Polycyclic aromatic hydrocarbons (PAHs), which are present in contaminated groundwater, have attracted increasing attention because of their serious harm to humans. In this study, the degradation performance of fluoranthene (FLT), a typical tetracyclic PAHs in organic contaminated sites, was investigated in the persulfate (PS)/Fe(II)/citric acid (CA) system. The effects of PS, CA, and Fe(II) doses on FLT degradation were tested. With the molar ratio at 60/20/5/1 of PS/Fe(II)/CA/FLT, FLT removal reached 96.3% in 120 min, much higher than 62% removal without CA at the same PS and Fe(II) doses, indicating that the addition of CA could remarkably enhance the FLT degradation. The water quality conditions (pH, anions and humic acid) were also investigated for their effects on FLT degradation. The results of probe tests, electron paramagnetic resonance detection and scavenging experiments showed that and acted predominantly on FLT degradation. The influence of surfactants on FLT degradation was examined. Furthermore, the primary degradation intermediates of FLT were detected by GC-MS and the possible degradation pathways of FLT were proposed. Finally, the effectiveness of the PS/Fe(II)/CA process for the FLT degradation in actual groundwater demonstrated that the process has a great prospect for the remediation of FLT-contaminated groundwater.

  • The enhancing effect of CA on FLT degradation was investigated.

  • and were the primary ROS in the PS/Fe(II)/CA system for FLT degradation.

  • The possible degradation pathways of FLT were proposed.

  • The effects of surfactants on FLT degradation were investigated.

  • Effective degradation of FLT in actual groundwater was achieved.

Graphical Abstract

Graphical Abstract

Polycyclic aromatic hydrocarbons (PAHs), which are a class of hazardous organic chemicals with two or more benzene rings, are widely present in the environment due to the incomplete combustion of coal, fuel and wood (Keyte et al. 2013). Fluoranthene (FLT), one of the typical tetracyclic PAHs with carcinogenic property and difficult degradation (Kadri et al. 2017), is on the priority list of PAHs regulated by the U.S. Environmental Protection Agency (USEPA) due to its high content and frequent detection in various countries, including China (Wang et al. 2017). Therefore, the elimination of PAHs, including FLT, from contaminated soil and groundwater has become an urgent task, and the corresponding technologies are diversely developed.

In recent years, in situ chemical oxidation (ISCO) has played an increasingly important role in the remediation of organics contaminated soil and groundwater (Baciocchi 2013; Qian et al. 2016). In Fenton and Fenton-like processes, oxidants including percarbonate (SPC), persulfate (PS), potassium permanganate and hydrogen peroxide (H2O2) (Ghauch et al. 2011) are commonly used to generate reactive oxygen species (ROS), which are essential for pollutant degradation. In particular, PS, which combines the advantages of H2O2 and other oxidants, is becoming more and more popular in ISCO (Lindsey & Tarr 2000). PS not only has strong stability but also produces and by chain reactions after activation (Lindsey & Tarr 2000), which can destruct hard-to-degrade pollutants, including PAHs (Peluffo et al. 2016). There are various activation methods for PS, such as thermal activation (Ghauch et al. 2015), UV activation (Ghauch et al. 2017), alkali activation and transition metals activation, among them transition metals activation is highly appreciated for its cost-effective and simple operating condition (Wang & Wang 2018). This activation method includes not only plated zero valent iron (ZVI), but also bimetallics (Ghauch et al. 2013), trimetallics (Ayoub & Ghauch 2014) or even iron scrap (Naim & Ghauch 2016). Compared with other transition metals, Fe-based catalysts (Fe(II), Fe(III), Fe0, MOFs, etc.) are commonly employed for their inexpensive, high-efficiency and environmentally friendly features (Rastogi et al. 2009a; El Asmar et al. 2021). The reaction stoichiometric efficiency (RSE) is the number of moles of probes degraded over the number of moles of PS consumed (Baalbaki et al. 2018; Tantawi et al. 2019). Generally, the % RSE values can vary a lot in different systems. For example, PS activated with iron powder showed % RSE around 5% while if Fe is used in MOF systems, the % RSE reached 30–40%, and the thermal activation showed an even greater value up to 70% and UV254 activation reached 55% (Table 1). They also correlated the % RSE with the total organic carbon (TOC) removed (Ghauch et al. 2017). Fe(II), as one of the transition metal ions, can catalyze PS to produce that degrades the target pollutant (Equation (1)):
(1)
Table 1

Comparison of % RSE in different systems for pollutant removal

Catalytic systemsRSE (%)pollutantReaction time (min)References
Fe0/PS 5.2 Sulfamethoxazole 120 Ghauch et al. (2013)  
Fe/Cu/PS 11 2,4-dichlorophenol 60 Fang et al. (2021)  
iFe/PS 72 Ranitidine 60 Naim & Ghauch (2016)  
MIL-88-A/PS 19.7–33.45 Naproxen 120 Asmar et al. (2021)  
UV254/PS 52 Chloramphenicol 60 Ghauch et al. (2017)  
Thermal/PS 7.5–68 Naproxen 90 Ghauch et al. (2015)  
Fe(II)/CA/PS 2.9 Fluoranthene 120 present work 
Catalytic systemsRSE (%)pollutantReaction time (min)References
Fe0/PS 5.2 Sulfamethoxazole 120 Ghauch et al. (2013)  
Fe/Cu/PS 11 2,4-dichlorophenol 60 Fang et al. (2021)  
iFe/PS 72 Ranitidine 60 Naim & Ghauch (2016)  
MIL-88-A/PS 19.7–33.45 Naproxen 120 Asmar et al. (2021)  
UV254/PS 52 Chloramphenicol 60 Ghauch et al. (2017)  
Thermal/PS 7.5–68 Naproxen 90 Ghauch et al. (2015)  
Fe(II)/CA/PS 2.9 Fluoranthene 120 present work 
However, it also has been found that Fe(II) can consume and when it is in excess (Equations (2) and (3)). In addition, when the pH of the solution is greater than 4, Fe(II) and Fe(III) are prone to form precipitates, causing a decrease in the concentration of dissolved iron ions (Gupta & Gupta 1981). Some researchers tried to overcome these drawbacks and boosted the removal of contaminants by adding chelating agents such as citric acid (CA) and ethylene diamine tetraacetic acid (EDTA) (Liang et al. 2004, 2009; Wu et al. 2014). Zhang et al. (2017) found that the CA addition could enhance the activation of PS by nano zero-valent iron (nZVI) effectively, thus promoting the 2,4,6-trichloroanisole degradation. Yan & Lo (2013) also discovered that ethylenediamine-N, N′-disuccinic acid (EDDS) and EDTA could enhance the PS/Fe(II) process to some extent. CA, a natural organic acid that is widely found in nature, is extensively used in environmental remediation because of its excellent chelating ability, biodegradability and environmental friendliness (Wu et al. 2014). So far, little research has been reported on PS oxidation for FLT degradation that is enhanced by Fe(II) coupled with CA. Further confirmation of the reaction mechanisms for FLT degradation in the PS/Fe(II)/CA system is still needed, and the possible degradation pathways of FLT need to be revealed. Further, the surfactants are frequently used as a pretreatment step in soil remediation due to their effective desorption effect to PAHs in contaminated soil. So it is also necessary to explore the effect of surfactants on this process.
(2)
(3)
Therefore, the objectives of this study are to: (1) investigate the efficiency of FLT removal in PS/Fe(II)/CA system and study the effect of dose of chemicals on FLT removal; (2) explore the effect of groundwater matrices on FLT degradation in the PS/Fe(II)/CA system; (3) identify the primary ROS for the FLT degradation in the PS/Fe(II)/CA system and propose possible degradation pathways for FLT by analyzing the intermediates produced in the PS/Fe(II)/CA system; (4) reveal the effect of surfactants on FLT degradation; and (5) evaluate the effectiveness of the PS/Fe(II)/CA system for FLT removal in actual contaminated groundwater.

Regents and chemicals

The details of materials used in this study are available in the Supplementary material.

Experimental procedures

The FLT stock solution (0.004 M) was prepared by employing 0.0202 g solid FLT with 25 mL acetone and then stored in the refrigerator at 4 °C. All FLT degradation experiments were conducted in a 250 mL cylindrical glass reactor (an inner diameter of 6.0 cm and a height of 9.0 cm) with two openings on the top for dosing and sampling in which a magnetic stirrer (600 r min−1) was used to keep the homogeneity of the aqueous solution. The temperature in all batch reactions was controlled at 20 ± 0.5 °C by a low-temperature water bath (DC, Ningbo, China) and the initial solution pH was unadjusted. According to the EPA DSSTox database, FLT has a relative molecular mass of 202.25 and a solubility of 0.20–0.26 mg L−1 in.water. The FLT concentration was set at 0.001 mM which corresponds to the concentration of FLT at actual PAHs-contaminated groundwater (Ogbuagu et al. 2011). 0.001 mM of FLT was prepared by adding 0.25 mL of FLT stock solution to a volumetric flask containing 1 L ultrapure water (groundwater used for actual groundwater tests) for dissolution for 8–10 hr. The addition of acetone was so small that it could not affect the reaction. First, the FLT solution was transferred into the reactor. Second, the predetermined amounts of FeSO4·7H2O and CA were added to the solution successively. After Fe(II) and CA were mixed homogeneously, a designed dosage of PS was added to initiate the reaction. At the predetermined time, 0.5 mL samples were taken out to a 5 mL vial pre-filled with 0.5 mL methanol to terminate the reaction. Afterwards, the samples were filtered by 0.22 μm millipore membrane for high-performance liquid chromatography (HPLC) analysis. To investigate the influence of the initial pH of the solution, the initial solution pH was adjusted by 0.10 M NaOH or 0.10 M H2SO4, otherwise, the initial solution pH was unadjusted.

For the experiments concerning the influence of groundwater matrixes, sodium bicarbonate, sodium chloride or humic acid was initially added into the reactor containing FLT, then other chemicals were added later as in the FLT experiments. The remaining content of FLT was determined by HPLC at the predetermined time. In the experiment exploring the effect of surfactants on FLT degradation, the surfactant was added first and other reagents were added later, then the FLT concentration was analyzed by HPLC at the predetermined reaction time. All tests were conducted at least in triplicate and the mean values were reported.

Analytical methods

The concentration of FLT was analyzed by HPLC (LC-20AT, Shimadzu, Japan) at the detection wavelength of 235 nm. The mobile phase was a mixture of ultrapure water and methanol (10:90 (v/v)). The oven temperature was 30 °C and the injection value was 100 μL.

The intermediates during FLT degradation were analyzed by GC-MS (Agilent, G2577A, USA) equipped with a flame ionization detector (FID) and an HP-5MS capillary column (Li et al. 2019a, 2019b). Electron paramagnetic resonance (EPR) tests for active radical analyses were conducted on a Bruker EMX-8/2.7C spectrometer using DMPO or TEMP as spin-trapping compound. The instrumental conditions are described in the Supplementary material.

The residual PS concentration was monitored using the KI colorimetric method (Liang et al. 2008). The concentrations of Fe(II) and total Fe in the aqueous solution were determined by o-phenanthroline spectrophotometry (Harvey et al. 1955). The explicit analytical methods used in this work are shown in the Supplementary material.

Comparison of FLT degradation in various systems

Figure 1 shows FLT degradation in different systems, among which the molar ratio of PS/Fe(II)/CA/FLT is 60/20/5/1. In the blank test, it can be seen that up to 4.7% FLT was lost in the solution, indicating that the volatilization of FLT was well controlled in the entire experiment. FLT degradations of 6.4 and 13.6% were achieved when only Fe(II) or PS along were added, indicating no direct reaction of Fe(II) with FLT when compared with the blank result, and the PS reaction with FLT in aqueous solution was also generally slow. For the PS/Fe(II) system, FLT was degraded rapidly in the initial 5 min (50% removal), then more than 12% was removed in the next 105 min. This two-stage degradation process was also observed in another report (Gan & Ng 2012) and it could be inferred that a large number of ROS were generated immediately with enough PS and available Fe(II) initially, but concurrently leading to massive consumption of Fe(II) and larger accumulation of Fe(III) in the following time, therefore losing the activation function. It is noteworthy that FLT degradation was increased dramatically from 62 to 96.3% in 120 min reaction after adding CA into the PS/Fe(II) system. Compared with the PS/Fe(II) system, the PS/Fe(II)/CA system had a persistent degradation of FLT in the whole reaction time, proving the significant effectiveness of CA in the PS oxidative process on FLT degradation. There may be several reasons for the enhancement of the FLT degradation by CA. On the one hand, the pH of the solution was decreased from 5.34 to 4.82 (Table 2) after adding CA, which could prevent the precipitation of irons and hold the soluble iron in the solution to a certain extent. On the other hand, CA may elevate iron solubility by forming Fe(II)/Fe(III)-CA complexes, and promote Fe(II) recycling by altering the redox conditions (Liang et al. 2004).

Table 2

Parameter values in PS/Fe(II)/CA system

Experimental conditions PS/Fe(II)/CA/FLTpH (initial/final)FLT removal (%)
60/20/0/1 5.34/4.06 62.0 
60/20/5/1 4.82/3.92 96.3 
*60/20/5/1 8.07/7.92 11.1 
*pH = 4 3.98/4.06 48.2 
*180/60/15/1 8.01/7.95 20.0 
*pH = 4 4.00/3.83 78.2 
*180/60/60/1 7.91/7.81 25.9 
*pH = 4 4.03/3.91 90.7 
*300/100/100/1 7.31/7.12 32.5 
*pH = 4 4.05/3.63 93.9 
Experimental conditions PS/Fe(II)/CA/FLTpH (initial/final)FLT removal (%)
60/20/0/1 5.34/4.06 62.0 
60/20/5/1 4.82/3.92 96.3 
*60/20/5/1 8.07/7.92 11.1 
*pH = 4 3.98/4.06 48.2 
*180/60/15/1 8.01/7.95 20.0 
*pH = 4 4.00/3.83 78.2 
*180/60/60/1 7.91/7.81 25.9 
*pH = 4 4.03/3.91 90.7 
*300/100/100/1 7.31/7.12 32.5 
*pH = 4 4.05/3.63 93.9 

*Experiments were conducted by using the actual groundwater.

Table 3

Parameter values in PS/Fe(II)/CA system in groundwater matrixes

Experimental conditionsPS/Fe(II)/CA/FLTpH (initial/final)FLT removal (%)
pH = 3.0 60/20/5/1 3.03/2.99 99.4 
pH = 5.0 4.98/4.00 95.1 
pH = 7.0 7.02/5.97 19.1 
pH = 9.0 8.99/8.40 6.3 
pH = 11.0 10.96/11.02 5.7 
Cl = 1 mM 60/20/5/1 4.69/3.98 96.3 
Cl = 10 mM 4.68/4.12 96.8 
Cl = 50 mM 4.86/5.97 96.1 
HCO3 = 1 mM 6.47/7.91 11.7 
HCO3 = 10 mM 8.30/8.42 3.1 
HCO3 = 50 mM 8.41/8.37 
HA = 0.1 mg L−1 60/20/5/1 4.36/3.84 93.1 
HA = 0.2 mg L−1 4.53/4.12 88.1 
HA = 0.5 mg L−1 4.56/4.09 80.2 
HA = 0.8 mg L−1 4.50/4.12 60.2 
HA = 1 mg L−1 4.58/3.93 51.6 
HA = 10 mg L−1 4.72/4.04 33.6 
Experimental conditionsPS/Fe(II)/CA/FLTpH (initial/final)FLT removal (%)
pH = 3.0 60/20/5/1 3.03/2.99 99.4 
pH = 5.0 4.98/4.00 95.1 
pH = 7.0 7.02/5.97 19.1 
pH = 9.0 8.99/8.40 6.3 
pH = 11.0 10.96/11.02 5.7 
Cl = 1 mM 60/20/5/1 4.69/3.98 96.3 
Cl = 10 mM 4.68/4.12 96.8 
Cl = 50 mM 4.86/5.97 96.1 
HCO3 = 1 mM 6.47/7.91 11.7 
HCO3 = 10 mM 8.30/8.42 3.1 
HCO3 = 50 mM 8.41/8.37 
HA = 0.1 mg L−1 60/20/5/1 4.36/3.84 93.1 
HA = 0.2 mg L−1 4.53/4.12 88.1 
HA = 0.5 mg L−1 4.56/4.09 80.2 
HA = 0.8 mg L−1 4.50/4.12 60.2 
HA = 1 mg L−1 4.58/3.93 51.6 
HA = 10 mg L−1 4.72/4.04 33.6 
Figure 1

FLT degradation performance in various systems. [PS]0 = 0.06 mM, [Fe (II)]0 = 0.02 mM, [CA]0 = 0.005 mM, [FLT]0 = 0.001 mM.

Figure 1

FLT degradation performance in various systems. [PS]0 = 0.06 mM, [Fe (II)]0 = 0.02 mM, [CA]0 = 0.005 mM, [FLT]0 = 0.001 mM.

Close modal

In order to determine whether the improvement of FLT removal was due to the effect of pH reduction or the effect of Fe(II)/Fe(III)-CA complexes, the initial solution pH of the PS/Fe(II) process was adjusted to 4.82 which was in line with the unadjusted pH of the PS/Fe(II)/CA system. Although the initial pH dropped to 4.82 in the PS/Fe(II) system, FLT removal was only 6.2% higher than the unadjusted one (Figure 2). In addition, the soluble Fe and oxidant concentrations during the reaction were determined. As illustrated in Figure S1b, the total Fe in the PS/Fe(II) system was significantly lower than that in the PS/Fe(II)/CA system, which confirmed that CA could increase the concentration of soluble iron by forming Fe(II)/Fe(III)-CA complexes. Besides, the PS consumption increased with the addition of CA, which implied the generation of more ROS, thus enhancing FLT degradation.

Figure 2

Effect of pH change caused by CA on FLT degradation. [PS]0 = 0.06 mM, [Fe (II)]0 = 0.02 mM, [CA]0 = 0.005 mM, [FLT]0 = 0.001 mM.

Figure 2

Effect of pH change caused by CA on FLT degradation. [PS]0 = 0.06 mM, [Fe (II)]0 = 0.02 mM, [CA]0 = 0.005 mM, [FLT]0 = 0.001 mM.

Close modal

Effect of chemical dosages on FLT degradation

To further explore the degradation performance of FLT in the PS/Fe(II)/CA system, the effect of chemical dosages was investigated. First, the experiments were performed by keeping the dosages of Fe(II) and CA at 0.02 and 0.005 mM, respectively, while the dosage of PS was changed from 0.02 to 0.12 mM to investigate the effect of PS dosage on FLT degradation. As shown in Figure 3(a), FLT degradation was improved significantly with the increase of PS concentration from 0.02 to 0.06 mM, which demonstrated that more ROS would be generated with the appropriate enhancement of PS concentration. Nevertheless, when the dosage of PS further increased to 0.12 mM, FLT removal had barely changed. This phenomenon may be due to the fact that the concentration increased with the PS concentration and the excess could react with , and , leading to some unnecessary loss of ROS (Equations (4)–(6)):
(4)
(5)
(6)
Figure 3

Effect of (a) PS dosage ([Fe(II)]0 = 0.02 mM, [CA]0 = 0.005 mM, [FLT]0 = 0.001 mM), (b) Fe(II) dosage ([PS]0 = 0.06 mM, [CA]0 = 0.005 mM, [FLT]0 = 0.001 mM), and (c) CA dosage ([PS]0 = 0.06 mM, [Fe(II)]0 = 0.02 mM, [FLT]0 = 0.001 mM) on FLT degradation.

Figure 3

Effect of (a) PS dosage ([Fe(II)]0 = 0.02 mM, [CA]0 = 0.005 mM, [FLT]0 = 0.001 mM), (b) Fe(II) dosage ([PS]0 = 0.06 mM, [CA]0 = 0.005 mM, [FLT]0 = 0.001 mM), and (c) CA dosage ([PS]0 = 0.06 mM, [Fe(II)]0 = 0.02 mM, [FLT]0 = 0.001 mM) on FLT degradation.

Close modal
Despite the fast reaction in the first 2 min, FLT degradation in the PS/Fe(II)/CA system within 2–60 min was in accordance with the pseudo-first-order reaction kinetic model (Equations (7)–(9)). As illustrated in Figure S2a and Table S3, the calculated rate constants (k) significantly increased from 0.0062 (R2 > 0.67) to 0.0326 min−1 (R2 > 0.92) when PS dosage was increased from 0.02 to 0.06 mM, and the FLT removal half-time (t1/2) was decreased from 111.8 to 21.3 min. However, upon increasing the PS dose from 0.06 to 0.12 mM, k showed a slight decrease along with a minor increase in t1/2, which indicated that k would not increase and t would not decrease indefinitely with increasing PS concentration. Therefore, considering the FLT removal and avoiding waste, the PS dosage was set at 0.06 mM in the following experiments:
(7)
(8)
(9)
Second, the amount of Fe(II) was varied to study the effect of Fe(II) on FLT degradation, and the amounts of PS and CA were controlled at 0.06 and 0.005 mM, respectively, and the results are shown in Figure 3(b), Figure S2b and Table S3. As the Fe(II) concentration rose from 0.005 to 0.02 mM, FLT degradation and k were increased from 73.5 to 96.3% and from 0.0105 to 0.0326 min−1, and t1/2 was reduced from 66.0 to 21.3 min, respectively. However, the adverse phenomenon of FLT degradation, k and t1/2 were observed when the Fe(II) concentration increased to 0.04 and 0.06 mM. FLT removal decreased by 5.6 and 11.5%, k reduced to 0.0308 and 0.0227 min−1, and t1/2 increased to 22.5 and 30.5 min, respectively, indicating that excess Fe(II) would in turn deplete the ROS in solution (Rastogi et al. 2009a).

Finally, further investigation was conducted to explore the effect of CA dosage on FLT removal. In the experiment, the doses of PS and Fe(II) were 0.06 and 0.02 mM, respectively. The consequences, which are presented in Figure 3(c), Figure S2c and Table S3, are similar to the effects of PS and Fe(II) doses on FLT degradation as described above. When CA was set as 0.02 mM, although FLT degradation was almost unchanged, k was significantly lower than that of 0.005 and 0.01 mM CA, and t1/2 was the opposite. This was mainly owing to the following two reasons. For one thing, ROS would react with CA causing unnecessary loss, and for another thing, excessive CA could form a very stable chelate with Fe(II), which was not conducive to the catalytic reaction of Fe(II) (Rastogi et al. 2009b). k and t1/2 did not change significantly when CA was increased from 0.005 to 0.01 mM, so the CA dosage was determined to be 0.005 mM considering the remediation cost. Therefore, in the following experiments, the doses of PS, Fe(II), CA and initial FLT concentration were set as 0.06, 0.02, 0.005 and 0.001 mM, respectively.

Effect of groundwater matrixes on FLT degradation

Effect of initial pH on FLT degradation

It is well known that the initial pH of the solution has a significant effect on ROS generation, thus the initial solution pH (3, 5, 7, 9 and 11) was adjusted by 0.1 M H2SO4 or 0.1 M NaOH to explore the effect of initial solution pH in the PS/Fe(II)/CA system and the results are shown in Figure 4(a) and Table 3. FLT removal reached 99.4 and 95.1% only at the initial pH of 3 and 5, while FLT removal plummeted to 19.1, 6.2 and 5.7% when the initial pH was increased to 7, 9 and 11, respectively, indicating that this system could only effectively degrade FLT in a limited acidic pH range. This result was ascribed in detail for the following reasons. First of all, as the solution pH increased, Fe(II)-CA chelation was converted to Fe(II)-(OH)-CA and Fe(II)-(OH)2-CA, which were more stable and difficult to release Fe(II) into the solution, thereby inhibiting the activation of PS (Kusic et al. 2011). Then, a high solution pH led to the low solubility of free Fe(II) and promoted the precipitation of Fe(OH)2 and Fe(OH)3, thus reducing the production of ROS (Han et al. 2015). What is more, would convert to under alkaline conditions (Equation (10)), leading to more generation of . However, SO42−, as one of the inevitable products of PS, would slightly inhibit the activity of to some extent (Liang et al. 2007). In conclusion, compared to neutral or basic conditions, the acidic condition was more favorable for FLT removal by the PS/Fe(II)/CA system:
(10)
Figure 4

Effects of (a) initial solution pH, (b) anions (Cl and HCO3) and (c) HA on FLT degradation in PS/Fe(II)/CA system. [PS]0 = 0.06 mM, [Fe (II)]0 = 0.02 mM, [CA]0 = 0.005 mM, [FLT]0 = 0.001 mM.

Figure 4

Effects of (a) initial solution pH, (b) anions (Cl and HCO3) and (c) HA on FLT degradation in PS/Fe(II)/CA system. [PS]0 = 0.06 mM, [Fe (II)]0 = 0.02 mM, [CA]0 = 0.005 mM, [FLT]0 = 0.001 mM.

Close modal

Effect of anions on FLT degradation

To further study the application of the PS/Fe(II)/CA system in practical scenarios, Figure 4(b) and Table 3 show the effect of two common anions in groundwater, HCO3 and Cl, on FLT degradation. To investigate the behavior of various concentrations of anion in this system, 1, 10 and 50 mM were set for each anion. The inhibitory effect was observed in the presence of all various concentrations of HCO3, and FLT removal was decreased from 96.3 to 11.7, 3.1 and 0% when increasing the concentration of HCO3 from 0 to 1, 10 and 50 mM, respectively, mainly because HCO3 would consume and (Equations (11) and (12)). Besides, the presence of HCO3 could act as a buffer to prevent drastic changes in solution pH, thus impairing FLT degradation (Dong et al. 2019):
(11)
(12)
The presence of Cl may affect FLT degradation in a negative way by complexing with Fe(II) and Fe(III) to form chloro-iron complexes (Equations (13)–(15)), and interacting with forming less reactive species, such as Cl2• and (Equations (16)–(19)):
(13)
(14)
(15)
(16)
(17)
(18)
(19)
In the presence of Cl, this system degraded at least 96% FLT within the concentration range surveyed in this study, which was not consistent with the recognized results raised above. It was concluded that although Cl was a scavenger of free radicals, more and were generated through complex free radical chain reactions to improve FLT removal. Monteagudo et al. (2015) found that although Cl could consume , it could also continue to produce more through the reaction Equations (20)–(22). Wang & Wang (2020) also discovered that the hypochlorous acid radicals formed from Equation (16) could be quickly converted to through Equation (23), indicating that there was less or no loss of and therefore no inhibition for FLT removal. In addition, the concentration of Cl may be several orders of magnitude higher than the concentration of due to the limited oxidant dose (Lian et al. 2017), which remedies the lower reaction between Cl and FLT:
(20)
(21)
(22)
(23)

Hu et al. (2019) reported that the representative anions, including sulfate and nitrate, had negligible effect on p-Nitrophenol removal in the persulfate/microwave heating system, and Diao et al. (2020) had the similar finding that no significant effect of sulfate as well as nitrate on bisphenol A degradation in a heterogeneous ultrasound-enhanced sludge biochar catalyst/persulfate process. In addition, the dose of PS was only set at 0.06 mM in this experiment, so the sulfate produced by decomposition was small enough to affect FLT degradation.

Effect of humic acid on FLT degradation

Actual groundwater is abundant in natural organic matter (NOM), and Lin et al. (2017) found that the electron-rich groups in NOM can react with and . Since humic acid (HA) is an important component of NOM, it was selected as a representative of NOM to study its effect on FLT degradation in the PS/Fe(II)/CA system. Generally, the concentration of HA substances in groundwater varies from 1 to 50 mg L−1 (Li et al. 2019a, 2019b). Considering the limited chemical dosing in the experiment, the HA concentration range was set to 0.1–10 mg L−1, and different concentrations were set in the range of 0.1–1 mg L−1 to carefully examine the effect of HA on the reaction. As shown in Figure 4(c), it is obvious that FLT degradation decreased with the increasing HA concentration, proving HA had a significant inhibitory effect on FLT degradation. On the one hand, as mentioned above, HA could consume and and compete with FLT, resulting in a lower FLT removal. On the other hand, while HA competed for and , HA also inhibited the regeneration of Fe(II) (Wang & Lemley 2004), which led to the reduction of ROS and FLT degradation.

Mechanism of FLT degradation in the PS/Fe(II)/CA system

Identification of ROS in the PS/Fe(II)/CA system

According to other research reports (Yu et al. 2018), several ROS (, and ) are generated in a PS system activated by Fe(II), which may be produced in the PS/Fe(II)/CA process as well. Nitrobenzene (NB) as probe compound, anisole (AN) as both and probe compound and carbon tetrachloride (CT) as probe compound to confirm the generated ROS in this process (see Supplementary material). As shown in Figure S3, all three probes were degraded to varying degrees and especially in the PS/Fe(II)/CA system, demonstrating the presence of , , , and the addition of CA could promote the generation of ROS. As far as we know, there was no suitable 1O2 probe due to the insolubility of some potential chemicals in water and insensitivity to 1O2 (Krasnovsky et al. 2008; Entradas et al. 2020). Therefore, EPR was used to determine whether there was 1O2 during FLT oxidation in the PS/Fe(II)/CA system.

To further confirm the presence of ROS, EPR detection was used in this study. As shown in Figure 5, DMPO-OH adducts, DMPO-SO4 adducts and TEMP-1O2 adducts were found in the PS/Fe(II)/CA system, indicating the presence of , and 1O2. However, probably due to the low concentration and instability of , no peaks were observed for the adducts composed of DMPO and (Fang et al. 2013).

Figure 5

EPR spectra at the reaction time of 10 min in the PS/Fe(II)/CA system, (a) DMPO as radical trap and (b) TEMP as radical trap.

Figure 5

EPR spectra at the reaction time of 10 min in the PS/Fe(II)/CA system, (a) DMPO as radical trap and (b) TEMP as radical trap.

Close modal

Isopropanol (IPA) acted as a scavenger for and , furfuryl alcohol (FFA) acted as a scavenger for and 1O2, while at the same time TBA and chloroform (CF) acted as scavengers for and , respectively (see Supplementary material). As shown in Figure 6, FLT could be degraded by 96.3% without scavengers, while FLT degradation drastically dropped to 50.6% after the addition of TBA, which revealed that was the dominant ROS in FLT degradation. Besides, FLT degradation was also significantly inhibited after the excess IPA addition, indicating that FLT removal also depended on . Notably, 27.9% of FLT was held back with CF addition, demonstrating that a certain amount of was generated and made a non-negligible contribution to FLT degradation. Finally, FLT removal was decreased by 32.3% after FFA addition compared to the inhibition of TBA, which elucidated that 1O2 also contributed significantly to FLT degradation. In summary, both and were dominant ROS for FLT removal, along which performed a more important role in FLT degradation. This was mainly due to a large amount of being converted from (Equation (10)).

Figure 6

Effect of scavengers on FLT degradation in the PS/Fe(II)/CA system. [IPA]0 = [TBA]0 = [CF]0 = [FFA]0 = 10 mM, [PS]0 = 0.06 mM, [Fe (II)]0 = 0.02 mM, [CA]0 = 0.005 mM, [FLT]0 = 0.001 mM.

Figure 6

Effect of scavengers on FLT degradation in the PS/Fe(II)/CA system. [IPA]0 = [TBA]0 = [CF]0 = [FFA]0 = 10 mM, [PS]0 = 0.06 mM, [Fe (II)]0 = 0.02 mM, [CA]0 = 0.005 mM, [FLT]0 = 0.001 mM.

Close modal

Possible FLT degradation pathways

The intermediates of FLT degradation were detected by GC-MS, which are shown in Table S6. Several chromatograms of intermediates are supplied in Figure S4. Based on the degradation products, the following possible degradation pathway was proposed (Figure 7). First, FLT was decomposed into o-dimethylbenzene (o-xylene) and naphthalene after being attacked by ROS (the attack position is shown by the arrow). Second, attacked naphthalene and made naphthalene undergo hydroxylation to form 1,4-naphthol (Onwudili & Williams 2007), which in turn formed 1,4-naphthoquinone under continued attack by (Bunce et al. 1997). After 1,4-naphthoquinone was produced from hydroxylation, it formed phthalic anhydride by ring-opening, which generated 1,4-benzoquinone through hydroxylation successively (Gu et al. 2018). In addition, the benzene ring of o-xylene was attacked by to form two dimethylphenol isomers, which were subsequently decomposed into carbon dioxide and water by ring-opening and completely mineralization (Xue et al. 2018).

Figure 7

Proposed FLT degradation pathways in PS/Fe(II)/CA system. [PS]0 = 0.06 mM, [Fe (II)]0 = 0.02 mM, [CA]0 = 0.005 mM, [FLT]0 = 0.001 mM.

Figure 7

Proposed FLT degradation pathways in PS/Fe(II)/CA system. [PS]0 = 0.06 mM, [Fe (II)]0 = 0.02 mM, [CA]0 = 0.005 mM, [FLT]0 = 0.001 mM.

Close modal

Effect of surfactants on FLT degradation

PAHs, including FLT, are commonly found in contaminated soil due to their low solubility in water and their high adsorption capacity to soil (Galiulin et al. 2002). Surfactants could act as solubilizers by reducing surface tension, which could desorb contaminants from the soil and transport them to the aqueous phase for further treatment (Lamichhane et al. 2017). However, some studies have shown that surfactants may affect the degradation of pollutants by advanced oxidation (Ying 2006). The effects of Tween 80 (TW-80), sodium dodecyl sulfate (SDS), polyoxyethylene octyl phenyl ether (TX-100) and polyoxyethylene lauryl ether (Brij-35) on FLT removal in PS/Fe(II)/CA system were investigated and their properties are listed in Table S2. The surfactant concentrations in actual contaminated site remediation are generally from 1.0 to 10 g L−1 (Besha et al. 2018), and the concentration was set as 1.0 g L−1, considering the limited dose applied in this system.

As shown in Figure 8, both k and FLT degradation showed a significant decrease after the addition of surfactants, which proved that surfactants could inhibit FLT removal in this system. For one thing, surfactant, as an organic compound that could be decomposed by oxidation, would compete with FLT for ROS (Mousset et al. 2014). Sun et al. (2021) found that TW-80, Brij-35 and TX-100 could inhibit trichloroethene (TCE) degradation by competing with TCE to consume through EPR experiments. For another thing, when the surfactant concentration was greater than 1.0 critical micelle concentrations (CMC), the surfactant would exist as micelles, which consist of the hydrophobic core and hydrophilic shell, and with regard to hydrophobic FLT molecule, it would tend to be stuck in the micelle core (Trellu et al. 2017). In the case of FLT molecules trapped in the micelle core, ROS need to attack the micelle and open it before they could degrade FLT. In addition, the effects on various concentrations of TW-80 and SDS, which are representatives of popular nonionic and anionic surfactants, were also investigated on FLT degradation. It is worth noting that the inhibition effect of TW-80 was slightly stronger than SDS at the same concentration, probably because the CMC of TW-80 is much smaller than SDS, so it could create more micelles in the solution thus requiring more ROS to open it.

Figure 8

Effect of (a) different surfactants ([TW-80]0 = [SDS]0 = [TX-100]0 = [Brij-35]0 = 1 g L−1), and (b) TW-80 and SDS concentrations on FLT degradation in the PS/Fe(II)/CA system. [PS]0 = 0.06 mM, [Fe (II)]0 = 0.02 mM, [CA]0 = 0.005 mM, [FLT]0 = 0.001 mM.

Figure 8

Effect of (a) different surfactants ([TW-80]0 = [SDS]0 = [TX-100]0 = [Brij-35]0 = 1 g L−1), and (b) TW-80 and SDS concentrations on FLT degradation in the PS/Fe(II)/CA system. [PS]0 = 0.06 mM, [Fe (II)]0 = 0.02 mM, [CA]0 = 0.005 mM, [FLT]0 = 0.001 mM.

Close modal

Removal of FLT in actual groundwater remediation

To investigate the applicability of the PS/Fe(II)/CA system for FLT degradation, experiments were conducted with actual groundwater instead of ultrapure water. The major characteristics of the actual groundwater are displayed in Table S5. As shown in Figure 9, when the molar ratio of PS/Fe(II)/CA/FLT was 60/20/5/1, FLT removal was only 11.1% in 120 min. The significant decrease in degradation was mainly due to the adverse effects of the high actual groundwater pH and the high concentration of HCO3. The effect of higher agent doses on FLT degradation was investigated and the results were less than satisfactory. Therefore, the initial solution pH was adjusted to 4 using 0.1 M H2SO4, and a significant increase of FLT degradation was unexpectedly acquired. At pH = 4, FLT degradation reached 90.7 and 93.9% at 180/60/60/1 and 300/100/100/1 molar ratio of PS/Fe(II)/CA/FLT (Table 2), respectively, mainly because H+ eliminated the inhibitory effect of HCO3 by reacting with it under acidic conditions. Considering that the pH of the solution after the reaction were all below 4, natural alkaline substances such as lime could be added to the restored groundwater (Long et al. 2014). Clearly, adjusting the initial solution pH after an appropriate increment of chemical dosage was a feasible way to remediate FLT in groundwater.

Figure 9

FLT degradation performance in actual groundwater in the PS/Fe(II)/CA system. [FLT]0 = 0.001 mM.

Figure 9

FLT degradation performance in actual groundwater in the PS/Fe(II)/CA system. [FLT]0 = 0.001 mM.

Close modal

In this research, 96.3% FLT removal was achieved in 120 min when the PS/Fe(II)/CA/FLT molar ratio was 60/20/5/1 and the % RSE in PS/Fe(II)/CA/FLT was 2.9%. Compared with the PS/Fe(II) system, the addition of CA could dramatically improve FLT degradation performance by chelating Fe(II)/Fe(III) and producing more ROS. The solution pH had a strong influence on the PS/Fe(II)/CA system, and the acidic condition was favorable for FLT removal. HCO3 and HA had a certain inhibition on FLT degradation, which became obvious as the concentrations increased, but Cl had almost no effect on FLT degradation. The results of probe tests, EPR detection and scavenging experiments indicated that and acted predominantly on FLT degradation. The possible FLT degradation pathway was proposed based on the detection of o-xylene and naphthalene during FLT degradation by GC-MS. The FLT molecule was attacked by ROS to split into o-xylene and naphthalene, where naphthalene and o-xylene eventually formed CO2 and H2O through hydroxylation, ring-opening, oxidation and addition reactions, ring-opening, and mineralization, respectively. The presence of surfactants inhibited FLT degradation and the inhibition was slightly pronounced for surfactants with low CMC at the same concentration. Experiments conducted in actual groundwater indicated that the PS/Fe(II)/CA system could be used to remediate FLT in actual groundwater.

This study was financially supported by a grant from the National Natural Science Foundation of China (No. 41977164).

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

All relevant data are included in the paper or its Supplementary Information.

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Supplementary data