The Influence of Sample Preparation on Observed Particle Size Distributions for Contrasting Soil Suspensions using Flow Field-Flow Fractionation

Abstract

Environmental Context. Colloids are an important environmental means for contaminant transport from land to water. Within the agricultural landscape, the mobilization of phosphorus species by soil leachate and drainage waters is a particularly important example. Colloidal material is, however, difficult to isolate and characterize, due in part to the dynamic nature of environmental systems and colloidal structure and in part of the lack of suitable analytical techniques. Field-flow fractionation is one emerging tool for the investigation of colloidal fractions in environmental matrices and for assessing how soil processes impact on water quality. Abstract. This paper reports the use of flow field-flow fractionation (FlFFF) as a suitable technique to characterize particle size distributions (PSDs) in the <1-μm fraction of ‘colloidal’ soil suspensions prepared from two contrasting soil types (one mineral rich and one organic rich). Both soils have been fully characterized and the <1-μm fraction obtained by gravitational settling of 1% w/v soil suspensions. The effect of shaking and settling times on observed PSDs was systematically investigated and revealed that gentle shaking for 16 h followed by gravity settling for 1 h gave reproducible results. Settling for longer periods resulted in re-aggregation of smaller particles and a shift in the PSDs to larger mean particle diameters. Changes in soil suspension concentration (0.25–1% w/v) had no significant effect on PSDs. Further fractionation of the gravitationally settled <1-μm fraction by centrifugation and filtration (to < 0.45 and < 0.2 μm) clearly showed that a significant component in each fraction was removed by the filtration process. This was true for both soil types and confirms that membrane filtration seriously underestimates the colloidal fraction in aquatic samples. These findings highlight the need for standardized and well-documented protocols for processing soil suspensions.