Indirect Grafting of Alkyl Ammonium Ions onto Glassy Carbon and Coal Surface. Enhancing the Performance of Heterogeneous Asymmetric Membranes

Glassy carbon (GC) and lignite coal powder surfaces are functionalized-either electrochemically or chemically-with alkylammonium groups. The modified coal powder is then embedded within cellulose acetate asymmetric membranes. The surface functionalization process involves the reduction of 2,6-dimethylbenzene diazonium salt (2,6-DMBD), which generates the 2,6-dimethylphenyl radical. Owing to steric hindrance, this radical does not react directly with the surface; instead, it undergoes a radical crossover reaction with brominated alkylammonium salts, generating alkylammonium radicals that subsequently graft onto GC and coal surfaces. In contrast, the reduction of 4-nitro- and 3,5-dicarboxybenzenediazonium salts produces aryl radicals that bind directly to the coal surface while simultaneously generating alkylammonium radicals. This dual mechanism enables the formation of mixed alkylammonium-aryl layers. On GC, the modification yields a surface layer of alkylammonium groups, as confirmed by cyclic voltammetry using redox probes and by X-ray photoelectron spectroscopy (XPS), particularly through the N 1s signal, which attests to the presence of alkylammonium groups. When coal modified with alkylammonium ions-either alone or in combination with aryl groups-is incorporated into heterogeneous asymmetric membranes, the resulting membranes exhibit enhanced productivity, product flux, and solute separation compared to both unmodified membranes and those modified without ammonium groups.