Tetracyclines are known as a group of broad-spectrum antibiotics that have a common basic structure and one of the most widely used these antibiotics is tetracycline. This antibiotic is used to treat humans and stimulate the growth of livestock and birds (Chopra and Roberts, 2001). According to the World Health Organization (WHO), the presence of this substance in nature is dangerous (Conde-Cid et al., 2020). Tetracycline finds its way to surface and groundwater through domestic and hospital effluents so livestock and poultry farms that it enters the human food cycle by drinking water and consuming fruits and vegetables (Fiaz et al., 2021). This antibiotic causes genetic resistance and accumulates in the skeletal tissues of the human body through the formation of a stable complex with calcium (Smith et al., 2015; D’Costa et al., 2011). Tetracycline is not biodegradable and disrupts in the biological treatment units, and also produces more toxic products in many chemical treatment processes such as chlorination and ozonation (Borghi et al., 2014; Park et al., 2007). On the other hand, the use of the surface adsorption technique only transfers tetracycline from one phase to another and does not eliminate its contamination (Homem and Santos, 2011). Therefore, it can be said that the common removal methods of organic contaminations are inefficient and troublesome for this antibiotic (Rodriguez-Narvaez et al., 2017). Instead, the advanced oxidation processes, especially photocatalytic systems, are a good way to remove these compounds and are based on the production of hydroxyl radicals (Hong et al., 2017; Al-Marzouqi et al., 2021; Chang et al., 2015). These radicals are able to oxidize all organic compounds to the stage of carbon dioxide and water production (Zhang et al., 2018). Most of the photocatalysts presented so far are efficient only in the ultraviolet range (Vasei et al., 2019; Yadav et al., 2018), and since only a small percentage of sunlight is made of ultraviolet light and most of that is visible light, so it needs to design and fabrication of the new generation of active photocatalysts in the visible range. In recent years, a lot of research has been done on semiconductors that have a suitable gap and can be used in the visible range (Zhang et al., 2020; Yu et al., 2020; de-Moraes et al., 2021). Graphite nitride carbon is one of the semiconductors that has a smaller bandgap (∼2.7 eV) than many other common photocatalysts such as titanium oxide and zinc oxide, and as a result, it is more efficient at absorbing sunlight (Dong et al., 2014; Zhang et al., 2021). This substance is non-toxic, cheap and easy to prepare (Cui et al., 2018). However, the most important weakness of g-C3N4, which reduces its performance in optical degradation, is the rapid recombination of electron-hole pairs (Shi et al., 2021; Liu et al., 2021). Increasing efficiency in the optical degradation operations depends on the degree of separation of electron-hole pairs. To produce active oxygen species, it is necessary that the electrons and produced holes can react separately with the water or oxygen molecules around them (Chen et al., 2020). There are two major ways to solve this g-C3N4 problem and achieve maximum performance; First, changes in the morphology and particle size of the photocatalyst: The particle size of a photocatalyst has a large effect on the energy gap. As the particle size decreases along with increasing the width of the energy gap, and so its oxidation-reduction strength will increase (Singh et al., 2018). Second, the decoration of g-C3N4 with other conductive and semiconductor metals; these metals increase the absorption of visible light and the efficiency of the photocatalyst by producing hot electrons and injecting them into the semiconductor (Wang et al., 2017).
In the present project, the above two techniques were used to fabricate an efficient photocatalyst in the visible range. In this regard, the g-C3N4 nanotubes were synthesized instead of g-C3N4 bulk and it cause to improve the energy band gap and reduce the recombination rate of the electron-hole pair. In addition, the use of g-C3N4 nanotubes increases the surface area and has a positive effect on photocatalyst performance. To implement the second approach and further enhance the photocatalytic properties, the surface of the nanotubes was decorated with a ruthenium complex. The ruthenium complexes have the ability to absorb light in the wavelength range of 400-800 nm and cause the formation and injection of electrons into the photocatalytic system. In the end, the photocatalytic ability of Ru(II)complex/g-C3N4 NTs was investigated in the decomposition of tetracycline antibiotic under visible light and the effect of factors such as radiation time, amount of photocatalyst, pH, and temperature on degradation efficiency was investigated. The advantages of this method are maintaining the photocatalyst stability and reusability in the heterogeneous system.