Conclusion
Longan shell, as a waste biomacromolecular substance,
demonstrates very high selectivity of Pb(II) and Hg(II) over
relatively light metal ions. The shell will not cause secondary
pollution, while most synthetic adsorbents and chemically
modified natural sorbents would probably cause unexpected
pollution. The longan shell has not only incomparable environmental
benignity and low-cost advantage compared with all
synthetic and seminatural sorbents but also simpler pretreatment
and stronger adsorbability toward Pb(II) and particularly Hg(II)
than most natural sorbents. Redox sorption mechanism of Pb(II)
and Hg(II) ions onto longan shell was revealed. The successful
elimination of the hazardous Pb(II) ions from both environmentally
polluted rivers and industrial wastewater demonstrates
that longan shell is the most promising in practical wastewater
treatment. The efficient utilization of longan shell waste for the
removal of harmful metal ions and retainment of the nutrition
ions represents a new form of using trash as treasure. Besides,
the entire pretreatment of the shell, which does not involve any
organic solvents or even acids or alkalis, achieves zero emission
and follows the principles of green chemistry. The shell should