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Does Hydrophilic Polydopamine Coating Enhance MembraneRejection of Hydrophobic Endocrine-Disrupting Compounds?Hao Guo, Yu Deng, Zhijia Tao, Zhikan Yao, Jianqiang Wang, Chuner Lin, Tong Zhang,

Baoku Zhu, and Chuyang Y. Tang*,

Department of Civil Engineering, The University of Hong Kong, Pokfulam, Hong Kong 999077ERC Membrane and Water Treatment Technology (MOE), Department of Polymer Science and Engineering, Zhejiang University,Hangzhou 310027, P. R. China

*S Supporting Information

ABSTRACT: Endocrine-disrupting compounds (EDCs), animportant class of micropollutants with potent adverse healtheffects, are generally poorly rejected by traditional thin filmcomposite polyamide membranes and thus pose significant risks inmembrane-based water reclamation. We hypothesize thatmembrane rejection of hydrophobic EDCs can be enhanced bya hydrophilic surface coating. Using polydoamine (PDA) as amodel hydrophilic coating layer, the PDA-coated NF90 membraneexperienced an up to 75% reduction in the passage of bisphenol Acompared to the control (NF90 without coating). Meanwhile, wealso observed a systematic increase in the level of rejection of threehydrophobic parabens with an increase in PDA coating time. Incontrast, there were no systematic changes in the rejection ofneutral hydrophilic polyethylene glycol, which suggests that the enhanced rejection of EDCs was due to weakened EDCmembrane hydrophobic interaction. Further sorption tests revealed that the hydrophilic PDA coating could effectively decreasethe rate of sorption of EDCs by the membrane, which is responsible for the improved rejection as predicted by the solutiondiffusion theory. This study reveals an exciting opportunity for engineering membrane surface properties to enhance the rejectionof targeted micropollutants, which has important implications in membrane-based water reclamation.

INTRODUCTIONThe grand challenge of water scarcity calls for more sustainablewater resource management.1,2 Reclamation from municipalwastewater, e.g., using membrane-based reverse osmosis (RO)and nanofiltration (NF), can play an important role inaddressing this challenge because of its practically unlimitedsupply.1,38 RO and NF membranes reject a variety ofcontaminants, including dissolved ions.9 Nevertheless, someorganic micropollutants with low molecular weights, neutralcharges, and/or high hydrophobicities can still pass throughRO/NF membranes.1013 One important class of micro-pollutants consists of endocrine-disrupting compounds(EDCs). Because of their ubiquitous occurrence in municipalwastewater1416 and potent endocrine disrupting effects even attrace concentrations,17 the presence of EDCs in reclaimedwater has been regarded as one of the most critical risksassociated with water reclamation.18

The existing literature on RO/NF membrane preparationcenters on the optimization of water permeability and saltrejection as well as antifouling performance.19,20 Generally, amodern RO/NF membrane based on polyamide chemistry caneasily achieve high salt rejection (e.g., 99% for NaCl by ROand 90% for divalent ions by NF).21,22 However, some studieshave reported insufficient rejection (e.g., 50%) of those

membranes for neutral and hydrophobic EDCs, and the lowrate of rejection was often attributed to the hydrophobicinteraction between the compounds and membrane sur-face.12,2325 Indeed, many studies have consistently reportedthat more hydrophobic compounds tend to have lower rates ofrejection under otherwise similar conditions (e.g., comparablemolecular size), citing their increased level of sorption by RO/NF membranes as a main cause.10,12,2328 On the other hand,fewer studies have systematically investigated the role ofmembrane surface hydrophilicity/hydrophobicity in therejection of trace organic compounds such as EDCs. Wetherefore hypothesize that the rejection of hydrophobic EDCscan be significantly enhanced by improving membrane surfacehydrophilicity. If this hypothesis were true, it would open a newdimension for improving membrane rejection of EDCs, e.g., bymembrane surface coating or grafting.In this study, we prepared a hydrophilic surface coating on a

commercial nanofiltration membrane and systematicallyinvestigated its effect on the removal of four hydrophobic

Received: July 12, 2016Revised: August 11, 2016Accepted: August 15, 2016

Letter

pubs.acs.org/journal/estlcu

XXXX American Chemical Society A DOI: 10.1021/acs.estlett.6b00263Environ. Sci. Technol. Lett. XXXX, XXX, XXXXXX

pubs.acs.org/journal/estlcuhttp://dx.doi.org/10.1021/acs.estlett.6b00263

EDCs. Polydopamine (PDA), widely studied for membraneantifouling modifications,29 was adopted as a model coatingmaterial because of its easy preparation and its ability to formstable films on a wide range of substrates.3034 Our results haveimportant implications for membrane design tailored for waterreclamation based on the nature of targeted organic micro-pollutants.

MATERIALS AND METHODSGeneral Chemicals. Unless described otherwise, all

solutions were prepared from analytical-grade chemicals anddeionized (DI) water. Dopamine hydrochloride (J&K ScientificLtd.) and tris (Acros Organics, Geel, Belgium) were used formembrane surface coating. Polyethylene glycol [PEG, averagemolecular weight (MW) of 200 (Aladdin)] was used toevaluate membrane rejection of the hydrophilic neutralcompound. Sodium chloride (Uni-Chem), hydrochloride acid(37 wt %, VWR, Dorset, U.K.), and sodium hydroxide (Uni-Chem) were used for solution chemistry adjustment. Optima-grade methanol (Fisher Scientific, Pittsburgh, PA) was used forultraperformance liquid chromatography coupled with tandemmass spectrometry (UPLCMS/MS) analysis and for EDCextraction.EDCs. Four EDCs were investigated in this study, including

ethylparaben (99%), propylparaben (99%), benzylparaben(99%), and bisphenol A (BPA, 97%). BPA was obtainedfrom Acros Organics, and the other EDCs were supplied bySigma-Aldrich (St. Louis, MO). The physicochemical proper-ties of the EDCs are summarized in Table 1. A stock solution ofeach EDC (1 g/L) was prepared by dissolving the compoundinto methanol and stored at 20 C.

Membrane. A commercial nanofiltration membrane, NF90,was provided by Dow Chemical Co. NF90 is a thin filmcomposite (TFC) membrane, with a fully aromatic polyamideas its rejection layer.38 The membrane was thoroughly rinsedwith DI water to remove any impurities and was soaked in DIwater for at least 24 h before further use.

Preparation of the PDA Coating. A hydrophilic PDAcoating was prepared by the self-polymerization of dopamine atroom temperature (25 C) following the proceduresdescribed by Lee et al.30 Briefly, a pristine membrane couponwas placed in a custom-designed container (SupportingInformation section S1) with only its rejection layer exposedfor coating. A 300 mL dopamine hydrochloride/tris solution[0.2 wt % dopamine hydrochloride, 10 mM tris solution (pH8.5)] was added to the container, and the self-polymerizationwas performed under moderate shaking for a predeterminedduration (0.5, 1, 2, and 4 h). Coated membranes are denoted asNF90-C0.5, NF90-C1, NF90-C2, and NF90-C4, respectively,in accordance with their coating time. The coated membraneswere thoroughly rinsed with DI water to remove any unreactedresidues. According to the published literature, the PDA coatingis very stable (e.g., under ultrasonic treatment39 or over longtime exposure40).

Membrane Characterization. Membrane surface mor-phology was characterized by a field-emission scanning electronmicroscope (FE-SEM, Hitachi S-4800). Vacuum-dried mem-brane samples were sputter-coated with a thin layer of gold(BAL-TEC SCD 005), and SEM micrographs were obtained atan acceleration voltage of 5 kV. Atomic force microscopy(AFM) was employed to determine the membrane surfaceroughness using a scanning probe microscope (Dimension3100, Veeco, Plainview, NY) with a scan area of 10 m 10

Table 1. Physicochemical Properties of the Four EDCs

aData obtained from ref 35. bData obtained from ref 36. cData obtained from ref 37.

Table 2. Membrane Surface Properties and Separation Performance of Uncoated and PDA-Coated NF90

surface energy (mN/m)

membranewater permeability (L m2 h1

bar1)aNaCl rejection

(%)awater contact angle

(deg)b total polar dispersiveroughness Ra

(nm)PEG rejection

(%)c

NF90 7.06 0.69 83.5 2.9 69.8 1.8 37.4 13.6 23.8 50.8 11.8 86.0 1.8NF90-C0.5 6.70 0.89 84.8 1.4 66.3 1.2 40.8 19.9 20.9 57.8 9.0 85.2 4.3NF90-C1 5.80 0.86 82.7 1.4 58.3 2.6 42.0 19.9 22.1 61.2 4.7 83.6 4.1NF90-C2 5.10 0.36 83.1 1.4 61.0 1.9 40.8 18.3 22.5 60.9 12.2 87.8 3.1NF90-C4 3.91 0.21 86.4 1.1 62.0 2.4 39.8 18.1 21.7 57.2 2.8 81.7 3.0

aExperimental condition: 10 mM NaCl, pH 6.6, and 25 C. The stabilized water flux was determined by weighing permeate water using a digitalbalance, and salt rejection was determined on the basis of the measured conductivity values of the feed and permeate water (Ultrameter II, Myron L,Carlsbad, CA). The results were calculated from at least three parallel experiments. bContact angle values have been corrected for the roughnesseffect using the Wenzel equation (see Supporting Information section S2). cExperimental condition: 200 ppm PEG 200, 10 mM NaCl, pH 6.6, and25 C. PEG samples from bulk solution and permeate were analyzed with a total organic carbon (TOC) analyzer (Aurora 1030, OI Analytical,College Station, TX). The results were calculated from at least three parallel experiments.

Environmental Science & Technology Letters Letter

DOI: 10.1021/acs.estlett.6b00263Environ. Sci. Technol. Lett. XXXX, XXX, XXXXXX

B

http://pubs.acs.org/doi/suppl/10.1021/acs.estlett.6b00263/suppl_file/ez6b00263_si_001.pdfhttp://pubs.acs.or

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