Colloids and Surfaces A: Physicochemical and Engineering Aspects 155 (1999) 287 – 310 Characteristic features of the major components of freshwater colloidal organic matter revealed by transmission electron and atomic force microscopy Kevin J. Wilkinson a, *, Eric Balnois a , Gary G. Leppard b , Jacques Buffle a a CABE (Analytical and Biophysical Enironmental Chemistry) Uniersity of Genea, 30 Quai Ernest Ansermet, Genea 4, CH-1211 Switzerland b National Water Research Institute, Aquatic Ecosystem Protection Branch, Burlington ONT L7R 4A6, Canada Received 3 August 1998; accepted 27 November 1998 Abstract Organic biopolymers such as humic substances and polysaccharides account for the majority of freshwater NOM. Their role in natural systems is largely dependent upon their supramolecular microscopic structure which cannot be determined by bulk chemical measurements alone. Microscopic techniques were developed so as to permit the systematic observation of several colloidal-sized organic macromolecules with variable structures. This paper describes the characteristic structures of some reference compounds representative of the major organic components of natural waters. Polyacrylic acid, alginic acid and schizophyllan in addition to humic substances and polysaccharides isolated from natural freshwaters were examined by transmission electron microscopy (TEM) and atomic force microscopy (AFM). The techniques were optimized for the observation of the aquatic biopolymers. Colloidal organic matter isolated from natural freshwaters was also observed by TEM and AFM and compared with the standard images of colloidal organic matter. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Colloids; Aggregates; Natural organic matter; Macromolecules; Humic substances; Polysaccharides; Transmission electron microscopy; Atomic force microscopy; Freshwater www.elsevier.nl/locate/colsurfa 1. Introduction In the aquatic environment, the transport and bioavailability of both nutrients and toxic com- pounds are regulated by the extent to which they are partitioned among the soluble, colloidal or particulate fractions of the water column, i.e. their chemical speciation [1,2]. Natural organic matter (NOM) will play an important role in element bioavailability, either by providing potential bind- ing sites for trace elements [3], by modifying the stability of the inorganic phases to which they are bound [4–6] or by binding directly to the target organism [7]. Indeed, while NOM may facilitate the transport of metallic and hydrophobic pollu- * Corresponding author. Tel.: +41-22-7026051; fax: +41- 22-7026069. E-mail address: kevin.wilkinson@cabe.unige.ch (K.J. Wilkinson) 0927-7757/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII:S0927-7757(98)00874-7