Carbonaceous Materials for Wastewater Treatment from Microwave-Assisted Pyrolysis of LignoForce Lignins

Microwave-assisted pyrolysis of kraft lignin was used to produce highly porous carbonaceous materials for environmental applications (biochars). LignoForce hardwood (LFHL) and softwood (LFSL) lignins were mixed with 30% potassium phosphate (K₃PO₄) and pyrolyzed at 450 °C to produce 45-47% biochar after 25 min of microwave irradiation. A 10 L min^-1 nitrogen flow was used for purging during the pyrolysis and cooling. Elemental analysis (CHNS-O) revealed that kraft lignins were highly deoxygenated, but nearly 26% of their original oxygen content remained after pyrolysis, suggesting retention of oxygenated compounds in the biochar structure. Nitrogen adsorption (BET), scanning electron microscopy (SEM), and X-ray diffraction (XRD) demonstrated the formation amorphous micro- and mesoporous carbonaceous materials with a high potential for water and soil remediation. Significant specific surface areas (142 and 202 m² g^-1 for LFHL-B and LFSL-B, respectively) were obtained for these lignin biochars, which were tested for methylene blue (MB) adsorption at pH 4.5, 6.0, 7.5, and 9.0. LignoForce lignin biochars presented an adsorption capacity of approximately 15.5 mg g^-1 and a removal efficiency higher than 94%, especially at higher pH levels (>4.5). However, MB adsorption was slightly faster for LFHL-B compared to LFSL-B. These results position kraft lignin as a valuable carbon-accumulating raw material for environmental applications and establish MAP as a fast and energy-efficient route for their thermal conversion.