Regenerable hydrogels based on poly(acrylic acid-sodium acrylate-acrylamide) modified by sodium humate for high removal of Pb2+ and Fe2+ ions: metal adsorption kinetics and thermodynamic studies

A novel superabsorbent hydrogel based on poly(acrylic acid-sodium acrylate-acrylamide)/sodium humate poly(AAc-SA-AM)/SH was applied as adsorbent to adsorb metal ions (Pb2+, Fe2+) from the aqueous solutions. The factors affecting adsorption capacity of the poly(AAc-SA-AM)/SH hydrogel such as contact time, pH, temperature, SH content (wt.%), initial concentration of the metal ion, and ionic strength were systematically investigated. Results from the experimental data revealed that an appropriate addition of SH (2.44 wt.%) not only increases the metal ion adsorption of poly(AAc-SA-AM) hydrogel but also improves its regeneration ability. The results showed that the adsorption equilibrium data fitted the Langmuir isotherm better than the Freundlich isotherm. The maximum binding capacity for Pb2+ ion was 198 and 164 mg/g/l for Fe2+ ion for per g of poly(AAc-SA-AM)/SH (SH4) hydrogel (high as compared to many other adsorbents). The changes in thermodynamic parameters were also calculated, and the negative Delta G degrees and Delta H degrees confirmed that the adsorption process was spontaneous and exothermic. The kinetic studies revealed that the adsorption process can be well described by the pseudo-second-order kinetic model.