Molecular weight cut-off (MWCO) of two different types of membranes (polyether sulfone (PES) and regenerated cellulose (RC) ultrafiltration (UF) membranes) has been investigated using 2, 4 and 6 kDa polyethylene glycol (PEG) solutes. The retention results of humic acid (HA), fulvic acid (FA), copolymers of dimethyl aminoethyl acrylate (CoAA) and poly dially dimethylammonium chloride (PDADMAC) is also presented in this paper. Retention of PEG solutions produced MWCO lower than stated by the manufacturers. The MWCO of P005F, C005F and P020F was 3.75, 3.35 and 12.1 kDa, respectively. The manufacturer values were 5 kDa for P005F and C005F and 20 kDa for P020F. The rejection of PDADMAC and CoAA was found to be lower than expected with respect to the MW specified by the supplier. The MW of PDADMAC and CoAA is 60-200 kDa and 10-12 intrinsic viscosity, respectively. The retention of such high MW substances should be ≫90%, but the results show a maximum retention of 91% and the other results are <90%. Nevertheless, the retention of HA and FA was as expected. The humic substances (HA and FA) were extracted from soil, which has a low MW i.e. <6 kDa. The maximum rejection of HA and FA was ~93 and ~84%, respectively. The lower rejection of FA is attributed to the lower MW of FA compared to HA. The observed rejection of PDADMAC and CoAA decreases with increasing the pressure, but the real rejection is constant. This is attributed to the increase in concentration at the wall of the membrane, hence increasing the mass transfer coefficient. The same trend is achieved when filtering HA and FA. The rejection of P005F is always higher than the rejection of C005F for all substances. Although the manufacturer stated the same MWCO of both membranes, the P005F MWCO was higher than C005F producing a higher rejection.
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