Facile Fabrication of Flocculent Magnesium Silicate for the Adsorption of Oxytetracycline.

The discharge of antibiotics and the potential threat to organisms posed by this have received increasing attention. In this work, flocculent magnesium silicate (FMS) was fabricated by a facile hydrothermal and freeze-drying process, and its adsorption behavior for antibiotic oxytetracycline (OTC) was investigated. FMS presented a sepiolite-type structure and the alkaline solution promoted its hydroxylation. A hierarchical pore structure ranging from micropores to macropores and a high specific surface area of 660 m2/g were exhibited. FMS exhibited a higher adsorption amount in neutral solution than in acidic or alkaline conditions as the physicochemical properties of FMS and OTC were significantly affected by the pH. Adsorption isotherm could be well-described by the Langmuir model, and the calculated saturated adsorption capacity was as high as 265 mg/g. Adsorption kinetics followed the pseudo-second-order kinetic model, and the adsorption rate-controlling step was intraparticle diffusion. Thermodynamic parameters indicated that the adsorption was a spontaneous physicochemical reaction. After five cycles, around 91% of the adsorption performance was still maintained, demonstrating the excellent reusability of FMS. The sepiolite-type FMS fabricated in this work could be applied to remove OTC from wastewater.

Introduction

Antibiotics are mainly synthetic analogues and secondary metabolites secreted by various microorganisms such as bacteria and fungi. Since antibiotics were introduced into modern medicine, they have played an irreplaceable role in the therapy of various bacterial or pathogenic microbial infections because of their powerful resistance. However, the excessive application and irregular discharge of antibiotics have led to a sharp increase in their concentration in natural water bodies and soil.[1,2] The presence of antibiotics in the environment threatens the ecological balance and people’s survival because they can kill microorganisms and lead to the generation of super resistant bacteria.[3,4] Therefore, it is necessary to remove excessive antibiotics in wastewater. As a common tetracycline antibiotic, the removal of oxytetracycline (OTC) from wastewater has received more attention because of its toxicity, interference with ecosystem, and poor natural degradability.[5]

To date, various methods have been explored to remove OTC from water such as electric irradiation, photocatalytic degradation, ultrasonic degradation, microbial degradation, flocculation, and adsorption.[6−8] Among them, adsorption method is considered to be the most feasible and effective and has received the most widespread attention because of its easy operation, low cost, and no secondary pollution to the environment.[9,10] Many adsorbents have been developed to treat OTC-containing water such as biochar, activated carbon, multiwalled carbon nanotube, graphene oxide, carboxymethyl cellulose, synthetic resin, activated sludge, hydroxyapatite, nano zero-valent iron, kaolinite, and montmorillonite.[11−15] For instance, cotton linter fiber-derived activated carbon prepared by fused NaOH activation exhibited a maximum adsorption capacity of 1340 mg/g for OTC at 323 K.[11] In another work, Harja and Ciobanu studied the adsorption performance of hydroxyapatite nanopowder toward OTC in an aqueous medium, and a maximum adsorption capacity of 291 mg/g was achieved.[14] Compared to other types of antibiotic adsorbents, clay minerals show bright prospect in the removal of OTC because of their merits of low cost, high adsorption capacity, and reusability.[16,17]

Sepiolite is an orthorhombic or monoclinic system magnesium silicate clay mineral composed of blocks of Si–O tetrahedral layers and Mg–O octahedral layers. Rich channels and abundant surface hydroxyl groups endow sepiolite with excellent adsorption performance.[18,19] Nevertheless, the application of natural sepiolite is limited by its impurity and unstable physicochemical properties. In view of this, the current work explores the fabrication of an effective sepiolite-type magnesium silicate adsorbent to remove OTC from aqueous solution. Porous flocculent magnesium silicate (FMS) was fabricated via the ingenious hydrothermal method and freeze-drying process. The micro-morphology, crystal structure, functional groups, and pore structure of FMS were characterized. The adsorption isotherms, adsorption kinetics, and thermodynamic parameters of OTC onto FMS were analyzed. Furthermore, the influence of the pH on adsorption and the adsorption reusability of FMS were also investigated. Based on the results of adsorption experiments, FMS showed broad prospect in removing antibiotics from water.

Results and Discussion

Fabrication and Characterization of FMS

The fabrication reaction of sepiolite-type FMS could be briefly described as follows: Mg2+, SiO32–, and OH– formed 2MgO·3SiO2·2H2O via the hydrothermal process, and the presence of ammonium chloride avoided the formation of Mg(OH)2 precipitation. The final collected FMS was 0.59 g. Figure a,b shows the scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of FMS, respectively. As shown in Figure a,b, FMS exhibited a rough and flocculent morphology, which was attributed to the “modeling” effect of tert-butanol during the freeze-drying process. FMS was filled with flocculent tert-butanol with different sizes under low temperature. After the sublimation of tert-butanol, flocculent pores were leaved. The energy-dispersive spectrometry (EDS) elemental mapping images of FMS are shown in Figure c. All elements were evenly distributed on the surface of FMS approximately, which meant that the chemical composition of the FMS was homogeneous.

Figure 1

SEM (a), TEM (b), and EDS elemental mapping (c) images of FMS.

Figure a shows the XRD pattern of FMS, which can be indexed as sepiolite-type magnesium silicate (2MgO·3SiO2·2H2O, JCPDS card# 02-0048). The diffraction peaks at 19.6, 27.9, 35.3, 53.2, 60.9, and 72.0° corresponded to the crystal faces of (020), (004), (201), (206), (060), and (402), respectively. The broad diffraction peak demonstrated that the FMS was an amorphous structure, which usually meant the large exposed surface of the material.[20]Figure b shows the Fourier transform infrared (FTIR) spectrum of FMS, which was consistent with the hydroxyl magnesium silicate. The characteristic bands of FMS could be attributed to the vibration of Mg–OH (3684, 3620, and 673 cm–1), O–H (3301 and 1659 cm–1), Si–O–Mg (1022 cm–1), and Si–O (465 cm–1).[21] Among various functional groups, Mg–OH was crucial to the adsorption performance of magnesium silicate.[22]Figure c,d shows the N2 adsorption–desorption isotherm and pore size distribution of FMS, respectively. The N2 adsorption–desorption isotherm of FMS showed a combination of type I, II, and IV, indicating that FMS contained the hierarchical pore structure from micropores to macropores. It could be seen from Figure d that the pore size of FMS was mainly distributed around 3.4 and 61.7 nm. The Brunauer–Emmett–Teller specific surface area and total pore volume of FMS were 660 m2/g and 0.96 m3/g, respectively, which provided FMS with enough adsorption surface and space.

Figure 2

XRD pattern (a), FTIR spectrum (b), N2 adsorption–desorption isotherm (c), and pore size distribution and cumulative pore volume (d) of FMS.

Adsorption of OTC

Effect of Solution pH

Figure a shows the effect of solution pH on the adsorption amount of FMS for OTC. The adsorption amount increased in the approximate range of pH 2–4 while decreased in the approximate range of pH 7–10. The pH at adsorption equilibrium was very close to the initial pH as the aqueous solutions of FMS and OTC showed weak basicity and acidity, respectively, which would not significantly affect the pH of the solution (Figure b). To explain the trend of the adsorption amount, the zeta potentials of FMS in the range of pH 1–7 were measured. As shown in Figure c, the zeta potential of FMS decreased with the increase of pH, and the isoelectric point presented at pH 1.7. When the pH was lower than 1.7, FMS was positively charged; on the contrary, when the pH was higher than 1.7, FMS was negatively charged. In addition, the solution speciation of OTC at different pH was also measured. As shown in Figure d, there could be four species of OTC in solution because of its dissociation, which were OTC+, OTC0, OTC–, and OTC2–. The increase in the surface negative charge of FMS enhanced its adsorption for OTC within pH 2–4. When pH was over 7, the OTC surface became negatively charged, resulting in electrostatic repulsion with the FMS and hence, a decrease in the adsorption amount was observed.

Figure 3

Effect of solution pH in the approximate range of 2–10 on the adsorption amount (a), the plot of equilibrated pH vs initial pH (b), zeta potential of FMS at solution pH of 1–7 (c), and solution speciation of OTC at different pH (d).

Adsorption Isotherms

Figure a shows the equilibrium adsorption amount of FMS for OTC with different initial concentrations. The adsorption data were fitted by Langmuir and Freundlich isothermal adsorption models to describe the adsorption interface and estimate the adsorption capacity, and the relevant parameters are listed in Table . According to the fitting curves and correlation coefficients (R2), the Langmuir model was more suitable to describe the adsorption of OTC onto FMS, indicating that the surface of FMS was homogeneous and the adsorption was monolayer. The adsorption capacity (qm) of FMS for OTC was 265 mg/g according to the linear fitting of the Langmuir model (Figure b). In addition, the isothermal parameters at adsorption temperatures of 293, 303, and 308 K are also listed in Table , and the R2 demonstrated that the adsorption was more suitable to be described by the Langmuir model as well.

Figure 4

Adsorption isotherms of OTC onto FMS within initial OTC concentrations of 50–400 mg/L (a) and linear fitting of the Langmuir model (b).

Table 1

Adsorption Isotherm Parameters of the Adsorption for OTC by FMS within Initial OTC Concentrations of 50–400 mg/L

isotherm adsorption models	Langmuir model			Freundlich model
temperature (K)	qm (mg·g–1)	KL (L·mg–1)	R2	KF	n	R2
293	273	0.139	0.997	68.45	3.349	0.841
298	265	0.123	0.997	60.892	3.131	0.706
303	260	0.103	0.998	52.764	2.91	0.794
308	252	0.116	0.997	55.88	3.101	0.802

Adsorption Kinetics

The variation curve of the adsorption amount of FMS for OTC with adsorption time is shown in Figure a. Rapid adsorption for OTC was observed within the initial 1 h. Subsequently, the adsorption rate went down gradually until the adsorption equilibrium was reached at 6 h. The adsorption data were fitted by pseudo-first-order and pseudo-second-order adsorption kinetic models to analyze the adsorption kinetic, and the relevant parameters are listed in Table . The equilibrium adsorption amount (qe) was 275 mg/g according to the linear fitting of pseudo-second-order kinetic model, which was closer to the experimental result (qexp) of 255 mg/g (Figure b). According to the closer fitted curves, higher correlation coefficients and smaller statistical indice (χ2), the pseudo-second-order kinetic model was more suitable to describe the adsorption of OTC onto FMS, indicating that FMS had saturated adsorption sites and the adsorption was dominated by chemisorption. The adsorption rate-controlling step was investigated using the Weber–Morris intraparticle diffusion model. As shown in Figure c, the slope of intraparticle diffusion stage was lower than the boundary layer diffusion stage meant that the adsorption rate was mainly controlled by the former.

Figure 5

Adsorption kinetic curves of OTC onto FMS within adsorption time of 5–360 min (a), linear fitting of the pseudo-second-order model (b) and Weber–Morris intra-particle diffusion plots (c).

Table 2

Adsorption Kinetic Parameters of the Adsorption for Cd2+ by RMS within Adsorption Time of 5–360 min

	pseudo-first-order model				pseudo-second-order model				Weber–Morris intra-particle diffusion model
qexp (mg/g)	qe (mg/g)	K1 (min–1)	R2	χ2	qe (mg/g)	K2 [g(mg·min)]	R2	χ2	kid [mg/(g·min1/2)]	c (mg/g)
255	170	0.012	0.976	42.5	275	1.221 × 10–4	0.998	1.455	10.292	83.688

Thermodynamic Parameters

The adsorption thermodynamics that reflects the spontaneity and absorbed/released heat property were investigated, and the thermodynamic parameters and their variation with adsorption temperature are shown in Figure and Table .[23,24] The free energy (ΔG) was negative and the value of ΔG were in the range of −80 to −20 kJ·mol–1, demonstrating that the adsorption was a spontaneous physicochemical process.[25] The increase of the absolute value of ΔG with the increase of temperature indicated that the spontaneity of adsorption increased with the increase of temperature. The negative value of enthalpy change (ΔH) and the positive value of entropy change (ΔS) showed that the adsorption was exothermic and randomly increased.

Figure 6

Linear fitting of the Langmuir model (a) and plots of ln Kcvs 1/T (b).

Table 3

Thermodynamic Parameters of the Adsorption for OTC by FMS within Adsorption Temperatures of 293–308 K

temperature (K)	Kc	ΔG (kJ·mol–1)	ΔH (kJ·mol–1)	ΔS (kJ·mol–1·K–1)
293	6.41 × 104	–26.96	–14.57	0.042
298	5.66 × 104	–27.11
303	5.34 × 104	–27.42
308	4.73 × 104	–27.56

Reusability of FMS

Reusability is an important factor in affecting the practical application of the adsorbent. The variation in the removal efficiency of FMS for OTC with the cycle times are shown in Figure a. After five cycles, the removal efficiency of FMS for OTC still remained 87%, indicating that the FMS has excellent cyclic stability.

Figure 7

(a) Reusability of FMS; (b) FTIR spectra of FMS (curve A), OTC (curve B) and, OTC adsorbed-FMS (curve C).

Adsorption Mechanism

The change of the surface charge state of FMS and OTC at different solution pH poses a significant effect on the adsorption amount, indicating that the electrostatic interaction is a factor controlling the adsorption process. To further study the adsorption mechanism, FTIR spectra of FMS, OTC, and OTC adsorbed-FMS were compared, as shown in Figure b. The FTIR spectrum of FMS after the adsorption of OTC had characteristic bands attributed to OTC at 3168, 2924, 2851 cm–1, from 1618 to 1082 cm–1, and from 865 to 499 cm–1, which confirmed the successful adsorption of OTC onto FMS. After the adsorption of OTC, the characteristic band of Mg–OH of FMS moved from 658 to 677 cm–1 and the characteristic band of Si–O–Mg moved from 1022 to 1016 cm–1, respectively, which indicated that the hydroxyl group of FMS played an important role in the adsorption process, and the electron cloud density decreased around the O atom and increased around the Mg atom.[26] After being adsorbed on FMS, the characteristic band of N–H of OTC moved from 773 to 768 cm–1, and the characteristic band of C–N moved from 1328 to 1334 cm–1, respectively, which indicated that the amino-group of OTC played an important role in the adsorption process, and the electron cloud density increased around the N atom and decreased around the C atom. The aforementioned FTIR analysis showed that the amino group of OTC was bound to the hydroxyl group of FMS, which was consistent with the conclusion reported in the literature.[22] In short, the adsorption mechanism of FMS for OTC was the synergistic effect of electrostatic attraction and hydroxyl bonding.

Comparison of the Adsorption Capacities for OTC with Other Clay Minerals

The adsorption capacities of FMS and other clay minerals for OTC are listed in Table . It can be concluded that the adsorption capacity of FMS for OTC was significantly higher than other clay minerals reported in the open literature, which demonstrated that this method is a feasible way to fabricate FMS with enhanced performance for removing OTC from water.

Table 4

Comparison of the Adsorption Capacities for OTC with Other Clay Minerals

adsorbents	adsorption capacity (mg·g–1)	references
kaolinite	15	(15)
attapulgite	33	(27)
sepiolite	41	(28)
halloysite	52	(29)
montmorillonite	59	(30)
zeolite	76	(31)
zeolite imidazole	149	(32)
palygorskite	207	(22)
FMS	265	this work

Conclusions

Sepiolite-type FMS was fabricated for the adsorption of OTC. Amorphous FMS with high specific surface area of 660 m2/g exhibited excellent adsorption performance for OTC. The adsorption of OTC onto FMS was monolayer and mainly controlled by chemisorption. Adsorption was spontaneous and lower temperature was more conducive to adsorption. Neutral solution was more suitable for the adsorption of OTC by FMS according to the adsorption amount at different solution pH. Adsorption mechanism analysis indicated that the adsorption was dominated by electrostatic attraction and hydroxyl bonding. Based on the high adsorption capacity and excellent reusability, FMS could find its application prospect in the treatment of OTC wastewater.

Materials and Methods

Materials

The introduction to materials acquisition and grade is given in the Supporting Information.

Fabrication of FMS

FMS was fabricated by the hydrothermal method and freeze-drying process. First, 1.28 g of magnesium nitrate, 4.5 g of ammonium chloride, and 0.1 mL of ammonia were added to 35 mL of water under magnetic stirring. Then, 20 mL of 0.35 M sodium silicate aqueous solution was added dropwise to the above solution and continue stirring for 1 h. The mixed solution was transferred to a polytetrafluoroethylene-lined reactor and reacted at 180 °C for 24 h. After hydrothermal reaction, the solid product was washed repeatedly with water and tert-butanol followed by freeze-drying at −60 °C for 24 h.

Characterization

The characterization of the physicochemical properties of the FMS is given in the Supporting Information.

Adsorption Experiments toward OTC

The adsorption experiments toward OTC were carried out in a temperature-controlled shaker with an oscillation rate of 160 rpm. Typically, 0.05 g of FMS was added to 50 mL of OTC solution with an initial concentration of 400 mg/L and adsorbed at 298 K for 6 h. Afterward, FMS was separated from the solution by centrifugation. The concentration of the OTC in the remaining solution was measured by a UV–vis spectrophotometer at 355 nm (maximum absorption wavelength of OTC), and the adsorption amount was calculated based on the concentrations of OTC at the initial and given time.

Different from aforementioned general experimental conditions, a certain condition was changed in the specific adsorption experiment below. For the adsorption isotherm study, the initial concentrations of OTC were set to 50–400 mg/L. For adsorption kinetics study, the solution was quickly separated at a specific adsorption time in the range of 5–360 min. For the adsorption thermodynamics study, the adsorption temperatures were set to 293–308 K. The introduction to isothermal adsorption models, adsorption kinetic models, and adsorption thermodynamic parameters are given in the Supporting Information. For revealing the effect of the solution pH on adsorption, the initial solution pH was adjusted to 2–10 by using hydrochloric acid or sodium hydroxide solution. For the reusability test, the OTC adsorbed-FMS was regenerated by calcining in a muffle furnace at 550 °C for 2 h. All adsorption experiments were performed in triplicate and the average was reported.