Co-pyrolysis of sewage sludge and sodium lignosulfonate: Kinetic study and methylene blue adsorption properties of the biochar

Co-pyrolysis of sewage sludge (SS) and lignin is a promising method to dispose of SS and obtain valuable biochar, however, the synergy effect has been rarely reported. In this paper, lignin of sodium lignosulfonate (SL) was blended with SS for co-pyrolysis. Thermogravimetric analyzer/differential scanning calorimetry (TGA/DSC) was used to investigate the reaction kinetics during co-pyrolysis. And the adsorption performance of co-pyrolysis biochar on methylene blue (MB) was determined. Results showed that the SS decomposition was promoted with SL addition. Kinetic analysis using the Coats-Redfern method indicated the reaction order model to be the primary pyrolysis kinetic mechanism. The synergistic effect of promotion was caused by catalytic metals, and inhibition was caused by ash and tar adhesion. The specific surface area of biochar was improved during copyrolysis from 62.53 to 243.22 m2 g-1, while SL ratio of 80% achieved the maximum specific surface area. Co-pyrolysis showed an inhibition effect on porosity formation when SL ratio was below 60%, while a promotion effect was found when SL ratio exceeded 60%. Co-pyrolysis elevated the MB adsorption capacity of SS-derived biochars from 12.48 to 154.06 mg g-1, and was indicated as a promising method for adsorbents production.