Effect and Mechanism of Bicarbonate Ion on Lead Absorption in Pontederia crassipes from Karst Water

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Effect and Mechanism of Bicarbonate Ion on Lead Absorption in Pontederia crassipes from Karst Water


The transmittance spectra of the Pontederia crassipes root after Pb uptake in karst water at 3, 4, and 5 mmol/L HCO3 molarities are shown in Figure 6. Helpful information about the functional group can be presented in the FTIR spectrum [45]. Figure 6 showed several intense feature bands of the functional groups combined with Pb by the Pontederia crassipes root. The spectral bands at 534, 1033, 1651, 2347, 2927, and 3453 cm−1 were, respectively, assigned to stretching vibration peaks of SO42−, C–O, protein C=O, C≡C, –CH3, and O–H [31,46,47].
As shown in Figure 6, the shapes of the FTIR spectra, and the intensity and shift of several spectral peaks were different after Pb uptake in karst water at 3, 4, and 5 mmol/L HCO3. The SO42− stretching vibration peak, respectively, shifted from 534 cm−1 to 540, 558, and 571 cm−1 in karst water at 3, 4, and 5 mmol/L HCO3. The peak shift of SO42− increased, indicating that SO42− worked more strongly with the increase in HCO3 molarities in karst water. No peak of protein C=O on the cell wall at 1651 cm−1 had any displacement; however, the peak intensity of C=O was the strongest when the molarity of HCO3 in karst water was 4 mmol/L, showing that structural changes to the cell wall occurred when protein C=O combined with Pb [32]; moreover, protein C=O had the strongest protective effect on the cell wall and reduced the Pb toxicity to Pontederia crassipes root to the greatest extent, thus resulting in the highest bioconcentration factor of Pontederia crassipes to Pb in karst water at 4 mmol/L HCO3. All peaks of C≡C at 2347 cm−1 shifted to 2345 cm−1 in karst water at 3, 4, and 5 mmol/L HCO3; however, the peak intensity reduced with the increase in HCO3 molarities in karst water, showing that the increase in HCO3 molarities in karst water decreased the C≡C oxidation. In this study, the results showed that the functions of SO42−, protein C=O, and C≡C all changed in the Pb uptake by Pontederia crassipes root with the increase in HCO3 molarities in karst water. The O–H stretching vibration peak at 3453 cm−1 shifted to 3459 cm−1 only in karst water at 3 mmol /L HCO3, indicating that O–H bonded with Pb only in karst water at 3 mmol /L HCO3. The C–O stretching vibration peaks at 1033 cm−1, respectively, shifted to 1043 cm−1 and 1056 cm−1 in karst water at 4 mmol/L and 5 mmol /L HCO3. The peak shift of C–O increased, suggesting that C–O interacted differently with Pb in karst water at 4 mmol/L and 5 mmol/L HCO3. The O-H stretching vibration and C–O stretching vibration suggest that there is alcoholic hydroxyl in Pontederia crassipes [47]. However, alcoholic hydroxyl in the Pontederia crassipes root had no significant effect on Pb uptake because there was only an O–H or C–O stretching vibration in karst water at each molarity of HCO3. The symmetric stretching vibration peak of –CH3 at 2927 cm−1 shifted to 2925 cm−1 only in karst water at 5 mmol/L HCO3, showing that –CH3 bonded with Pb and formed methyl compounds through the methyl transfer reaction only in karst water at 5 mmol/L HCO3. The methyl transfer reaction is closely associated with detoxification [48]. The results in this study suggest that a high molarity of HCO3 in karst water promoted the bonding between –CH3 and Pb and the methyl transfer reaction. Furthermore, a high molarity of HCO3 relieved the Pb toxicity to the Pontederia crassipes morphology through the methyl transfer reaction in Pb uptake.

The molarity of HCO3 in karst water played a vital role in regulating the functional group when Pontederia crassipes combined with Pb. Different kinds of functional groups worked in the Pb uptake by Pontederia crassipes; moreover, the same functional group in the Pontederia crassipes root from karst water at different HCO3 molarities worked distinctly (i.e., SO42−, C≡C, and protein C=O). Alcoholic hydroxyl in the Pontederia crassipes root had no significant effect on the increase in HCO3 molarities in karst water. Protein C=O on the cell wall was the most effective when the molarity of HCO3 was 4 mmol/L. A high molarity of HCO3 in karst water promoted the bonding between –CH3 and Pb and the methyl transfer reaction, which relieved the Pb toxicity to the Pontederia crassipes morphology.


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