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Keywords:
TiO
2
NPs; crystal; physicochemical; P removal; activated sludge
1. Introduction
Titanium dioxide nanoparticles (TiO
2
-NPs) are one of the most widely used
nanomaterials in industrial and commercial products, owing to their unique
physicochemical properties (Li et al., 2019a). Therefore, there is a high probability of
them entering the municipal sewage that flows into centralized wastewater treatment
plants (WWTPs) (Westerhoff et al., 2011
)
.
Such sewage systems are amongst the most
important pathways for the migration and transformation of TiO
2
-NPs in the
environment (Wang et al., 2017). Recently, Kiser et al. (2010) reported that raw
sewage may contain approximately 100–3000 µg/L of Ti, whereas effluent contains <
5–15 µg/L. The predicted concentrations of TiO
2
-NPs in an aeration tank are in the
range 121.9 ± 23.0 to 12,297.0 ± 1707.0 μg/L (Choi et al., 2018; Kiser et al., 2009),
and may reach up to 23.2 mg/kg in sewage sludge (Gottschalk et al., 2009).
Furthermore, TiO
2
-NPs with different crystal forms have been detected in WWTPs
because anatase and rutile TiO
2
-NPs have the same market value (Tong et al., 2015).
TiO
2
-NPs are toxic to some organisms (Jovanovic et al., 2018; Pinget et al., 2019;
Stapleton et al., 2018). Therefore, the potential effects of TiO
2
-NPs on the
performance of wastewater treatment systems are increasingly relevant.
Consequently, numerous studies have been undertaken into the effects of
TiO
2
-NPs on the biological treatment of wastewater, with the main focus being on
microbial activity and biomass (Zheng et al., 2015), organic degradation (Zhang et al.,
2016b), nitrogen (N) and phosphorus (P) removal (Li et al., 2015), and whole
community structure (Wang et al., 2017). However, further studies are needed to
address the adverse effects of TiO
2
-NPs with different crystal forms on organic and
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nutrient removal, as well as their mechanisms of toxicity, and the measures that are
necessary for their mitigation (Wang et al., 2017). In term of the effects of TiO
2
-NPs
on biological P removal, researchers have mainly concentrated on P removal rates and
the catalytic activity of P-cycle-related enzymes, such as exopolyphosphatase (PPX)
and poly-P kinase (PPK) (Li et al., 2017), and on functional microbial communities,
e.g., phosphate-accumulating organisms (PAOs) (Zheng et al., 2011). Exposure time
and dosage seem to be the major determinants of the inhibitory effects of TiO
2
-NPs
(Wang et al., 2017). The effects of TiO
2
-NPs on P removal are controversial, and
probably depend on different exposure conditions including various wastewater
treatment processes and microbial aggregation states (e.g., sludge and biofilm) (Li et
al., 2015; Li et al., 2017; Yang et al., 2018; Zheng et al., 2011). Furthermore, to the
best of our knowledge, there remains a lack of systematic research into the toxicity of
TiO
2
-NPs with regard to P-cycle-related metabolic substances or molecules and
functional dephosphorization microorganisms during anaerobic phosphorus release
and aerobic phosphorus absorption (Xu et al., 2016). In addition, some scientific
issues including the transformation and distribution of P among bulk liquids,
extracellular polymeric substances (EPS), and cell clusters, as well as the contribution
made by EPS to P removal after exposure to TiO
2
-NPs, remain obscure (Xu et al.,
2017). This is because a significant level of P accumulation in EPS substantially
enhances the ability of biofilms to absorb and remove P (Huang et al., 2015; Stewart
& Franklin, 2008).
However, now most TiO
2
-NPs
ecotoxicology studies are environmentally
irrelevant with regard to direct exposure to pristine NPs, because TiO
2
-NPs aging
occur inevitably, that is, the transformation of comprehensive characteristics
(including physicochemical transformation) after experiencing complex water
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environmental behaviors. For example, the TiO
2
-NPs in WWTPs may originate from
the detachment of new or aged facade paints by natural weathering, or may be
released from various commodities and industrial products by wear and tear
(Westerhoff et al., 2011). Inevitably, when they converge in WWTPs through sewage
pipelines, TiO
2
-NPs experience environmental aging according to the specific
chemical conditions present in the water, e.g., the presence of organic compounds,
inorganic salts, heavy metals, acids, and even microorganisms. Such aging takes place
through oxidation and reduction, dissolution, precipitation, adsorption, desorption,
combustion, biotransformation, and abrasion, among other biogeochemically driven
transformation processes (Lowry et al., 2012; Nowack et al., 2012; Sun et al., 2014;
Zhang et al., 2016a). Notably, the aging terms of nanomaterials including TiO
2
-NPs
and their aging experimental methods in different environmental media such as
soil/sediment or organic matter have been recognized by researchers (Fan et al., 2016;
Lei et al., 2016; Wang et al., 2015). These transformation pathways govern the fate,
transportation, and photo-reactivity of TiO
2
-NPs, and ultimately influence their
persistence, bioavailability/biouptake, and toxicity. Therefore, the nature and extent of
the transformations must be elucidated before significant progress can be made
toward understanding the environmental risks posed by TiO
2
-NPs in aquatic
environments (Nowack et al., 2012).
In view of the above discussion, we used pristine and aged anatase (TiO
2
-A) and
rutile (TiO
2
-R) NPs in a sewage environment to investigate: (1) the differential
responses of P-cycle-related metabolic substances (poly-β-hydoxyalkanoates (PHAs)
and glycogen) and key metabolizing enzymes (PPX and PPK) to NPs exposure; (2)
the fractionation and distribution of P in activated sludge systems (sludge cell residue
and EPS), and the contribution of EPS to P removal under anaerobic and aerobic
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conditions; (3) the responses of functional PAOs determined by high-throughput
sequencing; and (4) the comparative acute toxic effects of pristine and aged TiO
2
-NPs
with different crystal structures, and the potentially
inhibition mechanisms for P
removal in a sequencing batch reactor (SBR). These studies will be helpful to further
understand the effects of environmentally relevant (low concentration and aging)
TiO
2
-NPs on biological phosphorus removal in activated sludge.
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