MICROBIAL INACTIVATION BY COPPER-BASED DISINFECTION SYSTEMS: EFFICACY AND MECHANISM

Abstract

To control the pathogenic microorganisms threatening human health in drinking water, it is important to utilize proper physical or chemical disinfection processes at drinking water treatment plants. Various methods have been studied to efficiently control the microorganisms in water. While conventional chemical disinfectants, such as chlorine or ozone, are effective, by-products are produced, depending upon the water quality. However, metals, which are essential nutrient elements, exhibit antimicrobial properties, both to bacteria and viruses. Copper, which has been studied as an effective antibacterial, antiviral, and antifungal reagent, was selected in this work. In this study, E. coli and MS2 coliphage, the surrogates of bacteria and viruses, respectively, were treated by homogeneous or heterogeneous copper-based disinfection systems. The inactivation of E. coli and MS2 coliphage by Cu(II) is found to be significantly enhanced in the presence of hydroxylamine (HA). The addition of a small amount of HA (i.e., 520 μM) increased the inactivation efficacies of E. coli and MS2 coliphage by 5- to 100-fold, depending on the conditions. Dual effects were anticipated to enhance the biocidal activity of Cu(II) by the addition of HA, viz. i) the accelerated reduction of Cu(II) into Cu(I) (a stronger biocide) and ii) the production of reactive oxidants from the reaction of Cu(I) with dissolved oxygen (evidenced by the oxidative transformation of methanol into formaldehyde). Deaeration enhanced the inactivation of E. coli but slightly decreased the inactivation efficacy of MS2 coliphage. The addition of 10 μM hydrogen peroxide (H2O2) greatly enhanced the MS2 inactivation, whereas the same concentration of H2O2 did not significantly affect the inactivation efficacy of E. coli. Observations collectively indicate that different biocidal actions lead to the inactivation of E. coli and MS2 coliphage. The toxicity of Cu(I) is dominantly responsible for the E. coli inactivation. However, for the MS2 coliphage inactivation, the oxidative damage induced by reactive oxidants is as important as the effect of Cu(I). The Cu(II) and PMS combined system was examined to inactivate E. coli and MS2 coliphage in natural water. PMS is emerging oxidants commonly applied to the in situ treatment of organic contaminations for waste water and ground water. PMS is activated by the transition metal such as Co(II), Fe(II), Ni(II), producing sulfate radical or hydroxyl radical. In this study, the combined system varying the concentration of PMS with fixed Cu(II) concentration of 0.01 mM, exhibited a synergistic effect on both E. coli and MS2 coliphage inactivation in natural water. The presence of hydroxyl radical scavengers marginally inhibited the inactivation rate of the Cu(II)/PMS system, however, that presence almost inhibited the copper-chelating reagents, EDTA and DMP. The Cu(II)/PMS system is assumed to generate reactive oxidants rather than hydroxyl radical, and most reactive oxidants are due to the Cu(II) ion. Most reactive oxidants produced intracellular rather than extracellular region that is more lethal to E. coli. In particular, when E. coli was exposed to Cu(II) first, the sequential addition showed the highest efficacy. Thus, it is worth consideration to add sequentially for more effective application of the Cu(II)/PMS system. The biocidal effects of iron-based bimetallic nanoparticle, nFeCu, were tested to inactivate E. coli and MS2 coliphage, surrogate of bacteria and virus, respectively, and the mechanisms of production of the biocidal reagents were elucidated. In this study, nFeCu was synthesized via ferric ion reduction using sodium borohydride followed by the displacement of copper ion. The nFeCu showed enhanced biocidal effects compare to the single composite nanoparticles, nFe and nCu. nFeCu showed different inactivation mechanisms on E. coli and MS2 coliphage. In the absence of oxygen, the E. coli inactivation efficiency was greater than that observed in the presence of oxygen. In contrast to the E. coli inactivation results, MS2 coliphage inactivation was nearly inhibited in the presence of oxygen. nFeCu is assumed to generate Cu(I), which is a strong biocides, via a copper-catalyzed Fenton-like reaction and to produce reactive oxidants, such as •OH, Fe(IV) or Cu(III), from the Fenton or Fenton-like reaction with in situ generated H2O2. The E. coli inactivation efficiency was lowered more by the nCu/H2O2 system than by nFeCu, and was inhibited by the increase of H2O2 concentration. However, the MS2 coliphage inactivation efficiency was enhanced by the increase of H2O2 concentration; further, the higher concentration of H2O2 dramatically enhanced the inactivation rate. We concluded the E. coli inactivation was more likely induced by Cu(I) damaging culturability, rather than membrane damage. Meanwhile, the reactive oxidants produced by nFeCu exhibited oxidative damage on the culturability and antigenicity of MS2 coliphage.