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.