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Potentiometric titration, FTIR spectroscopy, 13C solid-state NMR spectroscopy and chemical analysis were used to study the gelation of technical lignins (oxidized hydrolysis, kraft, soda and lignosulfonates) and Pepper lignin in interaction with metal salts (CaCl2, Mg(NO3)2, FeSO4, Co(NO3)2, Pb(NO3)2, Fe(NO3)3, CuSO4, AgNO3). It has been found that compounds that are salts of weak base and strong acid possess an ability to gelation; the metal cation has a standard potential above a certain value, has moderate hydrolysis ability and the cation hydroxide formed in the hydrolysis reaction has a relatively low solubility. At the gel formation, a compounds of the composition R–COO–Mez +–OOC–R, z = 2, 3 were formed. A scheme of intermolecular bonds at the point of lignin hydrogel formation was proposed, according to which, adjacent lignin molecules bound to each other by three types of bonds: ionic between carboxyl groups of lignin and metal cation, coordination between metal cation and water molecules included in the coordination sphere, and hydrogen bonds, binding all components of the system into a single whole. Lignin hydrogels have a high sorption ability with respect to heavy metals such as lead, iron and copper, that, depending on the content of acid groups in lignin and the molar mass of the sorbate, is approximately 25–50% by weight of lignin. This opens up the possibility of using a new type of lignin hydrogels for wastewater treatment of chemical plants.

An analytical calculation method for the estimation of the contents of alkyl aryl ether bonds (α-O-4 and β-O-4) in lignin was developed. In the framework of the method, Alkyl–O–Aryl type bonds are described as coupled phenolic hydroxyls (OHphen). The method is based on the balance equation including the free and coupled OHphen contents in dissolved and residual lignins, on the one hand, and their respective contents in native lignin, on the other. The free OHphen content is calculated on the basis of the OHphen contents of dissolved and residual lignin, determined by the aminolysis method in the course of kraft cooking of softwood. The calculation results for soluble lignin preparations are in good agreement with the 13C NMR (nuclear magnetic resonance) spectral data for the solutions. The content of Alkyl–O–Aryl bonds in native softwood (pine, spruce) lignin was estimated at 79/100 PPU (phenylpropane unit). In isolated lignin preparations, the contents of these bonds decrease in the sequence: Freudenberg...

Lignin model dimers representing all typical interunit linkages in lignin (β‐O‐4, α‐O‐4, α‐O‐γ, β–5, 5–5, β–β, stilbene, enol ether) were characterized by differential pulse polarography (DPP) and cyclic voltammetry (CV) in DMSO. Polarographic activity was registered for α‐(C˭O)‐β‐O‐4 structures and conjugated double bonds in stilbene and enol ether structures. A structure–chemical reactivity correlation diagram is drawn based on the studies of one‐ and two‐unit model structures of lignin. Peaks in differential pulse polarograms of lignin were assigned to certain functional groups and bond types based on the comparison between the data obtained on model compounds and selectively chemically modified [NaBH4, LiAlH4, CH2N2, (CH3O)2SO2] lignin samples.

Electrochemical properties of compounds modeling typical structural units of lignin were determined by differential pulse polarography in dimethylsulfoxide with tetrabutylammonium perchlorate as a supporting electrolyte. p-Quinonemethides; derivatives of cinnamyl aldehyde and cinnamic acid; aromatic aldehydes, ketones, diketones, and carboxylic acids; isoeugenol; and coniferyl alcohol were studied. Structural effects on the reduction potentials of lignin units were estimated. The potentials, which correspond to the first polarographic peak, varied from -0.8 (p-quinonemethide) to -2.8 V (coniferyl alcohol). The compounds containing nonconjugated carbonyl or carboxyl groups were not polarographically active. The autoprotonation reaction had a profound effect on the electrochemical reduction of compounds containing hydroxyl groups.