CHAPTER 5: DISCUSSION
5.3 Factors influencing leaching
In this study, PVCs of equal size (approx. 5mm) except for PVC F (Figure 3-1) were leached at specific temperatures for specific periods. But all of these could have influenced the results, and this is explored in this section.
Several factors are known to influence leaching. These include temperature, concentration, and particle size (Bhargava et al., 2015). While this study was set up to mimic environmental conditions, it was not able to account for naturally turbulent conditions. Therefore, the role of several variables needs to be discussed. Along with seasonal changes, anthropogenic activities affecting water bodies, and varying levels of ingestion of plastic by organisms there is an expected change in all five of these parameters constantly.
5.3.1 Plastic size
One of the dangers of MPs is their small size. The increase in surface area vis a vis the volume of smaller MPs allows for higher sorption rates of pollutants. Environmentally, however, the size of the plastic also influences the distribution of the pollutants and harmful biota on the plastic.
Smaller plastic is also easily ingestible by many organisms, increasing the pathways of exposure.
A study done to test the toxicity of five different sized nano- and microplastics, found that there was a significant difference in the organism response to plastic size. The study concluded that this response had to do with the ability of the organisms to ingest the plastic particles easily (Lei et al., 2018). Furthermore, it was determined using chemical analysis that the detection
frequencies of EDCs of small plastics are much higher than that of larger plastics. Smaller-sized MPs had a 59% detection frequency for EDCs, whilst medium and large plastics had 34% and 50%, respectively (Chen et al., 2019).
5.3.2 Time
The leaching times of this study were chosen based on the knowledge that leaching reaches equilibrium after 24 h under isotropic conditions (Chen et al., 2019). The 48 h and 96 h leaching times were chosen to assess the influence of time on compound leaching and breakdown. The results of this study confirmed that there is a definite difference between the responses at 24 h and those at 96 h.
5.3.3 Temperature
Fluctuations in water temperature occur naturally due to seasonal changes and the flow rate of the water body. However, anthropogenic disturbances, climate change being the most prevalent, can also alter the temperature of a water body (Caissie, 2006). Furthermore, ingested plastics leach within the organism, and a mammal has an average body temperature of approximately 37°C (Morrison & Ryser, 1952) which is much higher than the experimental conditions chosen for the present study and can affect leaching. Water in bottles incubated at 20°C and 40°C were tested for EDCs. The water in the bottles that were incubated at the higher temperature appeared to have more EDCs (Aneck-Hahn et al., 2018).
5.3.4 Concentration
It is important to distinguish between the concentration of the MPs in the EtOH that were leached and the concentration of the compounds within the leachates. It was out of the scope of this study to determine the concentrations of specific compounds within the leachates. However, the concentrations reported are that of the amount of PVC leached in the EtOH. Trying to determine the exact concentration of microplastics within a body of water is not easy and also fell outside the scope of the study. The concentrations used in the reporter gene assays were therefore determined by the concentrations at which cytotoxicity was not a problem anymore.
5.3.5 pH
Finally, the pH of the leaching medium plays an important role. Ethanol was used as the leaching medium in this case, with a neutral pH of around 7.3. Acid and alkaline conditions are more damaging to MPs than neutral pHs (Mortula et al., 2021). The damage caused to the MPs in these conditions can influence the rate of leaching, but also can increase the number and amounts of compounds that leach from the plastic. Luo et al. (2020) found that chromium and lead
experienced enhanced leaching from MPs under acidic conditions. This poses a concern specifically for ingested MPs. Gastric acid can have a pH as low as 1.5. This means that there is a higher chance of harmful chemicals leaching within the gut of an organism than in the water.
In conclusion, there can therefore be little deductions made regarding the inability of these leachates to cause endocrine disruption under the present conditions. It is possible that the compounds that were tested individually were simply not leached due to the conditions set in this study. It is also possible that leached compounds were lost due to other conditions of this study such as the evaporation by nitrogen, or the storage method. All this, however, does not mean that these compounds would not be made bioavailable under different leaching conditions. Without chemical analysis, it is not possible to determine the content of the leachates. However, mixtures of compounds are known to respond differently than individual compounds due to additive, synergistic, and antagonistic effects on the receptor (Christen et al., 2012; Hansen et al., 2019).
This is a common trend amongst POPs which are readily found adsorbed to MPs. Studies have been done on individual POPs and their influence on the receptors, however, there is limited available data on the synergistic effects of POPs using reporter gene assays.