## 4 Toxicological Studies of Parabens Abatement  
As proven by the previous sections, the advanced oxidation processes can promote the total abatement of parabens (single or mixture), but some studies reveal that in some conditions, the total parabens removal cannot be achieved. Moreover, also the formation of by-products during these processes can interfere with the initial toxicity or even increase the toxic character of the treated solution [24, 30, 75]. In this way, it is important to consider the impact of treatments in target species, such as aquatic species, since the resulting by-products can be more toxic than the parent compounds [120]. With this, the toxicological study must be a parameter of consideration in the design and application of AOP technologies for wastewater treatment, since although the target substances may be eliminated, the solution toxicity can increase, causing negative impacts if discharged into the environment. Table 9 presents toxicity studies of initial and treated solutions of single or mixtures of parabens, using different AOP technologies. In general, all the treated samples had less toxicity compared to the initial ones, concluding that photolysis, photocatalysis, and ozone-based technologies can help in reducing toxicity. [PERSON] et al. [102] tested the abatement of MP, EP, and PP mixture at 1 mg/L each, by photocatalytic and catalytic ozonation, using TiO\({}_{2}\)/NT. For catalytic ozonation, the light inhibition (LI) of _A. fischeri_ decreased about 57%, while _L. sativum_ had a germination index (GI) increase of 90%, while for single ozonation, the toxicity of _A. fischeri_ decreased by 46% and occurred a GI increase of 79% for _L. sativum_ seeds. For both species, the toxicity decreased when the treatments were applied. [PERSON] et al. [121] treated 10 mg/L of each paraben in a five parabens mixture using photolysis and photocatalytic ozonation, and the toxicity was evaluated using _V. fischeri_, _C. fluminea_, and _L. sativum_. Regarding the _V. fischeri_, the treatment reduced the initial LI of 96% to 43% for the best catalyst, while _C. fluminea_'s total mortality was reduced to zero mortality in 72 h, except for 1%Pd-TiO\({}_{2}\) catalyst, which led to a mortality of 29%, _L. sativum_ had an initial GI of 42% and the best results showed a GI of 112%. Furthermore, [PERSON] et al. [26] removed a mixture of five parabens at 10 mg/L each by ozonation and catalytic ozonation using several doped TiO\({}_{2}\) catalysts. Toxicity decreased for all the treated samples, achieving a maximum of 53.7% of inhibition decrease for _V. fischeri_, 79% mortality decrease for _C. fluminea_, and an increase of 50% for _L. sativum_, while [PERSON] et al. [27] tested the same but using ozonation, photolysis, photolytic ozonation, and photocatalytic ozonation, achieving a maximum decrease for photocatalytic ozonation of 60% for _V. fischeri_, 100% for _C. fluminea_ (no mortality observed), and an increase of 70%, achieving a maximum of 112% for _L. sativum_. Moreover, [PERSON] et al. [91] used photocatalysis with different TiO\({}_{2}\) catalysts for the degradation of a mixture of five parabens, reaching the best results of decrease from 100% to 21% (79% decrease) in mortality of _C. fluminea_ and an increase from 40% to 70% (30% increase) for _L. sativum_, while _V. fischeri_ showed 80% inhibition after treatment. Comparing these studies, [PERSON] et al. [121] had a higher _V. fischeri_ LI decrease of 53%, which was similar to the results obtained by [PERSON] et al. [26] (53.7%) and [PERSON] et al. [27] (60%) for the same species. This represents an important behavior for the treatment technologies regarding toxicity removal, and the difference obtained by photocatalytic ozonation in [PERSON] et al. [121] and [PERSON] et al. [26] was similar to the ones obtained by catalytic ozonation of [PERSON] et al. [121] but better than the results obtained for photocatalysis performed by [PERSON] et al.