From alternative sources of cellulose to ultrafiltration membranes

Abstract

Cellulose, the most abundant biopolymer on the planet, presents a renewable alternative to petroleum-based polymeric materials. One of its applications is the production of polymeric ultrafiltration membranes, which can be used for water treatment, protein separation, and many other processes. Cellulose is considered an attractive membrane material due to its’ ability to form films, remarkable hydrophilicity and easily malleable charge on the surface, which can be helpful in the separation of charged compounds. Additionally, deriving cellulose from waste sources, such as agricultural and forest wastes and waste cotton textiles, adds additional advantages (e.g. reduced environmental impact and fuller utilisation of wastestreams and sidestreams). Moreover, if cellulose membranes are produced from alternative sources, such as wood chips or waste cotton textiles, their production can be understood as an upcycling procedure, contributing to a closed-loop economy. The utilisation of more sustainable solvents for cellulose membrane production is also required. The use of ionic liquids, which have high dissolution power over cellulose, and deep eutectic solvents, which demonstrate the ability to efficiently fractionate wood biomass in one step, is actively studied to develop and improve the process of cellulose membrane production.

This study was focused on the investigation of the use of wood biomass, in the form of wood chips, and waste cotton textiles as a source of cellulose for regenerated cellulosebased membrane production. Flat-sheet ultrafiltration membranes were prepared from wood biomass and cotton textile solutions in the 1-ethyl-3-methylimidazolium acetate −dimethyl sulfoxide mixture through a non-solvent-induced phase inversion technique. Variations in the casting procedure included tracing the effect of polymer composition, casting solution’s concentration, casting thickness, coagulation bath composition and presence of lithium chloride additive on the performance of the prepared membranes. The membranes’ performance and morphology were characterised using measurements of pure water permeance, retention of model compounds (polyethylene glycols of different molecular weight), contact angle and zeta potential, as well as Fourier-transform infrared and Raman spectroscopy. Additionally, the membranes were tested for the removal of phosphorus from the wastewater treatment plant’s effluent and dye retention from the model solution representing the dye industry’s wastewater.

The results revealed that membrane preparation was possible from both untreated wood biomass and cotton textiles. The applied wood delignifying treatments, such as treatment using deep eutectic solvent and bleaching chemicals, assisted the formation of membranes with better retention properties, as the chemical composition of wood biomass became more homogeneous. An alkaline coagulation bath as a primary medium for coagulation was introduced in this study and turned out to be an efficient tool for controlling the membranes’ permeance and surface charge. When lithium chloride was tested as an additive, an optimum concentration was found at which the solution’s viscosity was the highest, resulting in the formation of a membrane with the densest structure, highest retention and lowest porosity. The prepared membranes demonstrated the ability to separate compounds whose size is considerably smaller than the membranes’ molecular weight cut-off (e.g. residual phosphorus or dye molecules). Thus, this study identified two alternative sources of cellulose, reducing the dependence on commercially processed cellulose fibres in cellulose membrane production. Demonstrating interconnections between cellulose’s purity and membrane performance, this study suggested more efficient utilisation of wood biomass to produce membranes. Conducting experiments using alkaline coagulation bath and adding lithium chloride to the casting solution provided additional theoretical knowledge on the cellulose and wood biomass dissolution-regeneration process. The measured ability of the prepared membranes to remove residual phosphorus and dye molecules demonstrated the potential of the prepared membranes to be used as a part of reagent-free water treatment processes.