Co/Ni/Al-LTH Layered Triple Hydroxides with Zeolitic Imidazolate Frameworks (ZIF-8) as High Efficient Removal of Diazinon from Aqueous Solution

Today, high consumption and increasing use of pesticides and chemical fertilizers to control pests of agricultural products, and the entry of these pollutants into the environment, is one of the most important environmental and health problems. Their non-biodegradability, as well as their toxicity and carcinogenicity, have generally made these compounds one of the most dangerous pollutants that cause inevitable pollution of the environmental. Among the various methods used to remove agricultural pesticide residues from the water sources, the adsorption method has received more attention due to its simplicity, cost and higher efficiency. In this research, nanocomposite of Co/Ni/Al-LTH@ZIF-8 was synthesized by in-situ growth of ZIF-8 on the Co/Ni/Al-LTH and used for the removal of diazinon (DIZ) pesticide from aqueous solution. Characterizations of the nanocomposite were performed by various techniques, including Fourier transform infrared spectroscopy, X-ray diffraction, field emission scanning electron microscopy (FESEM), energy dispersive X-ray spectroscopy (EDX) and thermal analysis. Statistical evaluation was studied by BOX-Behnken design. In addition, the response surface methodology was used to optimize the factors affecting on the adsorption process. Parameters such as adsorbent dose (mg), pH, and contact time (min) were considered in this experiment. The results showed that the removal efficiency of diazinon is improved significantly (from 64 to 84%) by loading ZIF-8 on Co/Ni/Al-LTH. Statistical studies showed the optimum conditions achieved under pH = 6.9, adsorbent dosage 25 mg, and contact time 12 min.


Introduction
Crop production are essential for human nutrition. The Food and Agriculture Organization of the United Nations (FAO) defines food security as food availability, food access, and food usage. However, in order to rapid increase in food production, annually thousands tons of pesticides are used in the world [1]. Pesticides pollutions are considered an important concern in the world because many of such compounds are dangerous to both human health and the environment. Agricultural pesticides are one of the major pollutants, and this has raised concerns regarding the protection of health and the environment and hence must be controlled to minimize contamination problems [2,3]. Different types of pesticides such as organophosphorus and organochlorine are used in agriculture [4]. Organophosphate pesticide is one of an important category of pesticides that are mostly used in agriculture. Among organophosphorus pesticides, diazinon is widely used in agricultural for the struggle against pests. Diazinon is a kind of organophosphate pesticide. It is utilized as a control measure for pests in fruits, vegetables and field crops, but excessive consuming of this insecticide leads to contaminate the environment and consequently removal of this compound is essential concern. The LD50 for diazinon in male and female Rat oral are 1340 and 1160 mg/kg [5]. The existence of such toxins causes interaction with acetylcholinesterase enzyme and hence causes neurological disorders [6]. The solubility of diazinon in water is low, as a result, they remain in the soil for a long time and cause surface water and groundwater pollution [7].
In recent years, numerous and effective techniques have been used to identify or eliminate pollutants such as heavy metals and pesticides in the agricultural industry. For example, the manufacture of electrocatalytic composites based on intermediate metals such as cobalt and nickel has been developed as a technology for the disposal of wastewater in the agricultural industry [8][9][10][11][12]. Other methods such as electrochemical [13], photocatalytic degradation [14], electrochemical sensors [15], biodegradation [16], biosensor [17] and adsorption [18] have also been used for this purpose.
Among these, the adsorption method is a common and practical method due to its design, availability, simplicity, low cost and higher efficiency [19]. Removal of pesticides from aqueous solutions is performed by the various adsorbent, such as, activated carbon [20], carbon nanotubes [21], graphene oxide [22], zeolites [23], metal organic framework (MOF) [24] and carbon nitride nanosheets [25] are used to remove pesticides from aqueous solutions.
Layered double hydroxides (LDHs), have received a great attention in various fields of chemistry in recent years. LDH is known to have a layered construction composed of inorganic ions (either di-or trivalent) covered by hydroxyl ions. NO 3 − and Cl − are anions that are intercalated among the layers categorized as a portion of mud raw materials [26]. An LDH is represented by the general formula [M 2+ 1-x M 3+ x (OH) 2 ] x+ (A n-) x/n ·mH 2 O, where M 2+ and M 3+ are divalent and trivalent metals cations and A nis the interlayer anion [27]. Due to the unique molecular structure of LDHs, they exhibit desirable chemical and physical properties, such as high surface area, catalytic ability, anion exchange capability, high mechanical and thermal stability and tunable of interlayer spaces [28]. LDHs are widely used in adsorption process for removal of contaminant from water. The distance between the layers of LDHs are adjustable with access to the guest anions through the process of chemical synthesis. Therefore, these materials have led to a promising approach for removal of organic contaminants from wastewater and water purification. However, synthesized LDH nanosheets are usually not stacked in parallel and regularly [29,30]. As a result, the effective specific surface area and mass transfer space of LDHs are relatively small, reducing adsorption capacity [31]. On the other hand, the unsaturated coordination states of the surface cations of LDHs can be a suitable substrate for nucleation processes and direct growth of metal complexes [32]. Given that today the removal of organic pollutants such as pesticide residues from the environment is one of the main goals of wastewater treatment, so increasing the absorption capacity of LDHs is very important to achieve this. Recent research has shown that due to properties such as hydrophobicity or conjugated π, which are created by combining and modifying between layers or surfaces of LDHs, their ability to remove a variety of organic pollutants can be improved [33][34][35][36][37].
Porous materials such as metal organic frameworks (MOFs) are great interest to overcome this impediment for removal processing. The common strategy for the construction of MOFs is based primarily on the proper selection of transition metal cations as nodes and multidentate organic ligands containing O-or N-donors as linkers [38]. Zeolitic imidazolate frameworks (ZIFs) represent a unique class of (MOFs) in which the network topology and related properties vary greatly while core chemical connectivity is retained [39]. Research and development in the synthesis and use of metal-organic frameworks as multi-shell nanostructures in applications such as adsorptive removal of pollutants, catalytic conversion of gas pollutants, and high-performance batteries has been very rapid [40]. ZIFs are composed of tetrahedrally-coordinated transition metal ions (e.g. Fe, Co, Cu, Zn) connected by imidazolate linkers [41]. ZIFs have a higher surface area than zeolites, and they are more thermally and chemically stable than other MOFs [42]. As a result, these features, select ZIFs suitable for use in the field of adsorption [43]. Among these, imidazole zeolite frameworks (ZIF-8) are one of the most popular MOF-derived nanoporous carbons that have high stability [41] and in the absorption of environmental pollutants such as phthalic acid [44], sulfate [38], benzothriazoles [45], Rhodamine B [46,47] and tetracycline [48] has been used. ZIF-8 is richer and easier to synthesize than other MOFs. Also, ZIF-8, in addition to having high chemical and thermal stability, can maintain 1 3 its crystallization and porosity after being placed in various solutions such as water and organic solvents [49][50][51]. Currently, ZIF-8 materials are widely used in many forms such as powders, colloids, membranes or thin films, and in various important fields [52].
The unique physical structure and chemical properties of ZIF-8 make it a research target in adsorption and removal systems in the treatment of water pollutants. However, the ZIF-8 always suffers from some shortcomings. ZIF-8 nanoparticles tend to accumulate in water, which increases the particle size and transfer resistance and reduces the surface area, thereby reducing their adsorption performance. Also, because ZIF-8 nanoparticles are difficult to separate from aqueous solution, their reconstruction and reuse is very limited [53]. Compared to single ZIF-8, ZIF-8 composites can reduce adsorption accumulation, makes it easier for the adsorbent to separate from the water, increases active sites and adsorbent stability, demonstrating superior adsorption and better recycling ability than components during the treatment of water pollutants [54,55]. To address these shortcomings, multifunctional composites can be formed through the controllable integration of ZIF-8 and functional materials. These composites or hybrids show superior properties over individual components through the collective behavior of each functional unit. Many previous studies have proved the significance of this incorporation strategy. For example, ZIF-8 in combination with other nanoparticles in applications such as removal of sulfate from aqueous solution [38], CO 2 adsorption [56], adsorption several organic contaminants [47], bio-relevant fields [57] and pH-responsive drug release [58] has shown high potential.
It is generally reported that 3D structures show more adsorption sites and a higher specific surface area compared to 2D architectures [59]. As mentioned earlier, due to the fact that LDHs are excellent support materials for the growth of metal complexes, the design of functional composites by combining the advantages of 3D nanosheets of LDHs and hydrophobic/π-conjugated surfaces of MOFs can provide full use of functional surfaces and thus lead to significant performance improvements in the removal of organic compounds.
In this work, ZIF-8 and layered triple hydroxide (LTH) was successfully synthesized via a facile one-step in situ hydrothermal method. The as-synthesized Co/Ni/Al-LTH@ ZIF-8 hybrids were characterized by SEM, EDS, XRD, FT-IR, TGA, BET and Zeta potential methods. Subsequently, as-synthesized Co/Ni/Al-LTH@ZIF-8 nanocomposites were applied for the removal of diazinon pesticide from the aqueous solutions. Various factors influencing on the adsorption process including the adsorbent dosage, the pH of the solution, and contact time were investigated in detail. The Box-Behnken design was performed for optmization.  3 and diazinon were purchased from Merck Company. All chemicals compounds were used without further purification.

Reagents and Chemicals
Stock solution of diazinon (1000 mg/L) was prepared in mixture of methanol and water (50/50 by volume) and kept in a refrigerator at − 18 °C until use.

Instrument and Software
X-ray diffraction (XRD) patterns were recorded by an X-ray diffractometer (PHILIPS-PW1730) using Cu K α radiation (1.5 Å) in the range of 2θ 0.8º to 70º with a scanning rate of 0.05 degree/second. Field emission scanning electron microscopy coupled with energy dispersive X-ray spectrometer (FESEM/EDX) model TESCAN-MIRA III. (BELSORP MINI 2) was used to obtain the morphology of the synthesized samples. The FT-IR spectra of the prepared samples were recorded by Thermo/FT-IR AVATAR spectrophotometer in the range of 600-4000 cm −1 at room temperature. The porosity and surface area of the samples was studied by the Brunauer-Emmet-Teller (BET) method using nitrogen adsorption/desorption isotherms at 77 K on a BELSORP MINI II device. Electronic absorbance spectra were recorded using a UV-vis spectrophotometer (PG Instrument Ltd, double beam-Model T90 +) with spectral range of 190-350 nm. In order to find of the optimized conditions of the removal efficiency of diazinon by Co/Ni/Al-LTH@ZIF the Box-Behnken design method was performed (Design-Expert software, version 13, State-Ease, Inc., United States).
The response surface methodology (RSM) and BBD method was implemented to optimize the effect of different factors of the removal efficiency of diazinon by Co/Ni/ Al-LTH@ZIF. (Design-Expert software, version 13, State-Ease, Inc., 3 United States). Energy dispersive Xray (EDX) spectra were recorded on an EDX Genesis XM2 attached to SEM. The zeta potential of nanoparticles was measured by Horiba (SZ-100) at room temperature.

Synthesis of ZIF-8 Nanocrystals
ZIF-8 nanocrystals were synthesized with Zn 2+ : 2-methylimidazole: methanol molar ratio of 1: 80: 4000 according to the route reported previously with some modifications [60]. For this propose, two separate solutions were prepared as follows; solution A: 6 g of Zn(NO 3 ) 2 was dissolved in 100 mL methanol; solution B: 16 g of 2-methylimidazole was dissolved in 100 mL methanol. Then solutions A and B were mixed under vigorous agitation until the mixture turned cloudy. After 6 h, precipitate were collected by centrifuging (8500 rpm, 20 min), then washed with methanol for three times. Finally, the white product (ZIF-8) was dried overnight at 80 °C.

Synthesis of LTH@ZIF-8 Composite
12 g of Zn(NO 3 ) 2 .6H 2 O was dissolved in 50 mL methanol and gradually added in to a dispersed solution of Co/Ni/ Al-LTH (0.2 g in 10 mL methanol). Then 2-methylimidazole solution (4.5 g in 50 mL of methanol) was added to the above solution under vigorous magnetic stirring at room temperature for 4 h. The light brown precipitate was collected by centrifuging (8500 rpm, 20 min) and washed several times with water and ethanol, respectively and dried in oven at 80 ºC for overnight.

Measurement of Diazinon Uptake
Standard solutions of diazinon with different concentrations (5 to 50 ppm) were prepared by dilution of its stock solution (1000 ppm). Due to the low solubility of diazinon in water, all solutions were prepared using mixture of methanol and water (50/50 volume ratio). Calibration curve was constructed by plotting absorption (A) against concentration of the standard solutions. Absorption of the solutions was measured by UV-vis spectrophotometry at maximum wavelength (λ max ) 247 nm. The removal percentage was calculated according to following equation [61].
where C o and C e are initial and equilibrium concentrations (mg L −1 ), respectively. The adsorption capacity (q t ) of the adsorbents was calculated by Eq. 2 [62].
where, V is the volume (L) of the solution and W is the mass (g) of the adsorbent [63].

Characterization of Synthesized Nanocomposites
The synthesized samples were characterized with several techniques as follows:  Fig. 1a at 2θ of 19 • , a symmetrical peak with high intensity is observed, which can be caused by the crystallization of β-Ni(OH) 2 (JCPDS 14-0117) [65,66] (1) and β-Co(OH) 2 impurities [67][68][69][70]. The peaks from these plates are usually wide, asymmetric, and low in intensity. These are the characteristics of the hydrotalcite-like compounds [71]. Figure 134) and (044), respectively. By considering and comparison this result with the previous reports [72][73][74] it can be said, this compound was well synthesized. Figure 1c exhibits the XRD pattern of the Co/Ni/Al-LTH @ ZIF-8 nanocomposite, as can be observed this pattern almost consists of both peaks in XRD patterns of Co/Ni/Al-LTH and ZIF-8. Results reveal that the combination of Co/Ni/Al-LTH and ZIF-8 nanomaterials does not disturb their regular structure and the nanocomposite was successfully synthesized. No additional characteristic peaks were detected in the XRD patterns, indicating that no impurities were present in the synthesized products.  Figure 2a shows the FT-IR spectrum of the Co/Ni/Al-LTH. The narrow peak at 3636 cm −1 can be attributed to O-H bonds in the crystallite assembly of LDH. The broad peak around 3568 cm −1 can be assigned to the O-H stretching modes of the interlayer water molecules and the H-bound O-H groups and the peak at 1714 cm −1 is attributed to the bending mode vibration of the interlayer water molecule. The absorption bands appearing at 1363 cm −1 , 1419 cm −1 and 830 cm-1 are due to the vibration and bending modes of carbonate CO 3 2− intercalated in the interlayer of all Co/Ni/ Al-LTH materials [75]. Absorption bands observed in 1093, 1223 and 1363 cm −1 can be ascribed to interlayer nitrate groups [76]. Absorption bands around 1000 cm −1 are due to the stretching and bending vibrations modes of M-O and M-OH. (M = Co 2+ , Ni 2+ , Al 3+ ). Figure 2b shows the FT-IR spectrum of the ZIF-8 metal organic network. The bands observed at 689 and 755 cm −1 can be ascribed to the stretching vibrations modes of the Zn-N and Zn-O bonds, respectively [77]. In addition, the bands related to 1143, 1584 and 2927 cm −1 can be associated to the C-N, C = N and C-H stretching vibrations modes in the imidazole ring, respectively. Figure 2c exhibits the FT-IR spectrum of the Co/Ni/Al-LTH@ZIF-8 nanocomposite as can be observed this spectrum almost includes of both signals in spectra of (2a) and (2b).

Morphology Studies of the Synthesized Samples
To study the surface morphology and composition of Co/ Ni/Al-LTH @ ZIF-8 composite, Field emission scanning electron microscopy and EDX analysis were used.  Figure 3b shows images of ZIF-8 particles with a uniform polygonal structure measuring approximately 30-60 nm. Figure 3c exhibits the FESEM images of the Co/ Ni/Al-LTH @ ZIF-8 composite. The morphology of Co/Ni/ Al-LTH @ ZIF-8, has a moderate change with the Co/Ni/ Al-LTH images, transforming from hexagonal nanoplates to spherical shape Fig. 3a, b. EDX analysis was performed to evaluate the composition of the synthesized adsorbents. The EDX spectra with the tables of elemental composition of ZIF-8 and Co/Ni/Al-LTH @ ZIF-8 composite are presented in Fig. 3e, f. The EDX plot and the ratio of the number of the atoms of the elements constituting ZIF-8 are presented in Fig. 3e. This ratio is in agreement with C 8 H 10 N 4 Zn formula. The presence of both Co/Ni/Al-LTH and ZIF-8 in the composite structure was confirmed by its EDX result. In addition, the peaks show the specific energy values of Ni, Co, Al and O (Fig. 3f). The calculated ratio (Ni + Co)/Al based on the initial molar ratio of the atoms is equal to 4, which is confirmed and confirms its correct synthesis. The results of EDX analysis presented in other studies for Co/ Ni/Al-LTH composites with similar molar ratios [78,79] show the peaks corresponding to the energy values for Ni, Co and Al elements. The energy region for the Zn element from the Zif-8 EDX analysis synthesized in this study is also presented in Fig. 3e. By combining the two spectra of Co/ Ni/Al-LTH and ZIF-8 and comparing it with Fig. 3f (EDX spectrum of the Co/Ni/Al-LTH @ZIF-8), it can be said that the analysis regions of these elements are very close to each other but are independent of each other.

Thermal Analysis
Results of thermal analysis of Co/Ni/Al-LTH, ZIF-8 and Co/Ni/Al-LTH@ZIF-8 are presented in Fig. 4a-c. Figure 4a shows thermogram of Co/Ni/Al-LTH that contains two steps. In the first step, due to the evaporation of water molecules present on the surface and as well as interlayer water molecules, within the range of 50-200 °C temperature. In the second step, weight loss observed at a temperature range of 200-350 °C is ascribed the dehydroxylation of the brucite-like metal layers and the release of intercalated anions [80]. Figure 4b presents the thermogram of ZIF-8, a graduate weight loss owing to evaporation of water molecules absorbed on the surface and in cavities of the compound below 200 °C. The second weight-loss stage belongs to decomposition of the organic ligand 2-methylimidazole. Eventually, degradation of the metal organic framework is the reason for the weight loss of the third stage [81]. Figure 4c shows thermal behavior of the Co/Ni/Al-LTH@ZIF-8 composite. The weight loss of the first and second stages relate to the solvent extraction and dehydroxylation in Co/ Ni/Al-LTH nanoparticles and the third stage is related to the decomposition of the ZIF-8 metal organic framework in the Co/Ni/Al-LTH@ZIF-8 composite. Figure 5 shows the surface charge evaluation of the adsorbents synthesized in this study through the zeta potential. The zero-point charge (zpc) of nanocomposit particles is around 10. This value is very close to the zpc of the ZIF-8 [82]. This pronounces that Co/Ni/Al-LTH is covered by ZIF-8 particles and reconfirming above results. The adsorbent surface will be negatively charged surfaces when pH > pzc and in contrast will be positively charged when pH < pzc. Hence, the adsorption process is influenced by the repulsion or attraction forces between adsorbent and adsorbate, this phenomenon due to the pzc of the adsorbent (i.e., Co/Ni/Al-LTH@ZIF-8) and pK a of the adsorbate (i.e., diazinon) and also pH of the solution. As a result, to find higher removal efficiency, the pH value of the solution also should be optimized.

Adsorption Isotherm Studies
Figure 6a-c shows nitrogen adsorption/desorption (BET method) and pore size distribution curves (BJH method) of the Co/Ni/Al-LTH (a), ZIF-8 (b) and Co/Ni/Al-LTH@ZIF-8 composite, respectively. According to the IUPAC classification, the synthesized Co/Ni/Al-LTH can be categorized as type IV with H 4 hysteresis loop (Fig. 6a), which indicates the existence of slit-like mesoporous structures of the corresponding compound [83]. As shown in Fig. 6b, the ZIF-8 nanoparticle isotherm represents type I, which is characteristic of microporous materials. Figure 6c is similar to the Fig. 6b indicate that the nanohybrid of Co/Ni/Al-LTH@ZIF consists microspore, and also its isotherm is classified as type I. Table 1 presents the total surface area (S BET ) and total pore volume for the synthesized LTH, ZIF-8 and nanocomposites, respectively. Both surface area and total pore volume of Co/Ni/Al-LTH@ZIF-8 is very close to the ZIF-8, nevertheless both aforementioned parameters are significantly more than that for Co/Ni/Al-LTH.

Experimental Design and Optimization
An experimental design method was planned using Design-Expert 13 software in which the surface response design technique is employed based on the Box-Behnken method. The parameters of adsorbent dose, pH, and contact time were selected as the main affecting factors for removal of diazinon. Table 2 shows three levels [low (− 1), medium (0), and high (+ 1)]. Table 3 shows the Box-Behnken design matrix by considering three factorthree levels of input parameters and the corresponding output response for the removal of diazinon by the Co/ Ni/Al-LTH@ZIF-8.
In this study, results of the analysis of variance (ANOVA) were performed using Design Expert 13 software. In accordance with the conventional acceptance of statistical significance, at a 95% confidence level, the p-value must be less than 0.05 to be meaningful. If the p value for non-compliance is greater than the value selected for significance at a confidence level, it indicates that the model is desirable [84].
According to the data in Table 4 (ANOVA results), the Model F-value of 540.45 implies the model is significant. There is only a 0.01% chance that an F-value this large could occur due to noise. P-values less than 0.0500 indicate model terms are significant. In this case A, B, AB, BC, A 2 , B 2 are significant model terms. Values greater than 0.1000 indicate the model terms are not significant. The p-value for noncompliance in this design for diazinon removal is < 0.0001, which specifies the suitability of this model. The polynomial that expresses the relationship between the answer and the effective sentences is presented as Eq. (3). Where A, B, and C represent the adsorbent dose, pH, and contact time, respectively. The parameter that has the great positive coefficient value in the equation indicates a greatly positive influence on the response. The mean squares, the degree of freedom, and the sum of squares were also obtained. Moreover, F and p-values were used to display when these terms are significant in the quadratic model. The random pattern of residual distribution diagram allows effective uncontrollable factors to be detected during the experiments. The random and two-sidedness distribution of errors in each experiment indicates that the errors are not systematic, and the proposed model can be defined to perform defined experiments. Figure 8a shows the residual curve of each experiment to remove diazinon pesticide by synthesized adsorbents. As can be seen, the error in performing the experiments is a random error and is not systematic. Therefore, the prepared model is able to provide the percentage of removal of the desired pesticide from the solution. The Predicted R 2 of 0.9874 is in reasonable agreement with the Adjusted R 2 of 0.9967; i.e. the difference is less than 0.2. Figure 8b also shows the correlation between the experimental data and the predicted response by the RSM optimization approach. Data closer to the line show a slight difference between the experimental and the predicted responses, indicating that a good optimal point can be obtained from the designed experiment. Also indicated that the value of adequate precision is 74.789.

Response Surface
To better understand each response level, three-dimensional response surfaces curves and two-dimensional contour diagrams of effective parameters (adsorbent dose, pH and contact time) and their interaction on diazinon removal efficiency were plotted and analyzed (Fig. 9c). Figure 9a presents the effect on the interaction between the initial pH and the adsorbent dose on the removal efficiency of diazinon at the central point of the contact time.
According to the proposed model (Eq. 3), increasing the initial pH and the amount of adsorbent dose have a positive effect on the absorption percentage of diazinon. The removal of diazinon increases with increasing initial pH to about 7 and then the efficiency of the adsorption process (regardless of the amount of adsorbent) decreases. The pH of the base solution plays an important role in electrostatic absorption because it can affect the nature of the charge on the composite surface. The zpc value of Co/Ni/Al-LTH@ZIF-8 is about 10 that means at the pH < 10 its surface is positively charged, on the other hand, the pKa of the diazinon is 2.6 [85] indicating that the pH value higher 2.6 of the solution it has negative form. Consequently, it can be visualized that electrostatic attraction between the Co/Ni/Al-LTH@ZIF-8 and diazinon is one of the main effect for the adsorption process. Also, the decrease in diazinon removal efficiency by applying pH values above 8 may be attributed to the formation of Al(OH) 4− and the degradation of the LDH structure [86,87]. Also, according to the contour curve (Fig. 9a), due to the positive interaction of the two parameters, an increase in the removal percentage was observed with increasing the amount of adsorbent in the pH range between 5 and 9. It can be concluded that the suitable pH value for the absorption was 6.9, which could lead to the equilibrium of protonation and the stability of LTH/ZIF-8 structure. Figure 9b shows the response surface curve of the removal percentage under the influence of the parameters of contact time and the amount of adsorbent at the pH midpoint. According to the proposed model (Eq. 3), the contact time and its followup, the interaction of the two parameters does not show a significant effect on the amount of deletion. However, the adsorbent dose parameter alone has a positive effect on the removal percentage due to the ZIF-8 coating on the adsorbent surface. As the absorbent dose increases, the removal efficiency of diazinon increases almost linearly, but after values above 20 mg of the absorbent dose, the removal percentage becomes almost constant.
In fact, fast absorption occurs first and then the absorption rate decreases and finally reaches equilibrium. Due to the saturation and reduction of the number of active adsorption sites on the adsorbent surface over time, the adsorption of pesticides from the solution is limited.   Figure 9c shows the response curve of the removal percentage with contact time and pH.
The proposed model (i.e., Eq. 3) shows that the contact time of the effect is not significant, but the pH parameter is also positive in the pH range between 6 and 9 times according to the study described and is converted to the cationic form. Therefore, it creates a strong electrostatic bond between the adsorbent surface and the anionic form of diazinon and causes its removal. Also, due to the positive interaction of the two parameters, the removal percentage had a positive effect. Figure 10 presents the values of the desirability and optimal factors affecting the response under different conditions. Based on the above experiments, removal efficiency of 84% was obtained under following conditions: adsorbent dosage 25 mg, pH value 6.9 and contact time 12 min, which is very close to the actual value.

Study of Removal Efficiency at Optimal Levels
The removal of diazinon with an initial concentration (20 ppm) was performed by ZIF-8 and Co/Ni/Al-LTH@ ZIF-8 composites. UV-vis spectrum of diazinon solution before removal is shown in Fig. 11a. The UV-vis spectra of the diazinon solution after removal by Co/Ni/Al-LTH, ZIF-8 nanoparticles and Co/Ni/Al-LTH@ZIF-8 composite are presented in Fig. 11b, c respectively. Obviously, the intensity of the bands are reduced after adsorption process by the above adsorbates. Results revealed that the removal efficiency of the Co/Ni/Al-LTH@ZIF-8 (84%) is more than that of ZIF-8 (68%). ZIF-8 nanoparticles have a much higher specific surface area than other adsorbents. On the other hand, by synergizing with LTH materials, the composite is more stable in water, increasing the recyclability, porosity or pore size of ZIF-8 composites. As can be seen, the increase in the initial pH of the solution resulted in an increase in the adsorption efficiency of diazinon by the adsorbent, and after reaching the maximum value at pH 7, the amount of this yield decreased again. By observing of this trend, it can be concluded that increasing the pH leads to an increase in negative charge density at the adsorbent surface and may increase the adsorption of diazinon by the composite. According to Fig. 12, the diazinon molecule is non-ionic [88], which means that it is not in cationic and anionic form in acidic and basic medias. The adsorption capacity of LDHs is often considered to be limited due to the highly hydrophilic nature of clay surfaces for nonionic and highly hydrophobic pesticides [89]. It can be stated that the significant uptake of non-ionic and hydrophobic organic compounds by LDHs is related to the existence of "hydrophobic micro-sites" at the surface of the clay composition, i.e. base oxygen at the load sites in the clay mineral [90]. In such cases, to capture such relatively hydrophobic areas, effective competition between organic compounds such as diazinon and water molecules can be considered.
Therefore, it can be concluded that no dipole-dipole interaction can exist between the composite adsorbent and the adsorbed pesticide. On the other hand, the decrease in adsorption at pH less than 5 can be attributed to the partial dissolution of the mineral part of the composite structure (LDH) by acid hydrolysis. In addition, for pH greater than 7, carbonate contamination is more likely to occur, which due to the high affinity of LDHs for carbonate anions, may impair the absorption of diazinon [91].
Moreover, the π-π stacking interaction between the imidazole ring of ZIF-8 and the aromatic ring of organophosphate pesticide could be possible and hence this can be another reason for the uptake and removal of diazinon by the nanocomposite. Furthermore, the possible interaction between zinc metal in the ZIF-8 structure as Lewis acid and sulfur site of diazinon as Lewis base can influence on the adsorption process.
Results indicated that the removal efficiency increases with increasing dose of absorbent, since, the available sites are enhanced by increasing dosage of adsorbent. Contact time also is another factor that can influence on the response, obviously by increasing contact time the removal percent is increased however, after a given time it has no impact. In this experiment, the optimized values of adsorbent dosage, pH and contact time were obtained 24.85 mgr, 6.95 and 11.88 min. Figure 13 shows the UV-Vis spectra of diazinon solution measurements before the adsorption process and after removal by the Co/Ni/Al-LTH@ZIF-8 nanocomposite, based on the designed matrix. The results of removal percentage and equilibrium concentration of diazinon after adsorption by nanocomposite according to the designed model are shown in Table 5.

Adsorption Isotherm Studies
Langmuir and Freundlich models were used for isotherm studies of DIZ adsorption behavior by Co/Ni/Al-LTH@ where q m (mg g −1 ) is the maximum adsorption capacity, K L (L mg −1 ) is the Langmuir constant (L mol −1 ), C e (mg L −1 ) is pesticide concentration at equilibrium, and q e (mg g −1 ) is the amount of adsorbed pesticide at equilibrium. The plot of (C e /q e ) versus the equilibrium concentration (C e ) imputes a straight line with slope (1/q m ) and (1/K L q m ) as intercept (Fig. 14a). Freundlich isotherm model explains multilayer adsorption; it gives the possibility for the reversibility of the adsorption. The linear Freundlich equation as follows [93]: where C e is the equilibrium concentration of adsorbate (mg L −1 ), q e is the amount of adsorbed pesticide at equilibrium (mg g −1 ), K F is Freundlich constant, and n is a constant. Figure 14b show the diagrams of Freundlich adsorption isotherms for the DIZ.
The application of these equations was compared with the examination of the coefficient of determination (R 2 ). The parameters of the Langmuir and Freundlich isotherms models obtained from this study are given in Table 6 for diazinon pesticide. Based on these data, the adsorption of DIZ pesticide on the surface of Co/Ni/Al-LTH@ZIF-8 nanocomposite is in good agreement with the Freundlich model, and it proceeds via multilayer adsorption. Table 7 presents the comparison between the maximum adsorption capacity (qm) of diazinon onto the Co/Ni/Al-LTH@ZIF-8 nanocomposite and those of other compounds. As mentioned, among the various techniques for removing organophosphorus toxins from water and wastewater, the adsorption method is more common due to its simplicity, low cost and higher efficiency. Various adsorbents, for example, MCM-41 and MCM-48 mesoporous silicas [94], fly ash and loess, as alternative to activated carbon (LOESS) [95], Chitosan/Carbon Nanotube (CHN-CNT) [96], Montmorillonite modified with iron [97], acid treated zeolite (ATZ) and modified zeolite by Cu 2 O nanoparticles (MZ) [98] were used to remove diazinon organophosphate pesticides from aqueous solutions. Compared to various adsorbents to investigate the removal of phosphorus organic compounds from water, the use of LDHs and MOFs adsorbents has received much less attention. LDHs alone, in addition to structural strength, can act as high-level porous materials as good adsorbents of pollutants. The adsorption process in LDHs can take place through physical adsorption on surfaces or by Intercalation  with contaminant molecules. Compared to other adsorbents, MOFs exhibit more virtues of versatile framework compositions, exposed active sites, tunable pore sizes, and large specific surface. By combining MOFs with suitable raw materials, the synthesis kinetics, morphology, physical and chemical properties, stability and potential use of MOFs can be greatly improved. It is anticipated that the surface metal cations of LDH exist at an unsaturated coordination state, enabling them as active sites for the in situ nucleation and directed epitaxial growth of MOF [18]. Synthesis of nanocomposite, based on LTH adsorbent sheets as a substrate, in combination with hydrophobic/π-conjugated surfaces MOFs can, in addition to providing adsorption by easy and cost-effective synthesis method, provide full use of functional surfaces and thus improve performance Significantly, in the removal of organic compounds including pesticides.

Regeneration Studies
The reusability of Co/Ni/Al-LDH@ZIF-8 composite in the removal of diazinon under optimal conditions was investigated for four successive cycles and results are presented in Fig. 15. The main factor in the reusability of the adsorbent studied in the adsorption process is its stability. For this purpose, 25 mg of sorbent was added to 20 ml of diazinon pesticide at a concentration of 20 mg L −1 under a stirrer for 12 min. The adsorbent was then separated from the solution by centrifugation (8500 rpm, 20 min) and then washed several times with methanol and water, respectively and dried in oven at 80 ºC for overnight and used for another adsorption. This reconstruction method was repeated four times and the adsorption capacity of the Co/Ni/Al-LTH@ZIF-8 adsorbent composite was slightly reduced. Undoubtedly, in addition to fast adsorption speed and high adsorption capacity, regeneration is an important factor in an ideal adsorbent. As mentioned in the introduction, this is a difficult task, despite much research into the recycling and separation of ZIF nanoparticles as adsorbents, especially from aqueous solution. Fortunately, composite materials often combine competencies and reduce the shortcomings of both components, and recycling efficiency appears to have improved compared to either of the primary compounds alone. However, improving the efficiency of recycling efficiency requires further research in this area. The amount of adsorption and activation of reactive molecules can be adjusted by modifying the synthesis method and parameters and finally controlling the composition, particle size, morphology, dispersion and structure of the supported metal nanoparticles [99]. It is hoped that this study will focus on new composites based on LDH and MOF and that future work will be developed with better results in this area.

Conclusions
In brief, triple layer hydroxide (Co/Ni/Al-LDH) and Zif-8 imidazolate zeolite frameworks were synthesized by conventional hydrothermal method. In the next step, nanoparticles of ZIF-8 was successfully loaded on the surface of Co/Ni/ Al-LTH. The synthesized composite and adsorbents were characterized by various techniques such as FT-IR, XRD, FESEM/EDX, TGA and BET and the successful synthesis of the samples was confirmed by these techniques. Each of  Percentage of DIZ removal Cycle number Fig. 15 The recycle performances of CoNiAl-LTH@ZIF-8 the adsorbents was then used under the same conditions to remove the pesticide diazinon, and the UV-Vis spectroscopy method was used to quantify the amount of removal. The Box-Behnken Design (BBD) method and response surface methodology (RSM) were used to optimize the levels of variables in this study (adsorbent, contact time and pH of the solution). The optimal conditions of the determined factors for the removal of diazinon using the designed model of BBD were determined as pH 6.9 and the adsorbent dose 25 mg and contact time of 12 min, respectively. The model used showed that pH factors and the adsorbent dose can affect the adsorption conditions. The model designed using the real sample was also examined, and the predicted values were significantly closer to the actual values. The removal efficiency of diazinon by nanocomposite consisting of Co/ Ni/Al-LTH and ZIF-8 was 84%, which is significantly higher than LTH and ZIF-8 alone. Observing the evaluation of the obtained results, it can be concluded that the synthesis of nanocomposite consisting of LTH as a substrate for MOF, utilizes the adsorption mechanisms and the advantages of each of them, such as three-dimensional structure, hydrophobicity and conjugated π bonds, which makes it an adsorbent with fast, easy, cheap and efficient synthesis. In fact, Co/Ni/Al-LTH@ZIF-8 nanohybrid surface serves as efficient adsorbent, promoting the removal of diazinon from the wastewater. It is still worth noting that the ZIF-8 complexes are more stable and easier to be separated from water, which makes them more promising for the engineering applications.
Funding The authors declare that no funds, grants, or other support were received during the preparation of this manuscript.