Published on Journal of Cleaner Production, “Novel CdS/MIL-88A heterojunction coupled with H2O2/air-nanobubbles for enhanced visible-light driven photocatalytic performance” is the research of scientists from VNU University of Science.

Recently, nanobubble (NB) technology has gained much attention due to its special merits. The generation of reactive species enables it to address wastewater treatment and disinfection issues. In this research, for the first time, the combination of a heterojunction with NB and hydrogen peroxide (H2O2) for enhanced dye photodegradation was developed. Characteristics of CdS/MIL-88A photocatalyst were examined by XRD, SEM/EDX, TEM, FT-IR, UV–Vis-DRS, PL, and BET techniques. The influence of NB/H2O2 on photodegradation of direct blue 71 dye (DB 71) was systematically investigated by varying pH, H2O2 concentration, catalyst dosage, and DB 71 concentration. Results showed that DB 71 photodegradation was significantly improved by H2O2/NB assist due to high mass transfer and high generation rate of reactive species. About 99% of DB 71 (70 mg/L) were decomposed by CdS/MIL-88A/H2O2/NB system for 80 min of reaction time at ambient conditions. The DB 71 degradation process was best described by pseudo-1st-order kinetics model with R2 > 0.918 for all cases. CdS/MIL-88A photocatalyst was effectively recycled by using simple and environmental friendly process which permitted a DB 71 removal yield of over 78% after five regeneration cycles. The CdS/MIL-88A/H2O2/NB system generated both HO• and O2•- radicals responsible for degrading DB 71.

Nanobubbles (NB) are tiny spherical shaped bubbles in the water with diameters less than 1 μm. Recently, NB technology has gained much attention because of its special properties like long-term stability, large specific surface area, surface energy, high oxygen dissolution rate and high mass transfer efficiency (Kim et al., 2021; Yu et al., 2021). When nanobubbles burst, changes in gas–liquid interface would generate reactive species, including singlet oxygen, and hydroxyl, superoxide radicals (Liu et al., 2016). Nanobubbles technology allows to solve various environmental issues without causing secondary pollution. This is expected to create chemical-free green technology. Nanobubbles are widely used in different domains of research and technology, including cleaning and disinfection, smart agriculture, biotechnology, medical treatment, and so on due to its special properties (Gurung et al., 2016; Bhandari et al., 2017; Fan et al., 2021; Sha et al., 2020; Nam et al., 2022; Zhou et al., 2022). Recently, there are a number of studies on combining NB with different systems for various applications in environmental remediation (Li et al., 2009; Liu et al., 2018b; Kim et al., 2021; Hewage et al., 2021). Advanced oxidation processes (AOPs) using H2O2 have been being studied and widely applied to wastewater treatment. HO• and HOO• free radicals produced by the decomposition of H2O2 in combined systems such as H2O2/Fe2+ and UV/H2O2/O3 can effectively degrade organic matters (Buthiyappan and Raman, 2019; Scaria et al., 2021; Pan et al., 2022). H2O2-based process has the advantage of being easy to be used, high degradation efficiency and extensive application. Nevertheless, this system is quite costly and there is a risk of loss of H2O2 due to its easy decomposition at ambient temperature. Thus, the combination of H2O2 with other systems is expected to overcome these drawbacks.

Photocatalytic approach is one of the most effective methods among the AOPs to mineralize organic compounds to non-toxic forms, such as CO2 and water (Nguyen et al., 2018; Le et al., 2021; Zhang et al., 2021a; Swetha et al., 2022). Recently, cadmium sulfide (CdS) – a semiconductor with narrow bandgap energy (Eg < 3.0 eV) is increasingly attracting research attention (Cheng et al., 2018). However, CdS semiconductor also presents some intrinsic drawbacks like rapid electron-hole recombination and photo-corrosion (Ryu et al., 2007). To overcome such problems, several researches have been performed including coupling with other semiconductors, such as CdS–MoS2 (Chava et al., 2018), CdS–ZnFe2O4 (Fang et al., 2018), CoWO4/CdS (Cui et al., 2018). Metal–organic frameworks (MOFs), a well-developed porous class of materials, are built up from metal or metal cluster connecting nodes and organic ligands (Li et al., 2009). Because of their high surface area, well-distributed pore size and easily to be tailored to make specific materials, MOFs have attracted more and more attention for various applications such as gas storage, molecular sensing, adsorbents and catalysts (Huang et al., 2017; Zhang et al., 2017, 2021a, 2021b; Mohammadifard et al., 2019; Swetha et al., 2022). Nevertheless, many MOFs require harsh reaction conditions leading to increase in the production cost. Among these MOFs, MIL-88A is a 3D framework with cages and open channels that runs along the c axis, and composed of trimers of Fe(III) octahedra linked to fumarate dianions (Serre et al., 2004), in which MIL stands for Material from Institute Lavoisier. MIL-88A, a Fe-based metal-organic framework, could be synthesized with short time and low temperature, significantly reducing energy consumption for its production (Zheng et al., 2020). MIL-88A has a much larger volume swelling when exposed to polar solvents compared to other MOF photocatalysts, but it still fully maintaining its open-framework architecture. Mass transfer in photocatalytic processes is greatly facilitated by this significant swelling impact (Xu et al., 2014).

Herein, we synthesize novel CdS/MIL-88A composite to improve the visible light absorption, reduce the electron-hole recombination, and enhance its photocatalytic performance. Recently, there have been experimental studies on combining NB with AOPs (Wang et al., 2020), NB with H2O2 (Chen et al., 2021). Wang et al. (2020) revealed that the introduction of oxygen nanobubbles into the photocatalytic system under visible light irradiation enhanced oxytetracycline (OTC) photodegradation in solutions by a factor of 1.5 compared to the system without the presence of oxygen nanobubbles, with an increase in photodegradation efficiency from 40% to 60%. The same authors concluded that the improvement could be attributed to (i) the presence of more dissolved oxygen which boosted the formation of reactive radicals responsible for the antibiotic degradation, (ii) higher stable nanobubbles compared to the ordinary dissolved oxygen, and (iii) surface effect of nanobubbles leading to improvement of mass transfer the efficiency. Chen et al. (2021) reported that the combination of micro-nanobubble (MB) and hydrogen peroxide (H2O2) resulted in significantly enhancing tetracycline degradation efficiency. About 92% of tetracycline were removed by the system, corresponding to 9.44 and 3.94 times higher than that of MB and H2O2 alone. This enhancement was linked with (i) the rupture of MB led to high temperature and pressure, activating H2O2 to generate more oxygen-containing reactive species, especially HO• radicals which are main reactive species for MB degradation, followed by HO2• and O2•-. However, there have been no researches on simultaneously combining NB technology with H2O2 and photocatalytic processes to activate and generate more efficient free radicals, to the best of our knowledge. The main objective of this work is to develop a novel photocatalytic system based on CdS/MIL-88A heterojunction and H2O2/air-nanobubbles to study the DB 71 degradation under visible light irradiation.

The development of manufacturing and textile industry has resulted in increasing use of dyes, causing a major environmental contamination. Globally, about 70 millions tons of dyes have been annually produced, and over 10,000 tons of synthetic dyes have been used each year in the textile industry (Kishor et al., 2021). DB 71 is a common azo dye, with the molecular formula of C40H27N7O13S4. Unreasonable usage and very low biodegradability of this dye could potentially cause negative impacts on human health, animals, and environments. Dye concentrations in the effluents of textile industry are usually characterized by very high levels of chemical oxygen demand (COD), biochemical oxygen demand (BOD), total suspended solids (TSS), … (Yaseen and Scholz, 2019; Liu et al., 2022). In textile industry, about 50 g dyes are used to dye 1 kg of fabric while 15–20% of dye remains in the effluent (Babu et al., 2007). Water consumption for textile production can vary from 35 to 126 L/kg of product (Liu et al., 2022), suggesting that the concentration of dyes in the effluent would broadly very in the range of 59 mg/L to 286 mg/L. Although, there has been no report on the concentration of DB 71 in the textile industry waste water, concentration of other dyes usually varies from one report to another, and it is reportedly in the range of 10–800 mg/L, except for specific cases (Yaseen and Scholz, 2019).

Traditional waste water treatment techniques present different drawbacks such as relatively high cost, difficulty in completely removal the pollutants, and/or secondary pollution causes (Tan et al., 2015; Samaei et al., 2018; Bento et al., 2020). Thus, figuring out more efficient and eco-friendly treatment techniques to completely remove dyes, including DB 71 is of great importance.

In this research, we synthesized CdS/MIL-88A photocatalyst and characterized the as-synthesized material by means of several techniques, including UV–Vis-DRS, SEM–EDX, TEM, XRD, BET, FT–IR, and PL. CdS/MIL-88A photocatalytic system was then applied to study the photodegradation of DB 71 under visible light irradiation in the presence of H2O2/NB to enhance its photocatalytic performance. The influence of different factors on the DB 71 degradation efficiency was systematically examined. Also, kinetics models and radical quenching experiments to investigate synergistic effects of H2O2/air-NB on the DB 71 photodegradation were performed. The photocatalyst's stability and regeneration were explored, and finally analyses of limitations of and future development prospects were discussed.