## 7 Toxicity Assessment after PMS-Based Processes  
Toxicity is a vital parameter to be considered when studying the effectiveness of treatments to which wastewater has been subjected. In some cases, many intermediate products generated from pollutants present in treated water still possess strong biological toxicity and persistence in the environment, sometimes even greater than that of the original chemical, presenting a threat to human health and ecosystems [222; 223]. Therefore, it is of great importance to evaluate the toxicity of effluents from treated waters, especially in cases where their possible reuse is considered. There are numerous studies evaluating the toxicity of intermediate and by-products formed after PMS-based treatments [224; 225; 226]. Specifically analyzing those works in which Cu-based catalysts were used, it is found that a large number of authors have reported the relative toxicity calculated theoretically by applying Structure-Activity Ecological Relationship (ECOSAR) software (MS-Windows Version 2.2) [223; 227]. The ECOSAR program evaluates the acute (LC\({}_{50}\) and EC\({}_{50}\)) and chronic (ChV) toxicity of compounds using three model aquatic organisms: fish, _Daphnia_, and _green algae_. The toxicity values obtained are classified into four groups: non-harmful (LC\({}_{50}\)/EC\({}_{50}\)/ChV \(>\) 100 mg/L), harmful (10 mg/L \(<\) CL\({}_{50}\)/EC\({}_{50}\)/ChV \(<\) 100 mg/L), toxic (1 mg/L \(<\) LC\({}_{50}\)/EC\({}_{50}\)/ChV \(<\) 10 mg/L), and very toxic (CL\({}_{50}\)/EC\({}_{50}\)/ChV \(<\) 1 mg/L) [228; 229]. Some PMS-based studies in which this software was used to determine the toxicity of the contaminant as well as that of the intermediate products obtained during the degradation process include the study of [PERSON] et al. [230], where bisphenol A was treated with Cu\({}^{0}\)-Fe\({}_{3}\)O\({}_{4}\)@Biochar; the work of [PERSON] et al. [231], in which the Orange G dye was degraded with carbon supporting CuCo nanospheres; the article of [PERSON] et al. [232], where magnetic CuFe\({}_{2}\)O\({}_{4}\) was prepared for the removal of overtetracycline; and the study of [PERSON] et al. [233], where a newly compounded ZnO/CuCo\({}_{2}\)O\({}_{4}\) was applied as a catalyst to decompose enrofloxacin under simulated sunlight. In all these studies, several degradation intermediates were identified, and their toxicity was evaluated using ECOSAR. In general, some of the generated intermediates were more toxic than the original pollutant, but the presence of these intermediates was very brief, as they decomposed almost immediately to yield other, non-harmful degradation products. Moreover, a small number of articles evaluate toxicity using other techniques such as algal growth experiments, activated sludge inhibition tests, or bioluminescence tests. These tests are based on the direct assessment of the organism's inhibition of the treated solution to show its change in toxicity [228; 229]. In the study conducted by [PERSON] et al. [234], the potential toxicity of sulfamethoxazole and its intermediates after 60 min of treatment under a CuO@Al\({}_{2}\)O\({}_{3}\)(EPC)/PMS system using an activated sludge inhibition assay was evaluated. The results showed that the initial concentration of sulfamethoxazole (39.5 uM) produced a 42% inhibition in the oxygen absorption rate. In the first 20 min, the rate of oxygen absorption dropped sharply to 25%, and during the next 40 min, it gradually decreased to 16%. The results confirmed that the most toxic sulfamethoxazole was degraded into molecules of lower toxicity by the CuO@Al\({}_{2}\)O\({}_{3}\) (EPC)/PMS system. In addition, the increased concentration of inorganic salts released during the degradation process (SO\({}_{4}\)\({}^{2-}\), NO\({}_{3}\)\({}^{-}\), NO\({}_{2}\)\({}^{-}\), and NH\({}_{4}\)\({}^{+}\)) confirmed high mineralization. In the study of [PERSON] et al. [235], the acute toxicity of arsenilic acid and its oxidation by-products were evaluated by the bioluminescence of the marine bacterium _Vibrio fischeri_.