Sustainable Pb²⁺ adsorption using facilely separable alginate-derived metal-doped carbon aerogel beads

The growing contamination of water resources with toxic metals obliges the development of sustainable and efficient removal technologies. In this study, a facilely separable alginate-derived metal-doped carbon aerogel beads were synthesized and tested for Pb²⁺ adsorption from aqueous solutions. Sodium alginate was utilized as a renewable carbon source and crosslinked with cobalt (Co), zinc (Zn), or both (CoZn) to form aerogel beads, which were subsequently carbonized under an inert atmosphere. The resulting carbon aerogels were characterized using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), attenuated total reflectance Fourier-transform infrared spectroscopy (ATR-FTIR), X-ray photoelectron spectroscopy (XPS), and nitrogen adsorption analysis. SEM revealed a highly porous, three-dimensional structure, while EDX and XPS confirmed the presence of carbon, oxygen, and the respective metals. Among the samples, Co-Zn-co-doped carbon aerogel (Co-Zn-CA) exhibited the highest surface area (50 m²/g) and superior Pb²⁺ adsorption capacity (157.4 mg/g) according to the Langmuir model. Adsorption kinetics followed the pseudo-second-order model, suggesting chemisorption as the primary mechanism. The process was pH-dependent and showed minimal interference from ionic strength. Nearly complete Pb²⁺ removal (99.2 %) was achieved within 90 min using 1.0 g/L of Co-Zn-CA at pH 5.5 and room temperature. Thermodynamic studies confirmed the exothermic and spontaneous nature of the adsorption process. Mechanistic investigations via XPS revealed that Pb²⁺ removal occurred primarily through ion exchange and complexation with surface functional groups. The findings demonstrated that Co-Zn-CA is a promising, sustainable adsorbent for Pb²⁺ removal, offering high efficiency, straightforward separation, and potential for practical wastewater treatment applications.