Adsorption Behavior and Residue Degradation of Triazine Herbicides in Soil Amended with Rice Straw Biochar

Abstract

The removal of or decrease in pesticide residues in soil has attracted considerable attention, due to the serious pollution of pesticides in soil. The purpose of the study was to explore the adsorption behavior of biochar on pesticides and the impact on the degradation of pesticide residues in soil, providing a basis for the remediation of soil by biochar. Biochars were prepared via pyrolysis of rice straw at a high temperature (300 °C, 400 °C, 500 °C, 600 °C). The individual and competitive adsorption of three triazine herbicides, prometryn, atrazine, and simazine, on biochar was investigated, and the degradation of the herbicide residues in biochar-added soil was determined. The selected herbicides presented similar adsorption characteristics to rice straw biochar, and the amount of herbicides adsorbed increased with higher preparation temperature and the amount of biochar. The rice straw biochar adsorbed the studied herbicides simultaneously, and the adsorption amount decreased as follows: simazine > atrazine > prometryn. The competition adsorption of the selected herbicides on the biochar presented a lower adsorption affinity than that when they are adsorbed individually. The adsorption isotherm was best fitted by the Freundlich model. The half-lives of prometryn, atrazine, and simazine were 9.8~12.6 d, 5.2~8.1 d, and 3.7~5.6 d, respectively. Biochar addition increased the degradation of the evaluated herbicides in soil. The rice straw biochar could be the potential sorbents that can be implemented for the removal of pesticides.

1. Introduction

Pesticides are widely used to control diseases, insects, and weeds to increase agricultural production. With the widespread use of pesticides, large amounts of the chemicals are directly or indirectly transferred into the environment, causing serious hazards to the soil, water, beneficial organisms, and plants [1,2]. In addition, the enrichment of food chain chemicals will eventually threaten human health.

Triazine herbicides are extensively used as highly selective herbicides for controlling weeds in agricultural plants, mainly grasses [3]. However, they present the properties of good water solubility, a long half-life, relatively high mobility, and persistence in soils [4,5]. Therefore, their degradation, accumulation, toxicity, and hazards due to their environmental residues have caused considerable concern. Atrazine was reported to cause oxidative damage to rat erythrocytes [6] and neutropenia and lymphocyte reduction in female mice [7]. Prometryn was also shown to cause damage to the antioxidant system of Crayfish [8]. A trace amount of prometryn significantly affected the growth and development of tadpoles, resulting in deformities, and an association was observed between the extent of the deformities and the exposure time [9]. Therefore, prometryn could interfere with the normal reproductive activities of rats [10].

Biochar is a carbonaceous material prepared using biomass waste under oxygen-limited conditions [11,12]. Biochar’s physicochemical characteristics and structure depend on the different raw biomass materials. In addition, the pyrolysis temperature significantly impacts biochar structure [13,14]. In general, biochar with a large specific surface area presents a strong adsorption capacity for pollutants [15]. It has been reported that pesticide biodegradation in the soil was reduced due to adsorption mechanisms [16,17,18]. The adsorption of pesticides by biochar is of growing concern, such as sugarcane-bagasse-derived biochar as an adsorbent for the removal of chlorpyrifos from aqueous environments [19], cymoxanil adsorption by grape-pomace-derived biochar [20], and the remediation of fipronil and its metabolite contamination in aquatic systems by biochar derived from corn stalks [21]. Moreover, the addition of biochar may increase microbial populations, resulting in higher microbial degradation of pesticides [22,23]. On the one hand, pesticide biodegradation in biochar-enriched soils generally decreased, resulting in a decrease in the utilization rate of pesticides by soil microorganisms [24,25,26]. On the other hand, soil structure and microbes also affect biochar adsorption behavior [27,28].

In this study, the sorption–desorption of three triazine herbicides, prometryn, atrazine, and simazine, using rice straw biochar, was investigated. Biochar with higher adsorption capacity was added to the soil, and the degradation dynamics of the selected herbicides in the soil were determined. The study provides a practical basis for the removal of triazine herbicides in the soil through biochar application.

2. Materials and Methods

2.1. Apparatus

An Agilent 1290–6460 liquid chromatography–triple-quadrupole tandem mass spectrometry (Agilent Technologies, Santa Clara, CA, USA) was used to determine the analytes.

2.2. Chemicals and Reagents

Simazine, prometryn, and atrazine (purity > 97%) were purchased from the National Pesticide Quality Inspection Center (Beijing, Shanghai). Wahaha-purified water (Hangzhou Wahaha Group, Hangzhou, China) was used for the separation of the studied herbicides. Acetonitrile (HPLC grade) was purchased from Sigma-Aldrich (Shanghai, China). Analytical-grade methanol, acetone, sodium sulfate, dichloromethane, and petroleum ether (boiling point range 60~90 °C) were purchased from Aladdin Biochemical Technology Co., Ltd. (Shanghai, China). Biochar prepared from rice straw was purchased from the Jiangsu Biochar Engineering Center (Nanjing, China), and its physicochemical properties were measured. Biochar morphology was characterized via scanning electron microscopy (SEM). The rice straw biochar functional groups were determined using infrared spectroscopy (IRAffinnity-1s, Shimadzu, Japan, as indicated in Supplementary Materials).

2.3. Standard Solution Preparation

Stock standard solutions of atrazine, simazine, and prometryn (0.5 mg/mL) were prepared in methanol. A mixture of the three herbicides (0.05 mg/mL in methanol) was prepared by mixing the appropriate amount from each individual stock solution and was stored at 4 °C. Standard solutions with lower concentrations (25, 5.0, 2.5, 1.0, 0.5, 0.1, and 0.01 μg/mL) used for the calibration curve were prepared daily through serial dilution in methanol.

2.4. Adsorption and Desorption Behavior

2.4.1. Adsorption Behavior

The rice straw biochar was prepared via slow pyrolysis at different temperatures (300, 400, 500, and 600 °C, respectively). Three individual or mixed standard solutions of herbicides were sequentially added into 50 mL centrifuge tubes, which were shaken at room temperature (25 °C, 120 r.p.m) to reach the adsorption equilibrium. Subsequently, the solutions were centrifuged for 10 min at 4000 r.p.m, and then 1.5 mL of supernatant was collected and filtered through a 0.22 μm filter membrane for HPLC-MS/MS determination.

The adsorption of the selected herbicides on biochar was calculated using the following formula [29]:

$$C_s=\frac{(C_0-C_e)\times V}{m}$$

(1)

where C_S is the amount of the herbicide (mg/kg) absorbed on biochar, C₀ is the initial concentration (mg/L), C_e is the equilibrium aqueous concentration (mg/L), V is the volume of the solution (L), and m is the biochar weight (kg).

In this study, the Freundlich model (Equation (2)) was applied to fit the isotherm data [30]:

$$\log Q_e=\log K_f+n\log C_e$$

(2)

where Q_e is the adsorption amount at equilibrium (mg/kg), K_f is the sorption affinity (mg/kg), and n is the Freundlich exponent.

2.4.2. The Desorption Behavior

The selected herbicides’ desorption on biochar was investigated using the same samples after the adsorption characterization. After adsorption, the supernatant was removed, 10 mL deionized water was subsequently added into the tube, and the solution was shaken for another 24 h at room temperature. Then, the supernatant was collected and filtered through a 0.22 μm filter membrane to determine the analytes.

2.5. Degradation of the Selected Herbicide in Soil Supplemented with Biochar

2.5.1. Herbicide Degradation

Rice biochar prepared at 600 °C was selected to investigate the analyte degradation in soil. Brown soil used in the study was collected from the top layer of local lawn. It had a pH of 7.2 and organic matter content of 2.5%, with moderate fertility. The weight fraction of the added rice biochar in the soil was 0.1%, 0.5%, and 1%, respectively, with cultivation for one week to activate the microbial environment in the soil. The moisture content of the soil was controlled at 20%.

Subsequently, each of the studied herbicides was sprayed into the biochar-supplemented soil at 1.5-times the recommended doses, and herbicide-applied soil without biochar was used as the control. The soil samples were collected at 2 h, 6 h, 1, 2, 5, 10, 15, 20, and 30 d after herbicide application. The residual amount of the applied herbicides was determined using HPLC-MS/MS.

The residue dissipation in the soil of the studied herbicides was calculated using the following equation [31]:

$$C_t=C_0{e}^{-kt}$$

(3)

where C₀ is the initial concentration, C_t is the residual concentration at time t, k is the rate constant for dissipation, and t is the sampling time.

The half-lives of the analytes (t_1/2) were calculated using the k value with the following expression [32]:

$$t_{1/2}=\frac{\ln 2}{k}$$

(4)

2.5.2. Sample Preparation

Thus, 30 mL methanol/water (1:1, v/v) was added to a 5 g (±0.001 g) sample, which was extracted via ultrasonication for 30 min, followed by centrifugation of the solution at 4000 r.p.m for 5 min. The supernatant was collected, and then 20 mL methanol/water (1:1, v/v) was added into the remaining solution, which was successively extracted for a further 15 min. The supernatants were combined and evaporated to 35 mL and were then diluted using 50 mL of a 4% sodium sulfate aqueous solution.

The mixed solution was then extracted three times with dichloromethane/petroleum ether (60 °C~90 °C) (3.5:6.5, v/v), using 40 mL each time. The organic phases were combined and evaporated to almost dryness at 40 °C using rotary evaporator. The residue was redissolved in 2.0 mL methanol and filtered by a 0.45 μm filter membrane for further analysis.

The developed method’s accuracy and precision were determined using spiked samples. The spiked sample concentrations were 0.064 mg/kg, 0.32 mg/kg, and 0.64 mg/kg, respectively. Accuracy and precision were determined by calculating the average recoveries and relative standard deviation (denoted as RSD, n = 5) of the analytes at three spiked concentrations, respectively [33].

2.6. Analytical Conditions

HPLC-MS/MS separation of the samples was performed on an Eclipse Plus C₁₈ column, 2.1 mm × 100 mm (1.8 μm, Agilent Technologies Inc., Shanghai, China). Methanol (A) and formic acid aqueous solution (B) were used as mobile phases with gradient elution. The flow rate was 0.3 mL/min. The injection volume was 5 μL. The column temperature was set to 40 °C.

The gradient elution conditions for the analytes were as follows: the analysis started with 20% (v/v) of solution A, which was increased linearly up to 90% in 1.0 min, and then held for a further 2.0 min before being returned to 20% A in 0.5 min, followed by a re-equilibration time of 1.0 min.

Positive ionization mode was applied for MS/MS detection. The dry temperature was 350 °C, with a flow rate 9 L/min. The sheath gas temperature was 325 °C, with a flow rate 12 L/min. The capillary voltage was 4000 V. The analytes were determined using the MRM mode (multiple reaction monitoring mode) (

Table 1. MRM conditions for determination of the studied herbicides.

Analyte	Chemical Structure	Transitions, m/z	Fragment Voltage/V	Collision Energy/eV	Dwell Time/ms
Prometryn		241.36/124 a	35	23	300
		241.36/168	35	37	300
Atrazine		215.68/124 a	35	25	300
Simazine		201.66/124 a	35	25	300

^a: Quantitative ion pairs.

) [34].

3. Results and Discussion

The HPLC-MS/MS method was validated with spiked samples. There was a good linearity between the measured peak area and the analyte concentrations (linear range 0.01 mg/kg~10.0 mg/kg), with correlation coefficients above 0.9986. The analytes’ limits of detection (LOD, S/N = 3) and limits of quantitation (LOQ, S/N = 10) were above 0.004 mg/kg and 0.01 mg/kg, respectively. The average recoveries of the analytes at three spiked levels (0.064 mg/kg, 0.32 mg/kg, 0.64 mg/kg) ranged from 84% to 94%, with relative standard deviations (RSDs) between 4.7% and 11.2%. These results indicated that the proposed method achieved good accuracy and precision and, thus, was subsequently applied for analyte determination in the collected samples.

3.1. Adsorption and Desorption Behavior

3.1.1. Adsorption Test

The effect of the pyrolysis temperature for the preparation of rice straw biochar on the adsorption behavior of prometryn was investigated (Figure 1a). The amount of biochar used was 100 mg, and the adsorption of prometryn increased at higher pyrolysis temperatures, with the highest adsorption observed at 600 °C. Therefore, biochar prepared at 600 °C was used for the experiments.

Then, the influence of the biochar amount on prometryn adsorption was investigated (Figure 1b). Prometryn adsorption by rice straw biochar increased with higher amounts of biochar [35]. Therefore, the adsorption of prometryn by rice straw biochar increased with higher pyrolysis temperatures and when an increased amount of biochar was used.

Then, the adsorption properties of rice straw biochar on atrazine and simazine were also investigated. The adsorption of both herbicides similarly increased with the increase in the pyrolysis temperature and biochar amount (Figure 2 and Figure 3).

The individual adsorption behavior of the selected herbicides on rice straw biochar exhibited a similar trend. It was also reported that the biochar adsorption of organic pollutants increased with higher pyrolysis temperatures [36,37,38,39]. Within a certain range, the adsorption of non-polar and polar aromatic pollutants on biochar prepared from pine needles similarly increased with an increasing pyrolysis temperature [36]. The biochar prepared at higher pyrolysis temperatures exhibited higher adsorption capacity of organic pollutants due to the increased surface area, microporosity, and hydrophobicity [39]. In addition, the aromaticity of biochar increased with the higher pyrolysis temperatures, resulting in increased adsorption of hydrophobic pesticides in biochar [40].

The Freundlich model [30] was used to study the individual adsorption capacity of rice straw biochar prepared at different temperatures towards the studied herbicides. The rice biochar K_f value, 1/n, and correlation coefficient were calculated [41], as shown in

Table 2. Isothermal adsorption model of the selected herbicides adsorbed on rice straw biochar.

Parameter	Analyte	Prepared Temperature (°C)
		300	400	500	600
Kf	Prometryn	7.61	7.85	8.31	8.50
	Atrazine	7.40	8.35	8.68	9.06
	Simazine	7.85	8.61	8.93	9.50
1/n	Prometryn	0.025	0.024	0.020	0.017
	Atrazine	0.031	0.029	0.020	0.018
	Simazine	0.016	0.014	0.013	0.012
R2	Prometryn	0.99	0.97	0.98	0.99
	Atrazine	0.98	0.97	0.96	0.97
	Simazine	0.90	0.91	0.85	0.65

.

It was concluded that with the biochar preparation temperature increase, 1/n gradually decreased, indicating that the adsorption amount of the studied herbicides on biochar was enhanced. The value ranged from 0.01 to 0.03, indicating that rice straw biochar easily absorbed the herbicides. In contrast, the K_f value increased with the higher preparation temperature, indicating increased herbicide adsorption.

Competitive adsorption, when multiple pollutants are adsorbed by biochar, has been reported [38]. Hence, the mixed adsorption behavior of three triazine herbicides with the rice straw biochar prepared at 600 °C was investigated (Figure 4,

Table 3. Isothermal adsorption model of competitive adsorption of the studied herbicides on rice straw biochar.

Parameter	Analyte	Prepared Temperature (°C)
		300	400	500	600
Kf	Prometryn	2.38	2.44	2.48	2.91
	Atrazine	3.10	3.22	3.41	3.63
	Simazine	2.27	2.32	2.34	2.39
1/n	Prometryn	0.028	0.026	0.022	0.017
	Atrazine	0.062	0.055	0.049	0.043
	Simazine	0.022	0.018	0.017	0.012
R2	Prometryn	0.91	0.90	0.94	0.96
	Atrazine	0.93	0.94	0.94	0.94
	Simazine	0.90	0.95	0.94	0.96

). The Freundlich isotherms of the herbicides on biochars are shown in Figures S3–S5.

Prometryne and atrazine adsorption capacity increased rapidly in the first 20 h, while for simazine, it increased rapidly in the first 100 h. Subsequently, the adsorption capacity of the analytes increased slowly. The biochar prepared at 600 °C had strong aromaticity and could provide abundant π electrons [40]. Moreover, the N atom in atrazine had electronegativity, and the -Cl substituent had an electron-withdrawing property, resulting in π electron deficiency [42]. Therefore, the biochar could more easily adsorb atrazine, and the competition was stronger in the initial period. The adsorption capacity of the studied herbicides by the biochar decreased in the following order: Simazine > Atrazine > Prometryn. Compared with their individual adsorption behavior, the three herbicides exhibited competition for adsorption by rice straw biochar when they were mixed. The competition of the selected herbicides for the limited adsorption sites on the biochar resulted in a lower adsorption affinity than that when they were adsorbed individually [43].

3.1.2. Desorption Behavior of the Herbicides by Rice Straw Biochar

The desorption amounts after single and mixed herbicide adsorption were measured [41], and the results are shown in

Table 4. Desorption amounts of the selected herbicides after the individual herbicide adsorption experiment.

Prepared Temperature (°C)	Addition Amount (mg)	Desorption Amount (μg/kg)
		Prometryn		Atrazine		Simazine
		24 h	48 h	24 h	48 h	24 h	48 h
300	10	61 ± 0.1	55 ± 0.1	56 ± 0.6	56 ± 0.7	72 ± 0.4	70 ± 0.6
	20	60 ± 0.5	54 ± 0.5	52 ± 0.2	52 ± 0.4	70 ± 0.1	73 ± 0.9
	40	60 ± 0.3	53 ± 0.2	53 ± 0.3	51 ± 0.6	76 ± 0.2	71 ± 0.7
	60	58 ± 0.4	57 ± 0.1	56 ± 0.1	49 ± 0.5	76 ± 0.5	74 ± 0.5
	100	61 ± 0.2	56 ± 0.3	55 ± 0.4	51 ± 0.4	78 ± 0.8	72 ± 0.8
400	10	56 ± 0.7	53 ± 0.4	52 ± 0.2	56 ± 0.4	70 ± 0.9	65 ± 0.6
	20	60 ± 0.3	54 ± 0.8	53 ± 0.5	54 ± 0.3	78 ± 0.6	70 ± 0.3
	40	58 ± 0.4	58 ± 0.6	53 ± 0.7	49 ± 0.1	78 ± 0.3	65 ± 0.4
	60	58 ± 0.6	57 ± 0.4	55 ± 0.1	51 ± 0.2	78 ± 0.4	64 ± 0.5
	100	61 ± 0.2	54 ± 0.3	56 ± 0.6	55 ± 0.6	69 ± 0.7	66 ± 0.6
500	10	58 ± 0.1	56 ± 0.1	57 ± 0.8	49 ± 0.7	74 ± 0.8	69 ± 0.3
	20	56 ± 0.5	54 ± 0.5	53 ± 0.7	52 ± 0.5	73 ± 0.2	66 ± 0.2
	40	58 ± 0.3	55 ± 0.6	55 ± 0.6	52 ± 0.8	73 ± 0.4	67 ± 0.1
	60	57 ± 0.1	54 ± 0.7	56 ± 0.8	53 ± 0.3	69 ± 0.6	64 ± 0.4
	100	61 ± 0.4	56 ± 0.4	55 ± 0.9	54 ± 0.2	76 ± 0.2	70 ± 0.5
600	10	60 ± 0.4	56 ± 0.1	55 ± 0.2	51 ± 0.7	75 ± 0.4	67 ± 0.6
	20	56 ± 0.6	54 ± 0.7	56 ± 0.1	51 ± 0.6	75 ± 0.6	70 ± 0.3
	40	57 ± 0.5	57 ± 0.5	53 ± 0.5	51 ± 0.5	69 ± 0.4	65 ± 0.2
	60	57 ± 0.6	57 ± 0.5	55 ± 0.7	50 ± 0.4	76 ± 0.6	63 ± 0.4
	100	59 ± 0.2	53 ± 0.3	54 ± 0.3	51 ± 0.1	70 ± 0.4	63 ± 0.5

and

Table 5. Desorption amounts of the studied herbicides after the mixed herbicide adsorption experiment.

Prepared Temperature (°C)	Addition Amount (mg)	Desorption Amount (μg/kg)
		Prometryn		Atrazine		Simazine
		24 h	48 h	24 h	48 h	24 h	48 h
300	10	44 ± 0.8	37 ± 0.5	48 ± 0.7	38 ± 0.6	62 ± 0.9	43 ± 0.4
	20	43 ± 0.5	41 ± 0.3	47 ± 0.5	40 ± 0.6	58 ± 0.5	42 ± 0.5
	40	44 ± 0.2	39 ± 0.1	48 ± 0.3	40 ± 0.3	59 ± 0.4	43 ± 0.7
	60	46 ± 0.4	40 ± 0.5	49 ± 0.4	42 ± 0.2	61 ± 0.7	44 ± 0.6
	100	46 ± 0.7	40 ± 0.9	47 ± 0.1	41 ± 0.4	63 ± 0.8	39 ± 0.5
400	10	41 ± 0.6	44 ± 0.3	45 ± 0.2	42 ± 0.4	64 ± 0.6	38 ± 0.7
	20	42 ± 0.3	41 ± 0.2	46 ± 0.6	45 ± 0.7	62 ± 0.5	41 ± 0.9
	40	45 ± 0.1	42 ± 0.1	47 ± 0.8	39 ± 0.8	66 ± 0.4	43 ± 0.6
	60	45 ± 0.7	45 ± 0.7	45 ± 0.7	37 ± 0.5	59 ± 0.2	40 ± 0.3
	100	44 ± 0.9	45 ± 0.5	49 ± 0.7	41 ± 0.5	57 ± 0.3	39 ± 0.4
500	10	41 ± 0.4	47 ± 0.9	47 ± 0.5	42 ± 0.4	58 ± 0.4	39 ± 0.2
	20	42 ± 0.6	40 ± 0.7	47 ± 0.1	39 ± 0.3	61 ± 0.6	41 ± 0.5
	40	45 ± 0.2	39 ± 0.3	49 ± 0.2	44 ± 0.2	63 ± 0.4	43 ± 0.5
	60	47 ± 0.7	38 ± 0.4	42 ± 0.3	44 ± 0.2	62 ± 0.4	38 ± 0.5
	100	43 ± 0.5	41 ± 0.1	45 ± 0.5	37 ± 0.4	59 ± 0.9	37 ± 0.7
600	10	41 ± 0.6	40 ± 0.7	48 ± 0.7	42 ± 0.6	60 ± 0.3	41 ± 0.8
	20	40 ± 0.3	42 ± 0.2	41 ± 0.8	39 ± 0.8	60 ± 0.4	43 ± 0.4
	40	40 ± 0.4	40 ± 0.2	47 ± 0.9	41 ± 0.7	61 ± 0.3	40 ± 0.6
	60	45 ± 0.5	40 ± 0.5	46 ± 0.5	43 ± 0.5	63 ± 0.1	40 ± 0.4
	100	45 ± 0.4	39 ± 0.8	44 ± 0.1	39 ± 0.3	56 ± 0.2	39 ± 0.6

, respectively.

According to Table 4 and Table 5, the desorption amount of the herbicides was much lower than their adsorption amount [41]. The desorption amount after the competitive adsorption of the triazine herbicides was lower than that of the individual herbicides. In addition, the desorption amount of the herbicides in 24 h was slightly higher than that in 48 h. Their desorption amount in 24 h was approximately 0.02% of their adsorption amount, and the total desorption amount in 48 h was approximately 0.05% of their adsorption amount. Compared to the adsorption amount of the studied herbicides, their desorption amount was almost negligible, in agreement with a previous report [41]. The phenomenon was attributed to the following reasons: (1) irreversible binding or sequestration of the solute to the organic carbon content of the adsorbent [44]; (2) solute adsorption in the mesopore and micropore structures of the adsorbent [45], resulting in binding and structural hysteresis, respectively. In conclusion, rice straw biochar has a desorption hysteresis for the analytes and could be used for the removal of herbicide residues from the soil.

3.2. Effect of Biochar on Residue Behavior of the Herbicides in Soil

The rice straw biochar was added to the soil in a range of 0.1~1% (wt%) to study the effect of biochar on the degradation of the herbicides in the soil. The residue behavior of the herbicides in the soil is presented in

Table 6. Dissipation equations of the selected herbicides.

Herbicide	The Addition Amount of Biochar	Degradation Equation	K	R2	t1/2 (d)
Prometryn	CK	C = 27 × 10−0.055t	0.055	0.92	12.6
	0.1%	C = 27 × 10−0.058t	0.058	0.92	11.9
	0.5%	C = 27 × 10−0.064t	0.064	0.92	10.9
	1%	C = 27 × 10−0.071t	0.071	0.92	9.8
Atrazine	CK	C = 27 × 10−0.085t	0.085	0.96	8.1
	0.1%	C = 27 × 10−0.092t	0.092	0.96	7.5
	0.5%	C = 27 × 10−0.107t	0.107	0.97	6.5
	1%	C = 27 × 10−0.133t	0.133	0.97	5.2
Simazine	CK	C = 27 × 10−0.122t	0.122	0.98	5.6
	0.1%	C = 27 × 10−0.133t	0.133	0.98	5.2
	0.5%	C = 27 × 10−0.150t	0.150	0.98	4.6
	1%	C = 27 × 10−0.187t	0.187	0.99	3.7

. The degradation rate of the herbicides in the soil was accelerated with the addition of rice straw biochar. The degradation rates of the three herbicides increased significantly when the amount of rice straw biochar was higher.

The half-lives of the herbicides decreased with the increased amount of rice straw biochar added. Specifically, the half-lives of prometryn, atrazine, and simazine ranged between 9.8~11.9 d, 5.2~7.5 d, and 3.7~5.2 d, respectively, at rice straw biochar concentrations in the range of 0.1~1%.

Biochar derived from biomass waste was widely applied for the removal or degradation of pesticides in soils. It is reported that biochar could reduce pesticide biodegradation in soil due to its adsorption capacity [16,17,18]. An alkali-modified biochar (BC_NaOH)/graphitic carbon nitride (g-C₃N₄) was used as the remediation agent to degrade atrazine residue in paddy soil. Excellent degradation performance (above 60%) of the herbicide was achieved due to the construction of an intact electron transfer pathway via the π-π stacking structure [46]. Cassava-waste-based biochars were added to an agriculture soil to attenuate the migration of atrazine in soil. It is indicated that the biochar significantly enhanced the sorption capacity of soil for atrazine [47]. Therefore, the biochars are promising sorbents for pesticide elimination and soil remediation. Conversely, the addition of biochar into the soil may induce higher microbial stimulation, thereby increasing the microbial degradation rate of pesticides [22,23]. Therefore, the effect of biochar on pesticide degradation in soil depends on the combination of these two aspects and their interactions. According to the results, the addition of biochar into the soil enhanced the degradation of the herbicides (Table 6) because of the increasing biodegradation rate by the dominant microorganisms.

4. Conclusions

The adsorption behavior of three triazine-group herbicides on rice-straw-derived biochar was studied. The herbicides’ individual adsorption behavior was similar to that when they were present in a mixture, which increased with the higher preparation temperature and the increasing amount of rice biochar. The Freundlich isotherm adsorption model was the best fit for the selected herbicide adsorption. The rice biochar exhibited a strong desorption hysteresis effect on the studied herbicides.

The dissipation rate of the herbicides in the soil after biochar addition increased significantly and was positively associated with the amount of biochar added. The half-lives of prometryn, atrazine, and simazine were 9.8~11.9 d, 5.2~7.5 d, and 3.7~5.2 d, respectively, at rice straw biochar concentrations in the range of 0.1~1% in the soil.