Multilayer immobilizing of denitrifying Bacillus sp. and TiO2–AgNPs on floating expanded clay carrier for co-treatment of nitrite and pathogens in aquaculture

Abstract

Nitrite contamination and the spread of pathogens can seriously degrade water quality. To simultaneously control these factors, an innovative approach of fabricating a remediation agent that contained denitrifying bacteria and TiO₂–AgNPs co-immobilized on floating expanded clay (EC) was proposed in this study. The EC was fabricated from a mixture of clay and rice husk through pyrolysis at a high temperature of 1200 °C, followed by a rapid cooling step to create a porous structure for the material. TiO₂NPs were modified with Ag to shift the absorbance threshold of TiO₂–AgNPs into the visible region of 700–800 nm. The experimental results showed that the stirring speed of 250 rpm was suitable for immobilizing TiO₂–AgNPs on EC and achieved the highest Ti and Ag content of 639.38 ± 3.04 and 200.51 ± 3.71 ppm, respectively. Coating TiO₂–Ag/EC with chitosan (0.5%) significantly reduced the detachment level of immobilized TiO₂–AgNPs compared to that of the material with no coating. In particular, this functionalized material inhibited 99.93 ± 0.1% of Vibrio parahaemolyticus pathogen but did not adversely affect the denitrifying bacteria after 2 h of visible light irradiation. Based on the electrostatic bond between oppositely charged polymers, the denitrifying bacteria, Bacillus sp., in alginate solution was successfully immobilized on the chitosan-coated TiO₂–Ag/EC with a bacteria density of (76.67 ± 9.43) × 10⁷ CFU g⁻¹, retaining its nitrite removal efficiency at 99.0 ± 0.27% through six treatment cycles. These findings provide solid evidence for further investigating the combination of biodegradation and photodegradation in wastewater treatment.