Symmetry-reduction enhancement of nitrate removal on record-breaking layered yttrium hydroxide adsorbents

Abstract

A new class of layered metal hydroxide materials, namely layered yttrium hydroxides (LYH-X, X⁻ are anions such as Cl⁻ and Br⁻), are reported to be excellent adsorbents to capture nitrate-nitrogen from neutral water. More importantly, the adsorption properties are correlated with the crystal symmetry of adsorbents: both the adsorption capacity and rate constant of LYH-Cl (orthorhombic P2₁2₁2 space group) are almost twice of those of LYH-Br (monoclinic P2₁ space group). A comprehensive study combining multinuclear solid-state NMR spectroscopy and other multiscale characterization techniques was then performed to understand the origin of the symmetry-related adsorption behaviors. The results reveal an increasing trend of change in local environments of Y(OH)₇·H₂O, Y(OH)₈·H₂O, and Y(OH)₈, implying that the adsorbed nitrate anions are located within the pocket constructed by alternating Y(OH)₈·H₂O and Y(OH)₈. The splitting of the ⁸⁹Y NMR peak of Y(OH)₈·H₂O of LYH-Cl after adsorption is consistent with the reduction of crystal symmetry from P2₁2₁2 to its translationengleiche subgroup P2, providing an additional driving force for nitrate capture. This work thus not only discovers a nitrate adsorbent with a superior capacity of 44.56 ± 0.17 mg g⁻¹, but also demonstrates a new concept of symmetry-driven adsorption enhancement.

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New concepts

We propose a crystal-symmetry-driven principle to design and optimize the properties of layered rare-earth hydroxides, moving beyond conventional composition-focused strategies. We demonstrate that the symmetry reduction of LYH-Cl during nitrate adsorption enables a record-high adsorption capacity of 44.56 ± 0.17 mg g⁻¹ at pH 7, which is more than twice that of its low-symmetry analog of LYH-Br. In addition, the LYH-Cl adsorbent exhibits a doubled adsorption rate constant compared to the LYH-Br adsorbent. The results fundamentally challenge the conventional optimization strategies for layered metal hydroxides, which have historically prioritized the dimensional matching via interlayer spacing modulation or ionic radius selection, while overlooking the effect of crystal symmetry. The symmetry-driven principle thus offers a previously unexplored dimension for searching adsorbents with exceptional potential for environmental remediation, broadening their applicability in sustainable chemistry, materials science, etc.

Introduction

The escalating prevalence of nitrogen pollutants such as nitrate (NO₃⁻) in aquatic ecosystems has emerged as a critical environmental challenge, particularly in regions with intensive agricultural activities. In these areas, groundwater nitrate concentrations often exceed 100 mg L⁻¹, while the World Health Organization's (WHO) recommended safety limit for nitrate-nitrogen is only 11.3 mg L⁻¹.^1,2 This alarming situation highlights the urgent need for highly effective nitrate removal techniques. The widely applied techniques of biological denitrification,^3–6 chemical precipitation⁷ and electrochemical reduction⁸ face limitations like secondary pollution, high energy consumption and inadequate removal, especially when dealing with the complex water matrices and high concentration pollutants. As a promising alternative, the adsorption technique has gained attention due to its simplicity, cost-effectiveness, and high efficiency. Among various nitrate adsorbents such as activated carbon,^9–13 zeolites,^14–17 metal–organic frameworks (MOFs),^18–20 etc.,^21,22 layered double hydroxides (LDHs) and their derived materials can exhibit remarkable adsorption properties (Table S1, ESI†).^23–26 In particular, the benchmark Cl-LDHs-C500 adsorbent possesses a nitrate-nitrogen uptake of 15.83 mg g⁻¹ at pH 5 within 300 minutes.²⁷ However, searching novel adsorbents with improved adsorption capacity and kinetics is still desirable for nitrate removal.

In recent years, a newer class of layered metal hydroxides, namely layered rare earth hydroxides (LREH-X, in which RE³⁺ are rare earth cations and X⁻ are anions),^28–30 have been reported as promising adsorbents via anion exchange.^31–33 More importantly, the crystal symmetry of LREH-X can vary with the anions despite similar layered structures,³⁴ providing new opportunities to further tune the adsorption properties by selecting different X⁻ anions. Herein, we conducted a systematic study of nitrate adsorption performances on two representative layered rare earth hydroxides, namely LYH-Cl and LYH-Br, and then explored the effect of crystal symmetry on nitrate adsorption. The results obtained show that this work not only deliver adsorbents with high nitrate capture performances but also demonstrate a new concept of symmetry-driven adsorption enhancement.

Results and discussion

As shown in Fig. 1, LYH-Cl and LYH-Br share many structural and functional similarities to LDHs such as metal hydroxide layers and anion-exchange capacity, but exhibit higher intralayer heterogeneity. Distinct to the uniform octahedral M(OH)₆ (M = metal ions) in LDHs, LYH-Cl and LYH-Br consist of Y³⁺ ions in three coordination types: eight-coordinated Y(OH)₇·H₂O and Y(OH)₈, nine-coordinated Y(OH)₈·H₂O. These polyhedra share edges to construct quasi-hexagonal layers, with the coordinated water molecules oriented towards the intralayer region, which are different from the flat layers of LDHs.³⁴ However, LYH-Cl adopts a higher crystal symmetry of P2₁2₁2 than LYH-Br (P2₁). We have previously found that such a symmetry difference may lead to inequivalent anion-exchange behaviour, e.g., only the transformation from a high-symmetry phase (LYH-Cl) to a low-symmetry phase (LYH-Br) is favourable.³⁴

Fig. 1 Crystal structure of LYH-Cl and LYH-Br viewed perpendicular and parallel to the yttrium hydroxide layers, respectively. Yttrium ions are represented as black spheres, oxygen atoms as red spheres, chloride ions as bright green spheres, and bromide ions as dark green spheres. Hydrogen atoms are omitted for clarity. Color-coded polyhedra represent different crystallographically distinct yttrium coordination sites for visual clarity.

The anion-exchange characteristics of LREH-X compounds ensure them as promising candidates for nitrate removal with a high theoretical capacity of 43.01 mg g⁻¹ (nitrogen content) for LYH-Cl and 37.85 mg g⁻¹ for LYH-Br, respectively, if the nitrate adsorption is solely due to anion exchange.^{30,32,34–36} To confirm that, we first measured the nitrate adsorption performances of two materials under practical wastewater-treatment conditions (pH 7, 25 °C, 250 mg L⁻¹ initial nitrogen concentration, 12 h) and found that both materials exhibit exceptional nitrogen uptake (26.93 mg g⁻¹ for LYH-Cl and 16.28 mg g⁻¹ for LYH-Br, respectively). We then explored the pH-dependence of nitrogen uptake on LYH-Cl and LYH-Br (Fig. 2a). The monotonic decrease of nitrogen uptake between pH 5 and pH 10 for both materials is consistent with the trend of change in ζ-potential.³⁷ However, the nitrogen uptake of LYH-Br decays faster than that of LYH-Cl, especially at high pH. Despite similar ζ-potentials at pH 7 (≈ 10 mV), the uptake of LYH-Cl (44.56 ± 0.17 mg g⁻¹) is more than twice the uptake of LYH-Br (20.87 ± 0.24 mg g⁻¹). Kinetic studies (Fig. 2b) illustrate that the nitrate adsorption adopts a pseudo-second-order behaviour for both adsorbents, suggesting chemisorption as the rate-limiting step,^38–40 and the rate constant of LYH-Cl (k₂ = 0.0018 g (mg min)⁻¹) is twice that of LYH-Br (k₂ = 0.0009 g (mg min)⁻¹). The adsorption isotherms can be fitted well with the Langmuir model (Fig. 2c), with a fitted maximum capacity (Q_max) of 45.63 mg g⁻¹ for LYH-Cl. In contrast, LYH-Br exhibits a Q_max of 20.59 mg g⁻¹. The agreement with the Langmuir model validates a monolayer adsorption mechanism, where nitrate anions tend to bind to homogeneous surfaces without obvious intermolecular interactions (see the Freundlich model fitting shown in Fig. S1, ESI†). To eliminate the effect of possible impurities of LYH-Cl and LYH-Br, we further synthesized Y₂O₃ and Y(OH)₃ (Fig. S2, ESI†) and found the two materials exhibit very small nitrogen uptake under the same condition (< 5 mg g⁻¹). To further evaluate the suitability of LYH-Cl and LYH-Br as nitrate adsorbents in wastewater treatment, cyclic adsorption–desorption experiments were conducted by using saturated NaCl or NaBr (25 °C, 12 h) for desorption. A decreasing trend of adsorption capacity is observed for both adsorbents (Fig. S3 and S4, ESI†) due to incomplete desorption of nitrate anions and the CO₃²⁻ contamination from air, which is common for anion-exchangeable layered metal hydroxides. It is worth noting that these layered metal hydroxides can be regenerated by the calcination-rehydration process, which is more effective in restoring the pristine structure and adsorption capacity. The influence of competing anions (Cl⁻, Br⁻, SO₄²⁻, PO₄³⁻, and HCO₃⁻) was also examined with a concentration of 100 mg L⁻¹ for NO₃⁻–N and 200 mg L⁻¹ for the competing anion (Fig. S5, ESI†). It is striking that LYH-Cl retains more than 60% of adsorption capacity even in the presence of SO₄²⁻ and PO₄³⁻, in which the high charge density of these multivalent anions yields a strong electrostatic affinity to dominate site competition.¹⁹ And the leaching amount of Y in adsorption–desorption cycles was not significant (Fig. S6, ESI†).

Fig. 2 (a) Adsorption capacity of NO₃⁻–N on LYH-Cl (left) and LYH-Br (right) as a function of pH values (2.5 g L⁻¹ adsorbent, 500 mg L⁻¹ NO₃⁻–N, 12 h, 25 °C). (b) NO₃⁻–N adsorption capacity of LYH-Cl (left) and LYH-Br (right) as a function of time fitted with pseudo-first-order and pseudo-second-order kinetic models (2.5 g L⁻¹ adsorbent, 500 mg L⁻¹ NO₃⁻–N, 12 h, 25 °C). (c) Adsorption isotherm of LYH-Cl (left) and LYH-Br (right) fitted with the Langmuir model (2.5 g L⁻¹ adsorbent, C_e = 50–1400 mg L⁻¹, 12 h, 25 °C; error bars are enlarged by a factor of five for clarity).

To explore the molecular mechanism for nitrate adsorption, we systematically compared the structure of adsorbents before and after the reaction by powder X-ray diffraction (PXRD), scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS), X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared (FTIR) spectroscopy. As shown in Fig. 3a, most of the diffraction peaks were invisible after adsorption even at a very low nitrogen concentration of 50 mg L⁻¹, leaving only (00n) diffraction peaks characteristic of layered structures, which can be induced by displacement between adjacent yttrium hydroxide layers. In contrast, fine features were still present in the PXRD pattern of LYH-Br after adsorption even at the highest nitrogen concentration, implying a significantly lower stacking disorder of layers. As a result, the degree of stacking disorder seems to positively correlate with the nitrate adsorption capacity. Additional PXRD data (Fig. S7, ESI†) imply that the stacking disorder of LYH-Cl emerges immediately after adsorption, while such disorder can only be observed at a very long adsorption time for LYH-Br (Fig. S8, ESI†). SEM images show that both adsorbents retain their initial platelet morphology after adsorption (Fig. 3b) but the surface N/Cl atomic ratio is much higher than the surface N/Br atomic ratio (EDS mapping, Fig. S9–S11, ESI†). XPS analyses also confirm the significantly higher nitrogen uptake on LYH-Cl under the same conditions (Fig. 3c and Fig. S12–S14, ESI†). Different from EDS and XPS, the FTIR technique can provide insights into the structural changes of bulk materials. Fig. 3d indicates that the nitrate ions have been captured by two adsorbents by the emergence of strong NO₃⁻ vibrational modes at 1384 cm⁻¹ (ν₃ asymmetric stretch) and 1084 cm⁻¹ (ν₁ symmetric stretch).³⁶

Fig. 3 (a) PXRD patterns of LYH-Cl (left) and LYH-Br (right) before and after nitrate adsorption (C_e = 50–1200 mg L⁻¹, pH 7, 12 h). (b) SEM images of LYH-Cl (left) and LYH-Br (right) before and after nitrate adsorption (2.5 g L⁻¹ adsorbent, 1200 mg L⁻¹ NO₃⁻–N, pH 7, 12 h). (c) Cl 2p and N 1s XPS spectra of LYH-Cl (left) before and after nitrate adsorption, Br 3d and N 1s XPS spectra of LYH-Br (right) before and after nitrate adsorption (2.5 g L⁻¹ adsorbent, 1200 mg L⁻¹ NO₃⁻–N, pH 7, 12 h). (d) FTIR spectra of LYH-Cl (left) and LYH-Br (right) before and after nitrate adsorption as a function of time (2.5 g L⁻¹ adsorbent, 500 mg L⁻¹ NO₃⁻–N, pH 7).

As discussed above, routine characterization techniques such as PXRD, SEM-EDS, XPS and FTIR only provide limited insights into the structural changes after adsorption because they are not sensitive to or lack the resolution necessitated to distinguish local environments. To bridge this gap, we developed solid-state nuclear magnetic resonance (ssNMR) methodology to provide atomic-level structural information within the LYH-X materials, including site symmetry, displacement of hydroxide layers and anions, identity of hydrogen species and their spatial proximity, structural evolution during calcination, state of anions after exchange, etc.^{32,34,41–44} Herein, we first conducted ⁸⁹Y ssNMR experiments to probe the change of the local Y³⁺ environment within hydroxide layers. As shown in Fig. 4a, the ⁸⁹Y NMR spectrum of LYH-Cl exhibits dramatic and site-specific changes after adsorption. The ⁸⁹Y peak of the Y(OH)₇·H₂O environment (Y1 site) is only broadened, while the peaks of Y(OH)₈ (Y2 site) and Y(OH)₈·H₂O (Y3 site) display significant changes: the peak of the Y2 site moves to the shielded region and the peak of the Y3 site moves to the opposite (deshielded) direction, splitting into two peaks. As ⁸⁹Y isotropic chemical shift (δ_iso(⁸⁹Y)) values are very sensitive to the local environment of Y³⁺, decreasing with the increasing coordination number,⁴⁵ the shift of the Y2 peak from 125.8 ppm to 49.6 ppm suggests that this site must strongly interact with the adsorbed nitrate ions to form a quasi-nine-coordinated environment, with δ_iso(⁸⁹Y) similar to that of the Y(OH)₉ (68 ppm) in hexagonal Y(OH)₃.⁴⁶ Moreover, the splitting of the Y3 peak (14.7 ppm) to two ⁸⁹Y peaks with equal intensity (24.3 ppm and 29.7 ppm) unambiguously verifies the symmetry reduction of LYH-Cl during nitrate adsorption. According to crystallography, every space group can be facilely converted to its maximal subgroups by the corresponding transformation matrix:⁴⁷ translationengleiche subgroup(s) (t-subgroup) whose point group is decreased, and klassengleiche subgroup(s) (k-subgroup) whose point group is retained. As Scheme S1 (ESI†) illustrates, there are two t-subgroups of P2₁2₁2 (#18): P2₁ (#4) and P2 (#3). The symmetry reduction from P2₁2₁2 to P2₁ induces the splitting of Wyckoff position 4c to two Wyckoff positions 2a and 2a, giving rise to the splitting of the single ⁸⁹Y NMR peak of Y(OH)₇·H₂O for LYH-Cl to two ⁸⁹Y NMR peaks of Y(OH)₇·H₂O for LYH-Br. Similarly, the symmetry reduction from P2₁2₁2 to P2 induces additional splitting of Wyckoff position 2a to two Wyckoff positions 1a and 1d (2b to 1b and 1c), consistent with the splitting of the single ⁸⁹Y NMR peak of Y(OH)₈·H₂O (2b) to two ⁸⁹Y NMR peaks (1b and 1c) for LYH-Cl after adsorption. We should point out that the splitting of other ⁸⁹Y NMR peaks is not observed due to small δ_iso(⁸⁹Y) differences and/or significant peak broadening. In contrast, the ⁸⁹Y NMR spectrum of LYH-Br is almost not altered by nitrate adsorption except the peak broadening and the emergence of a weak and very broad peak centered at 66.7 ppm. Based on the relative intensity, it seems that some Y(OH)₈ spheres (Y3 site) are converted to quasi-nine-coordinated spheres with a δ_iso(⁸⁹Y) of 66.7 ppm, whereas the structural changes on other sites are negligible. We further monitored the replacement of anions by ³⁵Cl (Fig. 4b) and ⁷⁹Br (Fig. 4c) ssNMR experiments. The ³⁵Cl NMR signal decays dramatically after adsorption and becomes almost invisible at higher nitrogen concentration for LYH-Cl, whereas the ⁷⁹Br NMR signal persists even at very high nitrogen concentrations. Such observation is consistent with the adsorption capacity of two materials: all Cl⁻ anions can be substituted by NO₃⁻ in LYH-Cl and only half of the Br⁻ anions can be substituted by NO₃⁻ in LYH-Br.

Fig. 4 (a) ⁸⁹Y NMR spectra of LYH-Cl (left) and LYH-Br (right) before and after nitrate adsorption (C_e = 50–1200 mg L⁻¹, 2.5 g L⁻¹ adsorbent, pH 7, 12 h). (b) ³⁵Cl NMR spectra of LYH-Cl (*: NaCl) before and after nitrate adsorption (C_e = 50–1200 mg L⁻¹, 2.5 g L⁻¹ adsorbent, pH 7, 12 h). (c) ⁷⁹Br NMR spectra of LYH-Br (*: NaBr) before and after nitrate adsorption (C_e = 50–1200 mg L⁻¹, 2.5 g L⁻¹ adsorbent, pH 7, 12 h).

The molecular mechanism of nitrate adsorption on two LYH-X compounds can now be proposed as follows (Fig. 5): the NO₃⁻ anions insert into the interlayer space and replace existing Cl⁻ or Br⁻ anions during adsorption. The adsorbed nitrate anions reside within the pockets constructed by alternating Y(OH)₈ and Y(OH)₈·H₂O polyhedra, with weak coordination on the Y³⁺ ion of Y(OH)₈ and hydrogen bonding to coordinated water molecules of Y(OH)₈·H₂O. However, the crystal symmetry of LYH-Cl is reduced by the displacement between neighbouring layers accompanying with nitrate insertion. In addition, the increase of entropy of the LYH-Cl adsorbent by lowering its crystal symmetry from P2₁2₁2 to P2 can provide extra driving force for nitrate adsorption. This directional transition follows Bärnighausen's group-subgroup hierarchy described previously in the literature,⁴⁸ where asymmetric energy barriers are present between the structural transformation between a space group and its t-subgroup. Both factors contribute to the higher adsorption capacity and faster adsorption kinetics of LYH-Cl than LYH-Br.

Fig. 5 Schematic illustration of the nitrate adsorption mechanism of LYH-Cl (left) and LYH-Br (right).

Conclusions

This comprehensive study reports a novel layered metal hydroxide adsorbent (LYH-Cl) with a record-breaking capacity of 44.56 ± 0.17 mg g⁻¹ at pH 7. By comparing LYH-Cl (orthorhombic P2₁2₁2 space group) with its lower-symmetry analog LYH-Br (monoclinic P2₁ space group), it is found that the reduction of crystal symmetry during the anion-exchange reaction is responsible for the doubled capacity and rate constant of LYH-Cl, unraveled by a multiscale investigation integrating PXRD, SEM-EDS, XPS, FTIR, and especially ssNMR results. This work not only develops high-performance materials for environmental remediation, but also provides an additional symmetry-driven principle to design and optimize the properties of layered materials, opening avenues to broaden their applicability in sustainable chemistry, materials science, etc. And we will continue to work on the scalability and optimization of structuring/regeneration conditions of the LYH-Cl adsorbent to check its feasibility for large-scale applications.

Author contributions

Conceptualization: JX; methodology: XS; supervision: JX; investigation: XS, YW; formal analysis: XS; resources: JX; writing – original draft: XS; writing – review and editing: XS, JX; project administration: JX; and funding acquisition: JX.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the ESI.†

Acknowledgements

We thank the financial support from the National Natural Science Foundation of China (grant no. 22471128).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5mh01085c

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