**3. Results and discussion**

The results obtained from the exposure experiments are summarized in **Table 1** (young specimens) and **Table 2** (aged specimens). In the following text, the term significant refers to statistically significant based on the results of the statistical analysis carried out on the results obtained from the exposure experiments considering immobilization as the endpoint.

As for both young and aged individuals, exposure to PE-TiO2 is the most significant in terms of effects: it is very significant, especially in the presence of triton (both fasting and feeding). On the contrary, exposure to the PE-Zn mixture shows the minor effects (**Figure 2**). This result, not linear if we consider that there is no clearly identifiable trend in the toxicity levels of the NPs/MPs, leads to think that more than the actual toxicity of the individual elements of the mixtures, there is a significant additivity effect. This estimation would also be legitimized by the comparison with previously obtained results: PVC, in presence of surfactant, resulted the most toxic among tested dispersions for both neonates and aged *Daphnia magna* exposed [29].

On the other hand, in Renzi and Blašković [28], n-ZnO resulted less effective than n-TiO2 in leading to the target endpoints (immobilization and death) in exposed *Daphnia magna* individuals; in this sense the observations made in this study would confirm a higher toxicity contribution of n-TiO2 compared to n-ZnO. A further factor to be taken into consideration in the interpretation and future


*Action of Surfactants in Driving Ecotoxicity of Microplastic-Nano Metal Oxides Mixtures… DOI: http://dx.doi.org/10.5772/intechopen.99487*

#### **Table 1.**

*Percentages observed for the "immobilization" endpoint in young individuals, in relation to experimental exposure parameters.*


#### **Table 2.**

*Percentages observed for the "immobilization" endpoint in aged individuals, in relation to experimental exposure parameters.*

analysis of the results is the ability to interact at a chemical level between surfactant and MPs, which could be material dependent. This difference between plastics of different nature, already hypothesized [32], could explain the variability of the results obtained in terms of toxicity of different mixtures.

**Figure 2.**

*Comparison between young and adult organisms exposed in surfactant presence and fasting/feeding. Graphics report the mean percentages of mobile organisms at the target time normalized for negative controls.*

#### **3.1 Surfactant effect**

The effect of surfactant (Triton X-100) was significant in all the experiments carried out on young organisms, compared to the control batches without the exposure to surfactant. For this reason, and to simplify the factors considered, making the observed effects more evident, it was decided to always add surfactant to the mixtures of contaminants to which adult organisms were exposed.

**Figure 3** shows the contribution of surfactant presence/absence under different trophic conditions in young organisms exposed to NPs+MPs mixtures, in terms of percentages of mobile organisms at the target times. These effects could be because

#### **Figure 3.**

*Contribution of surfactant presence/absence and trophic condition in young organisms. Graphics report the percentages of mobile organisms at the target time normalized for negative controls.*

surfactants seem to improve the contact among microplastics and animals and therefore cause effects on exposed specimens [22]. In a previous study [29], exposure to microplastics + surfactant showed the highest toxicity on *D. magna*, also, supporting results obtained on tested complex mixture.
