# Anaerobic Digestion of Duckweed Used to Remediate Water Contaminated with Zinc and Ammonium
## 3 Results
### Zn Uptake and Zn/NH4\({}^{+}\)-N Removal by Duckweed  
Figure 2b shows the ability of duckweed to take up both Zn and NH4\({}^{+}\)-N. The tolerance index (TI) was well correlated with the Zn removal efficiency, while no obvious correlation was found between the TI and the NH4\({}^{+}\)-N removal efficiency. Lower TI values were found in the treatments with Zn and without NH4\({}^{+}\)-N, indicating the toxicity of Zn (T1 through T3, T4 through T6, and T7 through T9). The TI decreased with increasing Zn concentrations, however, the TI increased when NH4\({}^{+}\)-N was also present. This finding indicates that the presence of ammonium partly reduced the toxicity of zinc to duckweed growth. Duckweed in the treatments with Zn and without NH4\({}^{+}\)-N showed a good Zn recovery and removal efficiency. The Zn recovery efficiency of T2 was significantly different from that of T3 (\(p<0.05\)), while there was no difference in the Zn removal efficiency. The Zn recovery efficiency was not significantly different between the treatment with 20 mg/L NH4\({}^{+}\)-N or 40 mg/L NH4\({}^{+}\)-N (T5 vs. T8 and T6 vs. T9) at the same Zn concentration, while the Zn recovery efficiency decreased with an increasing Zn concentration. The Zn removal efficiency decreased with an increasing NH4\({}^{+}\)-N concentration (T2 vs. T3, T5 vs. T6 and T8 vs. T9). Minimal Zn removal efficiency was found in the presence of 20 mg/L NH4\({}^{+}\)-N (T6). NH4\({}^{+}\)-N removal also decreased with an increasing Zn concentration (T4 through T6 and T7 through T9), whereas the NH4\({}^{+}\)-N removal increased with an increasing NH4\({}^{+}\)-N concentration (T4-T6 vs. T7-T9), indicating that duckweed preferably took up more NH4\({}^{+}\)-N at higher ammonium concentrations.  
The uptake of Zn by the duckweed was further analyzed with SEM-EDX analysis (Figure 3a,b). EDX analysis confirmed the uptake of Zn by duckweed in treatments with Zn, while, as expected, no Zn was detected in duckweed grown in the absence of Zn. A higher percentage of Zn was detected in duckweed from treatments with Zn and without NH4\({}^{+}\)-N (T2 and T3), while a lower percentage of Zn was found in duckweed from treatment with NH4\({}^{+}\)-N (T5, T6, T8, and T9). EDX analysis further confirmed that the zinc accumulation ability of duckweed was weakened by the presence of NH4\({}^{+}\)-N, possibly due to the less bioavailable forms of zinc species.  
FTIR spectra of the duckweed biomass with and without Zn exposure were obtained to investigate the roles of various functional groups in Zn absorption (Figure 4). Similar FTIR spectra with slight peak/band shifts were found among the control and other treatments. Peaks at approximately 3279, 2926, 1622, 1545, 1387, 1317, and 1019 cm\({}^{-1}\) were observed in the spectrum of the control duckweed (Figure 4). These peaks correspond to the hydroxyl, hydrocarbons, carboxyl, and amide stretching of the C-H, O-H, C=O, and -NH functional groups [49]. Some of these are involved in zinc binding [50], and the strong peak value at 1019 cm\({}^{-1}\) represents the symmetric stretching vibration of -SO3. The intensity values of the peaks increased in the presence of 2.5 mg/L Zn. An increase in the peak intensity usually means an increase in the amount (per unit volume) of the functional group associated with the molecular bond. The maximum intensity values were found in the duckweed samples from T5, while the peak intensity values decreased with an increasing NH4\({}^{+}\)-N concentration, especially in the adsorption intensity of the amide group at 1622 and 1545 cm\({}^{-1}\), illustrating that the amide group played a main role in binding Zn under this condition. The variation in the peak position and intensity indicated that the functional groups, such as the hydroxyl and carbonyl groups, may be involved in the absorption of Zn and NH4\({}^{+}\).  
Figure 3: _Cont._