Insights into the Roles of Surface Functional Groups and Micropores in the Sorption of Ofloxacin on Banana Pseudo-Stem Biochars

Abstract

In the present study, banana pseudo-stem (BS) was pyrolyzed under anaerobic conditions without any physical or chemical modification. Their properties, as well as their sorption affinity to ofloxacin (OFL), were studied. As a result, oxalates and KCl formed at a relatively low temperature of 300 °C, while bicarbonates generally formed at a pyrolysis temperature above 400 °C. Surface functional groups of BS biochars facilitated OFL sorption mainly via specific interactions including electronic attraction (EA), π–π electron donor–acceptor (π–π EDA) interaction, the ordinary hydrogen bond (OHB), and the negative charge-assisted hydrogen bond ((−)CAHB). Except for (−)CAHB, these interactions all decreased with an elevated pH, resulting in overall decreased OFL sorption. Significant OFL sorption by BS biochars produced at 300 °C, observed even at an alkaline condition was attributed to (−)CAHB. Micropores formed in BS biochar prepared at 500 °C, with a specific surface area as high as 390 m² g⁻¹ after water washing treatment. However, most micropores could not be accessed by OFL molecules due to the size exclusion effect. Additionally, the inherent K-containing salts may hinder OFL sorption by covering the sorption sites or blocking the inner pores of biochars, as well as releasing OH⁻ into the solution. Thus, BS biochar produced at 300 °C is an excellent sorbent for OFL removal due to its high sorption ability and low energy. Our findings indicate that biochar techniques have potential win–win effects in recycling banana waste with low energy and costs, and simultaneously converting them into promising sorbents for the removal of environmental contaminants.

1. Introduction

Banana is the fourth most grown food crop around the world after rice, wheat, and corn [1]. In 2020, the global cultivation of bananas was up to 5.2 million hectares and the estimated production exceeded approximately 119 million tons [2]. Meanwhile, it was estimated that nearly 3 tons of residues such as banana pseudo-stem and leaf would be generated for each ton of banana fruit [3]. Most of these residues were abandoned randomly into fields, rivers, and lakes without any appropriate disposal, which would spread fungal plant disease and cause environmental pollution issues. One of the major environmental concerns is the emission of greenhouse gases (e.g., CO₂, CH₄, and N₂O) during the decay of banana waste over a very long time in a natural environment. It was reported that one ton of fruit and food waste may emit 4.14 tons of carbon dioxide equivalent during decomposition [4]. In the context of dual-carbon targets, it is, thus, urgent to explore a technology for quickly consuming and recycling such large amounts of biomass residues and converting them into valuable products, finally achieving sustainable development.

Biochars are carbon-rich solid particles generated from the incomplete combustion of the biomass under an oxygen-limited atmosphere at relatively low temperatures (<700 °C) [5,6,7]. The production of biochar itself is a carbon-negative process because it converts labile biomass into recalcitrant carbon that may persist in the environment for centuries [8]; moreover, the addition of biochar to soil also can mitigate greenhouse emissions. Previous research indicated that, compared with banana peel, the addition of its biochar to soil could significantly reduce the cumulative N₂O, CO₂, and CH₄ emissions from soils [9]. In addition, banana stem is abundant in cellulose (35–40%), hemicellulose (25–35%), and lignin (8–13%) [10], which can be considered as a favorable feedstock for producing biochar-based sorbents with a high fixed carbon content, a large specific surface area, and porous structures. Therefore, such techniques can efficiently recycle banana residues, mitigate greenhouse gas emissions, and simultaneously produce effective adsorbents for the treatment of environmental contaminants (e.g., antibiotics, pesticides, and heavy metals).

Antibiotics are widely used to treat or prevent microbial infections in humans and animals. They are commonly detected in urban wastewater or natural environments. Recently, the ongoing global epidemic of COVID-19, or the emergence of new pandemics caused by influenza, avian influenza, and swine flu, is leading to a growing global consumption of antibiotics. According to the estimation, the annual usage of antibiotics throughout the world will reach up to 105,600 tons by 2030 [11]. The continuously growing usage and unprocessed disposal of antibiotics has led to the widespread of antibiotic residues in aquatic and terrestrial environments, thus posing a long-term threat to the whole ecosystem and human health [12,13,14,15]. Fluoroquinolones (FQs) are broad-spectrum antibacterials, accounting for ~17% of the global antibiotic market [16]. FQs are considered emerging organic contaminants owing to their high solubility and mobility in water. Moreover, they are poorly digested or absorbed by humans or animals, thus up to 70% are excreted into environments unmetabolized [17]. The concentration of quinolones detected in the environmental compartments ranged from ppt to ppb levels [17]. Although the detection of FQs in environments is very low, residual antibiotics in environments can disrupt the ecological balance and promote the emergence of multidrug-resistant bacteria strains, ultimately weakening the efficacy of antibiotics [18].

Adsorption is considered one of the most effective methods for antibiotic removal because it is easy to design and operate, costs less, and does not generate toxic byproducts. Biochar-based sorbents have recently emerged as efficient materials for the removal of antibiotics owing to their rich functional groups, large surface area, and developed pore structure, as well as hydrophobic surface [19,20,21]. Compared with other antibiotics (e.g., sulfonamides and tetracyclines), the studies regarding the sorption of FQs by biochars were limited. For example, 375 and 823 articles were found during a search on the Web of Science with the keywords “biochar and sulfamethoxazole and adsorption” and “biochar and tetracyclines and adsorption”, respectively. However, only 54 articles were found with the keywords “biochar and ofloxacin and adsorption”. Moreover, controversial conclusions were obtained in these studies (Table S1). For example, Huang et al. stated that the sorption of biochars to ofloxacin (OFL) increased with pyrolysis temperature due to the pore-filling effect caused by the high surface area of biochars produced at higher temperatures [22]. On the contrary, other studies suggested that the sorption capacity of FQs on biochar was negatively correlated with the pyrolysis temperatures because abundant functional groups of biochar prepared at lower temperatures could facilitate the sorption through specific interactions such as hydrogen bonding, cation exchange, and π–π EDA interaction [23,24]. Liu et al. also excluded the contribution of the pore-filling effect on the OFL sorption [25]. These conflicting findings could be attributed to the varied properties of sorbents and different experiment conditions. Therefore, the role of surface functional groups and the porous structure of biochars in FQ sorption should be further confirmed with unified experiment conditions.

Although the sorption of antibiotics on biochar has been extensively studied, the sorption affinity of banana-residue-derived biochar to antibiotics was only evaluated in several studies [26,27,28,29], which could not provide enough theoretical support for their practical usage in wastewater management. Therefore, in this work, banana pseudo-stem (BS) was collected and pyrolyzed at temperatures of 300, 400, and 500 °C, respectively, and OFL was selected as model compound to study the sorption affinity of BS biochars to FQs. Additionally, according to the characterization, biochar produced at 300 °C has rich surface functional groups, while that produced at 500 °C possesses developed micropores. Thus, BS biochars could be suitable sorbents to study how the surface functional groups and pore structure affect the sorption of FQs. Moreover, the influence of the initial solution pH and inherent ash content on sorption was evaluated as well.

2. Materials and Methods

2.1. Materials and Chemicals

Ofloxacin (OFL, purity > 99.5%, CAS: 82419-36-1) was purchased from Aladdin Bio Chem Technology Co. (Beijing, China). The general physicochemical properties of OFL are as follows: solubility (Cs): 3400 mg L⁻¹; K_OW: 0.446; pK_a1: 6.10; pK_a2: 8.28 [30].

2.2. Preparation of Biochars

The banana pseudo-stem (BS) was collected from a local fruit market. The fresh BS was chopped into small pieces and oven-dried at 60 °C for 12 h, then ground and sieved with a 250 µm sieve before pyrolysis. The biomass was pyrolyzed for 4 h at temperatures of 300, 400, and 500 °C, respectively, in a muffle furnace under anaerobic conditions with N₂ flowing (flow rate was kept at 1 L min⁻¹) during the whole charring process. The charred residues were gently ground with agate mortar, and then passed through a 75 µm sieve. The original biochars were stored in a brown glass container, and named OBS3, OBS4, and OBS5 (suffix number of 3, 4, and 5 indicate 300, 400, and 500 °C, respectively), respectively. To investigate the role of intrinsic minerals in OFL sorption, the original biochars were washed with deionized water (DI) several times until the pH value of filter water was near neutral. Accordantly, the washed biochars were named WBS3, WBS4, and WBS5. To facilitate discussion, both OBS biochars and WBS biochars can be generally referred to as BS biochars.

2.3. Characterization of Biochars

The bulk elemental compositions of OBS and WBS biochars (C, H, N, S, and O) were measured using an Elementar analyzer (Elementar, Vario Micro Cube, Langenselbold, Germany). Ash contents were measured by heating samples at 800 °C in the presence of air for 4 h.

The specific surface area (SSA), total pore volume (V_t), and pore-size distribution were analyzed using N₂ (Micromeritics, 3Flex 5.02, Norcross, GA, USA). The SSA values were determined by the Brunauer–Emmett–Teller (BET) method. The V_t was estimated from a single-point adsorption of N₂ at a relative pressure of 0.989. The pore-size distribution was obtained by analyzing the N₂ desorption data based on the Barrett–Joyner–Halenda method.

The inorganic components were identified by X-ray diffraction (XRD) patterns: the samples were loaded into a glass holder and run on an X-ray diffractometer (Rigaku, smartlab9, Tokyo, Japan) equipped with Cu Kα radiation source at 40 kV and 40 mA. The diffraction patterns were recorded within a 2-Theta range of 10–80°.

The surface functional groups were characterized using a Fourier-transform infrared (FT-IR) spectrometer (Thermo Scientific, Nicolet-Is10, Waltham, MA, USA). Specifically, all the prepared particles were ground with potassium bromide (KBr, dehydrated in an oven at 105 °C for 24 h and ground into powder before use) at a weight ratio of 1:100 (biochar: KBr). The mixed powder was pressed into pellets and loaded on a FT-IR spectrometer. The spectra were collected in the range of 4000–400 cm⁻¹ with 32 replications and at a resolution of 4 cm⁻¹.

The morphological structure and surface elemental distribution of BS biochars were observed using scanning electron microscopy (Oxford Instruments, SEM, X-Max, Oxfordshire, UK), equipped with an energy-dispersive X-ray spectrometer (EDX). Raman spectra were obtained using a laser Raman spectrometer (Horiba, XploRA PLUS, Kyoto, Japan) with 514 nm incident laser light.

2.4. Sorption Experiment

Batch sorption experiments of OFL were conducted for all BS biochars in 4 mL brown glass vials sealed with a Teflon-lined screw cap. A stock solution of 50 mg L⁻¹ OFL was prepared in a background solution containing 0.02 M NaCl to maintain a constant ionic strength and 200 mg L⁻¹ NaN₃ as a biocide. This stock solution was diluted by the background solution to nine different concentrations (1–50 mg L⁻¹) with a final volume of 4 mL. The aqueous/solid ratio in the sorption experiment was 800:1 to ensure 20–80% sorption of initial OFL at equilibrium. The sealed vials were kept in the dark and shaken on an orbital shaker at 25 °C for 7 d. After being equilibrated for 7 d, all vials were centrifuged at 3000 rpm for 15 min, and then the supernatants were filtered through a 0.45 µm syringe filter for HPLC analysis. The experiments of pH effect on OFL sorption were conducted for WBS3 and WBS5 biochar with the same procedure as above except we adjusted solution pH to 6.0, 7.0, 8.0, and 9.0 with 0.05 M NaOH and 0.1 M HCl.

Sorption kinetics (aqueous/solid ratio of 800:1) was conducted for BS biochars in 100 mL brown glass containers sealed with a Teflon-lined screw cap at an initial concentration of 50 mg L⁻¹ OFL. The supernatant in each container was sampled and filtered with a 0.45 µm syringe filter for HPLC analysis at 0.25, 0.5, 1, 2, 4, 8, 24, 72, 120, and 168 h.

To study the effect of inherent minerals on the OFL sorption, experiments were conducted by mixing 5 mg WBS biochars with 4 mL OFL solution (50 mg L⁻¹) containing KCl or KHCO₃ with K⁺ concentrations of 300, 600, and 1000 mg L⁻¹, respectively. The supernatants were obtained after an equilibrium of 3 days. The follow-up experimental procedures were the same as in the batch sorption experiments.

All the sorption data points in the above experiments were run in duplicate.

2.5. Quantification of Dissolvable K in Biochars

An aliquot of 10 mg biochar was soaked with 8 mL DI water for 72 h. The supernatant was filtered through a 0.45 µm filter, and then quantified using a flame atomic absorption spectrometer (FAAS) (Varian AA240FS). The dissolvable K was quantified at the wavelength of 766.5 nm with a lamp current of 10 mA. Other parameters were set as: slit 0.5 nm, measuring time 5 s, airflow rate 13.5 L min⁻¹, and acetylene gas flow rate 2.0 L min⁻¹.

2.6. Detection of OFL

The concentration of OFL in the supernatants was quantified using HPLC (Agilent Technologies 1200, Santa Clara, CA, USA) equipped with a reverse-phase C18 column (5 µm, 4.6 × 150 mm) and a UV detector at 286 nm. The mobile phase was 10:90 (v:v) of acetonitrile and deionized water with 0.8% acetic acid. The injected sample volume was 10 µL with a flow rate of 1 mL min⁻¹.

2.7. Fourier-Transform Infrared Spectroscopy Characterization of Biochar and OFL-Loaded Biochar

FT-IR characterization was carried out for the biochar particles before and after the sorption of OFL. Specifically, 4 mL OFL solution (50 mg L⁻¹) was mixed with 5 mg of biochar and equilibrated for 7 d at the same condition as in the batch sorption experiment. After equilibrated for 7 d, the supernatant was removed as much as possible. Then, the residual biochar was washed with DI water and freeze-dried. All candidate biochars were characterized using FT-IR spectrometer (Thermo Scientific, Nicolet-Is10, USA).

2.8. Data Analysis

The pseudo-first-order model (PFOM), pseudo-second-order model (PSOM), and two-compartment first-order model (Two-PFOM) were selected to evaluate the sorption kinetics.

Pseudo-first-order model (PFOM):

$$\frac{q_{\mathrm{t}}}{q_{\mathrm{e}}}=1-e^{-k_{1}t}$$

(1)

Pseudo-second-order model (PSOM):

$$\begin{array}{l}\frac{q_{\mathrm{t}}}{q_{\mathrm{e}}}=\frac{k_2{q}_{\mathrm{e}}t}{1+k_2{q}_{\mathrm{e}}t}\\ \end{array}$$

(2)

Two-compartment first-order model (Two-PFOM):

$$\frac{q_{\mathrm{t}}}{q_{\mathrm{e}}}=f_{\mathrm{fast}}(1-e^{-k_{\mathrm{fast}}t})+f_{\mathrm{slow}}(1-e^{-k_{\mathrm{slow}}t})$$

(3)

In these models, q_t and q_e are solid-phase concentrations of sorbate at time t and sorption equilibrium, respectively; k₁ (h⁻¹) and k₂ ((mg kg⁻¹) h⁻¹) are the PFOM and PSOM rate constants, respectively; f_fast and f_slow are the fractions of the fast and slow compartments (f_fast + f_slow = 1); and k_fast and k_slow (h⁻¹) are the rate constants of two compartments.

The Freundlich and Langmuir models were employed to describe the sorption isotherms.

Freundlich model (FM):

$$q_{\mathrm{e}}=K_{\mathrm{F}}{C}_{\mathrm{e}}^N$$

(4)

Langmuir model (LM):

$$q_{\mathrm{e}}=\frac{q_{\max }{k}_{\mathrm{L}}{C}_{\mathrm{e}}}{1+k_{\mathrm{L}}{C}_{\mathrm{e}}}$$

(5)

C_e (mg L⁻¹) and q_e (mg kg⁻¹) are equilibrium liquid- and solid-phase concentrations, respectively; K_F is the Freundlich adsorption coefficient (mg kg⁻¹)/(mg L⁻¹) ^N; and N is the indicator of isotherm nonlinearity; q_max is maximum adsorption capacity (mg kg⁻¹); and k_L (L kg⁻¹) is Langmuir constant.

The single-point sorption coefficient was calculated as follows:

$$K_{\mathrm{d}}=\frac{q_{\mathrm{e}}}{C_{\mathrm{e}}};C_{\mathrm{e}}=0.001C_{\mathrm{s}} \mathrm{or} 0.01C_{\mathrm{s}}$$

(6)

3. Results and Discussion

3.1. Biochar Characterization

3.1.1. Elemental Characterization of Biochars

The bulk elemental composition, calculated atomic ratios, and ash contents of the original biochars derived from banana pseudo-stem (OBS) and de-ashed biochars with DI water (WBS) are listed in

Table 1. Elemental compositions, ash contents, atomic ratios and BET-N₂ surface areas.

Samples	Composition, wt%					Ash/%	Atomic Ratio			BET-N2
	C	H	O	N	S		H/C	(O + N)/C	O/C	SSA * (m2 g−1)	Vt * (cm3 g−1)	Dw * (nm)
OBS3	42.2	3.38	26.6	1.38	0.45	33.7	0.96	0.50	0.47	3.51	0.019	21.5
WBS3	51.8	3.69	25.6	1.92	0.26	10.3	0.85	0.40	0.37	4.75	0.026	21.7
OBS4	36.7	2.24	27.3	1.03	0.29	33.5	0.73	0.58	0.56	6.30	0.030	24.3
WBS4	59.9	2.83	21.5	2.05	0.24	9.5	0.57	0.30	0.27	9.53	0.035	14.7
OBS5	37.9	1.87	27.9	1.12	0.44	35.6	0.59	0.58	0.55	6.78	0.058	33.9
WBS5	70.3	2.59	22.1	2.32	0.26	7.69	0.46	0.20	0.25	390	0.198	2.03

*: SSA: specific surface area; V_t: total pore volume; D_w: average pore diameter.

. The ash contents of OBS biochars were generally high (>33.5%). After being treated with water, the ash in OBS biochars was remarkably eliminated. However, the residual ash content still in WBS biochars suggested that some inorganic components might be located deeper inside the inner pores or bounded with the organic moieties of biochars. Additionally, the relative percentage contents of bulk C, H, and N were generally elevated through de-ashing treatment, indicating the purification of the organic composition in biochars. On the contrary, the O and S contents significantly decreased, indicating that these two elements in the ash content were partially removed. When the interference from the ash content was eliminated, evolutional changes in the organic composition of biochar with pyrolytic temperature can be well described. According to the previous studies [5], the prominently increased C content and generally decreased H and O contents of WBS biochars with an increasing temperature were attributed to the carbonization, dehydration, and deoxygenation reactions caused by pyrolysis. In addition, the decreasing H/C, (O + N)/C, and O/C ratios indicated the increased aromaticity and decreased polarity of WBS biochars after pyrolysis.

3.1.2. BET Analysis and Pore Distribution

The N₂ adsorption–desorption isotherms and pore-size distribution of OBS and WBS biochars are shown in Figure 1, with the specific surface area (SSA), total pore volume (V_t), and average pore diameter (D_w) listed in Table 1. According to the IUPAC classification, the N₂ isotherms of all OBS biochars are of type III [31]. After water washing treatment, the N₂ isotherms of WBS3 and WBS4 still conformed to the type III isotherm, indicating the nonporous feature of BS biochars produced at pyrolytic temperatures below 500 °C [31]. Correspondingly, the SSA and V_t of OBS3 and OBS4 were generally very low and slightly increased after de-ashing treatment (shown in Table 1). It is worth noting that the N₂ isotherms of WBS5 increased dramatically when P/P₀ < 0.1 and exhibited an IUPAC classification of type I, which indicates a microporous structure [31], implying that the inorganic composition deposited in micropores of BS5 was released through the washing process. This result was confirmed by the fact that the D_w of WBS5 significantly decreased from 33.9 to 2.03 nm, while SSA and V_t greatly increased from 6.78 to 390 m² g⁻¹ and 0.058 to 0.198 cm³ g⁻¹, respectively. These developed micropores were mainly formed by the volatile components escaping from the biomass at high temperatures (e.g., ≥500 °C) [32] and the catalysis of the abundant K in the pristine materials. Based on the previous studies, the K content can react with carbon in the biomass and promote the formation of micropores [33]. Therefore, high-temperature biochars with well-developed micropore structures and a large SSA would be a potentially excellent sorbent for OFL removal.

3.1.3. The Inorganic Composition of Biochars Identified by X-ray Diffraction

The XRD patterns of the original and de-ashed BS biochars are shown in Figure 2. The OBS biochars are abundant in K-containing salts, including potassium chloride (KCl), potassium oxalate monohydrate (K₂C₂O₄·H₂O), and potassium bicarbonate (KHCO₃). KCl formed at a pyrolysis temperature of 300 °C and accumulated with increasing temperature, consistent with previous studies [21,34]. A small amount of K₂C₂O₄·H₂O was observed in OBS biochars of 300 and 400 °C, but it decomposed at 500 °C. This pyrolysis-dependent evolution was consistent with the formation of Ca-containing oxalate salts during the charring process found in the previous research [34]. KHCO₃ started to generate at a temperature of 400 °C, and its abundance increased greatly at 500 °C. The peaks of inorganic composition disappeared in WBS biochars, indicating that the potassium salts in OBS biochars were mainly water-soluble salts. Two broad bands were observed at a 2-Theta of 23.8° and 43.3° for all WBS biochar, indicating the formation of graphene sheets in the turbostratic carbon structure [34]. These results suggested that BS biochar is environment-friendly and cost-efficient because it does not need further acid treatments.

3.1.4. The Surface Functional Groups Analyzed by FT-IR

The FT-IR spectra of all investigated biochars in the range of 400–4000 cm⁻¹ are depicted in Figure 3. The solid and sharp peaks observed for the original biochars (Figure 3a) in the range of 400–1600 cm⁻¹ are characteristic peaks of inorganic components. In specific, the sharp peaks at 1597, 1312, 792, 618, and 526 cm⁻¹ are the characteristic peaks of K₂C₂O₄·H₂O [35], which are gradually eliminated with the increasing temperature. The OBS4 and OBS5 show strong peaks at 1627, 1402, 1008, 832, 701, and 662 cm⁻¹, indicating the formation of KHCO₃ at pyrolysis temperatures above 400 °C [36]. These results demonstrated that the formation of oxalic salt occurred at lower pyrolysis temperatures, while carbonate salt was usually generated at higher pyrolysis temperatures (above 400 °C), consistent with the XRD results. After the water washing treatment, the FT-IR characteristics of the carbon skeleton were revealed for the BS biochars (Figure 3b). The intensity of the broad band over a wide range of 2550–3700 cm⁻¹ corresponds to the stretching of –OH and –HN including phenols, aliphatic alcohols, and other H-bonding interactions, as well as atmospheric moisture tightly bonded to the biochar surface [27,37], which gradually weaken with the rising temperature, suggesting the dehydration and decomposition of aliphatic compounds or the decreasing hydrophilicity of the biochar surface after pyrolysis. The stretching at 2922/2854 cm⁻¹ was ascribed to aliphatic C–C, while the peaks of 1580 cm⁻¹ correspond to the aromatic C=C or C=O [22]. The prominent bands around 1700, 1450, and 1382 cm⁻¹ were associated with the stretching of –COOH [38]. The asymmetric stretching of C–O–C at 1102 cm⁻¹ indicates the presence of ether group [27]. These peaks gradually diminished with an increasing pyrolysis temperature, suggesting a decline in surface oxygen-containing functional groups and an increase in the aromaticity or hydrophobicity of BS biochars at elevated temperatures.

3.1.5. The Surface Morphology and EDX Analysis

The SEM and EDX analysis were employed to investigate the surface morphology and elemental distribution of BS biochars. As shown in Figure 4, the observed large particles preserve the fibrous cylindrical surface of banana pseudo-stem, and the small particles are irregular fragments generated during the grinding process. Generally, a rough surface was observed for OBS biochars (Figure 4a–c). The tiny particles randomly distributed/deposited on the surface or in the pores of biochars could be inorganic minerals. According to the element distribution obtained by EDX analysis, C and O are the primary elements of the biochar surface. It was found that K was abundant in the OBS biochars (13.4–23.4%) and their contents increased with the pyrolysis temperature, while the contents of other elements (i.e., Mg, Al, Si, and Ca) were all below 1%, indicating that the observed mineral crystalloids are mostly K salts. Through de-ashing treatment, most K salts were removed and the biochar surface turned out to be smooth (Figure 4d–f). These results were in line with the results of the XRD and FT-IR analysis. Thus, water washing treatment can efficiently eliminate the K-containing mineral in BS biochars, exposing more sorption sites for organic compounds.

3.1.6. Raman Analysis

The lattice structure and graphitic crystallite in the investigated biochars were identified by Raman spectroscopy [39]. As shown in Figure 5, two distinct peaks at 1351 and 1591 cm⁻¹ are assigned to the D and G bands, respectively. The D band is related to the disordered vibrations of sp³ carbon from the defects or edges, while the G band corresponds to the in-plane vibration of sp² carbon in graphitic crystallites [39]. The defects in the carbonaceous structures could be described by the intensity ratio of the D and G bands (I_D/I_G). For OBS and WBS biochars, the I_D/I_G ratios all increased with an increasing pyrolysis temperature, indicating that defects in the biochar were generated with pyrolysis. The results implied that turbostratic carbon structure (e.g., diamond-like lattice) gradually formed regarding the transformation of the sp² to sp³ carbon phase during pyrolysis [40]. The broad 2D band located between 2487 and 3500 cm⁻¹ was assigned to the multilayer graphene phase structure [41]. Interestingly, the 2D band was only observed for WBS5, indicating that the graphitic structure of BS biochar was highly developed at 500 °C. These observations indicated that condensed aromatic carbons were gradually generated in BS bicohars with increasing pyrolysis.

3.2. Sorption of OFL on Banana-Pseudo-Stem-Derived Biochars

3.2.1. Sorption Kinetics

The sorption kinetics of OFL on BS biochars before and after de-ashing treatment were investigated at the initial concentration of 50 mg L⁻¹. As shown in Figure 6, the sorption of OFL on all investigated biochars (except OBS5) increased fast at an initial stage, and then leveled off when the sorption approached the equilibration after 50 h. Three typical kinetic models, namely, PFOM, PSOM, and Two-PFOM, were employed to describe these curves, with the regression curves plotted in Figure 6 and fitting results listed in

Table 2. Fitting parameters of sorption kinetics of OFL on biochars.

Biochar	Pseudo-First-Order Model			Pseudo-Second-Order Model
	qe (mg kg−1)	k1 (h−1)	r2	qe (mg kg−1)	k2 ([(mg kg−1) h−1])	r2adj
OBS3	33,027.1	10.9	0.996	33,293.0	0.0015	0.999
OBS4	6191.5	4.8	0.893	6411.9	0.0013	0.940
OBS5	–	–	–	–	–	–
WBS3	38,699.6	10.5	0.997	39,016.7	0.0012	1.000
WBS4	11,114.5	2.2	0.773	11,785.8	0.0002	0.874
WBS5	4369.7	12.6	0.978	4384.2	0.0220	0.978
Biochar	Two-Compartment First-Order Model
	qe (mg kg−1)	Ffast	Fslow	kfast (h−1)	kslow (h−1)	r2adj
OBS3	33,509.3	0.953	0.047	13.9	0.191	0.999
OBS4	6957.4	0.751	0.249	9.3	0.061	0.995
OBS5	–	–	–	–	–	–
WBS3	39,087.3	0.945	0.055	14.1	0.478	1.000
WBS4	13,075.9	0.578	0.422	8.2	0.075	0.990
WBS5	4440.2	0.961	0.039	15.4	0.108	0.979

. PFOM and PSOM can afford a satisfying fitting to the OFL sorption on OBS3, WBS3, and WBS5, indicated by the high r²_adj values (>0.978). However, neither PFOM nor PSOM could precisely describe the kinetic data of OFL sorption on OBS4 and WBS4 because of the underestimation of the slow sorption at longer time points. By contrast, Two-PFOM, among the three models, exhibited the best fitting performance for all the kinetics (except WBS5), as evidenced by the r²_adj values (0.979–1.000). In addition, the experimental sorption capacity q_e,exp and the predicted q_e,cal at equilibrium based on Two-PFOM are almost identical, implying that Two-PFOM may provide more reliable fitting parameters compared with the other two models. More importantly, Two-PFOM could separately evaluate the sorption rate constant of the fast and slow compartments and simultaneously estimate the contribution of two compartments to the total sorption. Therefore, the present data were discussed based on the fitting results of Two-PFOM.

The two compartments could be distinguished by two varied sorption rate constants (k_fast and k_slow). As shown in Table 2, the k_fast and k_slow values of OFL sorption substantially decreased when the pyrolytic temperature was elevated from 300 °C to 400 °C, for both the original and de-ashed biochars, probably due to the decrease in the number of sorption sites on the surface of biochars prepared at 400 °C. According to the above characterizations, the oxygen content, O/C, as well as the surface oxygen functional groups on biochars remarkably decreased at temperatures above 300 °C, implying that the surface oxygen functional groups of biochar may play a key role in the sorption of OFL. As the pyrolytic temperature further increased to 500 °C, the k_fast and k_slow values of OFL sorption on WBS5 increased up to 15.43 and 0.108 h⁻¹, respectively, which could be attributed to the increased SSA and V_t of WBS5 as analyzed above. Based on the kinetics data shown in Figure 6 and fitted equilibria q_e,cal in Table 2, the sorption capacity of OFL significantly decreased with the rise in pyrolysis temperature for both OBS and WBS biochars, again revealing that the surface oxygen functional groups may be crucial for OFL sorption, which will be discussed extensively in the follow-up sections.

Unlike the other kinetic curves, the OFL sorption on BS5 exhibited a unique multi-stage process. Specifically, the sorption of OFL increased rapidly in an initial 24 h, followed by a remarkable decline in a later 48 h, and then reached re-equilibrium after 72 h. Therefore, the above three kinetic models failed to describe these data. In Zuo’s study [42], they reported a similar multi-stage process regarding the sorption between dibutyl phthalate (DBP) and chicken-feather-derived biochars, and attributed this phenomenon to the strong inter-molecular interactions of DBP. In the present study, a similar speculation could be proposed. Specifically, a strong negative charge-assisted hydrogen bond ((−)CAHB) may form between two OFL⁻ molecules under an alkaline condition (pH = 9.4 for OBS5 system). Thus, the rapid sorption of OFL at an initial stage was attributed to inter-molecular interactions of OFL, which may shield the negative charges of OFL and facilitate their sorption. However, the OFL sorption decreased when it reached a new equilibrium with the interaction time. Extensive discussions about this unique phenomenon are out of the scope of the present work, which will be further investigated in another work of ours.

3.2.2. Sorption Isotherms and Potential Sorption Mechanisms

The sorption isotherms of OFL on OBS biochars (Figure 7a) and WBS biochars (Figure 7b) which varied with the pyrolysis temperature were investigated. FM and LM were selected to fit the sorption isotherms, and the fitting results are shown in

Table 3. Isotherm fitting results of OFL on biochars by Freundlich and Langmuir models.

	Freundlich Model						Langmuir Model
Biochar	N	KF ([(mg kg−1)/(mg L−1) N])	r2adj b	SEE b	Kd (L kg−1)		Q0 (mg kg−1)	KL (L kg−1)	r2adj b	SEE b
					0.001 CS	0.01 CS
OBS3	0.342	9297.5	0.983	1049.3	4155.8	913.4	24,904.6	0.630	0.950	1823.3
WBS3	0.563	39,058.3	0.982	1692.4	22,880.1	8364.8	56,455.1	2.11	0.988	1389.2
OBS4	0.176	1394.5	0.942	257.4	508.7	76.3	2450.9	1.48	0.937	162.2
WBS4	0.198	7615.8	0.958	873.9	2854.1	450.3	13,041.9	3.12	0.920	1204.6
OBS5	0.145	1050.9	0.909	107.7	369.1	51.5	1633.2	2.26	0.934	91.6
WBS5	0.179	2172.7	0.966	244.9	795.5	120.1	3667.4	2.56	0.944	314.4
WBS3-pH 6.0	0.506	84,155.1	0.994	1007.1	45,947.7	14,715.1	55,107.7	10.49	0.966	2293.3
WBS3-pH 7.0	0.563	39,058.3	0.982	1692.4	22,880.1	8364.8	56,455.1	2.11	0.988	1389.2
WBS3-pH 8.0	0.484	16,000.6	0.992	979.9	8507.2	2591.7	41,373.7	0.82	0.985	1380.8
WBS3-pH 9.0	0.539	6733.5	0.996	570.97	3828.4	1323.2	36,473.9	0.20	0.986	1048.8
WBS5-pH 6.0	0.187	3856.5	0.937	602.9	1425.9	219.3	6843.3	1.51	0.856	908.3
WBS5-pH 7.0	0.180	2180.6	0.948	259.0	799.4	121.0	3700.0	2.48	0.914	332.2
WBS5-pH 8.0	0.166	1982.9	0.930	247.3	714.6	104.7	3207.3	2.95	0.898	299.7
WBS5-pH 9.0	0.219	1112.1	0.984	77.8	427.6	70.8	2272.6	0.880	0.885	205.5

^b: r²_adj denotes adjusted coefficient of determination, and SEE is standard error of estimation.

. In general, both models showed a satisfying fitting performance with r²_adj values mostly in the range of 0.90–1.00. However, FM was more appropriate for describing the actual sorption process of OFL on BS biochars, such as BS3 and WBS4 (shown in Figure 7), implying a multilayer sorption process [43,44] and an energetic preference of OFL for the heterogeneous sorption sites of BS biochars. The N values obtained from the FM were generally below 0.563, suggesting that the sorption energy of OFL on BS biochar was highly heterogeneous. In addition, for both OBS and WBS biochars, the N values significantly decreased with the rise in pyrolysis temperature, indicating increased sorption nonlinearity after pyrolysis [34]. This was attributed to the irregular pores and rigid aromatic structures formed at higher pyrolysis temperatures, which provided heterogeneous sorption sites for OFL.

As summarized in Table S1, the maximum adsorption capacity is in the range of 0.31–218.29 mg g⁻¹. The sorption capacity of OFL on BS biochars in the present study is comparable to most results in the literature. However, the sorption capacity cannot be directly compared due to different solute concentrations or sorption conditions used in these studies. In addition, the K_F values calculated by FM could also not be directly compared because the unit of K_F is determined by the nonlinearity of the isotherm. Therefore, the single-point sorption coefficient, K_d, was calculated at C_e = 0.001 C_s and 0.01 C_s (shown in Table 3) using the fitting parameters of FM to further study the sorption mechanisms of OFL. From the sorption isotherms shown in Figure 7 and the calculated K_d values listed in Table 3, it can be found that the sorption of OFL on BS biochars significantly decreased with increasing pyrolysis temperature either before (Figure 7a) or after (Figure 7b) water washing treatment, consistent with the above kinetic results, further confirming that the surface oxygen-containing groups of biochars played a vital role in OFL sorption. At C_e = ~3–5 mg L⁻¹, the K_d value of OFL sorption by WBS3 (22,880.1 L kg⁻¹) was at least ~5 times that by Ulva prolifera-derived biochar (4743 L kg⁻¹) [45] and ~31–497 times that by cassava-residue-derived biochar (46–732.49 L kg⁻¹) [22], indicating the excellent sorption performance of WBS3 to OFL. As previously discussed in the literature [30,46,47], various mechanisms could be involved in OFL sorption, such as electronic interaction, cation exchange, the hydrophobic effect, the pore-filling effect, π–π EDA interaction, ordinary hydrogen bond (OHB), and (−)CAHB, which will be discussed in detail in the follow-up section.

(i)

Electronic interaction

Previous studies indicated that the sorption of OFL on sorbents with various functional groups (e.g., biochar, activated carbon, or carbon nanotubes) is highly pH-dependent [24,27,30,48]. In the present study, the biochar surface is negatively charged due to the deprotonation of various oxygen functional groups (e.g., –COOH and –OH) at the current pH levels (7.22–9.4). In addition, having two pK_aS (pK_a1 = 6.10, pK_a2 = 8.28), the distribution of three species (OFL⁺, OFL^±, and OFL⁻) of OFL also varied with different pH conditions. As shown in Figure 7c, 82.3% of OFL existed as OFL^± in the sorption system of OBS3 (pH = 7.5), while 93.4 and 91.8% of OFL presented as OFL⁻ in those of OBS4 (pH = 9.4) and OBS5 (pH = 9.5), respectively. Hence, the OFL sorption was facilitated by the electronic attraction (EA) between the positive charge of OFL^± and the negative charge on the OBS3 surface, while it could be restricted by the electronic repulsion between OFL⁻ and negatively charged surface of OBS4 and OBS5. This is probably why the sorption of OFL by OBS3 was much greater than those by OBS4 and OBS5. Moreover, water washing treatment removed most potassium salts in OBS biochars, leading to a visibly declined solution pH in the WBS4 (pH = 7.4) and WBS5 (pH = 7.2) systems, and, thus, an increased fraction of OFL^± (84.9 and 83.8% in the WBS4 and WBS5 systems, respectively) (Figure 7c), thereby improving OFL sorption. Nevertheless, electronic interaction only partly explained the OFL sorption because OFL sorption markedly decreased with pyrolysis when the species distribution of OFL was identical in each sorption system of WBS biochars (Figure 7c).

(ii)

Cation exchange

The cation exchange between OFL and oxygen functional groups may be involved in OFL sorption (Equation (7)) and it could decrease with pyrolysis owing to the decreased acidic functional group. It was suggested that the cation exchange of cationic OFL was much greater than that of amphoteric OFL [45,49,50]. Thus, the contribution of cation exchange to the differential sorption behaviors of OFL on the investigated sorbents could be minimal because OFL⁺ only accounted for 3.83–6.21% of overall OFL concentrations in the sorption systems. Moreover, the phenomenon of H⁺ releasing after OFL sorption as indicated in Equation (7) was not observed in any batch sorption system of the current study, again confirming the above speculation.

BC − COOH/OH + OFL⁺/OFL^± → BC − COO/O − OFL + H⁺

(7)

(iii)

Hydrophobic effect

As discussed above, the decreased polarity and increased aromaticity, as well as diminished surface oxygen-containing functional groups, indicated the increased hydrophobicity of the biochar surface with pyrolysis. For both OBS biochars and WBS biochars, the generally decreased K_d values with the pyrolysis temperature implied that the hydrophobic effect could not govern the OFL sorption.

(iv)

Pore-filling effect

Furthermore, the above analysis assumed that increased SSA and V_t either with pyrolysis or after water washing treatment might provide more sorption sites for OFL. However, the order of OFL sorption was just reversed to the order of the SSA and V_t of biochars with pyrolysis, and the K_d value (C_e = 0.001 C_s) of OFL sorption by WBS3 was nearly 30 times that by WBS5, indicating that SSA and pore-filling were not the dominant factors determining the OFL sorption on BS biochars. This speculation was further supported by the results shown in Figure 7d,e, where the K_d values of OFL decreased with pyrolysis even more significantly after SSA normalization. Previous studies [51,52,53] suggested that the target molecules can effectively penetrate the pores only when the pore diameter is 1.7 times larger than the molecule’s second-widest dimension. The calculated molecular size of OFL is 1.43 nm × 8.4 nm [54], so the pores with a diameter smaller than 1.43 nm would be inaccessible for OFL. Although the SSA of WBS5 was up to 390 m² g⁻¹, some micropores smaller than 1.43 nm, especially the inner pores of WBS5, may not be accessed by OFL due to the size exclusion effect.

(v)

π–π EDA interaction

π–π EDA was usually involved in the interaction between ionic compounds and biochar because both of them have various functional groups [5,19]. OFL could be identified as a strong π-acceptor due to the strong electron-withdrawing ability of the fluorine on the benzene ring [46], while the biochars with electron-donating groups (e.g., –OH and –NH₂) can be a π-donor. With the rise in pyrolysis temperature, decreased –OH and –NH₂ on biochars weakened the π–π EDA interaction between OFL and biochars, leading to decreased OFL sorption. Although the increased polyaromatic structure at high temperatures may provide rich π electrons for the interaction with OFL [55], the inner surface cannot be accessed by OFL because of the size exclusion effect mentioned above.

(vi)

Hydrogen bond

The formation of hydrogen bonds between the functional groups of OFL and the oxygen-containing groups of biochar may facilitate the sorption of OFL on biochar. It is worth noting that the carboxyl groups of OFL molecules were mostly deprotonated at the current pH condition in the systems of OBS and WBS biochars. Thus, OHB with low-energy berries could not be involved in the carboxyl group of OFL. Instead, (−)CAHB may play a crucial role in OFL sorption [46,47]. Previous studies [47] indicated that, compared with OHB, (−)CAHB was a stronger hydrogen bond, which is generated when the hydrogen bond donor and hydrogen bond acceptor have similar pK_a values (|Δ pK_a| = |pK_a, hydrogen bond donor − pK_a, hydrogen bond acceptor| < 5), and it became stronger when the |Δ pK_a| approaches 0. According to the previous study, the pK_aS of –COOH on biochars could cover a wide pH range of 2–7, and the pK_aS of –OH could be around 9–11 [56]. Thus, the two pK_aS of OFL might be located in these pH regions. Thus, (−)CAHB may be involved in the sorption of OFL by biochars (Equations (8)–(11)). As discussed earlier in this work, surface oxygen-containing functional groups (e.g., –COOH and –OH) significantly decreased with pyrolysis, which could weaken the (−)CAHB between the OFL and biochar, and, thus, OFL sorption. Additionally, as shown in Figure 7f, an elevated pH was observed in the systems of WBS4 and WBS5 after the sorption of OFL, further confirming the formation of (−)CAHB (Equations (8) and (10)). Although the (−)CAHB between OFL and WBS3 could be stronger than the other systems, the buffer effect of abundant functional groups of WBS3 may counterbalance the pH variation caused by (−)CAHB. Similarly, the pH variation caused by (−)CAHB could not be seen in the OBS biochar systems because the hydroxide ions were consumed by potassium bicarbonate (Equation (12)) or other potassium salts.

OFL⁻ − COO⁻ + H₂O → OFL^±–COOH + OH⁻

(8)

BC–COO⁻/O⁻ + OFL^±–COOH → BC–COO⁻/O⁻∙∙∙H–OOC–OFL^±

(9)

OFL⁻–N + H₂O → OFL^±–N–H + OH⁻

(10)

BC–COO⁻/O⁻ + OFL^±–N–H → BC–COO⁻O⁻∙∙∙H–N–OFL^±

(11)

HCO₃⁻ + OH⁻ → CO₃²⁻ + H₂O

(12)

The above discussion indicated that BS3 biochars with abundant surface functional groups facilitated OFL sorption mainly via specific interactions including EA, π–π EDA interaction, OHB, and (−)CAHB. However, BS5 biochars with a developed micropore structure played a minimal role in OFL due to the size exclusion effect.

3.2.3. FT-IR Analysis of Biochars before and after OFL Sorption

FT-IR spectra characterized before and after OFL sorption were carried out to validate the interaction between OFL and the investigated biochars (Figure 7g–i). For WBS3, several new peaks were observed at 1616, 1446, and 1055 cm⁻¹ corresponding to the N–H bending vibration in the quinoline ring, stretching vibration of CH₂ in the benzoxazine ring, and C–F stretching of OFL [48,57,58], respectively, indicating the successful loading of OFL on WBS3. These changes were not found in the systems of WBS4 and WBS5, probably due to their much lower sorption amount. However, after OFL sorption, an evident blue shift from 3347 to 3362 cm⁻¹ and from 3351 to 3391 cm⁻¹ were observed for the hydrogen-bonded –OH groups of WBS4 and WBS5, respectively, with increased intensity. These changes suggested that strong hydrogen bonds, likely (−)CHAB, may form between OFL and these two biochars, consistent with the above discussions. As previously demonstrated by Zhang et al. [59,60], (−)CHAB can be directly evidenced by the blue shift of –OH to a higher frequency. The results in our study were not as obvious as those in their studies that could be ascribed to the low OFL-loading on WBS4 and WBS5; in addition, the (−)CHAB formed between OFL and BS biochars were not as strong as those in Zhang et al.’s studies. Unfortunately, the blue shift of –OH was not observed for WBS3 after OFL sorption despite the fact that the (−)CHAB formed between OFL and WBS3 could be the greatest among all systems of WBS biochars. This is because the changes might be inapparent which could be overlapped with the abundant functional groups of WBS3. Therefore, other techniques should be employed to further confirm the formation of (−)CHAB between OFL and BS biochars in future work.

3.2.4. Effect of pH, Surface Functional Groups, and Micropores on OFL Sorption

As discussed above, the OFL sorption was significantly affected by the solution pH owing to the varied OFL speciation and surface charge of biochars caused by pH variation. To further elucidate the effect of pH on the sorption of OFL by BS biochars, we selected two biochars, WBS3 (with abundant functional groups) and WBS5 (with a developed micropore structure), and simultaneously investigated the role of the surface functional groups and micropore structure in OFL sorption under different pH condition. The pH levels were set to obtain different distributions of three OFL species. Generally, a significant reduction was observed for OFL sorption by both WBS3 and WBS5 with the pH increase from 6.0 to 9.0. According to the calculation shown in Figure 8b, with the rise in pH, the fraction of OFL⁺ declined from 54.5% to 0.02%; that of OFL^± first increased to 85.0% at pH 7, and then decreased to 15.1% at pH 9; and that of OFL⁻ gradually increased from 0.25% to 84.9%. Based on the above discussion, EA, cation exchange, π–π EDA interaction, OHB, and (−)CAHB were involved in the sorption of OFL by WBS3 and WBS5. These proposed interactions, except for (−)CAHB, all decreased with an elevated pH, ultimately leading to a decreased OFL sorption. In addition, as can be seen from the sorption isotherms of OFL at different pH levels (Figure 8a,c) and K_d values obtained from the FM fitting (Figure 8d), the sorption of OFL by WBS3 was extraordinarily higher than that by WBS5 at each investigated pH level, revealing that surface functional groups other than micropores governed the sorption of OFL. Furthermore, even when the OFL⁻ molecules accounted for 85.0% at pH 9, the K_d values of OFL sorption on WBS3 were 9 and 19 times those on WBS5 at a low concentration and high concentration, respectively, suggesting that (−)CAHB was the dominant sorption mechanism for OFL by WBS3 at an alkaline condition, which could overcome the electron repulsion between OFL⁻ and negatively charged surface of biochars. These results demonstrated that, compared with the micropore structure, the surface functional groups were more important for the OFL sorption on BS biochars.

Although Huang et al. concluded that the pore-filling effect played a major role in OFL sorption, their present data were not sufficient to support this opinion. For example, they did not provide the O content of the investigated biochar and pH condition of the batch sorption experiments [22]. Both of them are essential for OFL sorption. As indicated by the characterization [61], the ash content of cassava-residue-derived biochar made at 750 °C (CW750, the same sorbent used in Huang et al.’s study) was as high as 30.56%. However, the investigators did not analyze the inorganic composition of CW biochars and discuss their influence on OFL sorption [22]. Therefore, their conclusions are not convincing. Furthermore, many studies found the importance of the surface functional groups of biochar in OFL sorption [1,23,24], again confirming our findings and conclusions.

3.2.5. Effect of K-Containing Salts on OFL Sorption

As discussed earlier, OBS biochars are abundant in K-containing salts, mainly including KCl, KHCO₃, and a small amount of K₂C₂O₄. The above relevant results indicated that these inorganic compositions hindered OFL sorption by covering the sorption sites or blocking the inner pores of OBS biochars, as well as releasing OH^– into solution. Moreover, the K⁺ and anions (Cl⁻ and HCO₃⁻) should be taken into consideration due to their co-existing with OFL in the sorption system, which may affect the sorption of OFL. According to the quantification (Figure 9), the concentration of K⁺ in the solution of OBS biochar systems increased from 266 to 310 mg L⁻¹ with the pyrolysis temperature, due to the accumulation of K salts at higher temperatures. After water washing treatment, the concentration of K⁺ decreased to 2.34 mg L⁻¹, indicating that minimal K⁺ co-exists with OFL in the systems of WBS biochars. Herein, two major K-containing salts KCl and KHCO₃ with varied concentrations (300, 600, and 1000 mg L⁻¹ quantified depending on K⁺ concentrations) were added to the systems of WBS biochars to investigate their influences on OFL sorption.

The impact of ions on FQs has been previously discussed. Huang et al. suggested that no significant effect on OFL sorption was observed for the K⁺ at a concentration of 0.01 M (390 mg L⁻¹) which is comparable with the concentration detected in the systems of OBS biochars. As can be seen from Figure 10, the addition of KCl and KHCO₃ arouses an adverse trend in the pH variation. Specifically, with the increasing amount of KCl, a general decline in the solution pH was observed, which was attributed to the cation exchange between K⁺ in the solution and H⁺ released from the acidic functional groups of biochars. Clearly, with an increasing pyrolysis temperature, the extent of reduction in the pH became weaker because the acidic groups or cation exchange capacity of biochars decreased with pyrolysis. The addition of KCl slightly increased OFL sorption by WBS3, which was caused by the decreasing solution pH. However, the addition of KCl to WBS4 and WBS5 caused little change to their sorption affinity to OFL, consistent with the results reported by Huang et al. [22]. He et al. also suggested that K⁺ had a relatively small effect on OFL sorption due to its negligible competition for the sorption sites [62]. In contrast, the solution pH generally increased with the increasing addition of KHCO₃, owing to the hydrolysis of KHCO₃, and, thus, released OH⁻ into the solution. Thus, an obvious decline in OFL sorption by WBS3 and WBS4 was caused by an elevated solution pH. However, the addition of KHCO₃ did not significantly affect the sorption of OFL on WBS5. A previous study also indicated that the hydrolysis of carbonate slats could produce a large amount of OH⁻ in the solution [63], thereby inhibiting the ciprofloxacin sorption. These findings suggested that the co-existing K⁺ did not significantly affect OFL sorption on BS biochars, while HCO₃⁻ may increase the solution pH, and thus decrease the OFL sorption.

4. Conclusions and Environmental Implications

Biochars with rich surface functional groups and developed micropore structures were directly pyrolyzed from the banana pseudo-stem without any modification. K₂C₂O₄·H₂O and KCl formed at 300 °C, but K₂C₂O₄·H₂O decomposed with the increasing of the pyrolysis temperature, while KHCO₃ was generated at a temperature of 400 °C with its abundance increasing greatly at 500 °C. Water washing treatment greatly increased the biochar surface area due to the release of the inner micropores. The sorption of OFL by BS biochars was closely related to the surface functional groups of biochars, mainly governed by the sorption mechanisms including EA, π–π EDA interaction, OHB, and (−)CAHB. However, the pore-filling effect resulting from the micropores of high-temperature biochars played a minimal role in OFL sorption due to the size exclusion effect. EA, π–π EDA, and OHB decreased with the rise in pH, ultimately weakening the overall OFL sorption. The inherent K-containing slats may hinder OFL sorption by covering the sorption sites or blocking the inner pores of biochars, as well as releasing OH⁻ into the solution. This study emphasized that surface functional groups other than micropores of BS biochars played an important role in OFL sorption. In the current study, BS biochar produced at a pyrolysis temperature of 300 °C showed an excellent sorption performance to OFL. Its production does not need high-energy input and extra acid washing or modification, which is a cost-efficient, low-energy, and low-carbon process. Thus, BS biochars are promising low-cost and environment-friendly sorbents for the treatment of wastewater containing FQ antibiotics.