Decoloration of Molasses by Ultrafiltration and Nanofiltration: Unraveling the Mechanisms of High Sucrose Retention

Abstract

Membrane technology provides a green approach to recover sucrose from cane molasses. However, the trade-off between color removal and sucrose permeation by membrane filtration debases the efficiency and limits the application. In this study, ten commercially-available and one self-made ultrafiltration (UF) and nanofiltration (NF) membranes were used for decoloration of molasses. By investigating the effect of membrane properties, molasses components, and operating parameters on the molasses filtration behaviors, the mechanisms of high sucrose retention were clarified. For polyether sulfone (PES) membrane, the high retention of pigments and sucrose was mainly caused by the serious irreversible fouling as an additional selective layer. Polyamide (PA) NF membranes showed high antifouling ability, and a suitable pore size (~ 500 Da) was important to achieve high color removal and sucrose permeation. Although regenerated cellulose (RC) membranes exhibited excellent antifouling performance to molasses, the severe membrane swelling induced by salts and high temperature (60 °C) limited their application. Reducing sugar in molasses produced negligible effect on the sucrose retention, while inorganic salts resulted in pore swelling and solute dehydration due to “salting-out” effect, thus improving the sucrose permeation. However, such positive effect was weakened or eliminated by the pigments in molasses, and irreversible fouling became more serious in the presence of the molasses salts, especially for PES membranes. Based on the underlying mechanisms, the pore swelling and sucrose dehydration effects were “reappeared” by simply removing the non-polar pigments via macroporous resin adsorption, thus decreasing sucrose retention. High temperature could accelerate the sucrose permeation, but it also attenuated concentration polarization layer, thus intensifying the sucrose retention increase induced by high permeate flux. If a loose NF membrane was selected for decoloration of molasses, minimizing fouling formation, reappearing salt effect on membrane and sucrose, operating at high temperature and low permeate flux could minimize sucrose retention.