Oxidation of various organic pollutants on a solar-powered semiconductor photocatalyst provides a clean and environmentally friendly process [1]. Among the existing semiconductor materials, titanium dioxide (TiO2) is a very active, low cost, and non-toxic photocatalyst. TiO2 with a wide gap assigned by Eg 3.20 eV can only be activated under UV light [2,3,4,5,6,7,8,9,10,11,12,13,14,15,16], and consequently, it is less active under visible light exposure. This deficiency restricts the application of TiO2 photocatalyst under low cost sunlight, which is mostly composed of visible light [2, 8, 11]. The enhancement of TiO2 activity under visible light by mono-doping [2,3,4,5,6, 9, 10] and co-doping [7, 8, 11,12,13,14,15,16] has been proven to be the most successful strategy. By doping process, the gap can be narrowed and thus decreasing the Eg of TiO2 [2,3,4,5,6,7,8, 10, 11, 13,14,15], emerging in the visible radiation. Thereby, the visible response and the activity of organic pollutant degradation of the doped TiO2 under visible light exposure are considerably enhanced [2, 3, 5, 9,10,11, 13]. Besides, the improvement of the photodegradation of various organic pollutants underdoped TiO2 is also contributed by the ability of the dopant in suppressing the recombination of electrons (e-) and holes (h+) photogenerated by TiO2 during UV irradiation [5, 9, 15], as presented as Eqs. (1) and (2).
$$ {\mathrm{TiO}}_2+ hv\to {\mathrm{TiO}}_2\left({e}^{-}+{h}^{+}\right) $$
(1)
$$ {\mathrm{TiO}}_2\left({e}^{-}+{h}^{+}\right)\to {\mathrm{TiO}}_2+\mathrm{heat} $$
(2)
Compared to metal elemental dopants, the non-metals are more interesting due to their smaller size allowing them to be inserted into the TiO2 crystal lattice facilely [2, 4, 9,10,11]. Furthermore, among the non-metal dopants, nitrogen has attracted extensive interest as it can be easily introduced into the TiO2 structure, due to its atomic size, which is comparable with that of oxygen, its low ionization energy, and high stability [2,3,4, 15]. In accordance, N-doped TiO2 demonstrates significant photocatalytic activity under visible light irradiation [2,3,4]. It was also reported that increasing the dopant loaded has improved the photodegradation of organic pollutants, but the further increase was found to show the opposite effect [2, 5, 8,9,10,11, 15]. This unexpected trend is generated by the turning role of the dopant from preventing becomes servicing recombination [5, 6, 11, 15]. Consequently, the fast recombination may proceed that declines the effectiveness of the photocatalytic organic degradation.
For solving such drawback, co-doping TiO2 with two different non-metal elements has received intensive attention [6, 7, 11,12,13,14,15,16], as with low loading co-dopants can deliver a significant effect on increasing the photocatalytic degradation of various organic pollutants under visible light [2, 15]. Moreover, some studies have also proven that co-doped TiO2 showed higher activity than the corresponding mono-doped [8, 11, 15].
Considering the advantages of using N dopant, as presented above, the double elemental dopants of N combined with C [7, 12], with P [11], as well as with S dopants [13,14,15,16] have been intensively studied. Due to the pronoun effect shown by double dopants of N and S in the improving photodegradation under visible light exposure, co-doping with N and S is growing of interest [13,14,15,16]. Several studies have investigated the co-doping TiO2 with N-S atoms for accelerating the degradation of residual anti-inflammatory drug [13], antibiotic residual [14], p-chlorophenol [15], and phenol [16] in the presence of the visible light, and high results have been obtained.
To the best of our knowledge, the co-doped TiO2-N,S has not been examined for oxidation of hazardous heavy metals such as Pb (II). The heavy metal become an environmental concern due to its characters including wider disposal sources, rapid accumulative in biotic tissues, and high hazard for human health [17,18,19,20,21,22,23]. To prevent the dangerous effect of Pb(II) ion on the ecosystem and humans, an effective detoxification method is urgently required to treat the corresponding wastewater before reaching the environment. Removal of Pb(II) ions from water has been frequently performed by adsorption techniques [17,18,19]. Unfortunately, at the end of the process, the adsorbent saturated with Pb(II) can generate undesired hazardous solid waste, which creates a new problem in the environment. The most suitable method is believed to be oxidation of Pb(II) resulting from the less toxic and handleable PbO2 [20,21,22,23]. The effective Pb(II) oxidation can be obtained through the photo-Fenton process [20] and photocatalysis over TiO2 photocatalyst under UV illumination [20,21,22,23].
However, detoxification of Pb(II) by using N,S-coped TiO2 in the presence of visible light so far is untraceable in the literature. The application of the N,S co-doped TiO2 for photo-oxidation of Pb(II) in the solution contributes plausibly a novelty in Pb(II) remediation as well as the application of TiO2-N,S photocatalyst. Under the circumstance, in the present research, photooxidation of Pb(II) over TiO2-N,S photocatalyst under visible light irradiation is addressed.
Concerning the co-doping N,S into TiO2, urea, and thiourea are the most employed as the sources of the N and S dopants [2, 3, 9, 10, 13, 14, 16] and other organic compounds [2, 11, 12]. However, using organic amine for N and S dopant sources can inevitably lead to organic residues on the photocatalyst surface [4, 5] that can decrease the activity of the photocatalyst. To avoid such weaknesses, in this paper, nitric and sulfuric acids are proposed as the simple inorganic N and S dopant sources respectively, into the TiO2 structure. The co-doping is performed in one step suggesting a fast and practice process. Furthermore, to reach the maximum Pb(II) photo-oxidation result, the influences of some important parameters controlling the effectiveness of Pb(II) oxidation such as dopant amount in the photocatalyst, photocatalyst mass, irradiation time, and solution pH are also evaluated.