Innovative valorization of spent liquors from hydrothermal carbonization for light-induced nanoparticle synthesis

To develop the biorefinery concept around the hydrothermal carbonization (HTC) process, it is essential to take advantage of the spent liquor (SL) produced. In general, SLs were reported to be treated by anaerobic digestion; however, some molecules present in SL are considered biological inhibitors. Nonetheless, this might be advantageous as these molecules exhibit antioxidant activity, opening a new valorization alternative as green reducing and capping agents for particle synthesis. Among the hydrothermally treated biomasses, the SL from Jatropha fruit husk (JFH) presented the highest antioxidant activity (TEAC ∼ 1597 µM g⁻¹), increasing its capability to form metal nanoparticles due to their reducing effect. Still, nanoparticle aggregation was observed, evidencing its lack of capping effect. In this sense, the present study provides a novel approach to recovering the organic compounds from HTC spent liquor (OC-HTCSL) in viscous concentrates by evaporating the remaining water. Surprisingly, the JFH sample showed the lowest antioxidant activity and phenolic content after concentration. Nevertheless, this concentrate reflected a capping effect for the LED-assisted synthesis of silver nanoparticles. Stable silver colloids containing nanoparticles with an average diameter of 17 nm were successfully obtained. These silver nanoparticles retained their original morphology after being stored for six months. This research suggests that a lower antioxidant activity and phenolic content of the OC-HTCSL concentrate might be more effective in forming silver nanoparticles.