Long-Term Toxicity of ZnO Nanoparticles on Scenedesmus rubescens Cultivated in Semi-Batch Mode.

The scope of this study was to investigate the toxic effects of zinc oxide (ZnO) nanoparticles (NPs) on freshwater microalgae, in long-term semi-batch feeding mode at two different hydraulic retention times (HRTs) (20 and 40 days). A freshwater microalgae, Scenedesmus rubescens, was employed and exposed to a semi-continuous supply of ZnO NPs at a low concentration of 0.081 mg/L for a period of 28 d. Experiments were conducted under controlled environmental conditions. Τhe impact of ZnO NPs on S. rubescens, which was assessed in terms of nutrient removal, biomass growth, and algal lipid content. Semi-batch mode cultures showed that low ZnO NP concentrations at an HRT of 40 d did not have any negative effect on microalgae growth after the fourth day of culture. In contrast, algal growth was inhibited up to 17.5% at an HRT of 20 d in the presence of ZnO NPs. This might be attributed to the higher flow rate applied and ZnO NPs load. A positive correlation between nutrient removal and microalgae growth was observed. The algal lipid content was, in most cases, higher in the presence of ZnO NPs at both HRTs, indicating that even low ZnO NPs concentration cause stress resulting in higher lipid content.

Long-term toxicity of ZnO nanoparticles to Scenedesmus rubescens cultivated in different media.

The aim of this work was to investigate the long-term toxic effect of zinc oxide (ZnO) nanoparticles (NPs) on freshwater microalgae, combined with the nutrient consumption in the culture. For this purpose, two common microalgae media (Blue-Green 11, BG-11, and Bold's Basal Medium, BBM) were used. Scenedesmus rubescens was used as freshwater microalgae model species and was exposed to ZnO NPs at different concentrations (0.081 to 810 mg/L) for a period up to 28 d. The experimental results revealed that microalgae growth was affected by the time of exposure and the NPs concentrations, but mainly the culture medium used. Differences in microalgae growth rates were observed and attributed to the selected culture medium. The toxic effect of ZnO NPs was higher on microalgae cultured in modified BG-11 compared to BBM, despite the fact that S. rubescens exhibited higher growth rate in modified BG-11 without the exposure of ZnO NPs.

Effect of cultivation media on the toxicity of ZnO nanoparticles to freshwater and marine microalgae.

The aim of this work was to investigate the effect of zinc oxide nanoparticles (ZnO NPs) on freshwater and marine microalgae cultivated in different media. Freshwater species Chlorococcum sp. and Scenedesmus rubescens were cultivated in modified Blue-Green medium (BG-11) and Bold's Basal Medium (BBM), and marine species Dunaliella tertiolecta, and Tetraselmis suesica, cultured in salt modified BG-11 and f/2 medium. The microalgae species were exposed for 96 h with a daily reading of algal growth rate, to different ZnO NPs concentrations (0.081-810 mg/L). Significant differences were observed on microalgae growth rates, with the marine being more sensitive than the freshwater species, as revealed by their half inhibitory concentration values (IC50). The IC50 values in freshwater species were affected by the culture medium. The lowest IC50 values (<2.57 mg/L) were observed in the marine species. S. rubescens showed the less toxic effect in cultures with modified BG-11, compared to BBM cultures, with IC50 values >810 mg/L and 14.27 mg/L after 96 h exposure time, respectively. ZnO nanoparticles appeared to have toxic effects in all species tested, depended on the species type, the exposure time, the NPs concentration, and mainly the culture medium.

Selection of microalgae for wastewater treatment and potential lipids production.

In the present study, ten microalgal strains found in fresh and saline waters were cultured, and used to conduct batch experiments in order to evaluate their potential contribution to nutrient removal and biofuel production. The growth rate of microalgae was inversely analogous to their initial concentration. Three freshwater strains were selected, based on their growth rate, and their behavior with synthetic wastewater was further investigated. The strains studied were the Scenedesmus rubescens (SAG 5.95), the Neochloris vigensis (SAG 80.80), and the Chlorococcum spec. (SAG 22.83), and higher growth rate was observed with S. rubescens. Total phosphorus removal at an initial phosphate concentration of 6-7 mg P/L in the synthetic wastewater, was 53%, 25% and 11% for N. vigensis, Chlorococcum spec., and S. rubescens, respectively. Finally, the lipid content was determined at 20th and 30th day of cultivation, and the highest amount was observed at the 20th day.

Virus inactivation in the presence of quartz sand under static and dynamic batch conditions at different temperatures.

Virus inactivation is one of the most important factors that controls virus fate and transport in the subsurface. In this study the inactivation of viruses in the presence of quartz sand was examined. The bacteriophages MS2 and ΦX174 were used as model viruses. Experiments were performed at 4°C and 20°C, under constant controlled conditions, to investigate the effect of virus type, temperature, sand size, and initial virus concentration on virus inactivation. The experimental virus inactivation data were satisfactorily represented by a pseudo-first order expression with time-dependent rate coefficients. Furthermore, the results indicated that virus inactivation was substantially affected by the ambient temperature and initial virus concentration. The inactivation rate of MS2 was shown to be greater than that of ΦX174. However, the greatest inactivation was observed for MS2 without the presence of sand, at 20°C. Sand surfaces offered protection against inactivation especially under static conditions. However, no obvious relationship between sand particle size and virus inactivation could be established from the experimental data. Moreover, the inactivation rates were shown to increase with decreasing virus concentration.