ABSTRACT
Selective removal of trace concentrations (mg/L to |Lig/L) of dissolved heavy metals from contaminated water and wastewater remains a challenging and a commonly encountered separation problem facing many industries and publicly owned treatment works (POTW). The use of beads or granules of chemically stable polymeric chelating exchangers in fixed-bed processes have, to a great extent, resolved the problem technically, but they are often found too expensive to justify their applications for heavy metals removal from water and wastewater. Both amorphous and crystalline forms of iron oxides, iron oxyhydroxides, and iron hydroxides have long been known to exhibit high sorption affinities toward dissolved heavy metal cations at alkaline pH [1-4 and many others]. Several researchers have confirmed the potential of various forms of precipitated iron oxyhydroxides (PIO) and iron oxides in removing dissolved heavy metals to a much lower level than achieved by precipitation [5-7]. Both electrostatic (i.e., ion exchange) and Lewis acid-base (i.e., metal-ligand) interactions are the underlying reason for heavy metals' high affinity toward PIO over
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alkali-and alkali-earth metal cations. In a simplistic way, the ion exchange type sorption of dissolved heavy metals (Me2+) on PIO may be presented as follows:
2(=SO-)Na + + Me2+(aq) <-> (=SO-)2Me 2 + + 2Na+(aq) (1)
The overbar denotes the solid phase and = S O ~ denotes a deprotonated surface site present in any of the iron oxyhydroxides. Figure 1 presents a visual interpretation of the forces involved in the metals sorption process where the oxygen atom (Lewis base) may donate a lone pair of electrons to a transition metal ion (Lewis acid).
