Desalination and lignin concentration in a lignin aqueous solution by nano-filtration process: Advanced γ-Al₂O₃ film-coated porous α-Al₂O₃ hollow fiber membrane

To remove the main barrier for lignin valorization, which is the development of actual lignin separation technology, we studied the ceramic membrane nanofiltration process using a dense, ultra-thin γ-Al₂O₃ film-coated porous α-Al₂O₃ hollow fiber (γ α HF) membrane for the simultaneous separation of alkali metals (Na⁺, K⁺, etc) and concentration of lignin from an alkaline media like black liquor with success as an advanced wastewater treatment process. The γ-AlOOH sol was prepared by sol-gel method and coated by dip-coating on α-Al₂O₃ hollow fiber (α HF) support. After calcination at high temperature, the mean pore sizes of the α HF support and the γ α HF membrane were found to be 0.2 µm and 1.6 nm. The rejection rate performance for various salts increased in the following order: Na₂SO₄<MgSO₄<NaCl<KCl<CaCl₂<MgCl₂<AlCl₃. Under the best condition, the removal rate of sodium and potassium ions reached about 92% and 85%, respectively, with 19.34% of the lignin being discharged. The γ α HF membrane presents good performance stability during a five day nano-filtration process.