Elisabeth Neubauer

Using FLOWFFF and HPSEC to determine trace metal–colloid associations in wetland runoff

Natural organic matter (NOM) and iron colloids can coexist in surface water. These colloids might exhibit different affinities to metals and metalloids. Previously it has been shown, that organic and inorganic colloids in the low nanometer range can be fractionated using Flow Field-Flow Fractionation analyzes (FlowFFF), but it is not yet understood how the presence of inorganic colloids influences results obtained by High Performance Size Exclusion Chromatography (HPSEC). Studies that compare the use of these size-separation techniques for the analyzes of organic and inorganic colloids and associated elements are needed in order to interpret results obtained by either of these methods. Therefore, associations between colloids from a small stream draining a wetland area and a selected range of elements (Fe, Al, Ti, Pb, Cu, Ni, As, U, and Rare Earth Elements (REE)) have been investigated. FlowFFF analyzes and HPSEC analyzes were combined with ultrafiltration, functional group titration and arsenic speciation analysis. NOM and, in a sample with a pH > 5.2, slightly larger iron organo-mineral colloids, were present in the <0.2 μm fraction in the surface water. Both exhibited notably different affinities for trace elements. Cu, Ni, Al, and the REE all showed similar modes (i.e. peak maxima) and size distributions to the NOM, while Pb and As showed a preferential association with iron organo-mineral colloids. It was not possible to differentiate between NOM and iron-organo mineral colloids with HPSEC. The differences in the results regarding the apparent molecular mass distributions obtained by FlowFFF and HPSEC are discussed.

The influence of pH on iron speciation in podzol extracts: Iron complexes with natural organic matter, and iron mineral nanoparticles

The quantities of natural organic matter (NOM) and associated iron (Fe) in soil extracts are known to increase with increasing extractant pH. However, it was unclear how the extraction pH affects Fe speciation for particles below 30 nm. We used flow field-flow fractionation (FlowFFF) and transmission electron microscopy (TEM) to investigate the association of Fe and trace elements with NOM and nanoparticulate iron (oxy)hydroxides in podzol extracts. For extracts prepared at the native soil pH (~4), and within a 1-30 nm size range, Fe was associated with NOM. In extracts with a pH≥7 from the E and B soil horizons, Fe was associated with NOM as well as with iron (oxy)hydroxide nanoparticles with a size of approximately 10 nm. The iron (oxy)hydroxide nanoparticles may have either formed within the soil extracts in response to the increase in pH, or they were mobilized from the soil. Additionally, pH shift experiments showed that iron (oxy)hydroxides formed when the native soil pH (~4) was increased to 9 following the extraction. The iron (oxy)hydroxide nanoparticles aggregated if the pH was decreased from 9 to 4. The speciation of Fe also influenced trace element speciation: lead was partly associated with the iron (oxy)hydroxides (when present), while copper binding to NOM remained unaffected by the presence of iron (oxy)hydroxide nanoparticles. The results of this study are important for interpreting the representativeness of soil extracts prepared at a pH other than the native soil pH, and for understanding the changes in Fe speciation that occur along a pH gradient.

Asymmetrical flow-field-flow fractionation coupled with inductively coupled plasma mass spectrometry for the analysis of gold nanoparticles in the presence of natural nanoparticles

Flow-Field-Flow Fractionation (Flow-FFF), coupled with online detection systems, is one of the most promising tools available for the analysis and characterization of engineered nanoparticles (ENPs) in complex matrices. In order to demonstrate the applicability of Flow-FFF for the detection, quantification, and characterization of engineered gold nanoparticles (AuNPs), model dispersions were prepared containing AuNPs with diameters of 30 or 100nm, natural nanoparticles (NNPs) extracted from a soil sample, and different concentrations of natural organic matter (NOM), which were then used to investigate interactions between the AuNPs and the NNPs. It could be shown that light scattering detection can be used to evaluate the fractionation performance of the pure NNPs, but not the fractionation performance of the mixed samples that also contained AuNPs because of specific interactions between the AuNPs and the laser light. A combination of detectors (i.e. light absorbance and inductively coupled plasma mass spectrometry (ICP-MS)) was found to be useful for differentiating between heteroaggregation and homoaggregation of the nanoparticles (NPs). The addition of NOM to samples containing 30nm AuNPs stabilized the AuNPs without affecting the NP size distribution. However, fractograms for samples with no added NOM showed a change in the size distribution, suggesting interactions between the AuNPs and NNPs. This interpretation was supported by unchanged light absorption wavelengths for the AuNPs. In contrast, results for samples containing 100nm AuNPs were inconclusive with respect to recovery and size distributions because of problems with the separation system that probably related to the size and high density of these nanoparticles, highlighting the need for extensive method optimization strategies, even for nanoparticles of the same material but different sizes.

Where is the nano? Analytical approaches for the detection and quantification of TiO2 engineered nanoparticles in surface waters

Detecting and quantifying engineered nanoparticles (ENPs) in complex environmental matrices requires the distinction between natural nanoparticles (NNPs) and ENPs. The distinction of NNPs and ENPs for regulatory purposes calls for cost-efficient methods, but is hampered by similarities in intrinsic properties, such as particle composition, size, density, surface chemistry, etc. Titanium dioxide (TiO2) ENPs, for instance, are produced in very large quantities but Ti also commonly occurs naturally in nano-scale minerals. In this work, we focus on utilizing particle size and composition to identify ENPs in a system with a significant background concentration of the target metal. We have followed independent approaches involving both conventional and state-of-the-art analytical techniques to detect and quantify TiO2 ENPs released into surface waters from sunscreen products and to distinguish them from Ti-bearing NNPs. To achieve this, we applied single particle inductively coupled plasma mass spectrometry with single-element (spICPMS) and multi-element detection (time-of-flight) spICP-TOFMS, together with transmission electron microscopy (TEM), automated scanning electron microscopy (autoSEM), and bulk elemental analyses. A background concentration of Ti-bearing NPs (approximately 5 × 103 particles per ml), possibly of natural origin, was consistently observed outside the bathing season. This concentration increased by up to 40% during the bathing season. Multi-element analysis of individual particles using spICP-TOFMS revealed that Al, Fe, Mn, and Pb are often present in natural Ti-bearing NPs, but no specific multi-element signatures were detected for ENPs. Our data suggests that TiO2 ENPs enter the lake water during bathing activities, eventually agglomerating and sedimenting. We found adhesion of the TiO2 ENPs to the air–water interface for short time periods, depending on wind conditions. This study demonstrates that the use of spICP-TOFMS and spICPMS in combination with other conventional analytical techniques offers significant advantages for discriminating between NNPs and ENPs. The quantitative data produced in this work can be used as input for modeling studies or as a benchmark for analysis protocols and model validations.