Quantitative Analysis and Adsorptive Removal of Glyphosate from Water Using MgO Nanostructures

INTRODUCTION
Adsorption is an important method for the removal of contaminants according to its high efficiency, flexibility, low cost, simplicity, and activity toward a wide range of contaminants [1,2]. Nanoparticles have a high affinity with adsorbents due to high porosity, their specific surface area, and active surface [3]. The adsorption method of pesticides on nano-surfaces is widely used.  Previous studies indicate that the Nano Metal Oxides have a high affinity for adsorption towards metals. So, they used to remove some toxic metals from water. Many studies show that MgO nanoparticles are traditional adsorbent substances, eco-friendly, low cost, and non-toxic [4,5]. Other studies have reported the superior characteristics of nano-materials as a result of their size and they show extremely superior performance in many fields [6-8]. The adsorption method of pesticides on Nano-surfaces is widely used. Industry and agriculture are the sources of introducing pesticides into the environment, so there are many studies that used Nanoparticles to remove pesticides from the environment by adsorption method as shown in Fig. 1. [9-11] 
Glyphosate, a non-selective and systemic herbicide, effectively controls weeds by preventing the creation of aromatic amino acids, which are vital for protein synthesis in vulnerable plants. Because of its strong adsorption to soil, glyphosate is unlikely to leach excessively or be absorbed by non-target plants.  [12-15]. Glyphosate is unlikely to enter the water through the surface because it binds strongly to the soil unless the soil is washed away by runoff [16]. The studies found that over 50% of the glyphosate was still present 14.4 km downstream due to its direct discharge into the waters of an irrigation canal [17]. Fig. 2 shows that the biodegradation pathway of glyphosate does not always lead to complete detoxification, as it produces AMPA, a persistent pollutant in the environment. The accumulation of glyphosate and its derivatives in soil and water disrupts microbial activity, reduces soil fertility, and negatively impacts non-target organisms such as earthworms and beneficial fungi. Glyphosate can also migrate into groundwater, increasing the risk of water pollution and affecting aquatic ecosystems by inhibiting the growth of algae and aquatic plants. Studies indicate that chronic glyphosate exposure can cause long-term environmental disruptions due to its slow degradation and the persistence of its intermediate products [18].

MATERIALS AND METHODS
Materials
All the chemicals are of high purity, commercially available AR grade. 

Synthesis of MgO nanoparticles 
The MgO nanoparticles were synthesis according to the method describe by Sim et al. [19]. MgO nanoparticles were prepared using the sol-gel method by dissolving magnesium nitrate in deionized water at pH 9.45 and gradually adding ammonia under continuous stirring at 80°C. The resulting precipitate underwent intensive washing and drying, then thermally calcined at 500°C for 2 hours and 20 minutes to ensure the final nanostructure.

Instrumentation and characterization 
To verify the properties of the prepared nanomaterial, several advanced analytical techniques were employed. X-ray diffraction (XRD) was employed to analyze the crystal structure and purity phase of the magnesium oxide flakes.
Electron refraction (EDX) microscopy, attached to the scanning electron microscope unit, was used. This technique allows for the determination of the atomic and weight ratios of the constituent elements of the magnesium oxide flakes and ensures they are free from chemical impurities resulting from the preparation process. For a deeper understanding of particle shape and morphological distribution, field emission scanning electron microscopy (FE-SEM) was used. The concentrations of glyphosate pesticide before and after adsorption were determined using UV-Vis spectrophotometry at a specific wavelength to ensure the accuracy of the removal efficiency study.

Procedure
The maximum wavelength (λmax) of Glyphosate is 224nm (Fig. 3) determined in Uv-visible spectroscopy. The calibration curve of glyphosate was determined by preparing five concentrations within the range (10 -200) mg.L-1 (ppm) (Fig. 4). Mix 0.1 g MgO with 10 ppm of the glyphosate solution for the procedure. Subsequently, the samples undergo shaking via a vibrator that maintains a constant temperature of 298K [20]. The sample was then filtered and within the range (298 - 318 K) the temperature effect is studied.

RESULTS AND DISCUSSION
Characterization of MgO nanoparticles
The properties of the prepared magnesium oxide nanoparticles (MgO NPs) were characterized using a combination of advanced analytical techniques: Utilizing X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM). XRD results as shown in Fig. 5 revealed a diffraction pattern that clearly confirmed the crystalline structure and high purity of the nanoparticles, with peaks conforming to standard specifications. FE-SEM images Fig. 6 provided a detailed view of the surface morphology and porous aggregates. The results from these three techniques complement each other, collectively demonstrating that the prepared nanoparticle surface possesses structural properties and an effective surface area, making it an ideal surface for glyphosate adsorption and achieving high removal efficiency. To confirm the elemental composition and purity of the prepared nanoparticles, electron refraction (EDX) microscopy was performed, as shown in Fig. 7. The results showed sharp, distinct peaks belonging only to magnesium (Mg) and oxygen (O), indicating the correct chemical composition of the MgO particles. The absence of any peaks belonging to impurities or foreign elements is also evident from the diagram, which supports the results obtained from the XRD analysis.

Adsorption studies 
The effect of time
The best contact time showed in (Fig. 8) the adsorption was at 20 min. due to the all adsorbate bonding on the adsorbent surface at the time due to the large number of active sites that are not occupied before that time [21].

The adsorption isotherms 
The study of adsorption isotherms gives important information about the adsorption process and its conditions. The general shape of glyphosate isotherm on the surface of MgO nanoparticles in Fig. 9 is comparable to the S1-type according to Giles classification. The S-type refers to the orientation of the active sites of the glyphosate that are inclined or vertically on the surface of the nanoparticles [22].
The Freundlich constants kf and n which are indicated adsorption capacity and adsorption intensity in respectively, while a and b are the Langmuir constants which are represented the capacity and binding energy respectively of adsorption. The R2 values refer to the goodness of fit for Freundlich and Langmuir. As seen in Table 1 and Figs. 10 and 11 Langmuir adsorption model was fitting for the glyphosate on MgO nanoparticles [23].

The effect of temperature
The adsorption nature is confirmed by calculating the thermodynamic parameters of the adsorption process at (298, 308, and 318) K. Gibbs free energy is evaluated by the Eq. 1 [24].

Where K, R, and T are equilibrium constant, the ideal gas constant and temperature respectively.
Equilibrium constant is calculated from the Eq. 2:

Qe is adsorption capacity in unit mg.g-1, Ce is equilibrium concentration in unit mg.L-1, m is weight of adsorbent in unit g. v is volume of adsorbant solution unit of L (Eq. 3) [25]:

 From Table 2 and Fig. 12 the enthalpy is calculated depends on Van Hoff- Arrhenius Eq. 3.
The entropy is calculated by Eq. 4:

From the data in Table 3, the negative value of the free energy of Gibbs ΔG indicates the spontaneous nature of the adsorption process. The adsorption process is exothermic because the value of the enthalpy ΔH is negative and the value of entropy ΔS is negative, which refers to the randomness being decreased during the adsorption process [23,25, 26].

CONCLUSION 
This study shows that MgO nanoparticles are useful to remove glyphosate pesticides from aqueous solutions in soil and decrease their toxicity in the environment. The adsorption process is cheap, easy, and not toxic to remove many contaminants from soil and water.

CONFLICT OF INTEREST
The authors declare that there is no conflict of interests regarding the publication of this manuscript.