2.1. Experimental materials
PCS was collected from a site contaminated by a chemical plant in Shenzhen, China (5 years after the oil leak, the total petroleum hydrocarbon [TPH] content was 256125 mg/kg), sealed, and stored in a plastic bag. The large particles of gravel and other debris were selected and removed for later use. Aniline (C6H7N, ≥ 99.5%) was purchased from Shanghai Runjie Chemical Reagent Co., Ltd. Sodium persulfate (Na2S2O8, ≥ 98%), sodium hydroxide (NaOH, ≥ 96%), tetrachloroethylene (C2Cl4, IR special chromatographic purity), glucose solution (C6H12O6·H2O, AR), mercury sulfate (HgSO4, AR), potassium dichromate (K2Cr2O7, AR), sulfuric acid (H2SO4, AR), sodium thiosulfate, isopropanol (C3H8O, AR), tert-butanol (C4H10O, AR), chloroform (CHCl3, AR), furfuryl alcohol (C5H6O2, AR), and ethylene diamine tetraacetic acid (C10H16N2O8, AR) were obtained from Sinopharm Chemical Reagent Co., Ltd. Methanol (HPLC) was acquired from Sigma (USA). Ultrapure water produced by an ultrapure water machine (Milli-Q) was used as experimental water.
2.2. Pyrolytic treatment of PCS
In brief, 10 g of PCS was placed in a quartz ark and then transferred to the tube furnace (OTS-1200X-S 1200A, Hefei Kejing Material Technology Co., Ltd., Hefei, China) with continuously flowing inert gas N2 (60 mL·min− 1) throughout the entire pyrolysis process. Temperature and time were recorded using the tube furnace thermocouple to ensure that the set conditions were reached. Prior to pyrolysis, N2 (> 100 mL·min− 1) was continuously introduced for 5 min to remove the air in the tube furnace and ensure the oxygen-free environment required for pyrolysis. Oxygen-limited pyrolysis temperature (300°C, 400°C, 500°C, 600°C, 700°C, and 800°C) and retention time (15, 30, 45, 60, 90, and 120 min) at a heating rate of 10°C·min− 1 were established. After the process ended, the system was first cooled down to 25 ± 2°C, and the nitrogen and tube furnace were then turned off. The collected soil was washed with ultrapure water, filtered to remove ash, frozen in the refrigerator for more than 24 h, transferred to a freeze dryer (FD-1A-50, Beijing Boyikang Experimental Instrument Co., Ltd., Beijing, China) for drying for more than 24 h, collected and mixed, and stored in a vacuum glove box. For the pyrolysis tube, a rubber ring was tightly sealed inside to prevent gas from escaping, and a gas outlet was placed at the right end to facilitate gas discharge. According to their differences in pyrolysis temperature and retention time, the obtained samples were named CSX−Y (X = temperature [°C], Y = retention time [min]).
2.3. CS-activated PS oxidation experiment
A series of batch experiments was conducted to evaluate the effect of CS-activated PS, pyrolysis temperature, retention time, and CS and PS dosages on AN degradation. All reactions in the AN degradation experiment were carried out at room temperature (25 ± 2°C), 100 mg·L-1 AN concentration, 100 mL solution, and 6 h reaction time unless otherwise specified. CS (dose: 0, 1, 2, 3, 4, and 5 g·L-1) was added to a sealed glass conical flask (150 mL) and mixed first with AN solution to prevent chemical oxidation and then with PS (dose: 0, 1, 2, 3, 4, and 5 g·L-1). The reaction was carried out at a constant temperature shaking box at a speed of 200 rpm·min-1, and all experiments were carried out in duplicate. Liquid samples were collected at different time intervals, and 10 g·L-1 sodium thiosulfate was added to terminate the reaction. The samples were filtered through a syringe filter (0.22 µm, Shanghai Anpu Experimental Technology Co., Ltd., Shanghai, China) and stored in a refrigerator at 4°C for subsequent testing.
With the addition of isopropanol (IPA), methanol (MEOH), tert-butanol (TBA), chloroform (CF), furfuryl alcohol (FA), ethylenediaminetetraacetic acid (EDTA) as probe materials, the types of free radicals were identified to determine their impact on organic matter degradation. IPA was used as the quencher for SO4−· and ·OH, MEOH for SO4−·, TBA for ·OH, CF for ·O2−, FA for 1O2, and EDTA for holes.
Recycling experiments were conducted to evaluate the potential applications of the recycled CS. After each cycle of PS activation by CS and AN degradation by PS, the reaction solution was centrifuged at 8000 rpm·min-1 for 3 min, the liquid was removed, and CS was recovered. Fresh AN solution was added to perform the shaking experiment again. The above steps were repeated several times to investigate the recycling effect of CS.
2.4. Measurement method and material characterization
AN concentration was determined by high-performance liquid chromatography (HPLC, 1260II, Agilent, USA), and a Waters Atlantis T3 column (2.1mm×50 mm, 3 µm) was used for separation. The mobile phase consisted of 70% methanol (mobile phase A) and 30% ultrapure water (mobile phase B). In brief, 5 µL of the sample was injected and measured for 4 min at a ultraviolet detection wavelength of 280 nm, a column temperature of 25°C, and a flow rate of 0.4 mL·min− 1.
The change in total organic carbon (TOC) content was used to reflect the mineralization degradation efficiency of AN, and the potassium dichromate oxidation spectrophotometric method (HJ 615–2011) was applied to determine the TOC content in the water samples as follows. First, 30mL of the water sample was placed in the digestion tube of the graphite furnace (Gr20 type, Shanghai Shengsheng Automatic Analytical Instrument Co., Ltd., Shanghai, China), added with 0.1 g of mercury sulfate and 5mL of potassium dichromate solution with a concentration of 0.27 mol·L− 1 and shaken well. Afterward, 7.5mL of sulfuric acid solution was added, the mixture was shaken gently, heated at 135°C for 30 min, removed from the heat, and cooled to room temperature in a water bath. Ultrapure water was added to each digestion tube to obtain a volume of 50mL and used after cooling to room temperature. The samples were measured at a wavelength of 585 nm in a fully automatic scanning ultraviolet-visible photometer (UV-3000PC, Shanghai Maple Instruments Co., Ltd., Shanghai, China).
TPH concentration was used as an indicator to measure the oil content of PCS and was determined by an infrared spectrophotometer (EP-900, Beijing Bohaixingyuan Instrument Co., Ltd., Beijing, China) as follows. In brief, 0.1 g (± 0.0001 g) of the original and pyrolyzed PCS samples were placed in a 50 mL Erlenmeyer flask and added with 10 mL of C2Cl4. The mouth of the bottle was sealed, and the bottle was shaken in a constant temperature shaking box for 30 min and allowed to stand for stratification. Glass fiber microporous membrane (diameter 60mm, pore size 0.45 µm) was used to filter the supernatant in a glass funnel (the soil remained in the Erlenmeyer flask) and collect the filtrate in a 50 mL colorimetric tube. Afterward, 10 mL of C2Cl4 was continuously added to the Erlenmeyer flask, and the above experimental steps were repeated. Finally, the glass rod and Erlenmeyer flask were rinsed with a small amount of C2Cl4, and their contents were poured into the funnel, filtered into the corresponding colorimetric tubes, and diluted to 25 mL volume.
The TOC content in the CS extracts was measured to determine the leaching risk of CS (Li et al. 2018). In brief, 2.0 g (± 0.0001 g) of soil was placed in a 50 mL Erlenmeyer flask and added with 15 mL of ultrapure water to extract the soluble organic matter (SOM) from the soil. The temperature of the constant temperature shaking box was set to 25°C, and the rotation speed was 200 rpm·min− 1. After 12 h of shaking, the supernatant was transferred to a plastic centrifuge tube and centrifuged (LG-20W, Beijing Jingli Centrifuge Co., Ltd., Beijing, China ) at a rotation speed of 8000 rpm·min− 1 for 10 min. The prepared water sample was subjected to potassium dichromate oxidation-spectrophotometry (HJ 615–2011) to determine the TOC of the CS extracts.
A portable water toxicity analyzer (BX-LID-P, Hunan Bixiao Technology) was used to detect the acute toxicity of PCS and CS extracts to determine the impact of pyrolysis conditions on the harmlessness of PCS. The specific method was as follows. The luminescent bacteria (Vibrio fischeri) freeze-dried powder was placed in the freezing layer of a refrigerator below − 12°C, removed, and immediately mixed with the bacterial resuscitation solution (refrigerated at 4°C) for recovery. The osmotic pressure of the samples (N, taken from the samples used in the SOM measurement) was then adjusted. Afterward, 0.5 mL of bacterial solution was added to each test tube (N + 1), 0.5 mL of negative control solution was added to the first test tube, and 0.5 mL of osmotic pressure-adjusted sample was added to the rest of the test tubes. After 15 min of mixing, the bioluminescence intensity was measured sequentially. After the test was completed, the relative inhibition rate was calculated to indicate the magnitude of toxicity, and the relative luminous intensity inhibition rate (IR%) was calculated by Eq. (1):
where IR% is the relative luminescence intensity inhibition rate, %; St is the sample luminescence intensity, the number of photons; S0 is the negative control luminescence intensity, the number of photons; and Cf is the correction coefficient.
The contents of C, H, O, N, and S in the soil samples were analyzed using an elemental analyzer (vario EL cube, Elementar, Germany). The valence states of the elements were determined by X-ray photoelectron spectroscopy (XPS) (Axis Ultra DLD, Thermo Fisher Scientific, USA). The high-resolution spectra of O1s and C1s were fitted by XPS Avantage 5.948 software to verify the structural characteristics of the material, and the Al Kα radiation was 1486.6 eV. The surface morphology of CS was observed under a scanning electron microscope (SEM-EDS, JSM-IT100, Japan JEOL), and its surface element composition was measured by an energy dispersive spectrometer (EDS). The functional group changes of CS were characterized using a Fourier transform infrared spectrometer (FT-IR, Nicolet iS50, Thermo Fisher, USA) (scanning wavelength from 4000 cm− 1 to 400 cm− 1) with KBr tablet technology for spectral analysis. The Raman spectrum of CS was obtained using the Horiba Evolution Raman spectrometer (Paris, France) at excitation wavelength of 532 nm and scanning wavelength range of 100–4000 cm− 1. Graphitization features were identified using the ratio (ID/IG) of the D (defect) band at about 1350 cm− 1 to the G (graphite) band at about 1580 cm− 1 in the Raman spectrum (ID/IG).