Effect of acid treatment on activated carbon: preferential attachment

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Materials and Interfaces

Effect of acid treatment on activated carbon: preferential attachment and stronger binding of metal nanoparticles on external carbon surface, with higher metal ion release, for superior water disinfection Pritam Biswas, and Rajdip Bandyopadhyaya Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b03844 • Publication Date (Web): 10 Oct 2018 Downloaded from http://pubs.acs.org on October 16, 2018

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Industrial & Engineering Chemistry Research

Effect of acid treatment on activated carbon: preferential attachment and stronger binding of metal nanoparticles on external carbon surface, with higher metal ion release, for superior water disinfection

Pritam Biswas, Rajdip Bandyopadhyaya* Department of Chemical Engineering, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India.

* Corresponding author: E-mail: [email protected] Tel: +91 (22) 2576 7209 Fax: +91 (22) 2572 6895 1

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract For ensuring complete, high throughput disinfection of water, superior interaction of silver nanoparticles (Ag-NPs) with contaminating E. coli cells in water is crucial. This is achieved by preferential attachment of Ag-NPs on the outer surface of activated carbon (AC),by either plasma or acid treated AC samples, termed as Ag-p-AC and Ag-a-AC hybrids, respectively. In subsequent flow-column experiments, Ag-a-AC showed 4 log reduction in E. coli cells in only 14 minutes of residence-time, which is much less compared to Ag-p-AC (23 minutes). This enhanced performance is due to: (i) attachment of more Ag-NPs on the outer surface of Ag-a-AC, providing better dispersion of individual Ag-NPs and (ii) altered surface topography of acid-treated AC, due to its higher surface roughness (increasing from 15.29 pm to 1.15 nm on acid treatment). The latter leads to release of more Ag+ ions and therefore, faster E. coli death in flow-column. Furthermore, Ag-NPs were much more strongly bonded to acid treated AC (944 kJ/mole adsorption energy), compared to plasma treated AC (667 kJ/mole), ensuring no Ag-NP detachment from Ag-a-AC surface during its use. Therefore, acid treatment is an efficient surface modification technique to continuously produce potable quality water at a high throughput of 2.66 L/h, presently demonstrated at least over 5 days.

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1. Introduction Activated carbon (AC) is extensively used in water purification. Usually, the welldeveloped pores and a wide range of surface functional groups present on AC leads to its very high surface area and adsorption capacity of various contaminants present in water.1,2 For further enhancement ofwater disinfection rate by use of metal nanoparticles(NP) with AC,preferential attachment of the NPs on the outer surface of such a host matrix is crucial. This will automatically ensure that the amount of NPs trapped inside the pores of AC or other such hosts is reduced significantly, leading to a potentially much higher disinfection rate with lesser NP requirement.This is becausemicroorganisms like E. coli are much larger than the small pores of AC.In fact, AC has a wide distribution of pores (from micron to nanometer size). In this context, most of the pathogens are having a larger size compared to the pore size of AC. Therefore, preferentialattachment is important for ensuring higher activity of the NP-AC hybrid. In the present work, silver nanoparticles (Ag-NPs) have been attached on the AC granules by room temperature acid treatment to demonstrate the above objective. In practical applications, others have attached NPs on a host-matrix to prevent their aggregation, ensuring reusability of the hybrid and to prevent its loss to the environment.3 This also helps to have a better contact and control over Ag+ ion release from the NP, in order to attack the bacterium. Conventionally, as the functional groups (like carboxylic acid and carbonyl groups) increases on the outer surface of the AC, more Ag-NPs can be preferentially attached on the AC surface.4–6 It has been reported that, AgNPs have strong interactions with carboxylic acid and carbonyl groups present on the AC surface, with the Ag+ ion forming Ag metal complexes with negatively charged functional groups.7 For achieving the goal of increasing the density of these functional groups on the outer surface of the AC - in order to ensure preferential attachment of Ag-NPs on the outer surface of the 3



AC - different surface modification techniques have been used so far, namely treatment with (i) plasma8,9 (ii) acids10,11 (iii) bases and other chemicals.7,12 Plasma treatment - in spite of having several advantages like, being a solvent free, benign, physical process to modify the AC surface - requires a high energy, very high instrument cost, supply of pure oxygen (99.9 % purity) and finally, only a small amount of AC can be treated in a single batch.4,8,9 Additionally, for efficient plasma treatment, vacuum must be generated before passing the oxygen. Therefore, the granules must have a larger size or be available in a pellet form. Therefore, plasma treatment is not a solution for production of large amount of surface modified AC, with smaller granule sizes. Conventional acid treatment methods with heating in presence of a concentrated acid, generates acidic functional groups on the AC. However, in this process, the surface area and pore size of AC decreases.2,13,14 Therefore, the inherent adsorption property of AC deteriorates. Basic (alkaline) treatment generates positively charged functional groups on AC, whereby the attachment of positively charged Ag+ ions and Ag-NPs deteriorates.2,13 Other chemicals used for surface modification of AC like, ammonia, hydrogen peroxide etc. are toxic in nature and therefore, not suitable for drinking water applications.4,12 Therefore, in the present work, acid treatment of AC was performed at room temperature as a replacement of other surface modification techniques, without affecting the porosity and pore volume of AC for subsequent Ag-NP attachment. For proper comparison, Ag-NPs have been attached on three types of AC samples - untreated, plasma treated and acid treated, to compare the Ag-NP attachment efficiency and their disinfection performance. To this end, the antibacterial activities and the ion release kinetics of all these hybrids were compared in a flow-column setup over a long duration. 4


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2. Materials and methods 2.1 Ag-NP synthesis by UV reduction method Silver nanoparticles (Ag-NPs) were synthesized following Biswas and Bandyopadhyaya.9 Typically, 7 ml of 0.1 M tri-sodium citrate was added to 70 ml, 0.01 M silver nitrate solution and the mixture was exposed to UV irradiation (λmax=365 nm) for 12 h. Tri-sodium citrate plays the role of a reducing as well as a coating agent. The Ag-NP sample was imaged using FEG-TEM (JEM-2100F, JEOL, Japan) and a size distribution was generated by measuring a total of 800-900 nanoparticles from three separate syntheses, by calculating diameters of 250-300 particles from each independent experiments.

2.2 Attachment of Ag-NPs on plasma and acid treated AC The activated carbon (AC) rings having granule size of 40 × 80 mesh sieve [with 10 % high density polyethylene (HDPE) binder] were plasma treated in presence of a controlled oxygen atmosphere, following Biswas and Bandyopadhyaya’s9 work. For acid treatment, the granules were crushed using a mortar pestle and sieved with a 20×40 mesh sieve. Subsequently, 100 ml of concentrated (69%) nitric acid was added to 5 g of AC and a mixing speed of 300 rpm was maintained for 48 h. The AC granules were washed and filtered using a ceramic filter. After every washing step, the AC granules were again mixed with Milli-Q water for 5-6 h, until the pH of the wash solution reached a value of 6.8-7. Subsequently, the AC granules were dried at room temperature for subsequent experiments. Acid treatment involves an extra pretreatment step, which although inexpensive compared to plasma or other chemical functionalization methods followed earlier, may require extra precaution,if it is to be applied inlarge scale operations.

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For identification of surface functional groups on the AC granules FTIR analysis [3000 Hyperion Microscope with Vertex 80 FTIR System, Bruker, USA] was performed and compared for untreated (un-AC), plasma treated AC (p-AC) and acid treated AC (a-AC). XPS analysis [AXIS Supra, Kratos Analytical (SHIMADZU group), UK] was also performed for all the three granules. Additionally, the raw data was deconvoluted to identify individual groups in the AC surface. Subsequently, for checking the wettability, contact angle was measured by placing a water drop on the AC surface for all the three types of AC samples (Goniometer: GBX Digidrop, France). Typically, 5 g of p-AC or a-ACgranule was individually added to 77 ml of Ag-NP solution and maintained at 300 rpm using an overhead stirrer for 12 h. Further, both the Ag-NP attached pAC (Ag-p-AC) and a-AC (Ag-a-AC) were filtered using a Whatmann filter paper (2 µm diameter ash-less filter paper) and dried at room temperature. FEG-SEM (JSM-7600F, JEOL, Japan)images were captured for both samples and the location of attachment of Ag-NP was also checked by elemental mapping. The pore volume was measured using a porosimeter (Autopore IV, Micromeretics, USA) for checking the extent of Ag-NP attachment inside the pores for un-AC, pAC, a-AC, Ag-un-AC, Ag-p-AC and Ag-a-AC. Concentrated nitric acid was added with the granules and kept for 72 h for measuring the Ag loading in the hybrid using ICP-AES (ARCOS, Simultaneous ICP Spectrometer, SPECTRO Analytical Instruments GmbH, Germany).

2.3 Cell-death experiments with Ag-p-AC and Ag-a-AC packing in a flow-column On the basis of contact time or residence time requirement for complete death of all the E. coli cellsduring batch mode cell-deathexperiments (Section A.1,SI), the flow rate was calculated using equation 1 from Biswas and Bandypopadhyaya’s9 paper. Accordingly, contaminated inlet water was passed through the Ag-a-AC packed column, at a flow rate of 2.66, 3.39 and 4.38 L/h 6


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Fig.1.Schematic diagram of continuous flow-column setup for water disinfection. Enlarged view shows, constituents of water in respective zones. The setup consists of a column packed with Ag-aAC granules, a peristaltic pump. The column has 80 mesh sieves on the entry and exit points for preventing outflow of granules during passage of water.

(equivalent to a residence time of 14, 11 and 8.5 minutes, respectively), for aflow-column of 25 cm height and 8 cm diameter(as shown in Fig. 1). Inlet water stream with an initial concentration of 104 CFU/ml of live E. coli cell was passed through the column and the samples were collected from the outlet after every 5 minutes. These samples were plated in an agar plate and incubated at 37º C for 24 h. The number of colonies were counted to get the concentration of live E. coli cells. The number of live cellswerealso quantified by fluorescence spectroscopy,usingthe calibration plot (Figs. S2 and 7



S3, SI). To establish the generality of the overall process of cell-killing, batch experiments were also performed for a gram positive bacteria, Basillussubtilis,(A.4,SI)following the same protocol mentioned in Section A.1,SI.

2.4 Agconcentration in the treated water in both batch and flow-column experiments After quantifying the residence time requirement for both Ag-p-AC and Ag-a-AC hybrids, the concentration of Ag was measured in the disinfectedwater in both batch and continuous mode runs. During batch mode experiments, typically, 200 mg of Ag-p-AC hybrid was dispersed in 100 ml of contaminated water (cells dispersed in PBS). The samples were collected after every 12 h of time interval up to 5 days. In flow-column, the samples were collected after every 12h of time interval using either Agp-AC or Ag-a-AC packed column, at a flow rate of 1.62 and 2.66 L/h, respectively, for 5 days. All samples were mixed with concentrated nitric acid and kept for 72 h for ensuring complete ionization of Ag. Subsequently, the Ag concentration was measured by ICP-AES.With the aim of checking the surface morphology of the AC after plasma and acid treatment, AFM (AFM; Asylum research, USA) measurements were performed. The surface topography of plasma and acid treated AC were characterized thereby and the effect of surface roughness on the Ag+ dissolution from the Ag-NP was assessed.

2.5 Quantification of energy of adsorption of Ag-NPs in thea-AC and p-AC granules With the aim of quantifying the interaction between Ag-NPs and AC, TGA-DSC (Thermogravimetry and differential scanning calorimetry)(STA 409 PS, NETZSCH, Germany) was performed for AC, Ag-p-AC and Ag-a-AC granules, in the temperature range of 23 to 1000 °C in 8


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N2atmosphere. Subsequently, the energy of adsorption of Ag-NPs from the AC surface was calculated for both Ag-a-AC and Ag-p-AC granules form the TGA-DSC plot.

3. Results and Discussion 3.1Characterization of Ag-NPs Experimental data Fitted curve

35

Relative frequency (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


30 25 20 15 10 5 0 0

20

40

60

80

Diameter (nm)

(a)

(b)

Fig. 2. (a) FEG-TEM image of discrete Ag-NPs formed; inset of Fig. (a) shows the diffraction pattern of Ag due its crystalline nature. Fig.(b) shows size distribution of Ag-NPs with a mean particle size of 27 nm. The size distribution was generated by measuring individual NPs from multiple TEM images. NPs were measured from three independent syntheses and at least 250-300 particles were measured from each single synthesis (measuring 800-900 particles to generate the bar graph). The dotted curve in Fig. (b) indicates the underlying size distribution of Ag-NPs by fitting the experimental histogram.

Fig. 2 (a) shows discrete, spherical Ag-NPs, with the inset having the diffraction pattern of crystalline Ag-NPs. Fig 2 (b) shows the NP size distribution based on the measurement (using 9



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ImageJsoftware) of a total of 800-900 Ag-NPs. The experimental data was fitted with a distribution to give the dotted curve as a trend line. The mean diameter of Ag-NP was found to be 30 nm having a standard deviation of 14 nm.

3.2Attachment of Ag-NPs on plasma and acid treated AC As final application of the hybrid is water disinfection, it is desirable to attach Ag-NPs preferentially on the outer surface of AC rather than inside the pores, since the pores (micro and meso pores) are much smaller than the E. coli cells [1.5-2.5 µm length and 0.8-1µm diameter, representative image is shown in Fig. S2 (a) in SI] and therefore, pores are not accessible by the cells. For Ag-un-AC, most of the Ag-NPs were attached on the pore-interiors, instead of the outer

Table 1. Total pore volume of different hybrids based on the intrusion volume measured using mercury porosimetry un-AC Pore volume (cc/g) Decrease in pore volume

2.20

Ag-un-AC

p-AC

Ag-p-AC

a-AC

Ag-a-AC

0.89

2.20

1.88

2.19

2.05

59.5%

14.4

6.3

(%) after Ag impregnation* *values were calculated by using the following formula:

(pore volume of AC- pore volume of AC after Ag-NP impregnation) 100 pore volume of AC

10


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surface of the AC [Fig. 3(a)], which is confirmed by a very high 59.5% decrease in pore volume after Ag-NP attachment (shown in Table 1). Significantly higher decrease in pore volume due to complete and partial pore blockage, as a result of attachment of Ag-NPs on Ag-un-AC hybrid was also evident form multiple FEG-SEM images (with varying magnifications) shown in Figs. S5 (a)(d) in SI. In contrast, pore volume decreases the least for Ag-a-AC (only 6.3%),compared to Ag-pAC (14.4%) and Ag-un-AC (59.5%) [Fig. 3 (d)], confirming attachment of maximum amount of Ag-NPs on the outer surface of the acid-treated AC rather than inside itspores (shown in Table 1). This can also be qualitatively seen in the varying amount of Ag-NP attached on AC surface, in the given representative images of different types of Ag-AC samples [Figs. 3 (b) and (c)]. Additional FEG-SEM images of Ag-un-AC, Ag-p-AC and Ag-a-AC are shown with lower magnification, in Figs.S6 (a)-(f) in SI, which proves the same point. Therefore, in further cell-death experiments (discussed later), Ag-a-AC is expected to provide a superior interaction between the Ag-NPs and E. coli cells and perform bestwater disinfection among all the three hybrids.Further, the pore volume was also measured for un-AC, p-AC, and a-AC (i. e. before Ag impregnation) using mercury porosimeter [Fig. 3 (d)]. The un-AC, p-AC and a-AC show almost similar pore volumes(2.20, 2.19 and 2.20 cc/g, respectively). Therefore, for our experiments, there is no detrimental effect of acid treatment on the pore volume of AC granules. This is in contrast to the case of acid treatment of AC with heating, followed by others in the literature, where a major impediment is resultant decrease in pore volume by 8.8 % (Marato-Valer et al.) to as much as 43.6 % (Chingombe et al.) .2,13,15,16

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(a)

(b) un-AC p-AC a-AC Ag-un-AC Ag-p-AC Ag-a-AC

0.25

0.20

Pore volume (cc/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.15

0.10

0.05

0.00

(c)

0

2

4

6

8

10

12

14

Pore diameter (µm )

(d) Fig. 3.FEG-SEM images of (a) Ag-NP attached untreated AC (Ag-un-AC), (b) Ag-NP attached plasma treated AC (Ag-p-AC) and (d) Ag-NP attached acid-treated AC (Ag-a-AC). Fig.(a) shows attachment of Ag-NP mostly inside the pores of the untreated AC surface.Figs.(b) and (c) show preferential attachment of Ag-NPs on the outer surface of the plasma and acid treated AC, respectively.(d) Comparison of pore volume for untreated AC, plasma treated AC, acid treated AC, Ag-NPattached untreated AC, plasma treated AC and acid treated AC.

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FTIR and XPS measurements are shown to identify if there is generation of any different surface functional groups (Fig. 4). FTIR spectraof un-AC, p-AC and a-AC are compared to that end in Fig. 4 (a). The peaks corresponding to wave numbers of 1460, 1550 and 1650 cm-1 are due to the -OH deformation of the carboxyl group, the stretching vibration of the C=O bond and the stretching 300 C1s

1.000 0.998 (COOH)

0.996 (C=O)

0.994 (COOH) (C=O)

0.990 1300

1400

1500

150

1600

1700

1800

C-OH or C-O-C

100

Un-treated AC Plasma-treated AC Acid-treated AC

0.992

C-C

200

Intensity (a.u.)

Transmittance (%)

250

COOH

C=O

50 0 284

1900

Wave number (cm -1)

286

288

290

Binding energy (eV)

(a)

(b)

100

100 C1s

C1s

80

80 C-C Intensity (a.u.)

C-C Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60 C-OH or C-O-C 40

C=O COOH

20

60 C-OH or C-O-C 40

C=O

COOH

20

0

0 282

284

286

288

290

282

Binding energy (eV)

284

286

288

290

Binding energy (eV)

(c)

(d)

Fig. 4. Comparison of FTIR spectra for untreated (AC), plasma treated (p-AC) and acid treated activated carbon (a-AC). Deconvoluted C1S peak from the XPS spectra for (b) untreated AC, (c) plasma treated AC and (d) acid treated AC, respectively.

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vibration of quinones, respectively [Fig. 4 (a)].8,9 The C=O stretching vibration of the carboxyl (COOH) group is seen at 1700 cm-1 8,10. Higher intensities of carbonyl and carboxylic acid groups are detected for plasma treated AC, compared to untreated AC. On acid treatment, intensities of both the groups further increased.During XPS analysis, the C1S peak was detected corresponding to a binding energy ranging from 281-291 eV [Figs. 4 (b)-(d)]. The peaks were deconvoluted to four different peaks, using the XPSPEAK41 software for un-AC, p-AC and a-AC samples. The peak due to the presence of graphitic carbon (C–C) bond is detected corresponding to a binding energy of 284.6 eV for all the three samples. The peaks at 285.8, 287.4 and 288.5 eV corresponds to hydroxyl (C–OH) or ether (C–O–C), carbonyl group in ketones and quinines (C=O) and the carboxyl (COOH) group, respectively8,17,18. The relative peak area due to C-OH, C-O-C and COOH groups increase after either plasma or acid treatment compared to un-AC (Table 2). By acid treatment we could achieve higher C=O (17.71 %) and COOH groups (7.31 %) compared to plasma treated AC. However, the C-OH and C-O-C groups are lower for a-AC (20.26 %) compared to p-AC (36.09 %). Hence, with acid treatment at room temperature (done by us), we could generate functional groups on the external surface, without any loss in pore volume.

Table 2 Comparison of relative peak area (%) of C1s peak from XPS spectra.

Untreated AC

C-C 77.41

C-OH and C-O-C 17.9

C=O 2.45

COOH 2.24

Plasma treated AC

54.8

36.09

2.51

6.59

Acid treated AC

54.7

20.26

17.71

7.31

14


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In addition, contact angle measurement with a water drop shows that, after acid treatment, the AC surface displays similar wettability as plasma treatment (Fig. S7 in SI) and thereby, better interaction between the AC surface and the E. coli cells in further cell-death experiments. On performing elemental mapping, it was reconfirmed that, acid treatment ensures attachment of AgNPs preferentiallyon the outer surface of AC (Fig. S8, SI). ICP-AES measurement shows similar Ag loading of 0.8 and 0.93 wt.% for Ag-p-AC and Ag-a-AC hybrid (with a standard deviation of 0.023 and 0.017, respectively), respectively, implying we can compare the performance of these two samples in terms of water disinfection capability. As standard deviation values are somewhat low, compared to mean values of Ag loading, the Ag loading can be considered more or less same, or maybe slightly more in case of Ag-a-AC, compared to Ag-p-AC. This is because, it is not possible to synthesize both Ag-a-AC and Ag-p-AC with exactly the same Ag loading. So, even if we consider slightly higher Ag loading in case of Ag-a-AC, it shows as much as 39.1% less contact time requirement, compared to Ag-p-AC. Therefore, Ag-a-AC truly shows significantly enhanced water disinfection efficiency, as otherwise one could have obtained only a slightly lower contact time only, not as much as 39.1 % lower contact time.

3.3 Time-dependent E. coli cell concentration during flow-column experiments usingAg-p-AC and Ag-a-AC packed column Using acid treated (Ag-a-AC) packed column, we could achieve zero live cell concentration from an initial cell concentration of 104 CFU/ml (i.e. 4 log reduction) in the treated outlet water fora residence time of only 14 minutes [Fig. 5 (a)]. In comparison, as reported by Biswas and 15



Contact time in min.

Initial concentration

*

with Ag-a-AC packing 8.5 11 14

8000

Contact time in min. with Ag-p-AC packing 23

4000

0 0

5

10

15

20

25

30

E.coli cell concentration (CFU/ml)

12000

12000

Contact time in min. with Ag-a-AC packing 8.5 11 14

8000

Contact time in min. with Ag-p-AC packing 23

4000

0 0

5

10

Time (min)

15

20

25

30

Time (min)

(a) (b)

0.3

0.2

wt% of silver released of total silver loaded

0.4 Silver concentration in the outlet (mg/L)

E.coli cell concentration (CFU/ml)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Ag-p-AC Ag-a-AC

Ag-p-AC Ag-a-AC

0.3

0.2

0.1

0.0 0

1

2

3

4

5

Permissible limit

TIme(days)

0.1

0

(c)

1

2

3

4

5

Time (days)

(d) Fig.5.(a)Quantification of numbers of live E. coli cells in the treatedwater from the flow-column, obtained by (a) shake flask testand (b) fluorescence spectroscopy using Ag-a-AC packing. A representative data for Ag-p-AC packing is also shown in both the plots. (c) Performance assessment of the flow-column setup with Ag-a-AC packing for longer duration, by measuring the cell concentration by shake flask testat a flow rate of 2.66 L/h, by disinfecting a total of 320 L of water over 5 days. Fig. (d) shows Ag concentrationin the water collected from the column outlet, with either Ag-p-AC or Ag-a-AC packing, at a flow rate of 1.62 and 2.66 L/h, respectively, over 5 days.Three independent sets of experiments were repeated to generate the error bars.

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Bandyopadhyaya9, Ag-p-AC required much higher residence time of 23 minutes to reach zero live cell concentration in the treated water. Therefore, Ag-a-AC shows a superior performance compared to Ag-p-AC, which automatically leads to a higher flow rate of 2.66 L/h [Fig. 5 (c)]. This has been possible for the currently developed acid treated sample because of preferential attachment of Ag-NPs (thereby better contact) and higher Ag+ ion release, which is discussed in the next section. Based on this superior performance of the Ag-a-AC packed column, it was selected also subjected to further long term experimental test at this high flow rate of 2.66 L/h over 5 days [Fig. 5 (c)]. In this regard, we found that, fluorescence spectroscopy data shows a very good match with the method of colony count to determine E. coli cell-death, data for both short [Fig. 5 (b)] and long term experiments (Fig. S9 in SI). This further validates our conclusion from two complementary methods of measurement.

3.4 Ag concentration in batch and flow-column experiments To investigate the reason behind the superior water disinfection performance of Ag-a-AC compared to Ag-p-AC hybrids (in spite of having almost similar Ag-NP loading), the Ag concentration was measured in the treated water for both hybrids, in both batch mode and flowcolumn experiments. In both batch (Fig. S10 in SI)and continuous mode (steady state concentration of 53 and 84.8 µg/L after 12 h for Ag-p-AC and Ag-a-AC packed columns, respectively) experiments,we find that the Ag concentration is higher for Ag-a-AC compared to Ag-p-AC (Fig. 5 (d)]. This results in superior antibacterial performance of Ag-a-AC, thereby displaying faster celldeath, compared to Ag-p-AC. Simultaneously, it was ensured that the Ag concentration was maintained below the maximum allowable limit of 100 µg/L for drinking water.19,20Finally, only 17



0.33 wt.% of the total Ag came out with treated water even after 5 days of continuous operation using Ag-a-AC, ensuring efficient water disinfection would be possible for a much longer period [inset of Fig. 5(d)].

3.5 Measurement of surface roughness for a-AC and p-AC granules To investigate the faster Ag release for Ag-a-AC, the surface topography of both plasmatreated AC (p-AC) and acid treated AC (a-AC) were measured and the average surface roughness was calculated for the overall surface area. For the p-AC sample, average surface roughness is 15.29 pm, which increased to a significantly higher value of 1.15 nm after acid treatment [Figs. 6 (a)-(b)]. This is because acid treatment erodes the surface of the AC, making the surface much rougher. The increase in surface roughness of the a-AC results in: (i) increase in specific surface area and (ii) incorporation of oxygen on surface peaks, which further leads to presence of more functional groups on the peaks.21Bouleghlimat et al. (2013) had also reported increase in surface roughness due to acid treatment for highly oriented pyrolytic graphite (HOPG).22 In their case, being in contact with concentrated nitric acid at room temperature, small blister like features were formed on the surface of the HOPG. Similar surface-features are observed in the present work on the AC surface, after nitric acid treatment at room temperature. The line profile also shows significant difference in roughness between p-AC and a-AC [Figs. 6 (c) and (d)]. Sukhorukova et al.21 has shown that, as the surface roughness of the hostmatrix increases, the release of Ag+ ion increases significantly due to more turbulence near the surface of 18


Page 18 of 32

Page 19 of 32

the AC during water flow. Therefore, in the present study too, increased surfaceroughnessof Ag-aAC

(b)

(a) 6

6

Plasma treated AC

Acid treated AC

4

4

2

2

Z (nm)

Z (nm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0

0

-2

-2

-4

-4

-6

0

200

400

600

800

1000

-6

0

200

X (nm)

400

600

800

1000

X (nm)

(d)

(c)

Fig.6. 3D topographical AFM images of (a) plasma treated and (b) acid treated activated carbon. Average roughness (Ra ) indicates the average roughness calculated on a 1 × 1 µm2 surface area in both the images. Line profile of (c) plasma treated and (d) acid treated AC surface.

19



hybrid resulted in generation of more functional groups on the outer surface of the AC, resultingin attachment of more Ag-NPs and higher release of Ag+ ions due to increased turbulence near the AC surface (with Ag-NPs attached on it). 3.6 Quantification of energy of adsorption of Ag-NPs in the a-AC and p-AC granules To measure the adsorption energy of Ag-NP to different AC surfaces, we have to first validate the measurement technique itself. For this, all the samples were heated under N2 atmosphere. Subsequently, on heating, the samples showed a significant decrease in sample mass,due to removal of moisture between the temperature range of 90-120°C (Fig. 7). The moisture content for all the three samples were quantified by multiplying the peak area obtained from the DSC plot (in the range of 90-120° C in Fig. 7) with the latent heat of vaporization of water. It gavemoisture contents of 12, 7.8 and 8.9 % for AC, Ag-p-AC and Ag-a-AC granules, respectively. The corresponding moisture content obtained from mass loss in the TGA data (in the same temperature range of 90-120° C in Fig. 7) are found to be 13.7, 8.7 and 9.9 % in AC, Ag-p-AC and Ag-a-AC granules, respectively, giving essentially the same trend in values as observed from DSC plots. This validates the method that, area under the DSC curve (over a certain temperature range of 480-540°C around the adsorption peak temperature of 512 °C, shown here in Fig. 7) can give a quantitative value of the adsorption energy of Ag-NPs on the AC surface. Therefore, using this validated method, we estimated the, respectively. So, it appears that,for both Ag-p-AC and Ag-a-AC, Ag-NP is chemisorbed (with an expected very large adsorption energy) on the AC surface. This is because typical chemisorption energy of individual, smallTGADSC measurement confirms that, for both Ag-p-AC and Ag-a-AC, the Ag-NPs aremoleculesisabout 80-100 kJ/moles. However, a large number of molecules are present in a single Ag-NP, leading to a 20


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much higher energy of adsorption, than individual molecules. Hence, chemisorbed on the ACsurface. Additionally, we concluded that, Ag-NPs were more strongly bound to the a-AC surfacecompared to the p-AC.This prevents any chance of leaching or detachment of Ag-NP from the a-AC surface

21



Fig. 7. Comparison of TGA-DSC curves for AC, Ag-a-AC and Ag-p-AC granules. The adsorption energy was calculated from the area under the curve by shown by the hashed area.

due to high shear generated because of turbulent flow near the surface of the a-AC.

3.7 Assessing the efficiency of water disinfection by comparing the death rate constant with previous works The water disinfection efficiency of Ag-NP attached hybrids from previous works were compared with the present work in terms of: (i) concentration of E. coli cells in inlet, contaminated water, (ii) quantity of Ag used per ml of contaminated water and (iii) death rate constant of E. coli cell (Table 3). Amount of Ag per ml of contaminated water was calculated by multiplying the Ag loading (wt. %) in AC and the concentration of solid AC per ml of water. The death rate constant value was found to be only 0.9 h-1 (i) for Tuan et al.’s, 7 work, which is 14 times lower than that of the present work (iv). This is because of the higher cell concentration and lower amount of Ag usage by them, compared to Ag-a-AC hybrid of the present work. Both Zhao et al.23 (ii) and Acevedo et al.24, (iii) have shown much lower death rate constant values, compared to the present Ag-a-AC hybrid (iv). In both cases, higher amount of Ag was used (3.03 and 4.56 times respectively), for a higher concentration of E. coli cells.

22


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Now, in all the previous reports [(i) – (iii)], the Ag-NP was attached both inside the pore and on the outer surface as well. Therefore, a significant amount of Ag in their cases, was not available for direct contact to the E. coli cells. For the present Ag-a-AC granules however, a significant amountof Ag-NPs were preferentially attached on the outer surface of the AC, facilitating faster cell-death with relatively lower Ag content. Thus, we have achieved 1.71 times higher death rate constant using the Ag-a-AC hybrid (iv), compared to Ag-p-AC (v). As explained earlier, Ag-a-AC shows ahigher Table 3 Comparison of E. coli death rate constant values calculated from batch mode E. coli cell-death data with previous works. Serial No.

Host matrix

Initial E. coli concentration (CFU/ml)

Quantity of Ag (mg)/ml of contaminated waterǂ

Death rate constant (h-1)

References

(i)

Ag-AC

106

0.02ǂ

0.90*

7

(ii)

Ag-AC

107

0.194ǂ

1.16*

23

(iii)

Ag-AC

107

0.354ǂ

2.62*

24

(iv)

Ag-a-AC

104

0.064

12.64

Present work

(v)

Ag-p-AC

104

0.064

7.41

Present work 9

(vi)

Ag-un-AC

104

0.064

3.58

Present work

(vii)

Ag-p-AC

4 x103

0.089ǂ

41.88

8

(viii)

Ag-un-AC

4 x103

0.096ǂ

3.72

8

23



ǂ Quantity of Ag (mg) in hybrids/ml of contaminated water was found by multiplying the amount of Ag-AC (mg)/ml of water and wt.% of Ag hybrid (wt.%). * In some of the previous literatures, the death rate constant of E. coli cells were not mentioned. For

those cases the death rate constant values were calculated using the E. coli cell concentration data with varying time mentioned in the literatures. A representative plot for fitting the death rate constant was shown in Fig.11, SI.

release of Ag+ ion, with better contact between cells and NPs, than that of Ag-p-AC, which finally results in superior antibacterial activity. In earlier work from our laboratory, Srinivasan et al.8 had reported death rate constant values for untreated and plasma treated AC and showed a significant enhancement in antibacterial performance due to plasma treatment of AC [(vii) and (viii)]. However, Srinivasan et al.8 could achieve such higher death rate constants in their work as the cell concentration was 10 times less for them, while the Ag content used by them was in fact higher compared to the present work. Therefore, to summarize, we could achieve maximum antibacterial activity for Ag-a-AC and thereby highest throughput of treated water in flow-column due to following reasons: (i) at room temperature acid treatment, the pore volume of a-AC remains intact and a significant number of surface functional groups were formed on the outer surface of the a-AC with better wetting of AC surface, (ii) preferential attachment of Ag-NPs on the outer surface of the a-AC (compared to poreinteriors), to ensure better contact and faster death of E. coli cells, (iii) increased surface roughness leading to possible increase in turbulence near the AC surface for Ag-a-AC, resulting in higher Ag+ ion release from the Ag-NPs in the Ag-a-AC hybrid, which provides better disinfection rate, and (iv) strong attachment of Ag-NPs on the a-AC surface, minimizing chances of detachment of Ag24


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NPs during any long period of use. Together all these culminates into production of disinfected water at a high flow rate of 2.66 L/h, starting with an initial cell concentration of 104 CFU/ml of E. coli cells, simultaneously ensuring Ag concentration (84.8 µg/L) to be below the maximum allowable limit of 100 µg/L for drinking water (WHO, EPA).19,20

4 Conclusions To achieve higher throughput of water decontamination, superior E. coli-nanocomposite (Ag-AC) interaction leading tofaster E. colicell-death(a gram-negative bacteria) is required. To this end, AC surface was functionalized by nitric acidtreatment at room temperature,resulting inpreferential attachment of Ag-NPs, mostly on the outer surface of AC. For demonstrating water disinfectionby utilizing faster cell-death, three kinds of AC samples - untreated (un-AC), plasma treated (p-AC) and acid treated (a-AC) AC samples were prepared and subsequently impregnated with Ag-NPs,the latter having a mean size of 30 nm. We ensuredamore or less same final Ag loading of 0.79, 0.8 and 0.93 wt.% in them, respectively, in order to compare their water treatment performance. Pore volume measurement of Ag-a-AC shows presence of most of the Ag-NPs to be on the outer surface (compared to pore-interiors),as evidenced by the minimum decrease (6.3%) in pore volume for this sample. This is in comparison to Ag-p-AC, where a higher amount of Ag-NPs areinside the pores, as suggested by the larger (14.4%) decrease in pore volume. These two values are in contrast to un-Ag-AC, which shows a very significant (59.5%) decrease in pore volume, due to attachment of most of the Ag-NPs inside the pores, which is due to the absence of any treatment and surface functionalization. The impact of this is seen in case of Ag-a-AC, where, presence of the

25



highest amount of Ag-NPs on the outer surface leads to thehighest(amongst present cases) celldeath rate of E. coli cells present in contaminated water. Therefore, on using 8 mg/ml of Ag-p-AC and Ag-a-AC in batch mode shake flask experiments, 4log reduction in live E. coli cell was achieved, in 25 and 15 minutes, respectively. The same 8 mg/ml of Ag-a-AC hybrid also showed 4 log reduction of B. subtilis (a gram positive bacteria) in just 3 minutes, proving the generality of this hybrid in eliminating both gram-positive and gram-negative microorganisms. Subsequently, the flow-column experiments (with 8 cm diameter and 25 cm height) show zero live cell concentration in the outlet water with a residence time of 23 and 14 minutes, using Ag-p-AC and Ag-a-AC packing, respectively, reflecting the superiority of acid treated samples again. Thus, using Ag-a-AC, 320 L of contaminated water was disinfected at a flow rate of 2.66 L/h over 5 days. This is supported by the fact that,in the treated water, Ag-a-AC (84.8 µg/L) shows 1.6 times higher Ag concentration compared to Ag-p-AC (53 µg/L). Therefore, both higher Ag+ ion release and presence of higher fraction of Ag-NPs on the outer surface of acid treated AC culminates into faster cell-death of E. coli, thereby, providing a much higher throughput of treated water in the column. It is also observed that due to acid treatment the surface roughness increases from 15.29 pm (for p-AC) to 1.15 nm (for a-AC). This may increase the turbulence in water flow near the AC surface for Ag-a-AC and thereby, increase the Ag release from the Ag-NPs. In addition, Ag-a-AC shows much stronger bonding of Ag-NPs with a-AC (adsorption energy of 944 kJ/moles) compared to Ag-p-AC (667.01 kJ/moles), ensuring no detachment during long term experiments. To conclude, we have achieved: (i) preferential attachment of Ag-NPs mostly on the outer surface of the a-AC with highest adsorption energy, ensuring stronger bonding of Ag-NPs on AC 26


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surface, (ii) productionof potable quality drinking water (with 4log reduction in E. coli cells), at a flow rate of 2.66 L/h with Ag concentration (84.8 µg/L) below the maximum allowable limit and (iii) loss of only 0.33 wt.% of Ag during 5 days of operation establishing Ag-a-AC hybrid to be a suitable hybrid for long term water disinfection. Therefore, we could meet all the essential requirements of enhanced cell-death and thereby resultin production of safe, drinking water. This has been explained by relating the cell-nanocomposite interaction due to change in surface topography of the carbon surface and the resultant higher binding energy of the Ag-NPs on the acid treated AC surface.

Supporting Information: (i) Batch mode cell-death experiments with Ag-p-AC and Ag-a-AC using E. coli (ii) Fluorescence microscopy imaging of E. coli cells (iii) Calibration plots based on the cell concentration and fluorescence intensity measured in fluorescence spectroscopy (iv) Batch mode cell-death experiments with Ag-p-AC and Ag-a-AC using B.subtilis (v) FEG-SEM images of Agun-AC hybrid showing complete and partial pore blockage due to Ag-NP attachment (vi) FEG-SEM images of Ag-un-AC, Ag-p-AC and Ag-a-AC with lower magnification (vii)Contact angle measurement using a water drop for un-AC, p-AC and a-AC (viii) Elemental mapping of un-AC, Ag-p-AC and Ag-a-AC hybrids (ix) Quantification of cell concentration during long term cell-death experiment (x) Quantification of Ag concentration during batch mode cell-killing experiment (xi) Death rate constant fitting based on the batch mode cell-killing data.

Acknowledgements 27



We thank the Department of Chemical Engineering, IIT Bombay for providing FEG-SEM, porosimetry and TGA-DSC facilities. We also thank sophisticated analytical instrumental facility (SAIF), IIT Bombay for FEG-TEM, FTIR and ICP-AES facilities. The authors also thank Department of Physicsand Department of Biosciences and Bioengineering, IIT Bombay for providing XPS facility andAFM facilities, respectively.

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Effect of acid treatment on activated carbon: preferential attachment

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