Dual response of various TiO specimens for the solar photocatalytic inactivation of and degradation of humic matter

Elucidation of in-situ produced organic matrix effect on the solar photo/photocatalytic inactivation of E. coli

Solar photocatalytic inactivation of Gram-negative bacteria with immobilized TiO2-P25 in a fixed-bed reactor was modeled with simplified kinetic equations. The kinetic parameters are the following: the photocatalytic inactivation coefficient (kd,QUV), the initial bacterial reduction rate (A) in the contact with the disinfecting agent, and the threshold level of damage (n) were determined to report the effect of QUV/TiO2-P25 on bacterial cultivability and viability and to compare the response of bacterial strains to photocatalytic treatment. In addition, the integration of the reactivation coefficient (Cr) in the photocatalytic inactivation equation allowed evaluating the ability of bacterial reactivation after photocatalytic stress. Results showed different responses of the bacteria strains to photocatalytic stress and the ability of certain bacterial strains such as Escherichia coli ATCC25922 and Pseudomonas aeruginosa ATCC4114 to resuscitate after photocatalytic treatment.

BackgroundControlling fish disease is one of the major concerns in contemporary aquaculture. The use of antibiotics or chemical disinfection cannot provide a healthy aquaculture system without residual effects. Water quality is also important in determining the success or failure of fish production. Several solar photocatalytic reactors have been used to treat drinking water or waste water without leaving chemical residues. This study has investigated the impact of several key aspects of water quality on the inactivation of the pathogenic bacterium Aeromonas hydrophila using a pilot-scale thin-film fixed-bed reactor (TFFBR) system.ResultsThe level of inactivation of Aeromonas hydrophila ATCC 35654 was determined using a TFFBR with a photocatalytic area of 0.47 m2 under the influence of various water quality variables (pH, conductivity, turbidity and colour) under high solar irradiance conditions (980–1100 W m-2), at a flow rate of 4.8 L h-1 through the reactor. Bacterial enumeration were obtained through conventional plate count using trypticase soy agar media, cultured in conventional aerobic conditions to detect healthy cells and under ROS-neutralised conditions to detect both healthy and sub-lethally injured (oxygen-sensitive) cells. The results showed that turbidity has a major influence on solar photocatalytic inactivation of A. hydrophila. Humic acids appear to decrease TiO2 effectiveness under full sunlight and reduce microbial inactivation. pH in the range 7–9 and salinity both have no major effect on the extent of photoinactivation or sub-lethal injury.ConclusionsThis study demonstrates the effectiveness of the TFFBR in the inactivation of Aeromonas hydrophila under the influence of several water quality variables at high solar irradiance, providing an opportunity for the application of solar photocatalysis in aquaculture systems, as long as turbidity remains low.

Solar photocatalytic inactivation of Fusarium Solani over TiO2 nanomaterials with controlled morphology—Formic acid effect

Solar photocatalytic inactivation (SPCI) of E. coli as the indicator microorganism using LaFeO3 (LF) has already been investigated under various experimental conditions, excluding any role of natural organic matter (NOM). However, comprehensive information about the behavior of E. coli and its inactivation mechanism in the presence of NOM, as well as the behavior of NOM components via solar photocatalysis using LF as a photocatalyst, has prime importance in understanding real natural water environments. Therefore, in this study, further assessment was devoted to explore the influence of various NOM representatives on the SPCI of E. coli by using LF as a novel non-TiO2 photocatalyst. The influence of NOM as well as its sub-components, such as humic acids (HA) and fulvic acids (FA), was also investigated to understand different NOM-related constituents of real natural water conditions. In addition to spectroscopic and mechanistic investigations of cell-derived organics, excitation emission matrix (EEM) fluorescence spectra with parallel factor multiway analysis (PARAFAC) modeling revealed further information about the occurrence and/or disappearance of NOM-related and bacteria-related fluorophores upon LF SPCI. Both the kinetics as well as the mechanism of the LF SPCI of E. coli in the presence of NOM compounds displayed substrate-specific variations under all conditions.

Solar disinfection of fungal spores in water: Kinetics, influencing factors, mechanisms and regrowth

Background. The development of alternative processes to eliminate pathogenic agents in water is a matter of growing interest. Current drinking water disinfection procedures, such as chlorination and ozonation, can generate disinfection by-products with carcinogenic and mutagenic potential and are not readily applicable in isolated rural communities of less-favored countries. Solar disinfection processes are of particular interest to water treatment in sunny regions of the Earth. Solar light may be used to activate a photocatalyst or photosensitizer that generates, in the presence of molecular oxygen dissolved in water, reactive oxygen species (ROS), such as the HO• radical, singlet oxygen (O21), or superoxide (O2•), which are toxic to waterborne microorganisms. Method of Approach. Wild and collection-type Escherichia coli have been selected as model bacteria. Inactivation of such bacteria by either TiO2 nanoparticles, water-soluble tris(2,2′-bipyridine)ruthenium(II) dichloride or Rose bengal (RB) subject to simulated sunlight have been compared. Although TiO2 is the prototypical material for heterogeneous photocatalysis, the other two dyes are known to generate significant amounts of O21 by photosensitization but have different chemical structures. The concentration of dye, illumination time, photostability, presence of scavengers, and post-treatment regrowth of bacteria have been investigated. Results. After 1hr of solar illumination the Ru(II) complex produced a strong loss of E. coli culturability monitored with solid selective agars. Both the collection- and wild-type bacteria are sensitive to the treatment with 2-10mgL−1 of dye. This photosensitizer showed a better inactivation effect than TiO2 and the anionic organic dye RB due to a combination of visible light absorption, photostability, and production of O21 and other ROS when bound to the bacterial membrane. A complete loss of culturability was observed when the initial concentration was 103CFUmL−1, with no bacteria regrowth detected after 24hr of the water treatment. At higher initial microorganism levels, culturability still remains and regrowth is observed. Scavengers show that the HO• radical is not involved in bacteria inactivation by photosensitization. Conclusions. A higher quantum yield of ROS generation by the sensitizing dyes compared to the semiconductor photocatalyst determines the faster sunlight-activated water disinfection of photodynamic processes. The homogeneous nature of the latter determines a more efficient interaction of the toxic intermediates with the target microorganisms. Solid supporting of the Ru(II) dye is expected to eliminate the potentials problems associated to the water-soluble dye.

Visible light photocatalytic response of Fe doped TiO2: Inactivation of Escherichia coli

Photocatalytic degradation of pharmaceutically active compounds (PhACs) in urban wastewater treatment plants effluents under controlled and natural solar irradiation using immobilized TiO2

Lanthanum orthoferrites are a versatile class of catalysts. Here, the photocatalytic bactericidal performance of LaFeO3 (LF) to inactivate pathogenic microorganisms, i.e., Escherichia coli (E. coli), in water under simulated solar irradiation conditions was investigated. Various competing and contributing factors were covered to visualize the reaction medium consisting of E. coli K12 cells, organic sub-fractions formed by cell destruction, and LF surface. LF solar photocatalytic inactivation (SPCI) kinetics revealed the highest inactivation rate in ultrapure water as expected, followed by distilled water (DW), aqueous solution containing anions and cations (WM) and saline solution (SS). Characterization of the released organic matter was achieved by UV-vis and fluorescence spectroscopic techniques as well as organic carbon contents (DOC). Upon SPCI, significant amounts of K+ along with released protein contents were detected expressing cell wall destruction and lysis. Under the specified experimental conditions, in the presence of released intracellular organic and inorganic components via cell lysis, a significant count of E. coli was still present in SS, whereas almost all bacteria were removed in other matrices due to various challenging reasons. Based on the presented data, SPCI of E. coli using LF as a novel photocatalyst was successfully demonstrated as an alternative and promising method for disinfection purposes.

BackgroundOutbreaks of infectious diseases by microbial pathogens can cause substantial losses of stock in aquaculture systems. There are several ways to eliminate these pathogens including the use of antibiotics, biocides and conventional disinfectants, but these leave undesirable chemical residues. Conversely, using sunlight for disinfection has the advantage of leaving no chemical residue and is particularly suited to countries with sunny climates. Titanium dioxide (TiO2) is a photocatalyst that increases the effectiveness of solar disinfection. In recent years, several different types of solar photocatalytic reactors coated with TiO2 have been developed for waste water and drinking water treatment. In this study a thin-film fixed-bed reactor (TFFBR), designed as a sloping flat plate reactor coated with P25 DEGUSSA TiO2, was used.ResultsThe level of inactivation of the aquaculture pathogen Aeromonas hydrophila ATCC 35654 was determined after travelling across the TFFBR under various natural sunlight conditions (300-1200 W m-2), at 3 different flow rates (4.8, 8.4 and 16.8 L h-1). Bacterial numbers were determined by conventional plate counting using selective agar media, cultured (i) under conventional aerobic conditions to detect healthy cells and (ii) under conditions designed to neutralise reactive oxygen species (agar medium supplemented with the peroxide scavenger sodium pyruvate at 0.05% w/v, incubated under anaerobic conditions), to detect both healthy and sub-lethally injured (oxygen-sensitive) cells. The results clearly demonstrate that high sunlight intensities (≥ 600 W m-2) and low flow rates (4.8 L h-1) provided optimum conditions for inactivation of A. hydrophila ATCC 3564, with greater overall inactivation and fewer sub-lethally injured cells than at low sunlight intensities or high flow rates. Low sunlight intensities resulted in reduced overall inactivation and greater sub-lethal injury at all flow rates.ConclusionsThis is the first demonstration of the effectiveness of the TFFBR in the inactivation of Aeromonas hydrophila at high sunlight intensities, providing proof-of-concept for the application of solar photocatalysis in aquaculture systems.

It is estimated that there are 748 million people throughout the world without access to improved water sources for drinking and many more rely on water that is not safe to drink due to contamination with pathogenic microorganisms. Furthermore, the reuse of wastewater is becoming increasingly important due to water scarcity throughout the globe, and it is vitally important to ensure that water for reuse is free from pathogenic microorganisms. Solar energy is free and ubiquitous on the Earth's surface. The combination of solar disinfection and photocatalysis technology offers real possibilities for removing lethal pathogenic microorganisms from polluted water and drinking water. The treatment time taken for the solar disinfection of drinking water can be greatly reduced by utilizing heterogeneous photocatalysis. This chapter reviews and compares the fundamental mechanisms of solar and solar photocatalytic inactivation of microorganisms. The complexity of the mechanisms, involving reactive oxygen species and their effects on the cell wall, membrane and internal components of microorganism, are discussed. Furthermore, the photocatalytic inactivation of antibiotic resistant bacteria is discussed. The development of visible light active photocatalytic materials for solar disinfection is reviewed and, finally, there is a short discussion concerning photo-reactors specifically for the solar photocatalytic disinfection of water.

Photocatalytic inactivation of E. coli in surface water using immobilised nanoparticle TiO 2 films

Photocatalytic disinfection is a promising technology with low cost and high efficiency. However, most of the current studies on photocatalytic disinfection ignore the widespread presence of natural organic matter (NOM) in water bodies, so the incomplete conclusions obtained may not be applicable. Herein, this paper systematically studied the influence of humic acid (HA), one of the most important components of NOM, on the photocatalytic inactivation of bacteriophage f2 with electrospinning Cu-TiO2 nanofibers. We found that with the addition of HA, the light transmittance of the solution at 550 nm decreased from 94 to 60%, and the band gap of the photocatalyst was increased from 2.96 to 3.05 eV. Compared with reacting without HA, the degradation amount of RNA of f2 decreased by 88.7% after HA was added, and the RNA concentration increased from 1.95 to 4.38 ng·μL-1 after the reaction. Hence, we propose mechanisms of the effect of HA on photocatalytic disinfection: photo-shielding, passivation of photocatalysts, quenching of free radicals, and virus protection. Photo-shielding and photocatalyst passivation lead to the decrease of photocatalyst activity, and the reactive oxygen species (ROSs) (·OH, ·O2-, 1O2, H2O2) are further trapped by HA. The HA in water also can protect the shape of phage f2 and reduce the leakage of protein and the destruction of ribonucleic acid (RNA). This work provides an insight into the mechanisms for the influence of HA in photocatalytic disinfection process and a theoretical basis for its practical application.

Visible-light-driven photocatalytic inactivation of bacteriophage f2 by Cu-TiO2 nanofibers in the presence of humic acid