Functionalized nanospheres for efficient sequestration of cadmium ions

Abstract

This manuscript reports the synthesis, characterization and the subsequent application of thiosemicarbazide functionalized nanosilica spheres in Cd(II) ion sequestration. The spheres are developed by the sol–gel method, activated with an organosilane precursor and then conjugated with thiosemicarbazide using a one-step Mannich reaction. The successful grafting is confirmed using FTIR, TGA, SEM and EDX analyses. Cd(II) ion sequestration from aqueous solutions is studied exhaustively in competitive and noncompetitive environments in batch experiments using the radiotracer technique. The sorption process is fast and can reach equilibrium in 40 minutes with 98% Cd(II) ion sequestration at pH 7 following first-order kinetics (K = 0.0465 min⁻¹). The sorption data follow the Langmuir, Freundlich and Dubinin–Radushkevich (D–R) isotherms and its characteristic constants were computed to be as follows: N = 10 mmol g⁻¹ and b = (0.1) × 10⁴ dm³ mol⁻¹, 1/n = 0.64 and of C_m = 0.04 mmol g⁻¹ and D–R constants, i.e. β = −0.00064 kJ² mol⁻², X_m = 1.8 mmol g⁻¹ and E_s = 8.8 kJ mol⁻¹. The sorption of Cd(II) ions onto nanospheres is spontaneous (ΔG = −1050.00 J mol⁻¹ K⁻¹) and endothermic in nature (ΔH = 2.45 J mol⁻¹ K⁻¹). The Cd(II) ions sequestration is highly reduced in the presence of cyanide, EDTA, aluminum and nickel ions. The sorbent is regenerable and can be used several times.

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1. Introduction

Due to their technological importance, heavy metals have extensive applications in electroplating, tanneries, as well as paint, metallurgy, chemical and pharmaceutical industries. The large scale production and disposal of these heavy metals poses serious health and environmental hazards because of their frequent appearance in wastewater. Many metal processing industries discharge their wastes without prior purification, which causes serious environmental problems. The concentration of these toxic heavy metals is often higher than the permissible limits. Trace metal determination and cleaning is essential in pharmaceutical, food, agricultural and chemical industries. Direct determination of metal ions at trace levels is difficult due to their presence below the detection limit and matrix interferences. Classical methods such as co-precipitation, ion exchange, solvent extraction, cloud point extraction and biosorption are used for the pre-concentration/separation of trace level impurities.^1,2 Due to their inherent limitations, a promising material for pre-concentration/separation remains in demand.

Functionalized sol–gel nanosilica is a prospective candidate for the cleaning of low level impurities from aqueous, petrochemicals and gaseous media. Silica tailored with organic functional groups is receiving special attention and used in various areas, including the adsorption of radiotracers,^3,4 ion exchange,⁵ biotechnology,⁶ catalysis⁷ and green chemistry.⁸ The surface of silica can be tailored with desired functional groups, which selectively directs the chelation of target metal ion towards the sorbent surface.^9,10

Cadmium is used in a variety of industrial applications and causes a variety of syndromes, renal dysfunction hypertension, lung and liver damage and teratogenic effects.¹¹ Considering the toxicological effects of cadmium and its strategic importance, there has always been a demand for preparing novel separating agents that could be effectively used for the sequestration of Cd(II) ions from aqueous media. Keeping this important purpose in mind, we report the synthesis of novel nanosorbents through the covalent grafting of an organic ligand, thiosemicarbazide (TSC), to nanosilica spheres. Thiosemicarbazide is one of the key compounds with N–S donor atoms and capable of complexation with a number of metal ions in pharmacological and biological applications,^12–14 and never used as a complexing agent on the surface of nanosilica spheres for the sequestration of Cd(II) ions. It has great potential and can be used for the sequestration of cadmium from aqueous and organic media.

2. Experimental

2.1 Reagents and chemicals

Tetraethylorthosilicate (TEOS, 97%), N-cetyl-N,N,N-trimethylammonium bromide (CTAB), thiosemicarbazide (TSC) and 3-aminopropyltrimethoxy silane (APMTS, 98%) and all other analytical grade reagents were used without any purification.

The ¹¹⁵Cd radiotracer was prepared by the irradiation of spec pure metal in PARR-1, a PINSTECH research reactor at a neutron flux of 7 × 10¹³ s⁻¹ and dissolved in concentrated HCl. The acid was removed by repeated evaporation with distilled water and finally 5.0 cm³ stock solution was made for further experiments. The radiochemical purity of the tracer was determined using a 25 cm³ Ge(Li) detector coupled with a 4k series of 85 Canberra, USA multichannel analyzer.

The buffer solutions of pH 1–8 were prepared by using appropriate volumes of potassium chloride and hydrochloric acid (pH 1–2), sodium acetate and acetic acid (pH 3–6), and boric acid and sodium hydroxide solutions (pH 7 and 8).

2.2 Apparatus/instruments

All pH measurements were made on a Metrohm-605 pH meter. The shaking was performed in a 30 ml Pyrex glass vial lined with a screw-type polythene cap. Gross gamma-ray detection was performed on a Tennelec; USA counting assembly equipped with a 25 cm³ NaI(Tl) crystal.

2.3 Synthesis of thiosemicarbazide functionalized nano silica spheres

Nano silica spheres were prepared using a sol–gel method as described in the literature.¹⁵ Functional groups such as –NH₂ were immobilized onto nanosilica as reported earlier.¹⁶ For the synthesis of thiosemicarbazide functionalized nanospheres, 5 g of TSC was dissolved in 50 ml of distilled water in a three neck flask and then placed on a hot plate fitted with a condenser and magnetic stirrer. 30 ml of 37% formaldehyde was drop-wise added to this solution, and the mixture was refluxed for 1 hour at 90 °C. The resulting mixture was distilled to remove unreacted water and formaldehyde. 10 g of NH₂–silica (APTMS) and 50 ml methanol was added to this solution and refluxed for 30 min with vigorous stirring at 90 °C. The resulting TSC-functionalized silica spheres were filtered, washed repeatedly with deionized water, ethanol and methanol and dried in oven at 45 °C for 12 hours. The synthesis scheme is given in Fig. 1.

Fig. 1 Synthetic scheme for the preparation of TSC functionalized nanospheres.

2.4 Procedure for cadmium ions sequestration

Cadmium sequestration was investigated in batch experiments using the radiotracer technique at room temperature (25 ± 1 °C) unless otherwise described. A known amount of radiotracer was added to a 5 cm³ solution of an electrolyte of a specific pH, in a 30 cm³ glass vial and mixed uniformly. Then, 1 cm³ solution was taken out for gross gamma counts (A₀). A₀ is considered the initial concentration of the Cd(II) solution. The remaining solution was equilibrated with a known quantity of nanospheres for a specific period of time in a Wrist action shaker (Griffin UK). After shaking, the solution was centrifuged for about 5 minutes and 1 cm³ of the aliquot was taken out for the radio assay (A_e). A_e is the equilibrium concentration of Cd(II) ions removed. Cadmium % sorption was calculated by using the relationship:

The distribution coefficient K_d is calculated as follows:

3. Results and discussions

3.1 Characterization of the sorbent

The specific surface area, total pore volume and pore diameter of sol–gel silica nanospheres was calculated from the Brunauer–Emmett–Teller (BET) equation. The nanospheres were porous in nature with a surface area of 920 m² g⁻¹, pore volume of 0.96 cm³ g⁻¹ and a pore diameter of 4.17 nm. The high surface area is an important factor for the immobilization of organic molecules. Fig. 2 shows the scanning electron microscopy (SEM) images of virgin (left) and functionalized silica nanospheres (right). The similarity between the two images reveals that the particles preserve their spherical morphologies after functionalization, suggesting that anchoring is a chemical process. The surface is fine-tuned with desired function groups without disturbing the basic morphology of the parent nanospheres. The nano size of the spheres was confirmed by the SEM images.

Fig. 2 SEM images of nanosilica spheres (left) and thiosemicarbazide functionalized nanospheres (right).

Fig. 3 shows the EDX analysis of the functionalized nanosilica spheres. The spectrum of the fine silica show signals indicative of the presence of oxygen, sulfur and nitrogen along with carbon and silicon, which originate from the organic moieties during the anchoring of silica spheres. The EDX spectra further confirm the successful immobilization of these organic groups on the surface. The adsorption processes are assumed to occur by the complex formation reaction between these hetero atoms and the Cd(II) ions.

Fig. 3 EDX spectra of thiosemicarbazide functionalized nanospheres.

The FT-IR spectra of the virgin (a) APTMS (b) and TSC (c) modified nanospheres is given in Fig. 4. A careful study of the figure shows that after functionalization with APTMS, nanosilica shows visible adsorption peaks at 1542 and 1637.37 cm⁻¹ (Fig. 4b), which agrees with the bending vibration of N–H group, as well as N–H (3200–3500 cm⁻¹) and C–N (1030–1230 cm⁻¹) stretching vibration overlap with the broad absorption band of the silanol group and Si–O–Si vibrations^17,18 (Fig. 4b). The bands at 2855.50 and 2927.1 cm⁻¹ are assigned to the symmetric and asymmetric vibrations of CH₂ groups. The appearance of these peaks is excellent proof for the immobilization of the amine moieties. After immobilization of TSC ligand some new peaks appeared at 1344.93 and 1489.57 cm⁻¹, which are assigned to the S=C–NH vibration of the TSC immobilized silica^19,20 (Fig. 4c). The C=S stretching vibration peaks (730–1089 cm⁻¹) overlap with the strong skeletal vibrations of silica. The appearance of some new peaks and the disappearance/modification of original peaks confirms the successful immobilization of the chelating ligand.

Fig. 4 FTIR analysis of virgin (a), NH₂ (b) and thiosemicarbazide (c) functionalized nanospheres.

Thermograms of the virgin and functionalized silicas were recorded over a temperature range of 30–700 °C at a constant heating of 10 °C min⁻¹ (Fig. 5). Virgin spheres showed high stability, and only physisorbed water and residual surfactant was removed. The 3-aminopropyltrimethoxysilane (APTMS) modified silica spheres exhibits weight loss from 30 to 200 °C (5%), from 200 to 500 °C (4%) and from 500 to 700 °C (6%), corresponding to physically adsorbed water, the decomposition of organic groups²¹ and the condensation of silanol groups to yield siloxane groups, respectively. The thiosemicarbazide modified nanospheres lose their mass in temperature range of 30–250 °C (3%), 250–500 °C (3%) and 500–700 °C (6%), which is attributed to the loss of water molecules,^22,23 decomposition of the –NH–C=S organic molecule and the condensation of surface silanol groups to produce siloxane groups, respectively.^24,25 The differences in thermal behavior of the materials show that they have different chemical compositions owing to the attachment of organic functional groups.

Fig. 5 Thermogravimetric analysis of virgin, APTMS and TSC functionalized nanospheres.

3.2 Evaluation of nanospheres for metal ions sequestration

3.2.1 Effect of pH on Cd(II) sequestration. The pH affects the sequestration process in several ways, such as increased solubility of metal ions, counter ion concentration and the extent of ionization of the adsorbate.²⁶ It also provides additional selectivity to the sorbent. Keeping this in mind, we examined the role of pH in the removal of Cd(II) ions from an aqueous solution in a pH range of 1 to 8, and the results are depicted in Fig. 6. Initially up to pH 4, no sorption occurred, which might be due to the protonation of thiol and amine groups. The sorption of Cd(II) ions starts at pH 5 and achieves a maximum at pH 7 while decreasing again at pH 8. In this pH range, neither the protonation of the ligand hetero atoms nor precipitation of the metal is expected.^27,28 The maximum sorption capacity of Cd(II) onto pyrocatechol functionalized XAD in the pH range of 5–8,²⁹ pH 4–5 for tiron³⁰ and aminophenol³¹ and between pH 5–6 for dithizone³² has been reported earlier. Above pH 5, the ligand coordination sites are deprotonated and more Lewis bases such as the –OH, –NH and =S– functionalities, which are present on the surface of the sorbent, are available for chelation with positively charged metal ions.³³ Until pH 8, the dominant species of the Cd(II) is Cd²⁺ (100%).²⁸ Above pH 8, the metal is prone to precipitation and hydrolysis³⁴ and yields Cd(OH)₂. Further sorption tests were not conducted, and pH 7 was selected as the optimum sorptive media for further experiments.

Fig. 6 Effect of pH on Cd(II) ion sequestration by TSC functionalized nanospheres.

3.2.2 Kinetics of Cd(II) ion removal. The influence of equilibration time on the rate of sequestration of Cd(II) ions onto TSC functionalized nanospheres is studied in the range of 5–60 minutes, and the results are presented in Fig. 7. It can be seen that the sorption kinetics was fast and about 85% of cadmium is removed within the first 10 minutes of agitation. The fast equilibration reflects the strong affinity of the material for binding metal ions, which broadens its scope for application on a large scale. A detailed kinetics evaluation of the process shows that the process is first-order with a rate constant K of 0.0465 min⁻¹. Furthermore, intraparticle diffusion is not a predominant process initially, and the intra particle diffusion rate, R_D, is 8.92 μmol g⁻¹ min^−1/2. The surface diffusion also plays a role in determining the kinetics.

Fig. 7 Cd(II) ions sequestration as a function of equilibrium time.

3.2.3 Effect of functionalized material quantity on sequestration of Cd(II) ions. The effect of the gel quantity on the Cd(II) ion sequestration has been examined from 2 to 12 mg ml⁻¹ of the TSC functionalized nanospheres at a constant Cd(II) concentration (7.39 × 10⁻⁵ M). The nanospheres offer its large surface area and vacant adsorption sites to the incoming metal ions. Initially increase in nano spheres dosage leads to increased sequestration. After some time when all the present sorbate ions are accommodated, the increase in the adsorbent dosage will have no effect on the sorption of metal ions and remain constant. The results show that a small amount of the nano-functionalized sorbent (4 mg ml⁻¹) is sufficient for accommodating all the present heavy metal ions in a given volume, which again shows the suitability of the sorbent on a large scale.

3.2.4 Effect of Cd(II) ions concentration on its sequestration. The effect of the initial concentration of Cd(II) ions on its removal from aqueous solution was examined over an 8-fold increase in the concentration (3 × 10⁻⁵ to 24 × 10⁻⁵ M) of Cd(II) ions at pH 7.0 using 20 mg of functionalized nano spheres, and the results are presented in Fig. 8. The initial metal ion concentration provides a driving force that helps overcome the mass transfer resistance of the accumulating material in an aqueous medium. This leads to an increase in the uptake capacity of the material.³⁵ At lower concentrations, the metal ions present in the system experience lower inter-ionic repulsion, are capable of interacting with the binding sites of the material. To explain the sequestration mechanism, the data were fitted to Freundlich, Langmuir and D–R isotherm models, as shown in eqn (3)–(6) , and the constants calculated from these equations are reported in Table 1.where C_e is the equilibrium concentration (mmol) of Cd(II) ions in solution, q_e is the amount of cadmium adsorbed at equilibrium (mmol g⁻¹), N is the maximum sorption capacity (mmol g⁻¹) and b is the Langmuir constant.where C_e and C_ads are the equilibrium concentration (mol L⁻¹) of Cd(II) ions in solution and sorbed on the nanospheres (mol g⁻¹), respectively. C_m and 1/n are the Freundlich constants showing maximum sorption capacity and sorption intensity, respectively.

ln C_ads = ln X_m − βε² (5)

where C_ads is the sorbed metal ion in mol g⁻¹ per unit mass of the sorbent, X_m is the maximum sorption capacity in mol g⁻¹ of the nanospheres, β is a constant in units of kJ² mol⁻² and ε is a polynomial potential. The mean sorption energy E_s is related to X_m and calculated by the equation:

Fig. 8 Cd(II) ions sequestration as a function of cadmium concentration.

Table 1 Langmuir, Freundlich and D–R isotherm constants of Cd(II) ions uptake by TSC functionalized nanospheres

Isotherm model	Constants	Values
Langmuir isotherm	N (mmol g^−1)	10
	b × 10^3 (L mol^−1)	0.1
	R^2	0.98
Freundlich isotherm	1/n	0.64
	Cm (mmol g^−1)	0.04
	R^2	0.99
D–R isotherm	β (kJ^2 mol^−2)	−0.0064
	Xm (mmol g^−1)	1.89
	Es (kJ mol^−1)	8.84
	R^2	0.98

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3.2.5 Effect of temperature on Cd(II) ions sequestration. Temperature plays an important role in metal ions removal by a solid surface, either increasing or decreasing the sorption of the metal ions. If sorption increases with a rise in temperature, it indicates a chemisorption nature of the sorption process and the reverse is true for physisorption. This increase is due to the creation of new sorption sites, acceleration of sorption steps or due to transport against an energy barrier.³⁶ The effect of temperature on Cd(II) ions sequestration by functionalized nanosilica spheres is studied over a temperature range of 283–323 K under optimized conditions and the results are presented in Fig. 9. The thermodynamic parameters like enthalpy change (ΔH) and entropy change (ΔS) were calculated using the following relationships.³⁷where , in which F is the fraction of metal ion sorbed at equilibrium; T is the temperature in Kelvin; and R is the universal gas constant in units of kJ mol⁻¹ K⁻¹. The plot of log K_c verses 1/T produces a straight line. The values of ΔH = 2.45 J mol⁻¹ K⁻¹ and ΔS = 16.96 J mol⁻¹ K⁻¹ were computed from the slope and intercept of the plot, respectively. The positive value of enthalpy indicates the endothermic nature of process. The large increase in entropy favors the stability of Cd(II)–TSC complex on the surface of the nanospheres. The Gibbs free energy (ΔG) was calculated by using the relationship:

ΔG = −RTln K_c (8)

Fig. 9 Effect of temperature on Cd(II) ions sequestration by TSC functionalized nanospheres.

The numerical value calculated for ΔG was found to be −1050.00 J mol⁻¹ at 293.16 K. The large negative value of ΔG reflects the spontaneous nature of the process.

3.2.6 Effect of interfering ions on cadmium sequestration. Common ions present in real life samples affect the sequestration of Cd(II) ions. This behavior is explained on the basis of the nature of interacting ions and the environment around the metal ion of interest. In this study, we investigated the effect of various counter ions on the uptake of Cd(II) under optimized conditions. Each salt was taken in 10 mg quantity and its effect on Cd(II) ions sequestration was studied and the results are summarized in Tables 2 and 3. In these tables, “nil” refers to the condition where no complexing ion except Cd(II) is present in the solution. Although all the ions added affected the Cd(II) ions sequestration, the thiosulfate, cyanide, EDTA, Cu²⁺, Ni²⁺ and Al⁺ drastically reduced it, as shown in Tables 2 and 3.

Table 2 Effect of anions on Cd(II) ions sequestration of by TCS functionalized nanospheres

Compounds	Interfering ion	Sorption (%)
Blank	Nil	98
KI	I^−	86
KF	F^1−	80
Na2B4O7	B4O7^2−	76
Na2SO4	SO4^2−	74
K2CrO4	CrO4^2−	70
C6H5O7Na3	C6H5O7^3−	67
Na2C2O4	C2O4^2−	62
Na2S2O3	S2O3^2−	45
KCN	CN^1−	20
EDTA	EDTA	02

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Table 3 Effect of cations on Cd(II) ions sequestration of by TCS functionalized nanospheres

Compound	Interfering ion	Sorption (%)
Blank	Nil	98
FeCl3	Fe^3+	86
SnCl2	Sn^2+	85
MnCl2	Mn^2+	82
LiCl	Li^1+	80
AgNO3	Ag^2+	79
CuSO4	Cu^2+	49
Ni(NO3)2	Ni^2+	15
Al(NO3)3	Al^2+	02

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3.2.6 Regeneration of the spheres. The Cd(II) ions were fully desorbed from the surface of the sorbent by treating with a 0.1 M HCl solution. After the regeneration, the nanospheres were conditioned in a pH solution and reused for the removal of cadmium. The material was stable enough that after three sorption–desorption cycles no significant change was observed in the sorption capacity of the thiosemicarbazide functionalized silica.

4. Conclusion

A functionalized nanosilica sorbent bearing nitrogen and sulphur functional groups is effectively employed as a Cd(II) scavenger from spiked cadmium samples in batch experiments. The nanospheres can adsorb 98% of Cd(II) ions in 40 minutes at pH 7. The sorption processes follow first-order kinetics and are characterized by surface and intra-particle diffusions mechanism. The sorption process follow the Langmuir, Freundlich and Dubinin–Radushkevich (D–R) isotherms with a maximum sorption capacity of 10 mmol g⁻¹. The positive value of entropy (16.96 J mol⁻¹ K⁻¹) shows the formation of a stable Cd–TSC complex and suggests the chemisorption nature of the process confirmed by E_s = 8.8 kJ mol⁻¹ computed from the D–R isotherm. The Cd(II) ion sequestration is highly reduced in the presence of cyanide, EDTA, aluminum and nickel ions. The sorbent is regenerable and can be used several times. These functionalized nanospheres are promising tools for the removal of a trace amount of cadmium from drinking and industrial water.

Acknowledgements

The authors highly acknowledge the Pakistan Institute of Nuclear Science and Technology (PINSTECH) for providing laboratory facilities and the Higher Education Commission (HEC) of Pakistan for providing financial assistance.

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