In-situ immobilization of ZIF-8 on sodium lignosulfonate/chitosan foams for efficiently removal of ciprofloxacin from water

doi:10.21203/rs.3.rs-1964271/v1

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Research Article

In-situ immobilization of ZIF-8 on sodium lignosulfonate/chitosan foams for efficiently removal of ciprofloxacin from water

https://doi.org/10.21203/rs.3.rs-1964271/v1

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published 16 Mar, 2023

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Ciprofloxacin (CIP) contamination poses a great threat to the environment due to it can induce the growth of antibiotic-resistant bacteria. Herein, a composite adsorbent material, sodium lignosulfonate/chitosan @ZIF-8 (SLS/CS@ZIF-8), was synthesized by introducing sodium lignosulfonate into chitosan foams (CS-FM) and then loading ZIF-8 onto the resulting foams (SLS/CS-FM) by in-situ immobilization to effectively remove CIP from water. The introduction of sodium lignosulfonate improved the mechanical strength of chitosan foams and provided more attachment sites for ZIF-8. The effects of pH, contact time, initial concentration and temperature for CIP adsorption were investigated. Overall, the adsorption capacity of SLS/CS@ZIF-8 for CIP was better than chitosan@ZIF-8 (CS@ZIF-8) in the pH range of 4 to 11, and SLS/CS@ZIF-8 kept high adsorption capacities at a wide pH range from 6 to 9. SLS/CS@ZIF-8 maximum adsorption capacity for CIP was 413 mg/g when the CIP concentration of 250 mg/L at 303K, which was better than most previously reported adsorbents. Pseudo-second-order kinetics and Langmuir isothermal model could better describe the adsorption process, indicating that the adsorption of CIP was mainly controlled by chemisorption mechanism and monolayer adsorption. Adsorption thermodynamic showed that the adsorption process of CIP on SLS/CS@ZIF-8 was spontaneous and endothermic. After 6 cycles, SLS/CS@ZIF-8 still had good adsorption capacity for CIP. According to FT-IR and XPS analysis, the adsorption mechanisms of CIP on SLS/CS@ZIF-8 was mainly electrostatic interaction, hydrogen bond interaction and π-π interaction. This work synthesized a potential adsorbent to effectively remove CIP from wastewater, developed a new low-carbon green chemical technology for potential applications of sodium lignosulfonate and chitosan.

sodium lignosulfonate

chitosan

ZIF-8

composite materials

mechanical strength

adsorption capacity

In recent years, antibiotics are frequently detected in wastewater, surface water, even ground water due to discharges from hospitals and drug manufacturers (Tang et al. 2019). Among the most frequently detected antibiotics, ciprofloxacin (CIP) as a broad-spectrum antibacterial have attracted increasing international attention to their serious threats to human health and aquatic ecosystems through inducing growth of antibiotic-resistant bacteria even at low concentration (Doorslaer et al. 2014; López-Serna et al. 2013; Zhou et al. 2011). Thus, efficient removal of CIP from wastewater is critical required. At present, there are many ways to remove CIP from wastewater, such as adsorption (Balarak et al. 2021; Theamwong et al. 2021; Zheng et al. 2021), biodegradation (Bhattacharya and Gupta. 2020; Hassan et al. 2020; Yang et al. 2017), ozonation (De et al. 2010; González-Labrada et al. 2020), and photodegradation (Chen et al. 2019; Chuaicham et al. 2021; Triquet et al. 2020). Among the above listed methods, adsorption is still one of the most effective methods for its characteristics of high efficiency, low cost, easy disposal and greenness.

Metal-organic framework is a kind of porous material formed by metal ions or metal clusters and organic ligands (Dai et al. 2021; Ahmed et al. 2021). Zeolitic imidazole frameworks-8 (ZIF-8) as a subset of MOFs has been attracted widely attention to remove CIP from wastewater (Dang et al. 2020; Li et al. 2018), due to its large surface area, good chemical stability and highly resistant to water degradation (Hong et al. 2021; Li et al. 2017). Although ZIF-8 has great potential as an adsorbent for removing CIP, it is difficult to separate from water because it usually obtained in powder form (Amo et al. 2021). To overcome the shortcoming, it is one of feasible way to combinate ZIF-8 with suitable substrate materials in varies forms. Among substrate materials, chitosan (CS) materials are one of the most prominent due to it can not only be used as a carrier for ZIF-8, but also as an adjuvant for hybrid adsorption. Xu et al. prepared a mixed matrix film by adding ZIF-8 into chitosan biopolymer matrix for methanol/dimethyl carbonate separation (Xu et al. 2022). MJ et al. fabricated chitosan polyethylene glycol@ZIF-8 nanocomposite films for utilization as the wound dressing materials (Mj et al. 2020). However, the CS materials have low mechanical strength and few attachment sites for ZIF-8 by the way of in-situ immobilization, which limits the practical application of CS and ZIF-8 composites to a certain extent. One of the methods to solve the problems of low mechanical strength and few attachment sites for CS materials is to composite them with other materials having supporting properties and abundant functional groups.

Lignin is the second most abundant renewable biomass next to cellulose in nature, and exists in woody biomass together with cellulose and hemicellulose (Kim et al. 2021). At present, most industrial lignin has been burned or directly discharged into rivers, which has become a great threat to the environment. It is of great significance to the research of high value application of lignin. Lignin and its derivatives contains a variety of functional groups and reactive sites (carbonyl, hydroxyl, methoxy, etc. (Zhou et al. 2021), which give they the potential to provide attachment sites for ZIF-8. Xu et al. prepared an adsorbent material by combining ZIF-8 with sodium lignosulfonate (SLS) for dye removal from water (Xu et al. 2018). As a kind of green and low-cost additive, lignin also has enough impact resistance and tensile properties to prevent the collapse of material structure and improve the adaptability of material (Rojo et al. 2015). Zhang et al. prepared a lignin-chitosan-PVA composite hydrogel for medical dressing, and found that the introduction of lignin effectively improved the mechanical strength of chitosan–PVA (Zhang et al. 2019). Zhang et al. founded that the introduce of lignin could effectively improve the mechanical strength of lignocellulose hydrogels (Zhang et al. 2019). So far, there is no report on introduction lignin to improve the performance of chitosan/MOFs adsorbents. In order to enhance the mechanical strength and increase attachment sites for ZIF-8 by the way of in-situ immobilization for efficiently removal of ciprofloxacin from water, SLS (C₂₀H₂₄Na₂O₁₀S₂), the by-product of sulfite pulping and papermaking process, was proposed to be introduced into chitosan foams.

Herein, a novel adsorbent (SLS/CS@ZIF-8) was synthesized by introducing sodium lignosulfonate into chitosan foams and then loading ZIF-8 onto the foams by in-situ immobilization (As shown in Scheme 1) to effectively remove CIP from water. The characterizations of samples were tested by electronic universal testing machine, SEM, FT-IR, XRD and automated surface area and porosimetry analyzer. The effect of pH on the adsorption capacity of CIP was investigated in detail. The adsorption kinetics, isotherms and thermodynamics were also studied for adsorption mechanism exploration. Moreover, the leading factors in the CIP adsorption process were discussed based on FT-IR and XPS analysis. Finally, the recycling potential of SLS/CS@ZIF-8 was studied by investigating the regeneration performance of the adsorbent. This research not only provided a novel adsorbent material for effectively removal of CIP from water, but also expected to realize a new way for the high-value application of lignin.

2.1 Materials

Sodium lignosulfonate (SLS, ≥ 96%), Chitosan (CS, viscosity average Mw190–310 kDa, 75–85% deacetylation), Ciprofloxacin hydrochloride (C₁₇H₁₈FN₃O₃•HCl, ≥ 99%), Sodium hydroxide (NaOH, ≥ 98%), Zinc nitrate hexahydrate (Zn(NO₃)₂•6H₂O, ≥ 99.0%) were purchased from Shanghai Maclin Chemistry Co., Ltd, China. Methanol (CH₃OH, ≥ 99.5%), Ethanol (CH₃CH₂OH, ≥ 99.5%), Acetic acid glacial (CH₃COOH, ≥ 99.5%), 2-Methylimidazole (C₄H₆N₂, ≥ 99.0%) and Hydrochloric acid (HCl, 36.0–38.0%) were purchased from Shanghai Aladdin Chemistry Co., Ltd, China. All reagents were analytical grade and used without further purification.

2.2 Synthesis of materials

Synthesis of CS-FM. First, CS (0.4 g) was dissolved in 20 mL acetic acid aqueous solution (2% V/W) and stirred continuously for 1h to obtain a uniformly mixed solution. Then the mixing solution was injected into porous silicone template with a syringe and freeze-dried for 24 h. The obtained CS-FM was washed several times by the deionized water to remove acetic acid in the foams and then freeze-dried for 18 h.

Synthesis of SLS/CS-FM. First, CS (0.4 g) and SLS (0.4 g) were dissolved in 20 mL acetic acid aqueous solution (2% V/W) and stirred continuously for 1h to obtain a uniformly mixed solution. Then the mixing solution was injected into porous silicone template with a syringe and freeze-dried for 24 h. The obtained SLS/CS-FM was washed several times by the deionized water to remove acetic acid in the foams and then freeze-dried for 18 h.

Synthesis of CS@ZIF-8. CS@ZIF-8 was synthesized by in-situ growth of ZIF-8 with CS-FM as carrier. First, the CS-FM was immersed in Zn(NO₃)₂·6H₂O solution (2 g of Zn(NO₃)₂·6H₂O in 40 mL of methanol) and left for 8 h. Then the containing Zn²⁺ of CS-FM was immersed in C₄H₆N₂ solution (1 g of C₄H₆N₂ in 40 mL of methanol) and left for 24 h. Finally, the obtained CS@ZIF-8 was repeatedly washed by the deionized water and ethanol to remove unreacted reagents on the surface of products followed by freeze-dried for 18 h.

Synthesis of SLS/CS@ZIF-8. SLS/CS@ZIF-8 was synthesized by in-situ growth of ZIF-8 with SLS/CS-FM as carrier. First, the SLS/CS-FM was immersed in Zn(NO₃)₂·6H₂O solution (2 g of Zn(NO₃)₂·6H₂O in 40 mL of methanol) and left for 8 h. Then the containing Zn²⁺ of SLS/CS-FM was immersed in C₄H₆N₂ solution (1 g of C₄H₆N₂ in 40 mL of methanol) and left for 24 h. Finally, the obtained SLS/CS@ZIF-8 was repeatedly washed by the deionized water and ethanol to remove unreacted reagents on the surface of products followed by freeze-dried for 18 h.

2.3 Characterizations

The mechanical properties of samples were determined by electronic universal testing machine (MTS CMT6103, USA). The micro-structure of samples was determined by the scanning electron microscopy (SEM Hitachi S4800, Japan). The crystallization of samples was determined by X-ray diffractometer (XRD Philips PW1830, Japan) at Cu Kα radiation with the scanning scope of 2θ from of 5° to 80°. The FT-IR spectroscopy of samples were obtained by Fourier transform infrared spectroscopy (FT-IR Fourier-380, USA) in the ranging from 400 to 4000 cm^− 1. An accelerated surface area and porosimetry analyzer (BET Micromeritics Quantachorme Corporation, USA) was used to determine the specific surface area of samples at 77 K using a nitrogen sorption-desorption isothermal analysis. The X-ray photoelectron spectroscopy of the samples were obtained on a Thermo Scientific K-Alpha electron spectrometer (XPS Thermo Scientific K-Alpha, USA).

2.4 Adsorption experiments

Batch experiments were conducted by put 0.025g adsorbent into 50 mL centrifuge tube and fill with 45 mL CIP solution. All sealed centrifuge tubes were placed in a shaker at a speed of 130 r/min. The adsorption kinetics were investigated with the contact time from 0 to 1440 min, when the CIP concentration was 250 mg/L, the temperature was 303 K and the pH was 7. The adsorption isotherms and thermodynamics were investigated with the CIP concentration from 25 to 250 mg•L^‒1 at the temperatures of 303 K, 318 K, 333 and 348 K, respectively, when the pH was 7 and the contact time was 900 min. The influence of pH on the removal capacity for CIP was examined at the pH range from 4 to 10, when the CIP concentration was 250 mg/L, the temperature was 303 K and the contact time was 900 min. After adsorption, the CIP solution was determined at 277 nm using a UV-visible spectrophotometer (UV-752, Shanghai), where the amount of CIP adsorption on SLS/CS@ZIF-8 was calculated by Eq. (1).

$${q_{\text{e}}}{\text{=}}\frac{{{C_0} - {C_{\text{e}}}}}{w} \times V$$

Where V is the volume of ciprofloxacin solution (L), w is the mass of the adsorbent SLS/CS@ZIF-8 used in the experiment, C₀ and C_e are the initial and equilibrium concentration of ciprofloxacin solution (mg/L), respectively.

2.5 Adsorbent regeneration experiments

To evaluate the regenerative capacity of SLS/CS@ZIF-8, 0.025 g SLS/CS@ZIF-8 were added into the sealed centrifuge tube containing 45mL CIP solution (250mg/L) at pH = 7. The sealed centrifuge tube was placed in a shaker at 130 r/min and 303K for 900 min. After adsorption and separation, the adsorbents were added into the mixture of 5 mL NaOH solution (5%) and 40 mL ethanol shaken at 130 r/min and 303K for 360 min, the above process was repeated three times, then the adsorbents were washed with the deionized water and freeze-dried for 18 h. Subsequently, the next adsorption process was carried out. In this study, six recycling tests were carried out.

3.1 Characterizations of samples

The mechanical strength is particularly important for macroscopic three-dimensional adsorbent materials because adsorbents are easily damaged by extrusion pressure in the real environment. The stress-strain tests result of CS-FM, SLS/CS-FM, CS@ZIF-8 and SLS/CS@ZIF-8 were presented in Fig. 1. It could be clearly seen that the mechanical strength of SLS/CS-FM and SLS/CS@ZIF-8 were significantly stronger than that of CS-FM and CS@ZIF-8. The improvement of mechanical strength showed that CS and SLS were well combined and compatible, SLS played a supporting role in the material. Meanwhile, it could be seen that after the in-situ immobilization of ZIF-8 on CS-FM and SLS/CS-FM, the mechanical strength of the obtained materials for CS@ZIF-8 and SLS/CS@ZIF-8 were slightly enhanced, indicating that the chemical bond between ZIF-8 and carrier foams helps to improve the mechanical strength of the material.

Digital photographs of CS-FM, SLS/CS-FM, CS@ZIF-8 and SLS/CS@ZIF-8 were presented in Fig. 2 (a, b, c, d). The color of CS-FM changed from white to brownish yellow after the introduction of lignin. CS-FM and SLS/CS-FM had a dash of white on their surface after loading ZIF-8, which was caused by the white ZIF-8 crystals. The SEM images of CS-FM, SLS/CS-FM, CS@ZIF-8 and SLS/CS@ZIF-8 at magnification 500 times were shown in Fig. 2 (e, f, g, h). CS-FM and SLS/CS-FM had similar structures, both had macroscopic three-dimensional cellular network pore structure. The reason for these pores were formed by the loss of moisture during the freeze drying (Wang et al. 2021). The three-dimensional aperture of SLS/CS-FM was significantly smaller than that of CS-FM. Hence, SLS/CS-FM could immobilization more ZIF-8 per unit volume than CS-FM due to it has more pore wall area to provide attachment sites for ZIF-8. The increased loading content of ZIF-8 in adsorbent materials would contribute to improve its adsorption capacity. In addition, when ZIF-8 was loaded on CS-FM and SLS/CS-FM, their pore macroscopic three-dimensional cellular network pore structure remains unchanged. The SEM images of CS-FM, SLS/CS-FM, CS@ZIF-8 and SLS/CS@ZIF-8 at magnification 100000 times were shown in Fig. 2 (i, j, k, l). The pore wall of CS-FM and SLS/CS-FM was uneven surface. After the in-situ immobilization ZIF-8 on CS-FM and SLS/CS-FM, their pores wall presented a large number of particles with smooth surface rhomboid dodecahedron morphology. These particles were the typical morphology for ZIF-8 crystals, indicating that ZIF-8 was successfully in-situ immobilization on CS-FM and SLS/CS-FM.

The crystalline structure of CS-FM, SLS/CS-FM, ZIF-8, CS@ZIF-8 and SLS/CS@ZIF-8 were investigated by XRD (Fig. 3). The XRD patterns of ZIF-8 show that its characteristic peaks were located at 7.43°, 10.42°, 12.86° and 18.20°. This was consistent with previous reports (Wang et al. 2020; Zhao et al. 2020), indicating that ZIF-8 had been successfully synthesized. The XRD patterns of CS-FM and CS/SLS-FM both preserved one wide diffraction peak at about 20°. Moreover, it could be seen that the typical ZIF-8 diffraction peaks in the XRD patterns of CS@ZIF-8 and SLS/CS@ZIF-8, which indicating that ZIF-8 was successfully in-situ immobilization on CS-FM and SLS/CS-FM, and the crystal structure of ZIF-8 did not change during the in-situ immobilization process.

The FT-IR spectra for CS-FM, SLS/CS-FM, ZIF-8, CS@ZIF-8 and SLS/CS@ZIF-8 were showed in Fig. 4. In the spectrum of CS-FM, the broad peaks at 3449 cm^− 1 were attributed to the stretching vibration of O-H and N-H bonds, the peaks at 2860 cm^‒1 and 2934 cm^‒1 belong to the symmetric and asymmetric vibrations of C-H bonds, respectively, the absorption peak at 1658 cm^− 1 belongs to amide I band stretching (Baran et al. 2020) After CS and SLS were prepared into foams composite (SLS/CS-FM), the peak of amide I band stretching peak in CS-FM moved from 1658 cm^− 1 to 1649 cm^− 1. It can be inferred that there was hydrogen bonding between CS and SLS. The vibrations peak of the asymmetric absorption vibration peak of -SO³⁻ was appeared in CS/SLS-FM at absorption peak of 1032 cm^‒1, the -SO³⁻ exist electrostatic interaction with − NH³⁺ in CS (Gu et al. 2019). In the spectrum of ZIF-8, the aliphatic stretching vibration and the aromatic stretching vibration of C–H bond on imidazole rings were described by peaks appearing at 2932 cm^‒1 and 3136 cm^‒1, respectively (Hoseinzadeh et al. 2021), the stretching vibrations of C = N bond was described by the peak appearing at 1580cm^‒1, and the stretching vibration of C–N bond in the methyl and imidazole rings was described by peaks at 995cm^‒1 and 1147cm^‒1, respectively (Cao et al. 2020). In addition, the spectrum of CS@ZIF-8 and SLS/CS@ZIF-8 showed the main peaks of CS-FM and SLS/CS-FM functional groups, respectively. Moreover, the spectrum of CS@ZIF-8 and SLS/CS@ZIF-8 both had the vibration peak of Zn-O bond at 756 cm^‒1 and the vibration peak of Zn-N bond at 420 and 690 cm^‒1 (Wang et al. 2019), which further indicated that ZIF-8 grown on CS-FM and SLS/CS-FM.

The N₂ adsorption/desorption isotherm curves of CS-FM, SLS/CS-FM, CS@ZIF-8 and SLS/CS@ZIF-8 were shown in Fig. 5. According to the IUPAC classification, the N₂ adsorption/desorption isotherms curves of CS-FM and SLS/CS-FM display the type Ⅲ, which indicated that the CS-FM and SLS/CS-FM were a typical of non-porous materials with weak affinities to nitrogen (Imran et al. 2019). When ZIF-8 was immobilized on CS-FM and SLS/CS-FM, their adsorption/desorption isotherm display the type Ⅳ and the specific surface area significant increased. The result confirmed the existence of microporous pores in the CS@ZIF-8 and SLS/CS@ZIF-8 (Imran et al. 2019). The parameters of surface area and porous structure of CS@ZIF-8 and SLS/CS@ZIF-8 were listed in Table 1, it could be seen that the BET specific surface area of SLS/CS@ZIF-8 was about 5 times that of CS@ZIF-8. The results suggested that SLS/CS-FM immobilized more ZIF-8 per unit volume than CS-FM, which was in line with SEM observation SLS/CS-FM could provide more attachment sites for ZIF-8 than CS-FM per unit volume.

Table 1

Textural parameters of CS@ZIF-8 and SLS/CS@ZIF-8
Materials	BET surface area(m²/g)	Total pore volume(cm³/g)	Average pore size(nm)
CS@ZIF-8	19.5123	0.015434	11.0176
SLS/CS@ZIF-8	108.3678	0.081055	12.2503

3.2 Effect of pH on adsorption capacity

The adsorption of CIP by CS-FM, SLS/CS-FM, CS@ZIF-8 and SLS/CS@ZIF-8 should be attributed to their chemical interaction. In this respect, considering the solution pH has a significant influence on dissociation of functional groups of adsorbents and the ionization forms of CIP, the pH_zpc (point of zero charge) of these four materials was tested by the potentiometric tiration and the effect of pH for CIP adsorption performance was carried out. The pH_pzc value of these four materials as shown in Fig. 6(a), meaning that these four materials surfaces had negatively charged when pH more than their point of zero charge, however, these four materials surfaces had positively charged when pH less than their point of zero charge. Combined with CIP molecules forms of existence (Fig. 6 (c)) and the hydrophobic interaction (Li et al. 2017; Wang et al. 2017), the electrostatic interaction and hydrophobic interaction between these four materials surfaces and CIP resulted in a larger adsorption capacity when 6༜pH༜9, which was consistent with the results of the adsorption experiment in Fig. 6 (b). In addition, the adsorption capacity was significantly improved when ZIF-8 immobilization on CS-FM and SLS/CS-FM, and the adsorption performance of SLS/CS@ZIF-8 was better than CS@ZIF-8.

3.3 Adsorption kinetics

The effect of contact time for SLS/CS@ZIF-8 adsorption capacity was investigated, and the experimental data was fitted by the quasi-first-order and quasi-second-order kinetic models. The adsorption data and fitting curve were shown in Fig. 7, and the parameters were listed in Table 2. The quasi-first-order kinetic and the quasi-second-order kinetic models are described by Eqs. (2) and (3), respectively.

$${q_t}{\text{=}}{q_e}(1 - {e^{{\text{-}}{k_1}t}}{\text{)}}$$

$$\frac{t}{{{q_t}}}{\text{=}}\frac{1}{{{k_2}q_{e}^{2}}}{\text{+}}\frac{t}{{{q_e}}}$$

Where, q_t (mg/g) and q_e (mg/g) are the CIP adsorption amount at different time intervals and the CIP adsorption amount at equilibrium, respectively; k₁ (min^− 1) and k₂ (g/mg/min) are the pseudo-first-order and pseudo-second-order rate constant, respectively.

As shown in Fig. 7, the CIP adsorption rate was fast at the initial 200 min, and then reaches equilibrium gradually with the increase of time. It could be known from Table 2, the quasi-second-order kinetic model had a higher R² and the maximum adsorption capacity (q_cal) had a good correlation with the experimental value (q_exp), implying that the rate-limiting steps of adsorption were mainly affected by chemisorption mechanism (Ghani et al. 2020).

Table 2

Kinetic parameters for CIP adsorption onto SLS/CS@ZIF-8
C₀(mg/L)	q_e(exp) (mg/g)	Pseudo-first order			Pseudo-second order
C₀(mg/L)	q_e(exp) (mg/g)	q_e(cal)(mg/g)	k₁(/min)	R²	q_e(cal) (mg/g)	k₂(g·mg^− 1/min)	R²
250	413	389	0.0012	0.973	426	4.080	0.993

3.4 Adsorption isotherms

In order to evaluate the adsorption performance of CIP on SLS/CS@ZIF-8, the CIP adsorption equilibrium data at different temperatures (303K, 318K, 333K and 348K) were fitted by Langmuir and Freundlich models in Fig. 8 (a, b), and the related parameters are listed in Table 3. The corresponding two adsorption isotherms models were expressed by Eqs. (4) and (5), respectively (Wu et al.2020).

$${q_e}{\text{=}}\frac{{{q_m}{K_L}{C_e}}}{{1+{C_e}{K_L}}}$$

$${q_{\text{e}}}{\text{=}}{K_F}{C_{\text{e}}}^{{1/{\text{n}}}}$$

Where, C_e is the equilibrium concentration of ciprofloxacin solution (mg/L), q_m (mg/g) is the maximum adsorption capacity, K_L (L/mg) and K_F (L/g) are the adsorption constants of Langmuir isotherm and Freundlich isotherm, respectively, n is the constant of intensity.

As could be seen from Fig. 8 (a, b), the adsorption capacity of SLS/CS@ZIF-8 for CIP increased with the increasing CIP concentration. The q_e also increased with the increasing of temperature, the maximum adsorption capacities (q_m) of SLS/CS@ZIF-8 for CIP was 413 mg/g, 430 mg/g, 439 mg/g and 449 mg/g at the temperatures of 303K, 318K, 333K and 348K, respectively. The relative parameters were listed in Table 3, it could be seen that Langmuir isothermal model was more suitable for describing CIP adsorption behavior on SLS/CS@ZIF-8 than Freundlich isothermal model because it had the highest R² value. Table 4 showed the comparison of CIP adsorption capacity between SLS/CS@ZIF-8 and other adsorbents. It could be seen that the adsorption capacity of SLS/CS@ZIF-8 for CIP was higher than that of most reported adsorbents.

Table 3

Isotherm parameters for CIP adsorption onto SLS/CS@ZIF-8
Isotherm	Parameters	303K	318K	333K	348K
Langmuir	q_m (mg/g)	470	482	490	509
	K_L(L/mg)	0.191	0.267	0.284	0.358
	R²	0.991	0.992	0.989	0.990
Freundlich	n	2.65	2.65	2.61	2.61
	K_F(mg/g(L/mg)1/n)	114	129	137	155
	R²	0.917	0.927	0.929	0.911

Table 4

The adsorption capacity for CIP onto various adsorbents
	Conditions
Adsorbents	Initial concentrations (mg/L)	Dose (g/L)	pH	T/(K)	q_m(mg/g)	References
SLS/CS@ZIF-8	50–250	0.5	7	303	413	Present work
Titanate nanotubes	3.3–26.7	0.1	5	298	153	Theamwong et al. (2021)
Ga₂S₃ and sulfur co-	100–190	0.25	3.23	298	330	Zheng et al. (2021)
modified biochar composites MIL-53(Fe-Cu)	—	1	6	293	190	Chatterjee et al. (2021)
Ordered mesoporous carbon	20–100	0.3	7	room	233	El-Bendary et al. (2015)
Bamboo-based carbon	20–100	0.3	7	room	362	El-Bendary et al. (2015)
Chitin-biocalcium	50-1800	0.5	4–10	298	2432	Antonelli et al. (2020)
KGM/ZIF-8 aerogels	100–1500	1	7	303	811.03	Yuan et al. (2018)

3.5 Adsorption thermodynamics

Thermodynamic parameters including enthalpy (ΔH⁰), free energy (ΔG⁰) and entropy (ΔS⁰) can be used to study the extent and driving force of adsorption process. The thermodynamic parameters of CIP adsorption on SLS/CS@ZIF-8 were obtained from the following Eqs. (7), (8) and (9).

$${K_0}{\text{=}}\frac{{{{\text{q}}_{\text{e}}}}}{{{C_{\text{e}}}}}$$

$$\ln {K_0}=\frac{{\Delta {S^0}}}{R} - \frac{{\Delta {H^0}}}{{RT}}$$

$$\Delta {G^0}=\Delta {H^0} - T\Delta {S^0}$$

Where K₀ is the thermodynamic equilibrium constant (L/mg), T is the absolute temperature (K), and R is the universal gas constant (8.314 KJ/mol).

Figure 9 shown the linear relationship between ln(K₀) and 1/T, the ΔH⁰ and ΔS⁰ could be obtained from the slope and intercept of the line. According to Table 5, the positive value of ΔH⁰ indicated that the CIP adsorption on SLS/CS@ZIF-8 was endothermic nature, and the ΔS⁰ value was positive indicates that the randomness of the solid-liquid interface increased when CIP was adsorbed on SLS/CS@ZIF-8, the ΔG⁰ values were all negative indicates that the adsorption process was spontaneous (Antonelli et al. 2020; Chatterjee et al. 2021).

Table 5

The determined values of thermodynamic parameters
ΔH ⁰(KJ/mol)	ΔS ⁰(J/(mol•K))	ΔG ⁰/(KJ/mol)
ΔH ⁰(KJ/mol)	ΔS ⁰(J/(mol•K))	303K	318K	333K	348K
1.22	60.53	-6.14	-7.04	-7.96	-8.86

3.6 Mechanisms of CIP adsorption on SLS/CS@ZIF-8

The FTIR and XPS were used to further explore the potential adsorption mechanism of CIP on SLS/CS@ZIF-8. Figure 10 showed the FT-IR spectra of SLS/CS@ZIF-8 before and after adsorption of CIP, respectively. In the spectrum of SLS/CS@ZIF-8, the absorption peak at 3449cm^‒1 belongs to O‒H and N‒H stretching, the absorption peak at 1600 cm^− 1 belongs to the C = C stretching in aromatic rings. After adsorption of CIP, the bond of O‒H and N‒H from 3449cm^‒1 shifted to 3457cm^‒1, suggesting the existence of a hydrogen bond and electrostatic interaction between CIP and SLS/CS@ZIF-8 (Zhao et al. 2020). The absorption peak at 1600 cm^− 1 shifts to 1617 cm^− 1 after the adsorption of CIP because of the π − π interaction. Figure 11(a) showed high-resolution spectra of C1s peak intensity of SLS/CS@ZIF-8 before and after CIP adsorption. The C–C/C = C and C–N/C–O were described by absorption peak appearing at 284.8 and 286.1 eV, respectively (Wang et al. 2019). After adsorption CIP, the two C1s peaks shift to 285.0 and 286.3 eV, respectively, indicating that the existence of π-π interaction between SLS/CS@ZIF-8 and CIP (Sca et al.). Figure 11 (b) and (c) showed high-resolution spectra of N1s and O1s peak intensity of SLS/CS@ZIF-8 before and after CIP adsorption. The peaks at 399.1, 400.5, 531.8 and 533.1 eV were corresponded to C‒N, C = N, Zn‒OH and O‒C = O, respectively. After adsorption of CIP, except for the binding energy of Zn‒OH peak, the binding energy of the other three peaks moved to higher values, which shows that there was hydrogen bonding interaction between SLS/CS@ZIF-8 and CIP and the hydrogen bonding interaction formed by the O and N functional groups was dominant. Figure 11(d) showed that the two binding energy peaks of Zn2p. The two peaks moved to lower values after adsorption of CIP, indicating that Zn²⁺ had electrostatic interaction with CIP. In summary, electrostatic interaction, π-π interaction and hydrogen bonding interaction dominated the adsorption of CIP on SLS/CS@ZIF-8, and the multiple adsorption mechanism of the SLS/CS@ZIF-8 towards CIP was shown in Fig. 12.

3.7 Reusability

The reusability efficacy of adsorbent is important for industrial application. As shown in Fig. 13, the adsorption capacity of SLS/CS@ZIF-8 for CIP decreased with the increase of adsorption cycles. However, SLS/CS@ZIF-8 was still have an excellent adsorption capacity after six adsorption-desorption cycles, which indicated that SLS/CS@ZIF-8 exhibits acceptable reusability as CIP adsorbent and has good industrial application value.

In summary, a novel adsorbent of SLS/CS@ZIF-8 was synthesized by introducing sodium lignosulfonate into chitosan foams (CS-FM) and then loading ZIF-8 onto the resulting foams (SLS/CS-FM) by in-situ immobilization. The introduction of sodium lignosulfonate improved the mechanical strength of chitosan foams and provided more attachment sites for ZIF-8. The adsorbent combines the good mechanical strength and easy recovery of SLS/CS with the excellent porosity and large specific surface area of ZIF-8, which makes the adsorbent easy to recycling and has high adsorption performance for CIP with wide pH rang. The adsorption mechanism between CIP and SLS/CS@ZIF-8 was controlled by electrostatic attraction, hydrogen bonding, π-π stacking interaction. This study provided a new adsorbent with good application potential for the removal of CIP from wastewater and a new low-carbon green chemical technology for the high-value utilization of lignin.

Ethics approval and consent to participate This article does not contain any studies with human participants or animals performed by any of the authors. In this experiment, we did not collect any samples of human and animals.

Consent for publication The authors consent for publication by the journal.

Availability of data and materials All data supporting the findings of this study are available within the article and its supplementary information files or from the corresponding author upon reasonable request.

Competing interests The authors declare that they have no competing interests.

Funding The National Natural Science Foundation of China (31660183), the Dean/Opening Project of Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification Technology (2020K003), Guangxi Zhuang Autonomous Region-level Guangxi University 2021 "College Students Innovation Training Program Project" (202110593188).

Authors' contributions Peiwen Wang, Hangyu Shen, Jinrou Wei, Yi Lan and Hui Fan did the main experiments and data analysis. Peiwen Wang and Congjin Chen conceptualized, designed，wrote and revised the manuscript. Xiufen Liao, Huayu Hu, Yanjuan Zhang and Zuqiang Huang put forward some opinions in the writing and modifying process of the manuscript.

Acknowledgments This work was supported by the National Natural Science Foundation of China (31660183), the Dean/Opening Project of Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification Technology (2020K003) and Guangxi Zhuang Autonomous Region-level Guangxi University 2021 "College Students Innovation Training Program Project" (202110593188)

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Scheme 1 is available in Supplementary Files section.

No competing interests reported.

Scheme.1.png
Scheme.1 Schematic diagrams of the preparation of the SLS/CS-FM and the growing ZIF-8

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In-situ immobilization of ZIF-8 on sodium lignosulfonate/chitosan foams for efficiently removal of ciprofloxacin from water

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Abstract

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1. Introduction

2. Experimental

2.1 Materials

2.2 Synthesis of materials

2.3 Characterizations

2.4 Adsorption experiments

2.5 Adsorbent regeneration experiments

3. Results And Discussion

3.1 Characterizations of samples

3.2 Effect of pH on adsorption capacity

3.3 Adsorption kinetics

3.4 Adsorption isotherms

3.5 Adsorption thermodynamics

3.6 Mechanisms of CIP adsorption on SLS/CS@ZIF-8

3.7 Reusability

4. Conclusions

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References

Scheme 1

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Supplementary Files

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