Chemical Engineering Journal 165 (2010) 225–233 Contents lists available at ScienceDirect Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej Photocatalytic inactivation of Escherischia coli and Pichia pastoris with combustion synthesized titanium dioxide Sharad Sontakke, Jayant Modak, Giridhar Madras ∗ Department of Chemical Engineering, Indian Institute of Science, Bangalore 560012, Karnataka, India article info Article history: Received 28 July 2010 Received in revised form 15 September 2010 Accepted 16 September 2010 Keywords: Photocatalysis Combustion synthesis TiO2 Ag/TiO2 E. coli Pichia pastoris abstract The photocatalytic inactivation of Escherischia coli and Pichia Pastoris was studied with combustion syn- thesized titanium dioxide photocatalysts. Three different combustion synthesized (CS) catalysts were used viz., CS–TiO 2 , 1% Ag substituted in TiO 2 and 1% Ag impregnated in TiO 2 . All the combustion synthe- sized catalysts showed higher activity as compared to the activity observed with commercial Degussa P-25 TiO 2 . The effect of various parameters like catalyst loading, different catalysts and initial cell concen- tration was studied. At the optimum loading, 1% Ag impregnated TiO 2 showed the maximum efficiency and complete inactivation of both the microorganisms was observed within an hour of irradiation. The morphology of inactivated cells was studied by inverted microscope and SEM. From the images obtained, it was hypothesized that damage to the cell wall was the main cause of cell inactivation. The initial cell concentration had a prominent effect on the inactivation. At a low initial cell concentration, the complete inactivation of E. coli and P. pastoris was observed within 10 and 20 min, respectively. This shows that P. pastoris has a stronger resistance towards photocatalytic inactivation than E. coli. The inactivation reac- tions were modeled with power law kinetics. The order of reaction in case of E. coli and P. pastoris were determined as 1.20 and 1.08, respectively. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Photocatalytic inactivation of microorganisms, using semicon- ductor catalysts, has received considerable attention in the past two decades. Matsunaga et al. [1] were the first to report the use of Pt/TiO 2 semiconductor powder for photochemical steril- ization of microorganisms. Since then, photocatalysis has been shown as an efficient process for the removal of a wide variety of microorganisms including pathogenic bacteria, fungi and viruses [2]. Many photocatalytic disinfection studies have been carried out to determine the effect of various parameters like light intensity [3,4], catalyst concentration [3–5], temperature [6], culture media [7], initial concentration of microorganism [4,6], pH [7], etc. on the inactivation of microorganisms to understand the underlying mechanism. Various hypotheses regarding the mechanism of microorgan- ism inactivation have been proposed. Matsunaga et al. [1] proposed that the direct photochemical oxidation of intracellular coenzyme A (CoA) was the main cause for inhibition in respiration of cells that led to cell death. Maness et al. [8] suggested that TiO 2 photocataly- ∗ Corresponding author. Tel.: +91 80 22932321; fax: +91 80 23601310. E-mail addresses: giridhar@chemeng.iisc.ernet.in, giridharmadras@gmail.com (G. Madras). sis promotes the peroxidation of polyunsaturated phospholipids and induces major disordering of cell membrane in Escherischia coli. Sunada et al. [9] have suggested, the cell wall of E. coli under- goes disruption by disordering outer membranes and the reactive species (e.g., HO • , HO 2 • ,H 2 O 2 ) penetrates the cytoplasmic mem- brane, leading to cell death. It is generally assumed that the reactive oxygen species (ROS) like H 2 O 2 ,O 2 • , etc. as well as hydroxyl radical (HO • ), which are generated on the surface of UV-illuminated TiO 2 , plays the major role for the inactivation of microorganisms [4,5,9]. Most of the inactivation studies are based on E. coli or other bac- terial species. However, the inactivation kinetics for yeast has not been investigated. TiO 2 is the most widely used catalyst in photocatalytic stud- ies because of its high photocatalytic activity, non-toxicity and wide availability. Most of the degradation studies have used com- mercially available TiO 2 , Degussa P-25 (DP-25) directly or in its modified form [4–7]. This commercial catalyst consists of 80% anatase and 20% rutile phase. However, anatase phase TiO 2 has been reported to possess higher photocatalytic activity than the rutile phase [10]. This catalyst has shown to remove a wide variety of organic and inorganic contaminants [11,12]. There are several methods for the synthesis of anatase phase TiO 2 [13–15]. Solu- tion combustion synthesis [16] is a single step process to produce pure anatase phase TiO 2 and the catalyst produced by this method has been shown to be superior to the commercially available DP- 1385-8947/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2010.09.021