Synthesis of Eco-Friendly Biopolymer, Alginate-Chitosan Composite to Adsorb the Heavy Metals, Cd(II) and Pb(II) from Contaminated Effluents

Abstract

Efficient removal of Cd(II) and Pb(II) from contaminated water is considered a fundamental point of view. Synthetic hydrogel biopolymers based on chitosan and alginate (cost-effective and eco-friendly) were successfully designed and characterized by highly efficient removal contaminants. The sorbents are characterized by FTIR, SEM-EDX, TGA, XPS analyses and textural properties which are qualified by N₂ adsorption. The sorption properties are firstly investigated by the effect of pH, sorption isotherms, uptake kinetics, and selectivity from multi-metal solution with equi-molar concentration. The sorbent with 1:3 ratios (of chitosan and alginate respectively) is the most effective for metal removal (i.e., 0.81 mmol Cd g^-1 and 0.41 mmol Pb g^-1). Langmuir and Sip's models fitted better the adsorption isotherms compared to the Freundlich model. Uptake kinetics was well fitted by pseudo-first-order rate equation, while the saturation was achieved within 40 min. The sorbent shows good reproducibility through duplicate the experiments with negligible decreasing efficiency (>2.5%). The sorbent was applied for water treatment on samples collected from the industrial area (i.e., 653 and 203 times over the MCL for Cd(II) and Pb(II) respectively according to WHO). The concentration of Cd and Pb was drastically decreased in the effluents as pH increased with removal efficiency up to 99% for both elements at pH 5.8 and SD equivalent 1 g L^-1 for 5 h.

Keywords: cadmium and lead contamination; contaminated water treatment; cost-effective biopolymers; eco-friendly sorbent.

Scheme 1

Synthetic route of the prepared hydrogel composite (the introduced color is referred to as the crosslinking of glutaraldehyde (GA) with chitosan and the ionotropic gelation of Ca²⁺ on alginate moieties).

Figure 1

FTIR spectra of CA#2, CA#3 and CA#4 (A) and CA#3, after loading with Cd and Pb, and after five cycles of sorption desorption (B) (wavenumber: 4000–400 (cm⁻¹)).

Figure 2

Overall XPS analysis for CA#3, after loading with Cd(II) and Pb(II) metal ions.

Figure 3

pH effect on Pb(II) (a) and Cd(II) (b) sorption using CA#2, CA#3 and CA#4 (C₀: 0.958 mmol Cd L⁻¹ for 1st Series and 0.967 mmol Cd L⁻¹ for 2nd Series; C₀: 0.48 mmol Pb L⁻¹ for1st Series and 0.51 mmol Pb L⁻¹ for 2nd Series; sorbent dosage, SD: 1 g L⁻¹; temperature, T: 23 (±3) °C; time: 48 h; agitation speed: 170 (±3) rpm).

Figure 4

Sorption kinetics profile of the pseudo-first (PFORE) for Cd (dark symbols/solid lines) and Pb (dark symbols/dotted lines) (C₀: 100 (±5) mg L⁻¹; SD: 250 mg L⁻¹, pH₀: 5).

Figure 5

Sorption isotherms using CA sorbents (a) CA#2 + Cd, CA#3 + Cd, and CA#4 + Cd; and (b) CA#2 + Pb, CA#3 + Pb, and CA#4 + Pb—Modeling with Langmuir (solid lines), and Sips (dotted lines) equations (C₀: 10–500 mg M L⁻¹; SD: 1 g L⁻¹; T: 23 (±3) °C; time: 48 h; agitation speed: 170 (±3) rpm).

Scheme 2

Suggested interaction mechanism between CA# sorbent and the metal ions.

Figure 6

Comparative studies of the removal efficiencies after treatment of water samples (pH_in, 2.07, 4.04 and 5.8; SD: 1 g L⁻¹; T: 23 (±3) °C; time: 5 h; agitation speed: 170 (±3) rpm) at different pH values.