# Enhanced decontamination of levofloxacin as toxic pharmaceutical residuals from water using CaO/MgO nanorods as a promising adsorbent

Sep 9, 2020

### Characterization

#### XRD investigation and structural properties

Diatomite skeletons formed mainly of opaline silica so its XRD pattern showed the distinguished broad peak of amorphous silica around 22° (Fig. 1A.A). The pattern of diatomite/MgO/CaO showed the broad peak of amorphous silica in addition to the characteristic peaks of CaO and MgO (Fig. 1A.B). The pattern of MgO/CaO NRs without the diatomite substrate revealed the dominance of calcium oxide and magnesium oxide as separated phases (Fig. 1A.C). The CaO peaks were observed at 32.24°, 37.6°, 55.41° and 67.14° which corresponding peaks to (111), (200), (202) and (222) planes that characterize the cubic crystal system of CaO (JCPDS card No. 00-004-0777). Also, the principal peaks of MgO were recognized at 43.22° and 74.7° which are the characteristic peaks of (200) and (311) planes that related to the cubic MgO (JCPDS; card No. 01-077-2364) (Fig. 1A.B). The calculated crystallite sizes of MgO and CaO according to Scherrer equation are 16.4 nm and 22.3 nm, respectively.

#### Chemical properties

The FTIR spectrum of diatomite substrate displayed the main bands of amorphous opaline silica (SiO–H (3437 cm−1), H–O–H (1638 cm−1), symmetrical Si–O–Si (799 cm−1), and asymmetrical Si–O-Si (1,092 cm−1 and 465 cm−1)21 (Fig. 1B.A). After loading it by MgO/CaO NRs, the main bands of opaline silica in addition to other bands related to both CaO (Ca–O (545 cm−1)) and MgO (Mg–O–Mg (867 cm−1)) were detected14 (Fig. 1B.B). After the dissolution of the diatomite substrate, the resulted spectrum revealed the disappearance of Si–OH and Si–O–Si that characterize the skeleton of diatoms and all the observed bands are of MgO and CaO structures (Fig. 1B.C). The Ca–O group was identified at 537 cm−1 and 718 cm−122 (Fig. 1B.C). For MgO, the Mg–O–Mg was recognized at 874 cm−1 while the higher and lower frequency stretching of MgO was identified by the bands at 600 cm−1 and 420 cm−1, respectively23 (Fig. 1B.C). The EDX result is of strong agreement with the FT-IR results (40% Ca, 14.7% Mg, and 45.3% O).

#### Textural properties

The nitrogen adsorption/desorption isotherm of MgO/CaO NRs was identified as type-III isotherm that is of the H3 hysteresis loop (Fig. 1C). This type related to plate-like particles of a nanoporous structure formed by the aggregation of the particles24 (Fig. 1C). The distribution curve of the present pores reflected the formation of the pores within the size range from 2 nm to about 13 nm and of 4.3 nm as average pore diameter which categorizes MgO/CaO NRs as materials of mesoporous structure (Fig. 1D). Additionally, the MgO/CaO NRs are of 0.103 cm3/g average pore-volume, 52.3 nm average particle size, and 112.8 m2/g surface area.

#### Optical properties

The optical properties and the bandgap energy of the synthetic MgO/CaO NRs are of vital role in the performance of it for further evaluation of the composite as photocatalyst in the degradation of organic pollutants. The pure phase of CaO NRs and the studied MgO/CaO NRs showed a broad absorption band extended from the UV area to the visible light area (Fig. 1E). This associated with an observable increase in the intensity of MgO/CaO NRs band and slight shifting of it towards the visible light region reflecting enhancing the photocatalytic properties by increasing the produced electron–hole pairs after the integration of CaO by MgO25,26 (Fig. 1E). The bandgap energies of both CaO NRs and MgO/CaO NRs were theoretically calculated from Tauc’s equation and the value is 2.1 eV and 1.8 eV, respectively which promising values for future investigation of the composite as photocatalyst (Fig. 1F).

#### Morphological studies

The SEM image of refined diatomite substrate showed the presence of pinnate diatomite frustules with their stunning oriented porous structure (Fig. 2A). After the incorporation of the diatomite frustules as the growing substrate, the SEM image revealed decoration of diatomite porous frustules by MgO/CaO nanorods that appeared on random orientation (Fig. 2B,C). After the dissolution of the siliceous diatomite substrate, the produced MgO/CaO NRs appeared as agglomerated fibrous or needle particles aggregated with each other (Fig. 2D). The HRTEM images confirmed the synthesis of MgO/CaO composite as well-developed needle particles or nanorods (Fig. 2E,F). The composite appear as single grains which reflects the homogeneity in the integration process between the two phases. The deep inspection for the main rods reflected their formation as bundles for minor nanorods. This might be related to the effect of the ultrasonic waves during the mixing processes in making the growth of the particles interlocked with each other. The MgO/CaO rods ranged in length from about 200 nm to about 450 nm and ranged in diameter from about 20 nm to about 50 nm.

#### Effect of the parameters and optimization

##### Effect of pH

The adsorption of LVX molecules by MgO/CaO NRs at different pH values showed a significant increase from 14.4 to 48.3% with increasing the pH of the solution from pH 2 to pH 7, respectively (Fig. 3A). The further increase in the LVX solution pH beyond pH 7 resulted in a reduction in the achieved removal percentage to 46.2% and 40.6% at pH 8 and pH 9, respectively (Fig. 3A). Therefore pH 7 was detected as the best value in the MgO/CaO NRs adsorption system for Levofloxacin which is reported also in other studies as Liu et al.27, Limbikai et al.28, and Yu et al.29.

The reported behavior can be explained based on the speciation of Levofloxacin at different pH values as well as the surface properties of the synthetic MgO/CaO NRs. The speciation diagram of LVX revealed the detection of it in three forms (cationic, zwitterionic, and anionic)27. The cationic form (protonated piperazinyl group) was detected at pH values lower than pH 5, the anionic form (deprotonated carboxyl group) was identified as the dominant species at pH values higher than pH 8.5, and the zwitterionic form was detected as the dominant species within pH range from pH 5 to pH 8.52,6. Thus the increase in pH associated with the continuous increase in the dominance of the zwitterionic form until achieving its maximum concentration at pH 7 (Yu et al.29). At the same time, the protonation of MgO/CaO NRs at the acidic condition and deprotonation of it at the alkaline conditions make its surface positively charged at lower pH values and negatively charged at higher pH values19,30. Therefore conducting the reactions at pH lower than or higher than pH 7 causes electrostatic repulsion with the LVX cationic form and anionic form, respectively.

#### Effect of time intervals

As most of the evaluated adsorption system, the uptake of LVX molecules by MgO/CaO NRs with time reflected two stages of different adsorption rates (Fig. 3B). The first stage is of significant changes in the LVX uptake rate and this was reported within a time interval from 30 to 360 min (Fig. 3B). The second LVX uptake stage is of very slight changes in the adsorption rate until attending the equilibrium point of no changes in the amount of adsorbed LVX molecules by MgO/CaO NRs. This was reported within the time intervals from 360 to 960 min achieving the equilibration after 720 min with 42.4 mg/g as experimental equilibrium uptake capacity (Fig. 3B). The observed stages reflected the continuous uptake of LVX molecules by the exposed sites of MgO/CaO NRs until the complete saturation of such sites31.

#### Effect LVX concentration

The uptake equilibrium of LVX molecules by MgO/CaO NRs is of vital effect in detecting the maximum adsorption capacity as well as the equilibrium behavior. The adsorption of LVX as a function of its initial concentrations displayed an obvious increase in the value was testing higher concentrations of it until achieving its maximum adsorption capacity without possible enhancement for the obtained value even with the further increase in the LVX concentration (Fig. 3C). The maximum LVX adsorption capacity (100 mg/g) was recognized after treating 80 mg/L of LVX which can be signified as the equilibrium concentration at which all the receptor sites of MgO/CaO NRs were occupied by the LVX molecules (Fig. 3C). The LVX adsorption equilibrium curve is of L-type (2L) type which identified for adsorbents that exhibit a very high affinity towards the studied pollutants32,33.

#### Effect of the used MgO/CaO NRs quantities

The influence of the used MgO/CaO NRs was studied for different concentrations (10 mg/L to 30 mg/L) and two selected time intervals (60 min and 120 min) (Fig. 3D). The results demonstrated an enhancement in the recognized removal percentages of LVX for all the studied concentrations and the selected time intervals with the controlled increase in the MgO/CaO mass from 0.02 to 0.1 g (Fig. 3D). This is a commonly reported phenomenon in the adsorption systems and related to the associated enrichment in the available receptor sites of MgO/CaO NRs as well as its exposed surface area with mixing higher masses of it with the solution19,31. The reported results reflected complete removal of 10 mg/L of LVX after 60 min and 120 min by using 0.08 and 0.06 g of MgO/CaO NRs, respectively (Fig. 3D). Also, 20 mg/L of LVX antibiotic can be completely removed after 60 min and 120 min by increasing the mass to 0.1 g and 0.06 g, respectively (Fig. 3D). Such effective results were recognized also for 30 mg/L of LVX and the complete removal was achieved after 120 min using 0.08 g of MgO/CaO NRs (Fig. 3D). Such removal percentages give the composite promising importance to remove high concentrations of LVX within short periods.

#### Kinetic, equilibrium, and thermodynamic studies

##### Kinetic studies

The kinetic behavior of LVX molecules in the MgO/CaO NRs adsorption system was addressed considering the assumptions of four theoretical (Pseudo-First order, Pseudo-Second order, Elovich, and Intraparticle diffusion models) (Fig. 4A–D). The representative equations of the inspected models were listed in Table S1. The fitting results reflected strong agreement for the kinetic behavior of LVX in MgO/CaO NRs adsorption system with the theoretical assumption of Pseudo-first and Pseudo-second order models as well as the Elovich model (Fig. 4A–C; Table 1). The pseudo-first-order model related normally to physical adsorption processes while the Pseudo-second order model related to several mechanisms of more chemical nature including electron sharing, surface complexation, electron exchange, and internal diffusion19. The Elovich model also suggested more chemical behavior for the adsorption of LVX within energetically heterogeneous materials which can be detected for composites materials as MgO/CaO NRs19,34. The fitting results suggested complex uptake mechanisms of LVX by MgO/CaO NRs that can be supported by the further equilibrium models.

Intra-particle-diffusion model also was addressed to introduce more physical explanations for the kinetic properties of the studied MgO/CaO NRs adsorption system for LVX molecules (Fig. 4D). The detection of the curves as three LVX uptake segments without intersection with the original point revealed the operation of more than one mechanism during the uptake of LAVX by MgO/CaO NRs and did not control only by the diffusion of the dissolved LVX molecules31 (Fig. 4D). The first segment related to the external adsorption of LVX by the external receptor sites of MgO/CaO NRs and this stage represents the dominant process. The second segment related to the layered adsorption stage of limited intra-particle diffusion for the target ions. The third segment reflected the equilibrium or the saturation state of MgO/CaO NRs which formed by forming thick layers of the adsorbed LVX molecules on its surface where the dominant mechanisms are inter-ionic attraction and molecular association35.

#### Isotherm studies

The equilibrium of MgO/CaO NRs adsorption system for LVX molecules was addressed considering the nonlinear fitting results with the suppositions of Langmuir, Freundlich, and Dubinin–Radushkevich models (Fig. 4E) and their represented equations were listed in Table S1. The Langmuir assumption suggested the adsorption of LVX as dissolved molecules on the surface of MgO/CaO NRs in a monolayer form by receptor sites distributed homogeneously on the surface of the synthetic nanorods36. The Freundlich assumption related to heterogeneous uptake of the LVX molecules in multilayer form37. The fitting results showed excellent correlation coefficient value with the Langmuir model as compared to the obtained value for the Freundlich model which was supported by the Chi-squared (χ2) values (Table 1). Additionally, the theatrical values RL parameter (0.044–0.313) and the theoretical LVX maximum uptake capacity reflected the applicability of MgO/CaO NRs to adsorb about 106.75 mg/g in monolayer from by homogeneous adsorption process38 (Table 1).

The Dubinin–Radushkevich (D-R) is an important model for its value in understating the nature of the controlling reaction during the uptake of LVX molecules (physical and chemical) considering based on the Gaussian energy19. The LVX adsorption data showed strong agreement with the D-R model and its related parameters displayed 99.14 mg/g as maximum uptake capacity and 0.2 kJ/mol as adsorption energy which very low value and suggested strong physisorption process (Table 1). The kinetic and isotherm studies suggested the adsorption of LVX molecules by MgO/CaO NRs by coulombic attractive forces that involves ion exchange process with very weak electrostatic attraction without formation or destruction of chemical bonds which explain the fitting of the results with Pseudo-second order kinetic model36.

#### Thermodynamic studies

The thermodynamic properties of MgO/CaO NRS adsorption system for LVX were studied considering the theoretical values Gibbs free energies (∆G°), enthalpy (ΔH°) and the entropy (ΔS°). The value of ∆Go was obtained from Eq. (1) while the values of ΔH° and ΔS° were estimated as theoretical parameters for the linear fitting with Van’t Hof equation (Eq. (2))36 (Fig. 4F).

$${Delta G}^{0}=-RT In {K}_{d}$$

(1)

$$mathrm{In }left({K}_{c}right)=frac{{Delta S}^{o}}{R}-frac{{Delta H}^{o}}{RT}$$

(2)

The reported negative signs for ∆Go within temperature from 303 to 318 K reflected the uptake of LVX by MgO/CaO NRs through spontaneous reactions of favorable properties and this declined with accomplishing the reactions at temperature beyond 318 K37 (Table 1). The negative value of (ΔH°) revealed the nature of the LVX uptake reaction as an exothermic reaction and its randomness properties decrease significantly with increasing the temperature which reflected in the negative sign of ΔS°19 (Table 1). The value of free energy and enthalpy supported the physisorption mechanism for the uptake of LVX by MgO/CaO NRs.

### Reusability

The reusability properties of MgO/CaO NRs as adsorbent to be reused in the removal of LVX several times were inspected for five reusing cycles (Fig. 5). The reusing test was performed using 0.1 g of the synthetic nanorods in the removal of 20 mg/L of LVX for 120 min at pH 8 and 30 °C as temperature (Fig. 5). The composite after each reusability test was washed extensively with distilled water five times and dried in an oven at 80 °C for 8 h as the regeneration technique. The composite displayed significant reusability as an adsorbent with removal percentages higher than 80% for the adsorption tests (Fig. 5). The achieved removal percentage by adsorption decreased by 100%, 96.3%, 91.4%, 85.3%, and 80.6% with repeating the reusing runs by run1, run 2, run 3, run 4, and run 5, respectively (Fig. 5). This gives the synthetic MgO/CaO NRs high reusability value which qualifies it for commercial applications.

### Effect of MgO integration

The influence of the integration between CaO and MgO as heterogeneous nanostructure in enhancing the adsorption removal of LVX molecules was studied for 30 mg/L of LVX using 0.02 g of MgO/CaO NRs at pH 7 for 120 min (Fig. 6). The study was conducted as a comparison study between CaO and MgO as single phases and the results plotted in Fig. 6. The adsorption results showed removal for the LVX molecules (20 mg/L) by 12.4%, 15.6%, and 23.4% using CaO, MgO, and MgO/CaO NRs, respectively. This demonstrates enhancement in the adsorption properties of MgO/CaO NRs by 11% and 7.8% than the achieved results by CaO and MgO, respectively (Fig. 6).

### Comparison study

The adsorption capacity of the synthetic MgO/CaO NRs for the removal of LVX molecules were compared with other studied adsorbents in the literature (Table 2). The synthetic composite displayed higher adsorption capacity than some natural clay adsorbents (kaolinite and montmorillonite), modified clay (iron pillared montmorillonite), natural, modified, and synthetic zeolite (mordenite, Zeolite-Y, and ZSM-5), carbon-based materials (charcoal and biochar), and some advanced synthetic materials (Fe3O4@SiO2 and MIL-100(Fe)) (Table 2). Such results demonstrate the excellent technical value of MgO/CaO NRs as adsorbent for the LVX molecules as toxic pharmaceutical residuals from water.