Synergistic removal of doxycycline antibiotics from water by green synthetic reduced graphene oxide and nano-zero iron complexes

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In this work, rGO/nZVI composites were synthesized for the first time using a simple and environmentally friendly procedure using Sophora yellowish leaf extract as a reducing agent and stabilizer to comply with the principles of “green” chemistry, such as less harmful chemical synthesis. Several tools have been used to validate the successful synthesis of composites, such as SEM, EDX, XPS, XRD, FTIR, and zeta potential, which indicate successful composite fabrication. The removal capacity of the novel composites and pure nZVI at various starting concentrations of the antibiotic doxycycline was compared to investigate the synergistic effect between rGO and nZVI. Under the removal conditions of 25mg L-1, 25°C and 0.05g, the adsorptive removal rate of pure nZVI was 90%, while the adsorptive removal rate of doxycycline by the rGO/nZVI composite reached 94.6%, confirming that nZVI and rGO. The adsorption process corresponds to a pseudo-second order and is in good agreement with the Freundlich model with a maximum adsorption capacity of 31.61 mg g-1 at 25 °C and pH 7. A reasonable mechanism for the removal of DC has been proposed. In addition, the reusability of the rGO/nZVI composite was 60% after six consecutive regeneration cycles.
Water scarcity and pollution are now a serious threat to all countries. In recent years, water pollution, especially antibiotic pollution, has increased due to increased production and consumption during the COVID-19 pandemic1,2,3. Therefore, the development of an effective technology for the elimination of antibiotics in wastewater is an urgent task.
One of the resistant semi-synthetic antibiotics from the tetracycline group is doxycycline (DC)4,5. It has been reported that DC residues in groundwater and surface waters cannot be metabolized, only 20-50% are metabolized and the rest is released into the environment, causing serious environmental and health problems6.
Exposure to DC at low levels can kill aquatic photosynthetic microorganisms, threaten the spread of antimicrobial bacteria, and increase antimicrobial resistance, so this contaminant must be removed from wastewater. The natural degradation of DC in water is a very slow process. Physico-chemical processes such as photolysis, biodegradation and adsorption can only degrade at low concentrations and at very low rates7,8. However, the most economical, simple, environmentally friendly, easy to handle and efficient method is adsorption9,10.
Nano zero valent iron (nZVI) is a very powerful material that can remove many antibiotics from water, including metronidazole, diazepam, ciprofloxacin, chloramphenicol, and tetracycline. This ability is due to the amazing properties that nZVI has, such as high reactivity, large surface area, and numerous external binding sites11. However, nZVI is prone to aggregation in aqueous media due to van der Wells forces and high magnetic properties, which reduces its effectiveness in removing contaminants due to the formation of oxide layers that inhibit the reactivity of nZVI10,12. The agglomeration of nZVI particles can be reduced by modifying their surfaces with surfactants and polymers or by combining them with other nanomaterials in the form of composites, which has proven to be a viable approach to improve their stability in the environment13,14.
Graphene is a two-dimensional carbon nanomaterial consisting of sp2-hybridized carbon atoms arranged in a honeycomb lattice. It has a large surface area, significant mechanical strength, excellent electrocatalytic activity, high thermal conductivity, fast electron mobility, and a suitable carrier material to support inorganic nanoparticles on its surface. The combination of metal nanoparticles and graphene can greatly exceed the individual benefits of each material and, due to its superior physical and chemical properties, provide an optimal distribution of nanoparticles for more efficient water treatment15.
Plant extracts are the best alternative to harmful chemical reducing agents commonly used in the synthesis of reduced graphene oxide (rGO) and nZVI because they are available, inexpensive, one-step, environmentally safe, and can be used as reducing agents. like flavonoids and phenolic compounds also acts as a stabilizer. Therefore, Atriplex halimus L. leaf extract was used as a repairing and closing agent for the synthesis of rGO/nZVI composites in this study. Atriplex halimus from the family Amaranthaceae is a nitrogen-loving perennial shrub with a wide geographic range16.
According to the available literature, Atriplex halimus (A. halimus) was first used to make rGO/nZVI composites as an economical and environmentally friendly synthesis method. Thus, the aim of this work consists of four parts: (1) phytosynthesis of rGO/nZVI and parental nZVI composites using A. halimus aquatic leaf extract, (2) characterization of phytosynthesized composites using multiple methods to confirm their successful fabrication, (3) study the synergistic effect of rGO and nZVI in the adsorption and removal of organic contaminants of doxycycline antibiotics under different reaction parameters, optimize the conditions of the adsorption process, (3) investigate composite materials in various continuous treatments after the processing cycle.
Doxycycline hydrochloride (DC, MM = 480.90, chemical formula C22H24N2O·HCl, 98%), iron chloride hexahydrate (FeCl3.6H2O, 97%), graphite powder purchased from Sigma-Aldrich, USA. Sodium hydroxide (NaOH, 97%), ethanol (C2H5OH, 99.9%) and hydrochloric acid (HCl, 37%) were purchased from Merck, USA. NaCl, KCl, CaCl2, MnCl2 and MgCl2 were purchased from Tianjin Comio Chemical Reagent Co., Ltd. All reagents are of high analytical purity. Double-distilled water was used to prepare all aqueous solutions.
Representative specimens of A. halimus have been collected from their natural habitat in the Nile Delta and lands along the Mediterranean coast of Egypt. Plant material was collected in accordance with applicable national and international guidelines17. Prof. Manal Fawzi has identified plant specimens according to Boulos18, and the Department of Environmental Sciences of Alexandria University authorizes the collection of studied plant species for scientific purposes. Sample vouchers are held at the Tanta University Herbarium (TANE), vouchers nos. 14 122–14 127, a public herbarium that provides access to deposited materials. In addition, to remove dust or dirt, cut the leaves of the plant into small pieces, rinse 3 times with tap and distilled water, and then dry at 50°C. The plant was crushed, 5 g of the fine powder was immersed in 100 ml of distilled water and stirred at 70°C for 20 min to obtain an extract. The obtained extract of Bacillus nicotianae was filtered through Whatman filter paper and stored in clean and sterilized tubes at 4°C for further use.
As shown in Figure 1, the GO was made from graphite powder by the modified Hummers method. 10 mg of GO powder was dispersed in 50 ml of deionized water for 30 min under sonication, and then 0.9 g of FeCl3 and 2.9 g of NaAc were mixed for 60 min. 20 ml of atriplex leaf extract was added to the stirred solution with stirring and left at 80°C for 8 hours. The resulting black suspension was filtered. The prepared nanocomposites were washed with ethanol and bidistilled water and then dried in a vacuum oven at 50°C for 12 hours.
Schematic and digital photographs of green synthesis of rGO/nZVI and nZVI complexes and removal of DC antibiotics from contaminated water using Atriplex halimus extract.
Briefly, as shown in Fig. 1, 10 ml of an iron chloride solution containing 0.05 M Fe3+ ions was added dropwise to 20 ml of a bitter leaf extract solution for 60 minutes with moderate heating and stirring, and then the solution was then centrifuged at 14,000 rpm (Hermle , 15,000 rpm) for 15 min to give black particles, which were then washed 3 times with ethanol and distilled water and then dried in a vacuum oven at 60° C. overnight.
Plant-synthesized rGO/nZVI and nZVI composites were characterized by UV-visible spectroscopy (T70/T80 series UV/Vis spectrophotometers, PG Instruments Ltd, UK) in the scanning range of 200-800 nm. To analyze the topography and size distribution of the rGO/nZVI and nZVI composites, TEM spectroscopy (JOEL, JEM-2100F, Japan, accelerating voltage 200 kV) was used. To evaluate the functional groups that can be involved in plant extracts responsible for the recovery and stabilization process, FT-IR spectroscopy was carried out (JASCO spectrometer in the range of 4000-600 cm-1). In addition, a zeta potential analyzer (Zetasizer Nano ZS Malvern) was used to study the surface charge of the synthesized nanomaterials. For X-ray diffraction measurements of powdered nanomaterials, an X-ray diffractometer (X’PERT PRO, the Netherlands) was used, operating at a current (40 mA), voltage (45 kV) in the 2θ range from 20° to 80° and CuKa1 radiation (\(\lambda =\ ) 1.54056 Ao). The energy dispersive X-ray spectrometer (EDX) (model JEOL JSM-IT100) was responsible for studying the elemental composition when collecting Al K-α monochromatic X-rays from -10 to 1350 eV on XPS, spot size 400 μm K-ALPHA (Thermo Fisher Scientific, USA) the transmission energy of the full spectrum is 200 eV and the narrow spectrum is 50 eV. The powder sample is pressed onto a sample holder, which is placed in a vacuum chamber. The C 1 s spectrum was used as a reference at 284.58 eV to determine the binding energy.
Adsorption experiments were carried out to test the effectiveness of the synthesized rGO/nZVI nanocomposites in removing doxycycline (DC) from aqueous solutions. Adsorption experiments were performed in 25 ml Erlenmeyer flasks at a shaking speed of 200 rpm on an orbital shaker (Stuart, Orbital Shaker/SSL1) at 298 K. By diluting the DC stock solution (1000 ppm) with bidistilled water. To assess the effect of the rGO/nSVI dosage on the adsorption efficiency, nanocomposites of different weights (0.01–0.07 g) were added to 20 ml of DC solution. To study the kinetics and adsorption isotherms, 0.05 g of the adsorbent was immersed in an aqueous solution of CD with initial concentration (25–100 mg L–1). The effect of pH on the removal of DC was studied at pH (3–11) and an initial concentration of 50 mg L-1 at 25°C. Adjust the pH of the system by adding a small amount of HCl or NaOH solution (Crison pH meter, pH meter, pH 25). In addition, the influence of reaction temperature on adsorption experiments in the range of 25-55°C was investigated. The effect of ionic strength on the adsorption process was studied by adding various concentrations of NaCl (0.01–4 mol L–1) at an initial concentration of DC of 50 mg L–1, pH 3 and 7), 25°C, and an adsorbent dose of 0.05 g. The adsorption of non-adsorbed DC was measured using a dual beam UV-Vis spectrophotometer (T70/T80 series, PG Instruments Ltd, UK) equipped with 1.0 cm path length quartz cuvettes at maximum wavelengths (λmax) of 270 and 350 nm. The percentage removal of DC antibiotics (R%; Eq. 1) and the adsorption amount of DC, qt, Eq. 2 (mg/g) were measured using the following equation.
where %R is the DC removal capacity (%), Co is the initial DC concentration at time 0, and C is the DC concentration at time t, respectively (mg L-1).
where qe is the amount of DC adsorbed per unit mass of the adsorbent (mg g-1), Co and Ce are the concentrations at zero time and at equilibrium, respectively (mg l-1), V is the solution volume (l), and m is the adsorption mass reagent (g).
SEM images (Figs. 2A–C) show the lamellar morphology of the rGO/nZVI composite with spherical iron nanoparticles uniformly dispersed on its surface, indicating successful attachment of nZVI NPs to the rGO surface. In addition, there are some wrinkles in the rGO leaf, confirming the removal of oxygen-containing groups simultaneously with the restoration of A. halimus GO. These large wrinkles act as sites for active loading of iron NPs. nZVI images (Fig. 2D-F) showed that the spherical iron NPs were very scattered and did not aggregate, which is due to the coating nature of the botanical components of the plant extract. The particle size varied within 15–26 nm. However, some regions have a mesoporous morphology with a structure of bulges and cavities, which can provide a high effective adsorption capacity of nZVI, since they can increase the possibility of trapping DC molecules on the surface of nZVI. When the Rosa Damascus extract was used for the synthesis of nZVI, the obtained NPs were inhomogeneous, with voids and different shapes, which reduced their efficiency in Cr(VI) adsorption and increased the reaction time 23 . The results are consistent with nZVI synthesized from oak and mulberry leaves, which are mainly spherical nanoparticles with various nanometer sizes without obvious agglomeration.
SEM images of rGO/nZVI (AC), nZVI (D, E) composites and EDX patterns of nZVI/rGO (G) and nZVI (H) composites.
The elemental composition of plant-synthesized rGO/nZVI and nZVI composites was studied using EDX (Fig. 2G, H). Studies show that nZVI is composed of carbon (38.29% by mass), oxygen (47.41% by mass) and iron (11.84% by mass), but other elements such as phosphorus24 are also present, which can be obtained from plant extracts. In addition, the high percentage of carbon and oxygen is due to the presence of phytochemicals from plant extracts in subsurface nZVI samples. These elements are evenly distributed on rGO but in different ratios: C (39.16 wt %), O (46.98 wt %) and Fe (10.99 wt %), EDX rGO/nZVI also shows the presence of other elements such as as S, which can be associated with plant extracts, are used. The current C:O ratio and iron content in the rGO/nZVI composite using A. halimus is much better than using the eucalyptus leaf extract, as it characterizes the composition of C (23.44 wt.%), O (68.29 wt.% ) and Fe (8.27 wt.%). wt %) 25. Nataša et al., 2022 reported a similar elemental composition of nZVI synthesized from oak and mulberry leaves and confirmed that polyphenol groups and other molecules contained in the leaf extract are responsible for the reduction process.
The morphology of nZVI synthesized in plants (Fig. S2A,B) was spherical and partially irregular, with an average particle size of 23.09 ± 3.54 nm, however chain aggregates were observed due to van der Waals forces and ferromagnetism. This predominantly granular and spherical particle shape is in good agreement with the SEM results. A similar observation was found by Abdelfatah et al. in 2021 when castor bean leaf extract was used in the synthesis of nZVI11. Ruelas tuberosa leaf extract NPs used as a reducing agent in nZVI also have a spherical shape with a diameter of 20 to 40 nm26.
Hybrid rGO/nZVI composite TEM images (Fig. S2C-D) showed that rGO is a basal plane with marginal folds and wrinkles providing multiple loading sites for nZVI NPs; this lamellar morphology also confirms the successful fabrication of rGO. In addition, nZVI NPs have a spherical shape with particle sizes from 5.32 to 27 nm and are embedded in the rGO layer with an almost uniform dispersion. Eucalyptus leaf extract was used to synthesize Fe NPs/rGO; The TEM results also confirmed that wrinkles in the rGO layer improved the dispersion of Fe NPs more than pure Fe NPs and increased the reactivity of the composites. Similar results were obtained by Bagheri et al. 28 when the composite was fabricated using ultrasonic techniques with an average iron nanoparticle size of approximately 17.70 nm.
The FTIR spectra of A. halimus, nZVI, GO, rGO, and rGO/nZVI composites are shown in Figs. 3A. The presence of surface functional groups in the leaves of A. halimus appears at 3336 cm-1, which corresponds to polyphenols, and 1244 cm-1, which corresponds to carbonyl groups produced by the protein. Other groups such as alkanes at 2918 cm-1, alkenes at 1647 cm-1 and CO-O-CO extensions at 1030 cm-1 have also been observed, suggesting the presence of plant components that act as sealing agents and are responsible for recovery from Fe2+ ​​to Fe0 and GO to rGO29. In general, the nZVI spectra show the same absorption peaks as bitter sugars, but with a slightly shifted position. An intense band appears at 3244 cm-1 associated with OH stretching vibrations (phenols), a peak at 1615 corresponds to C=C, and bands at 1546 and 1011 cm-1 arise due to stretching of C=O (polyphenols and flavonoids), CN -groups of aromatic amines and aliphatic amines were also observed at 1310 cm-1 and 1190 cm-1, respectively13. The FTIR spectrum of GO shows the presence of many high-intensity oxygen-containing groups, including the alkoxy (CO) stretching band at 1041 cm-1, the epoxy (CO) stretching band at 1291 cm-1, C=O stretch. a band of C=C stretching vibrations at 1619 cm-1, a band at 1708 cm-1 and a broad band of OH group stretching vibrations at 3384 cm-1 appeared, which is confirmed by the improved Hummers method, which successfully oxidizes the graphite process. When comparing rGO and rGO/nZVI composites with GO spectra, the intensity of some oxygen-containing groups, such as OH at 3270 cm-1, is significantly reduced, while others, such as C=O at 1729 cm-1, are completely reduced. disappeared, indicating the successful removal of oxygen-containing functional groups in GO by the A. halimus extract. New sharp characteristic peaks of rGO at C=C tension are observed around 1560 and 1405 cm-1, which confirms the reduction of GO to rGO. Variations from 1043 to 1015 cm-1 and from 982 to 918 cm-1 were observed, possibly due to the inclusion of plant material31,32. Weng et al., 2018 also observed a significant attenuation of oxygenated functional groups in GO, confirming the successful formation of rGO by bioreduction, since eucalyptus leaf extracts, which were used to synthesize reduced iron graphene oxide composites, showed closer FTIR spectra of plant component functional groups. 33 .
A. FTIR spectrum of gallium, nZVI, rGO, GO, composite rGO/nZVI (A). Roentgenogrammy composites rGO, GO, nZVI and rGO/nZVI (B).
The formation of rGO/nZVI and nZVI composites was largely confirmed by X-ray diffraction patterns (Fig. 3B). A high-intensity Fe0 peak was observed at 2Ɵ 44.5°, corresponding to index (110) (JCPDS no. 06–0696)11. Another peak at 35.1° of the (311) plane is attributed to magnetite Fe3O4, 63.2° may be associated with the Miller index of the (440) plane due to the presence of ϒ-FeOOH (JCPDS no. 17-0536)34. The X-ray pattern of GO shows a sharp peak at 2Ɵ 10.3° and another peak at 21.1°, indicating complete exfoliation of the graphite and highlighting the presence of oxygen-containing groups on the surface of GO35. Composite patterns of rGO and rGO/nZVI recorded the disappearance of characteristic GO peaks and the formation of broad rGO peaks at 2Ɵ 22.17 and 24.7° for the rGO and rGO/nZVI composites, respectively, which confirmed the successful recovery of GO by plant extracts. However, in the composite rGO/nZVI pattern, additional peaks associated with the lattice plane of Fe0 (110) and bcc Fe0 (200) were observed at 44.9\(^\circ\) and 65.22\(^\circ\), respectively .
The zeta potential is the potential between an ionic layer attached to the surface of a particle and an aqueous solution that determines the electrostatic properties of a material and measures its stability37. Zeta potential analysis of plant-synthesized nZVI, GO, and rGO/nZVI composites showed their stability due to the presence of negative charges of -20.8, -22, and -27.4 mV, respectively, on their surface, as shown in Figure S1A-C. . Such results are consistent with several reports that mention that solutions containing particles with zeta potential values ​​less than -25 mV generally show a high degree of stability due to electrostatic repulsion between these particles. The combination of rGO and nZVI allows the composite to acquire more negative charges and thus has higher stability than either GO or nZVI alone. Therefore, the phenomenon of electrostatic repulsion will lead to the formation of stable rGO/nZVI39 composites. The negative surface of GO allows it to be evenly dispersed in an aqueous medium without agglomeration, which creates favorable conditions for interaction with nZVI. The negative charge may be associated with the presence of different functional groups in the bitter melon extract, which also confirms the interaction between GO and iron precursors and the plant extract to form rGO and nZVI, respectively, and the rGO/nZVI complex. These plant compounds can also act as capping agents, as they prevent the aggregation of the resulting nanoparticles and thus increase their stability40.
The elemental composition and valence states of the nZVI and rGO/nZVI composites were determined by XPS (Fig. 4). The overall XPS study showed that the rGO/nZVI composite is mainly composed of the elements C, O, and Fe, consistent with the EDS mapping (Fig. 4F–H). The C1s spectrum consists of three peaks at 284.59 eV, 286.21 eV and 288.21 eV representing CC, CO and C=O, respectively. The O1s spectrum was divided into three peaks, including 531.17 eV, 532.97 eV, and 535.45 eV, which were assigned to the O=CO, CO, and NO groups, respectively. However, the peaks at 710.43, 714.57 and 724.79 eV refer to Fe 2p3/2, Fe+3 and Fe p1/2, respectively. The XPS spectra of nZVI (Fig. 4C-E) showed peaks for the elements C, O, and Fe. Peaks at 284.77, 286.25, and 287.62 eV confirm the presence of iron-carbon alloys, as they refer to CC, C-OH, and CO, respectively. The O1s spectrum corresponded to three peaks C–O/iron carbonate (531.19 eV), hydroxyl radical (532.4 eV) and O–C=O (533.47 eV). The peak at 719.6 is attributed to Fe0, while FeOOH shows peaks at 717.3 and 723.7 eV, in addition, the peak at 725.8 eV indicates the presence of Fe2O342.43.
XPS studies of nZVI and rGO/nZVI composites, respectively (A, B). Full spectra of nZVI C1s (C), Fe2p (D), and O1s (E) and rGO/nZVI C1s (F), Fe2p (G), O1s (H) composite.
The N2 adsorption/desorption isotherm (Fig. 5A, B) shows that the nZVI and rGO/nZVI composites belong to type II. In addition, the specific surface area (SBET) of nZVI increased from 47.4549 to 152.52 m2/g after blinding with rGO. This result can be explained by the decrease in the magnetic properties of nZVI after rGO blinding, thereby reducing particle aggregation and increasing the surface area of ​​the composites. In addition, as shown in Fig. 5C, the pore volume (8.94 nm) of the rGO/nZVI composite is higher than that of the original nZVI (2.873 nm). This result is in agreement with El-Monaem et al. 45 .
To evaluate the adsorption capacity to remove DC between the rGO/nZVI composites and the original nZVI depending on the increase in the initial concentration, a comparison was made by adding a constant dose of each adsorbent (0.05 g) to DC at various initial concentrations. Investigated solution [25]. –100 mg l–1] at 25°C. The results showed that the removal efficiency (94.6%) of the rGO/nZVI composite was higher than that of the original nZVI (90%) at a lower concentration (25 mg L-1). However, when the starting concentration was increased to 100 mg L-1, the removal efficiency of rGO/nZVI and parental nZVI dropped to 70% and 65%, respectively (Figure 6A), which may be due to fewer active sites and degradation of nZVI particles. On the contrary, rGO/nZVI showed a higher efficiency of DC removal, which may be due to a synergistic effect between rGO and nZVI, in which stable active sites available for adsorption are much higher, and in the case of rGO/nZVI, more DC can be adsorbed than intact nZVI. In addition, in fig. 6B shows that the adsorption capacity of the rGO/nZVI and nZVI composites increased from 9.4 mg/g to 30 mg/g and 9 mg/g, respectively, with an increase in the initial concentration from 25–100 mg/L. -1.1 to 28.73 mg g-1. Therefore, the DC removal rate was negatively correlated with the initial DC concentration, which was due to the limited number of reaction centers supported by each adsorbent for adsorption and removal of DC in solution. Thus, it can be concluded from these results that the rGO/nZVI composites have a higher efficiency of adsorption and reduction, and rGO in the composition of rGO/nZVI can be used both as an adsorbent and as a carrier material.
The removal efficiency and DC adsorption capacity for the rGO/nZVI and nZVI composite were (A, B) [Co = 25 mg l-1–100 mg l-1, T = 25 °C, dose = 0.05 g], pH. on adsorption capacity and DC removal efficiency on rGO/nZVI composites (C) [Co = 50 mg L–1, pH = 3–11, T = 25°C, dose = 0.05 g].
Solution pH is a critical factor in the study of adsorption processes, since it affects the degree of ionization, speciation, and ionization of the adsorbent. The experiment was carried out at 25°C with a constant adsorbent dose (0.05 g) and an initial concentration of 50 mg L-1 in the pH range (3–11). According to a literature review46, DC is an amphiphilic molecule with several ionizable functional groups (phenols, amino groups, alcohols) at various pH levels. As a result, the various functions of DC and the related structures on the surface of the rGO/nZVI composite may interact electrostatically and may exist as cations, zwitterions, and anions, the DC molecule exists as cationic (DCH3+) at pH < 3.3, zwitterionic (DCH20) 3.3 < pH < 7.7 and anionic (DCH− or DC2−) at PH 7.7. As a result, the various functions of DC and the related structures on the surface of the rGO/nZVI composite may interact electrostatically and may exist as cations, zwitterions, and anions, the DC molecule exists as cationic (DCH3+) at pH < 3.3, zwitterionic (DCH20) 3.3 < pH < 7.7 and anionic (DCH- or DC2-) at PH 7.7. В результате различные функции ДК и связанных с ними структур на поверхности композита rGO/nZVI могут взаимодействовать электростатически и могут существовать в виде катионов, цвиттер-ионов и анионов, молекула ДК существует в виде катиона (DCH3+) при рН < 3,3, цвиттер-ионный (DCH20) 3,3 < pH < 7,7 и анионный (DCH- или DC2-) при pH 7,7. As a result, various functions of DC and related structures on the surface of the rGO/nZVI composite can interact electrostatically and can exist in the form of cations, zwitterions, and anions; the DC molecule exists as a cation (DCH3+) at pH < 3.3; ionic (DCH20) 3.3 < pH < 7.7 and anionic (DCH- or DC2-) at pH 7.7.因此,DC 的各种功能和rGO/nZVI 复合材料表面的相关结构可能会发生静电相互作用,并可能以阳离子、两性离子和阴离子的形式存在,DC 分子在pH < 3.3 时以阳离子(DCH3+) 的形式存在,两性离子(DCH20) 3.3 < pH < 7.7 和阴离子(DCH- 或DC2-) 在PH 7.7。因此 , dc 的 种 功能 和 和 和 和 和 复合 材料 表面 的 相关 结构 可能 会 发生 静电 相互 , 并 可能 以 阳离子 两 性 和 阴离子 形式 , , dc 分子 在 pH <3.3 时 阳离子 阳离子 阳离子 阳离子 阳离子 (dch3+)形式存在,两性离子(DCH20) 3.3 < pH < 7.7 和阴离子(DCH- 或DC2-) 在PH 7.7。 Следовательно, различные функции ДК и родственных им структур на поверхности композита rGO/nZVI могут вступать в электростатические взаимодействия и существовать в виде катионов, цвиттер-ионов и анионов, а молекулы ДК являются катионными (ДЦГ3+) при рН < 3,3. Therefore, various functions of DC and related structures on the surface of the rGO/nZVI composite can enter into electrostatic interactions and exist in the form of cations, zwitterions, and anions, while DC molecules are cationic (DCH3+) at pH < 3.3. Он существует в виде цвиттер-иона (DCH20) при 3,3 < pH < 7,7 и аниона (DCH- или DC2-) при pH 7,7. It exists as a zwitterion (DCH20) at 3.3 < pH < 7.7 and an anion (DCH- or DC2-) at pH 7.7. With an increase in pH from 3 to 7, the adsorption capacity and efficiency of DC removal increased from 11.2 mg/g (56%) to 17 mg/g (85%) (Fig. 6C). However, as the pH increased to 9 and 11, the adsorption capacity and removal efficiency decreased somewhat, from 10.6 mg/g (53%) to 6 mg/g (30%), respectively. With an increase in pH from 3 to 7, DCs mainly existed in the form of zwitterions, which made them almost non-electrostatically attracted or repulsed with rGO/nZVI composites, predominantly by electrostatic interaction. As the pH increased above 8.2, the surface of the adsorbent was negatively charged, thus the adsorption capacity decreased and decreased due to the electrostatic repulsion between the negatively charged doxycycline and the surface of the adsorbent. This trend suggests that DC adsorption on rGO/nZVI composites is highly pH dependent, and the results also indicate that rGO/nZVI composites are suitable as adsorbents under acidic and neutral conditions.
The effect of temperature on the adsorption of an aqueous solution of DC was carried out at (25–55°C). Figure 7A shows the effect of temperature increase on the removal efficiency of DC antibiotics on rGO/nZVI, it is clear that the removal capacity and adsorption capacity increased from 83.44% and 13.9 mg/g to 47% and 7.83 mg/g. , respectively. This significant decrease may be due to an increase in the thermal energy of DC ions, which leads to desorption47.
Effect of Temperature on Removal Efficiency and Adsorption Capacity of CD on rGO/nZVI Composites (A) [Co = 50 mg L–1, pH = 7, Dose = 0.05 g], Adsorbent Dose on Removal Efficiency and Removal Efficiency of CD Effect of Initial Concentration on the adsorption capacity and efficiency of DC removal on the rGO/nSVI composite (B) [Co = 50 mg L–1, pH = 7, T = 25°C] (C, D) [Co = 25–100 mg L–1, pH = 7, T = 25 °C, dose = 0.05 g].
The effect of increasing the dose of the composite adsorbent rGO/nZVI from 0.01 g to 0.07 g on the removal efficiency and adsorption capacity is shown in Fig. . 7B. An increase in the dose of the adsorbent led to a decrease in the adsorption capacity from 33.43 mg/g to 6.74 mg/g. However, with an increase in the adsorbent dose from 0.01 g to 0.07 g, the removal efficiency increases from 66.8% to 96%, which, accordingly, may be associated with an increase in the number of active centers on the nanocomposite surface.
The effect of initial concentration on adsorption capacity and removal efficiency [25–100 mg L-1, 25°C, pH 7, dose 0.05 g] was studied. When the initial concentration was increased from 25 mg L-1 to 100 mg L-1, the removal percentage of the rGO/nZVI composite decreased from 94.6% to 65% (Fig. 7C), probably due to the absence of the desired active sites. . Adsorbs large concentrations of DC49. On the other hand, as the initial concentration increased, the adsorption capacity also increased from 9.4 mg/g to 30 mg/g until equilibrium was reached (Fig. 7D). This inevitable reaction is due to an increase in driving force with an initial DC concentration greater than the DC ion mass transfer resistance to reach the surface 50 of the rGO/nZVI composite.
Contact time and kinetic studies aim to understand the equilibrium time of adsorption. First, the amount of DC adsorbed during the first 40 minutes of the contact time was approximately half of the total amount adsorbed over the entire time (100 minutes). While the DC molecules in solution collide causing them to rapidly migrate to the surface of the rGO/nZVI composite resulting in significant adsorption. After 40 min, DC adsorption increased gradually and slowly until equilibrium was reached after 60 min (Fig. 7D). Since a reasonable amount is adsorbed within the first 40 minutes, there will be fewer collisions with DC molecules and fewer active sites will be available for non-adsorbed molecules. Therefore, the adsorption rate can be reduced51.
To better understand the adsorption kinetics, line plots of pseudo first order (Fig. 8A), pseudo second order (Fig. 8B), and Elovich (Fig. 8C) kinetic models were used. From the parameters obtained from the kinetic studies (Table S1), it becomes clear that the pseudosecond model is the best model for describing adsorption kinetics, where the R2 value is set higher than in the other two models. There is also a similarity between the calculated adsorption capacities (qe, cal). The pseudo-second order and the experimental values ​​(qe, exp.) are further evidence that the pseudo-second order is a better model than other models. As shown in Table 1, the values ​​of α (initial adsorption rate) and β (desorption constant) confirm that the adsorption rate is higher than the desorption rate, indicating that DC tends to adsorb efficiently on the rGO/nZVI52 composite. .
Linear adsorption kinetic plots of pseudo-second order (A), pseudo-first order (B) and Elovich (C) [Co = 25–100 mg l–1, pH = 7, T = 25 °C, dose = 0.05 g].
Studies of adsorption isotherms help to determine the adsorption capacity of the adsorbent (RGO/nRVI composite) at various adsorbate concentrations (DC) and system temperatures. The maximum adsorption capacity was calculated using the Langmuir isotherm, which indicated that the adsorption was homogeneous and included the formation of an adsorbate monolayer on the surface of the adsorbent without interaction between them53. Two other widely used isotherm models are the Freundlich and Temkin models. Although the Freundlich model is not used to calculate the adsorption capacity, it helps to understand the heterogeneous adsorption process and that vacancies on the adsorbent have different energies, while the Temkin model helps to understand the physical and chemical properties of adsorption54.
Figures 9A-C show line plots of the Langmuir, Freindlich, and Temkin models, respectively. The R2 values ​​calculated from the Freundlich (Fig. 9A) and Langmuir (Fig. 9B) line plots and presented in Table 2 show that DC adsorption on the rGO/nZVI composite follows the Freundlich (0.996) and Langmuir (0.988) isotherm models and Temkin (0.985). The maximum adsorption capacity (qmax), calculated using the Langmuir isotherm model, was 31.61 mg g-1. In addition, the calculated value of the dimensionless separation factor (RL) is between 0 and 1 (0.097), indicating a favorable adsorption process. Otherwise, the calculated Freundlich constant (n = 2.756) indicates a preference for this absorption process. According to the linear model of the Temkin isotherm (Fig. 9C), the adsorption of DC on the rGO/nZVI composite is a physical adsorption process, since b is ˂ 82 kJ mol-1 (0.408)55. Although physical adsorption is usually mediated by weak van der Waals forces, direct current adsorption on rGO/nZVI composites requires low adsorption energies [56, 57].
Freundlich (A), Langmuir (B), and Temkin (C) linear adsorption isotherms [Co = 25–100 mg L–1, pH = 7, T = 25 °C, dose = 0.05 g]. Plot of the van’t Hoff equation for DC adsorption by rGO/nZVI composites (D) [Co = 25–100 mg l-1, pH = 7, T = 25–55 °C and dose = 0.05 g].
To evaluate the effect of reaction temperature change on DC removal from rGO/nZVI composites, thermodynamic parameters such as entropy change (ΔS), enthalpy change (ΔH), and free energy change (ΔG) were calculated from equations. 3 and 458.
where \({K}_{e}\)=\(\frac{{C}_{Ae}}{{C}_{e}}\) – thermodynamic equilibrium constant, Ce and CAe – rGO in solution, respectively /nZVI DC concentrations at surface equilibrium. R and RT are the gas constant and adsorption temperature, respectively. Plotting ln Ke against 1/T gives a straight line (Fig. 9D) from which ∆S and ∆H can be determined.
A negative ΔH value indicates that the process is exothermic. On the other hand, the ΔH value is within the physical adsorption process. Negative ΔG values ​​in Table 3 indicate that adsorption is possible and spontaneous. Negative values ​​of ΔS indicate a high ordering of adsorbent molecules at the liquid interface (Table 3).
Table 4 compares the rGO/nZVI composite with other adsorbents reported in previous studies. It is clear that the VGO/nCVI composite has a high adsorption capacity and may be a promising material for the removal of DC antibiotics from water. In addition, the adsorption of rGO/nZVI composites is a fast process with an equilibration time of 60 min. The excellent adsorption properties of the rGO/nZVI composites can be explained by the synergistic effect of rGO and nZVI.
Figures 10A, B illustrate the rational mechanism for the removal of DC antibiotics by the rGO/nZVI and nZVI complexes. According to the results of experiments on the effect of pH on the efficiency of DC adsorption, with an increase in pH from 3 to 7, DC adsorption on the rGO/nZVI composite was not controlled by electrostatic interactions, since it acted as a zwitterion; therefore, a change in the pH value did not affect the adsorption process. Subsequently, the adsorption mechanism can be controlled by non-electrostatic interactions such as hydrogen bonding, hydrophobic effects, and π-π stacking interactions between the rGO/nZVI composite and DC66. It is well known that the mechanism of aromatic adsorbates on the surfaces of layered graphene has been explained by π–π stacking interactions as the main driving force. The composite is a layered material similar to graphene with an absorption maximum at 233 nm due to the π-π* transition. Based on the presence of four aromatic rings in the molecular structure of the DC adsorbate, we hypothesized that there is a mechanism of π-π-stacking interaction between the aromatic DC (π-electron acceptor) and the region rich in π-electrons onto the RGO surface. /nZVI composites. In addition, as shown in fig. 10B, FTIR studies were performed to study the molecular interaction of rGO/nZVI composites with DC, and the FTIR spectra of rGO/nZVI composites after DC adsorption are shown in Figure 10B. 10b. A new peak is observed at 2111 cm-1, which corresponds to the framework vibration of the C=C bond, which indicates the presence of the corresponding organic functional groups on the surface of 67 rGO/nZVI. Other peaks shift from 1561 to 1548 cm-1 and from 1399 to 1360 cm-1, which also confirms that π-π interactions play an important role in the adsorption of graphene and organic pollutants68,69. After DC adsorption, the intensity of some oxygen-containing groups, such as OH, decreased to 3270 cm-1, which suggests that hydrogen bonding is one of the adsorption mechanisms. Thus, based on the results, DC adsorption on the rGO/nZVI composite occurs mainly due to π-π stacking interactions and H-bonds.
Rational mechanism of adsorption of DC antibiotics by rGO/nZVI and nZVI complexes (A). FTIR adsorption spectra of DC on rGO/nZVI and nZVI (B).
The intensity of the absorption bands of nZVI at 3244, 1615, 1546, and 1011 cm–1 increased after DC adsorption on nZVI (Fig. 10B) compared to nZVI, which should be related to the interaction with possible functional groups of the carboxylic acid O groups in DC. However, this lower percentage of transmission in all observed bands indicates no significant change in the adsorption efficiency of the phytosynthetic adsorbent (nZVI) compared to nZVI before the adsorption process. According to some DC removal research with nZVI71, when nZVI reacts with H2O, electrons are released and then H+ is used to produce highly reducible active hydrogen. Finally, some cationic compounds accept electrons from active hydrogen, resulting in -C=N and -C=C-, which is attributed to the splitting of the benzene ring.

Post time: Nov-14-2022