As part of the ‘grow basket’, Singapore is planning to raise the local production of nutrients from 10% to 30% by 2030. Among these nutrients, 10% is the increase of proteins, including fishes and eggs. According to Singapore Food Agency (SFA), there are 123 fish farms that produced about 4,600 tonnes of fish in 2019, accounting for 9% of local fish consumption. To prevent the economic losses of fish farms, and increase aquaculture production in Singapore, advanced technologies to eliminate harmful pathogens and purify the water periodically are urgently required. Conventional technologies, such as coagulation, chemical oxidation, UV irradiation, etc., are not sustainable as they suffer from a high number of chemical usages, high energy consumption, etc. These technologies with a single function and high cost make them impractical in the aquaculture industry where the water environment is complicated.
Herein, multifunctional positively charged macroscale beads with high porosity were prepared by a facile method that involves a diffusion-driven layer by layer (dd-LBL) process using graphene oxide as the substrate and branched polyethyleneimine (bPEI) as the functional ingredient. The as-prepared beads then underwent a simple crosslinking reaction by which they were chemically stabilized and mechanically strengthened. Due to the properties of the dd-LBL self-assembly of graphene oxide, the size of the beads can be tuned from millimeter to centimeter level by adjusting the diffusion reaction time.
The developed crosslinked beads with negligible in vitro toxicity to 3T3 cells and zebrafish embryos can inactivate 99.9% of Escherichia coli (E. coli) in 30 min and, due to its chemical stability and mechanical strength, retain its antibacterial properties after 10 cycles of regeneration by a simple wash step. Meanwhile, the advanced porous beads with abundant functional groups show high adsorption capacity and good recyclability for many organic matters, including dye, fish fecal matter, organic solvent, oil, etc.
Further, the capability of removing total ammonia nitrogen (TAN) can be realized by adding zeolite nanoparticles, while preserving the antibacterial activity and biocompatibility. In addition, the facile preparation method makes the developed beads to be feasibly scaled up, and industrially economical. As calculated, the cost of raw material of one single bead is 0.01775 SGD. Given the recycling stability of the beads, the cost can be further reduced by recycling and reusing.
We believe that our developed multifunctional beads with economic cost are feasible to be applied and would benefit the modern aquaculture industry to deal with many issues, including purification of the recirculating aquaculture systems water, increasing the survival rate of both fresh water and marine food fishes, etc.