Why oxalic acid protects cellulose during ozone treatments?

Catalytic formation of oxalic acid on the partially oxidised greigite Fe3S4(001) surface

Greigite (Fe₃S₄), with its ferredoxin-like 4Fe-4S redox centres, is a naturally occurring mineral capable of acting as a catalyst in the conversion of carbon dioxide (CO₂) into low molecular-weight organic acids (LMWOAs), which are of paramount significance in several soil and plant processes as well as in the chemical industry. In this paper, we report the reaction between CO₂ and water (H₂O) to form oxalic acid (H₂C₂O₄) on the partially oxidised greigite Fe₃S₄(001) surface by means of spin-polarised density functional theory calculations with on-site Coulomb corrections and long-range dispersion interactions (DFT+U-D2). We have calculated the bulk phase of Fe₃S₄ and the two reconstructed Tasker type 3 terminations of its (001) surface, whose properties are in good agreement with available experimental data. We have obtained the relevant phase diagram, showing that the Fe₃S₄(001) surface becomes 62.5% partially oxidised, by replacing S by O atoms, in the presence of water at the typical conditions of calcination [Mitchell et al. Faraday Discuss. 2021, 230, 30-51]. The adsorption and co-adsorption of the reactants on the partially oxidised Fe₃S₄(001) surface are exothermic processes. We have considered three mechanistic pathways to explain the formation of H₂C₂O₄, showing that the coupling of the C-C bond and second protonation are the elementary steps with the largest energy penalty. Our calculations suggest that the partially oxidised Fe₃S₄(001) surface is a mineral phase that can catalyse the formation of H₂C₂O₄ under favourable conditions, which has important implications for natural ecosystems and is a process that can be harnessed for the industrial manufacture of this organic acid.

Boosting the effect of a laccase-mediator system by using a xylanase stage in pulp bleaching

Using an enzyme-based stage involving a xylanase (X) or laccase (as part of a laccase-mediator system, L) in a bleaching process can help reduce reagent consumption and hence its environmental impact. In this work, both types of enzymes were applied to eucalypt pulp. The influence of process variables in the laccase-mediator treatment (viz. laccase dose, HBT dose and reaction time) was assessed by using a three-variable sequential statistical plan. The effect of a pretreatment with X on the previous variables was also assessed. Kappa number and brightness models for the L stage and XL sequence were found to perform disparately, which suggests the formation of lignin derivatives interfering with brightness measurements. The L system oxidized readily accessible lignin within the first hours of treatment and affected the contents in cellulose and hexenuronic acids (HexA) of the resulting pulp. Xylanase facilitated access of the laccase-HBT system to lignin and HexA in cellulose fibres. The L treatment increased effluent properties such as Microtox toxicity, COD and colour, and led to strong inactivation of the enzyme. The increased toxicity of the effluents was due to HBT; based on statistical data, however, the effect can be reduced by lowering the mediator dose.