Benzo[a]pyrene (B[a]P) Degradation Enhanced by Soils Mixing Effects: Validation Study of Stirring Test and Discrete Element Method (DEM)

Abstract

To date, few studies have been carried out on the influence of the mixing effects of soils and remediation agents on the remediation effects of benzo[a]pyrene (B[a]P) in contaminated soils. In this study, the mixing effects of soils and remediation agents and the degradation effects of B[a]P under different stirring conditions were investigated by combining stirring tests with discrete element method (DEM) simulation. The results from the stirring tests indicated that the mixing effects of two-stage (C_Drill) drill bits were better than first-stage one-line (A_Drill) and first-stage cruciform (B_Drill) drill bits. The mixing quality of C_Drill at the drilling/raising rates of 2, 2.5, 3, 4, and 7.5 cm/min were 42.13%, 43.20%, 43.98%, and 43.30%, respectively. In terms of the results from the B[a]P oxidation remediation tests, the contaminated soils mixed with C_Dril have better remediation effects for B[a]p than those mixed with A_Dril and B_Dril, since B[a]p in contaminated soils stirred and mixed using C_Drill could not be detected after oxidative degradation. The present study results have proved that the mixing effects of soils and remediation agents could significantly affect the remediation effects of contaminated soils with polycyclic aromatic hydrocarbons (PAHs).

1. Introduction

As one of the most common PAHs, benzo[a]pyrene (B[a]P) is a hydrophobic, carcinogenic, and teratogenic compound with wide range of pollution sources and strong stability [1]. B[a]P has been classified as a Group I carcinogen by the National Cancer Institute [2]. Therefore, the degradation and removal of B[a]P in contaminated soils have attracted wide attention, due to its persistence and toxicity [3,4].

At present, soil remediation technologies mainly include electrokinetic remediation, in situ extraction, in situ thermal desorption, solidification/stabilization, chemical stabilization, cement kilns, washing, chemical oxidation/reduction, bioremediation, and phytoremediation [5,6]. Compared with other traditional remediation approaches, in situ chemical oxidation remediation technology has been most widely used in soil remediation engineering, with the advantages of lower cost and high efficiency [7,8]. In situ chemical oxidation remediation is essentially a coupling process of physical mixing and chemical reaction of contaminated soils and remediation agents [9]. To date, most scholars have mainly focused on B[a]P degradation by persulfate [10,11]. Other scholars are committed to developing various new remediation materials to improve the remediation effects of B[a]P contaminated soils [12,13]. Notably, if soils and remediation agents are not mixed evenly during in situ soil mixing processes, soil remediation effects will be significantly affected, which may lead to incomplete remediation or local contamination residues. Therefore, the accurate evaluation of the mixing effects is crucial for enhancing B[a]P degradation. However, studies have rarely considered the influence of the mixed effects of contaminated soils and remediation agents on the degradation effects of B[a]P in contaminated soils [14].

As a numerical calculation method, discrete element method (DEM) has been used to study the mixing processes and mechanisms of particle motion patterns [15,16]. In recent years, some scholars have begun to study the stirring effects of industrial equipment using DEM [17,18]. For example, Huan et al. used DEM method to analyze the effects of mixer form, blade length, and blade orientation on mixing, and built a mixing experiment platform to verify DEM accuracy [19]. Liu et al. used DEM to study the effect of the coefficient of variation on fertilizer uniformity [20]. However, few studies have used DEM simulation to study the influence of the mixing effects of soils and remediation agents on B[a]P degradation.

In these contexts, the main purposes of this study include: (1) soil stirring tests were carried out to study the mixing effects of B[a]P-contaminated soils and remediation agents; (2) DEM was used to simulate and verify the mixing effects of B[a]P-contaminated soils and remediation agents; (3) B[a]P degradation tests were conducted to study the impacts of soil mixing effects on B[a]P degradation. The results of this present study have important theoretical value and practical significance for significantly improving the remediation effects of PAHs-contaminated sites.

2. Materials and Methods

2.1. Chemicals and Soils

Benzo(a)pyrene (B[a]P, ≥99.0%) was purchased from Shanghai Titan Technology Co., Ltd (Shanghai, China). Acetone (C₃H₆O, >99.0%), sodium persulfate (Na₂S₂O₈, ≥99.9%), and ferrous sulfate heptahydrate (FeSO₄·7H₂O, analytically pure) were purchased from Sinopharm Group Chemical Reagent Co., Ltd (Shanghai, China). Clean soils were air dried with a soil layer of 2–3 cm under ventilation. The soils were often turned over and their large pieces were broken. After being completely dried, the soils were screened with a 2 mm mesh. In accordance with previous studies by Kong et al. [21], contaminated soils were placed in a fume hood until the acetone was completely volatilized, after evenly mixing 16.0 kg of soil with 100 mg B[a]p in an acetone solution. Afterwards, B[a]p contaminated soils were aged for one week in the dark. B[a]p concentrations in contaminated soils were measured to be 4.70 mg/kg. Green sands with the specific gravity of 2.67 kg/m³ were used to simulate agent particles, since their density was similar to that of soil particles. Considering that agent dosages affected soil mixing effects and their field application, the dosages of green sand particles and sodium persulfate were set to 3% and 2%, respectively.

2.2. Experimental Equipment

The mixing devices used in this study were mainly used to simulate the mixing effects of soil stirring and spraying equipment. The experimental devices were composed of drill bits, stirring vessels, stirring rods, stirring brackets, and power system. Detailed information about the drill bits, mixing containers, drill rods, and reducer are shown in Text S1, Text S2, Text S3 and Text S4, respectively. The physical pictures of drill bits, mixing containers, drill rods, and reducer are shown in Figures S1–S5, respectively. The key influencing factors such as configuration and drilling/raising rates of drill bits were investigated during the mixing process.

2.3. Experimental Design

2.3.1. Stirring Test

The soils were added into stirring vessels. Initially, about 13.5 cm of soils were filled into base cylinders. since the laboratory stirring tests did not have spraying operations, green sands were added in an insert manner. Five graduated cylindrical pipettes were evenly inserted into the filled soils. In accordance with the agent dosages, 5 cm of green sands were filled into the pipettes. Subsequently, middle cylinders with the height of 4 cm were added on the lower cylinders. The soils was filled to about 18.5 cm. After the soils and green sands were completely filled, the pipettes were slowly extracted, and then the upper cylinder was installed. The specific test process is shown in Figure 1a.

Once the stirring vessels were properly installed, the stirring devices and reducers, which were connected to the drill bits, were mounted on the upper cylinders. The schematic diagram of the complete stirring device is shown in Figure 1b. After the stirring device was fully installed, the power supply was connected, and the reverse switch was activated. The drill bits descended uniformly, while rotating clockwise at a speed of 20 r/min until a suitable height was reached. The stirring device along with the reducer, upper cylinder, and middle cylinder were sequentially removed. In each batch experiment, two sections were taken for image analysis to assess the mixing effects of soil and agent particles. Three parallel groups were set up for each batch experiment.

Three drill bits were used to explore the effects of their structure on soil mixing uniformity. The rotational speeds of first-stage one-line (A_Drill), first-stage cruciform (B_Drill), and two-stage (C_Drill) drill bits were set to 20 r/min. The drilling/raising rates of A_Drill, B_Drill, and C_Drill were set to 2, 2.5, 3, 4, and 7.5 cm/min, respectively. To investigate the mixing effects of drill bits in a water saturated state, stirring tests were carried out using B_Drill at a water-to-soil volume ratio of 1:2, with a rotational speed of 20 r/min and a stirring time of 3 min.

2.3.2. The Oxidative Degradation Test of B[a]p

The oxidizer was Na₂S₂O₈ with the dosages of 2%. FeSO₄ was used as the activation agent. The ratio of Na₂S₂O₈ to FeSO₄ was 2:1. B[a]p contaminated soils were added into the stirring vessels. About 8 cm of clean soil was initially filled into the base cylinders, followed by Na₂S₂O₈ addition using an insert method. A total of 5 cm of Na₂S₂O₈ was filled into each graduated cylindrical pipette. Then, another 8 cm of B[a]p contaminated soils wer added. After the oxidant and contaminated soils were filled, the pipettes were slowly withdrawn, and the upper cylinders were installed. Once the stirring vessels were set up, the stirring device and reducer connected to the drill bits were mounted on the upper cylinder. FeSO₄ solution was added to the stirring vessels at a water-to-soil ratio of 1:2. After the solution fully infiltrated the soils, the stirring devices were activated. The stirrer operated at 20 r/min, with thedrilling/raising rate of 3 cm/min. Upon the completion of stirring, the mechanical stirring device was removed, and the stirring vessel was sealed. B[a]p contaminated soil samples were taken from three different heights from top to bottom after one week. Three soil samples were collected at each height. Three parallel samples were carried out in each batch experiment.

2.3.3. Sample Analysis

A sieving method was used for soil and sand particles with sizes greater than 0.075 mm, while a densitometer method was used for those with sizes less than 0.075 mm. The specific gravity of soils and sand particles was measured using a pycnometer method. The concentration of B[a]p in the soils was determined by gas chromatography and mass spectrometry (8890-5977C) with a detection limit of 0.1 mg/kg.

2.4. Calculation Method of Mixing Quality

The post-processing function of EDEM software was used to calculate the number of soil and agent particles in grids. The mixing quality (M) of soil and agent particles was calculated as follows:

$$m={\sum }_{i=1}^n{m}_i$$

(1)

$$m_{\mathrm{a}\mathrm{g}\mathrm{e}\mathrm{n}\mathrm{t}}={\sum }_{i=1}^n{m}_{\mathrm{a}\mathrm{g}\mathrm{e}\mathrm{n}\mathrm{t},i}$$

(2)

$${\varnothing }_i={\displaystyle \frac{m_{\mathrm{a}\mathrm{g}\mathrm{e}\mathrm{n}\mathrm{t},i}}{m_i}}$$

(3)

$$\eta ={\displaystyle \frac{m_{\mathrm{a}\mathrm{g}\mathrm{e}\mathrm{n}\mathrm{t}}}{m}}$$

(4)

where n is the number of samples, m is the total mass of samples, m_agent is the total mass of agents, m_i is the mass of samples by detection, m is the mass of agents in the samples, Φi is the concentration of agents in the samples, and η is the total average concentration of agents.

$$\alpha =\sqrt{{\sum }_{i=1}^n{\left({\varnothing }_i-\eta \right)}^2\times {\displaystyle \frac{m_i}{m}}}$$

(5)

$${\alpha }_0=\sqrt{\eta \left(1-\eta \right)}$$

(6)

$${\alpha }_{0,ave}=\left({\alpha }_{0,1}+{\alpha }_{\mathrm{0,2}}+{\alpha }_{\mathrm{0,3}}\right)/3$$

(7)

$$\alpha ={\alpha }_{0,ave}\left(1-e^{-{\left[\left(n-1\right)/a\right]}^b}\right)$$

(8)

$$M=\left(1-{\displaystyle \frac{\alpha }{{\alpha }_{0,ave}}}\right)\times 100\%$$

(9)

where α is the mixing degree, α₀ is the initial mixing degree, α_{0, ave} is the average value of the initial mixing degree, and M is the mixing quality.

2.5. DEM Simulation

2.5.1. Device Model Construction

The three-dimensional modeling of the mixing device was completed using SolidWorks software. The device-based model was imported into EDEM software for simulation. In terms of the main mixing components, only the barrel and drills were retained in DEM model of soil mixing equipment, to improve the computational efficiency. The specific structure of soil mixing equipment is shown in Figure 2a. The outer and inner diameter of drill pipes in the center of drill bits were 2.4 cm and 1.7 cm, respectively. The single blade sizes of the upper and lower blades were 7 cm × 1.5 cm × 0.3 cm and 2.5 cm × 1.5 cm × 0.3 cm, respectively. The single blades were equipped with three nail teeth of 1 cm width with a tooth spacing of 1 cm. The horizontal inclination angle of the upper and lower blades was 30°.

In accordance with the detailed descriptions of DEM simulation from previous studies by Abdeldayem et al. and Tong et al. [22,23], the specific steps undertaken in generating soil and agent particles were as follows: (1) part of the soil particles were first generated; (2) agent particles were then generated at the specified position; and (3) soil particles were generated to completely cover the agent particles, and the static accumulation state was finally formed, as shown in Figure 2b. To clearly observe the mixing process, grey and red spherical particles were used to represent soil and agent particles, respectively. The stirring simulation started after particle formation was completed. During the mixing process, the drill bits first rotated in a clockwise direction at a speed of 20 r/min, while moving downward at a drilling speed of 3 cm/min. When they were lowered to the preset height, the drill bits were rotated in a counterclockwise direction at a speed of 20 r/min, and while being lifted upward at a rotational speed of 3 cm/min. The simulation process stopped, when the drill bits reached the top boundary of soil particles, as shown in Figure 2c.

2.5.2. Particle Model Construction

Similar to a previous study by Wang et al. [24], the general soil density and particle sizes were selected in DEM simulation. Both soil and agent particles were set as grey and red spherical particles, respectively, with diameters of 2 mm. The specific particle model is shown in Figure S7. The relevant parameter settings of soil and agent particles and drill bits are shown in Table S1. In addition, the Hertz–Mindlin with JKR contact model was used for simulation. The contact property parameters among the soil particles, agent particles, and drill bits are shown in Table S2.

3. Results and Discussion

3.1. Stirring Test Results

Figure 3 shows the mixing effects of three drill bits at five drilling/raising rates. The M values decreased as the grid number increased. The greater the M values were, the better the mixing effects were; that is, the higher the position of the fitting function curve was, the better the mixing effects were. Among the three configurations, B_Drill showed the best stirring and mixing effects at the drilling/raising rates of 2, 2.5, 3, 4, and 7.5 cm/min, while it exhibited the worst stirring and mixing effects at a drilling/raising rate of 2 cm/min. These results might be beausetoo many stirring cycles made larger-sized sand particles keep falling during slow stirring, considering the particle size differences between soil and agent particles.

The mixing effects of contaminated soils and agents using three drill bits were further investigated at the drilling/raising rates of 2.5 cm/min, 3 cm/min, 4 cm/min, and 7.5 cm/min. As shown in Figure 4, the M values of the three drill bits all first increased and then decreased, as the drilling/raising rates decreased. The M values of A_Drill at the four drilling/raising rates were 39.22%, 41.12%, 41.44%, and 40.74%, respectively. The M values of B_Drill at the four drilling/raising rates were 41.54%, 42.09%, 42.44%, and 42.11%, respectively. The M values of C_Drill at the four drilling/raising rates were 42.13%, 43.20%, 43.98%, and 43.30%, respectively. The M values of the three drill bits reached the maximum values at the drilling/raising rate of 3 cm/min.

3.2. DEM Simulation Results

3.2.1. Accuracy of DEM Simulation Results

The mixing effects in the stirring tests using C_Drill at a rotational speed of 20 r/min and a drilling/raising rate of 3 cm/min were verified with DEM simulation results. To more reasonably verify the accuracy of DEM model, the width of the grids was set to 4 mm in the Z direction without grid division. The same grid division was carried out in the X and Y directions as the image recognition of the stirring experiment. A mixing degree in the cross sections similar to the stirring experiment was obtained in DEM simulation. Three positions in the cross sections were taken to calculate M values. The calculated results of M values are shown in Figure 5a and

Table 1. Results from numerical simulation and experimental verification.

Grid Number	Mixing Quality (%)
	Test 1	Test 2	Test 3	Simulation 1	Simulation 1	Simulation 1
25	91.47	87.65	88.63	94.15	94.92	93.73
100	83.91	80.42	81.05	88.75	89.15	86.10
225	77.44	75.96	76.55	83.03	84.12	82.29
400	70.77	71.04	71.34	77.28	79.31	76.75
625	63.91	65.38	65.69	72.37	72.33	71.00
900	58.97	60.84	61.28	67.00	68.37	66.79
1225	53.63	56.14	56.11	62.29	62.80	60.68
1600	50.31	53.77	53.98	53.25	57.40	55.82
2025	47.11	50.16	50.43	49.54	51.25	50.60
2500	43.30	47.09	47.94	46.20	44.86	46.21

. The fitting function and M values obtained from the DEM simulation were similar to the experimental results, indicating that DEM simulation results were accurate.

3.2.2. Effect of Drilling/Raising Rates on Mixing Effects

The DEM simulation was also used to study the effect of drilling/raising rates on the mixing effects. The drilling/raising rates of drilling bits were set to 2 cm/min, 3 cm/min, and 4 cm/min, and their rotational speeds were set to 20 r/min in the DEM simulation. The DEM simulation results are shown in Figure 5b. The M values at the three drilling/raising rates were 45.80%, 45.76%, and 45.32%, respectively. The mixing effects were better at the lower drilling/raising rates, which was consistent with the stirring test results. In addition, the mixing effects were not influenced by the differences among soil particle sizes.

3.2.3. Effect of Blade Angles on Mixing Effects

To study the influence of the blade installation angle of drill bits on the mixing effects of contaminated soils and agents, the rotational speed of drilling bits was fixed at 20 r/min, and their blade installation angles in the DEM simulation were set at 30°, 35°, 40°, and 45°, respectively. As shown in Figure 5c, the M values at the four blade installation angles were 48.02%, 47.71%, 46.46%, and 45.76%, respectively. The results indicated that when the blade installation angle increased from 30° to 45°, the M values decreased. The M values at the blade installation angle of 30° were highest, because the axial and radial effects of the blades on the particles had better effects.

3.2.4. Effect of Multi-Stage Drill Bits on Mixing Effects

According to the above results, the mixing effects of C_Drill were better than those of A_Drill and B_Drill. Thus, the simulation of the multi-stage drill bits (D_Dril) was further carried out at the rotational speed of 20 r/min and the drilling/raising rate of 3 cm/min. D_Dril was composed of the mixing blades with staggered distribution in the vertical direction, as shown in Figure S2. The DEM simulation results of D_Dril are shown in Figure 5d. The M values of C_Dril and D_Dril were 45.76% and 46.95%, respectively. Thus, the mixing effect of D_Dril was better than that of C_Drill.

3.3. The Degradation Effects and Mechanism of B[a]p

As shown in Figure 6, after the oxidative remediation of B[a]p contaminated soils was carried out using mechanical stirring equipment, the B[a]p concentrations in the soils in three group tests decreased from 4.70 mg/kg to the values less than the second soil screening value of 1.5 mg/kg. B[a]p could not be detected in contaminated soils stirred and mixed using C_Drill, indicating the best remediation effects. When A_Drill was used to stir and mix contaminated soils with agents, B[a]p concentrations were 0.32 mg/kg and 0.18 mg/kg respectively, while no B[a]p was detected in the lower soil samples. The B[a]p concentrations were 0.11 mg/kg and 0.18 mg/kg in the middle and lower soil samples stirred and mixed using B_Drill, respectively, while no B[a]p was detected in the upper soil samples. These results indicated that the use of C_Drill could have the best remediation effects for B[a]p contaminated soils, which is consistent with the mixing effects of drill bits in the stirring tests. Therefore, the improvement of mixing effects plays key roles in soil remediation effects. Previous studies have reported similar results. For example, Ali et al. reported that the removal efficiency of BaP by thermally enhanced biodegradation (TEB) increased from 15.8% to 34.6% with the temperature increasing from 15 °C to 45 °C [25]. Lu et al. found that the removal efficiency of BaP via the combined remediation of pre-ozonation and bioaugmentation was 92.69–93.19% [26]. Li et al. indicated that the removal rates of BaP by constructing a solute transport–soil microbial fuel cell device ranged between 42.73% and 77.62% [27]. In addition, some studies have reported different degradation mechanisms of B[a]p in contaminated soils by sodium persulfate. For instance, Qu et al. found that ˙superoxide radicals (˙O₂⁻) and singlet oxygen (¹O₂) could play dominant roles in B[a]P degradation [28]. Qu et al. showed that ¹O₂ and electron transfer could play major roles in the degradation process of B[a]p in contaminated soils [29].

4. Conclusions

In this study, the influence of the mixing effects of contaminated soils and remediation agents on B[a]p degradation effects was investigated via the combination of stirring tests and DEM method. The results indicated that the mixing effects of the three drill bits increased first and then decreased with the decreases in drilling/raising rates. The decreases in mixing effects at the drilling/raising rate of 2 cm/min might be due to the larger green sand particles falling during the slow stirring process. In the DEM simulation, the lower the drilling/raising rates were, the better the mixing effects of the drill bits were. The M values decreased, as the blade installation angle increased. Under the same operating parameters, the mixing effects of D_Drill were better than those of C_Drill. In addition, the degradation effects of B[a]p using C_Drill were higher than those using A_Drill and B_Drill, due to non-detectable B[a]p in contaminated soils. Therefore, the mixing effects of soil and agents made dominant contributions to the enhanced degradation effects of B[a]p in soils. These findings provide important technical guidance for enhanced removal effects of PAHs in soil remediation engineering applications.