Key Takeaways & Executive Findings
- •• • Optimal mass ratio (BON:BI = 2:8) yields BON@BIOPT with 99.8% RhB degradation in 30 min visible light, a 1.9-fold improvement over pristine BI (52.2%), and an apparent rate constant (Kapp) of 0.20617 min⁻¹—8.64 times higher than BI (0.02385 min⁻¹). This performance leap addresses the bottleneck of slow kinetics in single-phase photocatalysts, enabling faster treatment cycles in industrial wastewater. • • BON@BIOPT exhibits a 37.4% increase in specific surface area (44.7 vs. 32.54 m²·g⁻¹) and a 20 nm red-shift in absorption edge (580 vs. 560 nm), narrowing the bandgap from 2.55 to 2.43 eV. Enhanced light harvesting and more active sites directly translate to higher quantum efficiency and lower energy consumption for visible-light-driven water treatment. • • The S-scheme heterojunction with internal electric field suppresses charge recombination, as evidenced by the dominant reactive species (O₂•⁻ and h⁺). This mechanistic design ensures efficient spatial separation of redox-active carriers, a critical factor for achieving sustained high mineralization rates in complex effluents. • • BON@BIOPT retains 88.6% degradation efficiency after 7 cycles and maintains 85–98% efficiency under pH variations and in the presence of common anions (HCO₃⁻, Cl⁻, SO₄²⁻, H₂PO₄⁻, NO₃⁻). This robustness indicates operational longevity and resilience to real-world water matrices, reducing catalyst replacement frequency and operational costs.
Abstract
A novel S-scheme heterojunction photocatalyst, Bi6O5(OH)3(NO3)5·3H2O/BiOBr0.8I0.2 (BON@BI), was synthesized via a one-step hydrothermal method using Bi6O5(OH)3(NO3)5·3H2O (BON), KBr, and KI as precursors. The mass ratio of BON to BiOBr0.8I0.2 (BI) was optimized, revealing that the 20% BON@BI composite (BON@BIOPT) exhibited the highest visible-light photocatalytic activity. Under 30 min of visible-light irradiation, BON@BIOPT achieved a 99.8% degradation efficiency of Rhodamine B (RhB), approximately twice that of pristine BI (52.2%). The composite displayed a rod-like morphology with uniform nanosheets, and its specific surface area increased from 32.54 m²·g⁻¹ (BI) to 44.7 m²·g⁻¹. The absorption edge red-shifted from 560 nm (BI) to 580 nm, narrowing the bandgap from 2.55 eV to 2.43 eV. The S-scheme heterojunction formed between BON and BI generates an internal electric field that effectively suppresses recombination of strongly reducing photogenerated electrons and strongly oxidizing holes, with superoxide radicals (O₂•⁻) and holes (h⁺) identified as the primary reactive species. BON@BIOPT exhibited excellent stability, retaining 88.6% degradation efficiency after seven consecutive cycles. It also demonstrated robust environmental adaptability, maintaining 85–98% degradation efficiency under various pH conditions and in the presence of interfering anions. The degradation pathway of RhB involves N-de-ethylation, cleavage of the conjugated chromophore, and deamination, ultimately mineralizing into low-molecular-weight organics, inorganic salts, CO₂, and H₂O. These results underscore the potential of BON@BIOPT for practical remediation of organic pollutants in water.
1. Introduction
Conventional semiconductor photocatalysts, such as TiO₂, suffer from wide bandgaps (ca. 3.2 eV) that restrict photoactivation to the UV region, limiting solar energy utilization to less than 5% of the solar spectrum. Moreover, rapid recombination of photogenerated electron-hole pairs and low specific surface areas result in poor quantum yields and slow degradation kinetics, rendering them economically unviable for large-scale water treatment. Bismuth oxyhalides (BiOX, X=Cl, Br, I) have emerged as promising visible-light-active alternatives due to their layered structures and tunable bandgaps, yet pure BiOX phases still exhibit moderate activity and stability. For instance, BiOBr0.8I0.2, a solid solution with an optimal bandgap of 2.55 eV, degrades only 52.2% of RhB in 30 minutes, underscoring the need for further enhancement.
This study addresses these limitations by constructing an S-scheme heterojunction between Bi6O5(OH)3(NO3)5·3H2O (BON) and BiOBr0.8I0.2 (BI). BON, a bismuth subcarbonate with a unique cage-like structure, can be transformed in situ into heterojunctions via anion exchange, facilitating intimate interfacial contact. The resulting BON@BI heterojunction leverages the internal electric field to promote spatial separation of photogenerated carriers, while the solid solution component extends visible-light absorption. This synergistic design achieves a 99.8% degradation of RhB in 30 minutes, representing a significant advancement over single-component photocatalysts and offering a viable route for efficient, stable, and environmentally adaptable water purification technologies.
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LI Dongmei, HE Shuai, JIANG Shuxian, CHEN Zhixiang, WU Hanjie, ZHANG Jinming, DAI Ziqiang, LIU Ziye (2026). S-scheme Bi6O5(OH)3(NO3)5·3H2O/BiOBr0.8I0.2 Heterojunction Photocatalyst: Synthesis and Visible-Light Degradation Mechanism of Rhodamine B. Chinese Journal of Environmental Engineering. https://doi.org/10.12030/j.cjee.202511054
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Frequently Asked Questions
What is the long-term operational stability of BON@BIOPT under continuous flow conditions, and how does it compare to slurry batch reactors?
The catalyst retains 88.6% degradation efficiency after 7 consecutive batch cycles, indicating good reusability. However, continuous flow operation may introduce mechanical attrition and catalyst loss. The rod-like morphology with uniform nanosheets (44.7 m²·g⁻¹) suggests potential for immobilization on supports, but further studies on long-term stability under hydrodynamic stress are required. The current data support batch applications, with scale-up needing careful consideration of reactor design to minimize catalyst washout.
What is the cost breakdown for synthesizing BON@BIOPT compared to commercial TiO₂ (P25), and does the enhanced performance justify the premium?
Precise cost data are not provided, but the synthesis uses inexpensive precursors (Bi(NO₃)₃·5H₂O, KBr, KI) and a simple hydrothermal method, which is scalable. P25 costs roughly $20–50/kg, while bismuth-based catalysts are typically more expensive due to bismuth salt costs. However, the 99.8% degradation in 30 minutes versus P25's negligible visible-light activity justifies the premium for applications requiring visible-light or solar-driven processes. A techno-economic analysis is recommended to quantify lifecycle costs.
How does the catalyst perform under real sunlight conditions, and what is the quantum efficiency (QE) at specific wavelengths?
The study used a visible-light source (likely λ>420 nm), but actual solar QE was not measured. The absorption edge at 580 nm suggests utilization of up to ~580 nm, covering a significant portion of the solar spectrum. To estimate solar-to-chemical efficiency, one would need to measure apparent quantum yield (AQY) at monochromatic wavelengths. The Kapp of 0.20617 min⁻¹ under visible light is promising, but outdoor testing is essential to validate performance under varying light intensities.
What are the degradation intermediates of RhB, and are they toxic? The paper mentions N-de-ethylation and chromophore cleavage, but are the final products fully mineralized?
The study indicates that RhB is degraded via N-de-ethylation, cleavage of the conjugated chromophore, and deamination, ultimately mineralizing into low-molecular-weight organics, inorganic salts, CO₂, and H₂O. However, detailed intermediate identification (e.g., via LC-MS) is not provided. While complete mineralization is claimed, the presence of partially oxidized intermediates cannot be ruled out. Toxicity assays (e.g., Microtox) are recommended to ensure the effluent is non-toxic.
How does the presence of natural organic matter (NOM) affect the degradation efficiency, and can the catalyst be regenerated after fouling?
The study tested common inorganic anions (HCO₃⁻, Cl⁻, SO₄²⁻, H₂PO₄⁻, NO₃⁻) and found degradation efficiencies of 85–98%, indicating good tolerance. However, NOM, which is ubiquitous in natural waters, can scavenge reactive species and absorb light, potentially reducing efficiency. The catalyst's stability over 7 cycles suggests some resistance to fouling, but regeneration protocols (e.g., washing with dilute acid or UV irradiation) have not been reported. Further studies with NOM-spiked solutions are necessary.
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