Reductive Sequestration of Toxic Bromate from Drinking Water using

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Reductive Sequestration of Toxic Bromate from Drinking Water using Lamellar 2D Ti3C2TX (MXene) Ravi P Pandey, Kashif Rasool, P Abdul Rasheed, and Khaled A. Mahmoud ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01147 • Publication Date (Web): 14 May 2018 Downloaded from http://pubs.acs.org on May 15, 2018

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ACS Sustainable Chemistry & Engineering

Reductive Sequestration of Toxic Bromate from Drinking Water using Lamellar 2D Ti3C2TX (MXene) Ravi P. Pandey1, Kashif Rasool1, P Abdul Rasheed1, and Khaled A. Mahmoud1,2* 1Qatar Environment and Energy Research Institute (QEERI), Hamad Bin Khalifa University (HBKU), Qatar Foundation, P. O. Box 34110, Doha, Qatar. 2

Department of Physics & Mathematical Engineering, Faculty of Engineering, Port Said University, 42523 Port Said, Egypt.

* To whom all correspondence should be addressed. Email: [email protected] Fax: +97444541528

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Abstract: Removal of toxic by-products such as bromate (BrO3−) from drinking water is a vital process. Two-dimensional titanium carbide also known as MXene (Ti3C2Tx) was proven a promising candidate for efficient reduction of metal ions. Herein, we first time report the simple method for efficient reduction of toxic bromate to bromide in water using 2D Ti3C2TX nanosheets. In this reduction, the Ti-C active layer of Ti3C2Tx was oxidized and formed TiO2 nanocrystals, while bromate reduced to bromide via self-reaction process. Reduction performance of the Ti3C2Tx nanosheets was evaluated with respect to the concentration of MXene, time, pH, and temperature. The MXene showed excellent reduction of bromate (~321.8 mg BrO3−/g Ti3C2Tx) within 50 min, at pH 7 and 25 °C. Furthermore, MXene nanosheets exhibited excellent sequestration performance towards bromate in comparison with other similar materials. The high resolution transmission electron microscopy (HR-TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) analysis confirmed the reduction of bromate and subsequent oxidation of MXene to form TiO2 nanocrystals and Br−. This makes it attractive reductant materials for the efficient removal of other toxic oxides present in water systems.

Key words: Ti3C2Tx (MXene), bromate, reduction, sequestration, water treatment


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Introduction Bromate (BrO3-) is a by-product formed when ozonation and chlorination processes are applied to disinfect drinking water.1 It is considered a potential carcinogen for humans and animals, cussing DNA damage and genetic mutations at very low doses.2-3 The World Health Organization (WHO) and the Environmental Protection Agency (EPA) have endorsed a maximum contamination level in drinking water of about 10 μg/L.4 The main source of BrO3− is Br− which may exist naturally in seawater, brines, salt lakes, and mineral sediments. Generally, BrO3− is generated from sequential oxidation of Br− during ozonation or radical-based oxidation processes, depending on the organic content, solution pH, ozone concentration, and other water characteristics. Due to the elevated levels of Br− in the desalinated seawater and subsequent treatment processes, BrO3− degradation and removal has become an issue of great concern.1, 5 Adsorption is a popular technique used for the removal of BrO3−.3, 6 However, this method can simply adsorb BrO3− from water and thus spent adsorbent is considered a hazardous waste, which may limit their practical use.7 To avoid hazardous waste, subsequent reduction of BrO3− to Br− has been proven an efficient method to control BrO3− pollution in water supply.8 Some classical processes have been proposed for the degradation of BrO3− such as hydrogenation reduction9,

10

and electron beam treatment11-12. Other combined approaches were proposed as more efficient methods for bromate removal such as, adsorption–reduction,13-15 chemical reduction,16 electrochemical reduction using modified carbon electrodes,2, 4, 17 microbial reduction,18 zerovalent metals (ZVM) reduction,13, 19 and catalytic reduction.20 Alternatively, free standing or supported TiO2 nanoparticles have been used for the photocatalytic reduction of BrO3− to Br− under UV light irradiation.21-22 However, photocatalysts usually suffer from rapid decay of catalytic activity due to charge recombination and narrow band gap. The integration of two-dimensional



graphene or graphene oxide nanosheets with the photocatalysts has improved the optical property of light absorption in the visible light region and thereby provided the higher degradation percentage of BrO3−.23-24 MXenes, 2D layered transition metal carbides, are receiving great attention recently due to their large specific surface area, superior electrical conductivity, hydrophilic surfaces, flexibility, well dispersion, and environmentally friendly characteristics.25 Ti3C2Tx (Tx = -O, -OH and/or –F) is the most utilized form of MXene. It is prepared from MAX (Ti3AlC2) via the selective etching of Al layer through a controlled acid etching process.26 Layered Ti3C2Tx exhibit charged surface, which is easily intercalated with different cations.27 Ti3C2Tx has been used as photocatalyst for CO2 reduction,28 anode material for lithium-ion batteries,29 substrates for surface-enhanced Raman spectroscopy,30 and reducing agent for oxygen reduction reaction.31 In environmental remediation applications, MXene and their composites have been used for the adsorption of toxic heavy metals,32-33 and phosphate removal.34 Ti3C2Tx have also been used as reducing agent for the in situ synthesis of noble nanoparticles.35-36 Moreover, in our recent study we confirmed the nontoxicity of MXene on the aquatic environment which can suggest a safe use and discharge of Ti3C2Tx MXene in the aquatic ecosystem at concentrations below 100 μg mL−1.37 Herein, we explore for the first time, the simple method for selective reduction and remediation of BrO3− from water using lamellar Ti3C2Tx (MXene). Reaction kinetics and effect of temperature and pH were evaluated to identify the optimal removal efficiency. The reaction products were evaluated by scanning electron microscopy (SEM), transmission electron microscopy (HR-TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). It is expected that the unique reducing properties of MXene could provide an efficient platform for fast reduction and removal of BrO3− ions from water.


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Materials and Methods Materials Sodium bromate (NaBrO3), potassium bromide (KBr), and lithium fluoride (LiF), were purchased from Sigma-Aldrich. Ti3AlC2 was purchased from Y-Carbon, Ltd., Ukraine. Other chemicals are of analytical grade and were used as received. In all experiments, deionized (DI) water was used. Preparation of delaminated MXene (Ti3C2Tx) nanosheets Ti3C2Tx was prepared by etching the aluminum atomic layers from MAX (Ti3AlC2) phase followed by delamination. Typically, etchant solution was prepared by adding LiF (80 mg) to 1 mL of 9 M HCl and kept under stirring for 5 min. MAX (50 mg) powder was slowly added to the above solution, and magnetically stirred for 24 h at room temperature. After, completing the etching process 2D multilayered MXene powders was collected by four times washing with DI water via centrifugation at 3500 rpm for 5 min, followed by decantation and freeze-drying. Finally, the dried multi-layered Ti3C2Tx was added in degassed water and sonicated using probe sonicator, under a flow of argon (Ar) gas for two hours. Subsequently, solution was centrifuged at 5,000 rpm for 30 min to settle down the multilayered MXene, decanted the supernatant and freeze dried to obtain the delaminated MXene nanosheets.38 Bromate reduction using Ti3C2Tx MXene. Batch experiments were performed to study the reduction of bromate using MXene. A number of experiments were conducted to evaluate the effect of MXene loading, pH, reaction time, and temperature on reduction of bromate to bromide by MXene. Typically, different amount of Ti3C2Tx sheets (0-23 mg) was added to a capped Erlenmeyer flasks (1000 mL) containing 1000 mL of bromate solution (0-70 μmol L-1) and placed on a shaker (VWR SHAKER MODEL 5000) at 250 rpm. After specified reaction time intervals, 5 ACS Paragon Plus Environment


solutions were filtered through a PVDF membrane (47 mm diameter, Millipore) with a pore size of 0.22 μm. Finally, filtrate was analyzed using an ion chromatography system (Dionex ICS-5000+, Thermo Fisher Scientific) to evaluate concentrations of the remaining bromate and the reduced to bromide at a given time. A Dionex™ IonPac™ AS19 IC analytical column (2 x 250 mm) connected to a Dionex™ IonPac™ AG19-4 um-IC guard column (2x 50 mm) was used. The unit was operated in auto suppression mode with 10 mM KOH eluent generator cartridge with flow rate of 1ml/min. Kinetics measurements A pseudo first order reaction model was applied to fit the experimental data and evaluate the reduction kinetics of bromate by Ti3C2Tx. This model had been successfully applied in studies of the reduction of bromate and nitrite in earlier studies20 and can be written as follows: 𝐶𝐶𝑡𝑡 ln � � = −𝑘𝑘𝑜𝑜𝑜𝑜𝑜𝑜 𝑡𝑡 𝐶𝐶0

(1)

where 𝐶𝐶0 and 𝐶𝐶𝑡𝑡 are the bromate initial and at any time (t) concentrations, respectively. Whereas, 𝑘𝑘𝑜𝑜𝑜𝑜𝑜𝑜 is the first order rate constant (min-1).

Analysis of real and co-existing anions samples Real sample analysis were performed by using tap water after spiking the 1 mg L-1 of KBr followed by ozonation treatment. Dissolved ozone stock solution was prepared by continuous bubbling of ozone gas for a duration of one hour in a 1L ultra‐pure deionized water that was maintained at 5 °C. Ozone was generated using a 4 g h-1 oxygen fed corona discharge generator (BMT 802 N). After stabilization, the dissolved ozone stock concentration was measured and it was found that 32 µg.mL-1. Then appropriate amount of ozone stock solution was added to make the concentration of ozone double than the amount of Br- in the Br- spiked samples. Then the concentrations of BrO3- in the ozonated water samples were calculated by ion chromatography (IC) method 6 ACS Paragon Plus Environment

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(Thermoscientific ICS-1100). Finally, batch experiments were performed to study the reduction of bromate in real sample using Ti3C2Tx. We also evaluate the selectivity and the effect of co-existing anions on the reduction efficiency of Ti3C2Tx for bromate. Different anions such as sulfate, nitrate and phosphate were mixed with bromate, at the same concentration (5 mg L-1), and study the selectivity of reduction of bromate by Ti3C2TX (15 mg L-1) using batch experiment. Characterization Wide angle X-ray diffractograms (WXRDs) were recorded using a Bruker D8 Advance (Bruker AXS, Germany) X-ray diffractometer with Cu-Ka radiation (λ= 1.54056 Å) at a voltage of 40 kV and a current of 15 mA with a step scan and scanning speed of 0.02°/step and 1°/min, respectively. X-ray photoelectron spectroscopy (XPS) analysis of given samples was performed with ESCALAB 250X (Thermo Fisher Scientific) using AlKα excitation radiation (25 W, hυ = 1486.5 eV) and 1 eV energy resolution. For the XRD and XPS analysis, the samples were prepared by depositing freeze dried nanocomposite films onto a silicon substrate. Transmission electronic microscopy (TEM) images of the neat oxidized MXene samples were recorded using a FEI Talos F200X TEM. The samples were dispersed in ethanol and mounted on a lacey carbon Formvar coated Cu grid. Study of Morphology of Ti3C2Tx and oxidized Ti3C2Tx were analyzed by scanning electron microscopy (SEM) using a FEI Quanta 650 FEG SEM, after gold sputter coatings on samples. The Brunauer–Emmett–Teller (BET) surface area of Ti3C2Tx was calculated by N2 adsorption/desorption isotherm at 77 K using a BET surface area analyzer (Micromeritics ASAP 2020). The Ti3C2Tx sample was degassed for 8 h at 573 K to eliminate the all moisture and measure the surface area using the manufacturer’s software to apply the BET equation to the adsorption data.



Results and Discussion Bottle-point batch experiments were conducted to evaluate the reduction of BrO3− on the surface of MXene as a function of residual BrO3− and formed Br− concentrations during the reduction reaction. The BrO3− reduction capacity was evaluated by reduction of bromate with different loading of MXene substrate (0-23 mg L-1) and reduction (%) of different concentration of bromate with constant MXene substrate (15 mg L-1) (Figure 1 (a & b)). As observed from Figure 1a, the reduction of BrO3− (37.741 μmol L-1) to Br− increased with increasing Ti3C2Tx substrate loading up to 15 mg L-1 at constant pH (7) and temperature (25 °C). Beyond 15 mg L-1 Ti3C2Tx, the reduction process reached saturation plateau. This result shows that the reduction capacity of MXene was about 321.8 mg BrO3-/g Ti3C2Tx (MXene) at pH = 7. As observed from Figure 1a, the concentration of reduced BrO3− is in stoichiometric equivalence with the concentration of formed Br−, indicating that adsorption plays a minor role in the removal of bromate. Further, BrO3− reduction capacity with MXene was evaluated at different concentrations of bromate (0-67.5 μmol L-1 ) and constant concentration of MXene substrate (15 mg L-1). As shown in (Figure 1b), up to ~37.5 μmol L-1 of bromate were reduced by ~ 100%. After this concentration the efficiency starts to decline and reached 45% reduction at 67.5 μmol L-1.


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Figure 1. (a) Effect of concentration of Ti3C2Tx on reduction of BrO3− (Time: 50 min, pH 7, and at 25 °C) and (b) Reduction (%) of different concentration of bromate using 15 mg L-1 Ti3C2Tx at pH 7 and 25 °C. The capacity of MXene towards bromate removal was found to be the highest compared to other adsorbent/reductant materials as described in Table 1. Reduction of BrO3− was strongly dependent on the reaction time and had a remarkable effect on the reduction capacity of MXene (Figure S1A). Particularly, the reduction of bromate rapidly increased with time up to 50 min, after that an equilibrium plateau was reached. The extent of reduction was ~53.97% in the first 15 min, for the second 15 min, reduction efficiency deceased to ~27.20 %, and finally in last 20 min (3050 min) slow reduction (~18.83%) observed. This decreased in reduction rate could be explain as, at first 15 min the surface of MXene reacted very fast and formed the TiO2 and amorphous carbon which acted as a passive layer reducing the active MXene sites from further reaction, thus decreased the rate of reduction of bromate. Figure S1b describes the total bromine mass of the two bromate and bromide species during bromate reduction process by MXene. The results suggested that bromide was the only product for bromate reduction, and the bromine mass balance was in



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the range of 96.71−100.00 % during the experimental course, indicating that MXene removes bromate by reduction rather than adsorption.

Table 1. Comparison of BrO3- Removal Performance of MXene with Reported Adsorbent/Reductant. Material

[BrO3-] (mg.L-1)

Adsorber/Re ductant (mg)

PH

Time (min)

capacity (mg.g-1)

Reference

Organomontmorillonite

0.128

250

6.3

1440

150.92

6

HFO-201

0.2

200

7

199.8

292.81

3

Pd/Fe3O4

50

200

6

60

-

20

ZSM5 (Cu1.5% 10 Pd3% )

500

10

-

39

Activated carbon 12.79 felt electrode

1980

2.2

10

-

2

ZV Fe

9.988

5000

3

7) inhibited bromate reduction, and the reduction efficiency started to decline. This could be explain by the ionization of both MXene and the bromate at higher pH values, resulting negative charge surface on both sides MXene sheets and thus repulsion with BrO3− decreased the reduction capacity.41 However, at acidic condition (pH < 7), a sharp increase in the reduction of bromate was observed. this can be explained by the presence of more proton can produce positively charged surfaces which leads to an increase in bromate adsorption and facilitate the bromate reduction reaction.7 Figure 2b, shows the reaction kinetics by plotting 𝑙𝑙𝑙𝑙(𝐶𝐶𝑡𝑡 ⁄𝐶𝐶0 ) versus t at different pH. The higher values of correlation coefficients (R2) (close to 1) indicated that BrO3- reduction

followed the pseudo-first order kinetic model (Table S1). The reaction rate constant 𝑘𝑘𝑜𝑜𝑜𝑜𝑜𝑜 increased

from 0.0296 min-1 at pH = 9 to 0.1121 min-1 at pH = 3, revealed that bromate reduction by MXene was accelerated with lower pH value. Thus, the acidic condition is more favorable for bromate reduction by MXene. Although at pH 3 MXene has a faster reduction kinetics to bromate, this highly acidic condition is not practical in water treatment process. Moreover, the reduction reached the same efficiency at pH 7 after 50 min. This means MXene can be used efficiently for removal of bromate at neutral pH, as compared with ZVM that require acedic conditions to inhibit the formation of a passivation layer of metal oxide.13, 19



Figure 2. (a) Effect of PH on reduction of BrO3− using 15 mg L-1 Ti3C2Tx substrate at 25 °C, (b) Plot of 𝑙𝑙𝑙𝑙(𝐶𝐶𝑡𝑡 ⁄𝐶𝐶0 ) versus time (min) of bromate reduction at different pH, (c) Effect of temperature

on reduction of BrO3− (15 mg L-1 Ti3C2Tx, PH 7), and (d) Plot of 𝑙𝑙𝑙𝑙(𝐶𝐶𝑡𝑡 ⁄𝐶𝐶0 ) versus time (min) of

bromate reduction at different temperatures.

The effect of temperature was also studied as an increase in temperature is predicted to increase the rate of reduction reaction. The temperature of surface water is varying seasonally, as during summer about 30 to 40 °C, and winter about 10 to 20 °C. Therefore, the effect of temperature was studied between 15 to 40 °C (Figure 2c). The reduction of bromate was significantly enhanced with increase in temperature from 15 to 40 °C by MXene. At 25 °C (normal temperature of


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drinking water), the reduction of bromate (37.741 μmol L-1) with 15 mg L-1 Ti3C2Tx was ~100% in 50 min. Moreover, enhancement in rate of reduction of bromate at higher temperature was determine by applying the pseudo first order rate law. Figure 2(d) displays the fitting results by the pseudo first order equation, and the corresponding correlation coefficients (R2) and 𝑘𝑘𝑜𝑜𝑜𝑜𝑜𝑜 . The pseudo first order rate constant 𝑘𝑘𝑜𝑜𝑜𝑜𝑜𝑜 values for BrO3− reduction increased from 0.0337 to 0.1066

min-1 with increasing temperature from 15 to 40 °C, respectively, indicating that temperature plays

a very important role in improving the BrO3− reduction kinetics (Table S1). Additionally, R2 values for all the tested temperatures were close to one, indicated that the BrO3- reduction kinetics followed the pseudo-first order kinetic model. The correlation between 𝑘𝑘𝑜𝑜𝑜𝑜𝑜𝑜 and temperature could expressed by using the Arrhenius equation as follows:7 𝑙𝑙𝑙𝑙𝑘𝑘𝑜𝑜𝑜𝑜𝑜𝑜 = 𝑙𝑙𝑛𝑛 𝐴𝐴 −

𝐸𝐸𝑎𝑎� 𝑅𝑅𝑅𝑅

(2)

where A is the pre-exponential factor (min-1), Ea is the activation energy, R is the gas constant (8.314 J K-1 mol-1), and T is the temperature in Kelvin (K). Ea was estimated by plotting the graph between 𝑙𝑙𝑙𝑙𝑙𝑙𝑜𝑜𝑜𝑜𝑜𝑜 vs 1000 T-1 (K-1) with well fitted by linear regression (Figure S2). The calculated Ea was 36.00 kJ/mol (error ± 0.20).

The excellent reduction capacity may be ascribed to the high oxidizing power of Ti-C groups present in MXene. Furthermore, the specific surface area of delaminated MXene (15 m2 g-1) provided additional reactive surface for BrO3− reduction. The changes in MXene chemical structure during reduction of BrO3− was examined by different spectroscopic methods. Figure 3 (a and b) shows the SEM micrographs, representing the morphology of the lamellar MXene, before and after reduction of BrO3− in aqueous media. After reaction, the apparent sheet-like structures of MXene were destroyed during oxidation and different shapes and sizes of TiO2 nanocrystals were formed (mainly square, rectangle, and rhomboid shapes) as observed in Figure 3b.42 The 13 ACS Paragon Plus Environment


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high-resolution (HR)-TEM image of the oxidized MXene in Figure 3c clearly shows the crystalline lattice fringes of an individual TiO2 nanocrystal on surface of MXene (diameter, ~8 nm; length, ~14 nm) with a single anatase crystallinity.43 The selected area electron diffraction (SAED) pattern, exhibited (002), (101), (004), and (200) diffractions with fringe spacing of 4.35, 3.57, 2.38, and 1.78 Å, of anatase TiO2 (Figure 3d).44 Figure 3e shows the XRD patterns of MXene, before and after reduction reaction. The broad diffraction peak at (002) was observed at lower angle (2θ = ~7°), and other characteristics peaks at (004), (006), (008), (0010), and (0012) confirming the successful delamination of multi-layered MXene.29,

45

After the successful reduction of BrO3− to Br−, (101), (004), (200) and (105)

diffraction peaks characteristic for TiO2 were observed and confirmed the oxidation of Ti3C2Tx.46

(a)

(b)

TiO2

200 nm

200 nm


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Figure 3. SEM images of delaminated Ti3C2Tx: (a) before reduction of BrO3− and (b) after reduction of BrO3−. (c) High resolution TEM image for delaminated Ti3C2Tx after BrO3− reduction, (d) selected area electron diffraction (SAED) pattern of oxidized Ti3C2Tx and (e) XRD pattern of Ti3C2Tx before and after reduction of BrO3− to BrThe survey and the individual core level high-resolution X-ray photoelectron spectroscopy (XPS) of MXene (before and after reduction reaction) are presented in Figure 4 and S3. Before, the reduction, MXene showed low-valence Ti species, at binding energy (BE) of ~455.17, ~456.18 and ~457.52 eV. After, reduction of BrO3−, MXene was oxidized and new strong peaks for oxidized Ti(IV) 2p3/2 and Ti(IV) 2p1/2 at ~459.51 eV and ~465.30 eV aroused, respectively, as an indication for surface oxidation of MXene (Figure 4c). Further, a difference in BE between two doublet of Ti 2p was observed at ~5.79 eV. The high-resolution XPS spectra for C1s showed two major peaks at ~281.96 and ~284.66 eV, corresponding to C-Ti-Tx (I, II, III, or IV) and graphitic C-C, respectively.



Figure 4 (a) XPS survey spectra of the delaminated Ti3C2Tx sheets and oxidized Ti3C2Tx, (b) XPS Ti 2p core level spectrum of Ti3C2Tx, (c) oxidized Ti3C2Tx, and (d) Br 3d for oxidized Ti3C2Tx. After reduction, two major contribution were observed from graphitic C-O.Hx/C-O (~286.67) and O-C=O (289.80) (Figure S3 a and c). Likewise, the O1s core level spectra after reduction showed contributions from TiO2, C-Ti-Ox and C-Ti-(OH)x (Figure S3 b & d).47 Moreover, two characteristics peaks for Br- were observed at 68.54 and 67.62 eV which correspond to the BE of Br 3d5/2 and Br 3d3/2, respectively (Figure 4d).48-49 This provides an evidence that BrO3− has been quickly reduced to Br- on the surface of MXene, where the later can act as the electron donor. The wastewater contains various ions, such as phosphate (PO43-), sulphate (SO42-), nitrate (NO3-), bicarbonate (HCO3-), chlorate (ClO3-), etc. Thus, it was essential to study the effect of coexiting anions on BrO3- reduction by MXene. SO42-, NO3- and PO43- were mixed with BrO3-, at the same concentration (5 mg L-1), and the selectivity and reduction of bromate by Ti3C2TX (15 mg L16 ACS Paragon Plus Environment

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1

) in presence of these ions was studies using batch experiment. The IC retention times of ions

before and after reduction with Ti3C2TX were given in Figure S4. As shown in Figure 5, ~100% reduction of BrO3− was observed in absence of co-existing ions, while in presence of co-existing ions about 92% reduction of BrO3- was achieved; this slight reduction prove a minimal impact interfering ions. Furthermore, the amount of SO42-, NO3- and PO43- after treatment with Ti3C2Tx were slightly decreased around 4-8%, may be due to the adsorption or reduction. This revealed the excellent selectivity of MXene towards BrO3− reduction.

Figure 5. Effect of co-existing anions on the reduction of bromate by Ti3C2TX The real sample application was investigated by attempting to remove BrO3- from domestic tap water. Concentration of Br− in the desalinated water sample was close to 0.2 mg L-1. Therefore, prior to the sequestration reaction, the sample was spiked with 1 mg L-1 of KBr salt as a source of Br- and was subjected to ozonation treatment to reach a 0.756 mg L- of BrO3−. Finally, batch experiments were performed to study the reduction of BrO3− using (3 mg L-1) Ti3C2Tx. As shown



in Figure S5, about 100% reduction was achieved with a reduction capacity of 302.4 mg BrO3-/g MXene, which was almost identical to those from the standard BrO3- reduction above. Based on the above results, BrO3- sequestration mechanism can be assumed as instant reduction of BrO3− to Br− on the surface of MXene. Ti-C group plays a special role in reduction of BrO3-.5051

Also, the surface Ti-OH sites of MXene could endow specific affinities toward target anions in

acidic media by forming inner sphere complexation.34, 52 Lin et al had to combine metal–organic frameworks together with sodium borohydride as source of H2 for complete reduction of BrO3− to Br−.10 Similarly, other 2D structures like graphene were only used as substrate to prepare photocatalysts for BrO3− reduction.22-24 In our case, MXene reached a very high reduction capacity for BrO3− without the aid of a catalyst or energy source. The changes occurred in the morphology and oxidation state of MXene lamellar is indicative of its strong reducing activities.30, 52 The oxidation of Ti2+ to Ti4+ provides enough electrons to catalyze the reduction of BrO3− into Br-. In summary, we reported a new facile method for the reductive sequestration of toxic BrO3− from water using lamellar Ti3C2Tx MXene. Excellent reduction capacity of BrO3(321.8 mg BrO3-/g Ti3C2Tx) was achieved at pH 7 within 50 min. The molar amount of bromide released into solution almost equaled to the reduced amount of bromate on MXene, indicating a reduction mechanism is dominating not adsorption. MXene showed high selectivity towards BrO3− in presence of common ions. Also, about 100% of BrO3− was successfully reduced from tap water. The SEM, TEM and XPS confirmed the gradual oxidation of MXene by forming TiO2/C during the reduction of BrO3−. In this case it is suggested that reusability of MXene after reduction may not be possible. However the recovered TiO2/C by-product can be useful for other catalytic application. Alternatively,


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new approaches for stabilizing MXenes are needed for more practical utilization as promising platform for efficient sequestration of water contaminants.

Associated Content Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Kinetic parameters of bromate reduction, bromine mass balance during removal of bromate, XPS C1s, and O1s core level spectra, and ion chromatography measurements.

Acknowledgements Authors acknowledge the financial support of Qatar National Research Fund (A member of Qatar Foundation) through the NPRP grants # 9-254-2-120. The authors are thankful to M. Helal, S. Suslov, and R. Essehli at the Core lab of QEERI/HBKU, Doha, Qatar for SEM, TEM and XRD analysis.



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TABLE OF CONTENTS (TOC) Lamellar 2D Ti3C2TX (MXene) was used for the efficient reduction of toxic bromate to bromide in water with excellent reduction capacity of (~321.8 mg BrO3−/g Ti3C2Tx) within 50 min, at pH 7 and 25 °C.


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Reductive Sequestration of Toxic Bromate from Drinking Water using

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