A study on microfluidic desalination using short-chain fatty acid by directional solvent extraction in a serpentine micromixer

Water shortage is an imminent issue in the foreseeable future while thermal desalination techniques are energy intensive. This study investigates a membrane-less low temperature technique known as directional solvent extraction (DSE) desalination in a serpentine microchannel at 45, 50, 55, 60 and 65 °C. Two lower fatty acids, heptanoic acid (HeptA) and nonanoic acid (NonA) were used as the solvents to recover water from a 35000 ppm total dissolved solids (TDS) seawater at 2, 3, 4 and 5 organic to aqueous flowrate ratios (O: aq). Custom-made polydimethylsiloxane (PDMS) – glass microfluidic device is fabricated via direct writing lithography with depth and length of 100 µm and 161 mm respectively while the width is varied from 400 - 800 µm. Droplets were generated during the extraction process to maximize diffusion by using a 90° T-junction where the organic solvent is the continuous phase and seawater is the dispersed phase. A maximum product water yield, Yw of 4 % v/v and 1.9 % v/v is achieved by HeptA and NonA respectively. The product water recovered from HeptA and NonA has salinity as low as 0.01% and 0.08%, respectively. The HeptA system produces water containing higher solvent residuals than the NonA system. HeptA is a better solvent as it can extract approximately two times more water than NonA under the same operating conditions. Both solvents are reusable.