Sustainable and exceptional Li+ /Mg2+selectivity through electrocoagulation enhanced triamino guanidine modified membrane

Abstract

Positively charged polyamide (PA) nanofiltration (NF) membranes with sub-nanopores show promise for lithium extraction from salt-lake brines with high Mg²⁺/Li⁺ ratios. This study introduces the use of triamino guanidine (TG), a highly positively charged molecule, to regulate grafting on monolayer PA membranes, resulting in dual-layer (PEI-TMC-TG) NF membranes. The monolayer PA membranes were created via spin-coating assisted interfacial polymerization between polyethyleneimine (PEI) and tricarbonyl chloride (TMC). The PEI/TMC-TG NF membranes exhibited several advantages over unmodified PEI-TMC NF membranes, including higher positive charge density, narrower pore size distribution, excellent hydrophilicity, and a more porous separation layer microstructure. Experimental results indicated that enhanced Donnan effect, size sieving, and dielectric repulsion within the sub-nanopores contributed to exceptional Li⁺/Mg²⁺ selectivity during filtration of East Taijiner brine (S_Li+_/Mg²⁺ = 64.5). Furthermore, the TG hydrophilic layer significantly improved water permeability (15.4 LMH/bar) and long-term stability. To reduce membrane fouling in the charge-positive NF process by removing organic matter from complex salt lake brine, we explored electrocoagulation as a pre-treatment method and identified optimal conditions for the EC-NF process. This strategy maximizes sustainable lithium recovery from brine in an economically viable green manner while enhancing the efficiency of NF membranes for lithium extraction from salt lakes.