Ultrasound assisted, thermally activated persulfate oxidation of coal tar DNAPLs

Journal of Hazardous Materials 318 (2016) 497–506

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Ultrasound assisted, thermally activated persulfate oxidation of coal tar DNAPLs Libin Peng, Li Wang, Xingting Hu, Peihui Wu, Xueqing Wang, Chumei Huang, Xiangyang Wang, Dayi Deng ∗ Guangdong Provincial Key Lab of Functional-materials for Environmental Protection, School of Chemistry and Environment, South China Normal University, Guangzhou, Guangdong 510006, China

h i g h l i g h t s • Coal tar PAHs can be effectively oxidized in biphasic and triphasic systems. • Individual PAH oxidation fits first-order kinetics in the biphasic system. • PAH degradability is related to individual molecular sizes and reactivity.

a r t i c l e

i n f o

Article history: Received 21 February 2016 Received in revised form 1 June 2016 Accepted 6 July 2016 Available online 6 July 2016 Keywords: Coal tar Dense non-aqueous phase liquid In situ chemical oxidation Activated persulfate Ultrasound

a b s t r a c t The feasibility of ultrasound assisted, thermally activated persulfate for effective oxidation of twenty 2–6 ringed coal tar PAHs in a biphasic tar/water system and a triphasic tar/soil/water system were investigated and established. The results indicate that ultrasonic assistance, persulfate and elevated reaction temperature are all required to achieve effective oxidation of coal tar PAHs, while the heating needed can be provided by ultrasonic induced heating as well. Further kinetic analysis reveals that the oxidation of individual PAH in the biphasic tar/water system follows the first-order kinetics, and individual PAH oxidation rate is primary determined by the mass transfer coefficients, tar/water interfacial areas, the aqueous solubility of individual PAH and its concentration in coal tar. Based on the kinetic analysis and experimental results, the contributions of ultrasound, persulfate and elevated reaction temperature to PAHs oxidation were characterized, and the effects of ultrasonic intensity and oxidant dosage on PAHs oxidation efficiency were investigated. In addition, the results indicate that individual PAH degradability is closely related to its reactivity as well, and the high reactivity of 4–6 ringed PAHs substantially improves their degradability. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Former manufactured gas plants (FMGPs) are very common waste sites that remain difficult to remediate [1–3]. These plants were historically used to produce artificial gas between early 1800s and 1950s, especially in the US and Europe [4]. For example, the US has ∼50,000 FMGPs with potential clean-up cost up to 128 billion dollars [5]. The main focus and challenge in the remediation of most FMGP sites are coal tars, which are viscous DNAPLs (dense non-aqueous phase liquids) that contain thousands of individual compounds, including many known or suspected carcinogens [4].

∗ Corresponding author. E-mail address: [email protected] (D. Deng). http://dx.doi.org/10.1016/j.jhazmat.2016.07.014 0304-3894/© 2016 Elsevier B.V. All rights reserved.

Especially, polycyclic aromatic hydrocarbons (PAHs), including all 16 PAHs on the US EPA’s priority pollutant list, are main components of coal tars [6,7]. Currently, coal tars in FMGPs present a serious environmental problem due to their persistence and toxicity, acting as long-term contamination sources of soil and groundwater [4,8]. In situ chemical oxidation (ISCO) with activated persulfate is an attractive option for rapid source destruction of coal tars without aboveground treatment [9]. With proper activation, persulfate can generate highly reactive radical species, mainly sulfate radical (E0 = 2.7 v) and hydroxyl radical (E0 = 2.8 v) that can oxidize most recalcitrant organic contaminants, including PAHs [9,10]. However, DNAPLs are a tough challenge for ISCO [11,12], including activated persulfate [13]. Specifically, the oxidation of PAHs, the primary contaminants of concerns in coal tars, is expected to be constrained by their limited availability in the aqueous phase, where PAHs

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oxidation is assumed to happen [9]. PAHs generally have low aqueous solubility, and their dissolution from coal tars is kinetically very slow due to the viscous nature of coal tars at ambient temperature [8,14–18]. This study investigated the feasibility of using ultrasound to accelerate the dissolution and mass transfer rate of PAHs from coal tar to the aqueous phase and consequently to assist persulfate oxidation of coal tar PAHs. When ultrasound is applied to two immiscible liquids, emulsion can be generated, and tiny droplets of the more viscous liquid are generally dispersed into the less viscous liquid [19]. For the biphasic tar/water system, ultrasonic emulsion of coal tar is expected to substantially increase the tar/water interfacial areas and consequently greatly raise PAHs dissolution rate. In addition, ultrasound induces intense mixing that can markedly enhance mass transfer in the bulk aqueous phase for both the oxidant and dissolved PAHs. Ultrasound is also an established subsurface application technology, often applied in oil industry for oil stimulation and enhanced oil recovery [20]. Subsurface ultrasound can fracture soil formation and break up soil aggregates, thereby enhancing soil permeability [21,22]. This will facilitate more even distribution of oxidants in the target zones. In addition, subsurface ultrasonic application results in heating of the treated zones [23]. The induced heating can reduce the interfacial tension and viscosity of coal tars, and consequently enhance PAHs dissolution and mass transfer from coal tar to the aqueous phase [8,18]. In addition, recent study [24] indicates that the induced heating can effectively activate persulfate and achieve rapid oxidation of PAHs in a biphasic soil/water system. In return, the rapid oxidation of dissolved PAHs will allow for continuous dissolution and consequent oxidation of PAHs from coal tars, potentially generating a positive feedback loop that induces efficient oxidation of coal tar PAHs. In this study, the feasibility of ultrasound assisted, thermally activated persulfate for effective oxidation of twenty 2–6 ringed coal tar PAHs was first investigated and established in a biphasic tar/water system, and the reaction conditions were optimized to achieve efficient PAHs oxidation. In addition, the oxidation kinetics of individual PAH was characterized and primary parameters controlling the rate constants were identified. Results from the biphasic study provide valuable information on how to optimize reaction conditions to achieve efficient oxidation of all coal tar PAHs, especially 4–6 ringed PAHs that are normally hard to oxidize due to their extremely low aqueous availability. Finally, the feasibility of the coupling process toward the oxidation of coal tar PAHs in a triphasic tar/soil/water system was evaluated and established. 2. Material and methods 2.1. Materials Coal tar (99%) was from Alfa Aesar. The main components of coal tar were characterized according to literature procedures [25] and illustrated in Table 1. Information about all other chemicals used is listed in Text S1. Surface soil samples were collected from an uncontaminated site near the campus at a depth of 20–40 cm. The soil was dried and grinded to pass a 35-mesh sieve. The properties of the soil were characterized according to literature procedures [26]. The soil contains approximately 42.8% well graded sand (<2.0 mm), 19.8% silt and 37.4% clay, and has an organic carbon content of ∼1.1 wt%, an iron content of ∼5.1 wt% and a pH of ∼4.3. 2.2. Oxidation of coal tar in the biphasic tar/water system ∼100 mg of coal tar was weighted and put as one droplet at the bottom center of a 40-mL EPA VOA vial, followed by the addition of

Table 1 Tar composition.

L. Peng et al. / Journal of Hazardous Materials 318 (2016) 497–506

20 mL solution with designated concentrations of sodium persulfate. The initial study was run at varied temperatures (20–80 ◦ C) in a temperature-controlled circulating waterbath to study the effect of reaction temperature on the oxidation of coal tar PAHs (ultrasound intensity = 60% maximum power capacity; [Na2 S2 O8 ]0 = 50 g/L). Ultrasound was generated by a direct immersion titanium probe (6-mm diameter) 20 kHz-sonicator (Model Scientz-IID, Ningbo Scientz Biotechnology Co., Ningbo, China). The probe was immersed ∼1.0 cm below the liquid surface at the center of the vial, and ultrasound was operated at a pulse mode (1 s on and 1 s off) at designated power levels (a maximum power capacity of 950 W). In addition, blank controls without oxidants were run under the same conditions. Comparative experiments in the absence of ultrasound were also carried out in a temperature-controlled shaking waterbath (∼150 rpm) at considered temperature. All experiments were run in triplicates. The effect of oxidant dosage on the oxidation of coal tar PAHs was investigated at 80 ◦ C by varying initial sodium persulfate concentration at 10, 25 and 50 g/L (ultrasound intensity = 60% capacity; reaction temperature = 80 ◦ C). On the other hand, the effect of ultrasound intensity was studied by varying the ultrasound intensity at 20, 40 or 60% capacity ([Na2 S2 O8 ]0 = 50 g/L; reaction temperature = 80 ◦ C). At designated intervals, sonication/shaking was stopped and the reaction vial was immediately put into an ice-water bath to stop the oxidation. Thereafter, residual coal tar and aqueous solution were centrifuged and separated according to work-up procedures in Text S2. The solution pH was measured with a pH meter, and residual persulfate concentration was determined by an automatic potentiometric titrator. The aqueous solution and residual coal tar were extracted with hexanes and hexane/acetone (1:1 v/v) respectively, according to procedures in Text S2. The extracted organic samples were analyzed with GC/FID (Agilent GC 7890A) and GC/MS (Agilent 7890A-5975C), and the concentrations of twenty targeted PAHs were measured. GC/MS was also applied to identify potential degradation intermediates. Detailed analytical procedures are provided in Text S2 as well. 2.3. Oxidation of coal tar in the triphasic tar/soil/water system 5.0 g of soil was weighted into a 40-mL EPA VOA vial, followed by the addition of one droplet of ∼100 mg of coal tar at the center of the soil solids, and the weight of coal tar was recorded. The vial was capped and kept for two weeks at an ambient temperature to let soil solids absorb the coal tar. Thereafter, 20 mL solutions ([Na2 S2 O8 ]0 = 50 g/L) were added. Ultrasound assisted oxidation (ultrasound intensity = 60% capacity) was carried out at 80 ◦ C, maintained with a temperature-controlled circulating waterbath. Blank control experiments without persulfate in the presence of ultrasound and comparative experiments without ultrasound in the shaking waterbath were also run. Sample work-up and analytical procedures were basically the same as in the tar/water system. To evaluate the feasibility of ultrasound induced, thermally activated persulfate for the oxidation of coal tar PAHs, another set of experiments were carried out without using a waterbath to provide the heating. Initial run was carried out with an ultrasound intensity at 60% capacity ([Na2 S2 O8 ]0 = 50 g/L), and ultrasonic intensity was later adjusted to 70% capacity to achieve more efficient oxidation of coal tar PAHs. 3. Results and discussion Twenty 2–6 ringed PAHs, including fifteen PAHs on the US EPA list of priority pollutants, two alkyl PAHs (1MN and 2MN) and one heteroatom PAH (DBF), were identified as the main extractable

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semi-volatile organic contaminants, as illustrated in Table 1. These PAHs account for ∼34.4% of the total mass of coal tar. This study focused on the oxidation of these PAHs, which are generally found as primary contaminants of concerns at FMGPs [2]. 3.1. Preliminary optimization of reaction conditions in the biphasic tar/water system The oxidation of coal tar PAHs is expected to be seriously limited by the slow dissolution and mass transfer of PAHs due to large viscosity of coal tar at an ambient temperature. To reduce the viscosity of coal tar, heating is a very effective approach [27]. Therefore, optimization of reaction temperature was first carried out. As illustrated in Fig. 1, elevated temperature is critical in achieving effective oxidation of coal tar PAHs. Initially, there was only minor PAHs oxidation at 20 ◦ C in 30 min, with ∼2.2%PAHs degradation. Further raising the reaction temperature to 60 ◦ C only resulted in slightly enhanced PAHs oxidation, with ∼11.8%PAHs degradation in 30 min. On the contrary, substantial oxidation of PAHs, including 5–6 ringed PAHs, was observed at 80 ◦ C, with ∼53.2%PAHs degradation in 30 min. Ultrasound may cause sonodegration of organic contaminants as well. However, results from blank controls at 80 ◦ C without persulfate (Fig. S1) confirm that potential removal of PAHs due to direct sonodegration or evaporation of PAHs from coal tars was minor, with ∼1.2% PAHs degradation in 30 min versus ∼53.2% with persulfate present. Therefore, persulfate is mainly responsible for the oxidation of PAHs in the ultrasound assisted persulfate oxidation system. However, persulfate in a waterbath shaker at 80 ◦ C (∼150 rpm) only achieved minor oxidation of coal tar PAHs (Fig. S2), with ∼8.9% PAHs degradation in 30 min. The results demonstrate that ultrasound substantially improves PAHs oxidation efficiencies. In addition, persulfate decomposition profiles in the presence of ultrasound or in the watherbath shaker (Fig. S3) shows negligible difference, implying that ultrasound enhances persulfate utilization efficiencies as well. Overall, preliminary results indicate that ultrasound assisted persulfate at 80 ◦ C can achieve effective oxidation of coal tar PAHs. In addition, GC/MS analysis of the hexane extracted and derivatized samples of the aqueous solutions show negligible accumulation of PAHs derived intermediates with the coupling process at 80 ◦ C, consistent with previous literature findings that activated persulfate can mineralize dissolved PAHs [24,28]. 3.2. Oxidation kinetics of individual PAH in the biphasic tar/water system The oxidation of twenty PAHs in a biphasic system is very complex. To better understand the oxidation processes and identify primary factors that determine the oxidation efficiency of each individual PAH, kinetic modeling and analysis were carried out. The oxidation of individual PAH progresses in two consecutive steps. As shown in Reaction 1 and Reaction 2, oxidation begins with dissolution and mass transfer of the considered PAH from coal tar to the aqueous phase, followed by oxidation with sulfate anion radical, the primary oxidative species for thermally activated persulfate, in the aqueous phase. PAHi refers to the considered individual PAH;PAHi,tar , PAHi in the coal tar; PAHi,aq , PAHi dissolved in the aqueous phase; Intermediatei,aq , the oxidized intermediate of PAHi . PAHi,tar → PAHi,aq PAHi,aq + SO4

•−

→ Intermediatei,aq + SO4 2−

Reaction 1 Reaction 2

When persulfate is in excess (as illustrated by representative persulfate decomposition time profiles in Fig. S3) and the reaction

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Fig. 1. Impact of reaction temperature to PAHs oxidation in the biphasic tar/water system: (a) residual PAHs concentrations and (b) PAHs degradation percentages (ultrasound intensity = 60% capacity; [Na2 S2 O8 ]0 = 50.0 g/L; PAHs destruction efficiencies at 30 min were compared in the figures).

temperature is high (i.e. 80 ◦ C), thermally activated persulfate oxidation of PAHi,aq in the bulk aqueous phase is very rapid [24], while slow dissolution of PAHi from coal tar, Reaction 1, is expected to be the rate limiting step. Previous literature study [18] indicates that the dissolution rate of PAHi from excess coal tar (i.e. which composition does not vary significantly within a short reaction period) to the aqueous phase can be described by the following equations: Vtar

Vtar ×

dCi,aq dCi,tar = Vaq × × dt dt

Vaq × Vtar ×

dCi,aq dt



= K1,i × a0 × Ceq,i,aq − Ci,aq

(dm2 ) is the total tar/water interfacial area; K1,i (dm/s) is the mass transfer coefficient. Due to rapid persulfate oxidation of PAHi,aq , Ci,aq can be neglected, compared to Ceq,i,aq . Consequently, Eq. (3) can be simplified as:

(1)



  dCi,tar = K1,i × a0 × Ceq,i,aq − Ci,aq dt

(2)

(3)

Vtar (dm3 ) and Vaq (dm3 ) are the volumes of the tar and the aqueous phase, respectively; Ci,tar (mol/L) and Ci,aq (mol/L) are the actual concentrations of PAHi at time t in the tar and the aqueous phase, respectively; Ceq,i,aq (mol/L) is the theoretical equilibrium concentration of PAHi in the aqueous phase according to Raoult’s law; a0

dCi,tar = K1,i × a0 × Ceq,i,aq dt

(4)

According to Raoult’s law, Ceq,i,aq = Xi × Sl,i , where Xi is the molar fraction of PAHi in coal tar and Sl,i (mol/L) is the aqueous solubility of PAHi at its pure liquid state at the considered temperature [18]. As Xi = Ci,tar /Ctotal,tar , where Ci,tar (mol/L) is the molar concentration of PAHi in coal tar and Ctotal,tar (mol/L) is the total molar concentration of all components in coal tar, Eq. (4) can be written as:

Vtar ×

K1,i × a0 × Sl,i dCi,tar = × Ci,tar Ctotal,tar dt

(5)

L. Peng et al. / Journal of Hazardous Materials 318 (2016) 497–506 Table 2 Kinetic analysis of individual PAH oxidation with ultrasound assisted persulfate oxidation at 80 ◦ C. PAHi

kobs,i min

NAP 2MN 1MN ACP DBF FLR PHE ANT FLT PYR BaA CHR

0.0727 0.0608 0.0636 0.0620 0.0291 0.0370 0.0258 0.0399 0.0213 0.0252 0.0168 0.0122

−1

R2

t1/2 (min)

0.943 0.948 0.933 0.675 0.992 0.991 0.983 0.781 0.989 0.940 0.898 0.940

9.5 11.4 10.9 11.2 23.8 18.7 26.9 17.4 32.5 27.5 41.3 56.8

At the initial reaction period, when Ctotal,tar can be considered as a constant, Eq. (5) can be written and simplified as: dCi,tar = kobs,i × Ci,tar dt where kobs,i =

K1,i ×a0 ×Sl,i Ctotal,tar ×Vtar

(6) . When the volume of the tar, persul-

fate concentration, ultrasonic conditions and reaction temperature are set, kobs,i is a constant for a considered system, and its value indicates PAHi mass transfer and oxidation efficiency. Accordingly, the mass transfer and oxidation of PAHi fit the first-order reaction kinetics. Integrating Eq. (6) gives:



ln

Ci,tar C0,i,tar



= −kobs,i × t

(7) Ci,tar

Based on the above analysis, the remaining fractions of PAHi ( C

0,i,tar

)

of individual PAHs versus reaction time (Fig. 2) were simulated with first-order kinetics. The results (Table 2) indicate that oxidation of PAHi in the first 30 min fits the first-order kinetics very well for most 2–4 ringed PAHs. The kobs,i (Table 2) generally decreases with the increase of ring number and molecular weight, as small PAHs generally have higher Sl,i at the considered temperature than larger PAHs. As a result, the half-life time (t1/2 ) generally increases with the molecular weight of PAHs, and the relative molar proportions (Ci,tar /Ctotal,tar ) of 5–6 ringed PAHs are expected to increase as the oxidation progresses, due to more rapid oxidation of 2–4 ringed PAHs. According to Eqs.

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(5) and (6), kobs,i of 5–6 ringed PAHs may keep increasing obviously. This may explain the deviation of 5–6 ringed PAHs from the simplified first-order kinetics, where kobs,i is assumed to remain constant. Results from Fig. 2 and Table 2 also show that some individual PAHs, i.e. ACP and ANT, do have much higher degradability than their structural isomers or comparative PAHs with similar molecular weights (ACP versus DBF and FLR; ANT versus PHE). This is closely related to the higher reactivity of ACP than DBF and FLR, and ANT than PHE, which is implied by the higher Ip-HOMO (energy of the highest occupied molecular orbital; Ip-HOMOs and aqueous solubility for the considered 2–4 ringed PAHs are listed in Table S1) of the former ones. Ip-HOMO of individual PAH is generally considered a good measure of the ionization potential of individual PAH [29]; PAHs with higher ionization potential are expected to have higher reactivity toward sulfate radicals, as sulfate radicals are electrophilic, which initiate the destruction of PAHs through one-electron transfer from the aromatic rings to sulfate radicals. As a result, dissolved PAHi with higher Ip-HOMO is expected to be more rapidly oxidized, which will facilitate the diffusion of fresh PAHi from coal tar to the aqueous phase and thereby contribute positively to raising its K1,i andkobs,i . In addition, Ip-HOMO and the reactivity of PAHs generally increase with PAHs size [29]. The greater reactivity of larger PAHs may substantially improve their degradability, as illustrated in Table 2. For example, the aqueous solubility of NAP (2-ringed PAH) is about 20,000-fold of that of CHR (4-ringed PAH) at ambient temperature (Table S1), but kobs,i of NAP is only about 6-fold of that of CHR (Table 2). 3.3. The contribution of ultrasound, elevated reaction temperature and persulfate to effective oxidation of coal tar PAHs The kinetics study indicates that PAHi oxidation efficiency is closely related to K1,i , a0 , and Sl,i . This helps further elucidate the contributions of elevated temperature, ultrasound and persulfate to effective oxidation of coal tar PAHs. 3.3.1. Effect of ultrasound Under mechanic shaking (∼150 rpm) in the waterbath at 80 ◦ C, coal tars still remained adhering to the vial bottom as one droplet. On the contrast, ultrasound caused coal tar droplet to disperse into

Fig. 2. Representative coal tar PAHs oxidation time profiles at 80 ◦ C (ultrasound intensity = 60% capacity; [Na2 S2 O8 ]0 = 50.0 g/L).

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Fig. 3. Impact of ultrasound intensity to PAHs oxidation in the biphasic tar/water system: (a) residual PAHs concentration and (b) PAHs degradation percentages (reaction temperature = 80 ◦ C; [Na2 S2 O8 ]0 = 50.0 g/L; PAHs destruction efficiencies at 30 min were compared in the figures).

many fine droplets, thereby substantially increasing the tar/water surface areas, a0 . In addition, ultrasound induces intense mixing that can greatly enhance K1,i . These two mechanisms operate simultaneously, and contribute to substantially enhanced oxidation of coal tar PAHs in the presence of ultrasound versus in the shaking waterbath, as discussed in Section 3.1. Considering that tar is very viscous and hard to disperse, ultrasonic intensity needs to reach a certain level to induce thorough dispersion of coal tar into fine droplets. Thereafter, further increasing the intensity will have much less impact on a0 and less enhancing effect on PAHs oxidation. As illustrated in Fig. 3, increasing the ultrasonic intensity from 20% capacity to 40% capacity did substantially improve PAHs oxidation, while further increasing the intensity to 60% only achieved minor improvement on PAHs oxidation. 3.3.2. Effect of elevated temperature As temperature increases from ambient temperature to 80 ◦ C, the viscosity and the interfacial tension of coal tar is expected to reduce substantially [27]. The reduction of interfacial tension will facilitate ultrasonic dispersion of coal tar into fine droplets, while the decrease of coal tar viscosity reduces the dispersed droplet sizes

[19]. Consequently, a0 is expected to substantially increase with temperature. On the other hand, increasing the reaction temperature and the resulting reduced tar viscosity will enhance mass transfer coefficients, K1,i . Furthermore, Sl,i also increases with temperature. Elevated temperature also plays a critical role in activating persulfate. Though ultrasound may be the main driver for persulfate activation at ambient temperature, literature studies [24,30] show that thermal activation is mainly responsible for persulfate activation at an elevated temperature (over ∼60 ◦ C) due to reduced cavitation effect at an elevated temperature. Effective activation of persulfate at 80 ◦ C results in rapid oxidation of dissolved PAHs, confirming by negligible accumulation of any individual PAH in the aqueous phase. This will further enhance mass transfer coefficients, K1,i , as literature studies indicate that oxidation of dissolved organic contaminants increases contaminant diffusion rate across the NAPL/water interface [31]. 3.3.3. Effect of oxidant dosage Raising the oxidant dosage at a considered temperature will result in more rapid destruction of dissolved PAHs. Consequently, K1,i is expected to increase with oxidant dosage, which will enhance

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Fig. 4. Impact of oxidant dosage to PAHs oxidation in the biphasic tar/water system: (a) residual PAHs concentration and (b) PAHs degradation percentages (ultrasound intensity = 60% capacity; reaction temperature = 80 ◦ C; PAHs destruction efficiencies at 30 min were compared in the figures).

PAHs oxidation efficiencies. As illustrated in Fig. 4, increasing the oxidant dosage from 10 g/L to 25 g/L results in minor enhanced oxidation of most PAHs, while increasing the oxidant dosage from 25 g/L to 50 g/L substantially increase the oxidation efficiency of PAHs in 30 min. Other dissoluble organic materials from coal tar (i.e. extractable acid or basic components in Table 1) and potential oxidized intermediates of the targeted PAHs may compete with dissolved PAHs for available sulfate radical anions. The impact levels of these competitions against PAHs oxidation are greater at low oxidant dosage than at high oxidant dosage, as illustrated in Fig. 4. 3.4. Oxidation of coal tar PAHs in the triphasic tar/soil/water system Based on the results from the biphasic tar/water system, oxidation of coal tar with persulfate at 80 ◦ C was carried out in the triphasic tar/soil/water system. Generally, the complication of soil matrix may seriously impair the oxidation of target contaminants. One issue is that soil natural organic materials and other oxidizable materials may consume portions of the oxidant and compete against coal tar PAHs, as implied by substantially enhanced persulfate decomposition in the triphasic system (Fig. S4) versus in the

biphasic system (Fig. S3). Another issue is that soil particles may trap coal tar inside their pores due to capillary forces. Therefore, oxidation of coal tar PAHs in the triphasic system is expected to be much more challenging than in the biphasic system. Persulfate decomposition profiles in the soil only control (Fig. S4) indicate that the impact of ultrasound to natural soil oxidant demand is minor, but ultrasound can shorten the reaction time required to meet the natural soil oxidant demand. Due to enhanced mass transfer of oxidizable material from the soil particles to the aqueous phase under an ultrasonic impact, much more persulfate was consumed at 30 min under an ultrasonic impact than in the waterbath shaker. But the oxidation consumption gap between ultrasonic impact and waterbath shaking was getting smaller as reaction time increases, and negligible difference in oxidant consumption at 120 min was observed. Representative oxidation profiles of PAHs are shown in Fig. 5. The results (Fig. 5) indicate that ultrasound assisted, thermally activated persulfate can achieve effective oxidation of 2–6 ringed coal tar PAHs, with ∼54.5% PAHs degradation at 30 min, ∼75.5% at 60 min, and ∼84.8% at 120 min. Control experiments with ultrasound alone showed less than 4% PAHs degradation at 120 min. On

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Fig. 5. Representative oxidation profiles of coal tar PAHs with ultrasound assisted persulfate at 80 ◦ C in the triphasic tar/soil/water system: (a) residual PAHs concentration and (b) PAHs degradation percentages (ultrasound intensity = 60% capacity; [Na2 S2 O8 ]0 = 50.0 g/L).

the other hand, comparative persulfate oxidation in the shaking waterbath only achieved ∼46.6% PAHs degradation at 120 min. Though PAHs oxidation efficiency was much less in the waterbath shaker at 120 min than that under an ultrasonic impact, the persulfate consumption at 120 min in the waterbath shaker was only slightly lower than that under an ultrasonic impact (Fig. S4). The results from Fig. 5 and Fig. S4 imply that ultrasound can enhance persulfate utilization efficiencies in the triphasic system as well. After persulfate is activated to generate sulfate radicals or other highly reactive species, they generally have very short life-time (∼milliseconds to a few seconds); they either oxidize target contaminants (e.g. dissolved PAHs herein) or be consumed through other non-productive pathways, including selfdecomposition to form O2 [32]. In addition, the decomposition of persulfate also result in acidification of the solution (pH <2.0 after 30 min with [Na2 S2 O8 ]0 = 50 g/L, both under ultrasonic impact and in the watherbath shaker), while strong acidic conditions generally accelerate the decomposition of persulfate to generate these reactive species [32]. Consequently, the enhanced mass transfer of PAHs from coal tar to the aqueous phase will favor the utilization of these reactive species toward the oxidation of coal tar PAHs versus non-productive consumption.

To assess the potential of ultrasound induced, thermally activated persulfate for the treatment of coal tar DNAPLs, oxidation of coal tars was run with persulfate in the presence of ultrasound without external heating. Preliminary trial with an ultrasound intensity at 60% ([Na2 S2 O8 ]0 = 50 g/L) indicated that ultrasonic impact could raise the slurry temperature to less than 70 ◦ C, and the oxidation efficiency only reached ∼39% PAHs degradation at 120 min. After increasing the ultrasonic intensity to 70% capacity, the slurry temperature gradually rose to ∼90 ◦ C after 30 min and remained around thereafter. Efficient oxidation of PAHs commenced in the latter case, and both 2–4 ringed PAHs and 5–6 ringed PAHs were effectively oxidized at 120 min, with ∼92.3% PAHs degradation (Fig. 6). As ultrasound breaks the soil aggregates into finer particles, the induced intense mixing and frequent colliding among the soil particles and coal tar globules generate evenly wetting and spreading of coal tar onto the soil particle surfaces, similar to literature observations about the interaction between coal tars and soil particles under acidic conditions [33]. Due to continuous ultrasonic impact, the tar-wetted particles remain well dispersed and mixed in the triphasic system. Consequently, the tar-coated surfaces of the fine soil particles are exposed to the aqueous phase, and the total

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Fig. 6. Representative oxidation profiles of coal tar PAHs with ultrasound induced, thermally activated persulfate the triphasic tar/soil/water system: (a) residual PAHs concentration and (b) PAHs degradation percentages (ultrasound intensity = 70% capacity; [Na2 S2 O8 ]0 = 50.0 g/L).

tar/water interfacial areas are expected to be much greater than those in the absence of ultrasound. According to discussion in Section 3.2, the ultrasonic enhanced mass transfer coefficients and the enhanced tar/water interfacial areas will greatly facilitating PAHs oxidation in this complex triphasic system.

guidance how to apply such equipments to achieve effective persulfate oxidation of coal tars.

3.5. Implication for potential field applications

In this study, the oxidation processes and mechanisms of coal tar DNAPLs by ultrasound assisted, thermally activated persulfate were systematically investigated. The results show that ultrasound accelerates the oxidation of coal tar PAHs in the biphasic tar/water system and the triphasic tar/soil/water system. Kinetic studies indicate that oxidation of individual PAH follows first-order kinetics, and ultrasound, elevated reaction temperature, and persulfate all play key roles in enhancing PAHs oxidation efficiency. In addition, PAH reactivity is another primary factor that regulates PAH degradability, and the higher reactivity of 4–6 ringed PAHs facilitates their effective oxidation. Finally, the feasibility of ultrasound induced, thermally activated persulfate toward the oxidation of coal tar PAHs in a triphasic tar/soil/water system was also evaluated and established. Overall, the results imply that the coupling process may offer a potential approach for source destruction of subsurface coal tar DNAPLs.

Activated persulfate technology and in-well ultrasound technology are both well-established subsurface technologies, and these two technologies may be integrated to remediate coal tar contaminated sites. After persulfate is injected to the target zones, in-well ultrasonic probe can be lowered to the target source zones through the original injection well. Following ultrasonic application provides an approach to enhance soil fracturing and permeability, the mass transfer and dispersion of target contaminants and oxidants, the activation of persulfate, mixing between persulfate and coal tars, etc. For potential field application, the soil/water ratio will be much higher than that in the batch study herein. This requires the application of high-power ultrasonic equipments, while application experiences using high-power in-well ultrasound [20] to stimulate and enhance oil well production can provide some

4. Conclusion

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