Hydrothermal synthesis of biobased carbonaceous composite from a blend of kraft black liquor and tannin and its application to aspirin and paracetamol removal

Abstract An alternative use for the Kraft Black liquor, the main byproduct of the pulping process, is presented in this work. It is a promising route for new biorenewable materials production, reducing dependence on petrochemicals. The synthesis of the novel biobased composites from a blend of tannin and Kraft black liquor involved a polycondensation reaction followed by a crosslinking process. This paper is the first one emphasizing the synthesis of a novel biobased composite from a blend of Kraft black liquor and tannin by performing the conventional and hydrothermal crosslinking. The hydrothermal conditions proved to be an effective strategy to obtain an oxygen-rich functional material. Besides being environmental-friendly, the hydrothermal process successfully replaced conventional curing, providing a higher yield and better chemical, functional, and structural properties. The role of two different processes, hydrothermal and pyrolysis, on the properties and adsorption performance of the final composites, was also assessed. RFA was obtained by hydrothermal conversion, processed at relatively mild conditions when compared to the pyrolysis performed to produce its carbon counterpart, RFC. Pyrolysis promoted a mesoporous structure on the RFC and the textural properties were improved. On the other hand, the severe thermal treatment degraded the RFC functional groups, increasing its basicity and decreasing the number of polar groups on the surface. RFA exhibited a smaller surface area than RFC, but it retained volatile organic matter and functional oxygen groups. These surface characteristics are known to be relevant in adsorption processes since the adsorbate-adsorbent chemical linkages and electrostatic interactions are favored. Paracetamol and aspirin adsorption onto RFA achieved the equilibrium at 480 and 720 min. On the other hand, the equilibrium for the adsorption onto RFC was achieved at 720 and 240 min. The maximum aspirin adsorption capacities onto RFA and RFC were 50.17 mg g−1 and 91.66 mg g−1, respectively. On the other hand, for paracetamol, the maximum adsorption capacities were 49.95 mg g−1 onto RFA and 73.58 mg g−1 onto RFC. The kinetic study suggested fast adsorption through a chemisorption process and the equilibrium implies the multilayer adsorption. Additionally, the major advantages of using the raw Kraft black liquor blended with tannin to synthesize the novel composites are their great availability and low cost, enabling the material large-scale production. Therefore, the results confirm the great potential of these new biomaterials as alternative adsorbents for wastewater treatment.