Estrogenic micropollutant adsorption dynamics onto nanofiltration membranes

Abstract

Nanofiltration (NF) can be used in water and wastewater treatment as well as water recycling applications, removing micropollutants such as hormones. Due to their potential health risk it is vital to understand their removal mechanisms by NF membranes aiming at improving and developing more effective and efficient treatment processes. Although NF should be effective and efficient in removing small molecular sized compounds such as hormones, the occurrence of adsorption onto polymeric membranes results in performances difficult to predict and with reduced effectiveness and efficiency. This study aims firstly at defining, understanding and quantifying the relevant filtration operation parameters and, secondly, in identifying the physical mechanisms of momentum and mass transfer controlling the adsorption and transport of hormones onto polymeric NF membranes in cross-flow mode. The hormones estrone (E1) and 17-β-estradiol (E2) were chosen as they have very high endocrine disrupting potency. The NF membranes used and tested were the NF 270, NF 90, BW30, TFC-SR2 and TFC-SR3 since they have a wide span of pore sizes. The first step is to experimentally acquire the knowledge of how fluid flow hydrodynamics and mass transfer close to the membrane affect hormone adsorption. The focus will be particularly on the effect of operating pressure, circulating Reynolds numbers (based on channel height, Re h) and hormone feed concentration. These hydrodynamic parameters play an important role in concentration polarisation development at the membrane surface. A Re h increase from 400 to 1400 for the NF 270 membrane caused the total mass adsorbed of E1 and E2 to decrease from 1.5 to 1.3 ng.cm-2 and 0.7 to 0.5 ng.cm-2 , respectively. In contrast, a pressure increase from 5 to 15 bar yielded an increase in the adsorbed mass of E1 and E2 from 1.0 to 1.8 ng.cm-2 and 0.5 to 0.7 ng.cm-2 , respectively. Moreover, increasing hormone feed concentration caused an increase in the mass adsorbed for both hormones. These observations led to the conclusion that adsorption is governed by the initial concentration at the membrane surface which, in turn, depends on the hormone feed concentration, operating Re h and pressure. Membrane retention, however, depends on the initial polarisation v ACKNOWLEDGEMENTS I would like to thank Professor Andrea Schäfer for giving me the opportunity to do a PhD and the conditions to have the financial support for it. Her support, constructive criticism, ability to make me think and perseverance were essential to the success of this work. The difficult moments also turned out to be helpful and a valuable contribution to my personal and scientific maturity.

Removal of adsorbing estrogenic micropollutants by nanofiltration membranes: Part B—Modeldevelopment

“…(8b)). As was shown in a previous study [13] this phenomenon causes an increase in solute concentration at the membrane surface, originating a concentration gradient from the bulk feed (C fb ) to the membrane surface (C mf ) as depicted in Fig. 1.…”

“…(3) takes into account experimental evidence where the membrane adsorption isotherm for both hormones was found to be linear [13] (Eq. (4)).…”

“…However, hormone observed retention varies greatly (from o10% up to 100%) [12]. This can be attributed to either different filtration conditions (e.g., pressure) [13] or to enhanced partitioning inside the membrane pore [14] which results in a lower performance than would be expected solely by size exclusion [15,16].…”

“…Furthermore it took into account a flux reduction element caused by sorption of the trace contaminant onto the membrane [29]. A flux decrease, however, does not always occur, possibly due to the low concentrations used ( omg L À 1 ) [13,22,28]. The addition of a convection term in combination with a transient term for adsorption into the contaminant mass transport considerably increases the problem complexity.…”

Assessment of the hindered transport model in predicting the rejection of trace organic compounds by nanofiltration

“…The lower-than-predicted rejection was attributed to the high hydrophobicity that is common for organic compounds [18]. A more hydrophobic substance tended to partition more in the membrane material and transported across the membrane with a higher rate, which in turn results in a lower rejection ratio [11,[19][20][21]. A later refinement of the DSPM&DE model by taking into consideration the solute−membrane affinity improved the prediction accuracy for some TOrCs, but not for others [22,23].…”