γ‑AlO(OH)/MgAl‑LDH Composite: Record‑High Adsorption of Methyl Orange and Sustainable Reuse

Abstract

We report a one‑pot hydrothermal synthesis that inserts nanoneedle γ‑AlO(OH) into MgAl‑LDH layers, yielding a heterostructure with exceptional adsorption of methyl orange (MO). The interlayer spacing expands from 7.6 Å to 8.77 Å, while abundant –OH groups enhance chemisorption kinetics. The composite removes 1000 mg L⁻¹ MO completely within 210 min, achieving a Langmuir capacity of 4681.40 mg g⁻¹ – the highest reported for any LDH‑based adsorbent. After four adsorption–desorption cycles the capacity remains above 760 mg g⁻¹, demonstrating robust recyclability.

Introduction

Organic dyes, ubiquitous in textiles, leather, and paints, pose severe environmental threats when discharged into water bodies. Their high polarity, low volatility, and resistance to biodegradation make them difficult to remove. Conventional methods (photocatalysis, biological oxidation, membrane filtration) are energy‑intensive or costly. Physical adsorption, especially with layered double hydroxides (LDHs), offers a simple, cost‑effective alternative due to their high anion‑exchange capacity and tunable surface area.

Recent advances have shown that inserting organic or inorganic species between LDH layers can further boost adsorption. γ‑AlO(OH), with its high specific surface area and plentiful hydroxyl groups, is an ideal candidate for intercalation. Here we demonstrate its successful integration into MgAl‑LDH, producing a composite that combines expanded interlayer spacing and enhanced –OH activity, yielding record adsorption performance.

Methods

Preparation of γ‑AlO(OH)/MgAl‑LDH

Mg(NO₃)₂·6H₂O (4.615 g) and Al(NO₃)₃·9H₂O (3.376 g) were dissolved in 50 mL DI water. NaOH (2.516 g) was dissolved in 25 mL DI water to maintain pH 10. The two solutions were added dropwise to 25 mL DI water under vigorous stirring at 60 °C, then sealed and heated at 140 °C for 10 h. The resulting slurry was washed, lyophilized, and stored. Pure MgAl‑LDH and γ‑AlO(OH) were synthesized identically for comparison.

Characterization

Phase identification: XRD (Cu Kα, 5–80°). Morphology: FESEM (S4800, 5 kV) and TEM (JEM‑2100F, 200 kV). Surface chemistry: FTIR (4000–400 cm⁻¹) and XPS (ESCALAB 250Xi). Porosity: N₂ adsorption–desorption (BET, BJH). Optical properties: UV–Vis and photoluminescence (Varian). Kinetic and isotherm studies employed standard equations for pseudo‑first/second order and Langmuir/Freundlich models.

Adsorption Experiments

50 mg of adsorbent was stirred in 50 mL of 1000 mg L⁻¹ MO solution (pH 3). At predetermined times, 3 mL aliquots were centrifuged; supernatant absorbance was measured at 463 nm. Equilibrium and instantaneous adsorption capacities were calculated via qₜ = (C₀–Cₜ)V/m and qₑ = (C₀–Cₑ)V/m.

Desorption and Regeneration

After adsorption, samples were washed with DI water, then with 30 % ethanol, and lyophilized. Repeated adsorption–desorption cycles assessed recyclability.

Results and Discussion

Structural Characterization

XRD confirmed the presence of both MgAl‑LDH and γ‑AlO(OH). The (003) peak shifted from 11.63° (7.6 Å) to 10.09° (8.77 Å), indicating expanded interlayer spacing. FESEM/TEM revealed nanosheet MgAl‑LDH layers interspersed with nanoneedle γ‑AlO(OH). HRTEM lattice fringes (0.235 nm, 0.152 nm) correspond to γ‑AlO(OH) (031) and MgAl‑LDH (110). EDX mapping showed homogeneous elemental distribution.

Effect of pH

Optimal adsorption occurred at pH 3; capacity declined at higher pH due to reduced electrostatic attraction. At pH 3, the solution turned clear after 210 min, confirming complete dye removal.

Adsorption Kinetics

All samples displayed rapid initial uptake. γ‑AlO(OH)/MgAl‑LDH reached an equilibrium capacity of 1000 mg g⁻¹ (MO concentration 1000 mg L⁻¹) versus 183.3 and 155.5 mg g⁻¹ for γ‑AlO(OH) and MgAl‑LDH, respectively. Pseudo‑second‑order fitting (R² > 0.99) indicated chemisorption as the rate‑determining step.

Isotherm Analysis

Langmuir fits yielded qₘ = 4681.40 mg g⁻¹, K_L = 4.62 × 10⁵ L mg⁻¹ for the composite, far surpassing γ‑AlO(OH) (1492.5 mg g⁻¹) and MgAl‑LDH (769.2 mg g⁻¹). Freundlich constants further confirmed strong adsorption affinity.

Adsorption Mechanism

BET surface areas (MgAl‑LDH = 14.1 m² g⁻¹; γ‑AlO(OH) = 95.9 m² g⁻¹; composite = 34.1 m² g⁻¹) show that capacity is not driven by surface area alone. Instead, the expanded (003) plane allows MO anions to intercalate via ion exchange, as evidenced by new XRD peaks and a 3.22 Å interlayer increase after adsorption. Zeta‑potential measurements (γ‑AlO(OH)/MgAl‑LDH = +43 mV at pH 3) indicate a highly positively charged surface that strongly attracts anionic dyes. FTIR and XPS analyses reveal hydrogen bonding and chemisorption onto –OH sites, consistent with pseudo‑second‑order kinetics.

Recyclability

After four adsorption–desorption cycles, the composite retained 762 mg g⁻¹ capacity (>76 % of initial), confirming durable performance.

Conclusion

We have engineered a γ‑AlO(OH)/MgAl‑LDH heterostructure via a single‑pot hydrothermal route that delivers unprecedented adsorption of methyl orange, achieving a Langmuir capacity of 4681.40 mg g⁻¹ and full dye removal within 210 min. The synergistic “space‑confined” growth of γ‑AlO(OH) and abundant hydroxyl groups underpin the superior kinetics and capacity. The material remains highly recyclable, retaining >76 % capacity after four cycles. Its strong positive surface charge and robust structure make it a promising candidate for treating anionic dye pollutants and potentially serving as a support for photocatalysts.