Few-layer graphene-hybrid sulfonate hydrogels for high-efficient adsorption of heavy metal (Pb2+, Ni2+, Cd2+) in water treatment

Heavy metal ions pose a significant environmental threat due to their non-degradability and accumulation to toxic levels. In addressing this challenge, we have designed two novel hydrogels through radical polymerization, an efficient and cost-effective method, using sulfonate groups. One hydrogel remained pristine, while the other was hybridized with few-layer graphene (FLG). The incorporation of FLG interacts with the polymeric network without compromising its thermal stability, as confirmed by Fourier-transform infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA). It also reduces pore size (from 42 to 35 µm), enhances mechanical properties (Young's modulus increased from 32 to 44 kPa), and increases the swelling degree (from 62 to 76), while maintaining a high adsorption capacity. The ability of both hydrogels to adsorb Pb²⁺, Ni²⁺, and Cd²⁺ ions from aqueous solutions was examined. These hydrogels demonstrated high adsorption capacity (q_e), with maximum uptake of 631.7 mg/g for Pb²⁺, 633.3 mg/g for Ni²⁺, and 373.1 mg/g for Cd²⁺ in the pristine hydrogels (VBS) and 540.6 mg/g for Pb²⁺, 615.1 mg/g for Ni²⁺, and 304.9 mg/g for Cd²⁺ in the hybrid FLG hydrogels (VBS_G). Adsorption kinetics studies indicated a fit to the pseudo-second-order model for all metal ions. Adsorption isotherms showed that Pb²⁺, Cd²⁺ and Ni²⁺ follow the Freundlich model. To demonstrate reusability and regeneration, hydrogels with adsorbed ions were introduced into acidic media. Evaluating their performance in various water sources, the hydrogels showcased potential as efficient adsorbents for water purification and agricultural applications, offering a promising solution for contaminated water treatment.