Adsorption kinetics of three fluoroquinolone carboxylic acids on red clay soil in five types of aqueous solutions

Antibiotics have garnered global attention due to their negative impacts on various ecosystems and human health (Yu & Wu 2020; Kumar et al. 2023; Oyekunle et al. 2023). One of the most significant concerns regarding antibiotics is their potential to promote antibiotic-resistant bacteria, posing a serious threat to public health (Zhao et al. 2021; Franklin et al. 2022). Antibiotics can affect the growth and reproduction of aquatic organisms, potentially causing a decline in aquatic biodiversity (Van Doorslaer et al. 2014; Mukhopadhyay et al. 2022). Furthermore, they can impact the growth of nitrifying bacteria in the soil environment, leading to reduced soil fertility and crop yields (Franklin et al. 2022). Fluoroquinolone carboxylic acids (FQCAs) are a class of fluoroquinolone antibiotics widely utilized as broad-spectrum antibacterial medicines for human and veterinary use (Bhatt & Chatterjee 2022). Among the FQCAs, ciprofloxacin (CIP), ofloxacin (OFL), and levofloxacin (LEV) are frequently used in human and veterinary medicine to treat a variety of bacterial infections, including respiratory, gastrointestinal, and urinary tract infections (Bhatt & Chatterjee 2022). Due to their moderately hydrophilic property with solubilities higher than 1 g/L in water, these FQCAs are frequently detected in wastewater treatment plants and various surface water bodies worldwide (Van Doorslaer et al. 2014; He et al. 2015; Bhatt & Chatterjee 2022).

Predicting and controlling potential environmental and health risks caused by FQCAs necessitates investigating the adsorption and desorption kinetics of FQCAs on natural soils (Franklin et al. 2022). Such investigations are essential for understanding their occurrence, removal, transport, and risk in the environment (Franklin et al. 2022). These investigations also aid in developing strategies aimed at conserving the global water environment (Franklin et al. 2022). Investigating the adsorption and desorption kinetics of FQCAs by soils can provide valuable insights for developing innovative, cost-effective, environmentally friendly, and sustainable soil-based adsorbents used to effectively mitigate FQCA pollution across various water bodies.

Numerous studies have investigated the adsorption behavior of FQCAs on various soils or soil components, including OFL on kaolinite (Li et al. 2017b), OFL on calcined Verde-lodo bentonite clay in Brazil (Antonelli et al. 2020), OFL on agricultural and forested soils located in the Penn State of the USA (Franklin et al. 2022), and OFL on soils in Yunnan Province of China (Pan et al. 2012). However, recent literature searches and reviews show that there are no studies reported on the adsorption and desorption behavior of CIP, OFL, and LEV on red clay soils. These soils exhibit a red color due to their high iron content and are extensively distributed globally, notably in the area of the Loess Plateau of China. Moreover, most prior research on antibiotic adsorption to soils has been conducted using pure water (PW) solutions spiked with antibiotics. It is well known that the primary routes through which antibiotics enter the soil are the land application of sewage sludge and animal waste, irrigation with treated or untreated wastewater, use of antibiotics-contaminated surface water for irrigation, infiltration from stormwater runoff (SW), and leakage from sewers and treatment plants (Revitt et al. 2015; Franklin et al. 2022). However, the impact of real-world water matrices on the adsorption behavior of FQCAs by soil is still unknown. Thus, current findings and conclusions regarding the adsorption of FQCAs to soils necessitate scrutiny using various real-world water matrices, such as treated wastewater (TW) effluent (TW), lake water (LW), and river water (RW), as well as SW.

Therefore, the objective of this study was to investigate the adsorption and desorption kinetics of CIP, OFL, and LEV on red clay soil within PW, TW, LW, RW, and SW solutions. It also aimed to illustrate the underlying mechanisms of FQCA adsorption onto red clay soil, considering the impact of varying water quality. This study provides new insights into the fate and transport of these FQCAs within the soil environment as well as risk analysis and offers valuable information for the development of red clay soil-based adsorbents for their removal from water.

CIP (≥ 98.0%), OFL (≥ 98.0%), and LEV (≥ 98.0%) were purchased from Aladdin Company, Shanghai, China, and used to prepare standard and specific solutions. NaN₃ (≥ 95.0%) purchased from the same company was used to prevent microbial activity during the adsorption of FQCAs onto red clay soil. All chemical reagents used in the adsorption and desorption experiments were analytical grade or higher. Ultrapure water was used as the solvent for the necessary solutions.

Samples of red clay soil were gathered from a farm in Changzhi City, Shanxi Province, China, situated at coordinates 36°50′26″ N latitude and 112°51′03″ E longitude. The collected soil samples experienced several preparatory stages. Initially, they were rinsed with ultrapure water and then dried in an oven at 105 °C until a consistent weight was attained. Once cooled to room temperature, the red clay soil samples were finely ground and sieved. Soil particles smaller than 0.1 mm in size were used for subsequent experiments as the adsorbent.

The pH and electrical conductivity (EC) of the red clay soil were determined using a FiveEasy Plus^TM pH meter (Mettler-Toledo FE28, Columbus, Ohio, USA) and an EC meter (Mettler-Toledo FE38, Columbus, Ohio, USA), respectively, with a soil-to-water ratio of 1:2 (m/v) at 25 °C (Li et al. 2022). The bulk density, the pH at the point of zero charge (pH_PZC), and the total organic carbon (TOC) of the red clay soil were measured using the methods provided by Li et al. (2022). The Brunauer–Emmett–Teller (BET) surface area, total pore volume, average pore diameter, and pore size distribution of red clay soil were determined using the N₂ physical adsorption method with a Micromeritics ASAP 2460 surface area and porosity analyzer (Micromeritics Corporation, USA). Powder X-ray diffraction (XRD) patterns were recorded in the range of 5–90° (2θ) using a PANalytical X'Pert PRO powder diffractometer (Empyrean, Malvern Panalytical Ltd, UK).

TW samples were collected at the endpoint of the ultraviolet disinfection chamber following tertiary treatment at a municipal wastewater treatment plant in Jinzhong City, Shanxi Province, China. LW was obtained from a lake located in Yingze Park, Taiyuan City, China. The water in this lake is mainly sourced from SW and groundwater. RWs were gathered from the Fen River in Taiyuan City, which is the second largest tributary of the Yellow River. SW was collected on the campus of Taiyuan University of Technology during a stormwater event in May 2024. Water samples were collected using a standard organic glass sampler at a depth of 30 cm below the water surface. On-site measurements of pH and EC were taken, and then the samples were placed into brown reagent bottles and stored in a refrigerated box at low temperatures before being promptly returned to the laboratory for filtration pretreatment. Following this, the samples were refrigerated for storage, and all subsequent experiments were conducted using these collected water samples. Furthermore, high-performance liquid chromatography (HPLC) analysis revealed the absence of the FQCAs under investigation in all water samples. The water sampling procedure, pretreatment, and quality analysis were conducted following standard methods provided by the American Public Health Association, the American Water Works Association, and the Water Environment Federation (APHA et al. 2023).

The adsorption kinetics were investigated in PW, TW, LW, RW, and SW under the following conditions: dosage = 2 g/L, C₀ = 10 mg/L, temperature = 25 °C, and adsorption time (t) = 0–240 min. The agitation speed was maintained at 200 rpm. After adding the red clay soil and FQCAs to the water samples, the pH values remained significantly unchanged. Thus, the pH in the adsorption experiments was consistent with that of the respective water samples.

Upon reaching adsorption equilibrium, the supernatant was removed. The red clay soil samples, loaded with FQCAs, were subsequently washed twice with ultrapure water and then air-dried for subsequent desorption experiments. The desorption experiments were performed in the same solution as adsorption with C₀ = 0 mg/L.

At a predetermined interval, a 5 mL sample was withdrawn from the reactor, with each sample being measured once to avoid variation in liquid-to-soil ratio with repeated sampling. The sample was immediately filtered through a 0.45 μm filter (Membrane Media: Nylon). The concentrations of CIP, OFL, and LEV in the filtrate were accurately quantified using HPLC (Agilent 1260II, USA). An Agilent TC-C18 column (250 × 4.6 mm) was used to separate the compounds. In the following detection conditions, the column temperature was uniformly set at 25 °C. The determination of CIP concentration was performed at a wavelength of 278 nm, using a mixture of 0.025 mol/L aqueous phosphoric acid (pH adjusted to 3.0 ± 0.1 with triethylamine) and acetonitrile (87:13, v/v) in the mobile phase at a flow rate of 1 mL/min. The OFL concentration was measured with a mobile phase consisting of acetonitrile and water (0.5% formic acid) in a ratio of 25:75, at a flow rate of 1 mL/min and a wavelength of 293 nm. The LEV concentration was determined by HPLC with an ultraviolet (UV) detector at 286 nm, using a mobile phase of 60:40 (v/v) acetonitrile anhydrous and formic acid at 0.1% in ultrapure water, with a flow rate of 0.75 mL/min. To ensure experimental precision, each experiment was conducted in triplicate.

One-way analysis of variance and t-tests were employed to compare the averages of variables among multiple groups and between two groups. The p-value less than 0.05 was regarded as statistical significance in this study.

The bulk density, pH, EC, and TOC of red clay soil were measured as 1.33 g/cm³, 8.26, 188.41 μS/cm, and 21.47 g/kg, respectively. The BET surface area, total pore volume, and average pore diameter of red clay soil were determined to be 54.236 m²/g, 0.062 cm³/g, and 7.557 nm, respectively. The XRD results showed that the predominant components in red clay soil included SiO₂, AlPO₄, KAl₂(AlSi₃O₁₀)(OH)₂ (Muscovite), and Fe₂O₃.

The water quality parameters of TW, LW, RW, and SW samples are listed in Table 1. The t-test results indicate that the pH in SW was significantly lower than in the other water samples. EC showed significant differences among the four water types, with the highest value in TW and the lowest in SW. Chemical oxygen demand (COD) was significantly higher in SW and lower in TW. Total suspended solids (TSS) were significantly greater in RW and lower in TW, while inorganic suspended solids (ISS) were also significantly higher in RW. Organic suspended solids (OSS) were highest in LW and lowest in SW. Total phosphorus (TP) levels were significantly higher in SW and lowest in LW. Finally, total nitrogen (TN) was significantly higher in RW and lower in TW.

It is important to note that, due to the complexity of real-world water matrices, the conclusions of the subsequent kinetic experiments should be generalized with reference to this batch of experimental samples.

The adsorption performance of red clay soil for FQCAs in PW was compared with the adsorbent materials reported in the literature, with the results presented in Table 2. The findings indicate that the adsorption capacity of red clay soil for FQCAs is superior to that of similar adsorbent materials and other synthetic materials reported in the literature. Furthermore, due to the easy availability of red clay soil and the lack of need for further processing, it has the potential to become an environmentally friendly and green adsorbent material for FQCA removal.

It is widely acknowledged that directly measuring the initial adsorption reaction rate in kinetic experiments is inherently challenging. Consequently, we utilized the average adsorption reaction rate over the first 20-s period (R₂₀, expressed in mg/g/s) as an indicator for the initial reaction rate. This was determined by dividing the adsorbed amounts of CIP, OFL, or LEV at the 20-s mark by the time interval of 20 s. Figure 1(b) illustrates that the R₂₀ for CIP was notably higher in PW (0.18 mg/g/s) compared to other water solutions. No significant difference was observed in the R₂₀ values for CIP in RW and SW. CIP exhibited a lower R₂₀ in TW and LW, with no significant differences observed in these two water solutions. For OFL, the R₂₀ was highest in PW (0.21 mg/g/s) and lowest in TW (0.11 mg/g/s). The R₂₀ of OFL was significantly greater in SW than in RW and LW. LEV showed a similar pattern, with its R₂₀ being significantly higher in PW (0.21 mg/g/s) than in other solutions, and notably higher in SW compared to TW, LW, and RW. No significant differences were found in the R₂₀ of LEV among TW, LW, and RW. These results suggest that the complex matrices of different water types can significantly influence the initial adsorption reaction rate of the three FQCAs.

FQCAs can exhibit various forms, including anionic, cationic, and zwitterionic forms, depending on the pH conditions in aqueous solutions (Xu et al. 2021; Chang et al. 2022). Specifically, CIP, OFL, and LEV predominantly exist in their cationic forms at pH values below 5.9, 6.1, and 6.0, respectively, and in anionic forms at pH values above 8.9, 8.3, and 8.2, respectively. In other pH ranges, they are predominantly found in their zwitterionic forms (Li et al. 2017a; Chen et al. 2019; Wei et al. 2021; Xu et al. 2021). The pH_PZC of red clay soil was determined to be 6.7, suggesting that the particle surfaces of red clay soil were negatively charged when the solution pH was above 6.7. As shown in Table 1, the pH values of TW, LW, RW, and SW all exceeded 7, suggesting that under these conditions, the soil particle surfaces were negatively charged. As the pH increased, the electrostatic repulsion between the anionic FQCAs and the negatively charged soil particle surfaces intensified, which might reduce the adsorption capacity and the initial adsorption reaction rate.

An elevated EC implies a higher concentration of ions in the solutions. The cations in solution were quite readily attracted to the soil particle surfaces due to electrostatic attraction, potentially leading to competitive adsorption with the FQCAs. This competition might hinder the adsorption of FQCAs by red clay soil and decrease the equilibrium adsorption amount and the initial reaction rate. Regarding the adverse impact of OSS on q_e and the initial adsorption rate, it was likely due to competitive adsorption between FQCAs and organic compounds for active adsorption sites. Further research is necessary to elucidate the detailed mechanisms of these interactions.

For CIP, q_t was significantly higher in PW compared to other water solutions, attributed to the favorable conditions for adsorption. Conversely, in TW and LW, q_t was significantly lower, which can be attributed to the negative impact of higher pH and EC on adsorption by red clay soil. In RW, q_t was significantly lower than in SW, likely due to the combined adverse effects of higher pH, EC, and OSS on adsorption. The complex compositions presented in TW, LW, RW, and SW consistently delayed the time required to reach adsorption equilibrium. Similar observations were noted for OFL and LEV (Figure 4).

Additionally, the initial factor (R_i) values for CIP, OFL, and LEV in PW, TW, LW, RW, and SW were determined as follows: 0.23, 0.28, 0.28, 0.14, and 0.27 for CIP; 0.13, 0.33, 0.29, 0.25, and 0.19 for OFL; and 0.12, 0.27, 0.25, 0.29, and 0.18 for LEV, respectively. All R_i values fall within the range of 0.1–0.5, indicating strong initial adsorption of FQCAs onto red clay soil across various water solutions (Wang & Guo 2022).

The findings from the desorption kinetic experiments show that the overall desorption efficiency of the three FQCAs from red clay soil varied across different water solutions. Specifically, the desorption efficiencies for CIP were between 1.35 and 3.09%, for OFL they ranged from 1.41 to 2.54%, and for LEV, the desorption efficiencies were between 1.40 and 2.32% (Table 4). These relatively low desorption efficiencies indicate a strong affinity of the FQCAs to the red clay soil, suggesting that the binding is not readily reversible. This characteristic is advantageous for the retention and removal of FQCAs in soil environments, potentially limiting their mobility and bioavailability. The implications of these results are significant for developing strategies to remediate and mitigate FQCA pollution in various environmental contexts.

The findings of the desorption study highlight the significant influence of water quality on the desorption dynamics of the three FQCAs from red clay soil across various aqueous solutions. Notably, higher desorption efficiencies were observed for CIP in TW and LW, for OFL in TW, LW, and RW, and for LEV in TW and LW. The ratio of desorption at 20 s to total desorption exhibited a consistent trend, with the lower q_e of the three FQCAs observed in TW and LW (Table 4).

Correlation analysis showed that pH, EC, and OSS were the potential key water quality parameters significantly affecting the total desorption, the desorption at 20 s, and the desorption q_e of FQCAs on red clay soil in various water solutions. For CIP, the PCC values with pH, EC, and OSS were 0.76, 0.90, and 0.66 for the total desorption, 0.89, 0.87, and 0.79 for the desorption at 20 s, and −0.98, −0.99, and −0.86 for the desorption q_e, respectively. Similarly, for OFL, the PCC values with pH, EC, and OSS were 0.94, 0.89, and 0.63 for the total desorption, 0.84, 0.76, and 0.75 for the desorption at 20 s; and −0.99, −0.99, and −0.82 for the desorption q_e, respectively. For LEV, the corresponding PCC values were 0.88, 0.97, and 0.73 for the total desorption, 0.82, 0.93, and 0.72 for the desorption at 20 s, and −0.99, −0.97, and −0.87 for the desorption q_e, respectively. The specific mechanisms by which these water quality variables influence the desorption of FQCAs from red clay soil require further investigation and understanding.

This study quantitatively assessed the adsorption and desorption kinetics of three FQCAs on red clay soil in various water solutions, including PW, TW effluent, LW, RW, and SW. The findings demonstrate that water quality poses a significant influence on the kinetics of both adsorption and desorption processes. Critical water quality variables, such as pH, EC, and OSS, were found to negatively affect the adsorbed mass of FQCAs at equilibrium following adsorption and the remaining mass after desorption. However, the underlying mechanisms still require further investigation to fully understand their impact on the adsorption and desorption processes. Furthermore, a broader range of real-world water matrices and water quality parameters should be investigated to gain a more comprehensive and in-depth understanding of the adsorption characteristics of FQCAs.

The adsorption and desorption processes were characterized by an initial rapid phase followed by a subsequent slower phase. Higher pH, EC, and OSS led to a significant reduction in the initial adsorption rates and overall adsorption efficiencies, while slightly improving the initial desorption and desorption efficiencies. The PSO model provided a better fit to the adsorption kinetics data compared to the PFO model, implying that the adsorption of FQCAs onto red clay soil in different water solutions likely involved chemical mechanisms. Analysis using the W–M model indicated that the rapid adsorption phase was predominantly influenced by external mass transfer, while the slower phase was mainly constrained by intra-particle diffusion, with both phases being subject to boundary layer effects. After desorption, a considerable portion of the FQCAs was found to persist within the red clay soil. These results highlight the red clay soil's substantial capacity to adsorb FQCAs, indicating its potential as an effective and environmentally friendly adsorbent for mitigating FQCA pollution in the environment. Further research should concentrate on the degradation of FQCAs following adsorption in the soil and their transport dynamics across diverse field conditions.

This work is supported by the Fundamental Research Program of Shanxi Province, China (Grant No. 202103021224082).

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.