Adsorption of Tricyclazole and 2,4-Dichlorophenoxyacetic Acid onto Biochar Produced from Anaerobically Digested Sludge

Abstract

Anaerobically digested sludge-derived biochar was produced through pyrolysis at 700 °C, called BC700. BC700 was characterized using a scanning electron microscope (SEM), X-ray diffractometer (XRD), Fourier-transform infrared spectroscopy (FTIR), and the Brunauer–Emmett–Teller (BET) method. The factors influencing the adsorption process and the mechanism involved in adsorbing tricyclazole and 2,4-D in single and binary systems were revealed. The theoretical maximum adsorption capacities of BC700 for tricyclazole and 2,4-D in a single system were 11.86 mg/g and 7.89 mg/g, respectively. In the binary system, the theoretical saturated adsorptive capacities of tricyclazole and 2,4-D were 5.27 mg/g and 3.20 mg/g, respectively. The adsorption of tricyclazole and 2,4-D by BC700, whether in single or binary systems, matched closely with the Freundlich isotherm and the pseudo-second-order model. This study indicates that anaerobically digested sludge-derived biochar is potentially valuable for removing pesticide contamination in surface water.

1. Introduction

As a leading agricultural nation, China has consistently been the largest user of pesticides globally [1]. The amount of pesticides used per hectare of arable land in China is approximately five times the world average [2]. This rough approach to production and use has exacerbated the pollution of surface water environments in China by pesticides [3]. These pesticides’ existence in surface water threatens biodiversity and raises significant public health concerns. Tricyclazole and 2,4-dichlorophenoxyacetic acid (2,4-D) are particularly notable among the various types of pesticides due to their widespread use and environmental persistence. Tricyclazole is a fungicide primarily used in rice cultivation [4]. It exhibits significant electron delocalization through the benzene ring, the heterocyclic ring, and the electron pair arc via the bridge S atom, with no polar functional groups, resulting in its highly stable properties. Therefore, it is known for its persistence in aquatic environments, posing a long-term risk to aquatic organisms and possibly making its way into the human food chain [5]. 2,4-D is a widely applied herbicide [6]. It has been associated with adverse effects on terrestrial and aquatic ecosystems, including hormone disruption in wildlife and potential health risks to humans through water contamination [7,8]. Different pesticides often coexist in aqueous solutions. Hence, exploring efficient methods for removing these coexisting pesticides from aqueous solutions is essential.

Biochar, a porous carbon material, is produced from biomass feedstocks through thermal combustion under limited oxygen conditions [9]. Biochar adsorption has become an effective and sustainable solution due to its high efficiency, low energy consumption, and ease of operation [10]. Recently, biochar has gained attention as a promising material for pollutant removal. However, one significant environmental challenge is the large volume of sludge generated from wastewater treatment plants, which is often disposed of in ways that contribute to environmental pollution, such as landfilling or incineration.

Using sludge as a raw material for biochar production presents a unique opportunity to address this waste management issue while creating a valuable adsorbent. In previous studies, the maximum adsorption capacity of the biochar, prepared from a broadleaf waste mushroom substrate using phosphoric acid as a chemical activator at 900 °C, for acetaminophen was 236.8 mg/g [11]. A mixture of KOH, bamboo, and sludge was pyrolyzed at 700 °C to produce biochar capable of removing tetracycline from aqueous solution, with a maximum adsorption capacity of 111.39 mg/g [12]. Similarly, biochar produced from chemical plant sludge was pyrolyzed at 800 °C for 4 h to adsorb ciprofloxacin, achieving a maximum adsorption capacity of 555.6 mg/g [13]. Sludge-based biochar produced at 800 °C was used to remove sulfamethoxazole and lincomycin from water, with adsorption capacities of 45.6 mg/g and 26.6 mg/g, respectively [14]. Biochar produced from steel wastewater sludge at 450 °C successfully removed tetracycline from aqueous solution, with a maximum adsorption capacity of 240.38 mg/g [15]. The above research indicates the feasibility of producing biochar using sludge as a raw material, particularly sludge with high inorganic content and relatively stable organic properties, such as anaerobically digested sludge [16,17].

Based on the above, biochar from anaerobically digested sludge was used to remove tricyclazole and 2,4-D in aqueous solutions effectively. The effects of different conditions (solution pH and contact time) on the removal performance of tricyclazole and 2,4-D by the biochar were examined. The kinetics and isotherm models were established to elucidate the adsorption process. Finally, the competitive mechanisms in the binary system were discussed and compared with those in the single adsorption system.

2. Materials and Methods

2.1. Materials

The anaerobically digested sludge taken from Shuanglin sewage treatment plant in Tianjin, China, was placed in a sealed container and refrigerated at 4 °C. The supplementary data indicate the structural formula and other properties of tricyclazole and 2,4-D (Table S1), as well as the sludge’s leading properties and heavy metal content (Table S2). Tricyclazole (CAS:41814-78-2) and 2,4-D (CAS:94-75-7) were acquired from Aladdin Biochemical Technology Corporation (Shanghai, China). To create the stock solution of the adsorbate, the necessary amount was dissolved in 1000 mL of DI water. The solutions were all formulated using premium-quality deionized water for maximum purity.

2.2. Preparation of Biochar Derived from Anaerobically Digested Sludge

The untreated sludge underwent a process of air-drying, followed by grinding and filtering through a mesh with a 0.15 mm aperture. The ensuing pyrolysis of sludge was carried out in a confined crucible using a laboratory muffle furnace at 700 °C, lasting for two hours [18]. The resulting biochar was labeled BC700 and used as an adsorbent in the following adsorption experiments.

2.3. Characterization of Biochar Derived from Anaerobically Digested Sludge

The superficial element compositions of BC700 were determined using an elemental analyzer (Vario EL Cube, Elementar, Rhine Main, Germany). The biochar’s zero point of charge (pH_PZC) was assessed using a laser particle size analyzer (Zetasizer Nano ZS90, Thermo Fisher Scientific, Norristown, PA, USA). An X-ray diffractometer (XRD, BRUCKER D8ADVANCE X’Pert3 Powder, Bruker, Billerica, MA, USA) was used to investigate the structural configurations. By utilizing SEM, the morphology of the biochar was analyzed. Fourier-transform infrared spectroscopy (FTIR, Nicolet IS10) was used to identify the surface functional groups. The gas adsorption method was used to determine the biochar’s specific surface area. It was determined through the adsorption–desorption of liquid nitrogen (−195 °C) by biochar and the analysis of adsorption data was conducted using the Brunauer–Emmett–Teller method (BET, ASAP 2460). The biochar’s pore size distribution was ascertained using Barrett–Joyner–Halenda method (BJH, St. Louis, MI, USA).

2.4. Batch Experiments

Batch experiments were used to investigate the adsorption performance with the use of a constant temperature shaker. Approximately 4 g/L of biochar was incorporated into a 25 mg/L adsorbate (tricyclazole or 2,4-D) in a single system. Approximately 8 g/L of biochar was incorporated into a 25 mg/L adsorbate (tricyclazole and 2,4-D at the same concentration) in a binary system. The mixture was oscillated at room temperature (T = 298 K) on a rotary shaker (MX-RL-E, Beijing Dragon Instrument Co., Ltd., Beijing, China) at 30 rpm and then passed through a 0.22 μm filter. Concentration analysis was performed using UPLC (MA-01757, Waters, Mumbai, MA, USA) (tricyclazole: detected at 226 nm peak, elution ratio 60:40 v/v, methanol: ultrapure water, retention time 1.59 min; 2,4-D: detected at 226 nm peak, elution ratio 80:20 v/v, methanol: ultrapure water, retention time 1.81 min) [19]. Equations (1) and (2) [20] were used to calculate the adsorption capacity at a given time (q_t) and the equilibrium adsorption capacity (q_e), respectively. The collected data represent the average of three consecutive tests, with a relative standard deviation of less than 5%.

$${\mathrm{q}}_{\mathrm{t}}={\displaystyle \frac{\left({\mathrm{C}}_{\mathrm{o}}-{\mathrm{C}}_{\mathrm{t}}\right)\mathrm{V}}{\mathrm{W}}}$$

(1)

$${\mathrm{q}}_{\mathrm{e}}={\displaystyle \frac{\left({\mathrm{C}}_{\mathrm{o}}-{\mathrm{C}}_{\mathrm{e}}\right)\mathrm{V}}{\mathrm{W}}}$$

(2)

where the initial concentration, denoted as Co (mg/L), and the concentration at equilibrium, denoted as Ce (mg/L), are used in conjunction with the volume of the solution, labeled as V (L), and the quantity of biochar, referred to as W (g).

2.5. Experimental Analysis Model

The experimental data were fitted to two isotherm models, Langmuir and Freundlich, to examine the reaction behavior of tricyclazole and 2,4-D with BC700 in single and binary systems. Additionally, two kinetic models—pseudo-first-order (PFO) and pseudo-second-order (PSO)—were used for the analysis.

2.5.1. Isotherm Models

The findings can accommodate two established models, namely Langmuir and Freundlich. According to Equation (3), the Langmuir model characterizes the adsorption of a single layer comprised of a finite number of homogenous active sites. In contrast, Equation (4) describes the Freundlich model, which proposes the adsorption of multiple layers [21,22].

$${\displaystyle \frac{{\mathrm{C}}_{\mathrm{e}}}{{\mathrm{q}}_{\mathrm{e}}}}={\displaystyle \frac{{\mathrm{C}}_{\mathrm{e}}}{{\mathrm{q}}_{\mathrm{m}}}}+{\displaystyle \frac{1}{{\mathrm{K}}_{\mathrm{L}}{\mathrm{q}}_{\mathrm{m}}}}$$

(3)

$${\mathrm{lnC}}_{\mathrm{e}}={\mathrm{lnK}}_{\mathrm{F}}+\left({\displaystyle \frac{1}{\mathrm{n}}}\right){\mathrm{lnC}}_{\mathrm{e}}$$

(4)

where the q_m value (mg/g) refers to the maximum adsorption capacity of a surface with complete monolayer coverage, while the Langmuir constant K_L (L/mg) indicates the adsorption strength. K_F denotes the adsorption capacity per unit concentration, and n⁻¹ describes the adsorption intensity, which can be either irreversible (n⁻¹ < 0), favorable (0 < n⁻¹ < 1), or unfavorable (n⁻¹ > 1) according to the isotherm.

2.5.2. Kinetics Models

To quantitatively determine the adsorption capacity and rate of the absorbent, the PFO (Equation (5)) and PSO equations (Equation (6)) were employed to fit the data on the kinetics of adsorption [23].

$$\lg ({\mathrm{q}}_{\mathrm{e}}-{\mathrm{q}}_{\mathrm{t}})=\lg {\mathrm{q}}_{\mathrm{e}}-{\displaystyle \frac{{\mathrm{k}}_{\mathrm{f}}}{2.303}}$$

(5)

$${\displaystyle \frac{\mathrm{t}}{{\mathrm{q}}_1}}={\displaystyle \frac{1}{{\mathrm{k}}_{\mathrm{s}}{\mathrm{q}}_{\mathrm{e}}^2}}+{\displaystyle \frac{1}{{\mathrm{q}}_{\mathrm{e}}}}\mathrm{t}$$

(6)

The variable q_t denotes the amount of adsorbent (mg/g) adsorbed at a specific time, whereas q_e represents the adsorption capacity (mg/g) at equilibrium. k_f denotes the rate constant (min⁻¹) for PFO adsorption, and t is the duration (min) of the process. k_s is the rate constant (g/(mg·min) ⁻¹) for PSO adsorption.

3. Results and Discussion

3.1. Characterization of BC700

The elemental composition of biochar is shown in the (supplementary data Table S3). Pyrolysis removes a significant number of constituent elements (C, H, O, N) from surface functional groups due to the evaporation of volatile matter. The mass percentage of carbon in BC700 is 11.86%. The O/C ratio was a good predictor of carbonization degree. The H/C ratio of sludge biochar remained below 0.14. The results revealed that biochar’s carbonization degree and aromaticity were extremely high. The structure was sturdy and difficult to decompose [24].

The pH_PZC of BC700 is 3.77 (Figure 1a). This indicated that in an acidic environment, the surface of the sludge biochar was positively charged, which would attract negatively charged pesticide pollutants through electrostatic interactions in the adsorption process. The surface charge properties of sludge biochar were favorable for the adsorption of pesticide pollutants with negatively charged or polar functional groups. However, under alkaline conditions, the biochar’s surface acquired a negative charge, which caused electrostatic repulsion with negatively charged substances or functional groups.

Analysis using the XRD, as shown in Figure 1b, showed that BC700 had the diffraction peaks of SiO₂ at diffraction angles 2θ of 20.915°, 26.705°, 36.629°, 39.525°, 50.203°, and 68.183°. The results indicate that SiO₂ was the principal constituent of the sludge biochar. Diffraction peaks of MgSiO₃ (2θ = 31.325°) and Mg₂SiO₄ (2θ = 59.537°) appeared in BC700. This was due to the ease with which SiO₂ heated with an alkaline substance melted to form silicates. The pH of the sludge biochar was alkaline, which provided the conditions for silicate formation [25].

The surface morphology of BC700 was assessed using the SEM (Figure 1c,d). The SEM images of the biochar revealed a remarkable amount of pores and voids on the surface. A refined and sophisticated pore structure was also observed on the biochar’s surface. This was due to the heat’s loss of volatiles and the gradual decomposition of ash [26].

Analysis using FTIR spectra in Figure 1e showed that the biochar possessed many functional groups that contained oxygen atoms. The FTIR spectra of the biochar contained peaks at 3410 cm⁻¹, 1620 cm⁻¹, 1450 cm⁻¹, 1036 cm⁻¹, 795 cm⁻¹, 685 cm⁻¹, and 470 cm⁻¹, which represented the structure of –OH, C=C, C–H, –COOH, and the aromatic ring, respectively [27]. The aromatic ring structure can provide π-electrons. Aromaticity facilitates the adsorption of tricyclazole by biochar [28]. The distribution of phenolic hydroxyl structures and C–H increased relatively, while the distribution of –C=O and–COOH decreased relatively. The oxygenated unsaturated functional groups (C=O) were reduced to a carbon–oxygen single bond, generating hydroxyl groups, and the long-chain hydrocarbon was cracked to produce more C–H bonds [29]. High heat could have accelerated the dehydrogenation reaction and increased the degree of carbonization [30].

Using the BET method, the N₂ adsorption–desorption isotherms and the specific surface areas and pore sizes of BC700 are shown in Figure 1f. The calculation results of BC700 were 41.83 m²/g and 12.96 nm. Biochar samples were found to exhibit type IV H3 hysteresis loops based on the IUPAC classification, indicating that the carbon materials were predominantly slit-like mesoporous structures internally. It can be seen that BC700 contains mainly mesoporous structures, measuring 15.6–32.8 nm. The tricyclazole molecules were smaller and were easily adsorbed by the larger surface area, while the larger and somewhat polar 2,4-D molecules were primarily influenced by the pore size distribution in their adsorption [31,32]. The size of a material’s pore volume and specific surface area were closely related to its adsorption capacity. The larger pore volume provided more space for adsorbate molecules, while the greater surface area offered more adsorption sites for the molecules. This was the reason for the enhanced physical adsorption capacity [33]. This indicated that high physical adsorption capacity could produce biochar through high-temperature pyrolysis.

3.2. Effect of Solution pH on the Adsorption of Tricyclazole and 2,4-D

The pH values ranging from 2.0 to 10.0 were adjusted using hydrochloric acid (0.1 N) or sodium hydroxide (0.1 N) solutions as needed. The adsorption efficiency of tricyclazole and 2,4-D was investigated under these pH conditions. Approximately 4 g/L of the adsorbent was stirred in a 25 mg/L tricyclazole (or 2,4-D) solution at 30 rpm on a mechanical shaker in a single system. 8 g/L of the adsorbent was stirred in a 25 mg/L tricyclazole and 2,4-D solution at 30 rpm on a mechanical shaker in a binary system. Filtration was then performed at the selected time (6 h), which was sufficient to ensure that equilibrium was reached.

The effect of solution pH on the adsorption of tricyclazole and 2,4-D using BC700 as an adsorbent was studied (Figure 2). The adsorption capacity of tricyclazole in both single and binary systems was unaffected by changes in solution pH, with a removal rate of approximately 90%. The highest adsorption capacity of 2,4-D in single and binary systems achieved a removal rate of 77.9%, which was reached at a pH of 2.0. Tricyclazole is a wholly aromatic compound with a stable planar geometric structure. It exhibits significant electron delocalization through the benzene ring, the heterocyclic ring, and the electron pair arc via the bridge S atom, with no polar functional groups, resulting in its highly stable properties. When the solution pH exceeded 2.64, 2,4-D existed in an anionic form. The pH_PZC of BC700 was 3.77 (Figure 1a). Therefore, when the pH was higher than 4, the surface of the sludge biochar carried a negative charge, which repelled the negatively charged 2,4-D anions. As the pH increased, the zeta potential of the biochar decreased, leading to stronger repulsion and a lower adsorption removal rate of 2,4-D. On the other hand, when the solution pH was less than 2.64, 2,4-D existed in a molecular form in the solution, which was highly hydrophobic, enhancing its suitability for adsorption by biochar.

3.3. Effect of Contact Time

3.3.1. Single System

Using a mechanical shaker, 4 g/L of the adsorbent was stirred in a 25 mg/L tricyclazole (or 2,4-D) solution at 30 rpm. Filtration was then performed at the selected time (6 h), which was sufficient to ensure that equilibrium was reached. Figure 3 illustrates the effect of contact time on the outcome in a single system. The adsorption of tricyclazole and 2,4-D by BC700 mainly occurred within the first 30 min of contact time, after which the adsorption efficiency declined. This was because, upon initial contact between the biochar and the adsorbate, the number of adsorption locations on the biochar surface was relatively abundant, allowing for the rapid adsorption of adsorbate. As the adsorption process progressed, the number of available adsorption sites diminished, reducing the adsorption rate until saturation was achieved. The equilibrium adsorption time for tricyclazole by BC700 was 180 min, with an equilibrium adsorptive capacity of 6.05 mg/g and a removal rate of 96.9%. The equilibrium adsorption time for 2,4-D by BC700 was 60 min, with an equilibrium adsorptive capacity of 4.77 mg/g and a removal rate of 76.3%.

3.3.2. Binary System

A mechanical shaker stirred 8 g/L of the adsorbent in a 25 mg/L tricyclazole and 2,4-D solution at 30 rpm. The effect of contact time on the outcome in a binary system is shown in Figure 3. The adsorption of tricyclazole and 2,4-D by BC700 mainly occurred within the first 30 min of contact time, after which the adsorption efficiency declined, the same as that of a single system. The maximum adsorption time for tricyclazole and 2,4-D by BC700 was 60 min, with equilibrium adsorptive capacities of 4.22 mg/g and 2.85 mg/g and removal rates of 87.5% and 66.5%, respectively. This indicated that the simultaneous adsorption accelerated the adsorption equilibrium time of tricyclazole by the sludge biochar. There was a mutual influence in the adsorption of the two pesticides, resulting in competitive adsorption, and tricyclazole had a more vital competitive ability for the adsorption locations present on the biochar surface compared to 2,4-D.

3.4. Adsorption Isotherm

3.4.1. Single System

The fitting of adsorption data using Freundlich and Langmuir’s isothermal adsorption equations is shown in Figure 4.

Table 1. Thermodynamic parameters of tricyclazole and 2,4-D adsorption on BC700.

Adsorbate	System Type	Langmuir			Freundlich
		qm (mg/g)	KL (L/mg)	R2	KF [L/(mg·g)]	1/n	R2
Tricyclazole	Single	11.86	4.91	0.988	5.38	0.236	0.998
	Binary	5.27	3.210	0.970	2.097	0.306	0.985
2,4-D	Single	7.89	0.945	0.985	2.76	0.352	0.998
	Binary	3.20	0.874	0.969	0.909	0.355	0.997

displays the results of fitting the experimental equilibrium adsorption data. The theoretical maximum adsorptive capacity of BC700 for tricyclazole was 11.86 mg/g. The theoretical maximum adsorptive capacities of tricyclazole in water by Moroccan clay and corn straw biochar were 2.45 mg/g and 1.96 mg/g, respectively [28,34]. Compared to other adsorption materials, sludge-derived biochar exhibited outstanding performance in adsorbing tricyclazole. The theoretical maximum adsorption capacity of BC700 for 2,4-D was 7.89 mg/g. The theoretical maximum adsorptive capacities of 2,4-D in water by activated carbon and switchgrass biochar were 3.69 mg/g and 13.3 mg/g, respectively [35,36]. The adsorption of sludge biochar for 2,4-D showed average performance relative to other adsorbents. However, sludge-derived biochar has low production costs and could effectively alleviate sludge disposal issues, making it a feasible method for sludge management.

The adsorption of tricyclazole and 2,4-D by BC700 was more consistent with the Freundlich isotherm equation. The 1/n values for the adsorption of tricyclazole and 2,4-D by BC700 were 0.236 and 0.352, respectively, indicating that the affinity of biochar for 2,4-D was weaker than for tricyclazole.

3.4.2. Binary System

In a binary system, the theoretical maximum adsorption capacities of BC700 for tricyclazole and 2,4-D were 5.27 mg/g and 3.20 mg/g, respectively (Figure 4). This suggested that during simultaneous adsorption, the adsorptive capacity of sludge biochar for both tricyclazole and 2,4-D decreased. This was due to the significant role of competition between the two pesticides for adsorption sites on the sludge biochar. When biochar was used to adsorb the pesticides atrazine and simazine and compared to the single system, the coexistence of the two pesticides in the solution significantly reduced the removal rates of atrazine or simazine by biochar [37]. This was consistent with the results of this study. The 1/n values for the simultaneous adsorption of tricyclazole and 2,4-D by BC700 were 0.352 and 0.355, respectively (Table 1), further confirming that the adsorption of tricyclazole by BC700 was more straightforward compared to 2,4-D.

3.5. Adsorption Kinetic

3.5.1. Single System

The effect of adsorption time on the removal of tricyclazole and 2,4-D by BC700 is shown in Figure 5. The adsorption kinetic parameters obtained by fitting the PFO and PSO adsorption kinetic equations are shown in

Table 2. Kinetic parameters of tricyclazole and 2,4-D adsorption on BC700.

Adsorbate	System Type	Pseudo-First-Order				Pseudo-Second-Order				C0(mg/L)
		qe, exp (mg/g)	qe, cal (mg/g)	kf	R2	qe, exp (mg/g)	qe, cal (mg/g)	ks	R2
Tricyclazole	Single	6.06	1.520	0.0134	0.96	6.06	6.1	0.0474	0.9999	25
	Binary	2.75	0.721	0.0104	0.975	2.75	2.757	0.0812	0.9999
	Single	7.08	1.890	0.0128	0.921	7.08	6.85	0.0435	0.9988	30
	Binary	3.25	0.852	0.0122	0.839	3.25	3.239	0.1033	0.9999
	Single	8.63	2.770	0.0126	0.963	8.63	8.63	0.0241	0.9995	40
	Binary	4.25	0.908	0.0105	0.936	4.25	4.205	0.0625	0.9995
2,4-D	Single	4.79	4.700	0.0283	0.914	4.79	4.76	0.0911	0.999	25
	Binary	2.09	0.661	0.0088	0.927	2.09	2.042	0.1318	0.9995
	Single	5.61	4.140	0.0348	0.975	5.61	5.7	0.0539	0.999	30
	Binary	1.85	0.709	0.0068	0.677	1.85	1.709	0.1597	0.9988
	Single	6.61	3.000	0.0278	0.947	6.61	6.25	0.0641	0.998	40
	Binary	2.93	0.758	0.0089	0.891	2.93	2.831	0.1085	0.9993

. The initial concentrations of tricyclazole (or 2,4-D) in this study were 25, 30, and 40 mg/L, and the pH was 7.0.

In the initial phase (0–30 min), the adsorption of tricyclazole (or 2,4-D) by the biochar increased rapidly, and after 30 min, the adsorption rate slowed down. The equilibrium adsorption times for tricyclazole and 2,4-D by BC700 were 180 min and 60 min, respectively. The PSO kinetic equation had a higher R² value than the PFO kinetic equation, suggesting that the adsorption of tricyclazole and 2,4-D by biochar involved a physical adsorption process and a significant chemical adsorption process. The parameter k_s value of BC700 increased with the initial concentration of tricyclazole, indicating a positive correlation between the rate of tricyclazole adsorption by biochar and the substrate concentration.

3.5.2. Binary System

Since both pesticides coexist in the solution, the adsorption conditions differ from those in the S-system, leading to changes. During the initial phase (0–30 min), the adsorption of tricyclazole and 2,4-D by biochar increased rapidly. After 30 min, the adsorption rate slowed (Figure 5). The simultaneous adsorption of tricyclazole and 2,4-D reached equilibrium after 60 min. The R² value for the PSO kinetic equation, ranging from 0.980 to 0.999, exceeded that of the PFO kinetic equation, which ranged from 0.677 to 0.975 (Table 2). This indicated that the PSO kinetic model demonstrated a better fit, suggesting that chemical adsorption played a significant role in the simultaneous adsorption of tricyclazole and 2,4-D. The ks value for tricyclazole adsorption was lower than that for 2,4-D, indicating that during the simultaneous adsorption process, although the saturation adsorption capacity of BC700 for 2,4-D was lower than for tricyclazole, the adsorption rate of 2,4-D on BC700 was higher than that of tricyclazole. This is because tricyclazole has higher solubility in aqueous solutions than 2,4-D, resulting in more excellent mass transfer resistance and, consequently, a slower adsorption rate.

3.6. Possible Adsorption Mechanism of Tricyclazole and 2,4-Dichlorophenoxyacetic Acid

Figure 6 illustrates the possible mechanisms of the adsorption of triazole and 2,4-D on BC700. The adsorption of triazole on BC700 was mainly physical adsorption, while the adsorption of 2,4-D was primarily physical adsorption with some chemical adsorption involved [27].

The adsorption mechanism is primarily reliant on the physicochemical properties of the adsorbent and the chemical constitution of the adsorbate. Tricyclazole has no polar functional groups and is wholly aromatic with significant electron delocalization via the benzene ring. Increased aromaticity on the surface of biochar results in an increase in electrons, which can improve the interaction between biochar and tricyclazole. This facilitates tricyclazole adsorption [28]. The principle of adsorbing organic pollutants relies on the distinct surface area of the adsorbent material [38]. The stronger the adsorption of pesticide pollutants, the smaller the pore size [39]. After the adsorption of tricyclazole, BC700 exhibits C-H stretching between 3030 and 3100 cm⁻¹, C=C stretching between 1450 and 1600 cm⁻¹, and C-H bending at 685 cm⁻¹ (Figure 7a), which is the diffraction peak of the aromatic ring [27,40], indicating that the aromaticity of the biochar surface is enhanced after the adsorption of tricyclazole. This is because tricyclazole is a wholly aromatic compound, and the aromaticity of the biochar surface is enhanced after adsorption. Additionally, the pore volume and specific surface area of biochar experience augmentation while the pore size undergoes a reduction. As a result, physical adsorption is the primary mechanism by which tricyclazole remains on the biochar’s surface. The adsorption of tricyclazole on Moroccan natural clays was studied, and it was concluded that the primary adsorption mechanism is physical adsorption. These conclusions are consistent with those of this paper. The presence of metallic elements in the adsorbent, particularly Fe (with a content of 2730 mg/kg), can enhance the charge attraction between the electron cloud and the lone electron pair of the aromatic and S atoms in tricyclazole, thereby improving the adsorbent’s capacity for tricyclazole [34]. According to this study, raw sludge contains a high concentration of heavy metals such as Al, Cu, Mn, Mg, Zn, and Fe, and these metal elements are enriched in sludge biochar with a high pyrolysis temperature as the biochar yield decreases. Therefore, we hypothesize that the metal elements in biochar are enriched through pyrolysis [41] and the electron conjugating effect of biochar on S atoms of tricyclazole is enhanced, providing strong support for tricyclazole adsorption. To summarize, the adsorption mechanism of sludge biochar on tricyclazole is primarily physical, and the metal elements in sludge biochar promote tricyclazole adsorption.

2,4-D belongs to the phenoxyacetic acid class, with a pK_a of 2.64, and is a weakly acidic substance. The pH affects the form in which 2,4-D exists in an aqueous solution. When the pH is more significant than 2.64, 2,4-D exists in an anionic form; when the pH is less than 2.64, 2,4-D exists in a molecular form with solid hydrophobicity. Hydrophobic interactions reduce mass transfer resistance, promoting the biochar’s adsorption of 2,4-D [35]. Therefore, the pH influences the adsorption process of 2,4-D by sludge biochar through hydrophobic interactions and electrostatic repulsion by affecting the form in which 2,4-D exists. In addition, the surface functional groups and specific surface area of sludge biochar are essential factors during adsorption. Biochar can enhance the adsorption performance for 2,4-D by increasing the specific surface area and micropores to create more active adsorption sites [42]. The aromatic nature and oxygen-containing functional groups on the biochar surface facilitated the adsorption of 2,4-D through π-π interactions and the formation of controlled hydrogen bonds. Carboxyl groups on the biochar surface undergo esterification reactions with the hydroxyl or carboxyl groups on 2,4-D, promoting the adsorption of 2,4-D [43]. BC700 showed a diffraction peak of the ester bond at a wavelength of 2360 cm⁻¹ after adsorbing 2,4-D, and the intensities of the phenolic hydroxyl peak at 3410 cm⁻¹ and the carboxyl peak at 1036 cm⁻¹ both decreased (Figure 7b), further proving the existence of the esterification reaction. To summarize, the adsorption mechanism of sludge biochar on 2,4-D is primarily physical, with some chemical adsorption also present.

As mentioned earlier, the aromatic groups on the biochar surface can enhance its adsorptive capacity for tricyclazole [28]. In the binary system, since 2,4-D is an aromatic compound, we hypothesize that the presence of 2,4-D in the solution increases the affinity of tricyclazole for the solution, thereby increasing mass transfer resistance and making it more difficult for sludge biochar to adsorb tricyclazole, leading to a reduction in the maximum adsorptive capacity for tricyclazole. Simultaneously, the pK_a of tricyclazole is 2.40 ± 0.40, so we hypothesize that under neutral conditions, tricyclazole exists as an anion in the solution, enhancing electrostatic repulsion with 2,4-D, leading to a decrease in the saturation adsorption capacity of 2,4-D by sludge biochar [44]. Finally, we hypothesize that the coexistence of tricyclazole and 2,4-D increases mass transfer resistance through electrostatic repulsion and the affinity between aromatic compounds, leading to a substantial reduction in the saturation adsorptive capacity of sludge biochar for both pesticides under dual-solute conditions. However, the actual competitive mechanism may be much more complex. Determining the competitive mechanism of tricyclazole and 2,4-D on sludge biochar requires extensive work, such as designing experiments to verify the impact of the number of positive and negative charges in the solution on adsorption and exploring the influence of solution aromaticity on mass transfer resistance.

4. Conclusions

The anaerobically digested sludge-derived biochar prepared through pyrolysis at 700 °C shows good removal efficiency for tricyclazole and 2,4-D in the single system. The Freundlich isotherm and pseudo-second-order models explain the adsorption results in single and binary systems well. The theoretical maximum adsorption capacities of BC700 for tricyclazole and 2,4-D in a single system were 11.86 mg/g and 7.89 mg/g, respectively. In the binary system, the theoretical saturated adsorptive capacities of tricyclazole and 2,4-D were 5.27 mg/g and 3.20 mg/g, respectively. However, the presence of 2,4-D shortens the time for tricyclazole to reach adsorption equilibrium. The proposed competitive mechanism is that both pesticides in the solution increase mass transfer resistance through electrostatic repulsion and the affinity between aromatic compounds, decreasing the maximum adsorptive capacity of BC700 for each pesticide under dual-solute conditions. Compared to a single system, the simultaneous adsorption performance of BC700 for tricyclazole and 2,4-D is generally lower. In summary, this study suggests that biochar produced from the pyrolysis of anaerobically digested sludge has potential application value in remediating pesticide contamination in surface water.