## 3 Results
### Field Experiment  
#### 3.2.1 Chemical Analysis  
During the 180 days of the experiment in the CW, the pH values of the wastewater from the gasification plant increased from 6.2 to 7.6 after the treatment with _P. australis_ (Table 1). The color of PL before the phytodepuration process was not detectable at a dilution of 1/40; after the biological treatment, the color was not detectable at a dilution of 1/20, and the unpleasant smell was also completely removed. These results are in agreement with the decreases of TSS, phenols, and hydrocarbons: at the exit of CW, the decrease of TSS was around 57% and 99.9% for phenols and total hydrocarbons, respectively (Tables 1 and 2).  
The determination of BOD\({}_{5}\) and COD showed a very high bio-treatment efficiency (\(-\)99.4 and -98%, respectively) (Table 1). The high removal of BOD\({}_{5}\) and COD is related to the deposition and filtration of settleable organics and biodegradation of organic compounds mediated by the heterotrophic microorganisms. In fact, the reduction of fats and oils was significantly high; the inflow concentrations were 9.5 ppm, and the outflow did not contain any recordable trace of this fraction. The degradation/removal of total surfactants after CW deprivation was assessed to be around 99.9% (Table 2).  
Regarding the nitrogen sources, a different trend was observed: nitrite was not recordable both in inflow and in outflow, while nitrate decreased by 88%, and ammonium increased from 0.65 ppm to 3.8 ppm. The results depicted in Figure 6 reported that denitrification process took place: this process is related to organic matter content and it is positively correlated with pH at 7-8. Total phosphorus decreased by around 94% (Figure 6). The high removal rate of phosphorus may be attributed to the long contact time within the wetland (4 days). Sulfites decreased in the inflow around 85% and sulfates around 40%. The level of detected chlorides was similar in inflow and outflow of CW; on the contrary, fluorides were not registered in outflow (Table 3). The chemical analysis of heavy metals showed traces of Fe, Ni and Zn with a significant reduction only for Zn (85%) (Table 4).  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameter** & **IN** & **OUT** & **u.m.** \\ \hline pH & 6.2 \(\pm\) 0.1 a & 7.6 \(\pm\) 0.2 b & pH \\ Color & N.p. 1/40 & N.p. 1/20 & - \\ Odor & unpleasant & not perceptible & - \\ Total suspended solids & 60.4 \(\pm\) 1.4 a & 34.5 \(\pm\) 2.7 b & mg/L \\ BOD\({}_{5}\) & 800.6 \(\pm\) 1.4 a & 4.8 \(\pm\) 0.3 b & mg/L O\({}_{2}\) \\ COD & 1413.4 \(\pm\) 3.8 a & 29.5 \(\pm\) 1 b & mg/L O\({}_{2}\) \\ \hline \hline \end{tabular}
\end{table}
Table 1: Physical and chemical characteristics of PL in inflow and outflow. Different letters indicated that the differences were significant (\(p\)-value \(<\) 0.05).  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Contaminants** & **IN** & **OUT** & **u.m.** \\ \hline Fats and oils & 9.5 \(\pm\) 0.8 a & \textless{}0.01 b & mg/L \\ Total hydrocarbons & 4.5 \(\pm\) 0.7 a & \textless{}0.01 b & mg/L \\ Total phenols & 27 \(\pm\) 3 a & \textless{}0.01 b & mg/L \\ Aldehydes & 8.2 \(\pm\) 0.8 a & \textless{}0.01 b & mg/L \\ Total surfactants & 1.38 \(\pm\) 0.45 a & \textless{}0.01 b & mg/L \\ Fats and oils & 9.5 \(\pm\) 0.8 a & \textless{}0.01 b & mg/L \\ \hline \hline \end{tabular}
\end{table}
Table 2: Organic contaminants previous and after phytodepuration. The values after the CW treatment were significantly different (\(p\)-value \(<\) 0.05).