3.1 Effects of dinotefuran dosage
Migration, transformation, and degradation of DNT all affect its dosage in the environment (Ma et al., 2023). Hence, it is important to explore the effect of the DNT dosage on the total content and chemical fractions of Cd in sediments (Fig. 1). With an increase in the DNT dosage from 0.5 to 200 mg, the Cd content decreased from 5.568 to 4.802 mg·kg-1, the F1 content ranged from 3.009 to 2.584 mg·kg-1, the F2 content reduced from 1.020 to 0.729 mg·kg-1, and the F3 and F4 content remained at about 0.488 mg·kg-1 and 1.004 mg·kg-1, respectively (Fig. 1(a)). At higher dose of 200 to 1000 mg DNT, no significant changes were observed in the total content of Cd (about 4.762 mg·kg-1) and individual chemical fractions of Cd (F1 ≈ 2.541 mg·kg-1, F2 ≈ 0.719 mg·kg-1, F3 ≈ 0.482 mg·kg-1, and F4 ≈ 1.019 mg·kg-1) in sediments.
Changes in the total content and chemical fraction of Cd in sediments in the presence of various doses of DNT and HA are shown in Fig. 1(b). The total content of Cd gradually descended from 5.154 (for 0.5 mg DNT) to 4.726 mg·kg-1 (for 200 mg DNT), and remained constant at 4.578 mg·kg-1 over DNT dose range of 200–1000 mg. The F1 content decreased from 2.903 mg·kg-1 at 0.5 mg of DNT to 2.564 mg·kg-1 at 200 mg of DNT and remained constant at 2.492 mg·kg-1 at DNT doses of 200–1000 mg. For F2, the content ranged from 0.998 mg·kg-1 at 0.5 mg of DNT to 0.879 mg·kg-1 at 200 mg of DNT, keeping about 0.801 mg·kg-1 at DNT doses of 200–1000 mg. In contrast, the F3 ( ≈ 0.508 mg·kg-1) and F4 ( ≈ 0.772 mg·kg-1) content did not change significantly with differing doses of DNT.
The total Cd content in sediments with both DNT and HA was lower than that with only DNT. Thus, HA promoted release of Cd from the sediment into the water. We also found the change of DNT dosage mainly affected the F1 and F2 chemical fractions of Cd in sediments. Interestingly, the F1 fraction of the total Cd content remained between 53.866% and 54.943% with exposure to DNT alone or with HA (Fig. 1(c) and (d)). In contrast, the F2 fraction of Cd in sediments slowly decreased with an increase in the dose of DNT from 0.5 to 200 mg. Therefore, DNT has a strong effect on F2 fraction. One possible reason is due to the notable impact of DNT on the pH, which affects Cd release and the Cd chemical fractions (Beesley et al., 2014; Wang et al., 2014). As the pH increased (Eh decreased) with addition of DNT, Mn(IV) oxides and Fe(Ⅲ) oxides were reduced to soluble Mn(II) and Fe(II), respectively (Guo et al., 1997). Stable fractions of Fe-Mn oxide-associated Cd in sediments were converted into mobile water-soluble and exchangeable Cd fraction. DNT dosage greater than 200 mg had no further effect on the chemical fraction of Cd. Therefore, 200 mg of DNT was selected as an appropriate dose for treated sediments in subsequent experiments.
Please insert < Fig. 1>
FTIR spectra (Fig. 2(a)) were used to obtain information on binding of functional groups when HA interacted with DNT. The characteristic absorption peak at 3440 cm-1 is related to the stretching vibration of the N-H bond of amino groups and the O-H bond of phenol (Rostami et al., 2023). The broad peak at 2440 cm-1 is attributed to the carboxyl group (Pan et al., 2021). In the DNT-HA system, the absorption peak at 1660 cm-1 corresponds to the amide I band and the C=O stretching of quinone or ketone (Zhao et al., 2022). These result show successful formation of complexes of HA and DNT. In the DNT-HA system, the characteristic absorption peak position was consistent with the results of Pan et al. (2021). The peak position at 1380 cm-1 was enhanced after the reaction, due to complexation of surface oxygen-containing and nitrogen-containing functional groups with Cd (Fig. 2(b)). The change in the FTIR spectra of DNT-HA before and after treatment of the sediment reveals the formation of Cd complexes (Fig. 2(b)).
Please insert < Fig. 2>
UV-vis spectra of interactions between HA and various DNT doses are shown in Fig. 3(a). As the dose of DNT increased, the UV-vis absorption in tensity for DNT-HA increased steadily, indicating increased formation of the complex, consistent with the FTIR spectra results. The fluorescence of HA was quenched by DNT; the quenching mechanism of HA fluorescence by DNT was verified using the Stern-Volmer equation [1] (Yu et al., 2015) (Fig. 3(b)). The Kq values were >2.0×1010 L·(mol·s)-1 (Xu et al., 2013) (Table 2), revealing that the quenching mechanism of HA and DNT was static quenching. The modified double logarithmic equations [2] and [3] (Veeralakshmi et al., 2017) were used to estimate the binding constant (log Ka) and the number of binding sites (n) for a binding process involving HA and DNT (Table 2). The binding constants decreased with increasing DNT doses of 0–150 mg, but increased with increasing DNT doses of 200–300 mg. However, the binding site values for the HA and DNT systems were all ~1, indicating there is only one binding site in the binding process involving HA and DNT
where F0 and F are the fluorescence intensities in the absence and presence of dinotefuran, respectively. Kq is the bimolecular quenching constant, τ0 is the average lifetime of the DNT-HA system of 5.75 ns (Pan et al., 2021), [Q] is the concentration of DNT, Ka is the binding constant, and n is the number of binding sites.
Please insert < Fig. 3>
Please insert < Table 2>
3.2 Effects of humic acid concentration
Figure 4 shows the effect of the HA concentration on the total content and chemical fractions of Cd in sediments. When DNT and HA were both present, the content of total Cd, F1, and F2 noticeably decreased with increases in the HA concentration, but the F3 and F4 content showed little change. Whereas for HA alone (Fig. 3(c) and (d)), an increase in the HA concentration considerably reduced the content of total Cd and F1, but had no significant effect on F2, F3, or F4 content. Unexpectedly, the percentage of F1 (about 53.035%) was almost constant, while the percentage of F2 significantly decreased with an increase in the HA concentration when both DNT and HA were present (from 21.860% of 25 mg-C·L-1 to 18.410% of 273 mg-C·L-1). The most common reaction between HA and the metal ions (Cd2+) was the cation exchange reaction, as shown in equation [4] (Helal et al., 2006). The main interaction of DNT with HA is through an anion exchange reaction (Eq. 5); however, H-bonding also occurs between the amino group of DNT and OH or C=O groups of HA (Helal et al., 2006). However, the exchange reaction between DNT and HA uses the same binding sites that bind HA to Cd. This was the main reason that the sediment released more Cd with HA alone than in the presence of both HA and DNT.
Please insert < Fig. 4>
3.3 Effects of pH
Fig. 5 shows the effect of pH on the total content and chemical fractions of Cd in sediments with DNT alone, HA alone, and with both DNT and HA. The total content of Cd in the different systems was as follows: DNT > DNT and HA > HA. In addition, the content of total Cd, F1, and F2 in sediments increased with increasing pH from 5 to 7, while remaining constant at pH 8–9. No significant differences in the F3 and F4 fractions were observed. Previous studies have demonstrated that the binding constants of DNT-HA at pH 5, 6, 7, 8, and 9 were 20.58×103, 41.95×103, 76.50×103, 25.10×103, and 34.45×103 L·mol-1 (Pan et al., 2021), respectively. The weak binding ability of HA and DNT under acidic conditions was apparent, with higher concentrations of HA promoting release of Cd from sediments. Lower pHs reduced the negative surface charge on the sediment and enhanced dissolution of Fe/Mn oxides and carbonates in sediments, increasing the release of Cd (Perez-Esteban et al., 2013). Under alkaline conditions, negatively charged HA created electrostatic repulsion, giving HA a strong affinity to DNT. Consequently, HA provided fewer binding sites for Cd in the sediment. The chemical fraction of Cd in sediments at different pHs in the same system exhibited only slight differences.
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3.4 Effects of the humic acid molecular weight
The basic properties of different molecular weight of the humic acid were shown in Table 3. The composition analysis showed that the largest proportion of the Mw (48.20 wt.%) consisted of UF5; UF4, UF3, UF2, and UF1 made up 9.65 wt.%, 11.60 wt.%, 13.90 wt.%, and 16.65 wt.%, respectively. The DOCs of pristine-HA and the five HA Mw fractions were 273.00, 82.20, 236.20, 481.40, 822.40, and 1956.80 mg·L-1, respectively. Lower Mw fractions of HA contained more phenolic and carboxylic functional groups than the higher Mw fractions. Specific UV absorbances (E2:E3) decreased and the absorbance at 280 nm (SUVA280) increased with an increase in the Mw, indicating that Mw fractions >100 kDa of HA had more aromatic components.
Please insert < Table 3>
Effects of the Mw of HA on the release of Cd from the sediments are presented in Fig. 6. Release rate of Cd from the sediments generally increased with increasing Mw and with increasing HA concentration. However, when the HA Mw was 1–10 kDa or <1 kDa HA, there were no significant changes in the release of Cd from the sediment with increasing HA concentration.
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3.4.1 Humic acid molecular weights more than 100 kDa
The total content of Cd in the sediment decreased gradually from 5.232 mg·kg-1 of 25 mg-C·L-1 to 4.021 mg·kg-1 of 1700 mg-C·L-1 for Mw > 100 kDa HA with 200 mg DNT (Fig. 7(a)). As the HA concentration increased from 25 to 1700 mg-C·L-1, the F1 and F2 content in the sediment decreased gradually from 2.709 to 1.976 mg·kg-1, and from 1.225 to 0.663 mg·kg-1, respectively. The F1 fraction accounted for approximately 51.381% of the total Cd content from 25 to 1700 mg-C·L-1 for HA Mw >100 kDa (Fig. 7(b)). In contrast, the F2 fraction of Cd decreased from 23.410% of 25 mg-C·L-1 to 16.490% of 1700 mg-C·L-1. After treatment with UF1-DNT, the F3 and F4 fractions of Cd in sediments were stable at content of 0.581 mg·kg-1 and 0.739 mg·kg-1, respectively. However, the percentages of F3 and F4 increased with an increase in the UF1 concentration (Fig. 7(b)), due to the decrease in total Cd content.
3.4.2 Humic acid molecular weights of 30–100 kDa and 10–30 kDa
For HA Mw of 30–100 kDa with 200 mg DNT, the F1 fraction of Cd in sediments decreased from 2.688 mg·kg-1 of 25 mg-C·L-1 to 2.302 mg·kg-1 of 700 mg-C·L-1. The F2 fraction of Cd decreased from 1.303 mg·kg-1 of 25 mg-C·L-1 to 0.824 mg·kg-1 of 700 mg-C·L-1. However, the F3 and F4 fractions of Cd maintain relatively constant content of 0.580 mg·kg-1 and 0.758 mg·kg-1, respectively (Fig. 7(c)). For HA Mw of 10–30 kDa with 200 mg DNT, the F1 fraction decreased from 2.911 mg·kg-1 of 25 mg-C·L-1 to 2.709 mg·kg-1 of 350 mg-C·L-1, the F2 fraction decreased from 1.263 mg·kg-1 of 25 mg-C·L-1 to 1.104 mg·kg-1 of 350 mg-C·L-1, and the content of the F3 and F4 fractions were approximately 0.584 mg·kg-1 and 0.756 mg·kg-1, respectively (Fig. 7(e)). The total Cd content decreased from 5.316 mg·kg-1 for 25 mg-C·L-1 to 4.481 mg·kg-1 for 700 mg-C·L-1 with 30–100 kDa HA, and was reduced from 5.504 mg·kg-1 for 25 mg-C·L-1 to 5.133 mg·kg-1 for 350 mg-C·L-1 with 10–30 kDa HA.
3.4.3 Humic acid molecular weights of 1–10 kDa and <1 kDa
As the UF4 concentration increased from 25 to 175 mg-C·L-1, the content of the F1, F2, F3, and F4 fractions of Cd in sediments were 2.962–2.887 mg·kg-1, 1.306–1.264 mg·kg-1, 0.566–0.548 mg·kg-1, and 0.752–0.737 mg·kg-1, respectively. For HA Mw of <1 kDa, the Cd content and chemical fraction in sediments did not change significantly with the HA concentration.
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3.4.4 Mechanism of interaction between various molecular weight humic acids and dinotefuran
Based on the above results, the content of total Cd, F1, and F2 in sediments decreased with increasing concentration of HAs with Mws of >100 kDa, 30–100 kDa, and 30–10 kDa. At these HA Mws, the F1 fraction of Cd in the sediments remained at ~52.100%, while the F2 fraction of Cd in the sediments decreased with increasing concentration of HAs. In contrast, HA with Mws of 1–10 kDa and <1 kDa in the presence of 200 mg DNT did not significantly influence Cd chemical fractionation or total Cd content.
For example, for 75 mg-C·L-1 HA with added DNT, a notable increase in the Cd content in sediments was observed with decreasing HA Mw from >100 kDa to <1 kDa (Fig. 8). Higher Mw HAs have more aromatic components and active adsorption sites; their higher logK (stability constant for complexation between Cd2+ and humic-like substances) reflects their stronger binding affinity for Cd2+ (Bai et al., 2018; Gao et al., 2022).
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FTIR and three-dimensional fluorescence spectra were used to further investigate the functional groups and interactions in the system. FTIR spectra of the DNT-HA system with various HA Mws are shown in Fig. 9. An N-H stretch at approximately 3300 cm-1 and C-H stretches at 2877 and 2951 cm-1 (Maha et al., 2017) were observed in the FTIR spectrum of DNT. Compared with the DNT-HA system, the appearance of the N-H peak at 3300 cm-1 and C-H peaks at 2877 and 2951 cm-1 indicates that a hydrogen bonding interaction occurred between DNT and the HAs. The FTIR spectrum of the HAs was characterized by aromatic C=C, COO-, and H-bonded C=O stretches at ~1630 cm-1 (He et al., 2016). The band at ~1400 cm-1 was assigned to COO- antisymmetric stretching of the HAs (Christl et al., 2000). The DNT-HA spectrum showed a weak shift at the 1630 cm-1 peak and disappearance of the 1400 cm-1 peak, indicating that hydrogen bonding interactions occurred between N-H of DNT and COO- of HA.
Please insert < Fig. 9>
To further investigate the intensity and positions of the fluorescence peaks, three-dimensional fluorescence spectra (Zhou et al., 2021; Ding et al., 2022) were obtained to characterize the pristine HA mixture and different Mw HAs. Dynamic changes in the structures and compositions of the different Mw HAs interacting with DNT were investigated. Three-dimensional fluorescence spectra of the pristine HA mixture and different Mw HAs (75 mg-C·L-1) with and without addition of DNT (200 mg) at pH 7.0 are shown in Fig. 10. Pristine HA without addition of DNT had a stronger peak intensity at the excitation wavelength (Ex) of 280 nm and the emission wavelength (Em) of 485 nm (Fig. 10(a)). The fluorescence intensities of the different MW HAs increased with decreasing Mw, potentially because the lower Mw HAs have higher concentrations of electron-donating groups, such as hydroxyl and methoxyl groups (Gao et al., 2022). Furthermore, the peak positions of different Mw HAs showed a detectable blue-shift with decreasing Mw, reflecting the reduced number of aromatic rings (Ren et al., 2017). This result agreed well with the UV-vis spectra trends.
With the addition of 200 mg DNT (Fig. 10(a1–f1)), the three-dimensional fluorescence spectra of HA showed maxima at λEx/λEm = 450/520 nm (UF0), λEx/λEm = 440/510 nm (UF1), λEx/λEm = 440/500 nm (UF2), λEx/λEm = 370/450 nm (UF3), λEx/λEm = 360/440 nm (UF4), and λEx/λEm = 350/400 nm (UF5). The fluorescence intensity of the HAs was quenched by DNT and the peak positions of the HAs showed a red-shift. The decrease in fluorescence and peak shifts indicate that DNT bonding with the HAs induced some micro-environmental changes in the HAs. The modified Stern-Volmer model (Gao et al., 2022) was used to calculate the Kq and Ka values of HAs binding with DNT (Table 4). These values increased with increasing HA Mw. The relatively high Mw HAs had stronger adsorption affinity and complexing capacity for DNT than the low Mw HAs.
Fig. 10 (a2–f2) displays the three-dimensional fluorescence spectra of HAs with 200 mg DNT after sediment treatment. The fluorescence intensity of DNT and HAs with various Mws was clearly quenched after the treated sediments. The decreased peak intensities and blue-shifted peak positions indicated that electron-donating groups were removed, indicating that the most active fluorophores in the DNT-HA system were binding with Cd from the sediment.
Please insert < Fig. 10>
Please insert < Table 4>