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Encapsulation of silica nanotubes from elephant grass with graphene oxide/reduced graphene oxide and its application in remediation of sulfamethoxazole from aqueous media Samson Akpotu, and Brenda Moodley ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02861 • Publication Date (Web): 14 Feb 2018 Downloaded from http://pubs.acs.org on February 15, 2018

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ACS Sustainable Chemistry & Engineering

Encapsulation of silica nanotubes from elephant grass with graphene oxide/reduced graphene oxide and its application in remediation of sulfamethoxazole from aqueous media Samson O. Akpotu1 and Brenda Moodley1, * 1

School of Chemistry and Physics, University of KwaZulu-Natal, Westville Campus, Durban, 4000, South Africa.

* corresponding author email: [email protected], First author email: [email protected] Telephone: +27 31 2602796 Fax: +27 31 2603091

Abstract Incorporation of graphene/graphene oxide on a low-cost support has been established to improve the adsorption performance. This study reports the synthesis of silica nanotubes (SNT) from elephant grass, its encapsulation by reduced graphene oxide (RGO)/graphene oxide (GO) and its application as a highly efficient adsorbent for the remediation of sulfamethoxazole from wastewater. The SNT was synthesized and encapsulated with GO and RGO, to obtain silica nanotubes graphene oxide (SNTGO) and silica nanotubes reduced graphene oxide (SNTG), respectively. The adsorbents were characterized with FTIR, CHN elemental analysis, thermogravimetric analysis, XRD, field emission scanning electron microscopy, and transmission electron microscopy. The FTIR, elemental analysis, and XRD results confirmed successful synthesis of the materials. SNTGO had an adsorption capacity of 125 mg/g which increased to 248 mg/g when it was reduced to SNTG in the adsorption of sulfamethoxazole. The pseudo-second order model best described the adsorption kinetics and the Freundlich isotherm was best fit for the equilibrium data. Thermodynamic studies showed the adsorption process was spontaneous and exothermic. The desorption studies revealed that the adsorbent could be regenerated and reused in the adsorption of sulfamethoxazole. SNTG was a better adsorbent compared to SNTGO for wastewater remediation. Keywords: elephant grass, graphene oxide, graphene, pharmaceutical, adsorption, remediation, biowaste

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Introduction Pharmaceuticals are classified as emerging organic pollutants.1 Antibiotics have been widely used by humans for over a century to treat and prevent diseases.2 Sulfamethoxazole (SMZ) (Figure S1 in the supplemental information (SI)) is part of a class of antibiotics used in livestock production, treatment of human ailments such as urinary tract infection3 and aquaculture.4 Due to its widespread use, disposal is a major problem. Studies have shown it gets discharged into the environment (soil and water)5 with concentrations of 2, 27.2 and 0.016 µg/L of SMZ found in municipal sewage treatment plants, hospital waste and surface water, respectively.6,7,8 In a wastewater treatment plant in South Africa, SMZ concentration ranged between 7.93-35.4 µg/L.9 SMZ has environmental risks associated with its usage and with pKa values of 1.7 and 5.7 it therefore can exist as cations, anions or zwitterions.10 Slow metabolism and poor degradation makes it persistent in the environment and results in undesirable impact on microbial communities, posing a risk to non-target species in the ecosystem.11 Pharmaceuticals are not easily removed by conventional water treatment systems.2, 12 Adsorption is the preferred method of choice for removal of SMZ and studies have investigated its removal from aqueous media by adsorption with manganese ferrite,12 biochar,13, sediment,14 soil,15 and activated carbon.16 Thus far, activated carbon (AC) is the most effective SMZ sorbent because of its large surface area, functional groups and delocalised electrons making it a fairly efficient adsorbent for pharmaceuticals. However, there are drawbacks associated with AC and these are complexity of the separation process,17 regeneration issues and high adsorbent cost.18 Carbon based nanomaterials such as carbon nanotubes, graphene (G) and graphene oxide (GO) are promising as sorbents because of their large surface area, pore volumes and variety of surface functional groups, and they are much more easily separated and cost less to regenerate as compared to AC.

Graphene gained prominence after it was isolated and characterised by Novoselov and coworker.19 It is a 2-dimensional nanomaterial that is one carbon atom thick, arranged in a honey comb lattice structure with high theoretical surface area (approximately 2630 m2/g), electron rich -system, and excellent mechanical, thermal and electrical properties.20,21,22 Despite graphene’s highly hydrophobic nature and ability to bind to organic pollutants through π- π 2


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interactions, the potential maximum adsorption capacity is not always achieved. This is because G nanosheets form aggregates in aqueous solution due to van der Waals forces and a very strong π - π interaction between the carbon layers. On the other hand, GO is made up of a single layer graphite oxide and on its surface, is an abundance of oxygen functional groups making it easily dispersed in water. The numerous oxygen functional groups make it an excellent precursor in the synthesis of GO based adsorbents. Nonetheless, GO has a low affinity for organic pollutants because of its weak π-electron structure and the influence of oxygen on van der Waals interactions, thereby rendering the GO surface hydrophilic.23

A potential way to make G/GO a highly efficient organic pollutant adsorbent is by loading G/GO on a low cost support,24 which enables maximum utilization of the carbonaceous material as an adsorbent. Silica-GO/G hybrid adsorbents has been utilized as chromatographic packing,25 solidphase extraction sorbent,23, 26 and in adsorption of uranium in water.20 Despite its high surface area, silica is not an efficient adsorbent for organic pollutants in aqueous media because of its hydrophilic nature. Silica is attracted to water molecules in solution when used as a sorbent.27 To make silica hydrophobic, it is coated with a highly hydrophobic material such as carbonaceous GO/G. This encapsulation serves 2 purposes: (1) helps with an improved dispersion of G/GO because of its hydrophobic nature and (2) creates better separation of the adsorbent and adsorbate in solution. In this study, silica nanotubes (SNT) were synthesized from elephant grass, an agrowaste material with no economic value, which is cheap to make as the grass was locally sourced the surrounding environment, and was encapsulated with reduced GO and GO.

To the best of our knowledge, this is the first report of synthesis of SNT from elephant grass, its encapsulation by RGO/GO and its application as highly efficient adsorbents for the remediation of sulfamethoxazole from simulated and real wastewater. Adsorbents were characterized using elemental analysis (EA), FT-IR spectroscopy, thermogravimetric analysis (TGA), x-ray diffraction (XRD) studies, textural analysis, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The relative low cost for the production of SNT from biowaste, coupled with the simple procedure of encapsulation and its high efficiency makes it an excellent adsorbent for the removal of pharmaceutical pollutants from simulated and real 3



wastewater. This study also comprehensively discussed the various adsorption mechanisms and adsorbent regeneration.

Experimental Materials

Natural graphite powder, methanol (HPLC grade), hydrazine monohydrate (80%), 3aminopropyl triethoxysilane (APTES, 99%), HCl (37%), KMnO4 and the pharmaceutical sulfamethoxazole were obtained from Sigma-Aldrich. Absolute ethanol (Merck), cetyltrimethyl ammoniumbromide (CTAB 99%+, Cabiochem), H2SO4 (98%), H2O2 (35%) and H3PO4 (80%) were obtained from Promark. All the reagents were analytically pure and were used without further purification. MilliQ water was used in the preparation of standard solutions and for the mobile phase. Elephant grass was obtained from the University of KwaZulu-Natal, Westville Campus, Durban, South Africa.

Synthesis of SNT Elephant grass (EG) was pre-treated with a modified method of Adam and co-workers.28 Approximately 40 g of a clean and ground EG was stirred with 1000 mL of 1.0 M HNO3 at room temperature for 24 hours in a plastic beaker with a magnetic stirrer at a speed of 300 rpm. Acid treated EG was washed with double-distilled water to a constant pH, dried in an oven at 100 °C for 24 hrs and calcined in a furnace at 600 oC to remove organics and form EG ash (EGA). Briefly, SNT was synthesized using a 1.2:1 molar ratio of elephant grass ash (EGA):CTAB. CTAB was first added to the sodium silicate solution whilst it was stirred at room temperature. After 10 minutes, 3.0 M nitric acid was added to bring the pH to 3, which signified the end of the reaction. The gel obtained was aged at room temperature for 5 days, filtered and washed vigorously to a neutral pH. The gel was washed with acetone, dried overnight at 100 °C. The product was calcined at 550 °C for 5 hrs and labelled as SNT.

4


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The detailed synthesis procedure for GO (Supplemental Information (SI 1),29 and SNT-NH2 (SI 2),30 are provided as part of the supporting information.

Synthesis of SNT-GO and SNT-G (SNT-reduced graphene oxide)

In this encapsulation process, SNT-NH2 (3.0 g) was dispersed and sonicated in water. GO (10 mg/mL, 150 mL) previously sonicated for 2 hrs was added to the suspension and the pH was adjusted to 7 by adding NH4OH whilst stirring. It was stirred continuously for 8 hrs and left to settle for an hour. The product was filtered, washed with water and dried in a vacuum oven for 6 hrs at 50 °C. This was labelled as SNTGO. In the synthesis of SNTG, a similar procedure to that of SNTGO was adopted but the temperature of synthesis was 80 °C and hydrazine monohydrate was added to the mixture to reduce GO to RGO. The mixture was stirred for 8 hrs, left to settle for an hour, filtered, washed with water and dried under vacuum. The solid black product was oven dried in a vacuum at 80 °C for 6 hrs. This was labelled SNTG.

Adsorbent characterization

The CHN/O analysis of the samples was carried out using a Thermo Scientific CHNS/O elemental analyser. N2 adsorption-desorption isotherms were obtained with a Micromeritics Tristar II 3020. Approximately 0.2 g of each sample was degassed at 90 °C for an hour and at 200 °C for 18 hrs. Textural properties were obtained from the BET equation. XRD analyses of the samples were carried out with a Bruker D8 XRD with Cu-Kα radiation at 40 mA and 45 kV. X-ray fluorescence (PAnalytical Axios Max) was used in the quantitative analysis of the chemical components of SNT. Surface morphology analysis of the adsorbent was carried out using a Zeiss 10 kV field emission scanning electron microscope (FESEM) and a JEOL JEM transmission electron microscope (TEM). About 5 mg of sample in a platinum pan was placed in a furnace (SDT Q 600 V 209 Build 20), from 25-900 °C at 10 °C/min in a N2 environment for thermogravimetric analysis (TGA). A fourier transform infrared (FT-IR) Perkin Elmer series 100 spectrometer in the range of 4000 – 400 cm-1 was used in the acquisition of FT-IR spectra. The

5



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point of zero charge (pHpzc) of SNTGO and SNTG were determined by an acid-base titration method by Li et al (SI 3). 31 Analytical method

SMZ was quantified using a HPLC system consisting of an Agilent 1200 SL (Agilent Technology, USA) with a pressure grade pump and a UV-Vis detector. Separation was carried out at room temperature with an Agilent Eclipse XDB C-18 (4.6 x 150 mm, 5 µm particle size) column and a mobile phase flow rate of 0.6 mL/min under isocratic conditions with a mixture of methanol/water (60:40; v/v). The pH of the water was adjusted with H3PO4 to 2.3. The sample volume injected was 5 µL and wavelength of detection was 270 nm.

Adsorption experiments

Adsorption of SMZ on synthesized adsorbents was studied in batch adsorption mode and was carried out in duplicate. It was done in PTFE screw cap vials covered with aluminium foil at room temperature, to determine effect of adsorption parameters such as effect of adsorbent dose adsorption time, effect of ionic strength, and pH and temperature. A 1000 mg/L stock solution of SMZ was prepared by dissolving 50 mg of SMZ in 3 mL methanol in a 50 mL volumetric flask and made up to mark with MilliQ water, from which several 15.0 mL working solutions of 12.5 mg/L SMZ were prepared. The pH of the solution was adjusted using 0.01 M NaOH or 0.01 M HCl to the desired pH. An amount (0.05 g) of the adsorbent was added to the solution and it was shaken with a mechanical shaker for a period of time. Thereafter, the solution was centrifuged at 5000 rpm for a minute in order to collect the supernatant. The supernatant was filtered with a 0.45 µm (cellulose acetate) filter disc and the concentration of SMZ in the solution was determined. The percentage adsorption was calculated using the equation below:

% adsorption =

( )

100

(1)

Where is the initial concentration (mg/L), is the equilibrium concentration after a set time in mg/L. Kinetic studies were carried out using 150 mg of adsorbents with 200 mL of 25, 50, 75 6


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and 100 mg/L of SMZ solution and the pH was adjusted to 2, using 0.01 M NaOH/0.01 M HCl solutions. Aliquots of the solution were withdrawn periodically (2 – 1440 mins), filtered, and the concentration of SMZ adsorbed was determined. In establishing the rate determining step of the adsorption process, 4 kinetic models were utilized; namely pseudo-first order (PFO), pseudosecond order (PSO), intraparticle diffusion (IPD) and Elovich models (Table S1). The adsorption capacity was calculated from equation 2:

=

( )

(2)

Where q is the amount of SMZ adsorbed by the adsorbents in mg/g, V is the initial volume of solution in L and m is the adsorbent mass in g.

The equilibrium data obtained were analysed with the following isotherm models; Langmuir, Freundlich, Temkin and Dubinin-Radushkevich models (Table S2).

Real water samples

Water samples were obtained from Blue Lagoon (BLG) (mouth of the Umgeni River into the Indian Ocean) with co-ordinates of 29° 48’ 41” S and 31° 02’ 12” E, and the Northern Wastewater Treatment plant (WWTP) inlet point with co-ordinates of 29° 48’ 27” S and 30° 59’ 51” E, in Durban, South Africa. Samples were filtered using a 0.45 µm filter disc and were kept in amber bottles, wrapped in aluminium foil and stored at 4 °C. The properties of the samples are shown in Table 1. The concentration of SMZ in the real samples was determined initially and was found to be below the detection limit of the HPLC. Thus, a 10 mL aliquot of real water sample was spiked with a 0.125 mL (1000 mg/L) SMZ standard solution to obtain a concentration of 12.5 mg/L. The samples were equilibrated overnight and 0.05 g of the adsorbents were added to the samples and shaken for 60 mins. The suspension was centrifuged and filtered and the removal efficiency and adsorption capacity for SMZ was evaluated with eqn (1) and eqn (2), respectively.

7



Table 1 Physical water quality parameters of the water samples Parameter

WWTP

BLG

pH

4.04

3.39

TDS (mg/L)

670

3100

Conductivity (mS)

1000.1

5200.3

Res (Ω)

729

150.7

Redox (mV)

93.5

181.3

Temp (°C)

22.0

22.5

Adsorbent regeneration

Regeneration of used adsorbent was by washing with acidic ethanol, dried in a vacuum oven at a temperature of 60 °C. Adsorption capacity was then calculated. The washing and recycling was carried out four times on the same adsorbent material.

Results and Discussion Characterisation of adsorbents Thermogravimetric analysis

TGA was carried out to determine the thermal stability, purity and volatile portion of SNT, GO, SNTGO and SNTG with a temperature range from 25 – 900 °C, using a mass of about 5 mg of samples placed in a platinum pan in the presence of nitrogen gas at a ramp rate of 10 °C/min. There profiles are shown in Figure 1. The weight loss between 100 - 130 °C was attributed to the evaporation of physisorbed water. In SNT, there was a further weight loss of 10% between 350 – 450 °C which was due to the decomposition of residual carbon and condensation of adjacent silanol.30,32 Thereafter, no mass loss was observed. In GO, there was a remarkable weight loss 8


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between 200 °C to 250 °C attributed to the removal of the oxygen containing functional groups (epoxy, carbonyl and carboxyl) which was observed by Liu and co-workers.17 In SNTGO and SNTG, there was a lower weight loss when compared to GO which can be attributed to the presence of GO and RGO on SNT, respectively. SNT had a higher thermal stability than GO and G on their own, hence, the possible encapsulation on silica helped in increasing GO and RGO thermal stabilities present on SNT. The weight loss before 250 °C observed in SNTGO and SNTG was insignificant and this is indicative of the stability of both materials. However, at 250 °C to 550 °C, SNTGO and SNTG lost 25% and 15% amount of its weight, respectively. This loss in weight is due to the thermal degradation of (carbon) GO and RGO on SNTGO and SNTG, respectively.

100

80 Weight (%)

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

GO SNTGO SNTG SNT

40

20 0

200

400

600

800

o

Temperature ( C)

Figure 1 TGA profiles of SNT, SNTGO and SNTG Elemental analysis (EA)

The elemental compositions of the samples are presented (Table 2). GO had a carbon content of 37.6%. Comparative evaluation of elemental compositions of SNT, SNTGO and SNTG, showed that the carbon content increased by 0, 17.0 and 21.4%, respectively. It should be noted that for SNTGO and SNTG, elemental composition did not take into account the amount of silica present 9



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and this assumption was necessary to calculate the O/C ratios. The lower H/C ratio of SNTG when compared to SNTGO shows that SNTG was more aromatic, whereas the lower O/C ratio of SNTG shows that SNTG was less hydrophilic than SNTGO as was also observed by Wang and co-workers,33 and Yao and co-workers.34 The presence of carbon in SNTGO and SNTG proves that RGO and GO had been successfully encapsulated on SNTG and SNTGO, respectively. There is still the presence of oxygen from the silica, however, SNTG and SNTGO now have higher C/O ratio due to the presence of the carbon which gives SNTG and SNTGO their hydrophobicity.

Table 2 Elemental composition and atomic ratios, and surface areas (SA) of SNT, GO, SNTGO and SNTG Sample

SNT

GO

SNTGO

SNTG

C/%

0.836

37.6

17

21.4

H/%

2.31

2.23

1.76

1.39

O/%

NA

49.5

79.9

75

N/%

0.128

0.58

1.4

3.00

H/C

NA

0.71

1.24

0.78

O/C

NA

1.75

6.25

4.66

SA(m2/g)

402

38

152

100

Pore volume (cm3/g)

0.67

0.017

0.47

0.52

Pore diameter (nm)

50

38.2

23.8

39

NA-Not applicable

Textural analysis

The nitrogen sorption isotherm results of GO, SNT, SNTGO and SNTG are shown in Table 2. All materials exhibited a distinctive type IV isotherm, typical of mesoporous materials (Figure 2).35 GO had a H3 hysteresis loop over a relative pressure of P/Po 0.4-1.0. This type of loop is characteristic of materials having agglomerate particles with slit shape pores of irregular size and shape. However, SNT, SNTG and SNTGO had a H2 hysteresis loop which typifies porous 10


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materials.36 This showed that the materials were regular and uniformly packed agglomerates.28 The textural properties from the N2 adsorption-desorption analysis are shown in Table 2. The decreased surface area in SNTGO and SNTG can be ascribed to their surface coverage by GO and RGO, respectively, causing the materials to have smaller surface area and porosity than SNT. This is in agreement with the elemental analysis which suggests that as more carbon is loaded on silica, its surface area decreases. The pore size distribution for GO showed it was made up of irregular shaped pores whereas SNT, SNTG and SNTGO had a monomodal pore

500

dV/dlogP(D) Pore Volume (cm3/g)

system.

-1 Quantity adsorbed cm3/g STP

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


(a)

400 300 200

SNT GO SNTG SNTGO

100 0 0.0

0.2

0.4

0.6

0.8

1.0

(b)

GO SNTG SNTG SNT 0

50

Relative Pressure/ P/Po

100 150 200 Pore volume/ nm

250

Figure 2 (a) N2 adsorption desorption isotherm and (b) Pore size distribution of GO, SNT, SNTG and SNTGO FT-IR analysis

The FT-IR spectra of SNT, GO, SNTG and SNTGO is shown in Figure 3. The peaks around 3380 cm-1, 3250 cm-1 and 3420 cm-1 are due to -OH stretching vibration in GO, SNTGO and SNT, respectively.30 In GO, the carbonyl vibration is seen at 1364 cm-1, the vibrations present at 1727 cm-1 and 1632 cm-1 are attributed to C=O stretching of the carboxylic acid substituent, the vibration at 1226 cm-1 is attributed to the C-O (epoxy) and the peak at 1065 cm-1 is attributed to C-O alkoxyl vibration. When compared to GO, SNTGO and SNTG had peaks which are distinctive for silica. This implied that SNT was successfully coated with GO and RGO. In 11



SNTGO, there were peaks at 1619 and 1415 cm-1 which corresponded to C=O stretching vibration. SNTGO had a similar profile to GO, which indicated the successful encapsulation of GO on SNT. In SNTG, there were no peaks due to the carbonyl, C-O-C and epoxy groups.29, 30 The band at 1040 cm-1 was attributed to the Si-O-Si/Si-O-C asymmetric vibration and Si-O-C was linked by covalent bonds, which was also observed in a similar study by Liu and coworkers.17 There was a reduction in the peak intensity at 1047 cm-1 for SNTG. This indicated that the carboxylic group was converted to a Si-O-C bond as was also observed in a study by Meng and co-workers.20 The peaks at 780 and 430 cm-1 were assigned as Si-O-Si symmetric and bending vibrations, respectively. In SNT, the peak at 1637 cm-1 for Si-OH bending vibration shifted to 1555 cm-1 which is Si-O-C in SNTG. In RGO, the peak at 1555 cm-1 is a –C=C- which is assigned as an vinyl group stretching vibration signifying the vinyl group was successfully introduced into SNTG.20 The results were clearly in agreement with the results from the elemental analysis.

SNTG

Transmittance (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

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GO SNTGO SNT -1 1637 cm

-1

3380 cm

-1 1555 cm -1 1047 cm

1000

2000

3000

4000

-1

Wavenumber (cm )

Figure 3 FTIR spectra of SNT, SNTGO, SNTG and GO X-ray diffraction and XRF studies

12


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The XRD patterns for GO, SNT, SNTGO and SNTG are shown in Figure 4. An intense peak (001) at 2(Ɵ) = 9.74 for GO the peak, typical for amorphous carbon, was observed which fits the inter-planar spacing of 0.87 nm. XRD profiles of SNT, SNTGO and SNTG had the amorphous nature of silica with a broad peak centred at 2(Ɵ) = 23.30 This showed that the silica matrix was present together with RGO and GO as seen in SNTG and SNTGO, respectively. In SNTG, the peak around 25.4 is typically the peak for RGO as was also observed in a similar study by Luo and co-workers.26 A similar very small peak at 2(Ɵ) = 26.20 is observed for SNTGO which is typical for that of graphite (002). This is because GO containing oxygen functional groups on the surface of SNTGO has been partially transformed to a certain degree as a result of the interaction between SNT and GO. This was also observed in a similar study by Li and co-workers.37 A 99.2% SiO2 was obtained from the XRF analysis with trace amount of metal oxides, which may be due to the source of the elephant grass.

(002)

SNTG 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


SNTGO SNT (001)

GO 10

20

30

40

50

60

2-Theta/degree

Figure 4 XRD profiles of GO, SNT, SNTGO and SNTG FEGSEM and TEM analysis

The micrographs (SEM and TEM) of GO, SNT, SNTG and SNTGO are shown in Figure 5. The surface of GO sheets obtained was smooth but also had pores and was a little transparent as seen on TEM image (5e). The SNT appeared as long webbed like tubes, with very visible large pores (SEM Figures 5b and TEM 5f). When SNTGO and SNTG were compared with SNT, there was a 13



visible change in the structure. In SNTGO, the silica matrix was encapsulated under thin layers of homogeneous graphene oxide sheets which appeared transparent and the GO sheet was well layered and bonded on the SNT. The SNT pores can be seen under the graphene oxide coating (Figure 5c (SEM) and 5g (TEM). In SNTG, the RGO layers appear as a thick stack on top of the silica matrix. It can be seen that total encapsulation occurs as the transparency was lost (Figure 5h) and with no appearance of reduced graphene oxide as free standing. This can be attributed to the increase in percentage carbon by the chemical reduction of GO to RGO by hydrazine monohydrate. This is in agreement with results obtained from the elemental analysis, and the decrease in specific surface area from textural analysis and FT-IR analysis. The SEM and TEM images suggest that the morphology of the synthesized SNTGO and SNTG depended more on the morphology of SNT rather than GO. It is also observed that there was no aggregation from the RGO sheets.

Figure 5 SEM of (a) GO (b) SNT (c) SNTGO (d) SNTG and TEM of (e) GO (f) SNT (g) SNTGO and (h) SNTG 14


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The effects of pH on adsorption of SMZ on SNTGO and SNTG

Figure 6 shows the effect of pH on adsorption of SMZ on SNTG and SNTGO. Liu and coworkers,38 predicted 3 ways in which pH affects the adsorption process in pharmaceuticals. Firstly, changes in pH may cause dissociation of hydrophobic neutral molecules to hydrophilic, negatively charged species, thereby reducing hydrophobic interaction. Secondly, electrostatic repulsion would increase with changes in solution pH, thereby suppressing electrostatic interaction between the adsorbent and the adsorbate. Also, a higher (basic) pH could increase the π donor ability of the adsorbate, thereby causing π-π interaction.39 In this study, the surface properties of SNTGO and SNTG, such as protonation-deprotonation transition, and surface charge of functional groups were affected by pH as the range varied from 2 - 10. The surface of SNTGO and SNTG contains a variety of oxygen rich functional groups and SNTG has a carbon rich surface (C=C) which are all responsible for adsorption. The adsorption of SMZ on SNTGO and SNTG decreased over the pH range by more than 80%. The removal of SMZ has been observed to be highest in an acidic pH.10 The pHPZC relationship played a crucial role in the adsorption behaviour between the adsorbate and adsorbents. The acid-base titration data for SNTGO and SNTG are shown (Figure S2). TOTH is the total concentration of protons consumed during titration and it is calculated as follow:

=

( )

(3)

Where Cb is the concentration of NaOH, V0 is the initial volume of the suspension, Vb is the total volume of -OH added at different titration points, while Veb1 is the NaOH volume added in the titration at the acidic side. The surface of SNTGO and SNTG contains a large amount of bonding sites and the surface site density of the adsorbents can be estimated from acid-base titration (details in SI 3) used in the pHpzc. The pHpzc for SNTGO and SNTG were 2.80 and 4.02, respectively. In solution, SNTGO and SNTG have a negatively charged surface and Li and coworkers had a similar observation.37 15



The pHpzc is used to gain insight and control the net surface charge of an adsorbent. If the pH of a solution is less than the pHpzc of an adsorbent, the net surface charge of the adsorbent becomes positive and when the solution pH is higher than the pHpzc of the adsorbent the net surface charge is negative. The pHpzc of SNTGO and SNTGO has a negative charge in the range of 2.80-10 for SNTGO and 4.02-10 for SNTG. SMZ has three pKa values of 1.7 (dominated by cationic species), 1.7-5.7 (dominated by neutral species) and > 5.7 (dominated by anionic species). At acidic pH (2-5.7), the adsorbate (SMZ) was dominated with neutral SMZ species and the adsorbent (SNTGO) was positively charged at pH < 2.8 but negatively charged at pH > 2.8. Thus, the attraction between the adsorbent (SNTGO) and SMZ was due to mainly hydrophobic effects and π-π interactions, which is shown by the lower percentage removal of SMZ.

A similar explanation can be used for the adsorption of SMZ with SNTG, however

because of the higher % C in SNTG (Table 2), SNTG has a greater hydrophobicity which has resulted in the greater adsorption of SMZ at lower pH values. Furthermore, SNTG also has a higher pore diameter and greater pore volume than SNTGO, which may have contributed to the higher percentage removal of SMZ with SNTG. A similar observation using GO for the adsorption of SMZ was recorded by Nam and co-workers.10 At pH > 5.7, the system is dominated by anionic SMZ species with a loss of proton from the SMZ. The adsorbent is now at a pH greater than its pzc and is therefore anionic. This results in electronic repulsion between the negatively charged adsorbents and the anionic SMZ, resulting in a decrease in adsorption. In addition, due to the polar functional groups on SMZ such as amine, hydroxyl and sulfonyl, it exhibits electron-withdrawing tendencies at basic pH. These groups can cause a rejection of aromatic rings in the adsorbents (electron acceptor). Hence, the removal of SMZ decreased as the solution became basic.

16


Page 16 of 34

Page 17 of 34

100 90

Percentage adsorption/ %

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


SNTGO SNTG

80 70 60 50 40 30 20 10 2

4

6

8

10

pH

Figure 6 Effect of pH on sorption of SMZ onto SNTGO and SNTG (Conditions: 15 mL of 12.5 ppm SMZ, equilibration time 180 mins, adsorbent dose 50 mg, temperature 25 °C, n=2).

The effect of adsorbent dose and ionic strength are shown in (SI 4 and Figure S3) and (SI 5 and Figure S4), respectively.

Adsorption isotherms

The isotherm parameters for both adsorbents are presented in Table 3. Adsorption isotherm plays a major role in analysing adsorption systems. Figure S5 shows the experimental adsorption isotherm plot of SMZ on SNTGO and SNTG. According to the Giles isotherm classification,40 the adsorption isotherms can be classified as type L. Therefore, it may be concluded from the adsorption isotherms that the adsorbed solute was not vertically oriented or there was no strong competition from the aqueous phase for the adsorption sites of SNTGO and SNTG.41’17 The data obtained from the experiments fitted both the Langmuir and Freundlich models for both adsorbents based on the high value of the correlation coefficients obtained. The Freundlich isotherm model is based on the phenomena of heterogeneous surfaces with several adsorption mechanisms involved, where KF and n are the Freundlich constants related to the adsorption 17



Page 18 of 34

capacity and adsorption intensity, respectively which is partly true for the adsorption on these adsorbents. SNTGO had a lower KF value (considered as a measure of adsorption capacity) as compared to SNTG; hence, it shows that a lesser amount of SMZ was adsorbed on it and this was could be attributed to the more hydrophobic nature of SNTG. This is consistent with the qm value from the Langmuir isotherm obtained for SNTGO and SNTG. Langmuir isotherm assumes a monolayer coverage of adsorbate over a homogeneous adsorbent surface and all sorption sites are identical with a further assumption of this theory being the uniformity of the energies of adsorption onto the adsorbent. The Langmuir adsorption capacity qm for SNTGO and SNTG was 125 and 248 mg/g, respectively which were favourable as compared to that for other studies (Table 4). Freundlich and Langmuir isotherm models both fitted the experimental data indicates the interaction between SNTGO, SNTG and SMZ took place between H-bonding and π-π interactions. These interactions can be explained based on the chemical structure of the adsorbents and the adsorbate, the presence of aromatic ring on SMZ favoured the increased adsorption of SNTG because of higher hydrophobic interaction and hydrogen bonding from the – OH groups can act as hydrogen bond acceptor with oxygen atom of SMZ molecules.

The favourability for adsorption can be quantified by the separation factor, RL and is defined as:

=

(4)

Where is the equilibrium constant (mg/L) from the Langmuir isotherm expressed in eqn 4, and is the adsorbate concentration (mg/L). The nature of the adsorption process is either unfavourable (RL > 1), linear (RL = 1), favourable (0 < RL < 1) or irreversible (RL = 0). The measured SMZ adsorption at equilibrium yielded values of 0.031 for SNTGO and 0.009 for SNTG, which indicated favourable adsorption on both materials. This is consistent with the N value obtained from the Freundlich isotherm, which gave values greater than 1 indicating favourable adsorption. The Freundlich isotherm N values > 1 are a pointer that the isotherm be considered as non-linear because N values obtained were out of the 0.95-1.05 range.42 They are usually higher for nanocomposites and this is an indication of a strong bond between the adsorbate (SMZ) and the adsorbents. N values less than 10, indicates favourable adsorption as can be seen in Table 3. This explanation is more suited for SNTG with the occurrence of 18


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aromatic-interaction and the positive relationship between the non-linear isotherm and aromatic carbons.43 The D-R model was used to describe adsorption on both homogeneous and heterogeneous surfaces.44 E values obtained from the D-R plots were below 8 kJ/mol, which indicated that the adsorption mechanism was not entirely a chemical process.

Table 3 Isotherm parameters for the adsorption of SMZ onto SNTGO and SNTG Isotherm

Parameter

SNTGO

SNTG

Langmuir

qm/mg/g

125

248

b/ L/mg

0.39

3.40

0.7356

0.7723

Kf (mg/g (mg/L) )

2.17

5.01

N

1.13

1.28

R2

0.9978

0.9929

E/ kJ/mol

2.45

5.00

qD/mg/g

118

215

B/ mol/kJ2

1.0 x 10-10

2.0 x 10-10

R2

0.9148

0.9959

B

21.5

35.3

b/ J/mol

11.2

70.2

A/ L/g

27.6

48.6

R2

0.946

0.928

R2 Freundlich

Dubunin-Radushkevich

Temkin

1/n

19



Page 20 of 34

The effect of temperature/thermodynamic studies are shown (SI 6, Figure S6 and Table S3). Table 4 summarises the SMZ adsorption capacities of different carbon-based nanocomposites. SNTGO and SNTG exhibited excellent adsorption capacities for SMZ. The results obtained compared favourably to other studies. SNTG had a significant adsorption of SMZ.

Table 4 Comparison of qm values to other studies of SMZ on different adsorbents Adsorbents

qm values/ mg/g

References

Mn-Ferrite activated carbon

217

12

Graphene nanosheet

103

45

Graphene

122

Rice biochar

1.83

46

Walnut shell

0.59

47

Activated carbon

185.19

48

CNT

21.4

45

49 49

CNT+SiO2

35.2

49 CNT+Al2O3

63.9 20


Page 21 of 34

SNTGO

125

This study

SNTG

248

This study

Adsorption kinetic studies

The effect of contact time on the adsorption of SMZ by SNTGO and SNTG was studied between 2 and 1440 min. The percentage removal of SMZ by SNTGO and SNTG increased with time (Figure 7). Equilibrium was obtained within 20 mins for both SNTGO and SNTG, and thereafter the percentage adsorbed remained constant. Adsorption was therefore very fast initially when there were available adsorption sites hence, the rate of removal of the pollutant was rapid and gradually decreased with time until equilibrium was reached. As adsorption sites became occupied, there were fewer sites available, and so sorption was considerably reduced. This was due to the unavailability of adsorption sites that were available for bonding. Thus with time, the vacant adsorption sites left became difficult to occupy as a result of repulsive forces between solute molecules in the solid and bulk phases.50 Although, the sorption process was fast for both adsorbents, higher removal efficiency was observed for SNTG because of the higher percentage of carbon ensuring its more hydrophobic nature. 25

20

qt (mg/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


SNTGO SNTG

15

10

5

21

0 0

400 800 1200 ACS Paragon Plus Environment

Time/ mins

1600


Figure 7 Effect of contact time on sorption of SMZ on SNTGO and SNT (Condition: 200 mL of 12.5 ppm SMZ, pH 2, equilibration time 1440 mins, dose 50 mg, temperature 25 °C, ionic strength, 20 mM NaCl) The kinetic parameter data of all the models and the R2 coefficient determined by non-linear regression analysis are presented in Table 5. Based on the high R2 values obtained, it was observed that the experimental data of SNTGO and SNTG best fitted the PSO model. This model is based on the assumption that a bimolecular interaction which involves ion-exchange between an adsorbate and adsorbent is the rate limiting step responsible for the SMZ adsorption.51,52 Based on the above premise, the adsorption of SMZ onto SNTGO and SNTG is thought to be through ionic interactions with the hydroxyl groups present on adsorbents surface. It was also noticed that k2 parameter of SNTG was larger than SNTGO and this could be attributed to the surface flatness and fewer -OH groups on SNTG which accelerated the adsorption process.45 Also, the calculated qe values (qe cal) of the PSO model are closer to the experimental values (qe exp). Boundary layer thickness of the adsorbents is likely to have impacted the adsorption of SMZ on the adsorbents and this is because the SNTG which is thicker and has RGO on its surface had a higher sorption capacity as compared to SNTGO. The Elovich model is dependent on the principle of chemisorption through bond sharing and interaction. This proves that a part of the adsorption mechanism of SMZ onto SNTGO and SNTG was via chemical interaction through the active groups on SMZ and SNTGO and SNTG. The adsorption process on porous solids can be separated into 3 stages: (1) transfer of adsorbate across liquid film to the exterior surface of the adsorbent otherwise referred to as film diffusion, (2) adsorbate transfer from the surface to the pores of the internal structure of the adsorbent otherwise called intra-particle diffusion, and (3) adsorbate is adsorbed onto the active sites on the inner and outer surface of the adsorbent.50, 53 The last step proceeds rapidly and cannot be the rate-determining step. The rate of adsorption is controlled by outer or inner diffusion or both. In a bid to determine the adsorption 22


Page 22 of 34

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mechanism, the experimental data obtained was modelled with the intra-particle diffusion model. A plot of qt versus t0.5 showed that an increase in initial concentration of SMZ increased the diffusion rate. The plot obtained from the IPD showed that it did not pass through the origin and was not linear, signifying that adsorption of SMZ was not solely through the intra-particle diffusion via the pores of the adsorbents and it is not the only rate-controlling step.50 A larger C value means, influence of surface adsorption in the rate limiting step. In the adsorption of SMZ, a mutual adsorption process of both intra-particle diffusion and surface adsorption was observed.

Therefore, SNTGO and SNTG adsorption of SMZ can be said to be a multi-step process.

Table 5 Kinetic parameters for the adsorption of SMZ from aqueous solution Model

Parameter

SNTGO

SNTG

Pseudo-first order

k1 /min-1

0.0160

0.0201

, eq/ mg/g

1.25

4.98

R2

0.9443

0.9218

, exp

mg/g

6.32

22.5

Pseudo-second order

k2 /g mg/g

3.18

4.94

/mg/g

6.42

22.7

0.9999

0.9999

0.14

0.33

C /mg/g

2.43

13.4

R2

0.531

0.453

R2 Intra-particle

Ki /mg/g min

-0.5

diffusion

23



Elovich

Page 24 of 34

A

1.00

2.60

B

0.02

7.01

0.8669

0.8591

R

2

Adsorption mechanism

Pharmaceuticals can be adsorbed onto carbon based adsorbents by the following mechanism; Hbonding, π-π interaction, electrostatic ion interactions, pore filling and the occurrence of one or more of these mechanisms simultaneously.54,55 The surface area of SNTGO and SNTG used in this study was relatively low with small pore sizes and few functional groups on their surface. The pH experimental results suggested adsorption was not much affected by surface chargedependent electrostatic forces. SMZ is an amphoteric molecule and has multiple charged/polar groups (ketones, enones, aminos and phenols) which are capable of electronic coupling. The sulfonamide group on SMZ has strong electron withdrawing abilities, therefore, the aromatic ring/unsaturated structure of the SMZ becomes electron deficient and thus could act as a πelectron-acceptor. At pH values > pzc, the adsorbent is negatively charged which means there are electrons present on the adsorbent that can be donated to the electron deficient SMZ. Therefore, SMZ can strongly interact with the adsorbents surfaces (π-electron-donor) through electron donor acceptance (EDA) and π-π EDA interactions. Adsorption of SMZ was also slightly influenced by electronic repulsion at pH>5.7. At this pH, the adsorbents and adsorbate were negatively charged leading to repulsion between the adsorbent and adsorbate and reduced percentage removal.

Hydrophobic effects also played an important role in the removal mechanism for SMZ from solution by SNTGO and SNTG. Despite SNTGO having a more negatively charged surface (at pH>pzc) and extensively functionalised with different oxygen groups, it showed lower adsorption of SMZ as compared to SNTG. Adsorption was most favoured at pH 2-4 because at this pH range SMZ exists as a neutral species and the adsorbents are negatively charged (pH>pzc). There is therefore no electrostatic attraction however, the hydrophobic surface of the SMZ interacts with the hydrophobic surface of the adsorbents leading to removal by 24


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hydrophobic interaction. Although, SNTG has fewer oxygen functional groups on its surface, it had a higher adsorption capacity compared to SNTGO at the same pH. This is indicative that in addition to π-π effects hydrophobic interactions and other adsorption mechanisms are responsible for the increased adsorption by SNTG. This increased adsorption may be due to the transformation in SNTG morphology.

Adsorption of organic pollutants onto RGO/GO nanomaterials can also be attributed to the following factors; high energy surface sites, defects, groove areas and edges 56 because adsorbed molecules would occupy these sites first with strong affinities before adsorbing onto other sites.57 Studies have indicated that G/RGO conformation might be altered from a smooth, flat surface to a wrinkled surface through the influence of chemical factors, in this case reduction.58 Therefore, the reduction of GO in SNTG led to a wrinkled surface where there were groove areas and surface defects into which the SMZ could fit. These surface defects also lead to non-uniform charge distribution, which have high chemical activity thus encouraging increased adsorption.57, 59, 60

These factors also accounted for the higher adsorption capacity of SNTG compared to

SNTGO. Therefore, there is a direct correlation in the morphology of the adsorbents, adsorption sites and adsorption capacity for SMZ on SNTG. Regardless of SNTGs lower surface area and similar surface functional groups to SNTGO, it had a better adsorption capacity for SMZ, which may be attributed to the presence of the aromatic ring encouraging its hydrophobic adsorption on SNTG. A study by Peng et al

54

observed that

more aromatic rings on a pharmaceutical molecule will result in a faster rate of adsorption by a more hydrophobic carbon based adsorbent. The SNTG also had a higher percentage of carbon (Table 2) and fewer oxygen atoms, which further contributed to the higher sorption capacity of SNTG for SMZ. This is also consistent with the reaction rate parameter (K1) in Table 5, where the adsorption rate was faster with SNTG as compared to SNTGO. Furthermore, SNTG had a higher pore volume and pore diameter than SNTGO, which will allow for more pore filling by SMZ leading to an increase in adsorption capacity of SNTG.

However, the dominant adsorption mechanism for SNTGO was hydrophobic effects, and SNTG adsorption was by hydrophobic effects and π-π interactions. Additionally, Freundlich values (N) 25



which indicates heterogeneity of the surface is higher in SNTG (1.28) as compared to SNTGO (1.13). Thus, the many different mechanisms discussed implied that the overall adsorption mechanism in this system was of a heterogeneous nature.

Environmental applications

Remediation of pharmaceuticals from wastewater could be hindered by the presence of competing/interfering pollutants thus making it critical to determine the efficiency of the adsorbents in real-life samples. Thus, both adsorbents were applied to real water samples obtained from a wastewater treatment plant (WWTP) and a second site close to the mouth of the river (BLG). Initially, the concentration of SMZ was determined to be below the detection limit of the instrument, in both samples, and for this reason the real samples were spiked with 12.5 mg/L SMZ and adsorption carried out thereafter.

The percentage removal of SMZ from BLG and WWTP samples by SNTGO was 22.5 and 26.8%, respectively. However, for SNTG it was 79.3% and 76.5% for BLG and WWTP, respectively. The less than modest removal rates of SMZ on SNTGO and SNTG may be attributed to competition for the differing active sites by other pharmaceuticals/cations present in the solution.61

The adsorption was particularly good as the pH of both samples were slightly acidic, hence it aided adsorption, which corresponded to our optimisation studies. Furthermore, the newly developed adsorbents have better separation from the aqueous solution compared to graphene and AC (SI 7 and Figure S7). This confirms that SNTGO and SNTG can successfully be used to remove SMZ from environmental samples with a high level of efficiency and substantiates the applicability of SNTGO and SNTG in the remediation of environmental samples with special reference to the pharmaceutical pollutant sulfamethoxazole.

Desorption efficiency (SI 8, Figure S8 and Figure S9) of 63-70.3% and 70.2-75.8% after four cycles for SNTGO and SNTG, respectively. 26


Page 26 of 34

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Acknowledgments

We gratefully acknowledge the School of Chemistry and Physics, University of KwaZulu-Natal, South Africa, for providing the laboratory facilities and instrumentation used in this research and also AAU small grant for funding (PC/6).

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For Table of Contents Use Only

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Synopsis: Silica nanotubes, synthesised from biowaste, was encapsulated with graphene oxide/reduced graphene oxide and used to remove sulfmethoxazole from wastewater

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Encapsulation of Silica Nanotubes from Elephant ... - ACS Publications

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