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.