Eco-inspired synthesis of a melamine formaldehyde-reinforced onion peel biochar/alginate composite for efficient lead ion removal from wastewater

Abstract

The present study focuses on the fabrication of a highly efficient, eco-friendly solid adsorbent, namely melamine formaldehyde-reinforced onion peel biochar/alginate composite (MCAg), along with onion peel-based biochar (C) and biochar/alginate composite beads (CAg). The synthesized adsorbents were characterized using various instrumental techniques. MCAg exhibited a surface area of 309.4 m² g⁻¹, a point of zero charge at pH 6.6, and the presence of multiple surface chemical functional groups as identified by FTIR analysis. The fabricated adsorbents were applied for the batch adsorption of Pb²⁺ ions under different operating conditions, in addition to desorption, reusability, and real-sample studies. MCAg showed the highest Langmuir adsorption capacity (348.9 mg g⁻¹) at pH 6, a solid dosage of 2.5 g L⁻¹, 20 °C, and 50 min of shaking time. The presence of coexisting ions in real polluted samples reduced the adsorption capacity for Pb²⁺ ions by an average of approximately 30%. Ethylenediaminetetraacetic acid was found to be the most effective desorbing agent for Pb²⁺ ions from the adsorbent surfaces. Kinetic and thermodynamic studies confirmed the applicability of the pseudo-first order and van’t Hoff models, indicating a physisorption mechanism and a spontaneous adsorption process. Column adsorption of Pb²⁺ ions was evaluated at three different bed heights (2, 3, and 4 cm) using an initial concentration of 80 mg L⁻¹ and a flow rate of 15 mL min⁻¹, achieving a maximum adsorption capacity of 232.7 mg g⁻¹ at a 4 cm bed height, with the experimental data fitting well with the Yoon–Nelson and Thomas models. The effective adsorption of Pb²⁺ by MCAg is mainly related to the synergistic action of surface functional groups enabling ion exchange, electrostatic attraction, and complexation with Pb²⁺ ions, highlighting the composite's strong potential for polluted water treatment.