Talanta 67 (2005) 144–154 Assessment of cross-flow filtration for the size fractionation of freshwater colloids and particles Frederic J. Doucet 1 , Leanne Maguire, Jamie R. Lead ∗ Division of Environmental Health and Risk Management, School of Geography, Earth and Environmental Science, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK Received 30 October 2004; received in revised form 19 January 2005; accepted 16 February 2005 Available online 23 March 2005 Abstract This research has evaluated the ability of cross-flow filtration (CFF) to perform correct size fractionation of natural aquatic colloids (materials from 1 nm to 1 m in size) and particles (>1 m) using scanning electron microscopy (SEM) combined with atomic force microscopy (AFM). SEM provided very clear images at high lateral resolution (ca. 2–5 nm), whereas AFM offered extremely low resolution limits (sub-nanometer) and was consequently most useful for studying very small material. Both SEM and AFM were consistent in demonstrating the presence of colloids smaller than 50 nm in all fractions including the retentates (i.e. the fractions retained by the CFF membrane), showing that CFF fractionation is not fully quantitative and not based on size alone. This finding suggests that previous studies that investigated trace element partitioning between dissolved, colloidal and particulate fractions using CFF may need to be re-visited as the importance of particles and large colloids may have been over-estimated. The observation that ultra-fine colloidal material strongly interacted with and completely coated a mica substrate to form a thin film has important potential implications for our understanding of the behaviour of trace elements in aquatic systems. The results suggest that clean, ‘pure’ surfaces are unlikely to exist in the natural environment. As surface binding of trace elements is of great importance, the nature of this sorbed layer may dominate trace element partitioning, rather than the nature of the bulk particle. © 2005 Elsevier B.V. All rights reserved. Keywords: Cross-flow filtration; Aquatic colloids and particles; Fractionation; Scanning electron microscopy; Atomic force microscopy 1. Introduction The chemical speciation, biological availability and res- idence time of trace metals in natural waters are primarily influenced by their interaction with and by the stability of colloids and particles. As a result, considerable effort has fo- cused on investigating such interactions with natural aquatic systems [1,2]. Natural aquatic colloids and particles have been defined as materials with sizes ranging between 1 nm and 1 m [3], and greater than 1 m, respectively. Colloids are ubiquitous in natural aquatic systems and are composed of phases, such as inorganic oxides (e.g. of aluminium, iron, ∗ Corresponding author. Tel.: +44 121 4148147; fax: +44 121 4145528. E-mail address: j.r.lead@bham.ac.uk (J.R. Lead). 1 Present address: Division of Food Sciences, School of Biosciences, Uni- versity of Nottingham, Sutton Bonington Campus, Loughborough LE12 5RD, UK. manganese and silicon), organic humic and fulvic substances and polysaccharides, carbonates, clays and microbes includ- ing viruses and bacteria. They are present in relatively low mass concentrations but at much higher number concentra- tions. The individual components are generally intimately associated with each other to form complex mixtures [4]. However, their heterogeneous character, their easily dena- tured structure, their instability, their small size and low con- centration are the main causes of the difficulty in sampling, separating and characterising them. Reliable, unbiased and minimally perturbing methods for their handling are there- fore primary requirements if accurate information is to be obtained. In recent years, a number of fractionation methods have been developed and used on natural systems (split-thin flow fractionation (SPLITT) [5], field-flow fractionation (FFF) [6,7] and cross-flow filtration (CFF)) [8–10]. In particular, CFF has become the most important and most widely used 0039-9140/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.talanta.2005.02.026