Green synthesis of Ag2O nanoparticles using Punica granatum leaf extract for sulfamethoxazole antibiotic adsorption: characterization, experimental study, modeling, and DFT calculation

Silver oxide (Ag2O) nanoparticles (NPs) were generated by synthesizing green leaf extract of Punica granatum, and afterwards they were used as adsorbent to remove the antibiotic additive sulfamethoxazole (SMX) from aqueous solutions. Prior of their use as adsorbent, the Ag2O NPs were characterized by various methods such as X-ray diffraction, Fourier transform infrared spectroscopy (FTIR), Brunauer–Emmett–Teller (BET), scanning electron microscopy/energy-dispersive X-ray (SEM-EDX), and transmission electron microscopy (TEM). The Ag2O NPs were found to be spherically shaped and stabilized by the constituents of the extract. Further, at SMX antibiotic concentration of 100 mg L–1, the Ag2O NPs achieved almost complete removal of 98.93% within 90 min, and by using 0.8 g L–1 of adsorbent dose at pH=4 and temperature T=308 K. In addition, the experimental data were well fitted with the theoretical Langmuir model indicating homogeneous adsorbed layer of the SMX antibiotic on the Ag2O NPs surface. The maximum uptake capacity was 277.85 mg g–1. A good agreement was also found between the kinetic adsorption data and the theoretical pseudo-second-order model. Regarding the thermodynamic adsorption aspects, the data revealed an endothermic nature and confirmed the feasibility and the spontaneity of the adsorption reaction. Furthermore, the regeneration study has shown that the Ag2O NPs could be efficiently reused for up to five cycles. The geometric structures have been optimized and quantum chemical parameters were calculated for the SMX unprotonated (SMX+/-) and protonated (SMX+) using density functional theory (DFT) calculation. The DFT results indicated that the unprotonated SMX+/- reacts more favorably on the Ag2O surface, as compared to the protonated SMX+. The SMX binding mechanism was predominantly controlled by the electrostatic attraction, hydrogen bond, hydrophobic, and π-π interactions. The overall data suggest that the Ag2O NPs have promising potential for antibiotic removal from wastewater.


Introduction
The main source of water pollution is the direct discharge of wastewater into water bodies without any prior treatment (Awual and Hasan 2015;Awual 2019aChowdhury et al. 2020;El Messaoudi et al. 2022b). The antibiotics are among the released water pollutants which pose serious threats even if they are present in water in small amounts. Such pollutants are chemical compounds having the ability to inhibit the life processes of selected microorganisms (Khameneh et al. 2019). Antibiotics can be classified based on several factors such as the chemical structure, the method, etc. the mode of action, the spectrum of action, and the route of administration (injection, oral and topical). One of the best-known, perhaps simplest classification of antibiotics is based on the presence in their structures of distinct functional groups, such as macrolides, tetracyclines, quinolones, sulfonamides, oxazolidinones, etc. (Berges et al. 2021;Grenni 2022). Sulfonamides have been widely approved and used to treat a wide range of different clinical infections. They have been used for decades due to their high potency, broadspectrum activity, and high serum concentrations (Fair and Tor 2014;Wang et al. 2019). Sulfamethoxazole (SMX) is a synthetic veterinary drug with a para-aminobenzene sulfonamide structure (Cheong et al. 2020). As a bacteriostatic sulfonamide antibiotic, SMX is used for the treatment and prevention of diseases in animals and humans Gao et al. 2019). It should be noted that the exposure to SMX to the environment can cause severe toxicity mainly to aquatic and nearby systems, bacteria, animals, and human health (Hwang et al. 2016;Xu et al. 2022). Therefore, the presence of antibiotics in water is unacceptable and there are currently several techniques to remove them, such as biodegradation, membrane separation, hydrolysis, and photodegradation (Reis et al. 2020;Wu et al. 2021;Yin et al. 2021). However, most of these techniques are still expensive, especially when applied at large scale for wastewater treatment. As a result, researchers are becoming more interested in adopting alternative adsorbents, which are quite low-cost, easily available, and extremely efficient.
In the recent years, nanoparticles (NPs) have received great attention from the scientific community not only because of their fascinating properties, but also due to their many technological applications (Khan et al. 2019;Shume et al. 2020). The NPs (MoO 3 , ZnO, WO 3 , MgO, Fe 2 O 3 , Ag 2 O, CuO, Si 2 O, etc.) showed great performance in antibacterial and were used as antibacterial agents and efficient adsorbents (Islam et al. 2020;Naseem and Durrani 2021) due to their high specific surface area, as well as, their unique chemical and physical properties (Kheyrabadi and Zare 2022;Damokhi et al. 2022). However, there are disadvantages to using these oxidants, such as their high cost and the production of large amounts of toxic waste, which largely reduce their industrial applications (Assal et al. 2018).
The synthesis of green nanoparticles from lignocellulosic biomass extracts have emerged as a cost alternative for high cost, commercial NPs. These green NPs reduce pollution as they promise effective adsorption due to their high efficiency, porosity, selectivity, and surface chemistry (Asghar et al. 2018;Khani et al. 2018;Si et al. 2020;Pai et al. 2021). Compared with other synthesis methods (sol-gel, hydrothermal, precipitation, ultrasound, etc.), the green synthesis of NPs is simple, inexpensive, non-toxic, and environmentally friendly (Marouzi et al. 2021). In addition, biosynthesis involving the use of plant extracts usually occurs in aqueous medium, which is cheap and offers no limitations in terms of applications (Elemike et al. 2017).
Ag 2 O nanoparticles have earned the most extensive applications due to their excellent antibacterial efficiency for a wide range of bacteria (Naseem and Durrani 2021). It should be noted that the Ag 2 O NPs have been used as an antiseptic component in many medical devices, food packages, and environmental purification processes (Kiani et al. 2018;Almatroudi 2020;Jeung et al. 2021). Further, the Ag 2 O NPs have great potential for treating contaminated water due to their unique properties, such as their large surface areas and low concentration (Rahman et al. 2019;Yu et al. 2021).
Here we report for the first time on the synthesis details, characterization, and adsorption efficiency of Ag 2 O nanoparticles made with Punica granatum leaf extract using the biosynthesis method. Further, in the present work we investigate also the use of Ag 2 O NPs as adsorbent for the sulfamethoxazole (SMX) antibiotic removal from aqueous solution. The assessment of functional groups, inner structure, constituent elements, thermal stability, and morphology of the Ag 2 O NPs used in this study were analyzed by corresponding techniques such as X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, Brunauer-Emmett-Teller (BET), scanning electron microscopy/energy-dispersive X-ray (SEM/EDX), and transmission electron microscopy (TEM) analyses. The kinetics and equilibrium isotherms, and the thermodynamics of SMX adsorption, were explored in-depth, in addition to the effect of operational parameters. Ag 2 O nanoparticles were regenerated and reused to remove SMX. The optimized geometries, highest occupied and lowest unoccupied molecular orbitals (HOMO-LUMO), and electrostatic potential (ESP) maps corresponding to the quantum chemical parameters of the SMX unprotonated (SMX +/-) and protonated (SMX + ) were investigated using calculations based on density functional theory (DFT) combined with the Lee-Yang-Parr correlation function (B3LYP) and the 6-31+g (d,p) under the solvation conditions. Finally, a possible mechanism of SMX adsorption on Ag 2 O NPs was proposed.

Chemicals and instrumentations
Hydrochloric acid (HCl), acetone, methanol, sodium hydroxide (NaOH), potassium nitrate (KNO 3 ), ethanol, and SMX antibiotic have been purchased from Sigma-Aldrich and they have analytical quality. Distilled water was used in all experiments. The characteristics of SMX are summarized in Table 1. XRD Experiments were performed on a Bruker D8 Advance Twin diffractometer to determine the nature of the Ag 2 O crystals. The Ag 2 O NPs FTIR spectrum has been recorded on IR JASCO 4100 spectrophotometer in the wavelength range 500-4000 cm -1 . The average pore size and the specific surface area of Ag 2 O NPs were determined by Barrett-Joyner-Halenda (BJH) and BET analyzes (Belsorp Mini II). The morphology characteristics and the chemical composition of Ag 2 O NPs were identified using SEM-EDX (JEOL, JSMIT200) and TEM (JEOL Ltd, Tokyo, Japan) analyzes. The point of zero charge (PZC) of Ag 2 O NPs has been determined according to the process reported by El Messaoudi et al. (El Messaoudi et al. 2022c).

Green synthesis of Ag 2 O NPs
The Punica granatum leaves (PGLs) were collected from a Tinghir area (Morocco). The PGLs were cleaned thoroughly using distilled water to eliminate dirt, dust, and other surface contaminants before sun drying. The cleaned PGL were then soaked in methanol for 24 h to remove the pigments and fat, washed with distilled water several times, and dried in the dryer for 12 h. The dry biomaterial was ground by a mechanical grinder and screened at 50 μm. 10g PGL powder was added to 50 mL of distilled water for 2 h at 70°C. The resulting mixture was filtered and centrifuged to separate the water extract and the residue of the PGL powder. Thereafter, 20 mL of the obtained extract was added to 40 mL of a solution consisting of AgNO 3 (0.2 M) and 5 mL of NaOH (0.1 M), stirring continuously for 5 h at 65°C. After precipitation, the sample was rinsed with distilled water multiple times and then dried in the dryer for 12 h at 70°C, and then calcined in an oven at 450°C for 3 h. The green synthesis process of Ag 2 O nanoparticles from Punica granatum leaf extract is detailed in Fig. 1.

Adsorption experiments
The experiment was performed by adding 40 mg of Ag 2 O NPs to 50 mL of the SMX solution (100 mg L -1 ), and this mixture was strongly shaken in a platform shaker to ensure its complete mixing at 308 K for 90 min. The adsorption performance of Ag 2 O NPs was evaluated for the removal of SMX to identify the effect of operating conditions, including adsorbent dosage (0.2-1.4 g L -1 ), initial SMX concentration (50-400 mg L -1 ), initial pH of the antibiotic solution (2-10), and contact time (5-180 min) at temperature (298-318 K). Evaluation of adsorption capacity was analyzed using the isotherm studies carried out at 298 K, 308 K, and 318 K by equilibrating Ag 2 O NPs (40 mg) with 50 mL of SMX solution (50-400 mg L -1 ) for 90 min at solution pH=4, while kinetics and thermodynamic experiments were conducted in a similar way at three different initial SMX concentrations 100 mg L -1 , 150 mg L -1 ,and 200 mg L -1 for 5-180 min, and at the above three temperatures, respectively. After each adsorption experiment, the antibiotic solution was filtered using a syringe filter (0.45 μm), and the concentration of the final solution was determined using an ultraviolet-visible spectrophotometer (UV-Vis 2300) at a maximum absorbance wavelength of 263 nm. The removal percentage (%) of the SMX solution was determined using Eq. (1): C i (mg L −1 ) and C e (mg L −1 ) refer to the initial and equilibrium SMX concentrations, respectively.
The amount of SMX adsorption at equilibrium, q e (mg L −1 ) was determined from Eq. (2): W (g) and V (L) are the mass of the adsorbent and the solution volume, respectively.

DFT computational
The electronic structure calculations were performed with Gaussian 9 software packages. The equilibrium structure of the SMX +/and SMX + , as well the parameters, E HOMO , E LUMO , ESP maps, energy gap (∆E gap ),dipole moment (μ), and parameters that give valuable information on the reactive behavior such as the electronegativity (χ) ionization, hardness (η), softness (σ) were determined using DFT/ B3LYP (Becke 1993;Stephens et al. 1994) and 6-31+g (d,p ) basis set (Kendall et al. 1992). All calculations were performed in the aqueous phase. The quantum parameters are presented by mathematical formulas as follows: Fig. 1 Green synthesis procedure of Ag 2 O NPs from Punica granatum leaf extract.

Ag 2 O NPs characterization
XRD is one of the most widely used methods for characterizing the structure of an Ag 2 O nanoparticle. XRD graph gives the most direct results such as crystallinity. The XRD results obtained ( The specific surface area and total pore volumes of Ag 2 O NPs were calculated using BET and BJH analyses (Fig. 2c). The BET surface area of Ag 2 O NPs was found to be 107.09 m 2 g −1 , and the result obtained revealed that Ag 2 O NPs have a higher porosity with a total pore volume and average pore diameter was 0.38 m 3 g −1 and 31 nm, respectively.
The morphology of the adsorbent was analyzed by using SEM analysis. Figure 3a shows the micrograph of Ag 2 O NPs. The spherical of the particles is clearly evident from the SEM image, and irregular structure with pores of different shapes and sizes, provide enough surface area for interaction with SMX molecules (Xu et al. 2013a;Abdalameer et al. 2021). The EDX spectra (  The TEM image of Ag 2 O NPs is obtained, as shown in Fig. 3c. This image presents the highly ordered porous structure of Ag 2 O NPs, and spherical in the shape of the particles (Ahmad and Majid 2018). The bioactive compounds trapped on the surface of Ag 2 O NPs can be attributed to the Ag 2 O NPs surrounding the thin layer (Camacho-Escobar et al. 2020). The size and average of nanoparticles ranged from 24 to 36 nm confirming that the Ag 2 O NPs synthesized were in the nanoscale range. Figure 4a displays the SMX speciation diagram as a function of the solution pH. The SMX displays a pKa couple of 1.97 (pKa 1 ) and 6.16 (pKa 2 ), between these values the SMX has the unprotonated form (SMX +/-), while charged positively (SMX + ) at pH < pKa 1 and negatively (SMX -) at pH> pKa 2 (De Oliveira et al. 2018). The effect of pH on SMX adsorption onto Ag 2 O NPs was investigated at different pH values ranging from 2 to 10 ( Fig. 4b) at the kept factorial values (time=180 min, temperature=298 K, concentration=100 mg L −1 , and Ag 2 O NPs dose =1 g L −1 ). Acidic pH promotes protonation of the Ag-O group (Ag-OH 2 + ) resulting in acidic adsorption sites. Excess of H + ions battle against the positive groups of antibiotic (SMX + ) for adsorption surface, resulting in repulsion among the Ag-O group (Ag-OH 2 + ) of adsorbent and SMX + and antibiotic adsorption is disfavored. Whereas basic pH causes deprotonation of Ag-O group (Ag-O − ) of adsorbent, more hydroxide ions render adsorption of SMX − , making the surface negatively charged of Ag 2 O NPS and antibiotic adsorption is also disfavored. The pH range 2-10 was selected for the sequestration of the SMX antibiotic. SMX adsorption depends upon charge and surface characteristics. The PZC of Ag 2 O NPs was found to be 5.3. At pH<PZC, the adsorbent is positively charged (Ag-OH 2 + ) while it is negative at pH> PZC (Kubra et al. 2021), the repulsive forces resulting between theAg-OH 2 + and Ag-Ogroups and SMX + and SMX -, respectively; therefore, the SMX adsorption is unfavorable. At pH near PZC (zwitterionic), the attractive forces result between the Ag-OH 2 + and Ag-Ogroups of adsorbent and SMX +/antibiotic, thus antibiotic adsorption is favored. The result obtained is similar to that reported by Zhang et al. (2010). The highest uptake quantity of SMX was 122.45 mg g −1 at pH=4.

Effect of adsorbent dose
The effect of Ag 2 O dosage on the SMX adsorption was examined in the range 0.2-1.4 g L −1 (Fig. 4c), while the other conditions are kept at the central value (time=180 min, temperature=298 K, concentration=100 mg L −1 , and pH=4). The SMX removal increased from 60.29 to 98.19% by increasing the adsorbent dose from 0.2 to 0.8 g L −1 because large nanoparticle production provides an outspread surface area of the adsorbent which provides numerous active adsorbent sites for the antibiotic to get adsorbed. Hence, more active site accessibility for the SMX molecules to adsorb (El Messaoudi et al. 2022a). After reaching maximum adsorption efficiency (>0.8 g L −1 ), an increase in dosage does not have much antibiotic sequestration efficiency. This is due to the gradient distribution between the Ag 2 O NPs and antibiotic as well as active sites saturation (Ogunleye et al. 2020). Adsorption capacity decreased (301.49−70.46 mg g −1 ) with the adsorbent dose from 0.2 to 1.4 g L −1 due to an increase in unsaturated sites with an increase in the adsorbent dose and a rise in SMX concentrations (Ahsan et al. 2018). Hence, 0.8 g L −1 of the adsorbent dose was selected for further experiments as higher concentrations did not yield significant results.

Effect of contact time
SMX antibiotic sequestration by Ag 2 O NPs is determined by finding the least time required for achieving equilibrium of antibiotic with the adsorbent in the range 5-180 min for three concentrations (100, 150, and 200 mg L -1 ) at experiment conditions (Ag 2 O NPs dose=0.8 g L −1 , T=298 K, and pH=4) (Fig. 5a). Noting strong adsorption of the SMX on the Ag 2 O NPs from the first minutes of contact between the antibiotic and the adsorbent is due to the number of active sites available on the surface (Guo et al. 2019). The balance of adsorption is reached in 90 min, beyond this time, it was found that the amount adsorbed for 100 mg L -1 is about 122.78 mg g -1 , 181.61 mg g -1 for 150mg L -1 , and 231.12 mg g -1 for 200 mg L -1 . After 90 min, the SMX adsorption capacity became de-escalate due to the occupation of active sites of Ag 2 O NPs . Therefore, 90 min is considered the equilibrium time optimum.

Effect of SMX concentration and temperature
The influence of initial SMS concentration at three temperatures 298 K, 308 K, and 318 K on SMX adsorption using Ag 2 O NPs is shown in Fig. 5b. To investigate the effect of initial antibiotic concentration on SMX adsorption, the antibiotic concentration was varied from 50 to 400 mg L -1 (Ag 2 O NPs dose=0.8 g L −1 , 90 min, pH=4). It is noted that the adsorption capacity of SMX increased from 60.96 to250.26 mg g -1 for 298K, from 61.57 to 260.14 mg g -1 for 308 K, and from 61.98 to 276.48mg g -1 for 318 K with SMX concentration from 50 to250 mg L -1 . This is because at a low concentration range the ratio between the Ag 2 O NPs surface and the SMX molecules in the solution is in abundance; therefore, all of the antibiotic particles are easily imbibed over the surface of the active adsorbed and get eliminated from the media (Guo et al. 2019). After 250mg L -1 of SMX concentration, the adsorption capacity was achieved. This is because at higher concentrations the driving force is also t 1/2 (min 1/2 ) 1 3 higher due to the concentration gradient and the adsorbed amount of the antibiotic per unit mass of the Ag 2 O NPs is also high which leads to the saturation of active sites ). Figure 5b also shows the effect of temperature on the adsorption capacity of SMX. It is evident that q e increases with temperature from 298 to 318 K, which may probably be due to the high rate of mass transfer from the bulk solution to a solid phase, widening of pore size, and/or activation of adsorption sites (Fulazzaky 2011). Additionally, an enhancement in the quantity adsorbed with temperature is descriptive of the endothermic nature of adsorption (El Messaoudi et al. 2021a), this is confirmed by thermodynamic and equilibrium isotherm studies subsequently.

Kinetics
The dynamics of the adsorption of SMX onto Ag 2 O NPs were investigated using Lagergren's pseudo-first-order (Lagergren 1898), pseudo-secondorder model proposed by Ho and McKay (Ho and McKay 1998), and intraparticle diffusion model (El Messaoudi et al. 2022c). The K PFO (pseudo-first-order constant) was calculated from the plot of log (q e -q t ) versus t (Fig. 6a), while the plot of t/q t versus t (Fig. 6b), and gives the value of K PSO (pseudo-second-order constant), which are tabulated in Table 2 along with R 2 values. Comparatively lower R 2 values for the pseudo-first-order model along with the evident deviation from linearity suggest incongruity of the model with the kinetic data thus ruling out the possibility of interaction of the antibiotic molecule with a single active site ). However, the linearity of the t/q t versus t plots and high R 2 values (0.999) reflects that adsorption conforms better to the pseudo-second-order kinetic model. Moreover, calculated equilibrium capacities q e,PSO are in complete agreement with that of experimental equilibrium capacities q e,exp . It is, therefore, concluded that the rate of adsorption of SMX onto Ag 2 O NPs for three concentrations (100, 150, and 200 mg L -1 ) is related to the square of the number of available active sites and a single antibiotic molecule is capable of interacting with two adsorption sites simultaneously (Bentahar et al. 2016;El Messaoudi et al. 2017). This is similar to reports on the adsorption of SMX onto the Fe-impregnated graphited biochar ) and montmorillonite (Wu et al. 2019) adsorbents. The intraparticle diffusion model controls the process when the three basic adsorption steps, that is, instant, gradual, and equilibrium adsorptions become indistinguishable (El Messaoudi et al. 2022c). The slope and intercept of q t versus t 1/2 plots give the values of K IPD (intraparticle diffusion constant) and C i (boundary layer thickness) at three different concentrations (Fig. 6c), which are tabulated in Table 2. From Fig. 6c, the SMX adsorption process on Ag 2 O NPs presents in three successive stages. The first stage describes the diffusion of SMX molecules from the aqueous phase to the external surface sites of Ag 2 O NPs (Ahmed et al. 2021). The second phase indicates the intraparticle diffusion of SMX species into the internal surface of Ag 2 O NPs with a rate-controlling step (Tavlieva et al. 2013). The values of C 2 (83.28-115.06 mg g -1 ) illustrate that liquid film diffusion plays a dominant contribution to the overall ratecontrolling step for three concentrations (100, 150, and 200 mg L -1 ). The third step corresponds to adsorption equilibrium because of the extremely low concentration of SMX (Yao and Chen 2017).

Equilibrium
To understand the mechanisms involved in the adsorption of SMX on Ag 2 O NPs at three temperatures 298 K, 308 K, and 318 K, four isotherm models were used such as Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich. The Langmuir model is very useful for the monomolecular adsorption of a solute by forming a monolayer on the surface of the adsorbent (Langmuir 1918). The Freundlich model is based on adsorption on heterogeneous surfaces (Freundlich 1907). Temkin isotherm model contemplates a linear decline in the heat of adsorption of molecules in the layer with surface coverage as an indirect consequence of adsorbate-adsorbent interactions, and the homogeneous distribution of binding energies characterizes the adsorption process (Johnson and Arnold 1995). Dubinin-Radushkevich isotherm is associated with multilayer adsorption with a tendency of adsorbate molecules to fill the micropores (Çelebi et al. 2007;Ayawei et al. 2017). It is also generally utilized for the prediction of the adsorption mechanism (Chen and Yang 2002;Gallego-Gómez et al. 2020).
The values of Q m (maximum adsorption capacity) and K L (Langmuir constant) at three different temperatures (298-318 K) were obtained from the slope and intercept, respectively, of C e /q e versus C e plots (Fig. 7a). The values of K F (Freundlich constant) and 1/n F (heterogeneity factor) were obtained from the slope and intercept, respectively, of the linear lnq e versus lnC e plots (Fig. 7b). A linear plot of q e versus lnC e (Fig. 7c) gives the values of Temkin constants, K T (Temkin constant), and b T from its slope and intercept, respectively. The linear curves of lnq e versus ε 2 (Dubinin-Radushkevich constant, ε=RTln(1+1/Ce)) ( Fig. 7d) were used to determine the Q m and β from the slope and intercept, respectively. From Table 3, the values of K L , which denotes the binding affinity or strength of antibiotic molecules with Ag 2 O NPs surface, show a gradual increment from 0.232 to 0.394 L mg -1 with raise in solution temperature indicating thereby an improved SMX-Ag 2 O NPs binding strength at an elevated temperature, the higher Qm, which is often considered a measure of the efficacy of an adsorbent (277.85-312.59 mg g -1 ) confirms the efficient adsorption performance of Ag 2 O NPs at all tested temperatures. An ascending trend in K F from 82.121 to 109.039 mg g -1 with rising in operating temperature from 298 to 318 K is illustrative of the endothermic adsorption of SMX. The calculated values of 1/n F (0.248-0.259) are positive but less than one which advocates favorable adsorption in the studied temperature range. The correlation coefficient R 2 values for the Langmuir model are higher (0.999) for three temperatures signifying a good fit of the equilibrium data at high temperatures. The constant K T increases from 6.903 to 16.014 L mg -1 with temperature further confirming the endothermic nature of SMX adsorption. The adsorption energy value (E=1/(2 β) 1/2 ) is functional in describing the nature of the adsorption mechanism (E <8 kJ mol -1 physisorption; E >16 kJ mol -1 chemisorption; and 8 ≤ E ≤ 16 kJ/ mol -1 : ion exchange) (Isiuku et al. 2021

Thermodynamic studies
The study of the thermodynamics of the adsorption process is useful in obtaining the information related to the feasibility/spontaneity, exothermic/endothermic, and disorder at the solid-solution interface based on the evaluated values of changes in Gibbs free energy (ΔG°), enthalpy change (ΔH°), and entropy (ΔS°) change, respectively. The values of ΔH° and ΔS °were obtained from the slope and intercept of the linear plot of lnK d versus 1/T (Fig. 8a)   where R (8.314 J mol -1 K -1 ) is the universal gas constant and T (K) is the absolute temperature, and K d is the distribution coefficient defined as q e /C e . The values of the thermodynamic parameters are given in Table 5. For the SMX adsorption onto the Ag 2 O NPs, the negative ΔG° at all studied temperatures infers the feasibility of the process for three concentrations (100, 150, and 200 mg L -1 ). Further, the increase of ΔG° with temperature shows that SMX removal is spontaneous and more promising at higher temperatures (Ahmed et al. 2021). This is supported by the positive ΔH°, which affirms the endothermic adsorption (Serna-Carrizales et al. 2021). Entropy change is a measure of the change in randomness at the solid-liquid interface. The positive ΔS° values (108.523-132.484 J mol -1 K -1 ) indicate the increase in randomness of SMX adsorption (Fierro et al. 2007).

Recyclability of Ag 2 O NPs
The reusability experiment gives useful insights into the possibility of recovery and operational behavior of the adsorbent. The recyclability of Ag 2 O NPs for the removal of SMX was tested after acetone and ethanol washing. The results are summarized in Fig. 8b. The difference in percentage removal between the first and five uses of Ag 2 O NPs adsorbent for removal of SMX was 22% at optimal conditions: 0.8 g L −1 of adsorbent dose, pH of 4, and 100 mg L −1 of SMX concentration at 308±1 K for 90 min. Still, further research is required for the usage of Ag 2 O NPs for antibiotics removal from wastewaters.

DFT results
The analysis of the HOMO and LUMO energy levels, as well as the gap energy, is very important in the study of the properties of organic molecules (Jodeh et al. 2022). The energies of the HOMO and LUMO frontier orbitals and the gap energy of the molecules calculated by the DFT/B3LYP 6-31+g (d,p) method under different pH conditions from the optimized structures are given in Table 6, an unprotonated and protonated state. From this table, we notice that the SMX + molecule has the highest energy gap ΔE gap =5.717 eV, therefore it is the most stable and the least chemically  active (Zhuang et al. 2020). In the case of SMX +/-, it has the lowest energy gap ΔE gap =5.197 eV, so it is the least stable and the most chemically active (Liu et al. 2020b). The importance of η and σ is to evaluate both reactivity and stability. The neutral form of SMX has the lowest value of chemical hardness (η=2.598eV), which means high reactivity compared to the protonated form (Alivand et al. 2019). Global reactivity plays an important role to describe the ability of SMX to interact with the surface of Ag 2 O NPs under different pH conditions. Optimized geometries, HOMO, LUMO frontier orbitals, and ESP maps corresponding to the two forms of SMX are presented in Fig. 9. This figure shows that the backbone of the SMX molecule in both the protonated and unprotonated forms is characterized by positive potential (i.e., the blue regions in Fig. 9d and g) (Obot and Obi-Egbedi 2010). According to these observations, the unprotonated molecule is reacting more favorably on the surface of Ag 2 O NPs. These results are in good agreement with those obtained experimentally.

Proposed adsorption mechanism
The conceptual adsorption mechanism of SMX onto the prepared nanoparticles is shown in Fig. 10. The FTIR and PZC of Ag 2 O NPs, and the influence of operational solution pH are mainly functional in understanding the type of interactions taking place between the surface of Ag 2 O NPs and SMX molecules. At pH<PZC, the Ag 2 O surface is positively charged (Ag-OH 2 + group) and will limit its electrostatic interactions with the SMX + form. However, at pH near PZC (zwitterionic), the SMX +/form can interact with available positive and negative sites at the Ag 2 O NPs surface, promoting the SMX adsorption at near-neutral pH conditions. This finding could justify and support the excellent SMX removal efficiency at near-neutral pH using Ag 2 O nanoparticles (Ninwiwek et al. 2019). Consequently, the anionic SMXcan show poor electrostatic interactions with a negatively charged Ag 2 O surface (Ag-Ogroup) under alkaline conditions. Based on FTIR Fig. 9. Optimized geometries, HOMO, LUMO frontier orbitals, and ESP (red, blue, yellow, and green: strongly negative, strongly positive, moderately negative, and moderately positive electrostatic potentials, respectively) maps for the SMX +/-(a-d) and SMX + (e-h), respectively.

Conclusions
The present work focused on the eco-friendly synthesis of Ag 2 O nanoparticles from Punica granatum leaf extract, and their use for the removal of SMX antibiotic from aqueous solutions. Prior to the adsorption investigation, the synthesized Ag 2 O NPs were characterized by various methods. Hence, the XRD method revealed the existence of high NPs crystallinity and a face-centered cubic phase structure. The FTIR spectroscopy confirmed the presence of the Ag-O group on the NPs surface. The BET method based on nitrogen adsorption onto the Ag 2 O showed a high surface area and small size of the NPs. Regarding the shape and chemical composition of the Ag 2 O NPs, the SEM and the TEM analyzes displayed spherical and circular disc-shaped morphologies, whereas the EDX method showed the purity of the particles. Thereafter, the optimum experimental conditions for the SMX adsorption were found to be pH=4, adsorbate-adsorbent contact time=90 min, and an initial SMX concentration=100 mg L −1 at adsorption temperature =308 K. Further, at the equilibrium, the adsorption isotherms were well fitted with the theoretical Langmuir model for temperatures ranging from 298 to 318 K. According to the study, SMX has a maximum adsorption capacity of 277.85 mg g -1 at 298 K. Good agreements were also found between the kinetic data and the pseudo-second-order model. The assessed results obtained from the temperature effect on the adsorption process and the associated calculated thermodynamic parameters confirmed that the reaction was spontaneous, feasible, and endothermic in nature. Moreover, the experimental results were in good agreement with those obtained by the DFT calculation. The overall data lead to conclude that the adsorption mechanism of the SMX onto the Ag 2 O NPs involves mainly hydrogen-bonding, hydrophobic, and π-π. Further, the Ag 2 O adsorbent as prepared from the leaf extract exhibited improved capacities of the SMX antibiotic removal from water. Therefore, the Ag2O NPs adsorbent can be used at large scale, as highly efficient material for sensitive detection and removal of SMX antibiotic from wastewater.

Data availability
The datasets used during the current study are available from the corresponding author on reasonable request.

Declarations
Ethics approval and consent to participate Not applicable.

Consent for publication Not applicable.
Competing interests The authors declare that they have no competing interests.