(583w) Determining the Structure of Hydrothermal Char and Its Effect on Adsorption Capacity

Hydrothermal chars (hydrochars) are a novel carbon material
made from the thermal degradation of biomass under liquid water, at moderate
temperature and pressure. The moderate conditions and ability to take
wet-biomass provide the advantage of allowing hydrothermal carbonization to be
a highly tunable process that has produced raw biochars that have been used as
fuel, soil amendments, specialty chemicals and adsorbents for heavy metals. To
best refine this material and take advantage of how tunable the process is, a
better understanding of the structure of the molecule, and its structural
kinetics is needed. Studies investigating hydrochar have been conducted using Infrared
spectroscopy, NMR spectroscopy, and the corresponding hydrochar model
structures are heavily instrument dependent. This points for a need to
reconcile hydrochar structures determined from different methods.

A common theme in hydrochar models is the presence of highly
oxygenated polycyclic aromatic hydrocarbons (PAHs). Using density functional
theory (DFT) to simulate the Raman spectra of PAHs we have investigated the
vibrational modes that make hydrothermal chars, such as the characteristic D
and G bands often attributed to graphitic content. We have examined the effects
molecular size (i.e., number of aromatic rings), presence of functional groups,
orientation of PAH rings, and type of aromatic content (arene or furan) on the
positions and intensities of different vibrational modes. Using this
information we have developed a fitting method for the Raman spectra of
hydrothermal chars, which provides information about the aromatic and aliphatic
content of the hydrochars.

We then synthesized several glucose based hydrothermal chars
at 180 °C and different reaction times. These hydrochars were analyzed using
Raman spectroscopy, Infrared spectroscopy, Nuclear Magnetic Resonance,
elemental analysis and surface area. The materials were also used to conduct
copper adsorption studies, to determine how structural changes in the char influenced
its performance as an adsorbate. Chars produced at lower reaction times
appeared to also have a highest surface area and capacity for copper(II). On the other hand, the affinity for copper(II) (measured as a function of adsorption per surface
area) was shown to increase with reaction time, reaching a maximum for char
synthesized with a reaction time of 12 hours. Raman analysis indicated that the
char structure changed the most from 2-8 hours. After 8 hours of reaction, the
char structure became very stable and changed very little from 8-24 hour
reaction time, in parallel with the observed changes in copper surface affinity.
We conclude that Raman spectroscopy is a useful tool for the structural
determination of hydrochar, and that hydrochar has strong affinity for
copper(II), but optimization of the material as a adsorbent depends heavily on
optimizing surface areas, which tend to decrease with increasing char yield.