Modification and characterization of ultrafiltration membranes for treatment of produced water

This contribution describes the surface modification of low molecular weight cutoff regenerated cellulose ultrafiltration membranes by grafting poly(N-isopropylacrylamide) (PNIPAAm)-block-poly(oligoethylene glycol methacrylate) (PPEGMA) nanolayers from the membrane surface using surface-initiated atom transfer radical polymerization. The objective of the work was to change the membrane surface properties in ways that limit foulant accumulation on these membranes and provide an easy, chemical-free way to remove any attached foulants during produced water filtration. ATR-FTIR spectra confirmed the successful grafting of both polymers from the membrane surface. Analysis of AFM topographical images indicated that membrane surface roughness decreased slightly from 2.6 nm for the unmodified membrane to 1.7 nm following PNIPAAm-b-PPEGMA modification. The performance of the modified membranes was tested by measuring water flux using synthetic produced water developed from an oil-in-water emulsion. Polymer grafting led to a roughly 40% decrease in the water flux, but the final flux was comparable to commercial membranes used for removal of organics with high salt passage. Furthermore, modified membranes showed slower flux decline than unmodified membranes, and, hence, the modified membranes allowed a 13.8% higher cumulative volume of water to be processed over a 40 h cross-flow filtration run. To test potential for chemical-free cleaning, filtration runs were carried out before and after a temperature-controlled water rinse. Flux recovery was better for the modified membranes after a cold water rinse. The flux recovered fully to initial values for both the PNIPAAm- and PNIPAAm-b-PPEGMA-modified membranes; while only ∼81% of the initial flux was recovered for the unmodified membrane. All three membranes exhibited poor salt rejection, as would be expected for ultrafiltration membranes. However, TOC removal efficiencies were higher than 97% for PNIPAAm-b-PPEGMA-modified membranes. Taken together, results from this work indicate that PNIPAAm-b-PPEGMA modified ultrafiltration membranes could be used to separate emulsified oils from large volumes of produced water at high flux.