Photocatalytic degradation of terbuthylazine: Modelling of a batch recirculating device Jérôme Le Cunff a, *, Vesna Tomaši  c b , Zoran Gomzi b a Xellia d.o.o., Slavonska avenija 24/6, 10000, Zagreb, Croatia b University of Zagreb, Faculty of Chemical Engineering and Technology, University of Zagreb, Trg Marka Maruli ca 19, 10000, Zagreb, Croatia A R T I C L E I N F O Article history: Received 13 August 2017 Received in revised form 12 November 2017 Accepted 13 November 2017 Available online 14 November 2017 Keywords: Recirculating reactor Photolytic/photocatalytic degradation Terbuthylazine TiO 2 /chitosan Photoreactor Triazines A B S T R A C T A thin layer photocatalyst using chitosan to immobilize TiO 2 on a glass fiber woven roving material was successfully used for photocatalytic degradation of terbuthylazine, s model s-triazine herbicide. The reaction was conducted in a photocatalytic recirculating reactor with the photocatalyst inserted as a removable module. The experimental reaction system employed in this study was composed of an annular photoreactor with the immobilized TiO 2 /chitosan layer and the radiation source and the second part of the reaction system only used for the aeration or the reaction mixture, both operating in unsteady conditions. The kinetic model is based on a simplified consecutive degradation of terbuthylazine to cyanuric acid through intermediate products. The model of the annular reactor is represented by a hyperbolic partial differential equation solved by method of characteristics; the model of the aeration vessel is given by an ordinary differential equation. The proposed model represents a simple way to describe a complex recirculating reactor system operating in unsteady conditions. © 2017 Elsevier B.V. All rights reserved. 1. Introduction Persistent organic pollutants (POPs), like pesticides, are not easily degraded by conventional degradation methods, making advanced oxidation processes like photocatalysis increasingly interesting to many researchers [1–6]. Research on photocatalysis is mostly based on TiO 2 suspended nanoparticles in fluid phase contaminated with organic pollutants, allowing for the largest surface area and efficient photocatalytic degradation. The lack of the photocatalysts selectivity allows a very wide range of their application [7–12]. Retrieval or separation of suspended photocatalytic nanoparticles from the fluid phase is the major drawback of this process [13–16]. Regeneration is also a challenge in the case of suspended nanoparticles making the photocatalyst poisoning another issue, as well environmental contamination [17–21]. Immobilization of TiO 2 is a common way to solve these issues [17,22]. The specific surface of a thin layer is very small compared to the reactor, which can, including the external mass transfer limitations, lead to a 70% reduction in photocatalyst performance compared to the suspension reactors. Immobilized layers offer the possibility of easier photocatalyst modifications, as well as regeneration [17,23–28]. Regardless, for industrial application, the photocatalyst needs to be removable, allow easy maintenance and off site regeneration. It also needs to withstand harsh operating conditions of industrial water and prepared from easily available and cost effective materials, also avoiding the risk of additional pollution. TiO 2 and chitosan as the photocatalyst binder are quite common and widely available materials, suited for such applications [29–42]. This study presents the development of a mathematical model of the photocatalytic recirculating reactor with an immobilized photocatalytic layer described in a previous paper [43]. The experimental reaction system comprised two parts. An annular photoreactor, with the immobilized TiO 2 /chitosan layer and the central radiation source. Since process variables were not only time dependent, but also dependent on the position along the reactor length, the reaction system could not be considered homogenous and solved as a batch reactor as it is usually done [44,45]. The annular part of the reactor system was described using a hyperbolic differential equation. The equation was simplified by variable substitution, expressing both the reactor length and reaction time as the residence time with the Courant number being equal to 1. The second part of the system was used for aeration of * Corresponding author. E-mail address: jerome.le-cunff@xellia.com (J.L. Cunff). https://doi.org/10.1016/j.jphotochem.2017.11.020 1010-6030/© 2017 Elsevier B.V. All rights reserved. Journal of Photochemistry and Photobiology A: Chemistry 353 (2018) 159–170 Contents lists available at ScienceDirect Journal of Photochemistry and Photobiology A: Chemistry journal homepa ge: www.elsev ier.com/locate/jphotochem