PHYTOREMEDIATION OF POTENTIALLY TOXIC METALS CONTAMINATED AGRICULTURAL SOIL USING PUTATIVE Brassica napus AND Raphanus raphanistrum IN UASIN GISHU COUNTY, KENYA

Abstract

Potentially toxic metals (PTMs), Arsenic (As), Cadmium (Cd), Chromium (Cr), Mercury (Hg), and Lead (Pb) naturally occur in soil within the environment, but human activities such as largescale farming have increased PTMs concentrations in agricultural soils resulting from the use of agrochemicals, sludge, and wastewater irrigation. This leads to PTM contamination of agrarian soil, making such soil a probable source of ecological and health risks. Hence, this study aimed to assess the concentrations and possibility of phytoremediation of PTMs in agricultural soils from selected farms in Kaprobu, Kosyin, Moiben, Naiberi, and Ziwa in Uasin Gishu County, Kenya. Brassica napus (canola) and Raphanus raphanistrum (wild radish) were used in this study. The selected PTM concentrations within the study areas, physicochemical parameters, and in vitro bioaccessibility in agricultural soils were determined using standard methods. In addition, wild Brassicaceae, Brassica napus, and Raphanus raphanistrum were identified and chemically treated for the possibility of phytoremediation of the polluted soils. Field surveys and completely randomized experimental designs were adopted to collect soil and seed samples. Standard lab procedures were applied to determine PTM concentrations in soil samples, physicochemical parameters of soil, in vitro bioaccessibility, seed germination rate, colchicine modification of seeds, and phytoremediation of PTMs in soils. Descriptive statistics, regression analysis, and analysis of variance (ANOVA) were used in analyzing the data, and the results are presented in tables and graphs. The mean concentrations of PTMs in agricultural soils ranged from 2.90 to 6.40 mg/Kg As, 0.06 to 0.13 mg/Kg Cd, 14.31 to 48.19 mg/Kg Cr, and 16.46 to 35.89 mg/Kg Pb, while Hg was not detected (ND). Chromium and Lead had relatively high concentrations across the study areas as Moiben recorded the highest of the two, 48.19 mg/Kg Cr and 35.89 mg/Kg Pb. Physicochemical parameters, pH, organic matter (%OC), Al+3 (Cmol+kg-1), and H+ (Cmol+kg-1) in agricultural soils from Moiben were low. In vitro bioaccessibility measured was low, 0.77% Cr and 11.88% Pb. Raphanus raphanistrum (RR) and Brassica napus (BN) were selected among locally identified Brassicaceae species and their germination rates were tested using germination agents. Gibberellic acid (GA3) gave an efficiency of 80% and 90% for RR and BN, respectively. The seeds were further treated with different concentrations of colchicine to heighten growth and morphological development in possible enhanced phytoremediation of PTMs to agricultural soil. The PTMs assessment results showed that Cr and As were above the USEPA agricultural soil regulatory standards. The assessed ecological risk indices ranged from low to extremely high Geo-Accumulation Factors for all PTMs, and low to moderate Ecological Risk Index. Health risks assessed via work-related exposures to agricultural soils posed no significant carcinogenic and non-carcinogenic risks. A negative correlation was recorded between the physicochemical parameters, soil pH, organic matter, and in vitro bioaccessibility of Pb and Cr. The putative mutant plants, B.napus and R.raphanistrum treated with a 0.50% dose of colchicine had hyperaccumulation potential at M1 and M2 generations for Cr and Pb decontamination. Both plants bioaccumulated high amounts of metals, Cr and Pb that could pose environmental and health risks. This study finding contributes greatly to enhanced phytoremediation techniques in environmental restoration that can be cascaded on different PTMs contaminated fields.