Abstract
Levels of Mercury (Hg), Lead (Pb), Cadmium (Cd), Chromium (Cr), Nickel (Ni) & Thallium (Tl) were established in wastewater & soil samples obtained from 8 sites in open drainage channels at Nairobi industrial area, Kenya. Ultra-trace inductively coupled plasma mass spectroscopy (ICP-MS) was used for metal analysis. Temperature, pH & turbidity of wastewater ranged from 16.75 to 26.05 °C; 7.28 to 8.78; 160.33 to 544.69 ppm respectively and within World Health Organization (WHO) allowable limits. Wastewater conductivities in 4 sites ranged from 770 to 1074 µS/cm and above WHO limits at 25 °C. The mean concentrations of the metals in wastewater ranged from 0.0001 to 0.015 ppm in an ascending order of Tl <Cd <Hg <Ni <Cr <Pb. Levels of Cd, Cr & Ni in wastewater were within WHO, World Bank (WB), United States Environmental Protection Agency (US EPA), China, Kenya and India’s Central Pollution Control Board (CPCB) limits while Hg & Tl were above US EPA limits. The mean Pb levels in wastewater (5 sites) were above WHO, US EPA and Kenya allowable limits. Mean levels of the metals in soil samples ranged from 0.085 to 199.99 ppm, with those of Hg, Pb, Cr, Cd & Ni being above WHO limits for agricultural soils. Positive correlations (P < 0.05) for Cd & Hg in soils; and Tl (wastewater) & Cd (soil) were noted. In conclusion, wastewater in open waste channels at Nairobi industrial area had elevated levels of Pb and Hg, while the soil from the same channels had high levels of Hg, Pb, Ni, Cr, and Cd. Good management of Nairobi industrial area effluents is inevitable since it borders densely populated informal settlements which are likely to suffer exposure to toxic wastes. Effective wastewater treatment and reuse is highly recommended.
Highlights
Heavy metals are defined as metallic elements that have a relatively high density compared to water[1]
This study showed that wastewater and soils samples from open waste channels in Nairobi industrial area contained heavy metals
The levels of Hg, Cr, Cd and Ni in wastewater samples were within the allowable limits set by World Health Organization (WHO), World Bank (WB), Kenya, China and India
Summary
Heavy metals are defined as metallic elements that have a relatively high density compared to water[1]. Heavy metals above allowable limits will often lead to disadvantageous effects in humans, other organisms and the environment at large[4]. Allowable safe limits of heavy metals in food samples are associated with low health risks in humans[5,6]. The adverse health effects that are associated with Hg and mercuric compounds in humans includes possible carcinogens; damage of the brain, lungs and kidneys; damage of developing fetuses; high blood pressure or heart rate; vomiting and diarrhea; skin rashes and eye irritation[8]. Hexavalent Chromium compounds including chromates of Ca, Zn, Sr, and Pb are highly soluble in water, toxic and carcinogenic[21,23]. The WHO recommended safe limits for Cr (hexavalent) in wastewater and soils used for agriculture are 0.05 and 0.1 ppm respectively[15,16]
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