Self-association and aggregation of kraft lignins via electrolyte and nonionic surfactant regulation: stabilization of lignin particles and effects on filtration

Understanding the aggregation behavior of lignins is relevant to their application, especially toward value-added products. Here we report on the self-association of kraft lignin derived from hardwood and softwood species under mild temperature and pH conditions and in the presence of various salts and surfactants. Besides characterization of lignin functional groups (NMR and others), aggregation (extent and rate) (turbidity, viscosity), surface tension, particle size (dynamic light scattering) and filtration efficiency were determined. Monovalent salt ions increased lignin aggregation while non-ionic surfactants enhanced the colloidal stability of the system by steric effects, depending on the concentration. The combination of salt and surfactant enabled the regulation of the colloidal aggregation and the rheological properties of the respective aqueous dispersions. While pH affects the colloidal stability of lignin in solution, it plays a minor effect on lignin particles dispersed in water, indicating contributions beyond those arising from the presence of carboxylic and phenolic groups. Filtration and separation of lignin from aqueous media (relevant to processing streams, wastewater, etc.), as well as applications that require the control over the stability of lignin can be addressed suitably by application of the simple methods presented here.