Tuning the carboxyl content of carbon quantum dots by an electrochemical exfoliation method to enhance Fenton-like reactions for contaminants removal in a wide pH range

Abstract

The complexation of CQDs with Fe(III) through -COO-Fe bond in the CQDs-Fe(III)/H2O2 system can accelerate the Fe(III)/Fe(II) cycle and broaden the pH range for application. However, the yield and carboxyl content of CQDs prepared by conventional electrochemical exfoliation were limited, which hampered the application of CQDs and the enhancement of CQDs Fenton-like process. Hence, in this study, carboxyl-rich and high-yield CQDs were successfully prepared by electrochemical exfoliation of graphite rods in hydrogen peroxide (H2O2) solution. The results suggested that the yield and carboxyl content of the optimal CQDs (CQD@5) were 1.49 and 2.01 times those of CQDs prepared by the conventional electrochemical exfoliation method (CQD@0), respectively. Subsequently, the CQDs were applied in CQDs-Fe(III)/H2O2 systems for phenol degradation. Due to 1.73 times the carboxyl of CQD@ 5 complexing with Fe(III) than that of CQD@ 0, the kinetic constant and removal of the CQD@ 5-Fe(III)/H2O2 system for phenol degradation were 2.36 and 1.44 times those of the CQD@ 0-Fe(III)/H2O2 system at pH= 6, respectively. Under the same condition, Fe(III)/H2O2 and Fe(II)/H2O2 showed negligible degradation performance. Furthermore, the removals of phenol in the CQD@ 5-Fe(III)/H2O2 system were 1.08–7.77 times those of the CQD@ 0-Fe(III)/H2O2 system at pH= 4–8, respectively. Finally, the effect of Fe(III), CQDs, and H2O2 dosages on the phenol degradation, reusability, and universality of CQD@ 5-Fe(III)/H2O2 were investigated individually. In summary, this work provides a new method for tuning the CQDs yield and carboxyl content, which enhances pollutant degradation in CQDs-Fe(III)/H2O2 systems at a wide pH range.