The Roles of Cell Components in the Photocatalytic Inactivation of Bacteria: Mechanisms and Interactions

摘要

Due to the increasing demand of clean and safe drinking water, numerous alternatives techniques for water purification have been developed. Recently, photocatalysis has been widely considered as the most promising solution for water purification due to its potential to use sun-light to drive the process with a solid catalyst and achieve disinfection with less or even no disinfection byproducts (DBP) formation. Under specific light irradiation on the photocatalyst, reactive charged and oxidative species (ROS) are generated and can cause fatal damage to microorganisms. However, the photocatalytic microbial inactivation mechanism has still not been well established.;Therefore, this study systematically investigated the roles of bacterial cell components from the outmost layer of capsular (extracellular polymeric substances) EPS, followed by the middle layer of cell membrane, finally to the most innermost core of the intracellular organic matters (IOM) in the photocatalytic inactivation and interaction between cell and nano-size photocatalyst (TiO2) using different approaches.;Firstly, we investigated the role of the EPS in the photocatalytic inactivation and photocatalyst-cell interaction mechanisms by comparing wild type Escherichia coli BW25113 and its isogenic mutants with up-regulated and down-regulated production of capsular EPS. The combined results of non-partition system and partition system suggested that, although the capsular EPS can protect the bacterial cell by consuming photo-generated reactive species, it also facilitated the photocatalytic inactivation by promoting the adhesion of TiO2 particles on the cell surface. The fluorescence microscopic and scanning electron microscopic analyses further confirmed that high capsular EPS density lead to the more TiO2 particles attaching onto cells forming bacteria-TiO2 aggregates. Furthermore, the interaction energy, represented by extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) potential, was also calculated to reveal the interaction profiles between cells and TiO 2 particles. Results indicated that the presence of capsular EPS enhanced the attachment of TiO2 particles towards bacterial cells via the dominating acid-base (hydrophobic) interaction.;Secondly, we investigated the interaction between TiO2 nanoparticles and biological membranes using bacterial ghosts (BGs) as a model and 2D-COS-FTIR techniques. Results indicated that functionalities of cell membrane protein preferentially interacted with TiO2 nanoparticles; whereas the interaction of TiO2 nanoparticles with C--OH (polysaccharides) and P=O (phospholipids) were very weak or insensitive. This result was corroborated with settling experiments using standard proteins, polysaccharides and phospholipids. Additionally, the asynchronous map of 2DCOS indicates a sequential order of functionalities bonded to TiO2 nanoparticles: COO- > aromatic C=C stretching > N-H, amide II > C=O, ketone.;Finally, we evaluated the released organic matter during the photocatalytic inactivation process using fractionation procedure that separate the bacteria cell into IOM and cell debris. Spectroscopic analysis including absorbance spectrum and fluorescence excitation-emission-matrix (EEM) combined with paralleled factor analysis (PARAFAC) were used to characterize the released organic matters. On the whole, results indicated that the released organic maters mainly derived from the intracellular during the photocatalytic inactivation process. Specifically, four fluorescence components were identified by the EEM-PARAFAC analysis during the photocatalytic inactivation of bacteria. Two components (C1 and C3) were associated with tryptophan-containing and tyrosine-containing proteins, and the other two (C2 and C4) were postulated to be the oxidation products of C1 and C3, respectively.;Besides, these released organic matters would be either absorbed on the surface of the photocatalysts or consume the photo-generated ROS to decelerate the inactivation processes reflected by a plateau in the inactivation curve ("tailing"). Moreover, the bulk total organic carbon (TOC) and total nitrogen (TN) measurement results implied that the mineralization of the released organic compound was much slower than complete inactivation of bacteria and no removal of TN would occurred, thus leading to high TN/TOC ratio in the resultant water.;This research revealed the roles of different cell components, from the outmost layer to the inner core of bacterial cells, in the photocatalytic bacterial inactivation and bacterial cell-photocatalysts interaction. The results obtained in this study have significant implications not only in the photocatalytic inactivation technology, but also in the fate and transformation of TiO2 nanoparticles and bacteria in natural and engineered system.