Ralf Kägi

Quantification of Carbon Nanotubes in Environmental Matrices: Current Capabilities, Case Studies, and Future Prospects

Carbon nanotubes (CNTs) have numerous exciting potential applications and some that have reached commercialization. As such, quantitative measurements of CNTs in key environmental matrices (water, soil, sediment, and biological tissues) are needed to address concerns about their potential environmental and human health risks and to inform application development. However, standard methods for CNT quantification are not yet available. We systematically and critically review each component of the current methods for CNT quantification including CNT extraction approaches, potential biases, limits of detection, and potential for standardization. This review reveals that many of the techniques with the lowest detection limits require uncommon equipment or expertise, and thus, they are not frequently accessible. Additionally, changes to the CNTs (e.g., agglomeration) after environmental release and matrix effects can cause biases for many of the techniques, and biasing factors vary among the techniques. Five case studies are provided to illustrate how to use this information to inform responses to real-world scenarios such as monitoring potential CNT discharge into a river or ecotoxicity testing by a testing laboratory. Overall, substantial progress has been made in improving CNT quantification during the past ten years, but additional work is needed for standardization, development of extraction techniques from complex matrices, and multimethod comparisons of standard samples to reveal the comparability of techniques.

Adsorption of arsenic on polyaluminum granulate.

The kinetics and efficiencies of arsenite and arsenate removal from water were evaluated using polyaluminum granulates (PAG) with high content of aluminum nanoclusters. PAG was characterized to be meso- and macroporous, with a specific surface area of 35 ± 1 m(2) g(-1). Adsorption experiments were conducted at pH 7.5 in deionized water and synthetic water with composition of As-contaminated groundwater in the Pannonian Basin. As(III) and As(V) sorption was best described by the Freundlich and Langmuir isotherm, respectively, with a maximum As(V) uptake capacity of ~200 μmol g(-1) in synthetic water. While As(III) removal reached equilibrium within 40 h, As(V) was removed almost entirely within 20 h. Micro X-ray fluorescence and electron microscopy revealed that As(III) was distributed uniformly within the grain, whereas As(V) diffused up to 81 μm into PAG. The results imply that As(V) is adsorbed 3 times faster while being transported 10(5) times slower than As(III) in Al hydroxide materials.

Solid Solutions between CrO4- and SO4-Ettringite Ca6(Al(OH)6)2[(CrO4)x(SO4)1-x]3*26 H2O

Chromate is a toxic contaminant of potential concern, as it is quite soluble in the alkaline pH range and could be released to the environment. In cementitous systems, CrO4(2−) is thought to be incorporated as a solid solution with SO4(2−) in ettringite. The formation of a solid solution (SS) could lower the soluble CrO4(2−) concentrations. Ettringite containing SO4(2−) or CrO4(2−) and mixtures thereof have been synthesized. The resulting solids and their solubility after an equilibration time of 3 months have been characterized. For CrO4-ettringite at 25 °C, a solubility product log K(S0) of −40.2 ± 0.4 was calculated: log K(CrO4−ettringite) = 6log{Ca2+} + 2log{Al(OH)4(−)} + 3log{CrO4(2−)} + 4log{OH−} + 26log{H2O}. X-ray diffraction and the analysis of the solution indicated the formation of a regular solid solution between SO4- and CrO4-ettringite with a miscibility gap between 0.4 ≤ XCrO4 ≤ 0.6. The miscibility gap of the SO4- and CrO4-ettringite solid solution could be reproduced with a dimensionless Guggenheim fitting parameter (a0) of 2.03. The presence of a solid solution between SO4- and CrO4-ettringite results in a stabilization of the solids compared to the pure ettringites and thus in an increased uptake of CrO4(2−) in cementitious systems.

Development of a mobile fast-screening laser-induced breakdown detection (LIBD) system for field-based measurements of nanometre sized particles in aqueous solutions

Laser-induced breakdown detection (LIBD) is a promising method to detect trace amounts of nanoparticles (NP, <100 nm) in aqueous suspensions. Based on available systems, we developed a mobile LIBD, designed for on-site and on-line measurements. We used the energy ratio of every laser pulse before and after passing the laser beam through the aqueous sample as a new method to detect laser-induced plasma events. The particle size and the particle number density are derived from recorded energy curves. Our LIBD is operated with a Nd:YAG laser at 100 Hz significantly reducing the measurement times compared to other LIBD systems operated at 20 Hz and increasing the capabilities for monitoring purposes. Long-term experiments on water samples revealed losses of NP up to 75% in 15 mL and 35% in 5 L sample containers after 3 months. The size of the particles remained constant (5 L) or slightly decreased (15 mL) indicating significant adsorption of NP to the walls of the sampling containers. Furthermore, we monitored the NP content of water after different purification steps at a drinking water plant (Maennedorf, Lake Zurich, Switzerland). Activated carbon filtration resulted in an increase of the particle size from approximately 20 nm to approximately 75 nm possibly caused by the release of organic fragments derived from the biology within the activated carbon tank. After the final ultrafiltration step the particle size was around 10 nm in agreement with the nominal cutoff of 100 kDa of the membrane. The results underline the strength of a fast-screening LIBD to detect relative changes in NP size and concentration.

Long‐term effects of sulfidized silver nanoparticles in sewage sludge on soil microflora

The use of silver nanoparticles (AgNPs) in consumer products such as textiles leads to their discharge into wastewater and consequently to a transfer of the AgNPs to soil ecosystems via biosolids used as fertilizer. In urban wastewater systems (e.g., sewer, wastewater treatment plant [WWTP], anaerobic digesters) AgNPs are efficiently converted into sparingly soluble silver sulfides (Ag2 S), mitigating the toxicity of the AgNPs. However, long-term studies on the bioavailability and effects of sulfidized AgNPs on soil microorganisms are lacking. Thus we investigated the bioavailability and long-term effects of AgNPs (spiked in a laboratory WWTP) on soil microorganisms. Before mixing the biosolids into soil, the sludges were either anaerobically digested or directly dewatered. The effects on the ammonium oxidation process were investigated over 140 d. Transmission electron microscopy (TEM) suggested an almost complete sulfidation of the AgNPs analyzed in all biosolid samples and in soil, with Ag2 S predominantly detected in long-term incubation experiments. However, despite the sulfidation of the AgNPs, soil ammonium oxidation was significantly inhibited, and the degree of inhibition was independent of the sludge treatment. The results revealed that AgNPs sulfidized under environmentally relevant conditions were still bioavailable to soil microorganisms. Consequently, Ag2 S may exhibit toxic effects over the long term rather than the short term. Environ Toxicol Chem 2017;36:3305-3313.