Toxicity of Three Crystalline TiO₂ Nanoparticles in Activated Sludge: Bacterial Cell Death Modes Differentially Weaken Sludge Dewaterability

Abstract

The eco-toxicities of different crystalline phases of TiO 2 -NPs are controversial, and the effects and mechanisms on activated sludge are unclear. Therefore, we assessed the acute-toxicities (8-h exposure) of P25, anatase, and rutile TiO 2 -NPs in activated sludge using flow cytometry under simulated sunlight (hereafter-sun) and evaluated the relationship between sludge dewatering and bacterial cell death modes using Pearson's correlation coefficients (r). Additionally, the response of the microbial community structure was examined by high throughput sequencing. Bacterial survival and death were observed by confocal laser scanning microscopy. Toxicity indicators (e.g., lactate dehydrogenase (LDH) and reactive oxygen species (ROS)) were determined. Overall, TiO 2 -NPs toxicity was concentration-dependent and crystalline-phasedependent. The responses of bacterial communities to crystalline phases were more obvious than that of dosage. P25-sun and anatase-sun caused necrosis-like cell death via strong photo-oxidation confirmed by 131%/123% (1 mg/L) and 301%/ 254% (50 mg/L) LDH released by the control, while rutile-sun induced apoptosis-like death via intracellular ROS production increased to 165% (1 mg/L) and 420% (50 mg/L) of the control. P25 and anatase NPs had higher protein and polysaccharide affinities, while rutile NPs exhibited stronger attachment onto phospholipids. TiO 2 -NPs-sun reduced activated sludge dewaterability. Specific resistance to filtration (SRF) showed the strongest positive correlation with tightly bound extracellular polymeric substances (EPS) and total soluble microbial byproducts (r = 0.974, p < 0.01) and was closely related to EPS content and composition, especially the increased bound water (BW) content and sludge protein concentrations. High Pearson correlation coefficients were observed between early apoptotic cells and BW content (r = 0.952, p < 0.01) resulting from massive polysaccharides and between necrotic (including late apoptotic) cells and SRF (r = 0.959, p < 0.01) resulting from high protein and EPS concentrations. Thus, in response to TiO 2 -NPs, bacterial cell death modes differentially weakened sludge dewatering.

Rapid preparation of terbium-doped titanium dioxide nanoparticles and their enhanced photocatalytic performance

“…Based on the concentration of the test solution and the mortality of zebrafishes, the acute toxicity of Tb–TiO 2 on zebrafishes was 23.2 mg l −1 (96 h-LC 50 ). According to the toxicity grading standards of China, Tb–TiO 2 exhibited a low toxicity on zebrafishes (LC 50 > 10 mg l −1 ) [40].…”

Copper, silver, and titania nanoparticles do not release ions under anoxic conditions and release only minute ion levels under oxic conditions in water: Evidence for the low toxicity of nanoparticles

“…Lastly, titania nanocrystals produced the least amount of reactive oxygen species over a dose-response relationship, e.g., as much as 675 ± 18% relative fluorescence units at 1000 μg/mL to as little as 210 ± 22% relative fluorescence units at 0.1 μg/mL. Taken together, these data suggest that the cumulative effect of production and consumption of oxygen would ultimately determine the concentration of reactive oxygen species upon irradiation of nanomaterials in water (Nasseri et al 2018;Irandost et al 2019;Li et al 2019).…”

“…Examples of zerovalent nanoparticles are copper and silver nanoparticles (Akaighe et al 2011;Gao et al 2019;Hedberg et al 2019). Similarly, metal oxide nanoparticles, e.g., titanium dioxide nanocrystals, are stable and remain intact when stored as dry powders (Sharma 2009;Li et al 2019). Metal and metal oxide nanoparticles suspended in aqueous solutions may transform when exposed to natural organic matter, oxic/anoxic conditions, and ultraviolet light (He et al 2011;Lowry et al 2012;Yin et al 2012;Sharma et al 2015;Wang et al 2017;Yin et al 2017a, b;Pham et al 2018;Hedberg et al 2019).…”

“…During this reaction nanoparticles may transform from their pristine engineered zerovalent state to surface-charged, ion-dissociated, agglomerated/ aggregated forms (Hou et al 2013;Sharma and Zboril 2017). These transformation processes may have negative effects on human health and ecological systems due to the possible release of toxic metal ions and reactive oxygen species (Georgantzopoulou et al 2018;Rai et al 2018;Wang et al 2018;Li et al 2019). The focus of the present paper is to understand the transformation of selected nanoparticles under different environmental conditions.…”

Attachment Efficiency of Nanomaterials to Algae as an Important Criterion for Ecotoxicity and Grouping

“…The adsorption of ENMs to algae is also influenced by surface modifications, as shown by the relatively stronger interactions between Ag nanoparticles and algae when the ENM was coated with polyvinylpyrrolidone rather than citrate [29]. Furthermore, P25 (14% rutile and 86% anatase) and anatase ENMs show higher affinity for proteins and polysaccharides whereas rutile ENMs attach more efficiently to phospholipids [37]. Anatase TiO 2 adsorbs more efficiently to bound EPS than rutile TiO 2 , which may explain its greater ecotoxicity [38].…”