As demonstrated in the Scheme (Fig. 1), first, 1000 mL pristine black liquor 12, the solid content was 49.5 wt% as measured after the black liquor was (pH dried in an oven at 100 oC for 24 h) was purged with carbon dioxide gas for around 30 min at 60 oC. After treatment with carbon dioxide gas, the pH of the solution