Uptake and Accumulation of Selected Potentially Toxic Metals by Wild Plants Raphanus raphanistrum and Brassica napus in Moiben, Kenya

Abstract

Intensive agricultural activities and prolonged use of inorganic fertilizers have raised concerns about soil contamination by potentially toxic metals (PTMs), which can adversely affect human and environmental health. This study aimed to: (1) determine the concentration of selected PTMs and pH levels in agricultural soil samples collected from selected farms in Moiben Sub-County, Kenya, and (2) determine the concentration of lead (Pb) in Raphanus raphanistrum and Brassica napus plants collected from the same area. Soil and plant samples were collected from nine farms using composite sampling and standard preparation protocols. PTM concentrations - specifically Chromium (Cr), Arsenic (As), Selenium (Se), Cadmium (Cd), Mercury (Hg), and Lead (Pb) were analyzed using Atomic Absorption Spectrophotometry (AAS), while soil pH was measured using a calibrated pH meter (1:2.5 soil-to-water ratio). The results revealed the presence of the five analyzed toxic metals (As, Se, Cd, Cr, Pb) in all the study sites except Hg. Mean PTM concentrations (mg/kg) in soil were as follows: Cr ranged from 14.31 to 26.48 mg/kg; As ranged from 2.94 - 7.34 mg/kg; Se ranged from 5.42-7.08 mg/kg; Pb ranged from 16.81-45.72 mg/kg; Cd was generally low, peaking at 0.186 mg/kg, while Hg was not detected in any samples. The concentrations of all metals were within World Health Organization (WHO) limits.Since lead concentration was found to be the highest, uptake and accumulation  of lead was determined in plant samples collected from the study sites, Raphanus raphanistrum (RR) recorded mean Pb of 19.4577 ± 6.104 mg/kg and 14.8183 ± 3.699 mg/kg in Brassica napus (BN) plants. The mean lead concentration in the leaves of the study plants was 6.29 ± 3.37 mg/kg for Raphanus raphanistrum (RR) and 4.76 ± 1.71 mg/kg for Brassica napus (BN). Although PTM levels were within FAO/WHO permissible limits for uncontaminated soils, the relatively high lead accumulation in plant tissues above FAO/WHO limits suggests potential human and environmental health risks. The results emphasize the need for integrated soil monitoring and demonstrate the phytoremediation potential of native wild plants.