Novel catalytic degradation of Diazinon with ozonation/mg-Al layered double hydroxides: optimization, modeling, and dispersive liquid–liquid microextraction

Abstract

Purpose

In this study MgAl- layered double hydroxides (MgAl-LDH) nanoparticles were prepared by a simple and fast co-precipitation method and used as a catalyst in the ozonation process to degrade diazinon from aqueous solutions.

Methods

The structure of the synthesized MgAl-LDH was investigated by X-ray diffraction pattern (XRD) and field emission scanning electron microscope-energy dispersive spectroscopy (FESEM-EDX). The response surface methodology (RSM) was used to investigate the effects of different parameters including of reaction time, initial diazinon concentration, pH, and LDH dose on the removal of diazinon by MgAl-LDH catalytic ozonation process. Central Composite Design (CCD) was employed for the optimization and modeling of the process. Dispersive liquid–liquid microextraction (DLLME) method was used to extract diazinon from aqueous samples. The GC-Mass analysis was performed to determine intermediate compounds during diazinon degradation reactions. To evaluate the process performance, TOC and COD removal were measured under optimum conditions.

Results

The highest removal efficiency of 92% was observed in optimum conditions as follow; initial diazinon concentration: 120 mg/L, pH: 8.25, LDH dose: 750 mg/L, and reaction time: 70 min. The quadratic model was obtained with a good fit. The removal of COD and TOC were 80% and 74%, respectively.

Conclusion

This process can be suggested and used in the treatment of various industrial wastewaters.

Supplementary Information

The online version contains supplementary material available at 10.1007/s40201-021-00687-w.

Introduction

Organophosphorus pesticides (OPPs) are extensively used in the world for agricultural and animal husbandry activities to control pests. However, OPPs are the known neurotoxin agents, which have been classified as moderately hazardous Class II by the World Health Organization (WHO) due to their toxic, carcinogenic, teratogenic, and mutagenic effects [1, 2]. Diazinon (O,O-diethyl O-(2-isopropyl-6-methylpyrimidin-4-yl)thiophosphate) is the most familiar insecticide in the organophosphate toxins [1]. The use of these toxins such as diazinon leads to the accumulation of undesirable residues in the food chain, soil and natural water resources. Consequently, their presence in the environment is a big scientific problem [3].

Many conventional purification processes are developed to remove pesticides from aquatic environments. The major current processes include adsorption [4], Sono-Fenton [1], electrocoagulation [5], biological [6], and photo-degradation reactions [3]. However, there are some limiting factors to using conventional methods. For example, in biological processes, long times are required to remove these compounds due to their toxicity and non-biodegradability effects. In the adsorption techniques, pollutants do not completely destroy and only convert from one phase to another [7, 8]. Currently, ozone with a redox potential of 2.07 V, as a strong oxidant, has drawn increasing attention in various environmental systems for its potential oxidation capacity. Ozone reacts rapidly with organic substances in aqueous solution in two ways: direct reaction with molecular ozone (O₃) or indirect reaction with the active radicals, such as hydroxyl radicals (^•OH) generated by ozone decomposition [9]. Hydroxyl radicals (redox potential of 2.33 V) react with most of the pollutants non-selectively, destroying them and converting into harmless organic compounds such as CO₂ and H₂O [7]. O₃ molecules react selectively with the electron-rich compounds, so reaction rate is quite slow for some other organic compounds such as pesticides and increases energy consumption for ozone production. These are examples of ozone application limitations for direct removal of pollutants [7, 10]. As a consequence, the application of some combined oxidation technologies, such as ozone/UV, ozone/H₂O₂, and catalyst/ozone can cause an increase in the production of hydroxyl radicals, leading to increased ozonation process efficiency and subsequent process cost reduction [7, 10, 11].

In heterogeneous catalytic ozonation process (COP), the decomposition rate of refractory compounds in water increases due to the presence of a solid as a catalyst, which produces destructive radicals. In addition, the mass transfer of ozone into liquid is improved at the COPs [12]. Depending on the chemical nature of the catalyst used or the nature of the reactions, the COPs can be classified into heterogeneous and homogeneous processes, with the heterogeneous COPs having a higher degradation efficiency [10].

So far, various catalysts have been investigated for AOPs such as COP, including metal oxides like Fe₂O₃, MnO₂, Ce₂O₃, WO₃, TiO₂, ZnO, CoO, Al₂O₃, MgO, CuO, N-doped TiO₂, zeolites, clay minerals, and activated carbon [10, 11, 13–17]. Recently, the synthesis and application of layered double hydroxides (LDHs) nanostructures in the various fields, especially water and wastewater treatment industry, has been considered as adsorbent and anionic exchanger due to their excellent ion-exchange properties [12]. Among the various catalysts, LDH nanoparticles was chosen for this study because of their distinct properties such as layered structure, reactive interlayer space, wide chemical compositions, mechanical and chemical stability, ion-exchange, variable layer charge density, and colloidal properties [12, 18].

layered double hydroxides are synthetic anionic clays having a two-dimensional (2D) lamellar structure consisting of both divalent M²⁺ and trivalent M³⁺ cations coordinated to six OH⁻ hydroxyl groups. The general formula of LDH can be described as bellow [18, 19]:

$$\left[{\mathrm{M}}_{1-\mathrm{x}}^{2+}{\mathrm{M}}_{\mathrm{x}}^{3+}{\left(\mathrm{OH}\right)}^{\mathrm{x}+}\right]{\left[{\left({\mathrm{A}}^{\mathrm{m}-}\right)}_{\frac{\mathrm{x}}{\mathrm{m}}}.\mathrm{n}\left({\mathrm{H}}_2\mathrm{O}\right)\right]}_2^{\mathrm{n}-}.\mathrm{n}{\mathrm{H}}_2\mathrm{O}$$ 1

Where M²⁺ and M³⁺ represent a divalent metal (Mg²⁺, Mn²⁺, Co²⁺, Fe²⁺, Ni²⁺, Cu²⁺, Zn²⁺, etc.) and a trivalent metal (Al³⁺, Mn³⁺, Fe³⁺, Cr³⁺, Co³⁺, etc.), and A^m represents a general anion such as Cl⁻, OH⁻, CO₃⁻, NO₃⁻, SO₄²⁻, etc. [18, 20]. The schematic of the LDH structure is shown in Fig. 1.

Fig. 1.

The schematic of the layered double hydroxides (LDHs) structure

Despite the growing interest in this field, relatively little attention has been paid to the use of LDH as catalysts in the ozonation process for refractory pollutants removal such as diazinon. Because the ozonation process is costly in industrial applications, therefore optimizing the process variables and determining a statistical model can be very cost-effective [21].

Many experiments are required in the optimization of a multivariable process such as ozonation technique with one-factor method. In addition, such methods do not demonstrate the combined effects of process variables and take prolonged time, which can be non-functional and unreliable in the industrial applications [22]. Therefore, the selection of an effective method for optimizing and modeling of the process is very important. Response Surface Method (RSM) is a mathematical and statistical methods used when the response variable is affected by several independent variables [23]. Central Composite Design (CCD) is one of the most popular RSM methods. CCD method applied to determine optimal conditions expressing the influence of each independent variable [21]. Hence, the novelty in this work is mainly focused on the design, optimizing, and modeling of diazinon removal from aqueous solutions by COPs using MgAl-LDH nanoparticles as catalyst. The optimized dispersive liquid–liquid microextraction (DLLME) method was used to extract diazinon from aqueous samples. The process variables including initial diazinon concentration, LDH dose, initial pH, and reaction time were optimized by CCD method and the best mathematical model was selected. Furthermore, intermediate compounds were identified during diazinon oxidation. This process has a high potential for the toxic pollutants degradation such as diazinon and can be suggested for purification and treatment of pollutants in the water and wastewater treatment plants.

Materials and methods

Materials

Diazinon (C₁₂H₂₁N₂O₃PS; 99.5%; Merck, Germany), Magnesium chloride hexahydrate (MgCl₂·6H₂O; 99.9%; Merck, Germany), Aluminium chloride hexahydrate (AlCl₃·6H₂O; 99.9%; Merck, Germany), Potassium iodide (KI; 99.5%; Merck, Germany), Sodium hydroxide (NaOH; 99.5%; Merck, Germany), Sulfuric acid (H₂SO₄; 99.5%; Merck, Germany), Sodium thiosulfate (Na₂S₂O₃; 99.5%; Merck, Germany), Starch ((C₆H₁₀O₅)_n-(H₂O); 99.5%; Merck, Germany), Methanol (CH₃OH; 99%; Merck, Germany), and Chloroform (CHCl₃; 99.5%; Merck, Germany) were purchased from Merck, Germany.

MgAl-LDH nanocatalyst synthesis

MgAl-LDH nanocatalyst was synthesized by co-precipitation method at room temperature (23 ± 5 °C). A solution (50 mL) containing 10. 165 g of magnesium chloride hexahydrate and 9.63 g of aluminum chloride hexahydrate was added dropwise under vigorous stirring to an aqueous solution of NaOH (4 M, 50 mL). The obtained suspension was transferred into a Teflon-lined autoclave (50 mL in volume) and maintained at 180 °C for 48 h in oven. As-prepared LDH was centrifuged and washed several times by deionized water. Finally, the white precipitate was dried at 60 °C overnight to obtain MgAl-LDH nanocatalyst [20, 24].

Characterization

The field emission scanning electron microscope-energy dispersive spectroscopy (FESEM-EDX) (TESCAN mira3, Czech Republic) was conducted to observe the surface morphology and the size of synthesized MgAl-LDH nanoparticles.

To characterize the structural properties (characterization of constituent phases and crystalline size) of nanoparticle, X-ray diffraction (XRD) patterns was recorded in the diffraction angle range 2θ = 10–80^○ by an Rigaku ultima iv (made in Japan). Also, the crystallite size of MgAl-LDH was estimated from the characteristic peaks in the XRD pattern through the Debye-Scherrer equation (Eq. 2) [13].

$$\mathrm{L}=K\lambda /\beta cos\theta$$ 2

Where L is the average size of LDH, λ is the X-ray wavelength (nm), K is the Scherrer constant related to crystallite shape (0.9), β is the line broadening at half maximum (FWHM) of the diffraction peak and θ is the Bragg’s angle [13].

Catalytic ozonation reactor and operation

The COP experiments were carried out using a pyrex cylindrical reactor of 300 mL, equipped with a sintered-glass diffuser at bottom, operated in the semi-batch mode. Ozone was produced using an ARDA ozone generator (model MOG). Silicone hoses which are resistant to ozone gas were used to the joints between reactor components. The amount of ozone gas entering the COP reactor was adjusted at 2.55 mg/min by the rotameter and iodometric method [25]. To prevent health damage, KI (20%) solutions were used to destroy excess ozone molecules. In order to enhance the ozonation process performance, the MgAL-LDH nanoparticles synthesized by the co-precipitation method was used as a catalyst in the COP process. A schematic of the COP reactor is shown in Fig. 2.

Fig. 2.

A schematic of the COP reactor for diazinon removal; 1. Rotameter, 2. Ozone generator, 3. Mg-Al LDH nanoparticles, 4. Sampling valve, 5. KI solution, 6. Gas outlet

Experimental design

Central composite design with four independent variables was used to investigate the effect LDH dose, diazinon initial concentration, reaction time, and solution initial pH and their interactions on the response (diazinon removal efficiency). Based on the CCD matrix, a total of 30 experimental runs were designed to calculate the coefficients of the second-order polynomial regression model for independent factors. The selection of independent variables was based on the previous literatures [9, 21, 26, 27].

Each variable was studied at five levels: -α, −1, 0, +1 and + α, the actual and coded levels used for CCD design are shown in Table 1.

Table 1.

Coded and real values of the independent variables used for CCD design

Independent variable	Coded symbol	Levels of variables					Unit of variables
		-α	-1	0	+1	+α
LDH dose	X1	0	250	500	750	1000	mg/L
pH	X2	3	4.75	6.5	8.25	10	–
Initial diazinon concentration	X3	50	87.5	125	162.5	200	mg/L
Reaction time	X4	10	30	50	70	90	min

The quadratic polynomial response model for predicting the optimal point is shown in Eq. 3 (9,21).

$$Y={\beta }_0+{\sum }_{i=1}^k{\beta }_i{X}_i+{\sum }_{i=1}^k{\beta }_{\mathit{ii}}{X}_i^2+{\sum }_{i=1}^{k-1}{\sum }_{j=i+1}^k{\beta }_{\mathit{ij}}{X}_i{X}_j$$ 3

where Y, β₀, β_i, β_ii, β_ij, X_i, X_j, and k are the response variable (diazinon removal efficiency), the constant coefficient, the linear coefficients, the quadratic coefficients, the interaction coefficients, the coded independent variables, and the number of input variables respectively [9, 21].

The analysis of variance (ANOVA) carried out to determine the effect of independent variables on the response variable and the optimal conditions by Design Expert 8.0 software.

Dispersive liquid–liquid microextraction procedure for diazinon determination

Briefly, parameters including: solvent to liquid sample ratio (0.1–0.4), centrifuge time (2–8 min), vortex time (2–8 min), extraction solvent to dispersion solvent ratio (0.125–0.5) were investigated to obtain the highest recovery rate. The optimized DLLME method [solvent to liquid sample ratio (0.3), centrifuge time (5 min), vortex time (5 min), extraction solvent to dispersion solvent ratio (0.3)] was used to extract diazinon from aqueous samples. Under these conditions, the recovery amount of 90% was obtained. In this method, a 3.5 mL aliquot of aqueous sample was placed into a Falcon 10 mL conical centrifuge glass tubes. 1500 μL of methanol as disperser solvent containing 500 μL of chloroform (extraction solvent) was quickly injected into the aqueous sample by 10 μL Hamilton syringe and then vortexed for 5 min. The fine droplets of chloroform were dispersed in the aqueous sample so that a cloudy solution was formed. The cloudy solution was centrifuged at 3000 rpm for 5 min. The chloroform solvent containing extracted diazinon was precipitated at the bottom of the centrifuge tube. Finally, 2 μL of centrifuge tube sediments were injected into the GC using a 10 μL Hamilton syringe (Bonaduz AG) [28, 29].

Analytical method

The residues concentration of diazinon in the solution were detected with a gas chromatography–flame ionization detector (GC-FID; YL Instrument 6500GC; Korea) equipped with a capillary column TRB-5 (30 m × 0.53 mm × 1.5 μm). H₂ gas was generated by a hydrogen generator (Model N°; PGH2–160) and used as a carrier gas at flow rate of 1 mL/min. The temperature program of GC oven was as follows: initial temperature of 50 °C held for 2 min and then increased at 20 °C/min to 280 °C where it was finally held for 10 min. The volume of 2 μL of extracted sample was injected in split-less mode at 280 °C. Detector and injector temperatures were set at 280 °C and 260 °C, respectively.

The intermediate compounds were determined with a gas chromatography/mass spectrometry detector (GC/MS; 7890A/7693 series, Agilent; USA) equipped with a HP-5MS column (L: 30 m; ID: 250 μm; FT: 0.25 μm) and He carrier gas with flow of 0.8 mL/min. The pH of solutions was adjusted by addition of 1.0 N of either H₂SO₄ or NaOH solutions. All experiments were carried out at room temperature (23 ± 5 °C).

The process was evaluated by measuring the reduction of chemical oxygen demand (COD) and total organic carbon (TOC) amounts under optimum conditions. The TOC amounts of the samples were measured at 800 °C in the furnace by Jena-C3100 TOC meter (Germany). COD analyzes carried out by using the recycled distillation based on C5220 method, the 20th edition of the “Standard Methods for the Examination of Water and Wastewater” book [30].

pH point of zero charges (pH_pzc)

The pH_pzc measurement of the prepared LDH nanoparticles was performed by the solid addition method. Briefly, the initial pH values of 100 mL of the KCl solution (0.1 mol/L) were adjusted at different pH values (2–12). Subsequently, 0.01 g of the MgAl-LDH nanoparticles were added to each solution. The obtained suspensions were agitated for 48 h until an equilibrium pH value were reached, then the final solutions pH were measured. The initial pH (pH_i) was plotted against the final pH (pH_f). The intersection point of pH_i = pH_f was chosen as pH_pzc [31].

Results and discussion

MgAl-LDH characterization

The XRD analysis of MgAl-LDH nanoparticles

The crystal structure and phase purity of the as-prepared MgAl-LDH nanoparticles were investigated by XRD analysis. The XRD pattern of MgAl-LDH nanoparticles is shown in Fig. 3.

Fig. 3.

The XRD pattern of MgAl-LDH nanoparticles

The XRD pattern of the MgAl-LDH showed the characteristic peaks of a layered crystal structure. The original main peaks at 2θ of 11.51^○, 23.17^○, and 35.08^○ related to the interlayer spacing of MgAl-LDH [20, 24]. The weak and broad peaks at 2θ of 14.48^○, 28.17^○, 38.37^○, 60.72^○, and 62.04^○ derived from γ-AlOOH (Boehmite structure, JCPDS 96–901-2252 and 96–901-2248,) were similarly detected in XRD pattern of MgAl-LDH. The peaks at 2θ of 60.72° and 2θ = 62.04° can be attributed to the ions arrangement along the plane of host layer [20, 24]. These sharp and strong diffraction peaks indicated that the synthesized nanoparticles of LDH were well crystallized and had a hydrotalcite-like structure with 3R layer, which was similar to the literatures reported by Shan et al. (2014) [32], Ai et al. (2011) [33], Yang et al. (2014) [34]. The average crystallite size for the MgAl-LDH nanoparticles prepared by the co-precipitation method was calculated using the Scherrer equation and the average crystallite size was 1.09 nm.

FESEM with EDX analysis

The surface morphology of the MgAl-LDH nanoparticles were investigated by FESEM analysis. Figure 4 clearly shows the FESEM images of synthesized LDH are smoothly, uniformly, and loosely aggregated fine particles with a spherical shape and average particle size of 45.8 nm [24, 33]. EDX gives the chemical composition of the as-prepared MgAl-LDH (Fig. 5). EDX element distribution reveals the presence of 47.74% O, 17.46% Mg, 15.72% Al, 10.19% Cl, and 8.89% C that confirm the purity and chemical structure of synthesized MgAl-LHD.

Fig. 4.

FESEM images of MgAl-LDH nanoparticles

Fig. 5.

EDX elemental spectrum and (inset) elemental composites of MgAl-LDH nanoparticles

Model fitting and statistical analysis

The design matrix for different experimental conditions and the response on experiments proposed by CCD for diazinon removal are given in the supplemental data (Table S1).

Based on these results, a second-order polynomial regression equation in terms of actual factors was obtained that demonstrates the empirical relationships between the response and independent variables in the coded units as below:

$$\mathrm{Y}=39.87157+0.018560{\mathrm{X}}_1-2.50689{\mathrm{X}}_2+0.156343{\mathrm{X}}_3+0.810454{\mathrm{X}}_4+0.001929{\mathrm{X}}_1{\mathrm{X}}_2-0.000206{\mathrm{X}}_1{\mathrm{X}}_4+0.003333{\mathrm{X}}_2{\mathrm{X}}_3+0.001125{\mathrm{X}}_3{\mathrm{X}}_4-0.000012{\mathrm{X}}_1^2\mathrm{X}32-0.001235{\mathrm{X}}_3^2\mathrm{X}42-0.002622{\mathrm{X}}_4^2\mathrm{X}52$$ 4

ANOVA analysis was carried out to evaluate the adequacy of the model. The results are shown in Table 2.

Table 2.

ANOVA results of response surface quadratic model for diazinon removal

Source	Sum of Squares	df	Mean Square	F-value	p value
Model	3197.36	14	228.38	174.30	< 0.0001	significant
X1-LDH dose (mg/L)	311.76	1	311.76	237.93	< 0.0001	significant
X2-pH	6.51	1	6.51	4.97	0.0415	significant
X3- diazinon concentration (mg/L)	94.01	1	94.01	71.75	< 0.0001	significant
X4-time (min)	2614.59	1	2614.59	1995.45	< 0.0001	significant
X1 X2	11.39	1	11.39	8.69	0.0100	significant
X1 X3	2.64	1	2.64	2.02	0.1762	Not significant
X1 X4	17.02	1	17.02	12.99	0.0026	significant
X2 X3	0.7656	1	0.7656	0.5843	0.4565	significant
X2 X4	1.89	1	1.89	1.44	0.2483	Not significant
X3 X4	11.39	1	11.39	8.69	0.0100	significant
${\mathbf{X}}_{\mathbf{1}}^{\mathbf{2}}$	14.88	1	14.88	11.35	0.0042	significant
${\mathbf{X}}_{\mathbf{2}}^{\mathbf{2}}$	5.58	1	5.58	4.26	0.0568	Not significant
${\mathbf{X}}_{\mathbf{3}}^{\mathbf{2}}$	82.71	1	82.71	63.12	< 0.0001	significant
${\mathbf{X}}_{\mathbf{4}}^{\mathbf{2}}$	30.18	1	30.18	23.03	0.0002	significant
Residual	19.65	15	1.31
Lack of Fit	16.65	10	1.66	2.77	0.1364	Not significant
Pure Error	3.01	5	0.6017
Core Total	3217.01	29
Standard deviation	1.14
Mean	78.26
C.V.%	1.46
R-Squared	0.9939
Adj R-Squared	0.9882
Pred R-Squared	0.9688
Adeq Precision	51.5810

From the ANOVA of the empirical second-order polynomial model, the p value for the model is <0.0001, which indicates that the model is highly significant. According to the correlation coefficient (R² = 0.9939), there is a 0.0061% chance that the total variation could not be explained by the empirical model for the degradation of diazinon by LDH COPs. Therefore, this model chosen as the best model to predict the diazinon removal efficiency.

Based on the results of F-values, reaction time, LDH dose, diazinon concentration, and initial pH of solution are the most effective parameters in COPs for diazinon degradation, respectively. Factors with p value≤0.05 were considered significant and retained in the model [35].

Similar studies reported by literatures. Kamani et al. (2018) conducted a study on modeling of enzymatic removal of direct red 81 by laccase-mediated system as a “green” technology using RSM. Their results indicated that the quadratic model with R-Squared of 0.9983, was the best model for the elimination of direct red 81 [23].

Al-Musawi et al. (2019) optimized the effective variables on the cephalexin degradation for the treating of pharmaceutical wastewater in Sono-Fenton process using Box-Behnken RSM. They reported that RSM is a significant and accurate methodology for optimization and modeling of the cephalexin removal by Sono-Fenton process [26].

Figure 6 represents the correlation between the observed and predicted values of the diazinon removal by LDH COPs. As shown, the points are placed very closely to the straight line indicating the validity and adequacy of the predicted model.

Fig. 6.

Predicted versus actual values of diazinon removal

Optimum conditions of the model

The optimized conditions for diazinon removal using MgAl-LDH COP were obtained by response optimizer. The values of optimized independent variables presented in Table 3.

Table 3.

The values of optimized independent variables

Diazinon removal efficiency (%)		LDH dose (mg/L)	pH	Diazinon concentration (mg/L)	Reaction time (min)	Desirability
Observed	Predicted
92	93.45	750	8.250	120	70	0.914

The main objective of optimizing the independent variables of this study was to achieve the highest diazinon removal by MgAl-LDH COP. Under optimal conditions, the predicted and actual efficiency of the model for diazinon removal were achieved 93.45 and 92%, respectively, indicating a strong correlation between predicted and observed values of this model.

Effect of operational parameters on diazinon removal efficiency

The effect of operational parameters including LDH dose (mg/L), diazinon initial concentration (mg/L), pH and reaction time (min) on diazinon removal efficiency by MgAl-LDH COP are shown in the three-dimensional (3D) response surfaces and linear plots, at central point values of other parameters. By using response surface plots, the removal efficiency of diazinon in different values of parameters can be predicted.

Effect of MgAl-LDH nanoparticles

The effect of LDH dose and interaction effect of reaction time and LDH dose on diazinon removal efficiency (%R) at central point values of other parameters is shown in Fig. 7. According to ANOVA analyses, the LDH concentration with a high F-value had a significant effect on the response y (diazinon removal) compared to other variables such as pH and diazinon concentration.

Fig. 7.

Effect of MgAl-LDH nanoparticles (a) and interaction effect of reaction time and LDH dose (b) on the removal efficiency of diazinon at central point values of other parameters (pH: 6.5, diazinon concentration: 125 mg/L)

It is obvious from Fig. 7 (a) that with increasing LDH nanoparticles concentration from 0 to 500 mg/L and reaction time to 90 min, the diazinon removal efficiency increased rapidly. The more increasing of LDH leads to reduce the removal efficiency. According to ANOVA analyses, reaction time with the highest F-value had a considerable effect on the removal of diazinon. Therefore, the removal efficiency of diazinon was increased with increasing the reaction time at all applied LDH concentration (Fig. 7 (b)).

In order to demonstrate the catalytic activity of MgAl-LDH on diazinon removal, four control processes were investigated under the same experimental conditions including pH: 8.25, initial diazinon concentration: 120, temperature: 23 ± 5 °C: (1) catalytic ozonation process (COP), (2) single ozonation process (SOP), (3) adsorption process without the presence of ozone, (4) aeration process without the presence of LDH. The results are shown in Fig. S1. (supplemental data).

As shown in Fig. S1., diazinon removal efficiency in the presence of LDH nanoparticles by MgAl-LDH COP was much more significant compare to the processes of SOP, adsorption, and aeration. As revealed in this figure, the effect of air on diazinon removal was completely negligible. In adsorption process, the maximum removal efficiency of diazinon was achieved 17% at 15 min then remained constant. In SOP and COP, with increasing reaction time from 0 to 70 min, the removal efficiency of diazinon increased rapidly, where the removal rate of 70 and 92% were obtained, respectively, at end of reaction time. Therefore, the high removal of diazinon in the catalytic ozonation process can be due to degradation but not adsorption.

Due to the presence of LDH nanoparticles as a solid substance in the ozonation process, the generation of ^•OH radicals derived from ozone destruction enhanced. Subsequently, the removal efficiency of diazinon increased. The chain reactions of ozone destruction in the presence of catalyst described by the following Eqs. 5–9 [36, 37]:

$${\mathrm{O}}_3+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{O}}_2+{\text{2HO}}^{•},{\mathrm{K}}_2=1.1\times {10}^{-4}{\mathrm{M}}^{-1}{\mathrm{s}}^{-1}$$ 5

$${\mathrm{O}}_3+{\mathrm{OH}}^{-}\to {{\mathrm{O}}_2}^{•-}+{{\mathrm{HO}}_2}^{•},{\mathrm{K}}_2=70{\mathrm{M}}^{-1}{\mathrm{s}}^{-1}$$ 6

$${\mathrm{O}}_3+{\mathrm{HO}}^{•}\to {{\mathrm{HO}}_{\mathbf{2}}}^{•}+{\mathrm{O}}_2\mathit{\leftrightarrow }{\mathrm{H}}^{\mathbf{+}}+{{\mathrm{O}}_2}^{•\mathbf{-}}$$ 7

$${\mathrm{O}}_3+{{\mathrm{HO}}_2}^{•}\leftrightarrow {\mathrm{HO}}^{•}+{\text{2O}}_2,{\mathrm{K}}_2=1.6\times {10}^9{\mathrm{M}}^{-1}{\mathrm{s}}^{-1}$$ 8

$${{\text{2HO}}_2}^{•}\to {\mathrm{O}}_2+{\mathrm{H}}^{+}$$ 9

Increasing the diazinon removal efficiency with the increase in LDH concentration in the COP can be attributed to the increase of active sites at the nanoparticles surface in react with ozone in solution. Consequently, the transfer of ozone from the gas phase to the solution and the rate of ozone destruction also increase [36, 37]. As a result, increasing ozone decomposition leads to an enhance in the concentration of radical hydroxyl in the surface and volume of the liquid, so increasing the process efficiency and the decomposition of organic compounds into minerals such as CO₂ and H₂O [21].

However, with further increasing LDH concentration from 500 to 1000 mg/L, diazinon removal was not significantly enhanced. It can be described that using excessive LDH nanoparticles might reduce the effective contact of O₃ molecules with the catalyst surface, so reduce the concentration of O₃ per unit area which resulted in the decrease of process efficiency.

In most advanced oxidation processes, increasing the concentration of nanoparticles to a certain amount increases the process efficiency. In the study conducted by Kamani et al. (2017), with the simultaneous increase of TiO₂ nanoparticles and contact time, the removal efficiency of erythromycin and metronidazole antibiotics by Sono-nano-catalytic process increased. With the consumption of 0.6 g/L nanoparticles and contact time of 90 min, the removal efficiency of erythromycin and metronidazole reached 91% [38].

Effect of pH

Figure 8(a) demonstrate the effect of solution pH on diazinon removal efficiency (%R) at central point values of other parameters. As can be seen in the Fig. 8, and according to ANOVA analyses, increasing the initial pH from 3 to 10, no obvious influence on diazinon removal. Therefore, the removal efficiency of the process is not so pH-dependent which can be related to the effect of pH on the properties of catalyst surface and on the ozone transfer from the gas phase to the solution and ozone destruction [10]. Many studies have reported that in alkaline conditions the performance of SOP increases. At alkaline pH, because of ozone decomposition generating free active radicals (such as superoxide ion (O₂^•), radical HO₂^•, and especially OH^• radicals), which are extremely oxidizing species and selectively react with organic and inorganic compounds in aqueous solutions. In contrast, ozone molecules are stable at acidic pH, so react with compounds through selective reactions [10, 39]. The lower efficiency of the ozonation process in acidic or natural conditions is a defect. The compounds such as diazinon, which react slowly with ozone molecules, decompose unselectively at high pH values.

Fig. 8.

Effect of pH (a) and interaction effect of reaction time and pH (b) on the removal efficiency of diazinon at central point values of other parameters (LDH dose: 500 mg/L, diazinon concentration: 125 mg/L)

In this study, due to the presence of LDH nanoparticles as a catalyst in the ozonation process, more free active radicals were formed via the decomposition of ozone molecules, which would enhance the process efficiency under acidic, natural and alkaline conditions [37, 40]. In addition, the high removal of diazinon in acidic pH can be related to pH_pzc of MgAl-LDH nanoparticles. The pH_pzc of MgAl-LDH was determined by the solid addition method to be 7.5. At pH below pH_zpc, the surface of LDH nanoparticles protonated, thereby increasing the chance of adsorption of diazinon ions [37, 41]. Figure 8(b) demonstrate the effect of solution pH and reaction time on the removal of diazinon. As can be seen in Fig. 8(b), the removal efficiency of diazinon gradually increased with increasing the reaction time at all applied pHs. But according to ANOVA analyses, interaction effect between reaction time and pH on the removal of diazinon was not significant (p value>0.05).

Effect of diazinon initial concentration

Figure 9 demonstrate the effect of diazinon initial concentration and interaction effect of reaction time and diazinon concentration on the response y (%R) at central point values of other parameters. In advanced oxidation process (AOP), the pollutant concentration in aqueous solutions plays an important role in the oxidation efficiency and the synergistic effects of AOP processes such as SOP and COP (42). Rey et al. in a study on the mineralization of metoprolol by photocatalytic ozonation process found that the synergistic effects of the process were greater at the higher pollutant concentrations compared with low concentrations [42].

Fig. 9.

Effect of diazinon initial concentration (a) and interaction effect of reaction time and diazinon concentration (b) on the removal efficiency of diazinon at central point values of other parameters (LDH dose: 500 mg/L, pH: 6.5)

As can be seen in Fig. 9, and according to ANOVA analyses, the initial concentration of diazinon has a negative effect on the response (removal efficiency). So that, the removal efficiency decreased slightly with increasing initial concentration of diazinon. This could be ascribed to the insufficiency of ozone molecules compared to diazinon molecules in the solution. Increasing the pollutant concentration leads to a reduce in the number of active sites to stimulate. In addition, a greater amount of intermediates can be generated in higher pollutant concentrations. Thereby, an increase in ozone consumption occurs [20, 43]. The presence of a catalyst in the ozonation process can reduce ozone consumption and increase the performance of the process. Similar results have been obtained from previous studies. In a study conducted by Alinejad et al. [44], the nanoparticles of MgO were prepared and used for degrading methotrexate drug from aqueous solutions by catalytic ozonation process. They reported that due to the low ^•OH radical production from O₃ depletion in the higher drug concentrations, degradation efficiency decreased [44]. According to Fig. 9(b), the removal efficiency of diazinon increased with increasing the reaction time at all applied diazinon concentrations.

Intermediate compounds, toxicity and degradation mechanism

In this section, intermediate compounds resulting from diazinon degradation were identified using GC-MS, and the proposed mechanism of diazinon decomposition were suggested from the other studies [2, 3]. The results of GC-MS analysis showed five intermediates including diethyl phosphonate, (2-isopropyl-6-methyl-pyrimidin-4-ol, IMP), diazoxon, hydroxydiazinon, and diazinon methyl ketone are represented in Table 4.

Table 4.

Identified compounds by GC/MS in the catalytic ozonation of diazinon.

Mechanisms of intermediates formation of doazinon degradation have been reported by various studies. They reported the diazoxon as an intermediate compound of diazinon degradation when atom of oxygen is replaced by atom of sulfur in the P = S bond [45, 46], which has more toxicity than diazinon. It is also, diazoxon and hydroxydiazinon have the same toxicity and are more toxic than the other metabolites and then IMP was formed by the hydrolysis of diazoxon by cleavage of the P_O bond(pyrimidine ring) [2, 3, 46]. Some researchers reported that hydroxydiazinon and 2-hydroxydiazinon have been formed by hydroxylation of primary and tertiary carbon atoms of propyl groups, respectively [45].

The next mechanism was concluded to be hydrolysis by the hydroxyl radicals attack on the O functional group to divide the diazinon into two compounds, namely diethyl phosphonate and IMP [47]. And finally the last mechanism was suggested as hydroxylation, where the hydroxyl radicals attached to the isopropyl group in diazinon and formed hydroxydiazinon [1].

Investigation on mineralization of diazinon

In order to investigate the performance of this process in the degradation of diazinon, TOC and COD analyses were performed under optimum operating conditions, for five different processes, i.e., SOP, MgAl-LDH COP, aeration and adsorption. Because the ozone gas was produced from dry air, the aeration process was also tested. The results are shown in fig. 10. For the initial concentration of 120 mg/L of diazinon and under the optimum conditions, the initial COD and TOC values of the diazinon sample were 9 g/L and 7 g/L, respectively. As shown in Fig. 10, using the SOP process and after 70 min, COD and TOC decreased by 70% and 62%, respectively. The removal efficiency of COD and TOC was 80% and 74% in the ozonation process with the presence of MgAl-LDH at a concentration of 750 mg/L (MgAl-LDH COP), respectively. The maximum removal efficiency of COD and TOC in adsorption process was 11% and 9%, respectively, which could be ignored during COP. Meanwhile, the aeration process had no significant effect on COD removal and TOC removal. These results confirmed that the addition of MgAl-LDH nanoparticles enhanced the process performance. Similar results were obtained by the literature [11, 27, 40, 48, 49].

Fig. 10.

COD and TOC removal efficiency in diazinon degradation (pH: 8.25, time: 70 min, initial diazinon concentration: 120 mg/L, and temperature:23 ± 5 °C)

Conclusion

In this study, the removal efficiency of diazinon in the catalytic ozonation process using MgAl- layered double hydroxides nanoparticles as a new catalyst was investigated. MgAl-LDH nanoparticles were synthesized by a simple and fast co-precipitation method. MgAl-LDH catalyst was characterized by XRD and FESEM-EDX analyses. The effects of process variables including initial diazinon concentration, LDH dose, initial pH, and reaction time on the removal of diazinon by MgAl-LDH catalytic ozonation process were studied. Central composite design was used for the optimization and modeling of the process. After optimizing the variables, for evaluation of the process performance, the total organic carbon (TOC) and chemical oxygen demand (COD) analyses were performed under optimum conditions. The highest removal efficiency of 92% was observed in optimum conditions as follow; initial diazinon concentration: 120 mg/L, pH: 8.25, LDH dose: 750 mg/L, and reaction time: 70 min. The quadratic model was obtained with a high degree of fit. The removal of COD and TOC were 80% and 74%, respectively. Also, optimized DLLME method [solvent to liquid sample ratio (0.3), centrifuge time (5 min), vortex time (5 min), extraction solvent to dispersion solvent ratio (0.3)] was used to extract diazinon from aqueous samples. Under these conditions, the recovery amount of 90% was obtained. As a result, MgAl-LDH COP, as an excellent practical alternative, has a high performance in removing pesticides such as diazinon from aqueous solutions.

Declarations

Conflict of interest

I, as the main corresponding author of this study declare that there is no known conflict and competing financial interests or personal relationships that can influence the work reported in this paper.