Adsorption Study of Hg ions onto polydopamine nanoparticles

A good number of industries are responsible for the release of heavy metals into the environment through their wastewater [1]. Some of these heavy metals are extremely toxic elements and can seriously affect many living species especially human; therefore their removal from wastewater is important. In recent years studies on biopolymers which bind metals ions have increased significantly [2-3]. polydopamine (PD) nanoparticles exhibit latent reactivity toward amine and some metal ions. Because of this fact, PD nanoparticles were used as an adsorbent for adsorption of Hg ions from aqueous solutions. The effect of pH, ionic strength, adsorbent dosage, temperature, initial concentration, foreign ions and contact time on adsorptin adsorbates, from aqueous solutions using PD nanoparticles was investigated through batch experiments. The experimental results were analyzed using Langmuir I, II, III, IV and V and Freundlich adsorption isotherms. The results obtained are well fitted in the linear forms of Freundlich and Langmuir adsorption isotherms but Langmuir isotherm is better than Freundlich isotherm. The Langmuir adsorption capacity (qm) were found to be 33/78mg Hg of the adsorbent. The batch sorption kinetics was studied for pseudo-first-order and pseudo-second-order reaction. The kinetic data were fitted well to pseudo-second-order model. Moreover the thermodynamic parameters of the adsorption (the Gibbs free energy, entropy, and enthalpy) were studied. Our results demonstrate that the adsorption process was spontaneous and exothermic under natural conditions. The adsorption of Hg (II) on PD nanoparticles is obviously influenced by the cation ions in the suspension. At pH=5 , the adsorption percentages of Hg (II) on PD nanoparticles under the same pH values are in the following sequence: Ag+ >Cu >Zn >Cd >Ni >Fe. This study revealed that PD nanostructure was an effective adsorbent for removal of Hg ions from aqueous solutions.