Impact of speciation on removal of manganese and organic matter by nanofiltration

The removal of manganese and humic acid (HA) by two nanofiltration membranes, TFC-SR2 and TFC-SR3, was investigated in order to highlight the influence of speciation on manganese and HA retention. Manganese speciation and complexation with HA were modelled to understand how speciation could affect NF removal mechanisms. The behaviour of the two membranes was dramatically different in terms of manganese retention. Manganese retention for TFC-SR3 was higher and dominated by size exclusion. Manganese retention for TFC-SR2 varied with pH. At pH 7 (i.e. the pH of most natural waters) manganese retention for TFC-SR2 was about 45% versus 90% for TFC-SR3, with fluxes of about 75 and 25 l m⁻² h⁻¹, respectively. Both membranes showed very high retention of HA (about 80%); therefore they are suitable for surface waters where no salt removal is required. Manganese deposit on both membranes was generally low (<10%), but increased at pH 10 and 12 as manganese deposited as precipitated MnCO₃, and the membranes showed a yellow-brownish layer.