Adsorption of Atrazine by Fe-Mn-Modified Biochar: The Dominant Mechanism of π–π Interaction and Pore Structure

Abstract

It is important to explore efficient materials to alleviate the negative effect of atrazine in soils or water. In this paper, four Fe/Mn-modified biochars were prepared to enhance atrazine removal. The batch adsorption experiment was conducted to explore the adsorption ability of biochar(DBC) and modified biochars (F₃M₁DBC, F₁M₃DBC, MnDBC, and FeDBC) on atrazine, and the adsorption mechanism was conducted by XRD, XPS, and FTIR. The modified biochar showed larger specific surface areas and zero-point charge than those of the original biochar. The increased oxygen functional groups (OH, C=C, and C=O) and the formation of Fe₃O₄, Mn₃O_4, and FeMnO₃ on modified biochar improved atrazine removal. The maximum atrazine adsorption by F₃M₁DBC was 4.3 times higher than that of DBC. The atrazine adsorption by modified biochar was not pH-dependent, and their removal of atrazine was dominated by adsorption rather than degradation. The desorption rate was 8.61% for F₃M₁DBC, 15.95% for F₁M₃DBC, 26.19% for MnDBC, and 29.83% for FeDBC, which were 29.1–79.5% lower than that of DBC, accordingly decreased the environmental risk. XPS and FTIR analysis proved that the adsorption mechanisms of Fe/Mn-modified biochars were mainly attributed to their strong π–π interactions between atrazine and oxygen functional groups, graphitic carbon, and Fe/Mn-oxides on the surface of biochar. In addition, the larger surface area and pore structure of modified biochar contributed to the adsorption and pore filling of atrazine on biochar. In general, the Fe/Mn-modified biochars can be used as effective adsorbents to remove atrazine from soils and waters.

1. Introduction

Atrazine (2-chloro-4-ethylamino-6-isopropylamino-S-triazine), as a commonly used herbicide in the world, can efficiently inhibit the growth of weeds in farmland and orchard. Its average half-life in soil is 40–43 days according to the ATSDR(Agency for Toxic Substances and Disease Registry). The solubility of atrazine in water is about 33 mg·L⁻¹. It easily migrates to groundwater and surface water because of its persistence [1] and low adsorption on soil [2]. As reported [3], the detection rate of atrazine in the surface water of the Yangtze River Basin, the Yellow River Basin, the Taihu Basin, and the Songhua River Basin is almost 90%. Atrazine has raised health concerns [4], as it can be an endocrine disruptor, and affect the immune system [5], and can even cause ovarian cancer and breast cancer [6] with long-term exposure. Therefore, an important issue remains to explore efficient and low-cost methods to alleviate the negative effect of atrazine in the soil.

Biochar is a carbon-rich, stable and microporous material pyrolyzed by biomass materials such as agriculture, forestry, and domestic waste under oxygen-depleted or anaerobic conditions [7]. Biochar is effective in the adsorption of heavy metals [8,9], some kinds of organic pollutants [10,11], and nitrogen and phosphorus nutrients [12,13], however, the original biochar has low adsorption capacity for anions [14] and electronegative organic molecule [15], such as Tetracyclines [16], because the surface of biochar shows negative charge. Therefore, it is useful to improve the electronegative organic pollutant (e.g., atrazine) adsorption capacity on biochar through modification or functionalization. There are many modification methods. Oxidation by acids, alkaline, metal oxides, physical, and natural oxidation on the surface area properties of biochar could increase the cation exchange capacity (CEC), micro-pores (MP), specific surface area (SSA), and oxygen-content functional groups (OFGs) of biochar [17]. Cao et al. [18] found the adsorption capacity of MgO-modified biochar was increased by 1.99–5.71 times. Biochar-supported iron oxides can not only improve the adsorption capacity of pollutants by increasing the surface charge and hydroxyl functional groups of biochar [19], but also make the material have certain magnetism, which is conducive to the recycling of materials [20]. Manganese oxide can be used as a catalyst to promote the pyrolysis of biomass, improve the pore structure of biochar, increase the pore size of biochar, and enhance its physical adsorption [21]. Chen et al. [22] prepared a biochar-supported Fe-Mn composite, which showed a high removal rate for acid red 88, which proved that bimetallic metal oxides-based systems may enhance the electron transfer, causing high catalytic reactivity. Due to the loading of iron or manganese on the surface of biochar and the change of structural characteristics during pyrolysis, the specific surface area, oxygen-containing functional groups and other physical and chemical properties of biochar may be improved to enhance the removal of organic pollutants. However, it is not clear whether the bimetallic metal oxides-based systems could improve the atrazine removal rate, and the removal mechanism of atrazine by bimetal-modified biochar is still unclear.

This study aims to (1) prepare the Fe/Mn modified biochar and deduce the physicochemical properties of modified biochar with different Fe and Mn addition ratios, (2) evaluate the adsorption and desorption capacity of atrazine by original biochar and modified biochar, (3) analyze the removal mechanisms of modified biochar by FTIR, XRD, and XPS.

2. Materials and Methods

2.1. Experimental Reagents

Atrazine (C₈H₁₄ClN₅, 97%) was purchased from Shanghai Maclean Biochemical Technology Co., Ltd., Shanghai, China. Analytical reagents (e.g., Methanol, FeSO₄·7H₂O, MnCl₂·4H₂O, NaOH, HCl, and Absolute alcohol) were purchased from Sinopharm Group Chemical Reagents Co., Ltd., Beijing, China.

2.2. Preparation of Biochar

Original biochar preparation: the rice straw was washed and crushed, dried to constant weight, and pyrolyzed at 500 °C, N₂ for 3 h and passed through a 100 mesh sieve to obtain the original rice straw-derived biochar, named DBC.

Preparation of modified biochar: 10.0 g crushed rice straw was soaked in 100 mL DI water (the liquid-solid ratio of 10:1), and then the different molar ratios of FeSO₄·7H₂O and MnCl₂·4H₂O were added to the biomass-water system. The four molar ratios of Fe²⁺ and Mn²⁺ were applied as 3:1, 1:3, 2:0, and 0:2, respectively. Ultrasonic 30 min, immersion 12 h, placed in a muffle furnace (subsequent conditions are the same as DBC) to produce modified biochar, named as F₃M₁DBC, F₁M₃DBC, FeDBC, and MnDBC, respectively.

2.3. Sample Characterization

The surface functional groups of the original and modified biochar were analyzed by Fourier transform infrared spectrometer (FT-IR, Thermo 6700, Thermofisher, USA). The specific surface area, pore size, pore volume, etc. of the biochar was determined using an automatic specific surface area tester (BET, V-sorb2800, Beijing Golden Eppo Technology Co., Beijing, China). Zeta potential measurement were carried out using a high-resolution potential and particle size analyzer (ZetaPALS, Brookhaven, USA). The morphological characteristics and element composition of the biochar were determined by scanning electron microscope (SEM, Quanta FEG 250, FEI Company, USA). An X-ray photoelectron spectrometer (XPS, Thermo Scientific EscaL-ab 250Xi, Thermofisher, USA) was used to analyze the binding energy of the elements on the sample surface. The atrazine content was determined by a high-performance liquid chromatograph (HPLC, LC-20AT, Shimadzu, Japan) [16].

2.4. Batch Sorption Experiments

Sorption kinetics experiments were conducted by adding 0.1 g biochar or modified biochar into 40 mL atrazine solution in which an initial concentration of atrazine was 10 mg·L⁻¹. The vials were shaken on a shaker at 150 rpm and 25 ± 1 °C until equilibrium. The atrazine solution without biochar or modified biochar was used as a control. The sampling times were set at 10, 20, 30, 60, 120, 240, 480, 720, 1080, 1440, 2160, 2880, 4320 min, all experiments were performed in triplicates.

The pseudo-first-order kinetic equation (Equation (1)), the pseudo-second-order kinetic equation (Equation (2)), and the intra-particle diffusion equation (Equation (3)) are used to describe the kinetic mechanism of adsorption [23,24,25]. The specific equation is as follows:

$$Q_t=Q_e\left[1-\exp \left(-\frac{k_1}{2.303}\right)t\right]$$

(1)

$$\frac{t}{Q_t}=\frac{t}{q_e}+\frac{1}{k_2{Q}_e^2}$$

(2)

$$Q_t=k_3{t}^{0.5}+C$$

(3)

where Q_t and Q_e are the adsorption amount of atrazine at time t and adsorption equilibrium, mg·kg⁻¹; k₁, k₂, and k₃ are respectively the pseudo-first-order adsorption kinetic rate constant, min⁻¹; the pseudo-second-order adsorption kinetic rate constant, g·(mg·min)⁻¹; the intra-particle diffusion rate constant, mg·(g·min^1/2)⁻¹; and C is the boundary thickness constant.

The isothermal sorption experiments were conducted using the above solution with initial concentrations of atrazine 0.5, 1, 5, 10, 20, and 50 mg·L⁻¹, respectively, which were shaken on a shaker at 150 rpm and 25 ± 1 °C for 24 h, other schedules were same as the Sorption kinetics experiment.

The Langmuir (Equation (4)) and Freundlich (Equation (5)) isotherm adsorption models were used to simulate the isotherm data [26,27], The specific equation is as follows:

$$Q_e=\frac{Q_m{K}_L{C}_e}{1+K_L{C}_e}$$

(4)

$$Q_e=K_F{C}_e{}^{1/n}$$

(5)

where Q_e and Q_m are the adsorption balance and maximum adsorption capacity of biochar, mg·kg⁻¹; C_e is the concentration at adsorption equilibrium, mg·L⁻¹; K_F and K_L are the correlation coefficients of Langmuir and Freundlich equations respectively; 1/n is the empirical coefficient of Freundlich.

2.5. Effect of pH on Adsorption Experiments

An amount of 0.1 g of biochar or modified biochar was added to 40 mL atrazine solution (10 mg·L⁻¹), and the initial pH of the solution was adjusted to 3.0, 5.0, 7.0, 9.0, and 10.0 by 0.1 mol·L⁻¹ HCL and NaOH, respectively. The solution was shaken in a 150 rpm shaker at 25 °C for 24 h under dark conditions. The following steps are the same as the adsorption kinetic conditions. To explore the influence of pH on the adsorption of atrazine by biochar.

2.6. Desorption Experiments

To investigate the atrazine adsorption stability and key adsorption mechanisms on original biochar and modified biochar, the desorption experiments were performed immediately after sorption. The 0.1 g pre-adsorption biochar or modified biochar was added to 40 mL methanol or DI water, respectively. The vials were subjected to ultrasonic desorption for 6 h. The desorption rate equation is as follows:

$$Desorption(\%)=\frac{\begin{array}{cc}\begin{array}{ccc}desorption& amount& in\end{array}& methanol/DI\end{array}}{\begin{array}{ccc}saturated& adsorption& amount\end{array}}$$

(6)

2.7. Atrazine Detection Method

At the end of each adsorption or desorption, each sample was centrifuged at 3500 rpm for 10 min, and filtered through a 0.22 μm filter, the residual content of atrazine was detected by a high-performance liquid chromatography (HPLC, LC-20AT, Shimadzu, Japan) system.

3. Results and Discussion

3.1. Characteristics of Original Biochar and Modified Biochar

The morphology of original biochar (DBC) and metal-modified biochar (F₃M₁DBC, FeDBC, F₁M₃DBC, and MnDBC) are revealed by SEM, EDS, and mapping (Figure 1). DBC and modified biochar show a rich pore structure and rough surface, which affect the adsorption capacity of organic pollutants [28]. The EDS elemental mapping showed that no obvious spots of Fe and Mn were found on the original biochar surface, but a significant increase of the Fe and Mn was found on the modified biochar surface, which was attributed to the loading of FeSO₄ and MnCl₂. Furthermore, the content of the O element on the modified biochar was higher than that of the original biochar, indicating the formation of metal oxides during the modification process.

The FTIR spectra of original biochar and modified biochar in the mid-infrared region with a wavelength range of 400–4000 cm⁻¹ are shown in Figure S1. Both original and modified biochar were rich in O-containing function groups. The peak at 1638 cm⁻¹ belonged to C=O in ketones and aldehydes [16]. The peaks at 3443 and 1410 cm⁻¹ were attributed to −OH and −COOH, respectively, which were responsible for the organic pollutant adsorption [29]. The stretching vibration of OH, C=C, and C=O groups of the four modified biochars was stronger than that of DBC, indicating that the modification increased the number of functional groups of biochar and was conducive to the adsorption of atrazine by biochar. A new peak at 776 cm⁻¹ assigned to the Mn-O bond was found in MnDBC, F₁M₃DBC, and F₃M₁DBC, while a new peak at 558 cm⁻¹ belonging to the Fe-O bond was found in FDBC, F₁M₃DBC, and F₃M₁DBC, which illustrated the formation of Fe oxides, Mn oxides or Fe-Mn oxides.

XPS spectra (Figure 2a–c) showed that the peaks for the original biochar and Fe-Mn modified biochar at 285.96 eV and 284.8 eV in the C1s correspond to C-O and C-C, and the peaks at 533.04 eV and 531.5 eV in the O1s correspond to C = O and −OH [30], respectively. Compared with the original biochar, the XPS full survey spectra showed new peaks of Fe 2P and Mn 2P on the surface of modified biochar, indicating that Fe and Mn were successfully loaded on the surface of biochar, which was confirmed by SEM and FTIR analyses as described above. For example, Figure 2b,c shows that the C-O of the original biochar increased from 21.5% to 25.08% for F₃M₁DBC, and the characteristic peak of metal oxide appeared at 530 eV in the O1s spectrum, indicating that the Fe-Mn element was involved in the pyrolysis process of the biochar and may form a metal oxide on its surface.

The XRD analysis (Figure 2d) was conducted to deduce the mineral crystal constituents in the original biochar and Fe-Mn modified biochar. There is no characteristic peak on the surface of DBC except for that of SiO₂ and CaCO₃. F₃M₁DBC and FeDBC diffraction patterns show that d-spacings are 30.12, 35.5, 43.1, 57, and 62.6 Å, which match the diffraction patterns for Fe₃O₄. A similar phenomenon was reported by Wang [31]. The peaks at d-spacings of MnDBC and F₁M₃DBC at 35.31 and 50.98 Å match Mn₃O₄ diffraction peaks. The Fe-Mn bimetallic modification process could form more metal oxides, such as the formation of Mn₃O₄ and FeMnO₃ for F₁M₃DBC, as well as Fe₃O₄ and FeMnO₃ for F₃M₁DBC, which provided more atrazine adsorption sites.

The specific surface area (SSA), average pore size, and pore volume determined by N₂ adsorption–desorption isotherms are shown in

Table 1. Specific surface area, pore size, pore volume, and pHpzc of biochar before and after modification.

Material	Specific Surface Area/(m2 · g−1)	Average Pore Size/nm	Pore Volume /(cm3 · g−1)	pHpzc
DBC	8.613	7.696	0.016574	2.29
F3M1DBC	148.155	4.282	0.158607	2.58
F1M3DBC	4.003	23.374	0.023397	2.63
MnDBC	5.040	13.355	0.016829	2.76
FeDBC	54.387	5.529	0.075186	2.56

. The average pore sizes of F₁M₃DBC and MnDBC were 23.374 and 13.355 nm, respectively, which were 3.04 and 2.38 times higher than that of DBC. The obvious pore size increase in manganese-modified biochar indicated that the participation of Mn in the biomass pyrolysis process changed the surface pore structure of the biochar [14]. Meanwhile, the SSA of F₃M₁DBC and FeDBC was 17.2 and 6.31 times higher than that of DBC, respectively, which is probably attributed to the attachment of iron oxide on the surface and in the pore in the impregnation and pyrolysis process [32]. The greatest SSA and pore volume for F₃M₁DBC and the greatest average pore size for F₁M₃DBC may be due to the synergistic formation of metal oxides by Fe and Mn in the pyrolysis process, which leads to the increase of SSA and pore volume of the biochar. The large surface area and pore structure can effectively enhance the adsorption and pore-filling of pollutants on biochar [33].

3.2. Zeta Potential Analysis

Zeta potential is an important affection in the mutual repulsion and attraction between charged particles and reveals the macroscopic phenomena of interaction between particles. The Zeta potential of the original biochar and four modified biochars under different pH is shown in Figure S2, and the zero charges (pHpzc) of the original biochar and modified biochar are shown in Table 1. The zero charge (pHpzc) of the original biochar (DBC) is 2.29, and the pHpzc of the modified biochar increased with the order as followed: MnDBC (2.76) > F₁M₃DBC (2.63) > F₃M₁DBC (2.58) > FeDBC (2.56). When the environmental pH was higher than the pHpzc value, the surface charge of the material was negative, so the surfaces of the original biochar and four Fe/Mn modified biochars were negative, which is not conducive to the adsorption of atrazine by the original biochar and modified biochar because of electrostatic repulsion.

3.3. Adsorption Kinetics

Time-dependent adsorption kinetics experiments were conducted to determine the adsorption speed of atrazine from the solution. The atrazine adsorption kinetics of the original biochar and four Fe-Mn modified biochars were fitted using the pseudo-first-order, pseudo-second-order (Figure 3a), and Elovich kinetic models (Figure 3b). The pseudo-first-order and pseudo-second-order model fit parameters are shown in Table S1, and the Elovich kinetic model fit parameters are shown in Table S2. The original biochar and 4 modified biochar were better fitted by the pseudo-second-order model with larger R², compared to the pseudo-first-order model (Table S1), which indicated that atrazine adsorption was controlled by chemical reaction [34,35]. The original biochar and modified biochar showed rapid adsorption for atrazine in the first 2 h and then experienced a slower stage till achieving the equilibrium. The adsorption equilibrium time of DBC was 36 h, while the modified biochar, except for FeDBC, shortened the adsorption equilibrium time to 12 h for F₃M₁DBC, 18 h for F₁M₃DBC, and 24 h for MnDBC, respectively. The adsorption rate constant (K₂) of F₃M₁DBC, F₁M₃DBC, MnDBC, and FeDBC was 0.00607, 0.00435, 0.00295, and 0.00311 g·mg⁻¹·min⁻¹, respectively, which was 4.63, 3.32, 2.25 and 2.37 times higher than that of DBC, indicating that modification improves the atrazine adsorption speed on biochar. The higher adsorption capacity by modified biochar was attributed to a large specific surface, pore volume, and abundant oxygen-containing functional groups on the surface of the modified biochar, which enhanced more atrazine adsorption sites.

The Elovich kinetic model (Figure 3b), closely correlated with heterogeneous diffusion [35], shows the atrazine adsorption by the original biochar and modified biochar was divided into three stages: diffusion of atrazine across the water membrane to the surface adsorption, the external liquid film diffusion, and the diffusion process in the particles [29]. The fitting parameters of the Elovich kinetic model are shown in Table S2. The rate constants (K₁, K_2, and K₃) usually indicated the adsorption speed. The lowest value for K₃ showed the channel diffusion process was the atrazine adsorption limited step for biochar [35,36].

3.4. Adsorption Isotherms

The sorption isotherms of atrazine by the original biochar and four modified biochars are shown in Figure 4. The equilibrium atrazine adsorption isotherms on the original biochar and four modified biochars fitted the Freundlich model (R²= 0.931–0.991) better than the Langmuir one (R² = 0.915–0.978) (Table S3), suggesting that the adsorption by the five biochars was saturated via multilayer adsorption and controlled by chemical adsorption [29]. According to the curves fitted by the Freundlich model, the saturated adsorption capacity (K_F) of the modified biochar was 4155.3 for F₃M₁DBC, 1899.8 for F₁M₃DBC, 1861.2 for MnDBC, and 1508.6 for FeDBC, which was 4.36, 1.99, 1.95 and 1.58 times higher than that of DBC, respectively. The higher K_f and Q_m exhibited by the modified biochar has better adsorption capacity than the DBC, and the F₃M₁DBC was the optimally modified biochar for atrazine adsorption. All 1/n values in the Freundlich model of five biochar were less than 0.5, indicating favorable atrazine sorption on the biochar [29,37].

3.5. Effect of pH on Adsorption Behavior

Figure 5 shows the atrazine adsorption capacity (Q_e) of the original biochar and four modified biochars in different pH values [38]. The Qe of the biochar and modified biochar was in an order of F₃M₁DBC > F₁M₃DBC > MnDBC > FeDBC > DBC at the pH value from 3 to 11. No significant changes of Qe for the modified biochar, especially for F₃M₁DBC at pH 3–11 were observed, indicating that the atrazine adsorption of modified biochar was less influenced by pH. There are two reasons for this phenomenon: (1) Atrazine was anion in the solution of pH = 3–11 because the pKa of atrazine is 1.68, and the surface of biochar was also negatively charged in the solution of pH = 3–11, which due to the zero charge of five biochar was lower than 3 (Table 1). Therefore, the interaction between biochar and atrazine should be electrostatic repulsion. However, the results of adsorption kinetics and adsorption isothermal experiments showed that both biochar and modified biochar could adsorb atrazine, and the atrazine adsorption capacity of modified biochar was higher than that of original biochar. It indicated that electrostatic adsorption is not the main adsorption mechanism of atrazine on biochar, especially for modified biochar, so atrazine adsorption by the modified biochar was not affected by pH. (2) The conflict between atrazine adsorption on the biochar and electrostatic repulsion indicated that some other adsorption mechanisms dominated the atrazine adsorption on biochar. The strong electron-absorbing N atom in atrazine molecules can be bonded with oxygen-containing functional groups on the surface of biochar by hydrogen bonding, and π-electron acceptors to form a π–π interaction with atrazine on biochar [39]. The F₃M₁DBC exhibited the largest atrazine adsorption capacity and was the least affected by pH, due to having the richest pore structure and strongest metal oxide bonds, which could provide more adsorption sites. The results were consistent with the study of Gao [29]. He thought that abundant porosity and high specific area can enhance the adsorption capacity of biochar.

3.6. Desorption Capacity

A study of desorption potential is usually conducted to evaluate the adsorption stability and deduce the adsorption mechanism. The desorption rates of atrazine from pre-sorbed biochar by DI water and methanol are shown in Figure 6.

After desorption, 42.1% of sorbed atrazine was desorbed by DI water in atrazine loaded DBC, suggesting the atrazine absorbed by the original biochar still has an environmental risk. Modified biochar displayed lower desorption of atrazine compared with the original biochar, indicating that Fe-Mn modification improved the fixation capacity of biochar in atrazine, thus reducing the environmental risk. The desorption rate was 8.61% for F₃M₁DBC, 15.95% for F₁M₃DBC, 26.19% for MnDBC, and 29.83% for FeDBC, which were 79.5%, 62.1%, 37.7%, and 29.1% lower than that of original biochar (DBC), respectively. The lower desorption of atrazine from the modified biochar indicated that the main sorption mechanism of the modified biochar was more stable.

Atrazine is easily soluble in methanol solution, so methanol can desorb the total absorbed atrazine amount from the atrazine-loaded biochar, while the difference between the saturated adsorption and the desorption amount of atrazine suggested the amount of atrazine degraded by the biochar. As Figure 6 shows, the desorption of atrazine by methanol from the original biochar and modified biochar was 97.68% for F₃M₁DBC, 98.01% for F₁M₃DBC, 98.20% for MnDBC, 98.16% for FeDBC, and 98.51% for DBC, suggesting that the removal of atrazine by the original biochar and modified biochar was dominated by adsorption rather than degradation.

3.7. Adsorption Mechanism

Figure 7 shows the FT-IR analysis of the five biochars before and after adsorption. A decrease of the −COOH amplitude at 1410 cm⁻¹ and −OH amplitude at 3443 cm⁻¹ can be found after adsorption by the original biochar and modified biochar. Previous studies have demonstrated that −OH, quinoid C=O groups, defects, and graphite-like structures in biochar can act as π donors increasing the π donor nature of the sorbent [40,41], while protonated carboxyl groups behave as strong π acceptors due to electronic resonance [41]. The highly electronegative N atoms in atrazine behave as π acceptors because they can donate the N lone electron pair [36]. Therefore, atrazine acts as a π-electron acceptor and adsorbs electrons from the surface of biochar [41]. The XPS spectra of DBC and F₃M₁DBC before and after adsorption also suggested the adsorption mechanism was the π–π electron donor acceptor interactions. The negligible changes in the areas of C-O and C-C of DBC and F₃M₁DBC after atrazine adsorption (Figure 8a) suggested that the adsorption of atrazine did not change the basic structure of the biochar. After adsorption, in the O1s spectra (Figure 8b), the content of −OH decreased from 51.6% to 37.7% for DBC, and decreased from 55.57% to 48.16% for F₃M₁DBC, while the content of C=O increased from 48.4% to 62.3% for DBC, and increased from 11.71% to 27.03% for F₃M₁DBC, indicating that the two oxygen-containing functional groups (OH, C=O) played a leading role in the atrazine adsorption process. Previous studies showed the oxygen functional groups could act as π-electron-donor (e.g., graphitic carbon, eOH group) and/or π-electron-acceptor (e.g., eCOOH, eC=O, eCOO−) sites, which could occur the via the π–π interactions between atrazine and the oxygen functional groups on the surface of the biochar [39,41]. Zhang’s research [16], showed that Fe/Mn-loaded biochar could promote the π-complex and H⁺/OH⁻ ion exchange capacity, and then improve the adsorption of Fe/Mn-loaded biochar on tetracycline. In addition, the higher atrazine adsorption capacity of modified biochar was also confirmed by FTIR (Figure 7) and XPS (Figure 8b–d). The peaks of Fe-O at 558 cm⁻¹ and Mn-O at 776 cm⁻¹ in FTIR (Figure 7) were significantly decreased for modified biochar after adsorption, which was due to Fe-Mn oxides providing more adsorption sites for atrazine, or the π–π interaction between Fe-Mn oxides and atrazine. Compared with DBC, a new peak of Fe/Mn-O bond by XPS (Figure 8b) was found in F₃M₁DBC and its content of Fe/Mn-O decreased from 32.72% to 24.81% after adsorption, confirming its strong π–π interactions between the atrazine and oxygen functional groups and graphitic carbon, and rich adsorption sites because of the abundance of Fe-oxides, Mn-oxides and Fe-Mn oxides on the surface of F₃M₁DBC [41,42].

Figure 8c and d show the spectra of Fe (2p) and Mn (2p) before and after the adsorption of F₃M₁DBC. The peaks at binding energies of 710.81 eV and 712.3 eV in Figure 8c correspond to Fe(II) and Fe(III), respectively. The obvious decrease of Fe(II) from 63.26% to 50.73%, while the increase of Fe(III) from 36.74% to 49.27% was found after adsorption by F₃M₁DBC, indicated that Fe(II) on the modified biochar assisted the electron mediation process. A slight change was found in the peak of Mn (II) at 641.46 eV and the peak of Mn (IV) at 642.7 eV, as shown in Figure 8d, before and after adsorption by F₃M₁DBC. In addition to π–π interaction, the electron transfer on FeOx/biochar, and MnOx/biochar contributed to the degradation of atrazine, which also was confirmed by the desorption experiment in Section 3.6. The results of desorption showed that 1.84–2.32% of atrazine was degraded by the Fe-Mn modified biochar, which was 21–56% higher than that of the original biochar (Figure 6). Tian [43] proved the recycling of redox pairs, such as Fe(III)/Fe(II), Mn(II)/Mn(IV) played an important role in the degradation of atrazine.

The adsorption capacity of the biochar was positively correlated with their SSA and porosity (Table 1, Figure 3 and Figure 4), indicating that the large surface area and pore structure can effectively enhance the adsorption and pore filling of atrazine on biochar.

4. Conclusions

In this study, the Fe/Mn modified biochar showed a higher specific surface area, pore size, pore volume, and zero-point charge than that of the original biochar. The original biochar and four modified biochars were better fitted by the pseudo-second-order model and Freundlich model. The adsorption capacity of Fe/Mn modified biochar increased by 1.58–4.36 times. The F₃M₁DBC exhibited the largest atrazine adsorption capacity and was the least affected by pH. The removal of atrazine by original biochar and modified biochar was dominated by adsorption rather than degradation. The adsorption mechanisms of Fe/Mn-modified biochar were attributed to its strong π–π interactions between atrazine and oxygen functional groups, graphitic carbon, and Fe/Mn-oxides on the surface of the biochar. In general, the Fe/Mn-modified biochar can be used as an effective adsorbent to remove atrazine from soils and waters.