AFM study on the adsorption and aggregation behavior of dissolved humic substances on mica

Abstract

Humic substances constitute the major organic fractions of soils, sediments and natural waters, and play a dominant role in the binding, mobilization, transport and ultimate fate of organic contaminants in subsurface systems. In this paper, two humic acid samples, Guanting and Tianjin, with different origin and chemical compositions have been investigated with AFM imaging for their adsorption and aggregation behaviors on mica. While the Tianjin humic sample is found to form small spheres with 250 to 330 nm in diameter at lower concentrations, irregular loop-chain assemblies of hundreds of nanometer in diameter with the chain width of about 40 nm are dominant for the Guanting humic sample, which may attribute to the more polar aliphatic fractions in the chemical composition in the latter. The heterogeneous and polydisperse nature of humic substances with multiple structural features, such as sponge-like structures, perforated sheets, aggregate of spheres, branches and chain-like assemblies etc., is apparent at higher concentrations for both humic samples, showing morphologically new evidence for the dominant view of the humic dual-mode sorption model. With naphthalene introduced, the assemblies of Guanting humic substances clearly become more compact with significantly narrowed branches and less porous the perforated sheet-like structures. It is indicative of that smaller nanometer scale rings present along the perforated assemblies could potentially represent hydrophobic domains, which may facilitate the adsorption and aggregation of naphthalene onto the natural particle surfaces and therefore lead to an important role of dissolved humic substances in the sorption of environmental pollutants.Keywords: atomic force microscopy (AFM), dissolved humic substances, morphology, aggregation, adsorption.Humic substances (HS) are a category of naturally occurring, biogenic, heterogeneous organic materials found in or extracted from soils, sediments, and natural waters that can generally be characterized as being yellow-to-black in color, of highly variable relative molecular masses, and refractory [1,2] . Derived from a variety of organic precursors (plant biopolymers such as lignin etc.), plant residues and animal debris via both transformation and synthesis processes [3] under the profound geochemical alteration, including the microbiological and chemical degradation, sedimentation and diagenesis as well, humic substances com-AFM study on the adsorption and aggregation behavior of dissolved HS on mica 257 prise highly complex composites of partially or completely degraded biomolecules, carbohydrates, lipids, and proteins etc. However, they cannot be classified into any of the normal, easily defined categories of discrete materials such as proteins, polysaccharides, and polynucleotides. As the major organic constituents of soils, sediments and waters, humic substances are ubiquitous in all terrestrial and aquatic environments. They can form molecular aggregates, dispersed in different colloidal states in solutio...

Aging effects on sorption–desorption behaviors of PAHs in different natural organic matters

“…And they proposed the likely mechanism that the non-labile fraction was a consequence of a change of conformation of the HA after the binding of pentachlorobenzene [17]. Ge et al observed a more compact conformation of humic substance assemblies after the sorption of naphthalene using atomic force microscopy [40]. As a result, it is possible that the conformation of DOM was changed after binding with solute, which resulted in resistant desorption.…”

Distribution of barium and fulvic acid at the mica–solution interface using in-situ X-ray reflectivity

“…However, the opposite effect occurs for anionic metal species such as (AsO 4 ) 3À or (SeO 3 ) 2À which compete with HS for sorption sites Grafe et al, 2001). The morphology of HS sorbed on mineral surfaces, especially mica, has been studied using atomic force microscopy (AFM) to reveal aggregation of organic matter with and without aqueous metal ions (Namjesnik-Dejanovic and Maurice, 1997;Balnois et al, 1999;Maurice and Namjesnik-Dejanovic, 1999;Plaschke et al, 1999;Liu et al, 2000;Ge et al, 2006;Guo and Ma, 2006;Gibson et al, 2007). Using a tapping-mode AFM technique in 20-100 mg C L À1 of NOM solution, Namjesnik-Dejanovic and Maurice (2000) observed that NOM adsorbed on the mica surface from a 100 mM LiCl solution as aggregates with flattened spherical shapes of lateral diameter 1.9-15 nm and thickness of 0.4-0.9 nm.…”

Assessing desorption resistance of PAHs in dissolved humic substances by membrane-based passive samplers

“…Also as shown in the NMR results, large amounts of aliphatic groups exist in Aldrich HA while FA is composed of relatively small oligomeric molecules containing predominantly polar and ionizable groups. It has been reported that larger macromolecules of HA that are held together by weak interactions can form a more compact conformation assembly after the sorption of PAHs and may form aggregated hydrophobic regions that are more effective in binding [13,34,35]. As stated above, we can draw the conclusion that the M W of HS might play an important role in binding of PAHs to dissolved humic and fulvic acids.…”