temperature and C12-MADS on individual PAHs oxidation and oxidant utilization efficiencies in the biphasic tar/water system; Figure S6, impact of C12MADS concentration on PAHs oxidation and oxidant utilization efficiencies in the tar/water biphasic system; Figure S7, impact of oxidant concentration on PAHs oxidation and oxidant utilization efficiencies in the tar/ water biphasic system; Figure S8, impact of surfactant compatibility on individual PAHs oxidation and oxidant utilization efficiencies in the biphasic tar/water system; Figure S9, two-phase kinetic fitting for naphthalene (NAP) and phenanthrene (PHE) oxidation in the tar/ water biphasic system; Table S1, formula and properties of representative nonionic and anionic surfactants; Table S2, clouding/precipitation of surfactants in the presence H


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of persulfate; Table S3, surface tension profiles of surfactants solutions in the presence of persulfate; Table S4, tar composition and viscosity profiles; and additional references (PDF)

AUTHOR INFORMATION

Corresponding Author

*Tel: +86-20-39310213; fax: +86-20-39310213; e-mail: [email protected] (D.D.). ORCID

Dayi Deng: 0000-0002-9964-7132 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge financial support from Special Project of Applicable Science and Technology of Guangdong Province (No. 2016B020240008) and National Natural Science Foundation of China (No. 41201304). Our deepest gratitude goes to the editor and the anonymous reviewers for their careful work and thoughtful suggestions that have helped improve this paper substantially.

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Compatibility of Surfactants and Thermally Activated Persulfate for

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