Pharmaceutical pollutant as sacrificial agent for sustainable synergistic water treatment and hydrogen production via novel Z- scheme Bi7O9I3/B4C heterojunction photocatalysts
Graphical abstract
Introduction
The realization of finite fossil fuel scarcity and environmental pollution has led the research community and environmentalists to search for alternative renewable and environmentally friendly green fuel [1], [2], [3]. Conversion of solar energy to storable chemical fuel from a clean source such as water is the key technology to the carry out the future energy demands [4]. Hydrogen has emerged as the most promising green fuel because of its high energy emission, zero hazard and most important can be produced from the water [5], [6], [7]. Electrocatalytic hydrogen evolution has been most popular route for hydrogen generation [8]. There are numerous issues including cost-effectiveness related to these techniques. Sustainable photocatalytic H2 production has been a promising and solution to tackle dual issues of the energy crisis and environmental pollution [9], [10], [11]. However, for efficient photocatalytic hydrogen generation, some external sacrificial agents or donors as alcohols, organic acids etc are generally employed for scavenging holes and diminishing recombination [12], [13]. Such addition is economically viable and practically suitable as the external sacrificial agents act as an energy supply or input and thus raise the cost of hydrogen evolution eventually [14].
Regarding sustainable and efficient hydrogen evolution, there are two important requisites. Firstly, the photocatalyst must be able to separate photogenerated electrons and holes efficiently along with numerous requisite reaction sites and high visible light activity [15], [16].The second challenge lies in recovery or generation of hydrogen energy from waste water for implementation of a environmental friendly and sustainable energy production-cum-water treatment with promising scale-up.
Among numerous contaminants of emerging concern, antibiotics and their residues have been most widespread because of their resistance to biological oxidation and other traditional methods [17], [18], [19]. Norfloxacin (NFN) belongs to class of fluoroquinlones which are one of the most common antibiotics [20], [21]. These are not completely metabolized by body and about 70% is excreted out [22]. Being reported in ng L−1 to μg L−1 in environment, their presence is surely a threat to aquatic ecosystem and humans [23] and lead to antibiotic resistance in microbes in longer run [24]. Thus utilizing these antibiotics as sacrificial agent in photocatalytic hydrogen evolution serves both the purposes of clean energy generation and wastewater treatment.
The semiconductor heterojunctions especially Z-schemes formed by choosing materials with suitable band structures are highly beneficial in the charge separation [25] which is an essential prerequisite for such purpose. Recently, bismuth oxyhalides have garnered extensive attentions owing to their significant sunlight response, built-in electric field and adequate chemical stability, such as (BiaObXc, X = Cl, I, Br) [26], [27], [28]. Among them, a new bismuth-rich Bi7O9I3 has been widely reported as one of most promising photocatalysts in the field of photocatalytic energy production and environmental remediation [29], [30]. It possesses excellent photocatalytic activity and high charge transfer ratio owing to the increased Bi and O content, deeply anisotropic p or hybridized sp states, and highly dispersed conduction band (CB) and valence band (VB) [31]. Nevertheless, the photocatalytic efficiency of Bi7O9I3 remains far from suitable for practical applications because the quantum yield of Bi7O9I3 is rather poor due to the rapid recombination of photogenerated electron–hole pairs and slow rate of photogenerated charge transfer [26], [32].
Boron carbide (B4C), is a stable and low-cost metal-free semiconductor with a strong absorption in solar light owing to mid-gap states and various structural distortions [33], [34]. It has low band gap, low density with an excellent resistance to oxidation resistance and tunable electrical properties [35]. Feng and co-workers (2013) utilized B4C in photocatalytic hydrogen evolution, with a decent hydrogen evolution activity originating from structural defects and distortions in [36]. Very few works have reported heterojunctions based on B4C for photocatalytic applications as B4C/TiO2 [35], B4C/C3N4 [37] etc.
As compared to Bi4O9I3, boron carbide has more strength and active sites for a robust heterojunction formation. In the current work, synthesis of Bi7O9I3/B4C heterojunction is reported for simultaneous photocatalytic hydrogen evolution and fluoroquinolone antibiotic degradation under visible light. The unique hierarchical structure of the heterojunction shows high visible light capture, high electron transport, highly suppressed electron-hole recombination with an effective Z-scheme mechanism. A 456.3 μmol g−1h−1H2 evolution with almost complete norfloxacin removal was achieved under anoxic conditions with retaining of high activity under oxic conditions too. An effective Z-scheme transfer facilitated by Bio (as found in XPS findings) was active for a higher charge separation and effective hydrogen evolution with NFN molecules acting as sacrificial agent by capturing directly holes as well as indirectly ●OH radicals. The activity was also tested with other pollutants as rhodamine B and tetracycline. This work paves way for converting different waste-water sources for a sustainable hydrogen evolution, water treatment and resource utilization. Utilizing pharmaceutical waste for hydrogen generation is essentially an important step towards realizing sustainable development and circular economy.
Section snippets
Preparation of Bi7O9I3/B4C heterojunction
Boron carbide (B4C) was synthesized by condensation reaction of polyvinyl alcohol (PVA) and boric acid (H3BO3) (detailed procedure in supplementary data). Bi7O9I3/B4C (BIBC) heterojunction photocatalyst with different ratios was prepared via a hydrothermal route. In a reaction vessel, 2.425 g Bi(NO3)3·5H2O was added into 50 mL ethylene glycol (EG) with constant stirring and sonication at 313 K until the solution becomes transparent. Subsequently, a certain amount (0.1–0.5 g) of as prepared
XRD analysis
The phase purity and crystallinity of the synthesized Bi7O9I3/B4C (BIBC), Bi7O9I3 and B4C were investigated by powder X-ray diffraction analysis, as shown in Fig. 2. The XRD pattern of Bi7O9I3, exhibits characteristic diffraction peaks at 2θ = 28.6°, 31.4°, 36.9°, 45.1°, 49.3° and 54.4° corresponding to (1 0 2), (1 1 0), (1 0 3), (2 0 0), (1 1 4) and (2 0 2) planes which are assigned to tetragonal phase of Bi7O9I3 [38]. It can be observed that the peak for (1 0 2) plane which is placed at 29.7° for BiOI
Conclusions
Focusing waste to energy conversion, this work presents research on wastewater purification coupled to photocatalytic hydrogen evolution. The findings confirm the role of antibiotic pollutant as a sacrificial agent or electron donor for hydrogen evolution on visible/solar light irradiation. Bi7O9I3/B4C (30 wt% B4C) i.e BIBC-30 heterojunction under visible light exhibits 812 μmol g−1h−1hydrogen evolution along with 94.2% norfloxacin antibiotic degradation which is manifolds higher than B4C and Bi
CRediT authorship contribution statement
Anamika Rana: Data curation, Formal analysis, Investigation. Amit Kumar: Conceptualization, Writing – original draft, Supervision. Gaurav Sharma: Writing – original draft. Mu. Naushad: Writing – review & editing. Chinna Bathula: Writing – review & editing. Florian J. Stadler: Writing – review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors are grateful to the Researchers Supporting Project number (RSP-2021/8), King Saud University, Riyadh, Saudi Arabia for the financial support.
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