Effect of pH, Dissolved Organic Carbon, and Sodium Dodecyl Sulfate to Degradation of Fenobucarb Insecticide in Paddy Soil-Water System

ABSTRACT In paddy rice farming systems, the degradation of insecticides from the soil reduces their occurrence as well as their levels in surface and groundwater. Degradation of insecticide fenobucarb in the paddy soil-water system was carried out by surface soil collected from paddy rice in the Hue province of Vietnam. Experiments were conducted using a batch technique, the fenobucarb-spiked paddy soils were shaken end-over-end with solutions of pH 7, pH 9, 25 mg L−1 DOC, and 1 critical micelle (cmc) sodium dodecyl sulfate (SDS). After each period (24, 48, 72, … 144 hours), the soil solutions were separated into the aliquot and the solid phase to analyze fenobucarb and intermediate metabolic products 2-sec-butyl phenol. The decrease of fenobucarb and the appearance of (2-sec-butyl-phenol) were the result of hydrolysis and biodegradation by fenobucarb-acclimatized microorganisms that were capable of degrading fenobucarb in the paddy soil. The degradation of fenobucarb in both the soil and water phases in the paddy soil-water system was enhanced in the experiment with pH 7, followed by those with pH 9, 25 mg L−1 DOC, and 1 cmc SDS. With pH 9, in addition to decomposition by bacteria present in the soil, fenobucarb was hydrolyzed under basic conditions. The presence of 25 mg L−1 DOC and 1 cmc SDS in the aqueous solution increased desorption and microbial activity in the soil and increased the biodegradation efficiency of fenobucarb in the paddy soil-water system. However, the degradation of fenobucarb in the 1 cmc SDS solution was inhibited because SDS solution may be preferentially utilized by microorganisms, and fenobucarb would have a competitive effect with SDS. Irrigation water for paddy fields often receives untreated domestic and industrial wastewater that may contain multiple compounds such as DOC and SDS. Therefore, it contributes to the elimination of fenobucarb insecticide in the paddy soil-water system.


Introduction
Rice is the main cereal crop in Vietnam, cultivated in irrigated rice fields with approximately 46% of the agricultural land.Fenobucarb (2-sec-butylphenyl-N-methylcarbamate) is an insecticide applied in large quantities in the rice-growing regions of Vietnam and other countries worldwide to control planthoppers and leafhoppers in rice plants (Berg 2001;Chapalamadugu and Chaudhry 1992;MARD 2015).This insecticide causes environmental problems as a result of its transport from rice paddy fields via effluent waters to the surface and groundwater (Phong et al. 2011).Fenobucarb reaches the soil after application to paddy rice fields and is then decomposed by microorganisms, hydrolysis, or photodegradation, which are the main pathways for removing insecticide residues from the soils (Guo et al. 2000).The degradation of carbamates in soil is affected by several factors, including volatility, leaching, soil moisture, absorption, desorption, pH, temperature, surfactant, photodecomposition, microbial degradation, and soil type, as well as their intrinsic properties (Ogle and Waren 1954;Yu, Zhu, and Zhou 2007).Bacteria in natural systems can degrade insecticide residues in the soil by co-metabolizing insecticides for nutrients and energy, thus reducing residuals in the soil and the environmental risk (Fenner et al. 2013;Park et al. 2003).Amine, alcohol, phenol, carbon dioxide (CO 2 ), and water are the final degradation products of the decomposition process of carbamate insecticides (Allan et al. 2012;Liu et al. 2003).Ueji and Kanazawa (1979) reported that the disappearance rate of fenobucarb is proportional to its concentration in the soil.In addition, fenobucarb was metabolized by 45 isolated fenobucarb-degrading bacteria from paddy soils (Kim et al. 2014) and produced an intermediate metabolite, 2-sec-butylphenol.This intermediate metabolite was also found to be degraded by the isolated bacteria.
In aquatic media, pH plays an important role in the degradation of carbamate insecticides because of hydrolysis and photolysis reactions.Carbamates are stable at neutral pH (5-8); however, they undergo rapid hydrolysis in an alkaline medium (pH 9) (Bobé et al. 1998;Faust and Gomaa 1972).
Surfactants have complex effects on the activity of insecticides in soil.Because of the special surface properties of anionic and nonionic surfactants, they can enhance the solubilization of pesticides and their participation in biological activity in the soil (Cheng and Wong 2006;Cheng, Lai, and Wong 2008;Gonzale et al. 2010) and act as a catalyzer during degradation processes (Haigh 1996).Dissolved carbon from organic soil resources enhances the desorption of organic contaminants and microbial populations and their activity in amended soils (Cheng and Wong 2006;El-Sharouny 2015;Gonzale et al. 2010).
Irrigation water for rice fields in Vietnam comes from irrigation canals and rivers, where they often receive untreated domestic and industrial wastewater (Cordero et al. 2012;Rao et al. 2011;Zhou et al. 2007).These irrigation water sources may affect to sorptiondesorption and decomposition of pesticides because they contain suspended organic matter (e.g.organic matter, surfactants, and organic acids) (Allan et al. 2012;Mo et al. 2008;Trinh et al. 2018).However, studies on the degradation of carbamate insecticides in paddy soils under effect of above factors still are limited.Therefore, in this study, fenobucarb-spiked soil was amended with pH 7, pH 9, DOC, and SDS solutions, and residual fenobucarb and 2-sec-butylphenol were analyzed in water and soil after each period of decomposition.This study aimed to (1) assess the effect of different environmental factors (pH, DOC content, and SDS) on the degradation of fenobucarb in paddy soil-water systems and (2) determine the intermediate metabolic products (2-sec-butylphenol) as a result of degradation.

Chemical
Stock solutions of fenobucarb and 2-sec-butylphenol with≥96% purity were purchased from Dr. Ehrenstorfer (Augsburg, Germany).A solution of anionic surfactant SDS concentration 1 critical micelle (cmc) was purchased from Merk (Darmstadt, Germany, purity>98%) with 1 cmc of 2.4 g L −1 SDS (Mukerjee and Mysels 1971).The pH solution was separately adjusted to 7 and 9 using 0.1 M hydrochloric acid or 0.1 M sodium hydroxide, respectively, and 0.1 M tris-buffer (tris-hydro methylamino methane) was procured from Merck, Sigma Aldrich.The organic solvents used were of analytical and GC grade.A DOC solution with a concentration of 25 mg L −1 was prepared from a 765 mg L −1 DOC solution isolated from Norway spruce forest soil with fulvic acid properties (Strobel et al. 2001;Trinh et al. 2017).The DOC concentration was determined prior to use.A silica gel cartridge (Sep-Pak VAC 2 g/12 mL; Waters Associates, Milford, MA, USA) was used to clean the extracts.

Paddy soil sample
Paddy soil samples were collected from rice fields (0-15 cm depth) in Huong Toan, Hue, Vietnam.The paddy soil sample was air-dried, ground, sieved through a 2 mm sieve, placed in polyethylene bags, and stored at −4°C.The soil was classified as textured silt clay loam (classification according to the WRB 2014) (Schad 2016), which comprised 29% clay, 8% sand, 63% silt.The soil had organic carbon content, pH (KCl) and cation exchange capacity was (45 meq/100 g) in the soil were 2.02%, 4.2 and 11.9, respectively.(Trinh et al. 2020).

Fenobucarb-spiked paddy soil sample
The fenobucarb concentration in the paddy soil samples was determined based on the fenobucarb dose applied for agriculture and its residue in the paddy soil.Alam et al. (2006) reported that the mean residue in paddy soil immediately after application on the first day was 2.3 mg kg −1 when the fenobucarb dose was 1500 g a.i/ha (Alam et al. 2006).Therefore, a fenobucarb concentration of 5 mg kg −1 dw in the spiked paddy soil was used in the present study and prepared according to previous studies (Zhou andZhu 2007a, 2007b).In brief, fenobucarb (5 mL, 100 mg L −1 in acetone) was spiked into a 5 g paddy soil aliquot (5 g soil in 20 mL acetone), and then acetone in the paddy soil aliquot was evaporated for 24 h in a fume hood.Finally, the spiked paddy soil was thoroughly mixed with full paddy soil samples (up to 100 g) to obtain the initial fenobucarb-spiked paddy soil samples (5 mg kg-−1 dw).

Desorption experiments in paddy soil-water systems
Desorption experiments in the paddy soil-water systems were conducted using a batch technique following the study of Trinh et al. (2017).Four grams of fenobucarb-spiked soil and 40 mL of each solution, 25 mg L −1 DOC, 1 cmc SDS, and pH values (7 and 9), were contained in 50 mL brown glass tubes with Teflon-lined screw caps.These tubes were shaken end-over-end at 25 ± 1°C (150 rpm) in the dark to avoid the photooxidation of fenobucarb.After 2,4,8,12,16,20,24,48,72,96,120, and 144 h of reaction, the sample was centrifuged at 2000 rpm for 15 min to separate the aliquot from the solid phase.

Extraction and analytical procedure
Fenobucarb and 2-sec-butylphenol in the aliquot phase were extracted as described by Kadokami, Jinya, and Iwamura (2009), with some modifications.Briefly, an aliquot (40 mL) was spiked with NaCl (0.5 g) and extracted twice using the liquid-liquid extraction method with dichloromethane (10 mL DCM).The extract was dried with nitrogen gas to approximately 5 mL, and hexane (10 mL) was spiked into the extract, re-concentrated to exactly 1 mL, and stored in a vial at −20°C until gas chromatography-mass spectrometry (GC/MS) analysis.Residual fenobucarb and 2-sec-butylphenol in the soil were also treated as previously described by Kadokami et al. (2012) and Trinh et al. (2020), with some modifications.Briefly, the soil samples were extracted by ultrasonication with 30 mL acetone (three times) for 20 min.The combined extract was extracted in the same manner as in the aliquot phase.Finally, 1 mL of the concentrate was cleaned up by applying to a silica-gel cartridge and eluted with 15 mL of 5% acetone-hexane.The eluate was concentrated to 1 mL with a N 2 stream, and the final concentrate was analyzed using GC/MS.
Fenobucarb and 2-sec-butylphenol were determined by GC/MS (ThermoQuest Trace GC 2000 Gas Chromatograph) with an auto-sampler and a capillary column ZB-5MSi (30 m × 0.25 mm × 0.25 µm; Phenomenex, Torrance, CA, USA), according to a previously described method (Trinh et al. 2018).Quality assurance and quality control of fenobucarb and 2-sec-butylphenol determination were performed using procedural blanks, repeatability tests, and recovery tests of the targets (100 μg L −1 each).The recovery of fenobucarb in water and soil samples was 97% and 96%, respectively, while the recovery of 2-secbutylphenol was 96% and 95%, respectively.The limit of detection for both fenobucarb and 2-sec-butylphenol was 4 µg L −1 in the water and 0.01 µg g −1 in the soil.

Effect of pH on degradation of fenobucarb in the paddy soil-water system
The concentrations of fenobucarb and 2-sec-butylphenol in the soil phase, aqueous phase, and total residual amount in the paddy soil-water systems at pH 7 are shown in Figure 1.The residual amount of fenobucarb or 2-sec-butylphenol was the total amount in both the solid and aqueous phases of the paddy 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 (Figure 1(a)).Desorption of fenobucarb from spiked soil to water reached equilibrium within 2 h and remained nearly constant until 20 h (Figure 1(b)).The concentrations of fenobucarb in water, soil, and the total residual amount in the paddy 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-secbutylphenol was found in both the water and soil phases of the paddy 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 paddy 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-secbutylphenol 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 paddy soil-water system, respectively (Figure 1(d-f)).
The fenobucarb concentration gradually decreased from 2 to 24 h in both the soil and water phases of the paddy 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 fenobucarb-acclimatized microorganisms were capable of degrading fenobucarb in the paddy soil-water system.Bacteria use fenobucarb as a source of carbon and energy, and the initially hydrolyzed product of fenobucarb is 2-secbutylphenol, after which this metabolite is continuously metabolized 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 degradepesticides, 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;Ou et al. 1988;Sahoo et al. 1998;Caracciolo et al. 2002).The results of this study showed that the paddy soil in the study area contained microbial communities that were capable of decomposing fenobucarb.
In experiments with pH 7 and pH 9, the residual amount of fenobucarb decreased rapidly, and more than 50% of the fenobucarb disappeared after 48 h in these experiments (Figure 2).After 120 h, the residual amounts of fenobucarb were 2.47 and 0.777 μg with pH 7 and pH 9, respectively.The disappearance of fenobucarb in both the soil and water phases occurred fastest in the experiment with pH 7, followed by pH 9.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.After 48 h, the degradation percentages of fenobucarb at pH 7 and pH 9 were 72.5 and 69.6%, respectively, and these percentages were 87.7 and 96.1% after 120 h (Figure 2 and Table 1).Due to insecticides biodegradation is a vital process that controls the fate of pesticides in the soil, rapid degradation of fenobucarb in the paddy soil-water system was observed in the experiment with pH 7 (at a temperature 25 ± 1°C) compared to pH 9, perhaps because these were favorable conditions for bacterial growth.
In the experiment with pH 9, 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 hydrolyzed 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).

Effect of DOC on the degradation of fenobucarb in the paddy soil-water system
In experiments with 25 mg L −1 DOC, the residual amount of fenobucarb decreased rapidly, and more than 50% of the fenobucarb disappeared after 48 h, the residual amount of fenobucarb was 8.54 μg after 120 h.The degradation rate of fenobucarb was fast in the first 72 h and then decelerated, in 120 h this rate was 93%, (Table 1 and Figure 2).
The presence of 25 mg L −1 DOC in the aqueous solution enhanced desorption and microbial activity in the soil, and increased the biodegradation efficiency of fenobucarb in the paddy soil-water system.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 (Gonzale et al. 2010;Luo et al. 2009;Maillard et al. 2011).

Effect of SDS on the degradation of fenobucarb in the paddy soil-water system
In experiments with 1 cmc SDS, after 48 hours, nearly half of the concentration of the fenobucarb was dissipated.The degradation rate of fenobucarb was fast in the first 72 h and then decelerated; after 120 h, only 55.6% of fenobucarb was degraded, and the residual amount of fenobucarb was 8.54 μg (Table 1 and Figure 2).
The presence of 1 cmc 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 paddy soil-water system (Figure 1 and Table 1).Consequently, the presence of 1 cmc SDS in the solution simultaneously affected desorption and biodegradation, and enhanced the desorption and degradation of fenobucarb.Insecticides 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 insecticides increased owing to the reduced interfacial tension, which led to an increase in the biodegradation efficiency of fenobucarb.
The disappearance of fenobucarb in both the soil and water phases in the paddy soilwater system occurred fastest in the experiment with pH 7, followed by those with pH 9, 25 mg L −1 DOC and 1 cmc SDS.The presence of 25 mg L −1 DOC and 1 cmc 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, Calvillo, and Alexander 1991) (Figure 2 and Table 1).However, the degradation of fenobucarb in the 1 cmc SDS solution was inhibited compared with that of the pH 7, pH 9, and 25 mg L −1 DOC solutions because 1 cmc SDS solution may be preferentially utilized 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, Calvillo, and Alexander 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, Jaffé, and Peters 1998).

Conclusions
This study focused on the effects of pH 7, pH 9, 25 mg L −1 DOC, and 1 cmc SDS solutions on the desorption and biodegradation of fenobucarb in a paddy soil-water system.At pH 7, the degradation of fenobucarb is mainly caused by microorganisms capable of decomposing fenobucarb in the soil, which produces 2-sec-butyl phenol as an intermediate metabolite.
The degradation of fenobucarb from contaminated soil in the paddy soil-water system was enhanced by pH 7, pH 9, 25 mg L −1 DOC, and 1 cmc SDS.The degradation of fenobucarb in the pH 7, pH 9, and 25 mg L −1 DOC solutions was greater than that in the 1 cmc SDS solution.The presence of DOC in the solution promoted the biodegradation of fenobucarb.However, in the presence of SDS in solution, desorption and biodegradation simultaneously affected the degradation of fenobucarb, in which fenobucarb biodegradation was inhibited, likely because of the preferential utilization of SDS by fenobucarb degraders.Irrigation water for paddy fields may contain multiple compounds such as DOC and SDS.Therefore, it contributes to the elimination of fenobucarb insecticide in the paddy soil-water system.The present study can potentially be useful for the removal of fenobucarb and carbamates from paddy soils.

Figure 1 .
Figure 1.Disappearance of fenobucarb and the appearance of 2-sec-butylphenol in the paddy soil-water system with pH 7 as a function of concentration from 24 to 144 hours.(a), (b) and (c) show fenobucarb concentration in soil phase, fenobucarb concentration in aqueous phase and the total residual amount of fenobucarb, respectively.(d), (e) and (f) show 2-sec-butylphenol concentration in soil phase, 2-secbutylphenol concentration in aqueous phase and the total residual amount of 2-sec-butylphenol.

Figure 2 .
Figure 2. (a), (b) and (c) shows fenobucarb concentration in soil phase, fenobucarb concentration in aqueous phase and the total residual amount of fenobucarb in the paddy soil-water system, respectively in experiments with pH 7, pH 9, 25 mg L −1 DOC, and 1 cmc SDS.

Table 1 .
Desorption and degradation percentage of fenobucarb in the paddy soil-water system in experiments with pH 7, pH 9, 25 mg L −1 DOC, and 1 cmc SDS.