Photocatalytic bacterial inactivation by polyoxometalates

The photocatalytic inactivation (PCI) of Escherichia coli (Gram-negative) and Bacillus subtilis (Gram-positive) was performed using polyoxometalate (POM) as a homogeneous photocatalyst and compared with that of heterogeneous TiO₂ photocatalyst. Aqueous suspensions of the microorganisms (10⁷–10⁸ cfu ml⁻¹) and POM (or TiO₂) were irradiated with black light lamps. The POM-PCI was faster than (or comparable to) TiO₂-PCI under the experimental conditions employed in this study. The relative efficiency of POM-PCI was species-dependent. Among three POMs (H₃PW₁₂O₄₀, H₃PMo₁₂O₄₀, and H₄SiW₁₂O₄₀) tested in this study, the inactivation of E. coli was fastest with H₄SiW₁₂O₄₀ while that of B. subtilis was the most efficient with H₃PW₁₂O₄₀. Although the biocidal action of TiO₂ photocatalyst has been commonly ascribed to the role of photogenerated reactive oxygen species such as hydroxyl radicals and superoxides, the cell death mechanism with POM seems to be different from TiO₂-PCI. While TiO₂ caused the cell membrane disruption, POM did not induce the cell lysis. When methanol was added to the POM solution, not only the PCI of E. coli was enhanced (contrary to the case of TiO₂-PCI) but also the dark inactivation was observed. This was ascribed to the in situ production of formaldehyde from the oxidation of methanol. The interesting biocidal property of POM photocatalyst might be utilized as a potential disinfectant technology.     

Concentration Evolution of Gas Species within a Collapsing Bubble in a Liquid Medium

In this study numerical methods are used to investigate the relationship between chemical concentration of gas species within a cavitating bubble, equilibrium radius of the gas bubble and pressure variations in the ambient liquid. For this purpose, governing equations are developed to describe the dynamic equilibrium of a bubble in a flowing fluid and mass transfer between gas and liquid phases, where it was assumed that gases undergo isothermal compression, obey the ideal gas law, Henry law. It is further assumed that the concentration of each phase within the bubble is uniform. The resulting nonlinear equations are solved using implicit Trapezoidal method with Newton iteration. Four gas species are modeled under various initial and ambient pressure variation conditions. These conditions maybe considered to represent typical cavitation events. The numerical results obtained are presented in terms of dimensionless numbers. These results indicate that chemical damage maybe an important component of cavitation surface damage, since high concentration profiles may develop within a collapsing bubble. Proposed formulation and numerical solutions are simple and cost effective to implement. The results presented in this study maybe used to benchmark experimental investigations or other more complex solutions, which are outside the scope of this study.     

Inactivation of Escherichia coli in the electrochemical disinfection process using a Pt anode

Recently, the electrochemical disinfection has gained a great interest as one of the alternatives to conventional chlorination due to its high effectiveness and environmental compatibility. Despite the extensive reports on electro-chlorination disinfection, few researches were reported on the systems without generating chlorine. This study mainly focused on the potential disinfecting ability of electro-generated oxidants other than chlorine with using an inert medium (chloride-free phosphate buffer solution), which was intended to exclude the formation of chlorine during the electrolysis, as the Escherichia coli as an indicator bacterium was disinfected by applying the current to a platinum anode. The electrochemical inactivation of E. coli without chlorine production was demonstrated to occur in two distinct stages. The first stage inactivation takes place rapidly at the beginning of electrolysis, which appears to be achieved by the electrosorption of negatively charged E. coli cells to the anode surface, followed by a direct electron transfer reaction. As the electrolysis continues further, the inactivation becomes slower but steady, in contrast to the first stage of inactivation. This was attributed to the action of reactive oxidants generated from water discharge, such as hydroxyl radical. Overall, this study suggests that the electrochemical disinfection could be successfully performed even without producing chlorine, recommending the potential application for disinfecting water that does not allow including any chloride ions (such as the production of ultra-pure sterilized water for semiconductor washing).     

Weak magnetic field accelerates chromate removal by zero-valent iron

Weak magnetic field(WMF) was employed to improve the removal of Cr(VI) by zero-valent iron(ZVI) for the first time. The removal rate of Cr(VI) was elevated by a factor of 1.12–5.89 due to the application of a WMF, and the WMF-induced improvement was more remarkable at higher Cr(VI) concentration and higher p H. Fe2+was not detected until Cr(VI) was exhausted, and there was a positive correlation between the WMF-induced promotion factor of Cr(VI) removal rate and that of Fe2+release rate in the absence of Cr(VI) at pH 4.0–5.5. These phenomena imply that ZVI corrosion with Fe2+release was the limiting step in the process of Cr(VI) removal. The superimposed WMF had negligible influence on the apparent activation energy of Cr(VI) removal by ZVI, indicating that WMF accelerated Cr(VI)removal by ZVI but did not change the mechanism. The passive layer formed with WMF was much more porous than without WMF, thereby facilitating mass transport. Therefore,WMF could accelerate ZVI corrosion and alleviate the detrimental effects of the passive layer, resulting in more rapid removal of Cr(VI) by ZVI. Exploiting the magnetic memory of ZVI, a two-stage process consisting of a small reactor with WMF for ZVI magnetization and a large reactor for removing contaminants by magnetized ZVI can be employed as a new method of ZVI-mediated remediation.