Polyethylene separator supported TFC nanofiltration membranes for arsenate removal: Impact of hydrolysis

This study explored the unique hydrolysis character of arsenate and counterintuitive concentration/rejection behavior using a low-cost polyethylene (PE) separator supported thin-film composite (TFC) nanofiltration membranes (PE-NF). The newly developed PE-NF membrane showed 99.6 % rejection for Na₂SO₄ and high water permeance of 18.2 LMH/bar; the high performance was attributed to the support's high surface porosity and the thin active layer with minimal transport distance as well as highly negatively charged surface and narrow pore size distribution. Increased rejection of As(V) from 73.8 % to 98.8 % as the feed concentration increasing from 0.13 to 57.3 mg/L was explained by the hydrolysis of HAsO₄²⁻ at a high concentration. Hydrolysis of HAsO₄²⁻ resulted to pH increase and enhanced Donnan exclusion by the PE-NF membrane. Simulation demonstrated severe external concentration polarization (ECP) due to high rejection, low diffusivity of As(V) ions, and high water permeance which in turn causes pH increase at the membrane surface. One stage NF permeate satisfied the surface water standard based on a synthetic underground water, showing great promise of present low cost NF membranes for efficient As(V) removal.