The influence of aggregation on the redox chemistry of humic substances

Abstract

Environmental context. The ability of humic substances (decaying plant and animal matter) to partake in redox reactions in the environment depends on the extent to which the various humic polymers aggregate in solution to form larger particles. This aggregation, in turn, is predicated on the solution conditions, especially ionic strength, the pH, and the types of cations present. Abstract. Aggregation and conformation play an important role in the aqueous redox chemistry of humic substances (HS). The reduction potentials of dissolved humic and fulvic acids vary with pH, ionic strength, and type of humate used, and depending on the solution conditions, they can abiotically reduce various species. Changes in HS reduction potential ranged from 60 to 140 mV on addition of divalent cations, whereas no significant changes were observed with equivalent additions of monovalent cations. Dynamic light scattering measurements showed that this behaviour paralleled the size changes obtained with humic aggregates under the same conditions. The effect was more pronounced at higher pH, where divalent cations caused a significant decrease in the average hydrodynamic radius, whereas monovalent cations did not. At pH 4, neither mono- nor divalent cations substantially affected aggregate sizes. Quinoid moieties, which are known to play an important role in the redox chemistry of HS, displayed fluorescence excitation–emission matrices with features related to changes in the reduction potential of HS. An increase in the reduction potential (Eh) induced by the addition of Ca2+, for instance, caused a red shift in the excitation–emission matrix maximum.

Partitioning of Humic Acids between Aqueous Solution and Hydrogel. 2. Impact of Physicochemical Conditions

“…Increasing the ionic strength apparently enhances the screening of intraparticulate HA charges and thus favors hydrophobic interactions, which generally leads to increased particle aggregation (Figure ). Other studies have revealed similar dependencies of the size of HA aggregates on electrolyte concentration. , …”

“…Other studies have revealed similar dependencies of the size of HA aggregates on electrolyte concentration. 19,26 Effect of pH on the Size Distribution of HA Particles. For an ionic strength of 10 mol m −3 , the changes in particle radius with varying pH are modest (Table 1).…”

The Effects of Conformational Changes on the Native Fluorescence of Aqueous Humic Materials

“…For this reason, fluorimetric investigations of humates involving strongly emitting probes are often more fruitful than those relying on native emissions [2]. One technique that does use the native fluorescence to good effect is total fluorescence spectroscopy in which excitationemission matrices are generated that provide fingerprint-like identification for mixtures of fluorophores [3].…”