1. Introduction
With the continuous development of modern medical technology and the improvement of living standards, people are paying more attention to their health. Antibiotics, as one of the most commonly used drugs worldwide, play an essential role in medical treatment due to their special bacteriostatic and bactericidal effects and are widely produced and used. From 2000 to 2015, global antibiotic consumption rapidly increased from 21 billion doses per day to about 35 billion doses, a growth of 65%. China is the major user and producer of antibiotics [
1]. It is estimated that China is the world’s largest antibiotic product market, consuming more than 25,000 tons of antibiotics annually [
2]. Tetracycline (TC) is widely used due to its activity against most Gram-positive and Gram-negative bacterial strains, protozoan parasites, as well as atypical organisms such as mycoplasmas, chlamydiae, and rickettsiae, and its relatively low price [
3]. The use of tetracycline antibiotics has significantly improved public health and promoted the development of the livestock industry but has also brought serious issues with antibiotic wastewater. The widespread use of antibiotics has led to their widespread presence in the environment, and raised concerns about ecological security. At the same time, the aging microplastics in the environment have become a potential pollutant carrier. These microplastics could not only adsorb a variety of pollutants including antibiotics, but also had the ability of long-distance transportation in the aquatic environment, which increased the risk of pollutant diffusion and accumulation, and posed a serious threat to the health of the aquatic ecosystem [
4].
Currently, antibiotic residues are widely present in wastewater and enter the aquatic environment through residential, medical, pharmaceutical, and agricultural sewage [
5,
6]. Antibiotics in the aquatic environment can cause long-term toxicity to aquatic organisms, impairing their growth, development, and reproductive functions [
7]. Effective removal of antibiotic contaminants from water is a hot topic in current environmental research. For high-concentration antibiotic wastewater from pharmaceutical factories, the main treatment methods include advanced oxidation [
8], biological treatment [
9], photocatalytic degradation [
10], and adsorption. In oxidation treatment, the oxidized products may have higher toxicity, causing secondary pollution [
11]; biological treatment processes are susceptible to environmental factors and may easily lead to the growth of antibiotic-resistant bacteria [
12,
13]. Therefore, adsorption is widely used due to its simplicity, low cost, and no generation of other harmful waste [
14]. Among various adsorption materials, biochar has broad application prospects due to its regenerability, simple production process, non-toxicity, and abundant raw material resources.
Antibiotic fermentation residues (AFRs) are waste generated during antibiotic production. According to relevant investigations, producing 1 ton of antibiotics generates about 10 tons of antibiotic residues, with an average annual production of 2 × 10
6 tons [
15]. The main components of AFRs include trace antibiotics, metabolites, bacteria of antibiotic-producing bacteria, insoluble components, culture medium, and other nutrients that have not been completely extracted. In addition, the moisture content of AFRs is high, ranging from 79% to 92%, most of which is bound water, with high viscosity, and easy to deteriorate after long-term storage [
16]. Antibiotic residues are classified as hazardous waste and can cause water, soil, and air pollution, and harm human health if not disposed of in a timely and effective way [
17]. Since 2008, China has listed the antibiotic fermentation residues in the national hazardous waste catalog (HW02-276-001-02) [
18]. If improperly treated, AFRs can destroy the ecological environment and endanger human health [
19]. For the treatment of AFRs, foreign countries generally use high-temperature incineration [
20]. However, due to the large scale of antibiotic production in China and the high cost of incineration, this method cannot be widely implemented. Composting, anaerobic digestion, incineration, and pyrolysis are the commonly used methods in China. The large volume and difficulty of treating AFRs make it a pressing issue to find safe, efficient, and mature treatment technologies to achieve reduction, harmlessness, and recycling of AFRs.
The main objective of this study was to develop a novel porous biochar (OSOB) derived from oxytetracycline fermentation residues (OFRs) and activated by KOH in one step, and its application for the efficient removal of tetracycline. A series of characterization techniques were used to analyze the biochars (OSOBs) and studied the effects of biochar dosage, initial tetracycline concentration, reaction time, and reaction temperature on the adsorption efficiency. Additionally, typical kinetic and isotherm models were used to fit the adsorption process to predict the adsorption mechanism. This study provided a simple, environmentally friendly method to prepare porous biochar from OFRs, with no secondary pollution, aligning with current energy saving and emission reduction policies, transforming waste into valuable resources, and providing an effective solution for recycling antibiotic fermentation residues.
2. Materials and Methods
2.1. Materials and Chemicals
Oxytetracycline fermentation residues (OFRs) were sourced from Qilian Mountain Pharmaceutical Co., Ltd. (Jiuquan, Gansu Province, China). The residues were dried to a constant weight at 105 °C, ground, and sieved through a 60-mesh screen to obtain fine powder for subsequent use. Tetracycline hydrochloride (TC) was obtained from Shanghai Aladdin Biochemical Technology Co., Ltd. (Shanghai, China). Potassium hydroxide (KOH) was purchased from Komio Chemical Reagent Co., Ltd. (Tianjin, Chnia). Hydrochloric acid was obtained from China National Pharmaceutical Group Corporation. High-purity nitrogen was provided by Kaifeng Wantong Gas Co., Ltd. (Kaifeng, China). All chemical reagents used were of analytical grade.
2.2. Preparation of the Activated Biochar
The dried OFRs were mixed with KOH at different mass ratios (1:1, 1:2, 1:3) and ground in an agate mortar until the mixture was uniformly fine and free of large particles. The ground mixture was then placed into a nickel crucible for subsequent pyrolysis. The grinding process should be completed quickly to avoid moisture absorption by KOH. The nickel crucible containing the mixture was placed into a quartz boat and then sequentially inserted into a tubular furnace. Under an N2 atmosphere (nitrogen flow rate of 25 mL/min), the temperature was increased at a rate of 2 °C/min to the preset temperature (500 °C, 600 °C, 700 °C) and maintained for 2 h. The samples were then naturally cooled to room temperature in the furnace. The samples were washed several times with 0.1 mol/L HCl until non-alkaline, then washed with deionized water until the pH level reached 7, and finally dried in a vacuum oven at 80 °C for 12 h. The obtained samples were named OSOB-X-Y, where X represents the mass ratio of KOH to OFRs and Y denotes the activation temperature.
2.3. The Selection of Different Proportions of Porous Biochar
Preliminary screening of OSOBs was conducted by placing 0.01 g of each porous biochar into a 50 mL centrifuge tube, adding 20 mL of a TC solution (100 mg/L), and placing the tube in a constant temperature shaker at 25 °C and 180 rpm for 24 h. Afterward, the solution was filtered through a 0.45 μm membrane, and the absorbance of the filtrate was measured at 360 nm using an ultraviolet-visible spectrophotometer (752 automatic, Shanghai Jinghua Instrument, Shanghai, China).
The adsorption capacity and removal rate were calculated using Equations (1) and (2), respectively:
where
C0 (mg/L),
Ce (mg/L),
V (L), and
m (g) are the initial concentration of TC solution, the equilibrium concentration, the solution volume, and the OSOB-3-700 amount, respectively.
2.4. Characterization of Materials
The surface morphology of the OSOB-3-700 was observed using scanning electron microscopy (SEM, Zeiss Merlin Compact, Jena, Germany). The mineral composition and crystal structure of OSOB-3-700 were determined by X-ray Diffraction (XRD, Bruker D8 Advance, Bruker AXS, Saarbruchen, Germany). The specific surface area and pore size distribution of OSOB-3-700 were determined using a specific surface area and pore size analyzer (BET, V-Sorb 2800P, Liaocheng, China). The elemental composition, proportion, and surface characteristics of the OSOB-3-700 were analyzed using X-ray photoelectron spectroscopy (XPS, Thermo Kalpha, Waltham, United States) and elemental analysis (Vario EL cube, Frankfurt, Germany). The weight change of the biochar was measured by the thermogravimetric analyzer (TGA, Mettler TGA/DSC1, Switzerland), which was heated from 30 °C to 900 °C at the heating rate 10 °C/min under an N2 atmosphere. The functional groups on the surface of OSOB-3-700 and OSOB-3-700-absorbed TC were analyzed by Fourier-Transform Infrared Spectroscopy (FT-IR, Nexus 470, Ettlingen, Germany).
2.5. Batch Adsorption Experiments
To simulate the real wastewater environments, the adsorption capacity of OSOB-3-600 for tetracycline was estimated by examining the effects of the adsorbent dosage (0.125–1.25 g/L), initial tetracycline concentration (250–500 mg/L), contact time (0–1440 min), and experimental temperatures (298 K, 308 K, and 318 K). All batch experiments were conducted in 50 mL centrifuge tubes with a solution volume of 20 mL. The centrifuge tubes were placed in a constant temperature shaker at 180 rpm for 24 h. After adsorption, the solution was filtered through a 0.45 μm membrane, and the absorbance of the filtrate was measured at 360 nm using an ultraviolet-visible spectrophotometer (752 automatic, Shanghai Jinghua Instrument, Shanghai, China). The adsorption capacity and removal rate were calculated using Equations (1) and (2), respectively.
2.6. Adsorption Kinetics and Adsorption Isotherms
In a 100 mL tetracycline solution (500 mg/L), 0.025 g of OSOB-3-700 was added to perform adsorption kinetics experiments. After shaking for 5–1440 min, the residual tetracycline concentration was measured. The adsorption capacity of the OSOB-3-700 at a given time (min) was calculated using Equation (3):
The pseudo-first-order (PFO) and pseudo-second-order (PSO) models were used to study the adsorption kinetics of tetracycline on OSOB-3-600. The non-linear forms of the PFO and PSO models are as follows (Equations (4) and (5)):
where
qe,
qt,
K1 (min
−1), and
K2 (g/(mg·min)) are the adsorption capacity (mg/g) of the OSOB-3-700 at equilibrium, at a certain time
t (min), and the adsorption rate constant of PFO and PSO, respectively.
When the adsorption process reached equilibrium, adsorption isotherms could further explore the concentration relationship between the adsorbent and the adsorbate. A mixture of 0.005 g of OSOB-3-700 and 20 mL of TC solution with different TC concentrations (200–500 mg/L) was agitated at 180 rpm for 24 h at temperatures of 25 °C, 35 °C, and 45 °C. The classical isotherm models (Langmuir, Freundlich, and Temkin) were used for analysis, represented by Equations (6)–(8), respectively:
where
qe (mg/g),
Ce (mg/L),
KL (L/mg),
qm (mg/g),
KF (mg/g),
n, bT (J/mol), and
kT (L/mg) represent the adsorption capacity of adsorbent, the concentration of TC solution at equilibrium, the Langmuir constant, the maximum adsorption capacity, the Freundlich sorption coefficient, the adsorption intensity in the adsorption process, the constant related to adsorption heat, and the Temkin isotherm constant, respectively. R (8.314 J/(mol·K)) is the gas constant, and T (K) is the temperature.
2.7. The Recyclability Assessment of OCOB-3-700
The recyclability of OSOB-3-700 was investigated through five consecutive adsorption–desorption cycles under the conditions of 0.25 g/L OSOB-3-700 and 250 mg/L initial TC concentration at 180 rpm for 24 h. The used OSOB-3-700 was regenerated using a 0.1 M NaOH solution until no detectable TC concentration remained in the solution. Subsequently, the OSOB-3-700 was rinsed using deionized water until pH = 7 and totally dried at 80 °C.
4. Conclusions
In this study, fresh oxytetracycline fermentation residues were used as raw materials, and KOH was used as a chemical activator to produce biochar with a good adsorption capacity through a one-step pyrolysis method. This material (OSOB-3-700) had characteristics such as high specific surface area, numerous micropores, and abundant surface active sites, making it very suitable for adsorption applications. The adsorption kinetics fitting results indicated that the adsorption of tetracycline on OSOB-3-700 conformed more to the chemical adsorption mechanism described by the PSO model. The experimental results showed that at 298 K, the maximum adsorption capacity of tetracycline on OSOB-3-700 reached 1096.871 mg/g. The adsorption process of tetracycline occurred on the surface of the OSOB-3-700, which was rich in active sites, promoting the adsorption process through electronic exchange and other chemical actions. The adsorption isotherm fitting results showed that the Langmuir model better described the adsorption process of tetracycline on OSOB-3-700. Tetracycline was adsorbed in a monolayer through chemical adsorption at specific homogeneous active sites, consistent with the kinetic conclusions. Therefore, the N-doped porous biochar derived from fresh oxytetracycline fermentation residues was an environmentally friendly material with excellent tetracycline adsorption capacity. This method reduced production costs through waste reuse, had certain economic benefits, and was simple to operate and environmentally friendly, with no secondary pollution. It aligned with current energy saving and emission reduction policies, transforming waste into treasure, and conforms to the concept of sustainable development by enabling the reuse of waste.