1 Introduction

In West Africa, Elaeis guineensis (an oil palm species) is widely used. Today, the aforementioned oil palms are grown as the most important economic plantation crop in tropical countries [3]. The majority of edible oils come from oil palm fruits, which can produce up to 38.5 106 t of oils per year in terms of oil palm biomass [18, 19]. The application of pesticides in agricultural areas can pose potential risks to the environment [2]. Imidazolinones, a family of herbicides, have attracted environmental concerns due to their high leaching potential, persistent nature, and high phytotoxicity [27, 30]. Characteristics of imazapyr, a member of this family are shown in Table 1. Imidazolinones can be used in the Clearfield® production system (CPS). Currently, the application of CPS is growing in paddy fields [5]. Therefore, consideration of effective solutions to remove their contaminations is critical.

Table 1 Physico-chemical characteristics of imazapyr herbicide [27, 30]
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There are a variety of techniques that have been used for environmental remediation mainly applying the process of degradation (e.g., bio-degradation, photolysis, and chemical degradation) [13, 26, 33], extraction (e. g. phytoremediation and electrokinetic) [31, 38], or stabilization [10, 35]. Mitigation of pollutants availability through increasing their sorption is a widely used technique to remediate the polluted media. A wide range of organic compounds including biochar can be used for immobilization of the soil and water contaminants [1, 36]. Biochar is a carbonaceous material produced during the pyrolysis process which is a thermo-chemical decomposition (< 700 °C) of biomass in the limited supply of O2 [23]. Biochar is produced as a cost-effective and eco-friendly sorbent. The ability of biochar to stabilize organic and mineral compounds is due to its Physico-chemical properties such as porous structure, expanded specific surface area, high organic carbon content, active functional groups, and high cation exchange capacity (CEC) [25]. Many studies have demonstrated immobilization of applied pesticides in agricultural fields amended with biochars which led to control their environmental risks [17, 20].

Based on Larsbo et al. [22], biochar can have opposing impacts on pesticides leaching depending on soil type and sorption strength of the compounds. Effects of biochar application on leaching were shown to be insignificant in loam soil. In clay soil, leaching of moderate mobile pesticides was reduced while that of immobile pesticides enhanced in the presence of biochar. It was concluded that material originating from the applied biochar facilitated the transfer of the immobile pesticides. Reduction of atrazine leaching in the presence of pine chip biochar was reported in both lab and field ([4, 12]).

Amendment of soil with biochar is predicted to decrease pesticides availability and the rate at which they are degraded in the soil [9]. However, some studies showed that pesticides micro-degradation can be stimulated by biochar in soil. Biochar can change the nature and activity of microbial communities and thus the soil microbiological characteristics (37, 28).

According to the available knowledge, hydrolysis is not generally considered an important process in the disappearance of imidazolinones [15]. It is shown that photolysis and bio-degradations are the main processes for imidazolinone herbicides losses in the environment [8]. Given the persistent nature of these herbicides, it is important to investigate the factors affecting their leaching and degradation in soil. There is no existing information on imidazolinones fate in the presence of biochars in soils. Therefore, the objective of this study was to investigate the effects of designed biochars application on the behaviour and fate of imazapyr in soil. Empty fruit bunch (EFB) of oil palm and rice husk (RH), the locally-available agricultural wastes, were used as sustainable and cheap pyrolysis materials in this study.

2 Materials and methods

2.1 Soil, biochars, and chemicals

Soil sample (0–15 cm depth) was collected from paddy fields located in the federal land consolidation and rehabilitation authority (FELCRA) Seberang Perak area which is the biggest Malaysian paddy estate applying CPS. The sample was taken from the plots with no previous history of imidazolinones application. The air-dried sample was passed through a 2-mm sieve and then characterized using standard methods as were applied in our previous works [39]. The soil texture was clay loam (37.99% clay). Its pH, total organic carbon, and CEC values were 6.36, 0.99%, and 12.55 cmol(+)/kg, respectively.

The collected EFB and RH biomasses were dried and pyrolyzed using a pure nitrogen-purged tube furnace (OTF-1200X-80, USA). The conditions were temperature of 300 °C, a heating rate of 3 °C/min, and reaction retention time of 1 h and 3 h for EFB and RH biochars, respectively. These conditions in which the maximum capacity for stabilization of imazapyr was achieved had been determined through our previous study [39]. The characteristics of the designed biochars are presented in Table 2.

Table 2 Physico-chemical characteristics of the designed EFB and RH biochars [39]
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The analytical standard of imazapyr (purity > 99.9%) was purchased from Sigma-Aldrich (Seelze, Germany). The herbicide stock solution (1000 mg/L) was prepared in a background electrolyte of 0.01 M calcium chloride (CaCl2) and 200 mg/L mercury chloride (HgCl2). Chemicals were purchased from Fisher Chemical (UK). Vacuum extraction manifold assembly and solid-phase extraction (SPE) cartridges were bought from Agilent Technologies (USA). Ultra-pure water was provided using a Millipore Milli-Q® system. High-performance liquid chromatography (HPLC) grade solvents were purchased from Merk (KGaA, Darmstadt, Germany).

2.2 Sorption-desorption experiment

The designed EFB and RH biochars were separately mixed into the soil (1.0% w/w) in plastic containers. Non-amended soil was also evaluated as control. Each media was wetted with de-ionized water to its field water holding capacity (FC) (21.61%) and then incubated at 25–30°C. The moisture contents were maintained constant during the experiment period. Sorption capacities of the media were measured just after incorporation of the biochars to the soil and at 10-day intervals until the readings reached constant amounts (70 days). Batch equilibrium technique [32] was applied for the sorption experiment during which different concentrations (0, 0.5, 1, 2, 4, 6, 8, and 10 mg/L) of the imazapyr solution (20 mL) were separately added to 10 g of each media in centrifuge tubes. The suspensions were shaken (250 rpm, 24 h, and 25 °C) and centrifuged (7000 rpm, 10 min). The supernatants were filtered by a polyvinylidene difluoride (PVDF) membrane filter (0.45 μm of pore size) and then analysed for the herbicide concentration. The conventional single-step decant-refill technique was applied for the desorption experiment [40]. After the sorption experiment, 10 mL of background solution was added to the remaining slurry of each tube to achieve the initial volumes (20 mL) of the herbicide solution. Then, the suspensions were shaken, centrifuged, filtered, and analyzed.

2.3 Leaching experiment

Leaching columns (24 cm height × 10 cm diameter) accompanied with a rainfall simulator were used to perform this experiment. The columns were filled with gravel (3 cm) then packed with each media uniformly to a height of 15 cm.

The background solution flow (6.8 mL/minute) was conducted from a reservoir to the top of each column through tubes by a peristaltic pump. The soil columns were saturated for 2 h. After an incubation period (48 h), the herbicide was applied to the media in an initial herbicide concentration of 0.2 μg/g in soil [5]. The columns were then subjected to the constant down-ward flows of background solution simulating the highest rain intensity in the area [34]. The leachates were collected in 200 mL fractions. Sample collection was continued until the herbicide concentration reached the lowest amount after 7 soil pore volumes (4000 mL eluted leachate). The soil in each column was divided into 2 equal parts in length (each 7.5 cm) and the amount of remaining herbicide in each section was also determined.

2.4 Photolysis and bio-degradation experiments

Photo-degradation of imazapyr herbicide was evaluated on the surface of the soil in the presence and absence of each EFB and RH biochar (1.0% w/w) for 70 days. 30 g of each biochar-free and biochar-amended soil was placed into glass Petri dishes (2.5 cm height × 14 cm diameter) and spread (2 mm depth). After autoclave-sterilizing, each media was uniformly spiked with the herbicide (0.2 μg/g). The solar irradiation was simulated using a high-pressure mercury vapour fluorescent lamp (400 W/m2) located at 30 cm above the samples [7]. A series of experimental units were kept in the dark at 25 °C to evaluate the possible impacts of chemical and bio-degradation throughout the experiment.

To conduct the bio-degradation experiment, 10 g dry weight of each media was placed in dark glass containers and moisturized to FC with de-ionized water. There were two sets for each media. One set was sterilized through autoclaving. The other set was non-sterilized and the containers were covered with perforated parafilm sheets. Each media was homogeneously spiked with imazapyr herbicide to give an initial concentration of 0.2 μg/g and then were incubated with constant moisture contents in darkness (25 °C) for 70 days. For each degradation experiment, the sample collection (10 g) and analysis were conducted at the intervals of 0, 7, 14, 28, 42, 56, and 70 days after the herbicide application.

2.5 Herbicide extraction and analysis

To extract the herbicide from leachates, the Bond Elut-PPL cartridge was conditioned with dichloromethane, methanol, and then ultra-pure water (pH = 2) (each 6 mL). Five liters of each aqueous sample (pH = 2) was loaded into the conditioned cartridge. The herbicide was then eluted by 6 mL dichloromethane. The solvent was evaporated to near dryness by the N2 evaporator. Then, an aliquot of 4 mL isopropanol was added and evaporated to get the volume of 1 mL.

To extract the herbicide from the media, each sample was mixed with 0.5 M sodium hydroxide in a ratio of 1:4 (soil: sodium hydroxide) and shaken for 1 h. The suspension was centrifuged (6000 rpm, 10 min) and then filtered through a glass fiber filter (GF/C, pore size 1.2 μm, 70 mm). C18 cartridge was primed with 5 mL methanol followed by 5 mL ultra-pure water and SCX cartridge was conditioned with 5 mL hexane, then 5 mL methanol followed by 5 mL ultra-pure water. Then, the soil extract (pH = 2) was passed through C18 cartridge. To elute the herbicide from C18 cartridge, the cartridge was stacked on top of SCX cartridge and the herbicide was eluted by 20 mL methanol:ultra-pure water (1:1). C18 cartridge was discarded and SCX cartridge was washed with 5 mL ultra-pure water. Elution of the herbicide from SCX cartridge was done by 20 mL 0.05 M phosphate buffer (pH = 2). The herbicide was partitioned by three vigorous washes with 15 mL dichloromethane. The solvent was evaporated to near dryness and then the herbicide was re-dissolved in 1 mL isopropanol.

Herbicide concentrations in the solutions were analyzed using HPLC (Agilent 1100 series). ZORBAX SB-C18 (150 mm × 4.6 mm, 5 μm particle size) was used as the detection column. Acetonitrile to 1.0 % acetic acid in ultra-pure water (35:65) was used as isocratic mobile phase with a flow rate of 1 mL/minute. The retention time for imazapyr herbicide was 2.3 minute.

2.6 Data analysis

The amount of the herbicide adsorbed on each media after the adsorption process Sads (mg/kg) was calculated by equation 1.

$$ {S}_{\mathrm{ads}}=\left({C}_0-{C}_{\mathrm{ads}}\right)\times V/M $$
(1)

where C0 (mg/L) and Cads (mg/L) are initial and final concentrations of the herbicide in the solution, respectively. V (mL) is the solution volume and M (g) is the mass of soil.

The amount of herbicide remaining adsorbed on the soil after the desorption process (Sdes (mg/kg)) was calculated by equation 2.

$$ {S}_{\mathrm{des}}={S}_{\mathrm{ads}}\hbox{--} \left[\left({C}_{\mathrm{des}}\hbox{--} 1/2\times {C}_{\mathrm{ads}}\right)\times V/M\right] $$
(2)

where Cdes is the herbicide concentration in the solution at desorption equilibrium.

To describe the sorption process, linearized form of the Freundlich model was used like the following equation [32]:

$$ \log\ {S}_{\mathrm{ads}}=1/n\times \log\ {C}_{\mathrm{ads}}+\log\ {K}_f $$
(3)

where Kf ((mg/kg)/(mg/L)1/n) is the Freundlich sorption coefficient indicating the soil adsorption capacity and n is a measure of the sorption intensity.

The hysteresis index (HI) was calculated according to equation 4 [40].

$$ \mathrm{HI}=\left({S}_{ads}\hbox{--} {S}_{des}/{S}_{ads}\right) $$
(4)

Breakthrough curves related to leaching of the herbicide in the media were obtained using the Excel® spreadsheet program.

A first-order model was used for the determination of the herbicide degradation rate constants (K) (per day) as shown in equation 5.

$$ \ln\ {C}_t=\ln\ {C}_0-K\times t $$
(5)

where Ct is the herbicide concentration at time t, C0 is its initial concentration, and t is the time.

After the determination of K values, the herbicide half-life was calculated using equation 6.

$$ {t}_{1/2}=\ln\ 2/K $$
(6)

Each experimental run was performed in triplicate. ANOVA was conducted by a statistical analysis system (SAS) (version 9.1 for Windows; SAS Institute, Inc. Cary, NC, USA). Duncan’s multiple range tests were applied to examine the significance between the sets of means. Correlation coefficients between the media Kf and each total amount of the herbicide leached, remained, and herbicide half-life was determined with a 95% confidence level.

3 Results and discussion

3.1 Adsorption-desorption of imazapyr herbicide

Imazapyr showed a high affinity to bind to the biochars. Table 3 presents the parameters related to the adsorption of imazapyr on the non-amended and amended soils. Data were fitted well to the Freundlich model with high R2 values (≥ 0.93). The value of soil Kf was 1.91. The value was increased (2.34-folds) in soil amended with EFB biochar and reached 6.38. The effect of RH biochar was found to be significantly lower (Kf = 4.74, 1.48-folds) than EFB biochar. Data obtained from the desorption experiments demonstrated that the sorption process was more reversible in non-amended soil compared to biochars amended soils (Table 3). HI, the value of soil amounted to 0.16. Among the biochars, a higher value was related to RH biochar (0.02) which had a lower affinity for the herbicide rather than EFB biochar (0.01).

Table 3 Freundlich parameters for adsorption and desorption of imazapyr on non-amended and EFB and RH biochars-amended soils
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Figure 1 presents the changes in Kf and CEC values of non-amended and amended soils during 70 days’ incubation experiment. Generally, capacities of the amended soils in sorption of the herbicide showed increasing trends during the first weeks of incubation. After that, the increases were continued insignificantly. The changes were more considerable in EFB biochar-amended soil. The maximum imazapyr removal was achieved after 50 days as the Kf value increased from 6.38 to 8.97 (40% increases). In RH biochar-amended soil, 37% increases in Kf value was observed. Changes in sorption capacity of the free soil were negligible and were less than 10% for the herbicide.

Fig. 1
figure 1

Changes in Kf (a) and CEC (b) values of non-amended and biochars-amended soils during incubation period

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Post-pyrolysis changes in sorption capacities of biochars can be controlled by soil factors, climatic conditions, and herbicidal properties [21]. Knowing about these changes is necessary for understanding the behaviour of biochar as a sorbent in the environment.

The investigation of changes in CEC values (Fig. 1b) revealed enhancement of the exchange capacities in presence of biochars in soils during the incubation period. It was observed that the initial CEC was 19.68 cmol(+)/kg in the soil mixed with EFB biochar and it reached 22.26 cmol(+)/kg at the end of the incubation period (13%). The increase was from 18.36 to 20.60 cmol(+)/kg in the RH biochar-amended soil (12%). CEC value of the non-amended soil stayed constant at about 12.5 cmol(+)/kg during the incubation time. Similar results were reported by Brodowski et al. [6] who indicated once biochar is applied to the soil, very fast oxidation and hydrolysis reactions happen on its surface during the first several weeks of incubation. However, they can be reduced later by mineral interactions and saturation of biochar active sites in the soils. During oxidation, the oxygen-containing groups like carboxylate and phenolate, increase on the external surface of biochar which can cause specific interaction between biochar and polar organic molecules such as imidazolinone herbicides. The chemisorption of oxygen increases biochar CEC and its surface hydrophilicity which influence biochar sorption behaviour [23].

3.2 Leaching of imazapyr

Breakthrough curves of imazapyr in non-amended and biochars-amended soils are presented in Fig. 2. Generally, in the biochar-free soil, imazapyr was leached out earlier in comparison with the biochars-amended soils. The maximum value of the relative concentration of imazapyr leached from biochar-free soil was 0.02 which was obtained after collection of 2200 mL leachate from the column, while the maximum values for EFB and RH biochars-amended soils were 0.0072 and 0.0047, respectively which were obtained at higher cumulative volumes of leachates, 3400 mL for EFB biochar and 2600 mL for RH biochar-amended soils. These results showed that the biochars applications delayed the leaching of imazapyr from the soils columns.

Fig. 2
figure 2

Breakthrough curves of imazapyr herbicide in non-amended and EFB and RH biochars-amended soils

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Based on the data are shown in Table 4, the highest percentage of imazapyr was leached out from non-amended soil (14.2%), followed by RH biochar- (4.0%) and EFB biochar-amended soils (2.8%). In all media, the higher amount of the herbicide was retained in the top 7.5 cm soil depth. Comparison of the amounts of remained herbicide showed that the soil amended with EFB biochar retained the highest amounts of the herbicide (97.1%) followed by RH biochar-amended soil (95.6%) and the lowest amount was measured in the biochar-free soil (85.3%). Amounts of the imazapyr herbicide retained in the soils were inversely related to the amounts of herbicide leached out from each column.

Table 4 Percentages of herbicide leached out from soil columns and herbicide remained in different depth of soils
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The results shown in Table 5 indicated that the amounts of the herbicide leached out from the soils were negatively correlated with the media sorption capacities. As Kf values increased, the percentages of the herbicide leached out from the columns reduced. The correlation between Kf values and the amounts of the herbicide that remained in the soils was positive, indicating that mobility of herbicide decreased with increasing media sorption capacities.

Table 5 Pearson’s r and level of significance (p value) for correlations between the media Kf values and the percentages of leached out and remained herbicides
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According to the data reductions of the herbicide leaching in the presence of the biochars was promising a finding to decrease the environmental threats of the applied herbicide. Many studies have also reported the enhanced sorption of pesticides in biochar-amended soils that resulted in a reduction of their leaching when compared to biochar-free soils [1, 39]. During a study conducted by Li et al. [24], low-temperature wood biochar was evaluated as a sorbent to decrease the mobility of 2,4-D and acetochlor herbicides using leaching columns. According to their results, the biochar had the potential to control the leaching of the herbicides significantly and could reduce the amounts of leached herbicides by half. In another attempt, Hagner et al. [16] showed that birch wood-derived biochar can decrease the leaching rate of glyphosate herbicide in soil. So, the addition of biochar can be considered as an effective strategy to decrease the impact of pesticides residue on the ecosystem.

3.3 Photolysis and bio-degradation of imazapyr

Photo-degradation reaction of the herbicide in all media followed the first-order kinetics (R2 ≥ 96%) (Table 6). Figure 3 shows the first-order rate plots of the process. Photolysis of imazapyr was reduced by biochars amendment, with rate constant reducing from 0.018 per day (a half-life of 38.5 days) in non-amended soil to 0.015 per day (a half-life of 46.3 days) in RH biochar-amended soil and 0.013 per day (a half-life of 53.3 days) in EFB biochar-amended soil. During the experiment period, the changes in concentrations of the herbicide in dark-incubated samples, if any, were very minimal and under the detection limits. This result plus the preliminary findings from the hydrolysis experiment indicated that any changes in the herbicide concentrations were only related to photo-degradation reaction and were not caused by bio-degradation or hydrolysis processes.

Table 6 First-order rate constants (K), half-lives (t1/2), and correlation coefficients (R2) of photo-degradation process for imazapyr herbicide
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Fig. 3
figure 3

First-order rate plots of photo-degradation process for imazapyr herbicide

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As a soil amendment, EFB biochar had a greater effect on decreasing the herbicide photo-degradation rate compared to RH biochar applied in the same dosage (1.0% w/w). This can be described by the higher ability of EFB biochar to stabilize the imidazolinone herbicides which is indicated by the greater Kf values of EFB biochar-amended soil for adsorption of the herbicide (Table 3). The higher surface hydrophilic nature, polarity index, and CEC made EFB biochar more efficient in the sorption of the herbicide (Table 2). The half-life of the herbicide was directly correlated to the media sorption capacity (Pearson’s r = 0.9810, p < 0.0190). As the Kf value increased, the half-life of the herbicide in the media enhanced mostly due to the reduction of its availability to the solar radiant.

It has been shown that low sorption of imidazolinone herbicides to soil can enhance photo-degradation processes. Curran et al. [11] investigated photolytic reactions of several imidazolinone herbicides on different soils and found that photolysis of these herbicides occurred readily in the soil having the lower sorption capacity because of the greater availability of the herbicides’ molecules to photochemical reactions. Similar results were obtained by Elazzouzi et al. [14] who indicated that the presence of organic matters in soil could significantly reduce the rate of imazethapyr photo-degradation in the environment.

Acceptable fits to first-order reaction kinetics (≥ 93%) were obtained for bio-degradation of imazapyr herbicide in the media over the incubation time of 70 days (Table 7). First-order rate plots of the process for the herbicide are presented in Fig. 4. Applications of the optimized EFB and RH biochars to the soil reduced bio-degradation of the imazapyr herbicide during the first weeks of the experiment period. After that, the herbicide bio-degradation accelerated significantly (p < 0.05) in the presence of the biochars in soil. Rate constants and half-life of the herbicide in the biochar-free and biochar-amended soils are shown in Table 7. The rate constant of bio-degradation in the free soil was 0.015 per day with a half-life of 46.2 days. The addition of EFB biochar in soil increased the rate constant to 0.024 per day with a half-life of 28.8 days. Further increase was obtained in the RH biochar amended-soil with the highest rate constant of 0.026 per day and the shortest half-life of 26.6 days. Concentrations of herbicides incubated in the sterilized condition remained unchanged during the incubation period indicating that the bio-degradation process was the only process responsible for the disappearance of the herbicide in the non-sterilized soils. This result also confirmed the preliminary findings of this study on the resistance of the imidazolinone herbicides to hydrolysis degradation. The capability of the optimized biochars in adsorbing the herbicide was the main reason for the decrease of their bio-degradation over the first weeks of incubation. Having the higher adsorption capacity, EFB biochar remained more effective than RH biochar in immobilizing the herbicide and thus reducing its bio-availability for soil microbes. Therefore, in comparison with RH biochar, EFB biochar showed smaller influences on the disappearance of the herbicides in the soil during the incubation time.

Table 7 First-order rate constants (K), half-lives (t1/2), and correlation coefficients (R2) of bio-degradation process for imazapyr herbicide
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Fig. 4
figure 4

First-order rate plots of bio-degradation process for imazapyr herbicide

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Many studies have also indicated the enhanced pesticides bio-degradation rate with the presence of biochars in the soil environment. Biochar can stimulate the activity of a variety of important soil microbes by providing them with soluble nutrients, water, and gases. Pores in biochar structure can also be suitable habitat for soil micro-organisms [29, 37]. Based on Zhang et al. [41], the application of biochars produced from maize straw and pig manure at 300 °C enhanced micro-degradation of thiacloprid pesticide due to increasing the abundance of soil microbes.

4 Conclusions

The designed biochars significantly increased soil capacity in sorption of imazapyr (EFB biochar = 2.34 and RH biochar = 1.48-folds) and decreased the herbicides reversibility. Capacities of biochars-amended soils in sorption of the herbicide showed an increasing trend during an incubation period due to enhancement of the biochars CEC values. Leaching of imazapyr can be a risk to the aquatic environment due to high persistence and biological activity. This study showed that application of the biochars can reduce herbicide leaching. So that the lowest percentage of the herbicide was leached out from EFB biochar-amended soil (2.8%). Higher amounts of the herbicides were retained in the top part of the soil columns and the biochars-amended soils retained the higher percentages of the herbicide (> 95%). Degradation experiments demonstrated that the photolysis rate of imazapyr in soil reduced in the presence of the biochars particularly EFB biochar. The half-life increased from 38.5 days in free soil to 53.3 days in the biochar-amended soil. In contrast, imazapyr bio-degradation accelerated significantly with the presence of biochars. RH biochar showed a greater effect. The half-life was 46.2 days in the free soil. The addition of RH biochar decreased it to 26.6 days.

In a conclusion, the designed EFB and RH biochars have the potentials to be used as eco-friendly and cost-effective bio-sorbent in the soil to reduce the threats of imidazolinone herbicides and protect the environment against their pollution. Further studies are suggested for a better understanding of biochar effects on the fate of pesticides in short- and long-term conditions.