Ecotoxicological evaluation of three tertiary wastewater treatment techniques via meta-analysis and feeding bioassays using Gammarus fossarum

Advanced treatment techniques, like ozone, activated carbon and TiO₂ in combination with UV, are proposed to improve removal efficiency of micropollutants during wastewater treatment. In a meta-analysis of peer-reviewed literature, we found significantly reduced overall ecotoxicity of municipal wastewaters treated with either ozone (n = 667) or activated carbon (=113), while TiO₂ and UV was not yet assessed. As comparative investigations regarding the detoxification potential of these advanced treatment techniques in municipal wastewater are scarce, we assessed them in four separate Gammarus-feeding trials with 20 replicates per treatment. These bioassays indicate that ozone concentrations of approximately 0.8 mg ozone/mg DOC may produce toxic transformation products. However, referred effects are removed if higher ozone concentrations are used (1.3 mg ozone/mg DOC). Moreover, the application of 1 g TiO₂/l and ambient UV consistently reduced ecotoxicity. Although activated carbon may remove besides micropollutants also nutrients, which seemed to mask its detoxification potential, this treatment technique reduced the ecotoxicity of the wastewater following its amendment with nutrients. Hence, all three advanced treatment techniques are suitable to reduce the ecotoxicity of municipal wastewater mediated by micropollutants and may hence help to meet the requirements of the European Water Framework Directive.     

The potential of perylene bisimide derivatives for the solubilization of carbon nanotubes and graphene

Carbon nanotubes and graphene are outstanding materials of the 21st century with a broad spectrum of applications. However, major challenges are faced such as the intrinsically low solubility of both sp2 carbon allotropes. To overcome this hurdle the potential of noncovalent functionalization is summarized with a special focus on the establishment of the perylene bisimide unit as aromatic anchor to the graphitic surface. Rational surfactant design is unmasked as the key to solubilization of the carbon allotropes, while at the same time tailoring their surface properties, or even electronic properties in a fully reversible fashion.     

On the construction, comparison, and variability of airsheds for interpreting semivolatile organic compounds in passively sampled air

Air mass origin as determined by back trajectories often aids in explaining some of the short-term variability in the atmospheric concentrations of semivolatile organic contaminants. Airsheds, constructed by amalgamating large numbers of back trajectories, capture average air mass origins over longer time periods and thus have found use in interpreting air concentrations obtained by passive air samplers. To explore some of their key characteristics, airsheds for 54 locations on Earth were constructed and compared for roundness, seasonality, and interannual variability. To avoid the so-called "pole problem" and to simplify the calculation of roundness, a "geodesic grid" was used to bin the back-trajectory end points. Departures from roundness were seen to occur at all latitudes and to correlate significantly with local slope but no strong relationship between latitude and roundness was revealed. Seasonality and interannual variability vary widely enough to imply that static models of transport are not sufficient to describe the proximity of an area to potential sources of contaminants. For interpreting an air measurement an airshed should be generated specifically for the deployment time of the sampler, especially when investigating long-term trends. Samples taken in a single season may not represent the average annual atmosphere, and samples taken in linear, as opposed to round, airsheds may not represent the average atmosphere in the area. Simple methods are proposed to ascertain the significance of an airshed or individual cell. It is recommended that when establishing potential contaminant source regions only end points with departure heights of less than ～700 m be considered.     

Chromium speciation in coal and biomass co-combustion products

Chromium speciation is vital for the toxicity of products resulting from co-combustion of coal and biomass. Therefore, understanding of formation processes has been studied using a combination of X-ray absorption fine structure (XAFS) spectroscopy and thermodynamic equilibrium calculations. The influence of cofiring on Cr speciation is very dependent on the type of fuel. Cr(VI) contents in the investigated fly ash samples from coal and cofiring average around 7% of the total chromium. An exception is cofiring 7-28% wood for which ashes exhibited Cr(VI) concentrations of 12-16% of the total chromium. Measurements are in line with thermodynamic predictions: RE factors of Cr around 1 are in line with volatile Cr only above 1400 °C; lower Cr(VI) concentrations with lower oxygen content and Cr(III) dissolved in aluminosilicate glass. Stability of Cr(VI) below 700 °C does not correlate with Cr(VI) concentrations found in the combustion products. It is indicated that Cr(VI) formation is a high-temperature process dependent on Cr evaporation (mode of occurrence in fuel, promoted by organic association), oxidation (local oxygen content), and formation of solid chromates (promoted by presence of free lime (CaO) in the ash). CaCrO(4)(s) is a probable chemical form but, given different leachable fractions (varying from 25 to 100%), different forms of Cr(VI) must be present. Clay-bound Cr is likely to dissolve in the aluminosilicate glass phase during melting of the clay.