An investigation of chromium and nickel uptake in tomato plants irrigated with treated waste water at the Glen Valley farm, Gaborone, Botswana

Abstract

The use of treated waste water for irrigation of vegetable crops is on the increase in Botswana especially in the Glen Valley farms, a peri-urban settlement of Gaborone city. However, the effects of this practice on heavy metals uptake by vegetable crops are uninvestigated. Chromium and nickel have been reported to be accumulating in Gaborone crop soils and cultivating vegetables in these soils with treated waste water could potentially lead to an increased bio-availability of the heavy metals in the vegetable crops. The main aim of this study was therefore to compare the uptake of chromium and nickel in tomato plants, a vegetable grown in sludge amended Glen Valley soils, to those grown in sludge absent Glen Valley soils using treated waste water at different pH values and tap water for irrigation. The high water uptake and high water consumption rate of tomato plants made it suitable for this study. Twenty five pots each containing 2.5 kg sludge amended Glen Valley soils and 5 pots each containing 2.5 kg sludge absent soils were utilized. Fresh treated waste water in a 50 L plastic container on a need by need basis was used. For the control experiments 5 pots each containing 2.5 kg standard commercial soils and fresh tap water were used. The potted tomato plants were cultivated from early May to middle of October 2009. One leaf and one fruit from each tomato plant was harvested and tested in this study. The highest uptakes of chromium (0.819 mg/L) and nickel (0.327 mg/L) were experienced in the leaves where the tomato plant were cultivated in standard commercial soil and irrigated with tap water at pH 7.0. The least uptake of chromium (0.052 mg/L) and that of nickel (-0.030 mg/L) was found in the fruits, where the tomatoes were grown in sludge amended Glen Valley soil and irrigated with normal Glen Valley treated waste water at pH 8.5. Increasing the pH of the treated waste water from 5.0 to 6.0 caused increased bio-accumulation of chromium and nickel in the leaves and the fruits of the tomato plants. Normal treated waste water (pH 8.5) and treated waste water at pH 9.0, however, reduced the chromium and the nickel uptake by the tomato plants. Treated waste water at pH 10.0 bio-accumulate more chromium and more nickel in the leaves and fruits of tomato plants. The pH variation experiments suggested that the fruit tissues accumulated more chromium and the leaf tissues accumulated more nickel. The mean chromium uptake in the tomato plants exceeded the Food and Agriculture Organization permissible limits but the Botswana Bureau of Standards effluent limit was not exceeded. The mean nickel concentrations were below the threshold limits for both local and international standards. Statistical analysis showed no significant difference between the mean chromium and the mean nickel concentration in the leaves and the fruits of the tomatoes at the 5% significant level. It can be concluded from this study that cultivating tomatoes with sludge amended Glen Valley soil combined with normal treated waste water at pH 8.5 could reduce the uptake of chromium and nickel uptake in tomato plants. However, an increase in the uptake of chromium and nickel in the leaves and fruits of the tomato plants could be triggered at slightly low pH (pH 5.0 and pH 6.0) and high pH (pH 10.0) of the treated waste water. It is recommended that the current practices of using treated waste water combined with sludge amended Glen Valley soil to cultivate tomatoes at the Glen Valley farm is good practice and should be continued. Nonetheless, further studies need to be carried out at the farm to establish possible phytotoxicity effects of these heavy metals on tomatoes when using treated waste water combined with sludge amended and sludge absent soils.