The Synthesis and Application of Solid-Supported Metal Borate Nanoparticles As Photo-Redox Catalyst for Environmental Remediation
Authors: Wu Bo-Yi , Zhu Yu-Lun , Keng Pei Yuin 
Published: December 2024

The burgeoning global population, coupled with increasing urbanization and industrialization, poses significant threats to environmental sustainability. Among these threats, the dissemination of hazardous substances—specifically, industrial dyes, pharmaceuticals, and agrochemicals—into natural ecosystems is a primary concern. This proliferation of toxic agents contributes to the diminishing availability of clean water, adversely affecting both aquatic life and human health. Concurrently, the shift in global demographics toward urban centers exacerbates the challenges posed by climate change, particularly concerning the availability and quality of freshwater resources. This issue is particularly pronounced in densely populated urban areas with limited access to fresh water. In this context, Advanced Oxidation Processes (AOP) emerge as a viable solution to mitigate these environmental challenges. AOP's appeal lies in its simplicity, efficacy, and versatility in addressing a broad spectrum of organic pollutants. Our laboratory has previously demonstrated the successful synthesis and photoredox capabilities of a novel, single-component metal borate catalyst. This catalyst showcases superior catalytic performance in degrading various organic contaminants. In comparison to emerging catalysts, which necessitate complex heterostructures for enhanced activity, the C, N-doped iron borate developed by our group exhibits significantly higher degradation rates—26-fold and 10-fold greater than those of pristine FeBO3 and Fe3O4 nanoparticles, respectively, under solar irradiation. Similarly, our cobalt pyroborate (Co2B2O5) nanoparticles catalysts have demonstrated marked improvements in degradation rates, achieving seven-fold and 2.7-fold increases, respectively, in solar-light-assisted sulfate radical-advanced oxidation processes (SR-AOP). Both metal borates possess a large absorbance in the visible light region and simultaneously exhibit high activity towards H2O2 and peroxymonosulfate (PMS), respectively. Consequently, the catalytic efficiency of both metal borate catalysts is notably augmented under solar light irradiation. This enhancement is primarily due to the photogenerated electrons that facilitate metal redox cycling, thereby accelerating the production of reactive oxygen species (ROS). These ROS play a crucial role in the degradation of organic pollutants present in wastewater. The enhanced catalytic activity of metal borates relative to their corresponding oxides can also be attributed to the synergistic effects associated with the borate ligand. Functioning as an electron donor, the borate ligand facilitates electron transfer to the metal center, thereby promoting the reduction processes of the transition metal. This interaction creates a more conducive environment for the efficient degradation of pollutants. Despite considerable advances in the development of new materials for AOP catalysts and their engineering applications in laboratories, there remains a critical need for efficient composite materials. These materials must not only possess high catalytic activity but also enable straightforward recovery through simple methods such as filtration or gravity sedimentation, thereby facilitating the practical application of AOP in treating real-world wastewater streams. This paper elucidates the synthesis of solid-supported iron borate and cobalt borate nanoparticles within mesoporous silica and KCC-1 frameworks, respectively. The formation of metal borate on these supports, necessitating high-temperature conditions, presents a challenge for achieving complete phase transformation of metal oxides without compromising the integrity of the support structure. We have systematically explored two immobilization strategies—hydrothermal reaction and impregnation—and examined the effects of varying the Si:Co and boron:metal precursor ratios. Through the hydrothermal method, a one-step reaction followed by high-temperature calcination is employed to produce solid-supported metal borate, necessitating precise adjustments to the Si:Co and B:Co ratios to ensure the preservation of the support structure and complete phase conversion. Our laboratory's findings indicate that the impregnation approach successfully yields cobalt pyroborate within the KCC-1 framework. The resulting Co2B2O5@KCC-1 structure exhibits a promising 41% degradation efficiency in SR-AOP conditions. Further research will focus on optimizing the Co:B ratio, impregnation temperature and duration, and degradation parameters to enhance this efficiency.