Adsorption of lead ions using a modified lignin hydrogel

Adsorbent composites comprising of bentonite/sodium lignosulfonate graft-polymerized with acrylamide and maleic anhydride (BLPAMA) were synthesized, characterized, and used to remove Pb2+ from aqueous solution with a large adsorption capacity. The fundamental adsorption behavior of the as-prepared BLPAMA was studied as a function of adsorbent dosage, initial pH, contact time, temperature, concentration of Pb2+, and additive electrolyte. Results showed that the adsorption of Pb2+ on BLPAMA was temperature-independent but strongly pH dependent. The Pb2+ adsorption kinetics was well described by a pseudo-second order model and the adsorption isotherms closely adhered to the Langmuir model, indicating a monolayer chemical adsorption. Thermodynamic parameters such as Gibbs free energy, enthalpy, and entropy changes were also evaluated. Their values demonstrated that the adsorption of Pb2+ on BLPAMA was spontaneous and endothermic in nature. The competitive adsorption of Pb2+, Cu2+, Cd2+ and Zn2+ in their quaternary metal solution was investigated and the results showed that Pb2+ was preferably adsorbed on BLPAMA. TGA and SEM characterization proved that Pb2+ was adsorbed onto BLPAMA. FTIR and XPS analyses further revealed that the adsorption mechanism of Pb2+ on BLPAMA were mainly through chelation and ion exchange mechanism.