Effects of ultrasound combined with ozone on the degradation of organophosphorus pesticide residues on lettuce

Abstract

Methamidophos (MDP) and dichlorvos (DDVP) are organophosphorous pesticides which are commonly used for pest control in agriculture to obtain better yields. The occurrence of OPP residues in vegetables is known to cause serious health problems to consumers. In the present research, a laboratory-scale experiment was performed to investigate the effects of ultrasound (US)/ozone (O₃) combination on the degradation of the two pesticides on lettuce. Various parameters including ozone flow rate, water temperature, treatment time and initial concentration of OPPs were chosen to identify their effects on the degradation rate of MDP under US/O₃ treatment. The degradation rate could reach up to 82.16% under the optimal conditions of ozone flow rate 75 mg min⁻¹, time 60 min, initial concentration 0.1–0.2 mg kg⁻¹, and water temperature 8 °C. It was observed that the treatments with O₃, US and US/O₃ had no obvious impact on the quality of the lettuce. Subsequently, the reaction kinetics of DDVP degradation in water were studied, and the dynamical equation of US/O₃ treatment was obtained as: , which confirmed the practicability and applicability of the US/O₃ degradation process.

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1. Introduction

Fresh vegetables are sources of nutrients and minerals and constitute an important part of a healthy diet.¹ Along with the rapid increase in population, the demand for vegetables is also growing quickly, and the use of pesticides is increasing as well.² Pesticides are a group of artificially synthesized substances used to fight pests and improve the agricultural production,³ among which organophosphorus pesticides (OPPs) are the most widely used.⁴ The use of OPPs has facilitated the mass production of high quality vegetables. However, trace amounts of OPPs residing in the human body for a long period can pose adverse risks to non-target tissues and organs.^5–8 Recently, the increasing demand for food safety has stimulated research regarding the risks associated with consumption of foodstuffs contaminated by OPPs.^9,10 Therefore, except for the control of OPPs application, there is a tendency to develop highly sensitive, simple and low cost techniques for the removal and degradation of OPPs residuals.¹¹

Due to the powerful, effective and nonselective oxidizing agent of ozone, ozone oxidation has increasingly developed into an attractive technique for the sterilization, virus inactivation, deodorization, decoloration and wastewater treatment for sanitation purposes, as well as for the decomposition of common pollutants including OPPs.^12,13 Ozone exerts oxidation roles mainly via two pathways: direct oxidation by ozone molecules or indirect oxidation by its decomposition products such as hydroxyl free radicals (˙OH).¹⁴ The low diffusivity and high cost of ozone, however, impede it from wider applications. In order to improve the utilization rate of ozone, several groups have made some advances in combing ozonolysis technique with other methods such as UV/O₃,¹⁵ H₂O₂/O₃,¹⁶ UV/H₂O₂/O₃,¹⁷ US/O₃ ¹⁸ to improve the advanced oxidization processes, among which US/O₃ has been proved to be a more effective technology with the advantages of using less energy,^19,20 being non-selective and leading to no secondary pollution.^21,22 Moreover, studies have shown US/O₃ combination to be more effective than US or O₃ alone treatment for the degradation of OPPs residuals,^23,24 it produces better utilization of the oxidant through promoting the dissociation of ozone. The mass transfer resistance, a major limiting factor for the application of ozone alone, is also eliminated due to the enhanced turbulence generated by ultrasound.²⁵ However, US/O₃ technique for the degradation of contaminants still remain in the laboratory stage so far, its application should be paid more attention and needs further explored.

The present study selects methamidophos (MDP) and dichlorvos (DDVP), two model OPPs, to investigate the degradation of OPPs residuals on lettuce by US/O₃ combination treatment. The degradation of MDP on lettuce was examined in order to get the optimization conditions, under which the reaction kinetics of DDVP degradation in water using US/O₃ treatment was investigated, aiming to provide theory and experimental data for the wider application of US/O₃ in the degradation of pesticide residues on lettuce. Furthermore, to ensure the practical significance and application value of the experiment, effects of the treatments on the lettuce quality was also evaluated and compared.

2. Materials and methods

2.1. Materials and apparatus

Uncontaminated lettuces were purchased from the academy of agricultural sciences. Standards of MDP (purity: 99.9%) and DDVP (purity: 99.9%) were supplied from Environmental Protection Institute of the Agriculture Ministry (Guangzhou, China). 50% MDP emulsifiable concentrates (EC) and 80% DDVP EC were obtained from the harvest of agricultural production materials Co., Ltd. (Guangzhou, China). All chemicals were of analytical grade, acetonitrile, acetone and methylene chloride were redistilled before use. The water used throughout was distilled water. The US/O₃ cleaning device for lettuce (Fig. 1) was designed to be suitable for our current experiment study.

Fig. 1 Schematic of experimental US/O₃ combination cleaning apparatus (JY-92-II ultrasonic generator, frequency rate 25 kHz, ultrasound power 0–1000 W; DHX-SS-1G ozone generator).

2.2. Sample pre-treatment

First, 50% MDP EC and 80% DDVP EC were respectively diluted to the required concentration in distilled water. Then the uncontaminated lettuces were completely immersed into the two OPPs solutions for 30 min to allow the pesticide to be absorbed in the lettuces. Subsequently, the lettuces spiked with OPPs were then air-dried for about 24 h under room conditions in basket.

2.3. Gas chromatographic (GC) analysis

According to guideline on organophosphorus pesticide residue detection in vegetables and fruits (NY/T 761.1-2004) at the Ministry of Agriculture standard,²⁶ the OPPs residues analysis on lettuce was carried out on a GC-FPD (flame photometric detector) (Shimadzu, Japan) equipped with an capillary column (Model GC2000 & Model DB-1701, 30 m × 0.25 mm. i.d, Shimadzu, Japan). The GC column was operated at a temperature of 120 °C for 2 min and then increased to 240 °C at 30 °C min⁻¹ and held for 5 min at 240 °C. The vaporizing chamber temperature was 260 °C and the detector was 270 °C with nitrogen as carrier gas at a flow rate of 120 mL min⁻¹, air of 120 mL min⁻¹, and hydrogen of 80 mL min⁻¹. The sample (1.0 μL) was injected in the split mode with narrow diameter at the diversion ratio of 1 : 1.

2.4. Effect of input time in water on ozone concentration

Ozone gas produced by ozone generator was bubbled into distilled water. The dissolved ozone levels were conducted via adjusting the bubbling duration and the flow rate, which were controlled via switching the ozone generator to valve I, valve II or valve III (Fig. 1, open one, two or three ozone generators, respectively). First, turned on the valve I, II or III (ozone flow rate of 25 mg min⁻¹, 50 mg min⁻¹ or 75 mg min⁻¹, respectively), and then kept bubbling duration for 10 min, 20 min, 30 min, 40 min, 50 min or 60 min, respectively. Finally, determined the dissolved ozone concentration in water on the basis of iodometry.²⁷

2.5. Degradation of OPPs residuals on lettuce

Degradation of OPPs on lettuce was studied from four single factors. Firstly, prepared four different initial concentration of MDP (13.243 mg kg⁻¹, 3.556 mg kg⁻¹, 1.083 mg kg⁻¹, 0.295 mg kg⁻¹, 0.082 mg kg⁻¹) and DDVP (20.698 mg kg⁻¹, 5.361 mg kg⁻¹, 1.402 mg kg⁻¹, 0.386 mg kg⁻¹, 0.103 mg kg⁻¹) on the entire leaves of lettuces, respectively. Turned on both ozone and ultrasound generator, bubbled into three different gas flow rate (25 mg min⁻¹, 50 mg min⁻¹, 75 mg min⁻¹) at four different initial water temperature (32 °C, 24 °C, 16 °C, 8 °C), and kept the duration of cleaning lettuce samples spiked with OPPs for desired time. Then, lettuce samples were taken out at regular time intervals (10 min, 20 min, 30 min, 40 min, 50 min, 60 min, 70 min or 80 min) and finally the OPPs residues on lettuce were determined using GC analysis. A control treatment was also conducted where the vegetables lettuce samples were immersed and rinsed in distilled water. Using this method, the definition of OPPs degradation ratio (K₀) is as follows:
where C₀ and C are the initial concentration and concentration at a given time of MDP or DDVP.

Based on the above results, Since the ozone flow rate, treatment time, initial concentration of OPPs and water temperature could impact the efficiency in OPPs removal, the better degradation efficiency could be obtained through optimizing these parameters, thus L9 (3⁴) orthogonal experimental design was applied to determine their influence on the degradation of MDP on lettuce. The four independent variables were ozone flow rate (A), treatment time (B), initial concentration of MDP (C) and water temperature (D), respectively. Furthermore, in order to further prove that US/O₃ has significant effect on the degradation of OPPs on lettuce, under the optimization of experimental conditions, degradation effects of the four different processing methods of US, O₃, continuous (Con) US/O₃ and intermittent (Int) US/O₃ were compared. All the experiments were performed in twice.

2.6. The influence of different processing methods on the quality of lettuce

The present study aimed to investigate whether using different processing methods could effectively remove OPPs residues on lettuces while there had as smaller influences on lettuce quality as possible, to this end, with reference to some relevant parameters as described by Lin et al.²⁸ We explored and compared the influence of US, O₃ and US/O₃ treatment on the lettuce quality from the aspects of titratable acidity, soluble sugar, ascorbic acid (VC), chlorophyll (Chl), carotenoid (Car) and nitrate.

2.6.1. Titratable acidity determination. The acid/base titration method for the determination of titratable acidity in lettuce was adopted.²⁹ First, lettuce was chopped and pulped, of which 25 g (m) pulp was diluted into moderate distilled water. After 30 min in a water bath at 75–80 °C, pulp was cooled and vacuum filtered, collected the effective filtrate with discarding the 25 mL of initial filtrate. And then, added 3–4 drops of 1% phenolphthalein into 50 mL (G) of the diluent in a 250 mL conical flask. That was next titrated with 0.1% (C) of NaOH standard solution to reddish until it did not fade in 30 s, finally, recorded the consumption volume (V₁) of 0.1% NaOH. The titratable acidity content (X) could be calculated from the following equations:

2.6.2. Soluble sugar determination. The concentrations of soluble sugar was measured using a modified anthrone–sulfuric acid method³⁰ with slight modifications. 1 mL of lettuce sample solution and 5 mL of 0.2% sulfuric acid anthrone solution were mixed in an ice bath, while 1 mL of distilled water was used as the control. After 7–9 min in a boiling water bath, the solution was cooled and the soluble sugar content was measured at 628 nm. Soluble sugar could be obtained from the following equations:
where SS = soluble sugar, C = sugar content obtained from the standard curve, μg; V₀ = total volume of sample solution, mL; V₁ = volume of sample solution for determination of access, mL; D = dilution multiples; W = weight of sample, g.

2.6.3. VC determination. VC content of the lettuce was determined referring to the 2,4-dinitrobenzene hydrazine colorimetric method.³⁰ Mixed 50 g of lettuce sample with equal amount of 2% oxalic acid, to 5 g of which 1% oxalic acid were added to the total volume of 100 mL, and the solution were subsequently filtered through filter paper. 10 mL of filtrate and 10 mL of 1% oxalic acid were added in some activated carbon and thoroughly mixed. To this mixed filtrate, 0.5 mL of 0.2% 2,4-dinitrobenzene hydrazine and a drop thiourea were added. After 3 h in the water bath at 37 ± 0.5 °C, Slowly added 2 mL of 85% sulfuric acid in ice-bath, after 30 min at room temperature, the optical density (X) was measured using a UV-1200 spectrophotometer at 540 nm. Concentrations of VC were determined using the following equation:
where C = sample solution concentration obtained from the standard curve, μg mL⁻¹; m = weight of sample, g; F = diluted multiples, V = sample volume consumed in fluorescence reaction, mL.

2.6.4. Chlorophyll determination. After being removed the big veins, 0.5 g dried sample lettuce were ground in acetone and the homogenate were filtered, then added the filtrate capacity to 10 mL using 80% acetone, which were diluted properly for the determination of Chl content by the means of Porra et al.³¹ Pretreated sample was applied to determinate the absorbance of Chl in acetone, the optical density was measured with a spectrophotometer (U-2000, Hitachi, Tokyo, Japan) at 663 nm (OD₆₆₃) and 645 nm (OD₆₄₅). According to Beer–Lambert law, concentrations of Chl were obtained from the following equations:

Chl (mg L⁻¹) = 20.29 × OD₆₄₅ + 8.02 × OD₆₆₃

2.6.5. Carotenoid determination. Carotenoid determination was performed according to the method described by Scott.³² Briefly, leaf samples (fresh weight, W) were ground and squeeze juice using gauze to which 3 g quartz was added, and the ground leaves were subsequently filtered with Buchner funnel. The filtrate was mixed with 20 mL of the extractant (chloroform : methanol = 2 : 1 (v/v)), and collected the organic phase of the bottom after vibration. All the extract liquor were merged, and then diluted to 100 mL, of which the maximum absorbance of determination (A) was determined at the wavelength of 445 nm.

TC (mg L⁻¹) = A × 20

where TC represents the total carotenoids.

2.6.6. Nitrate determination. Colorimetric method³³ is applied to determine the nitrate content. Briefly, 2 g of lettuce samples were ground, to which 10 mL of distilled water was added. After extraction for 30 min in a boiling water bath, the sample solution was cooled and filtered, then added distilled water to constant volume of 25 mL. 0.1 mL of the sample solution was mixed with 0.4 mL of 5% (w/v) salicylic acid (in pure H₂SO₄), after 20 min, slowly added 9.5 mL of 8% NaOH. Finally, the nitrate content was measured at 410 nm after the sample was cooled to room temperature. Nitrate content was calculated as follows:
where C = NO³⁻ − N content obtained from the standard curve, mg kg⁻¹; V = total weight of sample solution, g; W = fresh weight of sample, g.

2.7. Degradation kinetic of DDVP in water

Due to the practical applications, the degradation kinetics of DDVP in water was investigated under the mild optimum conditions. The experiment procedure similar to OPPs degradation on lettuce (reference to above 2.5) was carried out to investigate the effects of initial concentration of DDVP (10, 20, 30, or 40 mg L⁻¹), ozone flow rate (25 mg min⁻¹, 50 mg min⁻¹, 75 mg min⁻¹), and initial water temperature (32 °C, 24 °C, 16 °C, 8 °C). Fit the above data using the origin software to establish the appropriate degradation dynamics equation.

3. Results and discussion

3.1. Degradation of OPPs on lettuce in the combined US/O₃ system

3.1.1. Effect of input time on ozone concentration. Fig. 2A shows that the dissolved ozone in water all rapidly increased with the increase of input time in the first 40 min. The ozone concentration of the highest conversion rate at the gas flow of 75 mg min⁻¹ (valve III) could reach to 31.9 mg L⁻¹. It was noted that, ozone content basically remained constant up to 50 min exposure and that was just 32.6 mg L⁻¹ at 60 min at valve III. The phenomena are similar to a typical experimental data collected by Biń.³⁴ That mainly owned to the ozone concentration in solution influenced by the dissolution, reaction and decomposition of ozone would stop continually increasing when it saturated in certain ozone content as the input time prolonged and reached a dynamic balance in aqueous solution.

Fig. 2 (A) Effect of input time on ozone concentration, (B) effect of ozone flow rate on degradation rate of MDP, (C) effect of ozone flow rate on degradation rate of DDVP, (D) effect of water temperature on degradation rate of MDP, (E) effect of water temperature on degradation rate of DDVP, (F) effect of treatment time on degradation rate of MDP and DDVP, (G) effect of initial concentration on degradation rate of MDP, (H) effect of initial concentration on degradation rate of DDVP, (I) effect of different processing methods on degradation rate of OPPs.

3.1.2. Effect of ozone flow rate. Fig. 2B and C showed the MDP and DDVP degradation efficiency at different ozone flow rate controlled by the inlet concentration of ozone, respectively. It illustrated that the degradation rate of OPPs increased with the increase of ozone levels. In general, both OPPs were degraded very fast in the first 50 min, and then the speed was slowed down. In which the high ozone flow rate (75 mg min⁻¹) group improved the degradation efficacy in MDP removal to 79.32% at 8 °C, while DDVP 67.72%. The results were consistent with previous data studied by Yang et al.³⁵ Both findings indicated that increasing the flow rate induced a larger net surface area for mass transfer of ozone to the aqueous phase, accompanied with the increase of the volumetric mass transfer coefficient of ozone, enhancing the rate of mass transfer of ozone from air–ozone bubbles to the liquid phase.

3.1.3. Effect of water temperature. The degradation rate of both MDP and DDVP at the initial water temperature from 8 °C to 32 °C were significant difference (Fig. 2D and E), both decreased slowly with the increase of temperature. The results were contrast to what had been studied by Xu et al.³⁶ Temperature had complex effects on US/O₃ treatment. Oxidation is a temperature dependent process, for the combined treatment, ultrasound played roles mainly through promoting more ozone dissolution and changing the way of ozone oxidation. Although the increase of temperature could enhance the rate of mass transfer during ozone dissolution that is beneficial for chemical reaction between oxidants and substrates to promote reaction, too high water temperature could decrease the partial pressure of dissolved ozone in aqueous solution, further reduce the ozone concentration and the production of ˙OH.³⁷ In this study, the unfavorable effects caused by the increase temperature were superior to the improvement of mass transfer. Thus, the most appropriate temperature should be 8.0 °C.

3.1.4. Effect of treatment time. The effect of treatment time, from 10 to 80 min, on the degradation ratio was investigated. In both cases, the degradation ratio was increased by prolonging the degradation time. The degradation ratio could attain 80% in 60 min (Fig. 2F), in general, the OPPs were degraded very fast in the first 40 min, and then the speed was slowed down, after 60 min, the content of pesticide residues are no longer falling. That might be antioxidant capacity of OPPs initiated their protective roles by scavenging free radicals or reacting directly with ozone and hence resulting in the decrease of OPPs contents in lettuce samples.³⁸ At start, the content of OPPs on lettuce were relatively high, the utilization of ozone and ultrasound could sufficiently played roles in degradation, the reaction was accelerated, as prolonging the treatment time, OPPs were degraded gradually, leading to a slower reaction.

3.1.5. Effect of initial concentration of OPPs. The lettuce samples spiked with MDP and DDVP of different concentration were investigated. Previous studies had verified that the degradation percentage declined as the initial concentration of OPPs increased under both single US³⁹ and single O₃ treatment.⁴⁰ The similar trends were observed in present US/O₃ treatment and results were shown in Fig. 2G and H. This could possibly be attributed to the role of hydroxyl radical induced by ultrasound. The cavities and ˙OH approached saturation gradually in dissolve ozone solution with the increase in initial OPPs concentration, the higher initial OPPs concentration also induced in the generation of more inorganic anions, which would compete with carbonaceous organic substances for reaction with ˙OH.

3.2. Optimization of degradation

3.2.1. Optimization of degradation for MDP on lettuce using US/O₃. From orthogonal experiment results in Table 1, the factors order that had effect on the degradation of OPPs on lettuce was: A/ozone flow rate > C/initial concentration > B/treatment time > D/water temperature. The optimization conditions should be A₃B₃C₁D₁, which was, when bubbling 75 mg min⁻¹ of ozone to into the solution, after 60 min of US/O₃ combination treatment, at the initial concentration of 0.295 mg kg⁻¹ and temperature of 8 °C, the degradation rate of MDP could reach over 80%.

Table 1 Scheme and result for orthogonal experiment

Number	A (valve)	B (min)	C (mg kg^−1)	D (°C)	K0 (%)
1	I	20	0.354	8	54.12
2	I	40	1.396	16	43.46
3	I	60	5.513	24	36.17
4	II	20	1.396	24	45.91
5	II	40	5.513	8	38.95
6	II	60	0.354	16	60.76
7	III	20	5.513	16	56.19
8	III	40	0.354	24	70.18
9	III	60	1.396	8	68.89
K1	133.75	156.22	185.06	161.96
K2	145.62	152.59	158.26	160.41
K3	231.43	165.82	131.31	152.26
k1	44.58	52.07	61.69	53.99
k2	48.54	50.86	52.75	53.47
K3	77.14	55.27	43.77	50.75
R	32.56	4.41	17.92	3.24	A3B3C1D1

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3.2.2. Comparison of different processes for the degradation of OPPs. Comparison of water treatment, US, O₃ and Con US/O₃ and Int US/O₃ for the degradation of OPPs was presented in Fig. 2I. As had been reported by Xu et al.⁴⁰ and Yang et al.,³⁵ it was evident that US/O₃ was the most effective system for degradation on both MDP and DDVP, while single US treatment was the least effective one except rinsing in distilled water. Song et al.⁴¹ also reported that US and O₃ alone were less effective than the US/O₃ combination. The MDP and DDVP degradation after 60 min reaction were reached to 79.76% and 68.02% in the combined process, which was greater than 65.58% and 54.64% under single O₃ treatment, as well as the less than 50% under single US treatment. In addition, the DDVP degradation rate appeared to be distinctly lower than that of MDP.

The poor degradation effect of US treatment which was mainly resulted from acoustic cavitation might be attributed to the usual long contact time and the fact that low-frequency ultrasound could not promote the production of ˙OH.⁴² In the US/O₃ synergism system, the reaction pathways might be affected each other. On the one hand, myriad of tiny bubbles were produced due to the cavitation effect and mechanical action of ultrasound, enhancing the production of additional ˙OH for further oxidation of OPPs. The increase of mass transfer and decomposition processes of O₃ coupled with transient local ultra-high pressure and high-temperature upon the collapsing of cavities enabled more O₃ to enter the liquid phase or react on the gas–liquid interface.^43,44 On the other hand, aeration of O₃ might increase the turbulence of the aqueous solution, which would increase the migration of more related substances from the collapsing cavities into the bulk of the solution. In addition, the degradation effect of intermittent US/O₃ was significant as continuous US/O₃ processing mode; while intermittent US/O₃ could reduce ozone consumption and ultrasound function loss. Thus the intermittent US/O₃ should be taken serious attentions and applied to practice in future.

3.3. Influence of different processing methods on the quality of lettuce

3.3.1. Effects of different processing methods on titratable acid. Fig. 3A illustrated that although the initial titratable acid content on lettuce was only 0.073%, they appeared to increase slowly with the extension of time under the three processing methods. With 60 min of US, O₃ or US/O₃ treatment, the content of total acid increased by 6.85%, 14.67%, 18.2%, respectively. The production of free radicals (primally ˙OH) was enhanced by cavitation bubble of ultrasound and the decomposition of ozone.⁴⁵ As the primal organic acids in lettuce, oxalic acid which possesses two hydroxyls was so easily oxidized by free radicals to generate hydrogen ions, thus increasing the total acid.

3.3.2. Effects of different processing methods on soluble sugar. Zhang et al.⁴⁶ had reported that there was no significant difference between total sugar of fresh-cut celery treated with ozonated water and that of non-treated with further storage period from 3 to 9 days, which indicated that treatment of ozonated water had little effect on the content of total sugar in fresh-cut celery. A similar phenomenon of lettuce treated different processing modes were shown in Fig. 3B, the soluble sugar concentrations of lettuce did not exhibit significant differences under different treatment process, which were hardly affected by ultrasound cavitation and the ozone oxidation, and primarily kept at about 2.5 mg g⁻¹.

Fig. 3 (A) Effects of different processing methods on titratable acidity, (B) effects of different processing methods on soluble sugar, (C) effects of different processing methods on ascorbic acid, (D) effects of different processing methods on chlorophyll, (E) effects of different processing methods on carotenoid, (F) effects of different processing methods on nitrate.

3.3.3. Effects of different processing methods on ascorbic acid. Results of the ascorbic acid concentration of lettuce treated by different processing methods are shown in Fig. 3C. As the extension of processing time, the content of ascorbic acid under US/O₃ synergy treatment declined a little faster than O₃ alone, while a slight increase when processed by US alone. As an antioxidant, ascorbic acid has a strong reducibility susceptible to oxidation and is highly sensitive to various processing conditions.⁴⁷ In the synergy degradation system, the increase of VC content by ultrasound mechanical action was inferior to the production of free radicals accelerated by ultrasound, future improving the ozone oxidation ability.⁴⁸ Therefore, the VC content decreased in lettuces due to its scavenging of the free radicals formed during the decomposition of O₃.

3.3.4. Effects of different processing methods on chlorophyll. The concentration of chlorophyll showed no significant differences among different processing methods treatments in Fig. 3D, which generally kept at about 0.19 mg/100 g.

3.3.5. Effects of different processing methods on carotenoid. Fig. 3E showed that there was no significant influence on carotenoid content under the single O₃ treatment, which always maintained at about 8.5 mg L⁻¹. By contrast, ozone treatment alone made a reduction in carotenoids content by 9.3% that was further decreased by 15.4% under the US/O₃ treatment. These might be associated with the generation of additional ˙OH via both ozone oxidation and ultrasound cavitation effect, thus the synergy treatment presented the highest influence on carotenoid content.

3.3.6. Effects of different processing methods on nitrate. The trend of nitrate content that was consistent with the ascorbic acid was shown in Fig. 3F, as the influence on ascorbic acid treated by the three processes, more free radicals were induced via actions of ozone and ultrasound, which could oxidize the nitrate on lettuce, thus reducing the nitrate content.

3.4. Degradation kinetics

The trends of degradation effect on DDVP in water in different factors were generally consistent with those OPPs on lettuce. First, keeping other conditions constant, with the increase of DDVP concentration from the 10 mg L⁻¹ up to 40 mg L⁻¹, the degradation rate of DDVP in water was decreased from 86% to 58% after processing 60 min (Fig. 4A). Second, after 60 min of US/O₃ treatment at 8 °C, along with the ozone flow rate was raised from 25 mg min⁻¹ to 75 mg min⁻¹, the degradation rate of DDVP in water was increased from 55% to 86% (Fig. 4B). Third, as was shown in Fig. 4C. Degradation rate of DDVP varied in response to the different initial water temperature in the range of 8 °C to 32 °C, which was decreased with the increase of temperature.

Fig. 4 (A) Effect of initial concentration on degradation rate of DDVP, (B) effect of ozone flow rate on degradation rate of DDVP, (C) effect of water temperature on degradation rate of DDVP, (D) ln K_obs − T⁻¹ fitted curve, (E) ln K_obs − ln C₀ fitted curve, (F) ln K_obs − ln Q_O3 fitted curve.

The above data indicated that the degradation of DDVP fitted in the following pseudo-first-order reaction kinetic:^49,50

−ln(C/C₀) = kC

where, C₀ and C are the initial mass concentration of DDVP or at the reaction time t, mg L⁻¹, respectively; k is the pseudo-first-order reaction rate constant, min⁻¹. The simulated reaction rate constants are listed in Table 2, of which the linear correlation generally above 0.97.

Table 2 Fitted rate constants based on experimental results

Number	C0	QO3	T (K)	Kobs (min^−1)	r
1	10	25	281	0.0139	0.9845
2	10	50	281	0.0231	0.9923
3	10	75	281	0.0337	0.9938
4	10	75	289	0.0257	0.9887
5	10	75	297	0.0173	0.9876
6	10	75	305	0.0130	0.9750
7	20	75	281	0.0258	0.9896
8	30	75	281	0.0181	0.9924
9	40	75	281	0.0151	0.9918

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The previous results indicated that the reaction involved not only the thermal decomposition of ultrasound and the direct reaction of ozone molecules, but also the ˙OH reaction, so the K_obs was related to initial concentration of DDVP, ozone flow rate and water temperature, the degradation kinetic was adequately modeled on the Arrhenius equation:^51,52

K_obs = A × exp(−E_a,obs/RT)C₀^αQ_O3^β

ln K_obs = ln A(−E_a,obs/RT) + α ln C₀ + β ln Q_O3

where E_a is the activation energy, kJ mol⁻¹, R is the universal gas constant (8.314 J mol⁻¹ K⁻¹) and T is the absolute temperature, in Kelvin, K.

The multiple function regression analysis of ln K_obs − T⁻¹, ln K_obs − ln C₀ and ln K_obs − ln Q_O3 were conducted, the fitted curve of which were shown in Fig. 4D–F. The straight slopes of them were −E_a,obs/R = 3.4829, α = 0.5876, β = 0.7982, respectively. Thus, dynamics equation could be expressed as follows:

Furthermore, A = 0.0041 could be obtained after applying all experiment results to the above equation. As the result, the degradation dynamics equation was:

To verify the accuracy of dynamics equation, we compared the theoretical data with actual data. Under the condition of the water temperature at the range of 8–32 °C, initial concentration of DDVP at 10–40 mg L⁻¹ and the ozone flow rate at 25–75 mg min⁻¹, the scope of the deviations between the experimental data and theoretical data was kept within 18%, indicating a good applicability of the dynamics experience equation.

4. Conclusion

The US/O₃ combination was an effective way to degrade OPPs. The synergetic process showed a higher removal efficiency on both MDP and DDVP than that of using only distilled water, single US or O₃ treatment. Intermittent US/O₃ treatment would be a more potential candidate for the degradation of the lettuce pesticides residuals. At the high ozone flow rate of 75 mg min⁻¹, water temperature 8 °C, and 60 min US/O₃ treatment with the initial concentration of 0.1–0.2 mg kg⁻¹, the MDP degradation rate could reach 82.16%. Treatments of US, O₃, or US/O₃ combination all had no obvious influence on the quality of lettuce. The degradation efficiency of DDVP in water fitted the first-order reaction rate kinetic model for the US/O₃ and presented a potential applicability.

Funding sources

This project was supported by the National Natural Foundation of China (grant 21406074), Guangdong Province Science and technology plan project (2013B020311006), and Guangdong Provincial Bureau of ocean and fishery science and technology to promote a special (A201301B04).

Abbreviations

Car	Carotenoid
Chl	Chlorophyll
Con US/O3	Continuous US/O3
DDVP	Dichlorvos
EC	Emulsifiable concentrates
FPD	Flame photometric detector
GC	Gas chromatographic
˙OH	Hydroxyl free radicals
Int US/O3	Intermittent US/O3
MDP	Methamidophos
O3	Ozone
OPPs	Organophosphorus pesticides
US	Ultrasound
VC	Ascorbic acid

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