Biodegradation of radiolabelled synthetic lignin (¹⁴C-DHP) and mechanical pulp in a compost environment

Mineralization of radioactive synthetic lignin (¹⁴C-DHP) was studied in a compost environment at 35, 50 and 58 degrees C. Compost samples were successively extracted with water, dioxane and alkali, and the molecular weight distribution of some extracts was determined by gel permeation chromatography (GPC). Biodegradation of lignin-containing spruce groundwood (SGW) and pine sawdust was concurrently determined in controlled composting tests by measuring evolved CO₂. The temperatures were the same as in the ¹⁴C-DHP mineralization experiment and bleached kraft paper, with a lignin content of 0.2%, was used as a reference. The mineralization of ¹⁴C-DHP was relatively high (23-24%) at 35 degrees C and 50 degrees C, although the mixed population of compost obviously lacks the most effective lignin degraders. At 58 degrees C the mineralization of ¹⁴C-DHP, as well as the biodegradation of SGW and sawdust, was very low, indicating that the lignin-degrading organisms of compost were inactivated at this temperature. SGW was poorly biodegradable (<40%) in controlled composting tests compared with kraft paper (77-86%) at all temperatures, which means that lignin inhibits the degradation of carbohydrates. During the incubation, water-soluble degradation products, mainly monomers and dimers, and the original ¹⁴C-DHP were either mineralized or bound to humic substances. A substantial fraction of ¹⁴C-DHP was incorporated into humin or other insolubles.