Synthesis, characterization and adsorption of Pb(Ⅱ), Cd(Ⅱ) and Cu(Ⅱ) by red mud/polyacrylic acid/sodium carboxymethyl cellulose hydrogel

Heavy metal pollution in water and pollution hazards caused by red mud disposal are urgent environmental problems to be solved. Red mud waste can be utilized as resources by preparing hydrogels. By employing free radical polymerization, a red mud/polyacrylate/sodium carboxymethyl cellulose hydrogel (RMAAC) was synthesized using red mud (RM) as the main component, sodium carboxymethyl cellulose (SCMC) as the grafting substrate, N, N’-methylene bisacrylamide (MBA) as the crosslinking agent, and potassium persulfate (KPS) as the initiator. The proportions of MBA, KPS, and SCMC were optimized using the response surface method. The adsorption kinetics and isothermal adsorption model were applied to analyze the adsorption characteristics, while XRD, FTIR, SEM-EDS, and XPS characterization methods were employed to study the adsorption mechanism. The results demonstrated that the optimal proportions of MBA, KPS, and SCMC in RMAAC were determined to be 0.15 %, 0.2 %, and 1.5 %, respectively, and the addition of 0.5 % RM to enhance the adsorption performance of the hydrogel. The adsorption processes of Pb(II), Cd(II), and Cu(II) by RMAAC followed the pseudo-second-order kinetic model and the Langmuir model, suggesting chemisorption through a single molecular layer as the main adsorption mechanism. The maximum theoretical adsorption capacities of the three heavy metal ions at 25°C were determined to be 730.16, 292.71, and 215.37 mg/g, respectively. It was found that RMAAC exhibited a higher affinity toward Pb(II) compared to Cu(II) and Cd(II). Characterization analyses revealed that ion exchange and coordination chelation were the predominant mechanisms involved in the adsorption of Pb(II), Cd(II), and Cu(II) by RMAAC.