Valorization of Humic Acids by Hydrothermal Conversion into Carbonaceous Materials: Physical and Functional Properties

Humic acids (HAs) represent an economic and environmental challenge in water treatment, as they have the propensity to foul membranes and create toxic byproducts when interacting with chlorine. To overcome this, HAs were submitted to hydrothermal carbonization to convert them into an easy to remove, valuable carbon material. The result was a carbonaceous material which was easy to filter/dewater compared to HAs with a char yield of 49 ± 1.8 wt %, and with 46.6 ± 1.4 wt % ending up in the water phase, 2.2 ± 0.2 wt % in the tar, and the rest in the gaseous fraction. The molecular weight distribution of the organic matter in the water pre- and post-HTC indicated that the structure was broken into several different fragments with a lower molecular weight than that initially present. Physicochemical analysis of the material via elemental analysis, X-ray photoelectron spectroscopy, diffuse reflectance infrared Fourier transform spectroscopy, and solid-state nuclear magnetic resonance indicated that under hydrothermal carbonization, the aromatic structure of HAs condensed. Carboxylic acids groups were also lost from the surface of HAs, with ether and alcohols increasing because of their loss. The morphology of the obtained material had an amorphous macrostructure consisting of many smaller light lamellar carbon fragments. Finally, the hydrothermal treatment increased the surface area from 0.4 to 103.0 m² g^–1. The porosity is located in the mesoporous range of 10–80 nm with a maximum peak at 50 nm.