Titania‑Coated Silica Enhanced with Sodium Alginate: A Superior Sorbent for Cu(II), Zn(II), Cd(II), and Pb(II) Removal

Abstract

A novel bio‑hybrid composite—titania‑coated silica (ST20) further modified with sodium alginate (ST20‑ALG)—has been synthesized as an advanced adsorbent for heavy‑metal ions. The adsorption performance against Cu(II), Zn(II), Cd(II), and Pb(II) was evaluated across varying concentrations, pH, temperature, and contact time. Equilibrium data were fitted to Langmuir and Freundlich isotherms; kinetic data were analysed using pseudo‑first‑order, pseudo‑second‑order, intraparticle diffusion, and Elovich models. ST20‑ALG exhibited markedly higher sorption capacities: 22.44 mg g⁻¹ (Cu²⁺), 19.95 mg g⁻¹ (Zn²⁺), 18.85 mg g⁻¹ (Cd²⁺), and 32.49 mg g⁻¹ (Pb²⁺). Structural characterization via N₂ adsorption–desorption, ATR‑FTIR, SEM‑EDS, and pHₚzc analyses confirmed successful surface modification and provided insights into the adsorption mechanisms.

Background

Heavy metals released through industrial effluents and sewage pose significant environmental and health risks due to their toxicity and propensity for bioaccumulation. Conventional removal techniques—ion exchange, chemical precipitation, membrane filtration, and electrocoagulation—often suffer from high operational costs and limited efficacy. Adsorption, in contrast, remains the most cost‑effective and versatile approach for treating contaminated waters.

Silica, with its high surface area, low cost, and facile functionalization, has become a popular support for adsorbent development. Titanium dioxide (TiO₂) offers additional advantages: chemical inertness, non‑toxicity, high surface area, and strong affinity for a wide range of pollutants, including heavy metals. However, nanosized TiO₂ tends to aggregate, reducing its effective surface and complicating recovery. Immobilizing TiO₂ onto a silica matrix via sol‑gel synthesis mitigates aggregation while preserving the favorable properties of both oxides.

Biopolymers such as alginate, cellulose, and chitosan have been employed as carriers or modifiers to enhance sorption performance, improve mechanical stability, and provide additional functional groups for metal binding. Sodium alginate, in particular, offers carboxylate sites capable of chelating divalent cations and can cross‑link with calcium to form robust hydrogels. The combination of TiO₂‑silica composites with alginate represents a simple, inexpensive strategy to boost adsorption capacity and prevent nanoparticle aggregation.

Prior studies have demonstrated the efficacy of TiO₂‑based sorbents for metals like arsenic, antimony, lead, and cadmium, as well as the benefits of alginate‑modified systems for dye and metal removal. Building on this foundation, the present work synthesizes a titania‑coated silica–alginate composite (ST20‑ALG) and systematically evaluates its performance against Cu²⁺, Zn²⁺, Cd²⁺, and Pb²⁺, including kinetic, isotherm, and mechanistic analyses.

Methods

Materials

Fumed silica A‑50 (S = 50 m² g⁻¹) served as the silica precursor. Titanium isopropoxide (TTIP) (98 %) was used as the TiO₂ source. Sodium alginate (ROTH) and calcium chloride hexahydrate (CHEMPUR) provided the alginate matrix and cross‑linker, respectively.

Composite Synthesis

Silica A‑50 was dispersed in 2‑propanol at 40 °C and stirred. TTIP in 2‑propanol (heated to 200 °C) was added dropwise, followed by water to trigger hydrolysis. The mixture was then heated to 80 °C to form amorphous TiO₂, then to 110 °C to evaporate the solvent. Calcination at 800 °C for 1 h yielded the ST20 composite.

ST20 was subsequently modified by immersing it in 1 % sodium alginate, then dropwise into 2 % CaCl₂ under peristaltic pumping (2.5 cm³ min⁻¹). After 24 h, the resulting beads were rinsed with distilled water and designated ST20‑ALG.

Characterization Techniques

ATR‑FTIR (4000–400 cm⁻¹) confirmed functional groups. Nitrogen adsorption–desorption at 77.35 K (ASAP 2020) provided BET surface areas and pore volumes. SEM‑EDS (Quanta 3D FEG) and TEM (JEM100CX II) revealed morphology and elemental distribution. The point of zero charge (pHₚzc) was determined by the drift method, measuring pH change after equilibrium in NaCl solutions (0.01 M) across pH 1–14.

Adsorption Experiments

Batch studies used 0.1 g sorbent in 20 cm³ solutions (50–250 mg L⁻¹) of Cu²⁺, Zn²⁺, Cd²⁺, or Pb²⁺. Solutions were shaken (180 rpm, 7 mm amplitude) for 1–240 min, then analyzed by atomic absorption spectroscopy. Effects of sorbent dose, temperature (20–60 °C), and initial concentration were systematically varied. Kinetic models—pseudo‑first‑order, pseudo‑second‑order, intraparticle diffusion, and Elovich—were fitted to the data.

Results and Discussion

Adsorbent Characterization

BET surface area of ST20 was 53 m² g⁻¹, closely matching the raw silica. Pore size distributions differed between ST20 and ST20‑ALG due to polymer infiltration, as shown in the nitrogen isotherms (Fig. 1). ATR‑FTIR spectra confirmed the presence of Si–OH, Ti–O–Si, and C–O vibrations, with characteristic bands at 3605, 1067, and 935 cm⁻¹. SEM images revealed a “brain‑like” porous surface for ST20, which became more uniform and bead‑like after alginate coating. EDS mapping confirmed Ti and Si distribution.

Effect of pH

Adsorption increased steadily as pH rose from 2 to 6 for all metals on ST20‑ALG (Fig. 4). Maximum capacities (>95 %) were achieved for Pb²⁺ and Cd²⁺ between pH 5–6. The pHₚzc values were 7.8 (ST20) and 8.2 (ST20‑ALG), indicating that cation uptake is favored above these pH thresholds.

Kinetic Behaviour

Adsorption reached 80 % within 60 min and plateaued at 99 % after 240 min. Pseudo‑second‑order kinetics provided the best fit (R² > 0.999), pointing to chemisorption as the rate‑determining step. Intraparticle diffusion plots displayed three distinct stages, suggesting boundary‑layer diffusion, pore diffusion, and pore‑size limitation sequentially. The Elovich model was less suitable (R² ≈ 0.9).

Isotherm Modelling

Langmuir fitting yielded the highest R² values, supporting monolayer adsorption on a homogeneous surface. Maximum sorption capacities (mg g⁻¹) were: Cu²⁺ = 22.44, Zn²⁺ = 19.95, Cd²⁺ = 18.85, Pb²⁺ = 32.49 for ST20‑ALG; slightly lower for ST20 (20.26, 17.63, 16.73, 26.89). When expressed in mmol g⁻¹, capacities decreased with increasing atomic weight, highlighting the stronger affinity of Cu²⁺ and Zn²⁺ for the composite surface. Freundlich and Dubinin‑Radushkevich models offered lower correlation, reinforcing the Langmuir assumption.

Co‑existing Anion Effects

Cl⁻ and NO₃⁻ (100 mg L⁻¹) were tested for their influence on metal uptake. Cl⁻ significantly reduced Cu²⁺ adsorption, whereas NO₃⁻ had a minor effect. Zn²⁺ adsorption remained unchanged, and Cd²⁺/Pb²⁺ were unaffected by NO₃⁻ but decreased in the presence of Cl⁻. These observations underscore the complex interplay between electrolyte composition, surface charge, and diffusion processes.

Conclusions

The TiO₂‑silica–alginate composite (ST20‑ALG) demonstrates superior performance in removing Cu²⁺, Zn²⁺, Cd²⁺, and Pb²⁺ from aqueous solutions compared to the unmodified ST20. Key advantages include:

• Higher maximum capacities (up to 32.49 mg g⁻¹ for Pb²⁺).
• Rapid kinetics: 80 % removal within 60 min, equilibrium at 240 min.
• Robust adsorption governed by chemisorption, as confirmed by pseudo‑second‑order kinetics.
• Effective in the presence of competing anions, with predictable selectivity.

These findings confirm that simple, low‑cost modification of nanosized TiO₂–silica with sodium alginate yields a practical, high‑capacity adsorbent suitable for industrial wastewater treatment.

Abbreviations

ATR

Attenuated Total Reflectance

D‑RM

Dubinin–Radushkevich Isotherm Model

EM

Elovich Kinetic Model

FM

Freundlich Isotherm Model

FTIR

Infrared Spectroscopy

IPD

Intraparticle Diffusion Model

LM

Langmuir Isotherm Model

PFO

Pseudo First Order Model

PSO

Pseudo Second Order Model

S_BET

Specific Surface Area

SEM

Scanning Electron Microscopy