The role of colloidal phase in the measurement of pollutants in natural waters

Abstract

The components of natural water can be divided into three categories: dissolved, colloidal and particulate phases. The colloids are small particles with at least one dimension between 1.0 nm and 1.0 µm (IUPAC); they do not sediment when suspended in “solution”. The small size of colloidal particles causes this fraction to have a large surface area with potential to bind significant amounts of trace metals and organic pollutants and also allows colloids to migrate over long distances potentially increasing the spread of the contaminants. To better understand the behaviour of colloidal matter in natural waters, engineered spherical silica colloids of known size were made and chemically labelled so that the colloids can be sensitively tracked through the fractionation process of analytical filtration. The labels used to date have included the toxic metalloid arsenic and the fluorescent compound Rhodamine B, which had also been used to improve sensitivity. The behaviour of engineered labelled colloidal sols has been investigated during the preliminary filtration study in the laboratory. Artificial colloidal sols of the silicas in water were investigated to see how their filtration, employing a sequential filtration of a water sample through 1.6 (or 8 µm) and 0.1 µm cut-off filters, influenced a measurement of arsenic. The content of the filters after filtration were investigated by scanning electron microscopy and analysed for arsenic using atomic absorption spectroscopy. Ca. 50% and 65% of the silica particles were found on the large pore filters (1.6 µm and 8.0 µm filter), providing evidence of the aggregation and clogging effects on the fibre filter. The analysis of the Tamar estuary water samples revealed substantial amounts of arsenic, attributed to drainage from mining activities. Colloidal arsenic was found along with dissolved and particulate arsenic in freshwater. However, the saline water samples showed the arsenic content to be distributed mainly in the particulate and dissolved fractions. A suggested explanation of this phenomenon is that aggregation of colloids took place in the presence of saline water and therefore most of the colloidal arsenic was collected with the particulate fraction. Finally, a study of Southampton Water and Dart waters was performed by adding engineered colloidal particles to explore the aggregation of colloids in water of different salinity. Unfortunately, experimental problems prevented a definite verification of the influence of salinity in this case.

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ORCID for John R. Owen: orcid.org/0000-0002-4938-3693

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