Tuning La–O adsorption sites dispersion via hydrogen bond-capping organic–inorganic copolymerization strategy for enhanced phosphate removal

Abstract

Lanthanum (La) (oxy)hydroxides are promising adsorbents for efficient aqueous phosphate (P) removal. The incorporation of cationic hydrogel with La hydroxides represents an effective strategy to improve the dispersion of La–O active sites thereby favoring P adsorption. In this study, a hydrogen (H) bond-capping via organic–inorganic copolymerization strategy was developed for enhancing the dispersion of La–O active sites. This approach significantly enhanced the adsorption capacity of La hydroxide oligomer (LHO) copolymerized cationic hydrogel (LaCCH) to 308.2 mg_P g_La⁻¹. Fixed-bed experiments demonstrated that LaCCH effectively treated over 1098 bed volumes (BV) of synthetic wastewater (1.0 mg_P L⁻¹) containing co-existing ions. Combined analyses using FTIR, Raman, and XPS confirmed that the inner-sphere complexation and formation of LaPO₄·0.5H₂O were crucial to P adsorption. The results of MD simulation implied the weaker intermolecular H bonding between [La(OH)₃] in LaCCH results in a more favorable interaction between [La(OH)₃] and the hydrogel carbon chain. In summary, copolymerization significantly improved the dispersion of La–O active sites, which enhanced P adsorption and demonstrated a strong correlation between the fractal dimension of dispersion and adsorption capacity.

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Environmental significance

Organic–inorganic copolymerization is a promising method for the synthesis of environmental functional materials, particularly with regards to the development of adsorbents. In this study, a lanthanum-containing hydrogel (LaCCH) adsorbent was synthesized based on a hydrogen-bonding-capping mediated organic–inorganic copolymerization method. LaCCH achieved a maximum phosphate removal capacity of 308.2 mg_P g_La⁻¹. The result found that this hydrogen-bonding-capping pre-polymerization strategy enhanced the interaction energy (E_i) between the lanthanum oligomer and the hydrogel carbon chain, thereby strengthening the dispersion of La–O sites. In summary, this method significantly improved the dispersion of adsorption sites, which enhanced phosphate removal and demonstrated a strong correlation between fractal dimension and adsorption capacity.

1. Introduction

Phosphorus stands as a fundamental constituent of DNA, essential for sustaining life across terrestrial ecosystems. Moreover, the intricate process of phosphorus cycling mirrors a one-way flow, akin to a non-renewable resource.¹ Human reliance on phosphorus hinges upon P rock reserves, a resource constrained by its limited and geographically disparate distribution.^2,3 Notably, nearly 90% of phosphorus use is directed towards food production, a substantial portion of which eventually permeates surface water bodies as pollutants, primarily through agricultural runoff and wastewater treatment facilities.⁴ Therefore, the essential task of phosphorus removal and recovery from wastewater becomes crucial for enabling effective phosphorus cycling and optimal utilization of wastewater resources. In aquatic environments, phosphorus typically exists in particulate, colloidal (complexed), and ionic states.⁵ While conventional techniques effectively address the removal of particulate and colloidal phosphorus through coagulation precipitation or membrane filtration, the selective removal or recovery of ionic P, particularly at low concentrations, poses significant challenges.^6–8

A variety of methods, including adsorption, membrane technologies, electrochemistry, and more, are currently employed to selectively separate and remove low-concentration P from wastewater.^9–11 In these processes, phosphorus is separated and adsorbed onto solid substrates through ion exchange or physicochemical adsorption, allowing subsequent desorption to yield concentrated phosphorus solutions for recovery and reuse.^12–15 Various adsorbents, such as carbon-based materials,^16–18 metal oxides,^19–22 ion exchange resins^20,23–25 and others, have been developed for selective phosphorus enrichment and separation. Among them, La-based materials have emerged as promising candidates for selective adsorbing P, owing to the strong short-range interaction of La with phosphorus.^26–28 Due to the f orbitals being electron-deficient and having limited radial extension, La is naturally more inclined to bind with P compared to other transition metals.^29–31 Moreover, La exhibits a broader pH applicability range, maintaining heightened adsorption efficacy within the pH range of 4.5–8.5.^32,33 In contrast, the optimal pH applicability range of Fe(III) or Al(III) is narrower, with values of 3.5–4.5 and 5.0–6.5, respectively.³⁴ Additionally, La exhibits a broad-spectrum phosphorus-limiting activity against both P and organic phosphonates, with the stoichiometric interaction between La and phosphorus resulting in an exceedingly low solubility product of LaPO₄ (K_sp = 3.7 × 10⁻²³).³⁵

Besides the La–P adsorption process influenced by short-range forces, P can also be captured via long-range forces such as electrostatic interaction.^36,37 The synergistic effects resulting from the combination of long-range and short-range forces facilitate the accumulation of P at the interface of the adsorbent and water, especially at low concentrations.³⁷ Previous research has found that the cationic hydrogels (CH) are potential matrices for P removal. For example, Tang et al. (2012) and Rao et al. (2011) developed a magnetic cationic hydrogel (MCH) polymerized with (3-acrylamidopropyl)trimethylammonium chloride (APTMACL) with ζ-potential values ranging from 47 mV at pH 3 to 30 mV at pH 11, suggesting electrostatic attraction to oxygen-containing anions (such as phosphate ions) in water.^38,39 To further enhance the performance of La-modified adsorbents, it is crucial to develop methods to improve the dispersion of La–O adsorption sites on the carrier. He et al. developed a type of well-dispersed La(OH)₃-modified polyacrylonitrile via electrostatic spinning, achieving 172.2 mg_P g_La⁻¹ capture capacity in a 80 mg_P L⁻¹ solution.⁴⁰ Furthermore, Pan et al. developed a series of La modified resins by utilizing the hydrothermal method, including La@D201, which exhibited outstanding dephosphorization preferences from wastewater.^41–43 Due to the fundamental differences in thermodynamics and kinetics between the crystallization process of inorganic La oxides and the polymerization process of organic carbon chains, the La oxides in La-modified adsorbents tend to aggregate on the surface or pore channels of the matrix to minimize their surface energy, causing the inner regions of the resulting in inefficient utilization of the inner regions, even when facilitated by a dispersant or loaded into filters. Therefore, tuning the interface between organic hydrogels and La oxides to achieve uniformity during compositing is crucial for highly dispersed La–O adsorption sites.

A promising approach to achieve the mentioned objective is to promote the transition of La hydroxides from crystallization to oligomerization.⁴⁴ This transition facilitates the formation of a more compatible interface between the La hydroxides and polymer chains, enabling control over the dispersibility of inorganic La hydroxides within the organic matrix.⁴⁵ For example, Yu et al. utilized organic acrylamide monomers and inorganic calcium phosphate oligomers as precursors to prepare a uniformly structured polyacrylamide (PAM)–calcium phosphate copolymer via organic–inorganic copolymerization.⁴⁶ This copolymer material exhibited integration of organic and inorganic units at the molecular level, resulting in the complete elimination of interphase boundaries and the formation of a continuous and fully integrated hybrid network. Additionally, Liu et al. (2019) suggested that H-bond-based capping as an appropriate strategy, considering the presence of oxygen in most inorganic complexes, including La hydroxides.^46–48 Building upon this insight, it is conceivable to copolymerize La oligomers with CH precursors for a uniform inorganic–organic structure formation. This approach facilitates the distribution of La–O adsorption sites on the CH carrier, making them prime candidates for developing high-performance La-based dephosphorization agents.

In this study, we utilize triethylamine (TEA) to tune the hydrolysis process of La³⁺ in ethanol, leading to the formation of La oligomers by shielding the intermolecular H bonds of [La(OH)₃]. The organic–inorganic copolymerization strategy for synthesizing CH copolymerized with La oligomers (LaCCH) was developed to facilitate the distribution of La–O adsorption sites on the CH carrier, thereby enhancing the removal of P. The objectives of this work were to (i) develop highly La–O dispersed adsorbent that achieves direct and efficient P recovery from phosphorus-containing wastewater, (ii) clarify the effect of H bond capping and organic–inorganic copolymerization on the dispersion of [La(OH)₃] and the efficiency of P adsorption, and (iii) demonstrate the feasibility and universality of using the H bond capping approach to promote the dispersion of La–O sites within organic–inorganic copolymer systems.

2. Materials and methods

2.1 Materials

(3-Acrylamidopropyl)trimethylammonium chloride (APTMACL; 75 wt% in H₂O), acrylamide (AM), polyvinyl alcohol (PVA, M_w = 1750 ± 50), N,N′-methylenebis (acrylamide) (MBA), N,N,N′,N′-tetramethylethylenediamine (TEMED), potassium peroxydisulfates (KPS) and trimethylamine (TEA) were purchased from Sigma-Aldrich (Beijing, China). The LaCl₃·9H₂O, ethanol (99.7%), 1-hydroxy ethylidene-1,1-diphosphonic acid (HEDP), 2-phosphonobutane-1,2,4-tricarboxylic acid (PBTCA), amino trimethylene phosphonic acid (ATMP), ethylenediamine tetramethylenephosphonic acid (EDTMP) and polyamino polyether methylene phosphonae (PAPEMP) were purchased from Shanghai Macklin Biochemical Technology Co., Ltd. The P stock solution (1.0 g P[PO₄³⁻] per L) was generated by dissolving KH₂PO₄ into deionized water. All reagents were of analytical grade and used without further purification.

2.2 Preparation of La hydroxide oligomer (LHO) and LaCCH

LHO were prepared using an induced hydrolysis method. Initially, 0.01 mol LaCl₃·7H₂O was dissolved in 500 mL ethanol under magnetic stirring for 30 min. Subsequently, 0.2 mol TEA was added dropwise and stirred magnetically for 12 h at room temperature. The resulting product was obtained via centrifugation and three subsequent ethanol washes. The LHO were re-dispersed in ethanol (50 mg mL⁻¹) to form a uniform emulsion for future use. La-NP were produced using the same method as for LHO, but with ethanol replaced by an aqueous solution and TEA replaced by a 0.03 mol L⁻¹ NaOH aqueous solution, respectively.

LaCCH was prepared as follows. Initially, a 10 mL LHO slurry (50 mg mL⁻¹) was centrifuged for 10 min at 8000 rpm, and the resulting supernatant was discarded. Subsequently, 2.5 g of the APTMACL solution was uniformly mixed with the LHO precipitate to obtain a homogeneous emulsion. Then, 0.05 g of MBA, 20 μL of TEMED, and 1.0 mL of saturated KPS solution were introduced to the emulsion with vigorous stirring. Following copolymerization at 60 °C for 1 hour and a 24 hour aging process, the resultant gel was washed with ethanol and deionized water to remove any residues, and subsequently dried at 60 °C. The as-obtained brown product was stored under vacuum at room temperature. A pure CH was synthesized following the procedure outlined above, omitting the inclusion of LHO. Furthermore, the La–CH composite was prepared by replacing LHO with a La-NP solution. The La content measured by ICP-OES was 14.7% for LaCCH and 14.9% for La–CH.

2.3 Characterization and analysis

Transmission electron microscopy (TEM) samples were prepared using a cryo-ultramicrotome (EM UC7, Leica, Germany). TEM (Hitachi H800, Japan) and scanning electron microscopy (SEM; ZEISS GeminiSEM 300, Germany) were used to analyze the morphology and detect the surface elements of the materials. The used samples were examined by powder XRD (Bruker, Germany, radiation source Cu Kα) at a 2θ range of 10° to 80°, with a scan speed of 5° min⁻¹. Differences in the surface groups, valances of specific elements and molecular structure were analyzed by the combination of FTIR (Thermo Scientific, Nicolet 8700, USA), Raman (inVia-Reflex, Renishaw, England) and XPS (ESCALAB 250Xi, Thermo Fisher Scientific, USA). SAXS data were acquired at the 1W2A station of the Beijing Synchrotron Radiation Facility. The instrument had been previously calibrated by the beamline staff using a standard sample. The scattering vector magnitude q ranged from 0.08 to 4.00 nm⁻¹ for the reported experiment.

2.4 Batch adsorption experiments

To determine the adsorption isotherms, 0.015 g of the adsorbent was dispersed in 50 mL aqueous solutions with P concentrations ranging from 1.0 mg_P L⁻¹ to 50.0 mg_P L⁻¹. The initial pH values of aqueous solutions were adjusted to 7.0 ± 0.2 by 0.1 M NaOH solution, and stirred for 24 h at 300 rpm min⁻¹ (25 °C). The adsorption kinetics was performed in 500 mL of a 50.0 mg_P L⁻¹ solution at a dosage of 0.3 g L⁻¹ at pH 7.0 ± 0.2 (25 °C), and the samples were withdrawn at predetermined time intervals. The effect of pH on the P adsorption capacity was investigated at initial pH values ranging from 3.0 ± 0.2 to 11.0 ± 0.2. The impact of co-existing ions, including Cl⁻, SiO₃²⁻, SO₄²⁻, CO₃²⁻ and NO₃⁻, on the performance of LaCCH (at a dosage of 0.3 g L⁻¹) was studied at ion concentrations at 250 and 500 mg L⁻¹. The effect of organic phosphonates (HEDP, PBTCA, ATMP, EDTMP, and PAPEMP) on the adsorption of orthophosphate by LaCCH was studied. The orthophosphate concentration was fixed at 10.0 mg_P L⁻¹, and each organic phosphonate was tested at two concentrations: 2.5 mg_P L⁻¹ and 5.0 mg_P L⁻¹. The concentrations of P were determined by the molybdenum blue spectrophotometric method.

2.5 Molecular dynamics (MD) simulation

MD simulations of the interaction between APTMACL and LHO/La(OH)₃ dimers were performed using the Materials Studio 2020 software and COMPASS force field. Initially, 3D models of [La(TEA)(OH)₂]₂ and [La(OH)₃]₂ dimers were constructed and optimized to attain their most stable configurations. An amorphous cell was then generated based on the optimized configuration using the amorphous cell module. The configurations of [La(TEA)(OH)₂]₂ and [La(OH)₃]₂ with the (0 0 1) face were obtained employing a build layer tool, with the super crystal cell expansion and vacuum layers set to 3 × 3 × 1 and 1.0 Å, respectively. Interaction energies were subsequently calculated using the equation:

ΔE = E_total − (E_surface + E_polymer)

where ΔE is the interaction energy between the [La(TEA)(OH)₂]₂ or [La(OH)₃]₂ surface and the polymers, E_total is the total potential energy of the system, E_surface and E_polymer are the single point energies of the (0 0 1) surface and the potential energy of the polymer, respectively.

3. Results and discussion

3.1 Characterization

The TEM images of ultrathin slices of LaCCH and La–CH are depicted in Fig. 1a. The results showed that the La hydroxide was embedded in the network of CH (Fig. S1†), and the LaCCH demonstrated enhanced dispersion of La hydroxide compared to La–CH. The SEM results implied that the scale of La hydroxides in LaCCH and La–CH was ∼35 nm and ∼95 nm respectively (Fig. S2 and S3†). This highlights the role of organic–inorganic copolymerization. In Fig. S4,† TGA curve showed that the La₂O₃ content in LaCCH and La–CH was approximately 8.0% and 4.7%, respectively which was also consistent with the result of ICP-OES. The SAXS curves of LaCCH and La–CH further indicate a more homogeneous structure of LaCCH compared to La–CH, as evidenced by the lower slope of La–CH compared to that of LaCCH (Fig. 1b). The Raman spectra presented in Fig. S5† implied that the region of C–H vibrational stretching, spanning from 2800 to 3100 cm⁻¹, is assigned as bands representing the symmetric stretching of –CH₂ and –CH₃ of the samples, respectively.⁴⁹ Compared with La–CH, the reduced signal intensity observed in the C–H vibrational stretching in LaCCH indicated a gradually enhanced H bond interaction between cationic polymer chains and La–OH,⁵⁰ which was also consistent with the results of SAXS (Fig. 1b). Fig. S6† showcases the distribution of the La element within LaCCH and La–CH over an area of 500 μm × 500 μm. Notably, the distribution patterns of C⁺, CH₂⁺, and CH₃⁺ ions, which are attributed to the polymer chains, exhibit a high degree of similarity between LaCCH and La–CH. However, a distinct difference emerges when considering the behavior of La-related ions (La⁺, LaO⁺, and LaOH⁺) with respect to sputtering depth. Specifically, as the sputtering depth increases, La–CH displays a pronounced enhancement in the signal intensities of these La-related ions, indicating a concentration gradient or non-uniform distribution. In contrast, LaCCH exhibits a uniform dispersion of these ions throughout the sputtering process, highlighting the enhanced homogeneity of La dispersion within LaCCH compared to La–CH. This observation underscores the distinct chemical and structural characteristics of the two materials regarding their La distribution.

Fig. 1 The ultrathin slices TEM images of LaCCH and La–CH (a) (scan bar 50 nm). (b) SAXS curves of LaCCH and La–CH. (c) ZP values of LaCCH, La–CH and CH as a function of pH. (d) Solution pH over time after adding adsorbents.

The surface charges of the samples were determined and shown in Fig. 1c. The zeta potential (ZP) values of LaCCH, La–CH and CH remained positive within a pH range from 3.0 to 11.0. Within a pH range of 3.0 to 4.0, the ZP values of LaCCH stayed at 60.0 mV, sharply declining to 43.8 mV at pH 6.0, stabilizing until pH 8.0, and then rapidly decreasing to 26.9 mV as the pH rose to 11.0. This finding indicates that acidic protons boosted the ZP values of LaCCH, while alkaline OH groups shielded positive groups, lowering ZPs. Additionally, the hydrolysis of the APTMACL monomer (C–NH to C–O–) in alkaline suspension contributed to the ZP decrease. Compared with CH and La–CH, the ZP value of LaCCH was 42.9 mV at pH = 7.0, which was lower than that of CH (62.9 mV) and La–CH (58.8 mV). This phenomenon can be attributed to the H-bonding capping effect of TEA, which facilitates the expose of an increased number of hydroxyl groups on the LHO within LaCCH. Besides, the solution pH changed suddenly when the adsorbents were added (Fig. 1d). Specifically, the pH incremented by 0.77 mV and 1.18 mV upon the addition of LaCCH and La–CH, respectively. This phenomenon is intimately linked to the low ionic potential and robust basicity of LHO and La hydroxide, fostering a pronounced proclivity for hydroxyl group dissociation into hydroxide ions.

3.2 Batch P adsorption onto LaCCH

The P uptake by the LaCCH, La–CH and CH as well as their corresponding adsorption isotherms are depicted in Fig. 2a. The maximum adsorption capacities of LaCCH, La–CH and CH was 70.0, 56.9 and 32.2 mg g⁻¹, respectively. Compared with La–CH, the enhanced P adsorption capacity and La-usage efficiency of LaCCH can be attributed to the well-dispersed La–O sites and potential Donnan membrane effect.^51–53 The isotherms data of LaCCH, La–CH and CH were further analyzed by Langmuir, Freundlich, and Liu's models and the obtained parameters are reported in Table S1.† The Liu model provided higher correlation coefficient (R²) than the former two, thereby favoring monolayer adsorption. Moreover, the P adsorption capacity and P/La ratio of LaCCH in the present study was 308.2 mg_P g_La⁻¹ and 0.51, respectively, which was higher than that of La@D201, La(OH)₃/Fe₃O₄ and other reported La-based adsorbents.^{40,41,54–67} Furthermore, the initial concentration of P is the most important factor in determining the adsorption capacity. Compared with non-La-based adsorbents presented in Table S2,† LaCCH demonstrated outstanding P adsorption capacity even in lower P equilibrium concentration (C_e < 10 mg L⁻¹). The adsorption kinetics of LaCCH was investigated (Fig. 2c) and the data were fitted by the pseudo-first-order (PFO) and pseudo-second-order (PSO) models (Table S3†). The majority of the adsorption capacity was achieved in the first 2.0 h, then gradually reaching equilibrium and the fitted constant rate of the PSO kinetics model was obtained as 0.014 g mg_P⁻¹ h⁻¹ (the inset of Fig. 2c). In addition, the adsorption time to reach 83.1% of the ultimate adsorption capacity was within 0.5 h, due to the synergistic effect of chemisorption and electrostatic interaction. Furthermore, La oligomers with unsaturated bonds on the surface provide chemical adsorption sites for interaction with P, mainly through inner-layer complexation and hydroxyl exchange.⁶⁸

Fig. 2 (a) Adsorption isotherms of LaCCH, La–CH and CH. (b) The adsorption properties comparison between the LaCCH and various adsorbents for P uptake. (c) Adsorption kinetics of LaCCH using the pseudo-first-order and pseudo-second-order models. (d) Adsorption capacity of LaCCH obtained at pH = 3.0–11.0. (e and f) Effect of competing ions (i.e., bicarbonate, chloride, nitrate, and sulfate) (e) and organophosphates (f) on P adsorption by LaCCH.

The effect of pH on P adsorption by LaCCH is illustrated in Fig. 2d, where LaCCH exhibits a stable and efficient P adsorbs over a wide pH range (7–11). The removal of P by LaCCH in this pH range was mainly affected by the surface charge and morphology of La–P, and the corresponding adsorption process may be dominated by the synergistic effect of electrostatic attraction and inner sphere complexation. At pH = 3.2–4.5, H₂PO₄⁻ is the major species with part of H₃PO₄. With the increase of pH, the amounts of H₃PO₄ decreased, whereas those of the H₂PO₄⁻ and ≡La–OH forms increased. However, the electrostatic interaction between the protonated –N²⁺(CH₃)₃H and H₂PO₄⁻ was weakened, failing to enhance the beneficial effects increased ligand exchange. When the pH increased from 4.5 to 7.2, H₂PO₄⁻ and HPO₄²⁻ exhibited equal proportions initially (pH = 4.5). With the increase of pH, H₂PO₄⁻ decreased gradually while HPO₄²⁻ proportionally increased. As a result, the ligand exchange between HPO₄²⁻ and ≡La–OH was strengthened which benefited P adsorption. As the pH further increased to 11.0, The increased –OH group competed with HPO₄²⁻/PO₄³⁻ to bind with the La–O sites of LaCCH, thereby weakening the ligand exchange between HPO₄²⁻/PO₄³⁻ and ≡La–O. The electrostatic attraction between –N(CH₃)₃OH decreased because of the surface hydroxylation of LaCCH. However, the Lewis acid–base interaction between ≡La–O and HPO₄²⁻/PO₄³⁻ was strengthened under these conditions, this overcame the aforementioned adverse effects and maintained a relatively stable P adsorption capacity.

The effect of salinity and coexisting substances on P adsorption by LaCCH was evaluated and shown in Fig. 2e. Obviously, With the introduction of NaCl (8.3 g L⁻¹), the adsorption capacity of LaCCH exhibited considerable adsorption ability, which imply that P adsorption by LaCCH was basically unaffected by the competition of high salinity. Notably, the existence of 5.0 g L⁻¹ NO₃⁻ and SO₄²⁻ showed slight effects on P removal, but the same concentration of SiO₃²⁻ and CO₃²⁻ has a significant impact on P adsorption capacity of LaCCH. Especially, with 5.0 g L⁻¹ SiO₃²⁻, the adsorption capacity of LaCCH was substantially suppressed to 75% of its capacity under blank conditions. Comparatively, the competing abilities of these anions to P adsorption by LaCCH follow the order: SiO₃²⁻ > CO₃²⁻ > SO₄²⁻ > NO₃⁻ > Cl⁻. Organic acids, especially organic phosphonates, would intensively compete with P for the La–O sites. Fig. 2f illustrates the impact of the organic phosphonates (HEDP, PBTCA, ATMP, EDTMP and PAPEMP) on the adsorption. Compared with the control group, the presence of these organic phosphonates had varying degrees of adverse effects on the P adsorption of LaCCH. Among them, PBTCA and HDTMPA at a concentration of 5.0 mg L⁻¹ had the greatest impact, reducing the P adsorption amount to 55.70% and 28.84% of the control group's value, respectively. This is because PBTCA and HDTMPA have larger molecular weights and longer carbon chains compared to the other organic phosphonates, resulting in a stronger complexation effect with LaCCH. The interference of organic phosphonates on P adsorption increases with the rise of concentration, demonstrating completely different results compared to inorganic ion interference (Fig. 2e), which implies that the specific inner-sphere complexation and hydrophobic interaction between P and LaCCH surface play key roles in P adsorption.⁶⁹

Consecutive cycles of P adsorption and desorption were conducted to assess the reusability of LaCCH, with the corresponding results illustrated in Fig. 3a. Following the initial cycle, the P adsorption capacity of LaCCH decreased to 76.0% of its original value, and in the subsequent 2nd to 5th cycles, the adsorption capacity of LaCCH remained stable. Within the five cycles, the desorption agent concentrated to 290.2 mg_P L⁻¹, demonstrating the effectiveness of P separation. The XRD patterns of LaCCH after each adsorption cycle, from the 1st to 5th, are depicted in Fig. 3b. The typical peaks at 21.1 (−1 1 1), 28.5 (1 2 0), 30.9 (0 1 2), 40.7 (0 3 1), 41.7 (−1 0 3), 48.3 (3 2 0), 51.4 (−3 2 2) and 51.9 (1 3 2) were observed indicating the formation of LaPO₄ (PDF card no. 32-0493). Nevertheless, after the 5th cycle, the spectra closely resembled the XRD diffractograms of La(OH)₃ (PDF card no. 36-1481). This suggests that a significant number of La–O bonds failed to reform into La–O–P bonds in the 5th adsorption. Despite this, the adsorption capacity in the 5th cycle remained at 45.0 mg g⁻¹, only slightly lower than the 50.4 mg g⁻¹ observed in the 4th cycle. This unusual observation might be due to the LaPO₄ formed in the 4th cycle in LaCCH resisting reversion to La(OH)₃, while the CH in LaCCH exhibits significant electrostatic adsorption towards P, thus maintaining the P adsorption capacity in the 5th cycle. After the 5th P adsorption cycle, the XRD pattern of the sample reveals distinct La(OH)₃ crystals, while the amorphous structure of CH is absent. This absence is attributed to the stronger H bonding between [LaPO₄] molecules compared to [La(OH)₃] molecules. Consequently, interactions with CH are weakened, facilitating the dissociation of CH from LaCCH. Notably, the La hydroxide formed during the desorption process exhibits significantly reduced P adsorption activity compared to the original hydroxide present within the pristine LaCCH, owing to increased crystallinity and a decrease in the number of available La–O adsorption sites, thereby diminishing the re-adsorption capacity for P. Additionally, the P adsorption capacity of LaCCH after 5th cycle remained higher than that of reported adsorbents.^27,70 Furthermore, the fixed-bed adsorption was conducted in simulation wastewater that contained 1.0 mg L⁻¹ phosphorus and competing substances (Fig. S7†) and the result implied the LaCCH column can treat 1098 BV of the influent, whereas the effective treatment capacity of the column packed was reduced to 486 BV in the following cycle.

Fig. 3 P adsorption–desorption cycles (a) and XRD diffractograms of the used LaCCH after 1st to 5th adsorption cycles (b).

3.3 Adsorption mechanisms

To investigate the mechanism of P adsorption on LaCCH, the multiple analytical methods including TEM, FTIR, Raman spectroscopy and XPS were employed. In Fig. S9,† the SAED image of LaCCH after P adsorption revealed obvious annular diffraction pattern, indicating the LHO in LaCCH transitioned from a low-crystalline (Fig. S2†) to a high-crystalline state. This transformation might be attributed to the formation of LaPO₄ crystals resulting from the spatial overlap of La, O and P (Fig. S9†). Fig. 4a presents the FTIR spectra of LaCCH before and after P adsorption. The broad peak observed at 3440 cm⁻¹ corresponds to the –OH stretch vibration of the adsorbed water.⁷¹ The peaks at 2176 cm⁻¹, 1647 cm⁻¹, 1475 cm⁻¹, 1384 cm⁻¹, and 915 cm⁻¹ are attributed to the C–O, C=O, C–O, N–O and H–O stretching vibrations of the polymer chains in LaCCH, respectively.^72–76 Additionally, the strengthened adsorption peaks at 1077 cm⁻¹, 968 cm⁻¹, 620 cm⁻¹, and 534 cm⁻¹, corresponding to the asymmetric stretch vibration of P–O, were observed, indicating the capture of PO₄³⁻ by LaCCH which was further confirmed by the Raman spectrum (Fig. S8†).^77,78Fig. 4b–d depict the XPS spectra of P 2p, La 3d and O 1s before and after P adsorption, respectively. The deconvoluted peaks of LaCCH after P uptake at 133.1 eV and 132.1 eV are ascribed to the P species in the LaPO₄·0.5H₂O state and different positions of the inner-sphere complex, respectively, with corresponding fractions calculated to be 28.8% and 71.2% (Fig. 4b and Table S4†).^79,80 This result was also consistent with the result of the FTIR and Raman spectral analysis of LHO after P uptake, which confirmed that La–OH and La≡OH were involved and played a critical role in the hydroxyl exchange effect of P. The La 3d region in the XPS spectrum of LaCCH showed two sets of peaks at E_B[La 3d_5/2] = 835.2 eV and 839.0 eV, and E_B[La 3d_3/2] = 851.9 eV and 855.8 eV (Fig. 4c). After P adsorption, a slight peak shift to lower binding energy of the La 3d_5/2 (837.8 eV) was observed demonstrating a possible electron transfer from La to PO₄³⁻ and the formation of stable complexes (La–O–P).⁸¹ Furthermore, the satellite energy separation of La 3d_5/2 altered from 3.8 eV to 2.3 eV, suggesting a form conversion from La–OH to La–PO₄ (as evidenced in Table S5†), which is consistent with the formation of stable La–O–P bonds.⁶⁸ As shown in Fig. 4d, the broad and asymmetric O 1s XPS spectra correspond to three chemical states of O, including crystal lattice oxygen (O_L), hydroxyl oxygen (O_H), and oxygen species in the C–O state. The XPS signal of O_L is attributed to the La–O bond in the La(OH)₃ crystal lattice (peak position: 529.5 eV), and the O_H peak is related to the chemisorbed water (peak position: 531.7 eV) (Table S6†).⁸² The intensity of the –OH peak decreased and shifted after P adsorption, indicating a change in the chemical state of oxygen. This result suggests that inner-sphere complexation and the formation of LaPO₄·0.5H₂O were the main adsorption mechanisms of LaCCH for P.

Fig. 4 FTIR (a), P 2p (b), La 3d (c) and O 1s (d) peak deconvolution of LaCCH before and after P uptake.

To rigorously validate the efficacy of this approach, LHO was copolymerized with PVA to yield LaCPVA, and with PAM to yield LaCPAM, respectively. Additionally, for comparative purposes, La–PVA and La–PAM were prepared. They were obtained by coprecipitating La-NP with PVA and PAM gels, respectively, ensuring the same La content across all samples (section S2†). The maximum adsorption capacities of LaCPVA increased from 9.6 mg g⁻¹ to 26.4 mg g⁻¹, and LaCPAM increased from 15.2 mg g⁻¹ to 27.6 mg g⁻¹ (Fig. S10†). This result indicates that the H bond-capping organic–inorganic copolymerization possesses a certain degree of universality in enhancing P adsorption. To further evaluate the promotional effect of this pre-polymerization strategy on the dispersion of active sites, the TEM imaging of ultra-thin sliced samples (100 nm) was conducted and free path distribution in the La-containing samples were analyzed (Fig. S11 and S12†). Notably, the significant aggregates at the micrometer level were evident in the coprecipitated samples (La–CH, La–PVA and La–PAM), however, the adsorbents obtained through the pre-polymerization strategy (LaCCH, LaCPVA and LaCPAM) exhibited a significant improvement in La dispersion (Fig. S12†). The profiles of free-path spacing in LaCPVA, LaCCH, and LaCPAM exhibited multimodal distributions with peaks at approximately 1655, 50, and 70 nm, respectively, which are notably higher than those of La–PVA (50 nm), La–CH (10 nm), and La–PAM (20 nm) (Fig. S12†). The key principle is that ideal dispersion occurs when pair-wise interactions approach zero, thereby eliminating the driving force for agglomeration. The observed multimodal distribution in the pre-polymerization adsorbents was attributed to the capping effect of the TEA molecule embedded within the LHO. The fractal dimensions of the samples are shown in Fig. S13,† and the correlation between the fractal dimension and the adsorption capacity is shown in Fig. 5a. A linear correlation with a positive slope was observed between the adsorption capacity and the fractal dimension. Furthermore, the copolymerized hydrogels (LaCCH, LaCPAM and LaCPVA) demonstrated higher theoretical maximum adsorption capacity (x = 2, Q_e = 185.4 mg g⁻¹) than coprecipitated hydrogels (x = 2, Q_e = 68 mg g⁻¹). To elucidate the enhancing effect of the copolymerized strategy on the dispersion of La–O adsorption sites, the molecular dynamics interaction characteristics between LHO and hydrogels were compared under both H-bonding capping and original conditions. Since the dimer is the main component during the initial stages of hydrolysis of free La³⁺ ions to form dimers, the interaction characteristics between [La(OH)₃]₂ and APTMACL, [La(OH)₂(TEA)]₂ and APTMACL, as well as these dimers with PVA and PAM molecules, were investigated (Fig. 5b). Compared to [La(OH)₃]₂, [La(OH)₂(TEA)]₂ exhibits lower interaction energies (E_i) when interacting with the APTMACL, PAM and PVA. For example, the interaction energies (E_i) of [La(OH)₃]₂ and [La(OH)₂(TEA)]₂ with APTMACL were calculated as −44.1 kcal mol⁻¹ and −1038.6 kcal mol⁻¹, respectively, indicating that the TEA-induced La³⁺ hydrolysis assembly and H-bond capping process increased the interaction energy (E_i) between the La(OH)₃ dimers and APTMACL molecules, thus reducing the intermolecular H-bond effect of the La hydroxides.^83,84 In the interaction between [La(OH)₂(TEA)]₂ and APTMACL, the complete dissociation of TEA occurred within [La(OH)₂(TEA)]₂, which led to the encapsulation of [La(OH)₂(TEA)]₂ between the upper and lower layers of the C₆H₁₄N molecules via ionic bonding polymerization. This reveals that the addition of TEA promotes the dispersion of active La–O sites on the hydroxide surface, thereby increasing the number of La–O sites for P. During the interaction between LHO and the PVA polymer chain, a portion of LHO dissociated, resulting in the release of TEA molecules. Consequently, the PVA polymer chain, released TEA, and partially dissociated LHO formed sandwich-like structures, effectively preventing the aggregation of LHO in the PVA gel system. Conversely, the dissociation of LHO did not occur in LaCPAM, and the interaction between LHO and PAM primarily involved H-bond interactions between the tertiary amine and acyl amino groups. Overall, copolymerization and subsequent dissociation of LHO in the hydrogels, promoted the dispersion of La species, thereby increasing the utilization of La–O sites.

Fig. 5 (a) Correlation between the adsorption capacity of P and the fractal dimension of La in the adsorbents. (b) Interaction energies of [La(OH)₂(C₆H₁₄N)]₂ and [La(OH)₃]₂ between PVA, APTMACL and PAM molecules calculated from MD simulations. (c) Schematic diagram of the interaction effect between [La(OH)₂(C₆H₁₄N)]₂ and [La(OH)₃]₂ with PVA, APTMACL, and PAM from MD simulations.

Conclusion

In this study, a novel strategy for tuning the dispersion of La–O active sites was developed, utilizing H bond-capping via organic–inorganic copolymerization. The maximum adsorption capacity of LaCCH was 308.2 mg_P g_La⁻¹, with a pseudo-second-order kinetics model fitting constant rate of 0.014 g mg_P⁻¹ h⁻¹. LaCCH exhibited selective adsorption for P across a wide pH range (7.0 to 11.0), as well as under high-salinity conditions (8.3 g L⁻¹ NaCl) and in the presence of organic phosphonates. Spectroscopic analyses confirmed that the selective P adsorption by LaCCH was due to inner-sphere complexation and the formation of LaPO₄·0.5H₂O. ToF-SIMS and MD simulations further revealed that the TEA-induced La³⁺ hydrolysis assembly and H bond-capping process reduced the interaction energies (E_i) between LHO and the organic molecules. This reduction in intermolecular hydrogen bonding in [La(OH)₃] promoted the dispersion of La–O active sites and enhanced P removal. In summary, copolymerization significantly improved the dispersion of La–O active sites, which enhanced P adsorption and demonstrated a strong correlation between fractal dimension and adsorption capacity.

Data availability

Data for this article are available at ScienceDB at [https://www.scidb.cn/s/nQzAFv] (CSTR: 31253.11.sciencedb.12405; DOI: https://doi.org/10.57760/sciencedb.12405).

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors would like to thank all editors and reviewers for their constructive and helpful comments. This work was financially supported by the National Natural Science Foundation of China (52030003, 52100095 and 52170122) and the Fundamental Research Funds for the Central Universities (2023MS069).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4en00791c

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