In the past decades, substantial research efforts have been directed towards increasing the availability of renewable and recycled raw materials. Lignin, one of the most abundant natural polymers, constitutes a vast, renewable, and largely untapped source of aromatic structures. In addition, it is one of the most abundant renewable sources of carbon. Despite the countless research projects aimed at valorizing kraft lignin, the largest source of industrial lignin, relatively few commercial kraft lignin products have emerged. Simultaneously, lignosulfonates represent a commercially successful range of products with a steady and growing global market. This paper reviews the current outlook of technical lignin research, including common misunderstandings, and discusses various factors that have hampered the use of lignin as a renewable source of materials and chemicals.

Brownstock washing, a critical process in cleansing kraft pulp, removes dissolved lignin residues from the pulp after it has passed through the cooking digester. It plays a significant role in kraft pulp mills by enhancing economic efficiency and environmental sustainability. Improved washing efficiency leads to better pulp quality and more effective recovery of cooking chemicals. Our study aimed to better understand the impact of different chemical compositions in washing liquors on washing performance. We tested a range of washing liquors, including neutral solutions (deionized water, 1M NaCl, 3M NaCl, 1M Na2SO4) and alkaline solutions (tap water, washing liquor composed of 0.35M NaOH and 1M Na2SO4, and white liquor with 50 g[OH]/l and 8.77 g[HS]/l). These liquors were evaluated for their efficacy in maximizing lignin extraction. Our findings suggest that salt solutions generally reduce washing efficiency. Deionized water and white liquor proved to be the most efficient washing agents, while high-concentration salts and those with high ionic strength negatively impacted washing efficiency. This suggests that brownstock washing may not be operating at its full potential.

This work has focused on oxygen's role in the delignification process within the context of pulp production. We have investigated the role of oxygen in a complex set of chemical reactions taking place during this process, including both oxidative and non-oxidative reactions. This study explores the impact of pH changes during the oxygen delignification process and the characteristics of the resulting pulps. Additionally, this research examines the effect of oxygen, by comparing conventional oxygen delignification with trials using air and nitrogen. Industrial softwood kraft pulps with a kappa number of 35 were subjected to delignification for 20-120 min under alkaline conditions. The resulting pulps were assessed for kappa number, intrinsic viscosity, fiber charge, and ISO brightness. An important observation from this research is the reduction in lignin molecular weight upon exposure to oxygen and air, suggesting depolymerization reactions facilitated by oxygen species, whereas nitrogen exposure results in less pronounced changes. This finding underscores the impact of oxygen in altering lignin structure, thus informing the selectivity and effectiveness of the delignification process.

Lignin, a bio-originated polymer, is being explored as an alternative to nonrenewable fossil resources. It is obtained from biomass during pulping and is mostly burned for energy. In most kraft pulp lines, residual lignin in the pulp is oxidized and solubilized during an oxygen delignification step. This study proposes an isolation method for lignin solubilized during oxygen delignification, which we refer to as "oxlignin", and explores its structural characteristics and properties. The study found acid precipitation to be an effective method for partially isolating oxlignin from the oxygen delignification step. Various analytical methods were employed, including UV-vis absorption analysis, 31P NMR spectroscopy, FT-IR spectroscopy, SEC, and TGA. In addition, the solubility of the lignin was studied in four different solvents and compared to the commercial kraft lignins. The study found that oxlignin is a promising substitute for lignosulfonates in certain applications due to its hydrophilicity and high solubility in water, methanol, and ethanol. Compared to kraft lignins, oxlignin has a lower phenolic group content but higher carboxylic acid content.

Glucuronoxylan is the main hemicellulose in the secondary cell wall of angiosperms. Elucidating its molecular structure provides a basis for more accurate plant cell wall models and the utilization of xylan in biorefinery processes. Here, we investigated the spacing of acetyl, glucuronopyranosyl and galactopyranosyl substitutions on Eucalyptus glucuronoxylan using sequential extraction combined with enzymatic hydrolysis and mass spectrometry. We found that the acetyl groups are preferentially spaced with an even pattern and that consecutive acetylation is present as a minor motif. Distinct odd and even patterns of glucuronidation with tight and sparse spacing were observed. Furthermore, the occurrence of consecutive glucuronidation is reported, which adds to the growing body of evidence that this motif is not only present in gymnosperms but also in angiosperms. In addition, the presence of terminal galactopyranosyl units, which can be released by β-galactosidase, altered the digestibility of the glucuronoxylan by GH30 and GH10 xylanase and appeared to be clustered within the polymeric backbone. These findings increase our understanding of the complex structure of glucuronoxylans and its effect on the extractability and biological degradation of Eucalyptus wood.

Cellulose has inherent properties that are both hydrophilic and water-insoluble, which can create challenges in certain technical applications. One solution to these challenges is derivatization, however, the crystalline structure of cellulose limits its chemical reactivity. This study explores the reactivity of highly swollen cellulose produced by dissolving and reprecipitating microcrystalline cellulose. This extreme swelling of cellulose is expected to increase the accessibility and reactivity, however, upon drying the cellulose becomes hard and inflexible a phenomenon known as hornification. Different drying methods were used to overcome the problems of hornification, including freeze-drying, acetone-drying, and drying with glycerol as a spacer. The samples were carboxymethylated and the degree of substitution (DS) was used to assess reactivity, with freeze-dried samples showing the highest DS. The findings suggest that preserving the swollen structure through appropriate drying methods enhances cellulose reactivity, offering potential improvements in industrial cellulose ether production.

The inherent colloidal dispersity (due to length, aspect ratio, surface charge heterogeneity) of CNCs, when produced using the typical traditional sulfuric acid hydrolysis route, presents a great challenge when interpreting colloidal properties and linking the CNC film nanostructure to the helicoidal self-assembly mechanism during drying. Indeed, further improvement of this CNC preparation route is required to yield films with better control over the CNC pitch and optical properties. Here we present a modified CNC-preparation protocol, by fractionating and harvesting CNCs with different average surface charges, rod lengths, aspect ratios, already during the centrifugation steps after hydrolysis. This enables faster CNC fractionation, because it is performed in a high ionic strength aqueous medium. By comparing dry films from the three CNC fractions, discrepancies in the CNC self-assembly and structural colors were clearly observed. Conclusively, we demonstrate a fast protocol to harvest different populations of CNCs, that enable tailored refinement of structural colors in CNC films.

This study investigated self-heating and of-gassing of Scots pine (Pinus sylvestris) wood pellets made from sawdust generated from separated mature and juvenile wood. The pellets were produced at an industrial scale and stored in large piles of about 7.2 tonnes. The production process involved drying the sawdust using three diferent methods and to varying moisture contents. The results indicated signifcant infuences of both raw material type (F (6)=61.97, p<0.05) and drying method (F (2)=65.38, p<0.05) on the self-heating of the pellets. The results from the multiple regression analysis further showed that both the raw material type and pellet moisture content signifcantly infuenced the temperature increase, with strong correlations observed for pellets produced using low-temperature drying (F (3, 14)=83.52, multiple R2=0.95, p<0.05), and medium temperature drying (F (3, 13)=62.05, multiple R2=0.93, p<0.05). The pellets produced from fresh mature wood sawdust were found to be more prone to self-heating and of-gassing while steam drying the sawdust at high temperature and pressure led to a signifcant reduction in heat and gas generation across all materials. The heightened self-heating and of-gassing in mature wood pellet can be attributed to a higher proportion of sapwood in the raw material. The probable explanations to the observed diferences are in line with biological mechanisms for self-heating and of-gassing, as well as the chemical oxidation of fatty and resin acids.

Lignin is one of the most naturally occurring biopolymers on Earth and exists in a relatively large portion of the residual stream of the pulp and paper industry. Technical lignin is water-soluble, nontoxic, and rich in quinone-type groups; therefore, it could be a potential redox species for next-generation aqueous redox flow batteries (RFBs). Despite having attractive features, lignin does not show a reversible electrochemical behavior. Herein, we implemented a straightforward approach to modify the structure of soda-based lignin by oxidative depolymerization. The modified lignin showed good electrochemical activity through cyclic voltammetry with distinct redox peaks, which match lignin monomers, such as vanillin and acetovanillone. The modified lignin was used as the negolyte of the RFB setup with potassium ferrocyanide as the counterpart. The RFB was cycled for over 200 cycles with an average Coulombic efficiency of 91%. In addition, the modified lignin electrolyte maintained the (electro)chemical properties even after four months of storage, as proven by RFB tests.

Polyelectrolyte complexes (PECs) are polymeric structures formed by the self-assembly of oppositely charged polymers. Novel biomaterials based on PECs are currently under investigation as drug delivery systems, among other applications. This strategy leverages the ability of PECs to entrap drugs under mild conditions and control their release. In this study, we combined a novel and sustainably produced hemicellulose-rich lignosulphonate polymer (EH, negatively charged) with polyethyleneimine (PEI) or chitosan (CH, positively charged) and agar for the development of drug-releasing PECs. A preliminary screening demonstrated the effect of several parameters (polyelectrolyte ratio, temperature, and type of polycation) on PECs formation. From this, selected formulations were further characterized in terms of thermal properties, surface morphology at the microscale, stability, and ability to load and release methylene blue (MB) as a model drug. EH/PEI complexes had a more pronounced gel-like behaviour compared to the EH/CH complexes. Differential scanning calorimetry (DSC) results supported the establishment of polymeric interactions during complexation. Overall, PECs’ stability was positively affected by low pH, ratios close to 1:1, and the addition of agar. PECs with higher EH content showed a higher MB loading, likely promoted by stronger electrostatic interactions. The EH/CH formulation enriched with agar showed the best sustained release profile of MB during the first 30 h in a pH-dependent environment simulating the gastrointestinal tract. Overall, we defined the conditions to formulate novel PECs based on a sustainable hemicellulose-rich lignosulphonate for potential applications in drug delivery, which promotes the valuable synergy between sustainability and the biomedical field. Graphical abstract: (Figure presented.)