Longan shell as novel biomacromolecular sorbent for highly selective removal of lead and mercury ions

Strongly selective adsorbability of natural longan (Euphoria longan) shell as a biomacromolecular sorbent toward heavy metal ions from aqueous solutions has been discovered and successfully optimized by facilely controlling the shell dosage, sorption time, pH value, and ion concentration. The sorption well-fits the Langmuir isotherm and obeys the pseudo-second-order kinetics. The maximum adsorptivity of Pb(II) and Hg(II) is 99.3 and 99.1%, respectively, with short equilibrium sorption time. The redox sorption mechanism of Pb(II) and Hg(II) ions onto longan shell was proposed on the basis of IR and X-ray diffraction analyses. In particular, a slow but durative redox interaction between Pb(II) ions and longan shell was revealed even at a solid state, resulting in the formation of Pb(syn) crystals onto longan shell. Competitive sorption suggested that the shell demonstrated a preferential sorption to Pb(II) over Hg(II) in their mixture solution, despite higher adsorptivity of Hg(II) than Pb(II) in corresponding pure solutions. The shell displayed highly selective sorption to Pb(II) over light metal ions with an adsorbability order of Pb(II) > Hg(II) >> Zn(II) > Cu(II) > Ca(II) > Mg(II) >> Fe(III,II), indicating that the shell could efficiently purify drinking water and nutritious liquids by just eliminating the harmful metal ions and still retaining the nutritious metal ions. The printery sewage and polluted river water both meet the discharge standard after being treated once with longan shell, and even satisfy the standards for irrigation/fishery and drinking water, respectively, after being treated twice. A large amount of functional groups like hydroxyl, ether, and carbonyl in the longan shell, as well as its intrinsic hydrophilicity, are responsible for the strong adsorbability toward the heavy metal ions. Not only does the longan shell have low cost and have inherent environmental friendliness in contrast to all synthetic and even seminatural sorbents but it also has stronger adsorbability toward Pb(II) and particularly Hg(II) ions than the most natural sorbents reported, making longan shell attractive as a highly cost-efficient sorbent for the selective removal of hazardous heavy metal ions.