In the forest biorefinery, hydrolysis lignin (HL) is often dissolved with high concentration NaOH solution, followed by acid precipitation to obtain purified HL. For the first time, this study evaluates the effect of ultrasound (US) on the dissolution of industrially produced HL in aqueous NaOH solutions and the acid precipitation yield of HL. The solubility of HL in mild aqueous NaOH solutions was studied with and without US treatment at 20 kHz concerning the solid-to-liquid ratio, molecular weight of dissolved fractions and structural changes in dissolved HL. Results showed that the solubility of HL at 25 °C was strongly dependent on NaOH concentration. However, the US treatment significantly improved the solubility of HL, reaching a solubility plateau at 0.1 NaOH/HL ratio. US treatment enhanced the solubilization of HL molecules with higher MW compared to conventional mixing. The increase of HL solubility was up to 30 % and the recovery yield of purified lignin with acid precipitation was 37 % higher in dilute NaOH solution. A significant result was that the M_w of dissolved HL in homogeneous alkali solutions decreased with US treatment. SEC, HSQC and ³¹P NMR analyses of dissolved HL characteristics showed that both, the mechanoacoustic and sonochemical solubilization pathways contribute to the dissolution process. However, US does not cause major changes in the HL structure compared to the native lignin. Indeed, US technology has the potential to advance the dissolution and purification of HL in biorefineries by reducing the amount of chemicals required; thus, more controlled and environmentally friendly conditions can be used in HL valorization.

Forest biorefineries provide multiple new avenues for applied research. The main concept lies in the malleability of the processes and their stepwise organization. The core element of the biorefinery concept addressed in the present study is the pretreatment step; here, wood biomass is converted into free hemicellulosic sugars, lignin and cellulose. In traditional approaches, the pretreatment step is a starting point for isolating and separating lignin or cellulose through different processes. In this study, instead of performing any separation, a lignin-cellulose mixture was used as its own material, and the effects of ultrasound treatment with a probe system at 20 kHz, with various amplitude, sonication time and dry matter content were investigated with the aim of assessing the formation of a nanocellulose structure with a high lignin content (>30 %) and investigating the stability of the lignin-cellulose mixture under aqueous conditions. We demonstrated the importance of dry matter content for the specific particle size and water retention values for this mixture. US treatment of lignin-cellulose mixtures <4 % dry matter formed a gel-like material, with low particle size (90 % below 30 μm and smallest at nanoscale). Low dry matter loading led to better US transfer and higher conversion of cellulose to <100 nm nanoparticles. Our study can serve as a baseline for future developments in the field of stable emulsions, filtering materials or inputs for material synthesis.

Lignin is an abundant and renewable source capable of replacing different raw materials in the chemical industry. It can be obtained from lignocellulosic biomass (LCB) via different pretreatment methods. In the present study, hydrolysis lignin (HL) from the Sunburst^TM pretreatment technology was utilized to investigate its enzymatic conversion. At first, soluble HL fractions were obtained via alkali solubilization followed by acid precipitation, referred to as acid precipitated lignin (APL). Furthermore, the APL was tested with three different bacterial laccases to identify the optimal conditions for its conversion into small molecular weight fractions. Among the tested laccases, Streptomyces coelicolor A3(2) (ScLac) displayed the highest rate of APL conversion with a high lignin dosage and under extremely alkaline conditions, i.e., 50 g/L in 0.25 M NaOH solution, resulting in higher molecular weight fractions. The increase in the molecular weight and quantitative linkages before and after the enzymatic oxidation of the APL were characterized by size exclusion chromatography (SEC), Fourier-transform infrared spectroscopy (FT-IR), and two-dimensional heteronuclear single quantum correlation nuclear magnetic resonance (2D HSQC NMR) methods.

Lignin–cellulose mixtures (LCMs) generated as intermediates in wood biorefineries are commonly separated into lignin and cellulose. However, using ultrasound (US) to process these mixtures could create novel, valuable materials not possible with conventional methods. This study looked at how lignin affects the US modification of these mixtures. Crude and partially delignified LCMs were successfully prepared using aqueous solutions of EtOH, THF and dilute NaOH and then subjected to short, high-power US treatment. The resulting materials were characterised using FT-IR spectroscopy, particle size analysis, water retention value analysis, SEM and XRD. Sonication rapidly reduced the mean particle size, generating cellulose nanofibril-like structures in all samples according to SEM. The response depended strongly on lignin content, with samples containing lower amounts of lignin exhibiting substantially higher hydration capacity and stronger US responsiveness. At the molecular level, lignin removal exposes cellulose surfaces and enhances hydrophilic interface formation, increasing water uptake and suspension stability. Thus, results show that lignin limits accessible hydrophilic cellulose surface area rather than preventing fragmentation by sonication. US is therefore a chemical-lean strategy to tune the physicochemical properties of partly delignified LCMs and expand the product portfolio of integrated wood biorefineries towards novel advanced lignocellulosic materials.

Lignin-cellulose mixtures (LCMs) generated as intermediates in wood biorefineries are commonly separated into lignin and cellulose. However, using ultrasound (US) to pro-cess these mixtures could create novel, valuable materials not possible with conven-tional methods. This study looked at how lignin affects the US modification of these mixtures. Crude and partially delignified LCMs were successfully prepared using aqueous solutions of EtOH, THF and dilute NaOH and then subjected to short, high-power US treatment. The resulting materials were characterised using FT-IR spectroscopy, particle size analysis, water retention value analysis, SEM and XRD. Sonication rapidly reduced the mean particle size, generating cellulose nanofibril-like structures in all samples according to SEM. The response depended strongly on lignin content, with samples containing lower amounts of lignin exhibiting substantially higher hydration capacity and stronger US responsiveness. At the molecular level, lig-nin removal exposes cellulose surfaces and enhances hydrophilic interface formation, increasing water uptake and suspension stability. Thus, results show that lignin limits accessible hydrophilic cellulose surface area rather than preventing fragmentation by sonication. US is therefore a chemical-lean strategy to tune the physicochemical prop-erties of partly delignified LCMs and expand the product portfolio of integrated wood biorefineries towards novel advanced lignocellulosic materials.

Recent rapid developments in forest biomass valorisation have highlighted lignin as a key value driver for the economic sustainability of modern biorefining. Sugar-based biorefinery hydrolysis lignin (HL) from wood biomass has an immense potential as an alternative for fossil-based aromatic feedstock. However, impurities, heterogeneous structures and limited solubility in common solvents are the main challenges associated with HL functionalisation. Aqueous organic solvent fractionation represents a promising approach to improve on HL properties and subsequently increase its valorisation options. In this study, for the first time, biorefinery HL derived from birch was purified and fractionated with water-EtOH and -THF mixtures across the whole co-solvent concentration range at ambient temperature. The highest lignin solubility was achieved with 70 wt% THF mixture, which dissolved a remarkable 81 wt% of lignin available in HL. Moreover, fractionated lignin purity was further increased to 95 wt% when employing an extra water-washing step. In general, fractions soluble in THF mixtures had high dispersity, syringyl-to-guaiacyl (S/G) subunit ratio and aliphatic OH-groups, whereas EtOH fractions had narrower molecular weight distributions, lower S/G subunit ratio and higher concentration of phenolic OH-groups. A brief description of the solvent effects at the molecular level was given using molecular dynamics simulations which revealed specific solvation between the dissolved lignin and solvent mixture. Protocol presented in this paper enables to purify HL and control the physicochemical properties of the soluble lignin fractions by simply altering the composition of the solvent mixture, and therefore, expanding the possibilities in valorisation of HL.