Adsorption of Pyraclostrobin in Water by Bamboo-Derived and Pecan Shell-Derived Biochars

Abstract

Pyraclostrobin is a potent extensive-spectrum fungicide widely used in agricultural production but poses a substantial threat to aquatic life. Therefore, there is an urgent need to remove pyraclostrobin from the ecological environment. This study reports the adsorption of pyraclostrobin in water using pecan-shell biochar, bamboo biochar, and their deashing products. The kinetics and isotherms indicate that the pseudo-second-order kinetics and Freundlich model are the most suitable for both types of biochar. The thermodynamic results demonstrate that the adsorption process of biochar is spontaneous and exothermic. Combined with characterization and factor analysis experiments, it is revealed that the adsorption of pyraclostrobin on biochar is attributed to various mechanisms, including pore filling, hydrophobic interactions, π-π and p-π interactions, and hydrogen bonding. At the initial concentration of 0.5 mg·L⁻¹, the adsorption rates of pyraclostrobin of the four biochar samples (<0.075 mm) reached 67–80% within 5 min. These findings suggest that both pecan-shell and bamboo biochars are efficient pyraclostrobin adsorbents, with the former showing better outcomes. There is still an adsorption rate of >97% after 5 cycles of adsorption by two types of biochars. Deashing significantly enhances the adsorption efficiency of pecan biochar, but it has an insignificant effect on bamboo biochar. This study will aid in the selection of cost-effective and ecofriendly adsorbents to reduce the environmental risk associated with pyraclostrobin.

1. Introduction

Pyraclostrobin is one of the recommended quinone-outside-inhibitor fungicides and has significant antimicrobial activity against a wide range of fungi, such as oomycetes, basidiomycetes, and ascomycetes, especially against diseases caused by deuteromycetes; therefore, it is widely used in agricultural production [1,2]. However, with its increasing use, it is frequently detected in waterbodies [3,4,5]. Owing to its high toxicity to aquatic organisms [6,7] and persistence in the environment [8], it poses a huge risk to the ecological environment. Consequently, the identification of effective techniques to eliminate pyraclostrobin residues from water is crucial.

The general methods for removing organic contaminant residues include adsorption, flocculation, ultra-filtration, sedimentation, photodegradation, biodegradation, and advanced oxidation [9,10], among which the adsorption method is commonly employed because of its high availability, low cost, good reproducibility, simple preparation, and convenient process operation [11]. Biochar, activated carbon, graphene, carbon nanotubes, and bentonite are the commonly used adsorbents [12]. However, studies on the adsorption of pyraclostrobin onto these materials are limited. Previous studies have indicated that biochar has the noteworthy ability to sorb compounds, leading to its potential use in managing environmental contaminants [13]. Biochar is a solid product rich in carbon that results from the thermal decomposition of biomass under anoxic conditions [14]. Biochar is primarily obtained from agricultural and industrial waste such as corncob, wheat straw, municipal sludge, and rice husks [15,16,17,18]. Moreover, research has confirmed that biochar can exhibit effectiveness equal to that of activated carbon in certain situations while being notably more affordable [19].

Multiple mechanisms have been proposed to elucidate the adsorption of organic pollutants from water by biochar. Biochar provides abundant adsorption sites for contaminants owing to its developed pore structure and high specific surface area (SSA) and pore volume (PV), especially the dominant micropores, allowing pollutants to enter particles through pore filling [20]. A study has discussed the adsorption mechanism of pyraclostrobin onto mesoporous activated carbon derived from starch, suggesting a strong dependence on the electron-donating abilities of the oxygen-containing functional groups, nitrogen atoms, and the bonding network of the benzene rings [21]. Additionally, the adsorption capacity of biochar can be enhanced through specific interactions such as hydrophobic binding and electrostatic effects, which are facilitated by the aromaticity and surface charge of the biochar [22,23,24]. Biochar comprises carbonaceous and inorganic (ash) fractions [25]. Typically, ash hinders the accessibility of adsorption sites in the micropores and organic matter, thereby lowering the adsorption affinity of biochar for pesticides [26]. However, in the case of biochar obtained at 300–400 °C, the ash had a slight inhibitory effect on the adsorption of triclosan and even enhanced it [25]. Hence, further investigation is needed to explore the effect of deashing on the physicochemical characteristics of biochar as well as its adsorption capability for pyraclostrobin.

In addition to the ash content, the factors affecting the adsorption of pollutants by biochar generally include contact time, adsorbate concentration, temperature, biochar particle size, adsorbent dose, and ionic strength and pH of the solution [26,27]. To date, investigations of the adsorption mechanism of pyraclostrobin onto biochar are limited, and the effect of deashing on the adsorption of pyraclostrobin by different biochars remains unclear.

Thus, this study aimed to evaluate the adsorption performance of biochar prepared from pecan shells and bamboo for pyraclostrobin in water by analyzing adsorption kinetics, isotherms, thermodynamics, and various other factors, including particle size of the biochar, initial adsorbent dosage, inorganic ionic strength, and pH. In addition, the properties of the biochar before and after adsorption were evaluated. Furthermore, this study expounds on the adsorption mechanism of pyraclostrobin onto biochars and discusses the effect of the deashing treatment on adsorption.

2. Materials and Methods

2.1. Reagents and Chemicals

Basic information on pyraclostrobin (99.0%, Shanghai Pesticide Research Institute Co., Ltd., Shanghai, China) is shown in Table S1. Acetonitrile and methanol (>99%, chromatographic grade) were acquired from Merck (Darmstadt, Germany). Formic acid (>99%, chromatographic grade) was acquired from ANPEL Laboratory Technologies (Shanghai, China). NaCl (99%, analytical grade) was acquired from Sinopharm Chemical Reagent (Beijing, China). We used a Milli-Q (Bedford, MA, USA) reagent water system to prepare water for the experiment.

2.2. Biochars

Nut shells and bamboo are inexpensive wastes that have natural adsorption characteristics and can be used as precursors for producing activated carbon [28,29]. Pecan shells and bamboo chips were collected from a market in Hangzhou City (Zhejiang Province, China) and pulverized using a high-speed crusher to serve as raw biochar materials. The biomass material was pyrolyzed in a quartz boat (Zhonghuan Experimental Furnace Co., Ltd., Tianjin, China) in a tube furnace operated under a continuous flow of N₂. The heat was applied at 25 °C·min⁻¹, and pyrolysis was conducted at a temperature of 550 °C for 1.5 h. This procedure resulted in the creation of the original biochars, referred to as pecan-shell biochar (PBC) and bamboo biochar (BBC). The biochars were ground and screened using experimental screens to seven particle sizes, 1–2, 0.75–1, 0.5–0.75, 0.25–0.5, 0.125–0.25, 0.075–0.125, and <0.075 mm. An amount of 5 g of each biochar sample was mixed with 500 mL ultrapure water for 24 h to deash. Subsequently, the obtained biochar was dried in an oven at 80 °C overnight to obtain washed PBC (WPBC) and washed BBC (WBBC).

2.3. Characterization

An automatic elemental analyzer (VarioEL/micro cube, Elementar, Germany) was utilized to determine the overall C, N and H composition of the biochars, and the ash content of biochars was measured by heating the biochars for six hours at 750 °C in a muffle furnace [30]. The total O content was calculated according to the following formula [25]:

$$\mathrm{O}\left(\mathrm{\%}\right)=100\mathrm{\%}-\mathrm{C}\left(\mathrm{\%}\right)-\mathrm{H}\left(\mathrm{\%}\right)-\mathrm{N}\left(\mathrm{\%}\right)-\mathrm{A}\mathrm{s}\mathrm{h}\left(\mathrm{\%}\right)$$

A scanning electron microscope (SEM) (TM300 Tabletop Microscope, Hitachi, Japan) was utilized to examine the biochar samples’ structure and surface morphology. The Brunauer–Emmett–Teller (BET) method with N₂ adsorption was used to determine the SSA and PV of each biochar (ASAP 2460, Micromeritics, Norcross, GA, USA). Fourier-transform infrared spectroscopy (FTIR) (VERTEX 70, Bruker Optik Gmbh, Saarbrücken, Saar, Germany) was executed to determine the spectra of 400−4000 cm⁻¹ wavenumbers for all biochars, and an X-ray photoelectron spectrometer (XPS) (K-Alpha, Thermo Scientific, Cambridge, MA, USA) was employed to investigate the surface chemical components of BCs before and after pyraclostrobin adsorption.

2.4. Sorption Experiments

2.4.1. The Sorption Kinetic Experiment

Adsorption kinetics experiments of pyraclostrobin onto four types of biochar (<0.075 mm) were conducted. For each trial, a 50 mg quantity of adsorbent was mixed with a 50 mL volume of 0.5 mg·L⁻¹ pyraclostrobin solution in a 100 mL cone flask. The flask was sealed and agitated at 25 ± 1 °C using a reciprocating shaker in the dark (HCY-DB, Taicang Instrument, Suzhou, China) at 180 rpm. The sample bottle was removed from the shaker at prescribed times (2, 5, 10, 30, 60, 120, 360, 720, and 1440 min). To isolate the solution, the mixture was filtered using a syringe containing a 0.22-µm glass fiber membrane filter (ANPEL, Shanghai, China) because some filtration membranes have an adsorption effect on pyraclostrobin [31]. Control experiments were conducted under identical conditions, excluding adsorbents. Each experiment was performed in triplicate and the experimental error was expressed as the standard deviation.

2.4.2. The Sorption Isotherm Experiment

Except for the initial pyraclostrobin, concentrations ranged from 0.1 to 1.5 mg·L⁻¹ due to the low solubility of the adsorbate in water. The other experimental steps were the same as in the sorption kinetic experiment. Each sample was shaken for 24 h to reach the adsorption equilibrium. Control experiments were performed under the same experimental conditions, but without biochar.

2.4.3. The Adsorption Thermodynamics Experiment

Except for the temperature range of 25–45 °C, the other experimental steps are the same as in the sorption kinetic experiment. Each sample was shaken for 24 h to reach adsorption equilibrium. Control experiments were performed under the same experimental conditions, but without biochar.

2.4.4. The Equilibrium Adsorption Experiment

Various factors for the removal of pyraclostrobin from biochars have been investigated. The adsorption of pyraclostrobin onto biochar with seven different particle sizes was investigated. In addition, the effects of adsorbent content (5, 10, 20, 50, 100, and 200 mg), inorganic ionic strength (adding different amounts of NaCl to the initial solution to achieve a concentration of 1, 2, 5, 10, 20, and 50 g·L⁻¹), and pH (2–10) on the adsorption of pyraclostrobin by biochar were investigated. The duration of the particle size experiment was 72 h. All the other experimental conditions were identical to those described in the sorption kinetic experiment.

2.4.5. Adsorbent Regeneration

For the regeneration experiments, the spent adsorbents were quickly separated from the solution by suction filtration and then added to an equivalent volume of anhydrous ethanol. After shaking for 2 h, the adsorbent was separated and washed with anhydrous ethanol several times before drying at 60 °C for 12 h. The regeneration experiments were conducted 5 times in a sequence to evaluate the regenerable properties of adsorbents.

2.4.6. Testing Conditions

Subsequently, the filtered solution was diluted with chromatographic-grade methanol at a ratio of 1:1 and analyzed using ultra-performance liquid chromatography mass spectrometry/mass spectrometry using Xevo TQ-MS (Waters, Massachusetts, MA, USA), which was equipped with a reverse phase column of 2.1 × 100 mm × 1.7 µm (Acquity UPLC HSS C18, Waters). The mobile phase, consisting of acetonitrile and 0.1% formic acid aqueous solution in a volume ratio of 10%, was utilized at a running rate of 0.2 mL·min⁻¹. Quantitative ions (388.05 > 194.02) and qualitative ions (388.05 > 162.96) were analyzed using mass spectrometry. The linear equation within the range of 0.005–0.5 mg·L⁻¹ is y = 2,194,609.8304x + 2833.2255 (R² = 0.9997), and the LOD and LOQ of the pyraclostrobin on the instrument are 0.001 mg·L⁻¹ and 0.005 mg·L⁻¹, respectively.

2.5. Data Calculation

2.5.1. Adsorption Capacity and Removal Efficiency

The adsorption capacity of each biochar and the determination of removal efficiency W (%) were calculated as follows:

$$q_t=\frac{\left(C_0-C_t\right)V}{m}$$

(1)

$$W=\frac{C_0-C_t}{C_0}\times 100$$

(2)

where q_t represented the amount of pyraclostrobin adsorbed on per gram of biochar at time t (mg·g⁻¹); C₀ represented the initial pyraclostrobin concentration (mg·L⁻¹); C_t represented the solution concentration of pyraclostrobin at time t (mg·L⁻¹); V represented the volume of the aqueous biochar mixture (L); and m represented biochar mass (g).

2.5.2. Kinetic Models

Three models were used to fit the kinetic experimental data of pyraclostrobin adsorption by biochar [32]:

Pseudo-first-order:

$$q_t=q_e(1-e^{-k_1})$$

(3)

Pseudo-second-order:

$$q_t=\frac{k_2{q}_e^{2}t}{1+k_2{q}_{e}t}$$

(4)

Intra-particle diffusion model:

$$q_t=k_i{t}^{\frac{1}{2}}+C$$

(5)

where q_e represented the equilibrium adsorption capacity (mg·g⁻¹); k₁, k₂, and k_i represent the rate constants of the three models (min⁻¹) (g·mg⁻¹·min⁻¹) (g·mg⁻¹·min^−1/2), respectively. C represented the intercept (mg·g⁻¹).

2.5.3. Isothermal Models

The isothermal adsorption data were fitted to three isotherm models can be represented as follows [32]:

Linear model:

$$q_e=K_d{C}_e$$

(6)

Langmuir model:

$$q_e=\frac{q_m{K}_L{C}_e}{1+K_L{C}_e}$$

(7)

Freundlich model:

$$q_e=K_F{C_e}^{\frac{1}{n}}$$

(8)

where K_d, K_L, and K_F represented the linear, Langmuir, and Freundlich models adsorption capacity constant (L·mg⁻¹) (L·mg⁻¹) (mg^(1−1/n)·L^1/n·g⁻¹), respectively; q_m (mg·g⁻¹) represented the maximum adsorption quantity; 1/n represented Freundlich adsorption intensity constant.

2.5.4. Thermodynamics Models

The free energy change (ΔG), enthalpy change (ΔH), and entropy change (ΔS) were calculated with the following formulas [33]:

$$\mathsf{\Delta }G=-RTln{K}_F$$

(9)

$$ln{K}_F=\frac{\Delta S}{R}-\frac{\Delta H}{RT}$$

(10)

R represented the ideal gas constant (8.314 J·mol⁻¹·K⁻¹) and T represented the temperature (K).

2.6. Data Processing

Data processing was performed using Excel 2019 and SPSS 22, and chart drawing was performed using Origin 2021 software.

3. Results

3.1. Biochars Characterization

3.1.1. SEM

Figure 1 shows the microscopic surface characteristics of biochar. After carbonization, the surfaces of the two types of biochar were densely wrinkled, forming a large number of pore structures and providing a broad space for the loading material. The PBC had a hollow porous structure, whereas the BBC exhibited a fibrous porous structure. However, the pores of the PBC were significantly more abundant. Previous studies have shown that deashing generally improves the porosity of biochars because the removal of minerals and organic residues can expose hidden and blocked pores, thereby increasing their surface area [34]. Similarly, the particles on the surface of biochars were considerably reduced after deashing, the blocked pores were exposed, and the surface of the biochar was smoother.

3.1.2. Physical Properties

Table 1. Physical properties of different biochars (WPBC: wash pecan biochar, PBC: pecan biochar, WBBC: wash bamboo biochar, and BBC: bamboo biochar).

Biochar	Element (%)					H/C	O/C	(O + N)/C	SSA (m2/g)	PV (cm3/g)
	N	C	H	O	Ash
WPBC a	1.2663	63.8716	3.5504	23.5617	7.7500	0.0556	0.3689	0.3887	23.5818	0.0128
WPBC b	1.4184	62.3588	3.6246	24.3082	8.2900	0.0581	0.3898	0.4126	44.1823	0.0243
WPBC c	1.3860	60.7584	3.6486	24.7470	9.4600	0.0601	0.4073	0.4301	206.2498	0.1138
PBC a	1.2573	60.1070	3.6555	23.0003	11.9800	0.0608	0.3827	0.4036	9.4055	0.0084
PBC b	1.4398	57.4393	3.6323	26.2088	11.2800	0.0632	0.4563	0.4814	32.2835	0.0207
PBC c	1.4350	56.4132	3.5334	26.7684	11.8500	0.0626	0.4745	0.4999	178.0873	0.0986
WBBC a	0.5798	80.8845	3.1038	13.3120	2.1200	0.0384	0.1646	0.1717	1.6633	0.0005
WBBC b	0.7120	82.1940	3.0897	11.9543	2.0500	0.0376	0.1454	0.1541	4.0082	0.0040
WBBC c	0.7874	82.9020	2.4460	11.4546	2.4100	0.0295	0.1382	0.1477	127.2908	0.0731
BBC a	0.8178	79.1178	3.1213	12.8133	4.1300	0.0395	0.1620	0.1723	1.3871	0.0000
BBC b	0.6540	79.8697	2.7820	13.5443	3.1500	0.0348	0.1696	0.1778	1.7842	0.0012
BBC c	0.8413	81.0618	2.3288	11.2282	4.5400	0.0287	0.1385	0.1489	112.4005	0.0658

^a: 0.75−1 mm; ^b: 0.25−0.5 mm; ^c: <0.075 mm.

displays the basic properties of the biochar. The results indicated that BBC had a remarkably higher C content (>79%) compared to PBC (>56%), and its ash content was comparatively lower. There were slight variations in the O and H contents of biochar samples. Furthermore, deashing effectively reduced the ash content of the biochar, which was confirmed using SEM. The hygroscopicity, aromaticity, and polarity of biochar can be represented by O/C, H/C, and (O + N)/C ratios, respectively [26]. Deashing reduces the surface hydrophilicity and polarity and renders the biochar more hydrophobic. This result is consistent with previous studies [35]. In the case of WBBC, deashing did not considerably change the surface property of the biochar. Decreasing the particle size leads to an increase in SSA and PV, while a larger particle size after deashing is likely due to a reduction in pore blockage.

3.1.3. Fourier-Transform Infrared Spectroscopic Analysis

The Fourier-transform infrared spectroscopic (FTIR) analysis reveals that the washing treatment had no impact on the functional groups in the spectral bands, whereas the functional groups varied depending on the raw materials used (Figure 2). Specifically, PBC exhibited a broad peak at 3406 cm⁻¹ due to the stretching vibration of −OH. The peak at 1583 cm⁻¹ was associated with the stretching vibration of C=C or C=O in the aromatic ring [36]. Likewise, the peak located at 1381 cm⁻¹ and 1283 cm⁻¹ denoted the bending vibration of −COO and C−N [37,38], respectively, while the adsorption bands at 873 and 761 cm⁻¹ were related to the bending vibration of −CH in the aromatic ring [13]. Meanwhile, the analysis of the BBC reflected a broad peak band at 3442 cm⁻¹, while the mean at 1701 cm⁻¹ was related to the stretching vibration of C=O [38]. The peak at 1577 cm⁻¹ corresponds to the stretching vibration of C=C and C=O in the aromatic ring. The peak at 1091 cm⁻¹ denotes the bending vibration of C−O. Finally, the peaks at 747, 825, and 873 cm⁻¹ were caused by the bending vibration of −CH in the aromatic ring. After the adsorption of pyraclostrobin by PBC, the adsorption peak observed at 3406 cm⁻¹ moved to 3386 cm⁻¹, while the intensity decreased, indicating adsorption involving −OH. Meanwhile, the peak at 1583 cm⁻¹ decreased in intensity, accompanied by a small peak at 1549 cm⁻¹, indicating adsorption involving C=C and C=O. Similarly, the peak at 1283 cm⁻¹ moved to 1267 cm⁻¹ with increasing intensity, suggesting that C−N was involved in the adsorption. Additionally, the adsorption peak intensity at 873 cm⁻¹ weakened, indicating adsorption through the involvement of the aromatic −CH group. The peaks at 937 and 825 cm⁻¹ are attributed to the pyrazole ring and −Cl group, respectively [37]. After adsorption by BBC, the intensities of peaks at 3442, 1577, 1091, and 700–900 cm⁻¹ also changed. This indicated that hydrophobic binding, oxygen-containing functional groups, N atoms, and-bonding network of benzene promoted the adsorption.

3.1.4. X-ray Photoelectron Spectroscopy

A deconvolution analysis of the C1s in X-ray photoelectron spectroscopic spectra was performed. The high-resolution C1s spectra (Figure 3) were deconvoluted into three peaks after Gaussian curve fitting. The peak at approximately 284.7–284.8 eV was attributed to C=C/C−H bonds. The peak at 285.3–285.7 eV was attributed to C−O bonds. The peak at 288.3–290.1 eV was attributed to O−C=O bonds [39]. The adsorption of pyraclostrobin by biochar resulted in an increase in aromatic and aliphatic carbon contents from 53.07, 49.72, 53.74, and 48.20% to 59.95, 54.24, 57.93, and 59.97% in WPBC, PBC, WBBC, and BBC, respectively. This may be because the oxygen-containing functional groups on the biochar surface facilitate adsorption, leading to the aromatization of the biochar after adsorption.

3.2. Adsorption Kinetics

Figure 4a shows that the adsorption of pyraclostrobin by biochar was a time-dependent process. The addition of biochar to the aqueous solution resulted in an immediate decrease in the pesticide concentration. Within 5 min, approximately 80, 67, 77, and 80% of pyraclostrobin was removed by WPBC, PBC, WBBC, and BBC, respectively. WPBC is likely to adsorb pyraclostrobin faster than PBC because its surface is more hydrophobic and receptive to hydrophobic adsorbates. As the contact time increased, the concentration of pyraclostrobin in the solution gradually decreased until the adsorption equilibrium was reached. The two stages of fast and slow adsorption describe the entire adsorption process, and the time nodes of the two biochars were 120 and 30 min, respectively.

The adsorption kinetics of pyraclostrobin were simulated using three models, and the relevant parameters are listed in

Table 2. Adsorption kinetics data of different biochars (WPBC: Wash pecan biochar, PBC: pecan biochar, WBBC: Wash bamboo biochar, and BBC: bamboo biochar).

		WPBC	PBC	WBBC	BBC
Actual adsorption capacity		0.4762	0.4758	0.4161	0.4197
Pseudo-first-order	R2	0.9735	0.9408	0.9943	0.9963
	qe	0.4488	0.4329	0.4074	0.4123
	k1	0.7536	0.4275	1.2467	1.3348
Pseudo-second-order	R2	0.9929	0.9816	0.9985	0.9991
	qe	0.4601	0.4485	0.4121	0.4162
	k2	3.3642	1.6083	10.4746	12.6860
Intra-particle diffusion model	R2	0.9816	0.9897	0.9319	0.8902
	ki1	0.0321	0.0507	0.0071	0.0060
	C1	0.3181	0.2187	0.3684	0.3796
	R2	0.9299	0.9381	0.9857	0.9994
	ki2	0.0050	0.0072	0.0016	0.0013
	C2	0.4058	0.3600	0.3971	0.4035
	R2	0.9587	0.8749	0.8458	0.7995
	ki3	6.2732 × 10−4	0.0014	7.0506 × 10−5	8.1833 × 10−5
	C3	0.4537	0.4261	0.4137	0.4170

. All the biochars exhibited a better fit with the pseudo-second-order model, which yielded an R² value of >0.98, surpassing that of the pseudo-first-order model, and exhibited adsorption capacities that closely matched the actual concentrations (Figure S1). Therefore, adsorption is influenced by several mechanisms. Additionally, an intraparticle diffusion model was used to investigate the adsorption behavior of pyraclostrobin. The fitting curves of qt to t^1/2 of the four biochars were multilinear, indicating that multiple factors controlled the adsorption. According to the particle diffusion model, the adsorption process of pyraclostrobin onto the surface of biochar can be divided into three parts: liquid film diffusion, intraparticle diffusion, and adsorption equilibrium (Figure S2) [40]. The model for intraparticle diffusion did not intersect with the origin, implying that the adsorption of pyraclostrobin by the biochar was not solely governed by intraparticle diffusion, where K_i is the rate constant of the intraparticle diffusion, which reflects the ease of adsorbate diffusion inside the adsorbent. A higher Ki value indicates that adsorbate diffusion inside the adsorbent is facile. The intraparticle diffusion parameters revealed K_i1 > K_i2 > K_i3; thus, the rate of liquid film diffusion surpassed that of intraparticle diffusion and surface adsorption. The thickness of the boundary layer is typically determined using the C-value. A higher C₂ value suggests a thicker intraparticle diffusion boundary layer, which may be the primary rate-limiting factor for pyraclostrobin adsorption [41].

3.3. Adsorption Isotherms

Batch study experiments were used to examine the variation in the initial concentration of pyraclostrobin, ranging from 0.1 to 1.5 mg·L⁻¹ (Figure 4b). The results showed that an increase in the initial concentration increased the maximum adsorption but decreased the percentage removal as the adsorbent’s surface area and active sites became saturated. The linear, Langmuir, and Freundlich isotherm models were used to fit the pyraclostrobin adsorption isotherms. The results are shown in

Table 3. Adsorption isotherm data of different biochars (WPBC: Wash pecan biochar, PBC: pecan biochar, WBBC: Wash bamboo biochar, and BBC: bamboo biochar).

Adsorbates	T (K)	Linear		Freundlich			Langmuir
		Kd	R2	KF	1/n	R2	KL	qm	R2
WPBC	298	11.4491	0.9892	7.3812	0.8085	0.9933	N.A	N.A	<0.800
	308	14.9383	0.9973	11.3170	0.8916	0.9982	N.A	N.A	<0.800
	318	19.6723	0.9966	14.3411	0.8879	0.9975	N.A	N.A	<0.800
PBC	298	7.6697	0.9816	4.5369	0.7256	0.9975	N.A	N.A	<0.800
	308	10.5302	0.9981	8.3304	0.8955	0.9995	N.A	N.A	<0.800
	318	14.7769	0.9898	9.4655	0.8239	0.9919	N.A	N.A	<0.800
WBBC	298	2.8789	0.9842	2.3647	0.7445	0.9980	N.A	N.A	<0.800
	308	3.9101	0.9842	2.9256	0.8128	0.9956	N.A	N.A	<0.800
	318	3.9791	0.9915	3.0900	0.8186	0.9920	N.A	N.A	<0.800
BBC	298	2.9359	0.9706	2.3441	0.7104	0.9933	N.A	N.A	<0.800
	308	4.4135	0.9814	3.1408	0.7656	0.9879	N.A	N.A	<0.800
	318	5.4385	0.9931	4.0589	0.8220	0.9975	N.A	N.A	<0.800

Note: N.A: not applicable.

.

The Freundlich model best fits the adsorption process on the biochars, indicating multilayer adsorption on heterogeneous surfaces. The value of 1/n was <1, indicating that indicates that pyraclostrobin is easily adsorbed by biochar. The K_F in the model represents the size of the adsorption affinity, and the K_F of PBC and BBC verified previous results.

3.4. Adsorption Thermodynamics

The adsorption isotherms and parameters of pyraclostrobin adsorption onto the biochars at 298, 308, and 318 K are shown in Figure S3 and Table S2. The slope and intercept of the line can be acquired by plotting lnK to 1/T, and the ΔH and ΔS can be calculated. The thermodynamic parameters are presented in Figure S4 and Table S3. The negative value of ΔG proved that the adsorption process of pyraclostrobin was spontaneous. In addition, all ΔG values were negatively correlated with temperature, indicating that adsorption was more effective at higher temperatures. The positive value of ΔH indicated that the adsorption of pyraclostrobin by all biochars was endothermic. Therefore, an increase in temperature is beneficial for the adsorption process on biochars. An increase in temperature reduces the viscosity of the solution and increases the diffusion rate of molecules through both the inner pores and outer boundary layer of the adsorbent particles [42]. Additionally, the increase in adsorption with increasing temperature can be attributed to the increase in migration rate of the adsorbed molecules [43]. A positive value of ΔS indicates an increase in disorder and confusion in the pyraclostrobin mixed system during biochar adsorption [44].

3.5. Influencing Factors

3.5.1. Effect of Particle Size

The effect of particle size on the adsorption of pyraclostrobin was investigated (Figure 5a). These results demonstrated that PBC and WPBC contributed to better adsorption compared to BBC and WBBC. Moreover, the adsorption effect of WPBC was superior to that of PBC for all particle sizes, implying a more effective outcome for PBC after washing. As the particle size decreased, the difference in the adsorption effect between WPBC and PBC decreased rapidly because biochar adsorption in the aqueous solution was close to saturation. For WBBC and BBC, there were no significant differences in the adsorption effects of different particle sizes. As the particle size decreased, the adsorption capacity of all the biochar samples for pyraclostrobin increased, primarily because of an increase in SSA and PV [45]. Consequently, a biochar particle size of <0.075 mm was selected for subsequent experiments.

3.5.2. Effect of Initial Adsorbent Dosage

In Figure 5b, data were presented on the adsorption capacity of 0.5 mg·L⁻¹ pyraclostrobin solution with different adsorbent dosages, ranging between 5 mg and 200 mg. Results indicated that the adsorption capacity of WPBC outperformed PBC when the biochar dose was low. However, there was no noteworthy distinction between WBBC and BBC, corroborating prior outcomes. At a biochar addition amount of 50 mg, all biochars exhibited high adsorption capacity. While the adsorption capacity of WPBC and PBC slightly increased with an upsurge in their biochar addition. On the contrary, for BBC and WBBC, the adsorption capacity decreased slightly after adding biochar. The increase in adsorption capacity as the adsorbent concentration is increased can be attributed to the enhancement of both the surface area and the availability of adsorption sites. Nonetheless, a slight reduction in adsorption capacity may be attributed to biochar covering the active adsorption sites [46].

3.5.3. Effect of Inorganic Ions Strength

NaCl is an inorganic compound that is present in water. Various strengths (0–50 mg·L⁻¹) of NaCl were controlled to evaluate their effect on the adsorption of pyraclostrobin (C₀ = 0.5 mg·L⁻¹) by biochars in Figure 5c. Ionic compounds significantly promote the adsorption of pyraclostrobin onto biochars. In particular, the higher the NaCl concentration, the stronger the promotion of adsorption. Additionally, when the content of NaCl was 50 mg·L⁻¹, pyraclostrobin was almost eliminated from the solution. On the other hand, hydrophobicity is usually the primary driving factor for the adsorption of hydrophobic pollutants. Studies have shown that with an increase in ion concentration in the solution, the water solubility of hydrophobic compounds (such as polycyclic aromatic hydrocarbons) change owing to the salting-out effect, increasing the k_p value [47]. Similarly, the increase in the solubility of pyraclostrobin in the aqueous solution (log K_ow = 3.99) reduces its ionic strength, thereby promoting its hydrophobic adsorption by the biochar. Interestingly, in the presence of many ions in solution, the adsorption effect of BBC was greater than that of PBC.

3.5.4. Effect of Solution pH

The solution pH determines the surface charge of the biochar and the ionic species of the adsorbate, which significantly affect the adsorption capacity of the adsorbent [48]. As the pH increased, the adsorption capacity of pyraclostrobin onto biochar decreased. Figure 5d shows that a pH value of 2 was optimal for achieving the maximum adsorption capacity and removal percentage. Pyraclostrobin is a neutral compound and is not ionized by solution pH [49]. Therefore, it is possible that the groups on the biochar surface are protonated under acidic conditions, which makes the biochar surface positively charged and has an electronic dipole induction effect on pyraclostrobin that has a rich electronic structure, which increases its binding energy at low pH [50].

3.6. Adsorption Mechanism

Kinetics, isotherms, thermodynamics, batch experiments, and characterization analysis showed that the adsorption of pyraclostrobin by biochar was affected by various physical and chemical mechanisms. In addition, the adsorption mechanism of pyraclostrobin onto the two types of biochar was similar.

The adsorption capacity of biochar is closely related to its SSA. The adsorption capacity of pyraclostrobin onto the biochar increased as the particle size decreased. Interestingly, particles < 0.075 mm exhibited significantly higher adsorption capacities than particles of any other size. This can be attributed to their more developed pore structures and higher SSA, allowing for more active sites, suggesting that the adsorption of pyraclostrobin onto the small-sized biochar was mostly due to pore filling. When ∆S > 0 and ∆H > 0, hydrophobic effects were the dominant force during adsorption [38].

A possible chemical mechanism for biochar adsorption of pyraclostrobin is shown in Figure 6. First, because of the strong electron-withdrawing ability of Cl⁻ in the pyraclostrobin molecule, the related structure was electron-deficient; therefore, as an electron acceptor, it strongly interacted with the rich (electron donor) multi-aromatic surface of biochar [51]. Furthermore, due to its pure p-type nature and low electron occupancy, the nitrogen atom connected to the oxygen atom and the benzene ring has an electron-donating ability, which made it possible to bind to the π electron-acceptor on the benzene ring [37]. Moreover, the lone electron pair of N atoms in pyraclostrobin can form p-π electron donor–acceptor (EDA) interaction with the aromatic ring of biochar. Compared with the PBC, there were fewer or no p-π EDA interactions in the BBC (Figure 2). The C=O functional group of pyraclostrobin could serve as an acceptor to form hydrogen bonds with either the aromatic ring or −OH group of the biochar. Similarly, the aromatic ring of pyraclostrobin can serve as a hydrogen donor to interact with C=O on the biochar, thereby enhancing its adsorption affinity [31].

For PBC, deashing increased the SSA, PV, and adsorption capacity. This suggests that the inorganic content negatively affected the sorption of contaminants onto the biochars, and that the removal of ash improved the accessibility of the inner sorption sites. In contrast, for the BBC, deashing did not significantly change the adsorption capacity, indicating that ash could adsorb pyraclostrobin through specific interactions [52], which may be due to the different compositions of the ash components of the two biochars.

The main adsorption mechanisms of pyraclostrobin onto biochars include pore filling, hydrophobic, π-π, and p-π interactions, and hydrogen bonding.

3.7. Regeneration Analysis of Adsorbents

Recyclability is critical for assessing the practical application potential of the sorbent materials in water treatments [53]. Figure 7 shows that after five cycles, four types of biochar still exhibit an adsorption rate of >97% for pyraclostrobin. Therefore, biochar prepared from walnut shells and bamboo shows high recyclability and are promising and cost-effective adsorbents.

3.8. Comparison with Other Adsorbents

The study of the adsorption mechanism of pyraclostrobin onto biochar is limited; therefore, it is necessary to compare the biochar prepared in this study with other adsorbents. At an initial concentration of 0.1 mg/L for pyraclostrobin and an adsorbent dose of 1 g/L, the adsorption capacities of PS and PE reach 0.091 and 0.081, respectively [49]. In addition, when studying the adsorption performance of microfiltration membranes on pyraclostrobin, it was found that the maximum adsorption capacity of nylon filters for pyraclostrobin was only 0.724 mg/g [31]. Compared with the aforementioned adsorbents, biochar still exhibits a very significant adsorption capacity for pyraclostrobin.

4. Conclusions

The results revealed that biochar obtained from pecan shells and bamboo could effectively and economically eliminate pyraclostrobin from water, both adsorption rates are still > 97% after 5 cycles of adsorption. Compared to BBC, PBC has a larger surface area and higher adsorption capacity. The adsorption of pyraclostrobin by PBC and BBC was best described by pseudo-second-order kinetics and Freundlich model. Combined with characterization and factor analysis experiments, it is revealed that the adsorption of pyraclostrobin on biochar is attributed to various mechanisms, including pore filling, hydrophobic interactions, π-π and p-π interactions, and hydrogen bonding. Furthermore, deashing significantly enhances the adsorption efficiency of pecan biochar, but it has an insignificant effect on bamboo biochar. This study will aid in the selection of cost-effective and eco-friendly adsorbents to reduce the aquatic ecological risk posed by pyraclostrobin. Future research should focus on real-world environments.