Wood-Supported cationic polyelectrolyte membranes from a reactive ionic liquid for water detoxification

Abstract

Quaternised Wood Membranes (QWMs) are gaining prominence as cost-effective, sustainable alternatives for water detoxification, particularly the removal of hazardous oxoanions such as nitrate (NO₃⁻), sulfate (SO₄²⁻), and phosphate (PO₄³⁻). Pinewood has been quaternised using a polyelectrolyte-forming reactive ionic liquid (RIL), i.e., glycidyl triethylammonium chloride (GTEAC), in a water-free, one-step method. The optimised modification process at 90°C and for 1.5 h results in a substantial increase in the oxoanion removal efficiencies of the QWMs, with the most effective removal being achieved for SO₄^2-, followed by PO₄^3- and NO₃^–. The GTEAC can polymerise to yield long-chain cationic polyelectrolytes, which corroborates with a model synthetic study, the 1D and 2D NMR spectra, and the DSC and TGA/DTG thermal analyses. This grafted polyelectrolyte yields a high weight gain w_g = 40 % and the corresponding degree of quaternisation of DQ = 2.08 mmol/g, though screening effects yield a maximum ion exchange capacity of IEC_max = 1.07 mmol/g. Additionally, the regeneration is feasible after several filtration cycles and the QWM can withstand sufficient stress under operation conditions. Further, isothermal analyses indicate a Langmuir behaviour and Freundlich-like behaviour under equilibrium (zero flow) and flow conditions, respectively. This study highlights the potential of QWMs as a sustainable and cost-effective alternative to synthetic polymeric membranes in water treatment technologies for denitrification, desulfurisation, dephosphatisation and/or ultrafiltration applications. The process sustainability was quantified using the EcoScale approach of up to 73.6 (near excellent), yielding wood-based AEMs costing 50–60 times less than their synthetic polymeric peers.