Rapid and mild acid-induced recovery of high-yield carbon quantum dots from commercial black liquor

Abstract

Herein, carbon quantum dots (CQDs) are facilely recovered from commercial black liquor in high yield via a rapid and mild acid-induced strategy. Upon acidification with 60 wt% H2SO4 solution at 60 °C for 5 min, black liquor derived from a poplar/reed mixture (PR), a poplar/eucalyptus mixture (PE), and cornstalk (CS) afforded CQD yields of 22.0 wt%, 21.8 wt%, and 31.1 wt%, respectively. In addition, black liquor from PR produced 48.8 wt% lignin, resulting in a maximum combined CQD and lignin yield of 70.9 wt%. All synthesized CQDs exhibited excellent water dispersibility, favorable photoluminescent properties, and abundant surface functional groups, with quantum yields ranging from 8.09% to 15.04%. The photocatalytic application of CQDs was evaluated using modified TiO2 and g-C3N4 composites. The 1% CQDs-PE/TiO2 composite achieved a methylene blue (MB) degradation efficiency of 77% within 120 min, while CQDs/ g-C3N4 composites also exhibited enhanced degradation of the antibiotic ofloxacin (OFX). Furthermore, the Ru/CQDs catalyst demonstrated good electrocatalytic activity for the hydrogen evolution reaction (HER), requiring an overpotential of only 76.46 mV to reach a current density of 10 mA/cm2. This work provides a green, scalable, and economically viable approach for the valorization of black liquor, enabling the high-yield production of high-quality CQDs under mild conditions.