Adsorptive removal of Cd2+ from aqueous solutions

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second-order model showed good fitting to experimental data, whereas Langmuir and D–R ... possess uniform distribution of negative charge along the framework, making it an ideal candidate for .... CTF samples were dried over night at 100 1C and reused for ... MPD X-ray power diffractometer fitted with Cu Kα radiation.

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Adsorptive removal of Cd2+ from aqueous solutions by a highly stable covalent triazine-based framework Zahid Ali Ghazi,†a Abdul Muqsit Khattak,†b Rashid Iqbal,a Rashid Ahmad,c Adnan Ali Khan,c Muhammad Usman,d Faheem Nawaz,e Wajid Ali,a Zahra Felegari,f Saad Ullah Jan, a Azhar Iqbal*g and Aziz Ahmad *a Porous crystalline materials such as covalent organic frameworks (COFs) have gained tremendous popularity in multidisciplinary areas of science and technology. In this study, for the first time, we report a covalent triazine-based framework (CTF-1) as an efficient adsorbent for the removal of Cd2+ from aqueous solutions. CTF-1 offered excellent stability under extreme conditions for the effective removal of cadmium cations (Cd2+) from aqueous solutions. CTF-1 was first synthesized through an ionothermal method and then characterized by XRD, SEM, TEM and BET surface area measurements to confirm its highly crystalline and microporous nature. Batch adsorption experiments were systematically conducted under a wide range of pH, metal ions concentration, adsorbent dosage and contact time to investigate kinetics, thermodynamics and adsorption properties of CTF-1 towards Cd2+ ions removal. The pseudosecond-order model showed good fitting to experimental data, whereas Langmuir and D–R isotherms confirmed the chemical nature of the adsorption. Similarly, thermodynamic parameters indicated the

Received 12th April 2018, Accepted 4th May 2018

adsorption to be spontaneous and endothermic. Furthermore, our simulation results showed that CTF-1

DOI: 10.1039/c8nj01778f

the adsorption of cations together with the high stability in both acidic and basic pH. The strategies

possess uniform distribution of negative charge along the framework, making it an ideal candidate for adopted in this study will open a new avenue to design novel nanoporous functional materials for next

rsc.li/njc

generation adsorbents.

1. Introduction Water contamination by heavy metals, such as cadmium (Cd), mercury (Hg), chromium (Cr) and lead (Pb), has become a serious problem due to their high toxicity and non-biodegradable nature.1–3 Being among of the toxic heavy metals, Cd poses a

National Center for Nanoscience and Technology, University of Chinese Academy of Sciences, Beiyitiao No. 11, Zhongguancun, Beijing 100190, China. E-mail: [email protected] b Institute of Chemical Sciences, Gomal University, Dera Ismail Khan, 29050, Pakistan c Department of Chemistry, University of Malakand, Chakdara, Dir, Pakistan d Center of Excellent in Nanotechnology, King Fahad University of Petroleum and Minerals, Dhahran, 3126, Saudi Arabia e Balochistan University of Information Technology, Engineering and Management Sciences, Airport road Baleli, Quetta, Pakistan f Department of Chemistry, Science and Research Branch, Islamic Azad University, Tehran, Iran g Institute of Engineering Research, Hefei Guoxuan High-tech Power Energy Co., Ltd, No. 599, Daihe Road, Xinzhan District, Hefei 230011, P. R. China. E-mail: [email protected] † These authors contributed equally to this study.

severe health risks even at trace levels and can cause a number of acute and chronic disorders, such as emphysema, kidney failure, itai–itai disease, hypertension and testicular atrophy.2,4–6 Cd is widely used in various industries, such as those of protective coatings, batteries, PVC stabilizers, and fertilizers, and mining, and also used as an anticorrosive agent in alloys.3 Due to improper disposal, Cd finally makes its way to various domestic water bodies through different natural and human activities, thus causing water pollution. Since Cd is accumulative and non-degradable, its removal from drinking and domestic water sources is extremely important. Various strategies such as chemical precipitation,7 membrane separation,8 electrochemical techniques,7 ion exchange,9 evaporation and reverse osmosis10 have been exclusively used to remove heavy metals from wastewater. However, most of these methods are not suitable for commercial scale applications due to their high cost and/or complexity in synthetic and operational procedures. To this end, adsorption is recognized as an effective and economic technique for waste water treatment because of its low cost and eco-friendly nature.3,11 In the last several decades, tremendous research progress has been made to search novel and potential adsorbents.

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For example, various synthetic organic adsorbents, inorganic adsorbents, biosorbents and microbial adsorbents have been widely employed for the removal of heavy metals from wastewater.7 Modified activated carbon (AC) and other carbonaceous materials, such as biochar (BC) have been recognized as promising candidates for heavy metals removal due to their high surface area and abundant surface functional groups.12–16 However, these adsorbents lose their activity under adverse temperatures and pH due to the absence of inherited functionalities, thus making them unfavorable for practical applications.14,17 Covalent organic frameworks (COFs), a class of porous crystalline covalent organic polymers, have emerged as a new molecular platform for designing promising organic materials and have been exclusively used for numerous applications, such as gas storage,18,19 catalysis,20,21 optoelectronics22–24 and energy storage.25,26 The ordered nanoporosity, high surface area, controlled functionality, high physiochemical stability and the presence of lightweight elements could make COFs potential adsorbents for the uptake of heavy metals from wastewater. However, no considerable attention has been given to COFs for applications in adsorption. In this study, for the first time, we report the use of highly crystalline nanoporous covalent triazine-based framework (CTF-1) for the adsorption of Cd2+ from wastewater. As proof-of-concept, the continuous negative charge distribution along the COF framework and highly ordered nanoporous structure, originated from the stacking of the 2D planar sheets in one direction, could provide a large number of accessible active sites for the removal of Cd2+ ions together with the its high stability.17,25 We believe that the strategy demonstrated in this study will lead to the foundation for developing new adsorbents beyond conventional inorganic and amorphous organic polymer materials.

2. Materials and methodology 2.1

Materials

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salt in distilled water, from which working solutions of various concentrations (5–262 ppm) were prepared. Then, 0.1 g of the CTF-1 adsorbent was added to 40 mL of Cd2+ solution of desired concentration in pH range of 2–9 in 100 mL conical flasks. The samples were agitated at four different temperatures (298–318 K) on a mechanical shaker. Finally, the suspensions were filtered and Cd2+ concentration was determined using flame atomic absorption spectrophotometer (Perkin Elmer). The amount of Cd2+ adsorbed was determined with the help of following equation: qe ¼

VL ðC0  Ce Þ 1000m

(1)

where qe (mg g1) represents the maximum adsorption capacity, and Ce and C0 (mg L1) stand for equilibrium and initial Cd2+ concentrations, respectively. VL is the volume of the metal ions solution and m is the mass of the adsorbent. Similarly, adsorption kinetic experiments were conducted at 293 K using 1000 mL Cd2+ ions bulk solution at a concentration of 50 ppm. Samples at different time intervals were collected and analyzed to determine the adsorbed amount. 2.4

Desorption reproducibility studies

The adsorption–desorption experiments were performed to investigate the reversibility of the adsorption of Cd2+ on CTF-1. To investigate the reversibility of Cd-adsorbed CTF-1, desorption experiments were performed using HCl and distilled water. The native adsorbent was regenerated by shaking dried Cd-adsorbed CFT (pH 7 and 298 K) for 10 hours in 100 mL of 0.1 M HCl and H2O.28 This process was repeated seven times and after each cycle, the sample was filtered and analyzed via atomic absorption spectroscopy to determine the concentration of Cd2+ ions using eqn (1). Finally, the %desorption was calculated with the help of the following formula:   qe;des %desorption ¼  100 (2) qe;ads

1,4-Dicyanobenzene (99.0%), zinc chloride, sodium hydroxide, cadmium chloride and hydrochloric acid were purchased from TCI company and were used without further purification. Ethanol and acetone were provided by Beijing Chemicals Company.

where qe,ads and qe,des represent the amount of Cd2+ ions adsorbed (mg g1) and desorbed, respectively. The regenerated CTF samples were dried over night at 100 1C and reused for Cd2+ ions adsorption at 298 K and pH 7.

2.2

2.5

Synthesis of CTF-1

CTF-1 was synthesized according to the previously reported method by cyclotrimerization of 1,4-dicyanobenzene in molten ZnCl2 at 400 1C.27 Typically, 1.0 g ZnCl2 and 1.06 g dicyanobenzene were mixed in a 20 mL pyrex tube under Ar atmosphere, and the tube was vacuum sealed and heated to 400 1C at 5 1C min1 and maintained for 40 hours. Finally, the tube was carefully opened and the black product was ground and washed with excess water and acetone several times, followed by stirring in diluted HCl and NaOH for 15 hours, separately. The black powder of CTF-1 was then filtered, washed and vacuum dried for further use.

Surface morphologies were studied using a Hitachi S-4800 scanning electron microscope and FEI Tecnai G2 F20 U-TWIN transmission electron microscope, equipped with energydispersive X-ray analysis (EDAX). The X-ray powder diffraction (XRD) measurements were performed on Panaltical X’Pert-pro MPD X-ray power diffractometer fitted with Cu Ka radiation source (l = 1.54056 Å). The BET surface area and pore size distribution analysis were performed using a Micromeritics ASAP-2420 surface area analyzer (USA). 2.6

2.3

Batch-mode adsorption experiments

Stock solution of Cd2+ (having a concentration of 1000 ppm) was prepared by dissolving an optimum amount of cadmium

Characterization

Simulations

The carbon–nitrogen framework (CNF) geometry was built in Avagadro version 1.1.129 and then, the input file was generated in WxMacMolPlt v 7.6.2.30 The geometry optimization of monomer

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CNF and complex CNF–Cd was performed in GAMESS US code31 using the B3LYP-D332 method implemented in GAMESS US and employing the LANLDZ2-ECP external basis set.33 The ECP function was only employed on the Cd atom. Then, binding energy (Ebinding) was computed using the following equation:

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Ebinding = ECNF–CD  (ECNF + ECd)

(3)

where ECNF–CD, ECNF and ECd are the energies of complex and monomers, respectively. Natural bond orbital (NBO) charge distribution was determined at B3LYP/6-31G(d,p) levels of theory.25

3 Results and discussion 3.1

Structural characterization

Transmission electron microscope (TEM) and scanning electron microscope (SEM) images (Fig. 1b–d) clearly demonstrate the microporous nature of the as-prepared CTF-1, which was further

confirmed by nitrogen adsorption/desorption isotherms (Fig. 1e), with a high BET surface area of 490 m2 g1 and an average pore size of B1.3 nm (inset of Fig. 1e). X-ray powder diffraction (XRD) patterns of the CTF-1 revealed high crystallinity of CTF-1 (Fig. 1f), while the thermogravimetric (TGA) analysis showed that CTF-1 is highly stable up to 600 1C (Fig. 1g). The weight loss at around 150 1C is attributed to the removal of solvent/water molecules from the micropores.25,27,34 3.2

Structural stability of CTF-1

The stability of any adsorbent under a wide range of pH is very important for its sustainability and efficient adsorption. Generally, the adsorbent loses its activity due to degradation of the surface at extreme pH levels. In our present study, the chemical stability of CTF-1 was investigated at pH 2, 7 and 9, and the samples were characterized with SEM. Impressively, no surface corrosion was observed, suggesting high chemical stability of CTF-1 in both acidic and basic pH media (Fig. 2a–c).

Fig. 1 Schematic illustration of the CTF-1 and Cd2+ adsorption (a), characterization of CTF-1: TEM (b and c) SEM (d), BET surface area and pore size distribution (e), XRD patterns (f) and TGA (g).

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the adsorption capacity of CTF-1 depletes and saturation of the adsorbent surface occurs, suggesting monolayer coverage of metal ions on the surface.35 It is worth mentioning that the establishment of such a fast equilibrium can be attributed to the facile diffusion of Cd2+ into highly ordered micropores of CTF-1 and uniform distribution of negative charge. 3.3.2 Adsorption kinetic modeling. The kinetics of Cd2+ ions adsorption by CTF-1 was investigated by Lagergren pseudofirst-order and pseudo-second order models, separately, as given by eqn (4) and eqn (5):28

Fig. 2 Chemical stability of CTF-1 at different pH values: (a) pH 2, (b) pH 7, (c) pH 9 and (d) EDX images of CTF-1 after Cd2+ adsorption.

The energy-dispersive X-ray (EDX) analysis confirmed the successful adsorption of Cd2+ onto CTF-1 (Fig. 2d). 3.3

Adsorption studies

3.3.1 Effect of contact time. The effect of contact time on Cd2+ adsorption was first studied at 293 K and pH 7 for 500 minutes (Fig. 3a). As clearly seen in the figure, the adsorption significantly increases during the first several minutes until it reaches a steady state after 50 minutes, which corresponds to the establishment of equilibrium. After equilibrium,

Fig. 3

ln(qe  qt) = ln qe  kt

(4)

t 1 t ¼ þ qt k2 qe2 qe

(5)

where qt and qe (mg g1) represent the adsorption at time ‘‘t’’ and maximum adsorption capacity at equilibrium, respectively. The constants k1 (min1) and k2 (g mg1 min1) are rate constants for the pseudo first-order and pseudo second-order models, respectively. The kinetic data were successively subjected to eqn (4) (Fig. 3b) and eqn (5) (Fig. 3c). The quantitative analysis discloses that the R2 value (0.999) for the pseudo second-order model is higher than that for the pseudo-first order model (0.72) (Table 1). Moreover, Table 1 shows that the maximum adsorption capacity, theoretically calculated (qe,cal) from the pseudo-first order model, does not match with the experimentally observed maximum adsorption capacity (qe,exp). Whereas, the experimental and theoretical values derived from the pseudo-second order model are in close agreement. This further confirmed the adsorption of Cd2+ onto CTF-1 favoring pseudosecond order as well as its chemical nature.36,37

Kinetics and adsorption studies of CTF-1: effect of contact time (a), pseudo-first order model (b), pseudo-second order model (c) and pH (d).

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Paper Kinetic parameters of Cd2+ adsorption onto CTF-1 at 298 K and

Parameters Kinetic model

Rate constant (k1 and k2)

qe,cal qe,exp (mg g1) (mg g1) R2

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Pseudo-first order 0.01 (min1) 2.94 Pseudo-second order 0.015 (g mg1 min1) 12.50

11.32 11.32

0.724 0.999

3.3.3 Effect of pH. pH is another important parameter that deeply influences the adsorption process by altering solubility, surface charges, precipitation, and hydrolysis reactions.38 The effect of pH on the Cd2+ uptake by CTF-1 was investigated in the pH range from 2–9. The adsorption experiments were conducted using a fixed initial metal ions concentration of 50 ppm at 298 K (Fig. 3d). It can be seen that the uptake of Cd2+ ions increases with increase in solution pH. In fact, at lower pH, the concentrations of H+/H3O+ ions are high, which compete with Cd2+ ions for negative sites on the CTF-1 surface, resulting in lowered adsorption capacity. However, at pH 4 7 the surface becomes highly negative, resulting in enhanced sorption together with the precipitation process.37,39 3.3.4 Effect of temperature. The effect of temperature on the adsorption of Cd2+ onto CTF-1 was studied to calculate various thermodynamic parameters, such as Gibb’s free energy (DG), enthalpy (DH) and entropy (DS) of adsorption. The effect of temperature on the adsorption of Cd2+ by CTF-1 was investigated at 298, 303, 313 and 318 K (Fig. 4a). As described in Fig. 4a, the adsorption significantly increases with an increase

in temperature, suggesting its endothermic nature. By increasing the temperature, the mobility of the metal ions across the external boundary layer of the adsorbent increases, resulting in facile diffusion of Cd2+ ions into the ordered micropores of COF, thus leading to enhanced adsorption.35 3.3.5 Adsorption isotherm models. The adsorption of Cd2+ onto negatively polarized CTF-1 was modeled using Langmuir and Dubnin–Rudishkivich (D–R) models at different temperature at pH 7. The quantitative analysis of these adsorption tests showed a good fitting to equilibrium adsorption data. Each of these models is discussed below. (a) Langmuir model The adsorption data obtained at four different temperatures were first analyzed with the Langmuir model to investigate the nature of sorption and charge distribution along the COF framework, which may lead to a homogenous surface coverage by monolayer sorption.35,36 The Langmuir model is given by the following equation: Ce 1 Ce ¼ þ qe Kb qm qm

(6)

The constant Kb (L mg1) is called the Langmuir equilibrium constant and qm is the adsorption capacity (mg g1) at complete coverage of the surface. The linear plots of the Langmuir isotherm at different temperatures (298–318 K) showed good fitting to the adsorption data with high R2 values (Fig. 4b and Table 2), suggesting homogeneity of the surface and monolayer coverage during the adsorption.40 Further, the values of

Fig. 4 Adsorption isotherms: effect of temperature (a), Langmuir model (b), D–R model (c) and van’t Hoff plot (d).

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separation factor (RL) representing the strength of the adsorption capacity were also calculated according to following equation:

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RL ¼

1 1 þ Kb C0

(7)

where C0 is the initial metal ion concentration (mg L1) and Kb is the Langmuir constant. The positive values of RL (0.12–0.06) indicated the favorable nature of adsorption, whereas the increase in Kb values (Table 2) with an increase in temperature confirmed the endothermic nature of the sorption process.41 (a) Dubinin–Radushkevich isotherm model Dubinin–Radushkevich (D–R) model is generally used to predict the chemical or physical nature of an adsorption process. In the present study, D–R model was applied to the adsorption data in the following mathematical form:41–43 ln qe = ln qm  be2

(8)

where the term b is the Dubinin–Radushkevich constant (mol2 kJ2) and e is the Polanyi potential, which can be calculated as follows:   1 e ¼ RT ln 1 þ (9) Ce In eqn (7), R (8.314 J mol1 K1) is the gas constant, while T (K) and Ce represent the absolute temperature and the equilibrium concentration of the adsorbate, respectively. In the present study, the adsorption data was fitted to the D–R equation to obtain ln qe vs. e2 curves and the values of b and qm were calculated from the slopes and intercepts, respectively (Fig. 4c). As shown in Table 2, the values of R2 for the linear fitting curves are very high (0.99–0.97), suggesting good applicability of the D–R model to the adsorption data. Furthermore, to study the physical or chemical nature of Cd2+ adsorption by CTF-1, the values of mean free energy (E) were computed from the following equation:40,42,44 E¼

1 ð2bÞ0:5

(10)

Interestingly, the E values were found in the range of 10.9–12.3 kJ mol1, which confirmed the chemical nature of the sorption process (Table 2). 3.3.6 Adsorption thermodynamic studies. The van’t Hoff equation can be used to establish a relationship between the

Table 2 Langmuir and D–R parameters along with correlation coefficients for the adsorption of Cd2+ onto CTF-1 at pH 7

Langmuir parameters Temperature (K) Kb (L mg1) RL R2

D–R parameters

298 303 313 318

0.0042 0.0041 0.0036 0.0033

4208 4491 4841 5066

0.12 0.09 0.07 0.06

0.99 0.97 0.96 0.93

b (mol2 kJ2) E (kJ mol1) R2 10.90 11.0 11.80 12.30

0.99 0.98 0.96 0.97

Table 3 at pH 7

Thermodynamic parameters for adsorption of Cd2+ onto CTF-1

Temperature (K) DG (kJ mol1) DH (kJ mol1) DS (J mol1 K1) R2 298 303 313 318

20.3 21.1 22.2 23.0

69.3

164.6

0.93

adsorption coefficient (Kb) and temperature (T) to determine the values of various thermodynamics parameters as follows:45,46 (DG) = RT ln Kb ln Kb ¼ 

ðDGÞ DS DH ¼  RT R RT

(11) (12)

In the above equations, DS (J mol1 K1) stands for change in entropy, DH (J mol1) represents the enthalpy change and DG (kJ mol1) represents the Gibbs free energy change of adsorption. R is the gas constant and T is the absolute temperature. The values of DG were calculated from eqn (11), while eqn (12) was used to obtain the ln Kb vs. 1/T plot (Fig. 4d). The high R2 value (0.99) indicates the good linear fitting of the curve. DH and DS were derived from the slope and intercept of the plot, respectively. As can be seen from Table 3, the DH value is higher than 40 kJ mol1, suggesting the chemical nature of adsorption, whereas the negative values of DG and positive value of DS indicate that the adsorption of Cd2+ onto CTF-1 is thermodynamically feasible and entropy driven.35,47 3.3.7 Desorption reproducibility studies. To investigate the reproducibility of the CTF-1 adsorbent, adsorbent–desorption experiments were performed. When reused as the adsorbent for Cd2+ ions removal, CTF-1 was able to attain an adsorption capacity of 15.0 mg g1 after the first cycle and an impressively high adsorption capacity of 23.33 mg g1 after seven cycles (Fig. 5a), corresponding to 51.50% and 79.75% higher adsorption capacity compared to that of the pristine CTF, respectively (Fig. 5b). The %desorption was 1.34% and 55.25% after the first washing cycle with distilled water and HCl, respectively, which was further increased to 10.13% and 83.21%, respectively, after seven washing cycles (Fig. 5b), indicating good desorption ability and high reproducibility of CTF-1. 3.3.8 Effect of adsorbent dose and ionic strength. The effect of adsorbent dosage on Cd2+ ions adsorption by CTF-1 was investigated by varying the mass of the adsorbent from 0.1 to 1.5 g. The percent adsorption was found to increase from 27.25 to 94.83%. However, the adsorption per unit mass of the adsorbent decreased due to the incomplete saturation of the active sites with low metal ions concentration (Fig. 6a). Investigation of the effect of ionic strength on the sorption of heavy metals revealed a macroscopic method of concluding sorption mechanisms. Thus, taking into account the important decrease in Cd2+ removal in high competing electrolyte concentration, the progressive increase of cations of the strong electrolyte decreases the sorption of Cd2+ on the surface of the CTF-1 adsorbent. As an example, the adsorption isotherms of Cd2+ with various KCl concentrations on the CTF-1 surface

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Fig. 5 Adsorption–desorption isotherms of Cd2+ onto CTF-1: (a) adsorbed amount of Cd2+ left after washing with water (black), with HCl (red) and the amount of Cd2+ adsorbed by reusing CTF-1 (blue), and (b) %desorption with water (black), with HCl (red) and %adsorption by reusing CTF-1 (blue) after various washing cycles.

Fig. 6

(a) Effect of adsorbent dose and (b) ionic strength on Cd2+ adsorption onto CTF-1.

were recorded. There was a decrease in the uptake of Cd2+ when the ionic strength increased. This is because the ionic strength is a key factor in the control of electrostatic interactions. Therefore, these interactions, either attractive or repulsive, can be reduced by increasing the ionic strength of the solution. Moreover, the screening effect of the surface charge produced by the added salt could also lead to the decrease in heavy metals adsorption.48 The reduction in Cd ions adsorption due to the increase in ionic strength of the equilibrium solution could be attributed to the increased competition for adsorption sites between Cd2+ ions and K+ ions present in the background electrolyte.48 Additionally, the increase in solution ionic strength could have led to the formation of ion pairs between Cd2+ and other cations and anions (Cl)

Table 4 Comparison of adsorption capacity of Cd2+ ions with CTF-1 and other reported adsorbents

S. no.

Adsorbents

Adsorption (qmax)

Ref.

1 2 3 4 5 6 7

Natural phosphate Mesoporous activated adsorbent Capsicum annum Chemically treated laterite MOF-5 TiO2QGFO CTF-1

26.0 17.23 0.23 3.7 3.6 0.46 29.26

50 51 52 53 54 55 Present study

present in the electrolyte that reduced the activity of free Cd2+ ions in the solution, particularly in the multi-element system (Fig. 6b).49 The adsorption capacity of the CTF-1 adsorbent used in this study was compared to various adsorbents already reported in literature (Table 4). Table 4 shows the adsorbent used in the current study and comparatively proved that the efficiency of this newly developed material is either better or equivalent to that of the various adsorbents reported to treat cadmium affected effluents.

4. Conclusions We provide the following conclusions.  Highly crystalline and stable (above 600 1C) trizine based framework (CTF-1) having high surface area (490 m2 g1) with average pore size of 1.3 nm was successfully synthesized.  The kinetic studies indicated that the adsorption was pseudo-second-order, and was highly dependent on pH and temperature of the system.  The SEM images at different pH values confirmed that CTF-1 was chemically stable in both acidic and basic pH.  Langmuir and D–R isotherm models were fitted well to adsorption data, suggesting the favorable and chemical nature of the adsorption process.

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 Thermodynamic studies suggested the spontaneous and endothermic nature of the adsorption process.  From the above experimental results, it can be concluded that negatively polarized CTF-1 possesses high surface area and high physiochemical stability, and the strategies adopted in this study would help to design new functional materials for future adsorbents.

Conflicts of interest There are no conflicts to declare.

Acknowledgements The financial support from Chinese academy of sciences and Third world academy of sciences under the CAS-TWAS President’s PhD Fellowship Program is highly acknowledged.

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