Quantifying the structure of very small (<25 nm) natural aquatic colloids

Lead Research Organisation: Lancaster University
Department Name: Lancaster Environment Centre


Natural aquatic colloids are defined as solid phase material between the sizes of 1 nm and 1 um (10^-9 / 10^-6 m) in size and are thus extremely small and finely divided with very large surface areas. They are ubiquitous in the aquatic (and terrestrial) environment and composed of different types of material such as organic (humic substances and polysaccharides), inorganic (metal oxides) and biological (viruses and bacteria) and these phases are mixed together in complex ways. We know that colloids chemically and physically bind trace pollutants such as metals and that these metals such as mercury, cadmium, nickel etc., may be toxic. In addition, we know that these colloids affect trace metal fate and behaviour and control metal transport and bioavailability. Further, it is known that the very small fraction (less than approximately 25 nm) is very important in metal binding under many environmental conditions and plays a defining role in bioavailability. Despite this knowledge which is primarily qualitative rather quantitative, there is a great deal that remains unknown in this area. In particular, our knowledge of 'nano-colloidal' (< ca 25 nm) structure is poor and improving our knowledge base here is essential to further understanding trace element chemistry, transport and bioavailability. This project aims to address some of these uncertainties by validating a methodology coupling flow field-flow fractionation (FlFFF) and atomic force microscopy (AFM) to quantify the shape of nanocolloids and their permeability (to solute and solvent molecules). Information about these structural measures can be contained within a simple ratio, usable in further modelling studies on speciation and bioavailability are essential to better fundamental understanding of the environmental 'function' of nanocolloids in trace element behaviour. The area of investigation is analogous to research over the last century into the structure-function relationships of biological macromolecules such as proteins and genetic material.


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Description A miniaturized diffusive gradients in thin films (DGT) device with a sampling window which is 16 times smaller than the conventional DGT device was developed. Its main advantage is the reduced volume of solution necessary for deployment, which extends its potential applications to samples from FFF. The design of the miniaturized DGT device was also improved compared with the conventional DGT device, such that the area of the diffusive gel layer that allows the diffusion of metal towards the binding gel layer has the same diameter as the opening in the cap of the device and no additional metal is accumulated due to lateral diffusion as is the case for the conventional device. A good correlation between the metal concentration directly measured and the concentration estimated with the help of the DGT equation was obtained using this device in synthetic solutions (10 µg L-1 Cd, 0.01 M NaNO3, pH 6). The capacity of the device for Cd was ~20 µg, potentially allowing long term measurements to be performed. Metal accumulation increased linearly with deployment time up to 28 h and showed the theoretically expected dependence on diffusive layer thickness. The diffusive boundary layer thickness of ~0.2 mm in stirred solution was considered for accurate quantification of concentration. There was no difference between concentrations of labile metal measured in natural waters using the new miniaturised devices and the conventional devices.
Exploitation Route The miniaturized DGT devices can be used for chemical speciation measurements in small environmental samples and body fluids.
Sectors Agriculture, Food and Drink,Environment,Healthcare