Determination of Electrophoretic Mobilities and Hydrodynamic Radii of Three Humic Substances as a Function of pH and Ionic Strength

Abstract

Capillary electrophoresis (CE) and fluorescence correlation spectroscopy (FCS) were employed to determine electrophoretic mobilities and hydrodynamic sizes of three humic substances (IHSS aquatic fulvic acid (FA), IHSS aquatic humic acid (HA), and IHSS peat humic acid (PHA)) as a function of pH and ionic strength. A slight aggregation corresponding to the formation of dimers and trimers was observed at low pH using fluorescence correlation spectroscopy (FCS). For example, for the peat humic acid, diffusion coefficients decreased from 2.1 x 10(-10) m2 s(-1) at pH 4 to 2.4 x 10(-10) m2 s(-1) at pH 11. For all three humic substances, electrophoretic mobilities were also shown to decrease significantly below pH 6. Calculated zeta potentials observed at high pH of -69 mV (FA), -62 mV (HA), and -63 mV (PHA) decreased to -39, -50, and -47 mV, respectively, under slightly acidic pH (4.5-4.8) conditions. No evidence of ionic strength induced aggregation was found using fluorescence correlation spectroscopy (FCS); diffusion coefficients increased slightly (<25%) with increasing ionic strength (up to 1 M). Negative electrophoretic mobilities decreased to a maximum measured ionic strength of 0.18 M. Above this ionic strength, no peaks were observed due to an increased HS adsorption to the capillary wall and an important decrease in electroosmotic flow. Interpretation of electrophoretic mobilities determined by CE is complicated by the fact that under certain conditions, HS appeared to be complexed by CE buffer systems, including MES, BES, and AMPSO.

Contrasting Roles Of Natural Organic Matter On Colloidal Stabilization And Flocculation In Freshwaters

“…L'acide fulvique a une masse molaire moyenne de 711 g mol -1 (Goldberg et al 1995) avec un rayon hydrodynamique d'environ 1 nm selon le pH (Knöchel et al 1997;Fil et al 2000). Les AF sont considérés comme des oligoélectrolytes sous la majorité des conditions environnementales et sont négativement chargées (Hosse et al 2001).…”

“…3A) and typical average lifetimes of several hundred years. Due to the significant degree of branching of HS, their high charge density (above pH 5; (Hosse and Wilkinson 2001)) and degree of hydration (40% strongly bond water; (Buffle 1988)), they are less flexible than linear biopolymers such as proteins. In dilute solutions (< 10 mg L -1 ) at circumneutral pH and low ionic strength (≤ 10 -2 M), HS generally behave as small rigid globules, with diameters of 1-3 nm (Figs.…”

“…In dilute solutions (< 10 mg L -1 ) at circumneutral pH and low ionic strength (≤ 10 -2 M), HS generally behave as small rigid globules, with diameters of 1-3 nm (Figs. 3A, 3B; (Goldberg and Weiner 1994;Balnois et al 1999;Hosse and Wilkinson 2001)). HS aggregate formation is favored at high concentrations of electrolyte, protons (i.e.…”

“…8), in the presence of multivalent ions (Plaschke et al 2002), or in the solid state, such as in soils. Indeed, homoaggregates of HS have been observed by light scattering Tombacz et al 1997), turbidmetry (Senesi et al 1997), fluorescence spectroscopy (Engebretson et al 1996), atomic force microscopy ) and fluorescence correlation spectroscopy , even in the pH range 5-8 for which COOH groups of the HS are predominantly dissociated (Hosse and Wilkinson 2001). Under these conditions, HS can be considered as charged globules and DLVO theory would appear to be at least partly applicable (Tombacz and Meleg 1990), although for such small, chemically heterogeneous colloids, hydration effects, hydrogen bonding, non polar interactions, polyvalent cation interactions (Kam and Gregory 2001) and reversible aggregation must also be considered.…”

“…The IHSS fulvic acid has a number average molar mass of 711 g mol -1 (Goldberg and Weiner 1995) with a hydrodynamic radius of about 1 nm depending on the pH (Knöchel et al 1997;Balnois et al 1999;. The molecules are oligoelectrolytes under the majority of environmentally relevant conditions and negatively charged (Hosse and Wilkinson 2001).…”