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Water Research 108 (2017) 78e85

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Water Research

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Application and characterization of electroactive membranes based oncarbon nanotubes and zerovalent iron nanoparticles

Jorge E. Yanez H. a, Zi Wang a, Sascha Lege a, Martin Obst b, Sebastian Roehler c,Claus J. Burkhardt c, Christian Zwiener a, *

a Eberhard Karls Universit€at Tübingen, Center for Applied Geosciences (ZAG), Environmental Analytical Chemistry, H€olderlinstr. 12, 72074 Tübingen,Germanyb Universit€at Bayreuth, Bayreuth Center of Ecology and Environmental Research, Universit€atsstraße 30, 95447 Bayreuth, Germanyc NMI Natural and Medical Sciences Institute, Markwiesenstr. 55, 72770 Reutlingen, Germany

a r t i c l e i n f o

Article history:Received 13 May 2016Received in revised form29 August 2016Accepted 22 October 2016Available online 22 October 2016

Keywords:Electroactive membranesZerovalent iron nanoparticles (NZVI)AdsorptionPharmaceutical degradationTransformation product (TPs)Emerging pollutant

* Corresponding author.E-mail address: christian.zwiener@uni-tuebingen.

http://dx.doi.org/10.1016/j.watres.2016.10.0550043-1354/© 2016 Elsevier Ltd. All rights reserved.

a b s t r a c t

Carbon nanotube (CNT) membranes were produced frommulti-walled CNTs by a filtration technique andused for the removal of the betablocker metoprolol by adsorptive and reactive processes. The reactivity ofCNT membranes was enhanced by nanoparticulate zero-valent iron (NZVI) which was deposited on theCNT membranes by pulsed voltammetry applying defined number of pulses (Fe-CNT (100) and Fe-CNT(400) membranes). Surface analysis with SEM showed iron nanoparticle sizes between 19 and425 nm. Pore size distribution for the different membranes was determined by capillary flow porometry(Galwick fluid). Pore size distribution for all membranes was similar (40 nm), which resulted in a waterpermeability typical for microfiltration membranes. Metoprolol was removed by the CNT membrane onlyby sorption, whereas the Fe-CNT membrane revealed also metoprolol degradation due to Fenton typereactions. Further application of electrochemical potentials on both the CNT and the Fe-CNT membranesimproved the removal efficiencies to 74% for CNT membranes at 1 V and to 97% for Fe-CNT (400)membranes at 1 V. Seven transformation products have been identified for metoprolol by high-resolution mass spectrometry when electrochemical degradation was performed with CNT and Fe-CNTmembranes. Additionally, two of the identified transformation products (TPs) were also observed forFe-CNT membranes without the application of electrochemical potential. However, only 10% of thedegraded metoprolol could be explained by the formation of TPs.

© 2016 Elsevier Ltd. All rights reserved.

1. Introduction

Nanomaterials present a wide range of interesting propertiesand applications in different fields (e.g. environmental remediation,energy conversion and storage). Zerovalent iron nanoparticles(NZVI) are attractive nanomaterials; they present high reactivitytowards the transformation of pollutants and hence are one of themost extensively applied nanomaterials for remediation of soil,groundwater and treatment of hazardous waste. In recent years,NZVI were applied both in reductive (Parshetti and Doong, 2009;Xie and Hang, 2005; Klausen et al., 2003) and oxidative (Stieberet al., 2011; Forrez et al., 2011; Luo et al., 2011) processes ingroundwater systems. The mechanism of the reductive process

de (C. Zwiener).

involves the transfer of hydrogen or electrons from the aqueousFe(II) or from the surface of NZVI to contaminants.

Under aerobic conditions, NZVI present an oxidative behaviorproducing reactive oxygen species including hydrogen peroxide,hydroxyl and peroxy radicals (Joo et al., 2004, 2005; Keenan andSedlak, 2008). The oxidative pathway starts with the reduction ofdissolved oxygen by Fe(0) to form H2O2 (Eq. (A1) in Section S1 ofsupporting information). H2O2 may be reduced by Fe(0) to formwater (Eq. (A2)) or reacts under acidic conditions with Fe(II) toproduce OH

�radicals (Eq. (A3)), which are potent oxidants. At

pH > 6 Fe(II) is oxidized by oxygen and produces H2O2 (Eqs. A4 andA5). The desorbed H2O2 reacts with Fe(II) forming at this pH valueferryl ions which present high oxidative power (Eq. (A6)) (Joo et al.,2004, 2005; Keenan and Sedlak, 2008; Phenrat et al., 2007).

Despite the extensive use of NZVI and their combination withother metals for pollutant removal, their efficient application islimited. One main reason is the agglomeration tendency of

J.E. Yanez H. et al. / Water Research 108 (2017) 78e85 79

nanoparticles (NP) once synthesized, which considerably reducestheir reactivity (Bataineh et al., 2012). Furthermore, the toxicity ofNP discharged into the environment during and after their appli-cation has to be considered (Auffan et al., 2008). The problems ofagglomeration and discharge can be solved by designing NP withswitchable properties (Brandl et al., 2015) or by immobilization ofNP on solid-phase supports such as activated carbon, metal oxides,zeolites or membranes (Guan et al., 2015; Shujing et al., 2010; Liuet al., 2004; Shujing and Scurrell, 2003; Sun et al., 2003; Yanget al., 2014). For NZVI the use of cross-linked polymer mem-branes revealed the best results concerning NP size and segregation(Xu and Bhattacharyya, 2006, 2008). Polyacrylic acid (PAA) andpoly(ethylene glycol) (PEG) have been widely used as cross-linkersto immobilize nanoparticulate metals in cellulose acetate, nylonand polyvinylidene fluoride (PVDF) membranes, which have beensuccessfully used for the treatment of nitrobenzenes, TCE, PCBs andtrichloroacetic acid (Xu and Bhattacharyya, 2006, 2008; Tong et al.,2011; Wang et al., 2008; Parshetti and Doong, 2009). One majorproblem of these membranes is the limited stability and reuseunder conditions used for NZVI regeneration (Xu andBhattacharyya, 2008; Tong et al., 2011; Wang et al., 2008;Parshetti and Doong, 2009). Therefore, the resistance of alterna-tive support materials has to be investigated under typical regen-eration and application conditions.

Carbon nanotubes (CNTs) are interesting nanomaterials becausethey present high surface areas, excellent adsorption capacity,electrical conductivity and stability under physical and chemicalstress (heat, solvents, extreme pH conditions). Multi-walled carbonnanotubes (MWCNT) have been used for adsorption and removal ofvarious organic contaminants (pharmaceuticals, dyes, pesticides,phenols and other toxic organic contaminants) (Oleszczuk et al.,2010; Yu et al., 2014; Liang et al., 2014; Socas-Rodríguez et al.,2014).

CNT-membranes (buckypapers, BPs) are promising materialscomposed by MWCNT; they present high porosity and mechanicalrobustness in addition to the characteristics described for the CNT.They have been used in fuel cells (Hussein et al., 2011) and watertreatment (Fan et al., 2015; Liu et al., 2015; Rahaman et al., 2012;Vecitis et al., 2011a,b; Gao and Vecitis, 2011; Das et al., 2014;Oulton et al., 2015; Sears et al., 2010; Gao et al., 2014; Wanget al., 2012). In fuel cells BPs have been applied as support ofmetal nanoparticles (e.g. Pt) for catalytic purposes (Chiang andCiou, 2011). In water treatment they have been used as electro-chemical filters for oxidative and reductive degradation processesof organic pollutants (Liu et al., 2015; Gao and Vecitis, 2011; Vecitiset al., 2011b; Das et al., 2014; Gao et al., 2014), as well as for theinactivation of viruses and bacteria (Rahaman et al., 2012; Vecitiset al., 2011a) and as catalyst for efficient production of OH

�radi-

cals in ozonation processes (Oulton et al., 2015).There are several commonly used techniques to synthesize

metal nanoparticles, such as sol-gel, microwave, sputtering,reduction of metal ions and electrodeposition (Mohanty, 2011; Coteet al., 2003; Fu et al., 2007; Spatz et al., 1996; Garcia-Serrano andPal, 2003; Panigrahi et al., 2004). Electrodeposition is a usefultechnique for producing nanoparticles with controlled character-istics (size, morphology and composition). This process is fast,inexpensive and binder free, but it requires supports with electricalconductivity. The generated nanoparticles will be immobilized in oron the support and not in suspension for other uses. On the otherhand, this technique presents several advantages: the producednanoparticles present high purities (Mohanty, 2011), the size,morphology and crystallographic orientation can be controlledaccording to the operating conditions (Lu and Tanak, 1996; Huang,and Yang, 2005; Finot et al., 1999; Srinivasan and Weidner, 1997)and the generated nanoparticles will be immobilized on the

support of interest. Pulse voltammetric electrodeposition is a ver-satile technique for the generation of tailor-made nanomaterials(Puippe, 1986). It has been used for the preparation of nano-crystalline metals and alloys, for instance nano-Ni, nano-Pd, nano-Fe (Natter et al., 2000). Large bulk samples of high purity, lowporosity and enhanced thermal stability can be prepared by thistechnique. Furthermore, this electrochemical procedure allows toobtain desired physical and chemical properties by selecting thenanostructure (mainly grain size and grain size distribution). Thiselectrochemical synthesis process presents a two-step mechanism:first, the nuclei are formed in high quantity; second, these nucleigrow. The number and size of nuclei can be controlled by theapplied overvoltage (Budevski et al., 1996; L€offler et al., 2000) (seeEq. (B1) in S1 of supporting information): the higher the over-voltage, the smaller the nuclei radii. The higher the current density,the higher is the nuclei formation rate. The duration of such highdeposition rate is no longer than a few milliseconds (ton-time) dueto the depletion of the cation concentration in the vicinity of thecathode, then the process is diffusion controlled. In order tomaintain the grain size, of the potential has to be applied in pulses;between the pulses (toff-time) the cation can diffuse from the bulksolution to the vicinity of the cathode before the next pulse starts.Off-times are usually between 20 ms and 100 ms. Oswald ripeningoccurs during off-time and the particles grow. Therefore, there areseveral possibilities to control grain size: current density, off-time,temperature, pH-value, bath composition, hydrodynamic condi-tions and the use of special current shapes (Natter andHempelmann, 2000).

Due to the disadvantages of dispersed NZVI and considering theinteresting properties of the BPs, we present the production andcharacterization of a newmembrane based on MWCNTs as supportfor the electrochemical immobilization of NZVI. In addition, wedemonstrate the adsorption and oxidative degradation capability ofthese materials for the pharmaceutical metoprolol, which is awidely applied selective b1 receptor blocker and commonlydetected in the aquatic environment.

2. Materials and methods

2.1. Reagents

Commercially available chemicals with optima-LC-MS gradefrom Fischer Scientific were used without purification: water,methanol, acetonitrile, acetone, formic acid and ammonium ace-tate. Ammonium hydroxide (�25% in H2O, eluent additive for LC-MS) was purchased from Fluka. Iron (III) nitrate nonahydrate(98%) was purchased from Sigma-Aldrich. For the membrane pro-duction we used carboxylated functionalized MWCNTs (>95%, OD:30e50 nm) from US Research Nanomaterials INC. Triton™X-100was obtained from Sigma-Aldrich and TE36 membrane filters(PTFE, 0.45 mm pore size and 50 mm diameter) were purchasedfrom Whatman and the copper conductive tape from 3 M.

2.2. Membrane production. (CNT-membranes)

Carboxylated functionalized MWCNTs (100 mg) were mixedwith 200 mL of Triton™X-100 solution at 0.5%v/v, Triton™X-100 isa negatively charged surfactant which improves the dispersion ofthe MWCNTs in solution in order to avoid the agglomeration of thenanotubes. The mixture was stirred overnight (15 h) and immersedafterwards in an ultrasonic bath (Bandelin sonorex, model RK106)for 3 h. Afterwards, the homogenous dispersionwas centrifuged for20 min at 1233xG and the supernatant was filtered through a filtermembrane (TE36) using a zone of 40 mm of diameter and applying0.5 bar of underpressure. Once the complete supernatant was

J.E. Yanez H. et al. / Water Research 108 (2017) 78e8580

filtered, the obtained membrane was rinsed 2 times with 100 mLacetone and 3 times with 200mL water and dried for 8 h at 85 �C inan oven. To guarantee the reproducibility of the generated mem-branes (morphology, thickness and pore size distribution) theexperimental conditions were kept constant in every step duringthe membrane production: temperature (25 �C), mass of MWCNTs,Triton concentration, stirred/ultra-sonication time, centrifugationtime/speed, vacuum filtration pressure, cleaning and dry processes.

2.3. Electrochemical deposition of NZVI. Production of Fe-CNT (100)and Fe-CNT (400)

For electrochemical deposition of iron nanoparticles on thedried membrane, 50 mL of acetone were passed through themembrane followed by 150 mL of a 500 mMKCl solution containing10 mM iron (III) nitrate. Afterwards, a copper conductive tape wasfixed on the border of the membrane and the wet membrane wasfixed in an electrochemical cell as working electrode. Once themembrane was fixed, the cell was filled with Fe (III) solution. Themembrane was placed between two Pt meshes used as counterelectrodes and Ag/AgCl (3 M) was used as reference electrode. It isimportant to guarantee that the geometry of the electrochemicalcell provides a uniform electric field on the membrane.

The deposition of the nanoparticles was performed by pulsevoltammetry (In section S2 of supporting information Fig. A1) on amembrane area with 30 mm of diameter. 100 and 400 rectangularnegative pulses were applied. The pulse voltammetry was per-formed applying alternate pulses of -1 V per 0.2 s followed bypulses of 0 V per 1 s using a potentiostat EZstat-pro (NuVant Sys-tems Inc.; Indiana). A negative overpotential was used compared tothe potential of iron (III) to iron (0) of�37mV vs Ag/AgCl. The shortnegative pulses allowed a slow reductive deposition of iron ac-cording to the interaction of the solution with the membrane sur-face. With the first pulses several nuclei of Fe(0) are formed, uponwhich the selective deposition of iron continues according to thedifference in conductivity between the nucleus of iron (0) and theMWCNTs. In order to achieve reproducible nanoparticle sizes, allparameters except the number of pulses were constant during theexperiments (pH value, temperature at 25 �C, iron (III) nitrateconcentrations).

2.4. Physicochemical characterization of the generated membranes

2.4.1. Morphology and compositionScanning electron microscopy (SEM) was used for morpholog-

ical characterization. The images were acquired using a Carl ZeissAuriga 40 instrument at acceleration voltages of 5 and 3 kV. Nofurther coating of samples was necessary. A high topographiccontrast was achieved by an in-lens secondary electron (SE) de-tector. For the identification of iron particles on CNT membranes,energy-filtered imaging of backscattered electrons (EsB detector)was used featuring a higher material contrast than SE imaging. Thethickness and the morphology of the membranes, and the distri-bution and size of particles were measured at least 5 times indifferent membrane regions and also for different replicates. Thesurface areas of the membranes covered by nanoparticles wasmeasured upon thresholding the images using the open-sourceimage analysis software FIJI (Schindelin et al., 2012).

2.4.2. Pore size determinationThe pore size distribution was determined for the CNT mem-

brane and Fe-CNT (100 and 400 pulses) using a capillary flowporometer (Porous Materials, Inc., Ithaca NY, model CPF1500A)using Galwick (surface tension: 15.9 Dynes/cm) as wetting fluidand applying a pressure range between 59 and 3240 kPa. Pore size

distributionswere determined based on a simplified Young-Laplaceequation from wet and dry runs with a pressure step method.

2.4.3. Permeability and electrical resistanceThe conductivity was determined in Siemens (S) using a mul-

timeter (Extech instruments, model mn47) and measured 6 timeson different positions for each produced membrane; for measuringthe conductivity a small piece of copper tape was placed on twopoints of the membrane separated by 40 mm of distance (mem-brane diameter). The permeability of the membranes wasmeasured by passing several aliquots (30 mL) of Milli-Q waterthrough an area of 20 mm diameter and applying an underpressureof 0.5 bar for the produced membranes.

2.5. Metoprolol sorption

Metoprolol was selected as target compound for this experi-ment because it is a pharmaceutical compound commonly detectedin the aquatic environment. Besides, its oxidative degradation hasbeen well studied (Faber et al., 2014; Johansson et al., 2007;Rubirola et al., 2014). It is important to note that the adsorptionexperiments were performed in triplicates with 3 freshly-preparedmembranes (CNT and Fe-CNT membranes according to theexperiment).

Aliquots of 30 mL of a metoprolol solution (100 mg/L; pH ¼ 7)were passed through 20 mm diameter CNT and Fe-CNT (400)membranes. Once the sorption experiment was finished, thedesorption of metoprolol from the membranes (recovery) wasperformed passing 70 mL solution of acetone with 0.2% formic acidthrough themembranes. This recoverymethodwas stablished afterassessing different approached for metoprolol desorption. Thecollected solution was completely evaporated and reconstitutedwith LC-MS water for further analysis. The collected aliquots anddesorption samples were analyzed by high-performance liquidchromatography coupled to a triple quadrupole mass spectrometer(HPLC-QqQ) in order to determine the total concentration ofmetoprolol. In addition, some of the collected aliquots wereanalyzed by high-performance liquid chromatography coupled to aquadrupole-time of flight mass spectrometer (HPLC-QTOF) in orderto determine transformation products (TPs) of metoprolol. Detailedinformation on the analysis methods for quantification of meto-prolol and identification of TPs is given in the section S2 of sup-porting information.

2.6. Electrochemical oxidation of metoprolol

In order to evaluate the electrochemical oxidative performanceand to investigated the formation of TPs of the membranes, afiltration setup was used for passing through the membranes asolution of metoprolol (2 mg/L at pH ¼ 7). As initial step, four ali-quots of 30 mL of the metoprolol solution were passed through theCNT and Fe-CNT (400) membranes to guarantee the saturation ofthe sorption sites. Afterwards, the membrane was connected to apotentiostat and used as working electrode. Potentials of þ0.55 Vand þ1 V vs Ag/AgCl were applied on the membrane in differentexperiments, respectively using a Pt mesh as counter electrode andAg/AgCl as reference electrode. During the application of the po-tentials four 30 mL aliquots of the same solution were passedthrough themembrane and collected; these collected aliquots wereanalyzed by HPLC-QTOF-MS for the formation of TPs.

J.E. Yanez H. et al. / Water Research 108 (2017) 78e85 81

3. Results and discussion

3.1. Membrane production and physicochemical characterization

CNT membranes with a diameter of 40 mm were produced bythe filtration technique on a PTFE membrane as support accordingto the method described above. NZVI was deposited by pulsedvoltammetry (100 and 400 pulses of -1 V in a time window of 0.2 s)on CNT membranes using these membranes as working electrode(Fe-CNT (100) or Fe-CNT (400) membranes). SEM characterizationof the CNT membranes reveals a homogenous morphology of thesurface showing the network of nanotubes (Fig.1). It is important tonote that similar morphology has been found for five replicates ofthe produced membranes. The thickness of the different mem-branes was determined by the same technique and is about 60 mmfor all replicates (Fig. 1).

The CNT and Fe-CNT membranes have been characterized bySEM using the EsB signal which gives a clear compositional contrastbetween carbon and Fe on the surface (Fig. 2). On Fe-CNT mem-branes after applying 100 pulses Fe nanoparticles are not yetvisible. It is only possible to see some small particles which are notfrom Fe deposition, since they are already visible on the pure CNTmembrane. These small particles possibly originate from the syn-thesis of the CNTs and do not present any influence or activity in thenext experiments.

Fe-CNT membranes after 400 pulses show a clear presence ofnanostructures, which are not evenly distributed over the wholemembrane surface (Fig. 2, right). This figure shows that nano-particulate Fe occurs only on the electroactive zone of the mem-brane (right part was covered during electrodeposition). Thedensity of nanoparticles is directly related to the number of pulses.

The size distribution of nanoparticles deposited on the Fe-CNTmembrane modified with 400 pulses (S3 Fig. A2) is in the rangebetween 19 and 425 nmwith a distinct maximum of the frequencydistribution at 133 nm (Fig. 3). This is a promising size providinglarge surface areas for adsorptive and reactive processes.

The mean pore diameters of CNT and Fe-CNT membrane sur-faces were quite similar and between 37 and 41 nm and amaximum pore size distribution between 24 and 26 nm (Table 1).Largest pores are in the range of 1 mm. Therefore, surface pore sizesare not affected by Fe deposition. Water permeability of CNT andFe-CNT membranes was quite similar according to a t-test for twoindependent samples (a ¼ 5%) and about 220 L h�1 m�2 bar�1

(Table 2), which is a typical value for microfiltration membranes.This allows to apply CNTmembranes for water filtration. Due to thelower pore sizes of the CNT membranes compared to the support

Fig. 1. SEM images of a cross section (left, thickness 57 ±

we assumed that the permeability is dominated by the CNTmembrane itself. The electrical conductivity of CNT membraneswas increased with the iron deposition (Table 2). Conductivity is animportant feature for further use of these membranes in electro-chemical water treatment.

3.2. Metoprolol sorption and degradation

The sorption behavior of metoprolol on CNT and Fe-CNT mem-branes was investigated in flow-through experiments at a flow rateof 0.85 mL min�1. The mean hydraulic residence time of the liquidin the membrane was about 6 s. Complete breakthrough for theCNT membrane was reached after 600 mL (equivalent to 24 mg or0.09 mmol of metorpolol), 5% breakthrough after 150 ml (Fig. 4).Reduced flow velocity at 0.01 mL/minwas applied two times at 300and 450 mL for about 12 h each (boxes in Fig. 4) to reveal kineticlimitations duringmetoprolol sorption. No significant influence hasbeen found for metoprolol sorption. Solvent extraction of theloaded CNT membrane showed 79% recovery of metoprolol whichreveals only sorption as the dominant removal process. On theother hand, the breakthrough curve for the Fe-CNT membrane isshifted to higher volumes with a 5% breakthrough at 210 mL.Complete breakthrough was not yet reached until 900 mL (equiv-alent to 47 mg or 0.18 mmol of metoprolol). In this case sorption anddegradation processes can be hypothesized for metoprolol removaldue to reactions initiated by immobilized iron nanoparticles. Thefollowing data support the contribution of reactive degradation.First a gap in the mass balance after membrane extraction wasfound (only 56% of metoprolol recovered). Second, the furtherdecrease of metoprolol concentration on Fe-CNT membranes dur-ing reduced flow rates and therefore longer reaction times (blackboxes in Fig. 4) compared to CNT membranes. That clearly revealskinetic limitations of a process other than sorption. Third, theoccurrence of transformation products (TPs) of metoprolol is a clearindicator for its degradation. The TPs at m/z 226.1438 (desiso-propyl-metoprolol) and m/z 284.1856 (a-hydroxymetoprolol) havebeen identified as major degradation products by LC-MS (Fig. 5).However, the amount of single TPs was estimated to be in the rangeof less than 0.2% of metoprolol degraded, if the same analyticalresponse factor is assumed. No further quantification was per-formed due to the lack of pure standards of most TPs.

3.3. Electrochemical oxidation of metoprolol

In a further experiment electrochemical degradation of meto-prolol was investigated with CNT and Fe-CNT membranes as anode

19 mm) and a top view (right) of a CNT membrane.

Fig. 2. SEM images of CNT membranes using EsB detection, left: CNT membrane; right: Fe-CNT (400) membrane with a modified zone (left) and an unmodified zone (right).

Fig. 3. Fe-CNT membrane characterized by the size distribution of the diameter of ironnanoparticles. The image was processed for a membrane surface of 5.82 � 10�6 mm2

using the measurement function of Fiji.

Table 2Physical characterization of the membranes.

Membranes Conductivity [S/m] Permeability [L*h�1*m�2*bar�1]

PTFE support - 26,936 ± 97CNT (1.09 ± 0.19) 103 230 ± 80Fe-CNT (100) (1.30 ± 0.31) 103 210 ± 60Fe-CNT (400) (1.71 ± 0.41) 103 230 ± 70

J.E. Yanez H. et al. / Water Research 108 (2017) 78e8582

in a flow-through experiment with a similar hydraulic residencetime (6 s) compared to the adsorption and degradation experi-ments. The results show different efficiencies of metoprololdegradation of CNT and Fe-CNT membranes in dependence of theapplied potential. Membranes which were modified with Fenanostructures present a higher removal than without (Table 3).These observations can be explained by the role of the Fe nano-structures on the surfaces which generate oxidative species able todegrade metoprolol more efficiently (like in Fenton type reactions).

Metoprolol degradation also results in the occurrence of trans-formation products (TPs), which are formed in different quantitiesand patterns depending on the membranes (CNT or Fe-CNT) andthe applied electrochemical potentials. Seven TPs could be identi-fied by their mass fragmentation on a QTOF-MS instrument. TheTPs are formed from metoprolol by a-oxidation (benzylic position)at the methoxyethyl group to a-hydroxymetoprolol (TP284) and a-ketometoprolol (TP282), and by further oxidation to metoprolol-

Table 1Pore characterization by capillary flow porometry.

Membranes Mean flow pore diameter [nm]

CNT 39 ± 4Fe-CNT (100) 37 ± 5Fe-CNT (400) 41 ± 7

carboxylic acid (TP254-2) and metoprolol-benzylaldehyde(TP238), by O-demethylation to O-demethylmetoprolol (TP254-1),by N-dealkylation to desisopropyl-metoprolol (TP226), and finallyby cleavage of the alkylarylether bond to 3-(isopropylamino)pro-pane-1,2-diol (IPAP, TP134) (Fig. 5).

On a Fe-CNT membrane without an electrochemical potentialonly the TP 226 (desisopropyl-metoprolol) and TP284 (a-hydrox-ymetoprolol) were formed, whereas the application of an electro-chemical potential on bothmembranes (CNTand Fe-CNT) producedall seven TPs (Fig. 5) in different quantities. Only the relativeabundances of single TPs have been be directly compared in Fig. 6(lack of analytical standards for all TPs). With the exception of TP254-1 higher amounts of TPs were formed by the higher potential(1 V). In the presence of Fe (Fe-CNT membranes) at 1 V generallythe most TPs have been produced in highest quantities. a-hydrox-ymetoprolol (TP284), metoprolol-benzylaldehyde (TP238) and IPAP(TP134) were predominantly formed at 1 V. O-demethyl-meto-prolol occurred predominantly at 0.55 V andwas low or not presentat 1 V, which indicates further degradation at higher voltage. Thisbehavior generally reflects the predominance of electrochemicaland Fenton type processes.

A rough estimation of the TP quantity compared to the amountof removed metoprolol can be done under the assumption of anequal response factor for the quantification of metoprolol and allTPs. The results show that at 1 V the sum of the seven TPs is 10% ofremoved metoprolol for a Fe-CNT membrane and 5% for a CNTmembrane. At 0.55 V 4% TPs were found for Fe-CNT and 2.5% forCNT membranes. The remaining fraction of removed metoprololthat cannot be explained by the formation of TPs could be due toother transformation or mineralization products that cannot be

Diameter at maximum pore size distribution [nm]

25 ± 524 ± 326 ± 7

Fig. 4. Adsorption and degradation experiments: CNT membrane (blue circles); Fe-CNT(400) membrane (red triangles). Fractions at low-flow rates are indicated byblack squares, 5% compound break through by black circles (1 s error bars, s ¼ 3). (Forinterpretation of the references to colour in this figure legend, the reader is referred tothe web version of this article.)

Table 3Electrochemical degradation of metoprolol at different electrochemicalpotentials.

Membranes Metoprolol removal [%]

CNT at 0.55 V 43 ± 2CNT-Fe (400) at 0.55 V 55 ± 3CNT at 1 V 74 ± 3CNT-Fe (400) at 1 V 97 ± 5

J.E. Yanez H. et al. / Water Research 108 (2017) 78e85 83

detected with the applied LC-MS method. Furthermore, we wouldhave to point out that the real response factors of the generated TPsmay change the percentages considerably. Nevertheless, thedegradation results clearly demonstrate that TPs have to beincluded into the testing and evaluation process of new materialsand their applications.

Fig. 5. Transformation products (masses given for [MþH]þ ions) generated by oxidative detential (black triangles), on CNT membranes with a potential of 0.55 V (red circles) and on Fepropane-1,2-diol. Mass spectra of TPs are shown in section S3 of the supporting informatreferred to the web version of this article.)

4. Conclusions

The first results of the novel produced CNT and Fe-CNT mem-branes show promising properties for water filtration and com-pound degradation at rather low electrochemical potentials. Due tothe rather simple productionmethod and the possibility to producetailor-made surface modifications by electrochemical deposition ofnanoparticles from different metals with catalytic properties forreductive and oxidative processes, the membranes have a broadrange of applications, e.g. for point-of-use water treatment toremove different types of pollutants. Another possible applicationarea would be in analytical chemistry for the generation ofanalytical standards of TPs for further mass spectral characteriza-tion. The lack of analytical standards often impedes the detection ofTPs and their fate in the environment.

Author contributions

Jorge E.Yanez H.: Article preparation, guidance, experimentalplanning and work (membrane production, physical characteriza-tion, adsorption and electrochemical experiments and HPLC-MS

gradation of metoprolol on Fe-CTN (400) membranes without an electrochemical po--CNT (400) membranes with a potential of 1 V (blue squares). IPAP: 3-(isopropylamino)ion. (For interpretation of the references to colour in this figure legend, the reader is

Fig. 6. Peak areas of TPs produced on CNT and Fe-CNT membranes at 0.55 V and 1 V(each peak area of TP is normalized to that of caffeine used as internal standard; 1 serror bars, n ¼ 3).

J.E. Yanez H. et al. / Water Research 108 (2017) 78e8584

(QQQ and QTOF)) experiments/data evaluation.Zi Wang: Experimental work (membrane production, physical

characterization, adsorption and electrochemical experiments).Sascha Lege: HPLC-MS (QQQ and QTOF) experiments/data

evaluation.Martin Obst: SEM image processing and data interpretation.Sebastian Roehler: SEM measurements.Claus J. Burkhardt: SEM characterization/measurements and

data interpretation.Christian Zwiener: Guidance for the experimental work, data

interpretation, paper writing.

Notes

The authors declare no competing financial interest.

Funding sources

German Research Foundation (DFG), project ZW73-12 andGerman Federal Environmental Foundation (DBU) for a PhDscholarship for Sascha Lege.

Acknowledgments

We appreciate the technical support for sample preparation andLC-MS measurements by S. Merel and S. Nowak (Universit€atTübingen) and for membrane characterization by F. Saravia and F.Arndt (KIT, Karlsruhe). We acknowledge financial support by theGerman Research Foundation for the project ZW73-12 and theGerman Federal Environmental Foundation for a PhD scholarshipfor S. Lege.

Appendix A. Supplementary data

Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.watres.2016.10.055.

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