Removal of arsenic(III) via nanofiltration: contribution of organic matter interactions

The removal of arsenic(III) (As(III)) with nanofiltration (NF) was investigated with emphasis on the role of salinity, pH and organic matter on retention mechanisms. While no measurable impact of salinity on As(III) retention with NF membranes (NF270 and NF90) was observed, a significant increase in As(III) retention occurred from pH 9 to pH 12. This was explained by As(III) deprotonation at pH > 9 that enhanced Donnan (charge) exclusion. Of the five different organic matter types investigated at 10 mgC/L, only humic acid (HA) increased As(III) retention by up to 10%. Increasing HA concentration to 100 mgC/L enhanced As(III) retention by 40%, which was attributed to As(III)-HA complexation. Complexation was confirmed by field-flow fractionation inductively coupled plasma mass spectrometry (FFF-ICP-MS) measurements, which showed that the bound As(III) increased with HA concentration. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) showed that NF90, which exhibited lower permeability reduction than NF270, has accumulated a lower amount of As(III) in the presence of HA, where As(III)-HA complex was formed in the feed solution. This finding implies that As(III) retention with NF technology can be enhanced by complexation, instead of using other methods such as oxidation or pH adjustement.