Selective photocatalytic removal of procaine – comparison of molecularly imprinted and non-imprinted TiO2 material

Selective photocatalytic removal of procaine – comparison of molecularly imprinted and non-imprinted TiO2 material K. Tolić Čop*, D. Mutavdžić Pavlović*, L. Čakić*, I. Gabelica** *Department of Analytical Chemistry, Faculty of Chemical Engineering and Technology, University of Zagreb, Trg Marka Marulića 19, 10000 Zagreb, Croatia (E-mail: [email protected]; d[email protected]izg.hr) **Department of materials, Faculty of mechanical engineering and naval architecture, Ivana Lučića 5, 10000 Zagreb, Croatia Abstract Pharmaceuticals are ubiquitois in our daily lives and are therefore easily enter the environment. Such invisible small organic molecules pollute every part of environment; and constitute a major problem for the entire ecosystem. Conventional water treatment techniques are unsuitable for this type of pollutants, so advanced oxidation methods such as photocatalysis are being investigated. Due to the more economical and simpler photocatalytic performance, a catalyst was prepared in this work by depositing a TiO2 film on an Al2O3 ceramic foam substrate. The photocatalytic activity of procaine under UV-A light was investigated through experiments of optimization of reaction conditions by changing the pH and initial drug concentration. The obtained results indicated the procaine is degraded most rapidly in an alkaline medium (pH 10) and at a concentration of 10 mg/L. Considering that degradation is generally very slow at different concentrations and pH values (removal half-lives between 110 and 690 min), a different type of catalyst was used to improve the photocatalytic efficiency for procaine. Indeed, the photocatalyst was prepared in the same way as the previous one, only with the addition of a pollutant molecule to the reaction mixture. The acid-catalyzed sol-gel reaction led to the formation of imprinted procaine molecules on a TiO2-Al2O3 ceramic. The photocatalytic experiments were repeated under the same reaction conditions, and the results highlighted the importance of developing new catalysts such as molecularly imprinted materials, as these increased the selectivity of removal of non-degradable molecules such as procaine. Keywords: molecularly imprinted material; photocatalysis; procaine; sorption; water INTRODUCTION Since conventional methods for efficient wastewater treatment are not sufficient for the removal of small organic molecules, such as pharmaceuticals, advanced oxidation processes have been developing for many years. Heterogeneous photocatalysis is one of the promising technologies based on the photoactivation of semiconductors that act as an active catalytic surface with the aim of degradation and eventually mineralizing persistent organic pollutants (Patel et al., 2019; Mansouri et al., 2021; Kumari et al., 2023). To simplify the process and increase productivity, various composites are synthesized based on TiO2 immobilization. Solid substrates can be different; fibers, polymers, magnetic particles or, as in this work, ceramics such as Al2O3 (Antonopoulou et al., 2021). To overcome the problems of complex environmental matrices and to improve the selectivity and activity of TiO2 for the removal of organic micropollutants, molecular imprinting techniques are used to produce materials containing recognition sites for the target molecule. Various approaches are have been developed for the synthesis of these materials, ranging from polymerizations to numerous methods in which a template molecule is imprinted on an inorganic substrate, for example by sol-gel technique (de Escobar et al., 2018; Wang et al., 2016; Wang et al., 2024; Lulu et al., 2020). In this study, the target substance chosen was procaine, one of the oldest ester-type local anesthetics in use, characterized by high stability under environmental conditions and resistant to removal by conventional methods in wastewater treatment plants (detected in microgram quantities in industrial pharmaceutical wastewater) (Badea et al., 2002; Munzhelele et al., 2024; Queiroz et al., 2019). Therefore, the main objective of this work was to compare the capabilities of two TiO2-based materials (with and without imprinting template) for procaine removal from water. MATERIALS AND METHODS The photocatalyst used in this work was prepared by creating a highly porous ceramic Al2O3 foam using a simple and cheap replication method based on polyurethane impregnation. The nanostructured TiO2 film was deposited on the Al2O3 foam substrate, which was tailored to the dimensions of the reactor, by sol-gel assisted dip-coating. A detailed procedure for the synthesis of the basic photocatalyst (NIP – non-imprinted photocatalyst) is previously published (Švagelj et al., 2020). The molecularly imprinted photocatalyst (MIP) was prepared in the same way, only with the addition of the target molecule in the TiO2 sol. A MIP was ready for use after rinsing the template with a mixture of acetic acid and methanol (1:9) until the procaine peak was no longer visible in the chromatogram. The reaction system consisted of two UV lamps mounted above an open rectangular reactor, filled with 100 mL of procaine solution. Continuous mixing of the aqueous phase was ensured by a peristaltic pump with a flow rate of 30 mL/min. First, preliminary experiments were carried out to determine the hydrolitic (dark) and photolytic stability (UV only) of procaine. Subsequently, sorption experiments were performed to estimate the utilization of the active sites of both photocatalysts (TiO2 without UV). Prior to irradiation, the photocatalysts were exposed to procaine in the dark for 30 min to establish a sorption/desorption equilibrium. Photocatalytic experiments were performed to estimate the mutual influence of the reaction conditions on the removal rate by changing the pH (4 – 10) and the initial concentration of the pollutant (5 – 15 mg/L). To further estimate the reusability of the photocatalytic activity of the photocatalyst, three experiments on the cyclic degradation of procaine were performed. The experiments lasted 6 hours, during which samples (500 μL) were taken at specific time intervals and stored at − 4 °C in the dark. The samples were monitored using the Agilent 1100 HPLC-DAD system. RESULTS AND DISCUSSION Preliminary experiments showed that the procaine molecule is a hydrolytically and photolytically stable molecule (Figure 1), which emphasizes the need to use a photocatalyst and a light source to increase the removal of pollutants from an aqueous medium. Figure 1. Preliminary experiments The sorption experiments show a large difference in the selectivity of the photocatalysts used compared to procaine.. Approximately 86 % of the procaine sorbed onto MIP during the 240min sorption time, indicating an increased selectivity of this type of material for the removal 0 0,2 0,4 0,6 0,8 1 060 120 180 240 300 360 c/c0 t, min Hydrolysis Photolysis Sorption - NI Sorption - MI of procaine. The NIP composite showed a low sorption capacity for procaine binding (~ 3 % of sorption), so the main role of procaine removal by photocatalysis is expected after activation of the deposited TiO2 with UV light. Figure 2. Synergistic effect of sorption and photocatalysis on procaine removal with a) NIP and b) MIP The Figure 2 show the photocatalytic activity upon removal of procaine with a NIP and MIP photocatalyst, which carried out the reaction for 6 hours. Prior to irradiation, the photocatalysts were in contact with the pollutant for 30 min to allow the procaine molecules to reach sorption/desorption equilibrium prior to photocatalysis. Both figures show that, procaine is removed by more than 90 %, while in the case of MIP, the synergistic effect of, sorption and photocatalysis contributes to procaine removal. The sorption affinity of procaine is low (mostly less than 10 %) for NIP compared to MIP material produced with its imprint. Figure 3. Kinetic curves of procaine photocatalysis Figure 3 show the fastest degradation of procaine at pH 10 and 10 mg/L for both photocatalysts. It can also be seen that pH plays an important role as it affects the surface charge of the catalyst and the dissociation of the pharmaceutical. The negative surface charge of NIP at alkaline solution resulted in an increased kinetic rate of procaine removal, probably by some stronger electrostatic mechanisms, as procaine has a pKa close to 8.9. Procaine increased the kinetic rate with higher pH, indicating strong electrostatic forces between the neutral molecule and the possibly negatively charged photocatalyst (Mokhbi et al., 2014). An increased number of moles of procaine involved in degradation slowed down the removal due to a limited number of active sites on the catalysts. A significant acceleration of procaine degradation from water is evident when using the imprinted material; the half-lives of degradation with MIP ranged from 63.59 to 106.64 min. Acknowledgement This study was supported by the Croatian Science Foundation under the project number HRZZ-IP2022-10-4400 entitled Development of molecularly imprinted polymers for use in analysis of pharmaceuticals and during advanced water treatment processes (MIPdePharma). 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