Adsorptive removal of nickel and lead ions from aqueous solutions by poly (amidoamine) (PAMAM) dendrimers ([Formula presented])

This research is a first-time investigation into the use of poly (amidoamine) (PAMAM) dendrimers (Generation 4: [Formula presented]) as well-designed, highly-branched macromolecules with abundant active amine groups for the adsorptive removal of nickel (Ni[Formula presented]) and lead (Pb[Formula presented]) from aqueous phases. The adsorption capacity and efficiency were studied under various operating conditions including a pH range of 2–7, contact time ([Formula presented]) of 2–12 min, initial heavy metal (Ni[Formula presented] or Pb[Formula presented]) concentration ([Formula presented]) of 50–200 mg/L, and adsorbent dose (m) of 0.04–0.1 g/L were studied for the optimization and modeling of the process. The diameter, structure, and surface characteristics of PAMAM dendrimers were analyzed by Dynamic Light Scattering (DLS), Nuclear Magnetic Resonance (NMR), zeta sizer, Scanning Electron Microscope (SEM) and Fourier Transform Infrared (FTIR) spectroscopy. The adsorption mechanism and the relevant characteristic parameters were investigated by two-parameter isotherm models (Langmuir and Freundlich) and kinetic models (Lagergren's pseudo-first order and Ho and McKay's pseudo-second order). Adsorption thermodynamics of metal ions onto PAMAM dendrimers were also studied at various temperatures of 298, 308, 318, 328, and 338 K. The results showed that adsorption kinetics of Ni[Formula presented] and Pb[Formula presented] were very well described by Ho and McKay's pseudo-second order kinetic model ([Formula presented]). The adsorption isotherms of studied metal ions followed Freundlich model ([Formula presented]). According to the Langmuir model, the maximum removal capacity of PAMAM dendrimers for Ni[Formula presented] and Pb[Formula presented] ions were 1626 and 1783 mg/g, respectively. The thermodynamic evaluations demonstrated that the removal of Ni[Formula presented] and Pb[Formula presented] ions onto PAMAM dendrimers was endothermic with positive enthalpy changes of 17.048 and 16.995 kJ/mol and spontaneous with negative free energy values of −11.063 and −10.107 kJ/mol for Ni[Formula presented] and Pb[Formula presented] ions, respectively. The result indicated that PAMAM dendrimers could be applied a superior adsorbent for Ni[Formula presented] and Pb[Formula presented] removal from aqueous solutions.