Russian Journal of Applied Chemistry, 2012, Vol. 85, No. 9, pp. 1351−1354.
Pleiades Publishing, Ltd., 2012.
Original Russian Text © I.V. Eﬁ mova, S.L. Khil’ko, O.V. Smirnova, 2012, published in Zhurnal Prikladnoi Khimii, 2012, Vol. 85, No. 9, pp. 1436−1439.
AND CORROSION PROTECTION OF METALS
Antioxidant Activity of Humic Acids in Radical-Chain
I. V. Eﬁ mova, S. L. Khil’ko, and O. V. Smirnova
Litvinenko Institute of Physicoorganic and Coal Chemistry, National Academy of Sciences of Ukraine, Donetsk, Ukraine
Received January 23, 2012
Abstract—The behavior of humic acids in radical-chain oxidation processes was examined by the gas-volumetric
method using model hydrocarbons (cumene and ethylbenzene). It was shown that the liquid-phase oxidation of
hydrocarbons is inhibited in the presence of humic acids. The hydrocarbon oxidation rate depends on the humic
acid concentration. Owing to pronounced inhibitory properties, humic acids can be considered as promising natural
antioxidants for biomedical and technical (antioxidants for liquid fuels and oils) applications.
The search for new raw materials for manufacturing
chemical products is a pressing task. One of the
major classes of chemical products is represented by
antioxidants, oxidation inhibitors, which can be used for
medical and technical purposes. In medical applications,
much attention is dedicated to natural sources of
bioactive substances with antioxidant properties and to
development of new bioavailable drugs thereof . In
technical applications, much importance is attached to
development of efﬁ cient antioxidants from inexpensive
and readily available raw materials.
Formation of humic substances (humiﬁ cation) is the
transformation of organic matter in nature, exceeded
in scale only by photosynthesis. Humic substances
represent the most stable form of organic carbon
compounds existing outside of living organisms .
Today, humic substances are considered in the green
chemistry context as promising inexpensive and readily
available raw materials for manufacture of chemically
important products [3, 4].
In peats and brown coals, humic substances
constitute the predominant part of the organic mass,
%: up to 60 in peats, up to 20–60 in brown coals, and
up to 20 in soils. Chemically, humic substances from
various natural sources are polyoxypolycarboxylic acids
with the following elemental composition, %: C 50–60,
H 4–6, O 25–40 .
Humic acids (HAs) produced from peats and brown
coal are known to display pronounced biological
activity (hepatoprotective, antihypoxial, antitoxic, and
other properties) [6, 7]. The presence of active acidic
groups (–COOH, –OH) in the HA macromolecules
suggests their capability for antioxidant action. This
capability was revealed for humic acids from different
natural sources , but detailed published studies on
antioxidant properties of humic acids are lacking.
Here, we studied the antioxidant activity of humic
acids in radical–chain oxidation processes using model
Humic acids were prepared from an analytical
sample of brown coal from Aleksandriiskoe deposit
(Ukraine) by a single-step extraction procedure using
1 M NaOH, at the s : l = 1 : 8 and temperature of
20°C. Next, water-insoluble HAs were precipitated
from the “raw” extract by adding a 5% HCl solution
under permanent stirring to pH 1–2. The resulting HA
precipitate was separated from the supernatant liquid by
centrifugation and rinsed with distilled water to reach
neutral pH (6–7), after which humic acids were dried