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RESEARCH ARTICLE
Contents Vol 6(2)

The influence of aggregation on the redox chemistry of humic substances

Noel E. Palmer A and Ray von Wandruszka A B
+ Author Affiliations
- Author Affiliations

A Department of Chemistry, University of Idaho, Moscow, ID 83844-2343, USA.

B Corresponding author. Email: rvw@uidaho.edu

Environmental Chemistry 6(2) 178-184 https://doi.org/10.1071/EN08081
Submitted: 22 October 2008  Accepted: 6 March 2009   Published: 27 April 2009

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.

Additional keywords: abiotic reduction, dynamic light scattering, excitation–emission matrix.


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