Removal of Mercury (II) and Lead (II) from Aqueous Media by Using a Green Adsorbent: Kinetics, Thermodynamic, and Mechanism Studies

Abstract

In this study, a simple and highly efficient method for stabilizing the different combinations of citric acid and sodium hydroxide on the surface of a green adsorbent was presented as the bio-originated composite for the elimination of Hg(II) and Pb(II) ions from aqueous media. The removal behavior of the green adsorbent was assessed as a function of initial pH, changing the contact time, initial pollutants concentration, temperature, cleaning process, and amount of adsorbent. The synthesized bio-originated composite was characterized using different physicochemical techniques. The metals uptake of synthesized bioadsorbent increased as a result of chemical modification, and the highest adsorption capacity [61.54 mg g−1 for Pb(II) and 45.34 mg g−1 for Hg(II) ions] was achieved by citric acid (CA) and after further reaction with sodium hydroxide (NaOH) again (B─ CA─ NaOH). The sorption capacity increased in the order of B─CA─NaOH>B─NaOH─CA>B─NaOH>B─CA>pristine bone, with increasing metal concentration. The equilibrium sorption data obeyed a Langmuir–Freundlich isotherm type model. The kinetic data of the adsorption followed the mechanism of the pseudo second order model. The thermodynamic experiments indicated that the removal of metal ions was feasible, endothermic, and spontaneous. The findings of this investigation suggest that the combined modification of green adsorbent with CA and NaOH could persuade more active sites on the surface of the green adsorbent (the carboxylic acid groups of immobilized CA convert to their sodium salt form, −COO−Na+), which indicate very effective tendency in order to react with Pb(II) and Hg(II) ions. This study suggests that B─ CA─ NaOH has the potential to become an effective and economical adsorbent for the removal of heavy metal (HM) ions.