ANALYSIS OF FLUORIDE AND SELECTED HEAVY METALS IN KALES (Brassica oleracea) AND TOMATOES (Lycopersicum esculentum) FROM HORTICULTURAL FARMS IN NAKURU COUNTY, KENYA

Abstract

Recently field-grown vegetables including curly kales and endive have been reported to accumulate high fluoride levels in their leaves. As vegetables normally form an integral portion of the human diet, the occurrence of high fluoride (F) in their edible portions can cause significant human exposure to these fluoride residues. Nonetheless, chemical residues normally do not occur in isolation under natural environments and their availability is strongly controlled by their chemical and biochemical interactions with co-existing residues. In this respect, heavy metals and other contaminants have therefore been studied extensively especially in recent years. However, the interactions between metal contaminant ions with fluoride in polluted environments remains unclear. In this study levels of fluoride and selected heavy metals in kales and tomatoes from Nakuru County have been investigated and compared with the World Health Organization/Food and Agriculture Organization standards. Kales and tomatoes samples were obtained from selected horticultural farms from all the eight regions in Nakuru County. Fluoride in kales and tomatoes was extracted using a 6M sodium hydroxide solution and determined using the F ion-selective electrode. The levels of heavy metals in the two sets of samples were determined using atomic absorption spectrophotometry. The F and heavy metal measurements were based on calibrations prepared from respective standards in the linear ranges of 0.0–20.0 mg/L and 0.0 – 5.0 mg/L, respectively. The overall mean concentrations of F levels in tomatoes was 12.94 ± 0.35 mg/kg while the overall mean ranges for the selected heavy metals were: 0.23±0.00–0.38±0.06 mg/kg for Cu; 0.18±0.00–0.31±0.02 mg/kg for Zn; 0.69±0.20–0.91±0.01 mg/kg for Cr; 0.08±0.00–0.13±0.01 mg/kg for Mn and 0.71±0.05–0.87±0.04 mg/kg for Fe. In kales, the corresponding average F level was 13.33±0.29 mg/kg while mean concentrations ranges for the selected heavy metals were: 0.05±0.00–0.07±0.02 mg/kg for Cu; 0.20±0.01–0.37±0.03 mg/kg for Zn; 0.80±0.08–1.11±0.04 mg/kg for Cr; 0.10±0.01–0.13±0.01 mg/kg for Mn and 0.30±0.02–0.97±0.1 mg/kg for Fe. Fluoride concentrations in all the samples were above the WHO/FAO maximum allowable limits but those for the metals in the samples were within the recommended limits. There was a significant negative correlation between fluoride concentration with Zn and Cu concentration in tomatoes, whereas in kales significant positive correlation was found between fluoride concentration with Cr ion concentration.