The Speciation of Selected Trace Metals in Nairobi River Water, Kenya

Abstract

Metal ions form complexes with naturally occurring ligands released from industrial
effluent. The complexes are transported and enter the environment and biological
system leading to environmental degradation and health problems. This contribution
investigates the speciation of trace metals in water samples collected from Nairobi
River. Heavy metals (Pb, Mn, Cu, Fe, Cr and Zn) were determined using spectroscopic
techniques whereas sulfaver 4 method, diazotization, and titration methods were used
to determine the concentration of SO42+, NO2-
, F- and Cl- respectively. It was found that
~ 69.8% of total iron was in oxidation state III; the dominant species being Fe2+,
Fe(OH)SO4, [FeF4]-
, FeSO4, Fe2+ and [Fe(OH)2]+ during both the dry and wet seasons.
Manganese was found exclusively in oxidation state II (100%) in which some of its major
chemical forms were Mn2+ and [MnF6]4-
. Copper was present mostly in oxidation state
II (Cu2+) while lead and zinc existed chiefly as ([Zn(SO4)4]6- and [Pb(SO4)3]4-
)
complexes. Chromium was trivalent with its main complexes being [Cr4(OH)6]6+ and
Cr(OH)(SO4)). Traces of free metal ions (Cu, Fe and Mn) were found in Lenana section
of Nairobi River. Traces of free metal ions are the most aggressive water toxicants due
to their high solubility in biological systems.