Removal of Enantiomeric Ibuprofen in a Nanofiltration Membrane Process

A study of the behavior of R- and S-enantiomers of ibuprofen (R-IBU and S-IBU) in aqueous solution by nanofiltration (NF) membranes revealed that up to 23% of the pharmaceutical was adsorbed onto the stainless steel equipment of a flat-sheet experimental unit. Mass balances disclosed that IBU’s S-enantiomer was primarily responsible for the adsorption onto the equipment. Additional IBU adsorption was also experienced on the NF membrane coupons, verified by increased contact angle measurements on the surfaces. The IBU-equipment adsorptive relationship with and without the membrane coupon were best described by Freundlich and Langmuir isotherms, respectively. At a feed water pH of 4.0 units and racemic µg/L IBU concentrations, NF removal ranged from 34.5% to 49.5%. The rejection of S-IBU was consistently greater than the R-enantiomer. Adsorption onto the surfaces influenced NF rejection by 18.9% to 27.3%. The removal of IBU displayed a direct relationship with an increase in feed water pH. Conversely, the adsorption of IBU exhibited an indirect relationship with an increase in feed water pH.


Property
Value Molecular formula C13H18O2 CAS no.

Membrane Properties and Experimental Procedures
Prior to pressurized experiments, membrane operational properties were determined. The water flux coefficient (Lp) was evaluated by collecting permeate water flux at a pressure range from 25 to Anionic Ibuprofen 200 psi, which satisfies the typical NF pressure range in application. Water flux was determined as a ratio of the permeate flow over the area of the membrane, shown as Equation (S1) [7].
Virgin contact angle was determined upon receipt of the membranes, and compacted contact angle was determined after compaction with DI for 24 h, followed by drying for 48 h. A ramé-hart Model 100 Goniometer (Succasunna, NJ, USA) was utilized to determine membrane hydrophobicity via contact angle. Contact angle measurements were attained utilizing the sessile drop technique [8,9]. Membrane coupons were dried and inserted on the stage with the active layer facing up. A micrometer syringe delivered a droplet of DI water onto the membrane surface, and a contact angle was measured by the goniometer. To obtain a representative contact angle of the entire membrane surface, ten contact angle measurements were taken on various areas of the membrane coupon and averaged. The bench-scale, flat-sheet unit was operated by pumping pressurized feed water through a membrane coupon and producing a permeate and concentrate stream. During each experiment, permeate and concentrate streams were recycled back into the feed reservoir to simulate a constant feed composition. However, permeate tubes were taken from the feed reservoir and collected when appropriate.

Solid Phase Extraction Method
A solid phase extraction (SPE) method was utilized to extract and preconcentrate R-and S-IBU enantiomers [10]. Oasis HLB 3 mL, 60 mg cartridges were conditioned by gravity with 3 mL acetonitrile, 3 mL methanol, and then 3 mL HPLC grade water. Then, samples were loaded through the SPE cartridges under vacuum of approximately 4 psi and a constant flow rate of less than 2 mL/min. Then, sample bottles were washed with 6 mL of HPLC grade water, which was also sent through the cartridges. Next, cartridges were dried for 5 min under a vacuum pressure of 10 psi and eluted into sample tubes by gravity with 4 mL acetonitrile. Then, samples were evaporated using an Organomation N-EVAP nitrogen gas evaporator with water bath at 65 °C (Berlin, MA, USA) and then reconstituted with 1 mL methanol/formic acid (100:0.1 v/v) and manually agitated to dissolve the residue.

Adsorption Isotherm Modeling
Adsorption isotherms can be used to describe the relationship between the concentration of IBU adsorbed on a solid surface in relation to its surrounding aqueous content at a constant temperature and pressure. In this work, linearized forms of the Langmuir, Freundlich, and Temkin isotherm were used to model the relationship between the equilibrium concentration of IBU adsorbed on a solid surface, shown as Equations (S2) to (S4) [11][12][13]: where, q = concentration of IBU on solid surface (µg/cm 2 ) q = maximum adsorption capacity (µg/cm 2  Equipment stainless steel surface area was calculated as 10,247 cm 2 , and 10,331 cm 2 with the membrane coupons installed. Adsorption isotherms were considered for equipment-IBU and equipment-IBU-membrane relationships. Slopes and intercepts from the linearized forms of the isotherms were used to calculate appropriate parameters of each adsorption isotherm.

Determination of Rejection
Rejection (r) describes the ability of a membrane to remove solute from water and is calculated using Equation (S5) [7].
Equilibrated IBU rejection was calculated by collecting feed and permeate at 24 h. Hence, the overall rejection due to the adsorptive behavior of IBU could be compared to the actual rejection of the solute at quasi-equilibrium.