Appendix A. Supplementary information for ACS Sustainable Chemistry & Engineering

Size: px

Start display at page:

Download "Appendix A. Supplementary information for ACS Sustainable Chemistry & Engineering"

Wilfred Hoover
5 years ago
Views:

1 Appendix A. Supplementary information for 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. Akpotu and Brenda Moodley* School of Chemistry and Physics, University of KwaZulu-Natal, Westville Campus, Durban, 4000, South Africa. * corresponding author moodleyb3@ukzn.ac.za, First author samsonakpotu@yahoo.com Telephone: Fax: Supplementary information consists of 14 pages, including this page There are 9 Figures and 3 Tables and 8 additional SI discussion of results. Contents: List of Figures Figure S1: Structure of sulfamethoxazole Figure S2: Acid-base titrations of SNTGO and SNTG for the determination of PZC Figure S3: Effect of dose on sorption of SMZ onto SNTGO and SNTG Figure S4: Effect of ionic strength Figure S5: Experimental isotherm plots of (a) SNTGO adsorption of SMZ (b) SNTG adsorption of SMZ Figure S6: Effect of temperature on the adsorption of SMZ by SNTGO and SNTG Figure S7: Photographs of sorbents (a) agitated for a few minutes (b) after centrifugation Figure S8: Desorption efficiency of (a) SNTGO (b) SNTG Figure S9: FTIR spectra of SNTGO and SNTG after desorption S1

2 List of Tables Table S1: Kinetic models Table S2: Adsorption isotherm models Table S3 Thermodynamic parameters for the adsorption of SMZ onto SNTGO and SNTG Experimental and Results SI 1: Synthesis of GO SI 2: Synthesis of SNT-NH 2 SI 3: Surface charge properties SI 4: Effect of adsorbent dose SI 5: Effect of ionic strength SI 6: Effect of temperature SI 7: Consistency of adsorption of SNTGO and SNTG when compared with other sorbents SI 8: Desorption studies O O N O S N H H 2 N Figure S1 Structure of sulfamethoxazole SI 1: Synthesis of GO GO was synthesized using a modified Tour method. Briefly, a 9:1 mixture of concentrated H 2 SO 4 :H 3 PO 4 (360 ml: 40 ml) was added to a mixture of 3 g of graphite powder and 18 g of KMnO 4 and manually stirred in a beaker. It was then mechanically stirred on a hotplate at a temperature of 50 C for 12 h, 400 ml of ice was then added into the mixture and 4 ml H 2 O 2 S2

3 was added to stop the reaction. The product was filtered to remove the unoxidised graphite powder, the filtrate was centrifuged and the supernatant decanted. The remaining solid was washed with water, HCl and ethanol (twice) and filtered through a 0.45 µm filter. The solid product was transferred to a dialysis tube and kept there for a month to discharge acid, with the water in the dialysis tube holder constantly changed. The brown solid graphene oxide was finally dried under vacuum. SI 2: Synthesis of SNT-NH 2 The surface of the SNT obtained was modified by an aqueous easy one-step synthesis. Approximately 1 g of SNT was dispersed in 30 ml of ethanol, which was sonicated for an hour and stirred for another hour. Approximately 1000 µl of APTES was added to bind NH 2 to SNT. The reaction was stirred at 50 C for 8 hrs to obtain a monolayer and a positively charged SNT. The product was collected by filtration and vacuum dried at 60 C. SI 3: Surface charge properties The acid-base surface chemistry of the SNTGO and SNTG were determined using potentiometric titration. 1 The experiments were carried out in a capped 100 ml Teflon vessel which was enclosed with a glass jacket to keep temperature constant at C. Stirring was done with a Teflon bar and titration was carried out using a computer PC-titration system (G10S Automatic Titrator Mettler Toledo) with ph electrode. Care was taken to exclude CO 2 by bubbling argon successively through NaOH, HClO 4 and distilled water. Initially, about 2.0 g/l of SNTGO and SNTG were added to the Teflon vessel and purged for 2 h with argon and ph was lowered to about 3.5 by adding 1.51 M HNO 3. It was equilibrated for an hour and the suspension back titrated with small volumes of M NaOH to ph 11. The titrations were carried out with 0.01 M NaNO 3 background electrolytes. The values of the Gran function (G) were calculated as: On the acidic side: G a = (V 0 + V at + V b ) X 10 -ph X 100 On the alkaline side: G b = (V 0 + V at + V b ) X 10 - ( 13.8-pH ) X 100 (S1) (S2) S3

4 V b is the total volume of OH added and V at is the added volume of acid solution. The hydroxide ions added to the adsorbents were consumed by the steps in the Gran plots: neutralization of excess H + in the suspension (before V eb1 ), reactions with the adsorbents (between V eb1 and V eb2 ) and the system ph adjusted (after V eb2 ). The specific volumes V eb1 and V eb2 gotten from the results of linear regression analysis of the Gran plots which corresponds to the equivalent points. Table S1 Kinetic models Models Equation Reference Pseudo first-order =ln 2 Pseudo second-order = Intraparticle diffusion =. + 3 Elovich = + 4 Where and are the amount of SMZ adsorbed (mg/g) at equilibrium and time t (mins), respectively; is the rate constant of the pseudo first-order kinetic model (min -1 ). is the rate constant (g/mg/min) of pseudo second-order kinetic model for adsorption. For intraparticle diffusion model, C (mg/g) is the intercept and is the intraparticle diffusion rate constant (g/mg min/ 0.5 ) for adsorption. The Elovich coefficient is obtained from the plot of versus. The initial adsorption rate, a, and the desorption constant, b, were calculated from the intercept and slope, x is the initial adsorption rate (mg/g min). For thermodynamic studies, 0.05 g of the adsorbent was placed in the SMZ solution and adjusted to ph 2. The mixture was agitated in a thermostatic shaker bath at varying temperatures of 293, 303 and 313 K for 24 hrs. The solutions were centrifuged at 4000 rpm for 1 min and filtered with a 0.45 µm filter disc before the concentration of the filtrate was determined using HPLC. The S4

5 equilibrium data obtained were analysed with the following isotherm models; Langmuir, Freundlich, Temkin and Dubinin-Radushkevich models. The equations for these models are presented in Table S2. Thermodynamic parameters such as change in enthalpy ( ), change in Gibbs energy ( ), and change in entropy ( ), were also calculated. Experiments were carried out in duplicate and blanks were also carried out to ensure the integrity of the analytical procedure. Table S2 Adsorption isotherm models Model Equation Reference Langmuir = Freundlich ln = + 1 ln 3 Temkin = ln + 3 D-R ln = 5 In the D-R model, is a constant related to the mean free energy (mol 2 /kj 2 ), is the theoretical capacity and is the Polanyi potential defined as: = 1+ (S3) Where R is the gas constant in J/mol/K) and T is temperature K. The B value is the mean energy of adsorption. E is calculated using the equation: = (S4) E is the mean free energy E (kj/mol/l) of the adsorption per molecule of the adsorbate, is the equilibrium concentration of SMZ in solution (mg/l), b is the Langmuir constant related to the S5

6 free energy of adsorption (L/mg), is the theoretical limit of adsorption capacity when the monolayer surface is fully covered with molecules (mg/g), and n are Freundlich constants, in the Temkin isotherm, B = RT/b where b is Temkin constant in J/mol, A is Temkin isotherm constant (L/g). Results and discussion phpzc determination phpzc = phpzc = 4.02 TOTH TOTH SNTGO SNTG ph ph Figure S2 Acid-base titrations of SNTGO and SNTG for the determination of PZC SI 4: Effect of adsorbent dose The effect of SNTGO and SNTG dose on the adsorption of SMZ was studied. Adsorbent dose is important because the adsorbent surface provides the active sites for SMZ to adsorb on. An increase in adsorption will therefore result when there are more active sites present. Adsorbent masses of 0.01, 0.02, 0.03, 0.04, 0.05 and 0.06 g of SNTG gave a percentage uptake of 21.0, 35.0, 45.4, 48.3, 59.6 and 60.1%, respectively, while for SNTGO, the percentage uptake was 10.0, 10.1, 13.3, 17.4, 19.8 and 19.9% adsorption, respectively (Figure S3) for the same mass of SNTGO. It can be seen that the percentage uptake of SNTGO and SNTG increased linearly up to a mass of 0.05 g of adsorbent. This was due to an increase in adsorption sites due to the increase in adsorbent mass, thereby creating more available interaction for the SMZ molecules in solution. Any further increase in adsorbent dose resulted in the percentage adsorption capacity remaining constant for both adsorbents. Adsorption percentage of SMZ molecules was S6

7 unaffected by an increase in mass of SNTGO and SNTG after 0.05 g. Experiments thereafter, were carried out using a mass of 0.05 g for both adsorbents. Percentage adsorption/ % SNTG SNTGO Dose/ g Figure S3 Effect of dose on sorption of SMZ onto SNTGO and SNTG (Conditions: 15 ml of 12.5 ppm SMZ, ph 2, equilibration time 180 mins, adsorbent dose mg, temperature 25 C, ionic strength 20 mm NaCl) SI 5: Effect of ionic strength Wastewater effluent containing pharmaceutical contaminants often have high conductivity which may affect the behaviour of pharmaceuticals during remediation. This makes it necessary to study the effect of ionic strength on the adsorption behaviour of SMZ on SNTGO and SNTG because an increase in ionic strength may lead to a decrease in adsorption. The ionic strength was studied by adjusting 15 ml of 12.5 ppm SMZ solutions (ph 2) with 10, 20, 40 and 80 mm NaCl solution, and the result is presented (Figure S4). At 20 mm NaCl, there was a slight increase in percentage adsorption of SMZ with SNTG from 86.5% % and for SNTGO, from 29% %. At ph above 2 of this study, SMZ is neutral and the adsorbent has a negative charge. Therefore, any adsorption that occurs here is due to hydrophobic interaction via the π- electrons. When NaCl is added in solution, it dissociates into Na + and Cl -. The Na + is attracted to the π-electrons of the adsorbent pulling the electrons on one side of the π-bond towards it, leaving a high electron density on the other side. This result in increased π-interaction between the adsorbent and more adsorbate resulting in the increase in adsorption observed. However, S7

8 when more NaCl is added, Na + is attracted to more π-electons and hence the electron density is more evenly distributed resulting in a slight decrease in π-interaction between the adsorbate and adsorbent, leading to the slight decrease in adsorption. Any further increase in NaCl concentration, the Na + has no more π-electrons to interact with and thus there is no change in adsorption on π-interactions in the presence of cations. 100 SNTG Percentage adsorption/% SNTGO [NaCl]/mM Figure S4 Effect of ionic strength on sorption of SMZ onto SNTGO and SNTG (Conditions: 15 ml of 12.5 ppm SMZ, ph 2, equilibration time 180 mins, adsorbent dose 50 mg, temperature 25 ). 30 (a) 90 (b) Q (mg/g) 10 Q (mg/g) C e (mg/l) C e (mg/l) Figure S5 Experimental isotherm plot of (a) SNTGO adsorption of SMZ (b) SNTG adsorption of SMZ SI 6: Effect of temperature S8

9 An increase in solution temperature causes an increase in the volume of internal pores of the adsorbents, an increase in rate of diffusion across the internal and external boundary layers and a decrease in the viscosity of the adsorbate solution. 6 7 The temperature was studied between K. Figure S6 shows that the adsorption capacity decreased slightly as the temperature of the SMZ solution was increased for both adsorbents. At high temperature, the molecules become more energetic and move towards the pores of the adsorbent. However, when too many adsorbate molecules move towards the pores, the pores of the adsorbent tend to clog up, thus reducing the ability of the adsorbent to adsorb the adsorbate. Thus, the pore volume decreased and the equilibrium capacity of the adsorbent decreased with an increase in temperature. The decrease in adsorption may further be reduced due to a decrease in forces of collision responsible for adsorption. This effect of temperature on the adsorbents is indicative that firstly, at a point source the adsorbent can be used without a manipulation in temperature and secondly, the adsorbents can be used even at higher temperatures. The results revealed that the adsorption of SMZ decreased with increasing temperature. Thermodynamic parameters such as enthalpy change H and entropy change S were calculated from the linear plot of ln K d vs 1/T using equation S5 below and Gibbs free energy G was calculated from equation S6. Thermodynamic properties of SMZ adsorption on SNTGO and SNTG are presented in Table S q e / mg/g Temperature/ K SNTGO SNTG S9

10 Figure S6 Effect of temperature on the adsorption of SMZ by SNTGO and SNTG (Conditions: 15 ml of 12.5 ppm SMZ, equilibration time 1440 mins, adsorbent dose 50 mg). Table S3 Thermodynamic parameters for the adsorption of SMZ onto SNTGO and SNTG Adsorbent Temperature/ K G / kj/mol H / kj/mol S/ kj/mol SNTG SNTGO The change in Gibbs energy is calculated from the following equation: = ln (S5) Where is standard energy change (J/mol), R is the gas constant (8.314 J/mol/K), T is absolute temperature, and K d (L/g) is the equilibrium constant. A plot of ln K versus 1/T had a linear profile, and the values of S and H were obtained from the intercept and slope, respectively, using the Van t Hoff equation: ln = + (S6) The values obtained for both adsorbents were high and negative, which meant that the adsorption process was favourable and spontaneous. A decrease in the negative value of was observed as the temperature of the adsorbents was increased. A negative value was obtained for both adsorbents which indicated an exothermic adsorption process which corresponds to the decrease in adsorption at higher temperature. This also affirms that the interaction was weak, S10

11 being more physical than chemical. 8 The value obtained indicated increased disorderliness between the adsorbent and adsorbate during the adsorption process. The results obtained from the thermodynamics studies indicated that both adsorbents will be useful in the treatment of effluent from pharmaceutical industries. SI 7: Consistency of adsorption of SNTGO and SNTG when compared with other sorbents About 50 mg of SNT, GO, SNTGO, SNTG, reduced graphene and activated carbon (AC) were tested for their ability to separate properly from SMZ in solution after agitation with a concentration of 12.5 mg/l. The tests were performed at optimum conditions. The adsorption capacity was in this order SNTG (87.5%) > reduced graphene (60.2%) > SNTGO (28.5%) > activated carbon (26.1%) > (25.5 %) GO and the least being SNT (< 1%). However, worthy of note were the interesting separation profiles obtained. Figure S7 A and S7 B shows the separation profiles of the adsorbents before and after centrifugation at 4000 rpm for 5 minutes, respectively. The solutions containing the adsorbents were agitated mechanically for 5 minutes and pictures taken before centrifugation was carried out. From Figure S8 (pictures) it was observed that SNT remained cloudy before and after centrifugation and with its hydrophilic nature, it was no surprise it had the lowest adsorption. AC barely settled a little after centrifugation with particles clearly visible, on the other GO had a better separation profile when compared to AC, however, due to its highly dispersive nature the solution retained the brown GO colour. After centrifugation, reduced graphene gave a relative modest separation, however, small aggregates of reduced graphene were visible throughout the entire solution. In contrast to all the other adsorbents, SNTGO and SNTG after centrifugation resulted in a very clear solution with the adsorbent aggregated nicely at the bottom of the solution. The implication of the separation profile in practical water treatment is that based on issues such as aggregation and cloudiness, only SNTGO and SNTG can be used as appropriate adsorbents in the treatment of SMZ in water. S11

12 Figure S7 Photographs of sorbents (a) agitated for a few minutes (b) after centrifugation at 4000 rpm for 5 minutes: 1 (SNT), 2 (GO), 3 (SNTGO), 4 (SNTG), 5 (reduced graphene), 6 (AC) SI 8: Desorption studies Recycling and regeneration of adsorbents is important in the practical application of adsorbents. An ideal adsorbent is one with high adsorption capacity and excellent desorption ability. This would reduce the cost of the adsorbent and also prevent the introduction of secondary pollutants into the environment. After successive adsorption-desorption cycles, there was a slight decrease in adsorption capacity. Results showed good desorption efficiencies % for SNTGO after 4 cycles and % for SNTG, after 4 cycles (Figure S8). However, the FTIR spectra showed no obvious peak changes after 4 cycles of desorption (Figure S9) and surface area and pore volume were reduced when compared to the originally synthesized adsorbents. SNTGO and SNTG had a surface area of 125 m 2 /g and 82 m 2 /g and pore volumes of 0.41 and 0.43 cm 3 /g, respectively. The decrease in surface area and pore volume is because the SMZ molecule filled the pores and adhered to the surface of the adsorbents. The results are indicative that SNTGO and SNTG are reusable and stable. Desorption efficiency (%) (a) (b) 70 Desorption efficiency Desorption efficiency (%) 80 Desorption efficiency Cycle numbers Cycle numbers Figure S8 Desorption efficiency of (a) SNTGO (b) SNTG S12

13 SNTG Transmittance/a.u SNTGO Wavenumber/cm -1 Figure S9 FTIR spectra of SNTGO and SNTG after desorption References 1. Li, J.; Zhang, S.; Chen, C.; Zhao, G.; Yang, X.; Li, J.; Wang, X., Removal of Cu (II) and fulvic acid by graphene oxide nanosheets decorated with Fe3O4 nanoparticles. ACS Appl. Mater. Interfaces 2012, 4 (9), Ho, Y.-S.; McKay, G., Sorption of dye from aqueous solution by peat. Chem. Eng. J. 1998, 70 (2), Kushwaha, A. K.; Gupta, N.; Chattopadhyaya, M., Enhanced adsorption of methylene blue on modified silica gel: equilibrium, kinetic, and thermodynamic studies. Desalin. Water Treat. 2014, 52 (22-24), Han, R.; Zhang, L.; Song, C.; Zhang, M.; Zhu, H.; Zhang, L., Characterization of modified wheat straw, kinetic and equilibrium study about copper ion and methylene blue adsorption in batch mode. Carbohydr. Polym. 2010, 79 (4), Shao, Y.; Wang, X.; Kang, Y.; Shu, Y.; Sun, Q.; Li, L., Application of Mn/MCM-41 as an adsorbent to remove methyl blue from aqueous solution. J. Colloid Interface Sci. 2014, 429, Akpotu, S. O.; Moodley, B., Synthesis and characterization of citric acid grafted MCM- 41 and its adsorption of cationic dyes. J. Environ. Chem. Eng. 2016, 4 (4), S13

14 7. Khan, T. A.; Dahiya, S.; Ali, I., Use of kaolinite as adsorbent: Equilibrium, dynamics and thermodynamic studies on the adsorption of Rhodamine B from aqueous solution. Appl. Clay Sci. 2012, 69, Al-Khateeb, L. A.; Almotiry, S.; Salam, M. A., Adsorption of pharmaceutical pollutants onto graphene nanoplatelets. Chem. Eng. J. 2014, 248, S14

Efficient removal of heavy metal ions with EDTA. functionalized chitosan/polyacrylamide double network

Supporting Information Efficient removal of heavy metal ions with EDTA functionalized chitosan/polyacrylamide double network hydrogel Jianhong Ma a,b, Guiyin Zhou c, Lin Chu c, Yutang Liu a,b, *, Chengbin