Synthesis of Nanosilica for the Removal of Multicomponent Cd2+ and Cu2+ from Synthetic Water: An Experimental and Theoretical Study

Abstract

Copper and cadmium ions are among the top 120 hazardous chemicals listed by the Agency for Toxic Substances and Disease Registry (ATSDR) that can bind to organic and inorganic chemicals. Silica is one of the most abundant oxides that can limit the transport of these chemicals into water resources. Limited work has focused on assessing the applicability of nanosilica for the removal of multicomponent metal ions and studying their interaction on the surface of this adsorbent. Therefore, this study focuses on utilizing a nanosilica for the adsorption of Cd²⁺ and Cu²⁺ from water. Experimental work on the single- and multi-component adsorption of these ions was conducted and supported with theoretical interpretations. The nanosilica was characterized by its surface area, morphology, crystallinity, and functional groups. The BET surface area was 307.64 m²/g with a total pore volume of 4.95×10-3 cm³/g. The SEM showed an irregular amorphous shape with slits and cavities. Several Si-O-Si and hydroxyl groups were noticed on the surface of the silica. The single isotherm experiment showed that Cd²⁺ has a higher uptake (72.13 mg/g) than Cu²⁺ (29.28 mg/g). The multicomponent adsorption equilibrium shows an affinity for Cd²⁺ on the surface. This affinity decreases with increasing Cu²⁺ equilibrium concentration due to the higher isosteric heat from the interaction between Cd and the surface. The experimental data were modeled using isotherms for the single adsorption, with the Freundlich and the non-modified competitive Langmuir models showing the best fit. The molecular dynamics simulations support the experimental data where Cd²⁺ shows a multilayer surface coverage. This study provides insight into utilizing nanosilica for removing heavy metals from water.

Keywords: adsorption; cadmium; copper; heavy metals; multicomponent adsorption; nanosilica.

Scheme 1

Steps in the synthesis of the nanosilica using the sol–gel method.

Figure 1

XRD pattern for the produced nanosilica from a sol–gel method.

Scheme 2

Dehydroxylation of the silica at high temperature.

Figure 2

DSC of the nanosilica particles.

Figure 3

FTIR spectra of the silica NPs.

Figure 4

BET adsorption isotherm and H–J t-plot (inside) of N₂ at 77 K for the produced silica sample.

Figure 5

SEM image of the produced nanosilica.

Figure 6

Adsorption isotherms of copper and cadmium ions using the nanosilica (mass of adsorbent is 0.1 g, volume of solution is 50 mL, temperature is 25 °C, and equilibrium time is 72 h).

Figure 7

The Freundlich, Langmuir, and DR models’ fit for the adsorption equilibrium of (a) Cu²⁺ and (b) Cd²⁺ using the nanosilica (mass of adsorbent is 0.1 g, volume of solution is 50 mL, temperature is 25 °C, equilibrium time is 72 h, and confidence limit is 15%).

Figure 8

Effect of the equilibrium concentration of (a) Cd²⁺ and (b) Cu²⁺ on the multicomponent adsorption of copper (mass of adsorbent is 0.1 g, volume of solution is 50 mL, temperature is 25 °C, and equilibrium time is 72 h).

Figure 9

Cleaned hydrated silica structure: (a) Si (yellow), O (red), H (white), and (b) Cu and Cd ions.

Figure 10

Adsorption location of Cu²⁺ and Cd²⁺ within the silica crystal structure.

Figure 11

Variation in the silica structure’s energies during ions loadings.

Figure 12

Energy distribution and isosteric heat of adsorption for Cd²⁺ and Cu²⁺ by the nanosilica surface.

Figure 13

Theoretical equilibrium isotherms for Cu²⁺ and Cd²⁺.

Figure 14

Unit cell loading of cadmium and copper ions.