EDTA-Na₂ as a recoverable draw solute for water extraction in forward osmosis

Regeneration and reuse of draw solute (DS) is a key challenge in the application of forward osmosis (FO) technologies. Herein, EDTA-Na₂ was studied as a recoverable DS for water extraction by taking advantages of its pH-responsive property. The FO system using EDTA DS achieved a higher water flux of 2.22 ± 0.06 L m^-2 h^-1 and a significantly lower reverse salt flux (RSF) of 0.06 ± 0.01 g m^-2 h^-1, compared to that with NaCl DS having either the same DS concentration or the same Na⁺ concentration. The suitable pH range for the application of EDTA DS was between 4.0 and 10.5. A simple recovery method via combined pH adjustment and microfiltration was employed to recover EDTA DS and could achieve the recovery efficiency (at pH 2) of 96.26 ± 0.48%, 97.13 ± 1.03% and 98.56 ± 1.40% by using H₂SO₄, H₃PO₄ and HCl, respectively. The lowest acid cost for DS recovery was estimated from 0.0012 ± 0.0001 to 0.0162 ± 0.0003 $ g^-1 by using H₂SO₄. The recovered EDTA DS could be reused in the subsequent FO operation and the overall recovery efficiency was 94.4% for four reuse cycles. These results have demonstrated the feasible of EDTA-Na₂ DS and a potentially cost-effective recovery approach, and encouraged further exploration of using EDTA-based compounds as a draw solute for FO applications.