Optimized Fe³⁺‑Grafted BiOCl for Rapid Adsorption of Trace Cationic and Anionic Dyes in Aqueous Wastewater

Abstract

We report the synthesis, comprehensive characterization, and application of bismuth oxychloride (BiOCl) and Fe³⁺‑grafted BiOCl (Fe/BiOCl) as high‑performance adsorbents for removing low‑concentration (0.01–0.04 mmol L⁻¹) cationic dyes (rhodamine B, RhB; methylene blue, MB) and anionic dyes (methyl orange, MO; acid orange, AO) from water. Fe³⁺ grafting creates a more open, hierarchically porous structure and significantly increases the specific surface area. Both materials, with negatively charged surfaces, adsorb cationic dyes with near‑complete efficiency within minutes (99.6 % for RhB and 98.0 % for MB on BiOCl; 97.0 % and 98.0 % on Fe/BiOCl within 10 min). Fe/BiOCl outperforms BiOCl for anionic dyes, achieving 60 % adsorption of MO in 20 min. Adsorption behavior follows the Langmuir isotherm and pseudo‑second‑order kinetics for RhB. Electrostatic attraction, enhanced surface area, and open porosity jointly explain the superior performance. In mixed‑dye solutions, BiOCl shows higher selectivity for cationic dyes compared to Fe/BiOCl, highlighting its potential for real industrial wastewater treatment.

Background

Water contamination by dyes, organic pollutants, and heavy metals poses severe ecological and health risks worldwide. Conventional removal strategies—ion exchange, adsorption, precipitation, oxidation, biodegradation, and photocatalysis—vary in cost, efficiency, and environmental impact. Adsorption stands out for its simplicity, scalability, and low energy demand, making it a prime candidate for wastewater remediation.

BiOCl, a layered bismuth oxyhalide, has attracted attention as a photocatalyst. Its unique 3‑D flower‑like morphology offers a large surface area and porous architecture, which can potentially translate into high adsorption capacity. Prior work has improved BiOCl’s dye removal by surface modifications (e.g., CTAB, Ti, I) or by altering stoichiometry. However, systematic studies on Fe³⁺‑modified BiOCl for low‑concentration dye removal are scarce.

In this study, we synthesized Fe/BiOCl via a hydrothermal route, characterized the resulting nanostructure, and evaluated its performance against RhB, MB, MO, and AO at environmentally relevant concentrations. We also examined mixed‑dye adsorption, temperature, pH, and adsorption kinetics to elucidate the underlying mechanisms.

Methods

Synthesis of BiOCl and Fe/BiOCl

Bi(NO₃)₃·5H₂O, Fe(NO₃)₃·9H₂O, KCl, and glycerol (analytical grade) were used without further purification. For BiOCl, 0.776 g Bi(NO₃)₃·5H₂O was dissolved in 76 mL glycerol (solution A). 0.12 g KCl was dissolved in 4 mL deionized water (solution B). Solutions A and B were mixed, transferred to a Teflon‑lined autoclave, and heated at 110 °C for 8 h. The precipitate was washed with ethanol and water, dried at 80 °C, then calcined at 400 °C for 3 h to yield pure BiOCl. Fe/BiOCl was prepared identically, except that Fe(NO₃)₃·9H₂O was added to solution A in varying amounts to achieve Fe/Bi molar ratios (x). Samples are denoted Fe/BiOCl (x).

Characterization

X‑ray diffraction (Cu Kα, λ = 0.154 nm) confirmed the tetragonal BiOCl phase. Transmission electron microscopy (TEM) and high‑resolution TEM (HRTEM) on a JEM‑2010 (200 kV) revealed flower‑like microspheres composed of nanoplates. Scanning electron microscopy (SEM, Hitachi S‑4800, 15 kV) provided surface morphology details. X‑ray photoelectron spectroscopy (XPS, Thermo Escalab 250Xi, Al Kα) identified elemental composition and oxidation states. Nitrogen adsorption–desorption (Micromeritics ASAP 2020) measured BET surface areas and BJH pore size distributions. Zeta potential was determined with a DelsaTM Nano Zeta Potential instrument across pH 5–13.

Adsorption Experiments

Adsorption tests were conducted at room temperature in the dark. 50 mg of adsorbent was added to 50 mL of dye solution (0.01–0.04 mmol L⁻¹). Samples were stirred, and aliquots were withdrawn, centrifuged, and analyzed by UV‑vis spectroscopy (Hitachi U‑3900). Residual dye percentages and adsorption capacities were calculated using standard equations. Parameters such as contact time, initial concentration, temperature (25–85 °C), and pH (5–13) were varied systematically. Regeneration involved stirring the used adsorbent in 0.01 M NaOH/ethanol for 60 min, followed by washing and drying. Photocatalytic regeneration was performed under visible light for 60 min to assess the durability of the adsorbents.

Results and Discussion

Structural and Morphological Analysis

XRD patterns (Figure 1) show pure tetragonal BiOCl; Fe³⁺ incorporation does not alter the lattice, indicating Fe³⁺ remains as surface‑grafted species rather than dopants. SEM images (Figure 2) reveal that Fe/BiOCl microspheres are smaller (0.5–1 µm) and composed of thinner nanosheets (≈15 nm) compared to BiOCl (1–2 µm, 20 nm). TEM/HRTEM (Figure 3) confirm well‑defined lattice fringes (d = 0.276 nm, 0.344 nm). Energy‑dispersive X‑ray mapping (Figure 4) shows uniform Fe distribution. BET surface areas increase from 35.05 m² g⁻¹ (BiOCl) to 58.96 m² g⁻¹ (Fe/BiOCl 0.25); pore volumes similarly rise, reflecting the more open micro‑flower structure. Zeta potential (Figure 6b) indicates negative surface charge across pH 5–13 for both materials, with Fe/BiOCl slightly less negative due to Fe³⁺ surface complexes.

Adsorption Performance

BiOCl achieves >99 % RhB removal in 3 min and >98 % MB in 10 min; Fe/BiOCl (0.25) shows comparable performance. For anionic dyes, BiOCl adsorbs only ~30 % MO within 20 min, whereas Fe/BiOCl reaches ~60 % due to its higher surface area and porosity. Temperature (25–85 °C) has negligible effect on RhB adsorption but lowers MO uptake, indicating exothermic interactions for anionic dyes. pH studies (5–13) reveal optimal RhB removal at acidic pH 5; higher pH diminishes performance, likely due to BiOCl instability or reduced electrostatic attraction. MO adsorption declines sharply above pH 11.

Mechanism

Electrostatic attraction dominates cationic dye uptake, as evidenced by negative zeta potential and strong RhB/MB adsorption. Anionic dye removal is governed by increased surface area and porosity, compensating for electrostatic repulsion. The pseudo‑second‑order kinetic model fits the data best (R² ≈ 0.99), suggesting chemisorption as the rate‑determining step. Langmuir isotherms (Figure 11) yield maximum capacities close to experimental values, confirming monolayer coverage.

Mixed‑Dye Selectivity

In solutions containing equal amounts of RhB, MB, MO, and AO, BiOCl preferentially adsorbs cationic dyes (>90 % removal) over anionic ones (<30 %). Fe/BiOCl shows a broader but less selective uptake, reflecting its enhanced capacity for anionic dyes. Competitive adsorption reduces overall uptake compared to single‑dye systems, as expected.

Regeneration and Stability

Both BiOCl and Fe/BiOCl retain >80 % RhB removal after three cycles; Fe/BiOCl maintains ~50 % after five cycles. Photocatalytic degradation under visible light fully restores adsorbent color and adsorption capacity, demonstrating robust regeneration potential.

Conclusions

Fe³⁺ grafting transforms BiOCl into a highly porous, high‑surface‑area adsorbent that excels at removing trace cationic dyes rapidly while significantly improving anionic dye uptake. In mixed‑dye scenarios, pristine BiOCl offers superior selectivity for cationic dyes, whereas Fe/BiOCl balances performance across dye types. Both materials can be regenerated via mild chemical or photocatalytic processes, underscoring their promise for sustainable industrial wastewater treatment.