Role of Colloids in the Environement

• Collaborators
  • Water Studies Centre, Monash University, Australia
• Macaulay Contact
  • Charlie Shand (Soil Plant Microbial Interactions)

Colloids are microscopic particles with sizes in the range 1 to 0.01 µm. Because of their small size they tend not to settle out of suspensions, being influenced by Brownian motion and minor currents in the bulk of the solution. Colloids play an important role in the transfer of nutrients and pollutants in the environment over short and long distances. For example, colloids can transport nutrients such as phosphate through channels in soil to deeper horizons, as well as over much longer distances in surface waters. For chemical species strongly bound to soil, colloids can be the main vehicle for their transport. A knowledge of the physical and chemical composition of environmental colloids involved in nutrient and pollutant transport is important if the relevant processes are to be understood and accurately modelled.

Field-flow fractionation represents a family of techniques for the separation of particles, colloids and large molecules depending on a specific property. Sedimentation field-flow fractionation (sed-FFF) methods offers a means to fractionate colloids, with high resolution. In the normal mode of operation, separations are based on the buoyant mass and diffusion coefficients of the colloids, and on the differential parabolic flow of liquid in the thin channel of the sed-FFF separator (Figure 1). Monash University is a lead centre for such work, being equipped with state-of-the-art FFF devices and detectors such as a high resolution, inductively coupled, mass spectrometer (HR-ICP-MS). With a suitable interface the eluent from FFF devices can be analysed on-line by HR-ICP-MS, providing continuous information about elemental composition of colloids and their corresponding size/mass. Other variants of the FFF method include those based on charge (electrical-FFF), particle size (flow-FFF) and thermal diffusion (thermal-FFF).

To gain knowledge of the FFF methods and other procedures related to the characterisation of colloids, a 3-month visit, funded by the Institute and The Royal Society, was made to work with Dr R. Beckett and his group at the Water Studies Centre of Monash University in Melbourne, Australia. Experimental parameters were developed to allow sed-FFF separations of colloids derived from soils in Scotland. These soils had received sewage sludge contaminated with various heavy metals some years before and it was of interest to determine the distribution and availability of the heavy metals in different sized colloidal fractions. The size distribution of particles were determined by sed-FFF and a preliminary study was made of the elemental composition of the samples by graphite furnace atomic absorption spectroscopy on samples digested with aqua regia. This paved the way for experiments where the FFF device was connected on-line to the HR-ICP-MS. Samples of the colloids, before and after extraction with solutions such as dilute acetic acid (which is often used to determine metal bioavailability in acid soils) were analysed on-line and the distribution of elements determined and related to particle sizes. This work was carried out in collaboration with a research student. Although the amount of sample which can be loaded onto the FFF channel is small, discrete fractions of colloids eluting from sedimention-FFF can be dried and analysed by electron microscopy to determine their shape and by X-ray diffraction to determine mineralogy.

SPLITT is a separation technique closely related to sed-FFF and allows "splitting" of particles in suspension into groups of sizes (typical range 25 to 2 µm) on a continuous basis using a flat, thin channel (Figure 2) and differential migration under gravitational force. The technique has application for preliminary separation of particles for sed-FFF.

The Macaulay is equipped with sed-FFF and SPLITT devices.