## 5 Titanium Dioxide as a Photocatalyst for Water Remediation Applications
### Single Atom Doped TiO\({}_{2}\) Composites
Gold nanoparticles are stable, rather non-toxic, and biocompatible, which fits with the TiO\({}_{2}\) physicochemical properties. Au-TiO\({}_{2}\) nanoparticles have so far been applied in the decomposition of methanol [115] and degradation of phenol [116], methylene blue [117], methyl orange [118, 119], tartrazine [120], and various other organic dyes. Notably, while a rise in phototoxicity of doped nanoparticles is usually observed for reactions where some chemical groups are present, the same photodactally can exhibit poorer activity than expected [114]. [PERSON] et al. presented a broad study on the detoxification of parabens using UV-A enhanced by noble metal-TiO\({}_{2}\) supported photocatalysts [121]. In the study, titania doped with either palladium, platinum, silver, or gold were used in the degradation of a paraben mixture containing methylparaben, ethylparaben, propylparaben, butylparaben (BuPB), and benzylparaben (BePB). Au-TiO\({}_{2}\) was prepared by the sol-gel method, for another photocatalyst, 0.5 \(w\)/\(w\)% noble metals were photodeposited on the TiO\({}_{2}\) surface after UV-induced reduction in precursor ions. The paraben mixtures were prepared by mixing 10 mg \(\times\) dm\({}^{-3}\) of each paraben in ultrapure water. The results of the photodegradation test with Au-TiO\({}_{2}\) were analyzed against photolysis rate (4%) and compared with bare TiO\({}_{2}\). For all photocatalysts, the presence of noble metal dopants improved the rate of paraben degradation compared to bare TiO\({}_{2}\). Ag and Pd co-dopants proved to be the most effective, achieving over a 50% reduction rate in initial paraben concentration after 180 min of irradiation. Pd-TiO\({}_{2}\) led to 75% reduction rate in BePB. TOC and COD (chemical oxygen demand) removal rates were also studied. Initial TOC and COD values for the paraben mixture were 35 mg \(\times\) dm\({}^{-3}\) and 95 mg \(\times\) dm\({}^{-3}\), respectively. The best results were achieved with 0.5% of Pd-TiO\({}_{2}\) for which the mineralization rate was 25% and 19% for COD removal. Interestingly, Au-TiO\({}_{2}\) revealed the lowest activity among other noble metal photocatalysts in the study on the photodegradation of parabens. Supplemental toxicity assessment tests were carried out in the study on _Aliivibrio fischeri_, _Corbicula fluminea_, and _Lepidium sativum_ species. Acute toxicity test was carried out on _A. fischeri_ after 15 and 30 min of exposition to non-treated and treated samples. Bioluminescence inhibition rate was above 80% for all samples, showing that degradation rates achieved after 3 h of photocatalytic treatment were insufficient to lower the toxicity of the paraben mixture. However, toxicity tests with other species showed positive results. _C. fluminea_ and _L. sativum_ were more resistant species than _A. fischeri_ and were used to assess the toxicity of samples pre-treated with the three most efficient noble metal photocatalysts (Pt-TiO\({}_{2}\), Ag-TiO\({}_{2}\), and Pd-TiO\({}_{2}\)). A reduced toxicity in pre-treated samples against those species was found. Growth index of _L. sativum_ increased from 40% to 70% after treatment with Pd-TiO\({}_{2}\), and to around 60% for other doped TiO\({}_{2}\). Mortality of _C. fluminea_ claims decreased from 100% to 21% for the sample pre-treated with Ag-TiO\({}_{2}\) and was significantly lower for the other two photocatalysts. All in all, 0.5% Ag-TiO\({}_{2}\) and Pd-TiO\({}_{2}\) proved to be the most effective photocatalyst in the study for the removal of parabens from wastewater [121]. In the study presented by [PERSON] et al., gold and platinum nanoparticles were deposited on TiO\({}_{2}\) and used for the elimination of _E. coli_ from urban wastewaters [122]. All prepared noble metal photocatalysts showed bactericidal effects better than the bare commercial P25 TiO\({}_{2}\). The potential was evaluated for different light intensities and the light intensity of 120 W \(\times\) m\({}^{-2}\) revealed the highest elimination rates of _E. coli_ from wastewater.