Kinetic analysis of kraft lignin conversion via the Fenton process: process optimization and stochastic modelling†

Lignin is a macromolecule with a highly branched and complex structure, making it difficult to degrade. It is a by-product of the pulp and paper industry and extensive treatment is required to mitigate environmental issues associated with effluent discharge. As an alternative, lignin can be treated through advanced oxidative processes (AOPs) using the Fenton reaction, which involves hydrogen peroxide (H₂O₂) and iron ions. In this context, a rotational central composite design (RCCD) was conducted to optimize lignin degradation using different molar ratios of H₂O₂/Fe²⁺ and H₂O₂/Fe³⁺ to assess the synergistic catalytic action of ions. The reactions were conducted in a batch reactor (2 L capacity), and a kinetic study of lignin degradation was performed using a stochastic model to characterize the oxidative process. Optimized conditions for the Fenton reaction were predicted, adopting a molar ratio of H₂O₂/Fe²⁺ of 9.0 and H₂O₂/Fe³⁺ of 6.0. The optimal conditions resulted in a 47.3% reduction in total organic carbon (TOC), reaching a conversion of over 80% in the depolymerization process. A quadratic model performed for the response variable TOC reduction showed a correlation coefficient (R²) of 0.926, indicating the model's quality and its ability to predict the variable with the greatest influence on the lignin depolymerization process. Further, Pseudomonas putida exhibited growth on low-molecular-weight aromatic molecules after depolymerization of kraft lignin.