Åsa K. Jämting

Transformation of four silver/silver chloride nanoparticles during anaerobic treatment of wastewater and post-processing of sewage sludge

The increasing use of silver (Ag) nanoparticles [containing either elemental Ag (Ag-NPs) or AgCl (AgCl-NPs)] in commercial products such as textiles will most likely result in these materials reaching wastewater treatment plants. Previous studies indicate that a conversion of Ag-NPs to Ag2S is to be expected during wastewater transport/treatment. However, the influence of surface functionality, the nature of the core structure and the effect of post-processing on Ag speciation in sewage sludge/biosolids has not been investigated. This study aims at closing these knowledge gaps using bench scale anaerobic digesters spiked with Ag nitrate, three different types of Ag-NPs, and AgCl-NPs at environmentally realistic concentrations. The results indicate that neither surface functionality nor the different compositions of the NP prevented the formation of Ag2S. Silver sulfides, unlike the sulfides of other metals present in sewage sludge, were stable over a six month period simulating composting/stockpiling.

A review of the detection, fate and effects of engineered nanomaterials in wastewater treatment plants

Engineered nanomaterials (ENMs) are increasingly found in a wide range of products and processes, and consequently increasing loads are expected to reach wastewater treatment plants (WWTPs). To better assess the potential risk of ENMs to the environment via input through WWTP effluents, this review considers ENM detection methods, fate in WWTPs and potential effects on biota exposed to wastewater associated ENMs. Characterising ENMs in complex matrices presents many challenges, especially at low concentrations. Combining separation methods with techniques to assess particle size and chemical composition appears to be the most suitable approach for wastewater. In a range of studies, the majority of ENMs are removed from the aqueous phase by flocculation and sedimentation and remain in the sludge. However, ENM surface coating and the presence of organic matter and surfactants can alter removal. ENMs may affect biota via discharge of treated effluent to the aquatic environment or by application of sewage sludge to soil, although observed effects in laboratory studies only occurred at concentrations several orders of magnitude higher than the expected environmental levels. More realistic experimental designs with improved quantification of ENM properties under the selected test conditions are required to better understand the fate and effect of ENMs associated with WWTPs.

Behaviour of titanium dioxide and zinc oxide nanoparticles in the presence of wastewater-derived organic matter and implications for algal toxicity

The properties of engineered nanomaterials (ENMs), such as their small size and increased reactivity, enable their use in a wide range of products; however, these attributes may also pose an environmental hazard. Wastewater effluent is expected to be a significant source of ENMs to the aquatic environment. Little is known about the behaviour and effect of ENMs in this complex matrix. The aim of this study was to assess the effect of titanium dioxide (TiO2) and zinc oxide (ZnO) ENMs on microalgae Pseudokirchneriella subcapitata in wastewater effluent and compare with more commonly tested matrices, specifically Talaquil growth media and Suwannee River humic acid. To better understand the toxicity results, ENM concentration, size and streaming potential in the different experimental matrices were assessed. The different media types had a significant influence on TiO2 behaviour, with TiO2 settling out of solution within the first 24 hours in wastewater. However, as TiO2 was not toxic to algae at the studied concentration, no difference in effect was observed between media, humic acid and wastewater. In contrast, media types had less of an influence on ZnO behaviour, with growth inhibition observed in all three media types at 72 hours, although the presence of organic matter delayed inhibition slightly. The results demonstrate that organic matter properties can have a significant influence on ENM behaviour, and while this did not translate into an effect on algae in the current study, it is still important to consider ENM behaviour and fate when working in different matrices.

Nanoparticles and metrology: a comparison of methods for the determination of particle size distributions

Nanoparticles and products incorporating nanoparticles are a growing branch of nanotechnology industry. They have found a broad market, including the cosmetic, health care and energy sectors. Accurate and representative determination of particle size distributions in such products is critical at all stages of the product lifecycle, extending from quality control at point of manufacture to environmental fate at the point of disposal. Determination of particle size distributions is non-trivial, and is complicated by the fact that different techniques measure different quantities, leading to differences in the measured size distributions. In this study we use both mono- and multi-modal dispersions of nanoparticle reference materials to compare and contrast traditional and novel methods for particle size distribution determination. The methods investigated include ensemble techniques such as dynamic light scattering (DLS) and differential centrifugal sedimentation (DCS), as well as single particle techniques such as transmission electron microscopy (TEM) and microchannel resonator (ultra high-resolution mass sensor).

Size and shape distributions of primary crystallites in titania aggregates

The primary crystallite size of titania powder relates to its properties in a number of applications. Transmission electron microscopy was used in this interlaboratory comparison (ILC) to measure primary crystallite size and shape distributions for a commercial aggregated titania powder. Data of four size descriptors and two shape descriptors were evaluated across nine laboratories. Data repeatability and reproducibility was evaluated by analysis of variance. One-third of the laboratory pairs had similar size descriptor data, but 83% of the pairs had similar aspect ratio data. Scale descriptor distributions were generally unimodal and were well-described by lognormal reference models. Shape descriptor distributions were multi-modal but data visualization plots demonstrated that the Weibull distribution was preferred to the normal distribution. For the equivalent circular diameter size descriptor, measurement uncertainties of the lognormal distribution scale and width parameters were 9.5% and 22%, respectively. For the aspect ratio shape descriptor, the measurement uncertainties of the Weibull distribution scale and width parameters were 7.0% and 26%, respectively. Both measurement uncertainty estimates and data visualizations should be used to analyze size and shape distributions of particles on the nanoscale.

Characterisation of gold agglomerates: size distribution, shape and optical properties

The slow agglomeration of gold colloids of approximate diameter 30 nm in the presence of a small concentration of L-cysteine·HCl has been followed by multiple techniques, namely particle tracking analysis (PTA), differential centrifugal sedimentation (DCS), UV-visible spectroscopy (UV), second order spectroscopy (SOS) and transmission electron microscopy (TEM). The citrate-stabilized Au nanoparticles were characterized by PTA, DCS, UV and dynamic light scattering (DLS) prior to exposure to the cysteine. Hydrodynamic forces during centrifugation can cause the disintegration of weakly held agglomerates. TEM reveals small linear agglomerates that become open linked chains of fractal-like structures after several hours of agglomeration. Second order Rayleigh scattering observed at the harmonic wavelength of 680 nm was resonantly enhanced by surface plasmon excitation of the growing agglomerates. During the initial stage of the agglomeration process, when chainlike or quasi-chainlike agglomerates were the dominant species, the SOS signal goes up by a factor of about three before reaching saturation. This study of a model nanoparticle system provides insights into the information obtained from a range of measurement techniques, with recommendations for characterisation of agglomerating nanoparticles under end-use conditions.

Minimising the effect of nanoparticle deformation in intermittent contact amplitude modulation atomic force microscopy measurements

The results of systematic height measurements of polystyrene (PS) nanoparticles using intermittent contact amplitude modulation atomic force microscopy (IC-AM-AFM) are presented. The experimental findings demonstrate that PS nanoparticles deform during AFM imaging, as indicated by a reduction in the measured particle height. This deformation depends on the IC-AM-AFM imaging parameters, material composition, and dimensional properties of the nanoparticles. A model for nanoparticle deformation occurring during IC-AM-AFM imaging is developed as a function of the peak force which can be calculated for a particular set of experimental conditions. The undeformed nanoparticle height can be estimated from the model by extrapolation to zero peak force. A procedure is proposed to quantify and minimise nanoparticle deformation during IC-AM-AFM imaging, based on appropriate adjustments of the experimental control parameters.

Particle number density gradient samples for nanoparticle metrology with atomic force microscopy

Atomic force microscopy (AFM) can provide a link in the traceability chain between dimensional measurement techniques for nanoparticles, such as dynamic light scattering and differential centrifugal sedimentation, and the realization of the definition of the SI metre. Despite the size of nanoparticles being well within the resolution range of typical AFMs, the accurate measurement of nanoparticles with AFM presents a number of challenges. One of these challenges is the number density of particles deposited on substrates for AFM imaging and measurement. If the number density is too low, it is difficult to obtain adequate measurement statistics, whereas a number density that is too high can result in particle agglomeration on the substrate and make it difficult to image sufficient substrate area to obtain a reliable reference for height measurements. We present imaging and measurement results of 16 nm gold nanoparticles deposited on a substrate functionalized to produce a surface with a number density gradient across the sample. This substrate functionalization shows great potential for achieving reliable and efficient nanoparticle metrology with AFM.

Traceable nanoscale length metrology using a metrological Scanning Probe Microscope

We give an overview of the design and planned operation of the metrological Scanning Probe Microscope (mSPM) currently under development at the National Measurement Institute Australia (NMIA) and highlight the metrological principles guiding the design of the instrument. The mSPM facility is being established as part of the nanometrology program at NMIA and will provide the link in the traceability chain between dimensional measurements made at the nanometer scale and the realization of the SI meter at NMIA. The instrument will provide a measurement volume of 100 μm × 100 μm × 25 μm with a target uncertainty of 1 nm for the position measurement.

Quantification of nanoparticle concentration in colloidal suspensions by a non-destructive optical method

The estimation of nanoparticle number concentration in colloidal suspensions is a prerequisite in many procedures, and in particular in multi-stage, low-yield reactions. Here, we describe a rapid, non-destructive method based on optical extinction and dynamic light scattering (DLS), which combines measurements using common bench-top instrumentation with a numerical algorithm to calculate the particle size distribution (PSD) and concentration. These quantities were derived from Mie theory applied to measurements of the optical extinction spectrum of homogeneous, non-absorbing nanoparticles, and the relative PSD of a colloidal suspension. The work presents an approach to account for PSDs achieved by DLS which, due to the underlying model, may not be representative of the true sample PSD. The presented approach estimates the absolute particle number concentration of samples with mono-, bi-modal and broad size distributions with <50% precision. This provides a convenient and practical solution for number concentration estimation required during many applications of colloidal nanomaterials.