|
3. RESULTS AND DISCUSSION
3.1 Characteristics of the pristine and aged TiO
2
-NPs
As shown in Fig. 1, after 30 days of environmental sewage aging, high-resolution
transmission electron microscopy (HRTEM; JEM-2100F, Japan) and selected area
electron diffraction (SAED) revealed that the microstructures of the TiO
2
-NPs had
changed markedly. The clear lattice fringes had almost disappeared, and distinct
concentric circle outlines had appeared, probably caused by the inclusion of organic
matter (blue arrows) and the surface deposition of inorganic salt crystals from the
sewage. This was particularly noticeable in the case of the aged TiO
2
-A (aTiO
2
-A)
Journal Pre-proof
12
(Fig. 1Ab and Fig. 1Af). Table 1 shows that the trace metal basis values for aTiO
2
-A
and aTiO
2
-R decreased to 97.1% and 98.2%, respectively, based on inductively
coupled plasma mass spectrometry (ICP-MS) and total organic carbon (TOC) analysis.
Interestingly, there were more inclusions (the general term of organics and inorganic
salts etc. in sewage) on the surface of aTiO
2
-A, mainly due to its large surface area
with more adsorption area (namely adsorption sites) (Table 1). In addition, different
crystal forms with different crystal faces and functional groups (Fig. 1A) might have
different affinities for organic substances or metal ions etc. (Li et al., 2019a), which
ultimately led to differences in the amount of surface deposition between the two
aTiO
2
-NPs. The EDS images confirmed that the main elements in the surface
inclusions were Ti, O, Cu, Si, C, and Al. X-ray diffraction analysis using a Model D8
Advance X-ray diffractometer (Bruker, Germany; Fig. 1Ak and Fig. 1Al) proved our
conjecture that both titanium–metal (Al, Fe, Cu, Ca, etc.) oxides and halide (NaCl,
etc.) salt crystals formed on the surfaces of the aged TiO
2
-NPs. Both aged and pristine
TiO
2
-A and TiO
2
-R comprise internal monocrystalline structures with dominant
crystal surface indices of (101) and (110), respectively (the figures in parentheses
represent the Miller indices). This significantly affects the toxic behavior of faceted
TiO
2
-NPs in terms of their distinctive crystallographic facets (Liu et al., 2016).
Moreover, the crystallinity, grain size, and arrangement of the TiO
2
-NPs influence
their photocatalytic activity and selectivity (Vance et al., 2015), and are not affected
by aging.
Notably, diffuse reflectance spectra analysis revealed that sewage aging reduced
the band-gap energy (Eg) of TiO
2
-R from 3.02 eV (TiO
2
-R) to 2.97 eV (aTiO
2
-R),
whereas it increased the Eg of TiO
2
-A from 3.2 eV (TiO
2
-A) to 3.24 eV (aTiO
2
-A)
Journal Pre-proof
13
(Fig. 1Ba). This might have been due to comprehensive regulatory role of
“shell-photosensitization” effects of the organic macromolecules (such as humic
acids), the “sheltering phenomenon of deposited salt” on (core) TiO
2
-NPs in the
sewage water (He et al., 2016) and defects of both metal ion and non-metal elements
deposition (Fig. 1A) (Asahi et al., 2001; Srinivasan et al., 2006). One of which, Qin et
al. (2017) reported that the presence of salt crystals reduced the photocatalytic
efficiency, to an extent, depending on the nature or effects of the TiO
2
-NP structures
(Guillard et al., 2005). Moreover, the opposite changes in Eg might be closely related
to the thickness of the inclusion layer (even containing different compositions) (Table
1), and there is a threshold of coating percentage sensitivity to different light
wavelengths, which ultimately affects the photoreactivity and subsequent
phototoxicity of TiO
2
(He et al., 2016). In view of this, we used methylene blue
degradation rates to indirectly characterize the changes in acellular ROS yields of the
two TiO
2
-NPs before and after aging, i.e., to determine whether natural aging
passivates or sensitizes the photoactivity of TiO
2
-NPs. Figure 1Bb reveals that the
pristine TiO
2
-A and TiO
2
-R NPs had relatively high
K
ox
[h
-1
] values of methylene blue,
i.e., sufficient ROS production, whereas aging significantly (p < 0.05) reduced the
photoactivities of the two pristine TiO
2
-NPs and apparently reduced the essential
difference in their photosensitivity to a statistically non-significant level (p > 0.05),
even at 50 mg/L. Intriguingly, whether at 0.1 mg/L or 10 mg/L, aTiO2-R had slightly
greater peroxidation capability than aTiO
2
-A, but no significant difference (p > 0.05).
This may have been due to the smaller band gap of aTiO
2
-R (Fig. 1Ba) than that of
Journal Pre-proof
14
aTiO
2
-A, the broader visible light absorption range, that is, electrons are more easily
excited to form electron-hole pairs compared to aTiO2-A, making it relatively more
active under simulated sunlight irradiation (He et al., 2016). Generally, compared to
the pristine bigger bandgap being excited, the oxidizing ability produced by exciting
the aged smaller bandgap seems to be relatively weak, under the same light. Therefore,
here the reasons for the decrease of the photooxidation ability of two aTiO
2
-NPs are
that the electron hole pair of aTiO
2
-R with smaller band gap is easier to be excited,
but oxidation ability is not strong, while aTiO
2
-A is not easy to be excited due to the
larger band gap than that of pristine particles. Therefore, the photooxidation abilities
of two pristine TiO
2
-NPs are decreased by comprehensive effect of aging inclusions,
and reversing the oxidative capacity of both. Above results might suggest a related
weak and reversed biotoxicity of the two aTiO
2
-NPs in terms of photoreactivity.
Because their behavior in water and their particle size retention are the key
reasons of the toxicity of NPs (Wang et al., 2019), we examined the particle size
distributions and zeta potentials of the TiO
2
-NPs in Milli-Q water and the sludge
supernatant before and after aging. Table 1 reveals that TiO
2
-NPs had a smaller initial
particle size, but aging increased the negative charge on the surface of aTiO
2
-NPs
since the large amount of organic compounds with negatively charged functional
groups deposited on the surface of the particles (Li & Yu, 2014), which may have
made it more stable owing to the
electrostatic repulsion or steric hindrance resulted
from organic matter in the sewage or sludge (Gilbert et al., 2007). Figure 1Bc
confirms this; smaller aTiO
2
-NPs were comparatively stable in the sludge supernatant,
and were more uniformly dispersed. This was especially true in the case of aTiO
2
-R.
Journal Pre-proof
15
Do'stlaringiz bilan baham: | 
