In Situ Electrocoagulation: A Novel Approach for the Separation and Valorization of Lignin

Abstract

This dissertation presents an integrated, sustainable approach to the separation, characterization, and valorization of lignin and cellulose contents derived from two major byproducts of the pulp and paper industry: black liquor and paper mill sludge. The overarching goal is to advance the biorefinery concept through environmentally benign electrochemical and microwave catalytic processes that enable the generation of high-value products from industrial waste streams. In the first phase, a novel electrocoagulation (EC) system employing iron electrodes was developed as a greener alternative to conventional lignin recovery technologies such as LignoBoost and LignoForce. Unlike these conventional processes, which rely on harsh, environmentally unfriendly chemicals such as sulfuric acid and CO2 and generate harmful SOx emissions, the EC method operates under mild conditions and simultaneously promotes in situ Electro-Fenton oxidation. This dual mechanism not only facilitates lignin separation but also oxidizes it during treatment, enriching the recovered lignin with additional functional groups distinct from conventionally separated lignins. Electrooxidized lignin fractions were recovered through acid precipitation at various EC durations and systematically characterized using Gel Permeation Chromatography (GPC), 31P Nuclear Magnetic Resonance (NMR) spectroscopy, Modulated Differential Scanning Calorimetry (MDSC), and Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES). The analyses revealed that electrooxidation for 4 h tripled the aliphatic hydroxyl and quadrupled the carboxylic hydroxyl group content compared to untreated lignin, while reducing molecular weight and polydispersity and raising the glass transition temperature. These modifications produced a more uniform and functionally enriched lignin, enhancing its suitability for applications in advanced materials such as vitrimers, polymer composites, and resins. In the second phase, the electrocoagulated lignin was recovered by filtration, thoroughly washed, and oven-dried before being carbonized to synthesize Magnetic Mesoporous Activated Carbon (MMAC). The resulting MMAC exhibited a surface area of 125.37 m² g⁻¹ and an average pore diameter of 6.59 nm. Characterization by SEM, XRD, Raman spectroscopy, FTIR, and BET surface analysis confirmed the formation of a highly porous, magnetically active structure. The MMAC demonstrated excellent adsorption performance, removing up to 91% of methylene blue dye within 30 minutes while maintaining consistent efficiency over multiple regeneration cycles. These results underscore the potential of EC-derived lignin to produce high-performance adsorbents for wastewater purification and environmental remediation. Finally, the synthesized MMAC was evaluated as a microwave catalyst for the conversion of cellulose to furfural in aqueous media. Process optimization using Response Surface Methodology (RSM) based on a Box-Behnken design identified the optimal reaction conditions, 10 mg catalyst loading, 200 °C temperature, and 20 min reaction time, yielding a maximum furfural output of 48 mol% per mole of cellulose. The process was further validated using paper mill sludge as a cellulose source, yielding 31% furfural under identical conditions. The microwave process produced furfural with a competitive yield in much less time than conventional methods, e.g., the hydrothermal method. Overall, this work proposes a closed-loop valorization pathway that simultaneously recovers, transforms, and reuses lignin and cellulose from pulp and paper mill wastes. The integration of electrochemical oxidation, material synthesis, and catalytic conversion demonstrates a potential strategy for producing high-value carbon materials and bio-based platform chemicals, thereby advancing the development of a circular, low-carbon bioeconomy.