Conformational Arrangement of Dissolved Humic Substances. Influence of Solution Composition on Association of Humic Molecules

Abstract

Both the primary chemical structure and the conformational structure of humic substances are still a matter of debate. A traditional assumption is that humic substances are large polymers and may present linear or coiled conformations according to solution properties. We studied the conformational changes of humic and fulvic acids of different chemical nature by high-pressure size-exclusion chromatography (HPSEC) after dissolution in mobile phases differing in composition but constant in ionic strength (I = 0.05). Modification of a neutral mobile phase (0.05 M NaNO3, pH 7) by addition of methanol (4.6 × 10-7 M, pH 6.97), hydrochloric acid (<2 × 10-6 M, pH 5.54), and acetic acid (4.6 × 10-7 M, pH 5.69) produced, in the order, a progressive decrease in molecular size. Size diminishing was shown by increasingly larger elution volumes at a refractive index detector and by concomitant reductions of peaks absorbance at a UV−vis detector. The decrease of molecular absorptivity (the phenomenon of hypochromism) proved that size reduction of dissolved humic substances was due more to disruption of an only apparent high-molecular-size arrangement into several smaller molecular associations than to coiling down of a macromolecular structure. The most significant conformational changes occurred in acidic mobile phases where hydrogen bondings formation was induced, suggesting that the large and easily disruptable humic conformation was held together predominantly by weak hydrophobic forces. The size of molecular association varied with humic samples indicating a close relation between humic chemical composition and stability of conformational structure. Our results show that humic substances in solution are loosely bound self-association of relatively small molecules, and intermolecular hydrophobic interactions are the predominant binding forces. The stability of such a conformation in solution is attributed to the entropy-driven tendency to exclude water molecules from humic association and thus decrease total molecular energy. This model of dissolved humic substances based on the reversible self-association of small molecules rather than on the macromolecular random coil represents a new understanding that should contribute to predict the environmental behavior of contaminants in association with natural organic matter.

Facilitating Effects of Metal Cations on Phenanthrene Sorption in Soils

“…As a result, the sorption of phenanthrene to the samples was promoted and the nonlinearity of phenanthrene sorption was strengthened (10,13,24). At the low concentration (i.e., 0.01 mol/L CaCl 2), the added Ca 2+ cations might not result in the complete flocculation of DOC as well as the condensation of other rubbery OC on soil surfaces and a considerable amount of DOC still existed in the soil solutions.…”

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

“…In general, HS aggregation is favoured by high HS concentrations, high electrolyte concentrations (typically > 100 mM), relatively low pH values (typically pH < 3-4 for FA, pH < 6-7 for HA) or by the presence of divalent cations (Conte and Piccolo 1999;Leenheer et al 2001;Leenheer and Croue 2003). These conditions are similar to those which support the aggregation of the mineral particles.…”

“…HS aggregate formation is favored at high concentrations of electrolyte, protons (i.e. low pH), HS or divalent cations (Conte and Piccolo 1999;Leenheer et al 2001;Leenheer and Croue 2003). Although HS are sometimes represented as linear polyelectrolytes, the linear representation of the HS does not fit with either the experimental measurements discussed above or the more modern models that consider HS as highly branched molecules (Schulten et al 1998).…”

Sorption interaction of phenanthrene with soil and sediment of different particle sizes and in various CaCl2 solutions

“…Furthermore, the extent of HOC sorption shows a strong dependence on the solution parameters, such as pH [24][25][26][27][28][29], ionic strength (I ) [23,[26][27][28], electrolyte cation valence [26,29,30], humic concentration [26,31], cosolute [32], and others. The cation, such as H + and Ca 2+ , can interact with carboxyl/hydroxyl functional groups on dissolved humic substances and mineral surfaces and reduce the negative electrostatic repulsion between the humic molecules and mineral interfaces.…”

“…A decrease in the cation concentration will result in both dissolution and desorption of organic matter, which in turn affects the sorption of hydrophobic chemicals to natural solids [26,29]. A change in pH or ionic strength of the soil solution leads to different configuration of dissolved humics ("open" or "closed"), which may subsequently alter the structure of organic matter on the mineral surfaces [25,27,30]. Murphy et al [27] have reported that the sorption capacities of hydrophobic organic compounds to mineral-associated peat humic acid is larger at low I than at high I because the "open" configuration of adsorbed humic substances at low I facilitates the accessibility of hydrophobic sorption sites with HOCs.…”