High‑Performance Pr2CuO4 Nanosheet Adsorbent: Controlled Synthesis, Superior Selectivity for Malachite Green, and Mechanistic Insights

Abstract

Tetragonal Pr₂CuO₄ nanosheets, ~60 nm thick, were produced by coordination‑compound methods (CCMs) and demonstrate exceptional selective adsorption of malachite green (MG) in aqueous media. The material was comprehensively characterized by XRD, SEM, HRTEM, XPS, UV‑Vis DRS, and BET analyses. Adsorption isotherms (0.03–0.07 g adsorbent) at 298–338 K fitted best to the Langmuir model, yielding a maximum capacity (Q_m) of 3.52 g g^‑1 at 298 K. Deviations from the Langmuir prediction at very low (<0.03 g) and high (>0.07 g) doses were attributed to systematic mass loss and particle aggregation. Mechanistic studies—supported by CuO/Pr₂O₃ control experiments, competitive‑ion tests, and DFT calculations—indicate that MG adsorption proceeds via O–Pr coordination, H‑atom migration, and multilayer hydrogen bonding, explaining the record selectivity and capacity.

Background

Industrial dye effluents, especially malachite green (MG), pose severe ecological and health risks due to their non‑biodegradability and bioaccumulation [1–6]. Conventional adsorbents (e.g., ZnO, ZrO, mesoporous carbons, MOFs) suffer from reduced performance in real water matrices, underscoring the need for high‑capacity, selective sorbents [7–17]. Rare‑earth cuprates, such as Dy₂Cu₂O₅, have shown remarkable MG uptake (Q_m > 5.5 g g^‑1) but their mechanisms remain unclear. MG’s unique leucomalachite green (LMG) isomer possesses coordinatable oxygen atoms, suggesting a coordination‑bond–driven adsorption pathway akin to that in MOFs [4,17]. This study elucidates the adsorption behavior of Pr₂CuO₄ on MG, investigates deviations from Langmuir behavior, and validates a multilayer adsorption mechanism through experimental and theoretical analyses.

Methods / Experimental

Materials

Analytical‑grade Cu(OAc)₂·4H₂O, Pr(NO₃)₃·5H₂O, 3,4‑pdc, triethylamine, and MG were sourced from Sinopharm and Aladdin Industrial Corporation.

Synthesis

Precursor [PrCu(3,4‑pdc)₂(OAc)(H₂O)₂]·10.5H₂O was prepared per our previous protocol [21,22]. In a 1:1 H₂O/MeOH mixture, stoichiometric Cu, Pr, 3,4‑pdc, and triethylamine were dissolved, stirred 3 h, filtered, and left to crystallize blue polycrystals. Calcination in N₂ at 900 °C for 1 h yielded Pr₂CuO₄ nanosheets.

Characterization

XRD (Cu Kα, 0.05° s^‑1) confirmed tetragonal Pr₂CuO₄ (PDF # 22‑0245). SEM (Zeiss Supra 55) and HRTEM (FEI Tecnai F30) revealed well‑dispersed nanosheets (~60 nm thick) with octagonal facets. BET analysis (Builder SSA‑4300) gave a surface area of 11.6 m² g^‑1 and pore sizes 10–100 Å. XPS (PHI 5000 C) confirmed Pr³⁺ and Cu²⁺ oxidation states and distinct O species. UV‑Vis DRS (PERSEE T9) showed a 0.51 eV bandgap, indicating a dark‑blue, photoconductive material.

Adsorption Experiments

Batch studies (1000 mL, 0.1 g L^‑1 MG, 100 rpm) varied adsorbent dose (0.03–0.07 g). Equilibrium concentrations were measured by UV‑Vis (RF 5301). Adsorption capacity: q_e = ((C₀–C_e)V)/m. Isotherms were fitted to Langmuir and Freundlich equations; kinetics followed a pseudo‑second‑order model. Competitive‑ion tests (MO, RhB, Cl^‑, Na⁺, OAc^‑, Cu²⁺, Pr³⁺) assessed selectivity.

Theoretical Studies

DFT calculations (DMol³, DNP basis, PW91 GGA) modeled the Pr₂CuO₄ (001) surface. Two adsorption routes were examined: (1) direct O–Pr coordination of MG; (2) H‑atom migration from –OH to –NH₂ followed by O–Pr bonding and multilayer H‑bonding. Route 2 exhibited the strongest binding (83.3 kJ mol^‑1) and an additional 26.4 kJ mol^‑1 stabilization from H‑bond networks.

Results and Discussion

Structural Characterization

XRD patterns (Fig. 1) confirm phase purity at 900 °C; lower temperatures yield unreacted Pr₂O₃ and CuO, while >1100 °C reintroduces CuO impurities. SEM/TEM (Fig. 2) illustrate layered, octagonal nanosheets interconnected into a porous framework suitable for mass transport. HAADF and SAED verify single‑crystalline domains with (001) exposed facets. BET curves (Fig. 3) display a type III isotherm with negligible hysteresis, consistent with weak N₂–surface interactions.

Adsorption Performance

Equilibrium isotherms (Fig. 4) reveal a steep rise in MG uptake at low concentrations, plateauing at 3.52 g g^‑1 (298 K). The Langmuir model fits superiorly (R² > 0.99) compared to Freundlich, indicating monolayer coverage. Thermodynamic analysis yields Δ_rG^θ = –16.9 kJ mol^‑1 (spontaneous), Δ_rH^θ = –6.41 kJ mol^‑1 (exothermic), and Δ_rS^θ = 35.1 J mol^‑1 K^‑1 (entropy increase). Temperature elevation reduces Q_m (3.52→2.17 g g^‑1 at 338 K), confirming exothermicity.

Deviation Analysis

Plotting 1/q_e vs. 1/C_e (Eq. 2) revealed systematic deviations for doses <0.03 g and >0.07 g. Introducing a correction term m′ (Eq. 5) improved linearity, with an optimal m′ = 0.003 g—attributable to mass loss from particle aggregation. At high doses, visible dark‑blue spots on the vessel walls indicated multilayer adsorption and agglomeration, corroborated by reduced aggregation in a 1:1 H₂O/ethanol mixture.

Mechanistic Insights

DFT route 2 predicts O–Pr coordination (62.5 kJ mol^‑1) and, after H‑atom migration, a stronger bond (83.3 kJ mol^‑1) with a 2.99 Å O–Pr distance. Subsequent H‑bond formation between MG layers yields additional 26.4 kJ mol^‑1 stabilization, explaining the high capacity and multilayer behavior. Competitive‑ion experiments (Fig. 11) confirm that OAc^‑, Cu²⁺, and Pr³⁺ compete for coordination sites, whereas neutral dyes (MO, RhB) and simple salts (Cl^‑, Na⁺) have negligible effect—highlighting the specificity of coordination‑driven adsorption.

Conclusions

Pr₂CuO₄ nanosheets, prepared via CCMs, achieve an unprecedented Q_m of 3.52 g g^‑1 for MG at 298 K. Deviations from Langmuir behavior stem from systematic mass loss (m′ = 0.003 g) at low doses and particle aggregation at high doses. A multilayer adsorption mechanism—rooted in O–Pr coordination, H‑atom migration, and H‑bond networks—accounts for the exceptional selectivity and capacity, as validated by DFT calculations and competitive‑ion tests.

Abbreviations

3,4‑pdc

3,4‑pyridinedicarboxylic acid

CCMs

Coordination compound methods

HAADF

High‑angle annular dark‑field

MG

Malachite green

MOFs

Metal‑organic frameworks

Q_m

Maximum adsorption capacity

RhB

Rhodamine B

SAED

Selected area electron diffraction