materials

Article Effective and Green Removal of from Disinfected Water

Keren Trabelsi 1, Michael Meistelman 1, Rosaria Ciriminna 2 , Yael Albo 1,* and Mario Pagliaro 2,*

1 Department Chemical Engineering, The Center for Radical Reactions, Ariel University, Ariel 40700, Israel; [email protected] (K.T.); [email protected] (M.M.) 2 Istituto per lo Studio dei Materiali Nanostrutturati, CNR, 90146 Palermo, Italy; [email protected] * Correspondence: [email protected] (Y.A.); [email protected] (M.P.)

 Received: 21 December 2019; Accepted: 10 February 2020; Published: 12 February 2020 

Abstract: An innovative catalyst is reported for removing suspect carcinogen trichloroacetic acid (TCA) found in water after chlorination. SilverSil, a methyl-modified silica xerogel doped with Ag nanoparticles, shows remarkably high and stable activity as heterogeneous catalyst for the reductive dehalogenation of TCA with NaBH4 as reducing agent. and are the main products of the highly reproducible reductive dehalogenation. The low cost, high stability and ease of application of the SilverSil sol-gel catalyst to continuous processes open the route to the industrial uptake of SilverSil to free chlorinated waters from a probable human carcinogenic agent exerting significant genotoxic and cytotoxic effects.

Keywords: halogenated by-products; drinking water; trichloroacetic acid; environmental

1. Introduction In 1974 Rook, a chemist at Rotterdam Waters, discovered that the reaction of natural organic matter in water with during water disinfection leads to the formation of haloforms as unwanted by-products [1]. Regardless of low concentration in water, where most disinfection by-products are poorly soluble, chlorination by-product compounds which also include haloacetic acids, haloacetonitriles and halocarbonyl compounds, are toxic and tend to accumulate in the food chain [2]. Hence, guideline values exist in most world’s countries, with adsorption on activated carbons being the main method used for the removal of volatile trihalomethanes [2]. For example, the 0.1 mg/L guideline value for trichloroacetic acid (Cl3CCOOH) in drinking-water was first suggested by the World Health Organization (WHO) in 1993 as “provisional value” due to “the limitations of the available toxicological database” [3]. Trichloroacetic acid (TCA) is a suspect human carcinogen exerting significant genotoxic and cytotoxic effects, which accumulates in the body [4]. Analysis of TCA in urine samples from a reference population of 402 adults in the United States of America in 2003 showed presence of the acid in 76% of the samples analysed, with a 90th percentile concentration of 23 µg/L[5]. Almost all (81%) residents in urban areas, and a vast majority (62%) of residents in rural areas had urinary TCA, though with significant higher concentrations for residents in urban areas. The high toxicity and persistency of TCA have prompted extensive research concerning its degradation to environmentally safe products. Different technologies have been developed including biodegradation [6], electrochemical methods [7–10], radiation induced methods [11,12], photochemical processes [13], photosonochemical degradation [14] and chemical methods based on advanced reduction process based on carboxyl anion radical [15], as well as the removal of TCA (and other disinfection by-products) by granular and biological activated carbons [16,17].

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Materials 2020, 13, 827 2 of 9 However, despite the different suggested technologies, no water treatment method for TCA has However, despite the different suggested technologies, no water treatment method for TCA has been recommended by a regulatory body. According to the World Health Organization Guidelines for been recommended by a regulatory body. According to the World Health Organization Guidelines for Drinking-Water Quality published in 2004 “concentrations may be reduced by installing or optimizing Drinking-Water Quality published in 2004 “concentrations may be reduced by installing or coagulation to remove precursors and/or by controlling the pH during chlorination” [3]. optimizing coagulation to remove precursors and/or by controlling the pH during chlorination” [3]. We now report that SilverSil, a methyl-modified silica xerogel doped with Ag nanoparticles We now report that SilverSil, a methyl-modified silica xerogel doped with Ag nanoparticles of of exceptionallyexceptionally high high antibacterial antibacterial activity activity [18], [18], also also shows shows remarkably remarkably high high and and stable stable activity activity as as heterogeneousheterogeneous catalyst catalyst for thefor reductive the reductive dehalogenation dehalogenation of TCA of dissolved TCA dissolved in water inin high water concentration in high µ (229concentrationg/L) using (229 green µg/L) and using safe NaBH green4 andas reducingsafe NaBH agent4 as reducing (Scheme agent1). (Scheme 1).

Scheme 1. Reductive dehalogenation of trichloroacetic acid (TCA) with borohydride mediated Scheme 1. Reductive dehalogenation of trichloroacetic acid (TCA) with sodium borohydride by SilverSil. mediated by SilverSil. 2. Materials and Methods 2. Materials and Methods 2.1. Materials 2.1. Materials TrichloroaceticacidwasobtainedfromAlfaAesar. Tetraethylorthosilicate(TEOS),methyltriethoxysilane Trichloroacetic acid was obtained from Alfa Aesar. Tetraethyl orthosilicate (TEOS), (MTEOS) and silver nitrate were purchased from Aldrich. NaBH4 was bought from Strem Chemicals. Allmethyltriethoxysilane aqueous solutions were (MTEOS) prepared and silver from ni deionizedtrate were waterpurchased purified from byAldrich. a Millipore NaBH4 Milli-Qwas bought setup from Strem Chemicals. All aqueous solutions were prepared from deionized water purified by a (Darmstadt, Germany) with a final resistivity of >10 MΩ/cm. Millipore Milli-Q setup (Darmstadt, Germany) with a final resistivity of >10 MΩ/cm. 2.2. Syntheses and Characterization 2.2. Syntheses and Characterization The preparation of SilverSil organically modified silicas (ORMOSILs) by sol-gel hydrolytic The preparation of SilverSil organically modified silicas (ORMOSILs) by sol-gel hydrolytic co-polycondensation of TEOS and MTEOS in the presence of AgNO3 has been described elsewhere [18]. co-polycondensation of TEOS and MTEOS in the presence of AgNO3 has been described elsewhere The composition of the xerogels used throughout this study is summarized in Table1. [18]. The composition of the xerogels used throughout this study is summarized in Table 1.

Table 1. Composition of the different SilverSil materials. Table 1. Composition of the different SilverSil materials. TEOS MTEOS Ag(0) Load Specific Surface Area Specific Pore Volume SilverSil TEOS MTEOS Ag(0) Load Specific Surface Specific Pore SilverSil (mol%) (mol%) (mmol Ag) (m2/g) (cm3/g) (mol%) (mol%) (mmol Ag) Area (m2/g) Volume (cm3/g) 90:10 Ag 90 10 0.0325 394.036 0.9811 90:10 Ag 90 10 0.0325 394.036 0.9811 70:30 Ag 70 30 0.0325 582.475 0.9918 70:30 Ag 70 30 0.0325 582.475 0.9918 50:50 AgAg 50 50 50 50 0.0325 0.0325 506.046 506.046 0.9907 0.9907 30:70 AgAg 30 30 70 70 0.0325 0.0325 308.583 308.583 1.581 1.581 70:3070:30 5Ag5Ag 70 70 30 30 0.1625 0.1625 - - - - 70:3070:30 10Ag10Ag 70 70 30 30 0.325 0.325 - - - -

InIn a a typical typical preparationpreparation of of the the 70:30 70:30 SilverSil SilverSil (70 mol (70 percent mol percent TEOS and TEOS 30 mol and percent 30 mol MTEOS), percent 6.0 mL of aqueous HNO3 solution (0.2 M) was added dropwise to a solution consisting of TEOS (8.8 MTEOS), 6.0 mL of aqueous HNO3 solution (0.2 M) was added dropwise to a solution consisting mL), MTEOS (3.4 mL) and 13.3 mL of ethanol. After stirring for 10 min, 125 µL of of TEOS (8.8 mL), MTEOS (3.4 mL) and 13.3 mL of ethanol. After stirring for 10 min, 125 µL of (3-aminopropyl)triethoxysilane (APS) were added to the mixture. Hence, an aliquot (5.0 mL) of a (3-aminopropyl)triethoxysilane (APS) were added to the mixture. Hence, an aliquot (5.0 mL) of a 6.36 mM aqueous solution of AgNO3 was added dropwise, after which water (5.0 mL) and aqueous 6.36 mM aqueous solution of AgNO was added dropwise, after which water (5.0 mL) and aqueous ammonia (0.01 M, 5.0 mL) were added3 to promote the hydrolytic polycondensation. A wet gel was ammonia (0.01 M, 5.0 mL) were added to promote the hydrolytic polycondensation. A wet gel was quickly obtained which was dried at room temperature for approximately two weeks until a quicklyconstant obtained weight which of the was dry dried matrix at roomwas obtained. temperature The forxerogel approximately was crushed two into weeks a powder until a constantwith a weight of the dry matrix was obtained. The xerogel was crushed into a powder with a pestle, after which pestle, after which the powder was treated with aqueous NaBH4 (0.03 M, 100.0 mL) to promote

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the powder was treated with aqueous NaBH4 (0.03 M, 100.0 mL) to promote reduction of the entrapped Ag+ . At this stage the xerogel changed colour from white to light yellow characteristic to silver nanoparticles (NPs). The material obtained was air-dried again. The yellow xerogel was crushed in a mortar and the powder used as such in subsequent catalytic dehalogenation reactions of TCA. The textural properties of the SilverSil ORMOSILs were determined from N2 adsorption– desorption isotherms measured at liquid nitrogen temperature using a NOVA 3200E Quantachrome analyzer. The specific surface area was calculated from the linear section of the Brunauer–Emmett–Teller (BET) plot. The UV-vis spectra were measured using a Varian Cary UV Bio 50 spectrophotometer.

2.3. Catalytic Tests In a typical catalytic run, 2.8 mL of 0.010 M TCA was added to the suspension of 0.50 g of the SilverSil taken in 14.4 mL of deionised water in a 50 mL beaker kept at room temperature. The mixture was then stirred for a few minutes. A 2.8 mL aliquot of 0.060 M of aqueous NaBH4 was thus added, and the resulting solution further stirred for 2 h. After completion of the reaction the catalyst was recovered by filtration and the filtrate analysed by HPLC. The catalyst was washed extensively with water, dried in air at room temperature and reused as such in subsequent reaction runs under the same conditions. Each result reported is the average of at least three independent experiments. The error limit for the analytical results is 5%. ± Under said reaction conditions, no dehalogenation was detected without added catalyst (blank test). In addition, in the absence of NaBH4 but in the presence of a catalytic amount of SilverSil no reduction was observed (blank test). The dehalogenation of TCA was monitored by HPLC using a Dionex Ultimate 3000 chromatograph equipped with VWD (variable wavelength detector) flow cell in line with ISQ quadrupole mass spectrometer (MS, Thermo Fisher Scientific, Austin, TX, USA) The Ultimate 3000 chromatograph was equipped with Diode Array Detector by Thermo (λ = 210 nm) and a 4.6 mm 150 mm HPLC column × comprised of 5 µm organosilica microparticles (Agilent, Eclipse XDB-C18). The eluent, an aqueous H2O:CH3CN = 90:10 mixture kept at pH 2.0 due to the presence of 0.1 wt% H3PO4, was flowed at 1.0 mL/min rate.

3. Results and Discussion

3.1. Catalyst Characterization

The N2 adsorption–desorption isotherms (Figure1) are type IV curves characteristic of glassy mesoporous materials with uniform pores. In general, the SilverSil xerogels have narrow Barrett–Joyner–Halenda (BJH) pore size distribution, with average pore diameters nearly 4.0 nm for all compositions, except for the 30:70 SilverSil xerogel having an average pore diameter of 20 nm. The Brunauer–Emmett–Teller (BET) surface area and pore volume of the SilverSil matrices are summarized in Table1. Materials 2020, 13, 827 4 of 9 Materials 2020, 13, 827 4 of 9 Materials 2020, 13, 827 4 of 9

Figure 1. Pore size distribution of different SilverSil xerogels: A (90:10) (inset Brunauer–Emmett– Figure 1. Pore size distribution of different SilverSil xerogels: (A) (90:10) (inset Brunauer–Emmett–Teller Teller (BET) isotherm of the same matrix), B (70:30), C (50:50) and D (30:70). (BET)Figure isotherm 1. Pore ofsize the distribution same matrix), of different (B) (70:30), Silver (C) (50:50)Sil xerogels: and (D A) (30:70).(90:10) (inset Brunauer–Emmett– Teller (BET) isotherm of the same matrix), B (70:30), C (50:50) and D (30:70). 3.2. Catalytic Catalytic Activity Towards Dehalogenation of TCA 3.2. CatalyticFigure2 2 showsActivity shows that Towardsthat TCA TCA isDehalogenation entirelyis entirely decomposed decompos of TCA overed over all theall testedthe tested SilverSil SilverSil xerogels, xerogels, going going from fromrelativelyFigure relatively hydrophilic 2 shows hydrophilic that SilverSil TCA SilverSil 90:10, is entirely through 90:10, decompos hydrophobicthroughed hydrophobic over SilverSil all the 30:70. testedSilverSil The SilverSil same 30:70. plot xerogels,The shows same thatgoing plot all showscatalysts,from relatively that tested all catalysts, inhydrophilic 3 consecutive tested SilverSil reactionin 3 consecutive 90:10, runs, through exhibit reaction excellenthydrophobic runs, catalytic exhibit SilverSil stabilityexcellent 30:70. being catalytic The fully same reusablestability plot withbeingshows no fully that loss reusableall of activity.catalysts, with notested loss inof 3activity. consecutive reaction runs, exhibit excellent catalytic stability being fully reusable with no loss of activity. 100

10080

6080

4060

2040

% of TCA decomposition % of TCA 200 123 % of TCA decomposition % of TCA 0 9010 7030 5050 3070 123 Figure 2. Decomposition of TCA after 29010 h reductive7030 dehalogenation5050 3070 over different SilverSil xerogels Figure 2. Decomposition of TCA after 2 h reductive dehalogenation over different SilverSil xerogels with NaBH as reducing agent in 6:1 molar ratio, [TCA] = 1.4 10 3 M, [BH ] = 8.4 10 3 M. with NaBH4 as reducing agent in 6:1 molar ratio, [TCA] = 1.4 × 10−−3 M, [BH4−4] −= 8.4 × 10−3 M.− Figure 2. Decomposition of TCA after 2 h reductive dehalogenation× over different SilverSil× xerogels Thewith visualNaBH4 appearanceas reducing agent of the in SilverSil6:1 molar ratio, catalysts [TCA] prior = 1.4and × 10− after3 M, [BH 3 consecutive4−] = 8.4 × 10−3 reactions M. runs is The visual appearance of the SilverSil catalysts prior and after 3 consecutive reactions runs is shown in Figure3. shown in Figure 3. CatalystThe visual decolouration appearance observedof the SilverSil only forcatalysts the 30:70 prior SilverSil and after xerogel 3 consecutive of larger porosity reactions points runs tois shown in Figure 3. - likely surface modification of the nanoparticle upon the interaction with the reactants in solution (BH4 and TCA) within the much larger porosity of the latter xerogel.

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Figure 3. Visual appearance of different SilverSil xerogels prior (top) and after TCA dehalogenation

reaction with NaBH4 as reducing agent in 6:1 molar ratio (bottom).

Catalyst decolouration observed only for the 30:70 SilverSil xerogel of larger porosity points to Figure 3. Visual appearance of different SilverSil xerogels prior (top) and after TCA dehalogenation likelyFigure surface 3. Visual modification appearance of of the different nanoparticle SilverSil uponxerogels the prior interaction (top) and with after theTCA reactants dehalogenation in solution reaction with NaBH4 as reducing agent in 6:1 molar ratio (bottom). (BHreaction4- and TCA) with NaBHwithin asthe reducing much larger agent inporosity 6:1 molar of ratio the latter(bottom xerogel.). We remind that the yellow colour of ORMOSIL glasses functionalized with silver (or gold) WeCatalyst remind decolouration that the yellow observed colour only of for ORMOSIL the 30:70 glassesSilverSil functionalized xerogel of larger with porosity silver (orpoints gold) to nanoparticlesnanoparticles is is due due to to the the surface surface plasmon plasmon resonance resonance (SPR) (SPR) of the of entrappedthe entrapped metal metal nanoparticles nanoparticles [19]. likely[19]. Figuresurface 4modification displays the ofUV-vis the nanoparticle spectra for uponAg NPs the suspension interaction (aboutwith the 20 reactantsnm in size), in solutionfor 30:70 Figure- 4 displays the UV-vis spectra for Ag NPs suspension (about 20 nm in size), for 30:70 SilverSil, (BHSilverSil,4 and TCA)and for within the blank the much matrix. larger porosity of the latter xerogel. and forWe the remind blank that matrix. the yellow colour of ORMOSIL glasses functionalized with silver (or gold) nanoparticles is due to the surface plasmon resonance (SPR) of the entrapped metal nanoparticles AgNPs sol. [19]. Figure 4 displays the 0.9UV-vis spectra for Ag NPs suspension (about 20 nm in size), for 30:70 Blank Matrix SilverSil, and for the blank matrix.0.7 Ag Matrix 0.5 AgNPs sol. O.D. 0.90.3 Blank Matrix 0.70.1 Ag Matrix 0.5-0.1

O.D. 300 355 410 465 520 0.3 wavelength (nm) 0.1 Figure 4. UV-vis absorption spectrum for the 30:70 SilverSil xerogel, blank matrix and Figure 4. UV-vis absorption-0.1 spectrum for the 30:70 SilverSil xerogel, blank matrix and silver silvernanoparticles. nanoparticles. 300 355 410 465 520 wavelength (nm) TheThe reduced reduced intensity intensity in in the the light light absorption absorption in in the the 30:70 30:70 xerogel xerogel after after 3 3 reaction reaction runs runs may may be be 0 - duedue toFigure to formation formation 4. UV-vis of of surface-modified absorptionsurface-modified spectrum Ag Ag for0nanoparticles, nanoparticles, the 30:70 SilverSil due due to xerogel,to the the action actionblank of ofmatrix both both BHand BH4 4silverand- and TCA TCA hydrophobichydrophobicnanoparticles. moieties moieties adsorbed adsorbed and and concentrated concentrated at at the the surface surface of of the the sol-gel sol-gel cages cages entrapping entrapping the the silversilver NPs. NPs. ORMOSILs ORMOSILs indeed indeed are well are known well toknown act as chemicalto act as sponges, chemical adsorbing sponges, and adsorbing concentrating and reactantsconcentratingThe reduced are their reactants intensity inner surface are in their the [20 innerlight]. absorptionsurface [20]. in the 30:70 xerogel after 3 reaction runs may be 0 - due toFigureFigure formation5 displays5 displays of surface-modified the the large large e ff effectect of of theAg the ORMOSIL nanoparticles, ORMOSIL composition composition due to onthe theon action the product product of both distribution. distribution. BH4 and For TCA allFor SilverSilhydrophobicall SilverSil compositions compositionsmoieties adsorbed the the TCA TCA and decomposition decomposition concentrated is at complete.is the complete. surface The Theof morethe more sol-gel hydrophilic hydrophilic cages entrapping SilverSil SilverSil 90:10 90:10 the andsilverand 70:30 70:30NPs. xerogels xerogels ORMOSILs showed showed indeed the the highest highestare well amount amount known of of dichloroacetic dichloroaceticto act as chemical acidacid (DCA)(DCA) sponges, andand aceticadsorbingacetic acidacid (AA)(AA)and production.concentratingproduction. The The reactants lowest lowest are DCA DCA their formation formation inner surface is is obtained obtained [20]. at at the the surface surface of of the the more more hydrophobic hydrophobic (50% (50% and and 70%70% methyl-modified)Figure methyl-modified) 5 displays the silica silica large glasses. glasses. effect of the ORMOSIL composition on the product distribution. For all SilverSilThese resultscompositions show oncethe TCA again decomposition evidence of is the complete. key role The played more byhydrophilic the ORMOSIL SilverSil surface 90:10 hydrophilic-lipophilicand 70:30 xerogels showed balance the (HLB)highest on amount the catalytic of dichloroacetic activity of the acid functionalized (DCA) and acetic ORMOSIL acid (AA) [21]. Theproduction. 70% methyl-modified The lowest DCA xerogel formation 30:70 is SilverSil obtained is at able the to surface completely of the degrademore hydrophobic TCA almost (50% entirely and to70% monochloroacetic methyl-modified) acid silica (MCA, glasses. often called chloroacetic acid) and to fully de-halogenated product, acetic acid. Unlike DCA and TCA, monochloroacetic acid is not carcinogenic and does not show mutagenic action [22]. In 2004, the WHO recommended a guideline value for MCA of 20 µg/L in

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drinking-water, based on an allocation of 20% of the tolerable daily intake assuming a 60-kg adult Materials 2020, 13, 827 6 of 9 ingesting 2 litres of drinking water per day [23].

Figure 5. Product distribution after 2 h reaction in TCA reductive dehalogenation of different

SilverSil xerogels, and over the same 70:30 xerogel in increasing amounts, with NaBH4 as reducing

agent in 6:1 molar ratio, [TCA] = 1.4 × 10−3 M, [BH4-] = 8.4 × 10−3 M.

These results show once again evidence of the key role played by the ORMOSIL surface hydrophilic-lipophilic balance (HLB) on the catalytic activity of the functionalized ORMOSIL [21]. The 70% methyl-modified xerogel 30:70 SilverSil is able to completely degrade TCA almost entirely to monochloroacetic acid (MCA, often called chloroacetic acid) and to fully de-halogenated product, acetic acid. Unlike DCA and TCA, monochloroacetic acid is not carcinogenic and does not show Figure 5. Product distribution after 2 h reaction in TCA reductive dehalogenation of different SilverSil mutagenicFigure 5.action Product [22]. distribution In 2004, the after WHO 2 h reactionrecommended in TCA areductive guideline dehalogenation value for MCA of differentof 20 µg/L in xerogels, and over the same 70:30 xerogel in increasing amounts, with NaBH4 as reducing agent in 6:1 drinking-water,SilverSil xerogels, based and on over an theallocation3 same 70:30 of- 20% xerogel of thein3 increasing tolerable amounts,daily intake with assumingNaBH4 as reducinga 60-kg adult molar ratio, [TCA] = 1.4 10− M, [BH4 ] = 8.4 10− M. ingestingagent in2 litres6:1 molar of drinking ratio, [TCA]× water = 1.4 per × 10 day−3 M, [23]. [BH× 4-] = 8.4 × 10−3 M. Remarkably,Remarkably, the the high high reproducibility reproducibility of of the the TCA TCA catalytic catalytic reduction reduction over over the the SilverSil SilverSil xerogels xerogels is demonstratedis demonstratedThese results by by the show the almost almost once unvaried againunvaried evidence product product distributionof dithestribution key obtainedrole obtained played in two inby two catalyticthe catalytic ORMOSIL runs runs carried surface carried out hydrophilic-lipophilicinout batch in batch (Figure (Figure6). 6). balance (HLB) on the catalytic activity of the functionalized ORMOSIL [21]. The 70% methyl-modified xerogel 30:70 SilverSil is able to completely degrade TCA almost entirely to monochloroacetic acid (MCA, often called chloroacetic acid) and to fully de-halogenated product, acetic acid. Unlike DCA and TCA, monochloroacetic acid is not carcinogenic and does not show mutagenic action [22]. In 2004, the WHO recommended a guideline value for MCA of 20 µg/L in drinking-water, based on an allocation of 20% of the tolerable daily intake assuming a 60-kg adult ingesting 2 litres of drinking water per day [23]. Remarkably, the high reproducibility of the TCA catalytic reduction over the SilverSil xerogels is demonstrated by the almost unvaried product distribution obtained in two catalytic runs carried out in batch (Figure 6).

FigureFigure 6.6.Product Product distribution distribution after after 2 h reaction2 h reaction in TCA in reductiveTCA reductive dehalogenation dehalogenation of different of different SilverSil 3 xerogelsSilverSil withxerogels NaBH with4 as NaBH reducing4 as reducing agent in 6:1agent molar in 6:1 ratio molar in two ratio reaction in two runs,reaction [TCA] runs,= 1.4[TCA]10 =− 1.4M, × −3 − 3 −3 × [BH10 M,] =[BH8.44 ] =10 8.4 ×M. 10 M. 4− × −

Finally,Finally, FigureFigure7 7presents presents the the e effectffect ofof TCA:NaBHTCA:NaBH44 molarmolar ratioratio andand thethe pHpH onon thethe TCATCA decompositiondecomposition yields.yields. AsAs expected,expected, higherhigher levelslevels ofof TCATCA reductionreduction werewere obtainedobtained forfor higherhigher NaBHNaBH44 concentrations,concentrations, with with complete complete TCA TCA decomposition decomposition taking taking place alsoplace at pHalso 7 provided at pH 7 that provided borohydride that isborohydride used in 6:1 molaris used ratio in 6:1 with molar respect ratio to with the respect substrate. to the substrate.

Figure 6. Product distribution after 2 h reaction in TCA reductive dehalogenation of different

SilverSil xerogels with NaBH4 as reducing agent in 6:1 molar ratio in two reaction runs, [TCA] = 1.4 ×

10−3 M, [BH4−] = 8.4 × 10−3 M.

Finally, Figure 7 presents the effect of TCA:NaBH4 molar ratio and the pH on the TCA decomposition yields. As expected, higher levels of TCA reduction were obtained for higher NaBH4 concentrations, with complete TCA decomposition taking place also at pH 7 provided that borohydride is used in 6:1 molar ratio with respect to the substrate.

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100

99

98

97

% of TCA decomposition 96

95 NaBH4:TCA=2:1 NaBH4:TCA=6:1 NaBH4:TCA =6:1 NaBH4:TCA=2:1 pH=7 pH=7 pH=10 pH=10

Figure 7.7. DecompositionDecomposition of of TCA TCA after after 2 h2 reductiveh reductive dehalogenation dehalogenation over over 70:30 70:30 SilverSil SilverSil xerogel xerogel in TCA in

reductiveTCA reductive dehalogenation dehalogenation at different at different pH and withpH and NaBH with4 as NaBH reducing4 as agentreducing in 2:1 agent and 6:1in molar2:1 and ratio, 6:1 3 [TCA] = 1.4 10 M. −3 molar ratio, ×[TCA]− = 1.4 × 10 M. 4. Conclusions 4. Conclusions In conclusion, we have discovered that 30:70 SilverSil, a 70% methyl-modified silica xerogel doped In conclusion, we have discovered that 30:70 SilverSil, a 70% methyl-modified silica xerogel with Ag nanoparticles, shows remarkably high and stable activity as heterogeneous catalyst for the doped with Ag nanoparticles, shows remarkably high and stable activity as heterogeneous catalyst reductive dehalogenation of trichloroacetic acid (a suspect human carcinogen) with NaBH4 as reducing for the reductive dehalogenation of trichloroacetic acid (a suspect human carcinogen) with NaBH4 as agent. Non carcinogenic and non mutagenic chloroacetic acid is, along with non toxic acetic acid, reducing agent. Non carcinogenic and non mutagenic chloroacetic acid is, along with non toxic the main product of the reductive dehalogenation. The excess borohydride is rapidly decomposed to acetic acid, the main product of the reductive dehalogenation. The excess borohydride is rapidly boric acid by the powerful catalytic activity of the Ag nanoparticles. Found in virtually all vascular decomposed to boric acid by the powerful catalytic activity of the Ag nanoparticles. Found in plants (where boron exists in the equilibrium between boric acid [B(OH)3] and borate anion [B(OH)4−), virtually all vascular plants (where boron exists in the equilibrium between boric acid [B(OH)3] and boric acid (B(OH3) byproduct of the water treatment proposed in the aforementioned processes is a borate anion [B(OH)4−), boric acid (B(OH3) byproduct of the water treatment proposed in the weak Lewis acid (pKa = 9.25) mostly undissociated at neutral pH [23]. The compound naturally occurs aforementioned processes is a weak Lewis acid (pKa = 9.25) mostly undissociated at neutral pH [23]. in several fruit-derived beverages including wine and cider. The compound naturally occurs in several fruit-derived beverages including wine and cider. Following the discovery of ORMOSIL-entrapped Au nanoparticles in the reductive dehalogenation Following the discovery of ORMOSIL-entrapped Au nanoparticles in the reductive of mono- and tri-bromo acetic acids with sodium borohydride [24], the high activity of sol-gel entrapped dehalogenation of mono- and tri-bromo acetic acids with sodium borohydride [24], the high activity silver nanoparticles in the reductive dehalogenation of toxic bromoacetic acid (the most toxic of the of sol-gel entrapped silver nanoparticles in the reductive dehalogenation of toxic bromoacetic acid regulated trihalomethanes and haloacetic acids), formed during water chlorination was reported in (the most toxic of the regulated trihalomethanes and haloacetic acids), formed during water 2017 [25]. This means, in principle, that by using a single cartridge filled with 70% methyl-modified chlorination was reported in 2017 [25]. This means, in principle, that by using a single cartridge filled mesoporous SilverSil xerogel it will be possible to remove from disinfected waters both brominated with 70% methyl-modified mesoporous SilverSil xerogel it will be possible to remove from chlorination by-products and trichloroacetic acid. disinfected waters both brominated chlorination by-products and trichloroacetic acid. Future work aimed at evaluating the degradation of more highly brominated or Br/Cl acetic Future work aimed at evaluating the degradation of more highly brominated or Br/Cl acetic acids will be reported soon. In conclusion, the low cost, high stability and ease of application of the acids will be reported soon. In conclusion, the low cost, high stability and ease of application of the SilverSil sol-gel catalyst to continuous processes open the route to the industrial uptake of SilverSil to SilverSil sol-gel catalyst to continuous processes open the route to the industrial uptake of SilverSil remove a suspected human carcinogenic agent exerting significant genotoxic and cytotoxic effects from to remove a suspected human carcinogenic agent exerting significant genotoxic and cytotoxic effects chlorinated waters. ORMOSILs functionalized with different dopant species exhibit uniquely high from chlorinated waters. ORMOSILs functionalized with different dopant species exhibit mechanical, thermal and chemical stability which, along with their high functional activity and lack of uniquely high mechanical, thermal and chemical stability which, along with their high toxicity, originates manifold applications [20,26]. Hydrothermally stable ORMOSIL-based membranes functional activity and lack of toxicity, originates manifold applications [20,26]. highly resistant to NaClO in solution, for instance, are already commercialized for enhanced, low Hydrothermally stable ORMOSIL-based membranes highly resistant to NaClO in solution, energy separation processes [27]. Further research will address the development of one such column for instance, are already commercialized for enhanced, low energy separation processes [27]. reactor functionalized with an optimal SilverSil coating for the effective removal of haloacetic acids Further research will address the development of one such column reactor functionalized from chlorinated water at real plants. with an optimal SilverSil coating for the effective removal of haloacetic acids from Authorchlorinated Contributions: water atConceptualization, real plants. Y.A. and M.P.; methodology, R.C.; software, K.T. and M.M.; validation, R.C., Y.A. and M.P.; investigation, K.T. and M.M.; resources, Y.A.; data curation, Y.A., K.T. and M.M.; writing—originalAuthor Contributions: draft preparation,Conceptualization, Y.A. and Y.A. M.P.; and writing—review M.P.; methodology, and R.C.; editing, software, Y.A. and K.T. M.P. and visualization, M.M.; validation, R.C.; supervision,R.C., Y.A. and Y.A. M.P.; and M.P.;investigation, project administration, K.T. and M.M.; Y.A.; resources, funding acquisition,Y.A.; data curation, Y.A. All authorsY.A., K.T. have and read M.M.; and agreed to the published version of the manuscript. writing—original draft preparation, Y.A. and M.P.; writing—review and editing, Y.A. and M.P. visualization, R.C.; supervision, Y.A. and M.P.; project administration, Y.A.; funding acquisition, Y.A. All authors have read and agreed to the published version of the manuscript.

Materials 2020, 13, 827 8 of 9

Funding: This research received no external funding. Acknowledgments: This article is dedicated to L. Hessel Castricum, now at TU Delft, for all he has done for the development of ORMOSIL-based membranes for low energy separation processes. K.T. thanks her colleagues Yanna Gurianov, Olga Krichevski, Natalia Litvak, and Faina Nakonechny. Thanks to Sourav Mishra for making freely available at Pexels.com the photograph used to produce the Table of Contents graphics. Conflicts of Interest: The authors declare no conflict of interest.

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