Arsenic Redox Changes by Microbially and Chemically Formed Semiquinone Radicals and Hydroquinones in a Humic Substance Model Quinone

Abstract

Arsenic is a redox-active metalloid whose toxicity and mobility strongly depends on its oxidation state, with arsenite (As(III)) being more toxic and mobile than arsenate (As(V)). Humic substances (HS) are also redox-active and can potentially react with arsenic and change its redox state. In this study we show that semiquinone radicals produced during microbial or chemical reduction of a HS model quinone (AQDS, 9,10-anthraquinone-2,6-disulfonic acid) are strong oxidants. They oxidize arsenite to arsenate, thus decreasing As toxicity and mobility. This reaction depends strongly on pH with more arsenite (up to 67.3%) being oxidized at pH 11 compared to pH 7 (12.6% oxidation) and pH 3 (0.5% oxidation). In addition to As(III) oxidation by semiquinone radicals, hydroquinones that were also produced during quinone reduction reduced As(V) to As(III) at neutral and acidic pH values (less than 12%) but not at alkaline pH. In order to understand redox reactions between arsenite/arsenate and reduced/oxidized HS, we quantified the radical content in reduced quinone solutions and constructed Eh-pH diagrams that explain the observed redox reactions. The results from this study can be used to better predict the fate of arsenic in the environment and potentially explain the occurrence of oxidized As(V) in anoxic environments.

Absence of humic substance reduction by the acidophilic Fe(III)-reducing strain Acidiphilium SJH: implications for its Fe(III) reduction mechanism and for the stimulation of natural organohalogen formation

“…In contrast to microbial reduction experiments with FA and HA at neutral pH where high radical concentrations were found (Jiang et al 2009), in our setups where solutions of 10 mg/ml FA were incubated with strain SJH at acidic pH, no radical formation could be detected by ESR measurements (data not shown). Moreover, incubation of FA with H 2 /Pd at pH 2.3 also did not lead to increasing concentrations of radicals, although increasing Fig.…”

Environmental pH and ionic strength influence the electron-transfer capacity of dissolved organic matter

“…But as the swelling or shrinking of DOM molecules caused by pH or ionic strength variation shall also result in accessibility changes of the redox-active functionalities, the ETC of DOM are expected to change with the conformational alteration (swelling and shrinking) of DOM molecules induced by changes in the pH and ionic strength. During their research on the redox reaction between arsenic and semiquinone radicals, Jiang et al (2009) observed that the oxidizing capacity of semiquinone radicals is strongly pH dependent: more arsenite (up to 67.3 %) was oxidized at pH 11 than at pH 7 (12.6 % oxidation) and pH 3 (0.5 % oxidation). Quinone moieties are the main electron acceptors in DOM (Scott et al 1998).…”

A Quantum Statistical Approach to Remediation Effect of Humic Substances

“…Organic free radicals are studied by electron paramagnetic resonance (EPR) spectroscopy (Tollin and Steelink 1966;Jiang et al 2009;Lodygin et al 2007;Spagnuolo et al 2005;Bialk et al 2005). The EPR-parameters of free organic radicals of humic acids (HA) and fulvic acids (FA) were estimated for a variety of samples taken from different surface waters and soils (Paul et al 2006;Jiang et al 2009). These authors showed that spin concentrations of the free organic radicals ranged from 0:64  10 17 spin  g À1 to 3:93  10 17 spin  g À1 for HA, and 0:54  10 17 spin  g À1 to 5:41  10 17 spin  g À1 for FA.…”

“…Applications of HS already exist in medicine (especially in pharmacology), water purification techniques, and agriculture (Jiang et al 2009;Paul et al 2006). Examination of the influence of HS on reversible red cell aggregation, using addition of HS to blood in vitro (Inisheva et al 2008), showed their substantial effect on aggregation of red cells in blood in dependency on the method of HS extraction from peat and sapropel and the HS concentration applied to blood.…”