Permeation of nitrobenzene and 2,4-dinitrotoluene across a fully aromatic polyamide reverse osmosis membrane

Richard Ikechukwu Urama, Purdue University

Abstract

The permeation of nitrobenzene and 2,4-dinitrotoluene across a fully aromatic polyamide FT-30$\sp\circler$ membrane was investigated. Reverse osmosis transport equations (Spiegler-Kedem, 1966) include an advective contribution to solute transport. However, it is not very clear if the assumed advective solute transport is significant since an explicit assumption of negligible concentration polarization is often made. One of objectives of this study was to assess the contribution of concentration polarization, partitioning, diffusive and advective transport toward overall permeation of solute. A secondary objective was to investigate the effects of operating time, feed organic concentration, and temperature on membrane performance. Experiments were performed with a bench-top flat-leaf reverse osmosis unit. Feed solutions contained: NaCl (for the most part) as major solute (2000-6000 mg/L); nitrobenzene (0.8-32 mg/L), and 2,4-dinitrotoluene (3.8-62 mg/L) as trace components. Other operating conditions include: pH 6, temperature range of 10-25$\sp\circ$C, and operating pressure range of 100-400 psi. Observed solute rejections at 25$\sp\circ$C ranged from: 95 to 99.2 percent for NaCl, 99.8 to 99.9 percent for $\rm Na\sb2SO\sb4$, 20 to 60 percent for nitrobenzene, and 87 to 97 percent for 2,4-dinitrotoluene. The overall solute permeation was found to be mainly due to a combination of partitioning at water/membrane interfaces and diffusion across the membrane confirming the validity of the solution-diffusion model by Lonsdale and co-workers (i.e., negligible advective solute permeation). Solute permeation was affected by concentration polarization taking place primarily within a fouling film of corrosion products. Concentration polarization levels corresponded to solute concentrations next to the feed water/membrane interface ranging from 6 to 98 percent (NaCl), and from 2 to 174 percent (nitrobenzene) higher than bulk feed concentrations. The effect of membrane compaction on permeation coefficients is well described by an exponential model with the corresponding compaction rate increasing with molecular size. Temperature effect on permeation rates was well described by an Arrhenius type relationship. The resulting activation energies for permeation and diffusion revealed significant solute/membrane interactions in the permeation of nitrobenzene and 2,4-dinitrotoluene. Membrane performance was significantly affected by 2,4-dinitrotoluene concentration, reducing water flux by as much as 22 percent at the maximum feed concentration of 62 mg-DNT/L.

Degree

Ph.D.

Advisors

Marinas, Purdue University.

Subject Area

Civil engineering|Chemical engineering

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