Due to the increase in the world's population, it is predicted that in the coming years the demand for healthy water will increase by 55 % (Gopinath et al. 2020; Suhaimy et al. 2020). Although industries treat their wastewater with conventional processes, these are not efficient in degrading all pollutants, especially emerging organic compounds (EOCs) (De la Cruz et al. 2013). EOCs are those organic pollutants that have not been recognized by existing environmental legislation, but it has been shown that these pollutants are affecting aquatic ecosystems and their environment (Bueno et al. 2012; Giwa et al. 2021; Majumder et al. 2019; Phoon et al. 2020; Rasheed et al. 2019). In the case of antibiotics, studies by Klein et al. (2018) found that from 2000 to 2015 their consumption increased by 65 % and that some of these antibiotics are only partially metabolized allowing a fraction of them to be excreted from the body in an unchanged form after consumption (Rasheed et al. 2019; Ye et al. 2019). The mode of use, types, and concentrations of antibiotics are not the same for all countries, there are concentrations from ppm to ppt, even after the wastewater has been treated (Wen et al. 2014), other studies show that antibiotics are present in different ecosystems (Wang and Zhuan 2020), although in low concentration, but they remain biologically active, causing long-term resistance of bacteria, generating a negative impact on human and animal health (Hou et al. 2019; Phoon et al. 2020).
One of the most widely used antibiotics is amoxicillin (AMX, C16H19N3O5S) (PubChem 2021; Wang and Wang 2016). In traditional biological wastewater treatment, AMX exhibits stable chemical properties, biological toxicity, and a low rate of biodegradation (Song et al. 2016). Conventional methods for removing antibiotics from water include coagulation (Bratby 2016), ozone (O3) and O3 - H2O2 (Bavasso et al. 2020), biological systems as activated sludge, membrane and sequential bioreactor (Wang et al. 2020), inverse osmosis (Baheri et al. 2016), and adsorption by activated carbon (Perrich 2018), the disadvantage being that these contaminants are not mineralized, but only concentrated and transferred to produce new residues that require further processing to remove the new residues. In some works, amoxicillin mineralization has been reported by different methods: using TiO2 activated carbon composites, 50 to 100 % of 50 mg L− 1 of amoxicillin at pH 3 to 10 was removed with sunlight for 180 min (PubCHem 2021). Using TiO2 nanotubes with graphite and adding KBrO3 a degradation of almost 100 % is achieved (Gar et al. 2016). With hybrid processes: ultrafiltration membrane, activated carbon adsorption, and ultrasound irradiation in 10 ppm of amoxicillin, 99.5 % were removed (Secondes et al. 2014); and with adsorption, membrane and ultrasound irradiation for 0.1 mg L− 1, 99 % was removed (Naddeo et al. 2020).
Advanced oxidation process (AOPs) with modified electrodes are other promise technology, which shows high removal percentages of pharmaceutical compounds as paracetamol (Brillas et al. 2005), metoprolol (Dirany et al. 2012), sulfachloropiridazine (Cavalcanti et al. 2013), omeprazole (García-Segura et al. 2014), chloramphenicol (Olvera-Vargas et al. 2014), ranitidine (Salazar 2014), phantetra (Panizza et al. 2014) and amoxicillin (León et al. 2020). The oxidant power of AOPs is determined by the high oxidation overpotential to O2 evolution and the sorption enthalpy of electrogenerated hydroxyl radicals (Marcelino et al. 2017; Sopaj et al. 2015; Tan et al. 2020; Zha et al. 2014). AOPs are used because of their rapid reaction rate and strong oxidation capacity, which are effective for antibiotic degradation in aquatic environments (Benjedim et al. 2020; Moura et al. 2018; Seo and Park 2009; Wang and Zhuan 2020). In AOPs are used the dimensional stable anodes (DSA®), which are constructed by a thin film of transition metal oxides over metal as titanium (Ti) by its low cost; the Ti is sandblasted to increase the exposed area to include the metallic oxides, as it has been reported previously (Herrada et al. 2016, 2018, 2020; León et al. 2020).
In the case of nanostructured TiO2, it is used because it has a relatively high quantum value, easy accessibility, low toxicity, high physical/chemical stability, large surface area, fast degradation rates, is non-toxic, is biocompatible with the environment, and can be easily synthesized (Gopinath et al. 2020; Molina-Reyes et al. 2020). TiO2 nanotubes can be obtained by different synthesis methods, (a) hydrothermal (Subramanian et al. 2020), anodization (Diao et al. 2020; Suhaimy et al. 2020), microwave (Martínez-Sánchez et al. 2019), impregnation (Kulkami et al. 2016), sol-gel (Muswareen et al. 2019), solvothermal (Oh et al. 2019), template synthesis, and chemical reduction (Peng et al. 2021). Anodization is considered the most convenient and effective method for preparing high-quality TiO2 nanotubes due to its good controllability, simple operation, low cost, and environmental friendliness (Liu et al. 2013). Currently, there are several researches focused on modifying TiO2 nanotubes with different materials, for example TiO2-Ru (Gopinath et al. 2020), Co/Bi/TiO2 NTA (Ahmadi and Wu 2020), TiO2-S (Yang et al. 2021), Co-TNT (Caia et al. 2020), TiO2 NTA/Cu2O (Koiki et al. 2020), Fe-TNT (Subramaniam et al.2020), TNT/Fe3O4/TiO2 (Chen et al. 2018), [email protected] (Lei et al. 2018), TiO2/SiO2 (Raseed et al. 2019), g-C3N4/Ti3C2/TiO2 (Diao et al. 2020), TiO2/NTs/AgBr/BiOBr (Ye et al. 2018), Ag/TNA (Peng et al. 2021), CeO2 (Koiki et al. 2020), doped Zn (Xu et al. 2005), N2 (Lin et al. 2011), SnO2 (Tsai et al. 2017), CdS (Zhang et al. 2018), CdSe (Zhang et al. 2009), PbS (Zhang et al. 2016), and Pt (Zhou et al. 2018). Most of these nanomaterials have been used for the degradation of dyes or other organic compounds, these TiO2 doped nanotubes have not been proposed to degrade amoxicillin.
In this research, naked and modified surfaces with TiO2 nanotubes (TiO2,nt) electrophoretically modified with PbO2, IrO2, RuO2, and Ta2O5 were used to evaluate their efficiency in the electrochemical degradation of AMX in aqueous media, which is an example of the EOCs, and it has not been reported before. This pharmaceutical product has been electro-oxidized using IrO2-Ta2O5|Ti and RuO2-Ta2O5|Ti in acid (0.1 mol L− 1 H2SO4) and neutral (0.1 mol L− 1 Na2SO4) media in a previous study (Sopaj et al. 2015). On this occasion, the TiO2,nt modified with IrO2-Ta2O5, RuO2-Ta2O5, and PbO2-Ta2O5 are considered to electrooxidize AMX in acid media (0.1 mol L− 1 H2SO4).