3.1 The toxicity of MWCNTs to E. coli
The toxicity of three types of MWCNTs to E. coli was shown in Fig. 1. The bacterial survival didn’t vary greatly in the presence of three types of MWCNTs ranging from 0 to 200 mg∙L− 1. In contrast, SC-MWCNTs didn’t inhibit the growth of bacteria, while S-MWCNTs inhibited the bacterial growth with the concentration higher than 50 mg∙L− 1 slightly.
The toxicity of CNTs to bacteria varied greatly in different studies, which was related to the concentration, length, diameter, surface functional group, dispersion or aggregation, stiffness, aspect ratio, exposure duration, and so on (Kang et al., 2007, 2009;Young et al., 2012). The physical piercing on cell membrane was considered as the main reason of the toxicity of CNTs to bacteria. Roy et al. (2012) indicated that carboxylic acid functionalized water soluble carbon nanotube (wsCNT) shows no toxic effect against the growth of Escherichia-coli, which was similar to our results. The zeta potentials of the three types of MWCNTs were negative, so there was hardly direct interaction between MWCNTs with E. coli cells. In addition, the co-settling experiments of MWCNTs and bacteria were conducted to clarify the interaction between MWCNTs and bacteria. For S-MWCNTs, the co-settling ratio was similar to the addition of the two individual settling ratios, which indicated that there was less interaction between S-MWCNTs and bacterial cells (Fig. S1). For SO-MWCNTs and SC-MWCNTs, the co-settling ratio was a little higher than the addition of individual settling ratios, which was probably due to the better dispersibility of the SO-MWCNTs and SC-MWCNTs and thus the contact chance with bacteria increased (Figs. S2 and S3).
3.2 Effect of MWCNTs on the toxicity of Cd2+ to E. coli
MWCNTs could change the toxicity of Cd2+ by adsorbing Cd2+ in the suspension. MWCNTs with different functional groups had different adsorption abilities to Cd2+, and thus exhibited different effects on the toxicity of Cd2+. Figure 2 showed that the three types of MWCNTs reduced the toxicity of Cd2+ in different degrees. When Cd2+ concentration was fixed at 1 mg·L− 1, the combined toxicity of Cd2+ and SC-MWCNTs was reduced with the increasing SC-MWCNTs concentration. Although SO-MWCNTs reduced the Cd2+ toxicity, the combined toxicity didn’t decrease continuously with the increasing MWCNTs concentration. For Cd2+ from 1 to 10 mg·L− 1, the three types of MWCNTs of 100 mg·L− 1 reduced the Cd2+ toxicity to E. coli in varying degrees. SC-MWCNTs had the strongest ability to weaken the toxicity of Cd2+, followed by SO-MWCNTs, while S-MWCNTs had weak ability to weaken the toxicity of Cd2+.
In the case of coexistence of MWCNTs and Cd2+, MWCNTs would adsorb Cd2+ and thus influence the toxicity of Cd2+. The adsorption isotherm and fitting parameters were shown in Fig. 3 and Table 2.
Table 2
Fitting parameters of adsorption isotherm(Langmuir and Freundlich equations) for three type of MWCNTs
|
Langmuir
|
Freundlich
|
|
qmax
/(mg·g− 1)
|
kl
/(L·mg− 1)
|
R
|
n
|
kf
|
R
|
S-MWCNTs
|
2.481
|
0.376
|
0.999
|
1.664
|
0.612
|
0.996
|
SO-MWCNTs
|
8.097
|
0.241
|
0.999
|
1.388
|
1.481
|
0.989
|
SC-MWCNTs
|
10.741
|
0.440
|
0.999
|
1.482
|
2.938
|
0.989
|
The adsorption of Cd2+ on three types of MWCNTs could be fitted with both Langmuir and Freundlich equations well. It was seen that the adsorption of Cd2+ on the three types of MWCNTs varied greatly, which was related to the surface functional groups of MWCNTs. The adsorption capacity of SC-MWCNTs to Cd2+ was the strongest, followed by SO-MWCNTs, and the SMWCNTs had the weakest adsorption capacity to Cd2+. The adsorption mechanism of Cd2+ on the three types of MWCNTs may involve physical adsorption, electrostatic interaction, ion exchange and surface complexation (Duan et al., 2020; Yang et al., 2019), which could be explained by Fig. S4. The carboxyl group has stronger complexation ability with Cd2+ than hydroxyl group. In addition, the oxygen content was also very related to the adsorption of MWCNTs to Cd2+. The oxygen content of SC-MWCNTs was 1.5 times that of SO-MWCNTs, and twice that of SMWCNTs (Table 1), which was in accordance with the adsorption capacity of the three types of MWCNTs.
The combined toxicity of Cd2+ and MWCNTs was from the free Cd2+ and the piercing of the MWCNTs. The three types of short MWCNTs hardly inhibited the reproduction of the bacterium, and thus the free Cd2+ was mainly responsible for the combined toxicity of Cd2+ and MWCNTs. Based on the adsorption of Cd2+ on three types of MWCNTs, it was speculated that the combined toxicity of S-MWCNTs and Cd2+ was the strongest, followed by SO-MWCNTs, and finally SC-MWCNTs, which was in accordance with the toxicity results obtained in the experiments (Fig. 2).
3.3. Effects of SDBS on the toxicity of MWCNTs and Cd2+
The concentration of MWCNTs was fixed at 100 mg·L− 1, and Cd2+ concentration was fixed at 1 mg·L− 1 and 10 mg·L− 1, effects of SDBS on the toxicity of MWCNTs and Cd2+ were shown in Fig. 4. SDBS had little impact on the bacterial survival, and the toxicity of MWCNTs didn’t change with the increasing SDBS when SDBS was in the range of 0 ~ 200 mg·L− 1. However, SDBS enhanced the toxicity of Cd2+ to E. coli, especially for 10 mg·L− 1 of Cd2+. The combined toxicity of SDBS and Cd2+ was synergistic for both 1 mg·L− 1 and 10 mg·L− 1 of Cd2+ (Table 3). The enhancement of SDBS on the toxicity of Cd2+ to E. coli has been verified in our previous study (Li Mei et al., 2018). The coexistence of SDBS and Cd2+ increased the outer membrane permeability, whereas individual SDBS or Cd2+ didn’t (Fig. S5).
For the mixture of MWCNTs and Cd2+, the effect of SDBS on the combined toxicity varied with MWCNTs type and Cd2+ concentrations. For 10 mg·L− 1 of Cd2+, the combined toxicity of three types of MWCNTs and Cd2+ increased with the increasing SDBS concentration. For S-MWCNTs or SO-MWCNTs, their combined toxicity effect was additive for 1 mg·L− 1 of Cd2+ while synergistic for 10 mg·L− 1 of Cd2+. However, for SC-MWCNTs, the combined toxicity effect was synergistic for both 1 mg·L− 1 of Cd2+ and 10 mg·L− 1 of Cd2+ (Table 3).
Table 3
The judgement of combined toxicity effects of SDBS, MWCNTs, and Cd2+ to E. coli
Cd (mg·L− 1)
|
SDBS (mg·L− 1)
|
Determined toxicity (%)
|
Calculated toxicity (%)
|
Difference
|
Combined toxicity
|
1
|
0
|
32.8 ± 1.5
|
|
|
|
1
|
50
|
40.4 ± 5.7
|
22.3 ± 3.3
|
-18.1 S
|
SYN
|
1
|
100
|
44.7 ± 0.2
|
21.4 ± 5.7
|
-19.2 S
|
SYN
|
1
|
200
|
45.9 ± 0.1
|
27.6 ± 0.2
|
-18.3 S
|
SYN
|
10
|
0
|
50.7 ± 1.1
|
|
|
SYN
|
10
|
50
|
69.4 ± 4.8
|
40.1 ± 2.9
|
-29.3 S
|
SYN
|
10
|
100
|
79.5 ± 2.5
|
43.2 ± 5.3
|
-36.3 S
|
SYN
|
10
|
200
|
94.3 ± 0.5
|
45.3 ± 0.6
|
-48.9 S
|
SYN
|
Cd + SMWCNTs
|
|
|
|
|
|
1
|
0
|
29.4 ± 5.7
|
|
|
|
1
|
50
|
27.0 ± 1.7
|
18.9 ± 3.9
|
-8.1 I
|
ADD
|
1
|
100
|
23.4 ± 3.5
|
22.0 ± 1.5
|
-1.5 I
|
ADD
|
1
|
200
|
23.2 ± 6.6
|
24.1 ± 7.4
|
0.9 I
|
ADD
|
10
|
0
|
52.3 ± 2.5
|
|
|
|
10
|
50
|
64.2 ± 4.9
|
41.8 ± 0.8
|
-22.5 S
|
SYN
|
10
|
100
|
68.3 ± 0.8
|
44.9 ± 1.7
|
-23.4 S
|
SYN
|
10
|
200
|
92.5 ± 4.1
|
47.0 ± 4.3
|
-45. 5 S
|
SYN
|
Cd + SO-MWCNTs
|
|
|
|
|
|
1
|
0
|
33.5 ± 0.1
|
|
|
|
1
|
50
|
34.1 ± 2.5
|
22.9 ± 1.8
|
-11.2 S
|
SYN
|
1
|
100
|
36.0 ± 4.7
|
26.0 ± 4.2
|
-10.0 I
|
ADD
|
1
|
200
|
30.9 ± 0.8
|
28.1 ± 1.7
|
-2.8 I
|
ADD
|
10
|
0
|
45.7 ± 5.6
|
|
|
|
10
|
50
|
61.6 ± 4.1
|
35.1 ± 7.4
|
-26.5 S
|
SYN
|
10
|
100
|
72.9 ± 3.9
|
38.2 ± 9.8
|
-34.7 S
|
SYN
|
10
|
200
|
83.0 ± 3.8
|
40.4 ± 3.9
|
-42. 6 S
|
SYN
|
Cd + SC-MWCNTs
|
|
|
|
|
|
1
|
0
|
13.6 ± 3.4
|
|
|
|
1
|
50
|
26.2 ± 2.3
|
3.0 ± 1.6
|
-23.1 S
|
SYN
|
1
|
100
|
32.4 ± 2.2
|
6.1 ± 0.8
|
-26.2 S
|
SYN
|
1
|
200
|
22.2 ± 0.3
|
8.3 ± 5.1
|
-13.9 S
|
SYN
|
10
|
0
|
31.4 ± 4.6
|
|
|
|
10
|
50
|
39.5 ± 3.6
|
20.9 ± 6.4
|
-18.6 S
|
SYN
|
10
|
100
|
48.1 ± 0.2
|
24.0 ± 8.8
|
-24.1 S
|
SYN
|
10
|
200
|
69.4 ± 2.6
|
26.1 ± 2.9
|
-43.2 S
|
SYN
|
Notes: The concentration of MWCNTs was 100 mg·L− 1. ADD represents additive, SYN represents synergistic, ANT represents antagonistic;the toxicity data indicates bacterial mortality. |
The difference in toxicity came from the interaction among MWCNTs, Cd2+, and SDBS. In the presence of SDBS, the adsorption of Cd2+ on the MWCNTs was not only affected by the interaction between SDBS and Cd2+ but also affected by the adsorption of SDBS on MWCNTs (Fig. S4). SDBS in the solution and SDBS adsorbed on the surface of MWCNTs could interact with Cd2+ and resulted in the reduction of the dissolved Cd2+ concentration. In addition, SDBS could change the zeta potential of MWCNTs, and thus influenced the interaction between MWCNTs and bacteria.
The adsorption abilities of three types of MWCNTs to SDBS varied greatly (Figs. S4 and S6). The hydrophobic interaction and electrostatic attraction resulted in the adsorption of surfactants on MWCNTs (Zhong et al., 2013; Lopez-Lopez et al., 2016; Ostos et al., 2021). The hydrophobic interaction between S-MWCNTs and SDBS was the strongest, which resulted in the highest adsorption capacity of SDBS on S-MWCNTs. SDBS was anionic surfactant, which was repulsive with carboxy groups and hydroxyl groups on the surface of MWCNTs and resulted in the lower SDBS adsorption amounts for SC-MWCNTs and SO-MWCNTs.
Fig.5 shows the reduction of Cd2+ by SDBS and MWCNTs. For S-MWCNTs, the reduction of dissolved Cd2+ in the mixed system was much higher than the addition of Cd2+ reduced by SDBS and S-MWCNTs individually, which indicated that SDBS indeed promoted the adsorption of Cd2+ on S-MWCNTs. For SC-MWCNTs, the Cd2+ reduction in the ternary mixed system was higher than that reduced by individual SC-MWCNTs, but it was much lower than the addition of Cd2+ reduced by SDBS and SC-MWCNTs individually. For SO-MWCNTs, the Cd2+ reduction in the ternary mixed system was similar to the addition of Cd2+ reduced by SDBS and SO-MWCNTs individually. Although there was great difference in the Cd2+ adsorption for the three types of MWCNTs in absence of SDBS, the final dissolved Cd2+ concentrations in the mixture were similar for them in the presence of SDBS. However, the dissolved Cd2+ includes free Cd2+ and the Cd2+ complexing with SDBS(SDBS-Cd+/(SDBS)2Cd),the free Cd2+ and SDBS-Cd+ contributed more toxicity. The accurate concentration of each Cd2+ was difficult to determine, but the toxicity increased with the ratio of Cd2+/SDBS. For S-MWCNTs, the SDBS in the mixture was lower than that in the mixture containing the two other types of MWCNTs, and thus the combined toxicity was higher. The combined toxicity of the mixture containing SC-MWCNTs was still lowest.
Besides the toxicity from the dissolved cadmium, the direct contact between MWCNTs and bacteria was also considered. The adsorption of SDBS on the three types of MWCNTs resulted in the change of their zeta potentials. SDBS increased the negative charges on the surface of MWCNTs significantly, but the Cd2+ reduced the negative charges, especially for SC-MWCNTs (Table 4). Therefore, in the presence of Cd2+, the reduced negative charges on the surface of MWCNTs would promote the self-aggregation of MWCNTs. It could be seen from Fig. S3, in the presence of SDBS and Cd2+, the co-settling ratio for the three types of MWCNTs and bacteria was similar to the addition of settling ratio for MWCNTs and bacteria, which indicated that SDBS didn’t enhance the direct contact between MWCNTs and bacteria. Therefore, the direct contact between MWCNTs and bacteria in our study wasn’t the main toxicity source.
Table 4 Effect of SDBS and Cd
2+ on the zeta potential of MWCNTs
|
S-MWCNTs
|
SC-MWCNTs
|
SO-MWCNTs
|
Control
|
-13.34
|
-12.94
|
-12.92
|
+SDBS
|
-19.1
|
-18.15
|
-17.68
|
+Cd
|
-4.52
|
-6.37
|
-6.12
|
+Cd + SDBS
|
-6.16
|
-3.6
|
-5.98
|
Note: Both SDBS and MWCNTs were 100 mg∙L− 1, Cd2+ was 10 mg∙L− 1. |