Interplay of the forces governing steroid hormone micropollutant adsorption in vertically-aligned carbon nanotube membrane nanopores

Vertically-aligned carbon nanotube (VaCNT) membranes allow water to conduct rapidly at low pressures and open up the possibility for water purification and desalination, although the ultralow viscous stress in hydrophobic and low-tortuosity nanopores prevents surface interactions with contaminants. In this experimental investigation, steroid hormone micropollutant adsorption by VaCNT membranes is quantified and explained via the interplay of the hydrodynamic drag and friction forces acting on the hormone, and the adhesive and repulsive forces between the hormone and the inner carbon nanotube wall. It is concluded that a drag force above 2.2 × 10−3 pN overcomes the friction force resulting in insignificant adsorption, whereas lowering the drag force from 2.2 × 10−3 to 4.3 × 10−4 pN increases the adsorbed mass of hormones from zero to 0.4 ng cm−2. At a low drag force of 1.6 × 10−3 pN, the adsorbed mass of four hormones is correlated with the hormone−wall adhesive (van der Waals) force. These findings explain micropollutant adsorption in nanopores via the forces acting on the micropollutant along and perpendicular to the flow, which can be exploited for selectivity.


REVIEWERS' COMMENTS
Reviewer #1 (Remarks to the Author): [REDACTED].I am in favor of publicafion of the revised manuscript.

Reviewer #3 (Remarks to the Author):
the revision looks fine.

Reviewer #4 (Remarks to the Author):
In order to design befter membranes, it is necessary to understand the mechanisms of adsorpfion and transport in nanopores.This work looks like a mechanisfic study.The data obtained were analyzed in a systemafic and rafional manner.Membranes are mainly used for adsorpfion or retenfion of pollutants, but this work does not provide even basic retenfion or adsorpfion data.The innovafion of the paper is not sufficient, so I cannot recommend its publicafion in Nature Communicafions.

Reviewer #1 (Remarks to the Author):
[REDACTED].I am in favor of publication of the revised manuscript.

Thank you very much for your positive conclusion
Reviewer #3 (Remarks to the Author): the revision looks fine.

Thank you very much!
Reviewer #4 (Remarks to the Author): In order to design better membranes, it is necessary to understand the mechanisms of adsorption and transport in nanopores.This work looks like a mechanistic study.The data obtained were analyzed in a systematic and rational manner.Membranes are mainly used for adsorption or retention of pollutants, but this work does not provide even basic retention or adsorption data.The innovation of the paper is not sufficient, so I cannot recommend its publication in Nature Communications.No retention of E1, E2 and T was observed.For P, permeate concentration appeared to be level at 70 ng/L but this may be because of adsorption instead of retention.P (0.86 nm in diameter) is larger than E1, E2, and T (0.79−0.82 nm) but smaller than the VaCNT membrane pore diameter (~1.7 nm).

Thank you very much for your comments that
The adsorpfion of E1, E2 and T by VaCNT−MF membranes approached saturafion after 100 mL.The adsorpfion of P by both VaCNT−MF and MF membranes did not reach saturafion.The adsorbed mass of P by both MF and VACNT−MF membranes was higher than those of E1, E2 and T.

In conclusion, SH adsorption by the VaCNT membranes is specific to the SH type."
For Figure S9 "To determine whether the VaCNT membrane pore diameter in the nanometre range limits adsorption, the E2 breakthrough curves and adsorbed masses with VaCNT membranes that have three pore diameters (1.7, 2.6 and 3.3 nm) were compared in Figure S9.
the work is indeed a mechanistic study -and to us and membrane community this work conveys important knowledge.The data were analysed in a systematic and rational manner.Concerning the innovation not being sufficient, there is no other data available that covers this topic.If we are wrong, we would like to see it.If anything, we are probably ahead of current knowledge, based on the challenges we faced when discussing this work with very established colleagues, which is what an excellent journal ought to publish.As for your concerns about basic retention or adsorption data -and the statement 'that the work does not even basic retention or adsorption data' is outright wrong.Below we repeat what is already provided in the manuscript and supporting information in terms of (more than basic) retention and adsorption data.We trust that this will satisfy editor and reviewer.

Figure 5
Figure 5 in the main text shows the breakthrough curves at different fluxes, where apart from at the lowest flux (6 L/m 2 .h), the permeate concentration reached the value of the feed concentration within 65 mL, which indicates that the adsorption saturation is reached and there is no retention (otherwise the permeate concentration would be lower than the feed concentration).Figure 5 also shows the adsorbed masses of E2 as functions of permeate volume, which should qualify as adsorption data.The VaCNT membranes did not retain 17β-estradiol (E2) in all experiments.This is because the hydrodynamic diameter of the hormone molecule is smaller than the pore diameter (1.7−3.3 nm).

Figure 5
Figure 5 and relevant discussions are shown in lines 299−315, pages 12−13 as follows.