Acid solubilization of lignin and bioconversion of treated newsprint to methane

Abstract

Newsprint was pretreated with acetic-nitric acid reagent tosolubilize and remove the lignin component and improveits conversion to methane in anaerobic digestion. Aceticacid itself cannot dissolve lignin even at a concentrationas high as 80% at elevated temperature (in a boilingwater bath). In order to effectively dissolve significantamounts of lignin, nitric acid must also be added. At anacetic acid concentration of 35% with 2% nitric acid,about 80% of lignin was removed from newsprint(resulting in a weight loss of about 40%). Hydrochloricacid may partially but not completely replace the nitricacid. The methane yield from treated newsprintbioconversion increased nearly three times over that ofuntreated newsprint in a 60-day test. Treated newsprintgenerated about 75% as much methane as office paper inthe same 60-day test. Acid pretreatment producednewsprint with the same cellulose content (as apercentage of volatile solids) as office paper. Despitethese promising results, however, the cellulose/lignin ratioof newsprint was increased from 1.6 to only 9.9 by acidpretreatment, compared to 22.9 for office paper. Since thelignin was not completely removed, the cellulose-ligninassociation is considered to be the major limiting factoron long-term anaerobic digestion of both untreated andtreated newsprint. An additional limiting factor todevelopment of a practical pretreatment method based onthis approach, efficient recovery of acids following ligninsolubilization, was not pursued in this research.