Kraft lignin depolymerization by the Fenton process assisted by hydrodynamic cavitation†

Lignin is a highly branched and hardly biodegradable macromolecule that is mass-produced mainly as a byproduct of the pulp and paper industry. Lignin-containing wastewater generally confers high levels of chemical oxygen demand (COD) and presents a threat to aquatic life forms. As an alternative, lignin-containing effluents can be treated through Advanced Oxidation Technologies (AOTs) using the Fenton reaction, which involves the action of hydrogen peroxide (H₂O₂) and iron ions. In this context, a Box–Behnken 2³ experimental design was employed to optimize lignin degradation by the Fenton process assisted by hydrodynamic cavitation. Reactions were conducted in a batch reactor (V = 2.5 L), and a kinetic study of lignin degradation was performed in order to characterize and assess the efficiency of the oxidative process. The results revealed that the AOT assisted by hydrodynamic cavitation (HC) was highly efficient in lignin depolymerization. A quadratic model developed for the response variable lignin concentration (mg L⁻¹) exhibited a correlation coefficient (R²) of 0.918, indicating the model's quality and its ability to confidently predict the value of the variable with the greatest influence on the depolymerization process. Under the optimized conditions ([Fe³⁺] = 25.0 mM, [H₂O₂] = 272.9 mM, P = 1.78 atm), 98% conversion in the lignin depolymerization reaction was achieved in 2 hours of processing. The chemical structures of the depolymerized lignin were studied using pyrolysis coupled with gas chromatography-mass spectrometry (Py-GC/MS), aiming to identify and quantify the monomers and dimers obtained in the depolymerization process. The main compounds identified were benzyl alcohol, guaiacol, 2-methoxy-4-methylphenol, vanillic acid, and syringaldehyde. Furthermore, P. putida expressed growth in a culture medium supplemented with low-aromatic-weight lignin molecules after depolymerization. These results highlight that lignin utilization is crucial for the development of biorefineries and the optimization of the production chain. However, further research is needed to refine the process and recover the bioproducts derived from kraft lignin.