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2011-Sustainable Industrial Processing Summit
|Publisher:||Flogen Star OUTREACH|
|ISSN:||2291-1227 (Metals and Materials Processing in a Clean Environment Series)|
Contamination of water bodies by heavy metals is an established problem and several studies have been conducted to deal with it. South Africa is amongst those countries whose water systems are most affected as a result of intensive mining activities. This research was dedicated to the development of insoluble chelating polymers for use as adsorbents to abstract heavy metal ions from mining and industrial wastewater. Branched polyethylenimine, well known for its metal chelating potential, was cross linked by epichlorohydrin in order to convert it into a water-insoluble form for direct use as an adsorbent. Its surface was also modified for selective removal of specifically-targeted heavy metal ions. The binding affinity of the synthesized materials to heavy metal ions has been determined as well as their ability to be regenerated for reuse. These processes demonstrated that cross-linked polyethylenimine exhibited good complexation ability with high affinity to Cr and some divalent metal ions such as Fe, Zn, Ni, being observed, whereas the phosphonated polyethylenimine showed high selectivity for As, Mn and uranyl ions and the sulfonated polyethylenimine on the other hand exhibited high affinity to Se, and Hg. The difference in selectivity of different metal ions for each polymer can be explained based on their functional groups. The chelating group in cross-linked polyethylenimine is -NH where the nitrogen atom acts as a Lewis base and donates the electrons for metal cations. This mechanism implies the poor ability of polyethylenimine to bind oxo- anions in the case of Se and As while the sulfonated polyethylenimine showed high affinity to Se, attributed to the replacement of sulfate as the functional group. The high selectivity of phosphonated polyethylenimine to uranyl ions is due to the phosphate group being a hard ligand (hard base) and U is a hard metal (hard acid). The adsorption behavior of these polymeric materials involved more than one mechanism such as complexation, normal surface charge exchange, and anion replacement. So far, the developed materials showed promising results and good potential to be applied successfully for remediation of heavy metal-polluted waters.