Effective removal of heavy metals from aqueous solution using CS/OMMT nanocomposite synthesized by gamma irradiation

2.1 Materials

Pronova Biopolymer, Inc. (USA) supplied the chitosan (CS). We calculated that the degree of deacetylation and molecular weight to be 85% and 50,000 g/mol, respectively. Aldrich Chemical Co.) supplied the triethylamine employed in this work, and it was used in its as-received laboratory grade form. Sodium montmorillonite was the clay mineral tested. Samir Tech. Chem. Pvt. Ltd. supplied lead nitrate Pb(NO₃)₂ with a purity of more than 99% for use in the adsorption investigations. As received, metal salts were used.

2.2 Synthesis of reactive interposing agents for clay

In this investigation, we used Mulvancy and Chang's (Mulvaney and Chang, 1977) technique for synthesising vinyl benzyl triethyl ammonium chloride (VBTAC).

2.3 OMMT preparation

To prepare OMMT, We followed the cation exchange procedure using the previously described methods by Al-Sigeny et al (Al-Sigeny et al., 2009).

2.4 Nanocomposites of CS/OMMT synthesis

We made CS/OMMT nanocomposites by using radiation cross-linking. This method involves dissolving 0.5 g of CS into 30 mL of 1 wt% acetic acid solution. This yields a 3 wt% CS solution. The CS (5 mL) was mixed with several concentrations of nanoparticles (0% to 4% OMMT) and then dispersed using ultrasonic energy for around 30 min at room temperature. Then, after 10 min, the mixture was purged with nitrogen to get rid of the oxygen. The compound was subsequently exposed to radiation at the King Abdulaziz City for Science and Technology (KACST) in Riyadh, Saudi Arabia, using a 60 Co-ray source type GC-220 supplied by Nordion INT-INC, Ontario, Canada. The ingredients were allowed to cool and then cast onto a glass plate after the reaction was complete. After the solvent was removed, films were cast from the remaining samples (Abou Taleb et al., 2012).

2.5 Characterization

The JASCO- 4600 spectrometer was used to get the FT-IR spectra. Scanning electron microscopy (SEM) provided by JEOL, Tokyo, Japan, was used to look at the surface morphology of the copolymer.

2.6 Adsorption experiments

The wastewater was manufactured in a controlled environment. Analytical-grade CuSO₄·5H₂O was used to make the copper stock solution (0.5 M). The experiments employed recently prepared copper (II) working solutions derived from the initial solution 5 to 50 mg/L.

The technique of intake was performed in batches. For every adsorption test, the requisite adsorbent mass (0.15–0.5 g) is combined with 50 mL of copper solution. Initial pH was brought to the target range by adding hydrochloric acid (0.1 mol/L) and sodium hydroxide (0.1 mol/L) solutions, respectively. After that, you give the solution 10 min of room temperature stirring at a pace of roughly 150 revolutions per minute. The quantification of residual metal in the solution was performed by UV spectrophotometer. Three independent replicates of each experiment were conducted for each analysis. The amount of the ion Cu²⁺ removed was then calculated as: Metal removed (mg/g(=C₀–C₁/W

where C₀, C₁ are the concentrations of Cu²⁺ ions before and after uptake, respectively.

The study involved the preparation of multiple solutions containing 50 mg/L Cu²⁺ at an adsorbent dose of 0.3 g/L. The initial pH of the solutions was varied to drop within the range of 3–8. This experimental design enabled the investigation of the power of pH on the release of Cu²⁺ ions.

2.7 Reusability and stability

Solutions of HNO₃ and NaCl was used to perform the desorption experiments, where 10 mL of 0.01 mol/L nitric acid (HNO₃) and sodium chloride (NaCl) solutions were used under 1 min sonication to desorb Pb²⁺ from 50 mL of 50 mg/L Pb(II) solution. After the extraction of the nanocomposite from the desorption medium, an assessment was conducted to determine the concentration of Pb²⁺ ions. The efficacy of the CS/OMMT's reusability was evaluated through the utilisation of a 10 mL solution of 2 mol/L NaOH, which was subjected to 1 min of sonication and washed with deionized water (Abou Taleb et al., 2012). The aggregate level of uncertainty across all experiments was determined to fall within the interval of 3–4 percentage.

3.1 FTIR analysis

FTIR analysis of both CS and CS/OMMT nanocomposites are shown in Fig. 1. The FTIR spectra exhibits the band at approximately 3500 cm⁻¹, indicating the superimposition of the stretching frequency of N–H and O–H groups. The spectral features observed at 2930 and 2860 cm⁻¹ are associated to stretching of the C–H bond present in the –CH₂ groups of CS. The band at 1650 cm⁻¹ is attributed to the vibrational deformation of the amino (–NH₂) functional groups. The spectral feature observed at approximately 1580 cm⁻¹ is associated to the N–H deformation of –NH₂ moieties. The stretching vibration of C-N is attributed to the peak observed at 1425 cm⁻¹. This outcome validates the existence of CS. The benzene ring of OMMT is identifiable by the band at 1450 cm⁻¹, as reported in reference (El-Sigeny et al., 2014). The infrared spectra obtained through Fourier-transform infrared spectroscopy (FTIR) of the CS/OMMT composite material, as depicted in Fig. 1, demonstrate the amalgamation of the absorption bands that correspond to the existence of OMMT and amine groups of chitosan. The salient spectral features to observe in nanocomposites are the –NH₂ and Si–O moieties present on the OMMT architecture. The shifted bands of –NH₂ groups at 1549 cm⁻¹ and those at 1021–1080 cm⁻¹ linked with Si–O groups of -MMT support the hydrogen bonding connection between NH³⁺ and the negatively charged clay sites (Oviedo Mendoza et al., 2015). Fig. 1 shows that pure CS has no C-O bond stretching vibration at 1073 cm⁻¹.

Close

Fig. 1

FTIR spectra of CS and CS/OMMT nanocomposite.

Export to PPT

3.2 Microstructural analysis

Fig. 2a depicts the surface morphology of the CS/MMT nanocomposite, revealing a substantial layered structure with interlayer spaces. Fig. 2b depicts the minute particles present on the surface of the CS/OMMT. The Pb(II) ions can be ascribed to the small size of particles and the porous structure was observed.

Close

Fig. 2

SEM of the prepared nanocomposites CS/OMMT (a) before adsorption of Pb ions and (b)) after adsorption of Pb ions.

Export to PPT

3.3 Adsorption kinetics

The duration required for the remediation of metals present in water bodies is a crucial aspect from an economic standpoint. Fig. 3 shows an impact of contact time on the adsorption capacities (mg/g) of CS/OMMT nanocomposite at varying initial metal concentrations. The process of adsorption was observed for a duration of 120 min. Fig. 6 illustrates the kinetics of adsorption, whereby there was an initial rapid increase followed by attainment of equilibrium after 60 min. During the initial phases, there was a notable surge in rates owing to the greater availability of adsorption sites on the adsorbents. During the initial phases of the process, a significant quantity of functional groups (–OH and –NH₂) are present and can participate in complexation reactions with metal ions (Niu et al., 2017). The adsorption process experiences a deceleration over time owing to the increased diffusion range of metal into the inner surface of CS/OMMT. The gradual increase in the adsorption curve during later stages can be attributed to a slow diffusion, as stated in ref. (Azzam et al., 2016). The presence of repulsive forces renders the approach of Pb²⁺ ions towards the surface of CS/OMMT challenging. The duration of equilibrium exhibited a minor increase upon elevation in the metal ion concentration. This could be ascribed to the possibility of a greater number of Pb²⁺ ions competing for binding with CS/OMMT functional groups.

Close

Fig. 3

Effect of contact time on adsorption capacity using CS/OMMT nanocomposite at different initial concentrations of Pb(II). The adsorbent mass was 0.5 g/L at a temperature of 25 °C and a medium pH of 5.5.

Export to PPT

In order to clarify the adsorption characteristics of Pb²⁺ ions on CS/OMMT, the empirical data was subjected to fitting procedures utilising both pseudo 1st and 2nd order rate models. The kinetic models known as pseudo 1st and 2nd order are mathematically represented in reference (Rozaini et al., 2010; Abou Taleb et al., 2015) $$\log \left(q_e-q_t\right)=\log q_e-\frac{k_1}{2.303}t$$ $$\frac{t}{q_t}=\frac{1}{k_2{q}_e^2}+\frac{1}{q_{e}t}$$

Where q_e (mg/ g) represents the adsorption capacity, q_t (mg / g) is the adsorption capacity at any time t, and k₁ (min⁻¹) and k₂ (gm g⁻¹ min⁻¹) are the rate constants, respectively. The kinetic parameters obtained by fitting the data using the two models, as shown in Fig. 4, are presented in Table 1. The R² coefficients derived from the pseudo 2nd order rate model demonstrate a strong correlation, approaching a value of one. Furthermore, the computed q_e values exhibit a high degree of concurrence with the corresponding empirical values (q_e, exp). The findings of the experiment suggest that the adsorption mechanism of Pb²⁺ ions onto CS/OMMT nanocomposites is attributed to the chemisorption process, which is consistent with the hypothesis of the 2nd order model. The deduction postulates that the covalent bond formation transpires via electron sharing between the adsorbent and metal ion, as referenced in source 22. The findings suggest that a decrease in the competition among metal ions for the binding of functional groups in nanocomposites is indicated by higher values of k₂ at lower concentrations. (Futalan et al., 2011).

Close

Fig. 4

(a) pseudo-first-order kinetics and (b) pseudo-second-order kinetics for the adsorption of Pb(II) ions on CS/OMMT nanocomposite.

Export to PPT

3.4 Effect of adsorbent dosage

Fig. 5 shows that the adsorption capacity of lead ions exhibits a gradual increase with the rise in the quantity of adsorbent from 0.1 to 0.5 g. The adsorption capacity exhibits a notable increase with a corresponding rise in the quantity of CS/OMMT adsorbent. The observed increase can potentially be ascribed to the substantial surface area of the adsorbent and the existence of functional moieties, as reported in references (Ferrero et al., 2014) and (Dinu and Dragan, 2010). Furthermore, the augmentation of active adsorption sites within the adsorbent could contribute to the expeditious escalation in the adsorption capability. The findings indicate that the various levels of adsorbent dosage are sufficient for the elimination of Pb²⁺ ions from the wastewater.

Close

Fig. 5

Effect of adsorbent dosage on the adsorption of Pb(II) on CS/OMMT nanocomposite.

Export to PPT

Close

Fig. 6

Ph effect on the adsorption of pb²⁺ on CS/OMMT nanocomposite.

Export to PPT

3.5 Effect of pH

The sensitivity of the adsorption capacity is observed with respect to the pH of the solution containing metal ions. The investigation of the adsorption capacities of Pb²⁺ ions on the CS/OMM nanocomposite was conducted across a range of pH values (pH = 2 to 7). Fig. 6 illustrates a gradual increase in the value of qe, ranging from 30.78 to 42.93 mg g-1, as the pH of the medium increases from 2 to 5.5. Under acidic conditions, there is an increased availability of protons that can bind to the amino groups present in chondroitin sulphate (CS). The adsorption of Pb²⁺ ions onto the nanocomposite may be reduced due to the electrostatic repulsion between Pb²⁺ and NH³⁺ ions. A decrease in q_e is observed when the pH surpasses 5.5. The precipitation of Pb(OH)₂ commences when the pH value surpasses 6. The examination of Pb(II) ion adsorption was not conducted beyond pH 6. The findings presented herein are consistent with prior research on the adsorption of lead onto chitosan nanocomposites (Kanchana et al., 2012). This finding additionally corroborates the chelation of metal onto the chitosan composites that were prepared, as previously reported in ref. (Azzam et al., 2016; Sheshmani et al., 2015). The findings indicate that the CS/OMMT nanocomposite possesses promising adsorption properties for the remediation of heavy metal-contaminated water sources.

3.6 Adsorption isotherm

The initial Pb²⁺ concentration was manipulated from 50 to 150 mg L⁻¹ to generate adsorption isotherms to estimate CS/OMMT's maximum adsorption capability. The isotherm data was analysed utilising the Langmuir and Freundlich isotherms, both of which are widely recognised. The utilisation of Langmuir and Freundlich isotherm models is widespread in the characterization of adsorbent materials in relation to their adsorption properties (Niu et al., 2017). The following linear equation represents the Langmuir model: $$\frac{C_e}{q_e}=\frac{C_e}{q_{\max }}+\frac{1}{\mathit{KL}{q}_{\max }}$$

Where, q_m is the Langmuir isotherm (LI) constants and K_L is the rate of adsorption, respectively. The Freundlich model is expressed as, $$\ln q_e=\ln k_F+\frac{1}{n}\ln C_e$$

Where, K_f is the Freundlich constant and n is the heterogeneity factor, respectively. The slope and intercept of the linear plot of C_e/q_e vs. C_e (Fig. 7a) and lnqe vs. lnC_e (Fig. 7b) determine these constants. Table 2 lists all parameters. Table 2 shows that the Freundlich isotherm correlation coefficient R² (0.9889) is lower than Langmuir isotherm R² (0.999), which is confirming that it better fits for Pb²⁺ ions adsorption on CS/OMMT. Pb²⁺ ions adsorbing onto CS/OMMT formed a monolayer on a homogenous surface. Metal ions adsorb well because Pb²⁺,’s 1/n was between 0 and 1.

Close

Fig. 7

Langmuir (a), and Freundlich (b) adsorption isotherms for Pb²⁺ adsorption on CS/OMMT nanocomposite.

Export to PPT

3.7 Reusability and stability

The practical application of an adsorbent is contingent upon its desorption, regeneration, and reuse performance. The desorption experiments were conducted using solutions of HNO₃ and NaCl. The CS/OMMT nanocomposite was regenerated using NaCl due to its robust chelating capability, as documented in prior studies (Niu et al., 2017; Javanbakht et al., 2016). The phenomenon of desorption observed in the NaCl solutions is ascribed to the complexation of Pb²⁺ ions. The phenomenon of desorption within the HNO₃ solution is primarily ascribed to the ion exchange process. The graphical representation in Fig. 8 demonstrates a discernible reduction in the adsorption capacity of Pb²⁺ ions after each utilization. The fields of computer science and operations and materials management technology demonstrate a high level of effectiveness in the reutilization of resources for the purpose of purifying water contaminated with Pb²⁺ ions.

Close

Fig. 8

Desorption and reusability studies of the adsorbent (CS/OMMT).

Export to PPT