Insight into core -shell microporous zinc silicate adsorbent to eliminate antibiotics in aquatic environment under the COVID-19 pandemic

Abstract

Under the new crown pneumonia (COVID-19) epidemic, the intensive use of therapeutic drugs has caused certain hidden danger to the safety of the water environment. Therefore, the core-shell microporous zinc silicate (SiO₂@ZSO) was successfully prepared and used for the adsorption of chloroquine phosphate (CQ), tetracycline (TC) and ciprofloxacin (CIP) for eliminating the threat of COVID-19. The adsorption efficiencies of 20 mg L⁻¹ of CQ, TC and CIP by SiO₂@ZSO were all up to 60% after 5 min. The adsorption capacity of SiO₂@ZSO for CQ, TC and CIP can reach 49.01 mg g⁻¹, 56.06 mg g⁻¹ and 104.77 mg g⁻¹, respectively. The adsorption process is primarily physical adsorption, which is heterogeneous, spontaneous and preferential. Moreover, the effects of temperature, pH, salinity, and reusability on the adsorption of CQ, TC, and CIP on SiO₂@ZSO were investigated. The adsorption mechanism mainly involves electrostatic attraction, partitioning and hydrogen bonding, which is insightful through the changes of the elements and functional groups before and after adsorption. This work provides a solution to the problems faced by the treatment of pharmaceuticals wastewater under the COVID-19 epidemic.

Graphical abstract

1. Introduction

Since 2019, the new crown pneumonia (COVID-19) epidemic has swept the world. So far, it has not only deprived millions of lives, but also has a huge impact on the development of human society (Morales-Paredes et al., 2022; Peng et al., 2022). Massive amounts of anti-COVID-19 drugs (chloroquine phosphate (CQ, quinolines)) and antibiotics (tetracycline (TC, tetracyclines) and ciprofloxacin (CIP, quinolones)) are being poured into treatment to eliminate the threat of COVID-19 (Adebisi et al., 2021; Yacouba et al., 2021). However, only a small percentage of drugs will be absorbed into the human body to treat diseases, and most will be excreted into the environment through human excrement (Chen et al., 2021; Rajiv et al., 2021). Large quantities of CQ, TC and CIP residues in the ecological environment will affect the physiological functions of plants and animals by cumulative effect, promoting the generation of drugresistant bacteria and causing potential harm for ecosystem (Al-Musawi et al., 2021b; Yi et al., 2021). Therefore, the simple, rapid and efficient simultaneous removal of anti-COVID-19 and antibiotic drugs from water has become the focus of water treatment.

Multifarious strategies have been reported previously to effectively remove drugs from wastewater, such as biodegradation (J. J. Li et al., 2022), chemical oxidation (Balarak et al., 2021; Kyzas et al., 2022), electrochemical treatment (Ji et al., 2021), chemical coagulation (Gan et al., 2021; He et al., 2021), and adsorption (Xu et al., 2022b; Yilmaz et al., 2022). Among these methods, adsorption is the most competitive potential technology for the removal of drugs due to its simplicity, high efficiency, economic advantages and environmental friendliness (Z. Wei et al., 2022, Wei et al., 2022; Zhu et al., 2021). Although many nanomaterials such as metal–organic frameworks (MOFs) (Xu et al., 2022a), carbon nanotubes (Sajid et al., 2022), etc. are used to effectively remove drugs from water, traditional adsorption materials such as bentonite (Guan et al., 2022), kaolinite (Gao et al., 2022), diatomaceous earth (Kodali et al., 2022), activated carbon (Balarak and McKay, 2021) and clay minerals (Zhao et al., 2022) are still the mainstream adsorbents in the market.

Silicate minerals are the most abundant minerals in the earth's crust, and are composed of anionic groups of silicon-oxygen tetrahedra and metal ions located between layers or chains. Its economical, high chemical and thermal stability, and abundant hydroxyl groups on the surface make it an ideal candidate for adsorption of organic pollutants. For example, Zhu et al. used an one-pot hydrothermal method to prepare hierarchically porous sea urchin-like Cu_2−xSi₂O₅(OH)₃·xH₂O hollow microspheres for the adsorption and removal of the cationic dye methylene blue and achieved good adsorption activity (Zhu et al., 2021). Li et al. constructed a flower-like mesoporous magnesium silicate composites (MMSCs) by subjecting sepiolite to acid bleaching process and hydrothermal method to efficiently adsorb aflatoxin B₁ in water (Y. Y. Li et al., 2022). Tian et al. prepared hybrid silicate adsorbents via a simple one-step hydrothermal process for efficiently adsorb chlortetracycline and oxytetracycline (Tian et al., 2016). Deng et al. synthesized cost-effective hydrochar composite (MgSi-HC) by a simple one-step process using waste sawdust, inexpensive silicate and magnesium salts as raw materials. The results show that it has high adsorption activity for tetracycline (TC) in aqueous solution (Deng et al., 2020). However, the non-specific adsorption of organic pollutants by silicate adsorbents is rarely investigated.

Notably, zinc silicate has attracted extensive attention due to its unique structure such as variable morphology and crystal diversity (Dong et al., 2020; Park and Kim, 2021). Although the use of wastes as raw materials to prepare zinc silicate can realize waste utilization, the processes such as washing and sorting will increase the preparation cost, and the obtained adsorbents often have large impurities and low adsorption activity (Wang et al., 2016; Zhang et al., 2017). In addition, few studies have explored the non-selective adsorption of drugs by zinc silicate, especially chloroquine phosphate for the treatment of COVID-19. The development of low-cost, high-adsorption activity zinc silicate for emergency treatment of pharmaceutical wastewater is an interesting and promising research.

To address these challenges, a core-shell microporous zinc silicate (SiO₂@ZSO) with no selective adsorption of drugs was successfully prepared by a facile hydrothermal method. Broad-spectrum drugs such as chloroquine phosphate (CQ), tetracycline (TC) and ciprofloxacin (CIP) for eliminating the threat of COVID-19 are extracted from wastewater by synthetic SiO₂@ZSO. The control steps and energy conversion of the adsorption process of CQ, TC and CIP by SiO₂@ZSO were analyzed kinetically and thermodynamically, respectively. Different factors affecting the adsorption activity such as temperature, pH and dosage, salinity, etc. were investigated. In addition, the mechanism of SiO₂@ZSO adsorption of CQ, TC and CIP was discussed in depth by means of characterization methods such as FT-IR, XPS, TEM, Zeta potential, etc. This study provides a feasible strategy for the elimination of pharmaceuticals in the water environment through a rich source of silicate-based sorbents.

2. Experimental

2.1. Materials

The chemical reagents including Zn(NO₃)₂.6H₂O, NH₄Cl, ammonia, ethanol, tetraethylorthosilicate (TEOS), NaNO₃, NaCl, Na₂SO₄, NaHCO₃, Na₂CO₃, Na₃PO₃ and NaH₂PO₃, hydrochloric acid, and sodium hydroxide were purchased from Macklin Biochemical Co. Ltd. (Shanghai, China). Chloroquine phosphate (CQ, quinolines), tetracycline (TC) and ciprofloxacin (CIP) were obtained from Aladdin Reagent Co., Ltd. (Shanghai, China). All chemicals were analytical grade and were employed without further purification.

2.2. Synthesis

2.2.1. Preparation of SiO₂ pellets

Monodisperse, uniform particle size (about 500 nm) SiO₂ pellets were prepared by Stöber method. The preparation method is as follows: 50 mL of ethanol, 20 mL of deionized water, and 7 mL of ammonia water were separately measured and mixed in a beaker, and 6 mL of TEOS was added dropwise to the mixture under vigorous stirring. Then the mixture was stired at a constant speed (about 600 r/min) for 8h, until the white precipitate at the bottom of the beaker observed. Washed it with ethanol and deionized water for 3 times each, then dried at 60 °C for 12 h to obtain white SiO₂ pellet powder.

2.2.2. Preparation of core-shell structure ZnSiO₃

1.0 g SiO₂ balls were placed in 50 mL deionized water, ultrasonicated for 1h to make them evenly dispersed, and then 1 mL NH₃·H₂O and 10 mmol NH₄Cl were added to the above dispersion to make solution A. Under the action of magnetic stirring, 16 mmol of Zn(NO₃)₂.6H₂O was dispersed in 20 mL of deionized water and added to solution A to form solution B. The mixed solution B was continuously stirred for 30 min and then transferred to a 100 mL reaction kettle, placed in a blast heating drying oven, and reacted at 100 °C for 12 h. After the reaction, the precipitate was collected, centrifuged and washed three times with deionized water and absolute ethanol, and dried in a drying oven at 60 °C for 12 h. The obtained white powder samples are zinc silicate core-shell microspheres, marked as SiO₂@ZSO.

2.3. Characterization

The physical properties and chemical structure of the as-prepared adsorbent were obtained by SEM (ZEISS MERLIN Compact, U.K.), N₂-adsorption-desorption curve (BET, ASAP 2020, USA), XRD (XRD-6100, Shimadzu, Japan), FT-IR (IRPrestige-21, Shimadzu, Japan), HRTEM (JEOL F200, Japan), EDS-mapping, respectively. The changes of the adsorbent chemical composition before and after adsorption was analyzed by XPS (Thermo Scientific, ESCALAB 250xi, USA). Total organic carbon (TOC-2000, METASH, China) analysis of the purification effect of the adsorbent.

2.4. Adsorption experiment

The adsorption performance of SiO₂@ZSO was investigated by adsorbing pollutants in wastewater, including three typical broad-spectrum drugs (see Fig. S1 for structural formulas), namely chloroquine phosphate (CQ), tetracycline (TC) and ciprofloxacin (CIP). A certain mass of adsorbent was placed in a 100 mL conical flask containing target solution, and then placed in a rotary water bath shaker (rotational speed 200 r·min⁻¹) for adsorption experiments. Periodically, 5 mL of the supernatant was taken, filtered with a 0.22 μm water filter, and the concentration changes of pollutants were measured with a UV spectrophotometer (DR 6000, HACH, USA) or HPLC (5120, Hitachi, Japan). For details on the detection methods and conditions of antibiotics, see Text S1. All experiments were performed 3 times in parallel.

Batch experiments were carried out to investigate the effects of adsorption reaction time, initial concentration, adsorbent dosage, initial pH and temperature on the adsorption process. The pH value was controlled by 0.1 M HCl and 0.1 M NaOH using the equipment (HQ11d, HACH, USA). Due to the high salt content in drugs wastewater, seven common salts were placed in the adsorption system to investigate their effects on the adsorption of TC by SiO₂@ZSO, including NaNO₃, NaCl, Na₂SO₄, NaHCO₃, Na₂CO₃, Na₃PO₃ and NaH₂PO₃. The adsorption removal rate (η) and adsorption capacity (Q or q, mg·g⁻¹) of drug are shown in Equation (1) and Equation (2).

$$\eta =1-\frac{C_t}{C_0}$$ (1)

$$Q=\frac{\left(C_0-C_t\right)\mathrm{V}}{\mathrm{m}}$$ (2)

where C₀ (mg·L⁻¹) and C_t are the drug concentrations at time 0 and t, respectively; V (mL) is the drug solution volume; m (mg) is the adsorbent dose.

2.5. Adsorption models and thermodynamics

Adsorption kinetics were performed at an initial pollutant concentration ranging from 20 mg L⁻¹ to 100 mg L⁻¹ at a temperature of 20 °C, an adsorbent dosage of 1 g L⁻¹, and unadjusted pH. Pseudo-first-order kinetics and pseudo-second-order kinetics were used for nonlinear fitting of the data according to Equations (3), (4),

$$q_t=q_e\left(1-e^{-k_{1}t}\right)$$ (3)

$$q_t=\frac{k_2{q}_e^{2}t}{1+k_2{q}_{e}t}$$ (4)

where k₁ (min⁻¹) and k₂ (g·mg⁻¹·min⁻¹) are pseudo-first-order kinetic constants and pseudo-second-order kinetic constants, respectively; q_t and q_e are the adsorption capacity (mg·g⁻¹) at time t and equilibrium, respectively. In addition to the coefficient (R²), the normalized standard deviation (Δq) was calculated to describe the adsorption process by Equation (5):

$$\Delta q(\%)=\sqrt{{\sum }_{i=1}^N\frac{{\left[\left(q_{\mathit{exp}}-q_{cat}\right)/q_{\mathit{exp}}\right]}^2}{N-1}}\times 100\%$$ (5)

where q_exp and q_cal are the experimental and calculated adsorption capacities (mg·g⁻¹), respectively, and N is the experimental data point. The diffusion mechanism was revealed by intra-particle diffusion and liquid film diffusion models based on Equations (6), (7),

$$q_t=k_{id}{t}^{1/2}+C$$ (6)

$$\ln \left(1-\frac{q_t}{q_{\mathit{exp}}}\right)=-k_{f}t$$ (7)

where k_id (min^−0.5) and k_f (min⁻¹) are the intra-particle diffusion and liquid film diffusion constants, respectively, and C is the intercept related to the boundary layer.

Adsorption isotherms were investigated in batches at 20 °C, 25 °C and 30 °C with an adsorbent dosage of 1 g L⁻¹. Langmuir, Freundlich, Tempkin and D-R models were used to analyze the adsorption behavior from Equations (8), (9), (10), (11),

$$Q_e=\frac{q_m{K}_l{C}_e}{1+K_l{C}_e}$$ (8)

$$Q_e=K_f{C}_e^{1/n}$$ (9)

$$Q_e=\frac{RT}{b}\ln \left(K_t{C}_e\right)$$ (10)

$$Q_e=\frac{q_m}{e^{\beta E^2}}$$ (11)

where K_l, K_f, K_t and β are the adsorption equilibrium constants of the corresponding models, respectively; q_m (mg·g⁻¹) is the theoretical maximum equilibrium adsorption capacity; n is the adsorption intensity; b is a constant; R is the ideal gas constant of 8.314 J mol⁻¹ K⁻¹; T (K) is the reaction temperature; β is the parameter related to the adsorption energy; and $E=RT\ln \left(1+1/C_e\right)$ represents the adsorption potential which is related to the concentration of the adsorbate in the solution.

Thermodynamic parameters such as Gibbs free energy ΔG^°, enthalpy change ΔH ^° and entropy change ΔS ^° can be calculated by Equations (12), (13),

$$\Delta G°=-RT\ln K_C$$ (12)

$$\ln K_C=\frac{\Delta S°}{R}-\frac{\Delta H°}{RT}$$ (13)

where $K_c=\frac{Q_e}{C_e}$ is the adsorption equilibrium constant (L·mg⁻¹) when the solution is in equilibrium.

3. Results and discussion

3.1. Characterization

The phase structure of the adsorbent was obtained by XRD in Fig. S2. A hill peak is observed around 23.6°, which is typical for SiO₂. In addition, a weaker characteristic peak of ZnSiO₃ (JCPDS No. 34–0575) was observed in the SiO₂@ZSO structure. The XRD patterns show that all the prepared SiO₂@ZSO adsorbents exhibit poor crystallization. The morphologies and microstructures of the as-prepared SiO₂ spheres and SiO₂@ZSO were investigated by SEM and TEM in Fig. 1 . SiO₂ spheres exhibited smooth spherical shapes with a diameter of about 500 nm. In Fig. 1b, SiO₂@ZSO exhibit a core-shell structure composed of Zn²⁺ precipitated on the surface of SiO₂ spheres and reacted with it to form a zinc silicate shell and an inner core of SiO₂ spheres. To confirm that zinc silicate is supported on the surface of SiO₂ spheres, the element content and dispersion of the adsorbent were preliminarily obtained by SEM mapping in Fig. 1c. Zn, Si and O were observed and distributed in the SiO₂@ZSO structure, suggesting that the SiO₂@ZSO adsorbent was successfully synthesized. Solid spheres of SiO₂ with smooth surfaces were observed in Fig. 1d–e by HRTEM images. Clearly, zinc silicate appears on the surface of SiO₂ spheres, forming a core-shell structure in Fig. 1f–g. The insets in Fig. 1e and g do not observe a clearly bright ring, suggesting that SiO₂ and SiO₂@ZSO are present in an amorphous form, a conclusion that corresponds to XRD.

Fig. 1.

SEM images: SiO₂ (a) and SiO₂@ZSO (b); SEM-mapping image of SiO₂@ZSO (c); HRTEM images: SiO₂ (d-e) and SiO₂@ZSO (f-g) (Insets in e and g are the corresponding SAED images).

FT-IR and XPS provide the surface groups and element valence states of the adsorbents in Fig. 2 . In Fig. 2a, the broad bands at 1640 cm⁻¹ and 3443 cm⁻¹ are attributed to the bending vibration of O–H and the stretching vibration of hydroxyl groups, respectively (Zhu et al., 2022a). This suggests that the large number of functional groups on the surface of the adsorbent may be the active sites for reaction with organic molecules. The three peaks at 1101 cm⁻¹, 955 cm⁻¹ and 467 cm⁻¹ belong to the asymmetric stretching, symmetric stretching and asymmetric deformation vibrations of Si–O–Si, respectively (Huang et al., 2017; Wang et al., 2022). The narrow peak at 798 cm⁻¹ is usually attributed to the Si–OH stretching vibration in the unique silanol nests (Naini et al., 2022). In SiO₂@ZSO, the peak at 663 cm⁻¹ is attributed to the asymmetric stretching vibration model of Zn–O (Saberi Rise et al., 2022). Notably, the successful synthesis of zinc silicate may be due to Zn occupying the site of Si element in Si–OH. The full XPS spectrum of SiO₂@ZSO (in Fig. S3) contains Zn, Si and O elements, and their atomic ratios are about 1:2:6 (in Table S1). The Zn 2p narrow spectrum has two peaks corresponding to Zn 2p_3/2 and Zn 2p_1/2 (Zhu et al., 2022b). In Fig. 2c, the binding energy belonging to the Si–O bond is shifted by about 0.5 eV to lower binding energy in SiO₂@ZSO, which may be due to the influence of Zn²⁺. In the enlarged O 1s spectrum in Fig. 2d, it is observed that O–Si and adsorbed oxygen molecules emerge at 532.50 eV and 533.40 eV at the SiO₂ surface (Z. Wei et al., 2022). Unlike SiO₂, SiO₂@ZSO were additionally found a binding energy at 531.66 eV, which was attributed to the Zn–O bond (Khan et al., 2021). Through the above analysis, it can be seen that the hydroxyl-rich SiO₂@ZSO core-shell adsorbent was successfully prepared.

Fig. 2.

FT-IR spectrum of SiO₂ and SiO₂@ZSO (a) and XPS narrow spectra of Zn 2p (b), Si 2p (c) and O 1s (d).

The specific surface area and pore size distribution of SiO₂@ZSO were calculated by BET and BJH equations based on N₂ adsorption and desorption curves (in Fig. 3 ). The SiO₂ spheres and SiO₂@ZSO show the IUPAC type 4 curve with H3-type hysteresis loop. In Table S1, the specific surface area of SiO₂ is only 11.85 m² g⁻¹, the total pore volume is 0.028 cm³ g⁻¹, and the largest proportion of pore size is 4.67 nm. Moreover, the specific surface area of SiO₂@ZSO is 73.55 m² g⁻¹, which is 6.20 times that of SiO₂, and its largest proportion of pore size is 0.17 nm. The above conclusions indicate that SiO₂@ZSO has better pore structure, more adsorption sites and larger adsorption capacity compared with SiO₂.

Fig. 3.

N₂ adsorption-desorption isotherms and pore-size distribution curves (inset) of SiO₂ and SiO₂@ZSO.

3.2. Adsorption activity

3.2.1. Adsorption activity of SiO₂@ZSO

The adsorption activities of SiO₂ pellets and SiO₂@ZSO for CQ, TC and CIP were investigated in Fig. 4 a. At the onset of adsorption, the concentration of comtaminants dropped significantly due to the abundant active sites, and then slowly reached the adsorption-desorption equilibrium on the adsorbent surface over time. The SiO₂ spheres showed poor adsorption activities for CQ, TC and CIP, which were embodied in the adsorption activities of only 11.04%, 1.67% and 7.77% respectively after adsorbing the target pollutants for 2 h. Obviously, SiO₂@ZSO shows excellent adsorption activity for CQ, TC and CIP. The corresponding adsorption efficiencies are as high as 71.45%, 69.60% and 61.74% after adsorption for 5 min. Moreover, the adsorption capacities of CQ, TC and CIP are 17.37 mg g⁻¹, 18.15 mg g⁻¹ and 19.25 mg g⁻¹ for SiO₂@ZSO adsorption (Fig. 4b), which are 6.72, 49.65 and 9.62 times higher than those of SiO₂ pellets, respectively.

Fig. 4.

Adsorption activity (a) and adsorption capacity (b) of various adsorbents for CQ, TC and CIP. (Reaction conditions: dosage is 1 g/L; contaminant concentration is 20 mg/L; temperature is 20 °C; pH is not adjusted.)

3.2.2. Kinetics analysis

Pseudo-first-order and pseudo-second-order kinetic models were used to analyze the adsorption rate law and adsorption behavior of TC, CQ and CIP on SiO₂@ZSO, and the fitting results are shown in Fig. 5 and Table 1 . By comparing the coefficients (R²), we found that the adsorption process of TC, CQ and CIP by SiO₂@ZSO could well fit the pseudo-second-order equation, which is reflected in their R² is greater than 0.90 at different concentrations. Moreover, compared with the adsorption amount calculated by the pseudo-first-order, the value obtained by the pseudo-second order is more in line with the actual adsorption amount. Kinetic analysis showed that the adsorption processes were mainly related to the adsorption sites (Xia et al., 2022).

Fig. 5.

Pseudo-first-order (a-c) and pseudo-second-order (d-f) kinetics of adsorption of CQ, TC and CIP on SiO₂@ZSO. (Reaction conditions: contaminant concentration is 0–100 mg L⁻¹, dosage is 1 g L⁻¹, temperature is 20 °C, pH is not adjusted).

Table 1.

Kinetic parameters of adsorption of CQ, TC and CIP on SiO₂@ZSO.

	Conc. (mg·L‒1)	qexp	Pseudo-first-order model				Pseudo-second-order model				Liquid film diffusion
			k1 (min‒1)	qcat (mg·g‒1)	R2	Δq	k2 (g·mg‒1·min‒1)	qcat (mg·g‒1)	R2	Δq	kf (min‒1)	R2
CQ	20	17.372	0.688	16.963	0.9908	2.35%	0.083	17.492	0.9989	0.69%	0.060	0.8415
	40	32.140	0.413	28.286	0.9087	11.99%	0.020	30.104	0.9660	6.33%	0.026	0.9320
	80	38.759	0.556	33.352	0.8747	13.95%	0.022	35.445	0.9405	8.55%	0.024	0.9214
	100	46.406	0.608	40.744	0.8913	12.20%	0.021	43.110	0.9456	7.10%	0.030	0.9410
TC	20	18.164	0.457	16.978	0.9560	6.53%	0.042	17.859	0.9924	1.68%	0.042	0.9491
	40	35.069	0.329	31.590	0.9206	9.92%	0.015	33.661	0.9769	4.01%	0.033	0.9684
	80	56.784	0.108	50.516	0.9120	11.04%	0.003	56.127	0.9682	1.16%	0.044	0.9899
	100	62.463	0.151	55.975	0.9526	10.39%	0.003	61.633	0.9850	1.33%	0.028	0.9672
CIP	20	19.247	0.476	18.062	0.9596	6.16%	0.042	18.948	0.9938	1.55%	0.041	0.9272
	40	29.593	0.087	27.418	0.9180	7.35%	0.004	30.565	0.9655	3.28%	0.036	0.8033
	80	57.548	1.015	49.191	0.9050	14.52%	0.036	51.049	0.9321	11.29%	0.023	0.6111
	100	68.677	1.838	67.385	0.9960	1.88%	0.196	67.736	0.9966	1.37%	0.022	0.3733

To gain insight into the rate-limiting steps of adsorption of TC, CQ and CIP by SiO₂@ZSO, intra-particle diffusion and liquid film diffusion models were used for the analysis. In Figs. S4a–c, the adsorption process of organic pollutants by SiO₂@ZSO shows a typical multi-linear fitting, indicating that the adsorption mechanism is composed of different stages. The initial stage of adsorption is external diffusion adsorption, the second stage is internal diffusion of particles, and the third stage is equilibrium stage. Generally, the stage with the lower slope k _id is considered to be the rate control stage, but the third stage is not included because it occurs faster and cannot represent the rate determination step. The multi-stage results indicate that the intraparticle diffusion process is the main rate-determining step, but not the only ratecontrolling mechanism, in SiO₂@ZSO adsorption of antibiotics. In Figs. S4d–f, the liquid film diffusion model shows that all points behave linearly and do not pass through the origin, implying that intra-particle diffusion is not the only decision step again. Within a given adsorption time, the liquid film diffusion at the initial stage of adsorption is a rate-controlling step; once the pollutants cross the liquid film, intra-particle diffusion becomes a rate-controlling step until the adsorption equilibrium is reached.

3.2.3. Isotherms and thermodynamics

Adsorption isotherms at different reaction temperatures were studied in order to elucidate the mechanisms and interactions between adsorbent and adsorbate. The fitting results of the Langmuir, Fredunlich, Temkin and D-R models are shown in Fig. 6 and Table 2 . The correlation coefficients of the four isotherm fittings are all greater than 0.95, indicating that they can fit the data well. From the perspective of R², the Langmuir model can better explain the adsorption process of CQ, while TC and CIP are more suitable for the Temkin model. The maximum adsorption amounts calculated according to Langmuir were 49.01 mg g⁻¹, 56.06 mg g⁻¹ and 104.77 mg g ⁻¹ for SiO₂@ZSO adsorption of CQ, TC and CIP, respectively. Comparing the adsorption activities and capacities of the reported adsorbents for CQ, TC and CIP, SiO₂@ZSO shows outstanding advantages in Table S2. In the Fredunlich model, 0 < 1/n < 1, which implies that the adsorption process of SiO₂@ZSO for CQ, TC and CIP is preferential adsorption, and the adsorption process for CQ and TC is easy adsorption (0.1 < 1/n < 0.5). Temkin model also achieved good fitting results, which indicates there is a strong electrostatic attraction force during the adsorption process (Z. Wei et al., 2022). In D-R, the fitted heat of adsorption is < 1 kJ mol⁻¹, which is defined as the physical adsorption category (E < 8 kJ mol⁻¹). In general, it is not appropriate to consider only one model when R² is not much different. Therefore, we speculate that the adsorption process of SiO₂@ZSO for CQ, TC and CIP might be heterogeneous adsorption, and the adsorption process is closely related to electrostatic attraction.

Fig. 6.

Isotherm study of Langmuir (a), Fredunlich (b), Temkin (c) and Redlich342 Peterson (d) models for adsorption of CQ, TC and CIP on SiO₂@ZSO.

Table 2.

Isotherm parameters of adsorption of CQ, TC and CIP on SiO₂@ZSO.

Poll.	Temp.	Langmuir				Freundlich			Tempkin			D-R
		Kl (L·mg−1)	qm (mg·g−1)	R2	Δq	Kf (L·mg−1)	1/n	R2	Kt (L·mg−1)	b (kJ·mol−1)	R2	qm (mg·g−1)	E (kJ·mol−1)	R2	Δq
CQ	20 °C	0.15	49.01	0.9582	5.61%	14.00	0.29	0.9399	1.84	0.251	0.9512	42.55	0.36	0.9568	8.31%
	25 °C	0.15	46.78	0.9884	8.90%	13.48	0.29	0.9653	1.84	0.271	0.9792	39.78	0.38	0.9567	7.39%
	30 °C	0.15	46.52	0.9904	10.76%	13.44	0.28	0.9625	1.82	0.276	0.9786	39.61	0.38	0.9678	8.52%
TC	20 °C	0.19	53.68	0.9971	8.83%	15.76	0.29	0.9888	2.50	0.239	0.9988	43.67	0.61	0.8249	11.46%
	25 °C	0.20	55.41	0.9761	6.55%	15.67	0.31	0.9997	2.86	0.238	0.9941	43.96	0.78	0.7338	15.47%
	30 °C	0.23	56.06	0.9862	7.55%	17.19	0.30	0.9967	3.39	0.243	0.9984	45.21	0.85	0.7633	13.27%
CIP	20 °C	0.06	104.77	0.9906	16.53%	9.84	0.57	0.9880	0.55	0.102	0.9931	66.44	0.32	0.9815	26.10%
	25 °C	0.06	95.32	0.9841	8.55%	9.69	0.55	0.9875	0.57	0.113	0.9870	61.96	0.32	0.9598	29.44%
	30 °C	0.06	100.11	0.9890	13.55%	9.29	0.56	0.9919	0.53	0.110	0.9911	62.68	0.32	0.9621	28.90%

Thermodynamic behavior is determined by the thermodynamic parameters Gibbs free energy (ΔG^°, kJ·mol⁻¹), enthalpy change (ΔH ^°, kJ·mol⁻¹) and entropy change (ΔS ^°, J·mol⁻¹ K⁻¹). In Table 3 , the ΔG^° values of different pollutants at different temperatures are all negative, indicating that the adsorption process is spontaneous and favorable. The ΔH ^° of SiO₂@ZSO adsorbing CQ, TC and CIP are −11.03 kJ mol⁻¹, −12.22 kJ mol⁻¹ and −2.46 kJ mol⁻¹, respectively. These negative values indicate that the adsorption is an exothermic process and increasing of the temperature is not favorable for the adsorption to proceed, and the absolute value of ΔH ^° <20 kJ mol⁻¹ indicates physical adsorption. Further, the negative ΔS ^° indicates that the adsorption is entropy-decreasing and irreversible during this adsorption experiment.

Table 3.

Thermodynamic parameters of adsorption of CQ, TC and CIP on SiO₂@ZSO.

Poll.	Temp.	Thermodynamic
		$\Delta G^0$ (kJ·mol−1)	$\Delta H^0$ (kJ·mol−1)	$\Delta S^0$ (J·mol−1 K−1)
CQ	20 °C	−0.99	−11.03	−34.20
	25 °C	−0.87
	30 °C	−0.65
TC	20 °C	−2.13	−12.22	−34.50
	25 °C	−1.89
	30 °C	−1.79
CIP	20 °C	−1.27	−2.46	−4.00
	25 °C	−1.26
	30 °C	−1.23

3.2.4. Influence of external environment

A comparison of the adsorption activities of SiO₂@ZSO at different initial concentrations of different pollutants is given in Fig. 7 a. It was pointed out that SiO₂@ZSO had better adsorption activities for TC, CQ and CIP at low concentrations such as 20 mg L⁻¹, which are 91.56%, 79.21% and 83.52%, respectively. The adsorption capacities of SiO₂@ZSO at different initial concentrations of different pollutants are shown in Fig. 7b. The amount of adsorbent and the reaction temperature are important factors affecting the adsorption activity of the adsorbent. In Fig. 7c, the adsorption capacities of SiO₂@ZSO for TC, CQ and CIP increased with the increase of the dosage, but the growth rate of the adsorption capacity decreased significantly when the dosage is greater than 0.5 g L⁻¹. In Fig. S5, we can clearly see that the removal rate of TC, CQ and CIP can reach 50% after adsorption for 2 min by SiO₂@ZSO (contaminant concentration is 20 mg L⁻¹ and adsorbent dosage is 0.5 g L⁻¹), indicating that SiO₂@ZSO has a good application prospect. In Fig. 7d, we observed that the adsorption capacity of SiO₂@ZSO for TC, CQ and CIP is very weakly dependent on temperature, which indicated that temperature is not a factor limiting the adsorption of TC, CQ and CIP by SiO₂@ZSO.

Fig. 7.

The effect of initial concentration of organic pollutants (a-b), adsorbent dosage (c) and reaction temperature (d) on the adsorption activity of TC, CQ and CIP on 16- SiO₂@ZSO. (Reaction conditions: contaminant concentration is 20–100 mg L⁻¹, dosage is 0.25–1.25 g L⁻¹, temperature is 20–30 °C, pH is not adjusted).

3.2.5. Effect of pH

The structural stability of antibiotics is closely related to the pH in the solution, so it is necessary to investigate the effect of the pH change of the initial solution on the adsorption process. The pKa value of CQ is 8.40, it is a cation when pH < 8.40, and an anion otherwise (Yi et al., 2021). TC has three pKa values of 3.3, 7.7 and 9.7, corresponding to the existence of cation, zwitterion and anion, respectively (Álvarez-torrellas et al., 2016; Qiao et al., 2020). The pKa values of CIP are 6.10 and 8.70 corresponding to the presence of cations, zwitterions and anions, respectively (Al-Musawi et al., 2021a). As depicted in Fig. 8 a, the effect of initial pH range of 3–11 for CQ, TC and CIP solutions on adsorption capacity is presented. For pollutants with an initial concentration of 20 mg L⁻¹, the dosage of SiO₂@ZSO is 1 g L⁻¹, and the adsorption temperature is 20 °C, the adsorption capacities of SiO₂@ZSO for CQ, TC and CIP decreased with increasing pH. In addition, the Zeta potential (Zetasizer Nano ZS90, Malvern, U.K.) is given out to explore the pH_zpc and the surface charge properties of SiO₂@ZSO in Fig. 8b. With the increase of pH, the zeta potential value of SiO₂@ZSO becomes more and more negative, which implies that the pH_zpc of SiO₂@ZSO is less than 3.0 and has a negative surface charge in the pH range of 3–11. When the pH is alkaline, the three pollutants exist in the form of anions, which lead to electrostatic repulsion with the negatively charged SiO₂@ZSO, resulting in a decrease in the adsorption capacity. The above conclusions point out that the electrostatic force is a significant adsorption force during the adsorption of CQ, TC and CIP by SiO₂@ZSO.

Fig. 8.

Effects of pH on the adsorption activities (a) of CQ, TC and CIP on SiO₂@ZSO and zeta potential values (b) of SiO₂@ZSO.

3.2.6. Effect of salt environment

Drugs wastewater is one of the high salinity wastewaters, where the presence of salt can corrode equipment and affect the removal of antibiotics. Therefore, it is necessary to study the effect of salt on the adsorption process of antibiotics. Using seven kinds of salts widely present in water, the effect of SiO₂@ZSO on the adsorption activity of CQ, TC and CIP wastewater containing salt was analyzed. The effect of salt presence on the activity of SiO₂@ZSO to adsorb organic contaminants at concentrations of 5 mM, 10 mM, 20 mM and 50 mM is examined in Fig. 9 . Compared with the environment without adding any salt, the adsorption effect of SiO₂@ZSO on CQ remained at about 80% (the adsorption capacity was maintained at 17 mg g⁻¹). This indicated that the presence of the seven salts hardly changed the adsorption capacity of SiO₂@ZSO for CQ. Obviously, the presence of Na₃PO₄, Na₂CO₃, NaH₂PO₄ and NaHCO₃ inhibited the adsorption of TC and CIP by SiO₂@ZSO, and the adsorption capacity became weaker with the increase of their concentrations. This may be due to the differences in the ability of different pollutants to compete with anions for the adsorption sites of SiO₂@ZSO (Q. Wei et al., 2022).

Fig. 9.

Effect of salt on the adsorption activity of SiO₂@ZSO for CQ (a,d), TC (b,e) and CIP (c,f). (Reaction conditions: contaminant concentration is 20 mg L⁻¹, dosage is 1 g L⁻¹, temperature is 20 °C, pH is not adjusted).

3.3. Adsorption mechanism

According to the previous kinetic and thermodynamic analysis, there is a physical and chemical interactive adsorption mechanism in the process of SiO₂@ZSO adsorption of CQ, TC and CIP. Further, for in-depth analysis of the mechanism of action of the adsorption process, HPLC and UV scan spectra over time, FT-IR and XPS spectra before and after adsorption are provided. In Fig. 10 , it can be judged whether there are other adsorption intermediates in the adsorption process by the time-varying spectra of HPLC and UV. The conclusion shows that no new absorption peaks were observed during the adsorption of organic pollutants by SiO₂@ZSO, and the peak intensities of all absorption peaks gradually decreased with time. Further, the TOC data showed that most of the pollutants were adsorbed on the SiO₂@ZSO surface after 2 h of adsorption, resulting in water purification (in Fig. 10d).

Fig. 10.

Changes in the detection spectra of SiO₂@ZSO adsorption process of CQ (a), TC (b) and CIP (c) with time; and the removal efficiency of TOC after adsorption for 2 h (d).

The FT-IR and XPS spectra before and after adsorption are provided in Fig. 11 to further analyze the interaction process of SiO₂@ZSO and pollutant molecules. In Fig. 11a, the FT-IR spectra before and after adsorption are almost unchanged. But obviously, the adsorption of CQ, TC and CIP on SiO₂@ZSO can be observed through the stretching vibration of the benzene ring in the range of 1600-1400 cm⁻¹. The Si–OH vibration at 798 cm⁻¹ and the adsorbed –OH at 3443 cm⁻¹ were found to become weaker after adsorption of organic pollutants, indicating that the abundant –OH groups on the surface of SiO₂@ZSO were involved in the adsorption process. In addition, through the XPS full spectra (Fig. S6) before and after adsorption and the proportion of related elements (Table 4 ), the proportion of C and N elements increased after the adsorption of pollutants. In addition, Cl element was also observed after SiO₂@ZSO adsorption of CQ, which again confirmed the adsorption of pollutant molecules on the SiO₂@ZSO surface. The fine spectra of Zn 2p, Si 2p and O 1s show that the binding energies at the typical outgoing peaks after adsorption are shifted towards low in Fig. 11b, which indicates that the electron cloud of pollutants is shifted towards the SiO₂@ZSO structure.

Fig. 11.

Comparison of FT-IR spectra (a) and XPS narrow spectra of Zn 2p (b), Si 2p (c) and O 1s (d) of SiO₂@ZSO and SiO₂@ZSO after adsorption of CQ, TC and CIP.

Table 4.

Comparison of element content of SiO₂@ZSO and after adsorption of CQ, TC and CIP.

Sample	Atomic (%)
	C	N	O	Zn	Si	Cl
SiO2@ZSO	13.52	1.39	56.57	9.76	18.76	/
SiO2@ZSO -CQ	16.82	1.55	57.98	6.43	16.34	0.88
SiO2@ZSO -TC	16.47	1.65	57.07	6.55	18.27	/
SiO2@ZSO-CIP	16.82	1.64	57.04	5.74	18.33	0.43

The above conclusions show that the adsorption of pollutants by SiO₂@ZSO is mainly involves physical adsorption, involving three forces of electrostatic attraction, hydrophobic interaction and hydrogen bonding, as shown in Fig. 12 . The electrostatic attraction of negatively charged SiO₂@ZSO in water filled with cationic antibiotics is one of the main adsorption driving forces. This conclusion is confirmed by Fig. 8. When CQ, TC and CIP exist as cations or zwitterions, there is electrostatic attraction between them and the adsorbent. Conversely, electrostatic repulsion exists when both the pollutant molecules and the SiO₂@ZSO surface are negatively charged (under alkaline conditions), resulting in a decrease in adsorption capacity. According to the principle of similar compatibility, the hydrophobic interaction is an important adsorption mechanism when both the organic pollutant and the adsorbent surface are hydrophobic (Xu et al., 2021). The solubility (at 20 °C) of CQ, TC and CIP was 50 mg mL⁻¹, 50 mg mL⁻¹ and 35 mg mL⁻¹, respectively. Compared with the adsorption of CQ and TC by SiO₂@ZSO, the maximum adsorption capacity was obtained when SiO₂@ZSO adsorbed the more insoluble CIP under the same conditions (in Fig. 5), implying that the hydrophobic interaction is one of the adsorption forces. The abundant hydroxyl groups on the surface of SiO₂@ZSO are the prerequisites for hydrogen bond adsorption with pollutant molecules. From the molecular structures of CQ, TC and CIP, their hydrogen bond acceptor and donor numbers are shown in Table S3. In Fig. 11, the abundant –OH and Si–OH bonds on the surface of SiO₂@ZSO easily form hydrogen bonds such as –OH⋯O/−OH⋯N with the –OH bonds on CQ, TC and CIP molecules. It was confirmed from the FT-IR and XPS spectra of SiO₂@ZSO after adsorption of pollutants (in Fig. 11). In Fig. 12, after the core-shell adsorbent enters the polluted water body (left), the pollutants are adsorbed on the surface of SiO₂@ZSO under the action of intermolecular van der Waals forces such as electrostatic attraction, hydrophobic distribution and hydrogen bonding, so as to achieve the purpose of purifying the water body (right).

Fig. 12.

Possible mechanism of adsorption of pollutant molecules by SiO₂@ZSO. (Gray balls are C atoms, red balls are O atoms, blue balls are N atoms, green balls are Cl atoms, and rose-red balls are F atoms; the core-shell structure is SiO₂@ZSO).

3.4. Repetition and application potential of adsorbent

The repeated adsorption experiments are described as follows: First, after the adsorbent was saturation, the adsorbent was collected by centrifugation, washed with water for several times, and then dried for the next experiment. In addition, actual wastewater tends to be complex in composition, so it is important to consider the activity of the adsorbent when multiple drugs coexist. The results of repeated experiments and the activity of the adsorbents in complex waters are shown in Fig. 13 . The adsorption capacities of SiO₂@ZSO for CQ, TC and CIP decreased to 10.58 mg g⁻¹, 8.32 mg g⁻¹ and 13.65 mg g⁻¹, respectively, after four repeated experiments (Fig. 13a). This conclusion indicates that the reproducibility of SiO₂@ZSO is weak, possibly because the antibiotics adsorbed on the surface of SiO₂@ZSO are not completely desorbed, and they occupy part of the adsorption sites, thereby weakening the adsorption performance. This indicates that SiO₂@ZSO has great application prospects in dealing with emergencies, but further research is needed in terms of reusability. In Fig. 13b–c, SiO₂@ZSO exhibited excellent adsorption activity against complex wastewater (where OTC and CTC refer to oxytetracycline and chlortetracycline, respectively). The content of organic matter in the solution decreased by 80.24% after adsorption for 2 h. The conclusion points out that SiO₂@ZSO has the potential for practical application.

Fig. 13.

Reproducibility experiment of adsorption of CQ, TC and CIP by SiO₂@ZSO (a), UV–vis adsorption curves of complex wastewater adsorbed by SiO₂@ZSO (b) and the removal efficiency of TOC after adsorption for 2 h (c).

4. Conclusion

In conclusion, this work provides a new adsorption material as one of the solutions to the newly faced antibiotic contamination problem in the COVID-19 environment. Core-shell microporous SiO₂@ZSO was prepared using SiO₂ as a template, and SiO₂@ZSO exhibited nonspecific and excellent adsorption activity for CQ, TC and CIP. The adsorption capacity of SiO₂@ZSO for CQ, TC and CIP is 6.72, 49.65 and 9.62 times higher than that of SiO₂. Batch experiments indicated that the optimum adsorption condition for SiO₂@ZSO adsorption of CQ, TC and CIP is 0.5 g/L adsorbent dose, 20 mg/L initial concentration of contaminant and acidic conditions. Based on kinetics, the process of SiO₂@ZSO adsorption of pollutant molecules consists of intra-particle diffusion and liquid film diffusion as rate-controlling steps. Thermodynamic analysis indicated that the adsorption was heterogeneous, spontaneously and preferentially. The adsorption mechanisms mainly include: 1) electrostatic attraction, obtained from the effect of pH and zeta potential analysis; 2) partition effect, derived from pollutant molecules and structural properties of the adsorbent; 3) hydrogen bonding, based on functional group changes before and after adsorption. In addition, the salinity and repeated experiments show that SiO₂@ZSO is less affected by ions and has better reuse value, indicating that SiO₂@ZSO has excellent stability. Therefore, SiO₂@ZSO is an ideal and excellent adsorbent for mixed antibiotic wastewater, especially the complex organic wastewater brought by the COVID-19 epidemic.

CRediT authorship contribution statement

Xueli Hu: Conceptualization, Data curation, Formal analysis, Funding acquisition, Investigation, Methodology, Resources, Software, Validation, Visualization, Writing – original draft. Yuanhang Zhou: Investigation, Validation, Visualization. Yingying Zhou: Conceptualization, Investigation, Methodology. Yun Bai: Investigation, Validation. Ruiting Chang: Conceptualization, Investigation. Peng Lu: Supervision, Conceptualization, Formal analysis, Investigation. Zhi Zhang: Supervision, Project administration, Conceptualization, Investigation, Resources.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Handling Editor: Zhen Leng

Appendix A. Supplementary data

The following is the Supplementary data to this article:

Multimedia component 1

The support material includes: methods to detect changes in CQ, TC and CIP concentrations, XRD patterns, element content, physical properties of SiO₂@ZSO and SiO₂, fitting results of intraparticle diffusion kinetic model for adsorption of CQ, TC and CIP by SiO₂@ZSO, and comparison of the full XPS spectra of SiO₂@ZSO before and after adsorption.

Data availability

Data will be made available on request.