Influence of hemicellulose aggregate and gel layer formation on flux and retention during nanofiltration of alkaline solutions

Nanofiltration of process liquors from viscose-type textile fibre production was investigated using a laboratoryscale crossflow test apparatus. Feed solutions contained about 200 g/l sodium hydroxide and a high concentration of hemicellulose as a contaminant originating from the raw material wood. The effects of various pretreatments and the addition of reagents on flux, retention and irreversible fouling were studied. Reagent addition affected the solution state of hemicellulose thus purposely provoking aggregate formation. Dynamic light scattering and turbidity measurements were used to track the evolution of hemicellulose aggregates. Aggregation generally coincided with distinct flux decline during nanofiltration thus indicating that this phenomenon may result in the build-up of gel layers at the membrane surface. Partial neutralisation with sulfuric acid caused extremely severe fouling, whereas methanol addition had a minor effect. The impact of heat treatment and oxidative degradation of hemicellulose on nanofiltration performance was attributed to changes in molar mass distribution.

Dynamic light scattering experiments revealed that hemicellulose forms aggregates in strongly alkaline solutions. This process took place more slowly than in DMSO/water mixtures [3,4] due to repulsive forces between deprotonated carboxylic acid groups. Heat treatment of press liquor caused a degradation of hemicellulose and a decrease in average molar mass thus enhancing flux and reducing hemicellulose retention. Degradation by addition of hydrogen peroxide revealed similar effects. A slight reduction of flux at increased retention was observed after heat treatment of pilot plant permeate which was attributed to an increase in average molar mass resulting from re-condensation reactions. A decrease in relative amount of the high molar mass fraction resulted in lower irreversible fouling, possibly due to a decreased ability of hemicellulose to generate gel layers at the membrane surface.