Sustainable upcycling of industrial black liquor into carbon dots via one-pot hydrothermal synthesis: the role of precursor concentration

Black liquor (BL) is a major and high-volume byproduct of the pulp and paper industry, presenting a significant environmental disposal challenge. This study aims to develop a one-pot hydrothermal route for upcycling BL into carbon dots (CDs). Four samples of CDs were synthesized at 200 $${}^\circ{\rm C}$$ for 24 h using varying BL volumes (5, 10, 15, and 20 mL), denoted as BL-CDs1, BL-CDs2, BL-CDs3, and BL-CDs4. The effect of BL concentration on the optical characteristics, surface chemistry, and thermal stability of BL-CDs was investigated. Optical analysis revealed strong UV absorption (279–283 nm) and blue fluorescence with an emission peak at 472–487 nm. XPS analysis confirmed improved carbonization and enhanced oxygen functionalities, particularly in BL-CDs3. XRD and FTIR analyses revealed an amorphous structure with a characteristic (002) graphitic peak at 2θ = 20–21° and surface oxygen groups. Raman spectroscopy revealed ID/IG ratios ranging from 1.07 to 0.67, indicating variations in graphitization among the BL-CDs. The DSC curves of the samples exhibited endothermic peaks in the range of ~ 120–180 $${}^\circ{\rm C}$$. TGA assessment demonstrated the highest stability for BL-CDs3 with 17.4% residue at 998 $${}^\circ{\rm C}$$. TEM showed spherical to quasi-spherical morphology (15.09 ± 1.94 nm) and a hydrodynamic diameter of 43.82 ± 5.27 nm (PDI = 0.398) of BL-CDs3. The BL-CDs3 exhibits desirable properties, highlighting its potential for applications in chemical sensing, bioimaging, and photocatalysis.