The concentrations of fenobucarb and 2-sec-butylphenol in the soil phase, aqueous phase, and total residual amount in the soil-water systems at pH 7 are shown in Fig. 1. The residual amount of fenobucarb or 2-sec-butylphenol was the total amount in both the solid and aqueous phases of the soil-water system.
The fenobucarb concentration in the soil phase rapidly decreased during the first 2 h and then decreased gradually from 2 to 20 h (Fig. 1a). Desorption of fenobucarb from spiked soil to water reached equilibrium within 2 h and remained nearly constant until 20 h (Fig. 1b). The concentrations of fenobucarb in water, soil, and the total residual amount in soil-water systems after 20 h were 0.317 mg kg-1, 0.379 mg L-1, and 16.4 µg, respectively. Next, the fenobucarb concentration decreased rapidly in both the water and soil phases, with more than 50% of the fenobucarb dissipating after 48 h. A similar observation was reported in a previous study (Alam et al., 2006). Fenobucarb was degraded by 18.7%, 66.9%, 72.5%, and 87.7% after 24, 48, 72, and 144 h, respectively. Meanwhile, 2-sec-butylphenol was found in both the water and soil phases of the soil-water system. The 2-sec-butylphenol concentration first increased from 0 h to 20 h and then decreased. The 2-sec-butylphenol concentrations were 0.017 mg kg-1, 0.016 mg L-1, and 0.702 µg in the soil, water phase, and total amount in the soil-water system, respectively, after 20 h. The 2-sec-butylphenol concentration rapidly decreased from 20 h to 24 h and gradually decreased in both the water and soil phases. After 48 h, the concentrations of 2-sec-butylphenol were 0.01 mg kg-1; 0.01 mg L-1, and 0.328 µg in the soil, water phase, and total amount in the soil-water system, respectively.
The continuous disappearance of fenobucarb with the appearance of 2-sec-butylphenol indicated that the fenobucarb-acclimatised microorganisms were capable of degrading fenobucarb in the soil-water system. Bacteria use fenobucarb as a source of carbon and energy, and the initially hydrolysed product of fenobucarb is 2-sec-butylphenol, after which this metabolite is continuously metabolised to carbon dioxide and water (Kim et al., 2014). This observation was similar to that reported by Kim et al. ( 2014), which indicated that 2-sec-butylphenol increased over a period of 20 h in the culture medium and was completely degraded after 27 h of incubation. In addition, previous studies have shown that the soil is a major source of microorganisms that can degrade pesticides, and the microorganisms in the soil use carbamate insecticides and some of their metabolites as a source of carbon and nitrogen for growth (Baron and Merriam, 1988; Caracciolo et al., 2002; Ou et al., 1988; Sahoo et al., 1998). The results of this study showed that the paddy soil in the study area contained microbial communities that were capable of decomposing fenobucarb.
The fenobucarb concentration gradually decreased from 2 to 24 h in both the soil and water phases of the soil-water system and then rapidly decreased to 72 h. Additionally, 2-sec-butylphenol was not detected in the aqueous phase. The continuous disappearance of fenobucarb and absence of 2-sec-butylphenol observed in the experiments indicated that the degradation of fenobucarb occurred, including biodegradation and abiotic (hydrolysis) degradation.
The residual amount of fenobucarb decreased rapidly, and more than 50% of the fenobucarb disappeared after 48 h in all experiments except for the experiment with SDS solution (decreased by nearly half the concentration after 120 h (Fig. 1c)). After 120 h, the residual amounts of fenobucarb were 2.47, 0.777, 0.135, and 8.54 µg with pH 7, pH 9, DOC, and SDS solutions, respectively.
The disappearance of fenobucarb in both the soil and water phases occurred fastest in the experiment with pH 7, followed by those with pH 9, DOC, and SDS solutions (Table 1). After 48 h, the degradation percentages of fenobucarb at pH 7, pH 9, and DOC solutions were 72.5, 69.6, and 58.9%, respectively. Meanwhile, only 45.8% fenobucarb was dissipated in the SDS solution. In most samples, fenobucarb degraded in 120 h, in which the degradation rate of fenobucarb was fast in the first 72 h and then decelerated. In the experiments with pH 7, pH 9, and DOC solutions, 87.7, 96.1, and 99.3% of fenobucarb was degraded, respectively, and only 55.6% fenobucarb was degraded in the experiment with SDS after 120 h (Fig. 2d and Table 1).
Pesticide biodegradation is a vital process that controls the fate of pesticides in the soil. Rapid degradation of fenobucarb in the soil-water system was observed in the experiment with pH 7 solution and at a temperature ranging from 20°C to 25°C because these were favourable conditions for bacterial growth.
In the experiment with the pH 9 solution, 2-sec-butylphenol was only observed starting from 48 h in the soil phase with the highest concentration of 0.029 mg kg-1 and then gradually decreased at a concentration of 0.014, 0.01, and 0.01 mg kg-1 at 72, 96, and 120 h, respectively. In this experiment, in addition to decomposition by bacteria present in the soil, fenobucarb was hydrolysed under basic conditions, which caused fenobucarb to disappear rapidly. This observation was also consistent with previous studies, which indicated that carbamates were resistant to hydrolysis at neutral pH values (5–8), but underwent rapid hydrolysis under alkaline conditions (pH 9) (Bobé et al., 1998; Faust and Gomaa, 1972).
The presence of DOC in the aqueous solution enhanced desorption and microbial activity in the soil, and increased the biodegradation efficiency of fenobucarb in the soil-water system. (Table 1). This agreed well with the results reported by El-Sharouny (2015), which showed that the soil is amended with organic matter, and microbial activity of microorganisms in the soil will increase with the increase in organic compounds in water. In addition, when organic matter is added to the soil, these organic substances enhance the biological activity of the microorganisms in the soil and increase the biodegradability of pesticides (Suhail and Fahmi, 2020). It also enhances the desorption capacity of pesticides from soil into water (Gonzalez et al., 2010; Luo et al., 2009; Maillard et al., 2011).
Table 1
Desorption and degradation percentage of fenobucarb in pH 7, pH 9, DOC (25 mg L-1), and SDS (1 cmc) solutions in the soil-water system
Time (h) | Desorption percentage (%) | | Degradation percentage (%) |
pH 7 | pH 9 | DOC | SDS | | pH 7 | pH 9 | DOC | SDS |
24 | 70.6 | 67.9 | 56.2 | 47.3 | | 20.5 | 26.2 | 31.9 | 40.2 |
48 | 27.2 | 28.9 | 37.6 | 45.3 | | 72.5 | 69.6 | 58.9 | 45.8 |
72 | 14.3 | 17.1 | 19.4 | 44.4 | | 85.6 | 82.2 | 80.1 | 46.7 |
96 | 13.5 | 4.41 | 4.95 | 44.0 | | 86.2 | 95.2 | 94.7 | 49.1 |
120 | 12.0 | 3.56 | 0.35 | 40.9 | | 87.7 | 96.1 | 99.3 | 55.5 |
The presence of SDS in the solution enhanced the desorption of fenobucarb, which reached a maximum value of 0.267 mg L-1 with a desorption percentage of 53.4% in the first 2 h. Subsequently, the desorption ability gradually decreased and continued slowly after 24 h. Fenobucarb was continuously desorbed from the soil, and its concentration declined as biodegradation continued in the soil-water system (Fig. 1 and Table 1). Consequently, the presence of SDS in the solution simultaneously affected desorption and biodegradation, and enhanced the desorption and degradation of fenobucarb. Pesticides are adsorbed onto the hydrophobic cores of surfactant micelles in solution because of the increase in their concentration in the aqueous phase (Kile and Chiou, 1989; Wang and Keller, 2008). When SDS was added to the solution, the contact between microorganisms and pesticides increased owing to the reduced interfacial tension, which led to an increase in the biodegradation efficiency of fenobucarb.
The presence of DOC and SDS in the solution enhanced the desorption of fenobucarb from the soil into water. Therefore, microorganisms had more substrate to transform fenobucarb, which led to increased biodegradation of fenobucarb (Aronstein et al., 1991) (Fig. 2 and Table 1). However, the degradation of fenobucarb in the SDS solution was inhibited compared with that of the pH 7, pH 9, and DOC solutions because SDS solution may be preferentially utilised by microorganisms (Kim et al., 2014), and fenobucarb would have a competitive effect with SDS. This inhibition may be a result of a physical-chemical effect of the surfactant micelles interfering with substrate transport into the cell or with the activity of enzymes and other membrane proteins of the cell (Aronstein and Alexander, 1992; Aronstein et al., 1991). This inhibition may also be a result of the limited bioavailability of micellized fenobucarb, as it was reported that the addition of surfactant increased the degradation of polycyclic aromatic hydrocarbons; however, the bioavailable fraction of micellar-phase phenanthrene decreased with an increase in surfactant concentration (Guha et al., 1998).