Effect of Pumping Speeds on the Fate of Aniline in Different Soil Layer

Abstract

Helan Mountain is an important ecological safety barrier in northwest China. In this study, a heterogeneous site polluted by aniline on Helan Mountain was the research object, and the TMVOC (A Simulator For Multiple Volatile Organic Chemicals) model of aniline restoration by pumping was optimized by employing a column experiment. Four typical layers of the soil medium were selected to explore the influence of soil settlement caused by different pumping speeds on the fate of aniline in different zones. The results show that the optimal pumping speed at the site is 3.24 × 10⁶ m³/month and the latest remediation time is the 10th month after the start of the remediation. The larger the pumping speed is, the more obvious the sedimentation effect is. When the remediation is carried out at 5.18 × 10⁶ m³/month, the NAPL (Non-Aqueous-Phase Liquid) phase removal rate decreases by 33.75% and the distribution of aniline to the NAPL phase increases, compared to that without considering the soil settlement. The fate of aniline in the source zone is the least affected by sedimentation, while that in the vadose zone is the most affected. The phase redistribution phenomenon is the most obvious in the water table fluctuation zone, and the NAPL phase aniline changes into gas and liquid phases. In addition, the NAPL phase concentration in the water table fluctuation zone is two orders of magnitude higher than that at 0.2 m below the water table. NAPL is the most sensitive to the relative settlement in the aquifer. The simulation results can provide a technical reference for the future application of P&T (Pump-and-Treat) technology in the remediation of organically contaminated sites to facilitate the sustainable use of soil. It is suggested that more attention should be paid to the water table fluctuation zone during the remediation of contaminated sites.

1. Introduction

The sustainable development of water and soil resources is critical for human survival, and the relevant units advocate for the protection and restoration of environmental health. The volatile organic compound (VOC) aniline is often used as an intermediate product in the production of herbicides and dyes. The global production and consumption of aniline are mainly concentrated in Northeast Asia, Western Europe and North America, while the production of aniline in China is mainly concentrated in the East and Northeast. It is persistent and not easily decomposed, and can be found in large quantities worldwide, even in low concentrations of contaminated wastewater [1,2,3,4,5,6]. Due to its carcinogenic, mutagenic and toxic properties, once it leaks into the environment, it is harmful to the ecological environment and human health [7,8]. At present, aniline has been listed as a priority pollutant by the US Environmental Protection Agency (EPA), and the National Institute for Occupational Safety and Health (NIOSH) has identified it as a potential occupational carcinoid for humans and animals. It is also blacklisted as a priority control pollutant in China, with a maximum allowable concentration of 1 mg·L⁻¹ in wastewater [9,10,11]. However, due to accidental leakage and extreme pesticide use, 30,000 tons of aniline is released every year, making its control urgent [12]. Aniline is not easily oxidizable and is difficult to remove in situ [13]. Therefore, pump-and-treat (P&T) technology in ectopic remediation is often used to repair aniline-contaminated groundwater and reduce the quality of pollutants remaining underground [14].

Soil is a large reservoir and secondary source of environmental pollutants. Pores provide a preferential path for the movement of water, air and chemicals through soil, which is of great significance for the movement of solutes and pollutants in soil [15,16]. Changes in the water table during pumping lead to sediment compaction, soil settlement, and pore rupture and reduction [17,18,19,20]. This eventually leads to soil compression, trapping pollutants in pores [21], changing the permeability of soil and blocking the movement of water [22], air and chemicals throughout the soil. This process is often irreversible [23], which is particularly obvious in sand [24]. The pumping speed has a significant impact on the settlement rate [25,26]. As a result, the distribution ratio of aniline to the three phases (gas: Gas, aqueous: Aq; and non-aqueous-phase liquid: NAPL) also changes during the later stage of restoration, and pollutants firmly stored among soil particles are more difficult to remove, which is not conducive to the sustainable development of the environment. VOCs tend to diffuse and transfer vertically due to gravity [27]. The change in groundwater pressure affects the low-pressure area under the overburden and has different effects on various layers [28]. Due to the complexity of mechanical properties under different stratum conditions, the impact of subsidence on the movement of pollutants in different strata is different [29]. HS-GCMS analysis indicated that benzene content decreased significantly with the increase in soil depth [30]. Microplastics migrate from the surface layer of rice paddies and dryland to deeper soil layers, showing differences in different strata [31]. However, the behavior of aniline (Gas, Aq, NAPL) in the vertical profile under the influence of sedimentation has rarely been studied, especially the behavior of its three phases in the soil profile.

At present, most research on the effect of pumping treatment technology for aniline remediation is at the laboratory and field experiment stage, and its design and operation are mostly based on empirical formulas or limited site practice [32,33]. There are still many deficiencies in P&T numerical simulation research. The sub-module TMVOC of the TOUGH model has excellent performance in a multiphase flow simulation. Lari et al. used TMVOC to study the influence of SVE technology on the behavior of benzene series at a low temperature [34]. Lekmine et al. determined the dissolution characteristics of gasoline employing the TMVOC model [35]. It can simulate the three-phase flow of water, soil gases and VOCs in multi-dimensional heterogeneous media. It adopts a modular design and finite integral difference grid subdivision method to simulate the pollution problem of an organic solvent leakage in saturated and unsaturated regions, including soil adsorption, gas phase pumping, groundwater mining, etc. Additionally, it is a tool used to analyze the behavior of non-aqueous-phase liquids (NAPLs) below the vadose zone and groundwater table, which can be used for site-table simulations [36,37].

The Ningxia Helan Mountain is an important natural ecological barrier in the north of China and a natural laboratory for studying the biogeochemical cycle of arid mountain ecosystems [38,39,40]. It is sensitive to natural and human interference, and its ecosystem is very fragile [41]. Once pollution occurs, it will not only threaten local water safety, but also affect the surrounding environment of the reserve. In this study, it was assumed that 100 kg of aniline was released, and an indoor one-dimensional soil column experiment was established to simulate the effect of pumping sedimentation on aniline removal, verify the TMVOC model, build a site-scale aniline restoration site model, and analyze the destination, vertical distribution characteristics and phase redistribution law of aniline (Gas, Aq, NAPL) under different pumping speeds. By predicting the phase distribution of pollutants in different strata, aniline in different phase states can provide guidance for the formulation of remediation plans and solutions for the sustainable utilization of organic compound-contaminated sites.

2. Materials and Methods

2.1. Materials

The experimental soil utilized was from a pesticide-contaminated site in the Helan Mountain Basin, shown in Figure 1. According to the geological survey report, the structure of the inner layer within the depth range of 18 m below the natural ground in the site area can be divided into four layers according to lithologic characteristics: 0–1 m filled soil, 1–7 m silt, 7–14 m fine sand and 14–18 m silt–clay layer (relatively waterproof layer) from top to bottom. The regional groundwater is mainly quaternary loose lithologic pore water, and the phreatic water mainly occurs in the silt and fine sand layer. It is laterally supplied by atmospheric rainfall (annual average precipitation of 302.9 mm) and upstream groundwater.

Table 1. Physical and chemical properties of materials.

(a) soil
Soil type	D (kg/m3)	n	KXY (m)	KZ (m)	E (MPa)	PH	SOC (g·kg−1)
Loam	1890	0.350	1.21 × 10−11	1.210 × 10−13	20	7.34	41.3
Sand	1600	0.420	6.06 × 10−11	6.060 × 10−13	60	7.65	13.8
(b) aniline
S (mg·L−1)	MW (g·mol−1)	Kd (s−1)	Pvap (KPa)	B (K)	ρ (kg·m−3)	Tc (°C)	Pc (bar)	Koc (m3·kg−1)
3600	93.13	4.63 × 10−9	2.0	486.85	1000	525	53	0.06

D: soil density; n: void ratio; K: permeability coefficient; E: elasticity modulus; SOC: soil organic carbon; S: Solubility; MW: Molecular weight; Kd: Biological decay constant; Pvap: vapor pressure; B: boiling point; ρ: Compound density; Tc: Critical temperature; Pc: Critical pressure; Koc: Partition coefficient of compounds in organic matter.

outlines the physical and chemical properties of the site soil. Groundwater as a whole flows from west to east, with a large hydraulic slope between 0.08 and 0.15%. The aniline value exceeded the standard at many monitoring points, and the pollution plume formed was consistent with the direction of water flow due to the influence of the pumping water and groundwater flow direction. After the experimental soil was transported to the laboratory at a low temperature and protected from light, it was soaked for 12 h and then dried for 10 h. It was screened using an 18-mesh stainless steel sample sieve before use.

In this experiment, aniline (Macklin Biochemical Co., Shanghai, China.) was used as the characteristic pollutant, and its physical and chemical properties are shown in

Table 2. Correlation between experiments and models (R²).

	Test-Model 1 (Without Considering Sedimentation)	Test-Model 2 (Considering the Effects of Sedimentation)
1	0.968	0.981
2	0.960	0.985
3	0.953	0.983

. After aniline leaks on or near the surface, it migrates longitudinally and spreads horizontally [42,43]. According to the pollutant addition method, 100 mL of aniline (100 mg·L⁻¹) solution is injected at the sampling port, which promotes the downward migration of pollutants and reduces their volatilization, increasing the distribution uniformity of the pollutants in the soil [44].

2.2. Experimental Design

Figure 2 shows the device used to simulate pumping to restore aniline-contaminated soil. The aniline transport simulation device constituted a customized soil Perspex column with a 5 cm diameter and 60 cm height, a peristaltic pump (BT100S-1-DT115-44, LEADFLUID, CN), a soil liquid sampler (Macro Rhizon, Amsterdam, The Netherlands), a water tank and a filter cotton net. With the top of the soil column as the ground in order to prevent erosion, the filter cotton net and sand and gravel were placed at the bottom and top of the soil column. Then, 0~3 cm and 57~60 cm were filled with sand and gravel, and 3~57 cm was filled with soil. Four sampling holes with a 1.0 cm diameter were set on the side wall of the column, located at −10 cm, −20 cm, −30 cm, −40 cm and −50 cm (pollutant injection holes) of the soil column, respectively. The treated soil medium was filled into the experimental column in the layers and shaken every 5 cm. Fine sand was first laid in the soil column in the layers until 40 cm of the soil column was reached, and then silty sand was laid on the fine sand in the layers until 60 cm was reached. In order to prevent the excessive volatilization of pollutants, plastic wrap was used for sealing. Before the experiment, water pumping and injection experiments were carried out for 2 days, and after the capillary effect of the soil was satisfied, the experiment was carried out after 24 h of placement. The water speed-regulating device is a peristaltic pump, and its two ends are connected with the bottom of the glass column and the water tank. The soil solution sampler is inserted through the sampling mouth and buried inside the soil column, which is in direct contact with the contaminated soil.

In order to study the influence of sedimentation caused by different pumping speeds on the migration of aniline, three groups of pumping speed experiments were set up in this experiment, with pumping speeds of 1.30 × 10⁶, 3.24 × 10⁶ and 5.18 × 10⁶ m³/month, respectively. Water replenishment was realized through the water tank. To simulate the actual site situation, water was first injected to the target scale through a peristaltic pump, and samples were taken one day later as the initial concentration. Then, water was pumped through a peristaltic pump and sampled from different sampling holes every 10 min. The experimental methods of the three groups were identical. During sampling, the button of the soil sampler was opened, a disposable plastic syringe was connected to the tail of the soil sampler, and the solution in the soil was extracted via negative pressure and injected into the 10 mL centrifuge tube. After setting the centrifuge (Sigma2-16K, Osterode am Harz, Germany) at 1500 r·min⁻¹ for 5 min, the contents of the disposable syringe were passed through a 0.2 μm filter membrane and injected into a brown injection bottle (2 mL, Agilent Technologies, Santa Clara, CA, USA).

2.3. Numerical Model

The TMVOC module of TOUGH2022 software (Lawrence Berkeley National Laboratory, Berkeley, CA, USA) was used to establish the downward migration and transformation model of aniline in the study area under the sedimentation caused by water pump repair. The model assumed that the three phase states of aniline were always in chemical and thermal equilibrium. The simulation area was set as 100 m × 10 m × −5 m, and the x–z profile was divided into 20 rows and 21 columns, with 420 effective cells in total. The top grid was 0.001 m atmospheric boundary, and the first-type boundary of 0.001 m was set on the left and right sides. The site was composed of two media: 0 m to −1.5 m was silt, and −1.5 m to −5 m was sand. The burial depth of the initial groundwater table was 2 m on the left and 2.5 m on the right. The recharge of the contaminated site mainly originated from groundwater recharge and rainfall infiltration of 303 mm/a. The coordinates of the aniline leakage point were (47.5 m, 5 m, −0.8 m), and the leakage event was assumed to be 100 kg of leakage in 12 months. The leakage was set to end at the beginning of the repair period, the pumping well was 2.9 m away from the surface, and the left boundary was 49.5 m. The capillary pressure of relative permeability was calculated employing the three-phase function of Stone and Parker et al. [45,46]. In order to meet the capillary effect, the initial operation was carried out before the injection of pollutants, and the simulation was carried out until the convergence reached the static equilibrium and was then imported as the initial condition. The pump repair period of 2 years was created by changing the parameter conditions to create six simulation scenarios, outlined in

Table 3. Situational simulation.

Situation	1	2	3	4	5	6
Pumping speed (×106 m3/month)	1.30	1.30	3.24	3.24	5.18	5.18
Whether to consider sedimentation	Yes	No	Yes	No	Yes	No

. The restoration process of two consecutive years was discretized into 24 points on a monthly basis for simulation analysis. In scenarios 1, 2 and 3, the original geological parameters were always used in the restoration process. In scenarios 4, 5 and 6, after the end of each month of pumping, the porosity and permeability coefficient were re-calculated according to the stress effect generated by each month of pumping, which were used as the input values of the model parameters.

The porosity of soil after precipitation was calculated according to Equation (1). The Konzeny–Carman equation is a semi-empirical equation for predicting and estimating conductivity, which is widely used in biochemical oil exploitation, underground seepage and other fields. The calculation is outlined by Equation (2):

$$\mathrm{n}=\frac{{\mathrm{n}}_0-{\mathsf{\epsilon }}_{\mathrm{v}}}{1-{\mathsf{\epsilon }}_{\mathrm{v}}}$$

(1)

$${\mathrm{K}}_0=\frac{{{\mathrm{n}}_0}^3}{\mathrm{C}{(1-{\mathrm{n}}_0)}^2{\mathrm{S}}^2}$$

(2)

where ${\mathsf{\epsilon }}_{\mathrm{v}}$ is the bulk strain of the soil element after precipitation; is the initial, ${\mathrm{n}}_0$ is the initial porosity before precipitation; C is the K–C constant, taken to be 5 [47]; and S is the specific surface area of the medium (m⁻¹).

2.4. Sample Testing and Data Statistical Analysis

Instruments and equipment used included electronic scales (ME104, Mettler Toledo, Giessen, Germany); pH meter (FE-28, Mettler Toledo, Giessen, Germany); and HPLC MS (Agilent 1200, Agilent Technologies, Santa Clara, CA, USA).

For the determination of aniline, the HPLC MS system was configured as follows: Kromasil^® C18 column (φ: 4.6 × 250 mm, 5 μm) [48], wavelength of 254 μm, eluent consisting of a mixture of HPLC-grade methanol and ultra-pure water in a ratio of 30:70 (v/v) as the mobile phase, injection volume of 20 μL, flow rate of 1.0 mL·min⁻¹, column temperature of 30 °C, and analysis duration of 20 min.

For the statistical analysis, we used the Petrasim visualization platform (Lawrence Berkeley National Laboratory, Berkeley, CA, USA); Excel’s statistical analysis tool (Office 2003, Redmond, WA, USA); Tecplot 360 (EX2015 R1, Tecplot, Bellevue, WA, USA); and Origin (Gamma Design Software, Origin 6.1, LLC, Plainwell, MI, USA).

3. Results and Discussion

3.1. Model Verification

Table 2 displays the correlation between the removal rate of liquid aniline using the soil column experiment and numerical simulation. Furthermore, the correlation degree of the removal rate under the condition of soil settlement restoration (R² > 0.981) was higher than that without soil settlement (R² > 0.953). It was demonstrated that the simulation results of the optimization model considering settlement fit better with the real values, and reflected the real situation more accurately.

3.2. Migration Mechanisms of Aniline Caused by Pump Speeds

Previous studies generally adopted a single-layer soil system, which could not reflect the influence of soil heterogeneity on aniline migration behavior [49]. This experiment is a two-layer medium, and the six scenarios outlined in Figure 3 clearly demonstrate that the degree of vertical migration of aniline in sandy soil is increased, and the quality of aniline in soil is reduced regardless of sedimentation. Figure 3a,c,e show that when the pumping speed was 1.30 × 10⁶ m³/month, the range of aniline contamination plume was the widest, with a horizontal spread of 53 m and a vertical spread of 2.62 m, and the concentration area of the high value was located at −0.79 m to −1.22 m. When the pumping speed was 3.24 × 10⁶ m³/month, the range of aniline contamination plume was 27 m horizontally and 2.10 m vertically, and the concentration of the high value was at −0.8 m to −1.31 m. When the pumping speed was 5.18 × 10⁶ m³/month, the range of the aniline contamination plume was 14 m horizontally and 2.18 m vertically, and the high concentration area was located at −0.82 m to −1.36 m. As shown in Figure 3b,d,e, the higher the pumping speed was, the deeper the high concentration area of aniline was. Moreover, the more obvious the obstruction phenomenon of aniline migration caused by the sedimentation effect was, the smaller the range of aniline contamination plume was, resulting in the phenomenon that a large amount of aniline accumulates near the source area. A small part of aniline diffuses with the direction of water flow, and most aniline is intercepted in the pores of the soil in the source area, which affects the restoration effect and adversely affects the treatment of the soil–groundwater system.

3.3. Effect of Pumping Speeds on Aniline Fate and Interphase Partition

Figure 4a indicates that for the same repair time, the greater the pumping speed is, the more obvious the removal effect of aniline will be, and the sedimentation effect generated by it will have an increasing influence on the removal effect of aniline. When the pumping speed was 5.18 × 10⁶ m³/month, the settlement effect reduced the repair rate by 29.59%; therefore, controlling the pumping speed can reduce the influence of the settlement effect and within a certain repair time [50]. When the site was repaired with a pumping speed of 1.30 × 10⁶ m³/month and 3.24 × 10⁶ m³/month, the aniline removal effect under the effect of settlement was only 6.83% different, so pumping speed should be 3.24 × 10⁶ m³/month. The effect of sedimentation on the removal of aniline has a lag effect. When the removal effect of aniline decreases after the tenth month of pumping repair, pumping should be stopped in time before the treatment, and water injection should be carried out.

Figure 4b indicates that there are differences in the removal effect of aniline in three phases caused by different pumping speeds. The gas and liquid phases were more affected by the pore size between the soil particles than the NAPL phase. When the pumping speed was 1.30 × 10⁶ m³/month, the removal effect of each phase of aniline was reduced by sedimentation. The removal rates of the gas, liquid and NAPL phases were reduced by 12.03%, 2.28% and 0.02%, respectively. When the pumping speed was 3.24 × 10⁶ m³/month, the removal rate of aniline in the gas and liquid phases increased by 8.38% and 13.95%, respectively, and the removal rate of the NAPL phase decreased by 10.66%. When the pumping speed was 5.18 × 10⁶ m³/month, the removal rate of aniline in the gas and liquid phases increased by 14.04% and 7.6%, respectively, and the removal rate of the NAPL phase decreased by 33.75%. It can be seen that with the increase in pumping speed, the stress effect generated by it is stronger, which leads to the closure of or reduction in soil pores and the loss of porosity. Aniline is blocked in the pores [51] and there is a gravitational force between pollutants and soil particles, which is more difficult to remove in the restoration process due to the interaction between the molecules. As such, it is more difficult to control the soil–groundwater system.

3.4. Spatial Distribution Characteristics of Aniline in Vertical Soil

The distribution of aniline in different strata is shown in Figure 5. The saturation of aniline decreased with the deepening of the formation. In the stratum at −0.8 m, the concentration of aniline was the highest, mass fraction is 2.745 × 10³, and it was mainly distributed in the source region in the form of the NAPL phase. In the vadose zone of −2.0 m, although the diffusion coefficient at the edge of the capillary was lower [18], the pore water was less abundant and the movement space of aniline relatively large, so it was widely distributed in the vadose zone, and the molecular weight of aniline was only 5.41 × 10⁶. Because the pumping well was located at 49.5 m, the aniline concentration in the profile of the −2.6 m water-table fluctuation zone decreased sharply, and the water pressure difference led to the formation of a high concentration of aniline around the pumping well. Most of the aniline floated in the water-table fluctuation zone; even in the aquifer 0.2 m below the water surface, the aniline concentration decreased by two orders of magnitude, so the formation with a high aniline concentration was selected when pumping aniline. Affected by the flow direction, aniline migrated in the flow direction (X) and was most widely distributed in the aquifer.

3.5. The Vertical Distribution and Fate of Aniline in Soil

When repairing the contaminated site, the pollutant removal effect varies with the depth of the stratum [52]. The results are shown in Figure 6a. The pump-and-treat remediation method is to extract the water from the aquifer directly so as to extract the pollutants; it can be clearly seen that in the first 10 months of the restoration period, although the settlement of the aquifer is obvious, other strata are not obvious. The remediation effect of aniline in the aquifer at −2.8 m is the best, followed by the water table fluctuation zone at 2.6 m, the vadose zone at −2.0 m and the source at −0.8 m, showing a gradual deterioration from the aquifer to the ground. When considering sedimentation, except that the removal rate of aquifer at −2.8 m is reduced by 7.42%, sedimentation has a positive effect on the removal of aniline at −2.6 m, −2.0 m and −0.8 m, increasing by 4.06%, 36.55% and 7.0% respectively, the most obvious of which is at the −2.0 m layer of the vadose zone.

As shown in Figure 6b,c, after analyzing the removal efficiency of aniline in different strata, the sedimentation at the source had little effect on the removal efficiency and interphase distribution of aniline. In the tenth month of the restoration period, the removal effect of each phase of aniline in the vadose zone was obvious because most of the pores in the vadose zone were not supported by liquid, and the stress change had a great influence on the soil structure. Once sedimentation occurred, the removal efficiency of aniline in the vadose zone changed and the removal rate of each phase increased sharply, in which the gas phase distribution decreased and the change rate was 11.75%, but the increase rate of the liquid and NAPL phases was only 0.002%. When the water table fluctuated, the interphase redistribution changed most obviously, which was due to the unique NAPL phase property of aniline causing it to form an oil layer on the water surface. When the water table dropped, the soil particles did not have adequate space to capture NAPL aniline, and the NAPL phase was transformed into the gas phase. When the water table rose, the gas phase dissolved into the liquid and NAPL phases. Regarding sedimentation, the soil pores decreased or closed, and the reduction in the vadose zone caused gas phase aniline to transform into the liquid and NAPL phases, which were dissolved in water. It can be seen that the fluctuation in the water table had a great influence on the interphase distribution of VOCs. In the saturated layer, the content of aniline decreased under the influence of sedimentation, and the change in interphase distribution was the same as that of the wave layer, but this change was small.

4. Conclusions

Employing the column experiment and model simulation, the behavior of aniline in the pumping and treatment technology of a pesticide-contaminated site in the Helan Mountain area was studied as follows:

(1)

Considering the sedimentation effect, the pumping speed should be held at 3.24 × 10⁶ m³/month. The settlement effect had a lag. The site should therefore be restored within 10 months after the beginning of remediation to reduce the negative impact of the settlement effect.

(2)

The larger the pumping speed was, the more obvious the settlement effect was. When the restoration was carried out at 5.18 × 10⁶ m³/month, the removal rate reduced by 29.59% when considering the settlement effect. The removal rate of the NAPL phase reduced by 33.75%, and the distribution of the NAPL phase by aniline increased by 26.68% due to the sedimentation effect. Because the NAPL phase was hydrophobic and difficult to remove, it was not appropriate to incorporate a pumping speed that was too high in the process of site restoration, otherwise a large amount of NAPL aniline would have reduced the sustainable use of the site.

(3)

The removal rate of aniline in the vadose zone of the soil was most affected by sedimentation, which increased by 36.55%. The phase redistribution in the water-table fluctuation zone was the most obvious, and aniline in the NAPL phase escaped into the gas phase and dissolved into the liquid phase. Therefore, efficient control methods should be selected according to the existence of the aniline phase in different strata. With the increase in formation depth, the concentration of aniline decreased, and the concentration of aniline in the NAPL phase was two orders of magnitude higher in the water-table fluctuation zone than 0.2 m below the water surface. Therefore, when using the pumping treatment technology for restoration, wells should be set near the water surface with a high concentration.

In this study, the behavior of aniline under pump-induced subsidence was only considered below the surface. Because the subsidence caused the difficulty in the downward migration of aniline, it was blocked near the source area. Therefore, we suggest performing additional research on the volatilization flux of aniline at the surface.