| dc.description.abstract | Presence of heavy metals in our environment is of great concern since the heavy metals
bio-accumulate in food chains posing harmful health effects to humans who are
consumers of food grown in such environments. This study sought to determine the
concentration of selected heavy metals (Cd, Pb, Cr, Zn and Cu) in urban stream water,
soil and vegetables (spinach and kale) in Machakos municipality. Water and soil samples
were collected along the rivers under study. Vegetables (spinach and kale) grown along
the rivers, as well as vegetables sold in the market sites in the municipality were also
collected for analysis. All the collected samples were analyzed for heavy metal
concentrations using the inductively coupled plasma mass spectrometer (ICP-MS).
Before heavy metal analysis, the vegetable and soil samples were oven dried, ground and
digested using a tri acid mixture (HNO3, HClO4 and HCl). The water samples were also
digested in a tri acid mixture prior to heavy metal analysis. Physico-chemical parameters
for the river water (temperature, pH, total dissolved solids (TDS) and electrical
conductivity (EC)) were measured on-site using standard portable meters. The data was
analyzed using the Minitab Statistical software, version 19. One -way Analysis of
variance (ANOVA) was used to test for the significant difference (p≤0.05) of heavy
metal concentrations in vegetables, soil, and water among the different sampling sites.
Heavy metal concentrations were compared with acceptable standards by the World
Health Organization (WHO) for irrigation water, agricultural soil and vegetables for
human consumption. Carboxymethyl starch was synthesized and its ability to remove
heavy metals from water through chelation determined. The mean levels of the Physicochemical parameters ranged from 7.5 – 8.45 pH, 21.58 – 23.05 ºC temperature, 577.5 –
865.83 mg/L total dissolved solids (TDS) and 864.83 – 1778.5 µs/cm electrical
conductivity (EC). Mean values of pH were observed to be significantly different (p =
0.000) among the sampling sites. Temperature, pH and TDS were within the WHO
recommended limit for surface water while EC values exceeded the World Health
Organization (WHO) set limit. The mean concentrations (mg/L) for heavy metals in
water were found to be; Cd (0.0005 – 0.0011), Cu (0.0034 – 0.0055), Pb (0.0012 –
0.007), Zn (0.0232 – 0.1351) and Cr (0.0036 – 0.0292), while mean values (mg/kg) in
soil were; Cd (0.0058 – 0.0534), Cu (10.39 – 27.5), Pb (4.23 – 8.35), Zn (20.1 – 28.80)
and Cr (8.17 – 10.03). Mean concentration values (mg/kg) for the heavy metals in
vegetables were; Cd (0.004 – 0.243), Cu (0.909 – 14.5), Zn (9.05 – 26.7), Pb (0.098 –
0.0.867) and Cr (0.112 – 14.0). The levels of heavy metals in water and soil were found
to be within the WHO acceptable standards. The mean concentration of Pb, Zn and Cr in
vegetables from some of the sampling sites exceeded WHO acceptable levels. Pearson
correlation matrix showed positive correlations between heavy metals concentrations in
soil, water and vegetables. Carboxymethyl starch was found to chelate heavy metals to a
greater percentage; 92.31% Cd, 66.90% Pb, 66.19% Zn, 66.18% Cu and 14.37% Cr, thus
a potential material for remediation of heavy metals in water. Presence of heavy metals
(Zn, Pb, and Cr) in spinach and kales signifies a health hazard from consumption of these
vegetables. This study thus recommends proper treatment and disposal of effluent based
on best practices and continuous monitoring of heavy metals in urban stream water, soil
and vegetables as it is critical towards safe guarding public health. | en_US |
