Long-lasting and controlled-release borate as a biocide against microbial breeding in a recirculating cooling water system.

Based on the potential bactericidal properties of borate, we synthesized controlled-release borate (CRB) as a novel biocide to inhibit microbial proliferation in a recirculating cooling water system (RCS). In this study, toxicity experiments of CRB were conducted on the dominant bacteria and algae isolated from an actual RCS. The effects of CRB on biocidal performance and genotoxicity were evaluated in a simulated RCS. Our results showed that the inhibition rates of CRB on bacteria and algae reached 80.4-84.0 % and 55.0 %, respectively. CRB achieved a complete release of antibacterial factor, boron, within 20 days in the simulated RCS. The number of heterotrophic bacteria (HB) was reduced to 3.8 × 10³ CFU/mL (day 30), which met the requirement of the Chinese standard (GB/T 50050-2017). 16S/18S rRNA gene sequencing showed that CRB achieved a significant reduction of the predominant phyla, including Pseudomonadota and Chlorophyta. Flow cytometry analysis demonstrated that the algae-inhibiting behavior of CRB was mainly reflected in the inhibition of nucleic acid synthesis and photosystem II activity. Metatranscriptomic analysis revealed that the downregulated key genes were primarily annotated in the "photosynthesis-antenna proteins" and "large/small subunit ribosomal protein" pathways. It indicated that CRB might deteriorate microbial photosynthetic activity and protein synthesis, interfering with microbial growth. Additionally, CRB was associated with negative effects on reactive oxygen species metabolism and regulation of cell size. Moreover, CRB exhibited excellent scaling and corrosion inhibition properties. The determination of the biocidal mechanism of CRB will help enhance the practical application of multi-functional water treatment agents in the RCS.