Spent coffee ground-derived carbon quantum dot composite with metal oxides for photocatalytic degradation of carbaryl in water and antibacterial application

Spent coffee ground is a solid waste deriving from the production of coffee beverages and instant coffee in high amounts worldwide. The recycling of spent coffee ground is an approach to support the UN sustainable development goals (UN-SDGs) regarding waste reduction and recycling. The objective of this study was to synthesize carbon quantum dot (CQD) from the spent coffee ground using the alkaline peroxide-assisted hydrothermal method, and subsequently produce the composite photocatalysts by combining ZnO and TiO2 with CQD. The obtained CQD has an average particle size of 2.96 nm. The optical bandgap energies decreased after combining TiO2 and ZnO with CQD. TEM and XPS results confirmed the formation of composite photocatalysts. All photocatalysts were used in the removal of carbaryl (5 mg/L) in water under UVA irradiation (365 nm). The ternary composite of TiO2/ZnO/CQD exhibited the highest removal efficiency of 99.01 % under UVA irradiation for 90 min. After reusing the catalyst for five cycles, the efficiency dropped to 84.09 %. The half-life time of carbaryl in the presence of TiO2/ZnO/CQD under UVA was 12.16 min. This is 5.5 and 1.7 times lower than that of pristine TiO2 and ZnO. When the initial concentration of carbaryl increased from 1 to 7 mg/L, the removal efficiency showed a decreasing trend, which was expressed in terms of Langmuir-Hinshelwood rate equation. Furthermore, antimicrobial studies were conducted against both Gram-negative and Gram-positive bacterial strains. The results demonstrated that TiO2/ZnO/CQD is one of the promising photocatalysts for environmental remediation and antimicrobial applications.