Determining the Electron Storage Capacities of Black Carbon and Humic Acid Through Chemical Redox Titration
Date
2017-05
Authors
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Publisher
University of Delaware
Abstract
Carbonaceous constituents in soil and sediment play critical roles in biotic and
abiotic geochemical redox reactions. In particular, black carbon and humic substances
supplying and receiving electrons are of key interest to researchers for their role in
facilitating redox transformation of contaminants either directly (i.e., abiotically) or
through microbial activity in soil and sediments.
Black carbon (BC) is a category of solid carbonaceous materials formed by
pyrolyzing biomass in an oxygen-deficient environment. As a commercially available
black carbon that aims to promote soil fertility and microbial activity, it has been
hypothesized that redox-active functional groups, such as quinone, are presented in the
carbon matrix, and that they can exchange electrons with external bulk reactants. In
this work, titanium(III) citrate, a colored, one electron transfer reducing agent, was
evaluated as a titrant reducing a commercial black carbon and a standard humic acid.
Through a series of batch reduction experiments and continuous monitoring of the
UV-Vis absorbance at 400 nm, the measured electron storage capacity (ESC) of the
black carbon was 4.3(±0.1) mmol electron/gram at pH 6.4, 4.1(±0.1) mmol
electron/gram at pH 5, and 4.2(±0.4) mmol electron/gram BC at pH 7.5.
In addition, to standardize titanium(III) citrate and to demonstrate that it has
the capability to fully reduce quinone functional groups (the redox active component
in black carbon and humic acid), 99% purity 1,4-benzoquinone was used to
quantitatively titrate titanium(III) citrate stock solution to standardize the exact
concentration of titanium(III) used prior to the ESC experiment. We show that the
reduction of anthraquinone-2,6-disulfonate (AQDS) and benzoquinone (BQ) are
kinetically facile processes; therefore, the slow kinetic of black carbon reduction is
ascribed to slow intra-particle diffusion in black carbon particles.
The ESC of humic acid was also estimated using similar titration applied on
black carbon. Leonardite humic acid (LHA), one of the most studied standardized
humic acid forms, was titrated using a similar approach. By observing the net change
of absorbance at 250 nm upon mixing known amounts of titanium(III) citrate and
LHA in solution, the electron transfer from titanium(III) citrate to humic acid can be
studied. The ESC of Leonardite humic acid was determined to be 4.7(±0.4) mmol/g
LHA at pH 6.4, 3.1(±0.3) mmol/g LHA at pH 5 and 4.1(±0.4) mmol/g LHA at pH 8.
An approach of redox reduction of black carbon are hereby proposed, and upon
acquirement of additional research progress, a protocol of black carbon titration can be
developed and enables researchers to determine the electron storage capacity of black
carbon synthesized using various materials and approaches in an efficient and
affordable manner.
Description
Keywords
Chemical engineering, Black carbon, Humic acid, Electron storage capapcity