Degradation of polar and non-polar pharmaceutical pollutants in water by solar assisted photocatalysis using hydrothermal TiO2-SnS2
Graphical abstract
Introduction
Pharmaceuticals encompass a broad spectrum of compounds with various physico-chemical properties, polarities and mechanisms of biological action. Amongst them, diclofenac (DCF) and memantine (MEM) (Fig. 1(A) and (B), respectively) can be considered as pharmaceuticals on the metaphorical opposite ends of the physico-chemical spectrum. DCF is weakly acidic (pKa = 4.15), while MEM is weakly basic (pKa = 10.5) [1], [2]. Owing to the presence of chromophores, DCF has a quite strong absorption peak at λmax = 276 nm; thus, it is susceptible to UV photolysis. On the other hand, MEM is virtually transparent in the UV spectrum due to a lack of such moieties. According to the structure shown on Fig. 1(A), DCF can be characterized as a polar pharmaceutical, due to the presence of amide and carboxylic groups. The rigid, saturated diamandoid structure of MEM shown on Fig. 1(B) can be characterized as non-polar, but owing to the amino group, MEM readily forms highly water soluble salts. DCF is used as a non-steroidal anti-inflammatory drug worldwide in vast quantities, in excess of 940 tons globally, whereas MEM is a relatively recently approved neuroprotector in Alzheimer’s and Parkinson’s disease, with the potential to treat other disorders [3], [4], [5]. The detection of DCF in wastewater effluents and the environment has been widely reported in literature [6], [7], [8], [9], while only limited literature findings exist on behalf of MEM [10], [11]. However, one could expect growing use and presence of MEM in wastewaters due to the general aging of Western population and rising incidences of such neurological disorders [12]. Being only partially metabolized, MEM exacerbates the issue of wastewater contamination. Albeit limited studies on the environmental fate of MEM are available in literature [10], [13], [14], [15], MEM has the potential to be a persistent micropollutant in the environment as it is resistant towards oxidative degradation [14].
Photocatalytic advanced oxidation processes (AOPs) are considered as promising methods for the removal of pharmaceutical micropollutants from wastewaters [16], [17]. TiO2, especially in the form of Aeroxide P25 (TiO2-P25), is considered to be a superior photocatalytic material. However, it’s applicability in solar-driven photocatalytic processes is largely hindered by its prohibitively wide bandgap, thereby significantly limiting effective utilization of solar irradiation. Hence, the photocatalytic performance of the hydrothermally obtained TiO2-SnS2 composite photocatalyst (TiO2-SnS2-HT) was evaluated for the degradation of DCF and MEM in comparison to TiO2-P25. The TiO2-SnS2-HT composite photocatalysts should reap the advantages of TiO2, while providing a lower bandgap for improved photocatalytic activity under solar irradiation due to the inclusion of SnS2. However, the performance of a particular photocatalytic material is rarely evaluated on pharmaceuticals of different physico-chemical properties, usually being limited to a few, polar, target compounds. Herein, physico-chemical properties governing underlying phenomena of photocatalytic degradation of polar DCF and non-polar MEM were elucidated by density functional theory (DFT).
Section snippets
Chemicals
Diclofenac sodium salt (DCF, p.a., Sigma Aldrich) and memantine hydrochloride (MEM, p.a., Pliva), were used as water pollutants. Aeroxide P25 TiO2 (TiO2-P25, Evonik) was used as a reference photocatalyst. Tetra-butyl titanate (TBT, 97%, Acros Organics), tin (IV) chloride pentahydrate (98%, Sigma-Aldrich), thioacetamide (TAA, ≥99.0%, Sigma-Aldrich), ethanol (96%, EtOH, Gram-mol), and glacial acetic acid (AA, ≥99.7%, Carlo Erba Reagents) were used in a hydrothermal synthesis to prepare the TiO2
Results and discussion
Initial removals of DCF after achieving adsorption equilibrium in the dark (t = 0 min) using solar/TiO2-SnS2-HT process and benchmark solar/TiO2-P25 are shown on Fig. 2(A). It can be seen that during the dark period, significantly higher DCF removal extents have been achieved using composite material. The adsorption capacity of TiO2-SnS2-HT is positively affected by increasing SnS2 wt.% within the composite, while an increase in pH caused a significant decrease in DCF removal efficiency by
Conclusions
The photocatalytic performance of TiO2-SnS2-HT strongly depends on the adsorption extent and nucleophilic nature of the pollutant. Therefore, TiO2-SnS2-HT has shown improved photocatalytic activity in comparison to TiO2-P25 for the degradation of DCF at mildly acidic and circumneutral pH conditions, however the effectiveness towards MEM degradation was significantly lower due to a lack of adsorption. The notably higher DCF adsorption extent in comparison to MEM can be attributed to three
Acknowledgements
The research was funded by “Nano-sized Solar-active Catalysts for Environmental Technologies” (NaSCEnT, IP-2018-01-1982), Croatian Science Foundation and “Novel catalysts for solar driven water treatment; quantum-chemical and experimental approach” within the French-Croatian programme “Hubert Curien ‘Cogito’”. This work was granted access to the HPC resources of [CCRT/CINES/IDRIS] under the allocation 2018-2019 [A0040807031] made by GENCI (Grand Equipement National de Calcul Intensif). We also
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