Preparation-Height-Specific Surface Area of Flower-like ZnO for Norfloxacin Removal in Dialysis Wastewater

Abstract

According to statistics, the number of patients with kidney dialysis is increasing every year, especially in Taiwan. The high content of antibiotics in kidney dialysis wastewater can create an environmental burden if not properly treated. Therefore, in this study, a new design for a flower-like ZnO catalyst that can effectively treat norfloxacin (NF) in dialysis wastewater is presented and its NF treatment efficiency is investigated under different operating parameters (including different dosages, pH, ambient temperatures, and pollutant concentrations). Scanning electron microscope results indicate that the ZnO sample consists of flower-like nanostructures with diameters of about 4.97 μm. The surface area, pore volume, and pore size of the flower-like ZnO were estimated to be 46.45 m²g⁻¹, 0.132 cm³g⁻¹, and 19.50 nm, respectively. The total removal efficiency increased with the increase in the initial pH value of NF, when the initial pH value of NF increased from 3.5 (38.1%) to 7.5 (82.5%). However, the total removal efficiency decreased with an increase in the initial pH value of NF, when the initial pH value of NF increased from 9.5 (79.5%) to 11.5 (32.9%). The second-order kinetic simulation results show that the linear relationship is better than that of the first-order kinetic, and the Rc² values are all above 0.9.

1. Introduction

According to the statistics of the United States Renal Data System, Taiwan still ranks first in the world for the total number of people undergoing kidney dialysis [1]. According to research statistics, among the top ten causes of death in Taiwan in 2021, nephritis, nephropathy syndrome, and nephropathy rank ninth, and are rising. Among the three highest types of patients are obese individuals and smokers, and nearly 40% have abnormal kidney function issues. According to the 2021 nephrology annual report of the Taiwan Society of Nephrology, about 12% of Taiwan’s population suffers from kidney disease, and it is estimated that the number of people on kidney dialysis will exceed 90,000 [2]. In particular, new dialysis patients have increased from 11,184 in 2015 to 12,475 in 2019, and the number increases annually. Most of the initial symptoms of kidney disease are not obvious. If the patient is not aware of or not diagnosed correctly in time, once the kidney disease develops, it will lead to a rapid decline in function and evolve into end-stage renal disease. Without proper treatment such as hemodialysis or kidney transplantation, it can be potentially life-threatening. According to statistical reports, 79.6% of patients in Taiwan receive dialysis three times a week, only 2.4% patients receive dialysis twice a week, and a considerable number of patients (18%) receive dialysis more than three times a week [2]. These patients may still have residual renal function, but cannot bear the physiological work alone, so it must be supplemented by hemodialysis more than twice a week. Kidney dialysis wastewater contains antibiotics, blood, urine, urine protein, and glucose. It has a pollution concentration of 3 to 5 times the pollution intensity of general domestic sewage and is not easy to handle. If the wastewater from dialysis is not properly treated, it will cause environmental pollution. At present, no one has directly used kidney dialysis wastewater as a pollution source for research, and most of them use chemical agents to configure pollutant sources. The main reason is that kidney dialysis wastewater is infectious wastewater. Therefore, this study discusses antibiotic treatment in kidney dialysis wastewater.

Numerous studies have reported that the inhalation of pharmaceutical and personal care products (PPCPs) leads to malfunctions in the systems of humans and microorganisms [3,4,5]. PPCPs have become an important research target because they pollute rivers and bodies of water, impacting the organisms in them. PPCPs have been detected in surface water, such as river water [6], and in the fluent and effluent of a sewage treatment plant [7]. Among various pharmaceuticals, norfloxacin (NF) is the antimicrobial drug widely used in both human and animal husbandry [8,9]. It is difficult for these antibiotics to degrade biologically because of their antimicrobial activity [10]. Thus, in NF removal, photocatalysis processes are essential.

Photocatalysis [11,12], UV-H₂O₂ [13], catalytic ozonation [14], Fenton [15], sonolysis [16], and sonocatalysis [17] are the major advanced oxidation processes for the removal of PPCPs in an aqueous medium [18]. Of these techniques, photocatalysis has shown substantial potential for degrading antibiotic pollutants because it can penetrate into any water medium and offers high pollutant removal efficiency [7]. Nano-sized zinc oxide (ZnO) is one of the best semiconductors used in photocatalysis due to its unique characteristics [19]. ZnO has been used as a catalyst because of its larger specific surface area, wide band gap (3.37 eV), stability, durability, and low cost [20,21]. ZnO has numerous morphologies, including nanowires [22], nanospheres [23], nanotubes [24], nanoflowers [25], and urchin shapes [26]. Flower-like ZnO is thus expected to be a potential photocatalysis because its structure has a large surface area. Therefore, the objectives of this work are: (1) to prepare flower-like ZnO for developing a key reaction between NF and catalyst; (2) to obtain the properties of flower-like ZnO investigated through various techniques; (3) to obtain optimum operation conditions by assessing the effect of different flower-like ZnO mass on adsorbing arsenic; (4) to obtain optimum operation conditions by assessing the effect of pH and initial concentration on NF; and (5) to determine the kinetic parameters of NF removal.

2. Results and Discussion

2.1. Characterization of Materials

In order to investigate the specific surface area and pore size characteristics of the prepared flower-like ZnO, the pore size and specific surface area of flower-like ZnO were analyzed using a nitrogen isothermal adsorption–desorption instrument. The analysis results of the nitrogen isothermal absorption–desorption curve of the prepared flower-like ZnO are shown in Figure 1. The isotherms belong to type V according to the classification of IUPAC. The adsorption/desorption isotherm of type V is very similar to type III at low relative pressure, which may be due to the weak interaction between adsorbent and adsorbate. Capillary condensation and hysteresis will occur when the relative pressure is high. After calculation using the Brunauer–Emmet–Teller equation, the prepared flower-like ZnO has an average pore size of 19.50 nm, a specific surface area of 46.45 m² g⁻¹, and a pore volume of 0.132 cm³ g⁻¹. The specific surface area analysis results of the ZnO catalyst prepared in this study are 13.3 times larger than the specific surface area of commercial ZnO (3.5 m² g⁻¹) obtained by Moezzi et al. (2012) [27], as show in

Table 1. The surface characteristics of ZnO synthesized by others with the values obtained from this study.

Material	Surface Area (m2 g−1)	Ref.
Commercial ZnO	3.5	[27]
Flower-like ZnO	14.15	[28]
Hexagonally ring-like ZnO	17.12	[28]
Flower-like ZnO	35.01	[29]
Flower-like ZnO	18.7	[30]
Flower-like ZnO	46.45	This research

.

The morphology and microstructure of the ZnO nanostructures were determined using SEM. Figure 2a,b indicate that the ZnO sample consists of flower-like nanostructures with diameters of about 4.97 μm. In addition, flower-like ZnO exhibited a well-formed structure without any aggregation and uniformity across the substrate. The diameters of the flower-like ZnO catalyst prepared in this study are little larger than the diameters of the flower-like ZnO (4.5 μm) obtained by Luo et al. (2014) [31]. Additionally, this study also used EDS to analyze the distribution of the elements contained in the prepared flower-like ZnO catalyst. The results of the analysis are shown in Figure 2c,d. The distribution of Zn and O elements is very uniform, with atomic ratios of 41.2% and 58.8%, respectively, and the weight ratios are 74.1% and 25.9%, respectively. This result is consistent with the reason why the atomic ratio of oxygen and zinc in the ZnO prepared by Wang et al. (2012) [32] is not 1:1, which may be caused by the presence of oxygen ions on the surface of the material.

The X-ray powder diffraction patterns of flower-like ZnO are shown in Figure 3. The three characteristic peaks (100), (002), and (101) in the figure are sharp and strong, indicating that the crystal growth is complete, and the characteristic peak is 31.80° at the 2θ angle; 34.42°, 36.26°, 47.5°, 56.54°, 62.88°, 66.48°, 67.92°, 69.04°, 73.68°, and 76.90°, respectively, correspond to (100), (002), (101), (102), (110), (103), (200), (112), (201), (004), and (202) crystal. This crystal plane is consistent with the hexagonal wurtzite file (No. 36-1451) in the standard JCPDS database files of X-ray diffraction analysis. These results are consistent with the conclusions of Noureddine et al. (2008) [33].

In this study, to investigate the energy gap width of the prepared flower-like ZnO catalyst, UV-Vis was used to analyze the light absorption characteristics of flower-like ZnO at a wavelength of 300 to 400 nm. The results are shown in Figure 4. It is known that flower-like ZnO has its maximum absorption peak at 359 nm. If a tangent line is drawn at the inflection point of the light absorption characteristic curve, and the tangent line is extended to the X-axis intersection point, the maximum intercept of the light absorption characteristic curve can be estimated. After calculation, the energy gap width of the prepared flower-like ZnO was found to be 3.23 eV. This result is similar to the energy gap width of the ZnO catalysts prepared by Koao et al. (2018) [34] using the uniform precipitation method, Raji et al. (2018) [35] using the chemical precipitation method, and Ungula et al. (2018) [36] using the reflux precipitation method. Their energy gap widths were 3.29 eV, 3.18 eV, and 3.18 eV, respectively. The energy gap width of the zinc oxide catalyst prepared in this research is not much different from that of previous research, as shown in

Table 2. Comparison table of critical wavelength and band gap of ZnO.

Critical Wavelength λ (nm)	Band Gap Energy (eV)	Reference
380	3.29	[34]
389	3.18	[35]
390	3.18	[36]
384	3.23	This study

.

2.2. Effect of Catalytic Dosage

We compared the effect of different dosages of flower-like ZnO (0.1, 0.2, 0.3, and 0.4 g L⁻¹) on the removal efficiency of NF. When the initial concentration of NF is 25 µg L⁻¹, the pH value is 7.5, and the ambient temperature is 25 °C. The experimental results are shown in Figure 5. As the dosage of ZnO gradually increased, the total removal efficiency also increased gradually. When the dosage of ZnO was 0.1, 0.2, 0.3, and 0.4 g L⁻¹, the total removal efficiencies were 63.5, 74.1, 83.1, and 88.5%, respectively. The main reason for this is that when the initial concentration of NF is fixed, the NF molecules contained in the aqueous solution are also relatively fixed. As the dosage of ZnO increases continuously, the total active sites of ZnO will continue to increase, resulting in an increase in the collision probability of ZnO to NF in water. It also increases the generation of electron–hole pairs, and the total removal efficiency of NF also increases with an increase in ZnO dosage.

2.3. Effect of Initial NF Concentration

We next investigated the effect of different initial concentrations of NF (25, 35, 45, and 55 µg L⁻¹) on the removal efficiency of NF. When the dosage of ZnO is 0.3 g L⁻¹, the pH value is 7.5, and the ambient temperature is 25 °C. The experimental results show that as the initial concentration of NF gradually increases, the total removal efficiency gradually decreases. When the initial concentration of NF is 25, 35, 45, and 55 µg L⁻¹, the total removal efficiency is 83.1, 70.2, 45.4, and 29.1%, respectively, as shown in Figure 6. This is primarily because in the case of a limited catalyst dosage, the active sites on the surface of the catalyst are relatively limited. As the initial concentration of NF increases, the number of NF molecules also increases. Eventually, the active sites of the catalyst will be insufficient to effectively process the surplus NF molecules.

2.4. Effect of pH Values on Degradation Efficiency

We also investigated the effect on the removal efficiency of NF when various initial pH values of NF (3.5, 5.5, 7.5, 9.5, and 11.5) were changed, when the initial concentration of NF was 25 µg L⁻¹, the dosage of ZnO was 0.3 g L⁻¹, and the ambient temperature was 25 °C. The experimental results are shown in Figure 7. When the initial pH values of NF were 3.5, 5.5, 7.5, 9.5, and 11.5, the total removal efficiencies were 38.1, 68.5, 82.5, 79.5, and 32.9%, respectively. The total removal efficiency increased with an increase in the initial pH value of NF from an initial pH of 3.5 to 7.5. However, the total removal efficiency decreased with an increase in the initial pH value of NF, when the initial pH value of NF increased from 9.5 to 11.5. The main reason is that the pH value has a great influence on the catalyst, which not only affects the catalytic reaction but also changes the form of substances in the water. Generally speaking, NF has two dissociation constants, pKa₁ at 6.24 and pKa₂ at 8.75. The ionic state of the solution is mainly in the zwitterionic state when the initial pH value of the NF solution is between 6.24 and 8.75. When the initial pH value of the NF solution is less than 6.24 or greater than 8.75, the ionic state of the solution is mainly in the positive or negative ion state, as shown in Figure 8a [37]. The isoelectric point of ZnO is shown in Figure 8b. Its isoelectric point is pH 9.5 ± 0.3, which means that when the pH value of ZnO is less than 9.5, its catalyst surface is positively charged. When the pH value of ZnO is greater than 9.5, the surface of the catalyst is negatively charged. When the pH of ZnO is 9.5 ± 0.3, the positive and negative charges on the surface of the catalyst are balanced, which is consistent with the results of Lam et al. (2012) [38] and Sobana et al. (2007) [39].

Therefore, when the initial pH of NF was 3.5, the ion form of the NF solution was cationic and the surface of ZnO was positively charged, and the repulsive force between the two was strong, resulting in a poor overall removal efficiency. The ionic form of the NF solution is anion and the surface of ZnO is negatively charged when the initial pH value of NF is 11.5, and the repulsion force of the two is strong. However, under alkaline conditions, since it is easier to generate ^•OH radicals, the total removal efficiency is slightly higher than that of pH 3.5 and is highly efficient [40]. The ionic form of NF solution is half zwitterion and half cationic solution when the initial pH value of NF is 5.5, and the pH value of the solution is less than 9.5, which makes the surface of ZnO positively charged. Hence, the repulsion of the two is not as high as with pH values of 3.5 and 11.5, and the total removal efficiency is higher than at pH 3.5 and 11.5. However, the total removal efficiency is still not as high as that of NF when the initial pH value is 7.5. We judge that the change in the NF ion solution has a greater influence. The NF solution exists in the form of zwitterions and a small number of cations when the initial pH of NF is 7.5, and the surface of ZnO is positively charged. The charge tends to be balanced, and the total removal efficiencies of pH 7.5 and 9.5 are 82.5 and 79.5%, respectively, which are not far apart.

2.5. Effect of Different Temperature on NF Removal Efficiency

We investigated the effect on the removal efficiency of NF at various ambient temperatures (298, 308, 318, and 328 K) of NF, when the initial concentration of NF is 25 µg L⁻¹, the dosage of ZnO is 0.3 g L⁻¹, and the initial pH is 7.5. The experimental results show that the total removal efficiency increases as temperature increases, as shown in Figure 9. The total removal efficiencies were 73.1, 74.2, 79.5, and 84.4%, respectively, when the ambient temperatures of NF solution were 298, 308, 318, and 328 K. This may occur because as the reaction temperature increases, the chemical reaction rate of the reactants also increases, which accelerates movement between the catalyst surface and NF molecules, leading to an increase in the probability of collision reactions. These research results are consistent with those of Lei (2011) [41].

For comparison, the NF removal with various techniques, the various materials, and the advantages or disadvantages in which they were reported in the literature are summarized in

Table 3. Comparison of various techniques for NF removal.

Method	Material	Removal Eff. (%)	Advantages or Disadvantages	Ref.
Sonocatalysis	Multilayer-sheet-like ZnO	64	Low cost	[9]
Advanced oxidation	UV-H2O2	60	Low cost	[13]
Photodegradation	TiO2	79	Low cost	[42]
Catalytic ozonation	CuFe2O4	82	Hight cost	[43]
Fenton	MnFe2O4/H2O2	91	Hight cost	[44]
Photodegradation	Flower-like ZnO	83	Low cost	This research

. The flower-like ZnO synthesized in this study shows a good efficiency.

2.6. Kinetic Model Analysis Results

We explored the effect of different NF pollutant concentrations on the reaction rate. Kinetic model fitting was performed when the dosage of ZnO was 0.3 g L⁻¹, the initial pH was 7.5, and the first-order kinetic response fittings of different pollutant concentration processes were 25, 35, 45, and 55 µg L⁻¹. Figure 10 shows that the reaction process of the four different concentrations has a good linear relationship after fitting, with Rc² values all above 0.88. The simulation results show that the reaction rate constants of NF pollutant concentrations of 25, 35, 45, and 55 µg L⁻¹ are 0.0042, 0.0015, 0.0008, and 0.0007 min⁻¹, respectively, as shown in

Table 4. Kinetic constants of the first-order kinetic with different NF concentrations.

Concentration (ppm)	Correlation Coefficient Squared (Rc2)	Reaction Rate Constant k1 (min−1)
2	0.88	0.0042
4	0.98	0.0015
8	0.99	0.0008
12	0.98	0.0007

. However, the reaction rate constant decreases with an increasing concentration, and the reaction process in this study is not suited for description by the first-order reaction process at various concentrations. In addition, the fitting results of the second-order kinetic response are shown in Figure 11. The simulation results show that the linear relationship is slightly better than that of the first-order kinetic response, and the Rc² values are all above 0.9. The reaction rate constants of NF pollutant concentrations of 25, 35, 45, and 55 µg L⁻¹ are 0.0046, 0.0015, 0.0001, and 0.00009 L mg⁻¹ min⁻¹, respectively, as shown in

Table 5. Kinetic constants of the second-order kinetic with different NF concentrations.

Concentration (ppm)	Correlation Coefficient Squared (Rc2)	Reaction Rate Constant k2 (L mg−1 min−1)
2	0.90	0.00460
4	0.98	0.00070
8	0.99	0.00010
12	0.98	0.00009

.

3. Materials and Methods

3.1. Chemicals and Starting Materials

Zinc acetate (Zn(CH₃COO)₂, CAS 5970-45-6), hydrochloric acid (HCl, CAS 7647-01-0), sodium hydroxide (NaOH, CAS 1310-73-2), sodium sulfate (Na₂SO₄, CAS 7757-82-6), ethanol (C₂H₅OH, CAS 64-17-5), and glycine (C₂H₅NO₂, CAS 56-40-6) were purchased from ACROS Company (Geel, Belgium). In addition, norfloxacin was provided by Sigma Company (St. Louis, MO, USA). All the starting materials and chemicals were analytical and used without further purification.

3.2. Preparation of Flower-like ZnO

This research preferred solution-based methods to prepare flower-like ZnO because the method is simple, fast, and low cost. Moreover, flower-like ZnO has been used as a catalyst because of its larger specific surface area, wide band gap, stability, and durability.

In the experiments, 0.245 g of zinc acetate was added to a mixture of 15 mL deionized water and 10 mL absolute ethanol with vigorous stirring at room temperature. Then, 0.4 g of glycine and 0.4 g of sodium sulfate were added, and the mixture was stirred for 5 min. Next, 10 mL of 1 M sodium hydroxide solution was added to the above solution, which was then stirred for 1 h. The resulting white suspension was loaded into an 80 mL Teflonlined autoclave. The autoclave was sealed tightly and then heated at 140 °C for 18 h, before subsequently being cooled to room temperature. A white-colored product was obtained by filtering with deionized water. The flower-like ZnO catalyst was obtained after 24 h of drying at 60 °C. This study references the preparation method used by Luo et al. (2014) [31].

3.3. Materials Characterization

Nitrogen adsorption/desorption isotherms of adsorbent samples were carried out at 77 K on a Micromeritics ASAP 2020 N volumetric adsorption analyzer (Micromeritics, Norcross, GA, USA). The morphology of the material was measured through a scanning electron microscope (SEM) with energy dispersive X-ray spectroscopy (EDS) analysis (Hitachi S-4700, Tokyo, Japan). The crystal phase of the material was determined by an x-ray diffraction (XRD, X-Pert PRO, Malvern Panalytical, Techlink, Singapore) employing a Cu Kα radiation wavelength of 0.15405 nm. In order to explore the energy gap width of the prepared flower-like ZnO catalyst, this study uses a UV-Vis spectrophotometer (Hitachi High-Tech Corporation, Tokyo city, Japan, U-3900).

3.4. Performance Assessment

The kidney dialysis wastewater used in this study was obtained from the sewage treatment plant of the Far Eastern Memorial Hospitals in Taiwan, and the norfloxacin content of the wastewater ranged from 25 to 55 µg L⁻¹. In addition to norfloxacin antibiotics, other pollutants (such as blood, urine protein, and calcium nitrate) in kidney dialysis wastewater will directly affect the photocatalysis. Therefore, in this study, ceramic filter membranes were used to pre-treat dialysis wastewater to remove impurities. In addition, because the concentration of norfloxacin in the actual kidney dialysis wastewater is unstable, when adjusting the concentration of norfloxacin, the concentration of norfloxacin was adjusted to be low or high by diluting or adding drugs.

The experiments were conducted with flower-like ZnO catalyst at different dosages, contact times, initial NF concentrations, pH, and temperature. Photocatalytic experiments were carried out in a 1500 mL reaction system under UV light (365 nm, 8 W). Flower-like ZnO was mixed with 1000 mL of the appropriate concentration of the test NF solution, which was added for 100 min at 25 °C. The solution was filtered using a membrane filter (pore size 0.45 μm), and the residual NF in the aqueous solutions was determined using a UV–vis spectrophotometer (Hitachi Co., U-3900). The final NF concentrations were determined using λ_max (273 nm) of NF.

3.5. Kinetic Model

The pseudo-first-order equation is given as:

$$\frac{d{q}_t}{dt}{=k}_1(q_e-q_t)$$

(1)

where q_t is the amount adsorbed at time t (mg g⁻¹), q_e is the adsorption capacity at equilibrium (mg g⁻¹), k₁ is the pseudo-first-order rate coefficient (min⁻¹), and t is the contact time (min). The integration of Equation (1) with the initial condition, qt = 0 at t = 0, leads to the following equation [45]:

$$log\left(q_e-q_t\right)=log{q}_e-\frac{k_1}{2.303}t$$

(2)

The pseudo-second-order model is represented as:

$$\frac{{dq}_t}{dt}=k_2(q_e-{q_t)}^2$$

(3)

where k₂ is the pseudo-second-order rate coefficient (g mg⁻¹ min⁻¹). Integrating Equation (3) and qt = 0 at t = 0, the following equation is obtained [46]:

$$\frac{t}{q_t}=\frac{1}{k_2{q}_e^2}+\frac{1}{q_e}t$$

(4)

4. Conclusions

The flower-like ZnO catalyst exhibits good degradation of NF in kidney dialysis wastewater. The main factors affecting the degradation of NF are the specific surface area, active sites, and electron–hole pairs of the catalyst. The degradation capacity of flower-like ZnO catalyst rises with higher dosages and ambient temperatures under an initial pH of NF of 7.5. Conversely, the degradation capacity of flower-like ZnO catalyst falls as the NF influent concentration increases. The kinetic degradation behavior of the flower-like ZnO catalyst for NF is more suited to the second-order kinetic model.

The flower-like ZnO prepared in this research is a powder, and the kidney dialysis wastewater is taken back from hospital and then batch experiments are carried out in the laboratory, so there are many limitations. Therefore, this research is planned to be applied in the sewage treatment system of the hospital in the future, such as covering flower-like ZnO powder on the ceramic filter and combining with a continuous processing system to facilitate a practical application evaluation.