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

Abstract.

Mineralization of radioactive synthetic lignin (¹⁴C-DHP) was studied in a compost environment at 35, 50 and 58°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°C and 50°C, although the mixed population of compost obviously lacks the most effective lignin degraders. At 58°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.