Nanomaterials in Freshwater (Low-Matrix). Nanomaterials in High Salinity Water

Analysis of nanomaterials in freshwater such as surface water, groundwater, drinking water, aqueous ecotoxicological media, and wastewater effluents is often hampered by the low concentration of nanomaterials in the sample, which are expected to range between ngL^-1 and µgL^-1. Contrary, particulate matrix concentrations in freshwater samples commonly range between mgL^-1 and gL^-1. Conclusively, nanomaterial concentration remains orders of magnitude below the matrix concentration in freshwater. Therefore, the main aim of sample preparation is to adjust the nanomaterial concentration and eventually separate matrix and nanomaterials.

For example, Gondikas et al. applied onsite dead-end filtration to separate suspended solids from lake water. This pre-concentration step was applied to measure total elemental content in the suspended matter which was used to indirectly detect the presence of nanomaterials. The material collected on the filter was resuspended and analyzed with various analytical techniques to identify nanomaterials.

Ultrafiltration of surface water may be applied to separate large matrix constituents from nanomaterial. Nanomaterials associated in heteroaggregates which are larger than 0.45 µm are often retained during the filtration process. Crossflow filtration proved to be applicable to separate natural colloids from surface water. However, filtration conditions such as retentate-to-permeate flux have to be adjusted carefully to obtain high mass recoveries. FFF has been applied to study associations of trace metals with organic carbon or Fe-rich particle size fractions. Such associations are particularly relevant to elucidate enhanced transport of trace elements with organic matter. Almost direct analysis of the water sample is possible with high sensitivity methods, such as single particle ICP-MS, however, in this case, a filtration or centrifugation step is required to remove very large particles that may damage the sample introduction system of the ICP-MS.

Nanomaterials in High Salinity Water. Seawater contains high concentrations of salts that reduce the electrostatic repulsion forces between particles, thus favoring aggregation. The effect of seawater on particle stability is clearly observed in estuarine systems, where river water with high particle load and low salinity mixes with seawater. The combination of high particle numbers and salinity increase in the estuary creates ideal conditions for particles to form large aggregates which settle on the sediments. As a result, a very small fraction of particles in river water make it to the oceans and seas. Furthermore, oceans are hosts to the largest scale carbon transfer on earth due to the biogeochemical activity of a vast number of marine organisms, what is known as the “biological carbon pump". Therefore, marine waters are typically characterized by low particle numbers, high salinity, and large seasonal variations of organic matter content.

Sample pretreatment for such samples would typically aim at increasing the particle number content to levels that can be measured with analytical techniques. However, care must be taken, because higher particle numbers results in an increased chance of collisions and combined with the high salinity of seawater aggregation is more likely to take place. For example, centrifugation would increase particle numbers, without reducing salt concentration which creates ideal conditions for fast aggregation. A simple approach would be to filter the samples, thus removing most of the salt content and re-suspending the particles in deionized water with stabilizing agents.

However, with this approach, there is a risk of particles irreversibly attaching on the filtration membrane. Quantifying the amounts lost is a tedious task. Using field-flow fractionation is a more robust approach, but demanding in terms of method optimization. With flow FFF, the high salinity water will pass through the membrane, while particles will be concentrated on the membrane and surrounded by the eluent which typically contains stabilizing agents. With sdFFF, the sample is mixed in the eluent and particles will be retained on the outer channel wall under the influence of centrifugal force, while the high salinity water will be carried away.

FFF techniques have been successfully used for marine and estuarine sample treatment, using significantly larger sample volumes than with fresh water samples due to the low particle numbers. A similar approach that can be used to concentrate larger volumes of water than FFF is cross-flow ultrafiltration, where losses on the membrane are reduced by the parallel flow to the membrane surface. Nevertheless, the effect of membrane material and structure may still influence the selectivity of the method.