Variation of Pollutant Levels in Vegetables: A Case Study of

Abstract

There is growing public concern over the illegal cultivation of vegetables on soils amended with sewage sludge or irrigated with mixtures of sewage and sewage sludge. Effluents and wastewater from Bidii in Kitale treatment plant are used by vegetable farmers for the irrigation of their vegetables during dry season. These effluents may contain some toxic pollutants which bioaccumulate along the food chain. Moreover the uptake of such toxic metals by vegetables is governed by their availability and concentration in the soil. Therefore such vegetables may accumulate pollutants in excessive amount in their various parts. This may ultimately, adversely affect humans and other species that depend on such crops for food, hence the need to evaluate the variation of pollutant levels in vegetables samples in these areas. Samples of spinach and kales and top soils (0-20) cm were collected from the vegetable farms of Bidii and Taito area as control point. Random sample collections were made five (5) times during the period from the two plots between October to December 2012. The vegetables samples were dried in an oven at 80°C for 72 h. The dry samples were crushed in a mortar  and pestle and the resulting powder digested by weighing 1.0 g of oven-dried ground and sieved (<1 mm) into an acid-washed porcelain crucible and placed in furnace for 4 hours at 500°C. The crucibles were removed and cooled. The weighed vegetable samples were transferred to 300 ml digesting flasks and then 27 ml of a mixture of HNO₃ and concentrated H₂SO₄ were added to the sample. The mixture was heated on a hot plate for about half an hour until brown fumes disappeared. The mixture was cooled, and 3 ml of percloric acid were carefully added and then heating continued until white fumes disappeared. The digested sample was filtered into a 100 ml volumetric flask and made up to volume with de-ionized water. Soil samples were digested using aqua regia and 1.0 g of the soil sample was placed in a flask and 6 ml of concentrated nitric acid (HNO₃) was added first before adding 18 ml of concentrated HCl. The mixture was then heated until a clear digest was obtained. The digest was then cooled and a few drops of water were added before filtering. The filtrate was then diluted with de-ionized water to 100 ml. The concentrations of heavy metals which include, Cu, Zn, Cr, and Pb, were determined using atomic absorption spectrophotometer (AAS). Levels of some anions (nitrate and phosphate were determined using Spectroscan 30 UV-Vis spectrophotometer. The results revealed that Cr and Pb had the highest concentrations, while Cu had the lowest in the leafy vegetables studied. The order of heavy metals was found to be Zn>Pb>Cr>Cu in soils while Pb>Zn>Cr>Cu in vegetables. The concentration of NO₃^- ranged between 2.176 mg/kg to 3.202 mg/kg while PO₄^3- 2.897 mg/kg to 3.342 mg/kg in vegetables. Levels of NO₃^- in soils were 1.88 mg/kg to 2.06 mg/kg while PO₄^3- had 4.66 mg/kg to 4.99 mg/kg. The vegetables from Bidii area contained much higher concentrations than those from Taito area. The levels of Cr and Pb in vegetables exceeded WHO Maximum Limit (0.05 mg/kg for Cr and 0.3 mg/kg for Pb). The variation in the parameters determined were found to be statistically significant (p<0.05) as determined by one way ANOVA.

Key words: copper, lead, chromium, zinc, nitrates, phosphates, polluted soils, vegetables.