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Article

Complex Effects of Functional Groups on the Cotransport Behavior of Functionalized Fe3O4 Magnetic Nanospheres and Tetracycline in Porous Media

1
College of Environment and Climate, Jinan University, Guangzhou 511443, China
2
Guangdong Insitute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China
3
College of Life Science and Technology, Jinan University, Guangzhou 510632, China
*
Author to whom correspondence should be addressed.
Water 2025, 17(19), 2889; https://doi.org/10.3390/w17192889 (registering DOI)
Submission received: 11 September 2025 / Revised: 28 September 2025 / Accepted: 2 October 2025 / Published: 4 October 2025

Abstract

In this study, four types of Fe3O4-based magnetic nanospheres were functionalized with distinct surface groups to examine how surface chemistry influences their co-transport with tetracycline (TC) in porous media. The functional groups investigated are carboxyl (−COOH), epoxy (−EPOXY), silanol (−SiOH), and amino (−NH2). Particles bearing −COOH, −EPOXY, or −SiOH are negatively charged, facilitating their transport through porous media, whereas −NH2-modified particles acquire a positive charge, leading to strong electrostatic attraction to the negatively charged TC and quartz sand, and consequently substantial retention with reduced mobility. Adsorption of TC onto Fe3O4-MNPs is predominantly chemisorptive, driven by ligand exchange and the formation of coordination complexes between the ionizable carboxyl and amino groups of TC and the surface hydroxyls of Fe3O4-MNPs. Additional contributions arise from electrostatic interactions, hydrogen bonding, hydrophobic effects, and cation–π interactions. Moreover, the carboxylate moiety of TC can coordinate to surface Fe centers via its oxygen atoms. Molecular dynamics simulations reveal a hierarchy of adsorption energies for TC on the differently modified surfaces: Fe3O4-NH2 > Fe3O4-EPOXY > Fe3O4-COOH > Fe3O4-SiOH, consistent with experimental findings. The results underscore that tailoring the surface properties of engineered nanoparticles substantially modulates their environmental fate and interactions, offering insights into the potential ecological risks associated with these nanomaterials.

Graphical Abstract

1. Introduction

Engineered nanoparticles (ENPs) refer to nanoscale particulates or their aggregates, with the majority of individual particles occupying the 1–100 nm size range [1]. Owing to their high reactivity, ENPs hold substantial promise for in situ groundwater remediation. However, their mobility in natural porous media is often limited, presenting a major constraint for field implementation. Consequently, advancing understanding of the fate and transport of ENPs in subsurface environments and developing strategies to enhance their transport are essential research priorities [2]. Moreover, released ENPs may act as sources of emerging contaminants, raising concerns about potential ecological and human health risks [3]. In the context of groundwater exposure, the risk posed by ENPs is governed by their fate and transport within natural porous media [4]. Magnetite (Fe3O4) is among the most prevalent iron oxides found in nature. Fe3O4 magnetic nanospheres (Fe3O4-MNPs) exhibit notable properties, including strong magnetism, a large specific surface area, and good biocompatibility, enabling diverse applications in sensing, pigment production, biomedical imaging, ferrofluid technology, and environmental remediation [5,6,7]. Nevertheless, widespread use leads to their release into aquatic and terrestrial environments through various anthropogenic pathways [8]. The presence of Fe3O4-MNPs in the environment can pose risks to aquatic life and human health, potentially affecting microbial community structure, contaminant chemistry, as well as the bioavailability and transport of both contaminants and nutrients [9]. Interactions among nanoparticles and between nanoparticles and dissolved or colloidal water constituents can drive the formation or dissociation of aggregates, contingent on physicochemical properties such as morphology, surface charge, and distribution of functional groups. The formation and breakdown of aggregates constitute among the most critical surface-driven phenomena encountered in both aquatic and terrestrial environments, and they govern many key environmental processes [10].
Tetracyclines (TC) constitute a class of antibiotics extensively employed in agriculture, animal health therapy, and human medicine [11,12,13]. Consequently, TC are widely present in the environment. More than 70% of TC are excreted and released into the environment in their active form via urine and feces [11,12,13]. Their high hydrophilicity and low volatility confer significant persistence in aquatic environments, leading to environmental and potential human health concerns, including ecological risks [14]. In heterogeneous subsurface settings, processes such as migration and transformation render antibiotic contaminants in groundwater more prone to accumulation than in other aqueous systems, complicating remediation [15]. Additionally, TC residues can promote the evolution of drug-resistant microorganisms; prolonged exposure to low antibiotic levels in the environment can foster antibiotic resistance genes (ARGs) and resistant bacteria in humans, posing health threats [16,17]. TC is an amphiphilic molecule bearing multiple ionizable functional groups (e.g., carboxyl, phenolic, ketonic, and amino groups) that are expected to interact with charged and highly polar substances [18,19]. Significant residues of TC have been detected in surface water, groundwater, sediments, and soils [14]. As a polar, ionizable organic contaminant, the fate and behavior of TC in aquatic environments are largely governed by adsorption to fixed sediments and mobile colloids [20,21,22,23]. Consequently, there exists substantial potential for interactions between dispersed Fe3O4-NPs and residual TC in aquatic environments, which may affect their degradation and transport in porous media.
Several studies have separately investigated the transport of iron oxides and antibiotics in porous media. Evidence indicates that iron oxides with varying physicochemical properties (for example, aggregation behavior, distribution of surface functional groups, and surface charge) can modulate interactions with antibiotics. Zhang et al. [24] demonstrated that functional groups on microplastics influence their aggregation, transport, and interactions with contaminants in environmental media; however, there is a lack of systematic work on how different functional groups affect the migration of iron oxides in porous media. In environmental contexts, iron oxides tend to carry a positive charge due to a relatively high point of zero charge [25,26,27,28], making electrostatic attraction a key mechanism for tetracycline (TC) adsorption, particularly between anionic TC and positively charged iron oxides [29]. Additionally, metal cations can influence TC transport and deposition via cation-bridging mechanisms in aqueous environments [30,31]. To date, no studies have examined the co-transport behavior of Fe3O4-MNPs modified with distinct functional groups alongside TC in porous media. Carboxyl and amino groups are the two most common functional groups, representing negative and positive charges, respectively [24]. Considering hydrophilicity, epoxy and silanol groups have also been included in some studies [32]. The co-transport of two constituents is substantially more complex than the transport of a single component, and investigations into the co-transport of magnetic nanoparticles are comparatively scarce. Consequently, there is a need to develop more comprehensive and detailed experimental and modeling approaches to elucidate the transport behavior and underlying mechanisms of magnetic nanoparticles coexisting with antibiotics under various surface functionalizations.
Due to the extremely low mobility of positively charged Fe3O4-MNPs colloids in porous media, four variants of Fe3O4-MNPs functionalized with different groups were employed in this study: carboxyl (−COOH), epoxy (−EPOXY), silanol (−SiOH), and amino (−NH2). Under typical environmental conditions, all but the -NH2 modified particles carry a negative surface charge, which enhances their transport through porous media. Direct evidence for Fe3O4-MNPs–TC interactions was obtained from column experiments, complemented by characterization using scanning electron microscopy (SEM, Ultra-55), ultraviolet-visible spectroscopy (UV), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FTIR). Extended Derjaguin–Landau–Verwey–Overbeek (XDLVO) calculations were performed to assess the roles of magnetic interactions and gravity on both nanosphere–nanosphere and nanosphere–sand interactions. Given the limitations of DLVO theory for non-colloidal substances, molecular dynamics (MD) simulations were conducted to elucidate the contribution of non-DLVO forces to the deposition behavior of Fe3O4-MNPs and TC in porous media.

2. Materials and Methods

2.1. Experimental Materials

To investigate the effects of surface functionality, four Fe3O4-MNP variants bearing distinct groups were employed: carboxyl (−COOH), epoxy (−EPOXY), silanol (−SiOH), and amino (−NH2). The substrates comprised Fe3O4-MNPs (particle size ~100–200 nm, initial concentration 5 mg mL−1) and tetracycline (TC, purity ≥ 98%). For stock solutions, 100 mg of each chemical were dissolved in 1000 mL of deionized water to yield 100 mg L−1 stock solutions for both Fe3O4-MNPs and TC. The stock solutions were subjected to ultrasonic agitation for 2 h to ensure thorough dispersion and stored at 4 °C. Prior to experiments, the stock solutions were diluted with DI water to attain the required working concentrations.
The porous medium used in the column experiments was quartz sand obtained from Longxin Water Purification Materials Co., Ltd. (Gongyi City, China). The sand underwent a preparatory sequence comprising initial rinsing with tap water, immersion in 10% nitric acid to remove surface contaminants and activate the material, rinsing with deionized water until the effluent reached neutral pH, and oven drying at 105 °C prior to use.
To clarify the surface properties of functionalized Fe3O4-MNPs, their composition, morphology, and charge were analyzed via multiple techniques: Fourier transform infrared spectroscopy (FTIR, Nicolet iS50, Thermo Fisher Scientific, North Brunswick Township, USA) for functional group identification, X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo Scientific, Manchester, UK) for elemental composition, and scanning electron microscopy (SEM, JSM-7800 F, JEOL, Tokyo, Japan) operated at 2.0 kV accelerating voltage (selected to avoid particle surface damage) for morphological observation. Hydrodynamic diameters of Fe3O4-MNPs, tetracycline (TC), and their mixtures at varying mixing ratios were determined by dynamic light scattering (DLS) using a laser-based nanoparticle-size analyzer. In addition, the zeta potentials of Fe3O4-MNPs, TC, and their mixtures at the same mixing ratios, as well as the surface of the quartz sand, were measured by electrophoretic methods.

2.2. Cotransport Experiments

To elucidate the co-transport of Fe3O4-MNPs and tetracycline (TC) in saturated porous media under varying experimental conditions, column-based transport experiments were conducted. The experiments utilized a glass column with an inner diameter of 2.5 cm and a length of 20 cm (Figure S1). Prior to testing, the inner surface of the column was smoothed by sandpaper polishing to minimize flow heterogeneity and prevent preferential flow. Wet packing was adopted to ensure uniform quartz sand distribution (to avoid preferential flow). First, ultrapure water was added to the column. Afterward, quartz sand was added in columns (20 g per batch) while maintaining the water level above the sand surface. During each addition, a glass rod was used to stir the sand to expel air, and a small hammer was applied for mild compaction (controlling bulk density at 1.65 g·cm−3) until the column was fully packed.
To evaluate the uniformity of column packing and to exclude potential contaminants that could affect subsequent migration experiments, the packed column was flushed with 2 pore volumes of ultrapure water to remove residual impurities. A conservative tracer test using nitrate (NO3) was conducted to confirm complete packing and to ensure that no extraneous contaminants would influence the migration measurements. The column was sequentially injected with 1 pore volume of a 0.1 mmol·L−1 KNO3 solution, followed by 8 PV of ultrapure water, using a calibrated peristaltic pump. Effluent samples were automatically collected and archived by an autosampler throughout the experiment. KNO3 concentrations in the effluent were quantified by UV–visible spectrophotometry at 220 nm.
Following the completion of tracer experiments for each column, 2 pore volumes of background solution were pumped to re-establish equilibrium in the column water chemistry. Subsequently, 2 pore volumes of the target solution, comprising a mixture of Fe3O4-MNPs and tetracycline (TC), were injected, followed by 5 pore volumes of background solution to flush the system. Seven pore volumes of effluent were collected with an automated sampler. Concentrations of TC and the four different functional-group–modified Fe3O4-MNPs were quantified using UV–visible spectrophotometry at 275 nm and 430 nm, respectively. Method details for concentration determination are provided in the Supplementary Materials. The experimental conditions are summarized in Table S1.

2.3. Interaction Energy

The conventional Derjaguin–Landau–Verwey–Overbeek (DLVO) framework is employed to estimate the interaction energy between charged colloids and the surface of the medium, thereby assessing system stability (Figure S2). The DLVO interaction energy comprises van der Waals attraction (ΦVDW) and electrostatic double-layer repulsion (ΦEDL). Additionally, the magneto-gravitational energy (ΦVM) associated with magnetic nanoparticles is incorporated into the classical DLVO description to compute the interaction energy among Fe3O4-MNPs, enabling a quantitative analysis of colloidal aggregation and dispersion behavior [12].
Moreover, gravitational forces are recognized as important determinants of colloidal deposition on the surfaces of porous media [4,33]. The extended DLVO (XDLVO) framework is employed to quantify the interaction energies among Fe3O4-MNPs and between Fe3O4-MNPs and the porous matrix, with the corresponding derivations and numerical implementations detailed in the Supplementary Materials.

2.4. Molecular Dynamics Simulation

In this study, all-atom molecular dynamics (MD) simulations were conducted to investigate the chemical and physical adsorption of tetracycline (TC) on four surfaces of functional-group-modified Fe3O4-MNPs. Simulations were carried out using Materials Studio (MS) from Accelrys. The COMPASS force field—condensed-phase optimized molecular potentials for atomistic simulation studies—was employed to describe interatomic interactions. COMPASS is a general-purpose, all-atom force field developed with state-of-the-art ab initio and empirical parameterization techniques and has been validated across a broad range of systems. Owing to its capability to accurately model interactions in organic–inorganic environments, COMPASS was selected to characterize the adsorption reactions of TC on functional-group-modified Fe3O4-MNPs. This force field has been widely used in MD calculations for organic small molecules and metal oxides [33,34]. The functional form of the COMPASS force field is provided below [35,36]:
χ t o t a l = χ a + χ b + χ c + χ d + χ e + χ f + χ g + χ h + χ i + χ j + χ k + χ l + χ m
χ a = b K 2 b b 0 2 + K 3 b b 0 2 + K 4 b b 0 2
χ b = θ H 2 θ θ 0 2 + H 3 θ θ 0 2 + H 4 θ θ 0 2
χ c = V 1 1 c o s 1 0 + V 2 1 c o s 2 2 0 + V 3 1 c o s 3 3 0
χ d = χ K χ χ χ 0 2
χ e = b b F b b b b 0 b b 0
χ f = θ θ F θ θ θ θ 0 θ θ 0
χ g = b θ F b θ b b 0 θ θ 0
χ h = b b b 0 V 1 c o s + V 2 c o s 2 + V 3 c o s 3
χ i = b b b 0 V 1 c o s + V 2 c o s 2 + V 3 c o s 3
χ j = θ θ θ 0 V 1 c o s + V 2 c o s 2 + V 3 c o s 3
χ k = θ θ K θ θ c o s θ θ 0 θ θ 0
χ l = i > j q i q j ε r i j
χ m = i > j [ A i J r i j 9 B i J r i j 6 ]
The total potential energy (χtotal) contains the bond stretching (χa), angular bending (χb), torsion (χc), out-of-plane coordinates (χd), cross-coupling (χek), Coulombic interactions (χl), and van der Waals interactions (χm) terms. K, H, F, and V are the force field parameters; b, θ, ∅, and χ represent the bond length, bending angle, torsion angle, and out-of-plane angle, respectively; the parameters b0, θ0, ∅0 and χ0 are ideal values at zero energy.
The outermost layer of the oxide substrate was hydroxylated by terminating the surface oxygen atoms in the top layer with hydrogen. Subsequently, the hydroxyl groups were randomly replaced by four functional groups-carboxyl (−COOH), epoxy (−EPOXY), silanol (−SiOH), and amino (−NH2)-to simulate surfaces modified with these functionalities. The Fe3O4 (0 0 1) surface was constructed by cleaving the crystal along the (001) plane and subjected to geometry optimization via a two-stage scheme: an initial optimization using the Steepest Descent method with a convergence criterion of 1000 kcal·mol−1, followed by refinement with the Conjugate Gradient method to a convergence criterion of 10 kcal·mol−1. Convergence was achieved before 10,000 iterations.
In the initial configuration, five energy-minimized tetracycline (TC) molecules were placed on the Fe3O4 (0 0 1) surface, with full three-dimensional periodic boundary conditions applied in the simulation box and the z axis aligned with the surface normal. The height of the simulation cell was fixed at 20 Å. Solvation was achieved with approximately 150 water molecules, and the integration time step was set to 0.25 fs [37]. The Fe3O4 (0 0 1) surface was restrained prior to the simulation. MD simulations of TC adsorption on Fe3O4 (0 0 1) surfaces modified with different functional groups were conducted at 298 K under the NVT ensemble. Temperature was controlled by the Andersen thermostat, with a time step of 1.0 fs. A total MD duration of 2000 ps was performed, capturing the trajectories of all atoms in the TC–Fe3O4 system. Equilibration of the interfacial systems was achieved within this 2000 ps window. The interfacial interaction energy between TC and the Fe3O4 (0 0 1) surfaces was computed to quantify the adhesion strength.

3. Results and Discussion

3.1. Characterization of Materials

Figure 1 displays SEM images of four types of Fe3O4-MNPs modified with distinct functional groups. All particles are approximately spherical. The carboxyl (−COOH) and epoxy (-EPOXY) modified particles show relatively uniform dispersion with slight aggregation, whereas the silanol (-SiOH) modified particles appear more dispersed. In contrast, the amino (-NH2) modified particles exhibit pronounced agglomeration, in agreement with the observed hydrodynamic diameters. At equal concentrations, the hydrodynamic diameters are 1874.93 nm for -COOH, 1755.26 nm for -EPOXY, 783.31 nm for -SiOH, and 2523.18 nmfor -NH2. Zeta potentials and hydrodynamic diameters under varying conditions are provided in Table S2. Notably, the. significant agglomeration of Fe3O4-NH2 particles (Figure 1d) is a typical manifestation of the synergistic effect between their surface charge properties and the inherent magnetism of Fe3O4-NH2. According to the zeta potential data in Table S2, Fe3O4-NH2 is the only positively charged particle. The positively charged surface significantly weakens the electrostatic repulsion between particles compared to negatively charged particles. Meanwhile, Fe3O4-NH2 itself has strong magnetism. Under the dual effects, particles tend to agglomerate, forming obvious aggregates.
The FTIR spectral images of the four different functional group-modified Fe3O4-MNPs before and after TC adsorption are shown in Figure 2. The spectrograms under different aqueous solution mixing ratios primarily exhibit three bands at 1112 cm−1, 1637 cm−1, and 3421 cm−1. During the modification of Fe3O4-MNPs, their surfaces easily saturate with hydroxyl groups in aqueous environments [38]; therefore, the spectral bands at 1112 cm−1 and 3421 cm−1 are surmised to correspond to the bending of =Fe-OH and the stretching vibrations of -OH or -NH [34,39]. A weak COO- band intensity can be observed at 1403 cm−1, while the more intense band near 1637 cm−1 may be due to the stretching of carbon double bonds (C=O) [33]. A comparison of the two plots in Figure 2 indicates that the intensities of the spectral bands at 1112 cm−1, 1637 cm−1, and 3421 cm−1 decrease after the addition of TC to the Fe3O4-MNPs solution, suggesting that TC adsorbs onto the surface of Fe3O4-MNPs and indicates that a certain degree of chemical reaction occurs.
TC is an amphiphilic molecule with multiple ionizable functional groups (Figure S3). TC1, TC2, and TC3+ represent the three protonated functional groups in the TC molecule (the tricarbonylmethane group, phenol ketone group, and dimethylamino group), respectively. In an aqueous environment, these groups can undergo protonation-deprotonation reactions to form cationic species (pH<3.3), amphipathic species (3.3<pH<7.68), and anionic species (pH>7.68) [40]. The most characteristic peaks of TC are found between 1200 and 1700 cm−1. The bands at 1637 cm−1, 1614 cm−1, and 1587 cm−1 can be attributed to amides, C=O bond stretching at ring A, and C=O bond stretching at ring C, respectively. The spectrum also displays bands at 1514 cm−1 and 1456 cm−1, attributed to amide and C-C bond stretching vibrations. The bands at 1400 cm−1 and 1230 cm−1 correspond to -CH3 deformation and C-N bond stretching vibrations, respectively [31].
The elemental composition and functional groups present on the Fe3O4-MNPs modified with different functional groups were identified by X-ray photoelectron spectroscopy (XPS), with the XPS scans of the four different functional group-modified Fe3O4-MNPs depicted in Figure 3. Three main peaks corresponding to Fe, O, and C are present in the XPS results (Figure 3). Among them, the silanol-based modified Fe3O4-MNPs show weak Si atomic intensity at 99.8–103.7 eV, and the amino-modified Fe3O4-MNPs exhibit weak N atomic intensity at 397–412 eV. Focusing only on the O1s spectra, the four different functional group-modified Fe3O4-MNPs produce peaks of varying intensities at 530 eV, inferred to be caused by lattice oxygen (Fe-O) [41]. The second peak for carboxyl- and silanol-based modified Fe3O4-MNPs appears at 532 eV, with the subpeak of carboxyl-modified Fe3O4-MNPs inferred to correspond to C=O [42]. Simultaneously, the subpeaks of silanol-based modified Fe3O4-MNPs are attributed to Si-O-Si or Si-OH [43]. The second peak of epoxy-modified and amino-modified Fe3O4-MNPs appears at 533 eV, with subpeaks of epoxy-modified Fe3O4-MNPs deduced as arising from C-O [33]. The subpeaks of amino-modified Fe3O4-MNPs are attributed to -OH [44].

3.2. XDLVO Results

To quantify the stability of Fe3O4-MNPs in porous media, we applied the Derjaguin–Landau–Verwey–Overbeek (DLVO) theory, which describes interactions between charged colloids via van der Waals attraction (ΦVDW) and electrostatic repulsion (ΦEDL), and extended it to XDLVO theory by incorporating magnetic gravitational energy and gravity energy, as Fe3O4-MNPs’ strong magnetism significantly affects their aggregation.
To investigate the aggregation and dispersion behavior of strongly magnetic Fe3O4-MNPs modified with different functional groups, we calculate the XDLVO interaction energies between Fe3O4-MNPs and between Fe3O4-MNPs and quartz sand under various experimental conditions (Figures S4 and S5). Figures S4 and S5 illustrate two scenarios: one considering the effect of gravity and the other not. Due to the strong magnetic properties of Fe3O4-MNPs, the magnetic attraction is approximately four orders of magnitude larger than the classical van der Waals attraction (ΦVDW) and double electrostatic repulsion (ΦEDL). Consequently, Fe3O4-MNPs exhibit strong aggregation behavior in aqueous environments.
The results shown in Figures S4 and S5 indicate a general weakening of the magnetic attraction between Fe3O4-MNPs as the TC concentration increases, suggesting that TC enhances the dispersion of Fe3O4-MNPs in aqueous environments. When gravity is not taken into account, a large positive energy barrier exists between the Fe3O4-MNPs and quartz sand, which prevents Fe3O4-MNPs from easily depositing on the quartz sand surface. However, due to the strong magnetic mutual attraction among Fe3O4-MNPs, the agglomerates possess large diameters and masses, differing from the generally better-dispersed nano-colloids in aqueous environments. As shown in Figure S5, the positive energy barrier between Fe3O4-MNPs and quartz sand nearly disappears when considering gravity, making it significantly easier for Fe3O4-MNPs to deposit on the surface of quartz sand. This also elucidates the phenomenon of the extremely low mobility of Fe3O4-MNPs in porous media.

3.3. The Cotransport of Fe3O4 with Different Functional Group Modifications and TC

Previous studies have shown that the adsorption of TC on Fe3O4-MNPs is mainly driven by chemisorption rather than nonspecific physisorption [45]. The adsorption of TC on Fe3O4-MNPs is attributed to several mechanisms: ligand exchange, chemical complexation between ionizable carboxyl or amino groups of TC and hydroxyl groups of iron oxides, electrostatic interactions, hydrogen bonding interactions, hydrophobic effects, and cation-π interactions [46,47,48,49,50,51]. In addition to these mechanisms, the carboxyl group in TC can coordinate to the magnetite surface in either a monodentate or bidentate manner, with coordination occurring via oxygen atoms [47,52]. The keto groups in TC also interact with the magnetite surface through electrostatic interactions or inner-sphere complexation, further enhancing adsorption efficacy [53].

3.3.1. Monotransport of Fe3O4 and TC

Figure S6 shows the breakthrough curves (BTCs) for mono-migration of four different functional group-modified Fe3O4-MNPs at a concentration of 50 mg·L−1, as well as those for TC mono-migration at varying concentrations (10, 20, and 50 mg·L−1). The shapes of the BTCs for the four types of Fe3O4-MNPs with different functional groups are distinct. For epoxy-modified Fe3O4-MNPs, the C/C0 peak reached 0.89-significantly higher than the other three types, because their epoxy ring hydrolysis exposed -OH groups that reduced electrostatic attraction with quartz sand. Their BTCs showed steady C/C0 increase after 1 PV, leveling off at 2 PV, which reflects stable transport without obvious retention.
Combined with the XDLVO theoretical analysis (Figures S4 and S5), there is strong mutual attraction between Fe3O4-MNPs modified with different functional groups and the quartz sand surface when considering gravitational forces. The order of interaction strength between Fe3O4-MNPs and quartz sand is as follows: Fe3O4-SIOH < Fe3O4-EPOXY < Fe3O4-COOH < Fe3O4-NH2. However, as shown in Table S2, due to their strong dispersion in solution, the agglomeration diameter of Fe3O4-SIOH is much smaller than that of the other three types of Fe3O4-MNPs. This suggests that Fe3O4-SIOH, having a larger contact area, is preferentially deposited on the quartz sand surface, leading to a higher amount of deposition and thereby lower mobility.
Since the amino-modified Fe3O4-MNPs are positively charged, they exhibit strong electrostatic attraction with the negatively charged TC and quartz sand, resulting in considerable retention within the column and low mobility. The BTCs for TC follow a classical shape, with the peak value of C/C0 increasing with concentration.

3.3.2. Effect of TC on Fe3O4 Transport

Figure 4 shows the BTCs of the co-migration of Fe3O4-MNPs modified with different functional groups and TC in porous media, with the associated experimental results presented in Table S3. For the three negatively charged Fe3O4-MNPs, the carboxyl and silanol group-modified variants exhibit similar variations with respect to TC concentration. Low TC concentrations promote the mobility of Fe3O4-MNPs modified with carboxyl and silanol groups, whereas high TC concentrations inhibit their mobility. In contrast, the epoxy-modified Fe3O4-MNPs show a gradual inhibition of changes in TC concentration.
As shown in Table S2, the absolute values of the zeta potential of the mixture of Fe3O4-MNPs and TC initially increase, then decrease, and subsequently increase again with rising TC concentration. This suggests that the functional groups on Fe3O4-MNPs chemically complex with and electrostatically interact with ionized TC, thereby reducing the electronegativity of TC and the mixture. Once the available reaction sites for TC on the colloidal surface of Fe3O4-MNPs become gradually saturated, continued increases in TC concentration lead to a more negative zeta potential for the mixture. As the adsorption of Fe3O4-MNPs on TC slows, the relationship between them shifts from mutual agglomeration and co-precipitation to competition for the remaining adsorption sites on the quartz sand.
Fe3O4-COOH and Fe3O4-SIOH demonstrate a sudden rise with a small peak at 2.8 PV, likely due to saturation of adsorption sites on the surface of quartz sand. The epoxy group-modified Fe3O4-MNPs may undergo a nucleophilic reaction in the aqueous environment, leading to the breaking of the oxygen atom, which opens the ring structure and exposes two -OH groups. Therefore, it can be hypothesized that the epoxy group, compared to the carboxyl and silanol groups, is more reactive to TC and thus flows out of the column with TC, exhibiting higher mobility than the other functional group-modified Fe3O4-MNPs.
Since the amino-modified Fe3O4-MNPs are positively charged, there appears to be strong adsorption between Fe3O4-NH2 and TC, as indicated by the hydrodynamic diameters presented in Table S2 and the molecular dynamics simulation results illustrated in Table 1. Consequently, TC can act as a carrier to enhance the transport capacity of Fe3O4-NH2 in porous media.

3.3.3. Effect of Fe3O4 on TC Transport

Due to the stable inner-sphere surface complexation between TC and Fe3O4-MNPs, TC exhibits significant hysteresis in TC-iron oxide interactions [51,54]. The deposition of Fe3O4-MNPs colloids bound to TC hinders TC migration in porous media. Additionally, these Fe3O4-MNPs can inhibit TC transport by providing additional deposition sites.
The carboxyl group-modified Fe3O4-MNPs exhibit stronger inhibition of TC mobility as TC concentration increases. In contrast, the other three types of functional group-modified Fe3O4-MNPs inhibit TC transport at low concentrations but enhance it at high concentrations. At low TC concentrations, it is hypothesized that TC complexes and aggregates with Fe3O4-MNPs co-deposit on the surface of quartz sand, thereby occupying adsorption sites. At high TC concentrations, the adsorption sites available on the surface of Fe3O4-MNPs for TC complexation become saturated, leading to competition between Fe3O4-MNPs and TC for adsorption sites on quartz sand. Due to their strong magnetic properties, Fe3O4-MNPs can deposit more easily onto the surface of quartz sand. The surface of carboxyl-modified Fe3O4-MNPs has more available spots for TC reaction than the other three types of Fe3O4-MNPs and has not reached full saturation at 50 mg·L−1.

3.4. The Results of Molecular Dynamics Simulation

The potential energy of a system can be expressed as the sum of valence (or bond), cross terms, and non-bonding interactions. Table 1 presents the calculation results of the adsorption energies for four different functionalized Fe3O4-MNPs with TC. The order of adsorption energy strength for these Fe3O4-MNPs concerning TC is as follows: Fe3O4-NH2 > Fe3O4-EPOXY > Fe3O4-COOH > Fe3O4-SiOH. This suggests that the amino-modified Fe3O4-MNPs have the strongest adsorption capacity for TC, primarily due to the strong electrostatic interaction between positively charged Fe3O4-NH2 and negatively charged TC. The epoxy-modified Fe3O4-MNPs exhibit the second highest adsorption stability for TC. Our MD simulation results show adsorption energy order Fe3O4-NH2 > Fe3O4-EPOXY > Fe3O4-COOH > Fe3O4-SiOH, which closely match the hydrodynamic diameter data in Table S2 (Fe3O4-NH2 had the largest agglomeration diameter) and the reliability of the adsorption energy sequence. Core drivers of higher adsorption strength: (1) Surface charge-dominated electrostatic interaction-TC was negatively charged due to the dissociation of phenol-ketone groups. Fe3O4-NH2 was the only positively charged particle, and strong electrostatic attraction significantly enhanced the adsorption energy; (2) Differences in functional group reactivity. The epoxy ring of Fe3O4-EPOXY exposed two -OH groups after hydrolysis, which could form bidentate coordination and hydrogen bonds with the carboxyl groups of TC. In contrast, Fe3O4-COOH had electrostatic repulsion with TC (both negatively charged), relying only on weak hydrogen bonds; Fe3O4-SiOH had low hydroxyl density, and its interaction with TC was dominated by van der Waals forces, resulting in the weakest adsorption.
Figure 5 presents a schematic diagram showing the elements, the distribution of electrostatic potentials, and electron densities of different functional groups. The blue region indicates negative potential, while the red region indicates positive potential. Notably, the epoxy group features two -OH group sites, making it more reactive to TC. The molecular dynamics (MD) simulation, which models the surface adsorption of Fe3O4-MNPs modified with different functional groups interacting with TC (Figure 6), conforms to the experimental results.

4. Conclusions

This study provides direct experimental evidence for the interactions between functionalized Fe3O4 magnetic nanoparticles (Fe3O4-MNPs) and tetracycline (TC) through a range of analytical techniques, including column experiments, scanning electron microscopy (SEM), ultraviolet spectrophotometry (UV), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FTIR). The findings highlight that all four types of surface-modified Fe3O4-MNPs demonstrate significant aggregation due to their strong magnetic properties, with magnetic attraction surpassing classical van der Waals forces and double electrostatic repulsion by approximately four orders of magnitude.
Notably, under conditions of high TC concentration, the magnetic attraction among Fe3O4-MNPs appears to weaken, suggesting that TC facilitates the dispersion of Fe3O4-MNPs within aqueous environments. Despite this dispersion, the large diameter and mass of Fe3O4-MNPs agglomerates—resulting from their strong magnetic mutual attraction—contrast sharply with the typically well-dispersed nature of nanocolloids in solution. Additionally, the gravitational effects during the deposition of Fe3O4-MNPs onto quartz sand surfaces are significant. When these effects are considered, the positive energy barrier between Fe3O4-MNPs and quartz sand diminishes nearly to zero, resulting in their facile deposition and markedly reduced mobility within porous media.
The order of interaction strength between the modified Fe3O4-MNPs and quartz sand was found to be Fe3O4-SiOH < Fe3O4-EPOXY < Fe3O4-COOH < Fe3O4-NH2. However, due to the high dispersion of Fe3O4-SiOH in aqueous environments, its agglomeration diameter remains lower than that of the other functionalized MNPs. The examination of contact area effects reveals that increased deposition of MNPs onto quartz sand surfaces limits their mobility. Furthermore, the strong positive charge of amino-modified Fe3O4-MNPs enhances their electrostatic attraction to negatively charged TC and quartz sand, thereby facilitating significant retention within the column.
The adsorption mechanism of TC onto Fe3O4-MNPs predominantly involves chemisorption rather than nonspecific physisorption, with the carboxyl group’s coordination to the magnetite surface potentially occurring in monodentate or bidentate forms. Molecular dynamics (MD) simulations corroborate the experimental findings, indicating that the adsorption energies for TC on modified Fe3O4-MNPs follow the order: Fe3O4-NH2 > Fe3O4-EPOXY > Fe3O4-COOH > Fe3O4-SiOH. The amino-modified Fe3O4-MNPs exhibit the highest adsorption stability, attributed to the robust electrostatic interactions between their positive charges and the negatively charged TC, while the epoxy-modified variants demonstrate the second highest stability. Overall, this research underscores the importance of functional group modification on Fe3O4-MNPs in optimizing their interaction with TC, offering insights that could inform future developments in environmental remediation strategies.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/w17192889/s1, Text S1: Determining the concentrations of Fe3O4-MNPs and TC; Text S2: XDLVO theory; Figure S1: Experimental setup of co-transport of Fe3O4-MNPs with TC; Figure S2: Nanoparticles deposited on the surface of quartz sand; Figure S3: Molecular structural formula of TC (the dashed boxed regions represent the three functional groups TC1, TC2 and TC3+ associated with the corresponding acid dissociation constants pKa); Figure S4: XDLVO interaction energy without consideration of gravity: (a) XDLVO interaction energy between Fe3O4-COOH nanoparticles; (b) XDLVO interaction energy between Fe3O4-EPOXY nanoparticles; (c) XDLVO interaction energy between Fe3O4-SIOH nanoparticles; (d) XDLVO interaction energy between Fe3O4-NH2 nanoparticles; (e) XDLVO interaction energy between Fe3O4-COOH nanoparticle and quartz sand; (f) XDLVO interaction energy between Fe3O4-EPOXY nanoparticle and quartz sand; (g) XDLVO interaction energy between Fe3O4-SIOH nanoparticle and quartz sand; (h) XDLVO interaction energy between Fe3O4-NH2 nanoparticle and quartz sand; Figure S5: XDLVO interaction energy between four kinds of functional group-modified Fe3O4-MNPs and quartz sand considering the effect of gravity: (a) XDLVO interaction energy between Fe3O4-COOH nanoparticle and quartz sand; (b) XDLVO interaction energy between Fe3O4-EPOXY nanoparticle and quartz sand; (c) XDLVO interaction energy between Fe3O4-SIOH nanoparticle and quartz sand; (d) XDLVO interaction energy between Fe3O4-NH2 nanoparticle and quartz sand; Figure S6: (a) BTCs of four kinds of functional group-modified Fe3O4-MNPs; (b) BTCs of TC with different initial concentration; Table S1: Experimental conditions of co-transport of Fe3O4-MNPs with TC; Table S2: Zeta potential and hydrodynamic diameter of Fe3O4-MNPs under different aqueous conditions; Table S3: Experimental results of co-transport of Fe3O4-MNPs with TC.

Author Contributions

Y.C.: Conceptualization, methodology, writing—original draft; M.W.: conceptualization, methodology, writing—original draft, writing—review and editing, funding acquisition, project administration; M.C.: conceptualization, methodology; Y.H.: conceptualization, methodology. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Natural Science Foundation of Guangdong Province (2023A1515012228).

Data Availability Statement

Data will be made available on request.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. The scanning electron microscopy (TEM) results of four kinds of Fe3O4-MNP modified with different functional groups: (a) Fe3O4-COOH; (b) Fe3O4-EPOXY; (c) Fe3O4-SIOH; (d) Fe3O4-NH2.
Figure 1. The scanning electron microscopy (TEM) results of four kinds of Fe3O4-MNP modified with different functional groups: (a) Fe3O4-COOH; (b) Fe3O4-EPOXY; (c) Fe3O4-SIOH; (d) Fe3O4-NH2.
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Figure 2. FTIR spectral images: (a) FTIR spectra of four kinds of Fe3O4-MNP modified with different functional groups; (b) FTIR spectra of TC and four kinds of Fe3O4-MNP modified with different functional groups after TC adsorption.
Figure 2. FTIR spectral images: (a) FTIR spectra of four kinds of Fe3O4-MNP modified with different functional groups; (b) FTIR spectra of TC and four kinds of Fe3O4-MNP modified with different functional groups after TC adsorption.
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Figure 3. The scan results obtained by X-ray photoelectron spectroscopy: Fe3O4-COOH (a,b), Fe3O4-EPOXY (c,d), Fe3O4-SIOH (e,f), Fe3O4-NH2 (g,h).
Figure 3. The scan results obtained by X-ray photoelectron spectroscopy: Fe3O4-COOH (a,b), Fe3O4-EPOXY (c,d), Fe3O4-SIOH (e,f), Fe3O4-NH2 (g,h).
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Figure 4. BTCs of cotransport of Fe3O4-MNPs with TC: (a) BTCs of Fe3O4-COOH under different TC concentrations; (b) BTCs of Fe3O4-EPOXY; (c) BTCs of Fe3O4-SIOH; (d) BTCs of Fe3O4-NH2; (e) BTCs of TC under different initial concentrations (the concentration of Fe3O4-COOH is 50 mg·L−1); (f) BTCs of TC under different initial concentrations (the concentration of Fe3O4-EPOXY is 50 mg·L−1); (g) BTCs of TC under different initial concentrations (the concentration of Fe3O4-SIOH is 50 mg·L−1); (h) BTCs of TC under different initial concentrations (the concentration of Fe3O4-NH2 is 50 mg·L−1).
Figure 4. BTCs of cotransport of Fe3O4-MNPs with TC: (a) BTCs of Fe3O4-COOH under different TC concentrations; (b) BTCs of Fe3O4-EPOXY; (c) BTCs of Fe3O4-SIOH; (d) BTCs of Fe3O4-NH2; (e) BTCs of TC under different initial concentrations (the concentration of Fe3O4-COOH is 50 mg·L−1); (f) BTCs of TC under different initial concentrations (the concentration of Fe3O4-EPOXY is 50 mg·L−1); (g) BTCs of TC under different initial concentrations (the concentration of Fe3O4-SIOH is 50 mg·L−1); (h) BTCs of TC under different initial concentrations (the concentration of Fe3O4-NH2 is 50 mg·L−1).
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Figure 5. (a) Schematic diagram of different elements; (b) Electrostatic potential and electron density of carboxyl group (-COOH); (c) Electrostatic potential and electron density of epoxy group (-EPOXY); (d) Electrostatic potential and electron density of silanol group (-SIOH); (e) Electrostatic potential and electron density of amino group (-NH2).
Figure 5. (a) Schematic diagram of different elements; (b) Electrostatic potential and electron density of carboxyl group (-COOH); (c) Electrostatic potential and electron density of epoxy group (-EPOXY); (d) Electrostatic potential and electron density of silanol group (-SIOH); (e) Electrostatic potential and electron density of amino group (-NH2).
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Figure 6. MD simulations of TC adsorption on the surface of Fe3O4-MNPs: (a) Fe3O4-COOH; (b) Fe3O4-EPOXY; (c) Fe3O4-SIOH; (d) Fe3O4-NH2.
Figure 6. MD simulations of TC adsorption on the surface of Fe3O4-MNPs: (a) Fe3O4-COOH; (b) Fe3O4-EPOXY; (c) Fe3O4-SIOH; (d) Fe3O4-NH2.
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Table 1. Energies of four different functional group-modified Fe3O4-MNPs and TC adsorption systems.
Table 1. Energies of four different functional group-modified Fe3O4-MNPs and TC adsorption systems.
Energy Type-COOH/TC-EPOXY/TC-SIOH/TC-NH2/TC
Contributions to Total Energy (kcal/mol):
Total Energy−1,209,553.541−1,217,209.116−1,211,890.528−1,218,926.599
Valence Energy
(diag. terms)
9089.44815,453.36010,052.7103309.592
Bond2665.6523497.4703870.5292071.052
Angle6641.43712,153.5756560.1221677.012
Torsion−239.040−221.967−392.768−461.485
Inversion21.39924.28214.82723.013
Valence Energy
(cross terms)
−152.778−162.002−185.945−170.904
Stretch-Stretch28.019−1.092−5.566−5.356
Stretch-Bend-Stretch−127.546−107.638−129.826−114.045
Stretch-Torsion-Stretch−6.447−5.760−5.217−4.393
Separated-Stretch-Stretch1.0520.9800.8740.860
Torsion-Stretch−24.746−33.142−24.645−23.848
Bend-Bend2.1012.5842.0330.954
Torsion-Bend-Bend−15.906−13.288−13.174−10.278
Bend-Torsion-Bend−9.307−4.646−10.424−14.798
Non-bond Energy−1,218,490.210−1,232,500.474−1,221,757.293−1,222,065.287
Van der waals−949,359.830−965,758.372−953,107.261−958,078.039
Long range correction−164.921−167.228−166.904−159.944
Electrostatic−268,965.460−266,574.873−268,483.128−263,827.304
Adsorption energy−39,943.840−40,400.630−39,180.480−40,666.840
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Cui, Y.; Wu, M.; Chen, M.; Hao, Y. Complex Effects of Functional Groups on the Cotransport Behavior of Functionalized Fe3O4 Magnetic Nanospheres and Tetracycline in Porous Media. Water 2025, 17, 2889. https://doi.org/10.3390/w17192889

AMA Style

Cui Y, Wu M, Chen M, Hao Y. Complex Effects of Functional Groups on the Cotransport Behavior of Functionalized Fe3O4 Magnetic Nanospheres and Tetracycline in Porous Media. Water. 2025; 17(19):2889. https://doi.org/10.3390/w17192889

Chicago/Turabian Style

Cui, Yiqun, Ming Wu, Meng Chen, and Yanru Hao. 2025. "Complex Effects of Functional Groups on the Cotransport Behavior of Functionalized Fe3O4 Magnetic Nanospheres and Tetracycline in Porous Media" Water 17, no. 19: 2889. https://doi.org/10.3390/w17192889

APA Style

Cui, Y., Wu, M., Chen, M., & Hao, Y. (2025). Complex Effects of Functional Groups on the Cotransport Behavior of Functionalized Fe3O4 Magnetic Nanospheres and Tetracycline in Porous Media. Water, 17(19), 2889. https://doi.org/10.3390/w17192889

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