## 4 Discussion
### Phytoremediation Properties of Hydrocharis morsus-ranae
#### 4.3.2 Comparison of Potential Cadmium Uptake by _Hydrocharis morsus-ranae_ with Other Pleustophytes  
In the present study, the highest plant cadmium content (36.7 ug\(\cdot\)g\({}^{-1}\) d.w.) in the frogbit was recorded after 21 days (Table 2). Lower values have been noted in _Wolffia arrhiza_ for the same exposure time and comparable cadmium levels [72] (Table 13). In addition, at a similar cadmium concentration (130 ug/L), _Eichhornia crassipes_ absorbed 31 mg Cd\(\cdot\)kg\({}^{-1}\) d.w. after five days [101]. In the case of lemnids, the length of exposure to cadmium pollution in the aquatic environment ranged from 2 to 31 days. Despite their high metal uptake capacity, these plants only have a short time to function in a polluted ecosystem due to the short lifespan of their leaves (Table 14). \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Plant Species** & **Cd Form** & **Cd Level** & **Duration** & **Nutrient** & **Efficiency of Removal (\%)** & **Reference** \\ \hline \multirow{6}{*}{_Wolfia arrhiza_} & \multirow{6}{*}{Cd(NO\({}_{3}\))\({}_{2}\)} & 1; 10; 100; 100; 1000 uM & 7; 14 & medium—dilution to 1/50 & 100.0 = 0.055; 1000.0 = 0.083; 14 days: 1.0 Cd level = 0  
_Eichornia crassipes_ (water hyacinth) is the most commonly-used aquatic plant for water and wastewater treatment in Asia. It demonstrates high potential for accumulating both biogenic compounds and metals. Its efficiency index (E\({}_{\text{p}}\)) values for cadmium uptake vary depending on the length of exposure and the cadmium dose (Table 14). The highest E\({}_{\text{p}}\) index values were calculated for wastewater from anaerobic reactors (98%), followed by industrial wastewater (97.5%), and wastewater from steel production (82.8%) [102; 103; 104]. The exposure time of the plant ranged from 21 to 130 days. In these studies, cadmium content was found to fall in the plant as exposure time increased, resulting in a higher tolerance index. In the present study, at the initial concentration of 0.1 mg Cd/L in water, the E\({}_{\text{p}}\) of frogbit was found to be 83.5%, regardless of nutrient level. Elsewhere water hyacinth was found to demonstrate higher cadmium uptake at 2.5 times the water cadmium level compared to frogbit [103]. Another species from Asia used in the phytoremediation of aquatic ecosystems is _Pistia stratiotes_. As reported by [PERSON] et al. [105], this plant removes around 50% of cadmium from rainwater with varying levels of mineral nutrients after two months (Table 14). While the plant was found to absorb similar amounts of cadmium, a higher dry weight yield was observed at the lower EC level. Therefore, more cadmium could be removed from a pond with a slightly higher concentration of PNK. In the present study, E\({}_{\text{p}}\) was typically slightly lower at higher plant nutrient levels (81-82%) than at lower levels (67-100%), regardless of the time of exposure to cadmium. [PERSON] et al. [102] showed that under the same environmental conditions, water hyacinth (83%) was more effective than _Pistia_ rosettes (50%) (Table 14). _Lemna_ spp. also demonstrate fairly significant cadmium uptake, amounting to about 95% from mine, municipal and industrial wastewater after 31 days [108] (Table 14). Unlike our present findings on frogbit, the level of mineral nutrition was not found to influence the effectiveness of cadmium uptake. These differences may be due to differences in the cadmium concentration at the beginning of the phytoremediation process. In other studies based on prepared cadmium solutions, frogbit demonstrated lower removal efficiency, ranging from 42 to 84.8% [108], and shorter exposure, ranging from 2 to 22 days [107; 109] (Table 14). However, in both works, it can be seen that higher concentrations of the toxic element are associated with less efficient removal. _Salvinia_ spp. are characterized by an above-normative ability to take up heavy metals, thanks to their rapid growth and tolerance to toxic metals. [PERSON] et al.