Combining high performance fertiliser with surfactants to reduce the reverse solute flux in the fertiliser drawn forward osmosis process

Abstract

Solutions to mitigate the reverse diffusion of solutes are critical to the successful commercialisation of the fertiliser drawn forward osmosis process. In this study, we proposed to combine a high performance fertiliser (i.e., ammonium sulfate or SOA) with surfactants as additives as an approach to reduce the reverse diffusion of ammonium ions. Results showed that combining SOA with both anionic and non-ionic surfactants can help in reducing the reverse salt diffusion by up to 67%. We hypothesised that, hydrophobic interactions between the surfactant tails and the membrane surface likely constricted membrane pores resulting in increased rejection of ions with large hydrated radii such as SO₄^2-. By electroneutrality, the rejection of the counter ions (i.e., NH₄⁺) also therefore subsequently improved. Anionic surfactant was found to further decrease the reverse salt diffusion due to electrostatic repulsions between the surfactant negatively-charged heads and SO₄^2-. However, when the feed solution contains cations with small hydrated radii (e.g., Na⁺); it was found that NH₄⁺ ions can be substituted in the DS to maintain its electroneutrality and thus the diffusion of NH₄⁺ to the feed solution was increased.

Keywords: Fertiliser draw solution; Forward osmosis; Reverse salt flux; Surfactant.