Synthesis of zinc oxide/talc nanocomposite for enhanced lead adsorption from aqueous solutions

Abstract

In this study, talc was modified with zinc oxide nanoparticles to form a ZnO/talc nanocomposite. The nanocomposite was characterized using X-ray powder diffraction, Fourier transform infrared spectroscopy, nitrogen adsorption/desorption, field emission scanning electron microscopy. The characterization revealed that ZnO nanoparticles were well incorporated with the talc. The adsorption efficiency of the prepared nanocomposite was evaluated for Pb(II) removal from aqueous solution. The related parameters such as agitating time, dosage and pH, were optimized. Adsorption characteristics of the talc, and ZnO/talc nanocomposite were compared and the results showed that ZnO/talc nanocomposite had the highest adsorption capacity. The kinetic sorption data were found to fit the pseudo-second-order kinetic model. The experimental isotherm data of lead adsorption were examined using the Freundlich and the Langmuir models. The maximum lead adsorption capacity of the adsorbent was determined as 48.3 mg g⁻¹. The mechanism of adsorption was found to be controlled by electrostatic attraction on the nanocomposite. The overall results indicated the prepared nanocomposite can be employed as an alternative for Pb(II) removal from wastewater.

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1. Introduction

The level of lead production and consumption has increased worldwide due to a number of agricultural and industrial activities. Lead is a toxic heavy metal that cannot degrade and can easily accumulate in the human body. Lead is commonly used for various industrial applications such as battery industries, paints, fuels, pigment, photographic materials, metallurgical, coatings and automotive industries.¹ Drinking water and ingestion are the main source through which lead get into the body. Lead causes kidney disease, cancer, anemia, mental retardation and damage central nervous system.^2,3 The maximum contaminant level allowed by US Environmental Protection Agency is 0.015 mg L⁻¹ and the maximum contaminant level goal is zero.⁴ So, waters containing potentially lead metal need to be checked and treated before the discharge. Lead can pollute the environment from artificial sources as well as by natural geochemical processes.⁵

Lead ions from various medium have been removed using different techniques and materials from different researchers. Electrodialysis process was used for removal of lead.^6,7 The removal of lead by natural and pretreated clinoptilolite was reported. Magnetic magnetite (Fe₃O₄) nanoparticle was used for adsorption of lead(II) from water.⁸ A PSf/Fe₃O₄–talc membrane was also used for removal of lead.⁹ Two methods of electrochemical and chemical coagulation were combined to remove lead ions from wastewater.¹⁰ A new low-cost adsorbent from natural zeolite–kaolin–bentonite was used for lead removal.¹¹ An improved chitosan bead was employed for lead removal.¹² Adsorption is the most effective and cost-effective process for removal of heavy metal from waste water and serves as a substitute due to its advantage of adsorbent regeneration, high percentage removal, recovery of metal, reduction in the quantities of materials discharge.¹³ Zinc oxide is a unique material in nanotechnology because it exhibits unique physical and chemical properties such as high chemical stability, high electrochemical coupling coefficient, a broad range of radiation absorption and high photostability, which have to make it as a great potential in many applications.

Talc mineral is hydrated magnesium silicate with the chemical formula Mg₃Si₄O₁₀(OH)₂. It is the softest mineral. Talc is white clear material that is translucent to opaque with a density of 2.5–2.8. Talc does not dissolve in water. The talc layers are made of octahedral centered sheets coordinated Mg(OH)₂ in a trioctahedral arrangement sandwiched between coordinated silicate sheets with tetrahedral structure.¹⁴ Talc is applicable in many industries such as paper, plastic, rubber, food, pharmaceuticals, cosmetics, and ceramics. Important properties that make talc be used for such applications are natural abundance, low-cost, high surface area, chemical inertness, high thermal stability, good lubricity, low electrical conductivity and distinctive pore structure.¹⁵ The adsorption of uranyl ions onto talc from aqueous solutions and heavy metals rapid adsorption by Fe₃O₄/talc nanocomposite was evaluated.^6,17 Talc powder was employed for the removal of hexavalent chromium from aqueous solutions.¹⁸ Şener and Özyilmaz use sonicated talc for naphthalene adsorption.¹⁹

In this study, talc modified with ZnO nanoparticles was prepared by the heat-based method to form ZnO/talc nanocomposite. The obtained nanocomposite was characterized and its adsorption efficiency was explored for the removal of Pb²⁺. The ZnO/talc nanocomposite showed enhanced adsorption efficiency compared to ZnO and talc alone. The adsorption performance was evaluated in terms of kinetic properties and adsorption isotherms.

2. Materials and methods

2.1. Materials

Pb(NO₃)₂ and Zn(NO₃)₂·5H₂O were purchased from Bendosen. Talc and sodium alginate were purchased from Sigma-Aldrich. All the chemicals were of analytical grade.

2.2. Preparation of ZnO/talc nanocomposite

Zinc oxide nanoparticles were prepared by the reaction between zinc nitrate and sodium alginate. The aqueous solution of the prepared zinc nitrate was mixed with a solution of sodium alginate and vigorously stirred at 40 °C for 30 min. The nanocomposite was prepared by addition of the aqueous mixture of zinc nitrate and solution of sodium alginate into the aqueous suspension of talc and stirred for 24 h Fig. 1. The resulting solid phase was separated by decantation, washed several times with distilled water, and then dried at 105 °C for 24 h. The resulting powder was calcined at 500 °C for 1 h.

Fig. 1 Illustration for the preparation of ZnO/talc nanocomposite using sodium alginate.

2.3. Characterization

The X-ray powder diffraction (XRD) patterns were measured using X-ray diffractometer ShimadzuXRD-6000 instrument Cu Kα radiation (λ = 1.5406 Å, 30 kV, 30 mA) over a 2θ range of 5–80° with a resolution of 0.02°.

Field emission scanning electron microscopy morphology (FESEM) and the surfaces of samples were studied using field emission scanning electron microscopy spectroscopy (EDX) (JEOL JSM-7600F (SEM)) equipped with energy dispersive spectrometer (EDS) for elemental analysis. Fourier transform infrared spectroscopy (FTIR) was measured by Perkin Elmer 2000. The measurement was carried out in the spectral range 4000–400 cm⁻¹ using the KBr disc technique. Nitrogen adsorption/desorption was used to measure surface area and average adsorption pore width of the nanocomposite by Brunauer–Emmett–Teller (BET)-nitrogen gas analysis using a TriStar II Plus 3020 Automatic Physisorption.

2.4. Adsorption experiments

2.4.1 Preparation of the stock solution. Lead nitrate (Pb(NO₃)₂) (99% purity), was used to the prepared 1000 ppm stock solution of lead by dissolving 1.6 g of lead nitrate in 1 dm³ volumetric flask and made up to mark using distilled water respectively. Other standard solutions were prepared by diluting the stock solution.

2.4.2 Adsorption studies. The adsorption of Pb(II) onto the prepared ZnO/talc nanocomposite was studied using batch adsorption experiments. The percent removal of Pb(II) from aqueous solution was studied as a function of contact time, adsorbent dosage, solution pH and initial concentration of Pb(II). The effect of agitation time was investigated by varying the contact time for 120 min. Effect of dosage was varied from 0.01 to 0.1 g. The effect of metal ions concentration was studied from 50 to 100 mg L⁻¹, while the effect of pH was studied at pH of 1 to 5 by adjusting with 0.1 M HNO₃ and NaOH. The removal of lead from aqueous solutions was conducted at established optimum conditions.

The percentage removal and adsorption capacity were calculated and used to test the efficacy of the nanocomposite for removal of Pb(II) from aqueous solution. Calculation of % removal and adsorption capacity are shown below

where: C_o: initial concentration, C_e: equilibrium ion concentration and V: volume of solution. The experiments were performed three times and all data points are the mean of triplicate measurements.

2.4.3 Kinetic studies. The lead adsorption was investigated at various time intervals. In the kinetic studies, 25 mL lead solution (100 mg L⁻¹) and 0.08 g nanocomposite were agitated for 120 min until the adsorption equilibrium was attained. The remaining concentration of the lead ions in the solution was measured by AAS and the percentage removal of the adsorbent for the lead ions was calculated.

2.4.4 Equilibrium isotherm studies. The adsorption isotherm experiments were conducted at room temperature and optimum pH with initial lead ions concentrations between 50 to 150 mg L⁻¹ and 0.08 g of the prepared a nanocomposite (adsorbent) was added into 25 mL lead ion solutions with a different initial concentration. The adsorption process was carried out for 120 min and the initial and final lead ion concentrations in the solutions were determined with AAS.

3. Results and discussion

3.1. Characterization

FTIR study was used to analyze the minerals and functional groups that present in the talc and modified ZnO/talc. The FT-IR spectra of talc and ZnO/talc samples are presented in Fig. 2 and spectra were recorded in the range from 400 to 4000 cm⁻¹. The band at 1043 cm⁻¹ is attributed to Si–O in-plane stretching and bands at 524 and 617 cm⁻¹ are due to Si–O–Al bending and stretching vibrations, respectively.^20,21 The band at 464 cm⁻¹ is due to Si–O–Si bending vibrations. The bands at 3660 to 3665 cm⁻¹ correspond to the stretching and bending vibrations for the hydroxyl groups of water molecules present in the talc. The bands in ZnO/talc move to lower wave numbers that signify the interaction of ZnO nanoparticles with the talc this confirmed the presence of ZnO nanoparticles present in the talc matrix.

Fig. 2 FTIR spectra of talc and prepared ZnO/talc nanocomposite.

3.1.1 X-ray diffraction (XRD). XRD analysis was performed to confirm the crystal phase of the prepared ZnO/talc. The XRD pattern of talc and ZnO/talc nanocomposite shown in Fig. 3. The (001), (002), (101), (102) and (110) peaks of ZnO are obviously observed at 2θ = 31.5°, 34.8°, 36.5°, 47° and 56.74°, respectively. XRD patterns confirm the presence of hexagonal wurtzite structure ZnO in the talc. X-ray diffraction, especially small-angle X-ray diffraction (SAXRD) is a popular and efficient method to characterize clay materials. The change in the d spacing of talc after interaction with zinc oxide nanoparticles is negligible and these signify the nanoparticles are on the external surface of the talc. The peaks related to talc are observed at 2θ = 34.54°, 36.04°, 40.56°, 42.74°, 48.56°, 54.60°, 60.56°, 66.5°, 70.34° and 72.84°.

Fig. 3 (a) PXRD X-ray diffraction of talc and ZnO/talc nanocomposite for showing of ZnO nanoparticles crystals (b) X-ray diffraction of talc and ZnO/talc nanocomposite for determination of d-spacing.

3.1.2 Nitrogen adsorption/desorption. The nitrogen adsorption–desorption isotherms of talc and modified talc are shown in Fig. 4. There is no change in the shape of N₂ adsorption–desorption isotherms of talc and prepared ZnO/talc which revealed that the structure of the talc remained unchanged. The surface area and the pore volume decreases after modification. The specific surface area of talc decreases from 17.26 to 7.47 (m² g⁻¹). The presence of nanoparticles within the pore of the talc causes the decrease in the surface area of the nanocomposite or due to loss of micropores, which are in accordance with many kinds of literature.^22,23 The particle deposited makes it difficult for the nitrogen to enter the pores during adsorption measurement. The isotherm of talc and prepared nanocomposite are type IV.

Fig. 4 Nitrogen adsorption–desorption isotherms of (a) talc and (b) ZnO/talc.

3.1.3 SEM and EDX. Fig. 5 shows the SEM image and EDX spectra of the prepared ZnO/talc nanocomposite. It can observe that flaky shape of talc is covered with ZnO nanoparticles. The appearance of Zn in the EDX spectrum confirms the formation of ZnO/talc nanocomposite.

Fig. 5 (a) SEM image and (b) energy dispersive X-ray spectra for ZnO/talc.

3.2. Adsorption studies

3.2.1 Adsorption efficiency. The adsorption efficiency of raw talc, and prepared ZnO/talc nanocomposite were compared under optimum adsorption condition. The adsorption efficiency of talc increased with the incorporation of ZnO nanoparticles. The nanocomposite had the highest percentage removal of lead, around 95% compared to 46% removal of lead using raw talc. This could be explained by the formation of new properties as a result of the interaction between ZnO nanoparticles and talc.

3.2.2 Effect of dosage. Adsorbent dosage is an important factor because it determines the capacity of an adsorbent for a given initial concentration of the adsorbate. The results show that percentage removal increases as the adsorbent concentration increases, but the amount adsorbed per unit mass of the adsorbent decreases considerably. The decrease in unit adsorption with the increase in the dose of adsorbent is basically due to adsorption sites remaining unsaturated during the adsorption reaction.^24,25 The percentage removal increased from 51.64% to 92.87% when the dosage was increased from 0.02 g to 0.1 g. The maximum removal was found at adsorbent dosage 0.08 g L⁻¹ with 93% removal. The effect of dosage on Pb ion removal is presented in Fig. 6.

Fig. 6 Effect of adsorbent dosage on the removal of Pb²⁺ by ZnO/talc nanocomposite.

3.2.3 Effect of pH. The effect of percentage of Pb(II) and pH of the solution are represented in Fig. 7. Generally, in the batch adsorption experiments, pH is the main factor in as it is capable of influencing the adsorption process through changes in the surface charge distribution of adsorbents used.²⁶ The effect of pH of the solution on the percentage removal was studied under the optimum conditions: 100 mg L⁻¹ initial Pb(II) concentration, 0.08 g L⁻¹ adsorbent concentration, 90 min agitation time and pH of 1 to 5, the percentage removal was found in the range of 69–93%, as shown in Fig. 7. The difference of adsorption of lead ions at different pH values may be due to the formation of different lead ions species as well as competition by hydrogen ions and strong protonation of hydroxyl group that is attached to a silicon atom at low pH. The various species of lead ions present in aqueous solution at different pH of 1 to 6 exist in the forms of Pb²⁺, Pb(OH)⁺ and Pb(OH)₂. At lower pH Pb(II) mainly exist in the form of Pb²⁺ and its removal mostly relies on the adsorption efficiency.²⁶ The optimum pH was 4 which is the same with the research reported earlier.²⁷

Fig. 7 Effect of pH on the removal of Pb²⁺ by ZnO/talc nanocomposite.

3.2.4 Effect of agitation time. The effect of agitation on lead removal from aqueous solution by the nanocomposite is shown in Fig. 8. The time was varied from 15 to 120 min. There was an increase in percentage removal of the lead with increased in the agitation time. It can be seen that equilibrium of adsorption was attained within 90 min. Thereafter, no significant change in percentage removal showing that the reaction has reached equilibrium. The availability of abundant active sites on the surface of the nanocomposite is what causes the initial rapid adsorption rate.^28,29

Fig. 8 Effect of agitation time on the removal of Pb²⁺ by ZnO/talc nanocomposite.

3.3. Kinetics of adsorption

Adsorption kinetic is used to predict the rate at which lead ions is removed from the aqueous solutions. The adsorption data of Pb(II) at different time intervals are fit for to pseudo-first-order kinetic model and pseudo-second-order kinetic model. Pseudo first order kinetic model describe mechanisms of metal species adsorption by an adsorbent and can be expressed as

ln(q_e − q_t) = ln q_e − k₁t (3)

Pseudo-second-order kinetic model

where q_t is the amount of Pb(II) adsorbed at time t (mg g⁻¹); q_e is equilibrium solid phase concentration and k₁ and k₂ are first-order and second-order rate constant for adsorption (L min⁻¹) respectively. The adsorption kinetic studies of Pb(II) ions onto nanocomposite are shown in Fig. 9 and summarize in Table 1.

Fig. 9 Pseudo first order and second reaction kinetics for the adsorption of Pb²⁺ on ZnO/talc nanocomposite.

Table 1 The adsorption kinetic studies of Pb(II) ions on the nanocomposite

	Pseudo-first order			Pseudo-second order
	k1 (min^−1)	qe	R^2	k2 (g mg^−1 min^−1)	qe	R^2
Pb	1.59 × 10^−2	30.16	0.946	6.7 × 10^−4	40.0	0.999

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The theoretical adsorption capacity value (40.0 mg g⁻¹) for pseudo second-order kinetics is in good agreement with the experimental adsorption capacity value (38.8 mg g⁻¹). The regression coefficient R² of second order is higher than that of the first order and hence the data best fitted to pseudo-second-order than pseudo-first-order and hence confirming chemisorption as the rate limiting the reaction, Fig. 9.

3.4. Isotherm of adsorption

Isotherm of adsorption gives information on mechanisms of adsorption, properties of the surface and affinity of an adsorbent towards heavy metal ions.^30,31 The adsorption data were analyzed using Langmuir and Freundlich adsorption isotherm models and are presented in Fig. 10.

Fig. 10 Langmuir and Freundlich isotherms for the adsorption of Pb²⁺ on ZnO/talc nanocomposite.

3.4.1 The Langmuir isotherm. The Langmuir model assumes that the monolayer adsorption which happens at fixed numbers of homogeneous sites on the adsorbent.where C_e (mg L⁻¹) is the equilibrium concentration of lead Pb(II) in the solution, q_e (mg L⁻¹) is the amount adsorbed at equilibrium, b, and Q_max (mg g⁻¹) are the Langmuir constant related to the energy of adsorption and the maximum amount of the lead ion per unit weight of adsorbent respectively. Maximum adsorption capacity Q_max and value of b obtained are 48.3 mg g⁻¹ and 0.097 respectively, Fig. 10. In this study, the maximum amount of the lead ion per unit weight (Q_max) is higher than those found by 12.32 mg g⁻¹ using chitosan-coated sand and 0.241 mg g⁻¹ using grape stalk waste and lower than the value of 110.35 mg g⁻¹ reported.^5,32

The separation factor R_L was calculated and presented in Fig. 11 to confirm the adsorption process favorability. The adsorption of lead Pb(II) prepared ZnO/talc nanocomposite is favorable with value of R_L between 0 and 1

R_L = 1/(1 + bC_e) (6)

Fig. 11 Separation factor R_L for adsorption of lead ions on ZnO/talc nanocomposite.

3.4.2 Freundlich isotherm. Alternative commonly used empirical equation is Freundlich isotherm and can be expressed as

q_e = K_fC_eⁿ (7)

where K_f and n are empirical constants indicative of sorption capacity and sorption intensity, respectively. From the plot of ln q_e against ln C_e values of K_f and 1/n can be calculated. The value of the 1/n determines the adsorption is favorable.³³

The adsorption data best fitted Langmuir adsorption isotherm since regression coefficient (R²) obtained as is higher than from the Freundlich hence implying monolayer adsorption. The constants of Freundlich and Langmuir isotherms and correlation coefficients calculated from the adsorption data are given in Table 2.

Table 2 Langmuir and Freundlich isotherm parameters

	Langmuir isotherm			Freundlich isotherm
	Qmax (mg g^−1)	b	R^2	Kf	n	R^2
Pb	48.3	0.097	0.99	0.75	2.22	0.98

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The results obtained from this study were compared with other reported materials, Table 3. The capacity data indicated that the reported ZnO/talc nanocomposite has comparable efficiency to those reported materials. In addition, the ZnO/talc nanocomposite has the advantages of being a cost-effective and simple method of preparation, thus, easiness of scaling up the production process.

Table 3 Comparison of adsorption capacities (mg g⁻¹) of lead ions using different adsorbents

Adsorbent type	Adsorption capacities (mg g^−1) (Qmax)	Reference
Carbon aerogel	0.75	35
Zeolite/CuO NCs	45	36
Zeolite/Fe3O4 NCs	50
Modified palygorskite clay	62.10	37
Nanocomposite of carbon nanotubes/silica	13	38
Carbon derived from waste rubber	26	39
ZnO/talc	48.3	This research

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3.5. Mechanism of adsorption

Adsorption mechanism of lead ions onto the nanocomposite can be explained by the electrostatic attraction between the negatively charged sites on the surface of nanocomposite and the positively charged lead ions. The proposed mechanisms for the adsorption of Pb(II) onto ZnO/talc is illustrated in Fig. 12. The lead ions could be attracted by the negative sites on the zinc oxide nanoparticles and on the talc negative sites. The electrons on the surface can promote the formation of hydroxyl that increases the adsorption rate of metals from aqueous solution due to more interactions.^34,40,41 Another mean of attraction is between the lead ions and the hydroxyl groups that are attached to the edge surface of the talc sheets.¹⁶

Fig. 12 Proposed mechanisms for the adsorption of Pb(II) onto ZnO/talc.

4. Conclusions

In this work, ZnO/talc nanocomposite prepared from talc and zinc oxide nanoparticles was effectively used for removal of lead ions from aqueous solutions. The structure and morphology were confirmed by PXRD and FTIR. The ZnO/talc exhibited good adsorption efficiency for the lead ion removal from aqueous solutions comparing with the talc and ZnO alone. The sorption performances were affected by parameters such as agitating time, dosage and pH. The adsorption kinetics was fitted to pseudo-first order and pseudo-second-order rate models, revealing different lead adsorption mechanisms on the nanocomposite, which is attributed to the presence of reactive sites that functionalize the adsorbent surface. The application of the isotherm models to experimental results showed that the adsorption equilibrium data fitted very well to the Langmuir isotherm in the studied concentration range. The maximum adsorption capacity was found 48.3 mg of lead per g of the nanocomposite.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

The authors would like to acknowledge the financial support from Universiti Putra Malaysia (UPM, Serdang, Malaysia).

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