## 3 Findings and Discussion
### The Removal of Tetracycline in the CW Treatment System  
As seen from Figure 7, when the influent concentration of TC was stable during the period of operation, at 0.980 \(\pm\) 0.078 mg/L, 4.819 \(\pm\) 0.273 mg/L, and 10.481 \(\pm\) 1.070 mg/L, its effluent concentrations were 0.096 \(\pm\) 0.015 mg/L, 0.598 \(\pm\) 0.129 mg/L and 0.474 \(\pm\) 0.033 mg/L, respectively. Corresponding TC removal rates of 90.20 \(\pm\) 1.02%, 87.60 \(\pm\) 2.48% and 95.42 \(\pm\) 0.73% were also obtained from these trials, indicating that CW system was effective in eliminating TC from the influent water. It is well-known that the removal of TC in biologically-based treatment systems is related to a variety of physical, chemical and biological processes [40]. In this study, the removal of tetracycline as shown in Figure 7 may be achieved either through adsorption and/or biodegradation. However, it has been reported that TC was mainly removed via adsorption in biological systems while biodegradation can be completely ignored [41; 42]. In addition, [PERSON] [43] declared that TC is strongly adsorbed by soil and sediments, with a high potential for adsorption (kD = 70-5000 L/kg) [44]. Therefore, partitioning onto the biofilm and solid matrix in CW was expected to play an important role. In fact, although TC is a substance with high water solubility (231 mg/L) and a low \(n\)-octanol/water partition coefficient (logkow = \(-\)1.3), it is still considered to sorb strongly onto soil [44] and several materials such as clay, sediment, humic compounds, etc. [45]. Ionic interactions and the metal-complexing properties of TC have been proved to largely govern its adsorption characteristics [46]. Since suspended particles from the wastewater were retained in a wetland bed, sorption of dissolved organic contaminants in that organic matter and on the biofilm coating the CW bed could represent a significant mechanism for the removal of TC in CW, which was in agreement with the result which indicated that biosorption was the main mechanism of TC removal [47]. Figure 6: The effect of the HRT on effluent water quality. Another removal mechanism considered to be responsible for the removal of TC in this study is the photodegradation process. In fact, photodegradation is known as one of the main transformation reactions of tetracyclines in the environment [46]. Since the CW in these tests was uncovered, high photolability of TC may have occurred [8,48]. In fact, according to the study of [PERSON] et al. [8], the difference in TC removal efficiency between experiments with a covered control and an uncovered control was really clear, 15 \(\pm\) 8% and 89 \(\pm\) 3%, respectively. The improvement of the removal efficiency of TC may also be attributed to the presence of plants in the CW. Plants are generally considered favorable in a CW, as they act as an anchoring surface for biofilm, pump and release O\({}_{2}\) to the bottom of the systems, they retain suspended particles and insulate against low temperatures. It cannot be denied that plants actively contribute to TC removal via uptake. In fact, [PERSON] et al. [49] found that three antibiotics (ciprofloxacin HCI, oxytetracycline HCI, and sulfamethazine) could be taken up by plant roots and translocated to shoots, which was in agreement with the findings of [PERSON] et al. [50], who observed the similar phenomenon for highly water soluble organic compounds. From these results, those authors suggested that removal of such highly water-soluble organic compounds was most likely to be driven by the transpiration water stream in the plant uptake and the translocation within plant tissues. Furthermore, they also indicated a positive correlation between the antibiotic concentration and the levels of antibiotic accumulation inside plants [51]. In addition, [PERSON] et al. [52] studied the accumulation of TC in _Iberis sempervirens L_. and their results confirmed the plant's ability to take up TC.