Separation of sodium lactate from Span 80 and SDS surfactants by ultrafiltration

Abstract The ultrafiltration process for separation of sodium lactate from sorbitan monooleate (Span 80) and sodium dodecyl sulfate (SDS) surfactants using ZrO 2 flat-disc ultrafiltration membranes was studied in this work. The study is focused on the influence of the nominal molecular weight limit of the membrane (NMWL), the transmembrane pressure (TMP), and initial lactic acid concentration (C A ) on the permeate flux (J p ) and rejections observed to ion lactate (R A ) and SDS (R S ) using a full central composite experimental design and response surface methodology. Experiments were conducted in four stages: a first stage of lactic acid extraction with niosomes formulated with Span 80 (20 mol/m 3 ) and SDS (4 mol/m 3 ), a second back-extraction stage conducted by NaOH addition until pH > 12 for niosomes breaking and sodium lactate releasing, and a third and fourth ultrafiltration stages at 25 °C to separate the lactate ions from the mixed surfactants. Membrane NMWL, TMP and their interactions presented statistically significant influence on the permeate flux. Rejections to lactate ion and SDS were lower than 4.5% and higher than 86%, respectively, whereas Span 80 rejection was 100% in all range of experimental conditions tested. The optimal conditions were established for maximum values of permeate flux, and they were obtained for a 2 bar TMP and 15 kDa NMWL membrane. Under these conditions, the rejections of SDS surfactant and lactate ion were 87.3% and 4.31%, respectively, with a permeate flux of 42.63 L/m 2  h. The antagonistic effect between permeate flux and SDS rejection is also proved.