## 4 Discussion
### Alkylphenols and Chlorophenols Removal in CW Systems  
In this study, both alkylphenols NP and OP were completely removed, indicating that CWs were able to effectively remove a mixture of these compounds from the doped stream water. To our knowledge no studies have reported the removal of NP and OP in CW with vertical flow. Removals in CWs with other designs were in general lower: 54% NP removal in horizontal flow CW [49]; 58% for OP and 32% for NP removal in a hybrid CW [50]; 90% for NP and 85% for OP removal in free water surface CW [51]. For PCP slightly lower removal percentages (between 87 and 98%) than those of NP and OP were observed, removals similar to those reported in other studies [52], for instance, 89% of PCP removal from pulp and paper mill wastewater through VFCW planted with _Canna indica_. In CWs, alkylphenols and chlorophenol compounds may be removed by a number of processes, including adsorption and absorption, microbial degradation and uptake by plants [50; 52]. Considering the high log Kow (4.5, 5.3 and 5.0 for NP, OP and PCP,respectively) and low solubility (7 mg\(\cdot\)L\({}^{-1}\), 5 mg\(\cdot\)L\({}^{-1}\) and 14 mg\(\cdot\)L\({}^{-1}\) for NP, OP and PCP, respectively), high adsorption/absorption of the compounds to CWs substrate is expected [53; 54], as observed in other studies [52]. In the present study, NP was not detected in CW substrate, whereas PCP and OP were detected, indicating that adsorption occurred. However, other processes such as microbial degradation and plant uptake certainly occurred, leading to compound removal. OP was detected in CW substrate of both systems (irrigated with stream water not doped or doped), with no significant differences between them. This indicates that the presence of OP in CW substrate was probably not related to the water doping done weekly. Sediment collected in the estuary of Lima river-Viana do Castelo and used as substrate in CW after mixing with sand, was already contaminated with OP (0.53 \(\pm\) 0.03 \(\upmu\)g\(\cdot\)g\({}^{-1}\)) but not with NP (NP below detection limit). This result indicates that probably the new OP contamination (OP added to Ribeira de Joane stream water) was more easily available than the old contamination, the compound being more strongly bound in the latter due to significant diffusion into particle inner structure. Lower release from aged soil relatively to freshly contaminated soil could be due to differences in sediment-water partitioning (Kp) values and the compartmentation of sorbed molecules in soil matrix leading to nonlabile vs. labile sorbed fraction [55]. A similar behavior of OP desorption from sediment has been reported [56], showing that in the long-term OP can become more persistent. In addition, biodegradation may be limited due to slow desorption kinetics especially when dealing with aged sediments [57]. Several studies observed that bioavailability of contaminants in aged soil can be considerably lower than in spiked soil [58; 59; 60], which can also justify the detection of OP in CW substrate. Additionally, characteristics like acidity, alkalinity, polarity and polarization of organic contaminants also affect the adsorption behavior of charged substances. According to [PERSON] and [PERSON] [61], slightly acidic organic contaminants dissociate to form anionic substances repulsive to negative charges on soil surface in an alkaline environment, resulting in a poor adsorption of the contaminants by soil. The acidic characteristic of PCP (pKa = 4.7) and strong acidic characteristic of NP and OP (both pKa = 10) and the alkaline environment (pH = 8) of CWs systems might also not favor strong adsorption to CW substrate when a fresh input occurs. On the other hand, the concentration of organic matter in CW substrate might also affect this process [62]. In the case of PCP, adsorption of the fresh input occurred with all the mentioned processes contributing for its retention within the system.