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Article

Co-Transport of Aniline and TNT with Loess Colloid Particles in Saturated Loess Columns: Mechanism and Processes

by
Zhaohui Meng
,
Sihai Hu
,
Ran Sun
,
Chengzhen Meng
,
Yaoguo Wu
* and
Xiaofeng Sun
School of Chemistry and Chemical Engineering, Northwestern Polytechnical University, Xi’an 710129, China
*
Author to whom correspondence should be addressed.
Water 2024, 16(1), 180; https://doi.org/10.3390/w16010180
Submission received: 29 November 2023 / Revised: 28 December 2023 / Accepted: 30 December 2023 / Published: 4 January 2024
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Abstract

:
The transport of mobile colloidal particles with organic pollutants in porous media has attracted considerable attention. Aniline and 2,4,6-trinitrotoluene (TNT), as aromatic compounds and key components of energetic materials, are continuously released into the environment. This study compared the co-transport of loess colloidal particles with aniline and TNT, aiming to investigate the influence of structural and physicochemical properties of the pollutants. The colloids were prepared and characterized, and static adsorption and dynamic column experiments were conducted. The results indicate that the adsorption processes of aniline and TNT both conformed to the quasi-second-order kinetic and the intra-particle diffusion models, with aniline exhibiting higher rate constants than TNT. The main adsorption mechanism involved van der Waals force, hydrogen bonding, and electrostatic interaction. Response surface experiments indicated that the adsorption capacity increased with higher initial concentration of organic compound but decreased with larger particle size and higher Na+ concentration. In column experiments, the adsorption of loess colloid particles on aniline and TNT was strongly correlated with the concentration of loess colloid particles. Loess colloid particles could be used as carriers to enhance the co-transport, with aniline exhibiting a faster transport rate due to the differences in polarity and molecular structure compared to TNT. In summary, loess colloidal particles enhanced the transport behavior of aniline and TNT in saturated loess columns. The differences in polarity and molecular structure of aniline and TNT further affect their co-transport mechanism in loess.

Graphical Abstract

1. Introduction

In recent years, there has been an increasing demand for energetic materials in the global military industry, including military explosives, launchers, and rocket propellants [1,2]. Aniline and TNT are widely used aromatic organic compounds in the military industry, with aniline serving as a stabilizer in the manufacture of explosives [3,4], while TNT is widely used as a nitro-organic explosive in modern military munitions [5]. However, both aniline and TNT are characterized by high toxicity, difficult removal, environmental accumulation, bioaccumulation, and carcinogenicity [6]. Moreover, they are prone to accumulating in soil and groundwater environment [7], thereby posing threats to both the ecological environment and human health [8,9]. As a result, these issues have garnered significant attention from countries around the world [10,11].
Soil is a loose porous material composed of various granular minerals, organic matter, moisture, air, and microorganisms [12]. It acts as the primary recipient of organic pollutants, such as aniline and TNT, which can be released into the soil matrix through osmosis. Consequently, soil is the most direct and primary receptor of these organic pollutants, which can be adsorbed on the soil surface [13] and accumulated in shallow soils. However, previous studies have shown the presence of organic pollutants in deep soils and groundwater [14], indicating that colloidal particles present in the soil matrix exert substantial influence over the release and transport mechanisms of these pollutants.
In previous studies, the focus regarding organic pollutants predominantly revolved around the development of adsorption materials and wastewater treatment [15,16,17], with limited attention given to the environmental transport behavior of organic pollutants. In recent years, researchers have delved into the adsorption and transport characteristics of typical engineered nanoparticles and mineral colloids, such as biochar, carbon nanotubes, graphene oxide, silica, and ferrihydrite [18,19,20,21,22,23,24]. Hameed et al. found that the biocarbon (BC) colloids increased the transport of organic pollutants (oxytetracycline, atrazine, and phenanthrene) in soil, and the effect was related to carbonization temperature and colloidal particle size [25]. Lu et al. found that graphene oxide (GO) nanoparticles could provide adsorption sites to enhance the transport of dimethyl phthalate (DMP) at low ionic strengths, and the co-transport of GO and DMP varied in different porous media [26]. Furthermore, Wang et al. demonstrated the significant impact of illite and silica colloids on the transport of steroid estrogens (SEs) in saturated sand columns [27].
In fact, “intrinsic colloidal particles” are more common in natural soil and underground aquifer systems [28]. These particles possess intricate structures and exhibit distinctive physicochemical properties, such as greater specific surface area and specific surface energy [29], negative surface charge [30], rich surface functional group [31], and dispersion stability [32]. Consequently, soil colloid particles serve as excellent adsorption materials due to their strong affinity for organic pollutants. When soil colloid particles are released and transported in porous soil media, the less mobile adsorbable pollutants are easily transported. In other words, soil colloid particles can act as carriers to co-transport with pollutants. Therefore, the transport of soil colloid particles and pollutants in soil has received increasing attention [33,34].
The environmental behavior of organic pollutants in soil–water systems is greatly influenced by their chemical properties and structure [35]. Therefore, considering the structural and property differences among organic pollutants, this study focuses on aniline and TNT as the research subjects. This study aimed to determine the adsorption characteristics of the loess colloid particles to aniline and TNT with different physicochemical properties, and then to explore the mechanism of loess colloid particles’ release and co-transport with aniline and TNT. To a certain extent, this study can verify the possibility of pollutants in deep groundwater caused by the influence of vertical seepage on organic compounds in soil media. It provides a reference for predicting the distribution of organic pollutants in soil and their destination after entering soil.

2. Materials and Methods

2.1. Preparation and Characterization of Loess Colloid Particles

The loess samples used in this study were collected at 3 m underground of Northwestern Polytechnical University (34°14′ N, 108°55′ E) in Beilin District, Xi’an City, China. After the stones and plant debris had been removed from the samples, they were air-dried and ground, screened using a 2 mm (10 mesh) standard screen, and stored in sealed bags for later use. According to Stokes’ law, loess colloid particles were extracted by a siphon experimental device (Figure S1) with the natural settlement method (File S1), using the same methods as [36].
Particle size distribution and Zeta potential were analyzed by a nano laser particle size analyzer and a Zeta potential analyzer, respectively. The mineral composition of loess colloid particles was characterized by X-ray diffraction (XRD). Field emission scanning electron microscopy (SEM) was used to observe the microscopic morphology. The specific surface area and pore size distribution were characterized by BET (Brunauer–Emmett–Teller) and BJH (Barrett–Johner–Halenda) analyzers. A total organic carbon (TOC) analyzer was used to analyze the organic matter. The model of the instrument is shown in Table S2.

2.2. Adsorption Experiments

The adsorption experiment involved adsorption kinetics and response surface experiments. The dosage of loess colloid particles in the adsorption experiment was determined to be 6 g·L−1 based on the preliminary experiment. Furthermore, the adsorption kinetics experiment was conducted with organic compound concentrations of 10, 25, and 40 mg·L−1, respectively. The particle size used in the experiment was 1.1 μm. In the response surface experiment, the initial organic compound concentration, loess particle size, and Na+ concentration were selected as the influencing factors to study the adsorption performance of organic compounds on loess colloid particles. The experimental design is shown in Table S3. The method of the adsorption experiment is shown in File S2.
The calculation formula of adsorption rate and adsorption capacity of loess colloid particles for the two organic compounds is as follows:
q e = 1000 ( C 0 − C e ) c
C0 is the initial concentration of organic compound (mg·L−1), Ce is the equilibrium concentration of organic compound (mg·L−1), qe is the adsorption capacity of loess colloid particles to organic compound (mg·kg−1), c is the concentration of loess colloid particles (mg·L−1).

2.3. Column Experiments

The experimental device for dynamic soil columns (Figure S2) used the method of water intake from the upper part. In this device, a column of plexiglass (PMMA) was used with a height of 45 cm and an inner diameter of 8 cm. A section of PMMA tube was attached to the bottom of the column to collect the effluent from the soil column. Quartz sand, fine sand, and loess were used to fill the soil column from bottom to top (File S3). Prior to the experiment, the loess was saturated bottom-up with deionized water to expel air from the soil. It could reduce the disturbance of the soil structure and provide a stable initial environment [37,38].
The column experiments consisted of two groups, with both influent concentrations of aniline and TNT set at 40 mg·L−1. The column experiments included a dynamic adsorption experiment and a dynamic release experiment. The details of the column experiments are shown in File S4. Samples were collected at certain volume intervals and subsequently analyzed. Finally, the V − C/C0 curve was drawn, with the volume of effluent (V) on the X axis and the penetration ratio (C/C0) on the Y axis.

2.4. Methods for Determining the Concentration of Aniline and TNT

The determination of aniline was conducted using ultraviolet spectrophotometry of the borax–hydrochloride system, and the absorbance was measured at 230 nm. The determination of TNT adopted the ultraviolet spectrophotometry of the N-chlorocetylpyridine–sodium sulfate system, with absorbance measured at 540 nm. According to the above methods, the standard curves were drawn; see Figure S3.

2.5. Methods for the Analysis of Leachate

The volume, turbidity, Zeta potential, and particle size distribution of effluent were determined immediately after sampling. The turbidity was measured by a turbidimeter and converted to the concentration of loess particles, using the same methods as [39].
The formula for calculating the concentration of organic compound in particle adsorption state is as follows:
C 1 = C − C 2
C1 is the concentration of organic compound in particle adsorption state; C is the total organic compound concentration in untreated effluent samples; C2 is the concentration of soluble organic compound in solution, where C2 refers to the concentration of organic compound in effluent after centrifugal treatment of 15 min and passing through a 0.22 μm filter membrane; the treatment method is as per reference [40].

3. Results and Discussion

3.1. Basic Properties of Loess Colloid Particles

The particle sizes of the loess colloid particles were found to follow a normal distribution, as shown in Figure 1a. The median particle sizes (D50) were measured as 1.1 μm, 0.9 μm, and 0.7 μm, respectively. Correspondingly, the Zeta potentials were recorded as −19.2 mV, −15.1 mV, and −13.9 mV, respectively (Table S4). These findings indicate that the absolute value of Zeta potential increased with smaller particle size, suggesting improved dispersion stability of the particles. Consequently, the loess colloid particles possess the potential to transport with the water flow [41,42]. The XRD spectrum (Figure S4) revealed that the main components of loess colloid particles consisted of quartz (SiO2), albite, and calcite, while the main clay minerals identified were kaolinite, montmorillonite, and illite. The results of SEM (Figure S5) demonstrated the presence of a comparable laminar structure among the loess colloid particles. Some colloids in recent studies have been found to have similar morphology to loess colloid particles [27,43]. The analysis results of BET and BJH (Figure 1b) suggested that loess colloid particles were mesoporous materials, with a surface area of 40.4948 m2·g−1, a total hole volume of 0.0983 mL·g−1, and an average pore size of 9.7099 nm (Table S5). The pore size distribution ranged from 3–5 nm, and the distribution size of pore size was not uniform, indicating that the loess colloid particles exhibit the capability to absorb organic pollutants.

3.2. Adsorption Characteristics of Aniline and TNT on Loess Colloid Particles

3.2.1. Adsorption Kinetics

The adsorption kinetics curves of aniline and TNT on loess colloid particles are described in Figure 2. As the adsorption time increased, both organic compounds exhibited the trend of “an initial rapid increase in adsorption, followed by stabilization at adsorption equilibrium” [40]. Adsorption processes of aniline and TNT reached equilibrium at 120 min and 8 h, respectively. The results showed that when initial concentrations of the organic compound were 10, 25, and 40 mg·L−1, the aniline adsorption capacity were 120.84, 217.00, and 285.28 mg·kg−1 (Figure 2a), respectively. Correspondingly, the TNT adsorption capacity was 220.67, 334.28 and 468.39 mg·kg−1 (Figure 2b). These results suggested that the adsorption capacity of TNT on loess colloid particles was greater than that of aniline. The disparity in adsorption between TNT and aniline can be attributed to the presence of a certain amount of TOC (0.122%) in loess colloid particles (Table S6) and the logarithm of octanol–water partition coefficient (log Kow). TNT has a higher log Kow value (1.6) compared to aniline (0.94), making it easier for TNT to dissolve into the particles through partition. Furthermore, the adsorption capacity of both organic compounds increased with rising initial concentration, the reason being that a higher initial concentration could form a large concentration difference to provide the driving force for the adsorption process.
The adsorption kinetic model (Figure S6) indicated that the adsorption of aniline and TNT on loess colloid particles was more consistent with the quasi-second-order kinetic model (R22 > 0.99). The number of adsorption sites on the surface of loess colloid particles was the primary factor affecting the adsorption performance. The intra-particle diffusion model (Figure S6) showed that the adsorption process was mainly controlled by intra-particle diffusion, but it was not the exclusive determinant of the adsorption rate [44]. Notably, the quasi-second-order kinetic rate constant of aniline (2.03 × 10−2, 7.57 × 10−3, 6.43 × 10−3 kg·h−1·mg−1) was notably higher than that of TNT (2.04 × 10−3, 1.14 × 10−3, 1.08 × 10−3 kg·h−1·mg−1), and the intraparticle diffusion coefficient of aniline (Table S8) was greater than that of TNT (Table S9). These differences can be attributed to disparities in polarity, molecular dimensions, and structures.

3.2.2. Effect of Factors on Adsorption Capacity

The experimental results (Table S10) were analyzed by quadratic polynomial regression fitting through Design-Expert software. The quadratic regression model and response surface variance analysis data are given in Tables S11 and S12, respectively. The p-value of the two regression models was p < 0.0001 (<0.01), indicating that the models were extremely significant. The p-value of the fitting lock in the regression equation was p > 0.05, and it was insignificant. These results showed that the experimental method and model design were reliable and reasonable within the horizontal range. At the same time, the influencing factors were well fitted and statistically significant [45].
The significant effect of three factors on the adsorption capacity of aniline and TNT followed a descending order of initial concentration of organic compound, Na+ concentration, and size of loess colloid particle. In the 3D response surface diagram and the contour diagram (Figure 3 and Figure 4), the red surface indicates a higher adsorption capacity [46]. In the regression equation (File S5), the sign and absolute value of the coefficient represent the direction and magnitude of the influence on the adsorption capacity [47,48]. The analysis indicated that the adsorption capacity of both aniline and TNT increased with the increasing initial concentration of organic compound, and decreased with increasing particle size and Na+ concentration. A higher concentration of organic compound increased the possibility of collisions between organic molecules and the active sites of loess colloid particles in solution, resulting in greater adsorption capacity. Conversely, increasing particle size decreased the specific surface area of loess colloid particles and shortened the diffusion path of the organic compound, which hinders the adsorption process [49,50]. According to previous studies [51,52,53], the presence of Na+ could compress the bilayer thickness of the particles, causing aggregation and sedimentation of particles. As a result, the adsorption capacity of aniline and TNT was reduced.
Conditioning the maximum adsorption capacity of aniline and TNT, the optimal conditional parameters were obtained by the regression model. The optimal parameters of TNT were as follows: particle size was 0.77 μm, aniline concentration was 38.93 mg·L−1, and sodium ion concentration was 0.01 mol·L−1. Meanwhile, the optimal parameters of TNT were as follows: particle size was 0.71 μm, TNT concentration was 39.95 mg·L−1, and sodium ion concentration was 0.02 mol·L−1.

3.2.3. The Proposed Adsorption Mechanism

Through the analysis of the adsorption kinetic parameters and response surface experiments, it can be observed that there are differences in the adsorption characteristics of aniline and TNT on loess colloid particles under the same conditions. These differences are closely related to the physicochemical properties of organic compounds. The adsorption mechanism of aniline and TNT on loess colloid particles (Figure 5) is the result of a combination of van der Waals force, hydrogen bonding, and electrostatic interactions. The following factors contributed to the differences in adsorption between TNT and aniline: (1) The large specific surface area, specific surface energy, and pore size distribution (Table S5) of loess colloid particles enable the adsorption of both aniline and TNT through van der Waals forces, occurring on the surface and interior of the particles [54,55,56]. (2) The surface of loess colloid particles contains abundant functional groups, such as -OH and -COOH, which can act as hydrogen bonding donors, forming hydrogen bonds with nitrogenous functional groups in aniline and TNT [57,58,59]. Some previous studies on aniline and TNT have also conveyed similar hydrogen bonding interactions [60,61]. Since the electron cloud density of the amino group was higher than that of the nitro group, the hydrogen bonding strength of aniline was greater than that of TNT. (3) Electrostatic interaction mainly existed between loess colloid particles and organic compound containing polar functional groups [57]. -NH2 in aniline could be partially protonated (H+ + Ph-NH2 → Ph-NH3+) [40,62], and -NH3+ was adsorbed by electrostatic interaction to neutralize the negative charge of loess colloid particles. A recent study found the same electrostatic interaction of aniline [63]. The nitro group in TNT acted as a strong electron-absorbing group, resulting in an insufficient negative charge on the π orbitals11 and making TNT a strong electron receptor. As a result, it could complex with the functional group of loess colloid particles by charge transfer [5,15], which is consistent with previous research results [61]. The electron absorption capacity of -NH3+ was higher than that of -NO2, resulting in a stronger electrostatic interaction for aniline compared to TNT.

3.3. Co-Transport of Loess Colloid Particles with Aniline and TNT

3.3.1. Dynamic Adsorption of Aniline and TNT on Loess

Through the analysis of adsorption-release curve (Figure 6), it was evident that the dynamic adsorption-release modes of aniline and TNT in loess were generally consistent. Both adsorption curves (Figure 6a) showed a trend of slow adsorption, followed by rapid penetration and then reaching adsorption equilibrium. Compared to aniline, TNT exhibited a sequential backward movement of the penetration point (C/C0 = 0.1) and the penetration end point (C/C0 = 0.9), resulting in a decrease in the slope of the adsorption curve and an increase in penetration time. The adsorption rate of aniline was higher than that of TNT, while the adsorption capacity of TNT was larger than that of aniline. Furthermore, both release curves (Figure 6b) presented a pattern of rapid release, slow release, and sustained low-level release. When the volume of effluent reached 318.5 mL and 699.5 mL, there was no significant change in the concentration of aniline and TNT in the effluent. The concentration of aniline initially decreased to a lower stable value compared to TNT.

3.3.2. Mechanism of Loess Colloid Particles Release

In the adsorption-release processes of aniline and TNT, the trends of the release concentration of loess colloid particles were basically consistent. (Figure 7a). The trend involved a rapid decrease after an initial high concentration, followed by a slower decrease and finally stabilizing at a low level. It was consistent with the conclusions of previous studies [32,64]. Reasons for the high initial concentration of loess particles could be attributed to three factors. Firstly, water scouring generates shear stresses, which cause the release of loess colloid particles by vertical flow [15,65]. Secondly, changes in water content of soil aggregates affect surface tension, resulting in release of loess colloid particles from soil agglomerates. The release was associated with changes in the physical structure of the soil aggregates [66]. Lastly, in the bottom-up saturation process of the soil column with deionized water, some loess particles were already present in the soil pores and could transport with the water flow [67].
Effluent volumes of 26.5, 96.5, 168.0, 206.5, 245.0, and 322.0 mL were selected to analyze particle size distribution (Figure 7b) and Zeta potential (Figure 7c). The results suggested the particle size of loess colloid particles in the effluent ranged from 0.1 to 10 μm, and the absolute Zeta potential was greater than 10. Consequently, loess colloid particles are less likely to settle on the soil surface but have the potential to transport in the soil. Meanwhile, the findings showed that the loess colloid particles prepared in the static adsorption experiment were suitable and accurate. In brief, the results of static adsorption can indirectly reflect the transport of aniline and TNT on loess colloid particles in dynamic simulated column experiments.

3.3.3. Mechanism of Co-Transport of Loess Colloid Particles with Aniline and TNT

In the release experiment, the existence form of organic compound in effluent was separated (Figure 8). The results showed that the aniline and TNT in effluent were partially adsorbed on the loess colloid particles and existed in the particle adsorption state. It confirmed that loess colloid particles have an adsorption capacity for aniline and TNT, and the conclusions of adsorption characteristics in static experiments have the potential to be applied to the dynamic simulated column experiments. In addition, the changes in the concentrations of aniline and TNT on the particles were comparable to those of loess colloid particles (Figure 8a,b). The correlation fitting between particle adsorption state concentration of the two organic compounds and loess colloid particle concentration is illustrated in Figure 8c. The results of the fitting showed that the correlation analysis was significant, with correlation coefficients (R2) of 0.9796 and 0.9435, respectively. It is reasonable to conclude that loess colloid particles could act as carriers of aniline and TNT, enhancing the transport of organic pollutants through van der Waals force, hydrogen bonding, and electrostatic interaction along the water flow. A recent study confirmed the same, that clay colloids could serve as carriers for ciprofloxacin [68]. By analyzing the concentration of aniline and TNT on loess colloid particles and effluent volume at a low stable value, it was discovered that the transport rate of loess colloid particles with aniline was greater than that of TNT. This analysis was also consistent with the conclusion drawn from the static experiment. Therefore, the adsorption characteristics of aniline and TNT on loess colloid particles can provide an effective explanation for their transport mechanism. Combined with static adsorption kinetics analysis, the transport rate was mainly affected by the polarity, molecular size, and structure of the organic compound. The main reasons that aniline exhibited a greater adsorption rate than TNT were its larger polarity, smaller molecular weight, and smaller volume (Table S7).

4. Conclusions

In this study, the adsorption characteristics of loess colloid particles on aniline and TNT were investigated through batch tests firstly. Then, a dynamic process was conducted to determine the mechanism of release and co-transport of loess colloid particles with aniline and TNT. The loess colloid particles prepared in this study exhibited the ability to absorb organic pollutants and transport with water flow. The adsorption process of aniline and TNT followed the quasi-second-order kinetics model and the intra-particle diffusion model, and the quasi-second-order kinetics of aniline and the reaction rate constant of intra-particle diffusion were found to be higher than those of TNT. The adsorption mechanism of aniline and TNT on loess colloid particles was primarily driven by van der Waals force, hydrogen bonding, and electrostatic attraction. In addition, aniline showed stronger hydrogen bonding and electrostatic attraction on loess colloid particles compared to TNT. To enhance the adsorption capacity, three strategies can be implemented: reducing the particle size of loess colloid particles or the concentration of Na+, and increasing the initial concentration of organic pollutants.
During the dynamic transport process, loess colloid particles exhibited co-transport with aniline and TNT, and their transport behavior was significantly correlated with the released loess colloid particles (R2 was 0.9796 and 0.9434, respectively). Further analysis indicated that the transport rate of aniline was higher than that of TNT. The transport mechanism was influenced by polarity, molecular size, and structure, which in turn affected the adsorption rate. Overall, loess colloid particles displayed the capability to adsorb aniline and TNT, thereby promoting their transport in the environment. The differences in physical and chemical properties led to divergent transport behaviors of aniline and TNT in loess. These findings provide valuable insights for predicting the fate of organic pollutants upon entering the soil. Admittedly, as with the majority of studies, there are some possible limitations in this study. The main limitations are related to the experimental design, data simulation, and experimental conditions. Further research on the environmental behavior of the aniline and TNT composite pollutants, the combination of theoretical models and computer simulations, and field experiments would contribute to a more comprehensive understanding of the mechanism.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/w16010180/s1, File S1: Separation methods of loess colloid particles; File S2: The method of adsorption experiment; File S3: The filling method of soil column; File S4: The method of column experiment; File S5: Response surface regression equations of aniline and TNT; Figure S1: Schematic diagram of siphon separation experiment device; Figure S2: Schematic diagram of dynamic soil column experimental device; Figure S3: The standard curve of (a) aniline and (b)TNT; Figure S4: X-ray diffraction pattern of loess colloid particles; Figure S5: SEM image of loess colloid particles; Figure S6: Adsorption kinetics model of aniline: (a) pseudo-first-order kinetic model, (b) pseudo-second-order kinetic model, (c) intra-particle diffusion model. Adsorption kinetics model of TNT: (d) pseudo-first-order kinetic model, (e) pseudo-second-order kinetic model, (f) intra-particle diffusion model; Table S1: Sedimentation time of loess colloid particles with different particle sizes at room temperature; Table S2: Instruments for material characterization; Table S3: The design factors, levels and coded values of response surface experiment; Table S4: Particle size distribution and Zeta potential of loess colloid particles; Table S5: BET and BJH parameters of loess colloid particles; Table S6: Total carbon (TC), total inorganic carbon (IC)and TOC parameters in loess colloid particles; Table S7: Physical and chemical properties of aniline and TNT; Table S8: Fitting parameters of pseudo-first-order kinetic and pseudo-second-order kinetic models for the adsorption of loess colloid particles on aniline and TNT; Table S9: Fitting parameters of the intraparticle diffusion model for the adsorption of loess colloid particles on aniline; Table S10: Response surface experimental design and results of aniline and TNT; Table S11: Response surface variance analysis results of loess colloid particles to aniline adsorption capacity; Table S12: Response surface variance analysis results of loess colloid particles to TNT adsorption capacity.

Author Contributions

Conceptualization, Y.W.; methodology, Y.W.; formal analysis, Z.M.; investigation, Z.M., C.M. and Y.W.; resources, S.H. and R.S.; data curation, Z.M. and C.M.; writing—original draft preparation, Z.M.; writing—review and editing, Z.M., S.H., X.S. and Y.W.; supervision, Y.W.; funding acquisition, S.H. and R.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (No. 42077283, Yaoguo Wu; No. 41601338, Ran Sun; and No. 41502240, Sihai Hu), and the Natural Science Foundation of Shaanxi Province (No. 2020JM-110, Sihai Hu).

Data Availability Statement

The data presented in this study are available upon request from the corresponding author.

Conflicts of Interest

The authors declare no conflict of interest.

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Water 16 00180 g001
Figure 1. (a) Particle size distribution, (b) adsorption desorption isotherms and pore size distribution of loess colloid particles.
Figure 1. (a) Particle size distribution, (b) adsorption desorption isotherms and pore size distribution of loess colloid particles.
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Water 16 00180 g002
Figure 2. Adsorption kinetic curves of (a) aniline and (b) TNT on loess colloid particles.
Figure 2. Adsorption kinetic curves of (a) aniline and (b) TNT on loess colloid particles.
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Water 16 00180 g003
Figure 3. Three-dimensional response surface and contour diagrams of aniline adsorption capacity under interaction: (a) particle size and initial concentration of aniline, (b) particle size and Na+ concentration, (c) initial concentration of aniline and Na+ concentration. The color of the surface represents the adsorption capacity. Blue, green, and red indicate that the adsorption capacity of aniline increases sequentially.
Figure 3. Three-dimensional response surface and contour diagrams of aniline adsorption capacity under interaction: (a) particle size and initial concentration of aniline, (b) particle size and Na+ concentration, (c) initial concentration of aniline and Na+ concentration. The color of the surface represents the adsorption capacity. Blue, green, and red indicate that the adsorption capacity of aniline increases sequentially.
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Water 16 00180 g004
Figure 4. Three-dimensional response surface and contour diagrams of TNT adsorption capacity under interaction: (a) particle size and initial TNT concentration, (b) particle size and Na+ concentration, (c) initial TNT concentration and Na+ concentration. The color of the surface represents the adsorption capacity. Blue, green, and red indicate that the adsorption capacity of TNT increases sequentially.
Figure 4. Three-dimensional response surface and contour diagrams of TNT adsorption capacity under interaction: (a) particle size and initial TNT concentration, (b) particle size and Na+ concentration, (c) initial TNT concentration and Na+ concentration. The color of the surface represents the adsorption capacity. Blue, green, and red indicate that the adsorption capacity of TNT increases sequentially.
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Figure 5. Adsorption mechanism of (a) aniline and (b) TNT on loess colloid particles.
Figure 5. Adsorption mechanism of (a) aniline and (b) TNT on loess colloid particles.
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Water 16 00180 g006
Figure 6. (a) Adsorption and (b) release curves of aniline and TNT. 1, 3, 5, 7, 9 and 11 are sample sites with effluent volumes of 26.5, 96.5, 168.0, 206.5, 245.0, and 322.0 mL, respectively.
Figure 6. (a) Adsorption and (b) release curves of aniline and TNT. 1, 3, 5, 7, 9 and 11 are sample sites with effluent volumes of 26.5, 96.5, 168.0, 206.5, 245.0, and 322.0 mL, respectively.
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Water 16 00180 g007
Figure 7. (a) Concentration changes, (b) particle size distribution, and (c) Zeta potential change of loess colloid particles in effluent.
Figure 7. (a) Concentration changes, (b) particle size distribution, and (c) Zeta potential change of loess colloid particles in effluent.
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Water 16 00180 g008
Figure 8. Changes in the concentrations of (a) aniline and (b) TNT on loess colloid particles in influent, and (c) the relevant linear fitting result in the process of release.
Figure 8. Changes in the concentrations of (a) aniline and (b) TNT on loess colloid particles in influent, and (c) the relevant linear fitting result in the process of release.
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MDPI and ACS Style

Meng, Z.; Hu, S.; Sun, R.; Meng, C.; Wu, Y.; Sun, X. Co-Transport of Aniline and TNT with Loess Colloid Particles in Saturated Loess Columns: Mechanism and Processes. Water 2024, 16, 180. https://doi.org/10.3390/w16010180

AMA Style

Meng Z, Hu S, Sun R, Meng C, Wu Y, Sun X. Co-Transport of Aniline and TNT with Loess Colloid Particles in Saturated Loess Columns: Mechanism and Processes. Water. 2024; 16(1):180. https://doi.org/10.3390/w16010180

Chicago/Turabian Style

Meng, Zhaohui, Sihai Hu, Ran Sun, Chengzhen Meng, Yaoguo Wu, and Xiaofeng Sun. 2024. "Co-Transport of Aniline and TNT with Loess Colloid Particles in Saturated Loess Columns: Mechanism and Processes" Water 16, no. 1: 180. https://doi.org/10.3390/w16010180

APA Style

Meng, Z., Hu, S., Sun, R., Meng, C., Wu, Y., & Sun, X. (2024). Co-Transport of Aniline and TNT with Loess Colloid Particles in Saturated Loess Columns: Mechanism and Processes. Water, 16(1), 180. https://doi.org/10.3390/w16010180

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