Physicochemical aspects of adsorption efficiency of nanocrystalline ceria toward antibiotics, herbicides, and inorganic phosphates

Physicochemical aspects of adsorption efficiency of nanocrystalline ceria toward antibiotics, herbicides, and inorganic phosphates Jakub Ederer, * a Luboˇ s Vrtoch, b Petr Ryˇ s´ anek, b Matouˇ sB ´ arta, a Viktorie Neubertov´ a b and Zdeˇ nka Kolsk´ a b Nanoceria oxides (ceria, CeO 2 ) have emerged as promising materials for the effective adsorption of various pollutants, including antibiotics, herbicides, and inorganic phosphates, owing to their unique physicochemical properties. This study explores the adsorption efficiencies of nanoceria samples synthesized using various laboratory procedures. The adsorption behavior of cephalexin (CEF), 2,4dichlorophenoxyacetic acid (2,4D), and inorganic phosphate (IP) was evaluated using the Freundlich, Langmuir, and Langmuir–Freundlich isotherm models. The results showed that the adsorption of IP, 2,4D, and CEF followed the Freundlich and Langmuir–Freundlich isotherms with maximum adsorption capacities of 68.6 mg g −1 (IP), 65.9 mg g −1 (CEF), and 83.4 mg g −1 (2,4D), respectively. IP, 2,4D, and CEF adsorption on ceria samples follow pseudo-second-order kinetics, suggesting that the adsorption rate relies on the availability of adsorption sites and is controlled by chemisorption. The specific interaction of pollutants with ceria samples was evaluated by FTIR. The calculated Gibbs free energy (DG°) values indicate that the adsorption of pollutants onto the ceria materials is a spontaneous process. Among the tested samples, Ce-PER and Ce-AMN demonstrated superior adsorption capacity due to their high surface area. These results highlight the promise of ceria materials as highly versatile and effective adsorbents for removing various pollutants in the water treatment process. 1. Introduction The increasing contamination of water by antibiotics, herbicides, and inorganic phosphates has become a signicant environmental challenge due to their persistence, toxicity, and potential ecological problems. Compounds such as 2,4-dichlorophenoxyacetic acid (2,4D), cephalexin (CEF), and inorganic phosphate (IP) are frequently detected in wastewater and natural water bodies due to their widespread use in agriculture and healthcare. 1 Their persistence and potential toxicity have prompted research into novel adsorbents that can efficiently capture these contaminants. Antibiotics contribute to the spread of antibiotic-resistant bacteria, 2 while herbicides disrupt ecological balance and biodiversity. 3 Inorganic phosphates, on the other hand, lead to eutrophication, resulting in algal blooms and oxygen depletion in aquatic systems. 4 These pollutants cannot be completely removed by conventional water treatment methods such as chemical precipitation, coagulation, and ltration; therefore, the search for innovative materials and technologies capable of solving this problem is being developed. 5,6 Nanomaterials offer a promising solution to the limitations of traditional water treatment systems. Due to their high surface area, tunable surface chemistry, and unique catalytic properties, nanomaterials exhibit exceptional adsorption capacities. 7–9 Among various nanomaterials studied for environmental remediation, cerium oxide (CeO 2 , ceria) has attracted signicant attention due to its unique physicochemical properties, such as high surface area, tunable redox activity, and exceptional catalytic behavior. 10,11 Ceria is well known for its facile switchability between the Ce 3+ and Ce 4+ valence states, 12 promoting the high mobility of lattice oxygen and extending ceria from an ordinary adsorbent to a catalyst/reactive adsorbent 13 and making ceria a potentially highly effective adsorbent/ catalyst for removing various hazardous contaminants from water. 5,14,15 Nanoceria offers signicant advantages over other adsorbents and nanomaterials due to its unique redox properties, high oxygen storage capacity, and regenerative antioxidant behavior. A wide variety of materials have been studied for removing 2,4D, CEF and IP, including magnetite/ceria composites, 16 palygorskite, 17 zeolite/MnO 2 nanoparticles, 18 and Al 2 O 3 @Fe 2 O 3 (ref. 19) or other materials, 4,20,21 demonstrating diverse removal capacities and adsorption properties for CEF, 2,4D and IP depending on their composition and structural properties. a Faculty of Environment, Jan Evangelista Purkynˇ e University, Pasterurova 3632/15, 400 96 ´ Ust ´ ınad Labem, Czech Republic. E-mail: [email protected]; Fax: +420475-284-170; Tel: +420-475-284-111 b Faculty of Science, Jan Evangelista Purkynˇ e University, Pasteurova 3632/15, ´ Ust ´ ınad Labem, 400 96, Czech Republic Cite this: RSC Adv.,2025,15, 38391 Received 22nd July 2025 Accepted 7th October 2025 DOI: 10.1039/d5ra05301c rsc.li/rsc-advances © 2025 The Author(s). Published by the Royal Society of Chemistry RSC Adv.,2025,15,38391–38405 | 38391 RSC Advances PAPER Open Access Article. Published on 14 October 2025. Downloaded on 10/27/2025 10:03:53 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online View Journal | View Issue Unlike the previously mentioned nanomaterials, which oen suffer from insufficient adsorption capacity, complex synthesis (e.g., composite materials), or high economic costs, nanoceria exhibits a strong affinity for various contaminants, including heavy metals and organic pollutants 22–24 while maintaining excellent stability under different environmental conditions. Their prolonged reusability and enhanced efficiency of ceria in pollutant removal and catalysis further increase their attractiveness. Furthermore, their biocompatibility and low toxicity make them superior to other metal-based nanomaterials, making them highly suitable for applications in water purication, 15 biomedical applications, 25 and environmental remediation. 5,26 2,4D (Fig. 1A) is a widely used herbicide that targets broadleaf weeds in agricultural and residential environments. While highly effective, it poses environmental risks due to its potential to contaminate soil, water, and air through runoff, spray dri, and leaching. 27,28 The World Health Organization (WHO) classies 2,4D as moderately hazardous (class II) and limits its concentration in drinking water to 70 mgL −1 to protect human and animal health. 28 Runofffrom treated areas can pollute nearby water bodies, harming aquatic ecosystems by affecting non-target species and causing bioaccumulation in the food chain, as well as groundwater contamination. 29,30 Long-term human exposure has been linked to endocrine disruption and increased cancer risk, underscoring the importance of careful management. 3,31 Similarly, CEF (Fig. 1B), a rst-generation cephalosporin antibiotic used for treating bacterial infections, 17,32 poses environmental challenges when inadequately removed by wastewater treatment plants. 33 Residual CEF entering natural waters poses a threat to aquatic life and promotes antimicrobial resistance, a signicant global health concern. 34,35 IP (Fig. 1C), commonly found as phosphate salts such as calcium and sodium phosphate, is essential for biological functions, including energy transfer via ATP, nucleic acid formation, and bone health. 4 Environmentally, phosphate acts as a vital nutrient for plant growth, but excessive runoff from fertilizers and detergents leads to eutrophication, harmful algal blooms, oxygen depletion, and ecosystem damage. 19,36 Together, 2,4D, CEF, and IP illustrate how chemical contamination and nutrient overload create complex environmental and health challenges. Addressing these issues requires integrated approaches involving improved chemical use, advanced wastewater treatment, pollution monitoring, and regulatory measures to balance agricultural productivity with the protection of ecosystems and public health. In this study, a set of nanocrystalline ceria samples prepared using various synthesis methods was employed to evaluate their potential for removing 2,4D, CEF, and IP from a model water system. The mutual interaction of ceria with selected pollutants (IP) was studied using Fourier transform infrared spectroscopy (FTIR), X-ray uorescence spectroscopy (XRF), and powder X-ray diffraction (XRD). The prepared ceria materials were characterized using advanced analytical techniques, including scanning electron microscopy (SEM), XRD, determination of surface area and porosity, dynamic light scattering (DLS), electrokinetic potential measurements, and acid–base titrations. As shown, the prepared ceria samples exhibit good adsorption capacity toward IP, CEF, and 2,4D. This research may help drive the development of eco-friendly and effective water treatment technologies, thereby contributing to a cleaner and safer environment. The present study systematically investigates the adsorption of several classes of pollutants onto nanocrystalline ceria within a model aqueous system. Our goal is to elucidate the fundamental adsorbent–pollutant interactions, thereby creating a solid foundation for future research focused on the material's performance in real water and groundwater. Our study was designed to establish a baseline understanding of the adsorbent's intrinsic capabilities, providing a solid foundation for future research. The results highlight the importance of exploring specic applications of nanoceria for pollutant removal, which is crucial for developing efficient, scalable, and environmentally friendly water treatment strategies. 2. Experimental 2.1. Chemicals Analytical gradient-grade chemicals were used in this study. Cerium nitrate hexahydrate, cephalexin hydrate (CEF), 2,4-dichlorophenoxyacetic acid (2,4D), and ammonium bicarbonate were purchased from Sigma-Aldrich (Germany). Disodium phosphate (IP) was purchased from Lach-Ner Inc. For HPLC measurements, methanol, acetonitrile, and formic acid were used and purchased from VWR Inc. (Czech Republic). Deionized water from the GORO Pharmpur system (Goro, Prague, Czech Republic) with mixed-bed ion exchange purication was used. The Spectroquant® Phosphate test purchased from Merck Fig. 1 Structure of 2,4D (A), CEF (B), and IP (C). 38392 |RSC Adv.,2025,15,38391–38405 © 2025 The Author(s). Published by the Royal Society of Chemistry RSC Advances Paper Open Access Article. Published on 14 October 2025. Downloaded on 10/27/2025 10:03:53 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online Millipore (Germany) was used for the determination of free inorganic phosphate. 2.2. Ceria preparation Nanocrystalline ceria was prepared using wet chemical methods with cerium(III) nitrate hexahydrate and deionized water, as described elsewhere. 37 Samples were labeled by synthesis method: Ce-AMN (ammonium hydroxide precipitation), CeCARB (cerium carbonate precipitation and annealing), CeHMT (hexamethylenetetramine precipitation), Ce-PER (reux of peroxo-complexes), and Ce-UREA (urea precipitation and annealing). 2.3. Characterization of samples The FTIR spectra were obtained using VERTEX 70v Infrared spectrometer (Bruker, Germany) in diffuse reectance mode (DRIFT) within the 4000–400 cm −1 wavenumber range with 64 scans per spectrum and a 4 cm −1 resolution. Raw FTIR data were processed by OPUS soware (v. 8.7). The obtained data were further processed using MicrosoExcel 2021, OriginPro 2024, and Plot v2. X-ray diffraction analysis (XRD) was performed on the Panalytical X'Pert PRO diffractometer in symmetrical reection mode (Cu Ka=1.5418 Å radiation, 40 kV, 30 mA) and the X'Celerator 1-dimensional detector. Changes in the chemical composition of the samples were analyzed using a wave-dispersive X-ray uorescence spectrometer (XRF) Rigaku Primus IV with SQX soware and a standardless method of fundamental parameters. This method allows to measure the concentration of elements in the range F–U in concentration from ppm to 100%. The relative error of measurement is approximately 5%. Samples were analysed in the form of pressed tablets. To determine the sample-specic surface area (SSA) and pore volume, nitrogen adsorption/desorption isotherms were used. Samples were degassed at 50 °C for 24 hours. Aerwards, 66points adsorption and desorption isotherms were recorded with nitrogen (99.999%, Linde) at liquid nitrogen temperature using an Anton Paar Instrument NOVA 3200e. Surface morphology was analyzed using a scanning electron microscope SU5000 (Hitachi, Japan). See SI for more details. 2.4. Surface acid–base characterization and zeta potential determinations The slightly modied acid–base titration method published elsewhere 38 was used to evaluate the number of surface hydroxyl groups and pH(PZC) using an automatic titrator (794 Basic Titrino, Metrohm, Switzerland) with potentiometric endpoint determination. Zeta potential and particle size distribution of the synthesized ceria materials were analyzed using the Litesizer™500 (Anton Paar, Austria). Electrophoretic light scattering (ELS) was applied to determine the zeta potential, and dynamic light scattering (DLS) was employed to determine the hydrodynamic diameter and size distribution of the ceria nanoparticle suspensions. The system was integrated with a Metrohm automatic titrator featuring an 867 pH module and 846 dosing interface, all operated via Kalliope™soware. See SI for more details. 2.5. Batch adsorption experiment Adsorption studies were realized in batch experiment mode using a 100 mL Pyrex bottle with 50 mg of ceria sample and 50 mL stock solution of 2,4D, CEF, or IP (concentration ranges: 10–100 mg L −1 for IP and 2,4D, 5–125 mg L −1 for CEF). The experiment was carried out for 3 hours (IP) and 24 hours (CEF and 2,4D) at 25 ±1 °C to ensure that equilibrium was reached. The equilibration time was chosen based on our previous experience and adsorption kinetic measurements. 16,39 The free IP concentration was determined using a commercially available phosphate kit and the ammonium molybdate spectrophotometric method at 880 nm. The concentration of free CEF and 2,4D were measured using HPLC, and the concentration of IP/CEF/2,4D was calculated using a previously obtained calibration curve. The adsorbed amount of selected pollutant on the ceria samples at equilibrium q E (mg g −1 ) was calculated using the following equation (eqn (1)): qE¼ðc0cEÞV m(1) where c 0 and c E (mg L −1 ) are the initial and equilibrium concentrations, V(L) is the initial volume of the pollutant solution and m(g) is the mass of the ceria used in the experiment. 2.6. Adsorption kinetics To study adsorption kinetics, the Pyrex bottle (100 mL) was used along with 100 mg of ceria samples. In all experiments, a freshly prepared stock solution of pollutants (IP, CEF, 2,4D) was used with an initial concentration of 100 mg L −1 and a volume of 100 mL. At predetermined intervals for IP (5, 10, 15, 30, 60, 120, and 180 min) and CEF/2,4D (30, 40, 60, 120, 180, 240, 300, 360 and 1440 min), 1.5 mL aliquots were collected into Eppendorf vials (2 mL) and centrifuged (4 min/6000 rpm). The concentration of free IP was measured spectrophotometrically at 880 nm using a commercial phosphate kit. The concentration of free CEF and 2,4D were measured using HPLC. These concentrations were assessed based on a previously established calibration curve. The amount of adsorbed IP/CEF/2,4D at time q t (mg g −1 ) was calculated using eqn (2). qt¼ðc0ctÞV m(2) where c 0 and c t (mg L −1 ) are the initial and equilibrium concentrations in time t(min), V(L) is the initial volume of the phosphate solution, and m(g) is the mass of the ceria sample used in the kinetics experiment. 2.7. Analytical methods The HPLC analysis of CEF and 2,4D was realized using the LaChrom HPLC system (Merck Hitachi) consisting of a L-7100 pump, L-7400 variable wavelength UV/vis detector operating at © 2025 The Author(s). Published by the Royal Society of Chemistry RSC Adv.,2025,15,38391–38405 | 38393 Paper RSC Advances Open Access Article. Published on 14 October 2025. Downloaded on 10/27/2025 10:03:53 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online 262 nm (CEF) and 230 nm (2,4D), and a Rheodyne 7725i injection valve with a 20 mL sampling loop was used. CEF HPLC analysis was realized on Arion® Polar C18 column (100 ×4.6 mm, 5 mm) in the isocratic elution mode with the 15 mM NaH 2 PO 4 pH =3.3/methanol (65/35) as the mobile phase (1.0 mL min −1 ). The HPLC analysis of 2,4D was performed in isocratic elution mode with acetonitrile/water (50/50) as the mobile phase (1.0 mL min −1 , water contains 0.1% HCOOH), and the SIELC Newcrom A column (150 ×4.6 mm, 5 mm) was used. 3. Results and discussion 3.1. Characterization of samples The N 2 adsorption/desorption isotherms and pore size distribution are presented in Fig. 2B and Table 1. The nitrogen adsorption/desorption isotherms exhibit a typical type IV behavior, characteristic of porous and mesoporous materials with relatively small particle size (Fig. 2B). These isotherms exhibit hysteresis loops classied by IUPAC (1985) as types H4 and H2, 40 which is typical for microporous and mesoporous materials. The results align well with those published in ref. 37. It is evident that the annealing/drying temperature and synthesis procedure signicantly inuence the specic surface area (Table 1). The X-ray diffraction (XRD) patterns of the prepared samples are shown in Fig. 2A. A reduction in diffraction line sharpness is observed for Ce-HMT, Ce-AMN, and CePER samples. All diffraction lines correspond to the characteristic face-centered cubic uorite-type structure, with peaks assigned to the (111), (200), (220), (311), (222), (400), (331), and (420) planes, located at 28.761°, 33.281°, 47.748°, 56.561°, 59.157°, 69.594°, 76.729°, and 79.108°, respectively (ICDD PDF 34-0394). The average cubic crystallite size (CCS), ranging from 3 to 14 nm, was calculated from the broadening of diffraction lines using Scherrer's analysis. The calculated CCS aligns well with previously reported data for similar ceria materials. 25,37 The nanostructure morphology of the prepared ceria samples was studied by SEM (Fig. S1). Ce-UREA exhibits akelike particles assembled into bigger agglomerates without uniform shape. Ce-CARB exhibits plate-like aggregates formed by hexagonal-shaped particles. Ce-HMT, Ce-PER, and Ce-AMN show irregular aggregates with a random distribution of size and shape, consisting of very small primary nanoparticles (as conrmed by XRD). The chemical composition of ceria obtained by XRF measurements is summarized in Table S2. The XRF and XRD (Fig. S5) analyses conrmed the presence of P-containing moieties on the ceria aer IP adsorption. The phosphorus content in the ceria samples increased from an initial 0.0% to approximately 1.2 wt%. The complete elemental composition, as determined by XRF, is summarized in Table S2. Some other elements (e.g., F, SiO 2 ) were found only in minor amounts (see detailed results in Table S2). 3.2. Ceria surface acid–base characteristic The number of surface hydroxyl groups (q OH ) was evaluated from titration curves, while the pH(PZC) was obtained from TOTH curves, representing the total concentration of protons consumed in the titration process. Additional information can be found elsewhere, 38 and the calculated data are summarized in Table 2, Fig. S2A and B. Zeta potential determinations were used to evaluate the surface charge and stability of ceria samples in aqueous solution. Fig. S3A shows the ceria sample's Fig. 2 XRD patterns of ceria samples (A) and pore size distribution obtained from DFT analysis and the nitrogen-desorption BET isotherms of ceria samples (B). Table 1 The specific surface area (SSA) and total pore volume (V pore ) of prepared samples obtained from nitrogen-desorption using BET and DFT analysis. Mean cubic crystallite size of ceria samples calculated from XRD data Sample SSA (m 2 g −1 )V pore (cm 3 g −1 ) Cubic crystallite size a (nm) Ce-CARB 69.8 1.4 0.062 0.001 13.5 Ce-UREA 60.6 1.5 0.072 0.001 11.6 Ce-PER 179.7 0.6 0.136 0.001 3.1 Ce-HMT 30.0 1.4 0.076 0.004 8.7 Ce-AMN 132.4 5.4 0.142 0.003 4.6 a The average deviation is ±2.5 nm. 38394 |RSC Adv.,2025,15,38391–38405 © 2025 The Author(s). Published by the Royal Society of Chemistry RSC Advances Paper Open Access Article. Published on 14 October 2025. Downloaded on 10/27/2025 10:03:53 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online zeta potential as a function of pH, and the calculated data are listed in Table 2. The isoelectric point (IEP) of ceria samples was evaluated from the plot in Fig. S3A, and the values are summarized in Table 2. The hydrodynamic particle size distribution of ceria samples measured using dynamic light scattering (DLS) is presented in Fig. S3B. Table 2 presents the average particle size (diameter) and the polydispersity index (PDI). A PDI value below 0.30 indicates a uniform particle size distribution and good suspension stability of the ceria samples in water at their native pH. 41 See SI for more details. The nature and behavior of ceria in aqueous solutions is related to pH(PZC), which is the determining parameter for identifying the surface charge. From the pH(PZC) value, it can be determined whether the ceria surface will be positively or negatively charged at a given pH. The pH(PZC) remains similar, except for the Ce-HMT sample. The Ce-HMT higher pH(PZC) value could be associated with the remaining HMT residues. The calculated number of hydroxyl groups and pH(PZC) values nicely correlated with data published elsewhere. 37 3.3. Adsorption isotherm data The adsorption isotherms were used to describe 2,4D, CEF, and IP adsorption. The most widely used isotherm models, Freundlich (F), Langmuir (L), and Langmuir–Freundlich (LF) mathematical models were used in this work. Freundlich (eqn (3)), Langmuir (eqn (4)), and Langmuir–Freundlich (eqn (5)) isotherm models in non-linear form can be expressed by eqn (3)–(5): 42 q E =K F ×c E1/n F (3) qE¼qM KLcE 1þKLcE (4) qE¼qM ðKLFcEÞnLF 1þðKLFcEÞnLF (5) where q E is the equilibrium amount of given pollutant adsorbed per unit weight of ceria (mg g −1 ), q M is the maximum adsorption capacity (mg g −1 ), K F ,K L and K LF are the Freundlich ((mg g −1 ) (mg L −1 ) 1/n F ), Langmuir (L mg −1 ) and Langmuir–Freundlich (L mg −1 ) adsorption constants, respectively; n F is the adsorption intensity, and n LF is the heterogeneity parameter. The extrapolated experimental data with Freundlich, Langmuir and Langmuir–Freundlich mathematical models are presented in Fig. 3, and the data are summarized in Tables 3–5. The data obtained by non-linear tting are summarized in Tables 3–5, along with the maximum values of q M for each individual sample. The data suggested that the LF isotherms well-tted to measure data for CEF and 2,4D (Tables 4 and 5), which nicely correlated with information published elsewhere. 16 In contrast, the IP adsorption (Table 3) can be welldescribed by F (Ce-UREA, Ce-HMT, and Ce-AMN) and LF (CeCARB and Ce-PER) isotherm models considering R 2 under the concentration range studied. The heterogeneity parameter (n F ) is used to classify the adsorption process as chemical (n F < 1), physical (n F > 1), or linear (n F =1). A value of 1/n F < 1 or 1/n F > 1 indicates normal adsorption, whereas other values suggest cooperative adsorption. The measured values of n F > 1 and 1/n F < 1 conrm that the adsorption process is predominantly physical and that the Freundlich isotherm model is favorable for Ce-UREA, Ce-HMT, and Ce-AMN samples in the adsorption of IP. Using the R L parameter from the Langmuir isotherm, the favorable parameter K L c 0 was calculated (eqn (6)) KLc0¼1 RL1(6) The values of K L c 0 can be categorized into ve intervals, each corresponding to distinct adsorption isotherm shapes as described in ref. 43. When K L c 0 =0 isotherm is linear. For K L c 0 values between 0.1 and 1, the isotherm is considered pseudolinear, and between 1 and 10, the isotherm is considered favorable, while 10 < K L c 0 < 100 is classied as highly favorable, and 100 < K L c 0 < 1000 is pseudo-irreversible. 43 The calculated K L c 0 values (data not shown) for 2,4D and all ceria samples indicate a favorable and pseudo-linear isotherm. However, for IP and CEF, the K L c 0 values ranged from 1 to 770, suggesting favorable, highly favorable, and pseudo-irreversible isotherms, respectively. 43 The Langmuir constant K L was used to calculate the separation factor R L , and the data are presented in SI (see Fig. S4). The R L value indicates the nature of the adsorption to be either irreversible if R L =0, favorable if 0 < R L < 1, linear if R L =1, or unfavorable if R L >1. 43 The R L value below 1 indicates that the adsorption process becomes more favorable with increasing CEF, 2,4D, and IP concentration, conrming the high ceria affinity towards selected pollutants. The data summarized in Table 6 compare the adsorption capacities for 2,4D, CEF, and IP across various materials, Table 2 pH of isoelectric point (IEP) and average particle size, polydispersity index (PDI) calculated from DLS and the number of surface hydroxyl groups calculated from titration curves and pH(PZC) for the prepared ceria samples Sample pH (IEP) Average particle size from DLS SE (nm) PDI SE q OH SE (mmol g −1 ) pH(PZC) Ce-CARB 6.12 1667 46 29.9 1.4 0.190 + 0.007 4.6 Ce-UREA 6.87 2021 121 27.4 0.7 0.198 + 0.012 4.6 Ce-PER 9.40 754 21 28.1 1.6 0.137 + 0.010 4.5 Ce-HMT 8.21 1084 44 24.4 0.4 0.402 + 0.011 8.1 Ce-AMN 5.21 1261 69 21.4 1.2 0.112 + 0.004 4.9 © 2025 The Author(s). Published by the Royal Society of Chemistry RSC Adv.,2025,15,38391–38405 | 38395 Paper RSC Advances Open Access Article. Published on 14 October 2025. Downloaded on 10/27/2025 10:03:53 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online including those prepared and used in this study. The data for selected pollutants exhibited comparable or partly lower adsorption capacities relative to other materials. The higher q M value can be attributed to a comparable or higher BET surface area or to different adsorption mechanisms occurring in other samples. This is related to adsorption conducted under native conditions, i.e., no pH adjustment, thereby reecting the inherent properties of the ceria samples. 3.4. Adsorption mechanisms of pollutants The F model for IP adsorption suggested non-ideal and reversible adsorption at heterogeneous surfaces. Several studies 47–49 have linked the phosphate adsorption mechanism to the formation of insoluble CePO 4 species (conrmed by FTIR), resulting from the reaction of Ce 3+ ions present at defect sites within the ceria crystal lattice. As reported by Ko et al., 48 the cerium oxidation state changes from Ce 4+ to Ce 3+ with an increasing concentration of phosphate adsorbed on ceria, leading to the formation of CePO 4 through the reaction between Ce 3+ and phosphate. The XRF and XRD analyses conrmed the presence of P-containing moieties on the ceria surface, suggesting the formation of CePO 4 . Following the adsorption of IP, the phosphorus content in the ceria samples increased from an initial 0.0 mass% to approximately 1.2 mass%. The complete elemental composition, as determined by XRF, is summarized in Table S2. From the XRD pattern (see Fig. S5), the presence of crystalline CePO 4 and hydrogen phosphate is evident. The increased q M for IP in certain samples is likely associated with a higher number of Ce 3+ sites, which bind phosphate preferentially over Ce 4+ sites. 47 Our results show that Ce-UREA Fig. 3 Adsorption isotherms: Freundlich (––), Langmuir (/), and Langmuir–Freundlich (–-–) for IP, CEF, and 2,4D on prepared ceria samples. Table 3 Freundlich, Langmuir, Langmuir–Freundlich model constants and correlation coefficients for the adsorption of IP by nanoceria Sample Freundlich Langmuir Langmuir–Freundlich K F (mg g −1 ) (mg L −1 ) 1/n F n F R 2 K L (L mg −1 )q M (mg g −1 )R 2 K LF (L mg −1 )q M (mg g −1 )n LF R 2 Ce-CARB SE 10.5 4.89 0.7786 0.56 22.8 0.8669 0.61 21.8 1.35 0.8729 1.73 1.24 0.17 1.36 0.16 1.71 0.65 Ce-UREA SE 14.1 5.56 0.8154 1.67 25.4 0.4410 4.0 a 29.9 0.34 0.5640 1.86 1.22 1.29 2.33 0.0 12.4 0.55 Ce-PER SE 33.7 6.63 0.9127 5.92 52.3 0.9157 1.60 68.6 0.35 0.9351 2.93 1.28 2.51 3.11 3.89 23.2 0.18 Ce-HMT SE 11.5 8.24 0.9030 1.86 17.8 0.7288 4.0 a 20.3 0.36 0.7954 0.64 1.12 0.66 0.75 0.0 4.92 0.36 Ce-AMN SE 13.8 4.15 0.8808 0.57 32.5 0.5657 4.0 a 32.8 0.41 0.6161 2.02 0.76 0.48 4.07 0.0 7.30 0.37 a Fixed parameter. 38396 |RSC Adv.,2025,15,38391–38405 © 2025 The Author(s). Published by the Royal Society of Chemistry RSC Advances Paper Open Access Article. Published on 14 October 2025. Downloaded on 10/27/2025 10:03:53 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online and Ce-PER (12 at% Ce 3+ ) align with this observation. However, Ce-HMT sample (16 at% Ce 3+ ) does not follow this trend, which may be attributed to the inaccessibility of Ce 3+ sites, potentially hindered by the presence of –OH groups (Ce-HMT has the highest number of –OH groups; see Table 2). The elemental surface composition obtained by XPS measurement is summarized in Table S1. IP is likely to adsorb readily onto naturally positively charged samples, whereas the adsorption of negatively charged IP anions may be suppressed (on negatively charged ceria). 50 Conversely, the decrease in phosphate adsorption at higher pH levels is a well-documented phenomenon for sesqui(hydr)oxide adsorbents. 51 At higher pH levels, intense competition between PO 43− species and OH − ions is likely present, leading to signicant repulsion between phosphate and hydroxyl ions and thereby reducing phosphate adsorption. 19 This reduction can be explained by (1) the conversion of surface hydroxyl groups from the highly reactive M–OH 2+ form to the less reactive M–OH, where M represents a metal atom, and (2) the competitive interaction with OH − ions. 51 The higher adsorption capacities of Ce-AMN and Ce-PER for CEF and 2,4D are likely attributed to their high specic surface area to crystallite size ratio and bigger pore volume (V pore ), which can be benecial for adsorption. In contrast, Ce-HMT did not exhibit such a high adsorption capacity despite having the highest number of surface hydroxyl groups and being the only ceria with an alkaline value of pH(PZC). This suggested a partly negative effect of hydroxyl groups and the potential unavailability of Ce 3+ /Ce 4+ sites, which may play a crucial role in the adsorption process. At alkaline pH, CEF and 2,4D exist in their anionic forms, which could hinder adsorption due to electrostatic repulsion between the negatively charged ceria surface and the anionic forms of CEF and 2,4D. The LF model suggests that ceria behaves as a material with heterogeneous surfaces and unequal bonding sites at lower pollutant concentrations. However, at higher concentrations, the ceria surface exhibits identical and equivalent sites, with anite number of these sites available and monolayer adsorption capacity. The q M values for CEF increase in the following order: Ce-UREA < Ce-CARB < Ce-HMT < Ce-PER < Ce-AMN. The highest q M observed for Ce-AMN can be attributed to its secondlargest SSA (132.4 m 2 g −1 ), pore volume (0.142 cm 3 g −1 ), and smallest CCS (4.6 nm), all of which enhance the adsorption of both CEF and 2,4D. The primary mechanism driving CEF adsorption involves electrostatic interaction between the CEF zwitterion and positively/negatively charged –OH groups on the ceria surface. The solution pH and the surface charge of ceria play a crucial role in this process. According to Sutherland, 52 optimal adsorption occurs when CEF exists in its zwitterionic form. A similar adsorption mechanism, i.e., electrostatic Table 4 Freundlich, Langmuir, Langmuir–Freundlich model constants and correlation coefficients for the adsorption of CEF by nanoceria Sample Freundlich Langmuir Langmuir–Freundlich K F (mg g −1 ) (mg L −1 ) 1/n F n F R 2 K L (L mg −1 )q M (mg g −1 )R 2 K LF (L mg −1 )q M (mg g −1 )n LF R 2 Ce-CARB SE 9.81 5.56 0.8517 0.71 20.4 0.9263 0.56 21.9 0.73 0.9406 1.28 1.08 0.19 0.82 0.23 1.90 0.20 Ce-UREA SE 7.33 4.40 0.8578 0.27 19.8 0.9320 0.25 20.6 0.86 0.9348 1.18 0.83 0.06 0.90 0.09 2.09 0.26 Ce-PER SE 36.3 5.10 0.8459 4.28 68.3 0.9244 6.91 65.9 2.63 0.9608 4.41 1.01 1.12 4.10 0.70 2.99 0.70 Ce-HMT SE 23.9 9.94 0.6346 6.13 34.9 0.6225 4.14 40.7 0.36 0.6512 2.90 3.47 2.93 2.91 16.1 27.1 0.68 Ce-AMN SE 43.7 4.39 0.9301 3.53 81.7 0.9642 1.64 94.3 0.62 0.9845 3.64 0.58 0.78 3.74 0.70 7.58 0.10 Table 5 Freundlich, Langmuir, Langmuir–Freundlich model constants and correlation coefficients for the adsorption of 2,4D by nanoceria Sample Freundlich Langmuir Langmuir–Freundlich K F (mg g −1 ) (mg L −1 ) 1/n F n F R 2 K L (L mg −1 )q M (mg g −1 )R 2 K LF (L mg −1 )q M (mg g −1 )n LF R 2 Ce-CARB SE 8.77 2.11 0.7849 0.07 67.7 0.8579 0.15 46.7 3.19 0.9545 2.87 0.46 0.03 11.1 0.01 2.60 0.74 Ce-UREA SE 7.70 1.96 0.8834 0.06 70.5 0.9427 0.12 50.5 1.87 0.9761 1.97 0.31 0.02 8.02 0.01 3.21 0.31 Ce-PER SE 0.84 0.81 0.9226 0.10 a 50.6 0.4559 0.05 70.6 3.17 0.9944 0.44 0.11 0.0 8.63 0.002 3.65 0.28 Ce-HMT SE 4.92 2.54 0.9823 0.07 29.2 0.9566 4.4 ×10 −3 72.8 0.52 0.9843 0.42 0.15 0.01 1.81 0.01 62.3 0.14 Ce-AMN SE 0.29 0.50 0.9686 0.10 a 74.3 0.4445 2.8 ×10 −2 463.5 c (83.4 b ) 2.21 0.9691 0.14 0.05 0.0 13.7 5.3 ×10 −2 1370.9 c 0.77 a Fixed parameter. b Experimentally measured value. c Data obtained by non-linear regression using OriginPro 2024. © 2025 The Author(s). Published by the Royal Society of Chemistry RSC Adv.,2025,15,38391–38405 | 38397 Paper RSC Advances Open Access Article. Published on 14 October 2025. Downloaded on 10/27/2025 10:03:53 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online interactions, will be involved for 2,4D on materials with different structures and surface characteristics. 29,31 2,4D adsorption on Ce-AMN (LF model, Table 5) shows an unrealistic q M value of 1370.9 mg g −1 obtained by non-linear regression. The experimentally measured isotherm data do not display a plateau phase for Ce-AMN. Therefore, the experimentally obtained q M value corresponding to the highest q M value was added to Tables 5 and 6. Additionally, Ce-PER, which has the most defect-rich surface, 37 featuring Brønsted and Lewis sites, may further facilitate CEF, 2,4D and IP adsorption. The Ce-CARB, Ce-UREA, Ce-PER, and Ce-AMN exhibit comparable number of –OH groups and pH(PZC) values (see Table 2). The electrostatic interactions governing the adsorption process can be elucidated by comparing the pH(PZC) of the ceria adsorbent with the dissociation constants (pK a ) of the target pollutants. The ceria surface exhibits a positive charge at a solution pH below its pH(PZC), while it becomes negatively charged at pH > pH(PZC). Conversely, the pollutants CEF (pK a =2.56, 6.88), 52 2,4D (pK a = 2.73), 31 and IP (pK a =2.0, 6.8, and 12.3) 53 predominantly exist in anionic forms at pH > pK a . Therefore, in an acidic to neutral pH range where the pH is simultaneously above the pollutants' pK a and below the adsorbent's pH(PZC), a favorable electrostatic attraction is established between the anionic pollutant species and the positively charged ceria surface, driving the adsorption process. Therefore, it can be assumed that the number of surface hydroxyl groups is not crucial for CEF and 2,4D adsorption. In contrast, other physicochemical parameters, such as SSA, V pore , and CCS, are likely to play a signicant role in the adsorption of CEF and 2,4D. A study 38 highlights the importance of physicochemical parameters in the adsorption of inorganic phosphates on ceria samples annealed at various temperatures. The strong correlation of IP (Ce-UREA, Ce-HMT, and CeAMN) with the Freundlich model indicates that the ceria surface is heterogeneous. Adsorption occurs with multilayer coverage and involves interactions between the adsorbed phosphate molecules and already-created insoluble CePO 4 (ref. 54) (see eqn (7)). Insoluble form of CePO 4 is primarily formed by the Ce 3+ ions reaction with anionic form of phosphate (PO 43− , HPO 42− ,H 2 PO 4 − ) presented in ceria defects. 48 The proposed interaction mechanisms for IP, 2,4D, and CEF are presented in Fig. 7. Ce 3+ +PO 43− /CePO 4 Y(7) The efficiency of cerium-based materials in removing phosphate from water is signicantly affected by the presence of other coexisting anions, with the nature and degree of this interference depending heavily on the specic chemistry of the competing ion and the adsorbent used. 36,55 This is particularly evident with several anions that pose a signicant challenge. Silicates (SiO 32− ), for example, exhibit a profound inhibitory effect due to their chemical and structural similarity to phosphate, leading to intense competition for adsorption sites and a potential removal rate decrease of up to 82.88%. 55 Similarly, bicarbonates (HCO 3 − ) and carbonates (CO 32− ) interfere by both competing for binding sites and increasing the solution's pH to a less favorable alkaline state. 55,56 Other chemically homologous contaminants, such as arsenate (As(V)), which has a similar ionic structure to phosphate, and uoride (F − ), also act as strong competitors, with some adsorbents even showing a stronger affinity for arsenate over phosphate. 56–58 Conversely, many common simple anions show little to no negative inuence, which supports the hypothesis that the primary removal mechanism is chemisorption (inner-sphere complexation) rather than weaker electrostatic interactions. Ions such as Table 6 Comparison of maximum adsorption capacity, experimental conditions, and mathematical models for CEF, 2,4D, and IP adsorption by cerium materials and other metal oxides/composites Material Pollutant q M (mg g −1 ) Mathematical model a,b T(°C) Contact time (h) References Ceria (different synthesis) CEF 20.6–94.3 LF 25 24 This work Zeolite/MnO 2 nanoparticles 20.9 L 25 2 18 Biochar from corn bract 13.9 L 25 48 44 Palygorskite 112.33 L 28 24 17 Magnetite/ceria composite 28.1–110.7 LF 25 24 16 Ceria (different synthesis) 2,4D 46.7–83.4 LF 25 24 This work Magnetite/ceria composite 19.9–55.7 LF 25 24 16 Activated carbon from carbonized chest nut shell 0.93 L 35 4.2 20 Co–Al–Cl layered double hydroxide 27.2 L 25 1 21 Algal magnetic activated carbon 60.61 L 30 1 45 Ceria (different synthesis) IP 20.3–68.6 LF 25 3 This work CeO 2 nanoparticles 0.3–0.4 L NA c 24 36 Hydrous CeO 2 (annealed at 60–1200 °C) 6.5–99.8 L 25 24 46 MgO(100) functionalized cellulose sponge 26.8 L 25 2 4 Al 2 O 3 @Fe 2 O 3 composite 106.2 L 25 2 19 a L–Langmuir mathematical model. b LF –Langmuir–Freundlich mathematical model. c NA –not available data. 38398 |RSC Adv.,2025,15,38391–38405 © 2025 The Author(s). Published by the Royal Society of Chemistry RSC Advances Paper Open Access Article. Published on 14 October 2025. Downloaded on 10/27/2025 10:03:53 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online chlorides (Cl − ), sulfates (SO 42− ), and nitrates (NO 3 − ) typically have a minimal impact; in fact, chlorides and sulfates can sometimes even slightly enhance phosphate removal. 55,56 Ultimately, this means that while common electrolytes pose little threat, the performance of ceria-based adsorbents can be severely compromised by species like silicates, arsenates, and bicarbonates. 55 For this reason, experimental conrmation of an adsorbent's capacity in the presence of these specic interfering ions is a crucial and necessary step before its nal acceptance and deployment in real-world wastewater treatment scenarios. 3.5. Kinetics studies The experimental data were tted by the mathematical model for pseudo-rst (eqn (8)) and pseudo-second (eqn (9)) order kinetics 59,60 to evaluate the IP, CEF and 2,4D adsorption kinetics on the ceria samples. The pseudo-rst-order (PFO) and pseudosecond-order (PSO) kinetic models are represented by equations (eqn (8) and (9)), with corresponding data presented in Table 7 and illustrated in Fig. 4. q t =q E ×(1 −e −k 1 t )(8) qt¼qE2k2t qEk2tþ1(9) in eqn (8) and (9), q E and q t (mg g −1 ) correspond to adsorption capacity at equilibrium (can be dened by eqn (1) and (2)) and at any time t(min), respectively. The k 1 (min −1 ), k 2 (g mg −1 min −1 ) is the pollutant adsorption rate constant. From the data obtained from the PSO model, the approaching equilibrium factor R A was calculated (eqn (10)), which represents the characteristic of the kinetic curves of an adsorption system. RA¼1 1þk2qEtr (10) in eqn (10), t r corresponds to the longest adsorption time of the kinetic experiment, q E is the adsorption capacity at equilibrium (mg g −1 ), and k 2 (g mg −1 min −1 ) is the pseudo-second-order adsorption rate constant. The adsorption curve is described as “approaching equilibrium”when 0.1 < R A <1,“wellapproaching equilibrium”when 0.1 < R A < 0.01, and R A < 0.01 is “drastically approaching equilibrium”. 61 The values of R A indicate that the adsorption of IP and CEF is well-approaching equilibrium; in contrast, 2,4D is drastically approaching equilibrium. A larger CCS of the ceria adsorbent resulted in an increase in the R A value, especially for 2,4D. For other tested pollutants (CEF and IP) the effect was not observed. However, R A is inuenced not only by the particle size of the adsorbent but also by the properties of solution, adsorbent, and adsorbate. 62 In our experiments, the CCS, number of surface –OH groups, and SSA varied over the used samples, while other factors were held constant, such as adsorbent dosage, initial pollutant concentration, and temperature. The adsorption kinetics is a relatively fast process, according to data in Table 7 and Fig. 4. All samples exhibit rapid initial adsorption kinetics for IP, CEF, and 2,4D, with maximum adsorption achieved within 60 min for IP and 400 min for both CEF and 2,4D. With a further increase in the adsorption time, the sorption rate no longer changes and reaches adsorption equilibrium. Table 7 shows that the R 2 value is highest for the PSO model, with a value close to 1, indicating that CEF, 2,4D, and IP adsorption follow PSO kinetics. Moreover, the calculated q E value from the PSO model closely matched the experimental q E value for all pollutants, conrming the suitability of this model. The results align with the ndings for other adsorbents, such as magnesia/ceria composite, 16 Co–Al–Cl-layered double hydroxide, 21 zeolite/MnO 2 nanoparticles, 18 corn bract biochar, 44 MgO functionalized cellulose sponge, 4 Al 2 O 3 /Fe 2 O 3 composite 19 used for removing CEF, 2,4D, or IP, respectively. For some samples and 2,4-D, it is challenging to determine which kinetic model is more appropriate, i.e., PFO or PSO, for Ce-PER, CeUREA, and Ce-AMN, due to the close values of R 2 . The PSO model, commonly used for pollutant adsorption from aqueous solutions, 63 suggests that the adsorption of IP, Table 7 The adsorption kinetics parameters for IP, CEF, and 2,4D adsorption on the ceria samples Sample Pollutant Pseudo-rst order Pseudo-second order q E SE (mg g −1 )k 1 SE ×10 −2 (min −1 )R 2 q E SE (mg g −1 )k 2 SE ×10 −3 (g mg −1 min −1 )R 2 R A Ce-CARB IP 25.2 1.19 26.3 7.66 0.9165 26.9 1.02 16.6 5.60 0.9637 1.2 ×10 −2 Ce-HMT 47.4 1.65 20.2 3.60 0.9567 50.3 1.05 7.10 1.15 0.9892 1.5 ×10 −2 Ce-PER 57.1 2.77 10.3 1.89 0.9362 62.5 1.86 2.30 0.37 0.9838 3.7 ×10 −2 Ce-UREA 23.1 1.41 27.4 10.0 0.8931 24.3 1.39 20.1 10.7 0.9301 1.1 ×10 −2 Ce-AMN 50.8 3.63 5.48 1.35 0.9043 56.5 4.49 1.34 0.52 0.9309 6.8 ×10 −2 Ce-CARB CEF 27.4 1.18 1.60 0.26 0.9365 31.2 1.36 0.66 0.14 0.9624 3.3 ×10 −2 Ce-HMT 38.7 1.43 3.32 0.59 0.9252 41.8 1.45 1.26 0.31 0.9610 1.3 ×10 −2 Ce-PER 82.7 3.24 2.76 0.48 0.9220 91.0 2.65 0.43 0.08 0.9749 1.8 ×10 −2 Ce-UREA 24.5 1.53 1.78 0.40 0.8831 28.0 1.57 0.76 0.21 0.9433 3.2 ×10 −2 Ce-AMN 72.4 4.66 1.45 0.34 0.8720 84.9 4.20 0.19 0.04 0.9562 4.0 ×10 −2 Ce-CARB 2,4D 63.7 1.54 4.41 0.63 0.9643 67.0 1.51 1.31 0.28 0.9810 7.9 ×10 −3 Ce-HMT 79.4 1.27 4.40 0.42 0.9842 83.1 0.88 1.13 0.12 0.9957 7.3 ×10 −3 Ce-PER 98.4 0.24 11.9 3.18 0.9997 98.4 0.34 47.5 83.6 0.9996 1.5 ×10 −4 Ce-UREA 61.2 1.21 57.9 0.89 0.9747 63.1 1.65 2.48 0.91 0.9728 4.4 ×10 −3 Ce-AMN 84.0 0.52 7.12 0.68 0.9987 85.1 0.66 6.61 2.87 0.9981 1.2 ×10 −3 © 2025 The Author(s). Published by the Royal Society of Chemistry RSC Adv.,2025,15,38391–38405 | 38399 Paper RSC Advances Open Access Article. Published on 14 October 2025. Downloaded on 10/27/2025 10:03:53 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online