Development of copper based material systems for generating nitric oxide to control nitrifying bacterial biofilms

摘要

Biofilms, which are bacteria cells aggregates that exist within a matrix of extracellular polymeric substances, are known to cause problems in industrial water systems. In order to prevent biofilm formation and pathogens occurrence, disinfectants such as chloramine are usually added into the water. However, chloramine addition has been shown to trigger the growth of nitrifying biofilms which subsequently accelerate chloramine decay. Therefore, a new antibiofilm agent, namely nitric oxide (NO), is investigated for nitrifying biofilm control.Herein, NO was generated from catalytic reduction of nitrite in the presence of a copper(II) complex catalyst embedded in a poly(vinyl chloride) (PVC) matrix. Ascorbic acid was added into the solution as a reducing agent to aid the formation of the active copper(I) species that will react with nitrite to generate NO. The copper-nitrite-ascorbic acid combination showed enhanced NO generation compared to that generated in the presence of nitrite-ascorbic acid alone, and subsequently, enhanced biofilm suppression was observed. The catalytically generated NO was also found to be effective in dispersing pre-formed biofilm, with simultaneous biofilm cells killing observed when a high concentration of nitrite-ascorbic acid was used. An alternative reducing agent, namely Fe2+, was investigated for the potential to mediate reduction of copper(II) complex for generating NO. The amount of NO generated was found to be highly dependent on Fe speciation in different pH and buffer composition. Nonetheless, the NO generated in phosphate buffer pH 6 is still capable of dispersing pre-formed nitrifying biofilms, thus suggesting the robustness of NO-mediated biofilm dispersal. In the last part of the study, an iron complex, namely FeDTTCT, was synthesised and immobilised into PVC with the copper catalyst. The iron complex was found to facilitate copper(II)/copper(I) redox cycling, subsequently enabling NO generation from nitrite. Importantly, the mixed metal system exhibited a non-toxic antibiofilm activity, whereby biofilm formation was suppressed and bacterial growth was confined to the free-floating planktonic phase. These thus imply that the mixed metal system was capable of converting nitrite endogenously produced by nitrifying bacteria to NO, hence eliminating the need to add a reducing agent and NO precursor in solution form into the system.