Effects of nano-scale TiO2, ZnO and their bulk counterparts on zebrafish: acute toxicity, oxidative stress and oxidative damage

The acute toxicity and oxidative effects of nano-scale titanium dioxide, zinc oxide and their bulk counterparts in zebrafish were studied. It was found that although the size distribution of nanoparticles (NPs) was similar to that of the bulk particles in suspension, the acute toxicity of the TiO₂ NPs (96-h LC₅₀ of 124.5 mg/L) to zebrafish was greater than that of the bulk TiO₂, which was essentially non-toxic. The acute toxicities observed for ZnO NPs, a bulk ZnO suspension, and a Zn²⁺ solution were quite similar to each other (96-h LC₅₀ of 4.92, 3.31 and 8.06 mg/L, respectively). In order to explore the underlying toxicity mechanisms of NPs, ·OH radicals generated by NPs in suspensions and five biomarkers of oxidative effects, i.e. superoxide dismutase, catalase activities, malondialdehyde, reduced glutathione and protein carbonyl were investigated. Results showed that after the illumination for 96 h, the quantities of ·OH in the NP suspensions were much higher than ones in the bulk particles suspensions. The malondialdehyde content of zebrafish gills exposed to either illumination or dark were 217.2% and 174.3% of controls, respectively. This discrepancy indicates the occurrence of lipid peroxidation which is partly due to the generation of ·OH. In contrast, exposure to 5 mg/L ZnO NPs and bulk ZnO suspension induced oxidative stress in the gills without oxidative damage. Oxidative effects were more severe in the livers, where the protein carbonyl content, in the light and dark groups exposed to 50 mg/L TiO₂ NPs, was 178.1% and 139.7% of controls, respectively. The malondialdehyde levels in the liver of fish exposed to 5 mg/L ZnO NPs and bulk ZnO were elevated (204.2% and 286.9% of controls, respectively). Additionally, gut tissues exhibited oxidative effects after exposure to NP suspensions. These results highlight the importance of a systematic assessment of metal oxide NP toxicity mechanisms.     

A study of the mechanism of in vitro cytotoxicity of metal oxide nanoparticles using catfish primary hepatocytes and human HepG2 cells

Nanoparticles (NPs), including nanometal oxides, are being used in diverse applications such as medicine, clothing, cosmetics and food. In order to promote the safe development of nanotechnology, it is essential to assess the potential adverse health consequences associated with human exposure. The liver is a target site for NP toxicity, due to NP accumulation within it after ingestion, inhalation or absorption. The toxicity of nano-ZnO, TiO₂, CuO and Co₃O₄ was investigated using a primary culture of channel catfish hepatocytes and human HepG2 cells as in vitro model systems for assessing the impact of metal oxide NPs on human and environmental health. Some mechanisms of nanotoxicity were determined by using phase contrast inverted microscopy, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assays, reactive oxygen species (ROS) assays, and flow cytometric assays. Nano-CuO and ZnO showed significant toxicity in both HepG2 cells and catfish primary hepatocytes. The results demonstrate that HepG2 cells are more sensitive than catfish primary hepatocytes to the toxicity of metal oxide NPs. The overall ranking of the toxicity of metal oxides to the test cells is as follows: TiO₂ < Co₃O₄ < ZnO < CuO. The toxicity is due not only to ROS-induced cell death, but also to damages to cell and mitochondrial membranes.     

Oxidative stress responses of Daphnia magna exposed to TiO2 nanoparticles according to size fraction

Size is one of important factors determining titanium dioxide nanoparticle (TiO₂ NP) toxicity since penetration is eased with decreasing particle size and bioavailability is increased. The effect of particle size on oxidative stress against titanium dioxide nanoparticle (TiO₂ NP) exposure to Daphnia magna was investigated with both acute and chronic toxicity tests. Experiments on biochemical responses, repeatedly performed after size fractionation of the NPs using filtration, focused on the activities of four antioxidant enzymes: catalase (CAT), superoxide dismutase (SOD), glutathione peroxidase (GPX), and glutathione-S-transferase (GST). In the chronic bioassay, the mortality was significantly increased at TiO₂ NP concentrations of 5 and 10 mg/L; however, no reduction of the reproduction ability was observed. Biochemical measurements showed that TiO₂ NP exposure significantly increased the antioxidant enzyme activities in D. magna. CAT, GPX and GST, but not SOD, showed a concentration-dependent increase. In terms of size fraction, particles ranging from 400 to 800 nm exhibited an increase of antioxidant enzyme activities in GST and GPX. These biochemical level observations suggested that TiO₂ NP toxicity was mediated by reactive oxygen species (ROS) generation via oxidative stress in D. magna. The increased mortality at the concentration of 5 mg/L in the chronic bioassay was attributed to accumulated TiO₂ NPs in the intestine of D. magna, which might induce effects such as oxidative stress relating to the induction of antioxidant enzymes.     

Bacterial responses to Cu-doped TiO2 nanoparticles

The toxicity of Cu-doped TiO₂ nanoparticles (NPs, 20 nm), synthesized by a flame aerosol reactor, to Mycobacterium smegmatis and Shewanella oneidensis MR-1, is the primary focus of this study. Both doped and non-doped TiO₂ NPs (20 nm) tended to agglomerate in the medium solution, and therefore did not penetrate into the cell and damage cellular structures. TiO₂ particles (< 100 mg/L) did not apparently interfere with the growth of the two species in aqueous cultures. Cu-doped TiO₂ NPs (20 mg/L) significantly reduced the M. smegmatis growth rate by three fold, but did not affect S. oneidensis MR-1 growth. The toxicity of Cu-doped TiO₂ NPs was driven by the release of Cu²⁺ from the parent NPs. Compared to equivalent amounts of Cu²⁺, Cu-doped TiO₂ NPs exhibited higher levels of toxicity to M. smegmatis (P-value < 0.1). Addition of EDTA in the culture appeared to significantly decrease the anti-mycobacterium activity of Cu-doped TiO₂ NPs. S. oneidensis MR-1 produced a large amount of extracellular polymeric substances (EPS) under NP stress, especially extracellular protein. Therefore, S. oneidensis MR-1 was able to tolerate a much higher concentration of Cu²⁺ or Cu-doped TiO₂ NPs. S. oneidensis MR-1 also adsorbed NPs on cell surface and enzymatically reduced ionic copper in culture medium with a remediating rate of 61 µg/(liter?OD₆₀₀? hour) during its early exponential growth phase. Since the metal reducing Shewanella species can efficiently “clean” metal-oxide NPs, the activities of such environmentally relevant bacteria may be an important consideration for evaluating the ecological risk of metal-oxide NPs.     

In vitro evaluation of cytotoxicity of engineered metal oxide nanoparticles

The recent advances in nanotechnology and the corresponding popular usage of nanomaterials have resulted in uncertainties regarding their environmental impacts. In this study, we used a systematic approach to study and compare the in vitro cytotoxicity of selected engineered metal oxide nanoparticles to the test organisms — E. coli. Among the seven test nano-sized metal oxides, ZnO, CuO, Al₂O_3, La₂O_3, Fe₂O_3, SnO₂ and TiO_2, ZnO showed the lowest LD₅₀ of 21.1 mg/L and TiO₂ had the highest LD₅₀ of 1104.8 mg/L. Data of ¹⁴C-glucose mineralization test paralleled the results of bacteria viability test. After regression calculation, the cytotoxicity was found to be correlated with cation charges (R² = 0.9785). The higher the cation charge is, the lower the cytotoxicity of the nano-sized metal oxide becomes. To the best of our knowledge, this finding is the first report in nanotoxicology.     

Synergistic toxic effect of nano-TiO and As(V) on Ceriodaphnia dubia

Due to the active development and application of nanotechnology, engineered nanomaterials (ENMs) are becoming a new class of environmental pollutants that may significantly impact the environment and human health. While many toxicity investigations have been conducted, there is little information about the synergistic effect of ENMs and other toxic compounds in the environment. In order to extend this knowledge, the combined effect of TiO₂ nanoparticles (n-TiO₂) and As(V) were investigated. High concentrations of As(V) can accumulate on the n-TiO₂ surface. Cultured Ceriodaphnia dubia (C. dubia) species were used to examine the synergistic toxic effect through exposure to 1) n-TiO₂ suspensions, 2) As(V) solutions, and 3) mixtures of n-TiO₂ and As(V) suspensions. Results showed that n-TiO₂ alone was not toxic when the concentration was less than 400 mg/L and that the 24-hour median lethal concentration (LC₅₀) of As(V) alone was 3.68 ± 0.22 mg/L. However, in the presence of low concentrations of n-TiO₂, the toxicity of As(V) increased significantly. At the same initial As(V) concentration, the toxicity of n-TiO₂ first increased, reached a maximum, and then decreased with an increase in n-TiO₂ concentration. Hydrodynamic size and sorption capacity were most important parameters for toxicity.     

Release of phosphorous impurity from TiO2 anatase and rutile nanoparticles in aquatic environments and its implications

Phosphorus-bearing materials as an additive have been popularly used in nanomaterial synthesis and the residual phosphorus within the nanoparticles (NPs) can be of an environmental concern. For instance, phosphorus within pristine commercial TiO₂ NPs greatly influences the surface charge and aggregation behavior of the host TiO₂ in aquatic environments; however, it is unknown whether and how fast phosphorus is released. In this study, we focus on the phosphorus release kinetics from five types of TiO₂ NPs (i.e., 5, 10, and 50 nm anatase and 10 × 40, 30 × 40 nm rutile) under the influence of varying solution chemistries. The 50 nm anatase has the highest quantity of P (8.05 g/kg) and most leachable P dissolves within the first 2 h (i.e., 5.01 g/kg), which presents a potential pollutant source of P. Higher pH favors the phosphorus release (release order: pH 11.2 > pH 8.2 > pH 2.4), while variations in the environmentally relevant ionic strengths (0.01 M NaCl + 0.01 M NaHCO₃ and 0.04 M NaCl + 0.01 M NaHCO₃) and the presence of dissolved natural organic matter (10 mg/L) do not affect release rate greatly. X-ray Absorption Near Edge Structure results suggest that phosphate adsorbed on the pristine 50 nm anatase desorbs, and some dissolved phosphate again re-sorbs as a surface precipitate. The findings from this research may have important environmental implications such as accidental release of TiO₂ NPs and other nanomaterials that are synthesized using phosphorus containing chemicals as an ingredient.     

Cu-doped TiO(2) nanoparticles enhance survival of Shewanella oneidensis MR-1 under ultraviolet light (UV) exposure

It has been shown that photocatalytic TiO₂ nanoparticles (NPs) can be used as an efficient anti-microbial agent under UV light due to generation of reactive oxygen species (ROS), while Shewanella oneidensis MR-1 is a metal-reducing bacterium highly susceptible to UV radiation. Interestingly, we found that the presence of Cu-doped TiO₂ NPs in the cultural medium dramatically increased the survival rates (based on colony-forming unit) of strain MR-1 by over 10,000-fold (incubation without shaking) and ~ 200 fold (incubation with shaking) after a 2-h exposure to UV light. Gene expression results (via qPCR measurement) indicated that the DNA repair gene recA in MR-1 was significantly induced by UV exposure (indicating cellular damage under UV stress), but the influence of NPs on recA expression was not statistically evident. Plausible explanations to NP attenuation of UV stresses are: 1. TiO₂ based NPs are capable of scattering and absorbing UV light and thus create a shading effect to protect MR-1 from UV radiation; 2. more importantly, Cu-doped TiO₂ NPs can co-agglomerate with MR-1 to form large flocs that improves cells' survival against the environmental stresses. This study improves our understanding of NP ecological impacts under natural solar radiation and provides useful insights to application of photocatalytic-NPs for bacterial disinfection.     

Arsenic sorption on TiO2 nanoparticles: Size and crystallinity effects

Single solute As (III) and As (V) sorption on nano-sized amorphous and crystalline TiO₂ was investigated to determine: size and crystallinity effects on arsenic sorption capacities, possible As (III) oxidation, and the nature of surface complexes. Amorphous and crystalline nanoparticles were prepared using sol-gel synthesis techniques. For amorphous TiO₂, solute pH in the range of 4-9 had a profound impact on only As (V) sorption. As (III) and As (V) sorption isotherms indicated that sorption capacities of the different TiO₂ polymorphs were dependent on the sorption site density, surface area (particle size) and crystalline structure. When normalized to surface area, As (III) surface coverage on the TiO₂ surface remained almost constant for particles between 5 and 20 nm. However, As (V) surface coverage increased with the degree of crystallinity. X-ray absorption spectroscopic analysis provided evidence of partial As (III) oxidation on amorphous TiO₂ rather than crystalline TiO₂. The data also indicated that As (III) and As (V) form binuclear bidentate inner-sphere complexes with amorphous TiO₂ at neutral pH.     

In vitro evaluation of the cytotoxicity of iron oxide nanoparticles with different coatings and different sizes in A3 human T lymphocytes

The ever expanding use of engineered nanoscaled materials has brought about a commensurate growth in concern about their potential risks to human and environmental health. Toxicity of nanoparticles could vary with their physicochemical parameters. The dependence of cytotoxicity on particle size and surface coating of iron oxide nanoparticles was investigated in this in vitro study using the A3 human T lymphocyte as a model. Two different sizes (10 nm and 50 nm) and two different surface coatings (amine and carboxyl groups) of iron oxide (IO) nanoparticles were tested with fluorescein diacetate (FDA) assay and WST-1 assay. In the 1-h FDA assay with PBS, IO nanoparticles did not show size-dependent toxicity to A3 cells in terms of mass concentration; however, in terms of the number of particles per well and the total surface area, they did exhibit size-dependent toxicity. Fifty nanometer IO nanoparticles are more toxic than the 10 nm counterparts. The results of both the 24-h FDA and WST-1 assays in a complete growth medium indicate size- and surface coating-dependent toxicity to A3 cells in terms of mass concentration. IO nanoparticles of the smaller size are more toxic than those of the larger size. IO nanoparticles with the carboxyl group have a higher toxicity than those with the amine group. However, in the 24-h FDA assay, in terms of the number of particles per well and the resultant total surface area per well, the 50 nm IO nanoparticles are more toxic than those of size 10 nm. In terms of mass concentration, the number of particles per well and the total surface area, both of the 24-h assays showed the consistent results that IO nanoparticles with the carboxyl group have a higher toxicity than those with the amine group.     

Photoassisted reduction of metal ions and organic dye by titanium dioxide nanoparticles in aqueous solution under anoxic conditions

The photoassisted reduction of metal ions and organic dye by metal-deposited Degussa P25 TiO₂ nanoparticles was investigated. Copper and silver ions were selected as the target metal ions to modify the surface properties of TiO₂ and to enhance the photocatalytic activity of TiO₂ towards methylene blue (MB) degradation. X-ray powder diffraction (XRPD), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM) were used to characterize the crystallinity, chemical species and morphology of metal-deposited TiO₂, respectively. Results showed that the particle size of metal-deposited TiO₂ was larger than that of Degussa P25 TiO₂. Based on XRPD patterns and XPS spectra, it was observed that the addition of formate promoted the photoreduction of metal ion by lowering its oxidation number, and subsequently enhancing the photodegradation efficiency and rate of MB. The pseudo-first-order rate constant (k_obs) for MB photodegradation by Degussa P25 TiO₂ was 3.94 × 10^− 2 min^− 1 and increased by 1.4-1.7 times in k_obs with metal-deposited TiO₂ for MB photodegradation compared to simple Degussa P25 TiO₂. The increase in mass loading of metal ions significantly enhanced the photodegradation efficiency of MB; the k_obs for MB degradation increased from 3.94 × 10^− 2 min^− 1 in the absence of metal ion to 4.64-7.28 × 10^− 2 min^− 1 for Ag/TiO₂ and to 5.14-7.61 × 10^− 2 min^− 1 for Cu/TiO₂. In addition, the electrons generated from TiO₂ can effectively reduce metal ions and MB simultaneously under anoxic conditions. However, metal ions and organic dye would compete for electrons from the illuminated TiO₂.     

Toxicity of nanoparticles of CuO, ZnO and TiO2 to microalgae Pseudokirchneriella subcapitata

Toxicities of ZnO, TiO₂ and CuO nanoparticles to Pseudokirchneriella subcapitata were determined using OECD 201 algal growth inhibition test taking in account potential shading of light. The results showed that the shading effect by nanoparticles was negligible. ZnO nanoparticles were most toxic followed by nano CuO and nano TiO₂. The toxicities of bulk and nano ZnO particles were both similar to that of ZnSO₄ (72 h EC50 ~ 0.04 mg Zn/l). Thus, in this low concentration range the toxicity was attributed solely to solubilized Zn²⁺ ions. Bulk TiO₂ (EC50 = 35.9 mg Ti/l) and bulk CuO (EC50 = 11.55 mg Cu/l) were less toxic than their nano formulations (EC50 = 5.83 mg Ti/l and 0.71 mg Cu/l). NOEC (no-observed-effect-concentrations) that may be used for risk assessment purposes for bulk and nano ZnO did not differ (~ 0.02 mg Zn/l). NOEC for nano CuO was 0.42 mg Cu/l and for bulk CuO 8.03 mg Cu/l. For nano TiO₂ the NOEC was 0.98 mg Ti/l and for bulk TiO₂ 10.1 mg Ti/l. Nano TiO₂ formed characteristic aggregates entrapping algal cells that may contribute to the toxic effect of nano TiO₂ to algae. At 72 h EC50 values of nano CuO and CuO, 25% of copper from nano CuO was bioavailable and only 0.18% of copper from bulk CuO. Thus, according to recombinant bacterial and yeast Cu-sensors, copper from nano CuO was 141-fold more bioavailable than from bulk CuO. Also, toxic effects of Cu oxides to algae were due to bioavailable copper ions.To our knowledge, this is one of the first systematic studies on effects of metal oxide nanoparticles on algal growth and the first describing toxic effects of nano CuO towards algae.     

Effect of preparation methods on toxicity of fullerene water suspensions to Japanese medaka embryos

The physicochemical properties of fullerene water suspensions (nC₆₀) and their subsequent toxicity were influenced by different preparation methods. The nC₆₀ suspensions were produced by three methods: toluene exchange (Tol/nC₆₀), DMSO dissolving (DMSO/nC₆₀), and stirring overtime (Aqu/nC₆₀). The particle size, zeta potential, and nC₆₀ structure were strongly dependent on both the type of aggregates formed and the test medium addition. Specifically, Tol/nC₆₀ exhibited small and spherical closed aggregates, whereas DMSO/nC₆₀ and Aqu/nC₆₀ presented mesoscale aggregates of smaller spherical aggregates. These differences in the physicochemical properties of nC₆₀ determined the embryonic toxicity and oxidative stress of Japanese medaka (Oryzias latipes). The mortality and glutathione (GSH) induction of embryos were ranked in the order of Tol/nC₆₀ > DMSO/nC₆₀ > Aqu/nC₆₀, and the morphological malformations were in the order of DMSO/nC₆₀ > Tol/nC₆₀ > Aqu/nC₆₀. The mortality of Tol/nC₆₀ was attributed to its closely packed fullerene structure, which remained as largely underivatized C₆₀. The malformations of DMSO/nC₆₀ might have originated from the co-effect of organic solvent remaining in the fullerene colloid. To summarize, these findings clearly illustrated the need to consider the effect of preparation method on the physicochemical properties when assessing nC₆₀ toxicity.     

Aggregation and disaggregation of iron oxide nanoparticles: Influence of particle concentration, pH and natural organic matter

The surface coating, aggregation behavior and aggregate structure of unpurified iron oxide nanoparticles (NPs) at variable pH and in the absence and presence of natural organic matter (NOM, Suwannee River humic acid, SRHA) have been previously studied in Baalousha et al. [Baalousha, M., Manciulea, A., Cumberland, S., Kendall, K., Lead, J.R., Aggregation and surface properties of iron oxide nanoparticles; influence of pH and natural organic matter. Environ Toxicol Chem 2008; 27: 1875-1882.]. Here the aggregation behavior of iron oxide NPs at variable concentrations of NPs and SRHA, and the disaggregation behavior of iron oxide NP aggregates in the absence and presence of SRHA are investigated. The increase of NP concentration enhances their aggregation, particularly at pH values close to the point of zero charge (PZC). High concentration of SRHA (100 mg l^− 1) shifts the NP (100 mg l^− 1) PZC charge and aggregation maximum towards lower pHs, while low concentration (10 mg l^− 1) shows low or no effect. The disaggregation behavior of iron oxide NP aggregates was investigated at pH 7 and at increasing concentrations of SRHA. High concentrations (50 and 100 mg l^− 1) of SRHA induced the disaggregation of iron oxide NP aggregates with time, which was not the case at lower concentrations (10 mg l^− 1) or in the absence of SRHA. The disaggregation was triggered by the enhanced surface charge induced by the sorption of SRHA molecules. The disaggregation rate increased with SRHA concentration and decreased with time. Two regimes of disaggregation were identified, a fast regime of “fragmentation” at the first 15 days of the experiment and a slow regime of “erosion” afterwards. The formation of small aggregates of about 170 nm and surface coating of several nanometers of SRHA on iron oxide NPs confirm the role of NOM in the disaggregation process and indicate that NPs might mimic the behavior of natural colloids.     

Effect of sunlight irradiation on photocatalytic pyrene degradation in contaminated soils by micro-nano size TiO2

The enhanced catalytic pyrene degradation in quartz sand and alluvial and red soils by micro-nano size TiO₂ in the presence and absence of sunlight was investigated. The results showed that the synergistic effect of sunlight irradiation and TiO₂ was more efficient on pyrene degradation in quartz sand and red and alluvial soils than the corresponding reaction system without sunlight irradiation. In the presence of sunlight irradiation, the photooxidation (without TiO₂) of pyrene was very pronounced in alluvial and red soils and especially in quartz sand. However, in the absence of sunlight irradiation, the catalytic pyrene degradation by TiO₂ and the photooxidation (without TiO₂) of pyrene were almost nil. This implicates that ultra-violet (UV) wavelength range of sunlight plays an important role in TiO₂-enhanced photocatalytic pyrene degradation and in photooxidation (without TiO₂) of pyrene. The percentages of photocatalytic pyrene degradation by TiO₂ in quartz sand, alluvial and red soils under sunlight irradiation were 78.3, 23.4, and 31.8%, respectively, at 5 h reaction period with a 5% (w/w) dose of the amended catalyst. The sequence of TiO₂-enhanced catalytic pyrene degradation in quartz sand and alluvial and red soils was quartz sand > red soil > alluvial soil, due to different texture and total organic carbon (TOC) contents of the quartz sand and other two soils. The differential Fourier transform infrared (FT-IR) spectra of degraded pyrene in alluvial soil corroborate that TiO₂-enhanced photocatalytic degradation rate of degraded pyrene was much greater than photooxidation (without TiO₂) rate of degraded pyrene. Based on the data obtained, the importance for the application of TiO₂-enhanced photocatalytic pyrene degradation and associated organic contaminants in contaminated soils was elucidated.     

Degradation of the antibiotic oxolinic acid by photocatalysis with TiO2 in suspension

In the work presented here, a photocatalytic system using titanium Degussa P-25 in suspension was used to evaluate the degradation of 20 mg L⁻¹ of antibiotic oxolinic acid (OA). The effects of catalyst load (0.2-1.5 g L⁻¹) and pH (7.5-11) were evaluated and optimized using the surface response methodology and the Pareto diagram. In the range of variables studied, low pH values and 1.0 g L⁻¹ of TiO₂ favoured the efficiency of the process. Under optimal conditions the evolution of the substrate, chemical oxygen demand, dissolved organic carbon, toxicity and antimicrobial activity on Escherichia coli cultures were evaluated. The results indicate that, under optimal conditions, after 30 min, the TiO₂ photocatalytic system is able to eliminate both the substrate and the antimicrobial activity, and to reduce the toxicity of the solution by 60%. However, at the same time, ∼53% of both initial DOC and COD remain in solution. Thus, the photocatalytical system is able to transform the target compound into more oxidized by-products without antimicrobial activity and with a low toxicity. The study of OA by-products using liquid chromatography coupled with mass spectrometry, as well as the evaluation of OA degradation in acetonitrile media as solvent or in the presence of isopropanol and iodide suggest that the reaction is initiated by the photo-Kolbe reaction. Adsorption isotherm experiments in the dark indicated that under pH 7.5, adsorption corresponded to the Langmuir adsorption model, indicating the dependence of the reaction on an initial adsorption step.     

TiO2 nanoparticles effects on physical, thermal and mechanical properties of self compacting concrete with ground granulated blast furnace slag as binder

In this work, strength assessments and percentage of water absorption of self compacting concrete containing different amounts of ground granulated blast furnace slag and TiO₂ nanoparticles as binder have been investigated. Portland cement was replaced by 45 wt% of ground granulated blast furnace slag and up to 4.0 wt% TiO₂ nanoparticles and the properties of concrete specimens were investigated. TiO₂ nanoparticle as a partial replacement of cement up to 3.0 wt% could accelerate C-S-H gel formation as a result of increased crystalline Ca(OH)₂ amount at the early age of hydration and hence increase strength and improve the resistance to water permeability of concrete specimens. Several empirical relationships have been presented to predict flexural and split tensile strength of the specimens by means of the corresponding compressive strength at a certain age of curing.     

Synergetic effect between photocatalytic degradation and adsorption processes on the removal of phenolic compounds from olive mill wastewater

The photocatalytic degradation of two phenolic compounds, p-coumaric acid and caffeic acid, was performed with a suspended mixture of TiO₂ and powdered activated carbon (PAC) (at pH = 3.4 and 8). Adsorption, direct photolysis and photocatalytic degradation were studied under different pH and UV light sources (sunlight vs. 365 nm UV lamps). The potential for reusing this catalyst mixture in sequential photocatalytic runs was examined as well. Quantum yields for the direct photolysis of caffeic acid under solar and artificial 365 nm light were calculated (for the first time) as 0.005 and 0.011, respectively.A higher removal rate of contaminants by either adsorption or photocatalysis was obtained at a low pH (pH 4). Furthermore, the addition of PAC increased the removal efficiency of the phenolic compounds. Fast removal of the pollutants from the solution over three sequential runs was achieved only when both TiO₂ and PAC were present. This suggests that at medium phenolic concentrations, the presence of PAC as a co-sorbent reduces surface poisoning of the TiO₂ catalyst and hence improves photocatalysis degradation of phenolic pollutants.The adsorption equilibrium of caffeic acid or p-coumaric acid on TiO₂, PAC and the combined mixture of TiO₂ and PAC follows the Langmuir isotherm model. Experiments with PAC TiO₂ mixture and olive mill wastewater (anaerobically treated and diluted by a factor of 10) showed higher removal of polyphenols than of chemical oxygen demand (COD). 87% removal of total polyphenols, compared to 58% of COD, was achieved after 24 h of exposure to 365 nm irradiation (7.6 W/m²) in the presence of a suspended mixture of TiO₂ and PAC, indicating “self-selectivity” of polyphenols.     

Development of a TiO2 modified optical fiber electrode and its incorporation into a photoelectrochemical reactor for wastewater treatment

Electrochemical advanced oxidation processes (EAOPs) are used to chemically burn non biodegradable complex organic compounds that are present in polluted effluents. A common approach involves the use of TiO₂ semiconductor substrates as either photocatalytic or photoelectrocatalytic materials in reactors that produce a powerful oxidant (hydroxyl radical) that reacts with pollutant species. In this context, the purpose of this work is to develop a new TiO₂ based photoanode using an optic fiber support. The novel arrangement of a TiO₂ layer positioned on top of a surface modified optical fiber substrate, allowed the construction of a photoelectrochemical reactor that works on the basis of an internally illuminated approach. In this way, a semi-conductive optical fiber modified surface was prepared using 30 μm thickness SnO₂:Sb films on which the photoactive TiO₂ layer was electrophoretically deposited. UV light transmission experiments were conducted to evaluate the transmittance along the optical fiber covered with SnO₂:Sb and TiO₂ showing that 43% of UV light reached the optical fiber tip. With different illumination configurations (external or internal), it was possible to get an increase in the amount of photo-generated H₂O₂ close to 50% as compared to different types of TiO₂ films. Finally, the electro-Fenton photoelectrocatalytic Oxidation process studied in this work was able to achieve total color removal of Azo orange II dye (15 mg L⁻¹) and a 57% removal of total organic carbon (TOC) within 60 min of degradation time.     

Effect of ZnO particles on activated sludge: role of particle dissolution

The release of nanoparticles (NPs) into the environment, including wastewater treatment plants, is expected to increase in the future. Therefore, it is important to understand the potential effects of these NPs on activated sludge treatment processes. A pulse-flow respirometer was used to study the toxicity of nano-ZnO on activated sludge endogenous respiration, BOD biodegradation, and nitrification. In addition, toxicities of bulk ZnO particles and Zn ion (e.g. soluble Zn) were also studied. All three Zn forms were found to adversely impact the activity of activated sludge, with soluble Zn exhibited the greatest toxicity. The effects of nano-ZnO and bulk ZnO on activated sludge were caused by soluble Zn resulting from ZnO particle dissolution. The IC₅₀ values of soluble Zn on activated sludge endogenous respiration, BOD biodegradation, ammonia oxidation, and nitrite oxidation were 2.2, 1.3, 0.8, and 7.3 mg-Zn/L, respectively. Therefore, the first step of nitrification was most sensitive to Zn.