No abstract

Utilization of lignin-rich side streams has been a focus of intensive studies recently. Combining biocatalytic methods with chemical treatments is a promising approach for sustainable modification of lignocellulosic waste streams. Laccases are catalysts in lignin biodegradation with proven applicability in industrial scale. Laccases directly oxidize lignin phenolic components, and their functional range can be expanded using low-molecular-weight compounds as mediators to include non-phenolic lignin structures. In this work, we studied in detail recombinant laccases from the selectively lignin-degrading white-rot fungus Obba rivulosa for their properties and evaluated their potential as industrial biocatalysts for the modification of wood lignin and lignin-like compounds. We screened and optimized various laccase mediator systems (LMSs) using lignin model compounds and applied the optimized reaction conditions to biorefinery-sourced technical lignin. In the presence of both N–OH-type and phenolic mediators, the O. rivulosa laccases were shown to selectively oxidize lignin in acidic reaction conditions, where a cosolvent is needed to enhance lignin solubility. In comparison to catalytic iron(III)–(2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) oxidation systems, the syringyl-type lignin units were preferred in mediated biocatalytic oxidation systems.

Rate of sediment accumulation of extractable organic halogen (EOX) and of 12 trace elements (As, Cd, Co, Cr, Cu, Hg, Mn, Ni, Pb, Sn, V, and Zn) were analyzed retrospectively over 100 years in the recipient area of pulp mills in Lake Saimaa and in two nonrecipient forest lakes in southeastern Finland. The sediments were dated by 210 Pb and 137 Cs activities. From 1880s to 1940s, <1 mg of EOX-Cl accumulated m -2 year -1 in the sediment of southeastern Lake Saimaa, but accumulation increased to 1350 mg of EOX-Cl m -2 year -1 from the 1950s to the 1980s at 3 km and to 500 mg at 5 km from the discharge point of bleaching pulp mills. Abandoning elemental chlorine in bleaching and adopting biological treatment for wastewaters in the 1990s associated with a decreasing the sediment accumulation of EOX to <400 mg of EOX-Cl m -2 year -1 at both sites. The deposition rates of inorganic elements were similar in all three lakes from 1880s to 1940s, 5-40 mg m -2 year -1 for Mn and Zn, 1-5 mg m -2 year -1 for Cr, Cu, Ni, Pb, Sn, and V, and below 1 mg m -2 year -1 for As, Cd, Co, and Hg. After the 1950s, the sediment accumulation rates of Cd, Cr, Cu, Hg, Ni, Pb, Sn, Zn, and carbon increased 2-5-fold in the recipient lake and those of Pb, Hg, and Sn also in the nonrecipient lakes. From each ton of adsorbable organic halogen (AOX) discharged by the mills, 0.6-0.1 mg of EOX-Cl m -2 accumulated in the Lake Saimaa sediments at 3-5 km from the mills. From the slope of sediment accumulation vs time, the half-life of the EOX of pulp mill origin in the sediment was estimated as 60-80 years. The rates of sediment accumulation of Hg during the decades when organomercurials were used as slimicides show that sediment was not the main sink for the Hg.